Chenxing Yi, Lijie Zhou, Xiqing Wu,*, Wei Sun,*, Longsheng Yi, Yue Yang,*

1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

2 Beijing Battery Green Resources Technology Co., Ltd, Beijing 100081, China

Keywords:Spent lithium-ion batteries Graphite Anode materials Recycle

A B S T R A C T With the annual increase in the amount of lithium-ion batteries (LIBs), the development of spent LIBs recycling technology has gradually attracted attention. Graphite is one of the most critical materials for LIBs, which is listed as a key energy source by many developed countries. However, it was neglected in spent LIBs recycling,leading to pollution of the environment and waste of resources.In this paper,the latest research progress for recycling of graphite from spent LIBs was summarized. Especially, the processes of pretreatment, graphite enrichment and purification, and materials regeneration for graphite recovery are introduced in details. Finally, the problems and opportunities of graphite recycling are raised.

1. Introduction

With increasingly serious problems such as energy shortage and environmental pollution, the carbon-neutral policy has attracted more attention, and electric vehicles have gradually become the development trend in the next few decades [1-6].Meanwhile,as the continued expanded amount of electric vehicles and hybrid electric vehicles, the rising demand for batteries was found.Because of the high energy density, lack of memory effects,high-temperature resistance, and safety, the electronic appliance market had been dominated by Lithium-ion batteries (LIBs)[7-10]. Due to these advantages, LIBs were gradually replacing nickel-cadmium batteries, lead-acid batteries, and nickelhydrogen batteries [11,12].In china, the production of new energy vehicles exceeded 1.25 million in 2018 and reached 2.2 million in 2019 [10]. As shown in Fig. 1, according to calculation, it is expected that the power battery production will reach 650 W·h in 2025, and the Chinese production had reached 100 GW·h in 2019.The global LIB market price reached nearly 29.86 billion dollars in 2017, and the market size is expected to increase to 70 billion dollars by 2020[13-18].Due to the only 3-10 years life of LIBs,the increase of spent LIBs was found, which is expected about the spent LIBs of 6.76 million cars in 2035 [19-21].If the spent LIBs is not recycled effectively, it would not only lead to environmental pollution by harmful substances such as electrolytes in the spent LIBs but also lead to the loss of valuable materials in the spent LIBs,which would undoubtedly have a considerable impact on the environment and resources[22-27].Therefore,it is necessary to reduce the pressure of environmental pollution and waste of resources for the recycling of spent LIBs.

Fig. 1. (a) [10] Amount of global power battery and amount of (b) [13] Chinese power battery.

Currently, most electronic products in the market use LiFePO4,LiCoO2,LiMn2O4,or LiNixCoyMnzO2as the cathode electrode material,and most of them utilized graphite or other carbon material as the anode electrode materials [28-34]. Meanwhile, only 1% of spent LIBs were recycled, and the main concern in the industrial recycling process was the recovery of high-value metals (such as lithium, cobalt, nickel, manganese, aluminum, and copper elements)in spent LIBs[28,29,35-39].The recovery of anode graphite was often neglected or discarded as waste residuedue to their difficulty in regeneration [40]. In the majority of industrial procedures, both pyrometallurgy and hydrometallurgy, graphite is burned to the slag or as an energy source and reducing agent to feed into the chamber or separated by filtration as a residue.Spent graphite can only be burned at high temperatures or landfilled as waste residue. If the graphite was burned, it will produce CO2and poisonous gas, leading to the greenhouse effect and environmental pollution. Similarly, the direct landfill will cause heavy metal pollution in the soil,which destroys the environment.However, graphite has a mass ratio of approximately 10%-20% in LIBs,which is 11 times that of lithium, and it is also of recycling significance [28,41-43]. In addition, due to the high requirements of purity and crystal structure about anode graphite, the graphite ore needs to be complexly purified by pickling and anaerobic high-temperature graphitization at 2800-3300°C,resulted in high energy consumption and cost [44]. Meanwhile, only crystalline flakes graphite was elected as raw material,led to expensive prices of graphite. Currently, the market price of electrode graphite is 8-13USD·kg-1, and the value of electrode graphite will reach 29.05 billion dollars in 2022[40,44,45].Meanwhile,with the continuous consumption of graphite resources,graphite resources had became the key strategic resources of many countries [41,46-48]. Therefore, the spent graphite recovery is of great significance.

Up to now, the recycling of spent graphite has attracted great attention and many approaches have been developed. In this paper,we summarized the latest research progress in the recovery of graphite from spent LIBs, and look forward to the future trend,aiming to provide a reference for related research on the recovery of spent graphite.

2. Research Progress in the Pretreatment

In the recovery process of spent anode graphite, the pretreatment is an essential process, which separates graphite from other components. The pretreatment process usually includes the discharge process, the disassembly process and the binder removal process.

2.1. Lithium-ion battery components

For better recovering the graphite, it is critical to understand the components of the LIBs. Meanwhile, that of reaction and product also should be known during the charging and discharging process. LIBs consist of an anode electrode, a cathode electrode, a separator, electrolytes and a steel shell. The anode electrode is comprised of a polymer binder and carbon material coated on a copper foil, such as graphite and carbon fibre tubes [49-52]. The cathode electrode is comprised of acetylene black, polymer binder and lithium transition metal oxides coated on aluminium foil,including LiFePO4, LiCoO2, LiNiO2LiMn2O4, LNCM (LiNixCoyMnzO2),and LNCA (LiNixCoyAlzO2) [28,53-59]. The most commonly used binders in commercial LIBs were polyvinylidene fluoride (PVDF),copolymer and polypropylene (PP). Highly soluble solvents were usually poisonous and harmful, such asN-methyl-2-pyrrolidone(NMP) or NDimethylformamide (DMF). To explore more environmentally safe alternatives(such as water),subsequent studies used CMC as the binder and water as the solvent,with good results[60-65].The commonly used separators were microporous polyolefin membrane,such as polypropylene(PP),polyethylene(PE),and fluorinated polymers [66-68]. Meanwhile, different ion batteries used different ion electrolyte, like lithium ion battery electrolyte often used lithium salts such as lithiumhexafluorophosphate(LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluorome thanesulfonyl)imide (LiTFSI), or others, dissolved indipolar aproticalkyl carbonate solvents, such as ethylene carbonate/dimethyl carbonate (EC/DMC) [69-72].

In the charging and discharging process, the lithium-ions escaped from the cathode electrode, which passed through the separators and embedded between the graphite layers.And a complex protective film of lithium oxide was formed on the surface of graphite(solid electrolyte interphase).Meanwhile,due to the insertion and accumulation of lithium-ions,the irreversible and uneven expanding of graphite lattice was found.Moreover,the complexity and toxicity of electrolytes increase the difficulty of recycling spent graphite. Therefore, it is significant for the study of graphite recycling.

In the recycling process of spent lithium-ion batteries, the pretreatment process effectively and safely separates steel shell,plastic, diaphragm, positive and negative electrode materials or overcomes the bonding force between copper/aluminium foil and electrode materials to enrich and purify the cathode and anode electrode materials.

2.2. Discharge process

For preventing rapid exotherm, spontaneous combustion, and explosion in the dismantling and crushing process, it is necessary to release the residual electricity in the recovery process [73]. If the incomplete or unreasonable discharge is found, the spent graphite may be burnt during the recycling process, resulting in fire hazards and environmental pollution. Discharge methods include solution discharge, powder discharge and cryogenic freezing[74,75].The liquid discharge method means that the spent LIBs discharge in solution,for example, salt solution,acid,and alkali solutions[74,76].As confirmed by research,the discharge effect of salt solution is better than that of other media, and salt solution can promote electron transfer and absorb heat for discharge and protection. The powder discharge method generally refers to discharge with metal powder or graphite as a discharge meda.However, the heat conduction velocity of powder is slower than the release velocity,resulting in the sharp rise of the temperature,and caused danger. Although, the heat conduction velocity of graphite is matched with the release velocity,not leading to the overheating. Due to the fine and light graphite particles, the dust explosion was easily caused. The cryogenic freezing technology cannot be utilized for large-scale discharge processes, resulted from its high requirement and construction cost [76].

Currently,the most common discharge medium is sodium chloride(NaCl)in spent LIBs,but the metal shell could be corroded and destroyed from high concentration NaCl solution, leading to electrolyte leakage. Meanwhile, toxic lithium salts and organic compounds dissolve into the solution, which causes a severe threat to human health and the environment. Xiaoet al.[74] studied the effects of different salt solutions on the discharge efficiency and environment of spent LIBs, and compared NaCl, KCl, MgSO4,MnSO4and NaNO3solutions.The results showed that the discharge efficiency of NaCl and KCl solutions was the fastest,because of the corrosive Cl-,leading to damage of iron shell,and the electrolyte is leaked. The discharge efficiencies of the NaNO3solution are the slowest because there are no ions that react with the battery cell.The discharge efficiencies of MgSO4and MnSO4are relatively moderate,and the amount of suspended matter decreases significantly,as shown in Fig. 2. Moreover, after stirring and reducing the pH value, the reduced concentration of suspended matter could be found, which provided more environmentally discharge methods.Subsequently, Yaoet al.[76] studied the environmental effects of discharge in NaCl,MnSO4,and FeSO4solutions in detail.This shows that the NaCl solution not only damaged the battery structure but also reacted with electrolytes, resulted in producing small molecules of hydrocarbons and fluorinated phenyl groups, which can affect human health. Meanwhile, when using the MnSO4solution discharging, there are not suspended matters but existed the high nickel and aluminum contents in the supernatant, and it was not an environmentally friendly discharge solution.the pollutants produced during discharge in FeSO4solution are much purer and environmentally than NaCl solution, and the discharge efficiency is much higher than MnSO4in Table 1.

Table 1 Discharge effect and pollutant composition of different discharge methods [74-76]

Fig. 2. (a) Reprinted with permission from Ref. [74], Copyright 2019, Elsevier, and(b) reprinted with permission from Ref. [76], Copyright 2019, Elsevier, discharge effect of different solutions (after 24 h).

2.3. Mechanical separation process

In the process of spent graphite recovery, the battery shell is commonly separated by mechanical separation. Due to the simple operation, mechanical technology is widely used in the industrial recycling of spent LIBs, including crushing, grinding, screening,magnetic separation and classification [77]. After discharging, the machine breaks spent LIBs and separates the coarse particleplastic and steel shell. Val company [78,79] used mechanical processing equipment, automatic pretreatment-crushing, eddy current, screening, and magnetic separation automation. Although the mechanical process was economical and efficient,the materials are hardly separated completely. Zhanget al.[80] proposed a wet and dry grinding method to separate electrode materials more efficiently. However, these grinding methods cannot completely destroy the binder structure and separate the electrode materials.Costaet al.[81]studied the influence of grinding degree on subsequent electrode material recovery. Wanget al.[82] proposed a low-temperature grinding method, which significantly improved the electrode material recovery efficiency. The binder PVDF changes from a high elastic state to a glass state through liquid nitrogen cooling, which weakens the bonding performance and dramatically improves the stripping efficiency of electrode materials without producing toxic and harmful organic gas.However,this method is hard to apply on a large scale because of its high technical requirements and complex operation.Therefore,the research in the mechanical processing stage to improve efficiency and reduce costs is needed.

2.4. Separation of active materials from current collector

Heat treatment and solvent dissolution methods were commonly utilized for Separating active materials from current collector. The binder and residual electrolyte could be effectively removed by both methods. Heat treatment involves heating to a specific temperature to invalidate or volatilize the organic binder between graphite and copper foil. Heat treatment is widely used in the separation process of active materials from the current collector because of its removal effect.However,it also has disadvantages, such as high energy consumption and the production of dioxins and fluorinated organic compounds in the hightemperature process,which damaged the environment and human health[25,83,84].Zhanget al.[85]explored the pyrolysis products of electrode materials at high-temperature and found that the residual organic electrolyte volatilized at approximately 100 °C and that the organic binder completely decomposed at 500 °C.When the temperature continued rising, the pyrolytic carbon and fluorophenyl groups were generated and coated on the electrode surface,which affect the subsequent recovery of electrode materials. Some pyrolytic carbon and fluorophenyl groups were effectively removed by ultrasonic cleaning. Two-stage pyrolysis was proposed to improve the recovery of electrode materials [86].Wanget al.[87] proposed that the exhaust was adsorbed by alkali solution which prevents environmental pollution, but the HF gas released by PVDF decomposition can corrode the equipment,shorten the equipment’s service life, and increase the cost. Wanget al.[88] reported CaO reaction medium was added to promote the decomposition of PVDF while inhibiting the release of fluoride,thus reducing the pyrolysis temperature of PVDF to 300 °C and reducing energy consumption. The author also proposed that the deep eutectic solvent (DESs) decomposing PVDF, mixing choline chloride with glycerol in proportion, and the heat treatment with the electrode material further reduced the pyrolysis temperature of PVDF to 170 °C.

The solvent dissolution method was based on the principle of similar phase dissolution,using a suitable solvent at a specific temperature to dissolve the organic binder,reducing the viscosity,and is the best separation method in the case of a single binder. NMP,DMF, DMAC, DMSO, and ionic liquids commonly dissolve the binder PVDF and PTFE as solvents [28,89-92]. In the dissolution process, other ions are not added, and the separation effect is significant, which reduces the difficulty of subsequent treatments.Songet al.[93] found that the heating treatment could accelerate the dissolve rate of NMP. The recovery and separation efficiency of electrode materials can be improved through ultrasonic water bath heating [94-96]. Heet al.[97] studied a special compound aqueous stripping agent, namely, exfoliating and extracting solution (AEES). The anode materials could be effectively separated from the Cu foil by reducing the binder effect and interlaminar adhesion.A green and environmental method for removing binder in the recycling process was proposed [98]. Used citrus juice,which contained a variety of organic acids and flavonoid complex compounds, not only can effectively remove the binder, but also can reduce the metal impurities, and will not produce fluorinecontaining waste. Zenget al.[99] proposed a method to remove PVDF binder with heating ionic liquid(IL).However,due to the different manufacturing processes of LIBs, a variety of binders was found in spent LIBs,which reduces the separation effect.Additionally, the NMP has high cost and toxicity, which remained the potential danger to the environment and human health. Further research needs to find more suitable solvents.Due to the development of water system binders, an increasing number of anode materials have commercially used CMCs as binders, resulted in advantages in the IL method [100-103].

Obviously, the removal of binder by heat treatment is obvious and effective, but the production of waste gas should be purified by more effective methods. Solvent dissolution method is more targeted, but the added solvent is expensive and toxic. Hence,according to the actual condition, the separation methods with both of current collector and electrode material should be utilized appropriately.

3. Research on Enrichment and Purification of Spent Graphite

After the pretreatment, it is necessary to separate the anode graphite and cathode powder by effective methods to improve the subsequent recovery efficiency,achieving an enrichment effect.There are flotation, acid leaching, and roasting enrichment techniques. The flotation technique is used to separate graphite particles based on their hydrophilic/hydrophobic differences and opposite surface wettability to obtain pure anode and cathode materials[85,104-114].Leaching is the most commonly used recycling method for spent LIBs because this method can recover valuable metal ions well. According to the principle that graphite is insoluble in acid and base, powder separation of the anode and cathode is completed.However,owing to the changing of graphite interlayer structure, it could not achieve the desired recycling effect in recovery graphite,leading to the remain of acid waste liquid[77,86,115-124].In anaerobic calcination,graphite is used as a carbon heat reducing agent. Under certain conditions, electrode materials such as LiCoO2are transferred to cobalt or lithium and then separated by magnetic separation,or enrichment or recovery electrode materials with different heat resistance. There is also a high temperature to volatilize and precipitate impurities and to recrystallize graphite [19,116,125-128].

3.1. Flotation technology

Flotation combined with mechanical crushing could be used as a metallurgical process to recover graphite materials in Fig. 3. In the flotation, methyl isobutyl carbinol (MIBC), and n-dodecane as the foaming agents and collectors, the anode graphite was transported by bubbles to the top foam layer,due to its hydrophobicity.And the cathode lithium metal oxide was collected into the tailings, because of its hydrophilicity [104]. However, owing to the existence of organic impurities, the flotation becomes more complicated. Therefore, the flotation efficiency of electrode materials is improved by surface modification.

Fig.3. The recycling of flotation flowchart of electrode materials(a)Fenton reagent-assisted flotation,reprinted with permission from Ref.[114],Copyright 2017,Elsevier.(b)Pyrolysis and flotation,reprinted with permission from Ref.[105],Copyright 2020,MDPI.(c)Cryogenic grinding and froth flotation,reprinted with permission from Ref.[108],Copyright 2020, Elsevier. (d) Pyrolysis-ultrasonic-assisted flotation, reprinted with permission from Ref. [85], Copyright 2018, ACS Publications. (e) Mechanical crushing combined with pyrolysis enhanced flotation,reprinted with permission from Ref.[110],Copyright 2019,Elsevier.(f)Pyrolysis,wet-ball grinding and flotation,reprinted with permission from Ref. [109], Copyright 2020, ACS Publications.

Heet al.[114] reported that the electrode material surface was modified by the Fenton reagent. After discharging, the spent LIBs were broken into powder by the shear crusher and impact crusher.The fine powder electrode material with a size of 0.25 mm was enriched by sieving.Afterward,the surface of materials were modified by Fenton reagent.In this step,the Fenton reagent was used to remove the organic outer layer on electrode materials from spent LiBs and obtained the graphite and LiCoO2without impurities.Fenton reagent(Fe2+/H2O2)was prepared by FeSO4(0.1 mol·L-1),H2O2(0.1 mol·L-1) solutions, and deionized water. Finally, LiCoO2and graphite were separated by flotation. Organic outer layers coated on the surface of electrode materials could be removed with the Fenton reagent.During the reaction,macromolecule organics such as PVDF were degraded down into small molecules.Organic materials of small molecules were oxidized into CO2and H2O in the end.Fenton’s high-order oxidation process can breakdown organic binders and electrolytes. However, the Fenton reagent involves Fe2+,which remains on the electrode material’s surface, complicating the subsequent removal process.

Ruismakiet al.[107] proposed removing organic binders to improve the flotation efficiency of cathode and anode materials from spent LIBs through the pyrolysis process. After crushing and screening,spent cathode and anode electrodes were handled using pyrolysis combined with a mechanical-physical process. The remarkable improvement of flotation effect was found. However,the flotation efficiency of LiCoO2and graphite was affected,owing to the residual organic product and rough surface.Zhanget al.[85]reported pyrolysis-ultrasonic-assisted flotation. After pyrolysis,residual pyrolysis products were removed by ultrasonic cleaning.Additionally,the high content of fluorine in raw electrode particles was removed by pyrolysis and ultrasonication, and the content of fluorine decreases from 26.69% to 6.54%, which proves the significant removing of binder PVDF.However,there are still some residual binders, which affect the flotation effect. Follow-up [110]investigated the influence of pyrolysis temperature,pyrolysis time,and heating rate on subsequent flotation effect.The optimum flotation behavior occurred at a pyrolysis temperature of 550°C with a heating rate of 10 °C·min-1and a pyrolysis time of 15 min. The two-stage flotation process was used to increase product grade.However, multiple flotations increased the workload and wastewater, leading to more expensive costs and environment pollution.

Zhanget al.[109] and Liuet al.[108] proposed pyrolysis-wetball grinding flotation and cryogenic grinding flotation. For enhancing flotation efficiency, the residual pyrolytic carbon was removed by pyrolysis and wet ball grinding,and the contact angle of materials was changed by pyrolysis. Cryogenic grinding [108]gave rise to the binder and organics on the surface of electrode materials falling off, with spent graphite bringing out a new layer structure on the surface.The hydrophilicity/hydrophobicity of spent electrode powder was improved by cryogenic grinding,leading to the outstanding flotation separation.During the cryogenic grinding process,the grinding chamber was frozen by liquid nitrogen. It could prevent pollution, which causes electrolyte leakage.

Although the cathode and anode graphite achieve the enrichment effect,there are organic impurities and micro-other fine particles in concentrates,which are not pure materials.Therefore,the recovered graphite could not be cyclic utilized directly,due to their not enough purity. For improving the value of recovery graphite,the graphite of flotation needs to be reprocessed later.At the same time,the generated wastewater in the flotation process is reused to prevent secondary pollution.

3.2. Leaching technology

Leaching recovery is one of the most common methods in the spent lithium-ion battery industry because of its high selectivity and recovery. Hydrochloric acid,sulfuric acid, and other strong acids were used to leach the electrode materials as leaching reagents, and hydrogen peroxide was used as a reducing agent(Fig.4).The acid leaching process in anode electrode recovery usually removes impurities from graphite and recovers lithium in graphite intercalation.Guoet al.[120]proposed that hydrochloric acid(HCl)was taken as the leachate,and hydrogen peroxide(H2O2)was used as the reductant. Meanwhile, the effects of the hydrochloric acid concentration, HCl and H2O2volume ratio, solid-liquid ratio,time, and temperature were investigated. The author mentioned that the solid electrolyte interface film(SEI)on the graphite surface was formed by the interfacial effect of electrolyte decomposition during charge and discharge.The SEI films were mainly composed of inorganic components (including Li2CO3, LiF, Li2O) and organic constituents (including ROCO2Li, ROLi, (ROCO2Li)2) [129]. Thus,the main forms of lithium element in the spent anode graphite interlayer were Li2CO3, Li2O, LiF, ROCO2Li, and CH3OLi. Owing to their water-soluble, such as ROCO2Li, CH3OLi, and Li2O, the high leaching efficiency of lithium(84%) was found in deionized water.Other lithium compounds, such as ROCO2Li and LiF, are almost insoluble in water and inlaid in graphite layers, but they could be decomposed with HCl[130].Finally,high content of Li(approximately 30.07 mg·g-1Li) and pure graphite could be recovered in the anode of spent LIBs [120]. Usually, these binder agents, like polyvinylidene fluoride(PVDF)binders,all possess excellent chemical stability and mechanical performance, causing difficult react with most of the strong acids, which can lead to many problems during the separation of electrode materials after leaching and reduced the final recovery.So it is often combined with heat treatment. Yanget al.[86] collected graphite and Li2CO3by two-stage calcination, HCl leaching,and sodium carbonate addition.Approximately 100% lithium, copper, and aluminum dissolved the leach liquor by acid leaching under the conditions of 1.5 mol·L-1HCl,S/L = 100 g·L-1and leaching time 60 min. Lithium element in the leaching liquor was recovered by the carbonate precipitation method, the purity of the recovered lithium carbonate was over 99%, and the recovered graphite had a adequate electrochemical performance.

Fig. 4. The recycling of leaching flowchart of electrode material, (a) HCl and H2O2 leaching, reprinted with permission from Ref. [120], Copyright 2016, Elsevier. (b) HCl leaching, reprinted with permission from Ref. [86], Copyright 2019, Elsevier. (c) H2SO4 and H2O2 leaching, reprinted with permission from Ref. [124], Copyright 2019, ACS Publications. (d) Concentrated H2SO4 leaching, reprinted with permission from Ref. [123], Copyright 2020, Elsevier.

There is also a method of leaching the crushed cathode and anode powders in acid solution. The graphite material is separated based on the characteristic that graphite is hardly soluble in acid, and the impurity was removed at the same time. Maet al.[124] reported a recycling method for anode materials graphite from spent LIBs by a sulfate acid leaching process without separation steps. Firstly, spent LIBs were cut, shredded, and sieved to obtain the fine powder. The fine powder was leached by 5 mol·L-1sulfate acid and 35%(mass) H2O2at room temperature. Then, it was centrifuged, washed, and dried. After the release process, the graphite was roasted with NaOH powder at 500 °C for 40 min, washed with deionized water, and dried again.After acid leaching, the graphite layer spacing was expanded by strong oxidation. To reach the electrochemical requirement of reusing, recovered graphite was released and reacted with the fusion agent. The electrochemical performance of the recovered graphite was also obviously improved.Liuet al.[123]investigated the results of recycling spent graphite as an anode material for lithium-ion batteries(LIBs) and sodium-ion batteries(SIBs) after the reconstruction process. The spent graphite was infiltrated with concentrated H2SO4and stirred at 80 °C for 5 h. After filtration,the obtained graphite powders were treated at 750°C for 8 h under a N2atmosphere. The graphite powders, 8.3%(mass) FeCl2,and 10%(mass) glucose, were added into alcohol, stirred for 5 h,and then calcined at 800 °C for 8 h. Finally, the calcined powder was dissolved in a hydrochloric acid solution for 2 h to remove iron. The recovered graphite had an enlarged interlayer lattice distance, which may be suitable as an anode material for SIBs.Through electrochemical testing,the electrochemical performance of recovered graphite was found to be suitable for LIB and SiB anode materials. A SIB anode was prepared by expanding the layer spacing after acid leaching, which provides a new research idea for the recovery of anode graphite.

The purity of Li and graphite was excellent by the acid leaching method of recovery.But,owing to the acid resistance of the binder,the recovery process was complex, which combined with Hydrometallurgy and pyrometallurgy. In addition, due to the strong oxidation of acid-leached graphite, the graphite layer spacing expands, and the graphite crystal changes, which is not conducive to the reuse of lithium-ion battery anode materials, and subsequent research can be conducted on SIBs anode materials.

3.3. Anaerobic calcination

The method of anaerobic calcination refers to the in-situ reduction of these metal minerals with reducing agent graphite under high temperatures or recrystallizing the graphite at high temperatures to recovery in Fig. 5. Liet al.[127] recycled value metal and graphite by the anaerobic calcination and wet magnetic separation method. After crushing and screening, powders were oxygen-free roasted at 1000 °C for 30 min. Then, the roasted products were separated by wet magnetically for 48 h.Thus,mixed materials consisting of LiCoO2and graphite powders were converted into the products of metallic cobalt powder, Li2CO3powder, and graphite powder.The high-value cobalt powder was recovered by wet magnetic separation of the mixed powder due to its magnetism.Finally,the purity of recovered graphite in the process of magnetic reached 91.05%(mass).This method’s advantage was that no chemical solution was added in the process, saving the cost of treating secondary pollution.However,ball mills have a long working time and high energy consumption. Meanwhile, the recycled graphite had low purity and low utilization value.Gaoet al.[116]combined acid leaching with high-temperature calcination to recover highvalue graphite. The spent graphite was purified with sulfuric acid for 24 h.After leaching and filtration,the residue was washed with deionized water to neutralize and dried. Finally, the graphite recovered by high-temperature calcinated at 1500°C for 2 h under an Ar atmosphere. This method was not only for the excellent removal efficiency of impurities but also for low operating temperature,indicating low energy consumption.Therefore,nearly no wastewater was generated in the proposed recycling method,which is good for environmental protection.However,the method involved an acidic method and high temperature for high operation requirements. Yiet al.[19] directly recovered graphite by high-temperature treatment and screening, and the purity of the recovered graphite was more than 99.5%. The anode electrode was calcined under a N2atmosphere at 1400 °C for 4 h. Then, the Cu sphere and graphite powder were separated from each other particle size by simple ultrasonic vibration and sieving. The recycled graphite showed high electrochemical performance.However,the recovered graphite also had structural defects, resulting in the low initial coulomb efficiency. Fathimaet al.[128] recycled graphite by heat treatment at different temperatures, 500 °C, 650 °C,and 800 °C for 5 h under an Ar atmosphere. Spent graphite was recovered through lithium ion-based all-carbon dual-ion (ACDIB)applications. This research could be further extended to other alkali dual battery systems, such as sodium- or potassium-based systems.

Fig. 5. The anaerobic roasting flowchart of electrode materials (a) calcination at 1500 °C under Ar, reprinted with permission from Ref. [116], Copyright 2020, ACS Publications. (b) Thermal treatment at 800 °C under Ar, reprinted with permission from Ref. [128], Copyright 2021, Elsevier. (c) Smelting 1400 °C under N2, reprinted with permission from Ref. [19], Copyright 2020, Elsevier.

The method of anaerobic calcination for the graphite recovery effect is good, and the spent graphite heat treatment at high temperature exhibits better performance than the low-temperature treatment, which can be attributed to the improved crystallinity.However,due to the high energy consumption and the production of waste gas, it is difficult to apply in the industrial recycling of spent lithium-ion batteries. Therefore, there should be a more effective tail gas treatment method and high-value utilization method in the future.

3.4. Other methods

Caoet al.[131]reported an electrochemical method for the separation of graphite and copper foil in the anode material of spent LIBs. Copper foil-coated graphite was used as the negative electrode in the reaction pool, an inert electrode was used as the positive electrode, and the electrolyte solution was Na2SO4. The copper foil and graphite were completely separated at approximately 25 min under the optimal electrolysis conditions of an interelectrode distance of 10 cm, electrolyte concentration of 1.5 g·L-1and voltage of 30 V. The regenerated copper foil and recovered graphite were high integrity. However, the graphite in this method contained a small amount of binder residue, which affected its subsequent reuse value.

The SEM of recovered graphite by different methods was showed in Fig. 6. There are also significant differences in the amount of graphite recovered by different methods. There are obvious fine particles on the surface of the recovered graphite by flotation,which is the residue of flotation reagents.Recovered graphite by acid method shows the cleanest surface,but also exposes the largest interlayer spacing. The surface of recovered graphite had an obvious ablation hole by the calcination method, but the pellets are more compact. There are obvious impurity particles in the recovered graphite by other treatment methods,and the powder particle size was inconformity, which may be caused by organic residues and shedding graphite debris.

Fig.6. The SEM of different method collected anode graphite(a)flotation method,reprinted with permission from Ref.[85],Copyright 2018,ACS publications.(b)Flotation with Fenton agent method, reprinted with permission from Ref. [114], Copyright 2017, Elsevier. (c) H2SO4 leaching method, reprinted with permission from Ref. [123],Copyright 2020,Elsevier.(d)HCl leaching method,reprinted with permission from Ref.[86],Copyright 2019,Elsevier.(e)Calcination method,reprinted with permission from Ref.[19],Copyright 2020,Elsevier.(f)Electrolysis method,reprinted with permission from Ref.[131],Copyright 2021,Elsevier.(g)Water treatment method,reprinted with permission from Ref. [103], Copyright 2019, Elsevier.

4. Materials Regeneration of Spent Graphite

4.1. Regenerative graphite

After pretreatment and enrichment, the spent graphite was often processed into new anode materials for LIBs directly or by other treatments. Due to the small structural changes in the process of charge and discharge and recovery, the electrochemical properties of recycled graphite can be improved by coating treatment and high-temperature graphitization treatment. If the recycled graphite directly uses as active anode material, it is hardly comparable to commercial electrode graphite.Due to the low purity of graphite,large layer spacing,and the surface residual erosion,the electrochemical properties of graphite were degraded, such as cycle performance, capacity retention, and the initial cycle of the coulomb efficiency.Follow-up studies in the recycling process used carbon coating, recrystallization, and leaching modification to improve the electrochemical properties.

Direct using:After the hydrometallurgical and pyrometallurgical treatment, the recycled graphite can be separated from other components and could be directly used as a new active anode material for LIBs. Yanget al.[86] proposed a two-step heat treatment of spent anode material. After heat treatment, the spent graphite was recovered by acid leaching treatment to separate.And, the recovery rate of spent graphite was adjust by controlling different pH values. Due to the excellent impurity removal effect,the recovered graphite was pure. But, after acid leaching, the changes of graphite structure had occurred,leading to the reduced electrochemical performance, and the first discharge specific capacity was reduced to 172.6 mA·g-1. However, the capacity retention rate of the recycled graphite was 97.9% after 100 cycles,resulted from its purity and stable layered structure. Meanwhile,the recovery of copper and lithium was very effective,and the purity was higher than 99%. Maet al.[124] introduced a method of regenerating graphite by hydrometallurgy. The regenerated graphite was obtained by acid leaching again and with sodium hydroxide co-melting. This method subtracted the separation of active material and leached the broken spent LIBs directly, which reduced costs but complexed the subsequent impurity treatment in recovering graphite.In this study,the spent graphite was recovered well by acid leaching and strong alkali heat treatment,and the impurity removal effect was noticeable. There is a good crystal shape and good electrochemical performance in the recovered graphite. The discharge capacity after 50 cycles was 341.8 mA·h·g-1,the capacity retention rate was 96.64%, and the graphite recovery was approximately 60%. Wanget al.[103] proposed a method of water treatment to recycle graphite. In this method, hydrogen could be produced by the reaction of water and lithium in the spent graphite layer, and the SEI film could be separated from the graphite to remove impurities.In contrast,after multiple water washing, filtering, and drying, the phosphorus removal effect was apparent in washing for spent graphite. Moreover, the first cycle discharge specific capacity of graphite was 338.9 mA·h·g-1,which was increased compared to before washing. But, the first cycle of coulomb efficiency was 75.9%, which was relatively lower. The capacity remained at a rate of 75% after 100 cycles, much higher than before washing. This method is more environmentally friendly and convenient, but probably only for batteries of the water-based binder. Rothermelet al.[132] studied the technologies of regenerated graphite under three different extraction conditions of electrolytes. There are three technologies for recycling graphite:the first, recycling graphite from the volatilizing electrolyte, the second, recycling graphite with dynamic subcritical carbon dioxide auxiliary electrolyte extraction,and the third,recycling graphite with static supercritical carbon dioxide as the extractant separation electrolyte.Comparing with non-cycled cells and 70%state of health cells with three recovery methods,the critical separation method was beneficial to prevent the formation of impurities’ phosphorus and oxygen compounds on the graphite surface, while high-pressure CO2harmed the graphite crystal.The first cycle coulombic efficiency of graphite recovered from the critical CO2separation electrolyte was 82.9% and 81.6%, and the discharge capacity after 50 cycles was 372.7 mA·h·g-1and 379.9 mA·h·g-1, respectively. Moreover, the critical CO2auxiliary extractive electrolyte, including conductive salt, can be recovered by 90%. Maet al.[133] studied a method of microphysical reconstruction recovery of graphite. Graphite coexisting with sp2and sp3on the surface of carbon was prepared by microwave removal of impurities and spray drying.In the process of microwave cutting exfoliation, ethylene glycol was used as an expansion agent and dispersant to be treated in a 400 W microwave for 20 min.The graphite was recovered with a drying spray after being washed with ethanol. The first cycle discharge capacity of graphite recovered was 473.9 mA·h·g-1, with 81.6% of the first cooling efficiency,and the capacity retention rate was 106.1% after 100 cycles. This study provides new ideas and methods for the recovery of spent LIBs. Caoet al.[131] recovered graphite by electrolysis method,the initial discharge capacities and coulombic efficiencies of recovered graphite were 427.81 mA·h·g-1(81.92%), 409.15 mA·h·g-1(82.66%), 365.37 mA·h·g-1(81.34%), 325.65 mA·h·g-1(77.89%) at 0.1C, 0.2C, 0.5C, 1C respectively. The electrochemical performance of recycled graphite was stable,but it cannot achieve the commercial graphite standard, and other treatments were adopted for recycling in the subsequent recovery.

Carbon coating:carbon coating was the most commonly used electrochemistry modification technology in commerce. Zhanget al.[134] recycled graphite by H2SO4leaching and repaired by carbon coating. Recycled graphite was repaired with coated pyrolytic carbon from phenolic resin. Comparing the electrochemical properties with before and after carbon coating, the cycle performance could be improved by carbon coating, even over the midrange commercial graphite. The discharge capacity of regenerated graphite reached 342.9 mA·h·g-1and retention of 98.76% after 50 cycles. Compared to uncoated carbon graphite, the cyclic performance and the initial cycle coulomb efficiency were significantly improved. Liuet al.recycled graphite also by leaching and carbon coating, but the recovered graphite was coated with the glucose and doped the FeCl2. Due to the carbon coating and doping, the recovered graphite was restructured to a new structure, which also obtained an excellent electrochemical effect,and the rate performance and cycle performance were better.The discharge capacity of recycled graphite can reach 427.9 mA·h·g-1at 0.5C. In a word, carbon coating is an effective method of graphite modification.

Recrystallization:Recrystallization refers to the structural recovery of graphite in the anaerobic condition at high temperatures, also known as graphitization. Yiet al.[19] investigated a method for graphite recovery by high temperature, ultrasonic,and screening. The recovered graphite has stable cycle performance and rate performance with high charge and discharge capacity,and the discharge capacity of 100 cycles at 1C was maintained at 360.8 mA·h·g-1, with 99.8% of coulomb efficiency. But,the heating temperature did not reach graphitization temperature,leading to the incomplete graphitization, which resulted from the low initial coulombic efficiency, only 62.91%. Yanget al.[135,136]used different high-temperature to recover anode material. When the recrystallization temperature reached 2600 °C[135], the layered structure of graphite was uniform, with a high graphitization degree, and there is a small amount of graphene structure in the middle. The recycled graphite possessed high discharge capacity, after 50 cycles, the capacity of graphite remained 460.1 mA·h·g-1with retention of 101.1%. For reducing the recrystallization temperature of recovered graphite, when the roasting temperature of 1600 °C [136], the recycled graphite could achieve certain electrochemical performance indexes.It exhibited an excellent reversible capacity of 365.5 mA·h·g-1after 100 cycles at 0.1C.That is to say, high-temperature treatment was a very convenient and fast way to recycle graphite, but, for graphite graphitization,the recrystallization was raised to high, leading to huge energy consumption and expensive cost.

Modification:Gaoet al.[116] proposed a process of the recovered graphite repaired by acid leaching,heat treatment,and pyrolysis coating. The spent graphite was repaired by curing-acid leaching and high-temperature calcination, which reached the electrochemical of commercial graphite. The first cycle discharge capacity of recovered graphite was 343.2 mA·h·g-1, and the first cycle coulometry was 92.13%. The discharge capacity was 339.8 mA·h·g-1after 50 cycles, and the capacity retention rate was 99.0%. Ruanet al.[121] studied a method to repair the graphite anode by leaching, graphitizing, and pitch coating. The graphite recovered by this method had fewer impurities, fewer surface defects, a higher degree of graphitization, and a more complete structure. The recycled graphite exhibited a high initial coulombic efficiency of 90.64% and a high capacity of 358.1 mA·h·g-1, also showed high capacity retention, maintained around 344 mA·h·g-1for more than 100 cycles.

To sum up,as shown in the Fig.7 and Table 2,when recovering graphite by the hydrometallurgy method, the graphite layer spacing was changed, leading to the poor cycle of capacity in reusing.Subsequently, it was necessary to improve the electrochemical performance of the recovered graphite by utilizing hightemperature graphitization or carbon coating. However, when using the pyrometallurgy method,although the recovered graphite showed excellent cycle performance, its surface was ablated, and the surface groups were changed, resulting from the removed residual organic binder and impurity, resulting in low initial coulomb efficiency and poor rate performance. Other recovered graphite methods, such as the CO2critical method and microwave stripping method, the excellent electrochemical was exhibited by the recovered graphite. But, for better performance, the graphite structure was reorganized, leading to the complex operation, and the ICE was also reduced.

Table 2 Electrochemical performance of graphite by different recovery methods

Fig.7. Characterization and electrochemical performance of recycling graphite by different recovery methods.Direct recycling:(a)Reprinted with permission from Ref.[103],Copyright 2019,Elsevier.(b)Reprinted with permission from Ref.[133],Copyright 2018,Elsevier.And(c)reprinted with permission from Ref.[131],Copyright 2021,Elsevier.Recrystallization: (d) reprinted with permission from Ref. [19], Copyright 2020, Elsevier. Carbon coating: (e) reprinted with permission from Ref. [134], Copyright 2018,Elsevier. Combination: (f) reprinted with permission from Ref. [121], Copyright 2021, Royal Society of Chemistry.

4.2. Recycled graphene

In the charge and discharge process, the interlayer spacing of spent graphite was expanded, which weakened the interlayer van der Waals’ force, making it easier to form flake and sp2-hybridized carbon nanomaterials. Meanwhile, the attached oxygen-containing groups can prevent interlayer polymerization from occurring,and it was more conducive to the formation of graphite oxide [28,137-146].

Graphene was usually prepared by chemical and redox methods in Fig. 8. Hammers process was common preparation of graphene,Hammers process preparing graphitic oxide from graphite in what is essentially an anhydrous mixture of sulfuric acid,sodium nitrate,and potassium permanganate [147]. Yuet al.[137] reported the method recovered spent graphite into graphene by modified Hummers method. After breaking and screening, the graphite powder less than 75 microns was added to HNO3for 30 min by ultrasonic.Then, 2D graphene oxide can be prepared from spent graphite by the modified Hummers method. The modified Hummers method was that cryogenic mixed by adding H2SO4and KMnO4, then oxidated by mesothermal reaction, high-temperature reaction,cooling dilution and H2O2neutralization, finally, purified by vacuum drying and gradient centrifugation. The modified Hummers method can effectively remove these impurities and prepare multi-source graphite into graphene oxide with the same roasting degradation behavior, surface properties, crystal structure, and surface morphology.

Fig. 8. Characterization and property of recycling graphene by (a) Hummers method, reprinted with permission from Ref. [137], Copyright 2021, Elsevier. (b) Modified Hummers method,reprinted with permission from Ref.[146],Copyright 2018,Springer Nature.And(c)Hydrolysis and Micro-explosion,reprinted with permission from Ref.[142], Copyright 2021, Elsevier.

Zhaoet al.[146] proposed the recovery of soluble graphene nanoparticles from spent graphite. This method is to recover the spent graphite through the improved Hammers process to obtain graphene oxide sheets and by KOH and NaOH eutectic deoxidization, conduct high-temperature treatment to shrink the graphene surface, to produce good soluble graphene sheets.

The adding surfactant(sodium cholate)and ultrasound-assisted liquid-phase stripping method were used to prepare graphene oxide from spent graphite, and the peeling effect was 3-11 times that of natural graphite[144],and more than 60%of the recovered graphene had sizes over 1 mm and thicknesses less than 1.5 nm.

The main research method for preparing graphene from spent graphite was the Hummers process, which can oxidize the spent graphite into graphite oxide and remove the residual impurities in the process of charging and discharging. Then through ultrasonic, reduction, chemical, and other methods, exfoliated into a layer of graphene,and the effect of exfoliation was better than natural graphite[148,149].In the future,this will be a research direction of high-value utilization of spent graphite, but with a large amount of spent graphite, we needed more suitable industrial recycling methods.

4.3. Other materials

The high-value graphite in spent LIB anode materials cannot only be used in the battery field, but also many researchers have paid attention to the unique carbon structure,relatively pure components,and abundant surface functional groups in spent graphite,and then used in environmental protection and other electrochemical fields, and the spent graphite was recycled and processed into adsorption materials and other cathode materials [138,150-154].

Zhaoet al.[150]studied the adsorption of heavy metals in spent LIBs by recycling the anode material into a new adsorbent. In this study,the recycling graphite has the characteristics of large specific surface area and stable carbon structure and is used as the supporting carbon material for MnO2load. Firstly, the spent anode material was heat-treated to complete the separation of anode material and copper foil. Then, it was added to KMnO4solution,and MnO2loaded graphite was prepared by hydrothermal method.The removal effect of heavy metals in wastewater was analyzed by adsorption kinetics, adsorption, isotherm and adsorption amount.Lead, Cadmium, and Silver in the wastewater are removed by a load of graphite,and removal effects were 99.9%,79.7%,and 99.8%.

Caoet al.[151] found that the spent graphite had high-value application potential. They used different recovery methods to modify the graphite and prepared the recovered graphite into cathode materials of the electro-Fenton method to degrade organic pollutants in water. The recovered graphite was leached by HCl and NaOH, and the obtained acid-leached graphite and Alka-leached graphite have double electron oxygen reduction characteristics.Then the two kinds of recovered graphite were put into the electrochemical Fenton system to study the removal effect of bisphenol A from wastewater. In the electrochemical Fenton system, the removal rate of the acid-leached graphitic cathode was 100%within 70 min,and the performance did not decrease significantly after 10 cycles of repeated use.Other studies had treated spent graphite with surface modification, doping, calcination, and other modifications to treat phosphate [152,153], methylene blue[138], or heavy metal ions [154] in wastewater.

5. Policy and Regulations

The development of lithium-ion battery recycling is related to policies and regulations. The policies and regulations had come on the developed countries about spent LIBs recycling, which enhanced the value of LIBs. In the U.S, the regulations of echelon utilization and recycling were issued by the Environmental Protection Agency(EPA) and Battery Council International(BCI), which reduced the pollution of spent LIBs while improving that of subsequent value [155,156].

In China,‘‘ Interim Measures for the Management of Recovery and Reutilization of Batteries of New-Energy Vehicle”and‘‘Interim Regulations on Traceability Management for Recycling of Power Battery of New Energy Vehicles” were issued in 2018, which proposed the priority principle of ‘‘echelon utilization and recycling”,and the responsibility system of the Energy Vehicles(EV) batteries factory and battery enterprise was stipulated[157,158].In fact,the standard of LIBs recycling were released in 2008, such as ‘‘Recycling and Treatment Requirements of Lithium-Ion Battery for Telecommunication”(GB/T 22425-2008). The disposal of spent lithium-ion batteries has been strictly regulated,with direct incineration and landfills banned. The disassembly, crushing, and sorting processes were regulated before the metal materials of recovery.And,for preventing electrolyte leakage,a closed recovery system was required. Meanwhile, the three wastes (the liquid waste in the recycling of hydrometallurgy, and the waste residue and waste gas in pyrometallurgy) should be treated innocently.Subsequently, the ‘‘Methods for disposal and recycling of lithium ion battery material wastes” (GB/T 33059-2016) was drafted in October 2016.The mainly detailed recovery standard of acid leaching was proposed, focusing on recycling of metal material. Then,aiming at anode material recycling, the ‘‘Technical specification for recovery of anode materials for lithium-ion batteries-Graphite category”(T/SPSTS 004-2018)was published in the industry,which stipulated the recycling of graphite and copper material. Wherein the standard of recycling graphite referred to ‘‘Graphite negative electrode materials for lithium ion battery” (GB/T 24533-2009),according to preparation of electrode graphite. More mature policies and regulations will be proposed to make the lithium-ion battery recycling industry more advanced.

6. Conclusions and Prospects

In this paper,the recycling process of spent LIBs anode graphite material was summarized. The main products of spent graphite recovery include closed-loop recycling material, nano-materials such as graphene and adsorption material carrier. With the research on spent graphite recycling in recent years,spent graphite recycling has made great progress. However, in the follow-up research, it is necessary to combine industrial recycling methods,shorten the recycling process steps, and combine them with the recycling of cathode materials, so as to make the recycling more economic, effective, practical and environmental. Another direction for spent graphite recycling may change the application of spent graphite in sodium ion batteries, potassium ion batteries,nanomaterials such as graphene and expanded graphite. Through the closed-circuit cycle of spent graphite in the field of LIBs, and the extended application of high value-added materials, the clean recycling of graphite in spent LIBs will be realized.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2019YFC1907804 and 2019YFC1907801), National Natural Science Foundation of China (51904340) and Natural Science Foundation of Hunan(2020JJ4733), Outstanding Youth Fund Project of Hunan Natural Science Foundation(2011JJ20066).