Silicon negative electrode material and preparation method thereof

Abstract

The invention provides a silicon negative electrode material and a preparation method thereof, and relates to the technical field of lithium ion batteries. The preparation method of the silicon negative electrode material comprises the following steps: uniformly mixing a silicon source and a carbon source, performing high-speed ball milling to obtain a mixture, filling the mixture into a lithium metal bag, rubbing the lithium metal bag filled with the mixture to obtain a composite material, and washing the composite material to obtain the silicon negative electrode material. The silicon negative electrode material provided by the invention has excellent cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a silicon negative electrode material and a preparation method thereof. Background Art

[0002] With the development of the new energy industry, the energy density requirements for battery cells are getting higher and higher. The traditional graphite negative electrode has reached its limit. The high specific capacity of silicon negative electrode materials meets the pursuit of energy density for future battery cells. However, the cyclic stress expansion of silicon negative electrodes has greatly limited the further application of silicon negative electrodes. At present, the modification of silicon negative electrodes mainly includes nano-sizing, carbon coating, use of silicon oxide, oxide coating, etc. It takes a long time and consumes a lot of energy to coat the surface of traditional silicon negative electrode materials. In addition, the coating is not good and is easy to fall off. Summary of the invention

[0003] Based on the technical problems existing in the background technology, the present invention proposes a preparation method for in-situ coating of graphite material on the surface of a silicon source by lithium thermal reaction. The silicon negative electrode material prepared by the preparation method is economical and practical, and the prepared silicon negative electrode material has excellent cycle performance.

[0004] The present invention provides a method for preparing a silicon negative electrode material, comprising the following steps:

[0005] The silicon source and the carbon source are uniformly mixed and then subjected to high-speed ball milling to obtain a mixture, the mixture is loaded into a lithium metal bag, the lithium metal bag containing the mixture is rubbed to obtain a composite material, and the composite material is washed to obtain a silicon negative electrode material.

[0006] The present invention utilizes lithium thermal synthesis to in-situ prepare silicon negative electrode materials, in-situ graphite coating is formed on the surface of silicon materials, the reaction conditions are mild, the operation is simple, and the industrial feasibility is strong. The lithium thermal synthesis of the present invention is carried out in the friction stage after the copper plate is heated, the heated lithium bag is melted, and the friction causes the molten lithium to collide with the silicon source and carbon source mixture to trigger the reaction to synthesize the silicon negative electrode material.

[0007] Preferably, the silicon source is selected from one or more of elemental silicon and SiO2.

[0008] The role of the silicon source is to serve as the main material for the silicon negative electrode material prepared from silicon.

[0009] Preferably, the carbon source is selected from one or more of PVDF and PTFE.

[0010] The role of the carbon source is to generate graphite in the lithium thermal synthesis reaction and coat the surface of the silicon source.

[0011] Preferably, the mass ratio of the silicon source to the carbon source is (4-5):(5-6).

[0012] Controlling the mass ratio of silicon source to carbon source within a certain range helps to improve the coating quality, so that the prepared silicon negative electrode material has excellent cycle performance.

[0013] Preferably, the ball milling parameters are: rotation speed of 800-1000 r / min, ball-to-material ratio of 15:1, and ball milling time of 6-12 h.

[0014] By adjusting the parameters of ball milling, it is helpful to evenly mix the silicon source and the carbon source, and evenly coat the graphite material on the surface of the silicon source in situ.

[0015] Preferably, the mass ratio of the mixture to lithium metal is (1.5-2):1.

[0016] The mass ratio of the mixture to lithium metal is within a certain range, which is conducive to the occurrence of lithium thermal reaction and the preparation of silicon negative electrode material.

[0017] Preferably, the rubbing of the lithium metal bag containing the mixture is specifically: placing the lithium metal bag containing the mixture in a plate-type thermal compressor, heating a copper plate, and rubbing the lithium metal bag containing the mixture with the heated copper plate.

[0018] More preferably, the heating temperature of the copper plate is 200-250°C.

[0019] The heating temperature of the copper plate within a certain range helps ensure that the lithium metal bag can be completely melted. The copper plate only needs to be heated to a temperature of 200-250°C, and continuous heating is not required during the process, and the reaction time is short.

[0020] Preferably, the washing liquid is selected from one or more of methanol and hydrochloric acid.

[0021] The washing solution selected for washing helps to remove excess lithium metal and lithium fluoride impurities generated by the reaction.

[0022] More preferably, the concentration of the hydrochloric acid is 0.5-5M.

[0023] The present invention also provides a silicon negative electrode material prepared by the above preparation method.

[0024] The beneficial effects of the present invention are:

[0025] The present invention utilizes the instantaneous high temperature reaction of lithium thermal reaction to make the carbon source highly graphitized and coated on the surface of silicon source, thereby preparing high-performance silicon negative electrode material. The lithium thermal reaction process of the preparation method provided by the present invention is a self-propagating high temperature synthesis, which can be initiated by directly immersing the carbon source powder into molten Li in a transverse shearing manner. The heat released by the strong exothermic reaction generates a combustion wave and spontaneously propagates to the remaining reactants in a self-sustaining behavior. The graphite synthesized by instantaneous carbon vaporization and condensation under high temperature reaction has good graphitization crystallinity, which simplifies the graphitization process of carbon materials. At the same time, graphite coating can be formed in situ on the surface of silicon materials, and no external energy is continuously input during the entire synthesis process. The reaction time is short, which opens up an economical way for production. Compared with traditional methods, it has priority. Another important feature of the lithium thermal strategy is its feasibility in industrial-level material preparation, which can be achieved through tunnel kilns. Large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a SEM image of the silicon negative electrode material prepared in Example 1 of the present invention.

[0027] Figure 2 This is a cycle capacity retention rate curve diagram in Example 1 proposed by the present invention.

[0028] Figure 3 This is a schematic diagram of simulation of large-scale production using tunnel kiln;

[0029] 1- tunnel kiln, 2- copper plate, 3- lithium bag, 4- mixture of silicon source and carbon source.

[0030] Figure 4 This is the first charge specific capacity of the silicon negative electrode material prepared in Example 1. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is described in detail through specific embodiments.

[0032] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0033] Example 1

[0034] A method for preparing a silicon negative electrode material comprises the following steps:

[0035] 4g Si powder and 6g PTFE powder were placed in a star-shaped high-energy ball mill, and then zirconium balls were added with a ball-to-material ratio of 15:1. The mixture was subjected to high-speed ball milling at 1000r / min for 6h to obtain a mixture. 5g of the mixture was weighed in a vacuum glove box and loaded into a 9g lithium metal bag, which was placed in a plate-type thermal compressor. A composite material was obtained by rubbing the lithium bag with a copper plate heated to 200°C. The composite material was washed with methanol and 1M HCL solution to obtain the silicon negative electrode material Si@Gr.

[0036] Example 2

[0037] A method for preparing a silicon negative electrode material comprises the following steps:

[0038] 4g SiO2 powder and 6g PTFE powder were placed in a star-shaped high-energy ball mill, and then zirconium balls were added with a ball-to-material ratio of 15:1. The mixture was subjected to high-speed ball milling at 1000r / min for 6h to obtain a mixture. 5g of the mixture was weighed in a vacuum glove box and loaded into a 9g lithium metal bag, which was placed in a plate-type thermal compressor. A composite material was obtained by rubbing the lithium bag with a copper plate heated to 200°C. The composite material was washed with methanol and 1M HCL solution to obtain the silicon negative electrode material SiO2@Gr.

[0039] Example 3

[0040] A method for preparing a silicon negative electrode material comprises the following steps:

[0041] 4g Si powder and 6g PVDF powder were placed in a star-shaped high-energy ball mill, and then zirconium balls were added with a ball-to-material ratio of 15:1. The mixture was subjected to high-speed ball milling at 1000r / min for 6h to obtain a mixture. 5g of the mixture was weighed in a vacuum glove box and loaded into a 9g lithium metal bag, which was placed in a plate-type thermal compressor. A composite material was obtained by rubbing the lithium bag with a copper plate heated to 200°C. The composite material was washed with methanol and 1M HCL solution to obtain the silicon negative electrode material Si@Gr.

[0042] Example 4

[0043] A method for preparing a silicon negative electrode material comprises the following steps:

[0044] 4g SiO2 powder and 6g PVDF powder were placed in a star-shaped high-energy ball mill, and then zirconium balls were added with a ball-to-material ratio of 15:1. The mixture was subjected to high-speed ball milling at 1000r / min for 6h to obtain a mixture. 5g of the mixture was weighed in a vacuum glove box and loaded into a 9g lithium metal bag, which was placed in a plate-type thermal compressor. A composite material was obtained by rubbing the lithium bag with a copper plate heated to 200°C. The composite material was washed with methanol and 1M HCL solution to obtain the silicon negative electrode material SiO2@Gr.

[0045] The above negative electrode material was prepared into a negative electrode sheet, and assembled with a positive electrode sheet, a separator, and an electrolyte to obtain a lithium-ion battery. The capacity retention rate of the lithium-ion battery after 200 cycles was tested, and two sets of parallel experiments were performed. The test results are shown in Table 1. The test method is to charge from 2.8V to 4.35V at a rate of 0.5C, maintain a constant current charge of 4.35V, cut off the current at 0.05C, and discharge at a rate of 1C to 2.8V, and cycle this process for 200 cycles.

[0046] Table 1

[0047]

[0048] The assembled lithium-ion battery was tested for the first charge specific capacity and the first coulomb efficiency by first standing for 5 minutes, then discharging at a constant current of 0.06C to 0.005V, then discharging at a constant current of 0.01C to 0.005V, then standing for 5 minutes, and then charging at a constant current of 0.1C to 1.5V. The test results are shown in Table 2.

[0049] Table 2

[0050]

[0051]

[0052] From Table 1, Table 2 and Figure 2 , Figure 4 It can be seen from the data that the silicon negative electrode material prepared by the present invention has a higher cycle capacity retention rate, indicating that the silicon negative electrode material has excellent cycle performance and excellent first coulombic efficiency.

[0053] Figure 3 A schematic diagram of a large-scale production simulation using a tunnel kiln. The ball-milled silicon source and carbon source mixture are placed in a lithium metal bag, placed in a copper plate in a tunnel kiln, and then heated.

[0054] In summary, the silicon negative electrode material provided by the present invention has excellent cycle performance.

[0055] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a silicon negative electrode material, characterized in that: The following steps are involved: The silicon source and the carbon source are uniformly mixed and then subjected to high-speed ball milling to obtain a mixture, the mixture is loaded into a lithium metal bag, the lithium metal bag containing the mixture is rubbed to obtain a composite material, and the composite material is washed to obtain a silicon negative electrode material.

2. The preparation method according to claim 1, characterized in that: The rubbing of the lithium metal bag containing the mixture is specifically as follows: placing the lithium metal bag containing the mixture in a plate-type heat compressor, heating the copper plate, and rubbing the lithium metal bag containing the mixture with the heated copper plate.

3. The preparation method according to claim 1, characterized in that: The silicon source is selected from one or more of elemental silicon and SiO2.

4. The preparation method according to claim 1, characterized in that: The carbon source is selected from one or more of PVDF and PTFE.

5. The preparation method according to claim 1, characterized in that: The mass ratio of the silicon source to the carbon source is (4-5):(5-6).

6. The preparation method according to claim 1, characterized in that: The ball milling parameters are: rotation speed of 800-1000r / min, ball-to-material ratio of 15:1, and ball milling time of 6-12h.

7. The preparation method according to claim 1, characterized in that: The mass ratio of the mixture to lithium metal is (1.5-2):

1.

8. The preparation method according to claim 1, characterized in that: The washing liquid used for washing is selected from one or more of methanol and hydrochloric acid.

9. A silicon negative electrode material, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 8.

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