Negative electrode active material and preparation method thereof, secondary battery and electric equipment

Abstract

The invention provides a negative electrode active material and a preparation method thereof, a secondary battery and electric equipment. The negative electrode active material comprises a lithium carbon core and a coating layer coating the lithium carbon core, the structural general formula of the lithium carbon core is LiMCN, M + N = 1, M represents the atom number ratio of Li atoms, N represents the atom number ratio of C atoms, and M / N is greater than or equal to 1 and less than 2; the coating layer has a porous structure, and the coating layer comprises a halide electrolyte cross-linked conductive polymer. A lithium carbon material is used as an inner core, and coating modification is performed on the surface of the inner core, so that the coulombic efficiency and the cycle performance of the battery are further improved.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, in particular to a negative electrode active material and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0002] Market demands for driving range are constantly increasing. In lithium-ion secondary batteries, battery range is constrained by energy density. The anode material of lithium batteries has a significant impact on technical indicators such as battery energy density and cycle life. Metallic lithium anodes have a low redox potential and ultra-high theoretical specific capacity, but their practical applications are limited by shortcomings such as non-uniform lithium deposition, lithium dendrite growth, and low Coulombic efficiency. To more rationally apply carbon materials in electrode design, leveraging their lightweight, high conductivity, and tunable physical and chemical properties, they serve as ideal carriers for stabilizing lithium metal. Composite lithium-carbon materials can be used to address these issues.

[0003] Like other lithium-ion anode materials, the surface structure and properties of lithium-carbon anode materials significantly influence their electrochemical performance. The use of lithium-carbon materials in lithium batteries generates a large amount of side products, resulting in poor cycling performance. Summary of the Invention

[0004] The purpose of this application is to overcome the above-mentioned problems in the prior art and provide a new negative electrode active material with a lithium-carbon material as the core and a coating modification on its surface to further improve the energy density, coulombic efficiency and cycle performance of the battery.

[0005] To achieve the above objectives, the first aspect of the present application provides a negative electrode active material, comprising a lithium-carbon core and a coating layer coating the lithium-carbon core:

[0006] The general structural formula of the lithium carbon core is Li M C N , M+N=1, M represents the atomic number ratio of Li atoms, N represents the atomic number ratio of C atoms, and 1≤M / N<2;

[0007] The coating layer includes a halide electrolyte cross-linked conductive polymer; the structural formula of the halide electrolyte is Li x ZT y , Z includes at least one metal element selected from Al, Ga, In, Sc, Y, La, and Ho, T is a halogen, 0 <x≤15,1≤y≤17。

[0008] As an embodiment of the present application, the lithium-carbon core has the following structure: the lithium-carbon core includes porous carbon and lithium supported on the porous carbon.

[0009] As an embodiment of the present application, the particle size D of the lithium carbon core is V50 is A, in μm, the thickness of the coating layer is B, in μm, 0.2≤A / B≤0.5.

[0010] As an embodiment of the present application, the median particle size D of the negative electrode active material is V 50 is 5~400μm.

[0011] As an embodiment of the present application, the pore size of the coating layer is 2 to 20 nm.

[0012] As an embodiment of the present application, in the halide electrolyte cross-linked conductive polymer, the mass ratio of the halide electrolyte to the conductive polymer is (0.25-3):1.

[0013] As an embodiment of the present application, the conductive polymer includes at least one of epoxy resin, polyacrylonitrile, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacrylic acid, and polyacrylamide.

[0014] The second aspect of the present application provides a method for preparing the negative electrode active material according to the first aspect of the present application, comprising the following steps:

[0015] S1: Preparation of coating layer precursor

[0016] S11: adding lithium halide, metal Z halide, Lewis base, catalyst and surfactant to water to form a halide electrolyte slurry;

[0017] S12: uniformly mixing the conductive polymer monomer, hydrochloric acid, initiator, dispersant, and the halide electrolyte slurry prepared in step S11, initiating polymerization at 20-210° C., and performing the polymerization reaction for 16-48 hours to obtain a coating layer precursor;

[0018] S2: In-situ coating

[0019] The coating layer precursor and lithium-carbon core material obtained in step S1 are mixed and uniformly mixed, and then reacted at 0-20°C for 1-3 hours. The precipitated product is washed with water and dried, and then further dispersed in an organic solvent. A pore regulator is added, and the mixture is reacted at 30-110°C for 5-24 hours. The mixture is then calcined at 100-300°C for 2-48 hours to obtain the negative electrode active material.

[0020] As an embodiment of the present application, the surfactant includes sodium lauryl sulfate.

[0021] As an embodiment of the present application, the solid content of the halide electrolyte slurry is 50-80 wt %.

[0022] As an embodiment of the present application, the initiator includes at least one of ammonium sulfate, azobisisobutyronitrile, and benzoyl peroxide.

[0023] As an embodiment of the present application, the dispersant includes hydrogenated nitrile rubber.

[0024] As an embodiment of the present application, the organic solvent includes at least one of acetonitrile, tetrahydrofuran, dimethyl ether, N-methylformamide, and 1,2-dimethoxyethane.

[0025] As an embodiment of the present application, the pore regulating agent includes at least one of C5-C15 alkanes and fluorocarbons.

[0026] The third aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer provided on at least one side surface of the negative electrode collector, and the negative electrode active material layer includes the negative electrode active material described in the first aspect of the present application.

[0027] In a fourth aspect of the present application, an electrical device is provided, wherein the electrical device includes the secondary battery described in the third aspect of the present application.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] This application uses lithium-carbon material as the core and performs coating modification on its surface to further improve the coulombic efficiency and cycle performance of the battery. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0031] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions of the listed features.

[0032] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0033] The reagents and instruments used in this application without manufacturer indication are all conventional products that can be purchased commercially.

[0034] In a first aspect of the present application, a negative electrode active material is provided, comprising a lithium-carbon core and a coating layer coating the lithium-carbon core:

[0035] The general structural formula of the lithium carbon core is Li M C N , M+N=1, M represents the atomic number ratio of Li atoms, N represents the atomic number ratio of C atoms, and 1≤M / N<2;

[0036] The coating layer includes a halide electrolyte cross-linked conductive polymer; the structural formula of the halide electrolyte is Li x ZT y , Z includes at least one metal element selected from Al, Ga, In, Sc, Y, La, and Ho, T is a halogen, 0 <x≤15,1≤y≤17。

[0037] In the composite active material of the present application, a lithium-carbon composite material is used as the core, and a conductive polymer composite coating layer containing a halide electrolyte is coated on its surface, which effectively reduces the contact between the lithium-carbon core material and the electrolyte, reduces the side reaction between lithium-carbon and the electrolyte, and improves the cycle stability of the battery. In the lithium-carbon core, the carbon material has good conductivity, but during the charge and discharge cycle of the battery, the repeated insertion and removal of lithium ions will cause the crystal structure of the carbon material to change, thereby causing volume expansion; inserting lithium can increase the expansion performance of the carbon material, but inserting too much will cause the conductivity to deteriorate. Reasonable control of the ratio of lithium and carbon elements in the lithium-carbon composite material can work together with the coating layer, utilizing the appropriate expansion performance between the core and the coating layer to improve the conductive network of the negative electrode active material during the cycle, improve the ion transport performance, and further improve the cycle life and coulomb efficiency.

[0038] In some embodiments, the lithium-carbon core has the following structure: the lithium-carbon core includes porous carbon and lithium supported on the porous carbon. This structure of the lithium-carbon core can further improve the cycle stability of the battery.

[0039] In some embodiments, the particle size D of the lithium carbon core is V 50 is A, measured in μm, the thickness of the coating layer is B, measured in μm, 0.2 ≤ A / B ≤ 0.5, and the value of A / B can be any value selected from 0.5, 0.4, 0.3, and 0.2, or an interval consisting of any two values. When the lithium-carbon core and coating layer are within the above-mentioned suitable ranges, both excellent cycle stability and coulombic efficiency can be achieved.

[0040] In some embodiments, the median particle size D of the negative electrode active material V 50 is 5 to 400 μm. The negative electrode active material in this application, the lithium carbon core can be formed into particles after being coated with the coating layer. Therefore, the particle size of the negative electrode active material here refers to the particle size of the product after the lithium carbon core is coated with the coating layer. The median particle size D of the negative electrode active material V Specifically, 50 can be any one of 8 μm, 50 μm, 150 μm, 300 μm, and 400 μm, or an interval consisting of any two values.

[0041] In some embodiments, the pore size of the coating layer is 2 to 20 nm. The pore size can be any one of 3 nm, 40 nm, and 100 nm, or a range consisting of any two values. The pores in the coating layer can improve the wettability of the negative electrode active material with the electrolyte, which is beneficial to ion transport. The pore size within the above-mentioned appropriate range can make the coating layer have good flexibility and mechanical strength. While improving the wettability of the electrolyte, it also provides good protection for the lithium-carbon core, significantly improving the cycle life of the battery.

[0042] In some embodiments, in the coating layer material halide electrolyte cross-linked conductive polymer, the mass ratio of halide electrolyte to conductive polymer is (0.25-3):1. Specifically, it can be any one of 0.25:1, (3 / 7):1, 0.125:1, 1:1, (7 / 3):1, or an interval consisting of any two values. In the coating material of the coating layer, the conductive polymer has a certain flexibility, and the halide electrolyte is embedded in the conductive polymer chain segment. It can move with the polymer chain segment during the battery charge and discharge cycle, thereby improving the conductive network structure in the negative electrode active material and thus improving the cycle life of the battery. When the ratio of halide electrolyte to conductive polymer is within the above-mentioned appropriate range, the conductivity of the conductive polymer and the halide electrolyte, as well as the flexibility of the conductive polymer, can be fully utilized, significantly improving the cycle life of the battery.

[0043] In the embodiments of the present application, any conductive polymer commonly used in the art for secondary batteries can be used to prepare the negative electrode active material in the present application. The conductive polymer includes, but is not limited to, at least one of epoxy resin, polyacrylonitrile, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacrylic acid, and polyacrylamide.

[0044] The second aspect of the present application provides a method for preparing the negative electrode active material according to the first aspect of the present application, comprising the following steps:

[0045] S1: Preparation of coating layer precursor

[0046] S11: adding lithium halide, metal Z halide, Lewis base, catalyst and surfactant to water to form a halide electrolyte slurry;

[0047] S12: uniformly mixing the conductive polymer monomer, hydrochloric acid, initiator, dispersant, and the halide electrolyte slurry prepared in step S11, initiating polymerization at 20-210° C., and performing the polymerization reaction for 16-48 hours to obtain a coating layer precursor;

[0048] S2: In-situ coating

[0049] The coating layer precursor and lithium-carbon core material obtained in step S1 are mixed and uniformly mixed, and then reacted at 0-20°C for 1-3 hours. The precipitated product is washed with water and dried, and then further dispersed in an organic solvent. A pore regulator is added, and the mixture is reacted at 30-110°C for 5-24 hours. The mixture is then calcined at 100-300°C for 2-48 hours to obtain the negative electrode active material.

[0050] The present application is to mix the raw materials of the halide electrolyte with the polymer monomers of the conductive polymer. During the polymerization process of the conductive polymer, the halide electrolyte is in situ embedded in the molecular chain of the conductive polymer and the three-dimensional network structure formed by the entanglement of the molecular chains through covalent action or interionic charge force, thereby forming a tough coating layer precursor. The surface of the halide particles forms a covalent bond (such as a COZ bond, where Z is a metal element) with the polymer chain through a cross-linking reaction, significantly enhancing the interfacial bonding strength and reducing the contact impedance. The coating layer precursor material is then first uniformly dispersed and preliminarily coated with the lithium-carbon core material at a relatively low temperature (0-20°C), and then further reacted in an environment of an organic solvent and a pore regulator at a relatively high temperature (30-110°C) to strengthen the bonding force between the coating layer and the lithium-carbon core. Finally, the coating layer is further calcined in an environment of 100-300°C to generate a porous coating layer, thereby obtaining the negative electrode active material.

[0051] This application is to in-situ coat the halide electrolyte and conductive polymer onto the surface of the lithium-carbon core material, which can significantly improve the bonding force between the coating layer and the core material, significantly improve the cycle stability of the negative electrode active material, and thus improve the cycle life of the battery.

[0052] In some embodiments, the solid content of the halide electrolyte slurry is 50-80 wt %. The solid content within this range is conducive to uniform dispersion in the polymer monomer. In the electrolyte slurry of the present application, the amount ratio of lithium halide and metal Z halide is based on the structural formula Li x ZT y The design is adjusted, wherein the halogen T can specifically be a Cl element or a Br element.

[0053] In some embodiments, the surfactant includes, but is not limited to, sodium lauryl sulfate. The addition of the surfactant can uniformly disperse the components in the electrolyte slurry. The amount of the surfactant added is 0.1 to 1 wt % of the total mass of the lithium halide and the halide of metal Z.

[0054] In some embodiments, the Lewis base is a phosphorus trihalide, specifically at least one selected from PCl3 and PBr3. The amount of the Lewis base is 0.1-2 wt% of the total mass of the reaction monomers (lithium halide, metal Z halide).

[0055] In some embodiments, the catalyst comprises phosphorus pentasulfide. The catalyst is used in an amount of 0.1 to 1 wt % of the total mass of the reaction monomers (lithium halide, halide of metal Z). In some embodiments, the initiator for initiating the polymerization of the conductive polymer comprises, but is not limited to, at least one of ammonium sulfate, azobisisobutyronitrile, and benzoyl peroxide.

[0056] In some embodiments, the dispersant described in step S12 includes but is not limited to hydrogenated nitrile rubber.

[0057] In some embodiments, the organic solvent in step S2 includes but is not limited to at least one of acetonitrile, tetrahydrofuran, dimethyl ether, N-methylformamide, and 1,2-dimethoxyethane.

[0058] In some embodiments, the pore modifier includes, but is not limited to, at least one of a C5-C15 alkane and a fluorocarbon. Under the action of an organic solvent, the pore modifier can be uniformly loaded into or even infiltrated into the coating layer. During subsequent high-temperature calcination, the pore modifier decomposes to produce gas, which forms a pore structure on the surface of the coating layer. The pore structure of the coating layer can be adjusted by adjusting process parameters such as the amount of pore modifier added, the calcination temperature, and time.

[0059] In addition, it should be noted that, in the present application, in order to further improve the coating uniformity of the coating layer, the coating can be performed by multiple coatings or multi-layer coatings.

[0060] In a third aspect of the present application, a secondary battery is provided, comprising a positive electrode plate, a separator, a negative electrode plate and an electrolyte, wherein the negative electrode plate comprises a negative electrode collector and a negative electrode active material layer provided on at least one side surface of the negative electrode collector, wherein the negative electrode active material layer comprises the negative electrode active material described in the first aspect of the present invention.

[0061] In an embodiment of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material. The present application does not limit the type of positive electrode active material, and the positive electrode active materials commonly used in the art can be used to prepare secondary batteries in the present application. The positive electrode active materials include but are not limited to lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium nickel cobalt manganese oxide (general formula LiNi x Co y Mn z O2, wherein x+y+z=1, and 0<x<1, 0<y<1, 0<z<1).

[0062] In this application, there is no limitation on the types of positive electrode current collector, negative electrode current collector, separator, and electrolyte, and they can be selected according to the needs. Common current collector materials, separators, and electrolytes in the art can be used in this application.

[0063] In some embodiments, the negative electrode current collector may preferably be made of copper foil or carbon-coated copper foil.

[0064] The positive electrode current collector can be made of metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc.; carbon materials such as carbon cloth, carbon paper, etc.; composite materials formed by polymer and metal layer. In some embodiments, the positive electrode current collector is preferably made of aluminum foil.

[0065] In some embodiments, the type of solvent used to form the positive electrode slurry and / or the negative electrode slurry is not limited, as long as it can dissolve or disperse the positive electrode active material, the negative electrode active material, the conductive agent, the binder, and the dispersant.

[0066] In the secondary battery described in this application, the type of separator is not particularly limited and can be selected according to actual needs. The separator can be a polypropylene film, a polyethylene film, a polyvinylidene fluoride film, a spandex film, an aramid film, or a multi-layer composite film modified by a coating.

[0067] In the secondary battery described in this application, the type of electrolyte is not particularly limited and can be selected according to actual needs.

[0068] In some embodiments, the preparation of a secondary battery includes: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, placing the separator between the positive and negative electrode sheets to act as an isolater, and then winding them into a square bare cell, placing them in a battery casing, and then baking them at 65 to 95°C to remove water, injecting electrolyte, sealing, and obtaining a secondary battery after standing, hot and cold pressing, formation, clamping, and capacity separation.

[0069] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery may also be a soft shell, such as a bag-type soft shell. The soft shell may be made of a plastic such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited, and it may be cylindrical, square, or any other shape.

[0070] The fourth aspect of the present application provides an electrical device, which includes the secondary battery described in the third aspect of the present application. The electrical device can be an application device such as a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool. The vehicle can be a new energy vehicle, which can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft; the electric toys include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; the electric tools include metal cutting electric tools, grinding electric tools, assembly electric tools, and railway electric tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. The embodiments of the present application do not impose any special restrictions on the above-mentioned devices.

[0071] The following are specific examples of the present application, and the technical solutions of the present application are further described in conjunction with the examples, but the present application is not limited to these examples. The reagents, methods and equipment used in this application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0072] Example 1

[0073] Provide a negative electrode active material: the core is a structure with the general formula Li 0.6 C 0.4 Lithium carbon composite material, Li 0.6 C 0.4 The microstructure is a spherical structure in which lithium is loaded in porous carbon; the coating layer is a composite coating layer obtained by cross-linking "20 wt% LiAlCl4 + 80 wt% polyacrylonitrile", the ratio A / B of the particle size Dv50 (μm) of the core to the thickness B (μm) of the coating layer is 1:3, and the preparation method comprises the following steps:

[0074] S1: Preparation of coating layer precursor

[0075] S11: According to the stoichiometric ratio of the halide electrolyte LiAlCl4, the reaction monomers "LiCl, AlCl3" are mixed and dispersed in water, and then 0.1wt% of sodium lauryl sulfate, 0.5wt% of PCl3, and 0.1wt% of phosphorus pentasulfide are added to the water to form a halide electrolyte slurry with a solid content of 70wt%;

[0076] S12: According to the mass ratio of the halide electrolyte to the conductive polymer in the coating layer described in Table 1, the conductive polymer monomer (acrylonitrile in this embodiment), hydrochloric acid accounting for 5 wt% of the polymer monomer, azobisisobutyronitrile as an initiator accounting for 1 wt% of the polymer monomer, hydrogenated nitrile rubber accounting for 3 wt% of the polymer monomer, and the halide electrolyte slurry prepared in step S11 are uniformly mixed, and cross-linking polymerization is initiated at 90° C. The polymerization reaction is carried out for 20 hours to obtain a coating layer precursor;

[0077] S2: In-situ coating

[0078] The coating layer precursor and lithium carbon core material obtained in step S1 were mixed and then reacted at 15°C for 1.5h. The precipitated product was washed with water and dried, and then further dispersed in an organic solvent (acetonitrile). A pore modifier (hexane, C6H 14 ), react at 85° C. for 18 h, and then calcine at 260° C. for 10 h to obtain the negative electrode active material. The median particle size D V 50 is 8μm.

[0079] Examples 2 to 6

[0080] A series of negative electrode active materials were provided and prepared according to the method of Example 1. The difference from Example 1 was that the mass ratio of the halide electrolyte to the conductive polymer in the coating layer in step S12 was changed. The specific usage ratios are shown in Table 1.

[0081] Examples 7 to 10

[0082] A series of negative electrode active materials are provided, which are prepared according to the method of Example 1. The difference from Example 1 is that the halide electrolyte Li in the coating layer is changed. x ZT y The type, specific selection and dosage of Z and T are different. The chemical formula of the target product of the specific halide electrolyte is shown in Table 1;

[0083] In the specific preparation process, the type and amount of the reaction monomer in step S11 are adaptively changed according to the chemical molar ratio in the chemical formula of the target product of the halide electrolyte shown in Table 1.

[0084] Examples 11 to 13

[0085] A series of negative electrode active materials were provided and prepared according to the method of Example 1. The difference from Example 1 was that the type of conductive polymer in the coating layer in step S12 was changed:

[0086] In Example 11, the polymerizable monomer of the conductive polymer selected was acrylic acid, and the polymerization reaction was carried out at 80° C. for 20 h.

[0087] In Example 12, the conductive polymer monomer selected was aniline, and the polymerization reaction was carried out at 90° C. for 20 h.

[0088] The polymerizable monomer of the conductive polymer selected in Example 13 is pyrrole, and the polymerization reaction is carried out at 25° C. for 20 hours.

[0089] Examples 14 to 16, Comparative Examples 2 to 4

[0090] A series of negative electrode active materials were provided, which were prepared according to the method of Example 1. The difference from Example 1 was that the ratio of Li element to C element in the lithium-carbon core material was changed, and lithium-carbon core materials with different ratios of Li element to C element were selected.

[0091] It should be noted that the lithium-carbon composite material in the embodiments of the present invention is prepared by a conventional high-temperature solid-phase method in the art. Taking Example 1 as an example, the specific preparation includes the following steps:

[0092] The metallic Li and carbon material are fully mixed in a certain proportion and ground evenly, the mixed raw materials are placed in a mold, and tablets are formed under a pressure of 5 MPa to form a green body of the desired shape. The green body is placed in a high-temperature furnace, and the temperature is raised to 1000°C at a rate of 5°C / min in a nitrogen atmosphere, and calcined for 8 hours; Examples 14 to 16 and Comparative Examples 2 to 4 can change the usage ratio of metallic Li and carbon.

[0093] Examples 17-18

[0094] The negative electrode active materials of Examples 17 to 18 were prepared according to the method of Example 1. The difference from Example 1 is that only the mass ratio of the lithium-carbon core material to the coating layer precursor in step S2 was changed (see Table 1 for the specific mass ratio), and the other preparation processes were the same as those in Example 1 and remained unchanged.

[0095] Examples 19-20

[0096] The negative electrode active materials of Examples 19 to 20 are prepared by referring to the method of Example 1. By changing the reaction temperature in step S2 of Example 1, the rate at which the pore regulator decomposes into gas at high temperature can be adjusted. The higher the reaction temperature, the faster the decomposition rate of the pore regulator, and the larger the average pore size of the pores formed in the coating layer. The reaction temperature in step S2 of Example 19 is 95°C, and the reaction temperature in step S2 of Example 20 is 110°C.

[0097] Examples 21 to 24

[0098] The negative electrode active materials of Examples 21 to 24 were prepared according to the method of Example 1. The particle size of the negative electrode active material can be adjusted by changing the calcination time in step S2 of Example 1. Further, by sieving-assisted screening, the negative electrode active material products with the target median particle size in Table 1 can be obtained.

[0099] Comparative Example 1

[0100] The negative electrode active material of Comparative Example 1 was not subjected to coating modification.

[0101] Comparative Example 5

[0102] The negative electrode active material of Comparative Example 5 was prepared by referring to the method of Example 1. The halide electrolyte and the conductive polymer in the coating layer did not undergo a chemical reaction but were physically mixed. The preparation process differed from that of Example 1 in that:

[0103] In step S12, the halide electrolyte slurry prepared in step S11 is not added to obtain the coating layer polymer precursor; in step S2, the halide electrolyte slurry obtained in step S11, the coating layer polymer precursor obtained in step S21, and the lithium-carbon core material are mixed.

[0104] Comparative Example 6

[0105] The negative electrode active material of Comparative Example 6, in which no halide electrolyte is added to the coating layer, is prepared by referring to the method of Example 1. The difference from Example 1 is that step S1 is step S12, but no electrolyte slurry is added.

[0106] Table 1 Structural parameters of negative electrode active materials

[0107]

[0108]

[0109] Regarding the relevant parameters in Table 1, it should be noted that: (1) the pore size of the coating layer is obtained by testing using a scanning electron microscope. Specifically, the pore sizes of 5 holes are selected per unit area according to the "five-point sampling method", and the average pore size of these 5 holes is calculated, which is used as the average pore size of the coating layer. In the embodiments of the present application, during the preparation process, the pore regulator is ensured to be evenly dispersed. Therefore, the pores can be evenly distributed on the surface of the coating layer, and the average pore size per unit area can represent the average pore size of the entire coating; (2) the median particle size of the negative electrode active material can be directly measured using a particle size tester.

[0110] Application performance testing

[0111] The negative electrode active materials prepared in the above examples and comparative examples were prepared into secondary batteries, and then the charge and discharge cycle performance test was performed. The preparation of the secondary battery includes the following steps:

[0112] Positive electrode preparation

[0113] The positive electrode active material LiCoO2, conductive agent acetylene black, carbon nanotubes, dispersant sodium carboxymethyl cellulose (CMC), binder polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone solvent in a mass ratio of 98:0.5:0.05:0.1:1.35, and then coated on the positive electrode current collector Al foil. After drying, cold pressing, and stripping, the positive electrode sheet (the compaction density of the sheet is 2.50g / cm 3 );

[0114] Negative electrode preparation

[0115] The negative electrode active material, conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) prepared in the above embodiment or comparative example were mixed in a mass ratio of 97:0.5:1.3:1.2, and deionized water was added as a solvent. The mixture was stirred in a stirrer until the system was uniform to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, placed in an oven for drying, and rolled and cut to obtain a negative electrode sheet (the compaction density of the sheet was 1.60 g / cm 3 );

[0116] Electrolyte preparation

[0117] A solution prepared by mixing lithium salt LiPF6 with a non-aqueous organic solvent "obtained by mixing ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): vinylene carbonate (VC) in a mass ratio of 20:30:20:28:2" in a mass ratio of 8:92 is used as an electrolyte for a lithium ion secondary battery;

[0118] Assembly of secondary batteries

[0119] The prepared positive electrode sheet, separator (commercially available porous polyethylene separator), and negative electrode sheet are stacked in order with the separator placed between the positive and negative electrode sheets, then wound into a battery cell and placed in a soft-pack shell. After top and side sealing, liquid injection (electrolyte), formation, sorting and other processes, a lithium-ion secondary battery is obtained.

[0120] The performance test of secondary batteries is as follows:

[0121] (1) First coulombic efficiency (%):

[0122] At room temperature (25°C), charge at a constant current rate of 0.5C until the voltage reaches 4.53V. Then charge at a constant voltage of 4.53V until the current is lower than 0.02C, making it fully charged at 4.53V. At this time, the first charge capacity C1 is recorded. Then, discharge at a constant current rate of 0.2C until the voltage reaches 3.0V. The discharge capacity at room temperature is recorded as C2.

[0123] First coulombic efficiency = C2 / C1*100%;

[0124] (2) Cyclic performance test:

[0125] First, in an environment of 25°C, perform the first charge and discharge, charging at 3C to 4.25V with a cut-off current of 2C, charging at 2C to 4.35V with a cut-off current of 1.2C, and charging at 1.2C to 4.53V with a cut-off current of 0.05C, performing step-by-step charging until the upper limit voltage reaches 4.53V, then perform constant current discharge at a discharge current of 1C until the final voltage reaches 3V, and record the discharge capacity Cap1 of the first cycle and the volume V1 of the secondary battery at this time; then perform 100 charge and discharge cycles, and record the discharge capacity Cap100 of the 100th cycle and the volume V100 of the secondary battery;

[0126] Cycle capacity retention rate = (Cap100 / Cap1) × 100%;

[0127] Cycle expansion ratio = (V100 / V1) × 100%;

[0128] The test results are shown in Table 2.

[0129] Table 2

[0130]

[0131]

[0132] From the above results we can see that:

[0133] Lithium-carbon material is used as the core and its surface is coated and modified to further improve the coulombic efficiency and cycle performance of the battery.

[0134] In the negative electrode material of Comparative Example 1, the surface coating was not performed, resulting in contact between the lithium-carbon negative electrode and the electrolyte, and side reactions occurred during the cycle, resulting in a significant deterioration in the volume expansion rate and cycle capacity retention rate of the battery; the comparison results of Comparative Examples 2 and 3 with the embodiments show that appropriate doping of lithium in the lithium-carbon core material can significantly improve the cycle stability and cycle rate performance of the battery; in Comparative Example 4, lithium was not doped in the carbon core material, and the rate performance of the battery was poor; in the negative electrode active material of Comparative Example 5, the halide electrolyte and the conductive polymer were physically mixed in the coating layer on the surface of the lithium-carbon core, resulting in a significant deterioration in the rate performance and cycle performance of the prepared battery; in the negative electrode active material of Comparative Example 6, no halide electrolyte was added to the surface coating layer, and the rate performance and cycle capacity retention rate of the prepared battery were significantly deteriorated.

[0135] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A negative electrode active material, characterized in that Comprising a lithium-carbon core and a coating layer coating the lithium-carbon core: The general structural formula of the lithium carbon core is Li M C N , M+N=1, M represents the atomic number ratio of Li atoms, N represents the atomic number ratio of C atoms, and 1≤M / N<2; The coating layer includes a halide electrolyte cross-linked conductive polymer; the structural formula of the halide electrolyte is Li x ZT y , Z includes at least one metal element selected from Al, Ga, In, Sc, Y, La, and Ho, T is a halogen, 0 <x≤15,1≤y≤17。 2. The negative electrode active material according to claim 1, characterized in that The lithium carbon core has the following structure: The lithium-carbon core includes porous carbon and lithium supported on the porous carbon.

3. The negative electrode active material according to claim 1, characterized in that The particle size D of the lithium carbon core V 50 is A, in μm, the thickness of the coating layer is B, in μm, 0.2≤A / B≤0.

5.

4. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following characteristics: (1) The median particle size D of the negative electrode active material V 50 is 5~400μm; (2) The pore size of the coating layer is 2 to 20 nm.

5. The negative electrode active material according to claim 1, characterized in that In the halide electrolyte cross-linked conductive polymer, the mass ratio of the halide electrolyte to the conductive polymer is (0.25-3):

1.

6. The negative electrode active material according to claim 1, characterized in that The conductive polymer includes at least one of epoxy resin, polyacrylonitrile, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacrylamide, and polyacrylic acid.

7. The method for preparing the negative electrode active material according to any one of claims 1 to 6, characterized in that: The steps include: S1: Preparation of coating layer precursor S11: adding lithium halide, metal Z halide, Lewis base, catalyst and surfactant into water to form halide electrolyte slurry; S12: uniformly mixing the conductive polymer monomer, hydrochloric acid, initiator, dispersant, and the halide electrolyte slurry prepared in step S11, initiating polymerization at 20-210° C., and performing the polymerization reaction for 16-48 hours to obtain a coating layer precursor; S2: In-situ coating The coating layer precursor and lithium-carbon core material obtained in step S1 are mixed and uniformly mixed, and then reacted at 0-20°C for 1-3 hours. The precipitated product is washed with water and dried, and then further dispersed in an organic solvent. A pore regulator is added, and the mixture is reacted at 30-110°C for 5-24 hours. The mixture is then calcined at 100-300°C for 2-48 hours to obtain the negative electrode active material.

8. The method for preparing the negative electrode active material according to claim 7, characterized in that: Satisfy at least one of the following characteristics: (1) The surfactant includes sodium lauryl sulfate; (2) The solid content of the halide electrolyte slurry is 50 to 80 wt%; (3) The initiator includes at least one of ammonium sulfate, azobisisobutyronitrile, and benzoyl peroxide; (4) The dispersant comprises hydrogenated nitrile rubber; (5) The organic solvent includes at least one of acetonitrile, tetrahydrofuran, dimethyl ether, N-methylformamide, and 1,2-dimethoxyethane; (6) The pore regulator includes at least one of C5-C15 alkanes and fluorocarbons.

9. A secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode active material according to any one of claims 1 to 6.

10. An electrical device, characterized in that: The electric device includes the secondary battery according to claim 9.

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