The circular economy of LIBs consists of different phases, starting with the extraction of raw materials, through production and the use phase for disposal, reprocessing, recycling and the subsequent substitution of primary raw materials. In particular, the phases from disposal and collection to metallurgical processing into secondary raw materials play a central role in the recycling of the LIBs. In general, the end-of-life processes for LIBs contain most or all of the following process steps:

• Collection and sorting ^[1][2][[5][23^3][4],24,25,26,27];
• Disassembly and discharge ^[6][7][8][28,29,30];
• Thermal deactivation ^[9][10][11][31,32,33];
• Mechanical processing ^{[8][12][13][14]}[30,34,35,36];
• Metallurgical recycling ^[15][16][37,38].

The following sections present the current state-of-the-art in sorting, processing and recycling LIBs. The dismantling and discharging steps occur mainly in the recycling of LIBs from electric mobility ^[6][28]. Depending on the recycling route, thermal deactivation of LIB cells is also required ^[9][31].

LIB cells consist of two electrodes (anode and cathode), an electrolyte and a separator. The operation of an LIB cell is based on the principle of reversible lithium-ion intercalation and de-intercalation during charging and discharging ^[17][39]. Between the electrodes is an ion-conducting electrolyte containing a dissociated lithium-conducting salt, commonly used is LiPF₆. Other components of the electrolyte are organic carbonates, such as diethyl carbonate, dimethyl carbonate and others ^[17][39]. A porous plastic film separates the two electrodes from each other and acts as a membrane for lithium ions ^[18][20]. The cathode of commercial LIB cells usually consists of a compound that can accept lithium ions, such as transition metal oxides or polyanion compounds ^[19][40]. The most commonly used transition metal oxides are the following ^{[20][21][22][23][24][25][26]}[21,41,42,43,44,45,46]:

• Lithium cobalt oxide (LiCoO₂, LCO),
• Lithium manganese oxide (LiMn₂O₄, LMO),
• Lithium nickel cobalt manganese oxide (LiNi_XMn_YCo_ZO₂, NMC),
• Lithium nickel aluminum oxide (LiNi_0.8Co_0.15Al_0.05O₂, NCA).

Polyanion compounds include materials such as ^{[20][24][27][28][29]}[21,44,47,48,49]:

• Lithium iron phosphate (LiFePO₄, LFP),
• Lithium manganese phosphate (LiMnPO₄, LMP),
• Lithium cobalt phosphate (LiCoPO₄, LCP).

Aluminum foils are used as the current arrester for cathodes. In commercial use, most anodes use graphite as the anode active material ^[30][31][32][50,51,52] and a thin copper foil as the current arrester.

LIB are designed and built as individual cells. These cells are manufactured in various formats, and a distinction is made between button, cylindrical, prismatic and pouch cells ^[27][33][47,54]. Cylindrical cells are manufactured in different layers of anode, separator and cathode, which are rolled around a pin to a so-called jelly roll ^[34][55]. The jelly roll is further integrated into a steel shell, which is filled with the electrolyte, closed and welded ^[34][55]. Pouch cells are also manufactured with different layers of anode, separator and cathode sheets. These are stacked on top of each other, inserted into the pouch foil, a very thin aluminum-polymer compound, before being filled with the electrolyte and closed ^[34][55]. For cylindrical cells, the 18650 and 21700 cell formats, in particular, have become established ^[27][47]. The 4680 cell format is being discussed in particular by American car manufacturer Tesla and will be used there in the future ^[35][36][56,57]. The 18650 cell format is used for portable batteries in the consumer sector. The dimensions of this format are standardized by the American National Standards Institute in the ANSI C18.1M standard ^[37][58], while the dimensions of the prismatic and pouch cell formats for automotive applications are standardized by DIN SPEC 91252 ^[33][54]. Pouch cells used in the consumer sector for products such as laptops, tablets and phones are currently not subject to any standardization. Depending on the application, several cells can be connected in series and/or in parallel in a module ^[18][20]. Modules use a battery management system (BMS) to manage individual cells. The BMS determines the cell voltage and temperatures, monitors the current, and allows the battery system to be switched on and off ^[18][20]. For EV batteries, several modules are connected to a battery pack. Current trends also pursue the “cell-to-pack” approach, in which many cells are directly interconnected into a battery pack ^[38][59].

3. Collection and Sorting of Spent Batteries

Since the Battery Directive came into force in 1998, manufacturers and importers of batteries have been required to take back and recycle spent batteries and accumulators free of charge. The legislator provides so-called collection schemes for the return and proper recycling ^[39][62]. There are various collection schemes in Germany and other European countries, such as the following:

• Stiftung Gemeinsames Rücknahmesystem GRS-Batterien, today owned by Saubermacher AG ^[40][63];
• REBAT or REBAT+ ^[41][64];
• ^]
  DS Entsorgungs- und Dienstleistung GmbH, a subsidiary of the Landbell Group ^[42][65];
• Stibat B.V. ^[43][66].

In Germany, spent batteries as well as waste electrical appliances must be taken back by every retailer according to § 9 BattG and § 17(1) 2. ElektroG2 ^[44][45][67,68]. For the individual cells and battery packs, there are disposal boxes in which the consumer can separate the used batteries into LIBs and others. Since a mixed waste battery fraction with different chemical compositions cannot be recycled ^[39][46][62,69] and a sorted return by the consumer cannot be guaranteed, sorting based on the chemical composition of the waste batteries is necessary. Sorting spent batteries according to their chemical composition can be done in different ways. A distinction is made between mechanical, automatic and manual sorting ^[39][62]. Mechanical sorting of individual, general purpose portable batteries is carried out, for example, by sieving, where the spent batteries are sorted according to their size ^[47][70]. In manual (or optical) sorting, batteries are sorted by humans based on visual characteristics, such as type designation or manufacturer. Typical sorting speeds are around 300 kg of batteries per person per hour ^[39][62]. Currently, every battery sorting facility in Europe performs (partially) the manual sorting of battery cells or battery packs. Automatic sorting of spent batteries according to their chemical composition (Li-ion, NiCd, NiMH, etc.) can be done by sensor-based sorting. In practice, XRT sorting ^[47][70], VIS sorting and sorting by weight sensors are used ^[48][71]. Currently, sorting is only done based on the battery type but not further by CAM. Figure 1 shows state-of-the-art sorting concepts for EoL batteries. In some research concepts, LIB cells have been further sorted into cobalt-rich and cobalt-poor fractions, but good data on the results have not been published ^[49][72].

4. Mechanical Treatment of Spent Lithium-Ion Batteries

After collection and sorting, LIBs are pre-treated depending on the subsequent mechanical and metallurgical processing steps. These include the processes of deep discharging ^{[50][51][52][53]}[73,74,75,76], dismantling ^[54][77] and thermal pretreatment ^[9][31]. Due to its mostly manual execution, the process of deep unloading is currently only carried out for EV-LIB ^[55][78]. 3C-LIBs are largely thermally deactivated ^[9][31] or crushed in a wet environment ^[56][79], in which case deep discharge is not necessary. Current developments in automated deep discharge are also carried out exclusively for EV applications ^[57][80]. LIBs in module, pack or cell format represent a composite material consisting of many different components. To separate them for subsequent material recycling, the various plug-in, welded and bonded joints must be separated from each other. Size reduction has been a proven method in other waste streams, including LIB recycling. Depending on the pre-treatment measures implemented for the LIB, the following forms of mechanical pulping are used:

• Size reduction after thermal deactivation ^[58][81];
• Comminution in an inert gas atmosphere ^[59][[82⁶⁰,83];
• Wet grinding ^[56][60][79^[61],83,84].

Depending on the pre-treatment and mechanical shredding, the (residual) electrolyte components are removed from the shredder output. In the case of thermal deactivation of the cells by means of pyrolysis, the electrolyte components are no longer present in the material stream prior to shredding; thus, electrolyte removal is not necessary in this case. Various processes have been investigated and developed for electrolyte removal after mechanical comminution, such as extraction by thermal drying ^[62][85], solvent extraction ^[63][86] and extraction by supercritical CO₂ ^[64][87]. For further processing of the dry, non-organic material stream, the various materials need to be separated from each other. As in the processing of other waste streams, the aim is to sort the different materials as clean as possible. LIBs consist of many different components, some of which have very fine and complex structures and are therefore not easy to separate. An important sub-goal of material separation is the separation of the active materials for further metallurgical processing. Due to the small particle size of the active materials, screening is used ^[65][88]. This is typically used immediately after drying or (re)crushing. The separation and recovery of graphite from the active material mixture is performed by selective flotation ^[66][89]. This common process has been patented by Retriev Technologies ^[67][90] and the Warner Babcock Institute for Green Chemistry ^[68][91], among others. For the separation of ferromagnetic materials, a magnetic separator is usually used ^[69][92], as in other mechanical processing plants. The further separation of the conductor foils (consisting of aluminum and copper) from the less economically valuable separator foil, made from materials such as polyethylene or polypropylene, can be performed using different technologies. On the one hand, zig zag separators are used to exploit the density differences between the plastic and metal components ^[8][30]. In a two-stage separation process, the heavy metal components, such as housing parts, and then the light metal components, such as aluminum and copper foils, can be separated from the plastics. In wet mechanical processes, float-sink separation is another form of density separation to sort the metal and plastic components ^[56][79]. An alternative is separation with an eddy current separator. Here, the non-ferrous metals are induced by a rotating electromagnet and are thus separated from the non-magnetizable particles, such as the separator films ^[31][51].

5. Metallurgical Recycling

Due to the physical and chemical properties of the active materials, such as the small grain size, mechanical processes are not able to further separate the black mass by material components. Metallurgical processes can be used for these processing steps. Pyrometallurgy includes all processes aimed at recovering or refining metals at elevated temperatures ^[70][93]. In practice, either an electric arc furnace (EAF) or a shaft furnace (SF) are used to recycle EoL-LIB. These processes are part of extractive pyrometallurgy. Different pre-treatment processes are required for the pyrometallurgical processing of LIBs in EAF and SF. While only the black mass can be processed in EAF, whole LIB cells can also be recycled in SF ^[70][93]. Both processes use controlled reduction as the separation process so that the elements nickel, cobalt and copper end up in the metal phase and lithium, manganese, titanium, silicon, aluminum and iron in the slag phase. In this way, recycling efficiencies of around 60 percent can be achieved ^[70][93]. Hydrometallurgical recycling of LIBs primarily refers to the recovery of individual valuable metals from cathode active materials. This usually involves a combination of leaching and subsequent extraction processes. Leaching is a key step in the recovery process. The aim of leaching is to bring the metals of the cathode active materials into solution as ions. These can then be recovered via various extraction processes ^[71][94]. In hydrometallurgical recycling, a distinction is made between two feed streams depending on the pre-treatment. In the case of pyrometallurgical or thermal pre-treatment of the LIB or active mass, the alloy of copper, nickel, cobalt and iron is brought into solution by leaching. If no thermal pre-treatment is used, e.g., in the case of mechanical processing in an inert atmosphere ^[72][95], the metallic components are leached and the insoluble components such as graphite and, if applicable, binder are filtered off ^[73][96]. After leaching, the solution is first cleaned by hydroxide precipitation. In this process, copper and aluminum impurities are precipitated with the addition of, e.g., NaOH ^[74][97]. For the subsequent extraction of precious metals from the solution, either further precipitation steps or solvent extraction are used ^[75][98]. Chemical precipitation of precious metals results in the formation of insoluble compounds through the addition of suitable precipitants ^{[76][77][78][79][80][81][82]}[99,100,101,102,103,104,105]. Solvent extraction is a process in which a two-phase system, usually consisting of an organic and an aqueous phase, is introduced. Here, separation can be achieved by the unequal distribution of the two phases, where solvent extraction agents with high selectivity are used after leaching to separate specific transition metals from the leach solution ^[82][83][84][105,106,107]. The lithium then remains in the solution and can be precipitated, e.g., as lithium carbonate, by adding sodium carbonate ^[74][97]. In addition to pyrometallurgy and hydrometallurgy, the direct recycling of the active material is a third alternative. This process has been developed for the reuse of cathode active material from LIB recycling in the production of new LIBs. The process basically consists of two process steps: the recovery of electrode material from LIBs and the subsequent rejuvenation of the recycled electrode material ^[84][107]. In this context, the process of re-lithiation by the hydrothermal method, the electrochemical method and the direct calcination method has been extensively researched ^[85][86][108,109]. Success on a laboratory scale has already been achieved in various studies, but industrial implementation has not yet taken place.