Gradient-coated graphite composite anode material, its preparation method, and lithium-ion battery

Abstract

This invention discloses a gradient-coated graphite composite anode material, its preparation method, and a lithium-ion battery, belonging to the field of lithium-ion battery technology. The gradient-coated graphite composite anode material provided by this invention includes: a graphite core; a soft carbon layer coated on the surface of the graphite core; the soft carbon layer comprising soft carbon and a conductive agent; and a lithium lanthanum phosphate layer coated on the surface of the soft carbon layer; the mass ratio of the graphite core, soft carbon layer, and lithium lanthanum phosphate layer is (70~98):(2~10):(1~20). The soft carbon layer and the lithium lanthanum phosphate layer of this invention form a synergistic effect, effectively improving the ion and electron transport efficiency of the anode material; simultaneously, the gradient structure achieves stress release, reduces surface defects in the coating layer, optimizes interface stability, thereby improving the rate capability and cycle life of the lithium-ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a gradient-coated graphite composite anode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] A lithium-ion battery is a rechargeable battery that achieves charging and discharging through the reversible insertion and extraction of lithium ions between the positive and negative electrodes. During operation, lithium ions are extracted from the positive electrode and inserted into the negative electrode during charging, and are extracted from the negative electrode and return to the positive electrode during discharging. The negative electrode material, as the carrier of lithium ions, needs to have high specific capacity to support energy storage, excellent structural stability to ensure cycle life, compatibility with the electrolyte to form a stable solid electrolyte interphase (SEI) film, and good electronic / ionic conductivity to improve rate capability, fast charging, and low-temperature performance.

[0004] Graphite, with its layered structure, offers advantages such as high initial coulombic efficiency, structural stability, and low cost, making it a mainstream anode material. However, its slow interlayer lithium-ion diffusion leads to insufficient fast-charging and low-temperature performance. To improve graphite's kinetic performance, existing technologies mainly optimize it through surface coating (such as carbon coating and metal / metal oxide coating), morphology control (reducing particle size, creating pores, etc.), doping modification, and adjusting interlayer spacing during graphitization. Among these, carbon coating is the mainstream commercial solution. While soft carbon coating offers high electronic conductivity and tight bonding with graphite, it suffers from poor mechanical strength and susceptibility to structural changes at high temperatures. Hard carbon coating, while exhibiting better mechanical stability and ion transport, faces challenges such as low electronic conductivity and a large specific surface area that easily reacts with the electrolyte, leading to reduced initial efficiency.

[0005] Based on the high ionic conductivity, structural stability, and superior mechanical properties of solid-state electrolytes (SSEs), coating graphite with SSEs has become a new direction for improving the kinetic performance and cycle life of materials. Currently, related technologies mostly use traditional oxide SSEs such as aluminum-doped lithium titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum titanium oxide (LLTO) (with Young's modulus typically between 100-200 GPa). For example, coating is achieved through processes such as freeze-drying graphite with a titanium / lithium source solution followed by heat treatment, or spray drying and carbonization of a composite SSE with graphite. However, these high Young's modulus SSEs have significant limitations: during the charge-discharge process, the high hardness of the coating layer makes it difficult to adapt to changes in volume and stress, leading to cracking, electrolyte infiltration, and side reactions, reducing battery life and safety. Simultaneously, high-modulus materials are difficult to adapt to solid-phase coating processes (easily introducing defects), while liquid-phase coating processes suffer from complex steps, high costs, and significant environmental pressure related to organic solvent recovery. Therefore, developing a graphite composite anode material that can meet the requirements of flexible graphite coating and has high ionic / electronic conductivity has become a key challenge to improve the electrochemical performance of lithium-ion batteries. Summary of the Invention

[0006] In view of this, the present invention provides a gradient-coated graphite composite anode material, its preparation method, and a lithium-ion battery. The present invention achieves a synergistic improvement in the electron / ion transport efficiency of the graphite anode through the gradient coating structure, thereby effectively improving the rate capability, cycle life, and safety of the battery.

[0007] In a first aspect, the present invention provides a gradient-coated graphite composite anode material, comprising:

[0008] Graphite core;

[0009] A soft carbon layer is coated on the surface of a graphite core; the soft carbon layer comprises soft carbon and a conductive agent;

[0010] A lithium lanthanum phosphate layer is coated on the surface of a soft carbon layer;

[0011] The mass ratio of the graphite core, soft carbon layer and lithium lanthanum phosphate layer is (70~98):(2~10):(1~20).

[0012] Preferably, the particle size D50 of the graphite core is 8~20μm, and the mass ratio of soft carbon to conductive agent is (3~8):1; the conductive agent is selected from one or more of carbon nanotubes, graphene, conductive carbon black or carbon nanofibers.

[0013] Secondly, the present invention provides a method for preparing the above-mentioned gradient-coated graphite composite anode material, comprising the following steps:

[0014] Carbon source, conductive agent and graphite particles are mixed and coated, and then soft carbonization treatment is performed to obtain graphite particles coated with soft carbon layer.

[0015] Graphite particles coated with a soft carbon layer are mixed with lithium lanthanum phosphate, ball-milled, sintered, and cooled to obtain a gradient-coated graphite composite anode material.

[0016] Preferably, the carbon source is selected from one or more of asphalt, petroleum coke, needle coke, coal tar, or heavy oil.

[0017] Preferably, the mixing and coating temperature is 150~200℃, and the mixing and coating time is 30~60min.

[0018] Preferably, the temperature of the soft carbonization treatment is 800~1200℃, and the time of the soft carbonization treatment is 1~8h.

[0019] Preferably, the sintering temperature is 300~500℃ and the sintering time is 1~4h.

[0020] Preferably, the mass ratio of the carbon source, conductive agent and graphite particles is (2~10): (0.5~1.5): (88.5~97.5); the mass ratio of the graphite particles coated with the soft carbon layer to lithium lanthanum phosphate is (85~99): (1~15).

[0021] Preferably, both the soft carbonization treatment and the sintering process are carried out under an inert atmosphere.

[0022] Thirdly, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode comprises the gradient-coated graphite composite negative electrode material described above or the gradient-coated graphite composite negative electrode material prepared by the above preparation method.

[0023] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0024] (1) This invention constructs a gradient-coated graphite composite anode material (composed of a graphite core, a soft carbon layer, and a lithium lanthanum phosphate layer). The soft carbon layer provides high electronic conductivity, and the lithium lanthanum phosphate layer serves as a solid electrolyte with high ionic conductivity, forming a synergistic effect that effectively improves the ion and electron transport efficiency of the anode material. At the same time, the gradient structure realizes stress release, reduces surface defects of the coating layer, and optimizes interface stability, thereby improving the rate capability and cycle life of the lithium-ion battery.

[0025] (2) In this invention, the lithium lanthanum phosphate layer, as the outermost solid electrolyte, has a low Young's modulus (about 20 GPa). It can buffer the volume change of graphite during charging and discharging through its own elastic deformation, suppress the growth of lithium dendrites, reduce the risk of battery short circuit, and significantly improve the safety of the battery. Attached Figure Description

[0026] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0027] Figure 1 This is a schematic diagram of the structure of the gradient-coated graphite composite anode material in a specific embodiment of the present invention;

[0028] Figure 2 These are scanning electron microscope images of the gradient-coated graphite composite material obtained in Example 1 of this invention. Detailed Implementation

[0029] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, 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 invention pertains.

[0030] This invention provides a gradient-coated graphite composite anode material, comprising:

[0031] Graphite core;

[0032] A soft carbon layer is coated on the surface of a graphite core; the soft carbon layer comprises soft carbon and a conductive agent;

[0033] A lithium lanthanum phosphate layer is coated on the surface of a soft carbon layer;

[0034] The mass ratio of the graphite core, soft carbon layer and lithium lanthanum phosphate layer is (70~98):(2~10):(1~20).

[0035] Graphite's layered structure serves as a core carrier for reversible lithium-ion insertion / extraction, exhibiting high theoretical capacity (approximately 372 mAh / g) and good cycle stability, making it the mainstream choice for lithium-ion battery anodes. However, pure graphite suffers from two main drawbacks: first, lithium-ion insertion / extraction during charge / discharge leads to volume expansion, potentially causing structural fracture or interface detachment; second, the active sites on the graphite surface readily react with the electrolyte to form an unstable SEI film, increasing internal resistance and reducing initial coulombic efficiency. Therefore, an outer coating layer is needed for protection and performance optimization.

[0036] A schematic diagram of the structure of the gradient-coated graphite composite anode material of the present invention is shown below. Figure 1As shown, the soft carbon layer coats the surface of the graphite core, and its core function is to enhance electron transport and buffer stress. After high-temperature treatment, the soft carbon forms an ordered carbon structure with high electronic conductivity, which can establish an efficient electron conduction channel between the graphite core and the outer layer, reduce electron migration resistance, and improve the rate performance of the material. At the same time, as an intermediate transition layer, the soft carbon layer has a relatively flexible structure, which can partially absorb the stress generated by the volume expansion of graphite, avoid the cracking of the coating layer due to stress concentration, and maintain the stability of the overall structure. In addition, the soft carbon layer can also cover the microscopic defects on the graphite surface (such as edges and vacancies), reduce the direct contact area between graphite and electrolyte, inhibit the excessive growth of SEI film, and reduce irreversible capacity loss.

[0037] The lithium lanthanum phosphate layer, as the outermost coating layer, primarily functions as an ion transport and interface protection layer. Lithium lanthanum phosphate (Li3La(PO4)2, LLP) is a solid electrolyte material with high lithium-ion conductivity (typically above 10). -5 ~10 -3 Within the S / cm range, it can provide a fast channel for lithium ion migration on the electrode material surface, reduce ion diffusion resistance, and further improve the charge and discharge rate of the battery. At the same time, the LLP has a low Young's modulus (about 20 GPa) and has a certain elastic deformation capability. When graphite generates stress due to volume expansion, the LLP layer can absorb part of the stress through its own deformation, preventing the stress from being transferred back to the soft carbon layer or graphite core, thereby maintaining the integrity of the coating structure. In addition, as a solid electrolyte, the LLP layer can isolate the direct contact between graphite and electrolyte, reduce the decomposition reaction of electrolyte on the graphite surface (such as solvent reduction to generate gas or by-products), improve interface stability, and extend the cycle life of the battery.

[0038] The mass ratio of the graphite core, soft carbon layer, and lithium lanthanum phosphate layer is an optimized design based on the synergistic effect of the functional layers. The graphite core has the highest proportion, ensuring the main capacity and basic performance of the material; the soft carbon layer needs to be sufficient to cover the defects on the graphite surface and provide electron conduction, but should not be too thick to avoid increasing the internal resistance of the material or reducing the energy density; the lithium lanthanum phosphate layer needs to ensure sufficient ion conduction paths and stress buffering capacity, while avoiding excessive thickness that would lead to excessively long ion transport distances or increased material costs. By adjusting this mass ratio, the functions of each layer are balanced, ultimately achieving a comprehensive improvement in ion / electron transport efficiency, structural stability, and interface stability. More preferably, the mass ratio of the graphite core to the soft carbon layer is (85~98): (2~15); the mass ratio of the total mass of the graphite core and soft carbon layer to the mass of the lithium lanthanum phosphate layer is (80~99): (1~20), more preferably (85~99): (1~15).

[0039] In this invention, the particle size D50 of the graphite core is 8~20μm. The particle size D50 (median particle size) of the graphite core directly affects the specific surface area, compaction density, and lithium-ion diffusion efficiency of the material. If the particle size is too small, the specific surface area will be large, which may lead to an increase in side reactions with the electrolyte. At the same time, the reduced compaction density will affect the volumetric energy density of the battery, resulting in poor processing performance, easy agglomeration or sedimentation of the slurry, and low peel strength. If the particle size is too large, the diffusion path of lithium ions inside the graphite becomes longer, which may reduce the rate performance.

[0040] In this invention, the mass ratio of soft carbon to conductive agent is (3~8):1. Soft carbon, as the main material, needs to form a continuous coating layer to cover graphite surface defects and provide basic electronic conduction. The conductive agent, through its high conductivity, constructs a three-dimensional conductive network within the soft carbon layer, further improving electron transport efficiency. If the proportion of conductive agent is too high, it may disrupt the continuity of the soft carbon layer, leading to a decrease in stress buffering capacity; if the proportion is too low, a conductive network cannot be effectively formed, resulting in limited improvement in electron conduction efficiency. The conductive agent is selected from one or more of carbon nanotubes, graphene, conductive carbon black, or carbon nanofibers.

[0041] The present invention also provides a method for preparing the above-mentioned gradient-coated graphite composite anode material, comprising the following steps:

[0042] Carbon source, conductive agent and graphite particles are mixed and coated, and then soft carbonization treatment is performed to obtain graphite particles coated with soft carbon layer.

[0043] Graphite particles coated with a soft carbon layer are mixed with lithium lanthanum phosphate, ball-milled, sintered, and cooled to obtain a gradient-coated graphite composite anode material.

[0044] The graphite particles used in this invention are graphite particles that have undergone crushing and oxidation treatment, with a particle size D50 of 8~20μm. This invention does not impose special limitations on the specific crushing and oxidation treatment steps. This invention preferably uses air jet milling, and uses sulfuric acid or hydrochloric acid and hydrogen peroxide to oxidize the crushed graphite particles to remove surface impurities.

[0045] In this invention, the carbon source is selected from one or more of asphalt, petroleum coke, needle coke, coal tar, or heavy oil. The soft carbon formed after carbonization of these carbon sources has high electronic conductivity and a certain degree of flexibility, effectively undertaking the electron conduction and stress buffering functions of the intermediate layer. Furthermore, the aforementioned carbon sources have low softening points (approximately 70~180℃), which is beneficial for the subsequent mixing and coating process.

[0046] In this invention, the mixing and coating temperature is 150~200℃, and the mixing and coating time is 30~60 minutes. During the mixing and coating process, the carbon source softens and becomes a viscous liquid, which can uniformly wet the surface of the graphite particles and achieve tight adhesion. This invention does not impose special limitations on the equipment used in the mixing and coating process; however, a high-speed mixer is preferably used, and the mixing speed is preferably 500~1000 rpm.

[0047] In this invention, the temperature for the soft carbonization treatment is 800~1200℃. This invention does not impose special restrictions on the heating process to the soft carbonization treatment temperature. Preferably, a gradient heating method is used, first heating to 300~400℃ at a heating rate of 4~7℃ / min, and then heating to 800~1200℃ at a heating rate of 2~4℃ / min. The soft carbonization treatment time is 1~8h, more preferably 2~6h, for example, 2h, 3h, 4h, 5h, 6h, etc., to ensure sufficient carbonization of the carbon source while controlling the degree of graphitization of the soft carbon, balancing its electron conduction and stress buffering functions.

[0048] The present invention does not impose any special restrictions on the ball milling steps, and the ball milling steps commonly used in the art can be used. Preferably, the ball-to-material ratio is (10~20):1, the ball milling time is 4~10h, and the ball milling speed is 200~500rpm.

[0049] In this invention, the sintering temperature is 300-500℃. The purpose of sintering is to promote the interfacial bonding between the lithium lanthanum phosphate layer and the soft carbon layer, while avoiding damage to the material structure caused by high temperatures. Low-temperature sintering at 300-500℃ activates the active sites (such as hydroxyl and carboxyl groups) on the surface of both lithium lanthanum phosphate and soft carbon, enhancing the bonding strength through chemical bonds or van der Waals forces. Excessive temperature may lead to oxidation of the soft carbon layer or decomposition of lithium lanthanum phosphate; insufficient interfacial bonding may result in the detachment of the lithium lanthanum phosphate layer. The sintering time is 1-4 hours, sufficient for the interfacial reaction to complete, but excessively long sintering may lead to material performance degradation due to heat accumulation.

[0050] In this invention, the mass ratio of the carbon source, conductive agent and graphite particles is (2~10): (0.5~1.5): (88.5~97.5); the mass ratio of the graphite particles coated with the soft carbon layer to lithium lanthanum phosphate is (85~99): (1~15).

[0051] In this invention, both the soft carbonization treatment and sintering process are carried out under an inert atmosphere, such as argon or nitrogen, primarily to prevent the carbon source or soft carbon from oxidizing at high temperatures. Carbon materials readily react with oxygen at high temperatures to generate CO or CO2, leading to carbon source loss and damage to the soft carbon layer structure; although lithium lanthanum phosphate is a solid electrolyte, it may undergo oxidation reactions when in contact with oxygen at high temperatures (e.g., La). 3+The valence state change affects ionic conductivity. An inert atmosphere can isolate oxygen, ensuring that only the target reactions (carbon source decomposition, interfacial bonding) occur during carbonization and sintering, avoiding the negative impact of side reactions on material properties.

[0052] The present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode comprises the gradient-coated graphite composite negative electrode material described above or the gradient-coated graphite composite negative electrode material prepared by the above preparation method.

[0053] The gradient-coated graphite composite anode material provided by this invention effectively solves the problems of volume expansion, SEI film instability and insufficient rate performance of traditional graphite anodes through the synergistic effect of multiple functional layers. When applied to lithium-ion batteries, it can significantly improve the rate capability, cycle life and safety of the battery.

[0054] This invention does not impose any special limitations on the preparation method of the above-mentioned lithium-ion battery; any commonly used lithium-ion battery preparation method in the art can be used.

[0055] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0056] Example 1

[0057] This embodiment provides a gradient-coated graphite composite anode material and its preparation method.

[0058] (1) The raw coke was subjected to air jet pulverization, granulation and high temperature graphitization to obtain graphitized material with D50=12μm. It was ultrasonically cleaned for 40min with a mixture of 20wt% sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. It was washed with water and then vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0059] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 5:1:94 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 180℃, the mixing time is 50min, and the rotation speed is 600rpm to obtain mixture A.

[0060] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Hold for 3 hours to obtain soft carbon-coated graphite particles.

[0061] (4) Take the soft carbon-coated graphite particles from step (3) and mix them with the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 90:10. Place them in a planetary ball mill for ball milling. The ball-to-material ratio is 15:1, the ball milling speed is 300 rpm, and the time is 6 hours to obtain mixture B.

[0062] (5) Take the mixture B obtained in step (4) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2h. After cooling, a gradient-coated graphite composite material is obtained, which has a graphite core and a soft carbon layer and a solid electrolyte LLP layer on the surface in sequence.

[0063] The scanning electron microscope (SEM) image of the gradient-coated graphite composite material obtained in this embodiment is shown below. Figure 2 As shown, the graphite particles are flake-shaped with a particle size between 5 and 15 μm. The particle size distribution is relatively uniform, with no obvious agglomeration and uniform dispersion. The surface is relatively flat, without cracks or defects.

[0064] Example 2

[0065] This embodiment provides a gradient-coated graphite composite anode material and its preparation method.

[0066] (1) The raw coke was subjected to air jet pulverization, granulation and high temperature graphitization to obtain graphitized material with D50=12μm. It was ultrasonically cleaned for 40min with a mixture of 20wt% sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. It was washed with water and then vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0067] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 2:0.5:97.5 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 180℃, the mixing time is 50min, and the rotation speed is 600rpm to obtain mixture A.

[0068] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Hold for 3 hours to obtain soft carbon-coated graphite particles.

[0069] (4) Take the soft carbon-coated graphite particles from step (3) and the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 99:1, put them into a planetary ball mill for ball milling, the ball-to-material ratio is 15:1, the ball milling speed is 300 rpm, and the time is 6 hours to obtain mixture B.

[0070] (5) Take the mixture B obtained in step (4) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2h. After cooling, a gradient-coated graphite composite material is obtained, which has a graphite core and a soft carbon layer and a solid electrolyte LLP layer on the surface in sequence.

[0071] Example 3

[0072] This embodiment provides a gradient-coated graphite composite anode material and its preparation method.

[0073] (1) The raw coke was subjected to air jet pulverization, granulation and high temperature graphitization to obtain graphitized material with D50=12μm. It was ultrasonically cleaned for 40min with a mixture of 20wt% sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. It was washed with water and then vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0074] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 10:1.5:88.5 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 180℃, the mixing time is 50min, and the rotation speed is 600rpm to obtain mixture A.

[0075] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Hold for 3 hours to obtain soft carbon-coated graphite particles.

[0076] (4) Take the soft carbon-coated graphite particles from step (3) and the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 85:15, put them into a planetary ball mill for ball milling, the ball-to-material ratio is 15:1, the ball milling speed is 300 rpm, and the time is 6 hours to obtain mixture B.

[0077] (5) Take the mixture B obtained in step (4) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2h. After cooling, a gradient-coated graphite composite material is obtained, which has a graphite core and a soft carbon layer and a solid electrolyte LLP layer on the surface in sequence.

[0078] Example 4

[0079] This embodiment provides a gradient-coated graphite composite anode material and its preparation method.

[0080] (1) The raw coke was subjected to air jet pulverization, granulation and high temperature graphitization to obtain graphitized material with D50=12μm. It was ultrasonically cleaned for 40min with a mixture of 20wt% sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. It was washed with water and then vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0081] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 5:1:94 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 160℃, the mixing time is 30min, and the rotation speed is 500rpm to obtain mixture A.

[0082] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Hold for 3 hours to obtain soft carbon-coated graphite particles.

[0083] (4) Take the soft carbon-coated graphite particles from step (3) and the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 90:10, put them into a planetary ball mill for ball milling, the ball-to-material ratio is 15:1, the ball milling speed is 200 rpm, and the time is 4 hours to obtain mixture B.

[0084] (5) Take the mixture B obtained in step (4) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2h. After cooling, a gradient-coated graphite composite material is obtained, which has a graphite core and a soft carbon layer and a solid electrolyte LLP layer on the surface in sequence.

[0085] Example 5

[0086] This embodiment provides a gradient-coated graphite composite anode material and its preparation method.

[0087] (1) The raw coke was subjected to air jet pulverization, granulation and high temperature graphitization to obtain graphitized material with D50=12μm. It was ultrasonically cleaned for 40min with a mixture of 20wt% sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. It was washed with water and then vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0088] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 5:1:94 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 200℃, the mixing time is 60min, and the rotation speed is 1000rpm to obtain mixture A.

[0089] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Hold for 3 hours to obtain soft carbon-coated graphite particles.

[0090] (4) Take the soft carbon-coated graphite particles from step (3) and the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 90:10, put them into a planetary ball mill for ball milling, the ball-to-material ratio is 15:1, the ball milling speed is 400 rpm, and the time is 8 hours to obtain mixture B.

[0091] (5) Take the mixture B obtained in step (4) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2h. After cooling, a gradient-coated graphite composite material is obtained, which has a graphite core and a soft carbon layer and a solid electrolyte LLP layer on the surface in sequence.

[0092] Comparative Example 1

[0093] The difference between this comparative example and Example 1 is that steps (4) and (5) are not performed in this comparative example, that is, LLP coating is not performed. The specific steps are as follows:

[0094] (1) The graphite was air-jet pulverized to D50=12μm, and ultrasonically cleaned for 40min with a mixture of 98wt% concentrated sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. After washing with water, it was vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0095] (2) The asphalt, conductive carbon nanotubes and the treated graphite particles obtained in step (1) are mixed in a mass ratio of 5:1:94 and placed in a high-speed mixer for high-speed mixing and coating. The mixing temperature is 180℃, the mixing time is 50min, and the rotation speed is 600rpm to obtain mixture A.

[0096] (3) Take the mixture A from step (2) and put it into a rotary carbonization furnace for soft carbonization treatment. Use nitrogen as a protective gas, raise the temperature to 350°C at 5°C / min, and then raise the temperature to 1000°C at 3°C / min. Keep it at the temperature for 3 hours to obtain soft carbon-coated graphite particles with graphite as the core and soft carbon layer on the surface.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 1 is that steps (2) and (3) are not performed in this comparative example, that is, soft carbon coating is not performed. The specific steps are as follows:

[0099] (1) The graphite was air-jet pulverized to D50=12μm, and ultrasonically cleaned for 40min with a mixture of 98wt% concentrated sulfuric acid and 30wt% hydrogen peroxide in a volume ratio of 3:1 to remove surface impurities. After washing with water, it was vacuum dried at 100℃ for 6h to obtain the treated graphite particles.

[0100] (2) Take the graphite particles processed in step (1) and mix them with the solid electrolyte lithium lanthanum phosphate (LLP) at a mass ratio of 10:90. Then, put them into a planetary ball mill for ball milling. The ball-to-material ratio is 15:1, the ball milling speed is 300 rpm, and the time is 6 hours to obtain the mixture.

[0101] (3) Take the mixture obtained in step (2) and put it into a tube furnace for low-temperature sintering under nitrogen atmosphere. The temperature is increased to 400℃ at 4℃ / min and held for 2 hours. After cooling, the graphite composite material with LLP coating is obtained. It has graphite as the core and solid electrolyte LLP layer on the surface.

[0102] Comparative Example 3

[0103] Compared with Example 1, this comparative example differs in that the solid electrolyte lithium aluminum titanium phosphate (LATP) is replaced with the solid electrolyte lithium lanthanum phosphate (LLP), resulting in a gradient-coated graphite composite material with a graphite core and a soft carbon layer and a solid electrolyte LATP layer sequentially coated on the surface.

[0104] Comparative Example 4

[0105] Compared with Example 1, the difference in this comparative example is that in step (2) of this comparative example, phenolic resin is used instead of asphalt to finally obtain a gradient-coated graphite composite material with graphite as the core and a hard carbon layer and a solid electrolyte LLP layer successively coated on the surface.

[0106] Test case

[0107] 1. Ionic conductivity and electronic conductivity:

[0108] The ionic conductivity and electronic conductivity of the graphite composite anode materials of Examples 1-5 and Comparative Examples 1-4 were measured, and the test results are shown in Table 1.

[0109] Table 1. Ionic and electronic conductivity of graphite composite anode materials in the examples and comparative examples.

[0110]

[0111] As can be seen from Table 1, the graphite composite anode material of the embodiment has higher ionic conductivity and electronic conductivity compared to the material of the comparative example.

[0112] 2. Performance testing of soft-pack batteries

[0113] Preparation of soft-pack batteries: A negative electrode slurry was prepared by mixing graphite composite negative electrode materials (Examples 1-5 and Comparative Examples 1-4), conductive agent SP, binder SBR, and binder CMC in deionized water at a mass ratio of 96:1:1.5:1.5. This slurry was coated onto copper foil, dried, and then rolled to obtain a negative electrode sheet. A positive electrode slurry was prepared by mixing lithium iron phosphate, conductive agent SP, and binder PVDF in N-methylpyrrolidone at a mass ratio of 97:1:2. This slurry was coated onto aluminum foil, dried, and then rolled to obtain a positive electrode sheet. The sheets were stacked in the order of "negative electrode-separator-positive electrode," with the separator being a PP / PE composite membrane. After welding the tabs, an aluminum-plastic film was inserted, and the top and sides were pre-sealed. After vacuum drying the cell (80℃, 24h), an electrolyte (1M LiPF6 dissolved in a mixed solution of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2) was injected, and the injection port was sealed after vacuuming. A 3Ah soft-pack battery was produced.

[0114] The prepared soft-pack battery was tested for its charge-up performance, discharge performance, and low-temperature performance. The results are shown in Table 2.

[0115] Table 2 Performance data of pouch batteries

[0116]

[0117] As can be seen from the data in Table 2, the soft-pack battery assembled from the graphite composite anode material in this embodiment of the invention has better charge performance, discharge performance and low-temperature performance compared to the comparative example.

[0118] 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 gradient-coated graphite composite anode material, characterized in that, include: Graphite core; A soft carbon layer, which coats the surface of the graphite core; The soft carbon layer comprises soft carbon and a conductive agent; A lithium lanthanum phosphate layer is coated on the surface of a soft carbon layer; The mass ratio of the graphite core, soft carbon layer and lithium lanthanum phosphate layer is (70~98):(2~10):(1~20); The preparation method of the gradient-coated graphite composite anode material includes the following steps: Carbon source, conductive agent and graphite particles are mixed and coated, and then soft carbonization treatment is performed to obtain graphite particles coated with soft carbon layer. Graphite particles coated with a soft carbon layer are mixed with lithium lanthanum phosphate, ball-milled, sintered, and cooled to obtain a gradient-coated graphite composite anode material.

2. The gradient-coated graphite composite anode material as described in claim 1, characterized in that, The graphite core has a particle size D50 of 8~20μm, and the mass ratio of soft carbon to conductive agent is (3~8):1; the conductive agent is selected from one or more of carbon nanotubes, graphene, conductive carbon black or carbon nanofibers.

3. The method for preparing the gradient-coated graphite composite anode material as described in claim 1 or 2, characterized in that, Includes the following steps: Carbon source, conductive agent and graphite particles are mixed and coated, and then soft carbonization treatment is performed to obtain graphite particles coated with soft carbon layer. Graphite particles coated with a soft carbon layer are mixed with lithium lanthanum phosphate, ball-milled, sintered, and cooled to obtain a gradient-coated graphite composite anode material.

4. The preparation method according to claim 3, characterized in that, The carbon source is selected from one or more of asphalt, petroleum coke, needle coke, coal tar, or heavy oil.

5. The preparation method according to claim 3, characterized in that, The temperature for the mixed coating is 150~200℃, and the time for the mixed coating is 30~60min.

6. The preparation method according to claim 3, characterized in that, The temperature for the soft carbonization treatment is 800~1200℃, and the time for the soft carbonization treatment is 1~8h.

7. The preparation method according to claim 3, characterized in that, The sintering temperature is 300~500℃, and the sintering time is 1~4h.

8. The preparation method according to claim 3, characterized in that, The mass ratio of the carbon source, conductive agent and graphite particles is (2~10): (0.5~1.5): (88.5~97.5); the mass ratio of the graphite particles coated with the soft carbon layer to lithium lanthanum phosphate is (85~99): (1~15).

9. The preparation method according to claim 3, characterized in that, Both the soft carbonization treatment and the sintering process are carried out under an inert atmosphere.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The negative electrode includes the gradient-coated graphite composite negative electrode material according to claim 1 or 2, or the gradient-coated graphite composite negative electrode material prepared by the preparation method according to any one of claims 3 to 9.

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