There was research on the use of hammer crushing, vibrating screening and air separation combined process to achieve direct recovery of copper and carbon powder [30]. Subsequently, reference [31] used diesel as the collector and methyl isobutyl methanol as the foaming agent to conduct an experimental study on the flotation of graphite anode, and the recovery rate could reach 98.56%. Then, they reported a series of studies on the recovery of LiCoO₂ and graphite by flotation. First, the Fenton additive flotation method was used, and the H₂O₂/Fe²⁺ was 40/280 and the liquid-solid ratio was 99% [32]. The electrode material was modified under the optimum parameters of 25/100, and then separated by flotation, the recovery rate of LiCoO₂ reached 98.99%. The team then studied grinding flotation technology. The wettability of LiCoO₂ and graphite was different by grinding. The concentrate grades of LiCoO₂ and graphite after flotation were 97.13% and 73.56%, respectively, and the recoveries were 49.32% and 73.56%, respectively [33]. In addition, the organic binder can be removed by the pyrolysis-ultrasonic-assisted method, which increases the recovery rate of LiCoO₂ from 74.62% to 93.89% [34]. A schematic diagram of the recovery of this method is shown in Fig. 4. The flotation method realizes the simultaneous recovery of LiCoO₂ positive electrode and graphite negative electrode material, which simplifies the recovery process, is simple to operate, efficient, and has little pollution. However, the graphite recovered by this method contains many impurities, and the purity is difficult to meet the requirements of commercial graphite negative electrode materials (battery 9% and grade graphite mass fraction higher than 99.9%).

The prelithiated graphite was obtained by removing the organic electrolyte and the binder polyvinylidene fluoride (PVDF) with pyrrolidone (NMP), respectively. The first cycle capacity loss of the prelithiated graphite anode is reduced by about 10% (compared to the unused graphite anode) due to the retention of lithium-containing species that reduces the consumption of Li⁺ during the formation of the solid electrolyte interface (SEI) film. This method opens up a new avenue for the recycling of graphite anode materials [35].

To reduce the consumption of chemicals and energy, reference [36] proposed a simple, green water treatment method to recover graphite from spent LiFePO₄/graphite batteries. Due to the self-discharge during battery cycling, the corrosion of hydrofluoric acid, the dissolution of transition metal elements in the positive electrode and the deposition on the negative electrode material, the composition of the SEI film on the waste graphite negative electrode is very complicated [37,38]. Through water treatment, the residual Li in graphite reacts with water to generate H₂, which can separate the complex SEI film from graphite. Removal of the impurity results in the opening of Li⁺ transport channels, thereby restoring the electrochemical activity of graphite (345 mAhg⁻¹ capacity after 100 cycles). Water treatment technology does not require high-temperature operation, nor does it consume harmful chemicals, which is economical and environmentally friendly. In addition, the filtrate of water treatment can be recycled until the dissolved lithium salt is saturated, and then the lithium salt is recovered by precipitation method. However, this technique does not consider the removal of toxic binders such as polyvinylidene fluoride (PVDF).

Since the surface morphology and composition of graphite anodes change with the number of cycles, reference [39] prepared reconstituted graphite with sp²+ sp³ carbon surface by microwave exfoliation and reconstituted graphite on carbon surface (RG) method and neural network-like amorphous sp²+ sp³ carbon-coated graphite (AC@G). The results show that surface reforming is beneficial to improve the capacity of modified graphite, and the reversible capacities of RG and AC@G after 100 cycles at 0.1 C are 409.7 and 420.0 mA⋅h⋅g⁻¹, respectively. However, it is worth mentioning that due to the presence of surface activated carbon, the voltage plateau of AC@G increases sharply when the capacity is close to 300 mAhg⁻¹, resulting in a higher average discharge potential. Moreover, the porous structure will prevent a part of the intercalated lithium from being extracted, resulting in a decrease in the Coulombic efficiency in the first week.

Reference [40] took a novel, green approach to efficiently regenerate graphite, which consists of two key steps. The recovered anode material was first heat-treated in air for 1 h to remove the residual conductive agent acetylene black (AB), binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC), and then used phenolic resin The graphite surface was coated with 6.885% pyrolytic carbon at 950°C under N₂ atmosphere. The graphite coated with pyrolytic carbon exhibited an initial charge capacity of 347.2 mAhg⁻¹ and a coulombic efficiency of 92.07%, and all technical indicators exceeded those of the same type of mid-grade graphite, satisfying requirements for reuse. In addition, the method has simple equipment and high quality of recovered graphite, which is expected to realize large-scale application.

Reference [41] successfully regenerated pure graphite from waste lithium-ion batteries through a simple high-temperature smelting and screening process, as shown in Fig. 5. First, the negative electrode sheet was reacted at 1400°C for 4 h under N₂ atmosphere to melt the copper foil into spherical particles and separate from graphite. Then the copper and graphite are separated from each other by ultrasonic vibration and sieving, and 80.00% copper larger than 200 mesh and 77.53% graphite smaller than 300 mesh can be recovered. The purity of the recovered graphite is higher than 99.5%, and it has a higher specific capacity and a stable low-voltage discharge platform. However, this method consumes a lot of energy, and at a temperature of 1400°C, all the lithium salts are volatilized, resulting in a waste of precious lithium resources.

High-temperature heat treatment can effectively repair the structure of graphite and remove impurities, but this method has high energy costs, and the decomposition of polymers will produce toxic gases, and the recovered materials are single.

Studies have shown that the waste anode also contains lithium (30.07 mg g⁻¹), which is much higher than the environmental abundance, and most of them are composed of inorganic substances Li₂O, LiF, Li₂CO₃ and organic substances ROCO₂Li, CH₃OLi, (the form of ROCO₂Li)₂ exists in the SEI film. A small part exists in the graphite voids in the form of Li elemental substance. Among them, Li₂O, ROCO₂Li and CH₃OLi are water-soluble, while other substances are almost insoluble in water [42]. Based on this behavior, the research developed a hydrochloric acid leaching process to recover lithium in the anode. The results show that under the experimental conditions of 3 mol.L⁻¹ hydrochloric acid, 80°C temperature, 20 g.L⁻¹ solid-liquid ratio, and 90 min reaction time, the leaching efficiency of lithium is as high as 99.4%. Recently, Reference [43] used sulfuric acid solution as the leaching agent to study the effect of sulfuric acid concentration, leaching time, and leaching temperature on the leaching of lithium from the anode sheet and the separation of graphite and copper foil. The results show that when the leaching time is 5 min and the sulfuric acid concentration is 0.9 mol.L⁻¹, the copper foil and graphite can be completely separated. When the leaching temperature is 40°C, the sulfuric acid concentration is 1.8 mol.L⁻¹, the leaching time was 50 min and the solid-liquid ratio was 60 g.L⁻¹, and almost all the lithium in the graphite entered the solution.

The hydrometallurgical process has a low operating temperature and can effectively recover lithium salts in the negative electrode. However, due to the presence of insoluble lithium salts such as LiF, the process consumes a large amount of strong acids (sulfuric acid, hydrochloric acid) and produces more toxic hydrofluoric acid. Therefore, an effective solution for hydrometallurgical recycling is to combine the recycling of positive and negative electrodes, which can greatly simplify the recycling process and reduce secondary pollution caused by waste acid.

Reference [44] removed all residual cathode material, other metal impurities, most binders and alumina through sulfuric acid leaching and molten NaOH calcination steps, and regenerated high-quality graphite powder, which could reach 377.3 at 0.1 C The reversible specific capacity of mA⋅h⋅g⁻¹. This process merges the negative electrode recycling into the positive electrode recycling process, which greatly reduces the cost of battery separation and simplifies the recycling process. In addition, reference [45] adopted a two-step calcination and acid leaching method to achieve the recovery of graphite, Cu, Li and Al. Under 1.5 mol⋅L⁻¹ hydrochloric acid, S/L atomic ratio of 100 g⋅L⁻¹ and 1 h leaching time, 100% Cu, Li and Al can be leached out. By adjusting the pH value from 7 to 9, 99.9% Cu and Al can be extracted. After that, more than 99% of Li was recovered by adding Na₂CO₃ by precipitation method. The regenerated graphite also has excellent electrochemical properties. This technology is a sustainable method to achieve anode graphite regeneration and recovery of Li and Cu. Reference [46] removed the impurities by two-step sulfuric acid leaching and then calcined at 1500°C to obtain regenerated graphite. The schematic diagram of recovery is shown in Fig. 6. At high temperature, graphite is reduced to its original structure with a mass fraction of up to 99.6%, and exhibits an initial charge capacity of 349 mA h g⁻¹. The generated waste acid filtrate can be fed into the process of hydrometallurgical recovery of cathode materials, reducing secondary pollution and realizing lithium resource recovery.

The combination of heat treatment and hydrometallurgy is an economical and feasible technology to realize the efficient regeneration of graphite and recovery of lithium, and on the basis of solving the problem of waste acid pollution, this technology has great potential for commercialization in the future.

Another challenge for anode recycling is the disposal of the electrolyte. For digital waste lithium-ion batteries, most of the electrolyte is not recycled, and it is usually burned by fire. As a power source of lithium-ion batteries, the electrolyte accounts for about 15% of the battery cost, is rich in lithium ions, and has a high recovery value. Moreover, the commonly used electrolytes are generally carbonated organic solutions of LiPF6. In humid air, LiPF6 will react with water to generate harmful gas HF. Therefore, effective recovery of electrolyte not only has certain economic benefits, but also reduces harmful gas emissions.

Reference [47] used three different methods to remove electrolytes, namely direct thermal treatment, subcritical CO₂ and acetonitrile extraction, and supercritical CO₂ extraction. The results show that 90% of the electrolyte (including conductive salts) can be recovered by extraction with subcritical CO₂ and acetonitrile. However, the crystallinity of graphite decreases after extraction, which adversely affects its electrochemical performance as a negative electrode.

Reference [48] proposed a clean electrochemical method to “kill two birds with one stone” to recover graphite and copper foils from Li-ion batteries, and studied the effects of various parameters (voltage, inter-electrode distance and electrolyte concentration) on the electrolysis process. The schematic diagram of the recovery is shown in Fig. 7. The results show that under the optimum conditions of a pole distance of 10 cm, a concentration of Na₂SO₄ electrolyte of 1.5 g⋅L⁻¹ and a voltage of 30 V, the complete separation of copper foil and graphite can be achieved within 25 min of electrolysis. Li⁺ in the electrolyte can be further recovered by precipitation. In addition, the authors conducted an economic evaluation of this method, estimating a profit of approximately $10 per kg of lithium-ion battery anode material processed. However, due to the presence of binder PVDF and conductive agent, the mass fraction of recovered graphite (∼95%) needs to be improved.

Most of the recovered graphite is recycled as a negative electrode material for lithium-ion batteries. Table 1 shows the electrochemical properties of the recovered graphite in the literature reported in the past. The research shows that the recovered graphite maintains a good crystal structure, and the initial capacity can basically meet the requirements of reuse, but whether the recovered graphite meets the commercial standards depends on the particle size, density, specific surface area, purity, and the first week Coulomb efficiency, charge and discharge of the material, voltage platform, cycle stability and other aspects are comprehensively evaluated.

Reference [49] used mechanical treatment and sulfuric acid leaching process to recover anode materials and applied them to Li-ion batteries/capacitors. The results show that the recovered carbonaceous materials can achieve energy densities of 313 and 112 W⋅h⋅kg⁻¹ in Li-ion full batteries and Li-ion capacitors, respectively. However, due to the existence of disordered structure, the Coulombic efficiency of the recovered carbonaceous materials in the first week of Li-ion half cells is only 50%. Residual conductive agents, binders and thickeners can be removed by heat treatment in air, and the surface of graphite can be coated with pyrolytic carbon to improve the coulombic efficiency of the first week. The authors adopted this method to recover the coulombic efficiency of graphite. increased to 92.1%. In addition, Reference [50] proposed a water treatment-high-temperature calcination method to use the recovered graphite as the anode material for double carbon-lithium ion capacitors. It is calculated that the maximum energy density of recycled graphite reaches 185.54 W⋅h⋅kg⁻¹ under the power density of 0.319 kW⋅kg⁻¹ and the capacity retention rate is 75% after 2000 continuous cycles at 10°C and 25°C. This work opens up the possibility of recycling graphite as an electrode material for high-energy storage devices.

Reference [51] recovered the graphite anode material by a simple heat treatment method and reused it as a sodium/potassium ion battery anode. Under the current density of 0.2 A⋅g⁻¹, the reversible sodium storage capacity of the recovered graphite is 162 mA⋅h⋅g⁻¹, and even at 2 A⋅g⁻¹, the capacity retention rate after 1000 cycles is still as high as 94%. .6%. At a current density of 0.05 A⋅g⁻¹, a potassium storage capacity of 320 mA⋅h⋅g⁻¹ can be achieved. Recently, Reference [52] reported the application of graphite recovered by one-step acid treatment in Na-ion batteries. At the current density of 0.05 A⋅g⁻¹, the recovered graphite has a high reversible capacity of 127 mA⋅h⋅g⁻¹. In addition, it has an excellent long cycle life, with a reversible capacity of 106.8 mA⋅h⋅g⁻¹ maintained after 500 cycles at a current density of 2 A⋅g⁻¹. Applying recycled graphite to sodium/potassium-ion batteries not only solves the problems caused by waste lithium-ion batteries. The problem of waste pollution can also promote the development of next-generation batteries.

Reference [53] recovered graphite from spent lithium-ion batteries and explored its potential as a cathode for aluminum-ion batteries. Due to the large interlayer spacing of the recycled graphite, the intercalation/extraction of Al ions is effectively promoted during the charging and discharging process, so at a current density of 50 mA⋅g⁻¹, the Al ion storage capacity reaches 124 mA⋅h⋅g⁻¹, even after 6700 cycles at a high rate of 300 mA⋅g⁻¹, the capacity retained 81% of its initial capacity. This excellent aluminum ion storage property makes recycled graphite a promising cathode material, providing a feasible solution for the recycling of large amounts of graphite anode waste.

Due to the excellent properties of graphene in many aspects such as optics, electricity, mechanics, and heat, recycled graphite is often used to prepare graphene. Reference [54] prepared graphene by oxidizing graphite by Hummers method. Since the oxygen groups between the waste graphite layers can promote the oxidation reaction, the consumption of H₂SO₄ and KMnO₄ is reduced by 40% and 28.6%, respectively, but a large amount of toxic N₂H₄⋅H₂O is still required as a reducing agent. Therefore, Reference [55] used waste lithium-ion battery packaging materials (i.e., Al and stainless steel) as reducing agents to prepare reduced graphene oxide (rGO) in the presence of HCl, and studied the application of graphene in supercapacitors. At 0.5 A⋅g⁻¹, the material exhibits a specific capacitance as high as 112 F⋅g⁻¹. In another study, they also used Al reduction to prepare rGO, and transformed the separator into carbon hollow spheres (CHS). Under the conditions of 1.5 MPa and −196°C, the absorption of H₂ by rGO and CHS were 1.78% and 1.22% (mass fraction, the same below), respectively. The CO₂ uptake was 12% and 33%, respectively [56]. This study confirms the possibility of transforming the recycled separator and graphite anode into materials for efficient storage of H₂ and CO₂, providing a new idea for full-component recovery of Li-ion batteries.

Although the redox method is very effective, it consumes a large amount of strong oxidants (H₂SO₄, KMnO₄) and reducing agents (N₂H₄⋅H₂O), which not only causes environmental pollution, but also severely damages the crystal structure of graphene. Therefore, Reference [57] prepared graphene by exfoliating graphite in sodium cholate aqueous solution by ultrasonic-assisted liquid-phase exfoliation. Due to the weakening of the interlayer force after graphite cycling, the exfoliation efficiency of waste graphite is 3 to 11 times that of natural graphite, and more than 60% of the graphene flakes are larger than 1 mm in size and less than 1.5 nm in thickness. The conductivity of graphene is as high as 9100 Sm⁻¹, which can be applied to conductive ink. Reference [58] prepared graphene by combining H₂SO₄ leaching and shear mixing. The results show that battery cycling and acid treatment can expand the graphite lattice, thereby increasing the graphene yield to 10 times that of pristine graphite. This process can be seamlessly embedded into the current cathode recycling process. In addition, the authors further explored the effect of graphene on the mechanical properties of epoxy resins. Compared with the original epoxy resin, the flexural strength (122.5 MPa), elastic modulus (3.6 GPa) and toughness (2269.9 kJ⋅m⁻³) of the graphene/epoxy composites were improved, respectively 26.0%, 38.5% and 13.0%. This is mainly attributed to the functional groups generated on the graphite surface during battery cycling, resulting in better dispersion and stronger bonding between graphene and epoxy resin.

Recycled graphite also shows good prospects in the preparation of composite materials. The authors recovered and used graphite and separator materials (polyethylene, polypropylene) to synthesize polymer-graphite nanocomposite films. Compared with pure polymer films, the resulting nanocomposite films exhibited a 10-fold increase in tensile strength and a 5–6 orders of magnitude increase in conductance. In addition, they conducted a series of studies using waste anode carbon materials as adsorbents for wastewater treatment. They decorated the surface of waste graphite with nanostructured Mg(OH)₂ and explored its ability to adsorb phosphate in water. The obtained material exhibited a high phosphate adsorption capacity of 588.4 mg⋅g⁻¹ [59]. Similarly, the team fabricated Mg-rich carbon nanocomposites from waste graphite. The maximum phosphate adsorption capacity of the material reached 406.3 mg⋅g⁻¹. The adsorption mechanism is to remove phosphate through phosphorus precipitation on the surface to form MgHPO₄⋅1.2H₂O and Mg₃(PO₄)₂⋅8H₂O nanocrystals [60]. The team also prepared MnO₂-modified graphite adsorbents for the adsorption of heavy metal ions Pb²⁺, Cd²⁺ and Ag⁺ in wastewater. The results showed that the removal rates of Pb²⁺, Cd²⁺ and Ag⁺ were 99.9%, 79.7% and 99.8%, respectively [61]. The above research provides an economical and environmentally friendly method for waste recycling and heavy metal pollution treatment.