Double-coating-layer modified graphite negative electrode material as well as preparation method and application thereof

Abstract

The invention discloses a double-coating-layer modified graphite negative electrode material and a preparation method and application thereof.The negative electrode material comprises a graphite matrix, the surface of the graphite matrix is sequentially coated with a carbon layer and a lithium phosphate layer, the surface of the graphite matrix is directly coated with the carbon layer, the electronic contact resistance of graphite can be remarkably reduced, and the overall electronic conductivity is improved; the lithium phosphate layer on the outer layer has high lithium ion conductivity (10 <-6 >-10 <-4 > S / cm), can provide a rapid channel for lithium ion migration, and inhibits side reaction of an electrolyte and an electrode material at the same time. Through the double-coating design of the carbon layer and the lithium phosphate layer, the electronic conductivity and the lithium ion migration rate of the graphite negative electrode are synergistically improved, 10-100C high-current discharge is realized, the problem that the two are difficult to take into account by a single coating layer is solved, and the preparation method is simple in process, high in controllability, suitable for industrial production and suitable for industrial production. And when the modified graphite negative electrode is applied to the lithium ion battery, the lithium ion battery has high rate capability and cycling stability and is wide in application range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, and particularly relates to a double-coating layer modified graphite negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in consumer electronics, electric vehicles, energy storage systems and other fields due to their high energy density, long cycle life and other advantages. With the expansion of application scenarios, higher requirements are put forward for the high rate performance (i.e. large current charging and discharging capacity) of lithium ion batteries, especially in the fields of start-stop power supply, unmanned aerial vehicles, data center backup power supply, electric tools, electronic cigarettes and the like, the battery needs to withstand 10-100C large current discharge.

[0003] Graphite, as the mainstream negative electrode material of the current commercial lithium ion battery, has the advantages of high theoretical capacity (372 mAh / g) and good cycle stability, but has the problems of limited electronic conductivity and low lithium ion diffusion rate, and under high rate charging and discharging conditions, it is prone to phenomena such as accelerated polarization and rapid capacity decay, which is difficult to meet the demand for large current discharge.

[0004] In order to improve the high rate performance of graphite, surface coating modification is often used in the prior art: for example, coating a carbon layer can improve electronic conductivity, but has limited promotion effect on lithium ion migration; coating a metal oxide or phosphate can optimize lithium ion diffusion, but may reduce electronic conduction capacity. A single coating layer is difficult to solve the dual problems of electronic conduction and lithium ion migration, so it is of great significance to develop a modified graphite negative electrode material with high electronic conductivity and high lithium ion migration rate. SUMMARY

[0005] The purpose of the present application is to overcome the defects of insufficient high rate performance of graphite negative electrode in the prior art, and to provide a double-coating layer modified graphite negative electrode material, which simultaneously improves electronic conductivity and lithium ion migration rate through the synergistic effect of carbon layer and lithium phosphate layer, and realizes large current charging and discharging.

[0006] The double-coating layer modified graphite negative electrode material provided by the present application comprises a graphite matrix, and the surface of the graphite matrix is coated with a carbon layer and a lithium phosphate layer in sequence. The carbon layer is directly coated on the surface of the graphite matrix, which can significantly reduce the electronic contact resistance of the graphite and improve the overall electronic conductivity. The outer lithium phosphate layer has a high lithium ion conductivity (10 -6 -10 -4 S / cm), which can provide a fast channel for lithium ion migration and inhibit the side reaction of electrolyte and electrode material. The synergistic effect of the two layers enables the graphite negative electrode to maintain efficient electronic transmission and lithium ion diffusion under high rate, meeting the demand for 10-100C or more large current discharge.

[0007] Preferably, the carbon source of the carbon layer is selected from at least one of pitch, phenolic resin, glucose, and sucrose, which is easy to carbonize to form a continuous conductive carbon layer and has a low cost.

[0008] Preferably, the lithium phosphate layer is made of at least one of lithium dihydrogen phosphate and lithium hydrogen phosphate, which can form a stable lithium phosphate coating layer after calcination to ensure the lithium ion migration efficiency.

[0009] Preferably, the graphite matrix is natural graphite or artificial graphite with a particle size of 5-20 μm, which can balance the specific surface area and bulk density of the material to avoid the intensification of side reactions at high rates due to too small particle size.

[0010] A preparation method of a double-coating modified graphite negative electrode material, comprising the following steps: S1: carbon layer coating: dispersing the graphite matrix and the carbon source in a solvent, stirring to form a uniform suspension, drying, and then calcining at 600-1000°C for 2-6h under inert gas protection to obtain a carbon-coated graphite intermediate; S2: lithium phosphate layer coating: dispersing the carbon-coated graphite intermediate obtained in step S1 and the lithium phosphate raw material in a solvent, adjusting the pH to 6-8 (to avoid corrosion of the material in acidic or alkaline environments), stirring, drying, and then calcining at 400-700°C for 1-4h under inert gas protection to obtain a double-coating modified graphite negative electrode material.

[0011] Preferably, in step S1, the mass ratio of the carbon source to the graphite matrix is 1:10-1:50, which can ensure a moderate thickness (5-50 nm) of the carbon layer, and too thin a thickness cannot form a continuous conductive network, and too thick a thickness will reduce the specific capacity of the material.

[0012] Preferably, in step S2, the mass ratio of the lithium phosphate raw material to the carbon-coated graphite intermediate is 1:20-1:100, which can control the thickness of the lithium phosphate layer to be 10-100 nm, and too thin a thickness cannot effectively promote lithium ion migration, and too thick a thickness will increase the interfacial resistance.

[0013] Preferably, in steps S1 and S2, the solvent is deionized water, ethanol, or a mixture of the two, which facilitates the dispersion of the raw materials; the drying temperature is 60-120°C, and the drying time is 6-12h, which can fully remove the solvent; the inert gas is nitrogen or argon, and the gas flow rate is 50-200 mL / min, which ensures that the material is not oxidized during calcination.

[0014] The application of a double-coated modified graphite negative electrode material in the preparation of a high-rate lithium ion battery, including a positive electrode, an electrolyte, a separator and a negative electrode sheet made of the double-coated modified graphite negative electrode material, the battery can stably discharge at a high rate, and the capacity retention rate is not less than 80% after 500 cycles, having excellent high-rate performance and cycle stability.

[0015] The beneficial effects of the present application are: The present application improves the electronic conductivity of the graphite negative electrode and the lithium ion migration rate through the double-coating design of the carbon layer and the lithium phosphate layer, realizes 10-100C large current discharge, solves the problem that a single coating layer cannot simultaneously consider both, the preparation method is simple and controllable, suitable for industrial production, and the modified graphite negative electrode has high-rate performance and cycle stability when applied to a lithium ion battery, and has a wide range of applications. DETAILED DESCRIPTION

[0016] The present application will be further described below in conjunction with specific examples. EMBODIMENT

[0017] A double-coated modified graphite negative electrode material is proposed in the present embodiment, including a graphite matrix, the surface of the graphite matrix is sequentially coated with a carbon layer and a lithium phosphate layer, the carbon layer is directly coated on the surface of the graphite matrix, which can significantly reduce the electronic contact resistance of the graphite and improve the overall electronic conductivity; the outer lithium phosphate layer has a high lithium ion conductivity (10 -6 -10 -4 S / cm), which can provide a fast channel for lithium ion migration and inhibit the side reaction of the electrolyte and the electrode material, the synergistic effect of the two-layer structure enables the graphite negative electrode to still maintain efficient electron transmission and lithium ion diffusion at a high rate, meeting the demand for 10-100C or more large current discharge.

[0018] The carbon source of the carbon layer is selected from at least one of pitch, phenolic resin, glucose and sucrose, such carbon sources are easy to carbonize to form a continuous conductive carbon layer, and the cost is relatively low, the raw material of the lithium phosphate layer is selected from at least one of lithium dihydrogen phosphate and lithium hydrogen phosphate, which can form a stable lithium phosphate coating layer after calcination, ensuring the lithium ion migration efficiency, the graphite matrix is natural graphite or artificial graphite, and the particle size is 5-20um, which can balance the specific surface area and bulk density of the material, avoiding the intensification of side reactions at a high rate due to too small particle size.

[0019] A preparation method of a double-coated modified graphite negative electrode material, including the following steps: S1: Carbon layer coating: Disperse the graphite matrix and carbon source in a solvent, stir to form a uniform suspension, dry, and then calcine at 600-1000°C under inert gas protection for 2-6h. After cooling, the carbon-coated graphite intermediate is obtained. The mass ratio of carbon source to graphite matrix is 1:10-1:50, which can ensure a moderate carbon layer thickness (5-50nm). Too thin a carbon layer cannot form a continuous conductive network, and too thick a carbon layer will reduce the specific capacity of the material. S2: Lithium phosphate layer coating: Disperse the carbon-coated graphite intermediate obtained in step S1 and lithium phosphate raw material in a solvent, adjust the pH to 6-8 (avoid acidic or alkaline environments that can corrode the material), stir, and then dry. Calcine at 400-700°C under inert gas protection for 1-4h. After cooling, the double-coated layer modified graphite negative electrode material is obtained. The mass ratio of lithium phosphate raw material to carbon-coated graphite intermediate is 1:20-1:100, which can control the lithium phosphate layer thickness to be 10-100nm. Too thin a lithium phosphate layer cannot effectively promote lithium ion migration, and too thick a lithium phosphate layer will increase the interfacial resistance.

[0020] The solvent is deionized water, ethanol, or a mixture of the two, which facilitates the dispersion of the raw materials. The drying temperature is 60-120°C, and the drying time is 6-12h, which can fully remove the solvent. The inert gas is nitrogen or argon, and the gas flow is 50-200mL / min, which ensures that the material is not oxidized during calcination.

[0021] The application of a double-coated layer modified graphite negative electrode material in the preparation of a high-rate lithium ion battery includes a positive electrode, an electrolyte, a separator, and a negative electrode sheet made of the above-mentioned double-coated layer modified graphite negative electrode material. The battery can discharge stably at a high rate, and the capacity retention rate is not less than 80% after 500 cycles, with excellent high-rate performance and cycle stability.

[0022] Example 1 1) Carbon layer coating: Take 100g of artificial graphite (particle size 10μm) and 5g of pitch and mix them in a V-type mixer for 2h. Then, place them in a tube furnace under a nitrogen atmosphere (flow rate 100mL / min) and calcine at 800°C for 4h. After cooling, the carbon-coated graphite intermediate is obtained. 2) Lithium phosphate layer coating: Take 100g of the carbon-coated graphite intermediate from step S1 and 3g of lithium dihydrogen phosphate and disperse them in 500mL of deionized water. Adjust the pH to 7 with ammonia water, stir for 2h, and then dry at 80°C for 10h. Place them in a tube furnace under a nitrogen atmosphere (flow rate 100mL / min) and calcine at 500°C for 2h. After cooling, the double-coated layer modified graphite negative electrode material is obtained.

[0023] Example 2 1) Carbon layer coating: 100 g of natural graphite (particle size 15 μm) and 2 g of glucose were dispersed in 500 mL of ethanol, stirred for 2 h to form a suspension, dried at 100°C for 8 h, then placed in a tube furnace in an argon atmosphere (flow rate 150 mL / min), calcined at 900°C for 3 h, and cooled to obtain a carbon-coated graphite intermediate; 2) Lithium phosphate layer coating: 100 g of the carbon-coated graphite intermediate in step S1 and 5 g of lithium hydrogen phosphate were dispersed in 500 mL of deionized water, the pH was adjusted to 6.5 with phosphoric acid, stirred for 2 h, dried at 100°C for 8 h, then placed in a tube furnace in an argon atmosphere (flow rate 150 mL / min), calcined at 600°C for 3 h, and cooled to obtain a double-coated modified graphite negative electrode material (lithium phosphate layer thickness 30 nm).

[0024] Comparative Example 1 Artificial graphite in Example 1.

[0025] Comparative Example 2 Artificial graphite in Example 1 only coated with a carbon layer.

[0026] Comparative Example 3 Artificial graphite in Example 1 only coated with lithium phosphate.

[0027] The negative electrode materials of Examples 1-2 and Comparative Examples 1-3 were mixed with CMC, SBR, Super P at a mass ratio of 95:1.5:2:1.5, coated on a copper foil to form a negative electrode, assembled into a 26650-2 Ah cylindrical battery with lithium iron phosphate, a separator and an electrolyte (1.5 mol / L, ), and its performance was tested. Cycle test: 1C charge / 1C discharge at 25°C, cycle 3000 times, record capacity retention rate. Cycle test: 1C charge / 40C discharge at 25°C, cycle 500 times, record capacity retention rate.

[0028]

[0029] As can be seen from the above table, the negative electrode material of the application has a high cycle life at a high discharge rate, and at a conventional discharge rate, the cycle life is also greatly improved.

[0030] The above description is only a preferred specific embodiment of the application, but the protection scope of the application is not limited thereto, any person skilled in the art can make equivalent substitutions or changes within the technical range disclosed by the application according to the technical solution and inventive concept of the application, which should be covered within the protection scope of the application.

Claims

1. A double-coated modified graphite negative electrode material, characterized by, The graphite substrate is coated with a carbon layer and a lithium phosphate layer in sequence. 2.The double-coated modified graphite negative electrode material of claim 1, characterized in that, The carbon source of the carbon layer is selected from at least one of pitch, phenolic resin, glucose and sucrose, which is easy to carbonize to form a continuous conductive carbon layer and has low cost. 3.The double-coated modified graphite negative electrode material of claim 1, characterized in that, The raw material of the lithium phosphate layer is selected from at least one of lithium dihydrogen phosphate and lithium hydrogen phosphate, which can form a stable lithium phosphate coating layer after calcination to ensure the lithium ion migration efficiency. 4.The double-coated modified graphite negative electrode material of claim 1, characterized in that, The graphite substrate is natural graphite or artificial graphite with a particle size of 5-20 μm, which can balance the specific surface area and bulk density of the material to avoid the intensification of side reactions at high rates due to too small particle size.

5. A method for preparing a double-coated modified graphite negative material, characterized by, The method comprises the following steps: S1: carbon layer coating: dispersing the graphite substrate and the carbon source in a solvent, stirring to form a uniform suspension, drying, and then calcining at 600-1000 ℃ under inert gas protection for 2-6 h to obtain a carbon-coated graphite intermediate; S2: lithium phosphate layer coating: dispersing the carbon-coated graphite intermediate obtained in step S1 and the lithium phosphate raw material in a solvent, adjusting the pH to 6-8, stirring, drying, and then calcining at 400-700 ℃ under inert gas protection for 1-4 h to obtain a double-coating layer modified graphite negative electrode material.

6. The preparation of a double-coated modified graphite negative material according to claim 5, characterized in that, In step S1, the mass ratio of the carbon source to the graphite substrate is 1:10-1:

50.

7. The method for preparing a double-coated modified graphite anode material according to claim 5, characterized in that, In step S2, the mass ratio of the lithium phosphate raw material to the carbon-coated graphite intermediate is 1:20-1:100, which can control the thickness of the lithium phosphate layer to be 10-100 nm, and too thin cannot effectively promote lithium ion migration, and too thick will increase the interface resistance.

8. The method for preparing a double-coated modified graphite anode material according to claim 5, characterized in that, In steps S1 and S2, the solvent is deionized water, ethanol or a mixture of the two, which facilitates the dispersion of the raw materials; the drying temperature is 60-120 ℃, and the drying time is 6-12 h, which can fully remove the solvent; the inert gas is nitrogen or argon, and the gas flow is 50-200 mL / min, which ensures that the material is not oxidized during calcination.

9. Use of the double-coated modified graphite negative material according to any one of claims 1-4 in the preparation of high rate lithium-ion batteries, characterized in that, The method comprises a positive electrode, an electrolyte, a separator and a negative electrode sheet made of the above-mentioned double-coating layer modified graphite negative electrode material.

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