Lithium titanate composite coating modified graphite negative electrode material, preparation method thereof, lithium ion battery negative electrode and lithium ion battery

Abstract

This invention discloses a lithium titanate composite coating modified graphite anode material and its preparation method, relating to the technical field of lithium-ion battery anode materials. The material uses graphite as a matrix, with a 20-100 nm thick cation-anion dual-site co-doped lithium titanate composite coating on the graphite surface. The chemical formula of lithium titanate is given by [formula omitted], where M represents one or more of [types omitted] doped at titanium sites, and A represents one or two of [types omitted] doped at oxygen sites. In the preparation process, the graphite is first subjected to surface oxidation pretreatment. Then, a lithium source, a titanium source, and a dopant source are dissolved in a solvent according to stoichiometric ratio. After adding the graphite, a dry gel precursor is obtained by rotary evaporation and annealing at 650-850 °C under an inert atmosphere. This invention utilizes the synergistic effect of cations improving lattice rigidity and anions improving fracture toughness to improve the mechanical stability of the coating, while also considering fast charging, low-temperature safety, and high-temperature cycling performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to a lithium titanate composite coating modified graphite anode material, its preparation method, the anode, and a lithium-ion battery. Background Technology

[0002] Graphite-based carbon materials are currently the anode materials for large-scale commercial application in the lithium-ion battery industry due to their high theoretical specific capacity, low and stable lithium intercalation potential, wide availability of raw materials, low cost, and excellent cycle processing performance. They have been widely used in consumer electronics, new energy vehicles, energy storage power stations and other application scenarios.

[0003] As power batteries upgrade towards high-speed charging, wide temperature range, and long lifespan, the inherent defects of commercial graphite anodes are gradually being amplified: under high-current fast charging and low-temperature (≤0℃) charging conditions, lithium metal is easily deposited on the surface of graphite anodes, and the lithium deposition process is irreversible, posing serious safety hazards.

[0004] Lithium titanate has a much higher lithium intercalation potential than lithium metal deposition potential, which thermodynamically avoids the risk of lithium deposition. It also boasts long cycle life and high safety, making it a common choice for modifying graphite anodes. Existing technologies for lithium titanate-modified graphite mainly fall into two categories: bulk mixing and surface coating. Bulk mixing is simple, but lithium titanate has a low theoretical specific capacity, and mixing sacrifices the anode's energy density, while the graphite particles remain directly exposed to the electrolyte. Surface coating can improve the graphite surface exposure problem, but lithium titanate is a brittle ceramic phase with low fracture toughness, while graphite undergoes approximately 10% volume expansion during lithium intercalation. This deformation mismatch leads to microcracks in the coating during cycling, which gradually propagate and detach. Furthermore, existing coatings often rely on physical bonding, resulting in weak interfacial adhesion and easy debonding during cycling, leading to coating failure and electrochemical performance degradation.

[0005] Existing lithium titanate-modified graphite anode materials struggle to simultaneously achieve optimal performance in fast charging, low-temperature safety, and high-temperature cycling stability. Furthermore, their surface coatings exhibit problems such as cracking, debonding, and electrochemical performance degradation during long-term cycling. Therefore, providing a modified graphite anode material that combines excellent mechanical and electrochemical stability, suppresses lithium plating under fast charging and wide-temperature-range operating conditions, and maintains a long cycle life is a pressing technical challenge in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a lithium titanate composite coating modified graphite anode material and its preparation method, so as to solve the problems of easy cracking, easy debonding, electrochemical performance degradation, and difficulty in simultaneously achieving fast charging, low-temperature safety and high-temperature cycling stability in existing lithium titanate modified graphite anode materials.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a lithium titanate composite coating modified graphite anode material, using graphite as a substrate, wherein the surface of the graphite is coated with a cation-anion dual-site co-doped lithium titanate composite coating, the thickness of which is 20-100 nm; the chemical formula of the lithium titanate in the lithium titanate composite coating is [insert chemical formula here]. In the formula, M is a transition metal cation doped at the titanium site, selected from... , , , One or more of the following, where 0.025 ≤ x ≤ 0.1; A is a non-metallic anion doped at an oxygen site, selected from... , , One or two of the following, 0.12≤y≤0.36. This anode material uses graphite to provide high specific capacity and low lithium intercalation potential, and a dense lithium titanate composite coating separates the graphite from the electrolyte; the lithium titanate composite coating itself has a high lithium intercalation potential, which can thermodynamically prevent lithium metal deposition on the anode surface; the synergistic co-doping of cations and anions at titanium sites and oxygen sites respectively can simultaneously adjust the lattice rigidity and defect structure of the coating, so that the coating maintains high crystallinity while having good toughness, thus maintaining its integrity under the volume changes caused by repeated lithium intercalation and deintercalation of graphite.

[0008] Furthermore, the chemical formula of lithium titanate in the lithium titanate composite coating is: In the formula, M is selected from , , One or more of them, A is selected from , One or two of these high-valence transition metal cations can be incorporated into titanium sites to enhance the bond energy of titanium-oxygen bonds, improve the lattice rigidity and resistance to internal stress of the coating, and some of these cations can also introduce free electrons to improve the electronic conductivity of the coating. Doping helps maintain the structural stability of the coating lattice during cycling. After these low-electron-negative nonmetallic anions are incorporated into oxygen sites, This helps to introduce lattice dislocations and activate slip systems, thereby improving the fracture toughness of the coating. This allows for the formation of a lithium fluoride-rich interface layer on the coating surface, inhibiting the continuous decomposition of the electrolyte.

[0009] Further, the graphite is selected from one or both of artificial graphite and natural graphite. The surface of the graphite has oxygen-containing functional groups, and chemical bonds are formed between the lithium titanate composite coating and the oxygen-containing functional groups. The oxygen-containing functional groups include one or more of hydroxyl and carboxyl groups; during the sol-gel in-situ coating and subsequent high-temperature crystallization process, the hydroxyl groups generated by the hydrolysis of precursors such as titanium sources undergo a condensation reaction with the oxygen-containing functional groups on the graphite surface, thereby forming chemical bonds between the lithium titanate composite coating and the oxygen-containing functional groups, mainly metal-oxygen-carbon bonds (such as Ti-OC bonds), and may also include coordination bonds, so that the lithium titanate composite coating is chemically anchored to the surface of graphite. The oxygen-containing functional groups on the graphite surface can serve as nucleation and chemical bonding sites for the coating, enhancing the interfacial bonding force between the lithium titanate composite coating and graphite, and inhibiting interfacial debonding during cycling.

[0010] A second aspect of the present invention also provides a method for preparing the above-mentioned lithium titanate composite coating modified graphite anode material, comprising the following steps: S1. Graphite is subjected to surface oxidation pretreatment to obtain graphite with oxygen-containing functional groups on the surface; S2. Dissolve the lithium source, titanium source, cation doping source and anion doping source in the solvent according to the stoichiometric ratio of the above chemical formula to obtain a mixed solution. Add the graphite treated in step S1 and stir. Then remove the solvent by rotary evaporation to obtain the dry gel precursor. S3. The dry gel precursor is heated to 650~850℃ under an inert atmosphere and kept at that temperature for 3~6h, then cooled to obtain the lithium titanate composite coating modified graphite anode material.

[0011] This method employs sol-gel in-situ coating, which allows each dopant ion to be uniformly distributed on the graphite surface at the molecular scale along with the lithium titanate precursor. Then, a uniform and dense composite coating is formed by in-situ crystallization at high temperature. The oxygen-containing functional groups introduced by the oxidation pretreatment on the graphite surface further promote the chemical bonding between the coating and the graphite.

[0012] Further, in step S1, the surface oxidation pretreatment is as follows: placing graphite in a nitric acid solution with a concentration of 0.3~0.8 mol / L, stirring at 50~70℃ for 0.5~2 h, then washing until neutral and drying; or, placing graphite in a nitric acid solution with a concentration of 50~150 mg / L... Treat in an ozone atmosphere for 10-30 minutes.

[0013] Further, in step S2, the lithium source is selected from one or more of lithium hydroxide monohydrate, lithium nitrate, and lithium acetate; the titanium source is selected from one or more of tetrabutyl titanate, tetraethyl titanate, and titanium tetrachloride; and the solvent is selected from one or more of anhydrous ethanol, isopropanol, and ethylene glycol.

[0014] Further, in step S2, the cation doping source is selected from one or more of niobium pentachloride, tungsten hexachloride, tantalum pentachloride, and zirconium tetrachloride; the anion doping source is selected from one or more of urea, thiourea, and ammonium fluoride.

[0015] Further, in step S2, the lithium source, titanium source, cation doping source and anion doping source are dissolved in a solvent and stirred for 20-40 min to obtain a mixed solution; the graphite treated in step S1 is added and stirred for 1-3 h; the mass ratio of the graphite to the lithium titanate precursor solids is 92:8-98:2; the rotary evaporation is carried out in a water bath at 50-70℃.

[0016] Furthermore, in step S3, the heating rate is 3~8℃ / min; the inert atmosphere is one or more of argon and nitrogen; and after holding at the temperature, it is naturally cooled to room temperature.

[0017] A third aspect of the present invention also provides a lithium-ion battery anode comprising the above-mentioned lithium titanate composite coating modified graphite anode material.

[0018] A fourth aspect of the present invention also provides a lithium-ion battery comprising the above-described lithium-ion battery negative electrode.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: Firstly, this invention employs a lithium titanate coating modified with both cation and anion co-doping. Cation doping strengthens the Ti-O bond energy, improving lattice rigidity and coating hardness, and enhancing internal stress tolerance. Anion doping introduces lattice dislocations, improving the coating's fracture toughness and mitigating stress concentration during charge-discharge cycles. This invention balances high crystallinity and high toughness in the coating, fundamentally addressing the inherent brittleness of pure lithium titanate coatings and preventing electrochemical performance degradation caused by coating cracking.

[0020] Secondly, the anion and cation co-doping of this invention can simultaneously optimize both electron and lithium-ion conductivity: Nb 5+ W 6+ Cation doping introduces free electrons, improving the intrinsic electronic conductivity of the material; S 2- Anion doping reduces Li + Migration energy barrier, optimizing ion conduction efficiency; Zr 4+ Doping expands the lattice spacing, further accelerates lithium-ion diffusion, effectively improves the rate performance of the material, enhances the lithium-ion transport efficiency under high-current fast charging conditions, suppresses lithium plating phenomenon in graphite anodes during fast charging, and is suitable for high-power fast charging applications of lithium batteries.

[0021] Thirdly, the S of the present invention 2- F - Anion doping can improve the performance of Li at low temperatures. +Migration rate and electrode interface stability; combined with the inherent high potential characteristics of lithium titanate coating, lithium metal deposition can be suppressed from a thermodynamic perspective, enabling safe charging at temperatures of 0°C and below, broadening the temperature application range of graphite anode batteries, and solving the industry pain point of lithium deposition during low-temperature charging.

[0022] Fourth, the lithium titanate phase of this invention itself does not reduce to form an SEI passivation film, thus avoiding problems such as capacity decay, increased internal resistance, and lithium dendrite growth caused by excessive growth of the SEI film under high temperature conditions; F - The coating surface is enriched and a stable LiF-rich interface layer is formed, which inhibits the continuous decomposition of the electrolyte at high temperatures. The dual-site co-doping can enhance the intrinsic structural stability of the material, reduce the side reactions between the electrode and the electrolyte, significantly improve the cycle stability of the battery under high temperature conditions of 45℃ and above, and extend the overall service life of the battery.

[0023] Fifth, this invention adopts an intrinsic modification method of the material lattice, which is different from the compromise modification methods such as adding binders and carbon layers, and thoroughly improves the brittle defects of lithium titanate coating; thus enabling the graphite-lithium titanate composite material to synergistically improve multiple key performance indicators such as fast charging, low temperature, high temperature and long cycle, and providing a breakthrough technical solution for the next generation of high-safety and high-performance lithium-ion battery anode materials.

[0024] Sixth, this invention achieves comprehensive improvement in the graphite anode in terms of fast charging, low temperature, high temperature and cycle life through cation-anion dual-site co-doping overmodification. Moreover, the preparation method adopts sol-gel in-situ coating, which is controllable and produces a uniform and dense lithium titanate coating that is firmly bonded to the graphite matrix interface. Attached Figure Description

[0025] Figure 1 This is a schematic flowchart of the preparation method of the lithium titanate composite coating modified graphite anode material of the present invention. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments to facilitate a clear understanding of the present invention, but these descriptions do not constitute a limitation on the present invention.

[0027] The "scope" disclosed in this invention is defined in the form of a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. The scope defined in this way can include or exclude end values, and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a scope.

[0028] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined to form new technical solutions. All technical features and optional technical features of the present invention can be combined to form new technical solutions. All steps of the present invention can be performed sequentially or randomly.

[0029] The preparation process of the lithium titanate composite coating modified graphite anode material described in this invention is as follows: Figure 1 .

[0030] Example 1 This embodiment provides a lithium titanate composite coating modified graphite anode material, using artificial graphite as the substrate. The surface of the artificial graphite is coated with a lithium titanate composite coating with a thickness of approximately 60 nm. The chemical formula of lithium titanate is [insert chemical formula here]. .

[0031] Its preparation methods include: S1. Place the artificial graphite in a 0.5 mol / L nitric acid solution, stir at 60°C for 1 h, wash until neutral and dry to obtain artificial graphite with oxygen-containing functional groups on the surface. S2. Weigh lithium hydroxide monohydrate, tetrabutyl titanate, niobium pentachloride and urea according to the stoichiometric ratio of the above chemical formula, dissolve them in anhydrous ethanol and stir for 30 min to obtain a mixed solution, add artificial graphite treated in step S1, stir for 2 h, remove the solvent by rotary evaporation in a 60℃ water bath to obtain a dry gel precursor, wherein the mass ratio of artificial graphite to lithium titanate precursor solids is 96:4. S3. The dry gel precursor is placed in a tube furnace and heated to 750°C at 5°C / min under an argon atmosphere and held for 4 hours. It is then naturally cooled to room temperature to obtain the lithium titanate composite coating modified graphite anode material.

[0032] Example 2 This embodiment provides a lithium titanate composite coating modified graphite anode material, using artificial graphite as the substrate. The surface of the artificial graphite is coated with a lithium titanate composite coating with a thickness of approximately 20 nm. The chemical formula of lithium titanate is [insert chemical formula here]. .

[0033] Its preparation methods include: S1. Place artificial graphite in a solution with a concentration of 50 mg / Artificial graphite with oxygen-containing functional groups on its surface was obtained by treating it in an ozone atmosphere for 10 minutes. S2. Weigh lithium nitrate, tetraethyl titanate, tungsten hexachloride and ammonium fluoride according to the stoichiometric ratio of the above chemical formula, dissolve them in isopropanol and stir for 20 min to obtain a mixed solution, add artificial graphite treated in step S1, stir for 1 h, and remove the solvent by rotary evaporation in a 50°C water bath to obtain a dry gel precursor, wherein the mass ratio of artificial graphite to lithium titanate precursor solids is 98:2. S3. The dry gel precursor is placed in a tube furnace and heated to 650°C at 3°C / min under an argon atmosphere and held for 3 hours. It is then naturally cooled to room temperature to obtain the lithium titanate composite coating modified graphite anode material.

[0034] Example 3 This embodiment provides a lithium titanate composite coating modified graphite anode material, using artificial graphite as the substrate, with a lithium titanate composite coating approximately 100 nm thick coated on the surface of the artificial graphite. The chemical formula of lithium titanate is [insert chemical formula here]. .

[0035] Its preparation methods include: S1. Place the artificial graphite in a 0.8 mol / L nitric acid solution, stir at 70°C for 2 hours, wash until neutral and dry to obtain artificial graphite with oxygen-containing functional groups on the surface. S2. Weigh lithium acetate, titanium tetrachloride, tantalum pentachloride and thiourea according to the stoichiometric ratio of the above chemical formula, dissolve them in ethylene glycol and stir for 40 min to obtain a mixed solution, add artificial graphite treated in step S1, stir for 3 h, remove the solvent by rotary evaporation in a 70°C water bath to obtain a dry gel precursor, wherein the mass ratio of artificial graphite to lithium titanate precursor solids is 92:8. S3. The dry gel precursor is placed in a tube furnace and heated to 850°C at 8°C / min under an argon atmosphere and held for 6 hours. It is then naturally cooled to room temperature to obtain the lithium titanate composite coating modified graphite anode material.

[0036] Example 4 This embodiment provides a lithium titanate composite coating modified graphite anode material, using natural graphite as the matrix, with a lithium titanate composite coating of approximately 50 nm thickness covering the surface of the natural graphite. The chemical formula of lithium titanate is [insert chemical formula here]. .

[0037] Its preparation methods include: S1. Place natural graphite in a solution with a concentration of 150mg / Natural graphite with oxygen-containing functional groups on its surface was obtained by treating it in an ozone atmosphere for 30 minutes. S2. Weigh lithium hydroxide monohydrate, tetrabutyl titanate, zirconium tetrachloride and ammonium fluoride according to the stoichiometric ratio of the above chemical formula, dissolve them in anhydrous ethanol and stir for 30 min to obtain a mixed solution, add natural graphite treated in step S1, stir for 2 h, remove the solvent by rotary evaporation in a 60℃ water bath to obtain a dry gel precursor, wherein the mass ratio of natural graphite to lithium titanate precursor solids is 95:5. S3. The dry gel precursor is placed in a tube furnace and heated to 800°C at 5°C / min under an argon atmosphere and held for 5 hours. It is then naturally cooled to room temperature to obtain the lithium titanate composite coating modified graphite anode material.

[0038] Example 5 This embodiment provides a lithium titanate composite coating modified graphite anode material, using artificial graphite as the substrate. The surface of the artificial graphite is coated with a lithium titanate composite coating with a thickness of approximately 70 nm. The chemical formula of lithium titanate is [insert chemical formula here]. .

[0039] Its preparation methods include: S1. Place the artificial graphite in a 0.3 mol / L nitric acid solution, stir at 50°C for 0.5 h, wash until neutral and dry to obtain artificial graphite with oxygen-containing functional groups on the surface. S2. Weigh lithium nitrate, tetraethyl titanate, niobium pentachloride, tungsten hexachloride, and urea according to the stoichiometric ratio of the above chemical formulas, dissolve them in isopropanol and stir for 30 min to obtain a mixed solution, add artificial graphite treated in step S1, stir for 2 h, and remove the solvent by rotary evaporation in a 60°C water bath to obtain a dry gel precursor, wherein the mass ratio of artificial graphite to lithium titanate precursor solids is 94:6. S3. The dry gel precursor is placed in a tube furnace and heated to 780°C at 6°C / min under a nitrogen atmosphere and held for 4 hours. It is then naturally cooled to room temperature to obtain the lithium titanate composite coating modified graphite anode material.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that no cation or anion doping source is added during the preparation process, resulting in an undoped lithium titanate composite coating. The remaining components and preparation methods are the same as in Example 1.

[0041] Comparative Example 2 This comparative example uses uncoated and unmodified artificial graphite as the negative electrode material, and the artificial graphite used is the same as that in Example 1.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that no anion doping source is added during the preparation process, and the lithium titanate composite coating obtained is doped only at the titanium sites, with the chemical formula being: The remaining components and preparation methods are the same as in Example 1.

[0043] Comparative Example 4 The difference between this comparative example and Example 1 is that no cation doping source is added during the preparation process, and the lithium titanate composite coating obtained is doped only at oxygen sites, with the chemical formula being: The remaining components and preparation methods are the same as in Example 1.

[0044] Performance testing The negative electrode materials obtained in each embodiment and comparative example were respectively mixed with acetylene black and polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 80:10:10 to prepare a slurry, which was then uniformly coated onto copper foil. After drying, rolling, and cutting, lithium metal sheet was used as the counter electrode, and a mixture containing... A mixed solution of ethylene carbonate and diethyl carbonate was used as the electrolyte. A button half-cell was assembled in a glove box and tested according to the following method.

[0045] (1) First discharge specific capacity and first coulombic efficiency: The first charge and discharge was performed at 0.1C in the voltage range of 0.005~2.5V at 25℃, and the first discharge specific capacity and first coulombic efficiency were recorded.

[0046] (2) Rate performance: Charge and discharge at 0.5C and 3C respectively, and calculate the discharge capacity retention rate of 3C relative to 0.5C.

[0047] (3) Low temperature performance: Discharged at 0.5C at 25℃ and -20℃ respectively, and the discharge capacity retention rate at -20℃ relative to 25℃ was calculated.

[0048] (4) Room temperature cycling performance: 500 cycles at 1C at 25℃, and calculate the discharge capacity retention rate of the 500th cycle relative to the first cycle.

[0049] (5) High temperature cycling performance: 500 cycles at 1C at 45℃, and calculate the discharge capacity retention rate of the 500th cycle relative to the first cycle.

[0050] (6) Low-temperature fast charging lithium deposition observation: Charge the battery at -10℃ with a constant current of 2C until it is fully charged, and then disassemble the battery to observe the lithium metal deposition on the surface of the negative electrode.

[0051] (7) Charge transfer impedance: Electrochemical impedance spectroscopy was performed on the assembled half cell in the frequency range of 0.01 Hz to 100 kHz. The charge transfer impedance Rct was obtained by fitting the semicircle of the high frequency region in the obtained Nyquist plot.

[0052] (8) Coating integrity after cycling: After cycling at 25°C for 500 cycles, the battery was disassembled in a glove box, the negative electrode was removed, and the surface morphology of the lithium titanate composite coating was observed using a scanning electron microscope to evaluate whether there were cracks or peeling of the coating.

[0053] The test results of each embodiment and comparative example are shown in Table 1 and Table 2.

[0054] Table 1. Electrochemical performance test results of each embodiment and comparative example. Table 2. Cyclic and interface performance test results of each embodiment and comparative example. Based on the above results, the initial coulombic efficiency of the negative electrode materials obtained in each embodiment is no less than 91%, the rate capacity retention rate at 3C relative to 0.5C is no less than 86%, the low-temperature capacity retention rate at -20℃ relative to 25℃ is no less than 78%, the discharge capacity retention rate at 25℃ and 45℃ after 500 cycles is no less than 93% and 91%, respectively, and the charge transfer impedance Rct is no higher than 52Ω. Furthermore, no lithium metal deposition was observed on the negative electrode surface after charging at -10℃ and 2C, and the lithium titanate composite coating remained intact without obvious cracks after 500 cycles. Comparative Example 2 used unmodified artificial graphite, which showed significant lithium metal deposition on the negative electrode surface after low-temperature fast charging, and its rate capacity retention rate, low-temperature, room-temperature, and high-temperature cycle capacity retention rates were all lower. The lithium titanate composite coating in Comparative Example 1 was undoped, and obvious cracks and local peeling appeared after cycling. The lithium titanate composite coatings in Comparative Examples 3 and 4 were doped only at a single site, and a small number of microcracks appeared after cycling. The above three comparative examples all showed slight lithium metal precipitation after low-temperature fast charging, and the corresponding rate capability, low-temperature performance and cycle capacity retention were all lower than those of the embodiments, while the charge transfer impedance Rct was higher than that of the embodiments.

[0055] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit it. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application. Content not described in detail in this specification belongs to the prior art known to those skilled in the art.

Claims

1. A lithium titanate composite coating modified graphite negative electrode material, characterized by, Using graphite as a substrate, the surface of the graphite is coated with a lithium titanate composite coating with cation-anion dual-site co-doping. The chemical formula of the lithium titanate in the lithium titanate composite coating is , wherein M is a transition metal cation doped in a titanium site, selected from one or more of , , , , and 0.025≤x≤0.1; A is a non-metal anion doped in an oxygen site, selected from one or two of , , , and 0.12≤y≤0.

36.

2. The lithium titanate composite coating modified graphite anode material according to claim 1, characterized in that, The surface of the graphite has oxygen-containing functional groups, and chemical bonds are formed between the oxygen-containing functional groups and the lithium titanate composite coating. The oxygen-containing functional groups include one or both of hydroxyl and carboxyl groups.

3. The lithium titanate composite coating modified graphite anode material according to claim 1, characterized in that, The thickness of the lithium titanate composite coating is 20~100nm; the graphite is selected from one or two of artificial graphite and natural graphite.

4. A method for preparing the lithium titanate composite coating modified graphite anode material according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Graphite is subjected to surface oxidation pretreatment to obtain graphite with oxygen-containing functional groups on the surface; S2. Dissolve the lithium source, titanium source, cation doping source and anion doping source in a solvent to obtain a mixed solution. Add the graphite treated in step S1 and stir. Then remove the solvent by rotary evaporation to obtain the dry gel precursor. S3. The dry gel precursor is heated to 650~850℃ under an inert atmosphere and kept at that temperature for 3~6h, then cooled to obtain lithium titanate composite coating modified graphite anode material.

5. The method according to claim 4, characterized in that, In step S1, the surface oxidation pretreatment is as follows: placing the graphite in a nitric acid solution with a concentration of 0.3~0.8 mol / L, stirring at 50~70℃ for 0.5~2 h, then washing until neutral and drying; or, placing the graphite in a nitric acid solution with a concentration of 50~150 mg / L... Treat in an ozone atmosphere for 10-30 minutes.

6. The method according to claim 4, characterized in that, In step S2, the lithium source is selected from one or more of lithium hydroxide monohydrate, lithium nitrate, and lithium acetate; the titanium source is selected from one or more of tetrabutyl titanate, tetraethyl titanate, and titanium tetrachloride; and the solvent is selected from one or more of anhydrous ethanol, isopropanol, and ethylene glycol. The cation doping source is selected from one or more of niobium pentachloride, tungsten hexachloride, tantalum pentachloride, and zirconium tetrachloride; the anion doping source is selected from one or more of urea, thiourea, and ammonium fluoride.

7. The method according to claim 4, characterized in that, In step S2, the lithium source, titanium source, cation doping source and anion doping source are dissolved in a solvent and stirred for 20-40 minutes to obtain a mixed solution; graphite treated in step S1 is added and stirred for 1-3 hours; the mass ratio of graphite to lithium titanate precursor solids is 92:8-98:2; the rotary evaporation is carried out in a water bath at 50-70°C.

8. The method according to claim 4, characterized in that, In step S3, the heating rate is 3~8℃ / min; the inert atmosphere is one or more of argon and nitrogen.

9. A lithium-ion battery negative electrode, characterized in that, The lithium titanate composite coating modified graphite anode material as described in any one of claims 1 to 3.

10. A lithium-ion battery, characterized in that, It includes the lithium-ion battery negative electrode as described in claim 9.

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