State-of-the-Art Lithium-Ion Battery Pretreatment Methods for the Recovery of Critical Metals

Abstract

Today, lithium-ion batteries (LIBs) are widespread and play a vital role in advancing portable electronics (laptops and mobile phones), green energy technology (electrical vehicles), and renewable energy systems. There is about 30% off-spec scrap LIB production during manufacturing. This trend has caused the accumulation of a huge number of spent LIBs. In addition to containing chemicals that are harmful to the environment, these batteries also contain critical metals; their recycling will greatly help to maintain a green and sustainable economic transition. Therefore, this issue has forced researchers to seek cost-effective and eco-friendly strategies for recycling LIBs. The pretreatment of waste batteries is an essential part of LIB recycling. This article aims to comprehensively review the basic structure of LIBS and existing pretreatment methods in recycling critical metals from LIBs, with a special focus on recent innovations. This manuscript has been prepared to help researchers conduct cutting-edge and novel research in LIB pretreatment and recycling. This approach not only helps researchers to understand the concepts, but also helps to identify and evaluate the strengths and weaknesses of different pretreatment methods. Also, in addition to mentioning the existing research limitations, suggestions for future research perspectives and less investigated areas that need further research have been presented.

1. Introduction

1.1. Using Batteries as a Symbol of Sustainable Development

Today, electricity is referred to as a clean and important energy source, especially compared to fossil fuels [1]. The battery is a device consisting of electrochemical cells. It stores electrochemical energy, converts it to electrical energy, and provides portable energy for all kinds of electrical and electronic devices, from mobile phones to electric cars, which is of great importance [2]. Additionally, the consumption of electrical appliances and batteries has also enormously increased over the years [3]. Since the electricity produced by wind and solar power plants (as sources of clean energy) is discontinuous, batteries can solve the existing shortcomings to some extent. As a result, the use of batteries is an essential component in achieving the United Nations’ (UN) sustainable development goals (SDGs), and the use of new energies is increasing rapidly.

On the other hand, to reach a world with zero or near zero CO₂ emissions [4], as far as this goal is concerned, the use of electrical vehicles (EVs) instead of vehicles that run on fossil fuels is considered a necessity as part of the global decarbonization strategy [5]. Many developed economies, including the United States (US), China, and the European Union (EU), have recently planned to replace internal combustion engine (IEC) vehicles with EVs by 2050 [4], which in turn shows the varied role of batteries. Consumers show growing interest in EVs. In 2024, 2025, and 2040, global EV sales are expected to be 8, 10, and 55 million, respectively [6]. Rapid EV sales have also led to increased LIB consumption [7]. EV LIBs are expected to be the primary driver of LIB market growth. In 2025, about 2 million t/y of spent LIBs (S-LIBs) with a market value of USD 10 billion will be available for recycling. From 2035, annually, more than 9 million tons of S-LIBs will be available from EVs and consumer electronics [8].

1.2. Lithium-Ion Batteries (LIBs) and Their Overall Performance

Since the first commercialization of LIBs in 1991 by Sony Corporation, they have been continuously researched for several decades for the development of the future energy market, and major companies have introduced improved examples of them to the market one after the other. Their high energy density, long cycle life, and lightweight characteristics have made these types of batteries increasingly popular and widely used [9]. Although many efforts have been made to mass produce other types of batteries with lower costs (for example, sodium-ion batteries or anode-free sodium metal batteries [10]), LIBs continue to rank among the world’s most significant energy storage technologies [9].

In addition, with the fast development of EVs and energy storage system (ESS) technologies, the demand for LIBs is exponentially depleting cobalt (Co), lithium (Li), and nickel (Ni) resources. The Li content of EVs is growing in terms of more range and power generation. Li consumption has more than doubled in the past decade [11]. The demand for LIBs will increase by 40 times from 2015 to 2030, and the lithium demand will increase by 5.7 times from 2020 to 2030. The recovered Li and Co from recycling is lower than what is still available. According to the Indian Attero Recycling Ptv. Ltd., the world may run out of Li and Co resources during the 2030s if EoL and scrap LIB recycling are neglected [8].

There are different types of LIBs used in the industry today. LiCoO_2, LiMn₂O₄, LiFePO₄, LiNi_1-x-yMn_xCo_yO₂, and Li_0.8Co_0.15Al_0.05O₂ are common types of LIBs used in mobile phones, laptops, camareas, EVs, power tools, medical devices, energy storage, E-buses, E-bikes, and electrical power trains. The classification of LIBs and their components, functions, and application area details will be given in Section 2.

1.3. The Need to Recycle LIBs

Although a large volume of LIBs is still reused as second-life batteries [12], eventually they have to be decommissioned, and thus we end up with a huge amount of LIBs that have reached EoL. Therefore, to realize a circular economy, used batteries should be recycled after reaching the end of their useful lives [2]. Cradle-to-cradle (C2C) design is a sustainable approach that pays special attention to the environment and the lives of future generations [3,13]. C2C design is a regenerative design. This design is completely healthy for the Earth and its inhabitants. All power comes from renewable sources (such as solar energy, wind power, or water currents). Production only uses harmless technical or biological resources, inorganic or synthetic materials that can be reclaimed technically, and organic materials that decompose naturally without any harm to living things. C2C is a biomimetic approach to the design of products or systems that models nature’s processes. In the C2C approach, artificial materials made by man should be used many times without a loss of quality and remain in a continuous cycle [14]. New LIBs should be designed for the C2C approach.

LIBs, as an important component in lots of used equipment, are not exempt from this rule, and researchers’ efforts are focused on achieving the mentioned approach for LIBs. Meanwhile, to show the lifespan of batteries, the concept of the cycle life of batteries should be used, which is the number of charge and discharge cycles that a battery can complete before losing performance. But here, for the sake of simplicity, we use the duration of battery use as a basis for reaching conclusions on their average lifespan. Figure 1 presents the average lifespan of LIBs for different electrical devices.

As mentioned before, it is expected that the LIB market will continue to grow with the increasing demand for EVs and renewable energy systems. However, the industry must address existing resource and sustainability concerns to ensure that it can continue to responsibly and sustainably meet the world’s energy needs [15]. But the question is as follows: Is recycling LIBs economical?

In response to this question, it is necessary to examine the issue based on several aspects.

In terms of mineral resources, S-LIBs are rich in critical metals such as Co, Li, Ni, and Mn [16]. The percentage of critical elements and different compounds that are normally present in LIBs is presented in Figure 2. The Li content changes from 1.4% to 7.0%, Co 1.9% to 20.0%, Ni 5.0% to 15.0%, Cu 5.0% to 10.0%, Mn 1.8% to 19.4%, and Fe and Al. These contents are much higher than their concentrations in natural ores. LCO batteries may be made up of 20% Co, 2.7% Li, 0.2% Ni, and 8% Cu. In a 1 kg LFP battery, there is 7.6 g Li, 61 g Fe, 34 g/P, 38 g Cu, and 13 g stainless steel. In a 1 kg NCA battery, there is 86 g Ni, 16 g Co, 13 g Li, and 2.5 g Al. The NMC battery cathode contains 30 g Co and Ni, 3.6 g Mn, and 14 g Li, per kg of battery [8].

In addition to reducing the raw material shortage, recycling these important components, especially Li, Co, and Ni, in S-LIBs, can have a major positive social and economic impact. Furthermore, if not treated appropriately, the hazardous heavy metals and toxic organic solvents found in S-LIBs could pose major dangers to both human health and the environment [8,9].

But from an environmental point of view, used LIBs can be considered a threat to the environment in many ways [19]. The presence of different materials in the composition of LIBs can be a source of various environmental damages [19,20], but these damages are not limited to this amount. Figure 3 shows some of the possible risks associated with EoL LIBs.

Fire and bursting risks of LIBs via thermal runaway and short circuiting, environmental hazards of volatile organic compounds (VOCs), corrosive compounds, heavy metals, and health risks are critical. There is serious collection, logistical, transportation, technical, economic, regulatory, and legal problems and challenges in LIB recycling. High economic costs and poor operating economics require subsidies for keeping the recycling plants open. However, the environmental benefits and reduction in the imported critical raw material demand are very valuable. The recovery of these valuable metals through recycling will play a decisive role in regulating the market of raw material prices in the battery production industry [18].

Despite the aforementioned factors, huge amounts of S-LIBs have emerged worldwide due to their widespread consumption in electrical and electronic appliances, especially EVs [21]. It is expected that, with the increasing shortage of necessary raw materials, the recycling of S-LIBs will shortly become an essential part of a sustainable approach toward energy storage uses, considering the potential economic benefits and environmental aspects [22].

1.4. The Global Market of LIBs and the Need to Supply Raw Materials for Batteries

The global LIB market was valued at USD 118.2 billion in 2023 and is predicted to grow at a compound annual growth rate (CAGR) of 15.8% from 2023 to 2030, reaching USD 330 billion by 2030. In 2025, 4.5 million t/y of LIBs will be placed in the global market. A total of 70% of these are used in EVs. According to Bloomberg, by 2025 and 2030, 0.7 and 1.6 million tons of EoL battery packs will be available for recycling, respectively. A total of 160,000 tons of Co, Li, Ni, and Mn can be recovered from these wastes. Geographically, China, the EU, and the USA generate the highest LIB waste for second life. China and S. Korea recycle the most tons of LIBs globally. LCO- and LFP-type LIBs contribute the most recycling tonnages. The market value of the second-life batteries will increase by 5.7 times from 2024 to 2030 and reach USD 4 billion. By 2025, 15% of the global LIB capacity will reach EoL. From EoL LIBs, some 50% is recycled. In the LIB recycling market, the near-term drivers are consumer electronics, and in 2030, the long-term drivers will be EVs and energy storage [8,9,18].

The increasing demand for EVs in recent years is one of the factors influencing this increase. However, as the market grows, so do the associated challenges. One of the main challenges is the sources and sustainability of the raw materials used in the production of LIBs, such as Co and Li. Many companies are now focusing on developing new technologies or alternative materials to reduce their dependence on these limited resources [9].

1.5. The Most Important Limitations of the Research: Challenges and Obstacles in Previous Studies

One of the most important limitations of research in the field of the pretreatment and recycling of LIBs is the lack of comprehensive and up-to-date information about the environmental and economic effects of different methods. The existence of a competitive atmosphere among different industrial sectors in this field and efforts to obtain commercial innovation patents are some of the factors in the lack of updated information in this sector. Also, many previous studies have only investigated certain aspects of pretreatment processes in a limited way, and have rarely achieved a comprehensive and systematic approach. In addition, the lack of international standards and common methods in the analysis and evaluation of recycling methods has led to different and sometimes contradictory results in the research. Technological challenges such as the complete and economical recycling of valuable metals from S-LIBs are also important obstacles that require further research and the development of new technologies. Also, the complexity of the chemical and structural composition of LIBs makes it difficult to carry out detailed and complete research. Finally, although large companies have started investing in this field, in general, financial limitations and the lack of sufficient support from some of the related institutions can be some of the main obstacles to the progress of research in this field. Despite all these limitations and to improve the level of knowledge, in this article, a set of techniques has been used to achieve promising results, which means that this research constitutes a contribution towards the solution of this problem.

2. LIB Components and Their Functions

2.1. Different Components of Lithium-Ion Batteries

Figure 4 shows the various components of LIBs.

A brief description of the contents of the various components of LIBs is given below:

➢

Anode (Negative electrode): This includes anode active materials (generally hexagonal layered natural or synthetic graphite) that are placed on the Cu current collector foil [18,25]. Li₄Ti₅O₁₂, Si, Ge, Sn, and LiO₂ (amorphous) were also tested as anode materials. Anodes store and release Li-ions from the cathode, allowing for the passing of the current through an external circuit;

➢

Cathode (Positive electrode): This contains valuable Li-intercalated transition Co, Ni, and Mn metals and Li [18], which are placed on the Al current collector foil [18,23]. In the LMO structure, M and O₂ are strongly bonded to each other, while Li is free to move during charging;

➢

Separators: They act like a fuse and also prevent the free movement of electrons in the cell. Separators prevent contact between the anode and cathode. They are generally made of polypropylene (PP) and polyethylene (PE) [18,19,26]. The separator pore diameter is about 0.60 nm, and the thickness is 20–30 µm;

➢

Electrolyte solution: The lithium salts LiPF₆ and LiBF₆ with organic solvent ethylene carbonate (EC) (C₃H₄O₃) or dimethyl carbonate (DMC) (C₃H₆O₃) solutions are used [18,27];

➢

Adhesives or binder: Cathodic and anode active materials are generally connected to the electrodes via polymeric adhesives and are generally made of PVDF (polyvinylidene fluoride/-(C₂H₂F₂)_n-) and PTFE (poly-tetrafluoroethylene/(C₂F₄)_n)) [18,28];

➢

Conduction enhancers: Carbon black is generally used for increasing the conductance between active materials;

➢

Protective shells and covers (Fe/steel, Al, and plastic, etc.).

LIBs can be manufactured in cylindrical, prismatic, or pouch shapes. There are many types of LIBs using different cathode and anode chemistries. The anode is generally only made of graphite or silicon, but there are further differences in the chemistries used in the cathode, which are the main determinants of the cell’s characteristics [29]. It should be noted that due to the importance of the cathode in LIBs, the division of these batteries is also carried out based on the materials in their cathode, for example, LIBs are divided into several types (Figure 5), the most important of which are LCO, LMO, NCA, NCM, and LFP [3,18,24]. The chemical content, application areas, safety, cost, and performance of different batteries can be compared in Figure 5.

The first and most widely used type of layered transition metal oxide cathode in LIBs is lithium cobalt oxide (LCO/LiCoO₂) [18,32]. Electrolytes also have many different types, but in general, they are used in both solid and liquid forms in LIBs. Figure 6 shows the classification of electrolytes [33].

2.2. How Do Lithium-Ion Batteries Work?

Lithium-ion batteries, like other types of rechargeable electrochemical batteries, consist of two electrodes (an anode and a cathode), a separator, and an electrolyte material.

2.2.1. Cathode (Positive Electrode)

The cathodes are a source of Li-ions (containing an Al-foil current collector and Li transition metal oxide powder (such as LCO, NMC, or LFP, etc.)) and determine the capacity and the average voltage of a battery. The desired characteristics of cathode materials are a high discharge voltage, a high energy capacity, a high power density, a long cycle life, a low self-discharge, an absence of environmentally hazardous elements, being lightweight, and not reacting with electrolytes. Cathode behavior is affected by the preparation method, particle size, morphology, oxygen deficiency, and temperature. Cathode material should allow for the insertion or extraction of a large amount of Li to maximize the cell capacity and should induce few or no structural changes.

Table 1. Properties of the LIB cell type (compiled from [8,9]).

	LiCoO2	LiNiO2	LiMn2O4	LiFePO4	LiNiMnCoO2	LiNiCoAlO2
Theoretical capacity (Ah/kg)	274	274	148	170
Practical capacity (Ah/kg)	120–155	135–180	100–130	100–160
Theoretical energy density (Wh/kg)	570		400	544
Specific energy (Wh/kg)	110–200	100–150	90–140	90–120	150–200	200–260
Operating voltage (Li vs. Li+)	3.6–3.9	3.8	3.7–4.1	3.2–3.4	3.6	3.6
Density (g/cm3)	5.1	4.8	4.2	3.6
Cost (USD/kg)	25	13	0.5	0.23 (lowest)
Cycle	1000		820	300	850	950
Structure	Layered		Spinel/zigzag	Olivine
Generation	1st	1st	2nd	3rd
Rate capacity	Good	Medium	Poor	Poor
Cycle life	Good	Good	Fair	Good	Good
High-temperature property	Good	Good	Poor	Good
Thermal stability	Poor	Very poor	Good	Good
Environment	Toxic	Toxic	Green	Green
Synthesis	Easy	Hard	Hard	Hard
Application area	Consumer electronics Smartphones, laptops, and cameras	EVs	EVs	Power tools, EVs, e-bikes, medical, and hobbyist		Gaining importance in electrical powertrain and grid storage
Co content	55%–60%				10%–33%	10%
Intrinsic value (USD/t)	2946	523	636	312	1684
Charging					Ultra fast
Cathode performance improvement			Mesoporous oxide	30 nm nano-plates
Risk	When damaged or over-heated and fire			No overheating or fire and environment-friendly
Safety	Thermal runaway at 150 °C		Thermal runaway at 250 °C and inherently safe	Inherently safe up to 270 °C, with greater chemical stability	Thermal runaway at 210 °C	Thermal runaway at 150 °C
Self-discharge Range (km) Material required (kg) for production (Everbatt) Energy (MJ) inputs for production	0.377 Li2CO3 + 0.82 Co3O4 Electricity: 21.6			130–300	130–400 0.85 NaOH, 0.246 LiOH, 1.27 NiSO4 + 0.16 CoSO4, 0.16 MnSO4, 012 NH3OH Electricity: 28.8 + Natural gas: 42.6	330–500 0.84 NaOH, 0.25 LiOH, 0.04 Co3O4, 1.29 NiSO4, 0.25 CoSO4, 0.086 AlSO4, 0.35 NH3 Electricity: 28.8+ Natural gas: 42.7
Common brands	Panasonic and Tesla		AESC, EnerDel, GS Yuasa, Hitachi, LG Chem, and Toshiba	BYD, GS Yuasa, Sonnen, EnPhase, Lishem, and Valance	Tesla and LG Chem

summarizes the physical, chemical, environmental, and economic, etc., properties of the LIB cell types. The porous transition metal oxide CAM thickness changes from 65 to 130 µm. Recently, Co-free cathodes have been gaining importance. Over time, the NMC chemistry has evolved to reduce the amount of Mn and Co, but to increase the amount of Ni. The NMC₈₁₁ cathode is composed of a reduced amount of Co (80% Ni, 10% Mn, and 10% Co), which is beneficial as Co is scarce, expensive, and subject to ethical mining concerns. For these reasons, the long-term industry aim for NMC chemistry is to continue to reduce the use of Co with the potential for an NMC_9XX cell in the future.

Triclinic structured materials, spinel-type materials (LMO and LMNO), layer materials (LCO and vanadium oxide (VO)), olivine-structured materials (LFP and LMP), and rhombohedral-structured materials can be used as CAMs. Layered and spinel-structured materials are first-generation, zigzag-layered materials are second-generation, and olivine-structured materials are third generation cathode materials. Next-generation CAMs will include olivine, fluoride, and VO materials for higher power, energy, and safety. Olivine-based LiMPO₄, where M: Mn, Ni, can deliver more Li compared to the conventional LCO material. LiMnPO₄ has a potential of 4.3 V, and LiNiPO₄ has a potential of around 5.5 V. LiMn₂O₄ may be another potential candidate material if the Mn dissolution is suppressed [18].

The cathode reaction is a reduction reaction that consumes electrons. Cathode active material powder bonded on Al foil (12–15 µm) via PVDF binders with organic solvents (wet process) or spraying powder (dry process) at 250 °C and carbon black/nanotube conduction enhancers. Dry or wet coating follows the calendering between rolls to increase the density (70 vol.%) and remove surface imperfections. The following reactions occur in the cathode:

Li → Li⁺ + e⁻    (charge)    oxidation reaction

(1)

Li⁺ + e⁻  → Li  (discharge)  reduction reaction

(2)

The suitability of the powder to be used as a cathode in a battery depends on its chemical and phase composition, micro-structure, morphology, particle size, and degree and type of contamination, making quality control one of the most crucial aspects of manufacturing. These factors affect the electrochemical characteristics of the battery, including the life cycle and energy density, which influence the range of EV LIBs.

2.2.2. Anode (Negative Electrode)

Anodes contain hexagonal layered graphite and a Cu foil current collector. Graphite is the main component of LIBs by mass. The thickness of the porous graphite anode layer changes from 65 to 100 µm. An anode reaction is an oxidation reaction that releases electrons. In the anode, oxidation occurs, and electrons are lost through the external circuit. Anode active material (AAM) powder bonded on 8–15 µm-thick Cu foil via the above binder and conduction enhancers. Requirements for anode materials are a large capacity for Li adsorption, high efficiency of charge/discharge, excellent cyclability, low reactivity against electrolytes, a fast reaction rate, low cost, and being environment-friendly and non-toxic.

Higher energy density cells tend to blend silicon (silicon oxide) into the anode to increase energy performance. Whilst silicon has a very high storage capacity, it expands and contracts dramatically as it charges and discharges, so only small quantities (6%–10%) can be tolerated before the life of the cell is adversely affected. Some companies have begun to experiment with Si-dominant anodes in contrast to the small traces of silica currently used in some anodes. The benefits include a higher storage capacity, but limitations include the risk of cracks in the anode and poor performance, caused by swelling and shrinking when charging and discharging. Significant research, such as by Sila Nanotechnologies, has gone into stabilizing silicon anodes to reduce swelling and prevent cracking. Sila’s titan silicon anode powder consists of micrometre-sized particles of nanostructured silicon and replaces graphite in traditional LIBs. The Sila company, working with Tesla’s US battery supplier, has silicon-based technology that could soon give EVs 500-mile ranges and charge refills in just 10 min [34].

As with the cathode, the anode electrode is manufactured by mixing the active material into a slurry, coating, drying, and calendering. Natural or synthetic carbon-based anodes are the dominant type of anodes used. Today, natural graphite is 1,750 USD/t, synthetic graphite is 15,000 USD/t, and Si-added graphite is 20,000 USD/t [8,18].

Similar reactions shown in the following (Equations (1) and (2)) occur in the anode. Rocking chair mechanisms occur when charging and discharging LIBs. For LCO-type LIBs, the cathode, anode, and overall reactions are as follows:

LiCoO₂ ↔ CoO_1−xCoO₂ + xLi⁺ + xe⁻ (cathode)

(3)

C₆⁺ xLi⁺ + xe⁻ ↔ C₆Li_x (anode)

(4)

LiCoO₂ + C₆ ↔ Li_1−xCoO₂ + C₆Li_x (overall)

(5)

The charged anode has a low potential (V: 0.1V vs. Li). Hence, Li metal can deposit on the graphite surface at a high-rate charge. This can lead to the dendrite formation of Li and safety problems for the cell. Also, graphite layer exfoliation reduces the cycle life of the battery.

Future generation anode materials can be second-generation modified graphite (Sn-coated C; SnCoC composites, etc.); lithiated active carbon (C); non-graphite carbon; tin and silicon alloy; LTO (Li₄Ti₅O₁₂); pure metals/metal oxides (Sn, Si, Sb, and mixed Sn and Sb oxides); ATCO, SnO, SnO₂, SnP₂O₇, VSbO₄ (V_xSb_ySn)O_z, and CaSnO₃, etc.; CoO and NiO; SnO → Sn metal ↔ Li_4.4Sn (alloy); and CoO + Li ↔ Li₂O + Co for higher power, energy, and safety [18].

Charging process: In the LMO structure, M and O₂ are strongly bonded to each other, while Li is free to move during charging. Li ions move from the cathode to the anode and settle in the anode layer, a donor-type graphite intercalating compound (GIC). Li ions flow through the electrolyte. When this process is completed, the Li battery is charged, and electrical energy is stored in it. Charging is a nonspontaneous reaction. The cell volume expands while charging. In the cathode, Co³⁺ oxidizes to Co⁴⁺. In the anode, graphite + xLi⁺ + xe⁻ gives Li_xC₆ (Equation (4)) [8].

Discharge process: Li ions return from the anode to the cathode. The discharging process is spontaneous. When the battery is fully discharged, all the Li ions accumulate around the positive electrode or cathode. The cell volume contracts during discharge. During discharging, Co⁴⁺ reduces to Co³⁺ at the cathode. Li_xC₆ gives the graphite + Li⁺ + e⁻ reaction in the anode [8].

Movement of electron currents: When the Li battery is charged, the movement of opposite electron currents towards the Li ions occurs in the external circuit. According to the movement of these electrons, an electric current is produced. The potential and resistance difference between the electrodes causes the electric current to move. When a Li battery is charged through an external source, ion movement occurs (Figure 7).

2.2.3. Separator

The separator prevents contact and short circuiting between the anode and cathode. Separators are made up of polyolefin polymers such as polyethylene (PE) and polypropylene (PP) microporous membranes, with a porosity diameter of 0.60 µm, porosity of 40% to 43%, and thickness of 20–30 µm. Solid-state ceramic separators are also available today. Separators can be single or tri-layer (PP/PE/PP). In NCA LIBs, there are about 3% separators by weight. Separators are temperature sensitive. PE melts at 120–130 °C and PP melts at 160–170 °C. If PE/PP melts in the cell, short circuiting occurs, which leads to fire and the bursting (at 200–300 °C) of the LIB. Microvast produces polyamide-type separators that resist temperatures over 300 °C, which unlock greater safety in future EV batteries. Next-generation separators will be inflammable types [35].

Figure 7. Performance of lithium-ion batteries during charging and discharging. (a) Cross-section of an LCO-type LIB, (b) cell voltage (V) and discharge capacity (mAh) relations at different ampere currents, and (c) movement of Li⁺ and e⁻ (current) during charge and discharge [36].

Figure 7. Performance of lithium-ion batteries during charging and discharging. (a) Cross-section of an LCO-type LIB, (b) cell voltage (V) and discharge capacity (mAh) relations at different ampere currents, and (c) movement of Li⁺ and e⁻ (current) during charge and discharge [36].

2.2.4. Electrolyte

Electrolytes can be liquid, molten Li salt (LiPF₆ (generates solid electrolyte interface (SEI formation)/dendritic growth)/LiAsF₆ (poisonous)/LiClO₄ (explosive)), an amorphous polymer gel, crystalline polymer solids (PVDF/PVC-PVDF), and ionic liquids (IL). Traditional liquid electrolyte solutions (acid, base, and salt) allow an electrical charge in the form of Li ions to pass between the cathode and the anode. This generally comprises Li salts dissolved in organic carbonate (EC, PC, DMC, and DEC) solvents with small quantities of specialist additives for various purposes including extending cell lifetime. Liquid electrolytes can form a passivation layer instantaneously upon contact with the electrode. This SEI layer consists of insoluble and partially soluble reduction products of electrolyte components. Propylene carbonate (PC) prevents the formation of stable SEI. Electrolyte additives (like DMC, DEC, or EMC) increase bulk electrolytic solution conductivity, modify SEI chemistry, and improve the overcharging of the cell.

Liquid electrolytes: Liquid electrolytes have better conductivity (ethylene carbonate (EC)/dimethyl carbonate (DMC) with 1 M LiPF₆ ~ 10⁻² S/cm), but they are flammable. In terms of ionic conductivity, traditional liquid electrolytes such as those based on LiPF₄ in carbonate solvents exhibited high conductivities of ~ 10⁻²–10⁻³ S/cm at room temperature. Leakage, a narrow range of operation temperature due to the coagulation and evaporation of liquids, deformation, expansion, and exposition upon heating, and SEI formation, which reduces the capacity by 5%–10%, are liquid electrolytes’ main disadvantages. On the other hand, solid electrolytes have a twice-energy density and increased cycle life; are non-flammable; have excellent chemical and physical stability; are non-volatile; perform well as thin film (about 1 µm); and have simple fabrication (thin film processing). However, the low ionic conductivity of solid electrolytes is due to the reduced contact area and interface stress due to electrode charging and discharging. Some electrolyte additives are used for improving the ionic conductance of bulk electrolytes, for SEI chemistry modification, and for improving the overcharging of the cell.

A liquid electrolyte transfers Li ions from one side to another, and this liquid must be a very good conductor of ions and a bad conductor of electrons. The electrolyte, by not passing the electrons, forces them to move from the external circuit to the other side of the battery to transfer energy. For high ionic conductivity, we need the electrolyte to contain Li salts so that Li ions can easily move inside the battery. The movement of ions (through the electrolyte) and electrons (around the external circuit, in the opposite direction) is interdependent, and if one stops, the other will stop. If the ions are stopped due to the complete discharge of the battery, the electrons will also be stopped in the external circuit; therefore, the battery power runs out. Similarly, if you turn off the electric device, the flow of electrons stops, and therefore the battery discharge stops immediately (the energy discharge continues at a very low rate even with the electric device disconnected). Different from simpler batteries, the LIB has an electronic control system or battery management system (BMS) that controls how to charge and discharge energy and prevents overcharging or overdischarging, which may cause Li batteries to explode. Of course, it should be noted that the direction of movement of electrons in the circuit is opposite to the conventional current [8,18].

Liquid electrolytes can have better conductivity but higher flammability, and solid electrolytes have an increased energy density, good safety (non-explosive) and life cycle, simple fabrication as thin films (about 1 µm), and reduced flammability (non-volatile), but relatively lower conductivity due to the reduced contact area. Leakage, a narrow operating temperature due to the coagulation and evaporation of liquid, deformation, expansion, and explosion upon heating are other disadvantages of liquid electrolytes.

Solid electrolytes: Solid electrolytes can be inorganic (crystalline, amorphous, and mixed-phase inorganic materials called superionic conductors, such as sulfur-oxide-based garnet type; phosphate-based and sulfur-based glassy or glass–ceramic type; solid polymer (Li salts (LiClO₄, LiN(CFSO₂), LiCF₃SO₃, or LiBC₄H₈ dissolved in polymers); or composite electrolytes (sub-micron to nanoparticles of inorganic material dispersed in a polymer matrix and ceramic particles (Al₂O₃, TiO₂, and SiO₂) in PE oxide and Li salt) [8,18]. Solid polymer electrolytes (e.g., polyethylene oxide) generally have lower ionic conductivity in the range of 10⁻⁴–10⁻⁶.

A solid electrolyte is preferred if ion conductivity is high. Electrolytes should be inert and have high ionic conductivity, low viscosity, a low melting point, high thermal/chemical stability, and a low cost. They should be eco-friendly and non-toxic. Electrolytes are sensitive to moisture and high temperatures. They are difficult to prepare and purify.

Organic solution-type electrolytes will be replaced by solid polymer electrodes in the future. Promising solid-state electrolyte materials can be inorganic sulfide (Li₂S-P₂S₅(Li₂S-P₂S₅-MS_x); inorganic oxide (Li₇La₃ZrO₁₂/Li_3.3La_0.56 TiO₃)/Li₁₄Zn(GeO₄)₄); and organic polymer polyethylene oxide. Sulfide-type materials have excellent performance and poor safety; oxide-type materials have excellent safety and poor processing; and polymeric materials have excellent processing and poor performance. There are different types of solid-state batteries: all-solid-state, hybrid, and anode-free ones. The main benefit of solid-state batteries is improved energy density (2 to 2.5 times higher than current LIBs at the material level and a 30%–40% benefit at the cell level). Solid-state Li metal batteries have improved safety and a higher theoretical energy density (>400 Wh/kg compared to LIBs (250 Wh/kg)). These batteries can be used in electrified flight applications and are charged faster. Traditional LIBs are mature and relatively easy to manufacture. However, they have a modest energy density and safety concerns. Although all-solid-state batteries are safer, they are hard to make interfaces with and generate a lower energy density. Solid-state Li metal batteries have a 49% gravimetric and 80% volumetric energy density improvement compared to traditional LIBs. Solid electrolytes are suitable for gas sensors, fuel cells, and EV LIBs [8,18]. Solid electrolytes are growing fast as next-generation battery electrolytes because of the high power and energy density they promise, along with excellent safety features. However, they also need to process good electrochemical and mechanical properties for their commercialization. By using inorganic fillers in solid polymer electrolytes, composite organic–inorganic solid electrolytes are obtained, which are effective in drawing a balance between the electrochemical and mechanical properties.

2.3. Shapes of LIBs

Due to the wide variety of batteries, especially in terms of shape and size, which of course are appropriate for their type of use, Figure 8 shows the cross-section of the LIB [33].

Electrodes are rolled in cylindrical types and Z-folded in prismatic and pouch types of LIBs. Cylindrical cells (such as 4680, 18,650, and 21,700, etc.) have a higher mechanical stability, lower cost, and the most common availability. They are mainly used in power tools and EVs. Prismatic cells have a higher mechanical strength, high packing density, and lower energy density. They are generally used in smartphones. Pouch-type cells have low mechanical stability and a high packing density. They are used for drones. Thermal runaway, SEI formation, dendrite formation, and discharge capacity fade are some major LIB failure problems.

2.4. LIBs’ Improvements in the Future

LIBs are still a flagship technology, and there is still a lot of potential for their improvement. Their storage capacity could still be doubled. Companies or researchers are improving new batteries by reducing their charging time; increasing the amount of energy stored for the size and weight; increasing the life span and number of charges; and reducing production costs. The next generation of vehicles will be even more compact and efficient and have a long service life and high energy efficiency. In the future, significant improvements in EV LIBs will include the following [37]:

• Production cost reduction (Now: 130 USD/kWh for cell and 280 USD/kWh for pack; future: 50 USD/kWh for cell and 100 USD/kWh for pack);
• Energy density increase (Now: 700 Wh/L and 250 Wh/kg for cell; future: 1400 Wh/L and 500 Wh/kg for cell), which determines the range of EV;
• Power density increase (Now: 3 kW/Kg for pack; future: 12 kW/kg for pack), which determines the acceleration of EV;
• Safety (future elimination of thermal runaway at pack level to reduce pack complexity);
• First life (Now: 8 years for pack; future: 15 years for pack);
• Temperature range (Now: −20 °C to +60 °C for cell; future: −40 °C to +80 °C for cell);
• Predictivity (future full predictive models for performance and aging of battery);
• Recyclability (Now: 10%–15% for pack; future: 95% for pack);
• Self-healing mechanism of battery electrodes via polymer coating to prevent cracks and battery damage;
• There will be a global LIB recycling race.

2.5. Why Is LIB Processing Challenging?

High-capacity S-LIBs should be released during processing to reduce the risk of fire dangers or the generation of hazardous vapors. The risk of a thermal incident must be regarded as significantly larger than that of regular batteries when they are being stored. EU rules on batteries aim to make batteries sustainable throughout their entire life cycle—from the sourcing of materials to their collection, recycling, and repurposing. In the current energy context, the new rules promote the development of a competitive sustainable battery industry, which will support Europe’s clean energy transition and independence from fuel imports. In September 2006, the EU batteries directive (2006/66/EC) entered into force. Article 11(2) of Regulation (EU) 2023/1542 established derogations from removability and replaceability requirements for portable batteries incorporated in products, amending Directive 2008/98EC and Regulation (EU) 2019/1020 and repealing Directive 2006/66/EC. New battery regulations entered into force on 17 August 2023 [38]. For treatment facilities to process LIBs, the appropriate permissions must be obtained. The disposed-of battery undergoes a pretreatment procedure to separate the Li, Co, Ni, or Mn-containing CAM and graphite-containing anode from the peripheral components (plastic and polymer) because it is difficult to extract Li from the packed battery.

3. Method

3.1. Previous Classification Studies for Pretreatment

There is still no theoretical consensus among researchers regarding the classification of pretreatment methods. It should be noted that everyone has presented their pretreatment flowchart according to the process proposed for the separation of valuable material elements. Previous classification methods for LIB pretreatment are summarized in Figure 9.

3.2. Important Aspects of Methodology, Including Methods of Data Collection and Analysis

The LIB recycling process is divided into two general parts:

• The pretreatment stage.
• The stage of the battery’s active materials’ separation and purification.

The main purpose of the pretreatment process is to safely and effectively separate the various parts of LIBs and prepare the necessary ground material to carry out methods to recover valuable metals more effectively. What is certain is that it is necessary to carry out the pretreatment process in the recycling of LIBs, and in addition to increasing the recycling efficiency, this can also cause a reduction in energy consumption. It should be noted that the proposed pretreatment process, in addition to simplifying the solution treatment and facilitating the solvent extraction (SX) process, greatly reduces the production of disposable waste and improves the recycling rate of valuable metals. In addition, while reducing recycling costs, it prevents the discharge of highly hazardous wastewater [45].

In this study, the methodology is designed according to the different dimensions of the existing pretreatment methods and the new research. The statistical contents of this research include all kinds of articles by different experts, books, and websites of industrial companies in the field of recycling critical metals from used LIBs as part of various industrial and research sources. At first, the collected sources were comprehensively examined, and then a certain number of them were selected for more detailed analysis.

After collecting data from different sources and comparing them, the innovations made were also added to these findings. Also, in the data analysis stage, qualitative and quantitative content analysis methods were used, and the results were presented in the form of flowcharts. This comprehensive methodology has provided the possibility of obtaining accurate and valid results, and has helped to better understand the challenges and opportunities in the recycling process of LIBs comparatively. Also, an attempt has been made to identify the research gaps by using the available literature and evaluating the collected information, especially the innovations made in the field of the pretreatment of LIBs. The methodology of this research study is presented in Figure 10.

4. Results and Discussion

Pretreatment is a set of activities that are necessary to prepare the conditions for the final recycling of critical metals as much as possible [46]. One of the important points in LIB recovery is those elements that are less economically important, and impurities (Cu, Al, Fe, and C, etc.) should be removed from more valuable elements (such as Co, Ni, Li, and Mn) and separated as much as possible in different parts of the recycling process [16].

Since a significant percentage of these valuable elements is in CAMs, separating the cathode from the anode is one of the goals of pretreatment (as much as possible). Of course, this process will be impossible on an industrial scale. After that, separating the CAMs from the current collector can also be important for increasing efficiency. It should be noted that various pretreatment steps change according to the type of process considered for recovery in the industry, which means that some steps can be removed or added to some processes.

With this knowledge, in this research, all the steps that can be carried out in the pretreatment stage and to achieve a black mass (BM) suitable for entering the main recovery process have been discussed for more familiarity, and different steps will be selected from among them depending on the type of overall process. The steps for this purpose are as follows:

• Collecting;
• Sorting;
• Deactivation or discharging;
• Disassembly;
• Separating the electrolyte solution;
• Comminution;
• Particle size classification;
• Separation of AAMs and CAMs from current collectors;
• Screening;
• Separation of AAMs and CAMs from each other;
• Other steps include various other creative methods for pretreatment such as drying, heating, ultrasonic vibration, washing, and flotation-based separations.

4.1. Collecting

To create a suitable process for recovering LIBs, a suitable system for collecting the waste of batteries that have reached useful EoL should be defined first. Examining the environmental aspects and, of course, the commercial–economic aspects is very important [3,18]. The first challenge in this sector is the huge variety of batteries, especially in terms of design, shape, size, capacity, and the chemicals used in them. Premathilake et al. (2023) classified LIBs into three major categories (Figure 11) [3].

Thus, different collection paths should be created for each of these market types:

• Small-scale electrical equipment (SSEE) (household scale): This type of battery is used for portable/consumer electronic devices, has a lifespan of 3–10 years, and can be collected in places established by manufacturers and retailers;
• Stationary energy storage (SES): Fixed grid storage is used for energy storage systems. To collect these types of batteries, specialized and trained personnel are needed to dismantle them from the equipment before collection;
• EVs: Transportation devices such as cars, tracks, e-bikes, and aircraft, etc., use this type of battery. The lifespan of electric cars depends on many factors such as the charging frequency and condition, annual mileage, and battery type, and is considered to be about 8–10 years and 10–12 years considering the second life.

LIB recycling has serious logistic/collection and transportation problems and challenges. Currently, only a small fraction of batteries is properly collected and recycled, and the waste generation rate to waste collection rate is not balanced, resulting in a lot of e-waste ending up in landfills, which can be very dangerous from an environmental perspective, or even stored at home/plants [3]. These conditions may not change shortly, and for this purpose, more efficient planning and systems should be created for the systematic collection of batteries. For example, a system for the accurate identification of batteries from production to recycling or a mandatory plan for replacing batteries could be solutions.

Also, one of the modern methods that seems to have a special place in the future of electronic waste collection, including LIBs, is the intelligentization of e-waste collection using Internet of Things technology and geographic information systems (IoT and GISs). The IoT is a relatively new paradigm that is rapidly growing in modern wireless communication scenarios. The IoT enables different devices to communicate with each other through the internet daily. For this purpose, in the topic of collecting e-waste, especially LIBs, the way of working is that by placing sensors in the waste tanks, the type of waste is analyzed by examining factors such as its weight, volume, and even temperature. The data collected through these sensors will be sent to a platform through a cellular network (GPRS). In the next step, by recognizing the possibility of the presence of e-waste in a certain number of waste collection points, a list of these places is sent to the drivers of e-waste collection to specify their route to transport them to the collection centers. Plan collection and recycling uses a geographic information system (GIS) [47]. Figure 12 summarizes an e-waste collection flowchart using the IoT and GISs.

According to the EU 2023/1542 waste battery regulations [38], EV manufacturers are also the battery manufacturers in charge of the collection, treatment, and recycling of waste or damaged EVs [48], and this issue requires systematic coordination in all sectors, from the production and sale of the product to the collection and sending for the recycling of batteries. Therefore, it can be said that the collection of batteries will not only be a part of the recycling process, but is considered a requirement to achieve a circular economy in the eyes of most organizations.

Also, the urban mining approach uses recycling and circular solutions inspired by principles such as industrial metabolism, industrial ecology, and circular economy, which includes the stages of urban e-waste extraction and the potential of recycling value according to the principles of circular economy, location, and critical raw materials (CRMs) [49].

4.2. Sorting

Since what enters the recycling centers will be a stream of mixed waste from different batteries that must be sorted before any action is taken [50], the basic hand-sorting of used LIBs can help increase the efficiency of recycling the raw materials in these batteries. The concept of this was to increase profitability for recyclers and subsequently increase investment in additional recycling capacity [6]. One of the main reasons for this is that the nature of batteries, including the shape, size, type of chemicals used in them, and of course their use, is different, and for this reason, different techniques should be adopted in their recovery. Since a more uniform input set of batteries is required to achieve more effective and efficient recovery and higher-quality final products, sorting seems to be the first step in this process. Some countries have even developed labeling guidelines for LIBs to track the recovery of batteries [3].

It should be noted that most battery recycling plants are not designed to process different types of S-LIBs. In contrast, distributed or partitioned recycling can overcome the complexity of having different types of batteries with different raw materials. For example, isolated pretreatment plants can perform the initial stages of recycling pathways for all types of batteries [3].

Only laboratory analysis can determine the battery chemistry during the sorting process, which is a crucial pretreatment step. Therefore, proper battery labeling by the manufacturer can be very helpful in sorting [3]. The different criteria for this sorting are as follows:

• Sorting batteries based on their type and shape: Sorting batteries by size and shape can be performed due to size limitations in the processing equipment, for example, furnaces [23];
• Sorting batteries according to their size [50];
• Sorting batteries according to their state of health (SoH): Battery sorting can also be carried out based on their SoH. In this method, before deactivating the battery, information about the LIB current range, residual voltage, capacitance, and state of charge (SoC) should be known to establish a safe deactivation process [23];
• Sorting batteries according to the chemicals in their cathodes [50]: It should be noted that battery chemistry is constantly changing and trying to reconfigure battery storage stability, so for some specific material recycling processes, batteries can be categorized based on chemistry [23].
• LIB sorting can be carried out in two general ways.

4.2.1. Manual Sorting

Due to the heterogeneity of the battery structure and the high flexibility of the manual method, in the industry, manual sorting is usually used [23].

4.2.2. Automatic Sorting

In this method, sorting can be carried out in one step or multiple steps. Achieving a fully automated, one-step sorting process is challenging. Instead, multi-stage sorting can be adopted to increase efficiency. Considering the variety of batteries in size, shape, and the chemicals used in them, automatic sorting is one of the challenges in this sector and still needs research and development. Of course, efforts have been made in this field so far in the industry. Some automatic sorting methods use optical or X-ray sensors (Figure 13) [23].

Also, the presence of decentralized pretreatment plants for S-LIBs can be useful in increasing sorting, unloading, and dismantling. Thus, sorted, pretreated LIBs or battery parts can be assigned to recycling and recovery centers. It should be noted that policies play a critical role as a motivating factor for the recycling of S-LIBs. Establishing appropriate regulations can significantly increase the efficiency of recycling and recovery capacities [3].

However, because of this factor, sorting is compulsory for LIBs before any subsequent steps are performed. According to most researchers, the most appropriate sorting method for S-LIBs is their classification based on battery chemistry. The distinctive sorting of LIBs at the household level is somewhat ambiguous, and high collection rates are difficult to achieve. It also requires an additional workload on the part of the consumer, which is highly unlikely to be achieved in this case. Alternatively, the sorting of LIBs can be performed at recycling facilities in a reverse logistics process [3]. Sorting waste, especially batteries in e-waste, is a tedious and dangerous task. Employees responsible for the manual disposal and sorting of this type of waste are twice as exposed to health damage, and thus, this job is known as one of the most dangerous jobs.

Weigl and Young [6] have used a new method for the dynamic modeling of the LIB resource assessment (LIBRA) system to evaluate the impact of automated battery sorting technology in terms of the proportion of Li, Co, and Ni recovered through recycling. This analysis examines how the selective batching of profitable battery chemicals can enhance the advancement of the domestic LIB recycling industry and the recovery of Co, Li, and Ni for recirculation in newly installed batteries. The findings show that automated sorting has clear advantages over manual sorting methods, as it helps recyclers to process batteries with high critical metals.

Today, researchers are looking for mechanized sorting methods. Near-infrared (NIR) spectroscopy is suitable for polymer detection and is a fast, simple, and non-destructive analysis method that can be applied in an automated online WEEE sorting system. The NIR spectrum can be used in the detection and sorting of some materials, including polypropylene (PP) [52]. In NIR sorting, the recycling material is fed to the NIR sorting system via a conveyor belt. A light source installed above the conveyor belt irradiates the various recyclable materials with infrared light. This physical analysis technique uses infrared rays in the 760–2500 nm range. NIR spectroscopy can detect the specific molecular bonds in the plastic, and then automatically separate plastics by type. The high scan speed, high resolution, and ready amenability to on-line analysis are important advantages of NIR spectroscopy. NIRS, however, cannot be considered a primary method, and in order to function quantitatively, it requires a reference method.

But, with the continuous development of artificial intelligence information technology, under the guidance of cameras and computer systems trained to recognize specific objects, as well as the development of robots on moving conveyors, a promising perspective can be predicted in this sector.

By combining the use of various technologies as sensitive cameras in smart identification, including the identification of dimensions, size, volume, density, and, of course, other characteristics, including temperature and even chemical compounds, it is possible to determine the classification and separation of waste streams (such as used electronics, including batteries that have reached EoL).

Technology companies, including the Japanese company FANUC, have designed a set of automatic recycling technologies for waste-sorting robots, which includes artificial intelligence (a multi-layer neural network) and a sorting system. The visual system is used to obtain the visual information on the item, and the next step is to use artificial intelligence to identify the item. Sorting priority is determined by chemical composition, size, and value to ensure optimal results. After the judgment is completed, the robot can perform the sorting action continuously. It should be noted that since the robots are controlled by software, the mentioned system will be very compatible with the conditions and under control.

4.3. Deactivation or Discharging

The dismantling and transportation of S-LIBs are dangerous; S-LIBs should be discharged at the designated collection sites before transportation. EV LIB packs should especially not be transported over long distances.

4.3.1. Why Should LIBs Be Discharged?

Retired LIBs that have reached EoL cannot be directly recycled due to their serious parameter changes, especially in the capacity, resistance, and state of charge (SoC), which lead to aging, thermal runaway, and even bursting. One of the dangers in the battery recycling process is the presence of volatile and flammable electrolyte solvents inside them, which can ignite and sometimes explode if there is a short circuit between the anode and the cathode [29]. In other words, due to this short circuit and self-ignition, when the anodes are directly in contact with the cathodes, they release a lot of heat quickly and explode [28]. It should be noted that there are some reports of fires in used batteries, both during transportation and during pretreatment (i.e., shredding or crushing) for recovery [23]. The occurrence of fire during the recycling of LIBs damages the materials inside the battery and reduces the value of the critical metals that are recovered through recycling [23]. In the historical research conducted by Yao et al. [53], some of the experiences related to these fires during application have been identified, which are as follows:

• In 2011, the batteries of Chevrolet plug-in hybrids caught fire after a crash test;
• In 2013, the battery of a Tesla Model S electric car caught fire due to an accident;
• In 2016, cases of exploding batteries were reported in Samsung Galaxy Note 7 smartphones;
• In 2018, the luggage rack on a China Southern Airlines flight caught fire due to the explosion of LIBs in a power bank.

There are many reported fire and burst incidents for mobile phone or laptop LIB changing during repair services. All these cases indicate that there is a risk not only during transportation, but also in the process of recycling [53]. This possible fire could not only be dangerous for personnel, but also cause damage to the materials inside the batteries (especially the materials in the cathode), and in this sense, it could seriously disrupt the entire recycling process.

Therefore, it is necessary to prevent the following cases by deactivating LIBs or discharging them:

• Possible risk of self-ignition.
• Short circuiting.

For this purpose, various methods have been provided (Figure 14), such as short circuiting, the use of solid conductors, use of resistances, use of aqueous solutions, use of cryogenic cooling, or thermal methods.

4.3.2. The Types of Battery Discharge Methods

The purpose of discharging is to deactivate the LIB cell so that no violent reactions or severe irreversible damage can occur. There are three types of discharging: physical, chemical, and cryogenic discharging methods [53].

Table 2. Comparison of current discharging methods.

Chemical Discharging	Physical Discharging	Liquid (N2)/Cryogenic (−196 °C) Low-Temperature Deactivation
Immersing in salt solution	Use external electronic loads	Bonding is weakened
Chemical reaction occurs	Fast discharge	Bond is glass and fragile
Popular	No chemicals used	Environmentally friendly
Simple to operate	Process is cumbersome	Expensive
No battery type restriction	Discharge cabinets are used	No dust or gas emissions
Has high capacity	Cu and graphite powder use	No change in the crystal structure
Salt type and concentration	Contact problem	Simple
Environmental emission and discharge efficiency are important	Metal surfaces are readily oxidized	Physicochemical properties are the same
Additional auxiliary reagents are useful	Graphite dust bursting	Glass transition temp. of PVDF is −38 °C
Corrosion occurs	Inductive effect	No gas or dust emissions
Electrolyte leaks and pollution		Efficient
Discharge speed is low
Harmful gas emission (Cl2)
High efficiency

summarizes the properties of discharging methods.

The priority discharge of S-LIBs is decided before further treatments of S-LIBs, and the pretreatment process of discharging has recently gained more attention due to unintentional security incidents in LIB recycling plants [54,55,56,57]. Li ions depart from the anode, move into the electrolyte, cross the separator, and settle in the cathode during discharge. The electrons pass from the anode to the cathode through the external circuit. In the original cathode materials, the Li content was 28.95 mg/g, and in anode materials of undischarged S-LIBs, it was 32.49 mg/g. It will become much higher than in the undischarged batteries after complete discharging in salt solutions of different concentrations. Figure 15 shows the Li content of the cathode and anode at different voltages. The maximum voltage is attained at an 82.5% state of charge (SoC).

Other conductive liquids and solids are still being studied for their discharge and corrosion rates. As opposed to other types of batteries, LIBs frequently burst and burn during the recycling process due to oxidation. This is brought on by the mechanical shock of Li metal that results from the battery overcharging due to air exposure. This is extremely dangerous given the crushing and grinding procedures in use today. There are eight different types of battery discharge.

4.3.3. Battery Discharge via Short Circuit

It should be noted that in the short-circuit method, the battery discharge operation is carried out quickly, which can damage the active materials in the cathode and make them unsuitable for recovery [45]. Combining the short circuit with a cooling device provides energy recovery potential for larger batteries. The use of a cooling device is related to the possible rapid discharge of heat and, of course, the control of the inside pressure, which can lead to leakage and local overheating [23].

4.3.4. Battery Discharge Using Solid Conductors

If we are dealing with a limited number of batteries, using graphite and Cu powders as solid conductors to discharge them can be a solution. However, the discharge of LIBs using graphite is also fast, which may generate a lot of heat quickly, causing the battery to rise in temperature and be sensitive to explosion. Therefore, utmost care and safety should be taken when using this method. Experimental results indicate that discharge in metal powder causes a sharp increase in temperature because heat cannot be conducted as fast as it is released, while discharge in graphite powder has a milder discharge rate. In addition, conductivity decreases as a result of Cu powder’s easy oxidation [23].

4.3.5. Discharging the Battery Electrically Using Dynamic Resistance

An electronic load is usually applied to create a resistance to discharge in the S-LIB. The energy obtained from this method is lost as heat. Of course, modern electronic “recycling” banks are now integrated with an alternating/direct current (AC/DC) inverter, which allows energy to be recovered as AC. Because battery designs vary widely, automation is difficult with this method, and cables are used to manually link the battery and load, posing a serious safety risk to workers [58]. All these cases have made the industrial use of this method uneconomic, except in limited cases. At present, the use of this method is mainly used not for discharge purposes, but for troubleshooting batteries [23].

In an ideal process, the remaining charge would be recycled rather than wasted. However, such recovery is difficult to achieve because it requires removing the battery protection circuit, bypassing broken fuses, and making an electrical connection to recover the stored energy, which seems practically impossible on an industrial scale. It is difficult to determine the state of charge and health of a battery without electrochemical charging and discharging. Evacuation before isolation is common in the literature, but the exact details are often unclear [59]. Additionally, the terms full or full discharge may indicate an open circuit voltage (OCV) anywhere between 3.0 V and 0 V. It should be noted that the discharge of cells to 0 volts leads to the dissolution of Cu in the electrolyte when it is left to rest after discharge. At this time, the voltage increases, and Cu precipitates, causes the distribution of Cu throughout the cell, and unnecessarily contaminates downstream products (such as CAMs) [59].

4.3.6. Electric Discharge Through Inductive Effect

Due to the great attention from tablet and smartphone makers, inductive charging has recently been more popular in research as the inductive discharge process advances. With inductive charging, a coil with a current flowing through it generates a magnetic field, which in turn causes a second coil aligned with the first coil’s magnetic field to induce a current. With inductive charging, an oscillator and a rectifier in resonance are used to magnetically link the two coils via air, removing the requirement for a metal connection between them to transfer the magnetic field. The gadget is charged when the coils reach resonance, because the source current is then induced in the load. The current is stronger when the two coils are closer together. This method requires connecting a gadget to the battery terminals along with a coil and additional converters. A secondary coil, safety sensors, and power electronics are installed in a reception slab at the recycling facility to make sure that the battery is not harmed during the discharging process. With a few notable exceptions, however, wireless power exchange has not yet been widely implemented in EVs [60]. Inductive discharging has the best performance for fire hazard minimization, human interaction minimization, voltage rebound limited to 0.5 V, and sustainability. But the discharge time is not fast compared to metal powder or electronic load discharging [60]. Bidirectional wireless power transfer charging and discharging for the EV industry is a new revolution. There are important developments in static or dynamic charging batteries through inductive effects for EVs. But inductive discharging is in the crawling stage.

4.3.7. Battery Discharge Using Aqueous Solutions

These solutions are widely used to discharge S-LIBs chemically. By immersing the batteries in these conductive solutions, the negative and positive electrodes of the battery are connected [23]. These solutions are generally divided into three categories:

• Salt conductive solutions;
• Acid conductive solutions;
• Alkaline conductive solutions.

The energy within S-LIBs should be released before the shredding and milling processes by immersing them in unsaturated salt water (such as NaCl, KCl, NaNO₃, MnSO₄, FeSO₄, and MgSO₄) [61]. The overall discharge efficiency increases in the following order: MnSO₄ < FeSO₄ < NaCl, according to [62]. This most widely used stabilization technique lessens the anode’s tendency to experience exothermic reactions and short circuiting. For example, the highly flammable organic electrolyte solvents become inflamed due to Li’s reaction to oxygen (from air) and water (from humidity). Accordingly, S-LIBs must undergo an initial discharging treatment prior to hydrometallurgical recycling routes [63]. Using Cu and graphite powder for physical discharge in solid media could be a suitable large-scale discharge technique. FeSO₄ was proposed by [16] to be the most environmentally friendly discharge method.

This method of deactivation with salt solutions is the most reliable and cheapest method of discharging LIBs, despite problems such as the possibility of the corrosion of the battery shell and the leakage of materials into the solution. For this reason, despite its low efficiency, this method is used in the industry most of the time. Researchers are investigating a suitable salt solution with environmental conditions that are less corrosive than those of NaCl. It should be noted that salt solutions are less corrosive than acidic or alkaline solutions, especially for the battery shells. Considering the absorption of energy by the solution, this method is considered to have a very low risk. However, the possibility of the contamination of the solution and the presence of environmental hazards are very high.

Discharge in Salt Solution

Generally, 10% NaCl by weight concentration is considered a common conductive solution for discharging batteries, although sometimes ultrasound or a magnetic field can be used to speed up the discharge process. Also, adding other reagents such as ascorbic acid to the conductive solution helps to speed up the discharge process [23]. In this method, the lithiated anode of charged cells has the potential to react strongly with water if the cell coating is damaged. Because the anode reacts with the chloride ion, the cell coating can be corroded, so the discharge of salt water electrolyzes the water and produces hydrogen and chlorine gases, which can be flammable and toxic [59].

The chemical discharging method has gained predominant attention because of its dominant processing efficiency and tolerable safety [64]. The NaCl electrolyte solution was typically used as a discharging medium during discharging pretreatments. For instance, Li et al. [65] concentrated on the investigation of different parameters’ effects on the variation in the contents of metal, non-metal, and VOCs during the discharging processing using different NaCl dosages, and concluded that a 10 wt% NaCl salt solution concentration is optimal, with a high discharging efficiency and less emissions to the environment. Xiao et al. [62] and Yao et al. [53] compared the discharging processes of different salt solutions (NaCl, KCl, and FeSO₄, etc.) and found that the participation of ions in solutions can facilitate the discharging process, and that S-LIBs can be gently discharged in MnSO₄ solution to prevent serious galvanic reactions and organic leakage, providing a novel alternative to solve the corrosion issues during discharging [53,62].

The use of a 10 wt% NaCl salt solution has the advantages of simplicity and efficiency, but also has the disadvantage of environmentally hazardous and time-consuming operation. One of the points that has attracted the attention of researchers is the leakage of materials inside the battery cell case into the outside due to them being placed in salt solutions to discharge them. A rapid decrease in voltage due to the leakage of the battery cell case can lead to the contamination of the electrodes inside the case by the salt solution and the leakage of the electrolyte and other valuable metals into the salt solution, which in both cases, is one of the disadvantages of this method of battery discharge. However, because of its low costs and convenience, this method is the most widely applied method of discharging batteries in the industry. To reduce the harmful effects of this method, different solutions other than NaCl have been tested, including KCl, NaNO₃, MnSO₄, MgSO₄, FeSO₄, Na₂SO₄, and ZnSO₄. These tests are often performed under stirring, stagnant, with sacrificial metal, or without sacrificial metal conditions.

Fang et al. [64] found that the Zn(Ac)₂ solution holds an optimized discharging performance and corrosion resistance to shells compared to the other solutions via a comparison of the different salt solutions. Namely, 0.8 M salt solutions showed the following for discharging order: Zn(Ac)₂ > FeSO₄ > NaCl > KAc > MnSO₄ for S-LIBs. Zn(Ac)₂ achieved almost 0 V residual voltage in less than 40 min compared to 15 h for NaCl. Toribian et al. [66] studied the discharge of LIBs in 12%–20% NaCl, Na₂S, and MgSO₄ salt solutions via mixing or ultrasonication for safer storage, transportation, and recycling. They obtained the best discharging order as NaCl > Na₂S > MgSO₄. Sediment deposition and electrode degradation constituted the primary issues throughout the battery discharge process. Temperature variations did not produce notable variations. Nevertheless, the application of ultrasonication resulted in extremely favorable outcomes and a discharge period of less than 2 h. This is a result of ultrasonic waves’ ability to solve the sedimentation problem.

Wu et al. [67] developed an extensive assessment and new approach for battery discharge by avoiding thermal runaway during S-LIB recycling. They used inert electrodes and reductant ultrasonic systems for rapid discharge, which avoids cell corrosion. Without an ultrasonic system, they used conventional electrolytes of 0.5 and 1.0 M NaCl, (NH₄)₂SO₄, and MnSO₄ without reaching 1.5 V remaining voltage at 100 h of immersion time. Again, 2.0 M Na₂SO₄ and 0.5 M Na₂SO₄ + 5 g graphite did not reduce the voltage to 1.5 V at 100 h. With the ultrasonic system, 0.5 M NaCl and 1.0 M Na₂SO₄ + 1.0 M Na₂S₂O₃ did not achieve 1.5 V remaining voltage at 8 h. Moreover, 1.0 M Na₂SO₄ + 0.5 M FeSO₄ and 0.5 M NaCl + 0.5 M C₆H₈O₆ achieved less than 1.5 V remaining voltage in less than 6.5 h. NaCl + ascorbic acid achieved 1.5 V remaining voltage at 2.5 h. The ultrasonic system accelerated the movement of electrolytes in the solution, which also increased the discharge current. Thus, the remaining capacity of S-LIBs is released quickly by the reductant and ultrasonic system. Using graphite electrodes to discharge in reductant and ultrasonic systems not only avoids the corrosion of the cell, but also discharges faster and more thoroughly.

The discharging efficiency and corrosion behavior of various Na, K, and ammonium salts were thoroughly examined [68]. It was concluded that mild acidic solutions cannot cause remarkable corrosion, whereas halide salts (Cl⁻, Br⁻, or I⁻) indicate the rapid corrosion of positive polarities. By adjusting various parameters, the aforementioned studies mostly concentrated on increasing the corrosion resistance or discharge efficiency to reduce pollution emissions from S-LIBs. The discharging process, however, makes it impossible to prevent corrosion and pollution emissions, and changes both inside and outside the battery (such as metal ion migration) with theoretical underpinnings have seldom been documented. If soluble Li in the battery cannot be collected, the amount of Li available to the LIB industry will be significantly reduced by the high proportion of Li in the SEI layer that forms on the anode’s surface [69,70]. To comprehend the associated discharging problems, it is essential to elucidate the specific migratory mechanism and evolution destiny of the battery chemistry throughout the discharging process.

The analysis of various metal ion migrations within the battery, galvanic corrosion on the battery’s surface, and chemical development outside the battery is the primary focus of this work. First, S-LIBs were submerged and released into various salt solutions to reach the full discharging state. Various factors were examined to identify the best solution for effective discharging. The evolution fate of battery chemistry throughout the discharging process is then revealed by reviewing the exterior changes in the solute composition caused by organic leakage and galvanic corrosion, as well as the internal migration of metal ions, particularly Li ions, between the cathode and anode. This research is anticipated to provide clarity on the specific modifications both within and outside of the S-LIBs concerning their efficient discharging, with possible guidelines on metal recycling and secondary contamination control.

After fully discharging in the sulfate solutions, the cathode materials of S-LIBs have a larger Li content than in the chloride solutions because of the boosted impact of the corrosion degree on batteries. After fully draining the battery in a 5 wt% CuSO₄ solution, the Li content in the cathode materials is 48.7 mg/g, with a mobility of 60.8%; in contrast, the Li content in the battery’s cathode materials after fully draining in a 10 wt% KCl solution is only 39.00 mg/g, with a mobility of 30.94%. A 5 wt% CuSO₄ solution will take the smallest amount of time to discharge (66 min), while a 15 wt% Fe₂(SO₄)₃ solution would take the longest (420 min). The discharging time is dependent on the concentration of the salt solution. The primary factor influencing the corrosion degree is the pitting impact of Cl; after fully discharging, the corrosion degree in chloride solutions is significantly higher than in sulfate solutions, with a higher amount of total organic carbon (TOC) and more organic pollutants produced. The greatest concentration of TOC (503 mg/L) and transition metals was found in the supernatant in a 10 wt% KCl solution. In comparison to NaCl solutions, the supernatant of a 5 wt% CuSO₄ solution showed less TOC and fewer organic species after discharging, along with an intact surface of disassembled cathode materials. This suggests that rapid discharging in CuSO₄ solution can stop organic electrolyte leakage and speed up the transfer of Li from anode materials to cathode materials. The findings mentioned above corroborate the possibility of achieving a high rate of Li recovery during the discharging process. CuSO₄ solutions can be effective and environmentally friendly discharging media that promote Li migration, increase discharge efficiency, and leave a smaller environmental impact [19].

Figure 16 shows the contents of supernatants and sediments after discharging in different chloride and sulfate solutions. Li and Co did not dissolve during discharging. Only Al and Cu foils are dissolved slightly. The dissolution of Ni, Mn, and P was also negligible in both supernatants and sediments. After the complete oxidization of the cathode cap and safety valve, the voltage decreased rapidly, and the discharging efficiency was in the order of 10 wt% NaCl > 10 wt% KCl > 5 wt% AlCl₃. The discharging times were 60, 75, and 270 min, respectively, for 1.0 V residual voltage. Three CuSO₄ solutions with different concentrations show similar discharging curves with short discharging times. The discharging efficiency of salt solutions is in the order of 15 wt% CuSO₄ > 10 wt% CuSO₄ > 5 wt% CuSO₄ > 15 wt% Fe₂(SO₄)₃. The discharging times were 25, 27, and 75 min, respectively, for 1.0 V remaining voltage. Sulfate solutions achieved shorter discharging times [57].

Sediments of 10 wt% NaCl and 10 wt% KCl solutions had high levels of Fe and Al, suggesting that the galvanic corrosion effects can quickly encourage the formation of hydroxide precipitates after discharging. The minimal electrolyte leakage and lattice transition of Ni from cathode materials during the discharging process are indicated by the low concentrations of Co, Li, Mn, and P and the high concentration of Ni in sediments. The inability to identify the presence of metal ions, particularly Fe and Al, in the supernatant of NaCl and KCl solutions suggests that metal ions are mostly found as sediments. Fe ions are difficult to precipitate after complete discharging in a 5 wt% AlCl₃ solution, due to their low concentration and low solution pH of 3.80.

During discharging in the CuSO₄ medium, the following reactions occur:

Cu²⁺ + 2e⁻  → Cu

(6)

2H₂O − 4e⁻ → O₂ + 4H⁺

(7)

Galvanic corrosion occurs through the following equations:

Fe → Fe²⁺ → Fe³⁺
↓ ↓
Fe(OH)_2(s)  Fe(OH)_{3 (s)}

(8)

Al → Al³⁺ → Al(OH)_3(s)

(9)

Some organic compounds leak from the battery into the medium during the discharge. The severe leakage of electrolytes with high contents of TOC can be discovered in chloride salt solutions. A 5 wt% AlCl₃ solution held the highest TOC of 532 mg/L [57].

Aqueous solution discharging has the worst voltage rebound to 0.5 V, and a slow discharging time. Its sustainability and cost are moderate. Its fire hazards and human interaction are minimal. Metal powder discharging minimizes human interaction, and the fast discharging time and the voltage rebound are limited to 0.5 V; the fire hazards, sustainability, and cost efficiency are moderate. Electronic load discharging requires human interaction. The fire hazards are minimal. The voltage rebound, fast discharge time, sustainability, and cost efficiency are moderate. Inductive discharging minimizes fire hazards, human interaction, and the voltage rebound. The fast discharging time, sustainability, and cost efficiency are moderate [60]. It seems that aqueous discharging is better than metal powders, electronic loads, and inductive discharging. Wu et al. [71] studied the environmental hotspots and GHG reduction potential for different LIB recovery strategies. In pretreatment, heat treatment consumes over 2000 MJ of energy to obtain 1 kg of LCO from S-LIBs, whereas solvent soaking requires 50 and mechanical sonication requires 350 MJ of energy per kg of LCO treated. Solvent soaking consumes the most material.

Discharge in Acidic and Alkaline Solutions or Organic Solvents

Although they have a higher discharge speed, they often cause the Al shell of the battery to corrode. As a result, the electrolyte solution (such as LiPF₆) in the battery leaks from the battery and reacts with water, resulting in toxic HF formation [23].

4.3.8. Battery Discharge via Cryogenic Method

Today, in many LIB recycling companies, despite their high cost and the need for advanced equipment, advanced cryogenic freezing methods are used to discharge batteries. Thus, S-LIBs are usually exposed to very low cryogenic temperatures (−175 °C to −200 °C) by placing them in liquid nitrogen (N₂) to freeze and crystallize the electrolyte solution. The LIB becomes non-conductive, resulting in the temporary deactivation of S-LIBs, which has a similar effect to discharge. The low temperatures used in cryogenic cooling neutralize the Li [23]. Kar [72] compared discharge in a 10% NaCl solution with cryogenic discharge and found that cryogenic discharge was more efficient because of both shorter processing times and reliability. He found that cryogenic discharge and grinding without NMP solvent treatment were found to be the best operations to obtain black mass in the <2 mm size fraction in the lab scale. In the cryogenic freezing method, there is a high level of safety and, of course, the conditions are completely compatible with the environment, but both the initial cost and the operating cost of this method are high, and it is generally performed in the laboratory stage; the use of this method on a plant scale has not been suitable so far [48].

4.3.9. Battery Discharge via Thermal Method

This method is used in a controlled manner to provide the complete deactivation of LIB cells. The Accurec process uses thermal inactivation to discharge batteries. In this method, organic components (electrolyte, plastic, and adhesive) are completely removed via thermal decomposition during vacuum thermal recycling (VTR), and the state of the metal content remains unchanged. Finally, LIBs are discharged during this VTR process via the evaporation of the electrolyte; of course, a percentage of valuable metals is lost during this process and cannot be recovered. In addition, thermal inactivation eliminates the risk of fluorine compounds being released into the atmosphere or the electrolyte reacting with the atmosphere during mechanical pretreatment. At the same time, the process has an advanced particulate filtering system (HEPA), and therefore, no toxic gases or metals are released [23]. However, in addition to the high operating costs, some of the resources are lost due to the loss of graphite and Li. It is not possible to recover the electrolyte in the condenser system [73]. Figure 17 shows the comparison of all LIB discharge methods.

4.3.10. Water Pretreatment

Zhao et al. [69] used water for the precise separation of LIBs without discharging for recycling. The immersion in water of undischarged LIB cells is safe, efficient, profitable, fire-preventing, and oxygen-isolating. Upon immersion, the LiC_x in the anode reacts with water and forms soluble LiOH, and the Cu foil peels off easily. Soluble Li salts and electrolytes dissolved in the water can be processed for Li recovery and emission reduction. A non-destructive mechanical method was developed to separate components of jellyroll cells in water with a single step, avoiding uncontrollable anode reactions and electrolyte burning with only a limited anode Li reaction with water.

4.4. Corrosion of Batteries During Discharge in Solutions

Currently, chemical discharge is one of the most widely used methods to release residual energy from LIBs, and the problem of battery corrosion during discharge in solutions is considered one of the most important issues in deactivating batteries [74]. While battery corrosion in the process of chemical discharge and changes in the ion content in the electrolyte solution during discharge are often ignored, different solutions have very different rates of discharge and corrosion. One of the most important risks of corrosion when discharging batteries with solutions is the leakage of electrolytes into the solution and their reaction with water, which creates the formation of hazardous HF [68].

Wu et al. [74] used a new discharge method to solve the corrosion problem. By creating the NaOH + Na₂SiO₃ system, they formed aluminosilicate cement, which covers the Al shell with Al ions and silicates to prevent further corrosion of the Al shell. With this method, they were able to reduce the content of Al ions to less than 30 ppm. Also, with this technique, they were able to inhibit the corrosion caused by Fe, and they also achieved good results for battery discharge.

Some commonly available non-toxic solutions, namely bicarbonates and hydrogen phosphates, cause very little corrosion in LIBs when discharged. Other very common solvents, including NaCl, cause a lot of corrosion in a few minutes, especially at the end of the positive electrode. Shaw and Stewart et al. [68] tested 26 different ionic solutions with Na⁺, K⁺, and NH₄⁺ cations using constant weight percent concentrations. The results show good discharge for most salts without significant corrosion, while halide salts (Cl⁻, Br⁻, and I⁻) show the rapid corrosion of the positive terminal. Sodium thiosulfate solution (Na₂S₂O₃) also penetrates the cell. However, mild acidic solutions do not seem to cause significant damage to cells. However, the most alkaline solutions (NaOH and K₃PO₄) seem to penetrate the cell with no obvious damage to the battery terminals.

4.5. Reliable Voltage

The experimental results indicate that the complete discharge of the cells to 0 V causes the Cu foil on the anode to be oxidized and dissolved into the electrolyte, and then deposit more on the cathode and contaminate the cathode unnecessarily [23]. Therefore, achieving a suitable voltage that eliminates the risks related to fire and the possible explosion of LIBs, and on the other hand, does not increase the cost or possible damage, has always been discussed in most scientific and practical research in the recycling of LIBs. The experimental results of Wu et al. [67] showed that when the voltage was less than 1.5 V, it was far less than the battery discharge plateau (Figure 18). At this time, the remaining capacity of the spent Li ions is exhausted, which can be safely separated and broken.

4.6. Return Voltage

During battery discharge, we encounter a phenomenon called “voltage return,” which experimental results show occurs after discharging S-LIBs for 48 h. Wang et al. [75] found that when Na₂SO₄ and Na₂CO₃ solutions are used as discharge media, this phenomenon occurs more often. However, the batteries did not show much voltage return after discharge in NaCl solution and sheet graphite. Moreover, the reverse voltage was much lower than the safe isolation voltage. What is obtained from the experimental results is that if the battery is discharged to a safe voltage of 2.0 volts, and the voltage returns after 48 h, this phenomenon is called the voltage relaxation phenomenon in an electrical field, which can cause certain safety risks during battery separation and crushing.

4.7. Analysis of S-LIBs When They Are Disassembled Without Proper Discharge Operations

If LIBs enter the pretreatment process without proper discharge, after the possible short circuiting of the anode and cathode, the temperature will increase sharply and practically lead to thermal runaway. In addition, due to the insufficient discharge of the S-LIBs, Li ions accumulate in the anode in the form of lithiated graphite (Li-GICs). After separation and crushing, highly active Li-GICs react with water vapor in the air and generate a lot of heat, leading to intense thermal runaway [67].

The working principles of LIBs are that during charging, Li ions enter the anode through the separator from the cathode to form Li-GIC, and the anode is in a Li-rich state. The higher the SoC, the higher the degree of Li-GIC, and the more dangerous it is to destroy and break it, and while discharging Li-ions are transferred from the anode to the cathode in the fully discharged state, most of these ions are located at the cathode, so there is little risk of fire or explosion [67].

Experimental results indicate that when the battery is completely discharged, the anode still contains about 7% Li, although the Li content of the anode is proportional to the SoC of the battery and this is for the following two reasons:

• The first reason: During the long-term battery cycle, the anode is prone to the formation of Li dendrites, so this part of Li can hardly be transferred to the cathode during discharge.
• The second reason: The remaining electrolyte is lithium hexafluorophosphate (LiPF₆), which is formed on the surface of the anode [67].

In summary, the principle is always true that the lower the voltage of the S-LIB, the safer the separation, but due to the polarity of the Li battery, it is difficult for the voltage of most S-LIBs to reach 0 V. Long-term discharge not only increases time and cost, but also increases the risk of battery leakage. The battery voltage is the most visual indicator to reflect the battery’s remaining capacity [67].

4.8. Some Innovations in Discharging Batteries

Electrochemical discharge in conductive solutions has been widely investigated and used for discharge pretreatment, but there is no convincing evidence of its effectiveness. Wang et al. [75] proposed the flake graphite discharge method, which is a cleaner and more effective discharge method for S-LIBs [73]. The results of Wang et al. [75]’s study are presented in Figure 17. According to these tests and by examining the results, the following can be concluded:

• MnSO₄, Na₂SO₄, and Na₂CO₃ solutions do not have the necessary effect in discharging batteries;
• The 1.0 M NaCl solution shows an acceptable amount of discharge after about 5 h. But the NaCl solution causes corrosive damage in S-LIBs and produces large amounts of liquid waste and solid deposits;
• Flaky graphite resulted in zero emissions in the battery discharge process. According to the discharge process of batteries, the flaky graphite discharge method was suggested as one of the most effective and compatible discharge methods for S-LIBs [75].

Wu et al. performed LIB discharge in a regenerative ultrasonic system. They showed that in addition to increasing the discharge rate by more than 4 times in this system, graphite electrodes instead of tabs can prevent battery corrosion during discharge. Compared with the traditional electrolyte solution (NaCl), the battery voltage is more easily reduced to 1.5 V, and the total discharge time is shortened by 3 times [67].

Also, in another analysis, Yao et al. [53] evaluated different solutions along with conductive metal powder according to their discharge efficiency and stability, and then the gaseous, liquid, and solid pollutants resulting from the discharge process were also qualitatively and quantitatively analyzed. Among the tested solutions, the FeSO₄ solution had the most compatible conditions with the environment among the NaCl and MnSO₄ solutions. They designed and tested various models on a large scale, which resulted in a fast discharge model using the FeSO₄ solution, reducing the residual voltage to 1.0 V within 125 min. Meanwhile, in the full discharge model using the NaCl and FeSO₄ solutions, the residual voltage drops to 0.5 V or less in 183 min. In addition, the results of their tests are shown in Figure 19.

Based on the characteristics of the tested solutions, two types of large-scale discharge models were designed:

• The model fast discharges to 1.0 V voltage, which takes the least time to maintain the relative safety of the LIB for the isolation process.
• Full discharge model, which is for discharging LIBs to less than 0.5 V, which releases almost all the residual voltage.
• For this purpose, Yao et al. [53] designed a semi-closed discharge device with high efficiency (Figure 20). The device works in the following ways:
• The battery moves slowly with the conveyor belt and is immersed in the discharge solution inside the discharge device. (The speed of the conveyor belt allows each battery to remain in the solution for 125 min);
• A sediment catcher moves opposite to the batteries and pushes the sediment into the collecting funnel on the left;
• As the composition of the drain solution changes during the draining process, the device is drained through the drain outlet and refilled regularly through the water inlet;
• When the battery leaves the discharge solution, it enters the drip zone. In the drip zone, the solution on the surface of the battery can flow down through the holes in the belt;
• The air flows into the gap between the conveyor and the discharge device and forms a stable air flow [53];
• Then, the battery is dried through a process;
• The toxic gas is removed from the exhaust device by the gas collection hood.
• The following results are reported:
• Compared to the full discharge model, the quick discharge model is easier to use and less expensive because there is only one discharge device and no gas purification is required;
• Although physical methods have high discharge efficiency, they are not stable enough for large-scale plant applications;
• The active discharge time at 0.8 mol/L is 5 min for FeSO₄ and 30 min for NaCl, but the final voltage of FeSO₄ is slightly greater than that of NaCl [53];
• The 0.8 M NaCl solution and 0.8 mole/L FeSO₄ solution are the best chemical discharge solutions;
• The FeSO₄ solution was more environmentally friendly compared to the NaCl solution because of corrosion and leakage problems;
• The fast model reduces the residual voltage to 1.0 V in 125 min, and the full model reduces the residual voltage to 0.5 V in 183 min [53].

4.9. Disassembly

After discharging the batteries, the disassembly stage separates the battery components from each other. Dismantling removes the modules, cells, casing, glue, binder, and wire. This separation can be performed in three ways (Figure 21) [76]: automated, semi-automated, and manual disassembling.

The manual dismantling has a low speed but clean separation of electrolyte materials. During dismantling, VOCs may pose a threat to health and safety. Therefore, dismantling can be performed in an aqueous medium to reduce exhaust gas emissions. The automatic dismantling method is not mature yet [77]. For laboratory experiments, individual battery cells are frequently disassembled by hand; however, manual disassembly is costly for industrial processing. For industrial-scale operations, automated or robotic disassembly can be developed, though it is difficult due to the wide range of battery cell designs. As a result, the majority of processing flowsheets suggested for small LIBs entail shredding, followed by the separation of various components to enable material recovery. Typically, 212 µm of sieving is used to separate the Cu and Al foils containing the electrode active materials. Some of the active material might still adhere to the foils, though. As a result, 38 mm screening outperforms the Cu and Al foils in terms of Co and Li liberation and extraction. The organics’ evaporation step eliminates the binder and lowers the amount of active material adhered to the foils. It should be noted that in some cases, the disassembling step may be omitted according to the type of process considered for the purification step; there is no need to separate the different components of the battery from each other. The reason for this is that in the industry, the cost of separation is very high, and in addition, it is time-consuming and difficult. But what is certain is that the existence of this stage can have a significant effect on the overall efficiency of S-LIB recycling. Z-folded electrode-separator compounds are the automatic disassembly technology that Li et al. [50] proposed for the recycling of pouch-type LIBs. Without using any destructive forces, the cathode and anode sheets were extracted using the suggested electrode sorting strategy. Using specialized tool sets, the anode and cathode sheets attached on opposite sides of the Z-folded separators were scraped off by automatically stretching and feeding the separators. The Battery Identity Global Passport idea supports Z-folded approaches as well as automated dissembling, aiding battery identification through radio frequency identification (RFID) tags and quick response (QR) codes.

4.9.1. Manual Disassembly

At this stage, the different components of the battery are manually separated from each other so that the materials of the electrodes are exposed. It is expected that the different parts of the batteries are separated into four groups:

• Metal/plastic cover of the battery;
• The anode, including anode active materials (AAM) along with a Cu current collector;
• The cathode, including cathode active material (CAM) along with an Al current collector;
• Other parts of the battery, including the separator and electrolyte, etc.
• The manual disassembly method of batteries has some advantages and disadvantages:

Advantages:

• The removal of plastics and extra materials.
• Increasing the efficiency of the purification department.
• Disadvantages:
• Low efficiency;
• Low safety of the work environment, especially for employees, and the need to use personal protective equipment (PPE);
• The need for a lot of manpower;
• High cost, especially in areas with high manpower costs.

What does not seem far-fetched is the release of volatile substances inside the battery after opening its insulation pack. As an example, some researchers, including Li et al. [78] investigated volatile compounds during the disassembly of LIBs and identified two predominant VOCs, which were dimethyl carbonate and tent amyl benzene. Although efforts have been made by researchers to recover these volatile materials during disassembly, it seems that no favorable results have been obtained in this field. Moreover, due to the use of human power in the manual disassembly of batteries, the issue of safety remains one of the main challenges of this method.

4.9.2. Semi-Automated Disassembly

In this method, a combination of humans and machines is used to separate the cells. Cell separation is mainly manual, and the cells are opened using cutting tools to release the internal components of the cell, subsequently classified by human power in the desired sections such as the anode, cathode, and separator. This method is often used to open the cell for failure analysis and the determination of the failure mechanisms [59]. But it is possible to achieve higher speeds in this matter by adopting measures and by combining manual and automatic methods. For this purpose, robots usually perform superficial, repetitive, and time-consuming tasks, while human labor works on more difficult and complex activities.

4.9.3. Automated Disassembly

Due to the complexities of placing the different components of batteries together, as well as the many different types of batteries, battery disassembly is a very complicated task in a completely automatic way. Although many companies are trying to find special methods in this field, including the use of intelligent robots, it seems that ideal results for the mechanization of separation have not been achieved so far [79].

4.9.4. Some Innovations in Disassembly

Li et al. [50] succeeded in using the automatic disassembly methodology for the punch type of LIBs, which were named Z-folded Electrode Separator Compounds. In this automatic disassembly strategy, the cathode and anode electrodes can be extracted without applying destructive forces. By automatically stretching and feeding the Z-folded separators, the anode and cathode sheets attached to the opposite side of the separators are scratched using specialized toolsets. In this method, special handling tools were designed and assembled in a prototype automatic parts separation system that consisted of three modules. Their experiments showed that the main components of bagged LIBs can be separated by using this strategy and maintaining complete integrity by using the created cells. In this method, different components of the battery are separated in the form of anode sheets, cathode sheets, separators, and Al polymer film housing. Figure 22 shows how this separation works [50].

As the stock of EVs is increasing rapidly, there is urgent need for a large-scale automated approach to disassembling battery systems after vehicles’ operational life. For this reason, some countries are working on large-scale robot prototypes. In the future, flexible robotic cells for the fully automated disassembly of battery modules from battery systems will be developed. Novel tools and processes will be used for battery diagnoses, machine learning-based object recognition, loosening and removing fasteners, opening sealing, gripping components, separating cables and plugs, and removing the battery modules. Future work will focus on improving the process technologies for cycle time and robustness [18].

4.10. Separating the Electrolyte Solution

When the battery enters the disassembly stage, there is generally more attention paid to the recycling of valuable metals, and rarely to the collection of the electrolyte. In this process, many of its organic components evaporate in the air, which can be a serious threat to human health and the environment.

Therefore, electrolyte recycling is fully compatible with the environment [80]. It should be noted that the electrolyte is one of the most harmful and toxic substances in LIBs, which produces toxic substances such as HF in the event of contact with water. Therefore, some researchers are looking for ways to recover or remove the electrolyte in the early stages to provide more favorable conditions for the recovery of other valuable elements in LIBs. There are three electrolyte recovery methods (Figure 23): solvent extraction, supercritical CO₂ extraction, and low-temperature volatilization.

4.10.1. Solvent Extraction (SX)

Extraction via SX is one of the most promising methods for recycling organic electrolytes (DMC, DEC, and EC, etc.) in S-LIBs. The aged electrolyte is always dispersed into the pores of the electrode materials and separator. To extract the electrolyte, the electrode and separator parts are immersed in proper organic solvents, enabling the electrolyte to transfer into the organic solvent. After dissolving, the electrolyte is separated from the extraction solvents via distillation based on its different boiling point. However, there were too many impurities in the reclaimed electrolyte, making it impossible to reuse these electrolytes in new LIBs [8,18].

SX is a technique related to the initial stages of the recycling process, and its effect is not high. Aged electrolytes inside S-LIBs consist of volatile organic solvents and toxic Li salts, which can cause severe environmental pollution and safety issues without proper treatment. With organic SX, the aged electrolyte could be reclaimed from S-LIBs, which helped to increase the recycling value of S-LIBs and avoid the secondary pollution of the environment and human health caused by the decomposition of electrolytes. However, the yield of organic SX is relatively low, and a large amount of organic solvents will be consumed, which might arise in new emissions of organic waste from extraction solvents. In addition, the reclaimed electrolyte always contains a small amount of organic impurities from extraction solvents, leading to the purification costs and low performance of reclaimed electrolytes when reused in new LIBs [8,18].

4.10.2. Supercritical CO₂ Extraction

In this technique, supercritical CO₂ fluid is used as an extractor to separate the electrolyte from the electrodes. The ability of supercritical CO₂ to extract organic components is strong, and of course, the used CO₂ gas can be reused. The procedure has four stages:

• First, CO₂ enters the supercritical state by modifying its temperature and pressure in the supercritical reactor [80];
• The electrolyte of the LIB is dissolved in the supercritical fluid;
• Lastly, depressurization is used to separate the electrolyte and CO₂;
• Water and HF are removed from the waste electrolyte using weak alkaline anion exchange resins and molecular sieves to utilize recycled goods.

However, it should be noted that the removal of rare impurities such as acid-bound or fluorinated phosphates from the recycled electrolyte in this method was not satisfactory, so more thorough research should be conducted on its reuse (Figure 24).

4.10.3. Low-Temperature Volatilization (LTV)

In this method, electrolyte recovery is carried out using evaporation. As an example, Zhang et al. [81] subjected crushed LIBs to evaporation at a temperature of 120 °C for 150 min to remove and even recycle electrolytes. LTV resulted in the recovery of 99.32% of organic electrolytes and 99.93% of LiPF₆ through evaporation.

4.11. Comminution

Unlike the previous steps, comminution is completely mechanized and is conducted using specialized equipment. The noteworthy point is that regardless of whether the disassembly stage has been completed or not, the various components of the battery must be shredded to the desired size to be able to enter the next stages. It should be noted that the particle size distribution (PSD) and the liberation degree of the crushed product also have a direct effect on the subsequent recovery [80].

Most cell-breaking techniques appear to be continuous and generally involve rotary crushers. Various terms are used to describe these approaches, including hammer crushing, wet crushing, shear crushing, impact crushing, and cutting milling, all of which are rotational processes. In this sector, they produce materials of different sizes and shapes, which strongly influence downstream separation techniques. Therefore, before attempting a direct comparison of separation techniques in the literature, there is a need to distinguish the key parameters in crushing [59]. It should be noted that the purpose of creating a reduction is to achieve black mass enriched in metal content. Effective crushing operations ensure the following:

• Next separation stages and selective liberation of CAMs.
• Size reduction is carried out using a series of techniques.

4.11.1. Shredding

Shredding is widely used in the disposal of waste electrical equipment (WEEE) and batteries. Shredding is usually associated with high torque, and of course, the rotation of the blades is performed at a low speed so that the shredding operation can be carried out [8,18,59]. One- or two-shaft shredders are generally used for comminution. Shredding in a closed circuit with an inert N₂(g) medium is preferred.

4.11.2. Milling

Hammer mills have been used in various studies, especially publications that focus specifically on the industry, but the use of hammer mills for primary cell breakage and crushing in lab-scale studies is uncommon.

4.11.3. High-Shear Mixing

Recently, some studies have used a laboratory-scale mixer to separate electrode coatings from electrodes [59]. High shear mixing provides a simple lab-scale approach to separating electrodes from coatings.

4.12. Wet and Dry Grinding

LIB crushing operations can be performed by either wet or dry grinding.

4.12.1. Wet Grinding

Although wet milling more effectively reduces toxic gas emissions due to the erosive effect of water flow, it reduces each of the components in the S-LIB, including graphite, to fine particles. As a result, small products are obtained from this process, which includes all parts of the battery and complicates the recovery process. Therefore, in choosing this method, all aspects of the work should be carefully considered [80]. Wet grinding generally consumes 30% less energy than dry grinding and prevents dust generation. Wet ground material can be sieved more efficiently from screens than dry ground material. If we use an aqueous solution for discharging, it is more convenient to use the wet grinding of S-LIBs without the drying process. Wet grinding prevents over-crushing and results in the caking of fine materials, while with dry grinding, it is slower to achieve the same level of liberation between foils and coatings, but results in fewer Cu, Al, and polymer materials in the fine fractions.

4.12.2. Dry Grinding

In this type of grinding operation, reaching the same level of release between foils and coatings is carried out at a slower speed, but it leads to the reduction in Cu, Al, and polymer materials in small parts, which can increase efficiency [59]. Dry grinding can also provide selective grinding characteristics to improve the purity of the active ingredient and facilitate subsequent purification and regeneration. It should be noted that dry crushing in an inert gas (N₂) atmosphere can greatly improve purification efficiency [80]. Various laboratory and industrial studies have been conducted to devise a suitable method, some of which will be mentioned below.

On a laboratory scale, Zhong et al. [1], during their crushing experiment, claimed that it is possible to safely and cleanly crush S-LIBs even without discharging them using a refitted shear crusher. In this research, the remaining electric energy in the batteries was completely and safely purified via evaporation at a temperature lower than 120 °C. The XRD results showed that the cathodic and anodic active materials were not destroyed during this evaporation. Meanwhile, the XPS results in this research showed that a small amount of harmful electrolytes remained in the solid materials.

One of the examples that performs the crushing operation of LIBs on an industrial scale is the hydrometallurgical Batenus Process. The characteristics of this crushing center are as follows:

• The crushing is performed in an inert atmosphere of nitrogen;
• The first step of crushing is conducted with a low-speed rotary crusher;
• The second crushing is accomplished with a high-speed impact crusher;
• The hammer crushing, coupled with a two-blade rotor crusher, maximized the recovery of the black mass.

4.13. Some Innovations in the Crushing of S-LIBs Without Discharge

Zhong et al. [1] developed a method to crush S-LIBs without discharging them using a shear crusher installed inside the equipment, which reduces the generated heat and chemical reactions during the process. Firstly, electrolytes were evaporated at 120 °C for 150 min. In this method, the residual electrical energy in crushed LIBs, 99.32% of harmful organic electrolytes, and 99.93% of toxic LiPF₆ were safely treated. Figure 25 shows the schematic diagram of the installed crusher.

Anode plates, cathode plates, separators, and battery shells in S-LIBs are completely separated in the high-speed shredding process. The temperature of the chopped LIBs with high voltage and low voltage is about 29.5 °C and 24.3 °C, respectively. The crushed products’ temperature is too low for them to catch fire, and therefore, the retrofitted crusher is safe and efficient to crush S-LIBs without discharging them. No explosions or fires have been reported during the crushing process, which may be due to the following:

• A significant amount of nitrogen gas in the shredding chamber can slow down the rate of exothermic chemical reactions among shredded LIBs;
• The rapid movements between the crushed LIBs and the nitrogen gas cause the heat generated during comminution to dissipate quickly;
• The contact between the anode plates and the cathode plates is continuously reduced due to the rapid rotation of the rotor and N₂, which is useful for reducing the short circuit between them, resulting in less heat generation;
• Finally, the electrolyte pump causes the electrolytes produced during crushing to quickly leave the crushing chamber, which can reduce the chemical reactions on the electrolytes. They showed that the lower and upper valves, the inlet nitrogen gases, and the other repairs made to the crusher ensure that the crushing of S-LIBs is safe and clean. In this method, dangerous substances can be purified via an evaporation process at low temperatures. One of the features of this method is the possibility of using it in industry [1].

Wuschke et al. [82] also conducted investigations on the mechanical crushing of LIB cells and their separated solid components. They suggested that it is necessary to discharge the batteries to 0% SoC to avoid the risk of fire and explosion during crushing. They suggested the specific mechanical stress energy required to release the housing, separator foil, anode foil, and cathode foil was 4.5 kWh/t. They also stated that the crushing process should be adjusted in such a way as to avoid over-grinding and wasting valuable components by turning them into fine powders.

Additionally, Liu et al. [83] suggested a novel, highly selective cryogenic grinding technique to enhance electrode material liberation. Testing was carried out on the low-temperature + properties of both the present collector materials and the conventional binder material, PVDF. The findings indicate that the binder, PVDF, has a glassy transition temperature of roughly −38 °C. The binder is easily crushed at lower temperatures because it would transition from a highly elastic condition to a brittle state. Conversely, as the temperature drops, the impact strength of the current collector materials rises. Electrode plate grinding at low temperatures would therefore be more selective.

4.14. Physical Separation Methods

After comminution, different mineral processing separation methods can be used. Sieving/particle size separation can be used for coarse (plastics, Al, and Cu foils) and fine-sized material (electrode materials) separation. Gravity/density separation is used for separating heavy metals from light plastic materials/organic components. Vibrating screens, air separation, and heavy media separation can also be employed. Magnetic separation removes steel cases/frames and other ferrous metals from non-ferrous metals (Cu and Al, etc.). An electrostatic separator is effective for separating metals from non-metals (polymer) fractions. Eddy current separation separates non-metallic polymers from non-ferrous metals (Cu and Al). Reverse froth flotation is employed for hydrophobic anodic graphite separation from the hydrophilic cathode material. In reverse flotation, gang material (in this case (AAM)) floats easily and CAM sinks. Details of flotation separation are given in Section 4.20. After separation, a black mass is produced for direct or indirect LIB recycling.

4.14.1. Particle Size Classification/Separation (Sieving)

This step will be taken to achieve PSD. At this stage, the different components of the battery, whether separated from each other or mixed with the same sizes, are prepared for the next stage of separation. The output of this stage is called black mass [69]. The products of this separation include plastics, steel coatings, Al/Cu current collector foils, a BM, which usually contains materials from the positive and negative electrodes, graphite, carbon black, PVDF, metal oxides, and some Cu and Al flow collectors. BM separation processes, which are referred to as metallurgical or refining stages, are generally more complicated and therefore associated with larger amounts of pollutants [59]. But at this stage, we will talk about pre-purification, and thus, the separations performed in this stage after the comminution operation are generally physical and are as follows [80].

Primary separation is performed using the difference in the PSD of each component. For this purpose, a sieve is usually used as a primary separation device after crushing [80]. The black mass is separated through this process and is mostly composed of electrode-coating materials, graphite, and metal oxides such as Ni, Mn, and Li. Cu and Al are also found inside this mass, and their concentrations will be different depending on the crushing conditions. Current collectors should be removed from the slag before hydrometallurgical extraction to maximize the cost-effectiveness [59].

4.14.2. Magnetic Separation

The purpose of this separation is to separate the cathode materials, including Fe and Co, from non-magnetic materials such as plastic using magnetic techniques. Steel coatings are generally removed via magnetic separation. When the BM is converted into a slurry and subjected to wet magnetic separation steps of different intensities, several streams of the active material can be produced depending on the different magnetic susceptibilities of the component’s active materials and the different solvents in the slurry [59]. It should be noted that the use of this method for the direct recycling of cathode materials has not been proven, because the different ratios of non-metallic compounds are not easily separated. Some researchers have investigated the use of wet magnetic separation on a mixture of pristine cathode materials and have reported promising results [59].

As mentioned, waste LIBs contain both strategic metals, such as Co, Ni, Mn, and Li, and impurity elements, such as Al, Cu, and Fe, although there has been research on the use of techniques to reduce the use of expensive reductants such as hydrogen peroxide or ascorbic acid in the presence of these impurity elements (a method based on the effect of the presence of Fe as a catalyst on the reducing behavior of both Al and Cu, which ultimately leads to an increase in the leaching efficiency of Li and Co). However, in most methods, the removal of these impurities will be considered at various stages of pretreatment [84].

4.14.3. Density and Pneumatic Separation

Density separation is generally used to separate low-density plastics and papers from other high-density metallic parts of the mixture. This can be achieved using shaking tables, vibrating plates, a medium-density fluid, or air separation [18,59]. The separation of metal foils from the crushed product is achieved with pneumatic separation technology. The use of an intermediate-density carrier fluid, such as water, can be used alone or with a hydrocyclone to separate the lighter components from the heavier components. This method is successful in removing plastic from electrode materials and has also been reported for the separation of Al and Cu current collector foils. Sommerville et al. [59] described and suggested density separation in the form of air classification as a better method for the separation of cathodes and anodes, as well as Cu and Al.

4.14.4. Electrostatic Separation

An electrostatic separator effectively separates metals from non-metal (polymer) fractions. The separation of conductive materials (Cu and Al) is carried out with the electrostatic separation technique (when the particles are completely dry) [18].

4.14.5. Eddy Current Separation

Bi et al. [85] developed a new and eco-friendly technology for the separation of non-ferrous metal particles whose sizes range from 2 mm to 10 mm, and which can be used to separate Cu, Al, and plastic with coarse particle sizes. Eddy current separators can be used to separate ferrous metals, non-ferrous metals, and Al from Cu foils. However, to date, there is no specific case showing that these processes are used on an industrial scale for the separation of LIB components.

4.14.6. Gravity Separation

Gravity separation is chosen based on the density difference in materials [18]. It is worth noting that the effect of the above separation method is not sufficient for fine powders with very small particle sizes. Gravity/density separation is used for separating heavy metals from light plastic materials/organic components. Vibrating screens, air separation, and heavy media separation can be employed. Figure 26 shows two types of particle size classification/separation methods used in the recovery of LIBs.

4.15. Separation of Cathode and Anode Active Materials from Current Collectors

If it is assumed that the cathode and anode have been completely separated up to this stage of the pretreatment process, then in this stage of separation, the CAMs and AAMs should also be separated from the current collectors. As mentioned earlier, the CAMs and AAMs are connected to the current collectors (Cu in the anode and Al in the cathode) by the binder. Binder chemicals can reduce the effectiveness of subsequent washing processes and must be removed from the BM composition in some way [86]. Therefore, the removal of binders such as polyvinylidene fluoride (PVDF), which are used to adhere the cathode and anode powders to the current collectors, should be seriously considered. Meanwhile, other adhesives such as acetylene black (AB), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are also used. It should be noted that CMC is soluble in water and allows for the much easier separation of electrode coatings and foils; however, SBR is used as an emulsion in the manufacturing process and still needs to be removed from the BM after CMC removal [59]. For this purpose, the bond between the binders must be broken so that the active materials of the anode and cathode are separated from the current collectors. Despite the long charging and discharging cycles, the graphite structure of the anode is not destroyed, making it possible to recycle the spent material. According to several published research papers, the used anode material may be regenerated to be used again in batteries. The regeneration process removes AB, SBR, and CMC through heat treatment. The anode material is coated with pyrolytic carbon from phenolic resin. The technical indexes of the regenerated anode are higher than those of the same type of midrange graphite. Partial technical indexes are close to the unused graphite material, fully meeting the reuse criteria for LIBs. EcoGraf has achieved >99.95% purity carbon through its recycling program to recover graphite anode material to be able to reuse it in LIBs in EVs. The acid-treated graphite can be used directly in sodium-ion batteries (SIBs). Otherwise, structural reconstruction is necessary to be able to use graphite as an anode material in LIBs [8,18].

On the industrial level, Nouveau Monde Graphite Inc. in Canada and Lithium Recycling formed a collaboration agreement to recover recycled graphite for reuse as an anode material in LIBs. The recycled carbon electrodes achieve a specific capacity of 162 mAh/g in sodium-ion batteries and 320 mAh/g in potassium-ion batteries. It should be noted that certain methods are used for both anodes and cathodes, some of which are described below.

4.16. Separation of Anode Active Materials (AAM) from Anode Current Collector (Cu)

Considering that the anode binder and its collector are generally considered to be water-soluble organic substances, the process of separating these two from the AAM, which is graphite, will be more efficient and convenient [80]. Graphite recycling from the anode in LIBs is practically considered part of the treatment process. However, in this section, a brief description of this method will be provided. It should be noted that graphite has always been an integral part of LIBs [20]. Over the years, the main goal of the recycling industry has been to recover valuable metals from cathodes (such as Al, Co, Ni, Mn, and Li, etc.) and not graphite from anodes. However, today, due to the limited resources and the high production cost of anode active materials, the production of first-grade graphite for batteries has become a challenge to meet the ever-increasing needs of energy storage devices, and many companies, including Umicore and Inmetco, and of course other companies, have made many efforts to recover graphite of a suitable quality grade. Many of these companies follow a pyrometallurgical process in which graphite is burned in a furnace at a temperature above 1000 °C, which requires high energy for recovery. However, the hydrometallurgical process can also recover low-purity graphite based on SX and filtration techniques. However, some researchers are searching for newer ways to recover graphite from anodes. For example, Bhar et al. [87] presented an environmentally friendly technique to recycle graphite from S-LIBs via water washing followed by atmospheric plasma jet printing, the major advantage of which is that no binder or diluent is required. This technique does not have conductive parts. The excellent performance and cyclability of plasma-printed recycled graphite electrodes are due to the synergistic effect of oxidized graphite particles and their graphite matrix. In May 2024, a ton of natural graphite costs USD 1750, synthetic graphite USD 15,000, and Si-enriched graphite is USD 20,000 [18].

What is certain is that new methods are needed for the recycling of LIBs, because the existing recycling methods do not meet the needs of industries and have low efficiency and high waste production. This rule does not exclude the recycling of graphite from the anode, and for this reason, Zhao et al. [69] have claimed to have achieved a new method that can perform the isolation of gel LIB cells with high efficiency during a main step and in water (Figure 27). This precise mechanical, non-destructive method separates the components from the gel roll cell in water and prevents uncontrolled anode reactions and the burning of the electrolyte. It removes oxygen and allows the reaction between lithiated graphite and water to be controlled, while only allowing a limited portion of the Li anode to react with water. However, it seems that there is still a long way to go before the industrialization of this idea.

4.17. Separation of Cathode Active Materials (CAMs) from the Current Collector in the Cathode (Al)

The main research focus of the pretreatment process is on the separation of CAMs and their collector, i.e., Al. Therefore, special separation methods have been proposed to separate CAMs from the current collectors [78]. It should be noted that some of them are still on a laboratory scale and need more research and development. In general, the available methods can be divided into three general groups as follows:

• Mechanical separation;
• Thermal and chemical deactivation treatment methods;
• Mechanochemical methods.

Each of the above methods is performed using special techniques, some of which are mentioned in the following sections.

4.17.1. Mechanical Separation

The adhesion neutralization via incineration and impact liberation (ANVIIL) process is among the new methods that have been tested [88]. The work steps of this method are as follows:

• Pyrolysis of cathodes at 500 °C for 90 min and using the air jet impact process for 1 min to recover the coating powders with low organic contents [59].

In other words, in this process, by performing a calcination step, PVDF is decomposed, and the air jet impact separator separates the coating powder from the current collector foils, thus creating a purer material. However, this method caused pollution via toxic gases produced from the combustion of C and organic binders and released into the air [80].

4.17.2. Methods Based on Thermal and Chemical Treatment

According to the research, the adhesion strength of PVDF binders increases at high temperatures (500 °C), but after 15 min, it starts to decrease. For this purpose, various thermal and chemical techniques have been tested in this regard. Some of them are as follows [59]:

Purification at high temperature: The thermal deactivation method (Pyrolysis/Calcination) is used to remove C and the PVDF binder in the cathode [17,89]. In this method, the binder is placed at a high temperature (400 to 600 °C) and decomposed to reduce the adhesion force in the CAMs, and as a result, these materials are separated from Al [80].

Calcination pretreatment occurs at the 150–650 °C temperature range to remove conductive C, acetylene black, and organic material from the S-LIBs. In addition, by calcinating at 250 °C to 350 °C, the PVDF binder (which connects the active materials and metal foil) can be removed, thereby reducing the adherence of the active materials on the Cu and Al foils. Through the use of high-temperature binder decomposition, the thermal treatment reduces the bonding force between CAMs and AAM particles, allowing for easy screening separation. The cathode components were divided into tiny pieces by Yang et al. [10] and then heated to 550–650 °C in an inert N₂ gas atmosphere in a tube furnace [87]. After that, gravity separation made it simple to separate the CAM and the Al foil. However, calcination pretreatment is an energy-intensive procedure that can release hazardous gases and necessitate costly furnaces.

Two stages of heat treatment of the S-LIBs were used to separate the LCO CAMs. Following a 24 h drying process at 60 °C, the first thermal pretreatment was carried out for 1–2 h at 300–500 °C to remove the C, PVDF binder, and organic additives from the CAMs. The remaining unburned organic material was burned off during the second thermal treatment, which was carried out for 1.5–2 h at 700–900 °C to remove C. After LCO was first separated from the Al foil, PVDF and the C powder in the anode materials were eliminated using high-temperature calcining [88,89,90,91,92,93,94].

Sun and Qiu [95] suggested a method for separating CAM using vacuum pyrolysis. During the pyrolysis process, the electrolyte and binder evaporated or broke down, which decreased the adhesion between the CAM and Al foil. The CAMs did not separate from the Al foils if the pyrolysis temperature was less than 450 °C. The temperature increased with an improvement in the separation efficiency when it was between 500 and 600 °C. But above 600 °C, the Al foil grew brittle, making it challenging to extract the CAM from the Al foil. Yang et al. [89] suggested a reducing heat treatment procedure to separate the CAMs from the Al foil collectors. It was demonstrated that the separation of the CAMs from the Al foils is possible by adjusting the temperature of the reduction reaction. Additionally, this process modifies the molecular structures of CAMs, improving metal leaching throughout the leaching stages. Thermal treatments have large throughputs and are simple processes. Nevertheless, during the thermal treatment procedure, they release harmful gas emissions. Thermal treatment can also change the grinding characteristics of LIBs [96].

Immersion in organic solvent: In this method, organic solvents are employed to dissolve the organic binder and separate the Al foil from the CAMs, so the relatively intact structure of the CAMs is retained. To accelerate this operation, it can be carried out under little heat and with the help of ultrasonic cleaning. In this method, PVDF is dissolved in a mixture of organic solvents, and after separation, it is filtered and dried. He et al. [97] showed that N-Methyl-2-pyrrolidone (NMP) is the most effective cleaning solution for the separation of CAM from Al foil at 100 °C, and that a separation efficiency of 99% can be achieved with the help of ultrasonic cleaning. It is worth noting that NMP cannot be used to clean polytetrafluoroethylene (PTFE) because it is non-polar. PTFE binders need the use of various organic solvents at a temperature of about 100 °C. Furthermore, organic solvents are currently not widely used due to their high toxicity, high price, and subsequent waste disposal issues [80].

Some of the previously tested solvents for binder dissolution are presented in

Table 3. Solvents used for binder dissolutions in S-LIB recycling.

Solvent	Binder	Temp./Time /S/L	Material Removed	Material Remained	Toxicity/Env. Impact	Reference
NMP	PVDF (Solubility: 200 g/kg solvent; boiling point: 200 °C; price: 5–6 USD/kg)	<100 °C/1 h/10%	LiCoO2 and graphite	Al and Cu metals via filtration	Reprotoxic (Category 1B), high skin and eye irritant, env. hazard	[98]
NMP DMAC	PVDF binder PVDF (Boiling point: 165 °C; price: 21.5 USD/kg)	80 °C/2 h	LiCoO2	Al foil	Toxic, potential carcinogen, severe skin and respiratory irritant	[99,100]
DMF	Suitable for PVDF Not suitable for PTFE (Price: 0.9–2.4 USD/kg)	60–70 °C	NMC/LCO	Al foil	Reprotoxic, potential carcinpgen (IARC 2B), toxic to aquatic life	[101,102,103,104]
TFA DMSO Ethanol	PTFE acetic acid 15 v%, L/S: 8 mL/g with agitation (Boiling point: 71.8 °C)	40 °C/3 h	NMC	Al foil	Ethanol, flamable, low toxicity, DMSO, low acute toxicity but enhances skin absorption, TFA: corrosive	[92]
AlCl3 −NaCl molten salt	PTFE, 1/10 g/mL (price: 10–25 USD/kg)PVDF	160 °C/20 min	Cathode material	Al foil	Corrosive, AlCl3 highly reactive with water, high- temperature process may release harmful fumes	[105,106]

N-Methyl-2-pyrrolidone (NMP), N-N-dimethylacetamide (DMAC), N,N-Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), and TFA: trifluoroacetic acid.

, with temperature, time, S/L ratio, and material recovered/remained.

Because N, N-dimethylacetamide (DMAC) has an economic advantage over NMPP, Liu et al. [100] used it to separate the LiCoO₂ cathode active material from the Al foil. DMAC also dissolves PVDF, which is approximately 10%. LiCoO₂ and graphite conductive agents were filtered out of the solvent after the CAM was separated from the Al foil when the solid/liquid (S/L) ratios were adjusted between 1:4 and 1:5. Since DMAC has a boiling point of 165 °C, it can be evaporated by heating it for 12 h at 120 °C. N, N-dimethylformamide (DMF) was utilized by [102,104,105] and Xu et al. [103] to extract the LiCoO₂ and NMC cathode active materials from the Al foil. The cathode scrap was submerged in either DMF or a mixture of DMF and ethanol at 70 °C.

Wang et al. [106] used a low-temperature AlCl₃–NaCl molten salt system to separate the CAMs and the Al foil; this system performed better at peeling off than the frequently used NaOH or nitrate systems. At 160 °C for 20 min, an S/L ratio of 1:10 g/mL produced the optimal results. To separate the CAM and Al foil, the PVDF organic binder melted due to heat storage from the phase transformation of the molten salt AlCl₃–NaCl. In particular, a significant amount of heat is absorbed by the AlCl₃–NaCl molten salt mixture to change it from a solid to a liquid compound when the heating temperature rises above the phase transformation temperature of 153 °C. As soon as the temperature reaches 160 °C, PVDF melts and the CAMs and Al foil are effectively separated. Zhang et al. [92] used trifluoroacetic acid (TFA), a potent carboxylic acid with a 71.8 °C boiling point, to dissolve the PTFE binders in the NMC-based LIBs. With agitation, the ideal TFA concentration was 15 vol%, the S/L ratio was 1/8 g/mL, the reaction time was 180 min, and the temperature was 40 °C.

Solvent and ultrasonic-assisted binder elimination was also studied by some researchers. Because of the PVDF binder’s strong bond, removing CAM from Al foil during the S-LIB recycling process is quite difficult. Due to its powerful cavitation effect, ultrasonic-assisted treatment is regarded as one of the most effective ways to remove CAM from Al foil [97,107,108]. When molecules are cavitated, their bonds are broken. Li et al. [107] found that when mechanical agitation was used alone, the majority of the CAMs remained adhered to the surface of the Al foils when examining the effects of both agitation and ultrasonic treatment on the separation of CAMs. Only a portion of the CAMs were separated when ultrasonic treatment was utilized alone. On the other hand, nearly all of the CAMs could be removed from the foils when both techniques were applied simultaneously. This could be a result of the ultrasonic treatment’s cavitation effect, which can produce increased pressure to break down insoluble materials and disperse them in solution. The separation of CAMs from Al foils is further aided by the rinsing effect of mechanical agitation. According to He et al. [97], the cavitation effect, caused by sonication and the binder’s dissolution, was the mechanism by which CAMs were separated from Al foil via ultrasonic treatment. Based on this process, the CAM’s stripping efficiency was 99% when NMP was used at 70 °C, with 240 W of ultrasonic power, and 90 min of processing time. The following leaching procedures were greatly aided by the low degree of agglomeration shown by CAM, which had been ultrasonically treated to separate it from the Al foil. After discharging, manual dismantling, and separation, for cathode powders, NMP at 60 °C for 1 h and 700 °C for 5 h, and ultrasonication in NMP at 80–100 °C for 2 h and 700–800 °C for 2 h were tested.

Therefore, NMP is the best solvent for PVDF; the LCO material can also be separated from Al foils using other solvents including DMAC, DMF, N-dimethyl sulfoxide (DMSO/((CH₃)22SO), ethanol, or molten salt of AlCl₃–NaCl. In all solvents, the peel-off efficiency was less than 10% at a S/L ratio of 1:10 g/mL and 60 °C for 30 min. However, with the aid of the ultrasound, 100% peeling off was obtained and the peel-off efficiency rose by at least six times in the following order: ethanol < DMSO < DMF < DMAC < NMP [98]. One of the finest methods for removing cathode materials from Al foil is to use solvent dissolving in combination with ultrasonic treatment.

It should be noted that by using ultrasonic stirring in the binder removal process, the removal speed can be increased. In some research, among the above solvents, the use of NMP with the help of the ultrasonic technique has been reported as the most effective method despite its high cost and toxicity [59]. However, Kar [72] found that the use of NMP solution for binder removal caused problems in grinding due to the low evaporation rates. In LIB recycling, cathodes are usually degraded in contact with water.

Ionic liquids (ILs) and organic acids: The use of new and alternative solvents, including 1-butyl-3-methyl-imidazolium-tetrafluoroborate ([BMIm][BF₄]), has also been investigated [59]. Organic acids can also be used to separate Cu and Al current collectors from powder coatings. Cathodes can be dissolved from the anode collector using acids, a process that can be accelerated by adding hydrogen peroxide to the acid-washing process.

Alkaline solution dissolution: In this method, Al foil is dissolved in an alkaline solution, and with the subsequent calcination, the adhesive material is removed, resulting in a relatively pure active material [80].

In the alkaline solution method, Al current collectors can be separated from CAMs by using NaOH with the dissolution of some Li and, of course, the minimum dissolution of Co [59]. Nan et al. [109] separated the LiCoO₂-based CAM from Al foil using a 10 wt% NaOH solution as part of their CAM recovery procedure. Nearly 98% of the Al foil was dissolved in a 5 h leaching time at room temperature using a NaOH solution with a 100 g/L S/L ratio. The protective layer that covers the collector’s surface and the Al are both dissolved when the cathode’s Al foil is dissolved in a NaOH solution. However, this method has disadvantages, such as that Al foil cannot be recovered in a metallic form, and that in addition to high losses, it is prone to leaving impurities that affect the subsequent recovery and repair processes [78]. He et al. [99] recycled the LIBs containing LFP and NMC using an aqueous exfoliating and extracting solution that contained NaOH salts without strong acid and alkali components. By dissolving the Al foil, the cathode materials were separated from it. The Al foil and electrode materials’ resulting recovery efficiencies were nearly at maximum. Due to the amphoteric property of Al, which allows it to react with both acids and bases, leaching the cathode with an alkaline NaOH solution is a common method of separating the CAMs from the Al foil in proposed recycling processes. In the presence of water, the following reactions take place between Al and Al₂O₃ and NaOH:

2Al + 2NaOH + 6H₂O = 2Na[Al(OH)₄] + 3H₂

(10)

Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

(11)

The advantages of this NaOH dissolution method are its high separation efficiency and ease of use. Unfortunately, due to Al’s ionic form, the recovery of Al is challenging. Furthermore, the disposal of NaOH wastewater poses a significant environmental risk. Al dissolution can also be accomplished with a molten salt AlCl₃–NaCl mixture.

Ultrasonic-assisted acid scrubbing in facile acidic media: Chen et al. [110] were able to separate CAMs from Al foils in sulfuric acid and oxalic acid environments. Also, many efforts have been made to separate CAMs from the end using ultrasonic cleaning [86]. As a result, it should be noted that in other innovative plans, the strong oxidation of HO free radicals formed in an acidic ultrasonic solution has been used, which quickly destroys the adhesive and accelerates the pretreatment process. Also, the use of ionic liquids and low-temperature molten salts can facilitate the achievement of high efficiencies [20].

4.17.3. Mechanochemical Methods

One of the other methods of removing binders is using mechanochemical operations. A combination of chemical pretreatment and some mechanical pretreatment tools, such as crushing, sieving, or flotation, can also create high efficiencies in the separation of active substances [80].

Mixing electrode materials with a reagent in the grinding environment: Grinding for a long time due to the production of metal compounds that are soluble in water or acid can eventually lead to the dissolution of CAMs. This dissolution can be carried out using acid washing or water. Therefore, soluble metal chlorides can be synthesized from metal oxides. In some studies, PVC (polyvinyl chloride), NaCl, NH₄Cl, Zn₂Cl, and FeCl₃ have been used as chlorine donors to form soluble CoCl₂ and LiCl, which are then dissolved with water. In this way, there will be no need for hot, acidic conditions or the disposal of said acids. Wang et al. [20] also investigated EDTA as a complexing agent for the removal of Co and Li. Furthermore, 98% of Co and 99% of Li have been recovered via simultaneous grinding with LiCoO₂: EDTA powders, then dissolving with water and recovering CoO and Li₂CO₃ through sedimentation.

At the end of this section, it should be mentioned that most of the methods in this section are based on thermal decomposition, which ultimately causes the production of toxic HF. Also, techniques based on the use of harmful organic solvents lead to creating potential risks for the environment [111].

So far, no special attention has been paid to mechanochemical processes, and more research is needed to understand their importance. In these processes, additional chemicals in the milling process are used to extract metals without using acids or alkalis, and additional costs can be saved, even though these processes are also more suitable for the environment [59].

Figure 28 presents the classification of the separation methods of CAM from current collectors.

Finally, it should be noted that once the binder is removed, froth flotation can be used to remove any additives. In all cases, BM contamination from the primary crushing and grinding processes causes additional work and costs to purify the cathode and anode materials. This is because if its components were separated from the beginning, the level of contamination would be significantly reduced [59].

4.18. Some Innovations in the Separation of AAMs and CAMs from Current Collectors

Zhang et al. [112] proposed a shearing-enhanced mechanical exfoliation combined with mild-temperature pretreatment (SMEMP) method for the high-efficiency exfoliation of cathode active materials, in which the CAMs, after melting during pretreatment and gentle cleaning, remain firmly attached to PVDF. In addition, the heat treatment temperature was reduced by about 250 °C, its duration was also reduced to one-sixth of the original time, and finally, the exfoliation efficiency and product purity reached 96.9% and 99.9%, respectively. This study showed that the cathode material can be exfoliated with enhanced shear forces despite the weakening of the thermal stress. Compared to other traditional methods, employing high temperatures or chemicals contributes to air and water pollution; the superiority of this method lies in reducing the temperature and saving energy. The proposed SMEMP method is eco-friendly and cost-effective, and provides a novel route to recover CAMs from S-LIBs. Although the above research was still on the laboratory scale, it had significant potential for application in the industrial-scale recovery of valuable materials with layered structures [112].

Anode Active Material Exfoliation from Cu Foil

Graphite as an AAM can easily be separated/exfoliated from the Cu current collector foil via ultrasonic treatment. Water-soluble binders (CMC) are easier to remove in the anode. Figure 29 shows the ultrasonically treated anode electrode. Very wholly clean Cu foil was obtained in a very short time.

4.19. Screening

One of the points that should be taken into consideration is that AAM and CAMs (due to their nature and material) tend to decrease in size more than other metal and plastic compounds in the battery. Generally, by performing the crushing process, they are placed in a certain particle size distribution, which in turn makes it easier to separate these materials from other coarse plastic and metal materials [3].

Although screening should somehow be placed in the classification/separation section, here, the topic of discussion is to perform screening operations after separating the AAM and CAMs from the current collectors. Generally, different groups of materials in the battery are placed in certain particle sizes after crushing, for example:

• More than 90% of valuable metals (Li, Ni, Co, and even C) accumulate at the 212 µm size.
• However, more than 80% of the weight of Cu and Al in the battery is placed in the coarse size category above 1.4 mm.

It can be concluded that by creating a suitable sequence of screening methods, many components of the battery can be separated [3].

4.20. Separation of AAMs and CAMs from Each Other

Froth flotation is generally used to separate anode particles from the cathode. Flotation is a very efficient separation technology for fine particles based on the surface property differences (hydrophobicity or hydrophilicity) of materials. Figure 30 presents a possible froth flotation flowsheet for AAM and CAM separation. However, the entanglement of organic materials in the outer layer of the active material reduces the flotation selectivity [80]. Froth flotation works by exploiting the difference in hydrophobicity between two materials. The separation selectivity between graphite and LiCoO₂ was very high (>85%), with 2 kg of kerosene as the frother at a 1% w/w solid concentration. The froth product is mainly graphite with spherical particle shapes and particle sizes of 16–25 µm, and tailing consisting of pure LCO of uniform sizes (6–25 µm). The order of these operations is generally as follows [18,113]:

• Fine bubbles are introduced into the flotation cell containing the material for separation;
• Frothers stabilize the bubbles (i.e., aromatic alcohols and cyclic carbonates);
• Hydrophobic graphite is collected by the bubbles, transferred to the surface, and remains in a stable froth on the surface;
• Hydrophobic graphite components are recovered as a floating product, while hydrophilic CAM particles sink and are collected from tailing.

Reverse flotation is performed for anode and cathode active material separation. Since natural hydrophobic graphite is less valuable than desired and expensive CAM, it should be noted that due to the adhesion of hydrophobic PVDF materials to LiCoO₂ and graphite powders, weak separation between the cathode and anode powders is observed via the flotation method. But if PVDF is heated up to 500 °C to remove the binder from the BM, froth flotation can successfully remove C from the cathode material. This heat treatment breaks down the binder and may change the carbon in the sample. However, this decomposition may affect the surface of the metal oxide, or fluorine.

In some cases, instead of expensive heat treatment that generates hazardous gases, cryogenic milling has been studied by some researchers for the following reasons:

• The brittleness of the PVDF binder and the increased possibility of breaking it during the crushing stage can cause a better separation between the active materials of the cathode and the adhesive;
• Also, to maximize the separation using froth flotation, which takes advantage of the difference in the hydrophobicity of materials, surface modification through binder decomposition or surface treatment should be used to effectively improve the selection of froth flotation [59];
• To prevent the emission of toxic HF and P₂O₅ during the removal of the organic layer through heat treatment, Fenton’s reagent (Fe²⁺ + H₂O₂) can be used to oxidize and remove the layer, but more research is needed to remove Fe-containing impurities.

Some innovations in the separation of AAMs and CAMs from each other are given below:

Zhang et al. [114] used flotation technology to separate and purify CAMs and AAMs after removing organic binder materials via pyrolysis. Experiments showed that organic materials and their pyrolysis products can be sufficiently removed at the optimum pyrolysis temperature of 550 °C. After pyrolysis, the changes in the zeta potential and surface free energy show that the hydrophilic components of the cathode material increase while the hydrophobic components of the anode material increase. At a pH range of 1 to 11, the zeta potential of spent and pyrolytic anodes and cathodes decreased as the pH increased. The isoelectric point (IEP) of the cathode material increased from a pH of 1.5 to 3.2 for spent and pyrolytic materials, respectively. However, there was no IEP of the spent anode material, and the IEP of the pyrolytic anode material was pH: 1.6. Zeta potential changes in AAMs and CAMs demonstrated that the surface properties of electrode materials changed significantly with heat treatment. The induction time of CAMs rose from 190 ms to 650 ms, while the induction time of the AAMs decreased from 145 ms to 37 ms. The collector adsorption capacity of AAM is improved, and AAM is easy to incorporate with bubbles and will be collected in the froth product. Following a single flotation stage, the cathode material’s recovery rate is 83.8%, with a high grade of 94.7% [114].

Makuza et al. [115] developed a new method of recycling S-LIBs that consists of anode materials (graphite) and various combinations of cathode materials (LiCoO₂/LCO, LiMn₂O₄/LMO, and LiNiO₂/LNO), with the help of carbonated ultrasound-assisted water leaching (CUAWL). The effect of several factors, including roasting time, roasting temperature, grinding time, water leaching temperature, water leaching time, sonication, and CO₂ flow rate, on the metal leaching efficiency was tested. The results of the analyses show that the mixture of cathode and anode materials after reduction roasting in optimal conditions of 600 °C for 30 min was primarily converted into Li₂CO₃, Ni, CoO, Co, and MnO. However, the selective recovery of Li with leaching was low, and dry milling followed by CUAWL was employed to increase the recovery rate. The optimized results obtained up to 92.3% for selective Li recovery [115].

4.21. Other Steps, Such as a Variety of Other Creative Methods for Pretreatment

In some cases, heating, ultrasonic vibration, washing, calcination, and reduction roasting can be used separately or combined. Zhang et al. [81] demonstrated that in situ reduction pretreatment could dramatically improve the leaching efficiencies of valuable metals from cathodes. Specifically, calcination at below 600 °C without oxygen using an alkali-treated cathode can induce in situ reduction and collapse of the oxygen framework, which is attributed to the carbon inherently contained in the sample, and promote the following efficient leaching without external reductants. The tests revealed that during in situ reduction, high-valence metals such as Ni³⁺, Co³⁺, and Mn⁴⁺ can be effectively reduced to lower valence states, conducive to subsequent leaching reactions.

Yang et al. [45] were able to obtain better results during the recovery of metals by using reduction roasting as a part of pretreatment. After discharging and disassembling the battery from the shell and obtaining a mixture of positive electrodes, negative electrodes, and organic separators, they mixed it with a certain amount of starch uniformly and roasted it in a tube oven. Various roasting parameters, including roasting temperature (optimum roasting temperature is 600 °C), starch dosage (optimal starch dosage is 17.5%), and roasting time (30 min), were investigated to obtain the optimum operating conditions. Finally, they showed that with this method, they achieved positive results in the recovery rate of critical metals. In addition, research can be conducted in the field of using the available and remaining energy in batteries or even the heat obtained from the initial stages of recycling, especially for the energy supply of the next stages.

Finally, as mentioned before, the resulting product after size classification is a BM containing an enriched mixture of materials in the anode and cathode. This BM will go on to the next stage of recycling.

4.22. Suggested Flowsheets Related to Pretreatment in the Recovery of Critical Metals from LIBs

As mentioned, LIBs can vary greatly in the active material content of their cathode electrodes, as far as that the name of the batteries is mainly taken from the type of CAM. However, these differences are fewer in AAMs, and graphite is mainly used in them. The large difference in CAMs is the biggest challenge that recycling LIBs imposes on an industrial line. However, the current collectors, coating materials, and their connecting materials are mostly the same in different LIBs. Pretreatment involves separating the electrode material from the rest of the battery components so that the secondary treatment can recover the valuable material in the electrodes [3].

Although EV batteries mainly have a second life after the first use, in most LIBs, they spend a period of hibernation after their service, and this issue can be effective in collecting them. Batteries are not dead when they come to the end of their useful first life. Second-life batteries are batteries that can be applied for a different use (less demanding applications) after their initial lifecycle is over. Small PC and phone waste batteries are stored at homes for some time before going to be recycled.

Kim et al. [16] provided recommendations on the pretreatment steps involved in a pretreatment line for enhancing the recovery efficiency of valuable elements and reducing energy consumption. This review paper suggests that providing a solution for manual disassembly techniques is a necessity, and has not been properly addressed so far. They suggest a stabilization step of dismantling the S-LIBs in an inert environment using CO₂ or liquid N₂; discharging the LIB cells in NaCl or other solutions is the most popular stabilization method for most industrial LIB recycling plants. Furthermore, the paper shows the importance of multiple comminution steps and sieve sizes on the yield of BM, and involving several mechanical separation steps in the line. In addition, it suggests the use of the dissolution of the PVDF binder by NMP and the dissolution of Al foil by alkaline solution. For the complete recovery of the CAMs, a thermal treatment or pyrolysis at about 500 °C to effectively degrade PVDF binders is recommended at the expense of carbon-conductive agents. At lower temperatures, electrolytes are volatilized and carbons are burned off.

Discharging EoL LIBs before dismantling a track is widely accepted [3]. But, most industrial lines skip this step and use crushing or direct crushing [16]. The set of factors mentioned indicates that due to the wide range and variety of LIBs, it is almost impossible to propose pretreatment lines with similar conditions, and that each factory has a type of pretreatment line according to the type of incoming battery and its environmental strategy. Therefore, to become more familiar with the proposed lines, one suggested flowsheet is presented below.

Premathilake et al. [3] proposed a novel design that can be included in an industrial plant pretreatment line prior to the metallurgical steps to recycle LIBs [3]. Grinding, density separation, drying, second-stage grinding, heating with CaO, vibrating sieve, washing, and separation based on flotation were identified as the best sections to include in the listed order. Figure 31 shows the proposed flowsheet from these studies.

4.23. Metallurgical Steps

So far, the various pretreatment techniques of LIBs have been explained, but the use of each of these techniques in the industrial recycling process requires more investigation. Which technique to use and in what order depends on the general process of recycling. However, the main point is that the success of the separation operation is highly dependent on the choice of the pretreatment process [116].

After performing various pretreatment steps according to the designed recycling and metal extraction algorithm, we will enter the metallurgical step as the most basic step of recycling. There are various methods for recycling and extracting critical metals from S-LIBs. Feng et al. [116] classified separation methods in the following way:

• Pyrometallurgical and hydrometallurgical methods;
• Hydrometallurgical method;
• Direct recycling method;
• Biometallurgical method;
• Mechanochemical method;
• Combined methods.

Table 4. Comparison of different LIB recycling technologies with plant applications.

	Pyrometallurgy+ Hydrometallurgy	Hydrometallurgy	Direct Recycling	Biometallurgy	Mechano-Chemical Method	Combined Methods/Others
Mechanical pretreatment	Not required/optional	Required	Required	Required	Required
	-Direct feeding	(Discharging, dismantling, shredding, and sieving)	Perforation, supercritical, CO2 extraction
Discharging requirement	No	Yes		Yes
Allowance for heterogenity	Yes	No	No	No	No
Chathode morphology	Not maintained	Not maintained	Maintained
Cross contamination	Occure	Prevent	Prevent	Prevent
Separation for black mass (BM)	One(150–500˚C) or to– stage (1400–1700 ˚C) heat treatment +leaching, SX, precipitation	Shredding, low–temp. calcination, physical separation (gravity, magnetic, electrostatic, flotation, etc.), leach, SX, precipitation	Perfortation supercritical CO2 extraction, Shredding, density separation, froth flotation, relitiation, flash evaporation	Shredding, physical separation, (gravity, magnetic, electrostatic, flotation, etc.), bioleach, SX, precipitation
Materials recovered	Cu, Fe, Co, Ni, Mn	Cu, steel, Al, graphite, Co, Ni	Cu, steel, Al, graphite, plastics, LMOs, solvent of electrolytes and salts	Li, Co, Ni, Mn
Can be recovered	Optional expensive Li extraction from slag
Burned for energy	Graphite, plastics, electrolyte, PVDF	Electrolyte solvents and salts, plastics
Landfill	Al	Carbon black, PVDF
Recovery	50–70%	Up to 99%	Recover all components except separators
Energy/Power usage	High	Medium	Medium
Reaction time	Fast (hours)	Slow (hours)	Very slow (days)
Compatibility	Low	High
Control	High	Low	Complex purification stages
Reliability	High	High
Scale	Industrial	Industrial	Pilot
Eco-friendliness	Low	Medium	High	High
Capital cost	Worst	Medium	Medium
Operating/ Production cost	Best	Medium	Worst
Complexity	Best	Medium	Worst
Technology maturity	Mature
Products	Metal alloys	Metal salts
Waste & By-products	Slag	Wastewater (High), Na2SO4
Advantages	Simple, mature, without sorting & sizing, low capex	Low capex, low opex, low temperature, high recovery rates, low GHG emissions
Disadvantages	High opex, high temperature, low recovery, high GHG, lost slag (Li, electrolyte, graphite, and plastics), high safety risks, further process for alloys	High wastewater treatment cost, low safety risk
CO2 footprint	High	Medium	Medium
Plants	Sony/Sumitome, Umicore, Accurec	Bruno, Gem, GHTECH, TESAMM			Redux, Ecobat, Circu Li–ion	Lilnd.,Nth cycle
	Inmetco, Glencore, Redwood, BASF	Highpower Int., SungEel HiTeach			Akkuser, Librec, No Canary	Infinity Rec., Mecaware
	SNAM	Recuply, Batrec Ind. AG., Retriev, Albemarle, Battery Resources Primobius, ABTC, Li-Cycle, Fortum, Hydrovolt, BASF, SNAM, Asent, Redwood Mat., Veolia, Lithion, Accurec, RecycLiCo, Cirba, Solutions, Solvey, Duesenfeld			Stena Rec., Universe Energy, Posh	Aqua Metals

compares the LIB recycling methods from different operational points.

The pyrometallurgical method smelts some important metals (such as Ni, Co, and Cu) into alloys from the CAM at high temperatures using salts or reducing agents [2,117]. Despite its large processing capacity and relative simplicity, this process is energy-intensive and results in the decomposition or waste of several other components, including graphite and Li [55]. Furthermore, Li must be separated from the slag, and metal salts must be extracted from the alloy using further hydrometallurgical procedures [118].

Although pyrometallurgical processes are widely criticized for their environmental impacts and limited material recovery efficiency, they still dominate the industrial recycling of S-LIBs. This continued preference is due to several practical and economic factors [18]:

• Operational simplicity and flexibility: Pyrometallurgical methods can treat mixed battery chemistries and formats with minimal pretreatment, which is highly beneficial for large-scale processing without prior sorting or disassembly;
• Established infrastructure: Existing smelting plants, especially in the Cu and Ni sectors, can accommodate battery black mass with relatively low capital investment costs, and leveraging decades of metallurgical experience;
• Large processing capacity: These processes are suitable for continuous operation and high-throughput industrial-scale systems, making them attractive in regions with large volumes of battery waste.

In the hydrometallurgical process, acids or alkalis are used to leach valuable metals from S-LIBs into solutions, which are then extracted and purified vis SX and/or precipitation [43,119,120]. Finally, pure metals, metal hydroxides, and metal sulfates are recovered [121]. A highly efficient metal recovery technique, hydrometallurgy, produces comparatively pure metal compounds. However, the complexity of the process, the high consumption of reagents, and the seriousness of secondary pollution are major drawbacks of this method [17].

Pyrometallurgy and hydrometallurgical and hydrometallurgical methods are industrially used for LIB recycling. Capex is the most used, and Opex is the best for the Pyrometallurgy and hydrometallurgical method. But the metal yield and purity are lower than in hydrometallurgical treatment. Hydrometallurgical treatment methods alone are not as mature as pyrometallurgical and hydrometallurgical methods.

In contrast to pyro/hydrometallurgical methods, the direct recycling process, which uses physical separation techniques, enables the separation and recovery of individual battery components from S-LIBs while maintaining their original chemical structures for direct utilization in the manufacturing of new LIBs [22,96]. The purpose of the direct recycling process is not to extract metals, but to re-functionalize the spent active material via solid roasting or hydrothermal techniques, making it more economical, eco-friendly, and energy-efficient [122,123,124]. Based on this, this method is a promising development direction that has attracted much attention in the field of Li ion recovery in recent years. Direct cathode recycling delivers improved efficiency. Components are separated while retaining their structure, which can then be restored with initial properties and electrochemical capacity.

Direct recycling mainly aims to restore and reuse battery components—especially CAMs—without breaking them down into elemental forms. Recent studies have shown that processes like relithiation and structure repair via hydrothermal or solid-state treatments can successfully rejuvenate LiCoO₂ and NMC cathodes. While direct recycling offers high energy and cost efficiency and preserves material value, it faces limitations such as difficulty in sorting different cathode chemistries and the lack of standardized industrial protocols. Nonetheless, with increasing research and policy focus on sustainability, direct recycling is considered a promising component of future battery recycling systems [18].

4.24. An Overview of the Latest Advances in the Pretreatment of LIBs on an Industrial Scale

Recently, significant advancements have been made in the pretreatment of LIBs on an industrial scale, aiming to enhance the efficiency and environmental sustainability of subsequent metal extraction processes. Cutting-edge techniques such as mechanical crushing, pyrolysis, and hydrometallurgical methods have been optimized to better separate valuable components and reduce hazardous waste. Companies like Umicore and Duesenfeld have pioneered innovative pretreatment operations, with Umicore focusing on high-temperature smelting to recover Co, Ni, and Cu. At the same time, Duesenfeld employs a low-energy mechanical process combined with pyrolysis to minimize emissions and energy consumption. These diverse approaches underline the ongoing evolution and diversification of pretreatment strategies in the LIB recycling industry, setting the stage for more effective resource recovery and environmental protection. Figure 32 presents a comparison of pretreatment industrial operations in some LIB recycling companies.

Several industrial initiatives have demonstrated the successful large-scale implementation of LIB pretreatment and recycling technologies. For instance, Redwood Materials in the United States employs a comprehensive process involving thermal pretreatment, mechanical separation, and hydrometallurgical recovery to extract critical elements like Li, Co, and Ni from EoL LIBs. The company has partnered with major OEMs such as Panasonic and Ford to scale its operations, and aims to produce battery-grade materials directly from recycled sources. Similarly, in Europe, companies like Umicore and BASF have integrated mechanical and thermal pretreatment with closed-loop hydrometallurgical processes for the recovery of valuable metals from black mass, contributing to the development of a circular battery economy. These examples highlight the growing industrial relevance and scalability of pretreatment strategies in real-world recycling systems [8,18].

4.25. Recycling Batteries Is a Way to Reduce the Lack of Metal Resources

Sustainability in the battery supply chain for each country is important. There are some supply side constraints: limited accessible Ni-Co-Mn-Li geologies, geopolitical stability problems, community and regional water use and waste conflicts, the extent to which mining and processing can be made net-zero in terms of CO₂ emissions, poor metallurgical recoveries and yields, ensuring “green” processes are in place to produce Li, Ni, and graphite, and single or limited dominant players in the supply chain. The recovery of critical materials from LIB recycling can both reduce the demand for key raw materials and reduce the reliance on imported critical materials. In 2030, about 160,000 tons of Ni, Li, Mn, and Co can be recycled from scrap and waste LIBs. Recycling LIBs can significantly reduce hazardous waste amounts and GHG emissions. LIB recycling also reduces the negative local and social impacts from mining and refining. LIB recycling generates some economic income and employment. As a result, LIB recycling conserves scarce resources and prevents environmental pollution. S-LIBs are 9 times Li and 20 times Co richer than primary Li and Co ores. Thus, recycling LIB secondary resources decreases the cost by 40%, energy use by 82%, water use by 77% and SO₂ emissions by 91% [8,9,18].

Recycling metals containing used LIBs may be a suitable way to reduce the scarcity of metal resources. In the meantime, the management of used batteries from the consumer to recycling centers should be given more attention. In addition, the total environmental benefits of LIB recycling should be quantified and further investigated. Also, according to the type of LIB, more serious efforts should be made toward technological innovations to achieve cleaner management. There are also limitations in this field, such as the fact that there are many emerging technologies in the industry of LIB production. Methods with higher flexibility should be used to recover critical metals from LIBs. In addition, the long life of LIBs may strengthen the security of the metal supply in the future, but in any case, the feasibility of recycling huge LIBs should be evaluated from multiple dimensions, such as economic, technical, and social ones [15]. The recycling of LIBs is more complicated than simply developing a technique, and it is just as much an economic issue. Today, we are in a situation where we do not know, for example, where the volume of Li needed to produce LIBs will come from in 2030, but we know that we have to think of a suitable way to save our environment.

4.26. Issues with Existing LIB Recycling Technologies

The urban mining of LIBs has some problems and challenges. These are high operating costs and poor economics, high pollution and environmental costs, and a lack of raw materials and production flexibility. Thus, the LIB and EV industries urgently need a technical solution featuring both economic feasibility and strong environmental benefits. Today, there are serious battery collection, logistics, and transportation problems. The number of designated collector sites should be increased, and discharging should be performed at these sites. Environmental concerns about the existing industrial recycling technology can pose threats to the environment. Today, many players rely on government subsidies/incentives to survive. In addition, there are serious regulatory and legal problems, such as a lack of oversight from the government, a lack of industrial standards and operations, and insufficient laws and regulations enforcing measures.

Since the intrinsic values of LCO- and NMC-type batteries are highest, recycling them is plausible. The recycling efficiency of Co and Li can be more than 90%. The intrinsic values of different NCM types of batteries change: NCM₁₁₁ > NCM₆₂₂ > NCM₅₂₃ > NCM₈₁₁. LFP- and LMO-type batteries have low intrinsic value. Only Li is important in these types of batteries. In LIBs, the prices of materials concerning USD/kg change as follows: Li > Co > Ni > Cu > Al > Fe > Mn. All collected batteries should enter a recycling operation without any loss. Currently, many technologies are more efficient in treating specific types of LIBs. There is a lack of feedstock flexibility and compatibility. Today, LIB’s recycling efficiency is around 50%. It should be increased to 65% by 2028 and 70% by 2036. By 2031, Ni, Co, and Cu metal recoveries should be 90% and Li metal recovery 50%. By 2036, Ni, Co, and Co metal recoveries should reach 95% and Li metal recovery 80%. Battery passports and electronic records should be compulsory from now on for battery producers. The LIB recycling industry has an urgent need for a technical solution featuring both economic feasibility and environmental benefits.

Today, many gigafactories suffer from an above 30% scrap rate in LIB production. A 10% scrap rate reduction saves USD 200–300 million per annum for a 30 GWh plant, which largely pays off the initial recycling investments.

4.27. Directions for Future Research in LIBs

Future research directions in LIBs should encompass a holistic approach that spans from production to recycling. Key areas of focus should include the development of more sustainable and cost-effective materials for battery electrodes and electrolytes, enhancing battery energy density and lifespan, and improving safety features to mitigate risks such as thermal runaway. Additionally, research should prioritize the optimization of recycling processes, emphasising efficient pretreatment techniques and innovative methods for recovering critical metals like Co, Li, and Ni. Integrating circular economy principles into LIB production and disposal processes will also be crucial, promoting the reuse of materials and minimizing the environmental impact. Collaborative efforts between academia, industry, and government will be essential to drive these advancements and ensure the sustainable growth of LIB technology. LIB recycling and pretreatment technology should be used without dismantling and discharging, irrespective of battery chemistry and battery form and size. Zero liquid discharge is preferred. Figure 33 summarizes some directions for future research on LIBs.

5. Summary and Conclusions

In this article, the State-of-the-Art LIB pretreatment methods are comparatively and comprehensively covered, along with novel innovations. Although promising research has been conducted so far in the field of EoL LIB recycling, especially in the pretreatment stage, it must be acknowledged that we are still at the beginning of the road, and that there is a long way to go. For this reason, in addition to mentioning the research carried out in this field, suggestions for future research in the field of LIB production, pretreatment, and recycling are discussed. These suggestions include new ideas, novel research methods, and less explored areas that require further research:

• The development of new methods for separating cathodes from Al foil: Investigating and developing new mechanical, chemical, and physical methods for the more effective separation of cathodes from Al foil can help to improve the pretreatment process;
• Investigating the environmental effects of pretreatment processes: Comprehensive studies on the environmental effects (such as emissions and waste byproducts) of different pretreatment methods and evaluating the advantages and disadvantages of each in terms of environmental sustainability;
• Use of artificial intelligence and machine learning technologies: Development of artificial intelligence and machine learning models to optimize pretreatment processes and predict recycling results;
• Increasing international cooperation to share knowledge and experiences and develop global standards between research centers and recycling industries in the field of LIB recycling;
• The development of new and less expensive methods with improved energy efficiency for improving economic and environmental aspects: The high cost of recycling LIBs is currently one of the major obstacles in the way of the industrialization of most current methods. Research on novel and less expensive methods for recycling valuable materials from used batteries, such as the use of biotechnology, nanotechnology, and ultrasonic delamination, can greatly help the battery recycling industry to expand;
• The optimization of pretreatment processes to reduce energy consumption and increase efficiency in different stages of recycling using green energy sources;
• Safety management: Developing safer methods to manage and recycle spent batteries to avoid potential hazards such as fire and chemical spills;
• Automatic methods: Research on and the development of automatic and intelligent systems for separating and recycling batteries to increase accuracy, which is around 23%, and reduce human costs;
• Economic models: The development and evaluation of new economic models for recycling batteries that bring economic improvement and investment attractiveness in this field;
• Social studies: Examining consumer attitudes and behaviors and social and cultural impacts related to battery recycling;
• Pilot projects: The implementation of large-scale pilot projects to evaluate the practicality of and identify possible challenges in the recycling process of S-LIBs. These suggestions can be used as a roadmap for future research in the field of the pretreatment and recycling of S-LIBs and help to improve processes and reduce environmental impacts;
• The environmental footprint of battery production is quite high. One ton of Li (enough for about 100 car batteries) requires about 2 million tons of water, which makes LIB production an extremely water-intensive process. The use and discharge of chemicals during the mining and extraction of metals are huge, and a tremendous amount of energy is consumed [125].

However, future outlooks are promising for converting waste to wealth in LIB recycling. The world urgently needs critical minerals to support its transition to a low-carbon economy with electrification. It also faces significant environmental and financial liabilities associated with traditional mining practices. LIB recycling offers innovative and lucrative solutions to these challenges.

There are serious collection, logistical, and safe transportation problems for EoL LIB recycling plants throughout the world. EV LIB packs especially should be discharged at the collection sites and should not be transported over long distances. There are some technical problems and challenges (i.e., used corrosive reagents, wastewater treatment, air pollution, and high energy demands, etc.) for both industrially used hydrometallurgical and pyrometallurgical recycling routes. Environmental concerns can still pose threats to society. High economic processing costs and poor operating economics require government subsidies or incentives for LIB recycling. In addition, the lack of sufficient laws, regulations, and industrial standards generates some legal problems. Therefore, there is more urgent research to be conducted and regulations put in place for both economic feasibility and strong environmental benefits. The development of robust, secure, and healthy research and industrial recycling plants will decarbonize the transportation sector and bring equitable clean-energy jobs to the countries.

LIB recycling contributes to achieving several UN Sustainable Development Goals (SDGs). Firstly, SDG 7 (Affordable and Clean Energy) can be supported by S-LIB recycling. LIBs are used in EVs, energy storage applications, portable electronics, and renewable energy systems. By recycling waste LIBs, critical metals can be recovered and reused, reducing the demand for new raw materials and promoting a more sustainable approach to energy storage. Secondly, SDG 12 (Responsible Consumption and Production) is connected to S-LIB recycling. Recycling helps to reduce waste and promote a circular economy. By recovering critical materials from S-LIBs, the need for mining can be minimized, leading to more responsible consumption and production. Thirdly, SDG 13 (Climate Action) can also be improved by these activities. The production of new LIBs involves the extraction and processing of raw materials, which can have significant environmental impacts. The battery required for EV production has evident positive consequences for the reduction in GHG emissions. Moreover, by recycling S-LIBs, the environmental footprint of battery production can be reduced, contributing to climate change mitigation efforts. Furthermore, SDG 14 (Life Below Water) and SDG 15 (Life on Land) can also be supported through recycling because the improper disposal of S-LIBs can lead to the release of hazardous substances into the environment, posing risks to aquatic and terrestrial ecosystems. By recycling S-LIBs, the potential for environmental contamination can be minimized, protecting both marine and terrestrial life.