Composite negative electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of batteries, in particular to a composite negative electrode material as well as a preparation method and application thereof. The composite negative electrode material comprises a negative electrode base material and a fast ion conductor layer located on the surface of the negative electrode base material, the negative electrode base material comprises a silicon nanosheet and a carbon coating layer located on the surface of the silicon nanosheet, and the surface of the carbon coating layer is doped with element nitrogen; and the fast ion conductor layer comprises Li3PO4. According to the composite negative electrode material disclosed by the invention, through coordination and cooperation of all the layers, the problem of capacity fading of a pure silicon negative electrode in a circulation process can be relieved, stable long-cycle performance is realized, and a high capacity retention ratio is obtained.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to a composite negative electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries, as energy storage systems in applications such as electric vehicles and portable electronic devices, must meet the requirements of high capacity, long cycle life, and good safety performance. However, traditional lithium-ion battery anode materials have several difficult-to-solve issues, including the low theoretical specific capacity of carbon-based materials, volume expansion of silicon-based materials, low initial efficiency, and poor conductivity.

[0003] In the current carbon-based anode production process, most material manufacturers on the market use synthetic graphite composites through uniform coating and structural design. However, the theoretical specific capacity of graphite-based materials significantly limits their application in next-generation lithium-ion batteries. Anode materials are crucial to the cycle life and capacity of batteries. Silicon is abundant in the Earth's crust and widely available. It has a high theoretical specific capacity and is less susceptible to the formation of lithium dendrites than lithium metal, playing a key role in improving battery energy density and safety.

[0004] In contrast, when silicon anodes are used in lithium-ion batteries, they experience cracking, pulverization, and severe side reactions during cycling, leading to rapid capacity decay and deterioration in cycling performance. Therefore, improving silicon anodes to mitigate silicon volume expansion under high cycling pressures is crucial.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] One object of the present invention is to provide a composite negative electrode material that can improve the conductivity and stability of nano-silicon and reduce interface impedance.

[0007] Another object of the present invention is to provide a method for preparing a composite negative electrode material, which is simple and easy to implement, and the composite negative electrode material obtained by this method has excellent electrochemical properties.

[0008] Another object of the present invention is to provide a negative electrode sheet.

[0009] Another object of the present invention is to provide a battery.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted: A composite negative electrode material comprises a negative electrode base material and a fast ion conductor layer located on the surface of the negative electrode base material; the negative electrode base material comprises silicon nanosheets and a carbon coating layer located on the surface of the silicon nanosheets, the surface of the carbon coating layer is doped with elemental nitrogen; the fast ion conductor layer comprises Li3PO4.

[0011] In some embodiments, the silicon nanosheet has a thickness of 4 to 15 nm.

[0012] In some embodiments, the carbon coating layer has a thickness of 1 to 5 nm.

[0013] In some embodiments, the fast ion conductor layer has a thickness of 0.05-0.1 nm.

[0014] In some embodiments, the doping amount of the element nitrogen in the carbon coating layer is 5% to 15%.

[0015] In some embodiments, the carbon coating layer accounts for 5% to 10% by mass of the composite negative electrode material.

[0016] In some embodiments, the fast ion conductor layer accounts for 0.05% to 0.3% by mass of the composite negative electrode material.

[0017] The method for preparing the composite negative electrode material as described above comprises the following steps: (a) Obtaining silicon nanosheets.

[0018] (b) depositing a carbon material on the surface of the silicon nanosheet to form a carbon coating layer to obtain a silicon-carbon material.

[0019] (c) performing nitrogen doping on the silicon-carbon material to obtain a negative electrode base material.

[0020] (d) Mixing and grinding the negative electrode matrix material and the fast ion raw material to form a fast ion superconductor layer.

[0021] In some embodiments, the method for obtaining the silicon nanosheets specifically includes: treating calcium disilicide with sodium hydroxide and then treating it with 10-15 mol / L concentrated hydrochloric acid; or heating silicon tetrachloride liquid to 50-65°C and depositing the resulting gas on a hot silicon dioxide substrate under a hydrogen atmosphere.

[0022] In some embodiments, the carbon material includes at least one of methane, acetylene, ethylene, ethanol, and n-butanol.

[0023] In some embodiments, the temperature of depositing the carbon material is 600-800°C.

[0024] In some embodiments, the nitrogen doping treatment specifically includes: mixing a mixture of a nitrogen doping raw material and ammonia water with the silicon-carbon material, and performing a heat treatment.

[0025] In some embodiments, the nitrogen-doping raw material includes at least one of 1H-1,2,3-triazole and polyacrylonitrile.

[0026] In some embodiments, during the nitrogen doping process, the heat treatment temperature is 30-90°C.

[0027] In some embodiments, the fast ion raw material includes LiOH and NH4H2PO4.

[0028] In some embodiments, the mass ratio of the negative electrode matrix material to LiOH and NH4H2PO4 is (88~92):(4~6):(4~6).

[0029] A negative electrode sheet comprises the composite negative electrode material, or the composite negative electrode material prepared by the method for preparing the composite negative electrode material.

[0030] A battery comprises the negative electrode sheet.

[0031] Compared with the prior art, the present invention has the following beneficial effects: (1) In the composite negative electrode material of the present invention, silicon nanosheets mainly provide capacity, and they have stable chemical properties and structures, so that the volume expansion during the lithium insertion process of the negative electrode is suppressed, thereby improving the cycle stability of the electrode and the battery; carbon coating improves the cycle performance of the composite electrode, which can limit the volume expansion of silicon and maintain the stability of the electrode structure; nitrogen doping changes the conductivity type of the material, increases the free carrier density, and thus improves the conductivity and stability of the material, because the outermost layer of the nitrogen atom has only three valence electrons. If it is covalently bonded with the four surrounding Si atoms, three pairs of covalent bonds will have two electrons, while one pair of covalent bonds will have only one electron (provided by the Si atom), thereby forming a hole, which can improve the surface wettability of the carbon layer and the lithium ion adsorption capacity, and reduce side reactions; the fast ion conductor layer has high lithium ion conductivity, which can reduce the interface impedance; the carbon coating layer and the fast ion conductor layer synergistically enhance the ionic and electronic conductivity of the composite negative electrode material, forming a three-dimensional ion-conducting and electron-conducting path, thereby alleviating the capacity attenuation problem of the pure silicon negative electrode during the cycle, achieving stable long-cycle performance, and obtaining a high capacity retention rate.

[0032] (2) The preparation method of the composite negative electrode material of the present invention includes obtaining silicon nanosheets, carbon deposition, nitrogen doping, and fast ion layer coating. Through the coordination of each step, the obtained composite negative electrode material has excellent electrochemical properties.

[0033] (3) The battery of the present invention has high energy density, excellent long cycle performance and fast charging capability. DETAILED DESCRIPTION

[0034] The embodiments of the present invention will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention. Where specific conditions are not specified in the examples, the methods were performed according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments are not specified, they are all commercially available conventional products.

[0035] According to one aspect of the present invention, the present invention relates to a composite negative electrode material, comprising a negative electrode matrix material and a fast ion conductor layer located on the surface of the negative electrode matrix material; the negative electrode matrix material comprises silicon nanosheets and a carbon coating layer located on the surface of the silicon nanosheets, and the surface of the carbon coating layer is doped with elemental nitrogen; the fast ion conductor layer comprises Li3PO4.

[0036] In the composite negative electrode material of the present invention, silicon nanosheets mainly provide capacity, and they have stable chemical properties and structure, so that the volume expansion of the negative electrode during lithium insertion is suppressed, thereby improving the cycle stability of the electrode and battery; carbon coating improves the cycle performance of the composite electrode, which can limit the volume expansion of silicon and maintain the stability of the electrode structure; nitrogen doping changes the conductivity type of the material, increases the free carrier density, and thus improves the conductivity and stability of the material. Because the outermost layer of the nitrogen atom has only three valence electrons, if it is covalently bonded with the four surrounding Si atoms, three pairs of covalent bonds will have two electrons, and one pair of covalent bonds will have only one electron (provided by the Si atom), thereby forming a hole, which can improve the surface wettability of the carbon layer and the lithium ion adsorption capacity, and reduce side reactions; the fast ion conductor layer has high lithium ion conductivity and can reduce the interfacial impedance; the carbon coating layer and the fast ion conductor layer synergistically enhance the ionic and electronic conductivity of the composite negative electrode material, forming a three-dimensional ion-conducting and electron-conducting path, thereby alleviating the capacity decay problem of the pure silicon negative electrode during the cycle, achieving stable long-cycle performance, and obtaining a high capacity retention rate.

[0037] In some embodiments, the silicon nanosheets have a thickness of 4 to 15 nm, such as 4 nm, 6 nm, 8 nm, 10 nm, 15 nm, etc. Silicon nanosheets provide more open space, which helps accommodate expansion and reduces cracking associated with expansion. Silicon nanosheets have an appropriate thickness, allowing for isotropic expansion and preventing crack propagation caused by stress concentration.

[0038] In some embodiments, the carbon coating layer has a thickness of 1 to 5 nm, such as 1 nm, 2 nm, 3 nm, 4 nm, or 5 nm, etc. A suitable thickness can balance electrical conductivity and mechanical strength.

[0039] In some embodiments, the thickness of the fast ion conductor layer is 0.05-0.1 nm, such as 0.05 nm, 0.08 nm, 0.1 nm, etc. The fast ion conductor layer can better construct an ion network and reduce solid-solid interface impedance.

[0040] In some embodiments, the doping amount of the element nitrogen in the carbon coating layer is 5% to 15%, for example, 5%, 8%, 10%, 12% or 15%.

[0041] In some embodiments, the carbon coating layer accounts for 5% to 10% by mass of the composite negative electrode material, for example, 5%, 8%, 10%, etc.

[0042] In some embodiments, the fast ion conductor layer comprises 0.05% to 0.3% by weight of the composite anode material, for example, 0.05%, 0.08%, 0.1%, 0.2%, 0.3%, etc. A suitable fast ion conductor layer content can form continuous ion channels and optimize interface performance. This greatly mitigates structural damage caused by volume expansion and contraction during lithium removal and insertion in the silicon anode, ensuring the necessary ion and electron transport network within the anode and structural stability during cycling. Excessive fast ion conductor content can hinder electron conduction, reducing battery capacity and stability. Excessive fast ion conductor content can lead to insufficient interface optimization.

[0043] According to another aspect of the present invention, the present invention also relates to a method for preparing the composite negative electrode material as described above, comprising the following steps: (a) Obtaining silicon nanosheets.

[0044] (b) depositing a carbon material on the surface of the silicon nanosheet to form a carbon coating layer to obtain a silicon-carbon material.

[0045] (c) performing nitrogen doping on the silicon-carbon material to obtain a negative electrode base material.

[0046] (d) Mixing and grinding the negative electrode matrix material and the fast ion raw material to form a fast ion superconductor layer.

[0047] The preparation method of the composite negative electrode material of the present invention includes obtaining silicon nanosheets, carbon deposition, nitrogen doping, and fast ion layer coating. Through the coordination of each step, the obtained composite negative electrode material has excellent electrochemical performance.

[0048] In some embodiments, the method for obtaining the silicon nanosheets specifically includes treating calcium disilicide with sodium hydroxide, followed by treatment with 10-15 mol / L (e.g., 10 mol / L, 12 mol / L, 15 mol / L, etc.) concentrated hydrochloric acid; or heating silicon tetrachloride liquid to 50-65°C (e.g., 50°C, 55°C, 60°C, etc.), and depositing the resulting gas onto a hot silicon dioxide substrate under a hydrogen atmosphere. Silicon nanosheets can be obtained by chemical exfoliation or CVD deposition. In the chemical exfoliation method, NaOH removes calcium to form Si-H bond intermediates, which are then further exfoliated with HCl to yield a single-layer silicon sheet. High acid concentrations ensure complete exfoliation, avoiding residual calcium or SiOx impurities. In the CVD deposition method, SiCl4 is converted into elemental silicon in a reducing H2 atmosphere, which is then guided into two-dimensional growth on a hot substrate (e.g., SiO2). The layered structure of two-dimensional silicon nanosheets effectively mitigates volume expansion. Furthermore, their high surface area provides numerous active sites for lithium ion reactions, resulting in superior reaction kinetics.

[0049] In some embodiments, the carbon material comprises at least one of methane, acetylene, ethylene, ethanol and n-butanol. An amorphous carbon layer is formed by using a suitable carbon material.

[0050] In some embodiments, the temperature of depositing the carbon material is 600-800° C., for example, 600° C., 650° C., 700° C., 800° C., etc. A suitable temperature can balance the crystallinity and defect density of the carbon layer and optimize the performance of the carbon layer.

[0051] In some embodiments, the nitrogen doping treatment specifically includes: mixing a mixture of a nitrogen doping raw material and ammonia water with a silicon carbon material, and performing a heat treatment. The nitrogen doping raw material includes at least one of 1H-1,2,3-triazole and polyacrylonitrile, and the triazole is pyrolyzed in ammonia water to generate NH3, CN - Active nitrogen sources such as PAN form covalent bonds with the carbon layer; PAN is carbonized at high temperature to form a nitrogen-containing carbon skeleton, enhancing the uniformity of nitrogen doping throughout the carbon layer. During the nitrogen doping process, the heat treatment temperature is 30-90°C, for example, 30°C, 40°C, 50°C, 60°C, 80°C, 90°C, etc., and appropriate heat treatment is used to ensure the hydrothermal reaction effect and uniformly distribute the nitrogen element on the material surface.

[0052] In some embodiments, the fast ion raw material includes LiOH and NH4H2PO4. In some embodiments, the mass ratio of the negative electrode matrix material to LiOH and NH4H2PO4 is (88-92):(4-6):(4-6), for example, 88:6:6, 90:5:5, 92:4:4, etc. By limiting the mass ratio of the negative electrode matrix material to LiOH and NH4H2PO4, a uniform coating layer of appropriate thickness is formed on the surface of the negative electrode matrix material.

[0053] In some embodiments, the surface activity and ion transport capacity of the composite negative electrode material are improved by mixing LiOH, NH4H2PO4 and the above-mentioned negative electrode matrix material and ball milling, and preparing a SiO@Li3PO4 in-situ coating layer in a low-oxygen and high-temperature environment.

[0054] According to another aspect of the present invention, the present invention also relates to a negative electrode sheet, comprising the composite negative electrode material, or the composite negative electrode material prepared by the method for preparing the composite negative electrode material.

[0055] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on the surface of the negative electrode current collector, and the negative electrode material layer includes the above-mentioned composite negative electrode material.

[0056] In some embodiments, the preparation method of the negative electrode sheet includes: mixing the composite negative electrode material and the binder in a mass ratio of (95~97): (3~5), then adding the mixture to a solvent (such as dimethylformamide solvent), and obtaining a uniformly mixed negative electrode slurry by magnetic stirring. The multi-layer coated silicon nanosheets uniformly dispersed inside the electrode can inhibit the volume expansion of silicon during the cycle, and can also provide additional ion transmission channels for the electrode. The above-mentioned negative electrode slurry is prepared by drop coating or blade coating with a loading of 0.15-0.3 mg / cm 2 The electrode pieces between the electrodes are obtained to obtain composite silicon electrode pieces evenly distributed on the surface of the copper foil. The composite silicon electrode pieces are transferred to an oven for drying and heated to 140-160°C to remove the solvent. The dried electrode pieces are cut into composite silicon electrode pieces with a diameter of 8-15 mm using a punching machine. The surface loading is 0.15-0.3 mg / cm 2 , for example, it can be 0.15 mg / cm 2 , 0.2 mg / cm 2 wait.

[0057] According to another aspect of the present invention, the present invention also relates to a battery comprising the negative electrode sheet.

[0058] The battery of the present invention has high energy density, excellent long cycle performance and fast charging capability.

[0059] The following is further explained with reference to specific embodiments and comparative examples.

[0060] Example 1 A method for preparing a composite negative electrode material comprises the following steps: (a) 6 g of commercial calcium disilicide was rinsed with sodium hydroxide and then chemically exfoliated with 12 mol / L concentrated hydrochloric acid to obtain two-dimensional silicon nanosheets with a thickness of 5-10 nm.

[0061] (b) The obtained two-dimensional silicon nanosheets were coated with a carbon layer by CVD at 700°C using CH4 and C2H2 (mass ratio of 1:1) as carbon sources. The carbon coating layer accounted for 8% of the mass percentage of the composite negative electrode material. (c) Then, 1 g of 1H-1,2,3-triazole (C2H3N3) ligand was added to 10 mL of ammonia water and heated to 85 °C by hydrothermal method to obtain the negative electrode matrix material.

[0062] (d) The obtained negative electrode matrix material and the raw material of the fast ion conductor coating layer were mixed and ball-milled in a mass ratio of negative electrode matrix material: LiOH: NH4H2PO4 = 90:5:5 at a rotation speed of 500 rpm for 3 h to obtain a composite negative electrode material.

[0063] A method for preparing a negative electrode sheet, comprising: (1) The above-mentioned composite negative electrode material and PAA binder were weighed in a ratio of 95:5. 342 mg of silicon nanosheets and 18 mg of PAA binder were added to 2 mL of DMF and magnetic stirring was used to obtain a uniformly mixed negative electrode slurry.

[0064] (2) Take a clean copper foil and clean the surface with anhydrous ethanol. After the surface is air-dried, apply the above slurry on the surface of the copper foil by scraping (silicon loading = 0.20 mg / cm 2 The copper foil coated with the negative electrode slurry was then placed in an 80°C vacuum drying oven and dried for 4 hours to remove the DMF. After drying, the electrode sheet was removed and then punched into negative electrode sheets with a diameter of 10 mm using a punching machine.

[0065] Example 2 A method for preparing a composite negative electrode material, which differs from Example 1 in that: Silicon tetrachloride liquid is heated to 60°C, and the resulting gas reacts in a hydrogen atmosphere to generate silicon atoms, which are deposited on a heated silicon dioxide substrate to form two-dimensional silicon nanosheets with a thickness of 5 to 10 nm.

[0066] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, other conditions are the same as those in Example 1.

[0067] Example 3 A method for preparing a composite negative electrode material, which differs from Example 1 in that: The obtained negative electrode base material and the raw material of the fast ion conductor coating layer were mixed and ball-milled according to the negative electrode base material: LiOH: NH4H2PO4 = 88:6:6 (mass ratio) to obtain a composite negative electrode material.

[0068] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, other conditions are the same as those in Example 1.

[0069] Example 4 A method for preparing a composite negative electrode material, which differs from Example 1 in that: The obtained negative electrode base material and the raw material of the fast ion conductor coating layer were mixed and ball-milled according to the negative electrode base material: LiOH: NH4H2PO4 = 92:4:4 (mass ratio) to obtain a composite negative electrode material.

[0070] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, the composite negative electrode material:PAA binder = 97:3, and other conditions are the same as in Example 1.

[0071] Example 5 A method for preparing a composite negative electrode material, which differs from Example 1 in that: In step (c), polyacrylonitrile (PAN) and ammonia water are added to the material obtained in step (b), and the mixture is heated to 30° C. by a hydrothermal method.

[0072] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, other conditions are the same as those in Example 1.

[0073] Example 6 A method for preparing a composite negative electrode material, which differs from Example 1 in that: In step (c), the material obtained in step (b) is added to a mixture of C2H3N3, PAN and aqueous ammonia, and heated to 30°C for 5 hours, and then heated to 85°C for 5 hours.

[0074] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, other conditions are the same as those in Example 1.

[0075] Example 7 A method for preparing a composite negative electrode material, which differs from Example 1 in that: The carbon source in step (b) is replaced with methane, ethylene, ethanol and n-butanol in a mass ratio of 1:1:1:1.

[0076] A method for preparing a negative electrode sheet, except that the composite negative electrode material in this embodiment is used, other conditions are the same as those in Example 1.

[0077] Example 8 A method for preparing a negative electrode sheet differs from Example 1 in that: PAA was replaced by SBR and CMC, and the mass ratio of SBR to CMC was 2:1.

[0078] Comparative Example 1 A method for preparing a composite negative electrode material, which differs from Example 1 in that: Step (a) uses OD micron silicon powder.

[0079] The preparation method of the negative electrode sheet is the same as that of Example 1 except that the composite negative electrode material in this comparative example is used.

[0080] Comparative Example 2 A method for preparing a composite negative electrode material comprises the following steps: (a) Commercial calcium disilicide was rinsed with sodium hydroxide and then chemically exfoliated with 12 mol / L concentrated hydrochloric acid to obtain two-dimensional silicon nanosheets with an average thickness of 5-10 nm.

[0081] (b) The obtained two-dimensional silicon nanosheets were coated with a carbon layer by CVD at 700°C using CH4 and C2H2 (mass ratio of 1:1) as carbon sources.

[0082] (c) The material obtained in step (b) was mixed with the raw material of the fast ion conductor coating layer in a mass ratio of negative electrode matrix material: LiOH: NH4H2PO4 = 90:5:5, and ball milled to obtain a composite negative electrode material.

[0083] The preparation method of the negative electrode sheet is the same as that of Example 1 except that the composite negative electrode material in this comparative example is used.

[0084] Comparative Example 3 A method for preparing a composite negative electrode material, which differs from Example 1 in that: The negative electrode base material was obtained according to the method of Example 1, without performing the operation of step (d).

[0085] The preparation method of the negative electrode sheet is the same as that of Example 1 except that the composite negative electrode material in this comparative example is used.

[0086] Experimental example Batteries were prepared using the negative electrode sheets obtained in each embodiment and comparative example.

[0087] The Si / LPSC / Li-In all-solid-state battery was assembled in an argon glove box using a custom-made battery case equipped with a 10 mm diameter ceramic sleeve and two stainless steel presses to seal the battery.

[0088] The assembly process mainly includes four steps: electrolyte layer preparation, electrode layer composite, counter electrode preparation and battery packaging.

[0089] First, 100 mg of Li6PS5Cl solid electrolyte powder was transferred into the mold sleeve, and a pressure of 100 MPa was applied on the tablet press and maintained for 5 minutes to form a dense electrolyte layer.

[0090] Subsequently, the side of the electrode sheet carrying the active material was placed on the electrolyte layer, and a pressure of 380 MPa was applied to the electrode sheet and the electrolyte layer and maintained for 10 minutes to ensure that the electrode layer and the electrolyte layer were tightly bonded.

[0091] Next, a Li-In alloy counter electrode was assembled on the other side of the electrolyte layer, and a pressure of 100 MPa was applied and maintained for 1 minute to ensure good contact between the metal-electrolyte interface.

[0092] Finally, the assembled mold sleeve is placed in a special battery casing, and after installing the pressure sensor, an initial axial pressure of 100 MPa is applied. Finally, the assembled mold sleeve is placed in a special battery casing, and after installing the pressure sensor, an initial axial pressure of 100 MPa is applied.

[0093] The battery performance test results are shown in Table 1.

[0094] Table 1 Test results

[0095] From the above, it can be seen that the battery prepared by the composite negative electrode material obtained by the method of the present invention has a high first-cycle discharge specific capacity, a high first-cycle coulombic efficiency and an excellent capacity retention rate.

[0096] The capacity retention rate of the batteries obtained from the negative electrode materials of Comparative Examples 1 to 3 is poor.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite negative electrode material, characterized in that It includes a negative electrode base material and a fast ion conductor layer located on the surface of the negative electrode base material; The negative electrode matrix material includes silicon nanosheets and a carbon coating layer located on the surface of the silicon nanosheets, wherein the surface of the carbon coating layer is doped with elemental nitrogen; The fast ion conductor layer includes Li3PO4.

2. The composite negative electrode material according to claim 1, characterized in that Contains at least one of the following features (1) to (3): (1) The thickness of the silicon nanosheet is 4 to 15 nm; (2) The thickness of the carbon coating layer is 1 to 5 nm; (3) The thickness of the fast ion conductor layer is 0.05-0.1 nm.

3. The composite negative electrode material according to claim 1, characterized in that Contains at least one of the following features (1) to (3): (1) The doping amount of the element nitrogen in the carbon coating layer is 5% to 15%; (2) The mass percentage of the carbon coating layer to the composite negative electrode material is 5% to 10%; (3) The mass percentage of the fast ion conductor layer to the composite negative electrode material is 0.05% to 0.3%.

4. The method for preparing a composite negative electrode material according to any one of claims 1 to 3, wherein: The following steps are involved: (a) Obtaining silicon nanosheets; (b) depositing a carbon material on the surface of the silicon nanosheet to form a carbon coating layer to obtain a silicon-carbon material; (c) performing nitrogen doping on the silicon-carbon material to obtain a negative electrode matrix material; (d) Mixing and grinding the negative electrode matrix material and the fast ion raw material to form a fast ion superconductor layer.

5. The method for preparing a composite negative electrode material according to claim 4, wherein: The method for obtaining the silicon nanosheets specifically includes: treating calcium disilicide with sodium hydroxide and then treating it with 10-15 mol / L concentrated hydrochloric acid; or heating silicon tetrachloride liquid to 50-65°C and depositing the resulting gas on a hot silicon dioxide substrate under a hydrogen atmosphere.

6. The method for preparing a composite negative electrode material according to claim 4, wherein: Contains at least one of the following features (1) to (2): (1) The carbon material includes at least one of methane, acetylene, ethylene, ethanol and n-butanol; (2) The temperature of the deposited carbon material is 600-800°C.

7. The method for preparing a composite negative electrode material according to claim 4, wherein: Contains at least one of the following features (1) to (3): (1) The nitrogen doping treatment specifically includes: mixing a mixture of a nitrogen doping raw material and ammonia water with a silicon-carbon material, and performing a heat treatment; (2) The nitrogen-doped raw material includes at least one of 1H-1,2,3-triazole and polyacrylonitrile; (3) During the nitrogen doping process, the heat treatment temperature is 30-90°C.

8. The method for preparing a composite negative electrode material according to claim 4, wherein: Contains at least one of the following features (1) to (2): (1) The fast ion raw materials include LiOH and NH4H2PO4; (2) The mass ratio of the negative electrode matrix material to LiOH and NH4H2PO4 is (88~92):(4~6):(4~6).

9. A negative electrode sheet, characterized in that: A composite negative electrode material comprising the composite negative electrode material according to any one of claims 1 to 3, or a composite negative electrode material prepared by the preparation method of the composite negative electrode material according to any one of claims 4 to 8.

10. A battery, characterized in that: Contains the negative electrode sheet according to claim 9.

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