Carbon-coated metal-doped lithium titanate composite material, preparation and application thereof

Abstract

The application discloses a carbon-coated metal-doped lithium titanate composite material and a preparation and application thereof. The carbon-coated metal-doped lithium titanate composite material is prepared by using a proper amount of metal doping and carbon coating, so that the gas production problem of the lithium titanate material is improved, and the electronic conductivity of the lithium titanate material is enhanced, and the long service life of the lithium titanate material is fully utilized. In the application, a lithium ion battery is also provided, and the negative electrode material of the lithium ion battery is the carbon-coated metal-doped lithium titanate composite material. The lithium ion battery has excellent cycle stability, and the capacity retention rate can be kept above 90% after 8000 cycles of charging and discharging at a 3C rate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of lithium ion battery electrode material development, and particularly relates to a carbon-coated metal-doped lithium titanate composite material and a preparation method thereof, and a lithium ion battery using the composite material as an electrode material. BACKGROUND

[0002] As a renewable energy source, lithium ion batteries have the advantages of large capacity, long service life, environmental friendliness, etc., and are widely used in electric vehicles and portable electronic devices. With the continuous development of new energy technologies, the performance of lithium ion batteries is continuously developed, and researchers improve the performance of lithium ion batteries to meet the use requirements of consumers on lithium ion batteries.

[0003] In the development process of lithium ion batteries, the positive electrode material and the negative electrode material have always been the focus of research. In terms of positive electrode materials, high-voltage and high-energy-density core-shell materials are continuously developed. In terms of negative electrode materials, the currently widely used lithium intercalation graphitized carbon material has the characteristics of low price, simple preparation, and good performance, but has a large volume change during use, which poses a great safety hazard.

[0004] With the in-depth study of negative electrode materials, researchers have developed an excellent substitute for lithium intercalation graphitized carbon material, namely lithium titanate material. Lithium titanate material has the characteristic of "zero strain", which can avoid the destruction of the electrode structure caused by the expansion and contraction of the electrode material during use, thereby improving the cycle performance, service life and safety performance of the electrode, and also has good overcharge resistance and overdischarge resistance.

[0005] Although lithium titanate material has the above excellent performance, it has a serious problem of gas production during use. This is because the active components in the electrolyte, such as propylene carbonate (EC), diethyl carbonate (DEC), etc., are catalytically decomposed on the Ti 4+ surface of lithium titanate; and trace amounts of water and hydrofluoric acid exist in the electrolyte, which have a certain initiating effect on catalytic decomposition. Since the lithium titanate surface is not easy to form a SEI film, the above catalytic decomposition reaction will always exist, resulting in serious gas production of lithium titanate during use.

[0006] In order to solve the above-mentioned serious gas production problem, researchers usually adopt the method of coating carbon or metal oxide on the surface of lithium titanate. For example, the Chinese invention patent with the application number 201410808197.1 named "Preparation method of carbon and oxide composite modified lithium titanate material" adopts the method of microwave combination and ball milling to prepare modified lithium titanate material. The modified lithium titanate material prepared by the method overcomes the problem of poor conductivity of pure phase lithium titanate material, and the cycle performance and rate performance of the modified lithium titanate material are effectively improved. Although the performance of the modified lithium titanate material prepared by the method is greatly improved, in actual production, the microwave combination method has problems such as violent reaction and process difficult to control, and the coating uniformity of the ball milling coating method is poor, and it is difficult to form a uniform coating layer on the surface of the lithium titanate material.

[0007] In summary, the performance of lithium titanate material still has a lot of room for improvement. It is of great significance to find a suitable method to modify lithium titanate material and prepare a suitable coating layer on its surface to further improve the performance of lithium titanate material. SUMMARY

[0008] In order to solve the problems of the prior art, the present application provides a carbon-coated metal-doped lithium titanate composite material and its preparation and application. The metal ion doping amount in the lithium titanate composite material of the present application is 0.5-5% of the mass of the composite material, and the carbon coating amount is 1-10% of the mass of the composite material. Under the doping amount and coating amount, the lithium titanate composite material can maintain uniform and stable performance under the condition of small particle size of 50nm-2um. The appropriate amount of metal doping and carbon coating can improve the gas production problem of lithium titanate material, enhance its electronic conductivity, and fully exert the advantage of long service life of lithium titanate material. In the present application, the method of co-precipitation pyrolytic carbon is used to coat carbon on the surface of lithium titanate material, so that the coating effect is more uniform, and the influence of oxide impurities does not occur. The present application also provides a lithium ion battery, the negative electrode material of which is the above-mentioned carbon-coated metal-doped lithium titanate composite material. The lithium ion battery has excellent cycle stability, and the capacity retention rate can be maintained at more than 90% after 8000 cycles of charge and discharge at 3C rate.

[0009] The technical effects achieved by the present application are realized by the following scheme:

[0010] The present application provides a carbon-coated metal-doped lithium titanate composite material, which is composed of lithium titanate, metal ions and carbon. The metal ions are uniformly doped in the lithium titanate, and the carbon is coated on the outer surface of the lithium titanate. The metal ion doping amount is 0.5-5% of the mass of the composite material, and the carbon coating amount is 1-10% of the mass of the composite material.

[0011] Further, the carbon-coated metal-doped lithium titanate composite material has a particle size of 50 nm-2 μm.

[0012] The metal-doped and carbon-coated modification of the lithium titanate material can effectively improve the comprehensive performance of the lithium titanate material. The metal doping amount and the carbon coating amount have an important influence on the performance of the lithium titanate material. When the addition amount of metal and carbon is too large, it is not conducive to maintaining the particle size and morphology of the lithium titanate, and the excellent performance of the lithium titanate itself cannot be fully utilized. When the addition amount of metal and carbon is too small, it cannot have a good modification effect. In the present application, the metal ion doping amount is 0.5-5% of the mass of the composite material, and the carbon coating amount is 1-10% of the mass of the composite material. Under the doping amount and the coating amount, the lithium titanate composite material can maintain uniform and stable performance under the condition of a small particle size of 50 nm-2 μm, and can meet the requirements of production on the particle size, morphology, and compaction density of the modified lithium titanate material.

[0013] The present application also provides a preparation method of the above-mentioned carbon-coated metal-doped lithium titanate composite material, comprising the following steps:

[0014] S01, preparing a precursor: taking a lithium source and a titanium source into a solvent, stirring at 1000-1500 r / min for 20-30 min, then adding a metal salt solution, continuing to stir, and then adding an alkali solution, adjusting the pH value of the mixed solution to 8-12, and then standing to obtain a precipitate, which is filtered, washed, and dried to prepare the precursor;

[0015] S02, carbon-coating the precursor: taking the precursor prepared in S01 into a solvent, adding a carbon source, adjusting the reaction temperature to 90-120℃, stirring at 200-400 r / min for 30-50 min, and then standing and drying to prepare a carbon-coated precursor;

[0016] S03, calcining: grinding the carbon-coated precursor prepared in S02, and then calcining in an inert atmosphere to prepare the carbon-coated metal-doped lithium titanate composite material;

[0017] The molar ratio of lithium ions to titanium ions in the precursor is (0.75-0.95):1; and the molar ratio of the total amount of metal ions to lithium ions in the metal salt solution is (0-0.4):1.

[0018] Further, the solvent is at least one of pure water and anhydrous ethanol.

[0019] Further, the metal salt solution is at least one of a soluble metal salt ethanol solution and a soluble metal salt aqueous solution.

[0020] Further, the metal salt solution is one or more of nitrate, sulfate, and chloride of sodium, magnesium, aluminum, manganese, iron, cobalt, nickel, copper, zinc, and rare earth metal.

[0021] Further, the pH value of the alkali solution is greater than or equal to 13.

[0022] Further, the carbon source is one or more of glucose, polyvinyl alcohol, epoxy resin, phenolic resin and asphalt.

[0023] Further, the calcination condition is calcination at 500-1000 DEG C for 5-15h, or first calcination at 500-600 DEG C for 1-3h, then calcination at 650-750 DEG C for 0.5-2h, and finally calcination at 800-1000 DEG C for 8-10h.

[0024] The preparation method of the lithium titanate composite material in the present application is simple in process and low in cost, and has good practicability. In the prior art, a solid-phase ball milling method under inert gas protection is usually used to coat the lithium titanate material, and the coating layer prepared by the method has poor uniformity. In the present application, a carbon coating method of co-precipitation pyrolysis is used to coat carbon on the surface of the lithium titanate material, so that the coating effect is more uniform, and the influence of oxide impurities does not occur. At the same time, the preparation method in the present application can obtain lithium titanate composite material products with different thickness (nanometer level, micrometer level), different particle size distribution (nanometer level, micrometer level) and different compaction density, so as to meet the requirements of production on the physical and chemical properties of the lithium titanate composite material.

[0025] The present application also provides an application of the above-mentioned carbon-coated metal-doped lithium titanate composite material, i.e. a lithium ion battery prepared by using the carbon-coated metal-doped lithium titanate composite material as the negative electrode material. The lithium ion battery has excellent cycle stability, and the capacity retention rate can be maintained at more than 90% after 8000 cycles of charge and discharge at a 3C rate.

[0026] The present application has the following advantages:

[0027] 1. The lithium titanate composite material in the present application has a suitable metal doping amount and carbon coating amount, so that the lithium titanate composite material can maintain uniform and stable performance under the condition of a small particle size of 50nm-2um, and can meet the requirements of production on the particle size, morphology and compaction density of the modified lithium titanate material.

[0028] 2. In the present application, the carbon coating method of co-precipitation pyrolysis is used to coat carbon on the surface of the lithium titanate material, so that the coating effect is more uniform, and the influence of oxide impurities does not occur.

[0029] 3. The preparation method of the lithium titanate composite material in the present application is simple and low in cost, and is suitable for industrial production.

[0030] 4. The lithium titanate composite material in the present application has good electrochemical performance and a long service life, and the lithium ion battery prepared from the composite material has good cycle performance. Attached Figure Description

[0031] Figure 1 This is a cycle performance diagram of a lithium-ion battery prepared using carbon-coated metal-doped lithium titanate composite material as the negative electrode in this invention. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] 1. Preparation of carbon-coated metal-doped lithium titanate composite materials.

[0034] Example 1

[0035] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this embodiment includes the following steps:

[0036] S01, Preparation of precursor: Lithium hydroxide and tetrabutyl titanate were added to anhydrous ethanol (the molar ratio of lithium ions to titanium ions was 0.82:1), and stirred at 1200 r / min for 30 min to form a colorless and transparent sol. Then, an ethanol solution of aluminum nitrate (the molar ratio of aluminum ions to lithium ions in the aluminum nitrate ethanol solution was 0.2:1) was added, and stirring was continued. Then, a sodium hydroxide solution (the pH value of the sodium hydroxide solution was 13) was added. The pH value of the mixed solution was adjusted to 10 and allowed to stand to obtain a precipitate. The precipitate was filtered, washed, and dried to obtain the precursor.

[0037] SO2, carbon-coated precursor: The precursor obtained in SO1 is added to a solvent, glucose is added, the reaction temperature is adjusted to 95℃, and the mixture is stirred at 300r / min for 30min, then allowed to stand and dry to obtain the carbon-coated precursor.

[0038] S03, calcination: The carbon-coated precursor obtained in S02 is ground and then calcined in a tube furnace under a nitrogen atmosphere. The calcination conditions are 700℃ for 12h to obtain the carbon-coated metal-doped lithium titanate composite material.

[0039] In this embodiment, the metal ion doping amount is 3.1% of the mass of the composite material, and the carbon coating amount is 8.3% of the mass of the composite material.

[0040] Example 2

[0041] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this embodiment includes the following steps:

[0042] S01, preparation of precursor: lithium hydroxide and tetrabutyl titanate were taken into pure water (molar ratio of lithium ion to titanium ion was 0.82:1), stirred at 1200r / min for 30min, to form a white suspension, then an ethanol solution of ferric nitrate was added (molar ratio of iron ion in ferric nitrate ethanol solution to lithium ion was 0.3:1), continue to stir, then add sodium hydroxide solution (pH value of sodium hydroxide solution was 13), adjust the pH value of the mixed solution to 8, and then stand, to obtain a precipitate, which was filtered, washed, dried to prepare the precursor;

[0043] S02, carbon-coated precursor: the precursor prepared in S01 was added to a solvent, polyvinyl alcohol was added, the reaction temperature was adjusted to 100℃, stirred at 300r / min for 30min, then stood, dried to prepare the carbon-coated precursor;

[0044] S03, calcination: the carbon-coated precursor prepared in S02 was ground and calcined in a tube furnace under nitrogen atmosphere, the calcination conditions were first calcined at 500℃ for 3h, then calcined at 700℃ for 1h, and finally calcined at 800℃ for 10h, to prepare the carbon-coated metal-doped lithium titanate composite material;

[0045] In this embodiment, the metal ion doping amount was 4.6% of the mass of the composite material, and the carbon coating amount was 9.5% of the mass of the composite material.

[0046] Example 3

[0047] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this embodiment comprises the following steps:

[0048] S01, preparation of precursor: lithium hydroxide and tetrabutyl titanate were taken into anhydrous ethanol (molar ratio of lithium ion to titanium ion was 0.84:1), stirred at 1200r / min for 30min, to form a colorless transparent sol, then an aqueous solution of cerium nitrate was added (molar ratio of cerium ion in cerium nitrate aqueous solution to lithium ion was 0.2:1), continue to stir, then add sodium hydroxide solution (pH value of sodium hydroxide solution was 13), adjust the pH value of the mixed solution to 12, and then stand, to obtain a precipitate, which was filtered, washed, dried to prepare the precursor;

[0049] S02, carbon-coated precursor: the precursor prepared in S01 was added to a solvent, phenolic resin was added, the reaction temperature was adjusted to 100℃, stirred at 300r / min for 30min, then stood, dried to prepare the carbon-coated precursor;

[0050] S03, calcination: the carbon-coated precursor prepared in S02 was ground and calcined in a tube furnace under nitrogen atmosphere, the calcination conditions were first calcined at 500℃ for 3h, then calcined at 700℃ for 1h, and finally calcined at 800℃ for 10h, to prepare the carbon-coated metal-doped lithium titanate composite material;

[0051] The metal ion doping amount in this embodiment is 2.5% of the mass of the composite material, and the carbon coating amount is 5.1% of the mass of the composite material.

[0052] Embodiment 4

[0053] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this embodiment includes the following steps:

[0054] S01, preparing a precursor: lithium hydroxide and tetrabutyl titanate are taken and added to anhydrous ethanol (the molar ratio of lithium ions to titanium ions is 0.9:1), stirred at 1200 r / min for 30 min to form a colorless transparent sol, then an aqueous solution of copper chloride (the molar ratio of copper ions to lithium ions in the aqueous solution of copper chloride is 0.3:1) is added, continue to stir, then add a sodium hydroxide solution (the pH value of the sodium hydroxide solution is 13), adjust the pH value of the mixed solution to 11 and stand still, obtain a precipitate, and the precipitate is filtered, washed, and dried to prepare a precursor;

[0055] S02, carbon-coated precursor: the precursor prepared in S01 is added to a solvent, polyvinyl alcohol is added, the reaction temperature is adjusted to 100°C, stirred at 300 r / min for 30 min, then stand still and dry to prepare a carbon-coated precursor;

[0056] S03, calcination: the carbon-coated precursor prepared in S02 is ground and calcined in a tube furnace under a nitrogen atmosphere, the calcination conditions are 700°C for 12 h, and the carbon-coated metal-doped lithium titanate composite material is prepared;

[0057] The metal ion doping amount in this embodiment is 4.4% of the mass of the composite material, and the carbon coating amount is 5.8% of the mass of the composite material.

[0058] Embodiment 5

[0059] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this embodiment includes the following steps:

[0060] S01, preparing a precursor: lithium hydroxide and tetrabutyl titanate are taken and added to pure water (the molar ratio of lithium ions to titanium ions is 0.93:1), stirred at 1200 r / min for 30 min to form a white suspension, then an aqueous solution of magnesium chloride (the molar ratio of magnesium ions to lithium ions in the aqueous solution of magnesium chloride is 0.25:1) is added, continue to stir, then add a sodium hydroxide solution (the pH value of the sodium hydroxide solution is 13), adjust the pH value of the mixed solution to 10 and stand still, obtain a precipitate, and the precipitate is filtered, washed, and dried to prepare a precursor;

[0061] S02, carbon-coated precursor: the precursor prepared in S01 was added to a solvent, and glucose was added. The reaction temperature was adjusted to 95℃, and the mixture was stirred at 300r / min for 30min, then left to stand and dry, to obtain a carbon-coated precursor;

[0062] S03, calcination: the carbon-coated precursor prepared in S02 was ground and calcined in a tube furnace under a nitrogen atmosphere. The calcination conditions were: first calcination at 500℃ for 2h, then calcination at 700℃ for 0.5h, and finally calcination at 800℃ for 12h, to obtain the carbon-coated metal-doped lithium titanate composite material;

[0063] In this example, the metal ion doping amount was 3.8% of the mass of the composite material, and the carbon coating amount was 7.9% of the mass of the composite material.

[0064] Comparative Example 1

[0065] The preparation method of the carbon-coated metal-doped lithium titanate composite material in this comparative example included the following steps:

[0066] S01, preparation of precursor: lithium hydroxide and tetrabutyl titanate were added to anhydrous ethanol (the molar ratio of lithium ions to titanium ions was 0.82:1), and stirred at 1200r / min for 30min to form a colorless transparent sol. Then, an aluminum nitrate ethanol solution (the molar ratio of aluminum ions to lithium ions in the aluminum nitrate ethanol solution was 0.6:1) was added, and the mixture was stirred. Then, a sodium hydroxide solution (the pH value of the sodium hydroxide solution was 13) was added, and the pH value of the mixed solution was adjusted to 10. After standing, the precipitate was obtained, and the precipitate was filtered, washed, and dried to obtain a precursor;

[0067] S02, carbon-coated precursor: the precursor prepared in S01 was added to a solvent, and glucose was added. The reaction temperature was adjusted to 95℃, and the mixture was stirred at 300r / min for 30min, then left to stand and dry, to obtain a carbon-coated precursor;

[0068] S03, calcination: the carbon-coated precursor prepared in S02 was ground and calcined in a tube furnace under a nitrogen atmosphere. The calcination conditions were: first calcination at 500℃ for 2h, then calcination at 700℃ for 0.5h, and finally calcination at 800℃ for 12h, to obtain the carbon-coated metal-doped lithium titanate composite material;

[0069] In this example, the metal ion doping amount was 3.8% of the mass of the composite material, and the carbon coating amount was 7.9% of the mass of the composite material.

[0070] Comparative Example 2

[0071] Compared with Comparative Example 1, the difference between this comparative example was that the metal ion doping amount was 6.1% of the mass of the composite material, and the carbon coating amount was 12.3% of the mass of the composite material.

[0072] Comparative Example 3

[0073] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 7.2% by mass of the composite material, and the carbon coating amount is 13.5% by mass of the composite material.

[0074] Comparative Example 4

[0075] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 8.5% by mass of the composite material, and the carbon coating amount is 15.1% by mass of the composite material.

[0076] Comparative Example 5

[0077] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 10.3% by mass of the composite material, and the carbon coating amount is 16.4% by mass of the composite material.

[0078] Comparative Example 6

[0079] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 4.5% by mass of the composite material, and the carbon coating amount is 0.5% by mass of the composite material.

[0080] Comparative Example 7

[0081] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 4.5% by mass of the composite material, and the carbon coating amount is 0% by mass of the composite material.

[0082] Comparative Example 8

[0083] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 6.2% by mass of the composite material, and the carbon coating amount is 5.3% by mass of the composite material.

[0084] Comparative Example 9

[0085] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 7.5% by mass of the composite material, and the carbon coating amount is 5.3% by mass of the composite material.

[0086] Comparative Example 10

[0087] The difference between this comparative example and Comparative Example 1 is that the metal ion doping amount is 8.5% by mass of the composite material, and the carbon coating amount is 5.9% by mass of the composite material.

[0088] 2. Preparation of a lithium ion battery.

[0089] The carbon-coated metal-doped lithium titanate composite materials prepared in Examples 1-5 and Comparative Examples 1-10 were uniformly mixed with polyvinylidene fluoride (PVDF) and acetylene black at a mass ratio of 80:10:10, homogenously coated, dried, and punched into circular electrode sheets, with a lithium metal sheet as the positive electrode, to assemble CR2032 type button cells.

[0090] 3. Cycle performance test of lithium ion battery.

[0091] The cycle performance of the lithium ion battery was tested, and the test conditions were that the lithium ion battery was charged and discharged at a rate of 3C for 8000 cycles, and then the capacity retention rate of the lithium ion battery was measured.

[0092] 4. Analysis of test results.

[0093] The cycle performance graph of the lithium ion battery prepared with the composite material in Example 1 is shown in FIG. 1, where the abscissa represents the number of charge and discharge cycles, and the ordinate represents the capacity retention rate of the lithium ion battery. As shown in FIG. 1, the capacity retention rate of the lithium ion battery was 91.8% after 8000 cycles of charge and discharge at a rate of 3C. Figure 1 Figure 1 Figure 1

[0094] The metal doping amount and carbon coating amount of the composite materials in Examples 1-5 and Comparative Examples 1-10, and the capacity retention rate of the lithium ion battery prepared from the composite materials are shown in the following table.

[0095]

[0096] As shown in the above table, when the composite material prepared by the method of the present application has a metal ion doping amount of 0.5-5% by mass of the composite material and a carbon coating amount of 1-10% by mass of the composite material, the lithium ion battery prepared from the composite material has good cycle performance, and the capacity retention rate is greater than 90% after 8000 cycles of charge and discharge at a rate of 3C. When the metal doping amount and the carbon coating amount are not within the above ranges, the cycle performance of the lithium ion battery is significantly reduced. In summary, the carbon-coated metal-doped lithium titanate composite material of the present application has good electrochemical performance, and is suitable for preparing a lithium ion battery.

[0097] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the embodiments of the present application, but not to limit them. Although the embodiments of the present application have been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present application can still be modified or replaced by equivalents, and these modifications or replacements of the technical solutions should not make the modified technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.​​​

Claims

1. A method for preparing a carbon-coated metal-doped lithium titanate composite material, characterized in that, The process includes the following steps: S01, Preparing the precursor: Lithium source and titanium source are added to a solvent, stirred at 1000-1500 r / min for 20-30 min, then a metal salt solution is added, stirring is continued, then an alkaline solution is added, the pH of the mixed solution is adjusted to 8-12 and then allowed to stand to obtain a precipitate, and the precipitate is filtered, washed and dried to obtain the precursor; S02, carbon-coated precursor: The precursor obtained in S01 is added to a solvent, a carbon source is added, the reaction temperature is adjusted to 90-120℃, and the mixture is stirred at 200-400 r / min for 30-50 min, then allowed to stand and dry to obtain the carbon-coated precursor; S03, calcination: The carbon-coated precursor obtained in S02 is ground and calcined under an inert atmosphere to obtain the carbon-coated metal-doped lithium titanate composite material; the molar ratio of lithium ions to titanium ions in the precursor is (0.75-0.95):1; the molar ratio of the total metal ions to lithium ions in the metal salt solution is (0-0.4):1; The calcination conditions are as follows: calcination at 500-1000℃ for 5-15 hours; or calcination at 500-600℃ for 1-3 hours, then at 650-750℃ for 0.5-2 hours, and finally at 800-1000℃ for 8-10 hours; the pH value of the alkaline solution is greater than or equal to 13. The negative electrode material of the lithium-ion battery is the aforementioned carbon-coated metal-doped lithium titanate composite material. This lithium-ion battery has excellent cycle stability. At 3C rate, after 8000 charge-discharge cycles, the capacity retention rate can be maintained at over 90%. The particle size of the carbon-coated metal-doped lithium titanate composite material is 50 nm-2 μm; The metal salt is one or more of the following: nitrates, sulfates, and chlorides of sodium, magnesium, aluminum, manganese, iron, cobalt, nickel, copper, zinc, and rare earth metals. A carbon-coated metal-doped lithium titanate composite material is composed of lithium titanate, metal ions, and carbon. The metal ions are uniformly doped into the lithium titanate, and the carbon is coated on the outer surface of the lithium titanate. The amount of metal ion doping is 0.5-5% of the mass of the composite material, and the amount of carbon coating is 1-10% of the mass of the composite material.

2. The preparation method of the carbon-coated metal-doped lithium titanate composite material as described in claim 1, characterized in that: The solvent is at least one of pure water and anhydrous ethanol.

3. The preparation method of the carbon-coated metal-doped lithium titanate composite material as described in claim 1, characterized in that: The metal salt solution is at least one of a soluble metal salt ethanol solution or a soluble metal salt aqueous solution.

4. The preparation method of the carbon-coated metal-doped lithium titanate composite material as described in claim 1, characterized in that: The carbon source is one or more of glucose, polyvinyl alcohol, epoxy resin, phenolic resin, and asphalt.

5. A carbon-coated metal-doped lithium titanate composite material, characterized in that: Composed of lithium titanate, metal ions, and carbon, wherein the metal ions are uniformly doped into the lithium titanate and the carbon is coated on the outer surface of the lithium titanate; the amount of metal ion doping is 0.5-5% of the mass of the composite material, and the amount of carbon coating is 1-10% of the mass of the composite material; it is prepared by the preparation method described in claim 1.

6. A lithium-ion battery, characterized in that: The negative electrode material of the lithium-ion battery is the carbon-coated metal-doped lithium titanate composite material as described in claim 5.

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