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Johnson Matthey Technol. Rev., 2021, 65, (3), 431

doi:10.1595/205651320x15899814766688

湿法冶金对废弃三元锂离子电池正极材料的回收和直接再生：现状与最新发展

锂镍钴锰氧化物正极材料的经济回收法

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

Article Synopsis

1. Introduction

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.

Table I

Summary of Techniques for Recovery and Separation Valuable Metals from Spent Ternary Lithium-Ion Batteries

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)
	mechanochemical leaching
	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

^a+ / + means the corresponding parameters in the order of ‘reagents + solid/liquid, g l^–1 + temperature, K + time, min’ are absent

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.

Fig. 1

Process flow of hydrometallurgical recovery method for waste NCM cathode materials

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 (LiNi_x Co_y Mn_{1–x –y} O₂,0<x +y <1) (39–43). Of these, layered LiNi_x Co_y Mn_{1–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 (LiNi_1/3Co_1/3Mn_1/3O₂), NCM424 (LiNi_0.4Co_0.2Mn_0.4O₂), NCM523 (LiNi_0.5Co_0.2Mn_0.3O₂), NCM622 (LiNi_0.6Co_0.2Mn_0.2O₂) and NCM811 (LiNi_0.8Co_0.1Mn_0.1O₂) (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.

Table II

Chemical Properties and Potential Environment Pollution of Component Materials from Spent Ternary Lithium-Ion Batteries (45–48)

Material	Constitute	Content, %	Chemical properties	Potential environment pollution
Cathode material	LiNix Mny Co1–x –y O2	25–30	Reacting with acids and bases, and producing heavy metals	Heavy metals pollution
Anode material	Graphite/carbon	14–19	Producing toxic gases such as carbon monoxide and dust particles under combustion	Toxic gases and dust
Casing	Stainless steel/plastic	20–25	Hardly degradable	White pollution
Electrolyte	LiPF6, LiBF4, LiClO4, LiAsF6	10–15	Strongly corrosive, releasing toxic gases in contact with water or high temperatures	Producing toxic gases, polluting the air and damaging human health
Electrolyte solvent	Ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate	–	Producing carbon monoxide under combustion	Harm to human health through skin and respiratory contact with organic compounds
Copper foil	Copper	5–9	–	Heavy metals pollution
Aluminium foil	Aluminium	5–7	–	Heavy metals pollution
Separator	Polypropylene/polyethylene	–	–	Organic pollution
Binder	PVDF	–	–	Fluorine pollution

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.

Table III

The Advantages or Disadvantages of Pyrometallurgical and Hydrometallurgical Method (53–57)

Item	Pyrometallurgical method	Hydrometallurgical method
Process	Calcination	Leaching, purification, separation and extraction
Recycling methods	High temperature reduction, sulfating roasting, repair and regeneration	Acid leaching, bioleaching, chemical precipitation, solvent extraction, ion exchange, repair and regeneration
Operating temperature, K	573–1273	298–353
Cost	High	Low
Advantages	High efficiency, large processing scale, and easy to achieve industrial applications, no waste water and sludge	Low energy consumption, mild operating conditions, high recovery rate, multiple processing methods, high selectivity, low exhaust emissions
Disadvantages	High energy consumption, producing toxic waste gas (carbon monoxide/CO2/sulfur dioxide/hydrogen fluoride), low recovery rate and low selectivity, single processing method, high temperature operating system	Some chemicals are poisonous, waste liquid problem, long process, complex separation process

3.2 Recovery Process of Hydrometallurgical Method

3.2.1 Pretreatment

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.

Table IV

Parameters and Separation Rate of Pretreatment Methods for Spent Lithium Ion Batteries in Literature

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)
			313
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.

Fig. 2

Separation methods for leaching metals from active materials

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.

Table V

Optimal Leaching Parameters and Corresponding Leaching Rate of Metals from Active Material by Acid Leaching

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%.

Table VI

Leaching Rate of Metals from Active Material in Spent Lithium Ion Batteries with Bacteria

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)
LiFePO4			Li: 98	–
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.

Table VII

Summary of the Chemical Precipitation Parameters Investigated in the Literature (90)^a

Elements	Precipitates	Precipitants	pH
Lithium	Li2CO3	Na2CO3	–
	LiF	NH4F	–
	Li3PO4	H3PO4/Na3PO4	–
Cobalt	Co(OH)2	NaOH	10
	CoCO3	Na2CO3	9–10
	Co2O3·3H2O	NaClO	3
	CoC2O4·2H2O	H2C2O4/(NH4)2C2O4	1.5, 2
Nickel	Ni(OH)2	NaOH	8, 11
	NiCO3	Na2CO3	9
	NiC8H14N4O4	C4H8N2O2	5, 9
Manganese	Mn(OH)2	NaOH	12
	MnCO3	Na2CO3	7.5
	MnO2	KMnO4	2
Nickel/cobalt/manganese coprecipitation	Nix Coy Mnz (OH)2	NaOH	11
	(Nix Coy Mnz )CO3	Na2CO3	7.5–8
Iron/aluminium/copper impurities	Fe(OH)3	NaOH	3–6
	Al(OH)3	–	–
	Cu(OH)2	–	–

^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%.

Fig. 3

Process of chemical precipitation for hydrometallurgical recycling NCM material from spent LIBs

Fig. 4

Details and mechanism of recycle of previous metals in spent NCM cathode materials by reduction roasting combined acid leaching: (a)–(i) backscattering scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) mapping results of the cathode scrap after reduction roasting; (j) effect of CO₂ flow rate (L:S ratio 7.5 ml g⁻¹, 2 h, 25ºC); (k) X-ray diffraction (XRD) and SEM of LiCO₃ and residue; (l) XRD and SEM after the carbonation water leaching; (m) the comparison of leaching efficiency between roasted cathode scrap and unroasted cathode scrap. Reprinted from (93). Copyright (2018), with permission from Elsevier

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.

Fig. 5

Metals recovery process from the leachate of spent LIBs by combined solvent extraction and selective chemical precipitation

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.

Fig. 6

Regeneration flow of LiNi_1/3Co_1/3Mn_1/3O₂ cathode material from leaching liquid by coprecipitation

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):

(i)

Fig. 7

Resynthesis flow of LiNi_1/3Co_1/3Mn_1/3O₂ cathode material from leachate of spent LIBs by sol-gel method

Fig. 8

(a) Possible products and mechanism in the lactic acid leaching process; (b) XRD and SEM patterns of R-NCM and F-NCM samples; (c) electrochemical performances of R-NCM and F-NCM samples: charge/discharge profiles at 0.2 C; (d) cycling performances at 1 C; (e) rate performances at different currents and (f) Nyquist plots in the frequency range of 100 kHz to 0.01 Hz. Reprinted with permission from (108). Copyright (2017) American Chemical Society

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 (R_ct) of R-NCM (58.78 Ω) compared with fresh NCM333 (F-NCM, R_ct = 70.02 Ω), which can provide higher lithium ion diffusion coefficient (D_Li⁺) 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).

Fig. 9

(a) Diagram illustration of recycle and regeneration of NCM111 cathode materials under three different routes; (b) SEM image of DC-600 sample; (c) SEM image of SD-800 sample; (d) SEM image of BD-800 sample; (e) rate performances of DC samples at 0.2 C; (f) cycling performances of DC samples. Reprinted with permission from (109). Copyright (2016) American Chemical Society

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.

Table VIII

Regeneration Preparation and Electrochemical Properties of Cathode Materials from Leaching Solution

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
Reagent	–	NaOH	NaOH	Na2CO3	Na2CO3	–	NH4VO3	–
			NH3·H2O		NH3·H2O
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
Cycles	30	50	100	50	50	50	100	50
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
		0.5/170–168	0.2/149–146	0.5/150–144	0.2/240–235		0.2/175–173	0.2/145–132
		1/162–161	1/3/131–138	1/139–135	0.5/220–200		0.5/160–152	0.5/134–120
		2/158–159	0.5/139–177	2/130–127	1/190–175		1/135–131	1/117–115
			C/130–131		2/160–150		2/110–100	2/107–98
Back to original current density	–	168–170	–	148–152	230–222	–	173–175	132–155
Reference	(61)	(101)	(102)	(103)	(104)	(105)	(106)	(107)

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):

(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):

(iii)

Fig. 10

The separation of cathode materials and aluminium foil from spent LIBs by using choline chloride-glycerol DESs

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)):

(iv)

 

(v)

 

(vi)

 

(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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