Johnson Matthey Technol. Rev., 2021, 65, (3), 431
Recycling and Direct-Regeneration of Cathode Materials from Spent Ternary Lithium-Ion Batteries by Hydrometallurgy: Status Quo and Recent Developments
Economic recovery methods for lithium nickel cobalt manganese oxide cathode materials
- Lizhen Duan, Yaru Cui *, Qian Li
School of Metallurgical Engineering, Xi’an University of Architecture and Technology,
No. 13 Yanta Road, Xi’an, Shaanxi, 710055, China
- Juan Wang§
Xi’an Key Laboratory of Clean Energy, Xi’an University of Architecture and Technology,
No. 13 Yanta Road, Xi’an, Shaanxi, 710055, China
- Chonghao Man
Faculty of Engineering, University of New South Wales, Sydney, New South Wales, 2052, Australia
- Xinyao Wang
School of Metallurgical Engineering, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an, Shaanxi, 710055, China
The cathodes of spent ternary lithium-ion batteries (LIBs) are rich in nonferrous metals, such as lithium, nickel, cobalt and manganese, which are important strategic raw materials and also potential sources of environmental pollution. Finding ways to extract these valuable metals cleanly and efficiently from spent cathodes is of great significance for sustainable development of the LIBs industry. In the light of low energy consumption, ‘green’ processing and high recovery efficiency, this paper provides an overview of different recovery technologies to recycle valuable metals from cathode materials of spent ternary LIBs. Development trends and application prospects for different recovery strategies for cathode materials from spent ternary LIBs are also predicted. We conclude that a highly economic recovery system: alkaline solution dissolution/calcination pretreatment → H2SO4 leaching → H2O2 reduction → coprecipitation regeneration of nickel cobalt manganese (NCM) will become the dominant stream for recycling retired NCM batteries. Furthermore, emerging advanced technologies, such as deep eutectic solvents (DESs) extraction and one–step direct regeneration/recovery of NCM cathode materials are preferred methods to substitute conventional regeneration systems in the future.
In the 21st century, there is a need to deal with threats such as energy scarcity and environmental deterioration. The worldwide usage of fossil fuels accounted for 84.7% of global energy consumption in 2018, which is equivalent to 11.7436 billion tonnes of oil (1–2). Global CO2 emissions, especially from fossil fuels, will continue to grow rapidly. It is crucial to explore green and renewable energy systems, such as wind, tidal and solar energy, and energy storage such as batteries, to replace fossil fuels. LIBs, with excellent energy storage properties, safety and stability, are among the most promising clean and sustainable energy storage equipment. LIBs are widely used in zero-emission vehicles (mainly electric vehicles (EVs), and plug‐in hybrid electric vehicles (PHEVs)), computers and electronic communication devices (3–6). The newly emerging model for super-performance LIBs, assembled with specific complex three-dimensional (3D), porous and polyhedral geometric structures in cathode and anode materials, is perfectly suited to the scale-up requirements for zero-emission vehicles (3–5). Increasing demand for new energy vehicles contributes to the expansion of the LIBs market. It has been estimated that the world’s production of LIBs would increase by 520% from 2016 to 2020 (7), and about 50 million electric buses will run on the road by the end of 2027 (8). Therefore the number of expired LIBs, as major electronic wastes, will inevitably increase. In China, the weight of retired LIBs was predicted to reach 500,000 tonnes by the end of 2020 (9), and that of the European Union to reach 13,828 tonnes in 2020 (10). Since harmful substances may damage the environment and the metals contained in spent LIBs are precious resources, the recovery of retired LIBs is bound to gain considerable social, economic and environmental benefit.
The cathodes of retired LIBs are rich in valuable nonferrous metals such as lithium, nickel, cobalt and manganese, which are secondary resources worth recycling. Considering potential immense profit, researchers have been working hard to develop various technologies to recycle the metals in spent LIBs. Current recycling technologies mainly include pyrometallurgy and hydrometallurgy. Although pyrometallurgical recovery processes have the advantages of a short process, high efficiency and easy industrial application, high energy consumption and generation of toxic gases still limit their development (11–14). In contrast, hydrometallurgical recovery processes have attracted extensive attention due to low energy consumption, environmental friendliness and high recovery efficiency (15–19).
Leaching valuable elements by chemical reagents is the core of the hydrometallurgical recovery strategy. After leaching, valuable metals in the leachate are extracted by chemical precipitation, solvent extraction and ion-exchange (20–25). The conventional hydrometallurgical process for waste electrodes recovery can be illustrated as follows: (a) acid-reductant leaching and selective chemical precipitation; (b) sulfuric roasting, acid leaching and selective chemical precipitation; (c) mechanochemical activation leaching and selective chemical precipitation, as shown in Table I (26–34). However, sulfuric roasting or mechanochemical activation before leaching may complicate the recovery process and decrease overall leaching rate. Based on the principle: waste substance + waste substance → rebirth resources, Li et al. (35) developed an in situ recovery model for graphite, Li2CO3 and cobalt by oxygen-free roasting with an anode graphite + wet magnetic separation process, without pretreating or adding any other chemical reagents. However, this technique is not suitable for recovering complex electrode materials. Prabaharan et al. (36) investigated an electrochemical leaching system: lead as anode + electrode scraps as cathode + H2SO4 as leachate, in which the leaching rate of manganese, copper and cobalt exceeded 96% through adjusting pH value. Although it can perfectly achieve integrated recovery of valuable metals, the high electricity consumption and recovery cost limit the use of this method. Surprisingly, Gomaa et al. (37, 38) explored a new multifunctional recovery method to recycle ultratrace Co2+ (~3.05 × 10–8 M and 4.7 × 10–8 M respectively) with visible selective ion extraction-separation-detection. It can extract almost 100% Co2+ from leachate in only 10–15 min and the used ion-extractors after activation can be regenerated and reused in repeated adsorption-desorption processes. Following desorption by HCl eluting reagent, the overall recovery rates of cobalt from waste LIBs or printed circuit boards (PCBs) are 98% and 95.7% respectively.
|Type of LIBs||Leaching system (reagents + solid/liquid, g l−1 + temperature, K + time, min)a||Leaching rate, %||Extraction method (reagents) + recovered products (recovery rate/purity, %)||Reference|
|LiCoO2||Acid + reductant leaching + selective chemical precipitation||Co:96/Li:98||Selective precipitation (H2C2O4/H3PO4) + CoC2O4·2H2O (99/–) + Li3PO4 (93/–)||(26)|
|1.5 M H3 citric acid + 0.4 g g–1 tea waste + 30 + 363 + 120|
|2 M H3 citric acid + 0.6 g g–1H2O2 + 50 + 343 + 80||Co:98/Li:99|
|1.5 M H3 citric acid + 0.4 g g–1 phytolacca americana + 40 + 353 + 120||Co:83/Li:96|
|NCM523||Mechanochemical leaching + selective precipitation||Li: 95.10||Selective precipitation (Na2CO3) + Li2CO3 (–/99.96) + Ni0.5Mn0.3Co0.2(OH)2||(27)|
|Na2S·9H2O + 15 min + 600 rpm – water leaching + / + 298 + 30|
|LiCoO2||Mechanochemical activation leaching + selective precipitation||Co:98/Li:99||Selective precipitation (NaOH/Na2CO3) + Co3O4 (~94/–) + Li2CO3||(28)|
|mechanochemical activation (EDTA/600 rpm/240 min) + water leaching + / + 298 + /|
|LiCoO2||Acid + reductant leaching + selective chemical precipitation||Co/Li: ~99||Selective precipitation (H2C2O4/NaOH) + CoC2O4·2H2O (99/97.8) + Li3PO4 (88/98.3)||(29)|
|2% v/v H3PO4 and 2% v/v H2O2 + 8 + 363 + 60|
|Mixed-type||Sulfuric roasting + acid leaching||Overall recovery efficiency: ~Co:90.5/Li:93.2||–||(30)|
|before leaching: sulfuric acid, baking: 2M H2SO4 + 573 k + 30 min, leaching step one: H2O + / + 348 + 60, leaching step one: 1 M H2SO4 + 0.5 M HNO3 + plus glucose + / + 323 + 45||Ni:82.8/Mn:77.7|
|LiCoO2||Acid + reductant leaching||Co/Li: ~99||–||(31)|
|1.0 M citric acid + 8% v/v H2O2 + 40 + 343 + 70|
|LiCoO2||Acid + reductant leaching||Co:98/Li:96||–||(32)|
|3 M H2SO4 + 0.4 g g–1 plus glucose + 25 + 368 + 120||Co:54/Li:100|
|3 M H2SO4 + 0.4 g g–1 cellulose + 25 + 368 + 120||Co:96/Li:100|
|3 M H2SO4 + 0.4 g g–1 sucrose + 25 + 368 + 120|
|Spent LIBs||Acid + reductant leaching + selective chemical precipitation||Co:92/Li:99||Precipitation method (C4H8N2O2/H2C2O4/H3PO4) + Ni(C4H6N2O2)2 + CoC2O4 + Li3PO4||(33)|
|1.5 M citric acid + 0.5 g g–1 D-glucose+ 20 + 353 + 120||Ni:91/Mn:94|
|LiFePO4||Acid + reductant leaching + selective chemical precipitation||Co/Li: >95||Evaporation and precipitation (ethanol) + FePO4·2H2O + LiH2PO4||(34)|
|0.5 H3PO4 + / + 273 + 60|
NCM batteries consist of more valuable metals that are worth recycling compared with traditional LiCoO2 and LiFePO4 batteries. However, few reports have been made to systematically clarify recovery techniques for waste NCM materials. In order to avoid loss of valuable resources and risk of secondary pollution, it is urgent to construct a sustainable recycling model for valuable metals in cathodes of spent LIBs. This review aims to describe progress in hydrometallurgical recycling of cathode materials from spent NCM batteries. The hydrometallurgical recovery strategy of waste LIBs can be classified into three steps: (a) pretreatment or separation of active substances; (b) leaching or extracting the valuable metals from the active substances with appropriate solvents; (c) separation of valuable metals by selective extraction from leachate by different methods to obtain the metals or metallic compounds. The conventional process flow for recycling NCM materials from waste LIBs by hydrometallurgy is shown in Figure 1. The advantages, disadvantages, existing problems and current status of each treatment method are analysed. Furthermore, advanced recovery technologies, DESs extraction and crystal repair direct-regeneration/recovery technology are illustrated. The challenges and prospects for metals recovery from ternary cathode materials of used LIBs by hydrometallurgy are described. By comparing the advantages and disadvantages of different methods, it is expected that this information will contribute to exploring economic, green, sustainable, high-efficiency leaching, separation and regeneration recovery systems for closed-circuit recycling of LIBs.
2. Nickel Cobalt Manganese Cathode Materials for Lithium-Ion Batteries
The LIBs are mainly divided into five types depending on the composition of cathode materials: lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2) and lithium NCM oxide (LiNix Coy Mn1–x –y O2,0<x +y <1) (39–43). Of these, layered LiNix Coy Mn1–x –y O2(NCM) materials are preferred because of their excellent cycle and rate performance. Currently, there are five types of NCM cathode materials that have been commercialised: NCM333 (LiNi1/3Co1/3Mn1/3O2), NCM424 (LiNi0.4Co0.2Mn0.4O2), NCM523 (LiNi0.5Co0.2Mn0.3O2), NCM622 (LiNi0.6Co0.2Mn0.2O2) and NCM811 (LiNi0.8Co0.1Mn0.1O2) (44). It is predicted that NCM batteries will account for nearly 41% of the world’s LIBs market in 2025 (42). The chemical characteristics and potential hazards of each component in ternary LIBs are summarised in Table II (45–48). NCM cathode materials are rich in lithium, nickel, cobalt, manganese and other strategic essential metals. The heavy metals in waste LIBs will pose a huge threat to human health and the environment (49). Furthermore, the scarcity and high cost of nonferrous metals such as cobalt make it imperative to recycle ternary cathode materials from retired LIBs. It is also reported that the global lithium resource reserves in 2018 are over 62 million tonnes, but China only accounts for 7%, about 4.5 million tonnes (50). As a core raw material for LIBs, the value of cobalt is as high as AUD$115,000 per tonne (51, 52). Efficient extraction of these valuable nonferrous metals from retired LIBs is of great significance for green and sustainable development of the batteries industry.
3. Metallurgical Recycling Strategies for Nickel Cobalt Manganese
3.1 Recovery Process
The advantages and disadvantages of different technologies for recycling spent ternary LIBs are shown in Table III (53–57). Considering low energy consumption and high recovery rate, the hydrometallurgical recycling process is currently considered a preferred strategy to recover valuable metals from used cathode materials, while pyrometallurgical processes are usually used as a pretreatment for leaching in hydrometallurgical recycling.
3.2 Recovery Process of Hydrometallurgical Method
To prevent short circuit, spontaneous combustion and explosion during dismantling waste LIBs, predischarge work must be carried out before further processing (58–60). The cathodes consist of active material, current collector (aluminium foil) and binder (poly(vinyldiene fluoride) (PVDF)). Separating high-purity active material is the key pretreatment process. Table IV shows pretreatment methods and corresponding recycling effects of retired NCM batteries (61–66). Considering separation efficiency and purity of recovered products, ultrasonic treatment and solvent dissolution (SD) methods have distinct advantages compared with other methods. However, the ultrasonic method has high recovery cost due to the need for special equipment; and most organic solvents used in the SD methods are toxic and expensive. By contrast, the mechanical separation method has a high degree of automatic operation and is easy to implement. However, it is difficult to avoid the components being mixed together and unable to be separated fully in the next pulverisation process, which may lower the purity of recovered products. To improve extraction rate, reduction and roasting of cathode materials are usually applied to convert high valence cobalt and manganese into lower valences that can be easily leached by chemical reagents. Considering recovery cost and practical application, alkali dissolution and heat treatment are preferential pretreatment methods. The binders and current collectors of spent LIBs can be separated simultaneously through heat calcination, and the obtained active materials have high purity and excellent crystal morphology and electrochemical performance.
|Type of LIBs||Pretreatment methods||Reagents||Temperature, K||Recovery rate of scrap materials, %||Separated substance||Reference|
|LiNi1/3Co1/3Mn1/3O2||Solvent dissolution||Trifluoroacetic acid||–||–||Aluminium foil||(61)|
|LiNi1/3Co1/3Mn1/3O2||Heat treatment scraping and water leaching||–||393||–||PVDF aluminium foil||(62)|
|LiNi1/3Co1/3Mn1/3O2||Solvent dissolution||Dimethyl carbonate||–||99||Aluminium foil, PVDF||(63)|
|LiNi1/3Co1/3Mn1/3O2||Ultrasonic cleaning||N-methyl-2-pyrrolidone||343||99||PVDF, aluminium foil||(64)|
|LiNix Coy Mnz O2||Basic solution dissolution||1.5 M NaOH||–||–||Aluminium foil||(65)|
|Laptop LIBs||Heat treatment||–||523–573||–||Aluminium foil||(66)|
3.2.2 Leaching of Valuable Metals
Efficient leaching of valuable metals from active substances is the ultimate goal for hydrometallurgical recovery. Generally, acid leaching and bioleaching are applied to extract valuable metals (67), as shown in Figure 2.
The optimised process parameters and related leaching rates for extracting metals from waste ternary LIBs are listed in Table V (68–75). It can be seen that the ability of HCl to leach valuable metals is higher than that of other inorganic acids such as HNO3 and H2SO4 (68). Injecting a suitable reducing agent into the leaching system can significantly strengthen the leaching efficiency. Organic acids can effectively increase the leaching rate of different metals while decreasing the leaching time. Because it is environmentally friendly and does not introduce impurity ions, H2O2 is considered a popular candidate for reductants. However, H2O2 is easily decomposed under high temperature, which weakens its reduction effect (69). With better thermal stability, NaHSO3 may be a favourable substitute.
|Type of LIBs||Leaching reagents||Reductants||Solid/liquid, g l−1||Temperature, K||Time, min||Leaching rate, %||Reference|
|Lithium nickel cobalt aluminium oxide||4 M HCl||–||50||363||1080||Li, Ni, Co, Al: ~100||(68)|
|Mixed-type batteries||1 M H2SO4||–||50||368||240||Li:93.40/Ni:96.30Co:66.20/Mn:50.2||(69)|
|5 vol% H2O2||50||368||240||Li:93.40/Ni:96.30Co:79.20/Mn:84.60|
|0.075 M NaHSO3||20||368||240||Li:96.70/Ni:96.40Co:91.60/Mn:87.90|
|NCM311||1 M H2SO4||1 vol% H2O2||40||313||60||Li, Ni, Co, Mn: ~99.70||(70)|
|NCM311||2 M L-Tartaric acid||4 vol% H2O2||17||343||30||Li: 99.70/Ni :99.31Co:98.64/Mn:99.31||(71)|
|NCM311||2 M Formic acid||6 vol% H2O2||50||333||10||Li:98.22/Ni:99.96Co:99.96/Mn:99.95||(72)|
|NCM311||3 M Tricarboxylic acid||4 vol% H2O2||50||333||30||Li: 99.70/Ni: 93.00Co:91.80/Mn:89.80||(73)|
|LiNi1/3Co1/3Mn1/3O2||1.2 M DL-malic acid||1.5 vol% H2O2||40||353||30||Li:98.90/Ni:95.10Co:94.30/Mn:96.40||(74)|
|NCM||3.5 M Acetic acid||4 vol% H2O2||40||333||60||Li: 99.97/Ni: 92.67Co:93.62/Mn:96.32||(75)|
The leaching efficiency of valuable metals commonly depends on various factors, including leaching methods and agents, pH value, composition of leaching system and reductants. Great efforts have been made to optimise the leaching process and the subsequent separation and extraction processes. Liu et al. (76) developed a new method of pre-reduction roasting combined with two-step leaching to extract metals from waste NCM cathodes. Transition metal ions Mn4+ and Co3+ were converted into low-valence ions Mn2+ and Co2+ through pre-reduction roasting, with 93.68% leaching rate of lithium in H2O leaching step and almost 99.6% leaching rate of manganese, nickel and cobalt in H2SO4 leaching step. Zhuang et al. (77) studied the leaching property of mixed acid (phosphoric acid and citric acid) on valuable metals from used NCM materials. Without adding any reductants, the leaching cost of raw materials was 37.81% lower than that of single acid leaching studied by Sun et al. (74, 77). Under optimised conditions, the leaching efficiencies of lithium, nickel, cobalt and manganese were up to 100%, 93.38%, 91.63% and 92%, respectively.
Besides conventional acid leaching, much work has been done on biological leaching (78–80). Reported leaching rates of waste NCM cathodes by bacteria are summarised in Table VI (81–86). Bioleaching technology mainly relies on the organic or inorganic acids produced by bacterial decomposition to leach valuable metals. Bacterial leaching is characterised by extremely high selectivity for lithium, with leaching rate approaching 100%. Xin et al. (86) explored bioleaching on different metals in three leaching systems as shown in Table VI. It was found that the leaching rates of lithium and manganese in a mixed energy source-mixed culture (MS-MC) system were more than 95%, while that of cobalt and nickel were only 40%. The bioleaching mechanism revealed that lithium ions were released from NCM material through biogenic acid dissolution, while transition metal ions Mn4+, Co3+ and Ni3+ must be reduced to Mn2+, Co2+ and Ni2+ before acid leaching. It was also observed that Fe2+ in the MS-MC system contributed to reduce insoluble transition metal ions into soluble low-valence ions. That means the high-valence transition metal ions can be effectively dissolved through interaction of reduction and acid leaching by bacteria. Lowering the pH value of the leaching system is an effective means to accelerate cell growth during the bioleaching stage, which can ensure sufficient sulfuric acid and normal circulation of Fe2+/Fe3+ (86). Through controlling the system’s pH, the leaching efficiency of the above metals were all over 95%.
|Type of LIBs||Leaching reagents||Decomposing acids||Leaching rate, %||References|
|NCM batteries||Aspergillus niger (PTCC 5210)||Gluconic, citric, malic and oxalic acid||Li:100/Ni:54/Co:64/Mn:77||(81)|
|Spent coin cells||Acidithiobacillus thiooxidans (PTCC 1717)||H2SO4||Li: 99/Co: 60/Mn: 20||(82)|
|Laptop batteries||Acidithiobacillus thiooxidans, Acidithiobacillus ferrooxidans||H2SO4||Li:99.20/Ni:89.4/Co:50.4||(83)|
|NCM batteries||Aspergillus niger||gluconic, citric, malic and oxalic acid||Li:100/Ni:45/Co:38/Mn:72||(84)|
|NCM batteries||Aspergillus niger||gluconic, citric, malic and oxalic acid||Li: 95/Ni:38/Co:45/Mn:70||(85)|
|NCM batteries||Sulfur-oxidising bacteria (SOB)||Sulfur-A.t system, Pyrite-L.f system||NCM: >95||(86)|
|LiMn2O4||Iron-oxidising bacteria (IOB)||MS-MC system||Li: 95/Mn:96||–|
3.2.3 Extraction of Valuable Metals
There are three main separation methods, including selective precipitation, solvent extraction and ion-exchange, to extract valuable metals from leachate. Chemical precipitation is widely used due to its simplicity and easy industrial application. Lithium in leachate is usually recycled in the form of Li2CO3 by selective precipitation, while cobalt, manganese and nickel can be separated by chemical precipitation or solvent extraction (87–89), which can be reused as raw materials in LIBs or other fields. Literature values for chemical precipitation process parameters are summarised in Table VII (90). Meshram et al. (91) extracted valuable metals from H2SO4 leachate by selective precipitation. The CoC2O4·2H2O was extracted from leaching solution by oxalic acid, then precipitates of MnCO3, NiCO3 and Li2CO3 were obtained by controlling pH to 7.5, 9 and 14, respectively. Granata et al. (92) found that the order of different extractants affected the separation efficiency of metals in leachate. The best recovery rates reached above 90% with P204 di-(2-ethylhexyl)phosphoric acid (D2EHPA) to extract manganese first, followed by coextraction of cobalt and nickel with CYANEX® 272.
aReprinted with permission from (90) Copyright (2018) Royal Society of Chemistry
It was illustrated that reduction roasting is beneficial to improve the leaching rate of nonferrous metals. Zhang et al. (93) established a process of reduction roasting → H2O-CO2 leaching → H2SO4 leaching → evaporation and crystallisation to recover valuable metals from waste NCM materials, as shown in Figure 3. The related leaching mechanism and a graphical illustration of the recycling process are shown in Figure 4 (93). After reduction roasting, the phases in the cathode material were converted into Li2CO3, Ni, Co and MnO, and the corresponding leaching effect of different metals was improved simultaneously (as shown in Figure 4(m)). For leaching systems containing gas, CO2 had significant influence on the leaching rate of lithium. The solubility of lithium was improved by 47% after inputting CO2 to the leaching system. Through controlling the flow rate of CO2, pH and leaching temperature, lithium was selectively extracted as Li2CO3 by carbonic acid leaching combining evaporative crystallisation, while nickel, manganese and cobalt were deactivated and transformed into leaching residues. The reproduced Li2CO3, possessing high purity and submicron-scale stick morphology, can be directly used as a lithium source to prepare cathode materials of LIBs. After that, the other valuable metals in the leach residues were recovered and extracted in the form of sulfate (MeSO4, Me = manganese, nickel and cobalt) by H2SO4 leaching, with a high recovery rate of 96%.
In general, the leaching solution consists of complex components with different properties. Combined separation methods are adopted to improve the overall recovery efficiency (94). Chen et al. (94) developed an efficient process to extract valuable metals from leachate based on two-step precipitation combined with solvent extraction, as shown in Figure 5. C4H8N2O2 was first used to precipitate 98% nickel from hydrochloric acid leachate. Next, 97% cobalt was selectively precipitated as CoC2O4·2H2O with (NH4)2C2O4. Then, 97% Mn2+ was recovered as MnSO4 by extracting with D2EHPA solvent, and 89% of the lithium was precipitated by Na3PO4 to form Li3PO4.
4. Regeneration Technologies of Nickel Cobalt Manganese Cathode Materials
4.1 Regeneration of Nickel Cobalt Manganese from Leachate
Sustainable recovery and regeneration of NCM materials from leaching solution is the current main trend for recovery of exhausted LIBs. Common methods to synthesise cathode materials include chemical coprecipitation, high temperature solid phase, hydrothermal synthesis, sol-gel, microwave synthesis and electrostatic spinning method (95–100). The parameters of resynthesis methods to regenerate NCM cathode materials from leachate are summarised in Table VII (61, 101–107).
NCM cathode materials regenerated by a hydrometallurgical process hold superior crystal morphology and rate/cycle performance. Their electrochemical performance is comparable to that of commercial NCM materials (102, 104). The conventional hydrometallurgical regeneration process to prepare NCM materials is as follows: leaching liquid → coprecipitation → solid phase synthesis at high temperature, as shown in Figure 6. In the regeneration process, appropriate amounts of cobalt salts, manganese salts, nickel salts and lithium salts are added according to the stoichiometric composition of NCM materials.
Li et al. (108) resynthesised NCM333 (R-NCM) materials from spent cathode materials leaching liquids by a one-step sol-gel method, and the related details and recycling mechanism are shown in Figures 7 and 8 (108). In the recovery system, H2O2 is used as reducing agent, and lactic acid plays the role of leaching agent in the leaching stage and chelating agent in the regeneration stage respectively. The mechanism of leaching stage can be shown using Equation (i):
Under optimal conditions (lactic acid = 1.5 M, s/d = 20 g l–1, H2O2 = 0.5 vol%, temperature = 343 K, where s/d means solid/liquid (S/L) ratio) the leaching efficiency of the metals reached 98%. Electrochemical results proved that the R-NCM cathode material held excellent reversible discharge capacity of 138.2 mAh g–1 while capacity retention reached 96% at 0.5 C after 100 cycles (105). Due to low charge-transfer resistance (Rct) of R-NCM (58.78 Ω) compared with fresh NCM333 (F-NCM, Rct = 70.02 Ω), which can provide higher lithium ion diffusion coefficient (DLi+) and faster lithium-ion intercalation/deintercalation kinetic properties, the electrochemical performance, especially cycle capability, of R-NCM is self-evidently higher than that of F-NCM. It is indicated that the chelating agent lactic acid can efficiently recycle and resynthesise NCM materials by a sol-gel method with closed-loop recovery process.
4.2 Integrated High Value Utilisation of Electrode Scraps
In industrial production of LIBs, a large amount of cathode scraps are produced (89), which are difficult to utilise. Since those cathode scraps are not assembled into batteries, the active materials in them maintain superior electrochemical performance. Efficiently recycling the valuable components in these electrode scraps embodies both economic and environmental benefits.
Zhang et al. (109) developed a representative one-step recovery technology to regenerate NCM material from electrode scraps. The main recovery process includes detaching active materials from aluminium foil and directly repreparing NCM cathode materials by solid state reaction, as shown in Figure 9 (109). Following pretreatment with direct calcination (DC), SD and basic solution dissolution (BD) methods, the regenerated NCM samples present diverse properties including their electrochemical performance. Interestingly, it was found that different quantities of LiF compound emerged on the surface of regenerated NCM samples after pretreatment by DC/BD methods, as HF released from PVDF reacted with lithium. It can be seen that the reprepared cathode materials directly calcined at 600ºC (CD-600) exhibited uniform spherical particles and superior cycle performance, with initial discharge capacity of 145.4 mAh g–1 and capacity retention of 96.7% at 0.2 C after 100 cycles, correspondingly. The available literature shows that the emergence of a suitable amount of LiF contributes to hinder side reactions between NCM materials and electrolyte, which could strengthen the structural stability of the recovered NCM materials (110–113).
4.3 Potential and Challenges of Mediate/Direct Regeneration Method
The leaching-regeneration system has attracted attention because it can remove impurities and separate various metals using simple processes. Technologies such as coprecipitation, solvent dissolution and sol-gel are mostly mature, so the hydrometallurgical recycle pattern is feasible to achieve at industrial scale. As mentioned above, separation of active materials and leaching of valuable metals play an important role in the leaching-regeneration system. In fact, there is still a low content of impurities such as aluminium and magnesium in the leachate. One recovery idea is to employ such impurities as doped metals to modify regeneration NCM materials, avoiding the potential damage of impurities and purification treatment. Excellent electrochemical performance of regenerated aluminium/magnesium-doped NCM materials (105, 107) is shown in Table VIII. Furthermore, the problem of how to strictly control production conditions and identify the optimal residual content of impurities needs to be resolved. Thus, there is still a long way to go before building a recycle system with superior leaching rate and efficacious separation of all metals.
|Regenerated NCM materials||NCM 333||NCM523||NCM 333||NCM 333||Lithium-rich NCM||Magnesium-doped NCM333||V2O5-coated NCM333||Aluminium-doped NCM333|
|Resynthesis method||High temperature solid-state method||Coprecipitation||Coprecipitation||Coprecipitation||Hydrothermal method||Coprecipitation||Solid-state reaction||Coprecipitation|
|Calcination temperature, K||723–1173||773–1123||1173||773–1173||723–1173||–||623||–|
|Initial chargecapacity, mAh g–1||201||198.4||178||198.9||~330||175.4||–||~230|
|Initial dischargecapacity,mAh g–1||155.4||172.9||158||163.5||258.8||152.7||172.4||170|
|Capacity retention, %||83.01||93.8||80||94.01||87||94||90.6||~88|
|Rate performance: current density, C/discharge capacity,mAh g–1(approximation)||–||0.2/180–170||0.1/149–150||0.2/156–154||0.1/245–262||–||0.1/178–160||0.1/165–147|
|Back to original current density||–||168–170||–||148–152||230–222||–||173–175||132–155|
Another interesting recovery idea is using smelting slag as a coated material source to enhance the electrochemical performance of regenerated NCM materials. For instance, regenerated NCM samples coated with V2O5 produced from vanadium-containing slag (106) presented high cycle performance as shown in Table VIII. Meanwhile, the general acid-roasting process (15) combined with one-step or hierarchical leaching technology is worth consideration.
In addition to the hydrometallurgical leaching-regeneration system, direct regeneration technology, also known as reconstruction of crystal structure, is a non-damaged restoration technology. This integrated high value utilisation technology can effectively avoid attack of active materials from multiple chemical reagents during leaching and extracting processes. This simple and sustainable one-step recovery technology may also realise high value reuse and recycling of secondary resources, effectively cutting down the recovery cost. Last but most important, directly regenerated NCM samples hold excellent charge/discharge capacity and superior cycle/rate performance, even comparable to that of commercial NCM materials. Accordingly, one-step recycling or direct resynthesis of NCM materials are considered the most favourable and influential hydrometallurgical recycling strategies for retired LIBs or cathode scraps, at present. However, there are still immense challenges before successful industrial application. For example, green, simple technology must be further explored to efficiently detach active materials from aluminium foils, without destroying both crystalline particles and aluminium foils. In addition, new methods should be developed for direct regeneration of different types of NCM materials other than the high temperature solid phase method. Crucially, it is imperative to build a closed-circuit recycling mode for all valuable components, such as aluminium foils, PVDF and active materials via direct regeneration technology.
5. The Latest Recovery Technologies of Nickel Cobalt Manganese Cathode Materials
5.1 Extractive Methods Based on Deep Eutectic Solvents
Compared with traditional extractants, DESs have the advantages of being non-toxic, cheap, biodegradable and possessing extremely high dissolution and reduction capability for metal oxides (114, 115). However, there are few reports on recycling electronic waste using DESs. Tran et al. (116) first applied DESs to recycle cathode materials from waste LIBs. The adopted DESs were synthesised by choline chloride and ethylene glycol, and the possible synthesis reaction is shown in Equation (ii):
The aluminium foil and binder PVDF were recovered respectively while valuable metals were extracted by DESs. The results showed that the leaching rate of cobalt in LiCoO2 cathode material was as high as 99.4% at 180ºC, which was comparable to that of traditional leaching agents such as sulfuric acid (99.70%) (70) and formic acid (99.96%) (72). Cobalt in the DESs liquids can be recovered as Co3O4 by electrodeposition and calcination.
Wang et al. (117) developed a recycling process using DESs to detach active substances and aluminium foil of retired LIBs, as the recovery flow shown in Figure 10. The specific synthesis reaction of DESs is shown in Equation (iii):
The DESs attacked the hydrogen atoms in PVDF molecular chain, forming unsaturated double bonds, which were further oxidised into hydroxyl and carbonyl to construct an unsaturated ketone structure on the molecular chain. Therefore, PVDF was forced to deactivate and dissolve, the active materials and aluminium foil were separated successfully. Hence, it can be indicated that DESs have great potential in regenerating cathode materials. High selectivity of valuable metals is the key to promote leaching performance of DESs in recycling spent LIBs.
5.2 Nickel Cobalt Manganese Particles Recovery
Despite high recovery rates of R-NCM from leaching solution, electrochemical performance is damaged to some extent due to inevitable contact with organic solvent during the treatment process. In the future, recovery technologies with low efficiency must be avoided. Sieber et al. (118) directly recovered NCM particles from used cathode scraps. The active material was detached from the aluminium foil by strong stirring in water to maintain the electrochemical performance and morphological characteristics of NCM particles. However, during the separation process, side reactions occurred when the NCM cathode contacted with water, resulting in a rapid increase in pH value and degradation of NCM particles followed by forming of Al(OH)3 precipitate and LiAl(OH)4. The main chemical reactions during separation stage are as follows (Equations (iv–vii)):
Such side effects can be avoided by adjusting pH value with superior buffer solution, aiming to detach active materials from aluminium foil and hinder degradation of NMC particles. Under buffer solution control, the aluminium foil was successfully dissolved into solution while NCM particles were well-preserved.
6. Conclusions and Outlook
The waste cathode materials in spent LIBs contain valuable metals, which can be recycled by a hydrometallurgical process which offers low energy consumption, environmentally friendly and excellent recovery efficiency. Based on the present analysis and summary of hydrometallurgical recycling processes, the following conclusions are drawn. The following flow is considered the most competitive process to recycle valuable metals from waste cathode materials at industrial scale: alkaline solution dissolution/calcination treatment → reductant (H2O2) → sulfuric acid leaching → coprecipitation → resynthesis at high temperature. The leaching capability of organic acid with reducibility is greater than that of inorganic acid leaching united with reductants. Bacterial leaching shows high selectivity for lithium, which can be selected to recycle high purity lithium-containing products. A combination of processes for recovering valuable metals may complement each other, which contributes to improve the quality of the recycled products. The DESs separation technology can achieve efficient overall recovery of valuable components from spent LIBs, which is conducive to sustainable development. The highly efficient direct-regeneration or direct-recovery of NCM materials will face huge challenges and potential. In the future, both will become research hotspots in the field of spent LIBs recovery.
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This work was supported by Natural Science Foundation of China (No.51674186); Natural Science Foundation of Shaanxi Province, China (No. 2020JQ-679); Key laboratory project of education department of Shaanxi Province, China (No. 2018GY-166, 2019TD-019, 2019TSLGY07-04); Foundation of Xi’an Key Laboratory of Clean Energy, China (No. 2019219914SYS014CG036).
Lizhen Duan obtained her BS degree in Metallurgical Engineering, Xi’an University of Architecture and Technology, China, in 2018. She is currently a Master’s degree candidate at the School of Metallurgical Engineering, Xi’an University of Architecture and Technology. Her research interest is focused on cathode materials for LIBs.
Yaru Cui is Professor of the School of Metallurgical Engineering in Xi’an University of Architecture and Technology. She was conferred PhD degree in 2011, and was afforded the opportunity working in School of Materials Science and Engineering, University of New South Wales, Australia, as a visiting fellow for one year. Her main research interests covered metallurgical preparation of functional materials and recycling technologies for metallurgical residues. At present, she has authored or coauthored more than 70 journal publications.
Qian Li obtained her PhD in Metallurgical Engineering, the Central South University, China, in 2013. Then she worked as a lecturer in School of Metallurgical Engineering at Xi’an University of Architecture and Technology. She is mainly engaged in research and technology of solar thin film cells, functional materials, sodium-ion battery and LIBs. She has authored or coauthored more than 20 academic papers and five licensed patents.
Juan Wang is a Distinguished Fellow and a Team Leader at Shaanxi Key Laboratory of Nanomaterials and Nanotechnology, China. She received her PhD degree in materials science from Xi’an University of Architecture and Technology in 2009. She is currently a professor and doctoral supervisor of the School of Mechanical and Electrical Engineering, Xi’an University of Architecture and Technology. She has authored or coauthored over 50 papers in peer-reviewed journals related to the field of energy and materials.
Chonghao Man obtained his BS degree in condensed matter physics, Southeast University of China, in 2018. Currently, he is a MS candidate at the College of Engineering, University of New South Wales, major in information technology.
Xinyao Wang obtained his BS degree in metallurgical engineering, Xi’an University of Architecture and Technology in 2019. He is currently pursuing his Master’s degree at the School of Metallurgical Engineering, Xi’an University of Architecture and Technology under the supervision of Professor Yaru Cui. His main research is focused on recycling waste LIBs.