A gradient composite-coated graphite negative electrode material, a preparation method therefor, and an application thereof

Abstract

The present application relates to the technical field of battery materials, in particular to a gradient composite coated graphite negative electrode material and a preparation method and application thereof. The material comprises a graphite matrix and a first compact lithium salt inner layer, a composite lithium salt transition layer and a second compact lithium salt outer layer formed in situ on the surface of the graphite matrix from inside to outside. The present application also discloses a preparation method of the material. In the same closed reaction container, a lithium source and different precipitants are sequentially added, and three layers of gradient coatings are obtained by stepwise in-situ deposition. Then, solid-liquid separation, washing, drying and annealing treatment are performed. Through the synergistic effect of the three layers of coatings, the first coulomb efficiency, fast charging performance and high temperature cycle stability of the graphite negative electrode are significantly improved. The preparation process is simple, the cost is low, and the material is easy to be scaled up in industry, and is suitable for high energy density and fast charging lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a gradient composite coated graphite anode material, its preparation method, and its application. Background Technology

[0002] As the requirements for fast charging performance, high-temperature cycle life, and energy density of power batteries continue to increase in new energy vehicles, power-type artificial graphite has become the mainstream choice for lithium-ion battery anodes. However, existing graphite anodes still have three common pain points in the industry: 1. Insufficient fast charging performance: The lithium-ion desolvation energy barrier at the interface between the graphite anode and the electrolyte is high. During high-rate charging, lithium ions cannot be quickly embedded between graphite layers, and lithium is easily deposited on the surface of the anode, which not only leads to capacity decay but also poses serious safety hazards; 2. Poor high-temperature performance: At high temperatures of 60°C and above, the natural SEI film of power-type graphite is prone to thermal rupture, causing the electrolyte to continuously react with the fresh graphite surface, resulting in severe gas production, rapid capacity decay, and a significant reduction in cycle life; 3. Low initial coulombic efficiency: During the first charge and discharge process, the electrolyte decomposes on the graphite surface to form an SEI film, which consumes a large amount of active lithium from the positive electrode. The initial coulombic efficiency of conventional power graphite is generally only 90%-92%, directly resulting in a loss of battery energy density.

[0003] To address the aforementioned issues, the industry generally employs surface coating modification technology. However, existing technologies suffer from the following limitations: Single coating exhibits significant performance bottlenecks: While single LiF coating offers high lithium-ion mobility, its SEI stability is insufficient at high temperatures; single Li2CO3 coating stabilizes the SEI, but suffers from low lithium-ion mobility and high interfacial impedance; single Li3PO4 coating improves desolvation performance but fails to form a dense underlying interface, resulting in limited initial efficiency improvement; Bilayer coating has inherent defects: Existing bilayer coating schemes such as LiF+Li3PO4 and LiF+Li2CO3 suffer from lattice mismatch between the two layers, resulting in poor interlayer bonding and a tendency for coating layer detachment and failure during cycling, hindering long-term interfacial stability; Multilayer coating processes are complex and unsuitable for industrialization: The few existing three-layer coating schemes require multiple solid-liquid separations, material transfers, and reaction vessel replacements, leading to cumbersome processes, high production costs, and a high risk of coating layer damage and batch instability, failing to meet the demands of large-scale production.

[0004] The existing technology does not disclose the gradient structure design of the inner pure LiF-middle LiF+Li2CO3 composite transition layer-outer pure Li3PO4, nor does it have a preparation method to complete the gradient coating step by step in a single container. It cannot achieve the synergistic optimization of the whole process of desolvation-stable transport-dense lithium intercalation, and it is difficult to meet the multiple core requirements of power batteries for fast charging, high temperature cycling and high energy density at the same time. Summary of the Invention

[0005] Against this backdrop, the present invention aims to provide a gradient composite coated graphite anode material, its preparation method, and its application. Through the synergistic design of a three-layer gradient artificial SEI coating, it simultaneously addresses the industry pain points of poor fast-charging performance, unstable high-temperature cycling, and low initial efficiency of graphite anodes. At the same time, it provides a preparation method that can be completed stepwise in a single container, which is simple in process, has good batch stability, and is easy to scale up industrially.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a gradient composite coated graphite anode material, comprising a graphite substrate and a three-layer gradient composite coating deposited in situ from the inside to the outside on the surface of the graphite substrate; the three-layer gradient composite coating comprises a first dense lithium salt inner layer in direct contact with the graphite substrate, a composite lithium salt transition layer located outside the first dense lithium salt inner layer, and a second dense lithium salt outer layer in direct contact with the electrolyte.

[0007] In the above scheme, the first dense lithium salt inner layer is a continuous and dense inorganic coating layer, whose core function is to possess extremely high lithium-ion mobility (10). -6 (S / cm) to construct a low-impedance lithium-ion transport channel; at the same time, completely isolate the direct contact between the graphite matrix and the electrolyte, suppress irreversible side reactions during the first charge and discharge, improve the first coulombic efficiency, and form a dense SEI framework at the bottom layer.

[0008] The composite lithium salt transition layer serves as a buffer layer between the inner and outer layers. Its core function is to solve the lattice mismatch problem between pure LiF and pure Li3PO4, significantly improve the bonding force between the coating layers, and prevent detachment during cycling. At the same time, it synergistically forms a stable SEI film framework, inhibits solvent molecule co-intercalation and electrolyte decomposition, and significantly improves interface stability.

[0009] The second dense lithium salt outer layer is a uniform and continuous inorganic coating layer. Its core functions are: to directly contact the electrolyte, significantly reduce the desolvation energy barrier of lithium ions, accelerate the lithium ion interface transport speed, significantly improve fast charging performance, and suppress high-rate lithium plating; at the same time, it forms a high-temperature physical barrier, isolates the electrolyte from the internal active sites, inhibits electrolyte decomposition and transition metal dissolution at high temperatures, and significantly improves high-temperature cycling stability.

[0010] As a preferred embodiment of the present invention, the thickness of the first dense lithium salt inner layer is 2-5 nm; and / or, the thickness of the composite lithium salt transition layer is 2-5 nm; and / or, the thickness of the second dense lithium salt outer layer is 2-5 nm.

[0011] As a preferred embodiment of the present invention, the total mass of the three-layer gradient composite coating accounts for 1 to 5% of the mass of the graphite matrix.

[0012] As a preferred embodiment of the present invention, the first dense lithium salt inner layer is a LiF layer with a purity ≥99%; and / or, the composite lithium salt transition layer is a composite layer of LiF and Li2CO3 with a purity ≥99%; and / or, the second dense lithium salt outer layer is a Li3PO4 layer with a purity ≥99%.

[0013] As a preferred embodiment of the present invention, the mass ratio of LiF to Li2CO3 is 3:7 to 7:3.

[0014] In a preferred embodiment of the present invention, the graphite matrix is ​​dynamic synthetic graphite or natural graphite, with a D50 particle size of 5–20 μm and a compaction density of 1.3–1.7 g / cm³. 3 It meets the requirements for the use of power batteries.

[0015] Secondly, the present invention provides a method for preparing the gradient composite coated graphite anode material. The preparation method of the present invention is completed entirely within a single closed reaction vessel (single vessel), without the need for solid-liquid separation or material transfer. It achieves in-situ deposition of a three-layer gradient coating by sequentially adding reaction reagents, and includes the following steps: S1. Add a lithium source to the graphite dispersion, stir to dissolve, and then add the first precipitant dropwise to react in situ on the graphite surface to form the first dense lithium salt inner layer; S2. While keeping the reaction system closed and stirred, add lithium source, stir to dissolve, and then add composite precipitant dropwise to react and generate composite lithium salt transition layer; S3. Add lithium source again, stir to dissolve, and then add second precipitant dropwise to react and form second dense lithium salt outer layer; S4. Finally, after solid-liquid separation, washing, drying and annealing, gradient composite coated graphite anode material is obtained.

[0016] As a preferred technical solution of the present invention, in step S1, the preparation of the graphite dispersion includes: adding graphite matrix and anhydrous ethanol-deionized water mixed solvent (volume ratio 1:1 to 3:1) to a reaction vessel, ultrasonically dispersing at 25 to 30°C for 20 to 40 min, and then mechanically stirring for 30 to 60 min to obtain a uniform and stable graphite dispersion; wherein the solid-liquid ratio of graphite to mixed solvent is 1 g:(10 to 20) mL.

[0017] As a preferred technical solution of the present invention, in steps S1 to S3, the lithium source is lithium hydroxide monohydrate.

[0018] As a preferred embodiment of the present invention, in step S1, the first precipitant is a fluorine source, and the molar ratio of the lithium source to the fluorine source is 1:(1-1.5); and / or, in step S2, the composite precipitant is a mixed solution of a fluorine source and a carbon source, and the molar ratio of the supplemented lithium source, fluorine source and carbon source is 2:(0.7-1.6):(0.2-0.7); and / or, in step S3, the second precipitant is a phosphorus source, and the molar ratio of the supplemented lithium source to the phosphorus source is 3:(1-1.5).

[0019] As a preferred embodiment of the present invention, in step S1, the first precipitant is ammonium fluoride; and / or, in step S2, the composite precipitant is ammonium fluoride and ammonium carbonate; and / or, in step S3, the second precipitant is ammonium dihydrogen phosphate.

[0020] As a preferred embodiment of the present invention, in step S1, the dropping rate of the first precipitant is 2-5 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 2-4 h; and / or, in step S2, the dropping rate of the composite precipitant is 1-3 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 3-5 h; and / or, in step S3, the dropping rate of the second precipitant is 1-3 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 2-4 h; and / or, in step S4, the annealing treatment is carried out under an inert atmosphere, the annealing temperature is 300-400℃, and the time is 2-3 h. Annealing can enhance the crystallinity and interlayer bonding of each coating layer and remove residual organic impurities.

[0021] Thirdly, the present invention provides a lithium-ion battery comprising the gradient composite coated graphite anode material described above.

[0022] Compared with the prior art, the beneficial effects of the present invention include: 1. This invention employs a gradient design of inner LiF, intermediate composite layer, and outer Li3PO4 to precisely match the entire lithium-ion transport process from the electrolyte to the graphite interior: the outer Li3PO4 layer facilitates rapid desolvation of lithium ions, avoiding solvent co-intercalation; the intermediate composite layer constructs a stable SEI framework, suppressing side reactions; and the inner LiF layer provides a low-impedance transport channel, enabling rapid lithium-ion intercalation. The three layers work synergistically: the inner layer conducts lithium, the intermediate layer provides transition, and the outer layer stabilizes, optimizing the entire process from desolvation to stable transport and dense lithium intercalation. Through this synergistic effect, the anode material simultaneously achieves: an initial coulombic efficiency ≥94.5%, more than 3 percentage points higher than blank graphite; a capacity retention rate ≥85% during 8C high-rate charging, more than 40 percentage points higher than blank graphite, with no significant lithium plating; and a capacity retention rate ≥85% after 1000 1C charge-discharge cycles at 60℃, more than 35 percentage points higher than blank graphite, with no significant gas generation. The three-layer material creates a synergistic effect in terms of function, achieving a triple breakthrough in fast charging, high-temperature stability, and improved initial efficiency; it breaks through the performance bottleneck of existing single or double-layer coatings, achieving an effect of 1+1+1>3.

[0023] 2. This invention, through a LiF+Li2CO3 composite transition layer, perfectly solves the lattice mismatch problem between pure LiF and pure Li3PO4, improving cycle performance far superior to existing coating schemes.

[0024] 3. The entire process of this invention is completed in the same reaction vessel without the need for material transfer or multiple separation and washing. This not only greatly simplifies the process and reduces production costs, but also avoids damage to the coating layer caused by multiple processing steps. Batch stability is significantly improved, and it can be directly adapted to existing industrial production lines for anode materials without the need for large-scale equipment modifications.

[0025] 4. The reagents used in this invention are all commonly used industrial raw materials, free of toxic and harmful heavy metals. The reaction byproducts are ammonia, water and a small amount of ammonium salt, which are easy to wash away and have no environmental impact, meeting the requirements for large-scale production in the power battery industry. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0027] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0028] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0029] Example 1 This embodiment provides a gradient composite coated graphite anode material. The entire preparation method of this material is completed in the same closed reaction vessel without the need for solid-liquid separation or material transfer. The in-situ deposition of a three-layer gradient coating is achieved stepwise by sequentially adding reaction reagents. The specific steps are as follows: 1. Preparation of graphite dispersion 100g of dynamic artificial graphite (D50 particle size 10μm, compaction density 1.5g / cm³) was added to the reaction vessel. 3 1500 ml of anhydrous ethanol-deionized water mixed solvent (volume ratio 2:1) was ultrasonically dispersed at 25℃ for 30 min, and then mechanically stirred for 45 min to obtain a uniform and stable graphite dispersion.

[0030] 2. In-situ deposition of the inner dense LiF layer Keep the reaction vessel sealed and stirred. Add 1.02 g LiOH·H2O to the graphite dispersion and stir until completely dissolved. Heat to 55 °C and slowly add 100 ml of ethanol solution containing ammonium fluoride (1.08 g) at a rate of 3 mL / min. After the addition is complete, stir the reaction at a constant temperature for 3 h to form a 2.5 nm thick dense pure LiF inner layer on the graphite surface.

[0031] 3. In-situ deposition of the intermediate LiF+Li2CO3 composite layer Keep the reaction vessel sealed and maintain a constant temperature of 55°C with stirring. Add 1.43 g of LiOH·H2O to the above reaction solution and stir until completely dissolved. Slowly add the composite precipitant solution (100 mL of aqueous solution of 0.67 g NH4F + 0.76 g (NH4)2CO3) at a rate of 2 mL / min. After the addition is completed, stir the reaction at a constant temperature for 4 h. A 4.5 nm thick LiF+Li2CO3 composite transition layer (mass ratio LiF:Li2CO3 = 4.5:5.5) is generated in situ on the inner LiF surface.

[0032] 4. In-situ deposition of the outer Li3PO4 layer Keep the reaction vessel sealed and maintain a constant temperature of 55°C with stirring. Add 1.32 g of LiOH·H2O to the above reaction solution and stir until completely dissolved. Slowly add 100 mL of aqueous solution containing 1.21 g of (NH4H2PO4) at a rate of 2 mL / min. After the addition is complete, stir the reaction at a constant temperature for 3 h. A pure Li3PO4 outer layer with a thickness of 4.0 nm is generated in situ on the surface of the intermediate composite layer.

[0033] 5. Post-treatment and annealing strengthening After the reaction was completed, the reaction solution was directly subjected to solid-liquid separation. The obtained solid was washed four times with anhydrous ethanol to remove residual impurities. It was then placed in a vacuum drying oven and dried at 80°C for 12 hours to obtain the coated precursor. The precursor was then placed in a tube furnace and annealed at 350°C for 2.5 hours under an argon inert atmosphere, followed by natural cooling to room temperature to obtain the gradient composite coated graphite anode material. The total mass of the three coating layers accounted for approximately 3% of the mass of the graphite matrix.

[0034] Example 2 The difference from Example 1 is that the mass ratio of the transition layer LiF:Li2CO3 is 3:7, and the amounts of LiOH·H2O, NH4F, and (NH4)2CO3 are adjusted accordingly. All other conditions are the same as in Example 1.

[0035] Example 3 The difference from Example 1 is that the mass ratio of the transition layer LiF:Li2CO3 is 7:3, and the amounts of LiOH·H2O, NH4F, and (NH4)2CO3 are adjusted accordingly. All other conditions are the same as in Example 1.

[0036] Example 4 The difference from Example 1 is that the total mass of the three-layer coating accounts for about 1.5% of the mass of the graphite matrix, and the amount of all lithium sources and precipitants is reduced in proportion to the total mass. All other conditions are the same as in Example 1.

[0037] Example 5 The difference from Example 1 is that the total mass of the three-layer coating accounts for about 4.5% of the mass of the graphite matrix, and the amount of all lithium sources and precipitants is increased in proportion to the total mass. All other conditions are the same as in Example 1.

[0038] Comparative Example 1 (Blank Control Group) This comparative example provides a graphite anode material using a blank graphite matrix without the three-layer gradient coating of Example 1.

[0039] Comparative Example 2 (Existing double-layer coating group) This comparative example provides a double-layer coated graphite anode material, in which a LiF layer and a Li3PO4 layer are sequentially coated from the inside to the outside on the surface of a graphite substrate.

[0040] Comparative Example 3 (Group lacking inner LiF layer) This comparative example provides a graphite anode material, which differs from Example 1 in that only step 2 is omitted, while all other conditions are the same as in Example 1.

[0041] Comparative Example 4 (Li3PO4 group lacking outer layer) This comparative example provides a graphite anode material, which differs from Example 1 in that only step 4 is omitted, while all other conditions are the same as in Example 1.

[0042] Experimental Example The graphite anode materials prepared in the above embodiments and comparative examples were used to fabricate soft-pack lithium-ion batteries using a pouch stacking process. Lithium iron phosphate was used as the positive electrode. The specific assembly and testing methods are as follows: The LFP positive electrode, graphite negative electrode and polyethylene separator are stacked and assembled, then baked until the moisture content is qualified, electrolyte is injected, and after hot pressing formation, high temperature standing and encapsulation, after capacity testing and standing at room temperature, soft pack finished battery is obtained.

[0043] First Coulomb Efficiency: At 20–25°C, charge and discharge tests were conducted at a current density of 0.2C. First Coulomb Efficiency = (First Discharge Capacity / First Charge Capacity) × 100%.

[0044] 8C charging capacity retention rate: At 20–25°C, first perform standard charge and discharge at a 1C rate to measure the battery's rated capacity, then perform constant current charging at an 8C rate and record the charging capacity at that rate. 8C charging capacity retention rate = (8C charging capacity / rated capacity) × 100%.

[0045] Capacity retention rate (%) after 1000 charge-discharge cycles at 60℃ and 1C: At 60℃, 1000 charge-discharge cycles are performed. Capacity retention rate = (1000th discharge capacity / First discharge capacity) × 100%.

[0046] The test results are shown in Table 1.

[0047] Table 1 Based on the performance test data from Examples 1-5, Comparative Examples 1-4, and Table 1, the following conclusions can be drawn: Examples 1-5 (complete three-layer structure) significantly outperformed Comparative Example 1 (uncoated) and other comparative examples in three key indicators: initial coulombic efficiency, 8C fast-charge capacity retention, and capacity retention after 1000 cycles at 60°C. This demonstrates that the three-layer structure designed in this invention—inner LiF, middle LiF+Li2CO3 composite layer, and outer Li3PO4—has a significant synergistic effect. All examples were completed in the same closed reaction vessel, resulting in a simple process, good batch stability, and avoidance of coating layer damage caused by multiple transfers, making it suitable for industrial-scale production.

[0048] Comparative Example 2 (bilayer LiF + Li3PO4, without composite transition layer) showed a high-temperature cycling retention rate of only 60%, far lower than the 90% of Example 1, indicating that the intermediate composite layer effectively solved the lattice mismatch problem between LiF and Li3PO4, significantly improving the adhesion of the coating layer and the high-temperature stability of the interface. Comparative Example 3 (lacking inner LiF layer) showed an initial coulombic efficiency of 92.5% and a high-temperature cycling retention rate of only 67%, indicating that the inner LiF layer plays an irreplaceable role in constructing a low-impedance, dense interface and suppressing side reactions. Comparative Example 4 (lacking outer Li3PO4 layer) showed an 8C capacity retention rate of only 60%, far lower than the 85% of Example 1, indicating that the outer Li3PO4 layer plays a crucial role in reducing the lithium-ion desolvation energy barrier and improving fast-charging capability.

[0049] In summary, the gradient composite coated graphite anode material and its preparation method proposed in this invention successfully solve the common industry problems of graphite anodes in terms of fast charging, high-temperature cycling and first coulombic efficiency. The overall performance is significantly better than the existing technology, and it has clear technological advancement and industrialization prospects.

[0050] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A gradient composite coated graphite anode material, characterized in that, The material includes a graphite substrate and a three-layer gradient composite coating deposited in situ from the inside to the outside on the surface of the graphite substrate. The three-layer gradient composite coating includes a first dense lithium salt inner layer in direct contact with the graphite substrate, a composite lithium salt transition layer located outside the first dense lithium salt inner layer, and a second dense lithium salt outer layer in direct contact with the electrolyte.

2. The gradient composite coated graphite anode material according to claim 1, characterized in that, The thickness of the first dense lithium salt inner layer is 2-5 nm; and / or the thickness of the composite lithium salt transition layer is 2-5 nm; and / or the thickness of the second dense lithium salt outer layer is 2-5 nm.

3. The gradient composite coated graphite anode material according to claim 1 or 2, characterized in that, The total mass of the three-layer gradient composite coating accounts for 1 to 5% of the mass of the graphite matrix.

4. The gradient composite coated graphite anode material according to claim 1 or 2, characterized in that, The first dense lithium salt inner layer is a LiF layer with a purity ≥99%; and / or, the composite lithium salt transition layer is a composite layer of LiF and Li2CO3 with a purity ≥99%; and / or, the second dense lithium salt outer layer is a Li3PO4 layer with a purity ≥99%.

5. The gradient composite coated graphite anode material according to claim 4, characterized in that, The mass ratio of LiF to Li2CO3 is 3:7 to 7:

3.

6. The gradient composite coated graphite anode material according to claim 1, characterized in that, The graphite matrix is ​​either dynamic synthetic graphite or natural graphite, with a D50 particle size of 5–20 μm and a compacted density of 1.3–1.7 g / cm³. 3 .

7. A method for preparing a gradient composite coated graphite anode material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Add a lithium source to the graphite dispersion, stir to dissolve, and then add the first precipitant dropwise to react in situ on the graphite surface to form the first dense lithium salt inner layer; S2. While keeping the reaction system closed and stirred, add lithium source, stir to dissolve, and then add composite precipitant dropwise to react and generate composite lithium salt transition layer; S3. Add lithium source again, stir to dissolve, and then add second precipitant dropwise to react and form second dense lithium salt outer layer; S4. Finally, after solid-liquid separation, washing, drying and annealing, gradient composite coated graphite anode material is obtained.

8. The preparation method according to claim 7, characterized in that, In step S1, the first precipitant is a fluorine source, and the molar ratio of lithium source to fluorine source is 1:(1-1.5); and / or, in step S2, the composite precipitant is a mixed solution of fluorine source and carbon source, and the molar ratio of supplemented lithium source, fluorine source and carbon source is 2:(0.7-1.6):(0.2-0.7); and / or, in step S3, the second precipitant is a phosphorus source, and the molar ratio of supplemented lithium source to phosphorus source is 3:(1-1.5).

9. The preparation method according to claim 7, characterized in that, In step S1, the dropping rate of the first precipitant is 2-5 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 2-4 h; and / or, in step S2, the dropping rate of the composite precipitant is 1-3 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 3-5 h; and / or, in step S3, the dropping rate of the second precipitant is 1-3 mL / min, the reaction temperature is controlled at 50-60℃, and the reaction time is 2-4 h; and / or, in step S4, the annealing treatment is carried out under an inert atmosphere, the annealing temperature is 300-400℃, and the time is 2-3 h.

10. A lithium-ion battery, characterized in that, The gradient composite coated graphite anode material comprising any one of claims 1 to 6.

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