Highlights

3.1. Mechanical processing

The discharging process is often done by immersion in a concentrated saline solution. This step is required to reduce the risk of combustion, explosion, and toxic gas emissions for high voltage cells.

For EoL battery recycling, wet and dry crushing methods are used: wet crushing involves using a water or salt solution to create a cool, oxygen-free environment, but generates wastewater and may over-crush foils, while dry crushing produces fewer impurities [37], [38]. Sorting, sieving, and magnetic separation are then employed to separate fragments. Pre-treatment methods like ultrasonic [39], microwave [40], solvent [41], [42], or thermal treatment [43], [44], [45] were used before for separating active electrode material from current collector foil. Mechanical processes like milling or sieving may also be necessary to disrupt crystal structures [46], [47], [48].

3.2. Binder removal

Mechanical processes offer a low-cost, simple, and flexible alternative for binder removal and carbon black separation from cathode composites. Common methods include grinding, sieving, magnetic, electrostatic or pneumatic separation, and flotation [51]. Delamination involves crushing material into layers [44] utilized vibrating screens to separate black mass and graphite from Al and Cu foil, while [52] used high-powered ultrasound for rapid delamination. Also ionic liquids (ILs), like [C₄mim][BF₄], dissolve PVDF at lower temperatures with high efficiency [53].

Spouted bed elutriation is another method that implements separation of particles based on their size, shape and density, utilizing a stream of gas or liquid flowing in an opposite direction of sedimentation [54].

4.1. Pyrometallurgy

Pyrometallurgical technology is commonly used in the industrial recycling of LIBs as it employs thermal treatment to reduce metal oxides into an alloy. This process is advantageous due to its high processing capacity [16], simple operation, no need of chemical pretreatment. The primary environmental concerns associated with pyrometallurgy are the high-energy consumption and significant gaseous emissions. Furthermore, the process is unable to recover all battery components, as organic compounds are converted to pure carbon. The alloy produced by this process is mixture of different metals present in the LIBs and requires additional processing, to recover pure metals [72].

4.2. Direct recycling

Cathode materials can undergo regeneration through direct recovery pathways, such as restoring the crystal structure while simultaneously compensating for the loss of Li electroactivity in situ. This approach is motivated by the fact that the decline in battery performance often stems from an insufficient supply of active Li and irreversible structural transitions [73], [74]. In contrast, direct recycling is a low-cost alternative that retains the cathode structure of LIBs instead of dissolving, extracting, and reassembling materials. This method establishes its cost-effectiveness in relation to other methods by minimizing intervention on the active material [75]. It has been argued that full repurposing of battery requires a comparable number of steps as the leaching/re-synthesis process, potentially negating any economic advantage. Wang et al. [76]introduced an innovative ionothermal approach for the regeneration of NMC111 using ILs, specifically [C₂OHmim][Tf₂N]. Unlike conventional solid-state and hydrothermal methods, the ionothermal method offers the advantage of regenerating cathode active materials at relatively low temperatures (150–250 °C) and under ambient pressure. Notably, the combination of [C₂OHmim][Tf₂N] and LiCl at 150 °C yielded the highest discharge capacity retention (approximately 99 %) among the various regeneration techniques employed for NMC111. Shi et al. [73], demonstrated re-lithiation with LiNO₃:LiOH (3:2) deep eutectic solvent (DES) at 300 °C. Li content can be fully restored after 4 h. A short annealing step after re-lithiation is necessary so NMC523 particles can be completely restored to their original pristine composition. Without annealing step, cathode contains many amorphous regions which can decrease battery electrochemical performance.

Establishing and maintaining a direct recycling system can be expensive, especially when specialized equipment and processes are required for each battery type. The costs associated with collection, sorting, and processing may not always be economically viable [70].

4.3. Hydrometallurgy

Hydrometallurgical processes have the advantage of producing exceptionally pure products from low-grade ores and being low energy-intensive [41]. Traditional hydrometallurgical processes employ volatile organic compounds (VOCs) or multiple extracting ligands to extract valuable metals and separate different metals present in the leachates [77]. Although VOCs can be separated from the reaction medium, it can pose serious threats to living organisms. Exposure to VOCs through skin contact, inhalation, or ingestion can cause irritation, nausea, and dizziness, and long-term exposure may lead to liver, kidney, and central nervous system damage [78]. Additionally, VOCs present serious safety issues as they are highly flammable.

The pursuit of sustainable hydrometallurgical processes for LIBs processing using safer solvents has been subject of numerous studies in the last two decades.

Compared to conventional VOCs, ILs and DESs display high thermal and chemical stability, a wide electrochemical window, negligible vapor pressure and lower toxicity [81], [82]. Additionally, DESs offer benefits like easier preparation methods, enhanced purity of synthetic products, reduced costs, less corrosive impact on equipment, relatively straightforward biodegradation, and generally lower toxicity [82].

This review focuses on the recent advancements in the hydrometallurgical recovery of CRMs from LIBs using ILs and DESs, as depicted in Fig. 5. It is organized as follows: i) introduction of ILs and DESs and properties relevant to separation processes; ii) leaching and recovery of the CRMs by liquid-liquid extractions employing ILs and DES as receiving phases; iii) application of ILs/DES in supported liquid membranes; iv) status of industrial applications of separations employing ILs/DES; v) conclusions and outlook.

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Fig. 5

5.1. Ionic Liquids

ILs are salts formed by an organic cation and an organic or inorganic anion which are in the liquid state at temperatures below 100 °C [83]. The structure of some common anions and cations, for LIBs recycling, are shown in Fig. 6. When in the liquid state at room temperature, they are also called Room Temperature ILs (RTILs) [84].

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Fig. 6

Understanding the structure and thermodynamic characteristics of metal ion solvation within specific solvents [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95] and ILs [96], [97], [98], [99], [100], [101], [102] is crucial for a precise understanding of chemical phenomena within the liquid. This understanding is not only fundamental but also pivotal for practical applications [102], [103], [104], [105], [106]. The evolution in IL composition over the years demonstrates a growing sophistication in tailoring their properties to meet specific application requirements [107], [108], [109].

The physical, chemical, and thermal properties of ILs are significantly influenced by the selection of cations, including their nature, length, and symmetry, as well as the choice of anions, considering their structure and charge distribution [110], [111], [112]. For instance, variations in the anion composition within the IL structure can strongly impact its melting point, viscosity, thermal properties, and hydrophobicity [113], [114].

In hydrometallurgical solvent extraction processes, modifications in the composition of ILs can result in a variety of behaviors in metal interactions, thereby influencing the efficiency and mechanism of extraction [115], [116]. Depending on the extraction conditions, such as the composition of the aqueous feed solutions, and the presence of auxiliary extracting molecules in the IL phase, different mechanisms can be identified [117], [118]:

- Neutral extraction: This mechanism involves the formation and extraction of a neutral complex in the IL phase. While it shares similarities with conventional solvents, the detailed mechanisms can vary. In this mode, the IL functions as a polar non-aqueous solvent [119].

- Cation exchange: Unique to IL systems, this mode entails the IL acting as a liquid ion exchanger, transferring cationic complexes from the aqueous phase into the IL phase. Consequently, an IL cation shifts to the aqueous phase while a metal cation moves to the IL phase to maintain electro-neutrality [120], [121].

- Anion exchange: Also specific to IL systems, this mode involves the formation of over-neutralized anionic complexes that can be transferred to the IL phase. In exchange, an IL anion moves to the aqueous phase. It's worth noting that anion exchange is less common compared to cation exchange [122], [123].

The ion-exchange mechanisms pose a significant drawback in IL-based extraction systems, as a portion of the IL is lost into the aqueous phase, essentially acting as a sacrificial component. The primary factor influencing this exchange mechanism is the solubility of the IL component in the aqueous phase [124]. Furthermore, increasing the hydrophobicity, such as by extending the alkyl chain of the IL cation, can shift the mechanism from ion exchange to neutral complex extraction [125], [126].

5.2. Deep Eutectic Solvents

DESs were introduced as a family of solvents derived from ILs in the early 2000s by Abbott et al. [127], who described eutectic mixtures whose melting point is much lower than those of the pure components (Fig. 7). They are formed by a compound that acts as hydrogen bond acceptor (HBA), typically a quaternary ammonium salt, and either a metal salt or an organic compound that acts as hydrogen bond donor (HBD) [128].

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Fig. 7

Common HBAs and HBDs employed in the preparation of DESs are inexpensive starting materials, some of which are shown in Fig. 8 [129]. The latter features make DESs interesting in terms of economic sustainability and scaling of the process [130].

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Fig. 8

The properties of DESs, such as conductivity, density, melting point, polarity, and viscosity, are influenced by their known composition. By adjusting the HBA:HBD ratio, one can tailor these properties to achieve different properties and characteristics [131], [132]. DESs offer advantages similar to ILs, but they tend to exhibit higher viscosities, posing a limitation in separative applications [133].

6.1. Leaching

Microorganisms, such as bacteria and fungi, have been considered in recent years for their ability to extract metals from ores through a process known as bioleaching [194], [195]. This environmentally friendly technique utilizes the metabolic activities of microorganisms to dissolve metals from mineral ores, making them more accessible for extraction. Bioleaching offers several advantages over traditional chemical leaching methods, including lower energy consumption, reduced environmental impact, and the ability to process low-grade ores economically [196], [197].

6.2. Metal extraction processes with ILs

Several families of hydrophobic ILs have been tested as receiving phases in liquid-liquid metal extractions (see 5.1). The [P₆₆₆₁₄][TMPP] IL was applied in the recycling process of the NMC cathode powder [201], for the selective extraction of Co over Ni. In order to enhance the separation, a chelating extractant such as LIX 84-IC was employed [202]. Notably, [P₆₆₆₁₄][TMPP] IL demonstrates the ability to extract the metals through adduct formation with the phosphinate compound, especially when the acid concentration is relatively low [201]. This differs from another member of the phosphonium-based IL family, [P₆₆₆₁₄][Cl], where the exchange occurs between the chloride ions of the IL molecules and the anionic chloridometalates (MCl_xⁿ⁻) [203], [204], observed at high HCl aqueous concentrations.

Conversely, [P₆₆₆₁₄][SCN] IL was employed to separate Co from Li from concentrated H₂SO₄ solutions [208]. Co exhibited a remarkable capacity to preferentially form the tetra thiocyanate complex [Co(SCN)₄]²⁻ by interacting with the IL's anion. This interaction occurred through a split-anion extraction mechanism.

Nevertheless, the widespread use of ILs is hindered by excessive costs, environmental issues, and recycling problems. Bio-based ILs offer a greener alternative, as they are obtained from renewable sources like amino acids, sugars, or fatty acids. Anions derived from fatty acids, such as oleate and linoleate, remain immiscible with water, and tertiary amines and quaternary ammonium salts demonstrate effective anion exchange. Moreover, the use of bulky, long-chained tetraalkylphosphonium cations and hydrophobic oleate anions helps to prevent the loss of the IL to the aqueous phase [212]. Two hydrophobic fatty acid-based ILs were synthesized, [N₁₈₈₈][Ol], and [N₁₈₈₈][Linol]. These ILs were used in a single-stage extraction process to extract Co and Ni from an aqueous solution, demonstrating high efficiency even at low concentrations [213].

Nevertheless, amino acids have good prospects in industrial applications because of their high biocompatibility, low price, and excellent availability. Five amino acids (glycine, phenylalanine, serine, glutamine, and glutamic acid) were investigated in [N₄₄₄₄][HSO₄] IL on separation of Co and Ni from Mn [214].

Also ILs containing the [TTA]⁻ anion of ILs, have also been studied in the field of metal extractions [215], [216]. The [Omim][TTA] IL used for the selective recovery of Co from a LCO cathode leached using TAC [217] (Table 10) gave a high extraction rate for Co 88 % with all Li retained in the aqueous phase.

In a recent development, researchers successfully utilized a [TBP]-based IL, denoted as [TBP][DTPA], in aqueous sulfate solutions for extracting Li and Co from a spent LCO battery [220].

6.3. Metal extraction processes with DESs

PTSA is an organic acid, and when used as a HBA in DES, it imparts acidity of the solvent mixture. While, ChCl is a quaternary ammonium salt and contains a positively charged nitrogen atom, making it a potential proton donor that also impacts acidity of DES. Hydrated PTSA was employed by Roldán-Ruiz et al.[228] and Chen et al. [229]. By utilizing DES with hydrated PTSA, leaching temperature is almost two times lower, while leaching time decrease 280 times compared to dry PTSA [230]. Moreover, efficiency increase from 75 % to 94 %. Compared to organic acids, PTSA:ChCl-based DESs offered a significant reduction of the solute-to-solvent ratio needed for full Co dissolution. This brings economic and sustainability benefits and promising scalability due to low solvent volumes necessary. Using PTSA:ChCl-based DESs for leaching, Co recovery from spent LIBs reached 94.1 %. The process was finalized by the precipitation with either Na₂CO₃ or (NH₄)₂CO₃, and the final calcination to obtain Co₃O₄ [229].

PolyEG is a non-ionic polymer made up of repeating EG units that acts as HBD through its oxygen atoms, which can engage in hydrogen bonding in solution and does not impact DES acidity. In PTSA:ChCl DES, both compounds are acidic, which significantly helps in the leaching of LCO. This DES can leach LCO cathode in 15 min, while PTSA:EG DES needs 24 h to leach the cathode on similar temperatures. Suriyanarayanan et al. [231] have demonstrated a novel and efficient method for Co extraction from spent LCO-based LIBs using a DES made of N-methyl urea and acetamide. Previously, cathode was treated with N-Methyl-2-Pyrrolidone to separate aluminum foil from active cathode material. The product, after leaching in DES, was dissolved in 1 % CH₃COOH and 0.1 M KNO₃. Co was electro-deposited, dried, and calcinated before adding LiOH for fabrication of new LIBs. Other DES combinations, parameters, and leaching efficiencies are shown in Table 12 below.

Xu et al. [246] used microwave assisted ChCl:OXA DES for LMO cathode leaching. Best conditions were 15 min leaching at 75 °C at solid/liquid ratio 1:60. Under these conditions, 96 % of Li and Mn is leached.

Wang et al. [252] after successful leaching performed coextraction of Ni, Co, Mn and Li using 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507) in sulfonated kerosene as the diluent. H₂SO₄ was added as stripping agent to recover Ni, Co and Mn from P507 solution. The post-process economic analysis revealed significant benefits of NMC regeneration through recycled materials, notably in cost reduction. Synthesizing NMC cathodes from raw materials incurred a cost of $189.49/kg, whereas using recycled materials brought it down to $144.77/kg, saving $44.72/kg in cathode production costs.

Metals can undergo further separation through various techniques, enhancing the purity of the resulting materials. Two prominent methods for achieving this separation are precipitation using OXA [221], [227], [247], [257], [267], and high-temperature calcination to produce metal oxides [227], [242], [268].

6.4. Supported Liquid Membranes

In liquid-liquid separations, the hydrophobic phase (IL or DES) can be immobilized onto a supporting membrane, which separates a feed aqueous phase containing the metals to be separated into the IL/DES and a receiving phase where the separated metals are collected [269], [270], [271], [272]. One notable advantage of supported liquid membranes (SLMs) is the reduced amount of solvent employed with respect to liquid-liquid biphasic systems, which could be useful for the industrialization of separations involving ILs, given their relatively high cost. Moreover, extraction and stripping occur in a single step (Fig. 9). On the other hand, a drawback is the limited life due to the leakage of IL. To extend the lifetime of the membrane, polymer inclusion membranes (PIMs) have been proposed as an alternative [273], [274], [275], [276].

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Fig. 9

A recent work [276] studied the application of PIMs ([A336][Cl] and [P₆₆₆₁₄][Cl]) in the separation and recovery of Co and Li from LIBs, achieving good recovery performances with a long-term stability.

Co extraction using [P₆₆₆₁₄][Cl] in a SLM demonstrated up to 86 % extraction with high selectivity over Ni from a synthetic solution [277]. While stripping with deionized water is possible, reusability is hindered by IL leaching. While Zante et al. [278] assessed the potential for selectively recovering Li from aqueous solutions containing Na, Co, and Ni ions using a SLM created by a PVDF membrane support with a blend of [C₄mim][Tf₂N] and TBP as the carrier. Notably, their stability experiments revealed that the leakage of IL into the aqueous phase could be minimized by introducing salt into either the feed or the stripping phase.

DES-based SLMs exhibit distinct advantages, such as lower solvent loss and improved efficiency in liquid separation without loading limitations [279]. The chemical potential gradient in DES-based SLMs facilitates mass transfer on both sides, ensuring durability as the feed solute does not chemically react with the DES carrier during passive diffusion.

However, the fabrication process depends on parameters like DES type, composition, membrane support thickness, wettability, and additives [280], [281], [282]. Achieving long-term durability necessitates a reliable match between DES and its membrane support, an aspect requiring considerable investigation effort.

However, several challenges hinder their large-scale application of SILMs [272]. Current preparation methods are intrinsically inefficient as they basically rely on a trial-and-error approach. More systematic screenings involving molecular simulations [283] and machine learning techniques [284] could help to improve this aspect. Balancing permeability, selectivity and useful membrane lifetime remains a significant challenge. Innovations in interface regulation and mass transfer optimization through a more rational preparation would help in this direction.

6.5. Reusability of ILs and DES

In the quest for the reuse of metal-free ILs and DESs, specific procedures are indispensable, frequently involving multiple washing steps with strip solutions to eliminate any residual metals and solution [204], [285], [286], [287]. The recyclability of ILs and DESs hinges largely on their physico-chemical properties. Hydrophilic ILs, which encompass cations like ammonium and phosphonium along with anions such as [Cl]^–, tend to exhibit reduced recyclability due to the loss of anion during water washing. For instance, [A336][Cl] and [P₆₆₆₁₄][Cl] [288] displayed an 8 % decrease in extraction efficiency ( i.e., the ratio of extracted metal to the total amount in the feed) even after a single washing cycle.

On the contrary, hydrophobic ILs like [P₆₆₆₁₄][TMPP] and [P₆₆₆₁₄][Tf₂N], characterized by hydrophobic anions [289], demonstrate superior recyclability, retaining their extractability across multiple extraction-stripping cycles. This disparity in recyclability has posed significant challenges when scaling up IL-assisted solvent extraction processes, especially for chloro-complexes that involve ion-exchange reactions. The loss of IL molecules diminishes the availability of extractants, necessitating additional treatments to recover the dissolved IL molecules in aqueous solutions. Recognizing the importance of the recovery and reuse of ILs and DESs is fundamental for the successful commercialization of applicable technologies, as it plays a pivotal role in efficient resource utilization and environmental preservation [290], [291], [292]. The approach to recycling these compounds typically depends on their hydrophobic or hydrophilic nature. Consequently, researchers have been actively exploring the recycling of spent LIBs using ILs and DESs.

6.6. Transition to industrial applications

Despite the academic research on the application of ILs/DESs in the recovery of metals from LIBs for more than a decade (Fig. 2), and the promising performances achieved in selective extraction processes on the lab scale, currently, there are no commercial processes for metal recovery from LIBs employing ILs or DESs. More generally, it should be noted that also the full industrialization based on IL/DES-based processes for other metal extractions has not been reached yet. The only IL-based separation process known is related to rare earth recycling from permanent magnets [293], which is currently in an advanced demonstration stage (Ionic Technologies, former Seren Technologies [294]).

As for pilot scale plants, while some examples on LIBs recycling based on other technologies, such as multi-step solvent separation [295], smelting [296], or selective precipitation [171], are available, similar examples based on ILs/DESs are not reported to date. On the other hand several lab-scale plants relevant to LIBs recycling employing continuous bench-scale processes have been reported for Co/Ni separation [297], [298], also employing microfluidic reactors [299].

However, the interest in moving towards industrial applications exists, as the number of patents related to metal recycling from LIBs has constantly increasing in the last decade (a search on Google Patents reports 89 results from 2010¹).

One of the main barriers for the industrialization of processes employing ILs and DESs for separations is their cost, which is much higher than VOCs (Table 7) [301]. The prices of ILs and DES are strongly dependent on the specific solvent, and estimations based on chemicals catalogues may not be reliable as they are often referred to small quantities of research-grade products. Specific studies on economic feasibility of metals recycling from LIBs have not been published. However, some information can be obtained from other works on applications of ILs/DESs, where the cost of these materials has been estimated on an industrial scale.

A first study on techno-economic aspects of an industrial gas purification process involving ILs estimated a cost of IL ranging from 2.5 to 50 USD kg^–1, around 2–50 times higher than that of common VOCs employed in industry [302], [303]. Other analyses of applications of ILs and DESs for gas treatment estimated an average price of 6000 USD/t for some imidazolium-based ILs [304].

The first route to reduce the production cost is to optimize the synthetic procedures to prepare ILs. In this respect, several examples of studies where optimized syntheses are available [305], [306], [307]. Also, replacing more expensive ILs with cheaper ones can also contribute to reducing prices [308]. Besides improvements in chemical and process aspects, it should be taken into account that the ILs market is expected to grow, and the prices should decrease due to economies of scale as larger volumes of ILs are produced [309]. It has been reported that an increase of production from 1 kg to 5 tons for two imidazolium ILs can generate a decrease of the cost per kg of more than a factor of 3 [301].

DES are generally much less expensive than ILs as they are produced by simply mixing separate chemicals, which in some cases have an individual low cost (e.g., urea).

In a simulated process [310], the ChCl:urea (1:2) DES was considered with an estimated price of around 600 USD/t, 10-fold less than the IL employed in the same process [304]. A study [311] on the use of ChCl:urea (1:2) in a bio-refinery estimated 200 and 90 USD/t for the two components of the DES, respectively. A techno-economic analysis on biogas upgrading showed that DES-based process could be competitive in cost with current technology [312]. Also, natural DESs (NADEs), which have been proposed as solvents for bio-refineries, were considered suitable for an industrial process [313]. Therefore, the results of these studies demonstrate the economic feasibility and competitiveness of industrial processes based on DESs.

Additional factors which can help to mitigate the price issue are the use of low volumes of ILs and DESs (for example by coupling them with membrane supports), extending the operating lifetime and their recycling.