A Coating Modification Method for a Battery Anode Material

Abstract

The present invention discloses a method for coating and modifying a battery negative electrode material, comprising the following steps: taking needle coke raw materials, first performing graphitization treatment, then using a mechanical mixing method to coat a fluorine-containing polymer, then performing sintering, and finally depolymerizing the sintered material to obtain a fluorine-modified battery negative electrode material. In this application, needle coke is used as the raw material. After graphitization, a layer of fluorine-containing polymer is coated on the surface of the needle coke by a mechanical mixing method, then sintering treatment is carried out, and finally depolymerization is carried out to obtain a fluorine-modified negative electrode material, reducing the side reaction between the negative electrode material and the electrolyte, reducing the first irreversible capacity loss, and improving the specific capacity, first charge-discharge efficiency and cycle performance of the negative electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical fields of new materials, new energy vehicles, and lithium-ion batteries, and particularly relates to a method for coating and modifying a battery anode material. Background Art

[0002] A lithium-ion battery mainly consists of four major parts: a positive electrode material, a negative electrode material, an electrolyte, and a separator. Needle coke, as one of the main raw materials for artificial graphite negative electrode materials, is a special pitch-based carbonaceous material. It has a needle-like structure under an optical microscope and has a series of advantages such as high crystallinity, small thermal expansion coefficient, good orientation, high conductivity, and easy graphitization. However, the following problems exist during use: 1) The surface of the needle coke is prone to irreversible reactions with the electrolyte, resulting in a decrease in charge and discharge efficiency; 2) During the charge and discharge process, the reversible capacity of the battery decreases due to the co-insertion of solvents, and the cycle performance deteriorates. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for coating and modifying a battery anode material to solve the deficiencies of the prior art, aiming to improve the specific capacity, first charge and discharge efficiency of the anode material, and improve its cycle performance and rate performance.

[0004] The purpose of the present invention is achieved by the following technical solutions:

[0005] A method for coating and modifying a battery anode material includes the following steps: taking needle coke raw materials, first performing graphitization treatment, then using a mechanical mixing method for coating with a fluoropolymer, then performing sintering, and finally depolymerizing the sintered material to obtain a fluorine-modified battery anode material.

[0006] Preferably, the D50 particle size of the needle coke raw materials is 8 - 15 μm.

[0007] Preferably, the graphitization treatment step is: placing the needle coke raw materials in a graphitization device, heating at a rate of 4 - 6 °C / min to 2800 - 3200 °C, and then holding for 1 - 5 h.

[0008] Preferably, the fluoropolymer is one or a combination of poly(tetrafluoroethylene), perfluoro(ethylene propylene), perfluoroalkoxy, poly(vinylidene chloride), poly(vinyl fluoride), and poly(vinylidene fluoride).

[0009] Preferably, the mechanical mixing method uses one of ball milling, stirring milling, and high-speed gas flow impact method.

[0010] Preferably, the coating step of the fluoropolymer is as follows: putting the graphitized needle coke raw material and the fluoropolymer solution or suspension into a mechanical mixing device for mixing reaction; the mass ratio of the needle coke raw material to the fluoropolymer is (98.5 - 99.5):(0.5 - 1.5).

[0011] Preferably, the solvent of the fluoropolymer solution or suspension is one selected from DMF, DMAc, DMSO, NMP or purified water.

[0012] Preferably, the sintering temperature is not less than the melting point of the coated fluoropolymer, and the sintering is carried out under the protection of an inert gas.

[0013] Preferably, before sintering, there is also a drying step, and the drying method is one of spray drying, fluidized bed drying, and rotary kiln drying.

[0014] Preferably, the depolymerization is carried out by one of a classification crusher and a classification impact mill, and the D50 particle size of the depolymerized material is controlled to be 10 - 20 μm.

[0015] In this application, using needle coke as the raw material, after graphitization, a layer of fluoropolymer is coated on the surface of the needle coke by the mechanical mixing method, then sintering treatment is carried out, and finally depolymerization is carried out to obtain a fluorine-modified anode material, reducing the side reaction between the anode material and the electrolyte, reducing the first irreversible capacity loss, and improving the specific capacity, first charge-discharge efficiency and cycle performance of the anode material. Detailed implementation mode

[0016] The present invention provides a method for coating and modifying a battery anode material, including the following steps: taking a needle coke raw material, first carrying out graphitization treatment, then using the mechanical mixing method to coat a fluoropolymer, then carrying out sintering, and finally depolymerizing the sintered material to obtain a fluorine-modified battery anode material.

[0017] Conventional graphite anode materials have poor compatibility with electrolytes. During the charge and discharge process, solvent molecules enter the interlayer of graphite, causing the graphite layers to exfoliate, resulting in deteriorated cycling performance and decreased electrochemical performance. In this application, needle coke is used as the raw material for graphitization, which has the advantages of high crystallinity, small thermal expansion coefficient, good orientation, high conductivity, and easy graphitization. After graphitization, a fluoropolymer is coated on the outer layer of the material. The fluoropolymer has C-F bonds with high binding energy, showing good thermal stability and chemical stability, and helps to enhance the stability of the SEI film. The graphite anode material coated with fluoropolymer can improve the compatibility between the electrolyte (the main component of the general electrolyte is lithium hexafluorophosphate) and the anode material, contribute to the formation of a stable SEI film, effectively reduce the irreversible lithium consumption caused by electrolyte decomposition and SEI film formation, reduce the irreversible capacity loss caused by the reaction between the electrolyte and the anode material, thereby improving the reversible capacity of the graphite anode, increasing the initial Coulomb efficiency, improving the specific capacity of the anode material, and improving its cycling performance.

[0018] Moreover, the coating of the fluoropolymer has no adverse effect on the conductivity of the anode material. The specific analysis is as follows:

[0019] After the fluoropolymer is coated and sintered, the fluoropolymer will be in a molten state. The molten fluoropolymer will diffuse into the graphite layer, and F in the fluoride will undergo a micro-reaction with the graphite surface to form C-F bonds, which are relatively tightly combined. Coating the fluoride improves the compatibility between the electrolyte and the anode material, and the coating amount is small, so it does not affect the conductivity.

[0020] The mechanical mixing method is selected for the coating method of the fluoropolymer. This method has a short treatment time, easy control of the reaction process, and can be continuously produced in batches.

[0021] After sintering the molten fluoropolymer and the anode material, agglomeration often occurs, which affects the electrical performance. Therefore, in this application, depolymerization is used, which can disperse the agglomerated or caked anode material, fully ensuring the electrical performance of the anode material coated with fluoropolymer.

[0022] Therefore, in this application, needle coke is used as the raw material. After graphitization, a layer of fluoropolymer is coated on the surface of the needle coke by the mechanical mixing method, then sintering treatment is carried out, and finally depolymerization is carried out to obtain a fluorine-modified anode material, reducing the side reaction between the anode material and the electrolyte, reducing the initial irreversible capacity loss, and improving the specific capacity, initial charge and discharge efficiency, and cycling performance of the anode material.

[0023] Preferably, the needle coke raw material can be oil-based needle coke or coal-based needle coke. Since the particle size of the raw material is relatively large, which is not conducive to subsequent graphitization and coating treatment, therefore, it is crushed and classified, and its D50 particle size is controlled to be 8-15 μm.

[0024] Preferably, the graphitization equipment is one of Acheson furnaces and box-type graphitization furnaces. The specific graphitization treatment steps are as follows: Place the needle coke raw material in the graphitization equipment and heat it to 2800 - 3200 °C at a rate of 4 - 6 °C / min, then hold the temperature for 1 - 5 h. Under the above temperature and time conditions, the needle coke raw material can be fully graphitized.

[0025] Preferably, the fluoropolymer can be selected from one or a combination of polytetrafluoroethylene (PTFE), perfluoro(ethylene propylene) (FEP), perfluoroalkoxy (PFA), polychlorotrifluoroethylene (PCTFF), polyvinyl fluoride (PVF), and polyvinylidene fluoride (PVDF).

[0026] Preferably, the mechanical mixing method can be one of ball milling, stirring milling, and high-speed gas flow impact method. Further preferably, the stirring milling method is adopted.

[0027] Preferably, the coating step of the fluoropolymer is specifically as follows: Place the graphitized needle coke raw material and the fluoropolymer solution or suspension of the fluoropolymer in a mechanical mixing device and mix and react under wet conditions, which can make the fluoropolymer fully contact with the graphitized needle coke, facilitating coating; the mass ratio of the needle coke raw material to the fluoropolymer is (98.5 - 99.5):(0.5 - 1.5).

[0028] Preferably, the solvent of the fluoropolymer solution or suspension of the fluoropolymer is selected from one of DMF, DMAc, DMSO, NMP, or purified water. Further preferably, purified water is used as the solvent to disperse the fluoropolymer to form an emulsion, which will not cause organic solvent pollution.

[0029] Preferably, the rotation speed of mixing and dispersing the fluoropolymer and the solvent is 1000 - 2000 r / min; the dispersion time is 1 - 5 h.

[0030] Preferably, the mechanical mixing and grinding time of the fluoropolymer solution and the graphitized needle coke is 0.5 - 5 h.

[0031] Preferably, the sintering equipment can be a tube furnace, an atmosphere furnace, or a roller hearth kiln (the roller hearth kiln can be continuously produced with high automation and is suitable for large-scale preparation). For small-scale production, a tube furnace or an atmosphere furnace is preferably used, and for large-scale production, a roller hearth kiln is preferably used.

[0032] The sintering temperature can be adjusted according to the melting point of the fluoropolymer. Generally, the sintering temperature should not be less than the melting point of the fluoropolymer used to ensure that the fluoropolymer is completely in a molten state. Taking polytetrafluoroethylene as an example, the melting point of polytetrafluoroethylene is 340 °C, and it has high thermal stability and chemical stability at 487 °C, and has lipophilicity similar to graphite. When sintering above 340 °C, it is preferably sintered at 380 °C, and polytetrafluoroethylene will be completely in a molten state.

[0033] To fully ensure the combination of F in the fluoropolymer and graphite, the sintering time is preferably 2 - 5 h. During the sintering process, inert gas protection is adopted to prevent the increase in specific surface area caused by the oxidation of the negative electrode material, which affects the electrochemical performance.

[0034] Preferably, before sintering, it also includes a drying step, which is beneficial to sintering after drying.

[0035] The drying method can be one of spray drying, fluidized bed drying, and rotary kiln drying. Spray drying is preferably adopted, which has a short drying time, simple operation, can be continuously produced, and has a high degree of automation.

[0036] Preferably, the depolymerization is carried out by one of a classification crusher and a classification impact mill, and the D50 particle size of the depolymerized material is controlled to be 10 - 20 μm.

[0037] Example 1

[0038] Take the coal-based needle coke raw material, after crushing and classification, obtain a material with a D50 particle size of 9 μm, then carry out graphitization using a graphitization furnace, the graphitization temperature is 3000 °C, and keep the temperature for 4 h. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water, disperse it at a high speed of 1000 r / min for 3 h to form an emulsion. Place the emulsion and the graphitized particles in a stirred grinding machine for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 1.0:99), the grinding time is 3 h, then carry out spray drying. Transfer the dried material to a tube furnace and sinter at 380 °C for 3 h. Nitrogen is passed through for protection during the sintering process. After sintering, the material is sent into a classification crusher by a screw feeder, and the material is impacted and sheared by a high-speed rotating rotor. After being dispersed and classified, finally obtain a negative electrode material with a finished product particle size D50 of 15 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 361.7 mAh / g and 95.2% respectively.

[0039] Example 2

[0040] Take coal-based needle coke raw materials, crush and classify them to obtain materials with a D50 particle size of 9 μm. Then, use a graphitization furnace for graphitization at a graphitization temperature of 3000 °C and hold for 3 h. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water and disperse it at a high speed of 1000 r / min for 1 h to form an emulsion. Place the emulsion and the graphitized particles in a stirred mill for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 0.5:99.5), with a grinding time of 2 h. Then, perform spray drying. Transfer the dried material to a tube furnace and sinter it at 340 °C for 5 h. Protect the sintering process by passing nitrogen. After sintering, the material is fed into a classification crusher by a screw feeder. The high-speed rotating rotor impacts and shears the material. After dispersion and classification, finally obtain a negative electrode material with a finished product particle size D50 of 15 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 356.6 mAh / g and 93.5% respectively.

[0041] Example 3

[0042] Take coal-based needle coke raw materials, crush and classify them to obtain materials with a D50 particle size of 9 μm. Then, use a graphitization furnace for graphitization at a graphitization temperature of 3000 °C and hold for 1 h. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water and disperse it at a high speed of 1000 r / min for 4 h to form an emulsion. Place the emulsion and the graphitized particles in a stirred mill for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 1.5:98.5), with a grinding time of 4 h. Then, perform spray drying. Transfer the dried material to a tube furnace and sinter it at 400 °C for 4 h. Protect the sintering process by passing nitrogen. After sintering, the material is fed into a classification crusher by a screw feeder. The high-speed rotating rotor impacts and shears the material. After dispersion and classification, finally obtain a negative electrode material with a finished product particle size D50 of 15 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 350.2 mAh / g and 92.1% respectively.

[0043] Example 4

[0044] Take the coal-based needle coke raw material, crush and classify it to obtain a material with a D50 particle size of 9 μm. Then, use a graphitization furnace for graphitization at a graphitization temperature of 3000 °C and hold for 5 h. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water and disperse it at a high speed of 1000 r / min for 2 h to form an emulsion. Place the emulsion and the graphitized particles in a stirred mill for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 1.0:99), with a grinding time of 5 h. Then, perform spray drying. Transfer the dried material to a tube furnace and sinter it at 450 °C for 2 h. Nitrogen is introduced for protection during the sintering process. After sintering, the material is fed into a classification crusher by a screw feeder. The material is impacted and sheared by a high-speed rotating rotor, and after being dispersed and classified, a negative electrode material with a final product particle size D50 of 15 μm is finally obtained. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 358.8 mAh / g and 94.0% respectively.

[0045] The parameters and product performances involved in Examples 1 to 4 are shown in Table 1.

[0046] Table 1

[0047]

[0048] According to the above data, it can be obtained that:

[0049] 1. In Example 2, the content of the fluoropolymer and the sintering temperature are lower than those in Example 1, and the final specific capacity and efficiency are lower.

[0050] 2. In Example 3, the content of the fluoropolymer and the sintering temperature are higher than those in Example 1. However, due to the relatively high content of the fluoropolymer, the fluoropolymer agglomerates during the grinding process, and the coating effect is not good, resulting in a low specific capacity and efficiency.

[0051] 3. Compared with Example 1, in Example 4, the sintering temperature is higher and the sintering time is shorter, and the caking phenomenon is greater than that in Example 1, and the final specific capacity and efficiency are lower.

[0052] Comparative Example 1

[0053] Take coal-based needle coke raw materials, crush and classify them to obtain materials with a D50 particle size of 9 μm. Then, use a graphitization furnace for graphitization at a graphitization temperature of 3000 °C for 5 hours of heat preservation. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water and disperse it at a high speed of 1000 r / min for 2 hours to form an emulsion. Place the emulsion and the graphitized particles in a stirring mill for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 1.0:99), with a grinding time of 5 hours. Then, perform spray drying, send it into a classification crusher through a screw feeder, and use a high-speed rotating rotor to impact and shear the materials. After dispersion and classification, finally obtain a negative electrode material with a finished product particle size D50 of 15 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 348.5 mAh / g and 90.3% respectively.

[0054] Comparative Example 2

[0055] Take coal-based needle coke raw materials, crush and classify them to obtain materials with a D50 particle size of 9 μm. Dissolve the fluoropolymer polytetrafluoroethylene (PTFE) in purified water and disperse it at a high speed of 1000 r / min for 2 hours to form an emulsion. Place the emulsion and the crushed particles in a stirring mill for stirring and grinding (the mass ratio of the fluoropolymer to the needle coke in the grinding system is 1.0:99), with a grinding time of 5 hours. Then, perform spray drying, transfer the dried materials to a tube furnace, sinter at 450 °C for 2 hours, and protect the sintering process with nitrogen. After sintering, send the materials into a classification crusher through a screw feeder, and use a high-speed rotating rotor to impact and shear the materials. After dispersion and classification, finally obtain a negative electrode material with a finished product particle size D50 of 15 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 331.8 mAh / g and 86.5% respectively.

[0056] Comparative Example 3

[0057] Take coal-based needle coke raw materials, crush and classify them to obtain materials with a D50 particle size of 9 μm. Then, use a graphitization furnace for graphitization at a graphitization temperature of 3000 °C for 5 hours of heat preservation. After graphitization, send the materials into a crusher through a screw feeder, and use a high-speed rotating rotor to impact and shear the materials. After dispersion, finally obtain a negative electrode material with a finished product particle size D50 of 8 μm. Demagnetize the material and detect its electrochemical performance. The reversible specific capacity and the first Coulomb efficiency are 345.2.8 mAh / g and 87.6% respectively.

[0058] According to the above comparative examples, it can be obtained that:

[0059] 1. Comparative Example 1 was coated with a fluoropolymer but without a sintering process, which means that the fluoropolymer was coated on the surface of the particles but did not penetrate into the interior of the particles. During the subsequent electrochemical tests, it was prone to dissolution and shedding, resulting in poor electrochemical performance;

[0060] 2. Comparative Example 2 did not undergo graphitization and directly proceeded with the coating process. Since graphitization did not rearrange the internal structure of the needle coke, the electrical properties were very poor;

[0061] 3. Comparative Example 3 graphitized the needle coke but did not perform the coating and sintering process. Without the coating of the fluoropolymer, the specific capacity and first efficiency of the electrochemical performance of the material were low.

[0062] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments as well as all changes and modifications falling within the scope of the present invention. Obviously, those skilled in the art can make various changes and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A method for coating and modifying a battery anode material, characterized in that, It includes the following steps: Take the needle coke raw material, first carry out graphitization treatment, then use the mechanical mixing method for coating with fluoropolymer, then carry out sintering, and finally depolymerize the sintered material to obtain the fluorine-modified battery anode material; The mechanical mixing method adopts one of the ball milling method, stirring milling method, and high-speed gas flow impact method; The coating step of the fluoropolymer is: place the graphitized needle coke raw material and the fluoropolymer solution or the suspension of the fluoropolymer in a mechanical mixing device for mixing reaction; the mass ratio of the needle coke raw material to the fluoropolymer is (98.5 - 99.5):(0.5 - 1.5); The sintering temperature is not less than the melting point of the coated fluoropolymer, the sintering is carried out under the protection of inert gas, and the sintering time is 2 - 5 h; The D50 particle size of the needle coke raw material is 8 - 15 μm; The depolymerization adopts one of the classification crusher and the classification impact mill equipment, and the D50 particle size of the depolymerized material is controlled to be 10 - 20 μm.

2. The method for coating and modifying a battery anode material according to claim 1, characterized in that, The graphitization treatment step is: place the needle coke raw material in a graphitization device, heat it up to 2800 - 3200 °C at a rate of 4 - 6 °C / min, and then keep it warm for 1 - 5 h.

3. The method for coating and modifying a battery anode material according to claim 1, characterized in that, The fluoropolymer is one or more combinations selected from polytetrafluoroethylene, perfluoro(ethylene propylene), perfluoroalkoxy, polychlorotrifluoroethylene, polyvinyl fluoride, and polyvinylidene fluoride.

4. The method for coating and modifying a battery anode material according to claim 1, characterized in that, The solvent of the fluoropolymer solution or the suspension of the fluoropolymer is one selected from DMF, DMAc, DMSO, NMP, or purified water.

5. The method for coating and modifying a battery anode material according to claim 1, characterized in that, Before the sintering, it also includes a drying step, and the drying method adopts one of spray drying, fluidized bed drying, and rotary kiln drying.

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