A method for modifying waste graphite materials, the resulting modified graphite materials, and their applications.

Abstract

This invention relates to a method for modifying waste graphite materials, the resulting modified graphite materials, and their applications. The method includes the following steps: (1) mixing a transition metal compound and a carbon material with a solvent to obtain a composite interface layer precursor; (2) adding purified graphite to the composite interface layer precursor obtained in step (1) for surface coating, heating, and forming a gel; (3) subjecting the gel obtained in step (2) to drying, grinding, and heat treatment sequentially to obtain a modified graphite material; the carbon material includes phenolic resin. This invention uses a sol-gel method for coating. The composite interface layer coated recycled graphite anode material is based on carbon coating, with the addition of transition metal oxides, and is uniformly dispersed on the particle surface by the sol-gel method. This preparation method is simple and safe, and the modified graphite material obtained has high and stable capacity performance as an anode material for lithium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium battery recycling technology, and in particular to a method for modifying waste graphite materials, the resulting modified graphite materials, and their applications. Background Technology

[0002] Lithium-ion batteries (LIBs) have become the mainstream technology in the energy storage field due to their advantages such as low cost, low self-discharge rate, no memory effect, and good environmental adaptability. However, with the explosive growth in global demand for lithium-ion batteries, the accumulation of waste batteries poses a serious challenge to the environment. Graphite, as the core material of the LIB anode, accounts for approximately 15% to 20% of the total battery mass. Its efficient recycling is beneficial to conserving natural resources and achieving sustainable development.

[0003] Under the repeated insertion and extraction of lithium ions, the surface structure of decommissioned graphite ages, breaks down, and peels off, exposing numerous defect sites. During the first charge-discharge cycle, the electrolyte decomposes severely, forming a thick solid electrolyte interphase (SEI) layer. A large number of lithium ions are consumed in this irreversible reaction, leading to a significant decrease in initial coulombic efficiency. The unstable SEI that continuously grows on the damaged graphite surface not only results in low coulombic efficiency during cycling but also hinders lithium ion transport kinetics and exacerbates the degradation and aging process of electrochemical performance.

[0004] The prior art CN110137469A discloses a method for regenerating graphite anode material from waste lithium-ion batteries, including the following steps: (1) sintering the waste graphite anode material under a protective atmosphere to remove conductive agents, binders, and thickeners; (2) mixing the sintered waste graphite anode material with an inorganic acid solution, and obtaining graphite powder after filtration, washing, and drying; (3) mixing graphite powder, an inorganic titanium source, urea, and / or ammonia water, and obtaining a graphite anode material uniformly coated with titanium dioxide after calcination under a protective atmosphere.

[0005] Traditional recycling methods (such as high-temperature pyrolysis and hydrometallurgy) can separate graphite from impurities, but they struggle to repair microscopic damage, resulting in significantly lower electrochemical performance of recycled graphite compared to virgin commercial graphite. To improve the failed structure and protect graphite particles, a carbon layer or transition metal oxide layer is typically coated onto the surface of the recycled graphite particles to enhance structural stability and stabilize interfacial ion transport pathways. Previous repair methods primarily relied on carbon coating, using the mechanical strength of the carbon layer to protect the graphite particle structure and inhibit electrolyte ingress into the graphite, thus preventing side reactions. However, coated graphite materials often fail to achieve excellent initial coulombic efficiency because the surface carbon layer cannot inhibit SEI growth during cycling. Excessive SEI layer growth leads to capacity loss. Furthermore, an excessively thick or uneven coating layer can increase the particle surface area, reducing the contact between the graphite structure and the electrolyte, lowering lithium-ion insertion / extraction efficiency, and forming an unstable SEI film that causes irreversible capacity decay.

[0006] In summary, developing a new method for modifying waste graphite materials has become an urgent problem to be solved. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for modifying waste graphite materials, the resulting modified graphite materials, and their applications. Waste graphite materials are recycled through a composite interface layer composed of carbon materials and transition metal oxides. The precursor materials are synthesized using a sol-gel method, ensuring uniform coating. The resulting modified graphite material, as a negative electrode material for lithium-ion batteries, exhibits high and stable capacity performance.

[0008] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for modifying waste graphite materials, the method comprising the following steps: (1) The transition metal compound and carbon material are mixed evenly with a solvent to obtain a composite interface layer precursor; (2) The purified graphite is added to the composite interface layer precursor described in step (1) for surface coating, and heated to form a gel; (3) The gel obtained in step (2) is dried, ground and heat-treated in sequence to obtain modified graphite material; The carbon material includes phenolic resin.

[0009] The present invention provides a method for modifying waste graphite materials. A transition metal compound and carbon material are uniformly mixed with a solvent, which provides a liquid environment to enhance mixing efficiency, resulting in a composite interface layer precursor. Purified graphite is added to the composite interface layer precursor for surface coating, allowing the precursor to uniformly adhere to the surface of graphite particles. Heating is then performed to form a gel. The gel is subsequently dried, ground, and heat-treated to obtain surface-repaired modified graphite. Phenolic resin, acting as a carbon source for the composite interface layer precursor, also functions as a thickener, helping the solution form a sol at high temperatures. Subsequently, [the process is repeated]. Solvent evaporation forms a gel, and phenolic resin is thermally decomposed and carbonized through heat treatment to form an amorphous carbon coating layer. Transition metal oxides can enhance the chemical stability of the artificial interface layer. Through the synergistic effect of the two, the structure of graphite is sufficiently repaired at high temperature. In addition, the present invention uses the sol-gel method for coating, which forms a uniform distribution at the molecular level in a short time, so that the reactants achieve complete mixing at the molecular level. At the same time, this method can also ensure that the precursor is uniformly coated on the surface of graphite particles, forming a thin and dense interface layer, which improves its electronic conductivity and increases the ion transport rate at the interface.

[0010] In this invention, the metal elements in the transition metal compound include titanium and niobium.

[0011] Preferably, the titanium element is derived from tetrabutyl titanate, and the niobium element is derived from niobium pentachloride.

[0012] Preferably, when the metal elements in the transition metal compound are titanium and niobium, the molecular weight ratio of niobium pentachloride and tetrabutyl titanate is (1-3):1, for example, it can be 1:1, 1.2:1, 1.5:1, 1.7:1, 2:1, 2.2:1, 2.5:1, 2.7:1 or 3:1, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0013] Preferably, when the metal elements in the transition metal compound are titanium and niobium, the transition metal oxides formed after heat treatment include TiNbO4 and / or TiNb2O7.

[0014] In this invention, phenolic resin is converted into amorphous carbon after heat treatment.

[0015] In this invention, transition metal oxides TiNbO4 and TiNb2O7 are used. These materials have good ionic conductivity, which can improve the ion transport rate at the interface. They can also be used as an artificial SEI layer to reduce the loss of first coulombic efficiency caused by hard carbon coating.

[0016] As a preferred technical solution of the present invention, the solvent in step (1) includes ethanol.

[0017] Preferably, the mixing temperature in step (1) is 25-50°C, for example, it can be 25°C, 30°C, 35°C, 40°C, 45°C or 50°C, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] Preferably, the carbon material accounts for 2-5% of the mass of the purified graphite, for example, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8% or 5%, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 2-3%.

[0019] Preferably, the transition metal compound accounts for 1-10% of the mass content of the purified graphite, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5% or 10%, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0020] As a preferred technical solution of the present invention, the method for obtaining purified graphite in step (2) includes: pretreating waste graphite material with organic solvent and then reacting it with mixed alkali, and finally obtaining purified graphite by acid washing and oxidation treatment.

[0021] The method for obtaining purified graphite in step (2) specifically includes: adding waste graphite material into N-methylpyrrolidone (NMP) for filtration and separation, then repeatedly washing and drying with acetone, then adding it to a mixed alkali (KOH:NaOH=5:1) and heating at 180-300℃ for 0.5-8h in an air atmosphere. After the reaction is completed, rinsing with deionized water and drying are performed. Finally, the alkali-treated sample is added to a 2mol / L solution of H2SO4 (60mL) and H2O2 (40mL) and stirred at 60℃ for 2h. After leaching and filtration, the purified graphite is obtained by rinsing with deionized water and drying.

[0022] As a preferred technical solution of the present invention, step (2) further includes a step of impurity removal and shaping before adding the purified graphite to the composite interface layer precursor.

[0023] Preferably, the particle size of the waste graphite after impurity removal and shaping is 10-20 μm, for example, it can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, the heating temperature in step (2) is 60-90℃, for example, it can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃ or 90℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the heating time in step (2) is 8-15 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours or 15 hours, but not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] As a preferred technical solution of the present invention, the drying temperature in step (3) is 60-80℃, for example, it can be 60℃, 65℃, 70℃, 75℃ or 80℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the drying time in step (3) is 2-4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] As a preferred technical solution of the present invention, the temperature of the heat treatment in step (3) is 600-1000℃, for example, it can be 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, 950℃ or 1000℃, but is not limited to the listed values. Other unlisted values ​​in this range are also applicable.

[0029] Preferably, the heat treatment holding time in step (3) is 1-5h, for example, it can be 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h or 5h, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] This invention utilizes heat treatment to pyrolyze and carbonize phenolic resin to form an amorphous carbon coating layer. Transition metal oxides enhance the chemical stability of the artificial interface layer. Through the synergistic effect of these two components, the overall electrochemical performance of graphite particles is improved. The amorphous carbon coating layer is tightly bonded to the graphite material, resulting in better electronic conductivity and increased ion transport rate at the interface. It can also serve as an artificial SEI layer to reduce the loss of initial coulombic efficiency caused by hard carbon coatings. Without heat treatment, phenolic resin exists in a polymeric form and is non-conductive, which severely hinders electron and ion transport, leading to extreme battery polarization, inability to achieve full capacity, failure to form an effective protective layer, complete exposure of the graphite surface, uncontrollable SEI layer formation, and poor battery cycle performance.

[0031] In this invention, by limiting the heat treatment temperature and time, the formation of the composite interface coating layer and graphite structure is ensured, and the size of the oxide particles is controlled within 1-10 nm.

[0032] Preferably, the heat treatment in step (3) is carried out under an inert atmosphere.

[0033] Preferably, the inert atmosphere includes nitrogen or argon, with argon being the preferred gas.

[0034] In this invention, heat treatment is performed under an inert atmosphere to protect the graphite particles from oxidation.

[0035] In a second aspect, the present invention provides a modified graphite material prepared according to the method described in the first aspect.

[0036] Thirdly, the present invention provides an application of the modified graphite material described in the second aspect, wherein the modified graphite material is used as an active material in the field of lithium-ion battery technology.

[0037] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In the modification method of the present invention, the carbon source used in the composite interface layer precursor is phenolic resin. Compared with the asphalt used in the traditional carbon coating method, phenolic resin has stronger adhesion and can enhance the adhesion to the graphite surface. After high temperature heat treatment, phenolic resin is transformed into hard carbon material, which has better rate performance than soft carbon material. (2) The coating method of the present invention uses the sol-gel method for coating. Through a simple operation method, the characteristics of low viscosity solution are used to form a uniform distribution at the molecular level in a short time, so that the reactants can achieve complete mixing at the molecular level. At the same time, this method can also ensure that the precursor is uniformly coated on the surface of graphite particles to form a thin and dense interface layer. (3) The present invention adds transition metal compounds on the basis of carbon coating. The resulting TiNbO4 and TiNb2O7 have good ionic conductivity, which can improve the ion transport rate at the interface. They can also be used as artificial SEI layers to reduce the loss of first coulombic efficiency caused by hard carbon coating. (4) When the modified graphite material obtained in this invention is used as an active material to make a half-cell, the initial specific capacity is ≥331mAh / g, and the maximum is 370mAh / g. The first coulombic cycle efficiency is ≥79, and the maximum is 90%. Attached Figure Description

[0038] Figure 1 These are SEM images of the modified graphite material prepared in Example 1 of this invention before and after modification; Where a and c are particles before modification photographed at different magnifications; b and d are particles after modification photographed at different magnifications. Detailed Implementation

[0039] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0040] Example 1 This embodiment provides a method for modifying waste graphite materials and the resulting modified graphite materials, such as... Figure 1 As shown, the method includes the following steps: (1) Dissolve 0.162 g of niobium pentachloride in 100 mL of ethanol and stir to form solution A. Dissolve 0.102 mL of tetrabutyl titanate in 10 mL of ethanol and stir to form solution B. Mix solutions A and B evenly, then add 0.0682 g of phenolic resin and mix evenly at 40 °C to obtain the composite interface layer precursor. (2) 3.46g of purified graphite was poured into the composite interface layer precursor for surface carbon coating and heated at 80°C until a gel was formed. (3) The gel described in step (2) is dried at 80°C for 2 hours to form a dry gel. After grinding, it is placed in a tube furnace for heat treatment while argon gas is introduced. The calcination temperature is 800°C and the holding time is 3 hours to obtain the modified graphite material. from Figure 1 It can be seen that, Figure 1 Image 'a' is a scanning electron microscope image of the particles before regeneration and repair; the particles are of uneven size. Figure 1 b shows that the particle size remains uniform after regeneration and repair; Figure 1 c shows obvious cracks and damage on the material surface before regeneration and repair; Figure 1 The image shows that the surface structure of the regenerated and repaired particles remains unchanged, and they are uniformly coated with amorphous carbon and nano-TiNbO4 and TiNb2O7 particles.

[0041] Example 2 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The method includes the following steps: (1) Dissolve 0.0216 g of niobium pentachloride in 70 mL of ethanol and stir to form solution A. Dissolve 0.0272 mL of tetrabutyl titanate in 6 mL of ethanol and stir to form solution B. Mix solutions A and B evenly, then add 0.15 g of phenolic resin and mix evenly at 25 °C to obtain the composite interface layer precursor. (2) Pour 5g of purified graphite into the composite interface layer precursor for surface carbon coating, and heat at 60°C until a gel is formed. (3) The gel described in step (2) is dried at 70°C for 3 hours to form a dry gel. After grinding, it is placed in a tube furnace for heat treatment while argon gas is introduced. The calcination temperature is 600°C and the holding time is 5 hours to obtain the modified graphite material.

[0042] Example 3 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The method includes the following steps: (1) Dissolve 0.468 g of niobium pentachloride in 200 mL of ethanol and stir to form solution A. Dissolve 0.408 mL of tetrabutyl titanate in 15 mL of ethanol and stir to form solution B. Mix solutions A and B evenly, then add 0.5 g of phenolic resin and mix evenly at 40 °C to obtain the composite interface layer precursor. (2) 10g of purified graphite was poured into the composite interface layer precursor for surface carbon coating and heated at 90°C until a gel was formed. (3) The gel described in step (2) is dried at 60°C for 4 hours to form a dry gel. After grinding, it is placed in a tube furnace for heat treatment while argon gas is introduced. The calcination temperature is 1000°C and the holding time is 1 hour to obtain the modified graphite material.

[0043] Example 4 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Embodiment 1 is that the amount of phenolic resin used in step (1) is 0.034g (1%), and the rest is the same as in Embodiment 1.

[0044] Example 5 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Embodiment 1 is that the amount of phenolic resin used in step (1) is 0.204g (6%), and the rest is the same as in Embodiment 1.

[0045] Example 6 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that the amounts of niobium pentachloride and tetrabutyl titanate in step (1) are adjusted to 0.0104g and 0.00657mL (0.5%), respectively. The rest are the same as in Example 1.

[0046] Example 7 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that the amounts of niobium pentachloride and tetrabutyl titanate in step (1) are adjusted to 0.25g and 0.159mL (12%), respectively, while the rest are the same as in Example 1.

[0047] Example 8 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Embodiment 1 is that the heat treatment temperature in step (3) is 500°C, while the rest is the same as in Embodiment 1.

[0048] Example 9 This embodiment provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Embodiment 1 is that the heat treatment temperature in step (3) is 1100℃, while the rest is the same as in Embodiment 1.

[0049] Comparative Example 1 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that the carbon material phenolic resin in step (1) is replaced with asphalt, and the rest is the same as in Example 1.

[0050] Comparative Example 2 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that the preparation process of the composite interface layer precursor in step (1) is replaced by dissolving the carbon source (asphalt) in a solvent (toluene) to obtain the coating layer precursor. The rest is the same as in Example 1.

[0051] Comparative Example 3 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Comparative Example 2 is that the modified graphite material prepared in Comparative Example 2 is subjected to secondary coating. The secondary coating is carried out in accordance with the method of Example 1 of this invention: 0.162g of niobium pentachloride is dissolved in 100mL of ethanol and stirred to form solution A, and 0.102mL of tetrabutyl titanate is dissolved in 10mL of ethanol and stirred to form solution B. Solutions A and B are mixed evenly to prepare a coating precursor solution, and the precursor solution is uniformly coated on the surface of the treated recycled graphite to form a secondary coating layer, thereby obtaining the final modified graphite material.

[0052] Comparative Example 4 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference between this example and Example 1 is that the heat treatment described in step (3) is not performed, while the rest is the same as in Example 1.

[0053] Comparative Example 5 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that the transition metal compound in step (1) is replaced with an alkaline earth metal compound, that is, niobium pentachloride and tetrabutyl titanate in step (1) are replaced with magnesium acetate. All other aspects are the same as in Example 1.

[0054] Comparative Example 6 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference between this example and Example 1 is that no transition metal compound is added in step (1), while the rest are the same as in Example 1.

[0055] Comparative Example 7 This comparative example provides a method for modifying waste graphite material and the resulting modified graphite material. The difference from Example 1 is that no transition metal compound is added in step (1). After the gel is obtained in step (2), the transition metal compound is mixed with the gel and then step (3) is performed.

[0056] Application Examples 1-9 The modified graphite materials prepared in Examples 1-9 were used as active materials. After adding conductive agent (Super P), binder (PVDF) and solvent (NMP) and stirring, the mixture was coated onto copper foil and dried in a vacuum oven at 80°C for 12 hours to make a half cell.

[0057] Application Comparative Examples 1-7 The modified graphite materials prepared in Comparative Examples 1-7 were used as active materials. After adding conductive agent (Super P), binder (PVDF) and solvent (NMP) and stirring, the mixture was coated onto copper foil and dried in a vacuum oven at 80°C for 12 hours to make a half cell.

[0058] Test method: The modified graphite materials obtained in Examples 1-9 and Comparative Examples 1-7 were subjected to half-cell cycle performance tests. The initial specific capacity and first coulombic cycle efficiency test results are shown in Table 1.

[0059] Table 1 The test results show that: (1) As can be seen from Examples 1 to 3, the present invention recycles waste graphite materials by coating a composite interface layer composed of carbon materials and transition metal oxides. By using the sol-gel method to synthesize precursor materials, the uniformity of coating is ensured. When the modified graphite material is used as an active material to make a half battery, the initial specific capacity reaches 370 mAh / g and the first coulombic cycle efficiency reaches 90%.

[0060] (2) As can be seen from Examples 1 to 4-7, the present invention can achieve higher battery cycle performance by further limiting the mass content of carbon materials and transition metal compounds in purified graphite.

[0061] (3) As can be seen from Examples 1 to 8 and 9, the present invention converts phenolic resin and titanium niobium compound precursors into amorphous carbon and titanium niobium oxide through heat treatment. It can also reorganize and repair the structure of graphite at high temperature, which can achieve better battery cycle performance. When the heat treatment temperature is too low, the resin carbonization is incomplete, the conductivity of the coating layer is extremely poor, the structure is loose, and the mechanical strength is low. When the temperature is too high, it may cause nanoparticles to agglomerate into micron-sized large particles, which will damage the surface uniformity of the coating layer. The coating layer becomes brittle due to excessive graphitization, and the battery performance will decline.

[0062] (4) As can be seen from Example 1 and Comparative Example 1, the present invention uses phenolic resin, which has stronger adhesion than asphalt and can enhance the adhesion to the graphite surface. After high-temperature heat treatment, phenolic resin is transformed into hard carbon material, which has better rate performance than soft carbon material. Asphalt forms a sticky intermediate product during high-temperature pyrolysis and eventually forms highly graphitized soft carbon through liquid phase carbonization.

[0063] (5) As can be seen from Example 1 and Comparative Examples 2 and 3, in Comparative Example 2, only asphalt was used as the coating layer precursor, and in Comparative Example 3, the modified graphite material obtained in Comparative Example 2 was coated again with transition metal oxide. Since the soft carbon coating layer formed by the transition metal oxide layer and the asphalt was not well compatible, the interface phase was uneven, resulting in a decrease in battery cycle performance compared with Example 1.

[0064] (6) As can be seen from Example 1 and Comparative Example 4, the present invention enhances the mechanical properties and conductivity of graphite particles by thermally decomposing and carbonizing phenolic resin to form an amorphous carbon coating layer. The transition metal oxide can enhance the chemical stability of the artificial interface layer. Through the synergistic effect of the two, the comprehensive electrochemical performance of graphite particles is improved. The amorphous carbon coating layer is tightly bonded to the graphite material, making its electronic conductivity better and increasing the ion transport rate at the interface. At the same time, it can also serve as an artificial SEI layer to reduce the loss of initial coulombic efficiency caused by the hard carbon coating. Without heat treatment, the resin exists in a polymeric form and is non-conductive, which will seriously hinder electron and ion transport, leading to battery polarization, inability to utilize capacity, failure to form an effective protective layer, complete exposure of the graphite surface, uncontrollable formation of the SEI layer, and poor battery cycle performance.

[0065] (7) As can be seen from Example 1 and Comparative Examples 5 and 6, when the transition metal compound is replaced with an alkaline earth metal compound in Comparative Example 5 or no transition metal compound is added in Comparative Example 6, due to the lack of chemical stability provided by the transition metal oxidation to the interface layer, the coating layer may produce more interface side reactions, resulting in an excessively thick SEI layer, which leads to a reduction in its first efficiency and a significant decrease in battery cycle performance. This shows the importance of the selection of coating layer metal materials in this invention.

[0066] (8) As can be seen from Example 1 and Comparative Example 7, when the transition metal compound and carbon material are not mixed evenly with the solvent to obtain the composite interface layer precursor, but are mixed sequentially, the battery cycle performance will also decrease. The reason may be that the pre-mixed precursor can obtain more uniform transition metal oxide particles through the sol-gel method, which significantly improves the uniformity of the artificial interface layer, thereby obtaining better comprehensive electrochemical performance.

[0067] In summary, this invention recycles waste graphite materials by coating them with a composite interface layer composed of carbon materials and transition metal oxides, and ensures the uniformity of the coating by using a sol-gel method to synthesize precursor materials.

[0068] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for modifying waste graphite materials, characterized in that, The method includes the following steps: (1) The transition metal compound and carbon material are mixed evenly with a solvent to obtain a composite interface layer precursor; (2) The purified graphite is added to the composite interface layer precursor described in step (1) for surface coating, and heated to form a gel; (3) The gel obtained in step (2) is dried, ground and heat-treated in sequence to obtain modified graphite material; The carbon material includes phenolic resin.

2. The method according to claim 1, characterized in that, The metallic elements in the transition metal compound include titanium and niobium; Preferably, the titanium element is derived from tetrabutyl titanate, and the niobium element is derived from niobium pentachloride; Preferably, when the metal elements in the transition metal compound are titanium and niobium, the molecular weight ratio of niobium pentachloride and tetrabutyl titanate is (1-3):

1. Preferably, when the metal elements in the transition metal compound are titanium and niobium, the transition metal oxides formed after heat treatment include TiNbO4 and / or TiNb2O7.

3. The method according to claim 1 or 2, characterized in that, The solvent in step (1) includes ethanol; Preferably, the mixing temperature in step (1) is 25-50°C; Preferably, the carbon material accounts for 2-5% of the mass of the purified graphite, more preferably 2-3%; Preferably, the transition metal compound accounts for 1-10% of the mass of the purified graphite.

4. The method according to any one of claims 1-3, characterized in that, The method for obtaining purified graphite in step (2) includes: pretreating waste graphite material with organic solvent and then reacting it with mixed alkali, and finally obtaining purified graphite by acid washing and oxidation treatment.

5. The method according to any one of claims 1-4, characterized in that, Step (2) before adding the purified graphite to the composite interface layer precursor also includes a step of impurity removal and shaping; Preferably, the particle size of the waste graphite after impurity removal and shaping is 10-20 μm.

6. The method according to any one of claims 1-5, characterized in that, The heating temperature in step (2) is 60-90℃; Preferably, the heating time in step (2) is 8-15 hours.

7. The method according to any one of claims 1-6, characterized in that, The drying temperature in step (3) is 60-80℃; Preferably, the drying time in step (3) is 2-4 hours.

8. The method according to any one of claims 1-7, characterized in that, The heat treatment temperature in step (3) is 600-1000℃; Preferably, the heat treatment holding time in step (3) is 1-5 hours; Preferably, the heat treatment in step (3) is carried out under an inert atmosphere; Preferably, the inert atmosphere includes nitrogen or argon, with argon being the preferred gas.

9. A modified graphite material, characterized in that, Prepared by the method according to any one of claims 1-8.

10. A use of the modified graphite material according to claim 9, characterized in that, The modified graphite material is used as an active material in the field of lithium-ion battery technology.

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