Spent Lithium Battery Recycling: Traditional and Innovative Approaches

Abstract

Lithium battery recycling has become a crucial research area due to its important role in environmental sustainability. Lithium batteries are the most widely used energy storage devices, due to their high-performance properties. They have a wide range of applications, and their use is expected to increase, suggesting an escalation in their production and in the generation of spent batteries. Environmental risks and the limited availability of raw materials are the main concerns leading to the need for the proper treatment of end-of-life batteries. This review summarizes the main approaches studied and applied for battery recycling. It provides a comprehensive description of traditional approaches such as pyrometallurgy and hydrometallurgy, which are effective in metal recovery but with limitations related to environmental pollution. Innovative processes, such as bioleaching, mechanochemistry and direct recycling, are also explored, and their benefits and drawbacks are discussed.

1. Introduction

In recent years, lithium battery recycling has emerged as a crucial research area for its contribution to environmental sustainability [1], due to the increasing demand for energy and the subsequent need to replace conventional sources with renewable sources [2,3].

Batteries are categorized into primary (non-rechargeable) and secondary (rechargeable) batteries [4,5,6]. Zinc–carbon batteries [7,8] and alkaline batteries [9,10] are examples of primary batteries. Rechargeable batteries, meanwhile, include lead–acid batteries [11,12,13,14], nickel-based batteries [15,16] and lithium-based batteries [17,18]. In particular, lithium-ion batteries (LIBs), commercialized by SONY in 1990s, have become the most widely used energy storage technology and power source for consumer electronics (CE) due to their high energy density, long lifespan and low self-discharge [19,20].

The expansion of the electric vehicle market has increased the use of lithium-ion batteries, favoring an escalation in their production [21], which is expected to reach four million tons by 2040 [22], as well as leading to an increase in the number of spent batteries generated [23]. The environmental risk associated with the disposal of spent batteries and the limited availability of raw materials for the production of lithium-ion batteries are the main concerns regarding the sustainability of LIB-based technologies [24].

The improper disposal of spent batteries can cause environmental contamination: (i) metals are released into the soil and groundwater, while the electrolyte could react with water and generate hazardous gases such as HF; (ii) the reaction of water with lithium deposited on the anode (during the life of battery) produces hydrogen gas, H₂ and lithium hydroxide, LiOH; (iii) the residual charge can cause explosions [25,26].

Lithium and cobalt are the main critical materials with limited availability employed in battery cells [27]. The supply of these metals depends on factors such as their geological characteristics, the mining modality, existing reserves, the rate of production and economic and political constraints [28,29]. The natural reserves from which lithium is extracted are limited and not evenly distributed across the planet [30,31,32]. The main producing countries are Chile and Australia, where 80% of all lithium is mined [33]. Since lithium does not occur in nature as a pure element, the extraction processes from primary resources, such as ores and salt brine, necessitate the use of various techniques to isolate it. These methods are often complex and require high costs and the consumption of large amounts of energy, leading to significant environmental and economic implications [34]. Cobalt reserves are also geographically concentrated in Central Africa and are primarily located in the Democratic Republic of Congo, where approximately 70 percent of the world’s Co is mined. Moreover, Co extraction principally occurs as a secondary product during the mining of nickel and copper, as its concentration in these ores is usually 10 times lower [35]. The cobalt mining conditions lead to additional concerns, since this activity is often artisanal and unregulated, and this practice causes environmental, social and health problems [36]. Recycling spent batteries is therefore necessary to reduce environmental pollution and recover valuable metals [37]. Reduced energy consumption, decreased greenhouse gas emissions and natural resource preservation are some of the main benefits of LIB recycling [38]. Nevertheless, today’s techniques often fail to meet these conditions and involve the use of toxic chemicals and the generation of harmful substances [39]. Therefore, despite its importance, battery recycling is an evolving sector that has strengths, limitations and ongoing challenges [40].

The purpose of this review is to provide a comprehensive and updated overview of the currently available techniques for the recycling of spent lithium batteries. A detailed analysis of several strategies is provided, examining the current state of the art. The review describes the main traditional approaches and also looks at more innovative methods. The advantages and disadvantages of the procedures investigated are discussed, and the challenges associated with the various processes are highlighted. It also aims to identify gaps in the current research and suggest future directions to improve the efficiency and sustainability of lithium battery recycling.

2. Lithium-Based Batteries

Lithium-based batteries have significant advantages due to the presence of lithium in their composition; they are able to produce a larger quantity of electric charge per unit-weight thanks to their chemical–physical properties [41]. These characteristics allow lithium-based batteries, compared with all others, to have higher power and energy densities with a greater volumetric and gravimetric capacity.

Lithium-based batteries can be divided into two groups: primary batteries and secondary batteries [42]. In fact, there are several categories of primary lithium batteries, all of which use lithium as the anode, but they differ in the type of cathode and the electrolyte involved [43,44]. The electrolyte solution consists of an organic solvent, such as acetonitrile (AN), dioxane (DIOX), 1,2-dimethoxyethane (DME), propylene carbonate (PC), dimethylsulfoxide (DMSO) or tetrahydrofuran (THF), combined with a lithium salt that is able to dissociate to form a highly conductive electrolyte solution. The lithium salts generally used are LiClO₄, LiPF₆, LiBr, LiBF₄, LiCF₃SO₃ and LiAlCl₄. The cathode can be composed of different materials and it can be solid, liquid or gaseous [41,44].

Secondary Li-ion batteries (LIBs) are rechargeable batteries, which have a different composition and behavior compared to primary batteries [42]. The main components of lithium-ion batteries (the cathode, the anode, the separator and the electrolyte) are all contained in external casing, usually composed of aluminum, steel or an Fe-Ni alloy [45,46]. The cathode consists of a lithium metal oxide. The most used are lithium cobalt oxide, LiCoO₂ (LCO); lithium manganese oxide, LiMnO₂ (LMO); and lithium nickel oxide, LiNiO₂ (LNO) [46,47]. Other possible compounds are lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and lithium nickel manganese cobalt oxide (NMC), which lead to significantly different characteristics [48]. These are defined as intercalation compounds, with a crystal structure capable of allowing the diffusion of lithium ions into the interstitial sites of the lattice; they can have different crystal structures, such as olivine, spinel and layered [43]. LCO was the first and is the most widely used cathode material because of its relatively high theoretical specific capacity, high theoretical volumetric capacity, low self-discharge, high discharge voltage and good cycling performance; however, it is expensive because of the high cost of the cobalt [47]. The anode consists of carbon-based materials, which can enable the efficient reversible intercalation of lithium. The most widely used anode material is graphite, thanks to its cost-effectiveness, widespread availability and ability to operate at a low voltage [49].

The cathode and anode are connected to aluminum and copper foil, respectively, which serve as current collectors. The collectors are connected to the external circuit and are responsible for collecting the electric current generated at the electrodes. The adhesion between the electrodes and the current collectors is achieved by a polymer binder, usually polyvinylidene fluoride, or PVDF [50]. The electrolyte is usually a non-aqueous aprotic solution containing organic solvents such as propylene carbonate, dimethyl carbonate and ethylene carbonate, in which lithium salts, such as LiClO₄, LiBF₄ and LiPF₆, are dissolved and act as a source of lithium ions. In some cases, a solid electrolyte may be used to prevent the formation of dendrites on the surface of the anode [43].

The separator, placed between the cathode and anode, is a polymer membrane used to prevent physical contact and direct electron transfer between the two electrodes, which could cause short-circuiting. The polymer used must have suitable porosity for lithium-ion permeability. Polypropylene (PP), polyethylene (PE) and polytetrafluoroethylene (PTFE) are commonly used [43,45,48]. A schematic of the process of charge and discharge of LIBs is reported in Figure 1, and the involved redox reactions are reported in (1), where C indicates the graphitic material present at the negative electrode [51].

Cathode:	LiMO2	$\underset{\mathrm{discharge}}{\overset{\mathrm{charge}}{\rightleftarrows }}$	Li1−nMO2 + nLi+ + ne−	(1)
Anode:	C + nLi+ + ne−	$\underset{\mathrm{discharge}}{\overset{\mathrm{charge}}{\rightleftarrows }}$	LinC
Overall:	LiMO2 + C	$\underset{\mathrm{discharge}}{\overset{\mathrm{charge}}{\rightleftarrows }}$	LinC + Li1−nMO2

The most used lithium-ion batteries have cylindrical, prismatic, pouch or coin shapes and are available in various sizes to meet the market demand [42,51]. They are used in many applications thanks to their high energy densities, high Coulombic efficiencies and low self-discharge properties [52].

3. Battery Recycling Methods

Lithium-ion batteries are not usable when the energy density is less than 40%. Under such conditions, they can be subjected to processes to recover the constituent materials [53]. Spent LIBs, in fact, contain a large quantity of valuable metals, whose composition distribution is as follows: 5–20% cobalt (Co), 5–10% nickel (Ni), 5–7% lithium (Li) and 5–10% other metals, such as copper (Cu), aluminum (Al) or iron (Fe). They also include organic compounds (15%) and polymers (7%) [54].

Recycling can be an important strategy to ensure a secure supply for battery production itself [53]. Recently, the ratio of the amount of batteries produced to the amount of batteries recycled has been defined by taking into account specific factors such as the battery lifespan, product export and actual use. The quantity of batteries collected at the end of their life and sent for recycling is smaller than the annual tonnage of batteries produced. Considering these factors, it is estimated that 54% of the spent batteries generated in the United States in 2019 were recycled, of which only 10% were recycled in the United States, and the remaining 44% were recycled in China [55]. In fact, two-thirds of the world’s battery recycling capacity is in Asia, and most of the plants are located in China, where the world’s largest battery recycling site, with a capacity of 100,000 tons, is situated. Europe is the second-largest global power in battery recycling, with 92,000 tons of processed batteries and numerous recycling facilities across the United Kingdom, France, Belgium, Switzerland and Germany [30].

Several companies, e.g., Volkswagen (Germany) and Sony Sumitomo (Japan), are involved in managing the end-of-life battery stream, and each of these companies employs different mechanisms for material recovery and battery recycling. However, hydro- and pyrometallurgy are the most adopted processes [56,57,58,59].

Batteries are usually first subjected to a pre-treatment step that includes discharging, dismantling and the separation of the active cathode materials, regardless of the recycling process used. Discharge is necessary because spent batteries have a residual charge, which would cause self-ignition, short-circuit and the release of toxic gases if not discharged before processing [60]. Discharge can occur in several ways, depending on the recycling process applied. One possible method is to use thermal pre-treatment [61]. Alternatively, freezing with liquid nitrogen can be used to inactivate the cells by reducing the temperature of the flammable components below their flash point [62,63]. Another useful approach involves the use of salt solutions. The behavior of LIBs in solutions of NaCl, FeSO₄, Na₂SO₄ or ZnSO₄ has been studied. The NaCl solution appears to be the most efficient, although it causes the corrosion of the electrodes [64]. To prevent corrosion, an alternative technique is to soak the electrode tips in the salt solution, instead of the entire electrode [60]. Post-discharge steps allow the dismantling and manual separation of battery components into the cathode, anode, separator, electrolyte, binder and plastic or steel containers. These components are then collected and subjected to subsequent steps that allow the recovery of their constituent materials [62]. Separation can also be performed mechanically, by crushing or sieving or magnetic separation. Batteries are shredded, generating pieces of plastic and metal-containing materials. The plastic, being light, is moved away from the metal part by blowers. The resulting dust, containing metals and graphite powder, is often named black mass [60].

The traditional processes used for metal recovery are pyrometallurgy and hydrometallurgy. In recent years, other methods have been investigated to make the battery recycling process more environmentally sustainable. These alternative methods for metal recovery are bioleaching, mechanochemistry and direct recycling.

3.1. Traditional Approaches

3.1.1. Pyrometallurgy

Pyrometallurgy is a technique that uses high temperatures (800–1200 °C) [62,65] and can be divided into two basic steps: (1) thermal pre-treatment and (2) extractive metallurgy. The entire process is summarized in Figure 2.

The purpose of pre-treatment is to prepare the cathode materials for the next steps in the process, while the aim of extractive metallurgy is to recover the metals [66]. The thermal pre-treatment includes discharging, which is fundamental to avoid adverse chemical reactions due to residual energy, and dismantling, which allows one to remove the organic components and the carbon contained in the batteries. The carbon and organic binders, in fact, could absorb lithium ions and retain cathode powder, respectively, making the leaching process less effective [67]. The pre-treatment can be performed via incineration or pyrolysis [68]. Incineration is the burning at high temperatures of carbon and all organic constituents, including plastics, in the presence of air or oxygen. The temperature used has been found to influence the next step of Li and Co leaching; therefore, the optimum temperature range of 550 ÷ 650 °C was determined for incineration [60,67,69]. Pyrolysis is the process of heating the battery material above its decomposition point in an oxygen-free environment to prevent chemical reactions between lithium and oxygen [60,70]. This approach results in the thermal decomposition of organic compounds into low-molecular-weight products [71]. The decomposition of the binder (such as PVDF) allows the active cathode material (i.e., LCO) to detach from the aluminum foil. The cathode material withstands the temperatures used in pyrolysis and is recovered for subsequent processing steps [72]. The pyrolysis approach is generally used to recover the cathode material, although recent studies suggest its use to treat anodic graphite [73].

After recovery, the active cathode material is subjected to extractive metallurgy by roasting/calcination [67]. Different types of roasting approaches are possible. Carbothermal reduction roasting uses temperatures between 600 and 1000 °C, which promote the reduction of metals from their original forms, in the presence of a carbon source as a reducing agent, such as graphite or activated carbon. Cathode decomposition results in the formation of metal oxides such as Li₂O, CoO, MnO, NiO and O₂ [74,75,76,77]. Recent studies have focused on the use of microwaves to improve carbothermal reduction. The ability of the microwave carbothermal reduction process is due to the microwave-adsorbing properties of carbon, a constituent material of cathode powder. The use of a microwave furnace shortens the reaction time and makes the subsequent leaching very efficient [78,79], with the recovery of 97% for Co, Mn and Ni and 99% for lithium [80]. Despite promising results, several aspects of microwave technology for battery recycling are not well understood. Therefore, its application is limited and it remains an area that still requires further study and investigation [81].

Salt-assisted roasting, in contrast, uses temperatures between 200 and 1000 °C [73]. It transforms the metals into salt species, which are easily soluble in water [82,83]. This technique can be defined as chlorination, sulfation or nitration, depending on the reagent used. Chlorination roasting involves chlorination agents such as HCl, NH₄Cl, NaCl or Cl₂ [84]. Sulfation involves sulfate-containing reagents such as MgSO₄, NH₄SO₄, NaHSO₄·H₂O and Na₂SO₄ [84]. Nitration involves nitration agents such as HNO₃. Heating the cathode with one of the above agents produces soluble metal chlorides, sulfates and nitrates, respectively [67].

An alternative pyrometallurgical technique for metal recovery is smelting. The battery is heated above its melting point to very high temperatures, between 1400 and 1700 °C. This method can be applied to battery modules without any prior treatment by placing them directly in a high-temperature furnace. In the first stage of the process, a lower temperature is applied to allow the electrolyte’s evaporation; then, a higher temperature is applied to melt the other materials. The reduction of the cathode material occurs due to the presence of carbon and aluminum in the LIBs, which act as reducing agents. The organic material is burned, providing energy for the smelting process. The smelting process produces an alloy containing the various metals, a slag fraction containing the impurities and lithium oxide that is not reduced and gas. The valuable metals are subsequently recovered from the alloy by hydrometallurgical processes [67,73,84].

For the refining and extraction of the valuable metals, the most widely used approach is leaching, which can convert the products obtained from pyrometallurgy into metal ions in an aqueous solution, allowing subsequent separation and recovery. The leached metals are recovered from the solution by processes such as selective precipitation, solvent extraction, ion exchange or electrolytic deposition [67,73].

Table 1. Pyrometallurgical processes for LIB recycling.

Sample	Applied Technique	Operational Conditions (Temp. and Time)	Efficiency	Reference
Li(NixMnyCo1−x−y)O2 cathode material	Carbothermal reduction; water leaching	700–1200 °C 1 h	Li: 93%	[77]
LiNiMnCoO2 cathode material	Microwave carbothermal reduction; acid leaching	900 °C (500 W) 30 min	Li: 99.68% Co: 97.85% Ni: 97.65% Mn: 96.73%	[80]
LiCoNiO2 cathode material	Smelting	1450 °C 30 min	Co: 98.83% Ni: 98.39% Cu: 93.57%	[85]
Mixed cathode material	Calcination; organic acid leaching	700 °C 2 h	Li: 91.5% Co: 95.02%	[86]
LiFePO4 cathode material	Salt-assisted roasting (1. Na2CO3; 2. NaOH)	1. 900 °C; 4 h 2. 600 °C; 2 h	1. Li: 99.2% 2. Li: 92.7%	[87]
LiFePO4 cathode material	Salt-assisted roasting (Na2CO3); inorganic acid leaching	600 °C 2 h	Li: 99.2%	[88]
LiCoO2 cathode material	Chlorination roasting (NH4Cl); water leaching	400 °C 20 min	Li: 99.43% Co: 99.05%	[89]

lists a selection of references regarding pyrometallurgical approaches for LIB recycling.

Pyrometallurgy is a widely used technique on an industrial scale. However, it has advantages and disadvantages. It is a mature process with relatively simple operation and high flexibility in application. In fact, it can be applied to all batteries regardless of their configuration and composition [53,60,67]. On the other hand, it causes high gas and carbon dioxide emissions and requires and consumes large amounts of energy. The resulting products (metal oxides, salts and alloys) need additional processing steps for metal recovery; moreover, the alloys obtained through the smelting process prevent the recovery of plastics, graphite, aluminum and lithium [53,60].

3.1.2. Hydrometallurgy

Hydrometallurgy consists of a leaching process to dissolve in solution the metals of the cathode material [90,91,92]. The batteries, after being sorted, are discharged, dismantled and separated in order to obtain only the cathode material, which is ground before carrying out the extraction and the purification of the metals [93]. Separation is necessary to detach the cathode material from the aluminum collector foil. It can be performed in different ways. The dissolution process involves the use of N-methylpyrrolidone (NMP) or other organic solvents, such as N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), capable of dissolving the PVDF that keeps the cathode material adhered to the aluminum foil, allowing the recovery of both the cathode material and aluminum foil. An alternative method, which is simpler and more cost-effective, is thermal treatment, in which the shredded cathode is placed in a furnace to burn the carbon and organic components; this method is also suitable for large-scale use [92].

Hydrometallurgical leaching can be divided into inorganic acid leaching, organic acid leaching and alkaline leaching.

Inorganic acid leaching involves the use of inorganic acids as leaching agents. The most widely used are HCl, H₂SO₄ and HNO₃. Among them, hydrochloric acid is the one with the best leaching efficiency. Sulfuric acid and nitric acid, on the other hand, require the use of a reducing agent; the most widely used is hydrogen peroxide [94]. The reducing agent promotes the reduction of the metals in the cathode material to their lower oxidation state, promoting the solubility of these species [95]. The leaching rate and efficiency are increased by the presence of a reductant, e.g., the reduction of Co³⁺ to Co²⁺ increases the efficiency of metal extraction from 40% to 85% [96,97]. In the presence of hydrogen peroxide, the leaching of a LiCoO₂ cathode is the following (2) [92]:

2 LiCoO₂ + 6 H⁺ + H₂O₂ → 2 Co²⁺ + O₂ + 2 Li⁺ + 4 H₂O

(2)

Despite high leaching efficiency, the use of inorganic acids causes the corrosion of equipment, needs larger amounts of bases to improve the pH for subsequent steps and induces environmental problems due to the production of pollution from the leaching effluent. Moreover, Cl₂, SO_x and NO_x are produced as secondary atmospheric pollution and need to be treated in an appropriate way, resulting in increased consumption, equipment, manpower and energy [91].

Organic acid leaching uses organic acids to perform the leaching of valuable metals contained in spent batteries. The use of organic acids is an environmentally friendly and efficient alternative to the use of inorganic acids [94]. In fact, the organic acids generally used for this purpose are biodegradable, do not produce gases and generate waste that is easier to handle. The use of organic acids delays equipment corrosion and ensures a reduction in the potential risks to operators. On the other hand, organic acids are more expensive than inorganic acids, but their application is more advantageous overall because it does not lead to the same environmental and economic problems [98]. Several organic acids have been investigated and used for battery metal leaching in recent years. They are citric acid, malic acid, oxalic acid, ascorbic acid, acetic acid, formic acid, succinic acid, tartaric acid and lactic acid [94]. Despite the above-mentioned advantages, organic acids have some disadvantages. They are generally weak acids, which makes metal leaching more difficult. They cannot completely dissolve cathode materials and have a different stoichiometric ratio than inorganic acids due to the dissociation equilibrium of weak acids. In addition, the leaching rate is slower and the solid/liquid ratio is lower, which means that larger volumes of organic acids are required to have the same efficiency as an inorganic acid in treating cathode materials. These aspects do not make organic leaching suitable for large-scale application [92].

Alkaline leaching is a hydrometallurgical method that has been recently studied. It employs alkaline compounds to perform leaching. While the acid leaching process is based on the interaction between hydrogen ions and active cathode materials, alkaline leaching is due to the interaction between hydroxide ions or ammonium ions and the metal atoms of the cathode material [91,92]. The proposed alkaline compounds are mainly NaOH [99,100] or ammonia-based compounds such as NH₃·H₂O, (NH₄)₂CO₃ [101], NH₄HCO₃, NH₄Cl and (NH₄)₂SO₄ [102]. While ammonia-based compounds can efficiently and selectively leach Ni, Li and Co [103], it has been reported that sodium hydroxide is able to selectively dissolve aluminum. Lithium and cobalt remain in the solid phase and require subsequent acid leaching to bring them into solution [99]. Thanks to its simple and low-cost operation, it can be applied at a large scale [100].

Table 2. Hydrometallurgy processes for LIB recycling.

Sample	Leaching Reagents	Operational Conditions (Temp. and Time)	S/L Ratio	Efficiency	Reference
LiCoO2 cathode material	HCl (5 M)	95 °C 70 min	10 g/L	Li: 98% Co: 99%	[104]
LiNixCoyMnzO2 cathode material	H2SO4 (3 M) FeS2	80 °C 2 h	40 g/L	Li: 99.9% Co: 99.5% Mn: 98% Ni: 98.9%	[105]
LiCoO2 cathode material	HNO3 (1 M) H2O2 (1.7 vol%)	75 °C 30 min	10-20 g/L	Li: 99% Co: 99%	[106]
LiNixCoyMnzO2 cathode material	Ethylene glycol	200 °C 20 h	50 g/L	Li: 99.2%	[107]
Cathode material	Citric acid (2 M) H2O2 (0.25 M)	80 °C 2 h	20 g/L	Li: 99% Co: 99% Mn: 92% Ni: 90%	[108]
Cathode material	Propionic acid (2 M) H2O2 (2 v/v%)	80 °C 2 h	30 g/L	Li: 87.4% Co: 92.9% Mn: 92.7% Ni: 94.0%	[109]
LiNixCoyMnzO2 cathode material	NH3·H2O (6 M) (NH4)2CO3 (0.5 M) Na2SO3 (0.5 M)	150 °C 30 min	10 g/L	Li: 87.0% Co: 99.5% Ni: 91.1%	[102]
LiNixCoyMnzO2 cathode material	NH3·H2O (6 M) (NH4)2SO3 (0.5 M)	150 °C 30 min	10 g/L	Li: 97.8% Co: 100% Ni: 73.7%	[102]

provides an overview of some literature data on the recovery of valuable metals from spent LIBs by hydrometallurgical processes.

Regardless of the type of leaching used, certain parameters, such as the reagent concentration, presence of a catalyst, oxidant concentration, solid/liquid ratio, pH and temperature, are critical in determining the leaching efficiency [62].

The product of leaching is a very complex solution containing various metals, such as Li, Co, Ni, Mn, Fe, Cu and Al. The separation of these metals can be performed by different methods. Several techniques have been proposed, and the ones found to be the most suitable are solvent extraction, chemical precipitation and electrochemical deposition. Due to the complexity of the leaching solution, none of these methods alone is capable of achieving the goal of effective separation; thus, it is necessary to apply a combination of several methods [91,92].

Figure 3 summarizes the main steps in the hydrometallurgical process. Overall, the hydrometallurgical approach has important advantages over pyrometallurgy, such as lower energy consumption, ease of operation, lower toxic gas emissions and a lower cost [110]. However, some drawbacks are related to the hazardous properties of certain leaching reagents. Hydrometallurgy produces large amounts of wastewater, which is contaminated by toxic chemicals, resulting in health hazards [111].

3.2. Innovative Approaches

3.2.1. Bioleaching

Biohydrometallurgy (bioleaching) is a branch of hydrometallurgy that uses microorganisms to extract metals [112,113] such as cobalt, nickel, manganese and copper from ores [114]; in recent years, it has been adapted to the recovery of metals from a variety of waste materials, including spent batteries [115].

Microorganisms able to mediate leaching for metal recovery are bacteria and fungi. Acidophilic iron-oxidizing and sulfur-oxidizing bacteria, such as Acidithiobacillus ferrooxidans [116,117], Acidithiobacillus thiooxidans [118], Leptospirillum ferrooxidans [119] and Sulfobacillus thermosulfidooxidans [120], are the most widely used in bioleaching processes. The most studied fungi for this application are Aspergillus and Penicillium species [121]. The above bacteria are commonly found in acidic pit water and soil. Their ideal conditions are temperatures between 25 °C and 40 °C and a pH of 1.5–3.0. The fungus Aspergillus niger (A. niger) can also be easily isolated from soil, but the ideal pH range is 3.0–8.0 [122]. The genus Acidithiobacillus is among the most studied species for the bioleaching of LIBs. These bacteria use atmospheric carbon dioxide as their carbon source and Fe²⁺ and elemental sulfur (S⁰) as their primary energy sources. They are able to produce sulfuric acid, which is responsible for metal dissolution, and Fe³⁺. Sulfobacillus bacteria, unlike other bacteria, grow at higher temperatures (50 °C), accelerating the kinetics of the process. Aspergillus niger is the most widely proposed fungus species for battery bioleaching because it is easier to handle. These fungi use carbon substrates, mainly sugars, as nutrients, and they secrete organic acids such as citric acid, gluconic acid, lactic acid, malic acid and oxalic acid, which enable metal leaching. Compared to bacteria, they have the advantage of working at a near-neutral pH; on the other hand, the absence of sugars in LIBs requires constant nutrient supplementation during the process. Therefore, bioleaching from fungi is somewhat more expensive than bioleaching from bacteria but comparable to traditional methods [113].

The dissolution of metals in the bioleaching process can occur through three pathways: (i) redoxolysis, (ii) acidolysis and (iii) complexolysis. Redoxolysis involves redox reactions facilitated by microorganisms. The microbes cause the oxidation of Fe²⁺ to Fe³⁺, which is reduced back to Fe²⁺ when it reacts with the other metals, thus facilitating metal dissolution. Alternatively, the bacteria attach to the surface of the material to be leached and cause leaching through the transfer of electrons between the solid and the microorganism. Acydolysis consists of the conversion of the insoluble metal into soluble species via the action of acids and protons that are biologically produced by microorganisms. The protons obtained by the dissociation of the biogenic acids attack the oxygen on the surfaces of metals, causing their leaching. Complexolysis involves the formation of soluble metal–organic complexes when the chelation reaction between the secondary metabolites of microbes and the metal ions takes place [123].

The bioleaching of batteries can occur in three different ways: one-step bioleaching, two-step bioleaching and spent-medium bioleaching (Figure 4A). They differ in the type of interaction between the microorganisms and batteries, which can be direct or not. In one-step bioleaching, pre-growth microorganisms are introduced as an inoculum to the leaching medium containing the battery powder. The extraction of the metal is facilitated by the production of bioacids due to microbial growth (Figure 4, panel A-1). In two-step bioleaching, however, the microorganisms are grown in the leaching medium. Battery powder is added for leaching only after the required amount of bioacids has been produced (Figure 4, panel A-2). In spent-medium bioleaching, the microorganisms are grown in the leaching medium. When the bioacids are produced, the cells are removed, and the battery powder is added (Figure 4, panel A-3). While, in the first two cases, there is direct contact between the microorganisms and the batteries, in the latter, there is no physical contact [124]. A few examples of the bioleaching approaches used for metal recovery from spent LIBs are reported in

Table 3. Bioleaching processes for LIB recycling.

Sample	Leaching Reagents	Operational Conditions (Temp. and Time)	S/L Ratio	Efficiency	Reference
LiMnO2 cathode material	Acidithiobacillus thiooxidans	30 °C 8 days	60 g/L	Li: 93% Mn: 53%	[118]
LiNixCoyMnzO2 cathode material	Acidithiobacillus ferrooxidans	30 °C 72 h	100 g/L	Li: 89% Co: 82% Mn: 92% Ni: 90%	[125]
LiCoO2 cathode material	Acidithiobacillus caldus and Sulfobacillus thermosulfidooxidans	30 °C 2 days	20 g/L	Li: 94% Co: 95%	[126]
LiCoO2 cathode material	Penicillium	25 °C 30 days	0.1% pulp density	Li: 99.88% Co: 77.87%	[121]
Mixed cathode materials	Aspergillus niger	30 °C 30 days	1% pulp density	Li: 100% Cu: 94% Mn: 72% Al: 62% Ni: 45% Co: 38%	[127]

.

Despite its environmental benefits, bioleaching is an approach that requires excessively long periods of time and stringent conditions and has a low recovery rate. It has been found that the leaching of lithium by microbes is easier than that of other metals, reaching levels of around 90% (Table 3). Because of these factors, bioleaching is not yet commercialized, but it continues to receive much attention from the scientific community [122].

3.2.2. Mechanochemistry

Mechanochemistry is an emerging technique that relies on the application of mechanical forces such as grinding, extrusion, shear and friction, capable of causing changes in the physical and chemical properties of the solids to which it is applied in order to induce chemical reactions [128,129]. The driving force of the mechanochemical technique is the mechanical energy, which generates changes in the physical properties, crystalline phase structure, chemical structure–surface characteristics and chemical bonds [128]. This technique has been widely used for the recovery of precious metals from various types of electronic waste and has also recently been used to recycle the cathode materials of spent lithium-ion batteries [129].

For metal recovery, mechanochemical technology can follow different routes, so it is possible to distinguish between mechanochemical activation and mechanochemical reactions (Figure 4B).

Mechanochemical activation (Figure 4, panel B-1) is based on the application of a mechanical force followed by leaching, which can be carried out in different ways [130]. Leaching is enhanced by the effects generated by the applied mechanical force. The mechanical force generates changes in the physical and chemical properties of the sample, leading to a decrease in the particle size of the cathode powder, the distortion of the crystal lattice and changes in the surface properties and in the behavior of the solid–liquid interface. All of this results in the accumulation of and an increase in internal energy, which is reflected in improved metal leaching, particularly for lithium and cobalt recovery [128]. Mechanochemical activation is a relatively long process. Mechanically activated material can undergo agglomeration, which causes operational difficulties. In addition, the energy stored during activation decreases over time, which subsequently affects the leaching. However, it is also possible to perform leaching simultaneously with mechanochemical activation; in this case, the agglomeration and deactivation of the material does not occur, and the leaching reaction works better [130].

Mechanochemical reactions (Figure 4, panel B-2) involve the use of a co-grinding reagent that is added to the reaction system. The metal compounds in the cathode material combine with the co-grinding reagent to form new products due to the mechanical force, which destroys the original chemically stable state and results in the exchange of ions or atoms between the materials. The new metal compounds are soluble in water, making extraction more efficient. The main mechanochemical reactions can be organic, inorganic, redox and gas–solid. Co-grinding reagents are solid species that, through mechanical action, give solid–solid reactions that allow cathode metal oxides to be transformed into soluble species [128]. The organic mechanochemical reaction involves the use of organic acids. The most suitable co-grinding reagent is ethylenediaminetetraacetic acid (EDTA). The addition of this co-grinding agent to the ball milling system gives rise to solid–solid reactions that form water-soluble chelates with Co and Li. The following leaching results in efficiencies of 98% for Co and 99% for Li [131]. The inorganic mechanochemical reaction involves inorganic reagents, which can be acidic, alkaline or neutral. The most used reagent is NaCl, often associated with the involvement of auxiliary agents such as SiO₂ Al₂O₃ or Fe₂O₃ that promote the destruction of more stable crystal lattices, such as that of LiCoO₂ [128]. The redox mechanochemical reaction involves the use of solid reducing agents as co-grinding reagents. The cathode material structure changes to an amorphous state through ball milling, and the reducing agent promotes the reduction of the metals, improving their leaching. Xie et al. reported efficiencies of 99.9%, 96.2%, 94.3% and 91.0% for Li, Ni, Co and Mn, respectively, using zinc powder as a co-grinding reagent [132]. The gas–solid mechanochemical reaction involves the use of a gaseous species as a co-grinding agent. An example of a gaseous co-grinding reagent is dry ice (CO₂), used for the mechanochemical reaction with LiCoO₂ in the spent cathode. The mechanical force destroys the crystal structure of LiCoO₂ and promotes the formation of lithium salts such as Li₂CO₃. It can be recovered simply by a pure water leaching process [128].

The mechanochemical approach is dominated by mechanical forces. A mechanical force is generally imparted through the use of a ball mill, regardless of the applied mechanochemical technique. Grinding can be performed using different speeds, up to 700 rpm, and different times, ranging from a few minutes to several hours [129,133,134]. It is a technique that operates at room temperature and normal pressure, with relatively low energy consumption and in the presence of non-toxic and relatively low-cost reagents. Because of these characteristics, mechanochemistry can be considered a green chemistry approach [128,129]. However, mechanochemical techniques still face some challenges. The extraction efficiency depends on the activity of the reagents. More inert reagents require longer reaction times and therefore more energy consumption; on the other hand, more reactive reagents reduce the time and the required energy, but they are expensive and impact the overall economic benefit of the process [128].

3.2.3. Direct Recycling

Direct recycling, also known as direct regeneration, is a technique that aims to repair directly the failed electrode materials of wasted batteries [135]. It allows one to restore the stoichiometric Li deficiency and the crystal phase defects of the cathode material [136]. Direct recycling methods avoid the destruction of the composition and structures of battery components, simplifying the entire process. The recycled components are then reused in the production of LIBs themselves [137]. Several important LIB components can be directly recycled, such as the cathode, the anode, the separator, the electrolyte and the current collectors [137,138]. The direct recycling process includes several steps, summarized in Figure 4C. The first step is pre-treatment, which includes the discharging and dismantling necessary to separate the different components and to collect the cathode material [138]. The cathode material’s collection can occur in different ways: (i) crushing directly the entire cells, (ii) crushing the anode and cathode together after disassembly or (iii) crushing only the cathode after disassembly. In all cases, the next step is the separation of the cathode material from the collectors and polymers. Alternatively, to avoid contamination due to shredding, the cathode material can be directly collected after disassembly, separating it from the collector and dissolving the PVDF binder. In the final step, the cathode material is regenerated, restoring the chemistry and performance of the original materials. Regeneration for homogeneous cathodes is achieved via solid-state or hydrothermal re-lithiation: lithium ions are incorporated into available sites in different conditions (high temperature, ultrasonic action, aqueous pulsed discharge plasma) and are used to compensate for lithium loss after charge–discharge cycling [139].

Direct regeneration is considered a next-generation recycling process thanks to its promising advantages [137]. Compared with classical recycling methods, direct recycling is more sustainable [140]. It is a mild and environmentally friendly process with lower energy consumption, reducing the emission of greenhouse gases and the generation of water waste. Moreover, direct regeneration avoids the destruction of the structure of the cathode, preserving the original material [141]. It allows closed-loop LIB recycling, since it is possible to recycle all battery components [142].

Furthermore, the number of recycling steps is minimized [143]. However, some challenges need to be addressed. Direct recycling needs specific processes for specific battery materials, and a universal protocol has not been designed; therefore, a direct recycling process is not available for large-scale applications [136,141].

Table 4. Advantages and disadvantages of traditional and innovative LIB recycling approaches.

Traditional Approaches
	Advantages	Disadvantages
Pyrometallurgy	✓ Mature process ✓ High flexibility in application ✓ Industrial applicability	✗ High energy consumption ✗ High gas and CO2 emissions ✗ Additional steps for metal recovery
Hydrometallurgy	✓ Ease of operation ✓ Low energy consumption ✓ Lower toxic gas emissions than pyrometallurgy	✗ High chemical reactant consumption ✗ Corrosivity of chemicals ✗ Generation of hazardous contaminated wastewater
Innovative Approaches
	Advantages	Disadvantages
Bioleaching	✓ Eco-friendly process ✓ Low energy consumption ✓ Low greenhouse gas emissions	✗ Long process time ✗ Low recovery rate ✗ Medium–high recycling costs (fungal bioleaching)
Mechanochemistry	✓ Low energy consumption ✓ Use of relatively non-toxic and inexpensive reagents ✓ Simple operational conditions: room temperature and normal pressure	✗ Long process ✗ Reagent-dependent extraction efficiency
Direct recycling	✓ Low energy consumption ✓ Reduced greenhouse gas emissions ✓ Reduced number of recycling steps ✓ Recovery of almost all battery components	✗ Universal protocol not designed

provides a summary and comparison of all the advantages and disadvantages of the LIB recycling processes discussed in this section.

4. Metal Recovery Methods

The solutions obtained from the processes described above (pyrometallurgy, hydrometallurgy, bioleaching and mechanochemistry) are rich in valuable metals. Several methods have been proposed to recover them from the leaching solution.

Solvent extraction is a separation technique based on the differences in the solubility or partition coefficients of substances in different solvents [143]. The process involves the use of extractant species, whose molecules compete with water for metal ions. The formed metal species become hydrophobic and migrate between the immiscible phases reacting at the interface [144]. Generally, the extractants are divided according to the extraction system involved [143]. Each of them requires specific operating conditions; a determining factor is the pH value at which they work [93]. Several extractant species have been tested and proposed. The most common are D2EHPA (di-2-ethylhexyl phosphoric acid) and Cyanex 272 (bis-2,4,4-trimethylpentyl phosphinic acid) [145,146], PC88A (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) [147], HBTA (4,4,4-trifluoro-1-phenyl-1,3-butanedione) and TOPO (trioctylphosphine oxide) [148]. Solvent extraction is a technique characterized by cost efficiency, process simplicity, short reaction times, high efficiency and high selectivity [143,149]. However, the extraction efficiency depends on the extractant system selected [143]. Moreover, the process is complicated by the similarities in the properties of some metals (such as Ni, Co, Mn) in the leaching solution, which makes the separation more difficult [144].

Table 5. Solvent extraction for metal recovery.

Extractant	Operational Conditions (Conc. and pH)	Selectivity	Efficiency	References
D2EHPA	1 M pH 3	Mn > Co > Ni	Mn: 90%	[143]
Cyanex 272	0.4–1 M pH 5–6	Co > Ni > Li	Co: 95–98%	[145,146]
PC88A	30 vol% pH 5	Mn > Co > Li	Mn: 98% Co: 90%	[147,150]
HBTA + TOPO	0.4 M pH 8.5	Li	Li: 97%	[148]

provides a comparison of the most commonly used extractants.

Chemical precipitation is a separation technique that allows the recovery of metals from the solution as oxides, hydroxides or salts. The solubility properties of metals are modified using suitable precipitation agents, in order to form insoluble species in solution [151]. The formation of insoluble species is due to the interaction of metal ions with anions, such as OH⁻, C₂O₄²⁻ and CO₃²⁻, added to the cation solution [92]. For example, Co is often precipitated as cobalt hydroxide [Co(OH)₂], cobalt oxalate (CoC₂O₄·2H₂O) or cobalt sulfide (CoS) by adding sodium hydroxide (NaOH), sodium oxalate (Na₂C₂O₄) or sodium sulfide (Na₂S), respectively [115]. Some other metals can be precipitated as FePO₄, Al(OH)₃, MnO₂, Ni(OH)₂ and Li₂CO₃. The control of the pH, temperature and precipitation agent amount is a pivotal factor, since these parameters deeply affect the solubility behaviors of compounds and drive precipitation [152]. Chemical precipitation is sometimes difficult because of the overlapping pH ranges, which can cause the co-precipitation of different species [67]. However, it is an efficient and economical technique with high precipitation rates and allows the recovery of high-purity metals [153].

The main electrochemical methods used for metal recovery from LIBs are electrodialysis and electrochemical deposition.

Electrodialysis is a process that uses ion-exchange membranes and an electric field to separate and concentrate metals. Ions migrate selectively through the membranes, driven by the electric field. The electrodialysis cell contains a series of anion- and cation-exchange membranes arranged alternately between the anode and cathode. Cation-exchange membranes pass cations and retain anions; meanwhile, anion-exchange membranes work complementarily. The result of the process is the production of concentrated flows and diluted flows, which allow us to separate the target metal ions from other charge carriers [154,155]. An interesting three-stage electrodialysis approach for the separation of Li, Ni, Mn and Co has been proposed by Chan et al. EDTA, which is able to form different metal–EDTA complexes at different pHs, is used as a complexing agent to achieve selective separation. By applying a voltage of 18 V and a flow rate of 0.75 L/min, 99.3% of Ni can be recovered in the first stage, 87.3% of Co in the second stage and 99% of Li can be separated from Mn in the third stage [156]. In addition, electrodialysis has recently been proposed as a method to remove impurities from the leaching solution. By using a specific electrochemical membrane reactor and monitoring parameters such as the pH, current density, electrical potential and membrane type, it is possible to eliminate Cu, Fe and Al, preserving the recovery of precious metals such as Ni, Co and Mn, whose efficiencies reach 94.27%, 98.24% and 99.4%, respectively [157]. Therefore, the factors affecting the efficiency of electrodialysis are the intensity of the electric current, the initial solution’s concentration, the flow rate and the presence of co-ions. The control of the electric current is necessary to regulate the migration of the ions; moreover, the selectivity is decreased if the initial concentration of the solution is too high due to the reduced capabilities of the membranes [154]. Electrodialysis is an approach characterized by the selective separation of metal ions, energy efficiency and low chemical consumption. However, it strongly depends on the electrical energy, is limited to ionic species, requires high initial costs and is aggravated by membrane fouling and degradation [154,155].

Electrochemical deposition (or electrodeposition) allows one to separate the metal ions from the leaching solution by applying an electric current to the solution. The electric current promotes the reduction of the metal ions that are deposited on the electrode surface [158]. This technique is relatively effective in the separation of transition metal ions, which are recovered with high purity [158,159]. However, since the separation efficiency depends on the difference in the reduction potential of the species, metals with similar reduction potentials cannot be selectivity separated. For example, Co and Ni exhibit co-deposition, and additional steps for their separation are necessary before electrodeposition. An innovative strategy based on the involvement of surfaces functionalized by charged polymers has been proposed, which allows the control of the metal selectivity during deposition and leads to the recovery of Co and Ni with final purity of 96% and 94%, respectively [160]. The efficiency of electrochemical deposition is affected by several factors, such as the solution concentration, pH range, temperature and electric current density, which need to be adjusted to control the entire process [158,159,161].

Ion exchange is a technique in which the solution is passed through a bed of resin with high affinity for ions [67]. This process uses the selective adsorption properties of ion-exchange resins [162]. It is generally employed after solvent extraction or chemical precipitation in order to collect target metals [163]. Ion exchange has been studied not only to recover specific valuable metals in the battery recycling process but also to remove impurities from solutions. For example, using an aminomethylphosphonic acid functional chelating resin (Lewatit TP260), it was possible to remove Fe, Al, Mn and Cu from the leachate, leaving a 99.6% pure solution containing Co, Ni and Li [164]. However, the involvement of the ion-exchange approach in LIB recycling is relatively new [165]. Several variables affect the ion-exchange process; the most important are the nature of the ion-exchange resin, the concentration of the ions in the solution, the reaction time and the elution to collect metals. The resins employed are commercially available and can be anion-exchange resins, cation-exchange resins or chelating ion-exchange resins [163]. Ion exchange is characterized by reduced volatile organic solvent consumption, high selectivity and a wide operating pH range [165].

An overview of the advantages and disadvantages of the discussed metal recovery techniques is reported in

Table 6. Advantages and disadvantages of metal recovery methods.

	Advantages	Disadvantages
Solvent extraction	✓ Cost efficiency ✓ Process simplicity ✓ Short reaction time ✓ High efficiency ✓ High selectivity	✗ Extraction efficiency dependent on the selected extraction system ✗ Selectivity complicated by similar metal properties
Chemical precipitation	✓ Low cost ✓ High recovery rate ✓ Recovery of high-purity metals	✗ Co-precipitation due to overlapping pH range
Electrodialysis	✓ High selectivity ✓ Low chemical consumption	✗ High initial costs ✗ Membrane fouling
Electrochemical deposition	✓ Recovery of high-purity metals	✗ Selectivity affected by similar reduction potentials
Ion exchange	✓ High selectivity ✓ Reduced volatile organic solvent consumption ✓ Wide operating pH range	✗ High costs of resins

.

5. Scaling up of Recycling Methods

Although battery recycling is a priority area of research and many methods have been proposed, most of them are still only applicable at a laboratory level. Pyrometallurgy and hydrometallurgy are the only recycling methods that are mature enough for industrial use [166]. The major battery recycling companies are located in Asia, Europe and North America. In Europe and North America, battery manufacturers are often also involved in recycling [167]. The pyrometallurgical and hydrometallurgical processes employed by companies differ in the recycling routes involved, types of batteries processed, product quality grade, recovery rate and types of materials recycled. For example, North American companies (such as Glencore and Li-cycle, CA) use recycling routes that produce higher-value fractions than in Europe, where companies such as Akkuser (Finland), Duesenfeld (Germany), Redux (Germany/Austria), Euro Dieuze Industrie (France) and Batrec Industries (Switzerland) perform mechanical treatment to obtain black mass, which is then processed elsewhere. In contrast, other European companies, such as Umicore (Belgium) and Accurec (Germany), carry out the entire recycling process. In addition, some companies (Erasteel Recycling—France, SNAM—France and INMETCO—USA) do not deal exclusively with lithium batteries. Most Asian companies, such as Huayo Cobalt and GEM (China), prefer hydrometallurgical methods, which yield high-quality products, while Sony-Sumitomo (Japan) combines pyrometallurgy with hydrometallurgy to recover high-purity metals [168].

Despite success on a laboratory scale, several challenges still need to be addressed to achieve the industrialization of the most innovative processes. The scalability of a method is, in fact, influenced by several factors, such as environmental and economic sustainability, government regulations and the efficiency of the method, taking into account the quantity and quality of recycled materials [169]. Moreover, the small number of industrial facilities for the recycling of spent batteries necessitates more efforts involving manufacturers, consumers, recyclers and governments [166].

6. Economic and Environmental Issues of Recycling Methods

The feasibility of a recycling method depends on several factors; the most important are the economic evaluation and the environmental impact of the entire process (Figure 5).

The economic convenience of a method is determined by the cost that it requires and the profit that it generates. In principle, battery recycling cannot be considered a low-cost process because it involves several operational steps that can have fluctuating costs. The collection, transportation and processing of spent batteries are the main factors affecting the logistical costs of the process [170]. Moreover, the chosen recycling process, materials, chemical reagents and energy consumption are the major variables that determine the overall cost of recycling. Tian et al. analyzed five battery recycling processes (two common hydrometallurgical methods and three laboratory-level direct recycling methods) considering cost, revenue and profit. A minimum cost of USD 4.8/kg of recycled batteries was required and a profit of USD 41.7/kg was obtained for the best of the common hydrometallurgical approaches, while a profit of USD 24.6/kg was obtained in the best of the laboratory-level direct recycling methods. The economic differences between the approaches depend on the maturity of the process and on the different products and recycling rates [171]. Meanwhile, the costs of bioleaching are due to bioreactor construction, nutrient and reagent supplementation, operating costs, supply services and instrumental services [113]. Alipanah et al. tried to optimize a bioleaching process, showing that a profit of more than USD 92 million could be achieved by recycling 10,000 tons of black mass per year for 30 years [172].

Another important factor is the location of recycling plants. Regardless of the method employed, battery recycling is more expensive in the United States compared to South Korea and China. For example, recycling via the pyrometallurgical method for LCO requires USD 5.82/kg of recycled cells in the United States and USD 4.95/kg and USD 4.18/kg in South Korea and China, respectively [170].

Moreover, the chemical composition of battery cathodes affects the profitability of recycling methods. Currently, the recycling of batteries with low cobalt and nickel content appears to be less profitable [173]. The recycling of batteries containing lithium iron phosphate (LFP) cathodes is not economically feasible: (i) LFP has low content of valuable metals (Co, Ni and Mn) in comparison with other chemistries, such as the high-cobalt-content LCO cathodes, and (ii) LFP recycling only allows the recovery of Li and Fe. In addition, studies focusing on the recycling of this cathode are limited because of the high reactivity and affinity of Li for oxygen, which make its recovery difficult [174].

An additional issue in economic evaluation concerns raw material prices. The limited availability of raw materials needed for battery production contributes to the rising costs. In recent years, the price of metals has risen sharply, reaching values of USD 80,000/ton for cobalt and USD 14,000/ton for lithium. It has been estimated that the production of LIBs using recycled battery materials is cheaper than using virgin raw materials, resulting in a 20% cost reduction [175].

The environmental impact of the entire life cycle of a recycling process can be evaluated by life cycle assessment (LCA). This analysis tool is able to assess the potential environmental consequences associated with the production, use and recycling of batteries [176]. Several studies have been conducted using different models, but the current state of LCA is not complete, since many analyses do not provide quantitative data and some studies often focus on battery production and the life cycle while excluding recycling processes [177]. The indicators examined in LCA for the environmental impact include chemical consumption, energy consumption, wastewater discharge, gas emissions and solid waste disposal [171]. Different battery chemistries also affect the results of the LCA. For example, Yu et al. calculated the greenhouse gas emissions and water consumption in the recycling of lithium batteries consisting of NCM₁₁₁, NCM₆₂₂, NCM₈₁₁ (which differ in the relative lithium/nickel/cobalt composition) and NCA using three different recycling methods: pyrometallurgy, hydrometallurgy and direct recycling. In the production of LIBs using recycled materials from pyrometallurgical, hydrometallurgical and direct recycling processes, it was found that the latter had the greatest potential to reduce the greenhouse gas emissions, water consumption and manufacturing costs. In particular, direct recycling processes reduce the greenhouse gas (CO_2eq) emissions by 34.52% and the water consumption is reduced by 34.41% if compared with the use of virgin raw materials [178].

Wu et al. adopted a holistic LCA to evaluate the environmental impacts of various spent LIB recycling techniques, revealing that thermal treatment and alkaline leaching are the most resource- and energy-intensive processes. Moreover, they proved that combining a heating pre-treatment, an alkaline leaching step for metal separation and solvent extraction for metal recovery produced significant environmental problems. They also demonstrated that, when employing solvent dissolution, followed by chlorination and chemical precipitation, a lower environmental impact is obtained [179]. Additionally, considering the greenhouse gas emissions and the energy demand as indicators, the direct cathode regeneration approach was proven to be the most environmentally sustainable method. In fact, the greenhouse gas emissions ranged from 67 to 286 kg of carbon dioxide equivalents (CO_2eq) and the cumulative energy demand was 1164–4349 MJ per kg of LCO cathode material when applying hydrometallurgical or pyrometallurgical approaches; meanwhile, the direct recycling process emits 21–154 kg CO_2eq and consumes 267–2251 MJ per kg [179].

Finally, despite the limited number of LCA studies on innovative recycling methods, bioleaching appears to be a promising, environmentally sustainable approach in comparison with the other processes. Bioleaching produces 16–19 kg CO_2eq per kg of recovered cobalt, compared to 10–158 kg CO_2eq with other leaching methods. This advantage is mainly due to lower acid consumption [180].

7. Conclusions

Technological advancement and increasing energy demands have made lithium-ion batteries the most employed energy storage devices. Environmental pollution and the limited availability of raw materials are the main concerns related to the production and disposal of spent batteries. Battery recycling has, therefore, become indispensable to meet the world’s energy needs and mitigate the environmental impact. Battery recycling is still an uncommon practice, but it is also one of the most discussed topics of the last decade. Several approaches have been proposed to perform battery recycling and recover valuable materials. Among them, pyrometallurgy and hydrometallurgy are the most used processes, employed by many companies. They are mature processes with high effectiveness. On the other hand, they consume large amounts of energy and produce high levels of greenhouse gases and wastewater, contributing to environmental pollution. Alternative and innovative strategies have been developed. Bioleaching, mechanochemistry and direct recycling have been discussed in this review. Due to their greater environmental sustainability, they offer promising solutions. These processes are characterized by reduced energy consumption and low greenhouse gas emissions, but also by slow metal recovery rates. Moreover, they are not available and applicable at a large scale.

Several challenges need to be addressed to improve the current state of battery recycling. The development of simple and universal processes is difficult due to the complex compositions of batteries. Furthermore, the continuous evolution of battery chemistry and technological improvements in battery production necessitate new approaches and strategies for sustainable recycling. An additional challenge is related to the recycling costs, which are generally high. For the aforementioned reasons, lithium battery recycling is an evolving field, and future research must focus on the development of innovative, environmentally friendly and inexpensive processes.