Hard carbon-graphite composite negative electrode material and preparation method thereof, secondary battery

Abstract

The application discloses a kind of hard carbon-graphite composite negative materials and preparation method thereof, secondary battery, and it is related to secondary battery negative material technical field.By constructing negative material with three-layer structure, graphite core provides high electron conductivity and structural stability, as support framework guarantee material overall cycle durability;Hard carbon intermediate layer forms continuous, uniform and thickness controllable active layer on the surface of graphite, which significantly improves the specific capacity of the material;And carbon shell realizes dense coating on the surface of hard carbon intermediate layer, not only strengthens the interface bonding strength between graphite core and hard carbon intermediate layer, but also controls the closed porosity of negative material by directional capping and reconstruction of original pore of hard carbon intermediate layer, which can effectively inhibit the excessive infiltration of electrolyte and side reaction while maintaining sufficient ion transmission channel, thereby realizing the comprehensive performance improvement of the first coulomb efficiency, specific capacity and rate performance of the negative material.

Description

Technical Field

[0001] This invention relates to the field of secondary battery anode material technology, specifically to a hard carbon-graphite composite anode material and its preparation method, and a secondary battery. Background Technology

[0002] With the rapid development of new energy vehicles and portable electronic devices, the performance requirements for electrochemical energy storage devices such as lithium-ion batteries are increasing. As a core component of lithium-ion batteries, the performance of the anode material directly determines the battery's energy density, rate performance, and cycle life.

[0003] Currently, commercial lithium-ion batteries mainly use graphite-based materials as anodes, which have advantages such as low cost and stable voltage platform. However, the theoretical capacity of graphite anodes is relatively low (about 372 mAh / g) and the rate performance is poor, making it difficult to meet the needs of next-generation high-energy-density batteries.

[0004] Hard carbon materials have attracted widespread attention due to their high specific capacity (typically exceeding 400 mAh / g) and excellent rate performance. However, the large specific surface area and abundant open pore structure of hard carbon materials lead to severe side reactions with the electrolyte during charge and discharge, forming an excessively thick solid electrolyte interphase (SEI) film. This consumes a large amount of active lithium ions, resulting in low initial coulombic efficiency and poor cycle stability of hard carbon materials.

[0005] To combine the advantages of graphite and hard carbon materials, existing technologies employ physical mixing methods to prepare hard carbon-graphite composites. For example, hard carbon precursors are mechanically mixed with graphite and then directly carbonized to obtain hard carbon-coated graphite materials. However, existing hard carbon-coated graphite materials cannot simultaneously achieve high specific capacity, high initial coulombic efficiency, and excellent rate performance.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] After in-depth research, the inventors of this invention discovered that existing hard carbon-graphite composite materials have problems such as weak interfacial bonding between hard carbon and graphite, which easily leads to phase separation during cycling. Secondly, existing hard carbon-graphite composite materials cannot achieve controllable loading of hard carbon and graphite, and it is also difficult to accurately control the pore structure of the composite material, thus restricting the improvement of the electrical properties of hard carbon-graphite composite materials.

[0008] The purpose of this invention is to provide a hard carbon-graphite composite negative electrode material, its preparation method, and a secondary battery.

[0009] This invention is implemented as follows: In a first aspect, the present invention provides a hard carbon-graphite composite anode material, the anode material comprising multiple composite particles, the composite particles comprising a graphite core, a hard carbon intermediate layer and a carbon shell arranged sequentially from the inside to the outside.

[0010] The hard carbon interlayer has pores, and the carbon shell covers the surface of the hard carbon interlayer to form a closed-pore structure. The closed-pore rate of the negative electrode material is 10~25%, the average pore size of the closed-pore structure is 0.5~0.8nm, and the closed-pore volume is 0.1~0.2cm³ / g.

[0011] In a second aspect, the present invention provides a method for preparing a negative electrode material as described in any of the foregoing embodiments, comprising: adding graphite to a template agent dispersion and dispersing it evenly, then adding a phenolic compound, an alkaline catalyst and an aldehyde compound, and performing an in-situ polymerization reaction on the graphite surface to obtain a composite precursor; calcining the composite precursor under an inert atmosphere to obtain a carbonaceous composite material; and performing chemical vapor deposition on the carbonaceous composite material under a carbon-containing atmosphere.

[0012] Thirdly, the present invention provides a secondary battery, comprising a negative electrode material as described in any of the foregoing embodiments or a negative electrode material prepared by any of the foregoing embodiments.

[0013] The present invention has the following beneficial effects: This invention provides a hard carbon-graphite composite anode material, its preparation method, and a secondary battery. By constructing an anode material with a core-shell structure, the graphite core provides high electronic conductivity and structural stability, serving as a supporting framework to ensure the overall cycle durability of the material. The hard carbon intermediate layer, relying on its intrinsic high lithium storage capacity, forms a continuous, uniform, and controllable thickness active layer on the graphite surface, significantly improving the specific capacity of the material. The carbon shell achieves dense coating on the surface of the hard carbon intermediate layer, which not only strengthens the interfacial bonding strength between the graphite core and the hard carbon intermediate layer, preventing phase separation during cycling, but more importantly, by directionally sealing and reconstructing the original pores of the hard carbon intermediate layer, some open pores are transformed into closed pores. The closed-pore structure of this invention not only provides abundant ion storage sites, giving the material excellent specific capacity characteristics, but also the optimized pore network structure promotes rapid ion transport, endowing the material with excellent rate performance. Furthermore, the carbon shell of the negative electrode material of this invention can significantly suppress side reactions of the electrolyte during charging and discharging, resulting in a significantly improved first-time coulombic efficiency and effectively overcoming the technical bottleneck of low first-time efficiency in traditional hard carbon materials. This achieves a comprehensive improvement in the first-time coulombic efficiency, specific capacity, and rate performance of the negative electrode material. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0015] In a first aspect, the present invention provides a hard carbon-graphite composite anode material, the anode material comprising multiple composite particles, the composite particles comprising a graphite core, a hard carbon intermediate layer and a carbon shell arranged sequentially from the inside to the outside.

[0016] The hard carbon interlayer has pores, and the carbon shell covers the surface of the hard carbon interlayer to form a closed-pore structure. The closed-pore rate of the negative electrode material is 10~25%, the average pore size of the closed-pore structure is 0.5~0.8nm, and the closed-pore volume is 0.1~0.2cm³ / g.

[0017] The closed-porosity refers to the percentage of the total volume of closed pores (not connected to the outside) within the material to its apparent volume. It is calculated as follows: Closed-porosity = (2.26 - True density) / 2.26 × 100%. Wherein, true density is the true density of the negative electrode material, obtained according to the method in GB / T 24203-2024; 2.26 g / cm³. 3 ρ is the theoretical density of graphite, the theoretical limit value calculated based on its ideal crystal structure.

[0018] The method for testing the closed-pore size is as follows: Small-Angle Scattering (SAS) technique is used to characterize the closed-pore size in the material. Specifically, small-angle scattering (SAXS) technique using X-rays as the incident source is used to characterize the closed-pore size in the material.

[0019] Characterization methods can employ conventional approaches in the field. For example, during testing, the scattering behavior of X-rays (or neutrons) by the sample in a small angular range can be analyzed to obtain information on the closed-pore structure at the nanometer to micrometer scale within the material. Based on the relationship between scattering intensity and scattering vector in the scattering curve, the average pore size and its distribution of the closed pores can be calculated using Guinier fitting, Porod analysis, and pore structure models, thereby achieving quantitative analysis of the closed-pore size characteristics of the material.

[0020] The closed-pore volume is calculated as follows: The total pore volume (open pore + closed pore) is obtained using small-angle X-ray scattering (SAXS) technology. The open pore volume obtained from BET testing is then subtracted to obtain the closed-pore volume. The SAXS test method is based on ISO 20804:2022, and the BET test method is based on GB / T 19587-2017.

[0021] This invention constructs a three-layer anode material. A graphite core provides high electronic conductivity and structural stability, serving as a supporting framework to ensure the overall cycle durability of the material. A hard carbon intermediate layer, leveraging its inherently high lithium storage capacity, forms a continuous, uniform, and controllable-thickness active layer on the graphite surface, significantly improving the material's specific capacity. A carbon shell densely coats the surface of the hard carbon intermediate layer, not only strengthening the interfacial bonding between the graphite core and the hard carbon intermediate layer and preventing phase separation during cycling, but more importantly, by directionally sealing and reconstructing the original pores of the hard carbon intermediate layer, some open pores are transformed into closed pores. This closed-pore structure not only provides abundant ion storage sites, giving the material excellent specific capacity characteristics, but also, through the optimized pore network structure, promotes rapid ion transport, endowing the material with excellent rate performance. Furthermore, the carbon shell constructed in this invention significantly suppresses side reactions of the electrolyte during charging and discharging, resulting in a significantly improved first-coulombic efficiency, effectively overcoming the technical bottleneck of low first-coulombic efficiency in traditional hard carbon materials. This achieves a comprehensive improvement in the coulombic efficiency, specific capacity, and rate performance of the anode material for the first time.

[0022] In an optional embodiment, the interface between the graphite core and the hard carbon interlayer is seamless or the gap width does not exceed 100 nm. In this invention, a strong interfacial bond is achieved between the hard carbon interlayer and the graphite core, constructing a composite structure with strong interfacial bonding. This significantly improves the stability of the structure, facilitates the uniformity and increase of the load on the hard carbon interlayer, and also prevents phase separation during cycling, thereby improving the cycling performance of the material.

[0023] The size of the gap was observed using the following method: the gap between the graphite core and the hard carbon interlayer was observed using the FIB-SEM cross-sectional method. Specifically, conventional methods in the art can be employed. For example, a platinum or tungsten protective layer is first deposited at the target interface of the sample, followed by high-current coarse cutting and low-current polishing to obtain a smooth cross-section. The sample stage is tilted to 52°, and high-resolution scanning electron microscopy is used to directly measure the vertical width of the interface gap (with nanometer-level accuracy). This method clearly reveals the presence and specific size of the gap, avoiding interference from surface contamination.

[0024] In an optional embodiment, the carbon content in the negative electrode material is ≥95% by mass. Further, in an optional embodiment, doping elements such as nitrogen and phosphorus can be introduced as needed to further adjust the performance of the negative electrode material.

[0025] In an optional embodiment, the particle size Dv50 of the negative electrode material is 8~30μm, and the particle size distribution is uniform.

[0026] In an optional embodiment, the tap density of the negative electrode material is 0.8~1.2 g / cm³.3 In this invention, the graphite surface is uniformly coated with hard carbon to form a three-dimensional porous network structure. After the carbon shell is coated, the surface of the negative electrode material becomes denser and smoother.

[0027] In an optional embodiment, the specific surface area of ​​the negative electrode material is 3~10 m². 2 / g. With a suitable specific surface area, it significantly suppresses side reactions of the electrolyte during charging and discharging.

[0028] In an optional embodiment, the graphite core has a particle size Dv50 of 5-20 μm; the hard carbon interlayer has a thickness of 3-10 μm; and the carbon shell has a thickness of 10-50 nm. In this invention, the hard carbon interlayer has a porous framework structure, which is not subject to theoretical constraints. The thickness of the hard carbon interlayer is within the above range, laying the foundation for improving the capacity of the anode material.

[0029] To address the problems of weak interfacial bonding, low hard carbon loading, and uncontrollable pore structure in existing hard carbon-graphite composite materials, this invention provides a method for preparing a negative electrode material. This method achieves a high proportion of uniform loading of hard carbon on the graphite surface and precise control of the pore structure through a synergistic process of in-situ polymerization and chemical vapor deposition (CVD), thereby obtaining a negative electrode material with high specific capacity, high initial efficiency, and excellent fast-charging performance, as detailed below.

[0030] In a second aspect, the present invention provides a method for preparing a negative electrode material as described in any of the foregoing embodiments, comprising: adding graphite to a template agent dispersion and dispersing it evenly; then adding a phenolic compound, an alkaline catalyst, and an aldehyde compound; and conducting an in-situ polymerization reaction on the graphite surface to obtain a composite precursor; calcining the composite precursor under an inert atmosphere to obtain a carbonaceous composite material; and performing chemical vapor deposition on the carbonaceous composite material under a carbon-containing atmosphere.

[0031] This invention achieves molecular-level composite of hard carbon precursor and graphite substrate by in-situ polymerization of phenolic and aldehyde compounds on graphite surface, constructing a carbon composite structure with strong interfacial bonding. This fundamentally solves the problems of weak interfacial bonding and phase separation between hard carbon and graphite in traditional physical mixing methods, and significantly improves the structural stability and cycle life of the anode material.

[0032] Furthermore, this invention innovatively adopts a process route of "first constructing a porous structure and then controlling the pore characteristics". The composite precursor is first calcined to form a carbonaceous composite material with a high specific surface area. This carbonaceous composite material has a porous skeleton structure, which lays the foundation for improving the material capacity. Then, the pore structure of the carbonaceous composite material is precisely controlled by chemical vapor deposition technology to form a large number of closed-pore structures inside the material, thereby solving the performance contradiction between high capacity and high first-efficiency of the anode material.

[0033] This invention utilizes a closed-pore structure formed by chemical vapor deposition, which not only provides abundant ion storage sites, giving the anode material excellent specific capacity characteristics, but also promotes rapid ion transport through the optimized pore network structure, endowing the anode material with outstanding rate performance. Furthermore, the carbon shell formed by chemical vapor deposition effectively reduces the specific surface area of ​​the anode material, thereby suppressing side reactions of the electrolyte during battery charging and discharging, resulting in a significantly improved first-time coulombic efficiency and effectively overcoming the technical bottleneck of low first-time efficiency of traditional hard carbon materials. The method provided by this invention has advantages such as a reasonable process route, wide availability of raw materials, and controllable production costs, and possesses promising prospects for industrial application.

[0034] In an optional embodiment, the method for preparing the negative electrode material provided by the present invention includes the following steps: S01, Preparation of composite precursor S011. Preparation of template agent dispersion: The template agent is dispersed in a solvent to obtain a template agent dispersion.

[0035] The template agent includes at least one of polyvinyl alcohol, polyethylene glycol, P123, and F127. P123 and F127 are both nonionic triblock copolymer soft template agents, chemically classified as Pluronic surfactants with a polyoxyethylene-polyoxypropylene-polyoxyethylene (PEO-PPO-PEO) structure. P123 has the molecular formula EO. 20 PO 70 EO 20 The molecular formula of F127 is EO. 106 PO 70 EO 106 .

[0036] Preferably, the template agent is polyvinyl alcohol. Polyvinyl alcohol, as a template agent, not only assists in the dispersion of raw materials, but also, due to its plastic polymer properties, can form pores during carbonization and volatilization, thereby creating a porous structure in the hard carbon intermediate layer.

[0037] Furthermore, the solvent includes at least one of water, ethanol and isopropanol. Preferably, in order to ensure that the template agent can be fully dissolved, the solvent can be preheated, for example, by using hot water with a temperature ≥70°C.

[0038] Preferably, the mass-volume ratio of the template agent to the solvent is (1g:400mL) ~ (1g:600mL).

[0039] S012. Add graphite to the template agent dispersion and disperse evenly.

[0040] Preferably, the temperature for mixing graphite and template agent dispersion is 70~90℃, and the time is 1~6h.

[0041] Preferably, the mass ratio of template agent to graphite is (1:15) to (1:25).

[0042] Preferably, the graphite can be either artificial graphite or natural graphite.

[0043] S013. Phenolic compounds, alkaline catalysts, and aldehyde compounds are added sequentially to a graphite template dispersion, and an in-situ polymerization reaction occurs on the graphite surface to obtain a composite precursor.

[0044] In an optional embodiment, the reaction temperature of the in-situ polymerization reaction is 70~95℃, and the reaction time is 1~6h.

[0045] In an optional embodiment, the mass ratio of graphite to phenolic compounds is (1:1.2) to (1:1.75). For example, it can be a range of values ​​formed between any two of the following ratios: (1:1.2), (1:1.3), (1:1.4), (1:1.5), (1:1.6), (1:1.7), and (1:1.75).

[0046] Preferably, the molar ratio of phenolic compounds to aldehyde compounds is (1:1.5) to (1:2.2).

[0047] Preferably, the phenolic compound includes at least one of phenol, resorcinol, phloroglucinol, hydroquinone, catechol, and naphthol.

[0048] Preferably, the aldehyde compound includes at least one of formaldehyde, acetaldehyde, benzaldehyde, glyoxylic acid, and glutaraldehyde.

[0049] Preferably, the mass of the alkaline catalyst is 3 to 10% of the mass of the phenolic compound.

[0050] Preferably, the alkaline catalyst includes at least one of sodium hydroxide, potassium hydroxide, ammonia, and triethylamine.

[0051] Preferably, depending on the product performance requirements, the process further includes adding a dopant precursor to the template agent dispersion for in-situ polymerization; the dopant element in the dopant precursor includes at least one of nitrogen, phosphorus, sulfur, and boron.

[0052] S014. After the in-situ polymerization reaction, the reaction also includes acid curing of the in-situ polymerized reactants. After the acid curing is completed, the reaction system is separated and dried, and the composite precursor is collected.

[0053] Preferably, acid curing includes adding an acid curing agent to the in-situ polymerization reactant and adjusting the pH of the reaction system to 2-5.

[0054] Preferably, the acid curing agent includes at least one of phosphoric acid, hydrochloric acid, sulfuric acid, oxalic acid, and p-toluenesulfonic acid.

[0055] Preferably, the acid curing treatment temperature is 60~90℃ and the treatment time is 1~4h.

[0056] Preferably, the drying temperature is 100~120℃ and the drying time is 8~15h.

[0057] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 is calcined to obtain a carbonaceous composite material.

[0058] In an optional embodiment, the calcination temperature is 700~1000℃, the heating rate is 2~10℃ / min, and the holding time is 0.5~5h.

[0059] S03, Preparation of negative electrode materials The carbonaceous composite material obtained in step S02 was subjected to chemical vapor deposition in a carbon-containing atmosphere.

[0060] In an optional embodiment, the carbon-containing atmosphere includes at least one of methane, ethane, ethylene, acetylene, propane, and propylene.

[0061] Preferably, the deposition temperature of chemical vapor deposition is 500~1200℃ and the deposition time is 0.2~4h.

[0062] Thirdly, the present invention provides a secondary battery, comprising a negative electrode material as described in any of the foregoing embodiments or a negative electrode material prepared by any of the foregoing embodiments.

[0063] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0064] Example 1 This embodiment provides a method for preparing a hard carbon-graphite composite anode material, including the following steps: S01, Preparation of composite precursor S011. Preparation of template agent dispersion: Add 1.0g of polyvinyl alcohol (PVA) to 500mL of deionized water at 90℃ and stir mechanically until completely dissolved to obtain template agent dispersion.

[0065] S012. Add 20g of graphite to the template agent dispersion and stir at 300rpm for 45min to ensure that the graphite is fully dispersed.

[0066] S013. Add 35g of phenol, 1.75g ​​of sodium hydroxide and 50mL of formaldehyde solution to the dispersion from step S012 in sequence, and react in an 85℃ water bath for 3h to carry out in-situ polymerization to obtain the composite precursor.

[0067] S014. After the in-situ polymerization reaction is completed, add 20 mL of p-toluenesulfonic acid to the reaction system to adjust the pH of the reaction system to 3.5, and then continue stirring at 80°C for 3 h to carry out the acid curing reaction.

[0068] S015. The reactants obtained in step S014 are filtered, washed with deionized water until neutral, and dried in a vacuum drying oven at 100°C for 12 hours to collect the composite precursor.

[0069] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 700°C at a heating rate of 5°C / min under argon atmosphere protection. The temperature was held for 2 hours and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0070] S03, Preparation of negative electrode materials The carbon composite material obtained in step S02 was placed in a chemical vapor deposition furnace and deposited at 900°C for 1 hour with ethylene as the carbon-containing atmosphere. After deposition, it was cooled to room temperature under an argon atmosphere to obtain a hard carbon-graphite composite anode material.

[0071] Example 2 This embodiment provides a method for preparing a hard carbon-graphite composite anode material, including the following steps: S01, Preparation of composite precursor S011. Preparation of template agent dispersion: Add 1.0g of polyvinyl alcohol (PVA) to 400mL of deionized water at 85℃ and stir mechanically until completely dissolved to obtain template agent dispersion.

[0072] S012. Add 25g of graphite oxide to the template agent dispersion and stir at 300rpm for 40min to ensure that the graphite oxide is fully dispersed.

[0073] S013. Add 30g resorcinol, 1.8g potassium hydroxide and 45mL formaldehyde solution to the dispersion from step S012 in sequence, and react at 90℃ for 2h to carry out in-situ polymerization to obtain the composite precursor.

[0074] S014. After the in-situ polymerization reaction is completed, add 15 mL of phosphoric acid to the reaction system to adjust the pH of the reaction system to 3.0, and then continue stirring at 75°C for 3 h to carry out the acid curing reaction.

[0075] S015. The reactants obtained in step S014 are filtered, washed with deionized water until neutral, and dried in a vacuum drying oven at 110°C for 10 hours to collect the composite precursor.

[0076] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 800°C at a heating rate of 8°C / min under an argon atmosphere. The temperature was held for 1.5 hours and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0077] S03, Preparation of negative electrode materials The carbonaceous composite material obtained in step S02 was placed in a chemical vapor deposition furnace and deposited at 750°C for 1.5 h in an acetylene-containing atmosphere. After deposition, it was cooled to room temperature in an argon atmosphere to obtain a hard carbon-graphite composite anode material.

[0078] Example 3 This embodiment provides a method for preparing a hard carbon-graphite composite anode material, including the following steps: S01, Preparation of composite precursor S011. Preparation of template agent dispersion: Add 1.0g of polyvinyl alcohol (PVA) to 450mL of deionized water at 88℃ and stir mechanically until completely dissolved to obtain template agent dispersion.

[0079] S012. Add 22g of graphite oxide to the template agent dispersion and stir at 300rpm for 50min to ensure that the graphite is fully dispersed.

[0080] S013. Add 32g phenol, 1.6g sodium hydroxide, 42mL formaldehyde solution and 2g urea (as nitrogen source) sequentially to the dispersion from step S012. React in situ for polymerization and doping in a water bath at 88℃ for 2.5h to obtain the composite precursor.

[0081] S014. After the in-situ polymerization reaction is completed, add 18 mL of phosphoric acid to the reaction system to adjust the pH of the reaction system to 4.0, and then continue stirring at 85°C for 2 h to carry out the acid curing reaction.

[0082] S015. The reactants obtained in step S014 are filtered, washed with deionized water until neutral, and dried in a vacuum drying oven at 105°C for 11 hours to collect the composite precursor.

[0083] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 850°C at a heating rate of 6°C / min under argon atmosphere protection. The temperature was held for 1 hour and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0084] S03, Preparation of negative electrode materials The carbonaceous composite material obtained in step S02 was placed in a chemical vapor deposition furnace and deposited at 900°C for 1 hour with ethylene as the carbon-containing atmosphere to obtain a nitrogen-doped hard carbon-graphite composite anode material.

[0085] Comparative Example 1 This comparative example provides a negative electrode material, the preparation method of which is as follows: S01, Preparation of composite precursor 20g of graphite and 20g of thermosetting phenolic resin were physically mixed evenly to obtain a composite precursor.

[0086] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 700°C at a heating rate of 5°C / min under argon atmosphere protection. The temperature was held for 2 hours and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0087] S03, Preparation of negative electrode materials The carbon composite material obtained in step S02 was placed in a chemical vapor deposition furnace and deposited at 800°C for 1 hour in a carbon-containing atmosphere of methane. After deposition, it was cooled to room temperature in an argon atmosphere to obtain a hard carbon-graphite composite anode material.

[0088] Comparative Example 2 This comparative example provides a negative electrode material, the preparation method of which includes the following steps: S01, Preparation of composite precursor S011. Preparation of template agent dispersion: Add 1.0g of polyvinyl alcohol (PVA) to 500mL of deionized water at 90℃ and stir mechanically until completely dissolved to obtain template agent dispersion.

[0089] S012. Add 20g of graphite to the template agent dispersion and stir at 300rpm for 45min to ensure that the graphite is fully dispersed.

[0090] S013. Add 35g of phenol, 1.75g ​​of sodium hydroxide and 50mL of formaldehyde solution to the dispersion from step S012 in sequence, and react in an 85℃ water bath for 3h to carry out in-situ polymerization to obtain the composite precursor.

[0091] S014. After the in-situ polymerization reaction is completed, add 20 mL of p-toluenesulfonic acid to the reaction system to adjust the pH of the reaction system to 3.5, and then continue stirring at 80°C for 3 h to carry out the acid curing reaction.

[0092] S015. The reactants obtained in step S014 are filtered, washed with deionized water until neutral, and dried in a vacuum drying oven at 100°C for 12 hours to collect the composite precursor.

[0093] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 700°C at a heating rate of 5°C / min under argon atmosphere protection. The temperature was held for 2 hours and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0094] Comparative Example 3 This comparative example provides a negative electrode material, the preparation method of which includes the following steps: S01, Preparation of composite precursor S011. Add 20g of graphite to 500mL of deionized water at 90℃ and stir at 300rpm for 45min to ensure that the graphite is fully dispersed.

[0095] S012. Add 35g of phenol, 1.75g ​​of sodium hydroxide and 50mL of formaldehyde solution to the dispersion from step S011 in sequence, and react in an 85℃ water bath for 3h to carry out in-situ polymerization to obtain the composite precursor.

[0096] S013. After the in-situ polymerization reaction is completed, add 20 mL of p-toluenesulfonic acid to the reaction system to adjust the pH of the reaction system to 3.5, and then continue stirring at 80°C for 3 h to carry out the acid curing reaction.

[0097] S014. The reactants obtained in step S013 are filtered, washed with deionized water until neutral, and dried in a vacuum drying oven at 100°C for 12 hours to collect the composite precursor.

[0098] SO2, Preparation of carbonaceous composite materials The composite precursor obtained in step S01 was placed in a tube furnace and heated to 800°C at a heating rate of 5°C / min under argon atmosphere protection. The temperature was held for 2 hours and then naturally cooled to room temperature to obtain the carbonaceous composite material.

[0099] S03, Preparation of negative electrode materials The carbon composite material obtained in step S02 was placed in a chemical vapor deposition furnace and deposited at 900°C for 1 hour in a carbon-containing atmosphere of methane. After deposition, it was cooled to room temperature in an argon atmosphere to obtain a hard carbon-graphite composite anode material.

[0100] Comparative Example 4 This comparative example provides a negative electrode material, the preparation method of which is similar to that of Example 1, except that: in the SO14 process, after the in-situ polymerization reaction is completed, no acid curing reaction is carried out.

[0101] Experimental Example 1 The negative electrode materials provided in Examples 1-3 and Comparative Examples 1-4 were subjected to performance testing, and the results are shown in Tables 1 and 2.

[0102] The testing methods for each parameter are as follows: Secondary batteries were prepared using the negative electrode materials provided in each embodiment and comparative example as the positive electrode, lithium metal sheets as the negative electrode, (1 M LiPF6, EC:EMC=1:1) as the electrolyte, and a polypropylene separator. The first-cycle charge capacity, first-cycle discharge capacity, coulombic efficiency, 4C charge capacity, and cycle performance of the secondary batteries were tested using a Blue Battery Tester.

[0103] Specifically, electrochemical performance tests were conducted using a blue battery tester: the charge / discharge voltage range was set to 0.005~1.0 V (vs. Li). + ( / Li) (can be adjusted appropriately according to material properties). The first-cycle charge and discharge capacities were tested using a 0.1C constant current charge-discharge cycle, and the first-cycle coulombic efficiency was calculated from this (Coulombic efficiency = First-cycle discharge capacity / First-cycle charge capacity × 100%). The 4C charge capacity test involved constant current charging at a 4C rate to the upper limit voltage. Cyclic performance testing used a 1C / 1C charge-discharge rate, performing 50 constant current charge-discharge cycles within the same voltage range, and recording the capacity retention rate to evaluate cycle stability. All tests were conducted at room temperature (25±1℃).

[0104] Table 1 Performance of Anode Materials

[0105] Table 2 Performance of batteries made with different negative electrode materials

[0106] As shown in Tables 1 and 2, the negative electrode material provided in the embodiments of the present invention has a large closed-pore volume and a large closed-pore ratio, thereby effectively suppressing the continuous side reactions and interfacial by-product accumulation of the electrolyte during the charging and discharging process. The battery made from this negative electrode material has a high specific capacity, high initial efficiency and excellent fast charging performance.

[0107] In Comparative Example 1, the carbonaceous composite material prepared by physical mixing had low closed-pore volume and closed-pore ratio, resulting in low specific capacity and initial efficiency of the battery. Comparative Example 2 did not undergo CVD deposition, and Comparative Example 3 did not add a template agent, both of which resulted in low closed-pore ratio of the negative electrode material and low initial efficiency of the battery. Comparative Example 4 did not undergo acid curing reaction, resulting in a larger average pore size and smaller closed-pore volume in the closed-pore structure of the negative electrode material, thus significantly reducing the battery's electrical performance.

[0108] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A hard carbon-graphite composite anode material, comprising multiple composite particles, characterized in that, The composite particles comprise a graphite core, a hard carbon intermediate layer, and a carbon shell arranged sequentially from the inside out. The hard carbon intermediate layer has pores, and the carbon shell covers the surface of the hard carbon intermediate layer to form a closed-pore structure. The closed-pore ratio of the negative electrode material is 10~25%, the average pore diameter of the closed-pore structure is 0.5~0.8nm, and the closed-pore volume is 0.1~0.2cm³ / g.

2. The negative electrode material according to claim 1, characterized in that, The interface between the graphite core and the hard carbon intermediate layer is seamless or the gap width does not exceed 100 nm.

3. The negative electrode material according to claim 1, characterized in that, The raw material of the graphite core has a particle size Dv50 of 5~20μm; the thickness of the hard carbon intermediate layer is 3~10μm; and the thickness of the carbon shell is 10~50nm.

4. The negative electrode material according to claim 1, characterized in that, The carbon content of the negative electrode material is ≥95% by mass. And / or, the particle size Dv50 of the negative electrode material is 8~30μm; And / or, the tap density of the negative electrode material is 0.8~1.2 g / cm³. 3 ; And / or, the specific surface area of ​​the negative electrode material is 3~10 m² 2 / g.

5. A method for preparing a negative electrode material as described in any one of claims 1 to 4, characterized in that, include: Graphite is added to a template agent dispersion and dispersed evenly. Then, phenolic compounds, alkaline catalysts and aldehyde compounds are added, and an in-situ polymerization reaction occurs on the graphite surface to obtain a composite precursor. The composite precursor was calcined in an inert atmosphere to obtain a carbonaceous composite material. The carbonaceous composite material was subjected to chemical vapor deposition in a carbon-containing atmosphere.

6. The preparation method according to claim 5, characterized in that, The in-situ polymerization reaction is carried out at a temperature of 70-95°C for 1-6 hours. Preferably, after the in-situ polymerization reaction, the in-situ polymerization reactants are further subjected to acid curing. Preferably, the acid curing includes adding an acid curing agent to the in-situ polymerization reactant and adjusting the pH of the reaction system to 2-5; Preferably, the acid curing agent includes at least one selected from phosphoric acid, hydrochloric acid, sulfuric acid, oxalic acid, and p-toluenesulfonic acid; Preferably, the acid curing treatment temperature is 60~90℃ and the treatment time is 1~4h; Preferably, the process further includes adding a dopant precursor to the template agent dispersion for in-situ polymerization; the dopant precursor includes at least one of nitrogen, phosphorus, sulfur and boron.

7. The preparation method according to claim 5, characterized in that, The template agent dispersion is obtained by dispersing a template agent in a solvent, wherein the template agent includes at least one selected from polyvinyl alcohol, polyethylene glycol, P123, and F127; and the solvent includes at least one selected from water, ethanol, and isopropanol. Preferably, the mass-to-volume ratio of the template agent to the solvent is (1g:400mL) ~ (1g:600mL); Preferably, the mass ratio of the template agent to the graphite is (1:15) to (1:25); Preferably, the temperature for mixing the graphite with the template agent dispersion is 70~90℃, and the time is 1~6h.

8. The preparation method according to claim 5, characterized in that, The mass ratio of graphite to phenolic compounds is (1:1.2) to (1:1.75); the molar ratio of phenolic compounds to aldehyde compounds is (1:1.5) to (1:2.2). Preferably, the phenolic compound includes at least one of phenol, resorcinol, phloroglucinol, hydroquinone, catechol, and naphthol; Preferably, the aldehyde compound includes at least one of formaldehyde, acetaldehyde, benzaldehyde, glyoxylic acid, and glutaraldehyde; Preferably, the mass of the alkaline catalyst is 3-10% of the mass of the phenolic compound; Preferably, the alkaline catalyst comprises at least one of sodium hydroxide, potassium hydroxide, ammonia, and triethylamine.

9. The preparation method according to claim 5, characterized in that, The calcination temperature is 700~1000℃, the heating rate is 2~10℃ / min, and the holding time is 0.5~5h.

10. The preparation method according to claim 5, characterized in that, The carbon-containing atmosphere includes at least one of methane, ethane, ethylene, acetylene, propane, and propylene; Preferably, the deposition temperature of chemical vapor deposition is 500~1200℃ and the deposition time is 0.2~4h.

11. A secondary battery, characterized in that, It includes the negative electrode material as described in any one of claims 1 to 4 or the negative electrode material prepared by the preparation method as described in any one of claims 5 to 10.

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