Graphite-based composite material and preparation method thereof, negative pole piece, secondary battery and electric device

Abstract

The embodiment of the invention provides a graphite-based composite material and a preparation method thereof, a negative pole piece, a secondary battery and an electric device, and belongs to the technical field of secondary batteries. The graphite-based composite material is in a three-dimensional conductive network shape, the graphite-based composite material comprises graphite particles and porous carbon nanofibers coating the surfaces of the graphite particles, and the porous carbon nanofibers comprise nitrogen-doped porous carbon skeletons and metal nanoparticles embedded in the nitrogen-doped porous carbon skeletons. The metal nanoparticles comprise Co and Zn, the porous carbon nanofiber has a micropore-mesopore-macropore three-level pore channel structure, the porous carbon nanofiber is obtained by carbonizing a mixture of ZIF and a high polymer material, metal ions of the ZIF comprise Co < 2 + > and Zn < 2 + >, and ligands of the ZIF comprise dimethylimidazole and benzimidazole. The graphite-based composite material has excellent low-temperature and rate performance.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, specifically relating to a graphite-based composite material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Lithium-ion batteries are considered ideal for portable electronic devices and electric vehicles due to their high energy density and safety. Graphite is the primary power source for vehicles and renewable energy storage systems. Due to its high theoretical capacity, low cost, and low operating potential, it has become the most important negative electrode material for lithium-ion batteries. Furthermore, carbonate-based electrolytes are currently the most widely used electrolyte system in lithium-ion batteries, with advantages including excellent electrochemical stability, suitable physical properties, strong process compatibility, and cost advantages.

[0003] However, traditional graphite anode materials suffer from problems such as low ionic conductivity, strong anisotropy, and poor compatibility with electrolytes, resulting in poor rate performance. Especially at low temperatures, the viscosity of carbonate-based electrolytes increases, ionic conductivity decreases, and interfacial impedance increases sharply, causing a sharp drop in battery capacity or even failure to function properly.

[0004] To improve the rate performance of graphite anode materials, researchers have explored various methods, such as constructing porous structures, surface coating, and elemental doping. For example, one related technique involves adding soluble zinc salts and 2-methylimidazole to graphite to form a core-shell structure of 2-methylimidazole zinc salt coated graphite. High-temperature heat treatment in an inert atmosphere then forms a nitrogen-doped porous amorphous carbon layer, increasing the lattice spacing of graphite and improving the rate performance of Li. + The improved transport dynamics shorten the charging time of lithium-ion batteries and improve the high-rate charge-discharge efficiency. However, the above-mentioned technologies have limited effect on improving the rate performance of graphite anode materials, and do not address electrolyte systems used at low temperatures; the battery capacity retention rate is only about 25% at -20℃.

[0005] On the other hand, in order to improve the low-temperature performance of lithium-ion batteries, related technologies also optimize carbonate-based electrolytes. For example, fluorinated carbonates or fluoroethers (such as TTE) are added to carbonate electrolytes as additives. Although adding fluorinated carbonates or fluoroethers (such as TTE) can improve interface stability and flame retardancy, their effect on reducing low-temperature viscosity is limited. Summary of the Invention

[0006] This disclosure is based on the inventors' discovery and understanding of the following facts and problems: developing a graphite-based composite material and electrolyte, and organically combining the structure-optimized negative electrode material with a specific electrolyte to significantly improve battery performance, especially low-temperature and high-rate performance, is a systematic solution with significant scientific and commercial value.

[0007] This application aims to at least partially address one of the technical problems in the related art. To this end, embodiments of this application propose a graphite-based composite material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device.

[0008] In a first aspect, embodiments of this application provide a graphite-based composite material, which is a three-dimensional conductive network. The graphite-based composite material comprises graphite particles and porous carbon nanofibers coated on the surface of the graphite particles. The porous carbon nanofibers comprise a nitrogen-doped porous carbon framework and metal nanoparticles embedded in the nitrogen-doped porous carbon framework. The metal nanoparticles include Co and Zn. The porous carbon nanofibers have a three-level pore structure of "micropore-mesopore-macropore". The porous carbon nanofibers are obtained by carbonizing ZIF and a polymer material. The metal ions of the ZIF include Co. 2+ and Zn 2+ The ligands of the ZIF include dimethylimidazole and benzimidazole.

[0009] Optionally, the micropore size is 0.8 nm to 1.4 nm.

[0010] Optionally, the mesopore size is 3 nm to 10 nm.

[0011] Optionally, the pore size is 50 nm to 200 nm.

[0012] Optionally, the Co 2+ and the Zn 2+ The molar ratio is (0.5~1.5):1.

[0013] Optionally, the molar ratio of the dimethylimidazole to the benzimidazole is 1:(1~2).

[0014] Optionally, the polymeric material is polyacrylonitrile and / or polyvinyl alcohol.

[0015] Secondly, embodiments of this application provide a method for preparing a graphite-based composite material, the method comprising the following steps: S1. A metal salt is dissolved in methanol to form a metal salt solution, wherein the metal salt includes zinc salt and cobalt salt; a ligand is dissolved in methanol to form a ligand solution, wherein the ligand includes dimethylimidazole and benzimidazole; graphite powder is then added to the metal salt solution and ultrasonically dispersed to obtain a metal salt-graphite mixture; subsequently, under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture to obtain a raw material mixture; the raw material mixture undergoes a coordination reaction to obtain a solid-liquid mixture; the solid-liquid mixture is filtered, washed with methanol, and then dried to obtain a precursor, wherein the precursor is a ZIF-coated graphite core-shell structure; S2. Dissolve the precursor and polymer material in a first organic solvent to form a precursor solution; then electrospin the precursor solution to obtain an electrospun composite material. S3. The electrospun composite material is subjected to pre-oxidation and carbonization treatment to obtain the graphite-based composite material described in the first aspect.

[0016] Optionally, in step S1, the cobalt salt is cobalt acetate and / or cobalt nitrate, and the zinc salt is zinc acetate and / or zinc nitrate.

[0017] Optionally, in step S1, the concentration of the metal salt in the metal salt solution is 0.1 mol / L to 0.5 mol / L.

[0018] Optionally, in step S1, the molar ratio of the cobalt salt and the zinc salt in the metal salt solution is (0.5~1.5):1.

[0019] Optionally, in step S1, the concentration of the ligand in the ligand solution is 0.5 mol / L to 2.0 mol / L.

[0020] Optionally, in step S1, the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 1: (1~2).

[0021] Optionally, in step S1, the mass ratio of the metal salt to the graphite in the metal salt-graphite mixed solution is 1:(4~10).

[0022] Optionally, in step S1, the graphite satisfies at least one of the following conditions: (a) the Dv50 of the graphite powder is 5 μm to 19 μm; (b) the specific surface area of ​​the graphite powder is 0.7 m². 2 / g~2.0 m 2 / g; (c) The graphitization degree of the graphite powder is 91%~96%.

[0023] Optionally, in step S1, the specific conditions for ultrasonic dispersion are as follows: ultrasonic power is 150W~360W, and ultrasonic time is 20 minutes~40 minutes.

[0024] Optionally, in step S1, the molar ratio of the ligand to the metal salt in the raw material mixture is (3.6~5):1.

[0025] Optionally, in step S1, the specific conditions for the coordination reaction are as follows: the holding temperature is 40℃~80℃, and the reaction time is 12 hours~28 hours.

[0026] Optionally, in step S1, the sample is washed with methanol 1 to 3 times.

[0027] Optionally, in step S2, the first organic solvent is at least one of N,N-dimethylformamide, dimethyl sulfoxide, and N,N-dimethylacetamide.

[0028] Optionally, in step S2, the polymeric material is polyacrylonitrile and / or polyvinyl alcohol.

[0029] Optionally, in step S2, the concentration of the precursor in the precursor solution is 50 mg / mL to 150 mg / mL, and the concentration of the polymer material in the precursor solution is 50 mg / mL to 150 mg / mL.

[0030] Optionally, in step S2, the specific conditions for electrospinning are as follows: injection rate of 0.2 mL / h to 1.7 mL / h, positive electrode voltage of 13 kV to 23 kV, negative electrode voltage of -2.7 kV to -2.2 kV, and spinning distance of 10 cm to 22 cm.

[0031] Optionally, in step S3, the specific conditions for the pre-oxidation treatment are as follows: it is carried out in an air atmosphere, the holding temperature is 150℃~250℃, and the holding time is 1 hour~4 hours.

[0032] Optionally, in step S3, the specific conditions for the carbonization treatment are as follows: it is carried out in an inert gas atmosphere, the holding temperature is 450℃~900℃, and the holding time is 1 hour~3 hours.

[0033] Thirdly, embodiments of this application provide a negative electrode sheet, wherein the negative electrode sheet is the graphite-based composite material described in the first aspect or the graphite-based composite material obtained by the preparation method described in the second aspect.

[0034] Fourthly, embodiments of this application provide a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the negative electrode described in the third aspect; the secondary battery further comprises an electrolyte comprising a lithium salt, a second organic solvent, and a functional additive; wherein the functional additive is polyoxymethylene dimethyl ether (PODE). n The amount of the functional additive is less than 30%, based on the total mass of the electrolyte being 100%.

[0035] Preferably, the amount of the functional additive is 5% to 20% based on the total mass of the electrolyte (100%).

[0036] Optionally, the lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI), and the concentration of the lithium salt in the electrolyte is 1.0 mol / L to 1.5 mol / L.

[0037] Optionally, the second organic solvent comprises ethyl methyl carbonate (EMC) and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane (TTE), wherein the volume ratio of EMC to TTE is 1:1 to 3:1.

[0038] Fifthly, embodiments of this application provide an electrical device, which includes the secondary battery described in the fourth aspect.

[0039] Compared with related technologies, the advantages and technical effects of the embodiments of this application are as follows: (1) The graphite-based composite material of this application embodiment includes graphite particles and porous carbon nanofibers coated on the surface of the graphite particles. The porous carbon nanofibers include nitrogen-doped porous carbon skeletons and metal nanoparticles embedded in the nitrogen-doped porous carbon skeletons. Compared with graphite, the graphite-based composite material of this application embodiment has higher conductivity. Moreover, the porous carbon nanofibers in the graphite-based composite material of this application embodiment have abundant mesopores and macropores, which can provide a high-speed channel for ion transport and greatly alleviate the ion diffusion bottleneck at low temperature and high rate.

[0040] The metal nanoparticles include Co and Zn, and the metal ions of ZIF include Co. 2+ and Zn 2+ After ZIF carbonization, metal nanoparticles are formed that are embedded in a nitrogen-doped carbon framework, Zn 2+ Enhanced carbon framework graphitization, Co 2+ The synergistic effect of uniformly dispersed metal nanoparticles and porous carbon provides catalytic active sites and avoids concentration polarization and resistive polarization caused by local concentration of active material graphite, resulting in less voltage decay and higher capacity retention during high-rate charge and discharge of the battery.

[0041] ZIF's ligands include dimethylimidazole and benzimidazole, which have the following synergistic effects: dimethylimidazole and benzimidazole, respectively, interact with Co... 2+ and Zn 2+ By combining these components, four substances—ZIF-8, ZIF-9, ZIF-11, and ZIF-67—can be generated. The carbonization of ZIF results in diverse pore sizes, effectively controlling the hierarchical structure of the pores. Furthermore, the system contains conjugated π bonds and Co-N bonds of benzimidazole, forming a hydrogen bond network at the electrolyte-electrode interface. This leads to rapid ion transport at low temperatures and an increased lithium-ion diffusion coefficient, improving the rate performance of the graphite-based composite material. In addition, the conjugated π bonds of benzimidazole endow ZIF-9 and ZIF-11 with unique electron cloud distributions. After composite formation and carbonization with a graphite substrate, the carbon layer formed on the graphite surface is rich in defects and active sites, further enhancing the rate performance of the graphite-based composite material.

[0042] (2) The preparation method of this application uses ZIF in-situ growth and electrospinning followed by carbonization to construct a unique hierarchical porous graphite-based composite material. The porous carbon nanofibers derived from ZIF and electrospinning serve as a coating layer on the surface of graphite particles, which not only improves the conductivity of graphite materials, but also provides high-speed channels for ion transport through their abundant mesopores and macropores, greatly alleviating the ion diffusion bottleneck at low temperature and high rate.

[0043] (3) Since the graphite-based composite material described in the first aspect or the graphite-based composite material prepared by the preparation method described in the second aspect is used, the negative electrode sheet of the present application has all the advantages that the graphite-based composite material can bring, and will not be repeated here.

[0044] (4) The secondary battery of this application creatively incorporates PODE, which is commonly used as a diesel additive. n Introducing a lithium-ion battery electrolyte system. PODE n (n=2,3,4) possesses extremely low freezing points and viscosities; its addition significantly reduces the overall viscosity of the electrolyte system at low temperatures and improves ionic conductivity. More importantly, PODE n In synergy with TTE, it participates in and forms a more stable solid electrolyte interface (SEI) film with lower impedance and higher lithium-ion conductivity on the graphite anode surface. This SEI film can still maintain excellent ion conduction capability at low temperatures, thereby significantly reducing charge transfer impedance and solving the core problem of low-temperature performance degradation.

[0045] (5) The secondary battery of this application embodiment includes the graphite-based composite material of this application embodiment and a battery containing PODE. n The electrolyte, the structure of the graphite-based composite material is similar to that containing PODE nThe electrolyte exhibits a remarkable synergistic effect: the hierarchical porous structure of the graphite-based composite material provides low-viscosity PODE. n It provides good immersion and storage space, while PODE n This ensures rapid ion transport within the porous channels, resulting in material structure innovation and a synergistic effect. Therefore, the secondary battery of this application embodiment possesses high capacity and excellent rate performance at room temperature, and also maintains high rate performance at a low temperature of -20°C.

[0046] In addition, the graphite-based composite material of this application is a self-supporting negative electrode material that can be used directly as a negative electrode sheet without the need for a current collector. After cutting, it can be used normally as a button half electrode, eliminating the need for conductive agents and binders. At the same time, it has a certain degree of plasticity during use, making it more widely applicable and novel.

[0047] (6) Since the secondary battery described in the fourth aspect is used, the electrical device of the present application embodiment has all the advantages brought by the secondary battery, which will not be repeated here. Detailed Implementation

[0048] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion. In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0050] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific conditions, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this application, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when stating a condition as an integer ≥2, it is equivalent to disclosing that the condition is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0051] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0052] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces). In a first aspect, embodiments of this application provide a graphite-based composite material, which is a three-dimensional conductive network. The graphite-based composite material comprises graphite particles and porous carbon nanofibers coated on the surface of the graphite particles. The porous carbon nanofibers comprise a nitrogen-doped porous carbon framework and metal nanoparticles embedded in the nitrogen-doped porous carbon framework. The metal nanoparticles include Co and Zn. The porous carbon nanofibers have a three-level pore structure of "micropore-mesopore-macropore". The porous carbon nanofibers are obtained by carbonizing ZIF and a polymer material. The metal ions of the ZIF include Co. 2+ and Zn 2+ The ligands of the ZIF include dimethylimidazole and benzimidazole.

[0053] ZIF (Zeolitic Imidazolate Framework) is a class of porous crystalline materials formed by the self-assembly of metal ions and imidazolate ligands through coordination bonds. Examples of ZIFs include ZIF-8, ZIF-9, ZIF-11, and ZIF-67. ZIF-8 is formed by the self-assembly of zinc ions (Zn...). 2+ ZIF-9 is a porous crystalline material formed by the self-assembly of cobalt ions (Co) and 2-methylimidazole ligands through coordination bonds; 2+ ZIF-11 is a porous crystalline material formed by the self-assembly of zinc ions (Zn) and benzimidazole ligands through coordination bonds; 2+ ZIF-67 is a porous crystalline material formed by the self-assembly of cobalt ions (Co) and benzimidazole ligands through coordination bonds; 2+ Porous crystalline materials formed by self-assembly of 2-methylimidazole ligands via coordination bonds.

[0054] The graphite-based composite material of this application embodiment includes graphite particles and porous carbon nanofibers coated on the surface of the graphite particles. The porous carbon nanofibers include a nitrogen-doped porous carbon framework and metal nanoparticles embedded in the nitrogen-doped porous carbon framework. Compared with graphite, the graphite-based composite material of this application embodiment has higher conductivity, and the porous carbon nanofibers in the graphite-based composite material of this application embodiment have abundant mesopores and macropores. Therefore, it can provide a high-speed channel for ion transport and greatly alleviate the ion diffusion bottleneck at low temperature and high rate.

[0055] The metal nanoparticles include Co and Zn, and the metal ions of ZIF include Co. 2+ and Zn 2+ After ZIF carbonization, metal nanoparticles are formed that are embedded in a nitrogen-doped carbon framework, Zn 2+ Enhanced carbon framework graphitization, Co 2+The synergistic effect of uniformly dispersed metal nanoparticles and porous carbon provides catalytic active sites and avoids concentration polarization and resistive polarization caused by local concentration of active material graphite, resulting in less voltage decay and higher capacity retention during high-rate charge and discharge of the battery.

[0056] ZIF's ligands include dimethylimidazole and benzimidazole, which have the following synergistic effects: dimethylimidazole and benzimidazole, respectively, interact with Co... 2+ and Zn 2+ By combining these components, four substances—ZIF-8, ZIF-9, ZIF-11, and ZIF-67—can be generated. The carbonization of ZIF results in diverse pore sizes, effectively controlling the hierarchical structure of the pores. Furthermore, the system contains conjugated π bonds and Co-N bonds of benzimidazole, forming a hydrogen bond network at the electrolyte-electrode interface. This leads to rapid ion transport at low temperatures and an increased lithium-ion diffusion coefficient, improving the rate performance of the graphite-based composite material. In addition, the conjugated π bonds of benzimidazole endow ZIF-9 and ZIF-11 with unique electron cloud distributions. After composite formation and carbonization with a graphite substrate, the carbon layer formed on the graphite surface is rich in defects and active sites, further enhancing the rate performance of the graphite-based composite material.

[0057] Optionally, the Co 2+ and the Zn 2+ The molar ratio of cobalt salt to zinc salt is (0.5~1.5):1, for example, 0.5:1, 0.6:1, 0.8:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc. When only zinc metal salt is added, the lack of cobalt metal salt results in poor conductivity and fewer catalytic active sites in the material. Adding cobalt salt alone is expensive, which is not conducive to cost reduction and efficiency improvement. When the molar ratio of cobalt salt to zinc salt is within the above range, it is beneficial to both improve the rate performance of graphite-based composite materials and reduce costs and increase efficiency.

[0058] Optionally, the molar ratio of the dimethylimidazole to the benzimidazole is 1:(1~2), for example, 1:1, 1:1.1, 1:1.2, 1:13, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, etc. When dimethylimidazole is not added, ZIF-8 and ZIF-67 cannot be generated; after carbonization, the pores contain only a single-size pore, and pore hierarchical control fails. Without the addition of benzimidazole, ZIF9 and ZIF11 cannot be formed. After carbonization, the pores contain only a single size, and the hierarchical control of the pores fails. Furthermore, without the conjugated π bonds and Co-N bonds of benzimidazole, the hydrogen bond network at the electrolyte-electrode interface cannot be formed, hindering low-temperature ion transport and reducing the lithium-ion diffusion coefficient of the material, resulting in poor rate performance and low-temperature charging rate performance. In addition, the conjugated π bonds without benzimidazole endow ZIF-9 and ZIF-11 with a unique electron cloud distribution. After being composited with a graphite substrate and carbonized, the carbon layer formed on the graphite surface has fewer defects and active sites.

[0059] Optionally, the polymeric material is polyacrylonitrile and / or polyvinyl alcohol.

[0060] Secondly, embodiments of this application provide a method for preparing a graphite-based composite material, comprising the following steps: S1. A metal salt is dissolved in methanol to form a metal salt solution, wherein the metal salt includes zinc salt and cobalt salt; a ligand is dissolved in methanol to form a ligand solution, wherein the ligand includes dimethylimidazole and benzimidazole; graphite powder is then added to the metal salt solution and ultrasonically dispersed to obtain a metal salt-graphite mixture; subsequently, under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture to obtain a raw material mixture; the raw material mixture undergoes a coordination reaction to obtain a solid-liquid mixture; the solid-liquid mixture is filtered, washed with methanol, and then dried to obtain a precursor, wherein the precursor is a ZIF-coated graphite core-shell structure; S2. Dissolve the precursor and polymer material in a first organic solvent to form a precursor solution; then electrospin the precursor solution to obtain an electrospun composite material. S3. The electrospun composite material is subjected to pre-oxidation and carbonization treatment to obtain the graphite-based composite material.

[0061] This application describes a unique hierarchical porous graphite-based composite material constructed through in-situ growth of mixed-ligand ZIFs (including Co ions, Zn ions, dimethylimidazole, and benzimidazole) followed by electrospinning and carbonization. The ZIFs and the porous carbon nanofibers derived from electrospinning serve as a coating layer on the graphite particle surface, not only improving the conductivity of the graphite material but also providing high-speed channels for ion transport through their abundant mesopores and macropores. This significantly alleviates the ion diffusion bottleneck at low temperatures and high rates, optimizes the lithium-ion diffusion path, and substantially improves the low-temperature and rate performance of the battery.

[0062] Working principle: In step S1, a precursor with a ZIF-coated graphite core-shell structure is obtained by in-situ growth of metal ions and ligands on the graphite surface. After electrospinning in step S2, the precursor is used to obtain a fibrous conductive network of ZIF-coated graphite. After pre-oxidation and carbonization in step S3, metal ions are reduced to form metal nanoparticles, and ligands (containing C and N elements) are decomposed, cross-linked and graphitized, finally forming a nitrogen-doped porous carbon skeleton with carbon as the skeleton and nitrogen atoms doped in it.

[0063] Specifically, the preparation method of this application embodiment has the following key points: 1. Hierarchical porous structure design: The graphite-based composite material is formed by in-situ coating of graphite with a mixture of ZIF ions (including Co ions, Zn ions, dimethylimidazole, and benzimidazole), for example, a mixture of ZIF-8 and ZIF-9, or a mixture of ZIF-11 and ZIF-67. The large conjugated structure of benzimidazole promotes the formation of ZIF-9 / ZIF-11, with a pore size of approximately 0.8 nm to 1.2 nm; dimethylimidazole dominates the growth of ZIF-8 / ZIF-67, with a pore size of approximately 1.16 nm to 1.4 nm. Through the in-situ growth of the ZIF mixture (a mixture of ZIF-8 and ZIF-9, or a mixture of ZIF-11 and ZIF-67), a three-level structure of "micropore-mesopore-macropore" is formed on the graphite surface, improving the lithium-ion diffusion coefficient. The hierarchical porous structure shortens the lithium-ion diffusion path.

[0064] 2. Carbonization Process Regulation: During carbonization, metal ions are reduced to metal nanoparticles (such as Co) or partially volatilized (such as Zn), leaving "cavities" in the original ZIF channels, forming micropores. Additionally, gases generated during carbonization also form micropores, with pore sizes ranging from 0.8 nm to 1.4 nm. High-temperature carbonization causes the ZIF framework to shrink, forming mesopores with pore sizes of approximately 3 nm to 10 nm. After the ZIF layer on the graphite surface decomposes, macropores with pore sizes of approximately 50 nm to 200 nm remain, forming a tertiary diffusion channel. The composite material possesses a tertiary structure of "micropore-mesopore-macropore," with a pore size distribution range of 0.8 nm to 200 nm.

[0065] 3. Bimetallic synergistic effect: After carbonization, metal nanoparticles are formed and embedded in a nitrogen-doped carbon framework, Zn 2+ Enhanced carbon framework graphitization, Co 2+ The synergistic effect of uniformly dispersed metal nanoparticles and porous carbon provides catalytic active sites and avoids concentration polarization and resistive polarization caused by local concentration of active material graphite, resulting in less voltage decay and higher capacity retention during high-rate charge and discharge of the battery.

[0066] 4. Synergistic effect of dimethylimidazole and benzimidazole: The addition of dimethylimidazole and benzimidazole generates four substances: ZIF-8, ZIF-9, ZIF-11, and ZIF-67. After carbonization, the pore size is diversified, which effectively controls the hierarchical control of pores. In addition, after the addition of benzimidazole, the system contains the conjugated π bonds and Co-N bonds of benzimidazole, and a hydrogen bond network is formed at the electrolyte-electrode interface. This results in fast ion transport at low temperatures and an increased lithium-ion diffusion coefficient, which can improve the rate performance of graphite-based composite materials. Furthermore, the conjugated π bonds of benzimidazole endow ZIF-9 and ZIF-11 with a unique electron cloud distribution. After being composited with and carbonized with the graphite substrate, the carbon layer formed on the graphite surface is rich in defects and active sites, which can improve the rate performance of graphite-based composite materials.

[0067] 5. The presence of a conductive carbon network (graphite + ZIF carbon + conductive metal particles) significantly reduces the bulk resistance of the material. At the same time, the porous structure increases the electrode / electrolyte interface area, reduces charge transfer resistance, and enables the material to maintain efficient electron-ion transport even at high current densities.

[0068] Optionally, in step S1, the cobalt salt is cobalt acetate and / or cobalt nitrate, and the zinc salt is zinc acetate and / or zinc nitrate.

[0069] Optionally, in step S1, the concentration of the metal salt in the metal salt solution is 0.1 mol / L to 0.5 mol / L, for example, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, etc.

[0070] Optionally, in step S1, the molar ratio of the cobalt salt to the zinc salt in the metal salt solution is (0.5~1.5):1, for example, 0.5:1, 0.6:1, 0.8:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc. When only zinc metal salt is added, the lack of cobalt metal salt results in poor conductivity and fewer catalytic active sites in the material. Adding Co salt alone is expensive, which is not conducive to cost reduction and efficiency improvement. When the molar ratio of cobalt salt to zinc salt is within the above range, it is beneficial to both improve the rate performance of graphite-based composite materials and reduce costs and increase efficiency.

[0071] Optionally, in step S1, the concentration of the ligand in the ligand solution is 0.5 mol / L to 2.0 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L, 2.0 mol / L, etc. A ligand concentration within this range is beneficial for generating sufficient ZIF, thereby facilitating the production of sufficient porous carbon nanofibers.

[0072] Optionally, in step S1, the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 1:(1~2), for example, 1:1, 1:1.1, 1:1.2, 1:13, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, etc. When dimethylimidazole is not added, ZIF-8 and ZIF-67 cannot be generated; after carbonization, the pores contain only a single-size pore, and pore hierarchical control fails. Without the addition of benzimidazole, ZIF9 and ZIF11 cannot be formed. After carbonization, the pores contain only a single size, and the hierarchical control of the pores fails. Furthermore, without the conjugated π bonds and Co-N bonds of benzimidazole, the hydrogen bond network at the electrolyte-electrode interface cannot be formed, hindering low-temperature ion transport and reducing the lithium-ion diffusion coefficient of the material, resulting in poor rate performance and low-temperature charging rate performance. In addition, the conjugated π bonds without benzimidazole endow ZIF-9 and ZIF-11 with a unique electron cloud distribution. After being composited with a graphite substrate and carbonized, the carbon layer formed on the graphite surface has fewer defects and active sites.

[0073] It should be noted that the benzimidazole molecule contains a benzene ring (a six-membered conjugated ring) and an imidazole ring (a five-membered conjugated ring), which are linked by C-C bonds to form a continuous π-π conjugated system. During carbonization (e.g., 450℃~900℃), the CH bonds and some CN bonds in the ligand break, but the conjugated ring structure is not completely destroyed. Instead, it is reconstructed into a graphite-like carbon skeleton through "dehydrogenation-crosslinking-graphitization". Co has a boiling point as high as 2870℃, which is much higher than the carbonization temperature (e.g., 450℃~900℃). Therefore, Co does not volatilize but remains in the carbon skeleton in an atomically dispersed form. During the carbonization of the benzimidazole ligand, the N atom (containing lone pair electrons) in the imidazole ring does not completely escape but is transformed into stable forms such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. The lone pairs of electrons of these N atoms can form coordinate bonds (Co-N bonds) with the empty orbitals of Co. This coordination is thermodynamically stable at high temperatures (e.g., 450℃~900℃) and will not break due to carbon skeleton reconstruction. Therefore, the system contains benzimidazole conjugated π bonds and Co-N bonds.

[0074] Optionally, in step S1, the mass ratio of the metal salt to the graphite in the metal salt-graphite mixed solution is 1:(4~10), for example, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc. Meeting this condition is beneficial for graphite-based composite materials to achieve both excellent capacity and rate performance.

[0075] Optionally, in step S1, the graphite powder satisfies at least one of the following conditions: (a) the Dv50 of the graphite is 5 μm to 19 μm; (b) the specific surface area of ​​the graphite powder is 0.7 m². 2 / g~2.0 m 2 / g; (c) The graphite degree of the graphite powder is 91%~96%. Graphite-based composite materials made from graphite powder that meets the above conditions exhibit excellent rate performance and also achieve high capacity.

[0076] Optionally, in step S1, the specific conditions for ultrasonic dispersion are as follows: ultrasonic power is 150W~360W, for example, 150W, 160W, 180W, 200W, 220W, 240W, 260W, 280W, 300W, 320W, 340W, 360W, etc., and ultrasonic time is 20 minutes~40 minutes, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, etc. Meeting these ultrasonic dispersion conditions is beneficial for the uniform mixing of metal salt and graphite.

[0077] Optionally, in step S1, the molar ratio of the ligand to the metal salt in the raw material mixture is (3.6~5):1, for example, 3.6:1, 3.8:1, 4:1, 4.2:1, 4.4:1, 4.6:1, 4.8:1, 5:1, etc.

[0078] Optionally, in step S1, the specific conditions for the coordination reaction are as follows: the holding temperature is 40℃~80℃, for example 40℃, 50℃, 60℃, 70℃, 80℃, etc., and the reaction time is 12 hours~28 hours, for example 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, 28 hours, etc.

[0079] Optionally, in step S1, the graphite is washed with methanol 1 to 3 times. The purpose of methanol washing is to remove unreacted impurity components and protect the integrity of the core-shell structure of the ZIF-coated graphite.

[0080] Optionally, in step S2, the first organic solvent is at least one selected from N,N-dimethylformamide, dimethyl sulfoxide, and N,N-dimethylacetamide. The first organic solvent effectively dissolves the precursor and polymer material, facilitating subsequent uniform spinning.

[0081] Optionally, in step S2, the polymer material is polyacrylonitrile and / or polyvinyl alcohol, etc. These polymer materials serve as fiber-forming substrates, possessing good spinnability and enabling the formation of continuous, uniform fibers through electrospinning.

[0082] Optionally, in step S2, the concentration of the precursor in the precursor solution is 50 mg / mL to 150 mg / mL, for example, 50 mg / mL, 60 mg / mL, 80 mg / mL, 100 mg / mL, 120 mg / mL, 140 mg / mL, 150 mg / mL, etc., and the concentration of the polymer material in the precursor solution is 50 mg / mL to 150 mg / mL, for example, 50 mg / mL, 60 mg / mL, 80 mg / mL, 100 mg / mL, 120 mg / mL, 140 mg / mL, 150 mg / mL, etc. Meeting this condition is beneficial for forming continuous and uniform fibers through electrospinning.

[0083] Optionally, in step S2, the specific conditions for electrospinning are as follows: injection rate of 0.2 mL / h to 1.7 mL / h, positive electrode voltage of 13 kV to 23 kV, negative electrode voltage of -2.7 kV to -2.2 kV, and spinning distance of 10 cm to 22 cm. Electrospinning technology combines the active material graphite with a carbon fiber network to form a continuous conductive framework, improving the electrochemical performance of graphite-based composite materials. Meeting the above electrospinning conditions is beneficial for forming continuous and uniform fibers through electrospinning.

[0084] When the electrospinning step is omitted, the ZIF-coated graphite particles agglomerate severely, resulting in a reduced specific surface area of ​​the graphite-based composite material after carbonization. It also lacks a fibrous conductive network, increases the lithium-ion diffusion distance, and reduces the battery rate performance, as well as the low-temperature charging rate performance.

[0085] Optionally, in step S3, the specific conditions for the pre-oxidation treatment are as follows: It is carried out in an air atmosphere, with a holding temperature of 150℃~250℃, such as 150℃, 160℃, 180℃, 200℃, 220℃, 240℃, 250℃, etc., and a holding time of 1 hour~4 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, etc. Meeting these conditions is beneficial for achieving controllable thermal stabilization of polymer materials such as polyacrylonitrile, while protecting the integrity of the core-shell structure of ZIF-coated graphite, laying the foundation for subsequent carbonization to form high-performance hierarchical porous graphite-based anode materials.

[0086] Optionally, the specific conditions for the carbonization treatment are as follows: it is carried out in an inert gas atmosphere, with a holding temperature of 450℃~900℃, such as 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, etc., and a holding time of 1 hour~3 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc. Meeting these conditions facilitates precise control of carbon framework formation, metal behavior, and pore evolution, enabling the construction of a hierarchical porous structure and highly active electrode characteristics suitable for low-temperature batteries.

[0087] Thirdly, embodiments of this application provide a negative electrode sheet, wherein the negative electrode sheet is the graphite-based composite material described in the first aspect or the graphite-based composite material obtained by the preparation method described in the second aspect.

[0088] Since the graphite-based composite material is obtained by the preparation method described in the first aspect or the second aspect, the negative electrode sheet of the present application has all the advantages that the graphite-based composite material can bring, which will not be repeated here.

[0089] Because of the preparation method described in the second aspect, electrospinning can produce continuous nanofiber structures, and after coating with electrospinning technology, it can be prepared into a self-supporting negative electrode material that can be used directly as a negative electrode sheet without the need for a current collector. After cutting, it can be used normally as a button half electrode, eliminating the need for conductive agents and binders. At the same time, it has a certain degree of plasticity during use, making it more widely applicable and novel.

[0090] Fourthly, embodiments of this application provide a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the negative electrode described in the third aspect, and the electrolyte comprises a lithium salt, a second organic solvent, and a functional additive; wherein the functional additive is polyoxymethylene dimethyl ether (PODE). n The additive is at least one of n=2, 3, 4, such as at least one of dioxymethylene dimethyl ether (PODE2), trioxymethylene dimethyl ether (PODE3), and tetraoxymethylene dimethyl ether (PODE4). The amount of the functional additive is less than 30% based on the total mass of the electrolyte, such as 0.5%, 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, etc.

[0091] In low-temperature performance, electrolyte viscosity is also a crucial factor. Electrolyte viscosity increases sharply with decreasing temperature, directly hindering the diffusion of lithium ions into the pores of the negative electrode material. High viscosity of the prepared electrolyte at low temperatures can significantly impact material performance. Polyoxymethylene dimethyl ether (PODE) n It is usually used as a diesel additive, with low melting point and low viscosity, and is theoretically suitable for use in electrolytes at low temperatures. However, its application in lithium battery electrolytes, especially its synergistic effect with high-performance graphite anode materials, has not been systematically studied and reported.

[0092] The secondary battery in this application creatively incorporates PODE, commonly used as a diesel additive. n Introducing a lithium-ion battery electrolyte system. PODE n (n=2,3,4) possesses extremely low freezing points and viscosities; its addition significantly reduces the overall viscosity of the electrolyte system at low temperatures and improves ionic conductivity. More importantly, PODE n In synergy with TTE, it participates in and forms a more stable solid electrolyte interface (SEI) film with lower impedance and higher lithium-ion conductivity on the graphite anode surface. This SEI film can still maintain excellent ion conduction capability at low temperatures, thereby significantly reducing charge transfer impedance and solving the core problem of low-temperature performance degradation.

[0093] The graphite-based composite material described in the first aspect or the graphite-based composite material obtained by the preparation method described in the second aspect has a structure similar to the PODE-containing composite material described above. n The electrolyte exhibits a remarkable synergistic effect: the hierarchical porous structure of the graphite-based composite material is a low-viscosity PODE. n It provides good immersion and storage space, while PODE nThis ensures rapid ion transport within the porous channels, resulting in material structure innovation and synergistic effects. Therefore, the graphite-based composite material and electrolyte system of this application embodiment, when combined, enable the assembled lithium-ion battery to possess high capacity and excellent rate performance at room temperature, while also maintaining high rate performance at a low temperature of -20℃.

[0094] The graphite-based composite material has a tertiary channel structure similar to the one containing PODE mentioned above. n The electrolyte forms an "ion transport highway." The hierarchical porous structure with large pores (approximately 50 nm to 200 nm in diameter) provides ample wetting space for the electrolyte. TTE solvent molecules preferentially occupy the pore surface, forming low-resistance ion transport channels. The high dissociation of LiFSI ensures that the ion concentration remains at a high level (e.g., above 0.8 mol / L) at low temperatures. Furthermore, the solvent (e.g., EMC, TTE) and lithium salt (e.g., LiFSI) in the electrolyte undergo a reduction reaction at the negative electrode surface, forming a solid electrolyte interphase (SEI) film. Different substances produce different SEIs, affecting ion transport resistance and cycle stability. If traditional carbonate solvents, such as EC / PC, are used, they easily co-intercalate with lithium ions between graphite layers at low temperatures, causing the interlayer spacing to expand and leading to structural collapse. This electrolyte, through the synergistic design of a second organic solvent, lithium salt, and functional additives, solves the three core problems of ion transport, interfacial impedance, and solvent stability at low temperatures.

[0095] In some embodiments, based on the total mass of the electrolyte (100%), the amount of the functional additive added is 5% to 20%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc. When the functional additive PODE n When the content of (n=2,3,4) is too low, it is not conducive to reducing the low-temperature viscosity of the electrolyte, thus hindering the improvement of the low-temperature rate performance of lithium-ion batteries. However, when the content of the functional additive PODE is too low... n When the content of (n=2,3,4) is too high, it may lead to the precipitation of lithium salt, thereby reducing the conductivity and also hindering the improvement of the low-temperature performance of lithium-ion batteries.

[0096] Optionally, the lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI), and the concentration of the lithium salt in the electrolyte is 1.0 mol / L to 1.5 mol / L, such as 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, etc.

[0097] Optionally, the second organic solvent includes ethylene methyl carbonate (EMC) and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy) propane (TTE), wherein the volume ratio of EMC to TTE is 1:1 to 3:1. The combination of EMC and TTE can build a basic system of "high ionic conductivity + stable interface + wide low-temperature adaptability" for the electrolyte at low temperatures through the synergistic optimization of solvent polarity, viscosity, and interfacial properties.

[0098] The positive electrode sheet and the separator of the secondary battery according to the embodiments of the present application will be described in detail below.

[0099] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0100] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0101] The positive electrode film layer includes a positive electrode active material. The positive electrode active material can be selected from materials that can absorb and release lithium.

[0102] The specific type of the positive electrode active material is not specifically limited and can be selected according to requirements. As an example, the positive electrode active material may include, but is not limited to, lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), lithium iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel-type lithium manganese oxide (LiMn2O4), spinel-type lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 1 / 3Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 At least one of the following: O2, lithium-rich materials (e.g., lithium-rich nickel-cobalt-manganese oxide), manganese oxide (MnO2), vanadium oxide, sulfur oxide, silicate oxide, and their respective modified compounds. These materials may be used alone or in combination of two or more.

[0103] The modified compounds for the above-mentioned positive electrode active materials can be modified by doping, surface coating, or both doping and coating.

[0104] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0105] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0106] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0107] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0108] In some embodiments, the secondary battery further includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0109] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0110] Fifthly, embodiments of this application provide an electrical device, which includes the secondary battery described in the fourth aspect.

[0111] The electrical device of this application embodiment has all the advantages that the secondary battery of this application embodiment can bring, and will not be repeated here.

[0112] This application is described in detail below with reference to embodiments.

[0113] Example 1 Preparation of graphite-based composite materials (1) Dissolve cobalt acetate and zinc acetate in methanol to obtain a metal salt solution with a concentration of 0.1 mol / L; wherein the molar ratio of cobalt acetate and zinc acetate in the metal salt solution is 0.05:0.05.

[0114] (2) Dimethylimidazole and benzimidazole are dissolved in methanol to form a ligand solution with a concentration of 0.5 mol / L; wherein the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 0.2:0.3.

[0115] (3) Add graphite powder to a metal salt solution and disperse it by ultrasonication to obtain a metal salt-graphite mixture; wherein the graphite powder used has a Dv50 of 5 μm and a BET of 2.0 μm. 2 / g, graphitization degree of 92%, the mass ratio of metal salt to graphite in the metal salt-graphite mixture is 1:4, ultrasonic power is 360W, ultrasonic time is 20 minutes.

[0116] (4) Under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture; wherein the molar ratio of ligand to metal salt in the metal salt-graphite mixture is 5:1.

[0117] (5) The reaction temperature was controlled at 40℃ and the reaction time was 28h. After the reaction, the resulting solid-liquid mixture was filtered and washed twice with methanol. After drying, the precursor was obtained. The precursor was a ZIF-coated graphite core-shell structure.

[0118] (6) Dissolve the precursor and polyacrylonitrile in N,N-dimethylformamide and stir to form a precursor solution; wherein the concentration of the precursor in the precursor solution is 80 mg / mL and the concentration of polyacrylonitrile in the precursor solution is 90 mg / mL. (7) The precursor solution is loaded into a syringe and electrospun into a composite material using an electrospinning machine. The syringe advance speed is 0.2 mL / h, the positive electrode voltage is 20 kV, the negative electrode voltage is -2.7 kV, and the spinning distance is 15 cm.

[0119] (8) The electrospun composite material was first pre-oxidized in air at 220°C for 3 hours, and then carbonized in argon at 800°C for 3 hours to obtain the graphite-based composite material.

[0120] Application Example 1 The graphite-based composite material prepared in Example 1 was cut into circular pieces with a diameter of 14 mm and used directly as the negative electrode.

[0121] In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), LiFSI was dissolved in a mixed solvent of EMC and TTE (EMC:TTE volume ratio = 1:1) to prepare a solution with a concentration of 1.0 mol / L. Then, 10% of the total mass of the electrolyte, trioxymethylene dimethyl ether (PODE3) was added and stirred until homogeneous to obtain the electrolyte.

[0122] Lithium iron phosphate (LiFePO4) material, conductive agent carbon black, and binder polyvinylidene fluoride were mixed at a mass ratio of 70:20:10. A certain amount of N-methyl-pyrrolidone was then added and stirred until homogeneous, resulting in a positive electrode slurry with a solid content of 60±10%. This slurry was uniformly coated onto aluminum foil and then dried in an 80℃ vacuum oven for 12 hours. After drying, it was cut into 10mm diameter discs using a mold to obtain the positive electrode sheet. The lithium iron phosphate material loading in the positive electrode sheet was 2.8 mg / cm³. 2 .

[0123] Ceglard 2500 was used as the separator membrane.

[0124] In a vacuum glove box, the negative electrode, electrolyte, separator, and positive electrode are assembled together to form a lithium-ion battery.

[0125] Example 2 Preparation of graphite-based composite materials (1) Cobalt acetate and zinc nitrate are dissolved in methanol to obtain a metal salt solution with a concentration of 0.3 mol / L; wherein the molar ratio of cobalt acetate and zinc nitrate in the metal salt solution is 0.1:0.2.

[0126] (2) Dimethylimidazole and benzimidazole are dissolved in methanol to form a ligand solution with a concentration of 1.2 mol / L; wherein the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 0.6:0.6.

[0127] (3) Add graphite powder to a metal salt solution and disperse it by ultrasonication to obtain a metal salt-graphite mixture; wherein the graphite powder used has a Dv50 of 19 μm and a BET of 1.1 μm. 2 / g, graphitization degree of 93%, the mass ratio of metal salt to graphite in the metal salt-graphite mixture is 1:6, ultrasonic power is 200W, ultrasonic time is 25 minutes.

[0128] (4) Under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture; wherein the molar ratio of ligand to metal salt in the metal salt-graphite mixture is 4:1.

[0129] (5) The reaction temperature was controlled at 50℃ and the reaction time was 22h. After the reaction, the resulting solid-liquid mixture was filtered and washed twice with methanol. After drying, the precursor was obtained. The precursor was a ZIF-coated graphite core-shell structure.

[0130] (6) Dissolve the precursor and polyacrylonitrile in dimethyl sulfoxide and stir to form a precursor solution; wherein the concentration of the precursor in the precursor solution is 50 mg / mL and the concentration of polyacrylonitrile in the precursor solution is 50 mg / mL. (7) The precursor solution is loaded into a syringe and electrospun into a composite material using an electrospinning machine. The syringe advance speed is 0.5 mL / h, the positive electrode voltage is 15 kV, the negative electrode voltage is -2.2 kV, and the spinning distance is 10 cm.

[0131] (8) The electrospun composite material was first pre-oxidized in air at 180°C for 1 hour, and then carbonized in argon at 900°C for 2 hours to obtain the graphite-based composite material.

[0132] Application Example 2 The preparation method of the lithium-ion battery in this application embodiment is the same as that in application embodiment 1, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in embodiment 2; and the electrolyte is prepared by the following method: in a glove box filled with argon (H2O<0.1 ppm, O2<0.1 ppm), LiFSI is dissolved in a mixed solvent of EMC and TTE (EMC:TTE volume ratio = 3:2) to prepare a solution with a concentration of 1.4 mol / L. Then, 5% of the total mass of the electrolyte dimethyl ether (PODE2) and 3% of the total mass of the electrolyte trimethyl ether (PODE3) are added and stirred evenly to obtain the electrolyte.

[0133] Example 3 Preparation of graphite-based composite materials (1) Dissolve cobalt nitrate and zinc nitrate in methanol to obtain a metal salt solution with a concentration of 0.25 mol / L; wherein the molar ratio of cobalt nitrate and zinc nitrate in the metal salt solution is 0.15:0.1.

[0134] (2) Dimethylimidazole and benzimidazole are dissolved in methanol to form a ligand solution with a concentration of 0.9 mol / L; wherein the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 0.4:0.5.

[0135] (3) Add graphite powder to a metal salt solution and disperse it by ultrasonication to obtain a metal salt-graphite mixture; wherein the graphite powder used has a Dv50 of 8 μm and a BET of 1.5 μm. 2 / g, graphitization degree of 92.5%, the mass ratio of metal salt to graphite in the metal salt-graphite mixture is 1:5, ultrasonic power is 250W, ultrasonic time is 30 minutes.

[0136] (4) Under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture; wherein the molar ratio of ligand to metal salt in the metal salt-graphite mixture is 18:5.

[0137] (5) The reaction temperature was controlled at 60℃ and the reaction time was 12h. After the reaction, the resulting solid-liquid mixture was filtered and washed once with methanol. After drying, the precursor was obtained. The precursor was a ZIF-coated graphite core-shell structure.

[0138] (6) Dissolve the precursor and polyacrylonitrile in N,N-dimethylformamide and stir to form a precursor solution; wherein the concentration of the precursor in the precursor solution is 120 mg / mL and the concentration of polyacrylonitrile in the precursor solution is 100 mg / mL. (7) The precursor solution is loaded into a syringe and electrospun into a composite material using an electrospinning machine. The syringe advance speed is 1.2 mL / h, the positive electrode voltage is 18 kV, the negative electrode voltage is -2.4 kV, and the spinning distance is 22 cm.

[0139] (8) The electrospun composite material was first pre-oxidized in air at 150°C for 4 hours, and then carbonized in argon at 600°C for 1 hour to obtain the graphite-based composite material.

[0140] Application Example 3 The preparation method of the lithium-ion battery in this application embodiment is the same as that in application embodiment 1, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in embodiment 3; and the electrolyte is prepared by the following method: in a glove box filled with argon (H2O<0.1 ppm, O2<0.1 ppm), LiFSI is dissolved in a mixed solvent of EMC and TTE (EMC:TTE volume ratio = 2:1) to prepare a solution with a concentration of 1.3 mol / L, and then 5% of the total mass of the electrolyte, dimethyl dioxymethylene ether (PODE2), is added and stirred evenly to obtain the electrolyte.

[0141] Example 4 Preparation of graphite-based composite materials (1) Cobalt acetate, cobalt nitrate and zinc acetate are dissolved in methanol to obtain a metal salt solution with a concentration of 0.4 mol / L; wherein the molar ratio of cobalt acetate, cobalt nitrate and zinc acetate in the metal salt solution is 0.1:0.1:0.2.

[0142] (2) Dimethylimidazole and benzimidazole are dissolved in methanol to form a ligand solution with a concentration of 2 mol / L; wherein the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 0.8:1.2.

[0143] (3) Add graphite powder to a metal salt solution and disperse it by ultrasonication to obtain a metal salt-graphite mixture; wherein the graphite used has a Dv50 of 12 μm and a BET of 1.2 μm. 2 / g, graphitization degree of 94%, the mass ratio of metal salt to graphite in the metal salt-graphite mixture is 1:9, ultrasonic power is 150W, ultrasonic time is 35 minutes.

[0144] (4) Under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture; wherein the molar ratio of ligand to metal salt in the metal salt-graphite mixture is 5:1.

[0145] (5) The reaction temperature was controlled at 80℃ and the reaction time was 18h. After the reaction, the resulting solid-liquid mixture was filtered and washed with methanol three times. After drying, the precursor was obtained. The precursor was a ZIF-coated graphite core-shell structure.

[0146] (6) Dissolve the precursor and polyacrylonitrile in N,N-dimethylacetamide and stir to form a precursor solution; wherein the concentration of the precursor in the precursor solution is 150 mg / mL and the concentration of polyacrylonitrile in the precursor solution is 140 mg / mL. (7) The precursor solution is loaded into a syringe and electrospun into a composite material using an electrospinning machine. The syringe advance speed is 1.7 mL / h, the positive electrode voltage is 13 kV, the negative electrode voltage is -2.3 kV, and the spinning distance is 20 cm.

[0147] (8) The electrospun composite material was first pre-oxidized in air at 250°C for 2 hours, and then carbonized in argon at 450°C for 3 hours to obtain the graphite-based composite material.

[0148] Application Example 4 The preparation method of the lithium-ion battery in this application embodiment is the same as that in application embodiment 1, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in embodiment 4; and the electrolyte is prepared by the following method: in a glove box filled with argon (H2O<0.1 ppm, O2<0.1 ppm), LiFSI is dissolved in a mixed solvent of EMC and TTE (EMC:TTE volume ratio = 5:2) to prepare a solution with a concentration of 1.1 mol / L. Then, 10% of dimethyl oxyacetaldehyde (PODE2) and 10% of tetramethyl oxyacetaldehyde (PODE4) of the total mass of the electrolyte are added and stirred evenly to obtain the electrolyte.

[0149] Example 5 Preparation of graphite-based composite materials (1) Cobalt acetate, cobalt nitrate, zinc acetate and zinc nitrate are dissolved in methanol to obtain a metal salt solution with a concentration of 0.5 mol / L; wherein the molar ratio of cobalt acetate, cobalt nitrate, zinc acetate and zinc nitrate in the metal salt solution is 0.2:0.1:0.1:0.1.

[0150] (2) Dimethylimidazole and benzimidazole are dissolved in methanol to form a ligand solution with a concentration of 1.8 mol / L; wherein the molar ratio of dimethylimidazole and benzimidazole in the ligand solution is 0.6:1.2.

[0151] (3) Add graphite powder to a metal salt solution and disperse it by ultrasonication to obtain a metal salt-graphite mixture; wherein the graphite powder used has a Dv50 of 9 μm and a BET of 1.3 μm.2 / g, graphitization degree of 93.5%, the mass ratio of metal salt to graphite in the metal salt-graphite mixture is 1:10, ultrasonic power is 300W, ultrasonic time is 40 minutes.

[0152] (4) Under stirring conditions, the ligand solution is slowly added dropwise to the metal salt-graphite mixture; wherein the molar ratio of ligand to metal salt in the metal salt-graphite mixture is 9:2.

[0153] (5) The reaction temperature was controlled at 75℃ and the reaction time was 26h. After the reaction, the resulting solid-liquid mixture was filtered and washed once with methanol. After drying, the precursor was obtained. The precursor was a ZIF-coated graphite core-shell structure.

[0154] (6) Dissolve the precursor and polyacrylonitrile in N,N-dimethylformamide and stir to form a precursor solution; wherein the concentration of the precursor in the precursor solution is 100 mg / mL and the concentration of polyacrylonitrile in the precursor solution is 120 mg / mL. (7) The precursor solution is loaded into a syringe and electrospun into a composite material using an electrospinning machine. The syringe push speed is 1.5 mL / h, the positive electrode voltage is 23 kV, the negative electrode voltage is -2.6 kV, and the spinning distance is 18 cm.

[0155] (8) The electrospun composite material was first pre-oxidized in air at 200°C for 1 hour, and then carbonized in argon at 700°C for 1.5 hours to obtain the graphite-based composite material.

[0156] Application Example 5 The preparation method of the lithium-ion battery in this application embodiment is the same as that in application embodiment 1, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in embodiment 5; and the electrolyte is prepared by the following method: in a glove box filled with argon (H2O<0.1 ppm, O2<0.1 ppm), LiFSI is dissolved in a mixed solvent of EMC and TTE (EMC:TTE volume ratio = 3:1) to prepare a solution with a concentration of 1.5 mol / L. Then, 6% of dimethyl oxyacetaldehyde (PODE2) and 4% of trimethyl oxyacetaldehyde (PODE3) and 2% of tetramethyl oxyacetaldehyde (PODE4) of the total mass of the electrolyte are added and stirred evenly to obtain the electrolyte.

[0157] Example 6 The preparation method of the graphite-based composite material in this embodiment is the same as that in Example 4, except that the concentration of the metal salt in the metal salt solution is 0.04 mol / L and the concentration of the ligand in the ligand solution is 1 mol / L.

[0158] Application Example 6 The method for preparing the lithium-ion battery in this application example is the same as that in application example 4, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in example 6.

[0159] Example 7 The preparation method of this embodiment is the same as that of Example 5, except that the concentration of the metal salt in the metal salt solution is 0.8 mol / L and the concentration of the ligand in the ligand solution is 1 mol / L.

[0160] Application Example 7 The method for preparing the lithium-ion battery in this application example is the same as that in application example 5, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in example 7.

[0161] Comparative Example 1 The preparation method of the graphite-based composite material in this comparative example is the same as that in Example 1, except that... (1) Dissolve zinc acetate in methanol to obtain a metal salt solution with a concentration of 0.1 mol / L.

[0162] (2) Dissolve dimethylimidazole in methanol to form a ligand solution with a concentration of 1.5 mol / L.

[0163] Application Comparative Example 1 The preparation method of the lithium-ion battery in this comparative example is the same as that in Application Example 1, except that the graphite-based composite material prepared in Comparative Example 1 is used.

[0164] Comparative Example 2 The preparation method of this comparative example is the same as that of Example 2, except that (6) and (7) are omitted.

[0165] Application Comparative Example 2 The preparation method of the lithium-ion battery in the comparative example is the same as that in application example 2. The difference is that the graphite-based composite material prepared in comparative example 2 is mixed with conductive agent SP, thickener CMC and binder SBR in the following mass ratio: graphite-based composite material: SP: CMC: SBR = 95.7: 1.0: 1.5: 1.8. N-methylpyrrolidone is added and stirred. After stirring evenly, a negative electrode slurry with a solid content of 50±10% is obtained. The negative electrode slurry is coated on the current collector copper foil and dried under vacuum for 12 hours. After being taken out, it is cut into a circular piece with a diameter of 14 mm as the negative electrode sheet.

[0166] Comparative Example 3 The preparation method of this comparative example is the same as that of Example 3, except that steps (1) to (5) are omitted.

[0167] Application Comparative Example 3 The preparation method of the lithium-ion battery in the comparative example is the same as that in application example 3, except that the negative electrode sheet is prepared using the graphite-based composite material obtained in comparative example 3.

[0168] Application Comparative Example 4 The preparation method of the lithium-ion battery in this application comparison is the same as that in Application Example 1, except that the electrolyte is prepared by the following method: in a glove box filled with argon (H2O<0.1 ppm, O2<0.1 ppm), LiPF6 is dissolved in a mixed solvent of EC, EMC and DEC (EC:EMC:DEC volume ratio = 1:1:1) to prepare an electrolyte with a concentration of 1.0 mol / L.

[0169] Application Comparative Example 5 The preparation method of the lithium-ion battery in this comparative application is the same as that in Application Example 2, except that the electrolyte does not contain dimethyl oxymethylene (PODE2) and trimethyl oxymethylene (PODE3).

[0170] Application Comparative Example 6 The preparation method of the lithium-ion battery in this application comparison is the same as that in application example 3, except that 1,3-dioxolane (DOL) is used instead of dioxymethylene dimethyl ether (PODE2) when preparing the electrolyte.

[0171] Application Comparative Example 7 The preparation method of the lithium-ion battery in this application comparison is the same as that in application example 4, except that when preparing the electrolyte, 15% of dioxymethylene dimethyl ether (PODE2) and 15% of tetraoxymethylene dimethyl ether (PODE4) by mass of the total electrolyte are added.

[0172] Material information for each embodiment: It should be noted that the graphite with different properties (average particle size / specific surface area / degree of graphitization) used in the embodiments of this application were all purchased from Shanghai McLean Biochemical Technology Co., Ltd.

[0173] Test method: 1. The particle size Dv50 test method is as follows: The particle size distribution (PSD) was tested according to the detection method described in GB_T 24533-2019 for graphite anode materials of lithium-ion batteries. The specific method is as follows: Graphite powder was taken and tested using a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK, with a wet sampler to obtain the particle size distribution data of the graphite powder.

[0174] 2. The specific surface area test method is as follows: The specific surface area was tested according to the test method described in GB_T 24533-2019 for graphite anode materials of lithium-ion batteries. The specific method is as follows: Graphite powder was taken and a 3H-2000A dynamic specific surface area tester from Best Instruments Technology Co., Ltd. was used. The sample was pretreated by vacuum degassing at 200°C for 2 hours, and then the specific surface area data of the graphite powder was obtained.

[0175] 3. The method for testing the degree of graphitization is as follows: The degree of graphitization was tested according to the detection method described in GB_T 24533-2019 for graphite anode materials of lithium-ion batteries. The specific method is as follows: Take graphite powder, add a certain amount of Si standard sample, and grind it in an agate mortar. Place the well-mixed sample into the groove of a glass slide and flatten it. Use an X-ray diffractometer to perform XRD tests on the sample on the glass slide. Set the starting angle of the scan to 10°, the ending angle to 30°, and the step size to ≤0.02°. The scanning mode is continuous scanning. Select the diffraction peak 2θ=26.5° of the graphite (002) crystal plane. Use the position of the diffraction peak of the Si (111) crystal plane for calibration to obtain the position of the diffraction peak of the graphite (002) crystal plane after calibration. Calculate the degree of graphitization of the graphite powder using formulas 1 and 2.

[0176] Formula 1: 2dsinθ=nλ; Formula 2: Degree of graphitization = (3.44 - d) 002 ) / (3.44-3.354)*100%.

[0177] 4. The electrochemical performance testing methods are as follows: 4.1. Assemble the CR2032 button cell: Lithium hexafluorophosphate (LiPF6) was dissolved in a solvent to obtain a test electrolyte. The solvent was a mixture of ethyl methyl carbonate (EC) and ethyl methyl carbonate (EMC). The concentration of LiPF6 in the test electrolyte was 1 mol / L, and the volume ratio of EC to EMC was 3:7.

[0178] The graphite-based composite materials obtained in Examples 1-7 and Comparative Examples 1 and 3 were cut into 14 mm diameter discs using a cutting machine and used directly as negative electrode sheets. Ceglard 2500 was used as the separator, and lithium metal sheets were used as the counter electrode. The negative electrode sheets, along with the test electrolyte, separator, and counter electrode, were assembled into a CR2032 button cell in a vacuum glove box.

[0179] In addition, the graphite-based composite material prepared in Comparative Example 2 was mixed with conductive agent SP, thickener CMC, and binder SBR, wherein the mass ratio of graphite-based composite material:SP:CMC:SBR was 95.7:1.0:1.5:1.8. N-methylpyrrolidone was added and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 50±10%. The negative electrode slurry was coated onto a current collector copper foil and dried under vacuum for 12 hours. After drying, it was cut into circular pieces with a diameter of 14 mm as negative electrode sheets. Ceglard 2500 was used as the separator, and lithium metal sheet was used as the counter electrode. In a vacuum glove box, the negative electrode sheet, along with the above-mentioned test electrolyte, separator, and counter electrode, was assembled into a CR2032 button cell.

[0180] 4.2. Test methods for initial reversible lithium intercalation specific capacity and initial coulombic efficiency The initial reversible lithium insertion capacity and initial coulombic efficiency were tested according to the testing method described in GB / T 24533-2019 for graphite-based anode materials for lithium-ion batteries. The specific method is as follows: On the Shenzhen Xinwei button cell testing system, at 25℃, the CR2032 button cell assembled in step 4.1 was discharged at 0.1C to 0.005V, allowed to stand for 120 minutes, and then discharged at 20μA to 0.005V to obtain the initial lithium insertion capacity of the graphite-based composite material. After standing for 120 minutes, it was charged at 0.1C to 2.0V to complete the first cycle, obtaining the initial reversible lithium extraction capacity of the graphite-based composite material. The ratio of the initial reversible lithium extraction capacity to the initial reversible lithium insertion capacity is the initial coulombic efficiency. The results are shown in Table 1.

[0181] 4.3. Test Methods for 3C Constant Current SOC On the Shenzhen Xinwei button cell battery testing system, the CR2032 button cell battery whose initial lithium delithiation capacity was tested in step 4.2 was used for a room-temperature 3C constant-current SOC test. The measurement procedure was as follows: at 25℃, the CR2032 button cell battery whose initial lithium delithiation capacity was tested in step 4.2 was discharged at a 3C constant current to 0.005V, and the lithium intercalation capacity of the graphite-based composite material under this condition was obtained. The ratio of this lithium intercalation capacity to the initial lithium delithiation capacity measured in step 4.2 is the room-temperature 3C constant-current SOC, reflecting the 3C fast charging capability. The results are shown in Table 1.

[0182] 4.4 Test method for low-temperature charging rate On the Shenzhen Xinwei button cell battery testing system, low-temperature charge rate tests were conducted on the lithium-ion batteries assembled in the above application examples and application comparison examples. The test methods are as follows: At -20℃, the assembled lithium-ion battery was fully charged at 0.1C and fully discharged at 1C 10 times. Then, the battery was fully charged at 0.1C. The negative electrode was then removed, and the lithium deposition on its surface was observed. If lithium deposition still occurred, the same lithium-ion battery was selected, and the process was repeated 10 times with a full charge at 0.08C and a full discharge at 1C. The battery was then fully charged at 0.08C, and the negative electrode was removed again to observe the lithium deposition. If lithium deposition still occurred on the negative electrode, the maximum charge rate of the battery at -20℃ was <0.08C. If no lithium deposition occurred, the maximum charge rate of the battery at -20℃ was 0.08C.

[0183] At -20℃, the assembled lithium-ion battery was fully charged at 0.1C and fully discharged at 1C 10 times. Then, the battery was fully charged at 0.1C. The negative electrode was then disassembled, and the lithium deposition on the surface of the negative electrode was observed. If no lithium deposition occurred on the negative electrode at this point, the charging rate was increased in increments of 0.02C, and the test was repeated until lithium deposition occurred on the negative electrode surface. The test was then stopped. The maximum charging rate of the battery was the charging rate obtained by subtracting 0.02C from the charging rate at this point.

[0184] Table 1. Preparation conditions of graphite-based composite materials in the above embodiments and comparative examples.

[0185] Table 2. Preparation conditions of graphite-based composite materials in the above embodiments and comparative examples.

[0186] Table 3. Electrochemical performance of CR2032 button batteries assembled from graphite-based composite materials in the above examples and comparative examples.

[0187] Table 4. Preparation conditions of lithium-ion batteries for the above application examples and comparative examples.

[0188] Table 5. Electrochemical performance of lithium-ion batteries in the above application examples and comparative examples.

[0189] Based on the first reversible lithium intercalation specific capacity, first coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite materials prepared in Example 1 and Comparative Example 1, it can be seen that, compared with Example 1, the first reversible lithium intercalation specific capacity, first coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite materials prepared in Comparative Example 1 are all lower. Based on the low-temperature charge rate test results of the lithium-ion batteries applied in Example 1 and Comparative Example 1, it can be seen that, compared with the lithium-ion batteries applied in Example 1, the low-temperature charge rate test results of the lithium-ion batteries applied in Comparative Example 1 are lower. This is because: (1) When only zinc metal salt is added, the lack of cobalt metal salt makes the conductivity of silicon-based composite material worse and the number of catalytic active sites less; (2) When benzimidazole is not added, ZIF9 and ZIF11 cannot be generated. After carbonization, the pores contain only a single size pore, and the pore hierarchical control fails; and without the conjugated π bond and Co-N bond of benzimidazole, the hydrogen bond network at the electrolyte-electrode interface cannot be formed, the low-temperature ion transport is blocked, the lithium ion diffusion coefficient of the material decreases, resulting in a worse rate performance and a worse low-temperature charging rate performance; in addition, the conjugated π bond without benzimidazole gives ZIF-9 and ZIF-11 a unique electron cloud distribution. After being composited with graphite substrate and carbonized, there are fewer defects and active sites in the carbon layer formed on the graphite surface.

[0190] The results of the first reversible lithium intercalation specific capacity, first coulombic efficiency, and 3C constant current SOC tests of the graphite-based composite materials prepared in Example 2 and Comparative Example 2 show that, compared to Example 2, the first reversible lithium intercalation specific capacity, first coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite material prepared in Comparative Example 2 are all lower. The results of the low-temperature charging rate tests of the lithium-ion batteries using Example 2 and Comparative Example 2 show that, compared to Example 2, the low-temperature charging rate test results of the lithium-ion battery using Comparative Example 2 are lower. This is because, without the electrospinning step, the carbon layer@graphite particles after ZIF carbonization agglomerate severely, the specific surface area of ​​the graphite-based composite material decreases, there is no fibrous conductive network, the lithium-ion diffusion distance increases, the battery rate performance decreases, and the low-temperature charging rate performance also deteriorates.

[0191] The results of the first reversible lithium intercalation capacity, first coulombic efficiency, and 3C constant current SOC tests of the graphite-based composite materials prepared in Example 3 and Comparative Example 3 show that, compared to Example 3, the first reversible lithium intercalation capacity, first coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite materials prepared in Comparative Example 3 are all lower. The results of the low-temperature charge rate tests of the lithium-ion batteries applied in Example 2 and Comparative Example 2 show that, compared to Example 3, the low-temperature charge rate test results of the lithium-ion batteries applied in Comparative Example 3 are lower. This is because: due to the lack of a ZIF carbonization layer coating, the graphite edges are prone to react with the electrolyte to form an unstable SEI film; the mesoporous / macroporous structure without ZIF pyrolysis reduces the lithium-ion diffusion coefficient; and the absence of a ZIF carbonization coating layer also reduces the restriction on graphite volume expansion.

[0192] The initial reversible lithium intercalation capacity, initial coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite materials prepared in Examples 4 and 6 show that, compared to Example 4, the initial reversible lithium intercalation capacity, initial coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite materials prepared in Example 6 are all lower. The low-temperature charge rate test results of the lithium-ion batteries using Examples 4 and 6 show that, compared to Example 4, the low-temperature charge rate test results of the lithium-ion batteries using Example 6 are lower. This is because when the metal salt concentration is too low, ZIF nucleation is insufficient, the "micropore-mesopore-macropore" tertiary structure is reduced, the specific surface area decreases, the active sites are insufficient, the catalytic effect is weak, and the battery performance deteriorates.

[0193] The results of the first reversible lithium intercalation capacity, first coulombic efficiency, and 3C constant current SOC tests of the graphite-based composite materials prepared in Examples 5 and 7 show that, compared to Example 5, the first reversible lithium intercalation capacity, first coulombic efficiency, and 3C constant current SOC test results of the graphite-based composite material prepared in Example 7 are all lower. The results of the low-temperature charging rate tests of the lithium-ion batteries using Examples 5 and 7 show that, compared to Example 5, the low-temperature charging rate test results of the lithium-ion battery using Example 7 are lower. This is because when the metal salt concentration is too high, ZIF grows excessively into a thick shell layer, graphite particles agglomerate, the specific surface area decreases, the lithium-ion diffusion path is prolonged, and the rate performance decreases. A uniform coating layer is very important for the structure of graphite-based composite materials.

[0194] The results of the low-temperature charging rate test of the lithium-ion batteries in Application Example 1 and Application Comparative Example 4 show that even if the graphite-based composite material of this application is used as the negative electrode material, its low-temperature rate performance is difficult to fully realize if an unsuitable electrolyte is used, especially at low temperatures.

[0195] The low-temperature charge rate test results of the lithium-ion batteries in Application Example 2 and Application Comparative Example 5 show that the electrolyte in Application Comparative Example 5 only lacks the core functional additive PODE3; the other components (LiFSI salt, EMC solvent, TTE diluent) are the same as in Application Example 2. n The lack of a specific electrolyte in Comparative Example 5 resulted in a higher low-temperature viscosity than that in Application Example 2, leading to significantly worse low-temperature rate performance of the lithium-ion battery in Comparative Example 5 compared to Application Example 2. This is due to PODE. n The addition of this ingredient played a crucial role in bringing the dish to life, rather than contributing to the overall effect of other components.

[0196] The low-temperature charge rate test results of the lithium-ion batteries in Application Example 3 and Comparative Example 6 show that PODE in the electrolyte is the cause. n The unique structure (oligoether, symmetrical methyl end capping) provides irreplaceable advantages, which not all ether solvents (such as 1,3-dioxolane (DOL)) can achieve the same effect.

[0197] The low-temperature charge rate test results of the lithium-ion batteries used in Application Example 4 and Comparative Example 7 show that excessively high concentrations of PODE in the electrolyte... n It can adversely affect low-temperature performance; excessive addition may lead to lithium salt deposition, thereby reducing conductivity. Comparative Example 7 was used to demonstrate the presence of PODE in the electrolyte. n There is an optimal concentration range for the addition of [the substance], and more is not necessarily better, which demonstrates the value of the preferred formulation determined by this application through a large number of experiments.

[0198] In this application, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0199] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A graphite-based composite material, characterized by, The graphite-based composite material has a three-dimensional conductive network, and the graphite-based composite material comprises graphite particles and porous carbon nanofibers coated on surfaces of the graphite particles, the porous carbon nanofibers comprise a nitrogen-doped porous carbon framework and metal nanoparticles embedded in the nitrogen-doped porous carbon framework, the metal nanoparticles comprise Co and Zn, the porous carbon nanofibers have a "micropore-mesopore-macropore" three-level pore structure, the porous carbon nanofibers are obtained after carbonization of ZIF and a polymer material, metal ions of the ZIF comprise Co 2+ and Zn 2+ , and ligands of the ZIF comprise dimethyl imidazole and benzimidazole. Optionally, the micropore diameter is 0.8 nm-1.4 nm; Optionally, the mesopore diameter is 3 nm-10 nm; Optionally, the macropore diameter is 50 nm-200 nm; Optionally, the Co 2+ and the Zn 2+ molar ratio is (0.5-1.5):

1. Optionally, the molar ratio of the dimethyl imidazole and the benzimidazole is 1: (1-2); Optionally, the high polymer material is polyacrylonitrile and / or polyvinyl alcohol.

2. A method of producing a graphite-based composite material, characterized by, The method comprises the following steps: S1. Dissolving a metal salt in methanol to form a metal salt solution, wherein the metal salt comprises a zinc salt and a cobalt salt; dissolving a ligand in methanol to form a ligand solution, wherein the ligand comprises dimethyl imidazole and benzimidazole; then adding graphite powder into the metal salt solution, ultrasonic dispersion, to obtain a metal salt-graphite mixed solution; then slowly dropping the ligand solution into the metal salt-graphite mixed solution under stirring to obtain a raw material mixed solution; after coordination reaction of the raw material mixed solution, a solid-liquid mixture is obtained; the solid-liquid mixture is subjected to suction filtration and washed with methanol, and then dried to obtain a precursor, wherein the precursor is a core-shell structure of ZIF coated graphite; S2. Dissolving the precursor and a high polymer material in a first organic solvent to form a precursor solution; then electrospinning the precursor solution to obtain an electrospun composite material; S3. Pre-oxidizing and carbonizing the electrospun composite material to obtain the graphite-based composite material.

3. The preparation method according to claim 2, characterized in that, In step S1, the cobalt salt is cobalt acetate and / or cobalt nitrate, and the zinc salt is zinc acetate and / or zinc nitrate; Optionally, the concentration of the metal salt in the metal salt solution is 0.1 mol / L-0.5 mol / L; Optionally, the molar ratio of the cobalt salt to the zinc salt in the metal salt solution is (0.5-1.5):1; Optionally, the concentration of the ligand in the ligand solution is 0.5 mol / L-2.0 mol / L; Optionally, the molar ratio of the dimethyl imidazole to the benzimidazole in the ligand solution is 1: (1-2).

4. The production method according to claim 2, characterized by, In step S1, in the metal salt-graphite mixed solution, the mass ratio of the metal salt to the graphite is 1: (4-10); Optionally, the graphite satisfies at least one of the following conditions: (a) the Dv50 of the graphite powder is 5 μm ~ 19 μm; (b) the specific surface area of the graphite powder is 0.7 m 2 / g ~ 2.0 m 2 / g; (c) the graphitization degree of the graphite powder is 91% ~ 96%. Optionally, the graphite satisfies at least one of the following conditions: (a) the Dv50 of the graphite powder is 5 μm ~ 19 μm; (b) the specific surface area of the graphite powder is 0.7 m 2 / g ~ 2.0 m 2 / g; (c) the graphitization degree of the graphite powder is 91% ~ 96%. Optionally, the specific conditions of the ultrasonic dispersion are as follows: the ultrasonic power is 150 W-360 W, and the ultrasonic time is 20 minutes-40 minutes; Optionally, the molar ratio of the ligand to the metal salt in the raw material mixed solution is (3.6-5):1; Optionally, the specific conditions of the coordination reaction are as follows: the incubation temperature is 40℃-80℃, and the reaction time is 12 hours-28 hours; Optionally, the washing with methanol is performed 1-3 times.

5. The preparation method according to claim 2, characterized in that, In step S2, the first organic solvent is at least one of N,N-dimethylformamide, dimethyl sulfoxide and N,N-dimethylacetamide; Optionally, the high polymer material is polyacrylonitrile and / or polyvinyl alcohol; Optionally, the concentration of the precursor in the precursor solution is 50 mg / mL-150 mg / mL, and the concentration of the high polymer material in the precursor solution is 50 mg / mL-150 mg / mL; Optionally, the electrostatic spinning is performed at an injection rate of 0.2 mL / h to 1.7 mL / h, a positive electrode voltage of 13 kV to 23 kV, a negative electrode voltage of -2.7 kV to -2.2 kV, and a spinning distance of 10 cm to 22 cm.

6. The preparation method according to claim 2, characterized in that, In step S3, the pre-oxidation treatment is performed in an air atmosphere at a holding temperature of 150 DEG C to 250 DEG C for 1 hour to 4 hours. Optionally, the carbonization treatment is performed in an inert gas atmosphere at a holding temperature of 450 DEG C to 900 DEG C for 1 hour to 3 hours.

7. A negative electrode sheet characterized by comprising: The negative electrode sheet is the graphite-based composite material of claim 1 or the graphite-based composite material obtained by the preparation method of any one of claims 2 to 6.

8. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the negative electrode sheet is the negative electrode sheet of claim 7, and the electrolyte comprises a lithium salt, a second organic solvent and a functional additive; wherein the functional additive is oligomeric formaldehyde dimethyl ether PODE n , at least one of n = 2, 3, 4; the addition amount of the functional additive is 30% or less, based on 100% of the total mass of the electrolyte.

9. The secondary battery according to claim 8, characterized by The functional additive is added in an amount of 5% to 20% based on 100% of the total mass of the electrolyte; Optionally, the lithium salt is lithium bisfluorosulfonylimide (LiFSI), and the concentration of the lithium salt in the electrolyte is 1.0 mol / L to 1.5 mol / L. Optionally, the second organic solvent comprises methyl ethyl carbonate (EMC) and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane (TTE), and the volume ratio of EMC to TTE is 1:1 to 3:

1.

10. An electrical device, characterized by The power consumption device comprises the secondary battery of claim 8 or 9.

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