Literature Review, Recycling of Lithium-Ion Batteries from Electric Vehicles, Part I: Recycling Technology

: During recent years, emissions reduction has been tightened worldwide. Therefore, there is an increasing demand for electric vehicles (EVs) that can meet emission requirements. The growing number of new EVs increases the consumption of raw materials during production. Simultaneously, the number of used EVs and subsequently retired lithium-ion batteries (LIBs) that need to be disposed of is also increasing. According to the current approaches, the recycling process technology appears to be one of the most promising solutions for the End-of-Life (EOL) LIBs—recycling and reusing of waste materials would reduce raw materials production and environmental burden. According to this performed literature review, 263 publications about “Recycling of Lithium-ion Batteries from Electric Vehicles” were classiﬁed into ﬁve sections: Recycling Processes, Battery Composition, Environmental Impact, Economic Evaluation, and Recycling & Rest. The whole work reviews the current-state of publications dedicated to recycling LIBs from EVs in the techno-environmental-economic summary. This paper covers the ﬁrst part of the review work; it is devoted to the recycling technology processes and points out the main study ﬁelds in recycling that were found during this work.


Introduction
The ever-increasing number of requirements placed on decarbonization in the transportation sector result in an ever-increasing demand for electric vehicles (EVs) [1,2]. Lithium-ion batteries (LIBs) are the dominant energy storage technology for powering these vehicles due to their high volumetric and gravimetric energy density, long calendar life, low maintenance, and high-power capability. Thus, LIBs can meet the demands of the user in terms of high torque and speed, as well as range [3]. In the coming decade, the demand for EVs is expected to further increase; thus, the production on the lithium-ion market is predicted to grow exponentially [2]. Consequently, the demand for raw material will rise [4], while concerns regarding the availability of critical materials, in particular lithium (Li) and cobalt (Co), will further increase [5].
Therefore, ways for reducing the consumption of raw materials are being sought. Although their reuse in secondary applications extends the battery lifespan and increases benefits of their usage, one of the most promising ways for handling the End-of-Life (EOL) LIBs seems to be recycling [6][7][8][9]. Recycling allows for the recovering of valuable metals, securing the alternative materials supply chain, or gaining independence from exporting economies [10]. Therefore, all parameters of the techno-environmental-economic character are affected.
Recent studies describe that the recycling processes of retired automotive LIBs usually start with the pretreatment method, including discharging, dismantling, and any form of The resulting publications were individually filtered by titles and clustered into the following five sections described by specific keywords: Recycling Processes, Battery Composition, Environmental Impact, Economical Evaluation, and Recycling & Rest. The keywords used for selection criteria and the descriptions of individual sections are summarised in Table 1. The five selected sections were arranged to completely cover the entire techno-environ mental-economic impact of recycling EV-retired LIBs. Because the individual parts of this investigation are intertwined, there is an overlap in the techno-environmental-economic scope of the resulting sections that is captured by Table 2. All the publications were one-by-one reviewed for exclusion based on the same selection criteria as shown in Table 1. In total, 263 publications corresponding to the requirements of this literature review were selected. From these, 89 publications were analyzed according to the main topic of this paper and are listed in Appendix A. The current state of research & development (R&D) in focus on the technology of recycling processes of LIBs from EVs was discussed, and the main directions identified in this field of study were pointed out.

Results
All the selected 263 publications were divided into sections according to the methodology described in Section 2. Many of these papers thematically fall into several categories. The exact number of publications per category (labeled as a keyword), the percentage in the whole literature review, and their overlap are summarized in Table 3. Table 3. Distribution of publications, the methodology of performed literature review.

Section
No.  37 19 In total, 89 papers were selected from these publications (all sections) that characterized as the "Recycling Technology". The representation and overlapping in the individual categories are shown in Figure 2. In this figure, a connection between the categories is visible. For example, the hydrometallurgical processes are also linked to direct recycling (6), special methods (4), recycling retired automotive LIBs (3), and active cathode materials (2). Only one publication was found that was devoted directly to electrolyte processing in this literature review.
Of the selected 89 publications about the technology of recycling processes of LIBs from EVS, there were 53 Articles, 31 Reviews, 4 Proceeding Papers, and 1 Editorial Material. The distribution according to the type of publication in the individual category related to the technology of the processes is shown in Figure 3.

Pretreatment of the Recycling Processes
The techniques of the recycling pretreatment process were described in a total of 4 publications according to the performed literature research as shown in Figures 2 and 3; there are 2 reviews and 1 article in the "Pretreatment" category, and 1 review in "Recovery of Material" one. These papers are summarized in Table A1 of Appendix A, containing the type, publication year, and a keyword summary of the primary content.
The pretreatment recycling techniques for lithium-ion batteries can improve the efficiency of recovering the valuable materials from the LIBs and lower the energy consumption in the following processes. These processes are used for the separation of the cathode materials from the battery casing, separator, current collector, electrolyte, additives, and connections. Approaches to pretreatment are different, but generally can be divided into experimental/laboratory-scale or large/industrial-scale methods. Laboratory-scale methods, which excel in the separation of active materials and process efficiency, are mainly focused on leaching and/or subsequent metal recovery. The industrial-scale processes are higher in process capacity and throughput, but separation of metals is less refined [11,13].
There is no well-defined designation for each pretreatment process category; thus, many variations have been used in recent years. The previously used division into mechanical separation, mechanochemical processes, thermal treatment, and dissolution processes [21,22] can be described in more apt categories yet. The usual three-step division of battery into discharging, dismantling, and separation (corresponding to laboratory-scale methods) was in [11] extended to a seven-step process: discharging, dismantling, comminution, classification, separation, dissolution, and thermal treatment, that fully captures the techniques applied in the industrial-scale processes.

Laboratory-Scale Pretreatment Methods
The most precise laboratory preparation process is described by a three-step method [11,13] usually consisting of:

Discharging
The lithium-ion battery cell is discharged by soaking in a saturated brine solution for 24 h. This process reduces the generation of short-circuiting and exothermic reactions of material deposits in the anode. For example, the response of lithium (Li) with oxygen and water leads to the inflammation of the highly combustible organic solvent [13].

Dismantling
The battery casing is manually disassembly to regain the cathode active material and the aluminum (Al) current collectors [13].

Separation
For cathode active material separation from the current collectors, binding reagents, and conductive additives, one of the following procedures can be used:

•
High-temperature calcination is carried out between 350 • C and 600 • C to decompose the organic binders, additives, and electrolyte and release the active material in a powder form; • Dissolution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) supported using heat and/or sonication, subsequently following processes of drying and filtration; • Dissolving the Al current collector using a strong base [13], e.g., sodium hydroxide NaOH [23].
NMP dissolution is nonpolar; thus, it is not applied on electrodes that contain a poly (tetrafluoroethylene)-based binder. Instead, a polar PVDF is used [13].

Industrial-Scale Pretreatment Methods
The classification, presented in [11], summarizes in detail the industrial pretreatment, which is divided into seven steps and is illustrated in Figure 4. Every step of this approach fully respects the initial stage of following recycling processes.

Discharging
The retired LIBs are in industrial processes treated variously. However, the discharging, as in laboratory-scale methods [13] is carried out by soaking for 24 h in one of the solutions: • Distilled water; • Sodium chloride (NaCl) water-based solution (using 10 wt% NaCl solution ensures the best discharging conditions; the extraction of the valuable metals is maximal); • Alternative research-focused solutions such as potassium chloride (KCl), sodium nitrate (NaNO 3 ), manganese(II) sulfate (MnSO 4 ), magnesium sulfate (MgSO 4 ), and iron(II) sulfate (FeSO 4 ) [11].

Dismantling (disassembly)
The individual retired cells are usually manually dismantled or disassembled with knives and saws. These techniques adopt simple protective measures to fulfil the safety requirements [11].
There is one non-destructive automatic disassembly methodology used to recycle pouch-type LIBs, named Z-folded electrode-separator compounds. Using automatically stretching the Z-folded separators, the cathode and the anode are scraped off by the suitable toolset [11,23].

3.
Comminution (mechanical treatment) The dismantled LIBs need crushing and grinding to release the materials of the electrodes. The comminution step is essential for the recycling of the hydrometallurgical process [11].
The mechanical techniques differ according to the required particle size for further processing. Comminution can be mainly divided into [11]: • Dry processes-crushing is conducted in a gastight unit in an inert atmosphere, generally in a two step-method in a low-speed rotary mill and high-speed impact mill [23], or a combination of the hammer crushing combined with a two-blade rotor crusher that can maximize the efficiency of this process [11]; • Wet processes-the comminution equipment is a blade crusher with a water-based medium; firstly, the batteries are cut into pieces in a shear crusher, and the outputs are then crushed using the impact crusher. Then, water feeds into an entrance of the crusher and the particles in the form of a slurry carry the broken fractions through a selective sieve [11,23].
To enhance the efficiency of leaching of materials (extractions of Li and Co) in the processes, a mechanochemical reduction using planetary ball mill, attrition scrubbing, or grinding flotation is often used. The repetition of the crushing procedure increases the yield of the black mass, which is the shredded materials consisting of high amounts of lithium, cobalt, manganese, and nickel metals [11,24].

4.
Classification (sieving) According to the size of produced fragments from the LIBs, the material composition of obtained products can be determined. For example, products smaller than 850 µm consisted of the Lithium Cobalt Dioxide (LCO) particles and carbon, whilst materials as plastic, steel, aluminum (Al), and copper (Cu) formed enlarged compounds [11].
The particle size also affects the overall efficiency of the recycling process [11], where, e.g., a 500 • C optimum pyrolysis temperature was determined according to the maximum amount of the 0.2 mm size fraction in the pyrolytic cathode [25]; material extraction of the hydrometallurgical process was the most Fe (65%) was contained in the particles cruder than 6.7 mm, the particles of Al and Cu were mainly observed for the size of 2 and 6.7 mm, and the electrode elements, including Li, Co, manganese (Mn), and nickel (Ni), set from 92 to 98% in the particles finer than 6.7 mm [26].

Separation
The crude separation depends on the size of particles; more purified separation techniques, such as magnetic, eddy current, electrostatic, gravity separation, and froth flotation, ultrasonication, agitation, air separation, and density separation being used. The first three mentioned processes are described below, while the rest of the methods are listed in detail in the review [11]. Different approaches for the separation process are provided in [11,12].

•
The magnetic separation removes the iron (Fe)-containing components and separates the cathode that contains active materials, the Al current collector, the anode, the steel casings, and the packaging [27].

•
The eddy current separates the electrical conductors from the non-conductors or the minimally conductive materials. This method provides a high-ranking separation between Al and Cu in the electromagnetic fraction and Co and Li in the nonelectromagnetic fragments [28].

•
The differences in material electrical properties are utilized in the electrostatic separation. When an electric field is applied, charged or polarized particles are being moved and sorted from the LIBs crushed mass [29].

Dissolution
Even at this stage of the process after the classification and separation stage, some active materials are still fixed to the current collectors. The binders connect the other detached active materials.

7.
Thermal treatment At this stage, the binder materials that bring together the active materials and carbon conductive agents are still fixed to the current collectors [11]. For removal, some of the following thermal treatments can be used:

•
A two-step thermal treatment followed by calcination. The furnace temperature range varies from process to process and affects the overall duration. The first thermal step can be conducted between 150-500 • C and lasts for 1-2 h; the second is between 500-900 • C and lasts for the same length [30,31]. The calcination at 600-700 • C lasts about 5 h [11].

•
The vacuum pyrolysis, which is carried out in a vacuum furnace during temperatures 500-600 • C [32]. The cathode materials are directly placed into the furnace to reduce the material loss [11].
The pyrolysis method can be combined, e.g., with ultrasonication or microwave methods [11].

Recycling Processes
The recycling processes were reported in 85 publications according to the literature search; the total number of publications is given by the sum of papers in individual categories: 9 in the "Metallurgy/Mechanical", 18 publications in the "Pyrometallurgy", 23 in the "Hydrometallurgy", 17 in the "Direct Recycling", and 18 in the "Special Method". These papers are described in more detail in relevant Tables A2-A6 of each category in Appendix A that contains the type, publication year, and the keyword summary of the primary content.
Overall, the recycling process brings an advantage in material savings (especially for valuable metals) and has a positive effect on energy consumption and the protection of the environment. Material production needs high energy for virgin resources' extraction and causes the release of greenhouse gases (GHGs) from transportation and smelting processes. Both the energy consumption and the amount of released GHGs can be reduced by reusing and recycling [13,33].
Recycling methods are generally used in industry, but there is still a lot of ongoing research carried out on the experimental/laboratory-scale to make recycling processes more efficient [33]. In Table 4, lithium-ion battery recycling facilities worldwide are listed according to [13,[33][34][35]. For all companies, their location and their pretreatment and/or recycling process are described. The main products presented are for European facilities. For example, the German company Accurec developed a facility which recycles almost all the components from LIBs. Firstly, the batteries are disassembled into components, electronic parts, cables, and plastics; next, the remaining cells are sent to pyrolysis carried out at 250 • C. The subsequent mechanical processes separate ferromagnetic steel, aluminum cases, and Al and Cu foil from electrodes. The vacuum pyrolysis recovers lithium in the pure metallic form by evaporation and distillation or in the form of lithium oxide using selective gas evaporation of Li [18,36].
The recycling process from the French company Recupyl can be characterized by mechanical processing under an inert atmosphere of carbon dioxide (CO 2 ) and further hydrometallurgical recycling. LIBs are classified and crushed in the atmosphere containing a mix of CO 2 and argon (Ar) in an inert chamber. The mechanical crushing is two-step using a rotary mill at less than 11 RPM and an impact mill at less than 90 RPM. Meanwhile, the water neutralizes the off gas of these processes. Large-size fractions such as metal cases, paper, plastics, and foils are sorted out from electrode materials by the mill. The high induction magnetic separator splits up steel pieces. The rest of the fraction represents non-magnetic materials, which are segregated according to their densities. Separated active minerals are leached (hydrometallurgical processed) in heavily stirred water under an atmosphere with low oxygen levels, from where the Li, Co, and Mn are recovered. This process is very similar to the one used by the Swiss company BatRec [13,37].
As stated in Table 4, the most-used industrial recycling techniques include metallurgical and mechanical procedures such as the pyrometallurgical process and the hydrometallurgical recycling method. Moreover, the direct recycling process is under intensive research. These methods have been detailed described in this review in Sections 3.2.1-3.2.5. Section 3.2.5 is devoted to presenting unique methods and techniques of recycling.

Metallurgical and Mechanical Processes
The metallurgical and mechanical process was described in a total of nine publications according to the review; there are four articles and one review in the "Metallurgical and Mechanical" category, three articles in the "Cathode", and one article in the "Hydrometallurgy". These papers are summarized in Table A2.
As outlined in the next sections, the recycling processes are created by long and complex process chains to accomplish the main recycling aims fulfilling the high environmental and economic requirements. In the past few years, various recycling procedures have been developed. These techniques can be divided into two general process routes: pretreatment of the processes (the mechanical treatment before the metallurgy) and a straight pyrometallurgy or hydrometallurgy technique, or their combination, e.g., the pyrometallurgy and subsequent hydrometallurgical treatment. Direct recycling is still in a phase of industrial implementation [10,21,33,38].
As described in Section 3.1.2, the LIBs can be pretreated mechanically, often before a thermal treatment. After the initial comminution, the materials and components are classified by the physical properties (particle size, form, density, and electric and magnetic properties). Afterward, the outputs are processed in further metallurgical treatments. Typical outputs of these processes are Al/Cu fractions (conducting foils), non-ferrous metals (casings), ferrous metals (casings and screws), and a fraction called black mass (active electrode materials). The black mass can be either processed by pyrometallurgy methods or treated directly in hydrometallurgy procedures. Before hydrometallurgy, thermal pretreatment is required for the removal of organic components and concentration of the metal content [10,39].
Using the pyrometallurgical method, Co, Cu, and Ni alloys (metallic phase) or matte (sulfidic phase), Al, Mn, and Li slag (oxidic phase), and flying ash are produced [7,10,40]. These products can be further treated by hydrometallurgical procedure/direct recycling to recover the individual metals. In hydrometallurgy, Co, Li, Mn, Ni, and graphite can be recovered [10,38].
Nowadays, requirements on a combination of mechanical methods with pyrometallurgical and/or hydrometallurgical procedures targeted to processing of black mass increase and are under intensive research and development [24]. The reason is that the precious active electrode materials, such as Co, Li, Ni, Mn, and graphite, are deposited in the black mass. Additionally, conducting salts or elements such as Al, Cu, and Fe can be found there as well [10,33,41]. The concentration ranges of the black mass components produced from layered oxide chemistries are given in Table 5 according to [10]. Table 5. Concentration ranges of major black mass components produced from layered oxides chemistries, according to [10].

Elements Content [wt %]
Aluminum (Al) 1-5 Cobalt (Co) 3-33 Graphite approx. 35 There are many approaches for the mechanical pretreatment of recycling LIBs. Thermal treatment can be applied before or after the mechanical methods or may be omitted altogether. The thermal treatment is typically formed by pyrolysis at around 500 • C to reduce the energy and eliminate the organic and halogenic content. The pyrolyzed LIBs can be safely mechanically processed without fire hazards [10,36]. In the other cases, specific safety measures are mandatory to prevent explosion and ignition during the processes. The non-thermal-treated procedures include for instance crushing under inert atmosphere of nitrogen (N 2 ), carbon dioxide (CO 2 ), or argon (Ar), followed by recovery of the volatile components by vacuum distillation or drying at moderate temperatures, or comminution in a solution, e.g., in a slightly alkaline medium [10,42].
Monitoring the effect of mechanical procedures has been investigated in many experiments [10]; for example, the influence of a second crushing on process and product was observed in [21]. In this experiment, a cutting mill with a discharge screen of 10 mm was used, and two different routes with/without the second crushing method were compared. Because of the comminution of the black mass fragments and the decomposition of inclusions, the second crushing increases the yield of black mass from 60% to 75%, as shown in Figure 5 from [39].
The black mass can be industrially processed by pyrometallurgical routes, described in Section 3.2.2, or treated by hydrometallurgical methods, described in Section 3.2.3. Due to the significant reduction of Al, other organic compounds, and excessive Co and Ni concentrations, the pyrometallurgy is preferred for black mass treatment. However, the F and Li content lead to corrosion, and the recovery efficiency of Li is not optimal [10,18,43]. On the contrary, Mn, graphite, and Li can be effectively recycled by hydrometallurgical processes. Nonetheless, there are still many issues of this procedure, such as F recovery, Mn recycling control, the production of graphite products, or the sensitiveness to organics compounds that easily contaminate process water [38,44].

Pyrometallurgical Process
The pyrometallurgical recycling technique was described in a total of 18 publications according to the performed search; there are 3 articles and 1 review in the "Pyrometallurgy" category, another 4 reviews and 2 articles in the "Recycling LIBs", 4 reviews in the "Special Method", 3 reviews in the "Recycling EV LIBs", and 1 article in the "Hydrometallurgy" category. These papers are summarized in Table A3.
The pyrometallurgy recycling technique uses a high-temperature furnace to reduce valuable metals and refine them through physical and chemical transformation. As temperature increases, at first structural changes, as phase transitions, occur. Later, at already high temperatures, the dominating process is chemical reactions, resulting in batteries being smelted. The required heat is typically provided by exothermic reactions, combustion, or electrical power, depending on various specifying factors such as temperature and processing time [9,43].
Thermal pretreatment is significant for the pyrometallurgical process, and is described in detail in Section 3.1.2 [11]. Usually, these techniques are based on incineration, pyrolysis pretreatment, or other variations of these types [11,43].
The mainly used pyrometallurgical options conducted for recycling LIBs are roasting, calcination and smelting. These processes can be sorted according to the atmosphere, where the process is carried out and the extraction mechanisms used, as illustrated in Figure 6 [43].
Roasting is an exothermic mechanism, which at elevated heat involves reactions between gas and solid particles. Materials such as carbon, charcoal, or coke then act as a reducing agent, which heats up the cathode material, resulting in a mix of alloys and carbon residue. A lower valence state is then reached by the lithiated metal oxide. Additionally, further oxide reductions can happen by using the carbothermic reduction carbon at the required temperature [45,46]. The amount of metal oxide reduction in this process is then controlled by the quantity of carbon participating [43,45,46]. The roasted mechanism was for example, realized by [43] where a mixture of Lithium Cobalt Dioxide (LCO) and graphite were treated under a nitrogen atmosphere for 30 min at a temperature of 1000 • C. The residue after roasting was water leached [43]. The recovery rates were 98.93% for Li, 95.72% for Co, and 91.05% for graphite [43,47]. In another study [48], a temperature of 800 • C was used to roast Lithium Manganese Dioxide (LMO) for 45 min. For the LMO reduction, graphite was used in the mix of lithium carbonate (Li 2 CO 3 ) and manganese(II) oxide (MnO) subsequent with water-based leaching and mechanical separation. Recovery of this process was 99.13% Li and 95.11% manganese(II,III) oxide (Mn 3 O 4 ) [43,48].
A change from carbothermic reduction roasting to salt-assisted reduction roasting is recently applied. The salt-assisted procedures can reduce costs by lowering the excessive energy needs for evaporative crystallization. Moreover, salt-assisted roasting can presumably increase the efficiency of the whole recycling process, reducing acid consumption and toxic gas emissions by the production of water-soluble salts. According to the used reagent, other roasting procedures, such as chlorination, sulfation, and nitration, can be classified [43].
The smelting process allows the recycling of various EOL LIBs based on different chemistries [9]. By smelting, the material of the retired battery is heated above its melting point and subsequently the metals are separate in the liquid phase by reduction and subsequent formation of immiscible molten layers [43,49]. Moreover, this process eliminates the necessity for a previous passivation step, and the battery cells can be directly thrown into the smelting furnace. The process of smelting usually consists of two complementary phases [43]: • At first, the battery material is heated at a lower temperature to slowly evaporate the electrolyte [14]; • Secondly, the temperature is increased to melt the material's feeds [43].
The organic material is burnt out to provide energy for the process. The reduction of the active cathode material is regulated in a blast or electric furnace. After the smelting, the transition metals are concentrated into a molten alloy phase. For the recovery of valuable metals, the hydrometallurgical technique is consequently used [34,43].
Pyrometallurgical recycling techniques have many advantages, such as that the batteries do not have to be pre-treated (e.g., before smelting), and the material recovery yields are high (more than 90%) [43]. However, this process is costly [48,50]. Further advantages and disadvantages of this process are listed according to [51] in Table 6. Table 6. Advantages and disadvantages of pyrometallurgical process according to [51].

Advantages Disadvantages
Application flexibility; all battery compositions, and configurations

Hydrometallurgical Process
The hydrometallurgical recycling method was described in a total of 23 publications according to literature review; there are 7 articles and 1 review in the "Hydrometallurgy" category, 4 reviews and 2 articles in the "Recycling LIBs", 3 articles and 1 review in the "Special Method", 1 review and 2 proceeding papers in the "Recycling EV LIBs", and 1 article and 1 review in the "Cathode" category. These papers are summarized in Table A4.
Various hydrometallurgy techniques were developed in recent times for recycling cathode active materials of different chemistries LIBs including Lithium Cobalt Dioxide (LCO), Lithium Manganese Dioxide (LMO), Lithium Nickel Manganese Cobalt Oxide, (NMC), Lithium Nickel Cobalt Aluminum Oxide (NCA), and Lithium Iron Phosphate (LFP) [52], to recover valuable metals such as Co, Ni, Mn, and Li [53,54]. The whole hydrometallurgical process is created by physical and chemical methods proceeding in liquid media, which allow the high recovery of these metals [26,55,56].
The physical operations include the mechanical pretreatment methods such as sieving and magnetic separation described in Section 3.1.2. [11]. The LIBs are discharged and separately treated to improve the safety and recovery rate of the whole process [55,57]. This operation allows the conservation of the minerals and efficiently separates the individual metals [55].
The chemical operations can be classified according to the procedures and final products [46] as illustrated in Figure 7 from review [58].

•
The first stage, "Leaching", contains dissolving or leaching of the valuable metals by acid or basic agent in an oxidizing or reducing medium in leaching tanks [55,57].

•
The second stage "Impurity removal" is created by solid-liquid separation, which clarifies the leach solution by filtration or centrifugation [55,62].

•
The last stage "Ni, Co, Mn, and Li recovery" is devoted to the final recovery of valuable metals in hydroxide or metal salts [55]. This process includes, for example, solvent extraction [63], electrochemical techniques [64], selective precipitation [65], and separation by ion exchange resins [55].
Different leaching techniques are used for various battery chemical compositions to achieve the most efficient recovery of materials. The most promising current state treatment reagents belong to hydrochloric acid (HCl), used for LCO batteries [52,[66][67][68][69]. According to the experimental results of [66][67][68], the leaching efficiency of valuable metals (Co and Li) is achieved 100% for 2 M and 4 M HCl at a temperature of 80 • C in 90 min and 60 min-long extraction, respectively. In other experiment, the leaching efficiency was higher than 99% of all materials (Co, Ni, Mn, and Li) by using 4 M HCl at 80 • C for 60 min [69].
Many other studies ( [52,70,71]) were to the leaching reactions of nitric acid (HNO 3 ) devoted. For example the 100% recovery of Li and 95% recovery of Mn was achieved by leaching using 2 M HNO 3 at 80 • C within 120 min [70]. The combination of 1 M HNO 3 and 1.7 vol% hydrogen peroxide (H 2 O 2 ) leached at 75 • C for 30 min increased the recovery of Co and Li up to 99% [71].
Leaching sulfuric acid (H 2 SO 4 ) procedures are usually slow and the metal recovery efficiency is lower. By using 2 M H 2 SO 4 at 70 • C, just 76% efficiency was observed [71]. A detailed description of other procedures using reagents such as oxalate, ammonia, DL-malic acid, phosphoric acid, etc., additionally supplemented with overview tables, is provided, for example, by reviews [13,52,72].
Hydrometallurgical recycling techniques have many advantages such as the high recycling process efficiency and pureness of the final products [51]. However, the LIBs must be subjected to both procedure steps of the hydrometallurgical method-physical pretreatment and chemical recovery [52]. Further advantages and disadvantages of this process are shown in Table 7 according to [51]. Table 7. Advantages and disadvantages of hydrometallurgical process according to [51].

Advantages Disadvantages
Application flexibility; all battery compositions and configurations Necessary to crush the batteries; high safety requirements Flexibility of separation process; a desired product (metal) can be obtained

Uneconomical for Lithium Iron Phosphate (LFP)
High efficiency of the recycling process (especially for Li) High volume of waste water; necessary disposal or further recycling High purity of products Impossibility of recycling anode materials (graphite, conductive additives) Emission-free High operating costs

Direct Recycling Process
The process of direct recycling was totally described in 17 publications according to the performed review; there are 3 articles in the "Direct Recycling" category, 5 articles in the "Cathode", 1 article and 1 review in the "Anode", 1 article and 1 review in the "Hydrometallurgy", and 3 reviews and 2 articles in the "Recycling LIBs. These papers are summarized in Table A5.
The direct recycling process handles the total recovery of LIBs materials for reusing in a new battery production instead of dissolving the active material entirely by hydrometallurgical techniques [13,58]. This process consists of various physical and chemical steps with low temperature and energy requirements used for battery separation. Therefore, the procedure should be lower in costs than leaching because the intervention of the material is minimized [58]. Direct recycling is not commercialized yet [51,58]; there are only some published processes from recycling companies. Selected companies are listed in Table 4. Detailed summaries of direct recycling processes of individual companies are provided by reviews [58,73,74].
For instance, the process patented by company Retriev with a capacity recovery of 95% consists of crushing and screening mechanisms followed by thermal treatment at 500 • C for the carbon surface modification and liberation of polyvinylidene fluoride (PVDF) [75]. For carbon removal, selective flotation is implemented; the calcination at 500-800 • C is realized using a fraction mixture and lithium hydroxide solution [58,75]. Alternatively, the mechanism of the American facility of OnTo Technology is executed in the atmosphere of supercritical carbon dioxide (CO 2 ), as an alkaline solution is used with lithium hydroxide (LiOH), and the calcination is performed at 400-900 • C [58].
As shown in Tables 5 and 6, the pyrometallurgical and hydrometallurgical process are not optimal for recycling Lithium Iron Phosphate (LFP) batteries. Therefore, direct recycling research is aimed at their recycling [58].
The cathodes of the pretreated LFP batteries with sorted components were submitted to thermal reactivation processes at 500 • C under nitrogen flow [76]. These processes decompose the polyvinylidene fluoride (PVDF) binder and the active LFP material from other features. The recovered cathode material has a low electrochemical capacity versus Li 0 (reaching 136 mAh/g at 0.1 C) [76]. Consequently, this process was improved by [77,78] at the pilot experiment by insertion a Li precursor and a reducing gas during the thermal treatment; hence, the electrochemical capacity versus Li 0 exhibits higher values (145 mAh/g at 0.1 C). The process flow chart of selective leaching process is shown in Figure 8 [58]. Moreover, the process integrated a two-temperature-washing-step with dimethyl carbonate (DMC) to remove organic compounds before component separation, milling, and thermal treatment at 200 and 500 • C in a nitrogen (N 2 ) atmosphere [78]. Regenerated LFP provides the highest capacity versus Li 0 (150 mAh/g at 0.1 C), but Al contamination at 0.5% was observed and had to be minimized to prevent accumulation in the recycled material [58]. Other direct recycling processes are being applied; for example, in a simple patented process by [79], very high-frequency ultrasound (>900 kHz) for up to 30 min was applied to the whole (not damaged) battery. Thereby the surface of the cathodes was cleaned out of absorbed phosphorous compounds that come from the dissociation of electrolyte lithium hexafluorophosphate (LiPF6) during cycling. Another leaching procedure was developed by [80], where the cathode materials are cleansed off residual Cu and Al in an alkaline solution (a complexing agent (5 M ammonium hydroxide NH 4 OH) and 1 M lithium hydroxide (LiOH) with O 2 (g) bubbling to promote oxidation) after separation from other components. After 12 h, these contaminants are completely solubilized. Using the direct recycling procedure, any type of LIB can be treated. However, LFP batteries must comply with the specific pH requirements for process safety [58].
The direct recycling process is complicated in terms of pretreatment techniques that are necessary for battery material separation. On the other side, all the battery materials can be recycled with sufficient energy efficiency. Other advantages and disadvantages of the process of direct recycling are described according to [51] in Table 8. Table 8. Advantages and disadvantages of the process of direct recycling according to [51].

Difficult mechanical pretreatment, necessary material separation
Suitable for Lithium Iron Phosphate (LFP) The mix of materials reduces the quality of the process Energy efficient Low quality of output products Production residues can be recycled Not yet fully industrially applied

Special Recycling Methods
The special methods describing recycling processes were characterized in a total of 18 publications according to the search; there are 11 articles, 2 reviews, 1 proceeding paper, and 1 editorial material in the "Special Method" category, and 2 articles and 1 proceeding paper in the "Life Cycle Assessment (LCA)". Many studies in these publications have been devoted to a precisely characterized process. Therefore, this essential content is summarized in Table A6.
The processes described in previous sections (Sections 3.2.2 and 3.2.3) have been summarized mainly at the industrial level; thus, a review gives a more detailed presentation of some studies from the experimental/laboratory-scale, not focusing only on direct recycling (Section 3.2.4), but supports the research of mentioned metallurgical procedures.
The pretreatment process before the recycling procedure in the laboratory-scale was described in detail, for example, in [11] or [13]. A review summarizing all the laboratory-scale recycling procedures was not found during this literature review. Only one article, published in 2001 [67], outlined "A laboratory-scale lithium-ion battery recycling process". This paper refers to experimental testing of commercial, cylindrical 18650 sizes LiCoO 2 /LiC x retired batteries, which were treated by crushing and riddling to the extraction of the active materials. Next, the active materials were selectively separated using N-methyl pyrrolidone (NMP) at about 100 • C for 1 h; from the product, the Al and Cu foils, and lithium cobalt oxide (LiCoO 2 ) and carbon powder were filtrated. The dissolution of lithium cobalt oxide was achieved by treating the separated residual powders with a small volume of 4 M hydrochloric acid (HCl) for 1 h at about 80 • C, where the acid/sample ratio of cobalt and lithium from the oxide was 10. As a side product, carbon powder was removed using the solution decantation. The cobalt dissolved in the hydrochloric solution was recovered as cobalt hydroxide Co(OH) 2 by the addition of one equivalent volume of a 4 M sodium hydroxide (NaOH) solution. Consequently, the precipitation due to the Co(OH) 2 would be obtained for pH between 6-8, ensuring NaOH solution treatment [67].
Existing industrial techniques are currently being extended under a laboratory scale. For example, using molten salts within a temperature range of 800-900 • C as electrolytes and reaction media is being applied as a chemical recycling procedure. Different eutectic mixtures of molten salts such as sodium, potassium, lithium, and calcium borates and chlorides, sodium, and potassium carbonates are being tested to provide an optimized alternative to the hydrometallurgical or pyrometallurgical processes of metal recovery. The final metal purity for single-metal oxides for these processes is around 98-99% [92].
An innovative mechanochemical method for the cathode materials (C/LiCoO 2 ) of retired LIBs and waste polyvinyl chloride (PVC) was examined. The mixture of LiCoO 2 /PVC/Fe was co-grinded and further water leached. As a result of this procedure, recoverable lithium chloride (LiCl) from Li by the dechlorination of PVC and magnetic cobalt iron oxide (CoFe 4 O 6 ) from Co were generated. This study can also be classified as environmentally friendly because it was found that the chlorine atoms in PVC are mechanochemically transformed into chloride ions that bond to the Li in lithium cobalt oxide (LiCoO 2 ) to form LiCl. This resulted in the reorganization of the Co and Fe crystals to create the magnetic material CoFe 4 O 6 . Therefore, including this recycling process in the existing chain would have both environmental and economic benefits [93].

Discussion and Conclusions
Electric vehicles are experiencing significant growth globally; hence, the need for Lithium-ion batteries is increasing as well. However, the best environmental and economic beneficial strategies of treating the retired LIBS are still being sought. Nowadays, the best scenario seems to be the combination of secondary applications of retired LIBs at the end of their primary lives in EVs (e.g., in stationary storage), followed by the recycling of End-of-Life (EOL) waste batteries [67,96,97].
In the last years, the recycling process of LIBs has been a much-discussed topic; the recycling procedure addresses the still-growing amount of EOL batteries and reduces the consumption of raw materials used in the new battery production. Recycling recovers valuable metals such as lithium (Li), cobalt (Co), nickel (Ni), and manganese (Mn) and other used materials, for example, aluminum (Al), copper (Cu), or iron (Fe); thus, it secures alternative materials supply chains and gains independence from exporting economies. Therefore, it can be stated that the choice of technological process of recycling will have a strong environmental and economic effect [10,11,13].
Thus, this comprehensive literature review, based on 263 publications, about "Recycling of Lithium-ion Batteries from Electric Vehicles" was performed providing a technologicalenvironmental-economic summary of the current state of this topic. The whole work was divided into two separate parts: (i) the technology of recycling processes and (ii) their environmental and economic effects. This paper describes in review only the first part, namely the technological point of view of recycling processes and industrially and laboratory used strategies. A techno-environmental-economy overlap of recycling EOL LIBs focusing on the summarized content of this part of the whole review is shown in Figure 9. All 89 articles devoted to recycling technology were reviewed according to their content and divided into six categories: pretreatment of the recycling processes; and recycling processes including metallurgical and mechanical processes, pyrometallurgical process, hydrometallurgical process, direct recycling, and special recycling methods. The summary content is listed in Tables A1-A6 of the Appendix A of this paper.
There is no well-defined designation for pretreatment processing of retired LIBs. The previously used four-step classification of pretreatment procedure appears to be insufficient for currently used industrial techniques. This process was best captured in [11], where the pretreatment was divided precisely, according to the methods and techniques, into the seven following steps: discharging, dismantling, comminution, classification, separation, dissolution, and thermal treatment. The intensity, methodology, and procedure choice of mechanical processing affect the quality and amount of output products (fractions of active materials called black mass). According to [39], the addition of the second crushing step into the mechanical pretreatment process increased the mass of black mass obtained by 15% compared to the single crushing process. Thus, the total process efficiency will be positively affected.
The pre-treated LIBs are subsequently metallurgically processed. The most industrially used techniques are pyro-and hydrometallurgy. These methods are in the right combination (pyrometallurgical method followed by the hydrometallurgical process) used for very effective recycling black mass, which can reach up to almost 100%. In the pyrometallurgical procedure, Co, Cu, and Ni alloys in the metallic or sulfidic phase, Al, Mn, and Li slag in oxidic phase, and flying ash can be produced. In the hydrometallurgical process, Co, Li, Mn, Ni, and graphite can be recovered [10,98].
The pyrometallurgy recycling reduction of valuable materials is based on the physical and chemical transformation of thermally treated LIBs. The lower temperature treatments secure structural changes, including phase transitions; the higher temperatures provide chemical reactions using roasting, calcination, or smelting. The efficiency of pyrometallurgical process in the recycling of electrode materials, LIB lithium slag, or scraps reached over 80-98.9%. The purity of recovered metals ranged between 98 and 99.95%. The hydrometallurgical technique requires only mechanical pretreatment of the discharged batteries, such as sieving and separation. Then, the batteries are chemically treated in liquid acids or basic agents, which allow a high recovery of metals. The efficiency of those processes is around 76-98.2%, the purity is set between 96.5-99.7%. When recycling anode materials by leaching, the overall efficiency was almost 100% with the resulting purity of the obtained products around 99.9% [17,38,98,99].
The direct recycling process represents another recycling technique. This procedure is created from many physical and chemical steps with low temperature and energy requirements used for battery separation. Thus, the procedure cost should be lower in comparison to hydrometallurgical leaching. Furthermore, the efficiency of the process still be higher than 95%, and the purity of recovered materials should be comparable to metallurgical processes. Direct recycling is not yet fully commercially available. The implementation is soon envisaged because treating this method allows for the complete recycling of LFP batteries, which is problematic using current processes. Nowadays, direct recycling is explained in procedures published by a few recycling companies or laboratories, which cooperate for full application [58,67].
Although the overall efficiency of the mentioned processes is mostly above 90%, their implementation is limited by their technological complexity, required output products, further processing, predicted, or already monitored environmental burden, and costs that can be provided for line equipment and operation. Even though the batteries can be pyrometallurgical recycled on existing facilities, the operating costs of the whole process are high (mainly because the process requires high temperatures). In addition, toxic gases and water are released. Pyrometallurgy does not allow the recycling of electrolyte or graphite from anode active materials. On the contrary, the equipment of the hydrometallurgical line is costly, but the operating costs are much lower (depending on the chemicals used). As an intermediate product, only toxic wastewater is discharged and is further treated for reuse. Even hydrometallurgy does not allow the recycling of electrolytes from batteries [33,100,101].
The currently used metallurgical processes have several disadvantages. For their optimal integration into the entire life cycle of LIBs, it would be necessary to reduce their environmental impact, whether in the form of generated emissions (pyrometallurgy) or wastewater (especially hydrometallurgy). These restrictions are complex and very expensive. Thus, the recycling integration could be reduced.
A solution that could encourage the expansion of recycling facilities is expected to implement the direct recycling process fully. The input costs for direct recycling line equipment are expected to be high. However, operating costs, waste intermediates and the environmental burden will be lower compared to the existing processes and therefore sustainable in the long-term run. Moreover, all types of LIBs can be recycled by direct recycling. The main shortcoming of the currently implemented direct recycling procedures is the resulting purity of the obtained materials, which ranges between 70-80%. The use of lower quality input products could fundamentally affect the characteristics of new batteries; thus, the effectiveness of metal recovery must be increased. This could be achieved by applying some steps of established metallurgical processes [33,102].
Research in the coming years shall focus on the complete application of the direct recycling process, increasing its metal recovery potential and lowering the total process costs. Within that, the optimization of current metallurgical processes would be desired because reducing the environmental burden could encourage further use of these recycling techniques.
Thereby, the main directions of current-state publications in the field of "Recycling of Lithium-ion Batteries from Electric Vehicles" are summarized in this review paper. The next work will be devoted to reviewing the rest of the publications, which are discussing the environmental and economic fields and provide answers to questions concerning environmental impact; comparison of Life Cycle Assessments (LCA) of LIBs; the contribution of recovery materials; and economic evaluation, mainly including cost-benefit analysis with consideration of extended lifetime in the secondary applications of retired LIBs. Acknowledgments: This review was discussed and supported by EEB/1 HV-Battery Systems and VAP eMobility Business Development Aftersales Škoda Auto, a.s.

Conflicts of Interest:
The authors declare no conflict of interest.