Future Technologies for Recycling Spent Lithium-Ion Batteries (LIBs) from Electric Vehicles—Overview of Latest Trends and Challenges

: The work describes the construction of lithium-ion batteries, with particular emphasis on metals that can be obtained as secondary raw materials. The work presents the latest trends in the recycling of lithium-ion batteries, using pyro-and hydrometallurgical methods, or their combination. The ecological aspect of the impact of the recycling processes on the environment is shown, as well as the challenges and expectations for the future in the ﬁeld of recycling processes.


Introduction
Electric vehicles are becoming ever more popular, although they are still expensive to purchase and to use.It should be taken into account that the number of electric cars will increase systematically.Along with this, there will be a need to manage the used lithium-ion batteries (LIBs).LIBs are the next-generation power source for electric vehicles (EVs), because of their superior performance, with aspects such as a high energy density, high capacity, long cycle life, no memory effect, low self-discharge rate, wide operatingtemperature range, and environmental friendliness [1][2][3].We can observe that, in the past ten years, the application of LIBs has developed rapidly around the world [4].According to published reports [5], the global sales of electric vehicles reached over 6.75 million in 2021.The use of LIBs is constantly increasing; therefore, their recycling and regeneration have become essential.It is expected that the amount of LIB waste will increase to over 460,000 tons in 2025, and about 11 million tons by 2030, worldwide [4].
The production of LIBs requires strategic elements.Cobalt and lithium are needed to produce these batteries, because lithium cobalt oxide LiCoO 2 is the main component of cathode materials.However, due to the high price of metallic cobalt, lithium cobalt oxide is replaced by other compounds, such as lithium manganese oxide and lithium iron phosphate, especially for vehicle use.The mentioned components are less expensive and safer alternatives but, unfortunately, are less chemically stable than LiCoO 2 [6][7][8].
Waste, in the form of spent batteries, can be an important source of valuable metals.Nevertheless, we should remember the very important 3R system: reduce, reuse, and recycle.The waste management hierarchy is as follows: (1) prevention, (2) reuse, (3) recycling, (4) recovery, and (5) disposal.Various electrical devices requiring batteries should be designed to use the smallest amounts of critical metals possible.The ability to reuse electric vehicle batteries from electric cars is very important.At present, electric-vehicle batteries are dismantled by hand, for reuse or for recycling.The efficient dismantling of electrical vehicle battery waste with the use of appropriate tools is still a major challenge for the future.The automation of battery disassembly is the subject of a pilot study.This is a difficult challenge, owing to the lack of standardization in LIB design.Reuse is definitely better than recycling, while the stockpiling of spent LIBs is unsafe.Therefore, if the reuse of waste batteries is not possible, they must be recycled.Today, research on the recycling of spent lithium-ion batteries is mainly focused on the recovery of cobalt, nickel, and lithium, as important raw materials that are useful in numerous industrial processes.It is well known that the purpose of recycling is to recover components for reuse in battery production [2,3,5,9,10].The recycling of batteries is complicated by the large variety of constructions of this type of battery: different types of cathodes (five types), anodes, electrolytes, etc.These batteries contain a variety of materials, and differ in size and weight.For example, batteries from electric vehicles contain electrolytes and cathodes, which have an approximately 15 kg equivalent of lithium carbonate per battery [6].As a consequence of their diversity, different methods are suitable for their recycling: the pyrometallurgical and hydrometallurgical methods, and a combination of the two.

Lithium-Ion Batteries-Composition, Reactions, Advantages, and Disadvantages
Since the introduction of lithium-ion cells into the market in the 1990s, their importance as storage and energy sources has been increasing, which is reflected in the constant growth in the rate of their production and sales.The dynamically developing electric vehicle industry brings with it a constant rise in the demand for lithium-ion batteries.It is estimated that the LIB market will reach more than USD 75 billion in 2025 [11].
Lithium-ion cells are produced in various sizes and shapes, depending on their application.A single cell basically consists of four main components: the anode, cathode, separator, and electrolyte.
The anode is a carbon material (usually graphite) that does not contain lithium, so its source must be the cathode material.For this purpose, lithium-intercalated compounds, e.g., layered oxide LiCoO 2 , are used as cathodes.The principle of the operation of a lithium-ion cell is based on the migration of lithium ions through the electrolyte between two electrodes, separated by a separator, during the reversible charging and discharging processes (Figure 1). is a difficult challenge, owing to the lack of standardization in LIB design.Reuse is definitely better than recycling, while the stockpiling of spent LIBs is unsafe.Therefore, if the reuse of waste batteries is not possible, they must be recycled.Today, research on the recycling of spent lithium-ion batteries is mainly focused on the recovery of cobalt, nickel, and lithium, as important raw materials that are useful in numerous industrial processes.It is well known that the purpose of recycling is to recover components for reuse in battery production [2,3,5,9,10].The recycling of batteries is complicated by the large variety of constructions of this type of battery: different types of cathodes (five types), anodes, electrolytes, etc.These batteries contain a variety of materials, and differ in size and weight.For example, batteries from electric vehicles contain electrolytes and cathodes, which have an approximately 15 kg equivalent of lithium carbonate per battery [6].As a consequence of their diversity, different methods are suitable for their recycling: the pyrometallurgical and hydrometallurgical methods, and a combination of the two.

Lithium-Ion Batteries-Composition, Reactions, Advantages, and Disadvantages
Since the introduction of lithium-ion cells into the market in the 1990s, their importance as storage and energy sources has been increasing, which is reflected in the constant growth in the rate of their production and sales.The dynamically developing electric vehicle industry brings with it a constant rise in the demand for lithium-ion batteries.It is estimated that the LIB market will reach more than USD 75 billion in 2025 [11].
Lithium-ion cells are produced in various sizes and shapes, depending on their application.A single cell basically consists of four main components: the anode, cathode, separator, and electrolyte.
The anode is a carbon material (usually graphite) that does not contain lithium, so its source must be the cathode material.For this purpose, lithium-intercalated compounds, e.g., layered oxide LiCoO2, are used as cathodes.The principle of the operation of a lithium-ion cell is based on the migration of lithium ions through the electrolyte between two electrodes, separated by a separator, during the reversible charging and discharging processes (Figure 1).During the operation of a typical LiCoO2/graphite cell, the following reversible electrochemical reactions take place [12]: Anodic reaction (oxidation): During the operation of a typical LiCoO 2 /graphite cell, the following reversible electrochemical reactions take place [12]: Anodic reaction (oxidation): Cathodic reaction (reduction): The most important and valuable elements of batteries, from a recycling point of view, are the cathodes.Commercially available lithium cobalt oxide is typically used as the cathode material.Recently, a progressively higher amount of its derivatives have been obtained for the production of cathodes, in order to replace some or all of the cobalt with nickel, manganese, or aluminum, so as to optimize the performance of the cells, while reducing the consumption of raw materials [13].Manganese-based compounds have a true specific capacity similar to that of LiCoO 2 .They are also characterized by increased availability, and a much lower price.Due to the excellent chemical and thermal stability that guarantees the safety of Li-ion cells, lithium iron phosphate is an attractive and popular cathode material.The main, and basically only, disadvantage of this compound is its extremely low electrical conductivity, which prevents the full use of its theoretical capacity.A new class of cathode materials is silicate systems of the Li 2 MSiO 4 type, where M = Fe, Mn, Ni, Co.These systems, owing to the content of two lithium ions in their structure, have a high theoretical capacity; however, similar to lithium-iron phosphates, they are characterized by a very low electrical conductivity [13].
A widely used anode material in commercial lithium-ion cells is graphite, which has the ability to intercalate lithium ions.Research has also been carried out on the modification of graphite electrodes, as well as other carbon materials, such as carbon nanofibers, fullerenes, graphene, or composite carbon materials [14].Another alternative to carbon anodes in lithium-ion cells are metal oxides, e.g., TiO 2 , which are stable during the intercalation/deintercalation of lithium ions, as well as lithium-metal anodes, which have a higher energy density compared to batteries with graphite anodes [14].
Mixtures of an organic solvent and a lithium salt are employed as electrolytes in lithium-ion cells.The electrolyte solution has to be able to freely transport lithium ions, which requires a high dielectric constant, as well as a low viscosity.Most lithium-ion cells contain a liquid electrolyte with a dissolved lithium salt, such as LiPF 6 , LiBF 4 , or LiClO 4 .Most of the time, the solvent is a mixture of organic compounds.A gel electrolyte, as well as a solid conductive polymer, can also be used [15].The usage of polymer electrolytes improves the mechanical stability of the cell, making it safer.
Currently, depending on the cathode material used, there are five types of lithiumion batteries on the market [16].In electronic devices such as laptops, mobile phones, and cameras, there are the commonly used lithium cobalt oxide (LCO) batteries.On the other hand, lithium manganese oxide (LMO) batteries, which are less expensive and safer alternatives to LCO batteries, are most commonly used in hybrid vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs).In EVs, lithium iron phosphate (LFP) batteries are also used.Finally, lithium nickel cobalt aluminum oxide (NCA) and lithium nickel cobalt manganese oxide (NMC) batteries are intensively developed, because of their relatively high energy density.
The cathode of the NCA battery, which was invented in the early 1990s, has a very good energy density, and high power.It is the preferred choice of Tesla.Other EV car manufacturers prefer the NMC solution.In this technology, an increase in the nickel content in the cathode material raises the energy density of the battery.The most popular and alreadyused solutions on the market are NMC-111 and NMC-622 [17,18].Currently, electric vehicle manufacturers are very interested in cells produced using NCM 811 technology [17,18].These are characterized by a low content of cobalt (10%) and manganese (10%), and a higher proportion of nickel (80%).
The main disadvantage of Li-ion batteries is their low power density, which manifests in long charging times for electric vehicles, and a lack of resistance to overcharging and over-discharging.These batteries may pose a safety risk because of improper handling, design, or assembly, or mechanical damage, which may result in the battery catching fire [12].
Another important problem is the growth in the production of large-sized car batteries, which may result in the accumulation of a large amount of hazardous waste.If they are not recycled and reused, they will have a huge impact on environmental degradation, and accelerate the depletion of mineral resources.
A set of Li-ion car batteries (a so-called battery pack) consists of many individual modules, which in turn are made of several individual cells (Figure 2).
Another important problem is the growth in the production of large-sized car batteries, which may result in the accumulation of a large amount of hazardous waste.If they are not recycled and reused, they will have a huge impact on environmental degradation, and accelerate the depletion of mineral resources.
A set of Li-ion car batteries (a so-called battery pack) consists of many individual modules, which in turn are made of several individual cells (Figure 2).For example, in the Volkswagen ID.3, for an energy-storage system with a capacity of 58 kWh, each battery cell has a mass of 1.101 kg.A single 24-cell module weighs 30.9 kg, while a 12-cell battery pack that includes coolant weighs 375 kg [19].The 75 kWh Tesla Model 3 battery pack contains as many as 4416 cells, and its total weight is 480 kg [20].
As can be seen in Figure 3, approximately 40% of the total weight of the car battery is the aluminum case holding the individual components of the pack, and other elements controlling the operation of the battery, together with the coolant.The remaining amount of Al is used as the cathode current collector.The next heaviest component is copper, which is the anode current collector.The active electrode materials, from which we obtain electricity, comprise about 37% of the total weight of the battery.For example, in the Volkswagen ID.3, for an energy-storage system with a capacity of 58 kWh, each battery cell has a mass of 1.101 kg.A single 24-cell module weighs 30.9 kg, while a 12-cell battery pack that includes coolant weighs 375 kg [19].The 75 kWh Tesla Model 3 battery pack contains as many as 4416 cells, and its total weight is 480 kg [20].
As can be seen in Figure 3, approximately 40% of the total weight of the car battery is the aluminum case holding the individual components of the pack, and other elements controlling the operation of the battery, together with the coolant.The remaining amount of Al is used as the cathode current collector.The next heaviest component is copper, which is the anode current collector.The active electrode materials, from which we obtain electricity, comprise about 37% of the total weight of the battery.
Another important problem is the growth in the production of large-sized car batteries, which may result in the accumulation of a large amount of hazardous waste.If they are not recycled and reused, they will have a huge impact on environmental degradation, and accelerate the depletion of mineral resources.
A set of Li-ion car batteries (a so-called battery pack) consists of many individual modules, which in turn are made of several individual cells (Figure 2).For example, in the Volkswagen ID.3, for an energy-storage system with a capacity of 58 kWh, each battery cell has a mass of 1.101 kg.A single 24-cell module weighs 30.9 kg, while a 12-cell battery pack that includes coolant weighs 375 kg [19].The 75 kWh Tesla Model 3 battery pack contains as many as 4416 cells, and its total weight is 480 kg [20].
As can be seen in Figure 3, approximately 40% of the total weight of the car battery is the aluminum case holding the individual components of the pack, and other elements controlling the operation of the battery, together with the coolant.The remaining amount of Al is used as the cathode current collector.The next heaviest component is copper, which is the anode current collector.The active electrode materials, from which we obtain electricity, comprise about 37% of the total weight of the battery.The basic raw material composition of a car battery based on NCM cells, together with the approximate costs of each component presented in Figure 4, clearly indicates that the most expensive structural element in Li-ion cells is the cathode materials.Therefore, an effective and inexpensive method of recovering metals such as Co, Ni, and Li, as well as aluminum and copper from Li-ion batteries, is particularly important.Furthermore, Gaines et al. compared the cost of cathode manufacture in different types of lithium-ion batteries [22], as shown in Table 1.
The basic raw material composition of a car battery based on NCM cells, together with the approximate costs of each component presented in Figure 4, clearly indicates that the most expensive structural element in Li-ion cells is the cathode materials.Therefore, an effective and inexpensive method of recovering metals such as Co, Ni, and Li, as well as aluminum and copper from Li-ion batteries, is particularly important.Furthermore, Gaines et al. compared the cost of cathode manufacture in different types of lithium-ion batteries [22], as shown in Table 1.Currently, only a limited number of Li-ion batteries are recycled or remanufactured.Most of them end up in landfills, or are collected in households [24].Additionally, battery technology continues to evolve, and current recycling processes [25] may need to change over time.

Characteristics of Current Industrial Technologies for Recycling LIBs
The International Energy Agency anticipates that the total amount of LIB waste generated by 2040 will be approximately 8 million tons [26].It is estimated that every year, about 800 thousand tons of vehicle batteries are shipped to the European market [27].These batteries contain many important metals.
Companies try to apply environmentally friendly technologies to conduct the efficient recovery of the valuable components of waste lithium batteries.According to the European Commission, cobalt and graphite have been classified as critical elements.It is  Currently, only a limited number of Li-ion batteries are recycled or remanufactured.Most of them end up in landfills, or are collected in households [24].Additionally, battery technology continues to evolve, and current recycling processes [25] may need to change over time.

Characteristics of Current Industrial Technologies for Recycling LIBs
The International Energy Agency anticipates that the total amount of LIB waste generated by 2040 will be approximately 8 million tons [26].It is estimated that every year, about 800 thousand tons of vehicle batteries are shipped to the European market [27].These batteries contain many important metals.
Companies try to apply environmentally friendly technologies to conduct the efficient recovery of the valuable components of waste lithium batteries.According to the European Commission, cobalt and graphite have been classified as critical elements.It is well known that natural metal resources, especially cobalt, are severely limited [26].Table 2 shows the composition of spent LIBs [28].The recovery of metals from used batteries can be performed using pyrometallurgical, hydrometallurgical, and biometallurgical methods, as well as a combination of hydrometallurgical and pyrometallurgical methods [3,5,9].Generally, the treatment and recovery of metals from LIBs is a multistep process.We can distinguish the three stages in recycling processes as: (1) pretreatment, (2) metal extraction, and (3) final product application [3].
The first step in this process is the dismantling of the battery.The initial stage of LIB recycling is very similar for different types of batteries, and comprises the following steps: crushing, firstly magnetic, then gravity separation, and sieving.After pretreatment, usually, three fractions are obtained: ferromagnetic (steel castings), diamagnetic (plastics), and the paramagnetic fraction containing other metals, such as cobalt, nickel, lithium, copper, etc.
The main purpose of this review is to present the recent advances in recycling lithiumion batteries, by means of various processes, including pyro-and hydrometallurgical methods.With the rising number of lithium-ion cells in use, the need to recycle them and recover their valuable components is becoming progressively more important.This is due, on one hand, to economic factors related to the increasing demand for this type of battery and, on the other hand, to environmental factors, i.e., the rising demand for raw materials for their production, in addition to the growing number of used batteries in waste streams [3,26,29,30].
The problem of developing suitable recycling methods has been of interest since the advent of LIBs in widespread use; that is, since the 1990s [26].The issue is still relevant today and, although a whole range of pyro-and hydrometallurgical methods for reprocessing spent lithium-ion batteries has been proposed, new technologies for recovering the valuable components from LIBs are still being sought.The new methods are expected to be more efficient, environmentally safer, and less expensive than those used today.In addition to the technology for separating valuable metals from spent Li-ion batteries, it is possible to perform regeneration, which involves restoring the relevant electrochemical properties of the electrode materials, and thus restoring the performance of the entire battery.The regeneration method is often associated with the need to restore the stochiometric composition of the electrodes, which is usually achieved by introducing additional amounts of lithium [26,31,32].The undoubted advantages of such a procedure are its non-destructive nature, and low consumption of chemicals and energy, and a reduction in secondary waste generation.Nonetheless, the application of this method is limited to a relatively small number of batteries.In other cases, it is necessary to subject the LIBs to a destructive recycling procedure, allowing the selective separation and recovery of valuable components and energy.
Given the differences in the structure of, and the type of materials used in, electrode construction, a variety of metals (both in metallic form, and relevant chemical compounds) can be expected to appear in the LIB waste stream: lithium, cobalt, nickel, manganese, iron, aluminum, and others, occurring in varying proportions [3,31,[33][34][35][36][37][38].The recycling of lithium-ion batteries, regardless of the technology used, involves several main steps that include preliminary operations, the recovery of metals and their compounds from the mass of the battery, and the separation and isolation of the final products.The preliminary operations usually include, among others, electrical discharge, disassembly, comminution (crushing, grinding), and the separation of the individual components according to the type of material, using various physical methods, which include sieving, flotation, gravity, magnetic, or electromagnetic separation.The result is the separation of the active electrode materials from the other components.The use of pretreatment operations results in a higher purity in the final products, and a reduced energy consumption throughout the process [39][40][41].
Once the active components have been separated, they are usually subjected to thermal treatment, the purpose of which is to dispose of the organic electrolyte and other organic components at high temperatures.Such operations have the advantage of being simple and relatively low cost.Organic lithium compounds, which are the main components of the electrolyte, behave slightly differently under heating, but generally undergo a two-stage thermal decomposition process.The first stage is associated with the removal of free HF or HCl, and usually occurs at temperatures < 100 • C. In the second stage, which occurs at temperatures in the order of several hundred degrees Celsius, the lithium salts are converted to low-molecular-weight substances.On the basis of the characteristic decomposition temperatures, the stability of the selected lithium salts is arranged in a series: LiClO 4 > LiCF 3 SO 3 > LiTFSI > TEAB > LiBF 4 > LiPF 6 [42].
The thermal treatment of electrolytes can be carried out using distillation, pyrolysis, thermolysis, and combustion processes.The main purpose of distillation is to recover and reuse electrolytes and, therefore, the temperature of this process must be lower, to prevent the pyrolysis of the electrolyte.As the temperature increases, the decomposition processes, i.e., pyrolysis under anaerobic conditions or combustion in an oxidizing atmosphere, become more important [43,44].In similar processes, binders connecting active materials to current collectors, e.g., PVDV, graphite, etc., may also be removed from active materials.Typically, pyrolysis processes are carried out at temperatures between 500 and 900 • C.
The recovery of metals and their compounds from electrode materials is usually performed using pyro-or hydrometallurgical, or mixed, processes (Figure 5).

Pyrometallurgical Methods
However, a review of the literature shows that most of the recycling processes for spent LIBs from electric vehicles are currently based on pyrometallurgical processes, despite the efforts made to develop hydrometallurgical technologies for recycling LIBs [6,26,39].
Pyrometallurgical processes, carried out at companies such as Sony, Umicore, Accurec, Onto, and Inmetco involve the high-temperature melting of the electrode material, typically resulting in an alloy of metals, such as cobalt, nickel, and iron, and slag containing a major amount of lithium.This type of process may also involve the combustion or pyrolysis of organic components, which are incompletely separated in the initial stage.These operations are usually accompanied by the emission of gases (CO 2 , SO 2 , NO x , and volatile hydrocarbons).The resulting products can be used directly in the obtained form, as metal alloys, or further processed (slag) to separate the desired components, e.g., lithium compounds [3,26,39,42,45,46].The main disadvantages of pyrometallurgical technologies are the following: the production of toxic gases, high energy costs, and the limited amount of recycled materials.

Pyrometallurgical Methods
However, a review of the literature shows that most of the recycling processes for spent LIBs from electric vehicles are currently based on pyrometallurgical processes, despite the efforts made to develop hydrometallurgical technologies for recycling LIBs [6,26,39].
Pyrometallurgical processes, carried out at companies such as Sony, Umicore, Accurec, Onto, and Inmetco involve the high-temperature melting of the electrode material, typically resulting in an alloy of metals, such as cobalt, nickel, and iron, and slag containing a major amount of lithium.This type of process may also involve the combustion or pyrolysis of organic components, which are incompletely separated in the initial stage.These operations are usually accompanied by the emission of gases (CO2, SO2, NOx, and volatile hydrocarbons).The resulting products can be used directly in the obtained form, as metal alloys, or further processed (slag) to separate the desired components, e.g., lithium compounds [3,26,39,42,45,46].The main disadvantages of pyrometallurgical

Hydrometallurgical Methods
Hydrometallurgy-based separation processes are an important stage in LIB recycling.After mechanical sieving to separate the steel from the electrode material, leaching is conducted, using various reagents, such as inorganic or organic acids [7,8].To date, the dominant view has been that "(. ..)Pyrometallurgical processes are expensive, not adaptable for recycling lithium batteries from electric vehicles, consume too much energy and may not be applicable to recover future generations of electrodes" [6].The essence of hydrometallurgical processes is the transfer of the electrode material components into a solution in the leaching step, using mineral or organic acids.The advantage of using solutions of strong inorganic acids (HCl, HNO 3 , H 2 SO 4 ) is the relatively high speed and high efficiency of the dewetting reaction, while a disadvantage is the possibility of releasing toxic gases (e.g., Cl 2 , SO 2 , NO x ).For transition metals in high oxidation states, the oxides of which are more difficult to dissolve, it is necessary to add reducing agents, which include, for example, H 2 O 2 , NaHSO 3 , Na 2 SO 4 , Na 2 S 2 O 3 , glucose, and ascorbic acid.
Inorganic acids are strong acids, with which leaching reactions proceed at high rates.In addition, an increase in the leaching rate can be achieved in these solutions by adding reducing agents.Acid leaching can be performed directly with the pretreated electrode material, or as a subsequent step after, for example, alkaline leaching.The reagents in this group employed for leaching include sulfuric acid (VI), hydrochloric acid, nitric acid(V), etc.In these cases, a common leaching aid is the addition of hydrogen peroxide.Examples of this type of process are discussed below.
Nan et al. [47] investigated the leaching of spent LIB cells using sulfuric acid solutions, and obtained process efficiencies of 98% for Li and Co in about 6 h.On the other hand, the work of Swain et al. [48] found a reduction in the leaching time in the presence of hydrogen peroxide, with a decrease in the efficiency of the process.Sun and Qiu [49], on the other hand, found the almost complete recovery of lithium and cobalt when leaching with a 2M sulfuric acid solution at 80 • C for 60 min.The course of leaching with sulfuric acid solutions in the presence of hydrogen peroxide can be represented by the following equation: Takacova et al. [50] used hydrochloric acid solutions in electrode material leaching processes at temperatures of 60-80 • C for 90 min, and obtained yields of almost 100% for lithium and cobalt.The reaction occurring between lithium compounds and hydrochloric acid can be represented by the following equation: The inconvenience of using solutions of this acid is the release of toxic chlorine, as shown in the reaction equation above.
Similarly, high leaching efficiencies can be achieved using nitric acid solutions.Castillo [51,52] and Li [53] reported the achievement of an approximately 100% leaching efficiency in Li and Co from spent LIBs, using HNO 3 solutions with or without the addition of hydrogen peroxide.
Solutions of certain organic acids also have a high efficiency, indicating that they can be used as leaching agents.These acids include, for instance, malic acid, citric acid, succinic acid, oxalic acid, etc.In these cases, additional substances, such as oxidants and/or reducing agents (e.g., H 2 O 2 ) are also necessary.Organic acids tend to have a higher leaching selectivity than mineral acids [38,[54][55][56].Leaching results in multicomponent solutions, containing mainly cobalt and lithium ions, but also other metals present in the electrode material (nickel, manganese, iron, copper, etc.).Consequently, to obtain high-purity end-products (metals or their salts), it is necessary to perform the prior separation of the components of the solution after leaching.For this purpose, a solvent extraction process using various extractants, in the form of single compounds or their mixtures, is often used.The most common extractants for the efficient separation of cobalt or nickel ions from lithium ions include the organic esters of phosphoric acids, e.g., D2EHPA (di(2-ethylhexyl)phosphoric acid), or Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid.On the other hand, typical hydroxyoxime derivatives are utilized to separate copper ions [30,35,38,[57][58][59][60][61][62][63][64][65][66][67][68][69][70].
The final step of hydrometallurgical lithium-ion battery recycling technologies is usually the separation of the compounds of the individual metals, via crystallization or precipitation (Table 3).
The recycling of spent LIBs is an important part of a sustainable approach to metal recovery, because of the economic and environmental advantages.Currently, we can find a number of examples of LIB battery-recycling technologies using both hydrometallurgical and pyrometallurgical processes.Hydrometallurgical processes are applied on an industrial scale by companies such as Recupyl, Accurec, and Toxco [3,26].
Pyrometallurgical processes are implemented on an industrial scale by companies such as Sony, Umicore, Accurec, Onto, and Inmetco [3].The pyrometallurgical process can be combined with the hydrometallurgical process.For instant, in the Val' Eas process adopted by Umicore, at the beginning, the spent LIBs are smelted, in order to obtain the alloy containing cobalt, nickel, copper, and iron.In the next stage, the non-ferrous metals are recovered from this alloy via hydrometallurgical processes, including leaching, extraction, and separation [2].
Up to now, a large number of researchers have reported results on the laboratory-scale recycling of LIBs.The laboratory method developed by Zou et al. [93] for the recycling of mixed-cathode materials, including LiCoO 2 , LiMn 2 O 4 , LiNi 0.33 Mn 0.33 Co 0.33 O 2 , and LiFePO 4 , provides a high recovery rate for Ni, Co, and Mn in solution, and is suitable for commercial use.Lithium is often recycled in the last step of hydrometallurgical methods, resulting in low purity and recovery rates.In recent times, a more efficient way to recycle lithium has been developed.A laboratory pathway for recycling mixed cathode materials using the waste electrolyte from spent lead-acid batteries has also been proposed and tested [94].
Recently, Chen et al. [58] reported an important and very interesting method for the recycling of the spent LIBs, using a novel extraction-precipitation process for the separation of lithium and non-ferrous metals (manganese, cobalt, nickel), using p-tertylphenoxyacetic acid (POAA).The results indicated that the selective extraction of Li(I) from Ni(II), Co(II) and Mn(II) was possible using one-step precipitation with 0.2 mol/L POAA solution.The lithium was recovered in the form of Li 2 CO 3 , with a purity level of 97.7%.

Environmental Impact-Challenges of LIB Recycling
A sharp rise in the number of waste batteries from electric cars is projected to occur 10-15 years after their production, and it will be one of the fastest-growing waste streams, creating a demand for landfill sites around the world.This carries a serious risk of increasing the number of harmful chemicals that could be released into the environment.Therefore, it will be extremely important to have an established recycling industry.The recycling of end-of-life LIBs can protect the supply chains of energy resources essential to the production of new batteries.
Lithium, nickel, and manganese are available raw materials, but some elements used in the production of LIBs, primarily cobalt and graphite, are not mined on a large scale.Forecasts show that even with the continuous increase in demand for electric vehicles, there should be enough lithium until 2050, without taking into account its recycling.Currently, lithium recycling is technically feasible, but the cost is still relatively high [95].It should additionally be borne in mind that obtaining lithium is also expensive, time-consuming and, owing to the use of a large amount of fresh water and electricity, unfriendly to the environment.Cobalt that originates mainly from the Democratic Republic of the Congo, which is often a side effect of nickel mining, could become a difficult element to obtain, because the country in which it is mined does not have a stable political and economic situation.The recycling of aluminum is favored by environmental and economic factors, as the primary aluminum production is associated with much higher emissions than the secondary production [96].Smelting existing aluminum requires only 5% of the energy needed to produce new aluminum, resulting in significant energy savings, and reductions in CO 2 emissions.The savings from battery recycling can range from 43% to 90%, compared to a battery made entirely from virgin materials [97,98].
Neither the production or recycling of LIBs can be performed without affecting the environment, as a result of the large number of production stages, related mainly to the recovery of metals.For correct planning of the LIB recycling process, it is crucial to recognize the physical and chemical properties of the employed materials.In addition, attention should be paid to the hazardous waste generated as a by-product in physical, and pyro-and hydrometallurgical processes, which, due to its nature, may pose a particular threat to the natural environment.
The main types of pollution that can be generated during the different steps involved in the recycling of LIBs are shown in Table 4.
Before opening, LIB cells are subjected to a stabilization process, to prevent the release of hot, toxic, and corrosive compounds, and possibly recover the electrolyte and plastics [99,100].An overview of the stabilization methods is presented in [101].
To remove the electrolyte, binder and separator, the pyrolysis process is utilized [102,103].During anaerobic roasting, as a result of the decomposition of organic binders and electrolytes, the main products are CO, CO 2 , HF, and C 2 H 4 , as well as small amounts of C 2 H 5 OH, CH 3 CHO, and C 6 H 6 [104,105].The use of thermal pretreatment before comminution creates problems related to the removal of the separated organic substances.Electrolyte extraction with supercritical or subcritical CO 2 [106,107], used by various authors, allows electrolyte recovery, purification, and reuse.
In the step of drying and grinding the material in an inert atmosphere, an off-gas is produced, containing various components of organic electrolytes, mostly dimethyl carbonate, ethyl, and methyl carbonate, and CO 2 .HF may form in a humid atmosphere.Crushing batteries in water avoids harmful gas emissions, and also prevents fires, when LIBs are opened and crushed, generating liquid waste that needs to be treated.If we use flotation processes to separate the anode and cathode material, we produce much more wastewater [108].
In general, the mechanical methods for recycling LIBs release toxic gases, and make it impossible to separate all the battery components, but they are simple, and generate relatively small amounts of wastewater and solid waste.The key challenge to be faced in industrial grinding is the emission of the organic electrolyte.
To meet strict environmental regulations, LIB pyrometallurgical recycling requires an off-gas treatment system.A well-designed furnace with a flue gas cleaning system is essential to preventing the emission of toxic gases, e.g., dioxins and furans [109].A chemical hazard may be caused by toxic gaseous products, such as carbon monoxide and hydrogen fluoride, formed as a result of the decomposition of the conductive salt, lithium hexafluo-rophosphate LiPF 6 , and the PVDF binder [105].Small amounts of wastewater are produced in the flue-gas-cleaning process.Despite the use of high-temperature processes in the industrial recycling of LIBs, the literature lacks information on the formation of secondary waste, its characteristics, and an assessment of the environmental risk of the products left after combustion and pyrolysis.Lithium is not recovered in the pyrometallurgical process, as it is lost in the fly ash.Metal recovery from slag is energy-intensive and uneconomical; therefore, slag is generally disposed of, or sold as a building material [110].In order to prevent contamination and implement the environmentally friendly pyrometallurgical recycling of spent LIBs, a strict system for monitoring and treating waste gases and volatile solid wastes should be established.
Hydrometallurgical treatment is the preferred recycling technology for LIBs, over pyrometallurgical processing, as it offers advantages such as a low energy consumption, and the high purity recovery of valuable components.In hydrometallurgical processes, less off-gas is emitted, and less solid waste is generated (the main phase of the leached residue is graphite).Nonetheless, attention should be paid to the large amounts of wastewater produced by the chemical processes of leaching, co-precipitation, and washing.In the process of extraction of metal ions with inorganic acids, toxic gases are released, such as Cl 2 , SO 3 , and NO x [111], which pose a serious threat to the environment; the waste solution is harmful and requires neutralization, which increases the recycling costs.Although organic acid leaching does not generate any harmful by-products that have an environmental impact, its low strength, low leaching rates, and low purification efficiency make it difficult to use in industrial recycling [112].More research is needed on wastewater treatment, water reuse, or water reduction in the process of reducing or eliminating wastewater and the associated costs.
The use of the life-cycle assessment (LCA) method to compare the potential impact of various recycling technologies of used LIBs on the environment makes it possible to obtain information to reduce the adverse impact of the analyzed processes on the environment.
The results of the eco-balance analysis presented in [113] indicate that the technology in the hydrometallurgical recycling of used LIBs generates the greatest benefits in the category related to human health, i.e., 50% of all the identified environmental benefits.The climate change and resource use categories account for 18% and 19% of all the identified environmental benefits, respectively.At the slightly lower level of 13% are the benefits that result from saving primary raw materials.
Concerning the life-cycle assessment, the available studies show positive results for LIB recycling compared to primary production in most impact categories [108].However, most of the results are based on lab-scale process data, and require industrial-scale verification.
In addition, comparative studies would be interesting, but are difficult to generate, owing to the sensitivity of the industrial processes concerned, complex process pathways, and co-processing with other primary or secondary materials.

Summary and Outlook
After a brief overview of the recycling processes of lithium-ion batteries from electric vehicles, we can summarize that future recycling technologies will combine pyrometallurgical and hydrometallurgical processes, to recover useful materials for the production of lithium-ion batteries.Recycling is a necessity that brings economic and ecological benefits.It is also a challenge, because of the expected huge amount of waste resulting from the growing popularity of electric cars.
In recent years, various physical, mechanical, and physicochemical methods have been proposed, to separate the valuable components of spent LIBs.Technologies have been developed that take into account the principles of a closed-loop economy, and incorporate environmental protection requirements.Pyrometallurgical processes have found their way into industrial practice, but they require significant energy use, and pose a risk of gaseous emissions.It now appears that hydrometallurgical processes, which are seen as more environmentally friendly, and have a relatively low energy consumption, and high recycling rates for individual Li-ion battery components, may be a good alternative.In addition to this, the end-products can be utilized to produce new electrode materials for LIBs, and can be employed for other purposes.Nevertheless, this type of technology also poses environmental risks associated with the use of aqueous and organic solutions, and thus the danger of contaminating natural waters.When comparing the advantages and disadvantages, it seems that hydrometallurgical processes are more promising.Considering also the growing demand for lithium-ion batteries, and the prospects for the development of these technologies, it can be concluded that there is still a need to seek new technological solutions that are more economically beneficial and environmentally friendly.
To meet strict environmental regulations, LIB pyrometallurgical recycling requires an off-gas treatment system.Despite the use of pyrolysis/combustion in industrial recycling, there is a lack of information on the secondary waste generation, waste characterization, and environmental risk assessment of products resulting from incineration and pyrolysis.
In hydrometallurgical processes, attention should be paid to the large amounts of waste water produced as a result of the chemical processes of leaching, co-precipitation, and washing.More research is needed on their treatment, water reuse, or water reduction in the above-mentioned processes.
It is also necessary to develop a more efficient lithium recycling process, and to set a limit value for lithium discharge in the waste water from battery recycling factories.

Figure 1 .
Figure 1.Diagram of the working principle of a Li-ion battery.

Figure 1 .
Figure 1.Diagram of the working principle of a Li-ion battery.

Figure 3 .
Figure 3. Content of components in the battery pack, with a total weight of 400 kg.Authors' compilation, based on [11,21].

Figure 3 .
Figure 3. Content of components in the battery pack, with a total weight of 400 kg.Authors' compilation, based on [11,21].

Figure 3 .
Figure 3. Content of components in the battery pack, with a total weight of 400 kg.Authors' compilation, based on [11,21].

Figure 4 .
Figure 4. Approximate costs of the basic raw materials in an NMC battery, with a total weight of 400 kg.Authors' compilation, based on [17,21,23].

Figure 4 .
Figure 4. Approximate costs of the basic raw materials in an NMC battery, with a total weight of 400 kg.Authors' compilation, based on [17,21,23].

Table 1 .
[22]arison of the cathode manufacturing costs of different types of LIB.Authors' compilation, based on[22].

Table 1 .
[22]arison of the cathode manufacturing costs of different types of LIB.Authors' compilation, based on[22].

Table 3 .
Chosen hydrometallurgical methods used to separate and recover LIB components from pregnant leach solutions (PLSs).

Table 4 .
Main types of contaminants that can be generated during the different steps involved in LIB recycling.