Lithium ion battery negative plate and preparation method thereof

Abstract

The invention discloses a lithium ion battery negative plate and a preparation method thereof, and the preparation method comprises the following steps: dispersing an active substance, a conductive agent, a binder and a first solvent to obtain a suspension; defoaming the suspension in vacuum and then coating the surface of a current collector with the suspension to form a wet film; transferring the current collector on which the wet film is formed into a second solvent, standing and curing to form an active layer to obtain a cured pole piece; and taking out the cured pole piece and drying to obtain the lithium ion battery negative pole piece. Compared with the traditional solvent evaporation, the porous negative electrode prepared by the phase inversion method provided by the invention has a gradient pore structure, and the structure can effectively improve the porosity and tortuosity of the negative electrode plate, improve the electrolyte infiltration rate, and improve the rate capability and capacity retention rate of the battery at the same time; the method is simple in preparation process and suitable for large-scale production.

Description

Technical Field

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

[0002] With the rapid development of global economy and technology, human demand for energy is increasing day by day. Lithium-ion batteries have a series of excellent characteristics such as small size, light weight, long cycle life and environmental protection, and have gained wide attention and application in the fields of electronics, electrical and new energy vehicles.

[0003] Especially in the automotive field, due to the limited space for accommodating batteries, higher requirements are placed on the quality and volume energy density of batteries. Under the condition of fixed material system and single cell size, there are two main ways to improve the energy density of lithium-ion batteries: 1. Improve the energy density by increasing the lightweight level of battery cells; 2. Improve the energy density by increasing the percentage of active materials and reducing the proportion of current collectors. However, with the increase of solid content, the rate performance and cycle performance of lithium ions show a significant decline. This is mainly attributed to the fact that the electrodes of lithium-ion batteries are mainly composed of disordered porous structures composed of active material particles, binders and conductive agents. The disordered pores have a high tortuosity factor, which affects the Li + The diffusion rate in it is easy to cause concentration polarization. Therefore, improving the compaction of the electrode while not weakening the diffusion of lithium ions is the key direction involved in battery electrodes.

[0004] To this end, many workers in the field of lithium-ion batteries have disclosed a large number of new technologies to improve the design of thick pole pieces. The Chinese patent application document with publication number CN111312985A discloses a pole piece with a graded porosity distribution and a preparation method thereof. A double-layer coating method is used to coat the active material, conductive agent, binder, pore-forming agent and solvent in a certain ratio to make a slurry, which is sequentially coated on the foil. The entire preparation requires multiple coating, drying and rolling, and the process operation is complicated. Therefore, optimizing the electrode microstructure to eliminate the diffusion resistance of lithium ions is a research direction with great commercial prospects. Summary of the invention

[0005] The technical problem to be solved by the present invention is how to simply improve the rate performance of lithium-ion batteries.

[0006] The present invention solves the above technical problems through the following technical means:

[0007] A method for preparing a negative electrode sheet for a lithium ion battery comprises the following steps:

[0008] S1, dispersing the active material, the conductive agent, the binder and the first solvent to obtain a suspension;

[0009] S2, vacuum degassing the suspension and applying it on the surface of the current collector to form a wet film;

[0010] S3, transferring the current collector having formed a wet film into a second solvent and allowing it to stand and solidify to form an active layer to obtain a solidified electrode;

[0011] S4, taking out the solidified electrode sheet and drying it to obtain the negative electrode sheet of the lithium ion battery.

[0012] Preferably, in S1, the mass ratio of the active material, the conductive agent, and the binder is 80-96:0.5-10:3.5-10.

[0013] Preferably, in S1, the mass proportion of the first solvent in the suspension is 40% to 60%.

[0014] Preferably, in S1, the binder is a high molecular polymer that is soluble in the first solvent and insoluble in the second solvent.

[0015] Preferably, in S1, the active material is at least one of graphite, carbon microspheres, lithium titanate, silicon oxygen and silicon carbon; the binder is at least one of polysulfone, polyethersulfone, polyvinylidene fluoride and polytetrafluoroethylene; and the conductive agent is SP.

[0016] Preferably, in S1, the first solvent is selected from at least one of N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide and N,N-dimethylformamide.

[0017] Preferably, in S1, dispersion is performed by one or more of magnetic stirring, ball milling and ultrasonic dispersion.

[0018] Preferably, in S3, the second solvent is at least one of water, methanol, ethanol and isopropanol.

[0019] The present invention also provides a lithium ion battery negative electrode sheet, which is prepared by adopting the preparation method of the lithium ion battery negative electrode sheet.

[0020] Preferably, the active layer on the surface of the current collector has gradient pores, and the pore size of the gradient pores gradually decreases in a direction away from the current collector.

[0021] The traditional lithium-ion battery negative electrode preparation process is to use solvent evaporation to remove the solvent in the wet film, and the negative electrode structure formed is winding and disordered. The present invention discloses a method for preparing a negative electrode sheet for a lithium-ion battery, firstly, the active material, the conductive agent, the binder and the first solvent are dispersed in a certain proportion to obtain a uniformly dispersed suspension; vacuum degassing is performed, and the suspension is coated on the surface of the current collector; then the wet film is transferred into a second solvent, and the first solvent and the second solvent are exchanged with each other to undergo phase inversion to obtain a solidified lithium-ion battery negative electrode sheet. Compared with traditional solvent evaporation, the porous negative electrode prepared by the phase inversion method provided by the present invention has a gradient pore structure, and the gradient pore diameter gradually decreases in the direction away from the current collector. This structure can effectively improve the porosity and tortuosity of the negative electrode sheet, so that the battery can increase the electrolyte infiltration rate after liquid injection, and at the same time increase the lithium ion diffusion rate during charging and discharging, reduce the concentration polarization in the battery, and improve the rate performance by 40% at a large rate.

[0022] The preparation method of the present invention only requires adding a solution exchange process to the traditional coating process, and has the advantages of simple equipment investment, convenient operation, low cost, etc., and is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a process flow chart of a method for preparing a negative electrode sheet for a lithium-ion battery according to an embodiment of the present invention;

[0024] Figure 2 This is a SEM photo of the cross section of the negative electrode sheet prepared in Example 3 of the present invention;

[0025] Figure 3 This is a SEM photo of the cross section of the negative electrode sheet prepared in Example 4 of the present invention;

[0026] Figure 4 This is a SEM photo of the cross section of the negative electrode sheet prepared in Example 7 of the present invention;

[0027] Figure 5 This is a SEM photo of the cross section of the negative electrode sheet prepared in Comparative Example 1 of the present invention;

[0028] Figure 6 The cycle performance of batteries assembled from negative electrode sheets prepared by the traditional preparation method in Comparative Example 1 of the present invention and the preparation method provided in Example 3 of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the test materials and reagents used in the following examples can be obtained from commercial sources.

[0031] If no specific techniques or conditions are specified in the examples, they can be carried out according to the techniques or conditions described in the literature in the art or according to the product instructions.

[0032] The present invention provides a method for preparing a negative electrode sheet for a lithium ion battery, and the specific process flow chart is as follows: Figure 1 As shown, the specific embodiments are described as follows:

[0033] Example 1

[0034] A method for preparing a negative electrode sheet of a lithium ion battery comprises the following steps:

[0035] (1) Graphite, conductive agent SP, and binder polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 80:10:10, and N-methylpyrrolidone was used as a solvent with a designed solid content of 50%. The mixture was then put into a ball mill and ball-milled at 500 rpm for 4 h to obtain a uniformly mixed slurry;

[0036] (2) After vacuum degassing the uniformly mixed slurry for 30 minutes, the uniformly mixed slurry is coated on a copper foil to form a wet film;

[0037] (3) Immerse the copper foil with the wet film in deionized water at 25° C. and let it stand for 30 minutes to completely cure it to obtain a cured electrode;

[0038] (4) The cured electrode sheet is placed in an oven at 70-100° C. and dried for 4 hours, and then vacuum dried for 6 hours to obtain a graphite negative electrode, namely, the negative electrode sheet of the lithium-ion battery.

[0039] Embodiment 2-7

[0040] The difference between Examples 2-4 and Example 1 is that the mass ratios of graphite, conductive agent SP and PVDF are different; the difference between Examples 5-6 and Example 3 is that the solid content is different; the difference between Example 7 and Example 3 is that the second solvent is different, and the specific differences are shown in Table 1.

[0041] Table 1

[0042]

[0043] Comparative Example 1

[0044] A method for preparing a negative electrode sheet of a lithium ion battery comprises the following steps:

[0045] (1) Graphite, conductive agent SP, and binder polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 95:1:4, with N-methylpyrrolidone as solvent and a designed solid content of 50%, and then put into a ball mill and ball milled at 500 rpm for 4 h to obtain a uniformly mixed slurry;

[0046] (2) After the uniformly mixed slurry is subjected to vacuum degassing for 30 minutes, the uniformly mixed slurry is coated on a copper foil to form a wet film; the slurry is placed in an oven at 70 to 100° C. and dried for 24 hours, and then vacuum dried for 6 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0047] The negative electrode sheets prepared in the above examples and comparative examples were assembled into button-type batteries and subjected to the following room temperature rate discharge performance test. The test results are shown in Table 2.

[0048] Specific capacity test

[0049] The lithium-ion battery was charged to 3.65V at 0.5C constant current and constant voltage, and then discharged to 2.0V at 0.33C / 0.5C / 1C / 2C constant current, and the specific capacity was calculated.

[0050] The negative electrode sheets prepared in the above Example 3 and Comparative Example 1 were assembled into button-type batteries and the following room temperature cycle performance tests were performed. The test results are shown in Figure 6 .

[0051] Normal temperature cycle performance test

[0052] The lithium-ion battery was charged to 3.65V at 1C constant current and constant voltage, and discharged to 2.0V at 1C current for a long cycle test.

[0053] Table 2

[0054]

[0055]

[0056] By comparing Examples 1-7 with Comparative Example 1, optimizing the pore structure of the electrode and reducing the pore tortuosity factor will significantly reduce the liquid phase diffusion resistance of lithium ions and improve the battery specific capacity.

[0057] The cross-sectional morphologies of the negative electrode fragments prepared in Examples 3, 4, and 7 are as follows: Figures 2 to 4 As shown in Figure 2, we can observe the gradient pores from top to bottom, and the pore size of the gradient pores gradually decreases away from the current collector, which is consistent with the Figure 5The structure of the conventional negative electrode sheet prepared in Comparative Example 1 shown is significantly different.

[0058] The long cycle test results of the button batteries corresponding to the negative electrodes prepared in Example 3 and Comparative Example 1 are as follows: Figure 6 As shown, due to the presence of gradient pores, the concentration polarization of lithium ion transmission is effectively reduced, which facilitates the insertion and extraction of lithium ions in the negative electrode, reduces the formation of lithium dendrites, and significantly improves the cycle performance of the battery.

[0059] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a negative electrode sheet for a lithium ion battery, characterized in that: The following steps are involved: S1, dispersing the active material, the conductive agent, the binder and the first solvent to obtain a suspension; S2, vacuum degassing the suspension and applying it on the surface of the current collector to form a wet film; S3, transferring the current collector having formed a wet film into a second solvent and allowing it to stand and solidify to form an active layer to obtain a solidified electrode; S4, taking out the solidified electrode sheet and drying it to obtain the negative electrode sheet of the lithium ion battery.

2. The method for preparing a negative electrode sheet for a lithium ion battery according to claim 1, characterized in that: In S1, the mass ratio of the active material, the conductive agent, and the binder is 80 to 96: 0.5～10：3.5～10。 3. The method for preparing a negative electrode sheet for a lithium-ion battery according to claim 1, characterized in that: In S1, in the suspension, the mass proportion of the first solvent is 40% to 60%.

4. The method for preparing a negative electrode sheet for a lithium-ion battery according to claim 1, characterized in that: In S1, the binder is a high molecular polymer that is soluble in the first solvent and insoluble in the second solvent.

5. The method for preparing a negative electrode sheet for a lithium-ion battery according to claim 1, characterized in that: In S1, the active material is at least one of graphite, carbon microspheres, lithium titanate, silicon oxygen and silicon carbon; the binder is at least one of polysulfone, polyethersulfone, polyvinylidene fluoride and polytetrafluoroethylene; and the conductive agent is SP.

6. The method for preparing a negative electrode sheet for a lithium-ion battery according to claim 1, characterized in that: In S1, the first solvent is selected from at least one of N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide and N,N-dimethylformamide.

7. The method for preparing a negative electrode sheet for a lithium-ion battery according to claim 1, characterized in that: In S1, dispersion is performed by using one or more of magnetic stirring, ball milling and ultrasonic dispersion.

8. The method for preparing a negative electrode sheet for a lithium ion battery according to any one of claims 1 to 7, characterized in that: In S3, the second solvent is at least one of water, methanol, ethanol and isopropanol.

9. A negative electrode sheet for a lithium-ion battery, characterized in that: The negative electrode sheet of a lithium-ion battery is prepared by the method for preparing the negative electrode sheet of a lithium-ion battery as described in any one of claims 1 to 8.

10. The negative electrode sheet for a lithium-ion battery according to claim 9, characterized in that: The active layer on the surface of the current collector has gradient pores, and the pore size of the gradient pores gradually decreases in the direction away from the current collector.

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