A coating method of a lithium ion battery positive electrode sheet

Abstract

This invention discloses a coating method for a positive electrode sheet of a lithium-ion battery, comprising the following steps: S1, preparing a positive electrode slurry; S2, coating the prepared positive electrode slurry onto the surface of a foil, and simultaneously performing electrospinning on the foil coated with the positive electrode slurry; S3, drying the foil containing the spun fibers and the positive electrode slurry before the spun fibers fall onto the foil surface, so that the spun fibers are suspended and fixed between the upper and lower surfaces of the positive electrode slurry. This invention rationally utilizes electrospinning technology, adding spun fibers during the coating process to form a double-layer structure, improving the overall structural connectivity of the electrode sheet, preventing electrode powder shedding, and simultaneously improving the electrode sheet's conductivity.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode coating technology, specifically relating to a coating method for a positive electrode sheet of a lithium-ion battery. Background Technology

[0002] Currently, lithium-ion batteries are pursuing high capacity and high rate performance. As the content of binders and conductive agents in the electrodes continues to decrease and the coating density continues to increase, the electrodes are prone to performance degradation such as coating powder shedding and cycle failure.

[0003] Currently, two coating methods are commonly used: one is to increase the conductivity and adhesion of the electrode by adding novel conductive agents and binders to the slurry, such as the coating process of a positive electrode slurry disclosed in CN106953066A. This patent improves the rate performance and capacity of lithium-ion batteries by adding novel conductive agents and binders to the slurry, but it is costly and the long development cycle of new materials affects product progress. The other method is to perform surface treatment on the foil, pre-coating a layer of conductive material before normal coating to ensure electrode performance, such as the preparation method of a lithium-ion battery disclosed in CN109103503A. This patent uses physical vapor deposition and pre-corrosion treatment processes on the foil to improve the energy density and cycle stability of lithium-ion batteries. Although surface treatment of the foil can improve electrode performance, the overall improvement is relatively limited and the process is cumbersome. Neither of these methods can effectively guarantee the improvement of production efficiency while ensuring product performance at present, so the development of new processes is necessary. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the present invention aims to provide a coating method for a positive electrode sheet of a lithium-ion battery, which makes reasonable use of electrospinning technology, adds spinning to form a double electric layer structure during the coating process, improves the overall structural connectivity of the electrode sheet, prevents the electrode sheet from shedding powder, and improves the electrode sheet conductivity.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0006] This invention provides a coating method for a positive electrode sheet of a lithium-ion battery, comprising the following steps:

[0007] S1. Prepare the positive electrode slurry;

[0008] S2. The prepared positive electrode slurry is coated onto the surface of the foil, and electrospinning is performed on the foil coated with the positive electrode slurry.

[0009] S3. Before the spun yarn falls onto the foil surface, the foil containing the spun yarn and the positive electrode slurry is dried so that the spun yarn is suspended and fixed between the upper and lower surfaces of the positive electrode slurry.

[0010] Preferably, the preparation method of the positive electrode slurry is as follows: the positive electrode material, the first conductive agent and the first binder are mixed evenly, and then dispersed in the first organic solvent and stirred evenly; wherein, the mass ratio of the positive electrode material, the first conductive agent and the first binder is (90-98):(1-5):(1-5).

[0011] More preferably, the positive electrode material is selected from any one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, ternary nickel cobalt manganese, or ternary nickel cobalt aluminum; the first conductive agent is selected from one or more of acetylene black, carbon black, metal fiber, vapor-grown carbon fiber, carbon nanotube, graphene and its mixed conductive slurry, and conductive graphite; the first binder is selected from one or more of polyvinylidene fluoride, polyacrylic acid, sodium carboxymethyl cellulose, and styrene-butadiene rubber; and the first organic solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and ethylene glycol dimethyl ether.

[0012] Preferably, the solid content of the positive electrode slurry is 64±3%.

[0013] Preferably, the coating speed of the positive electrode slurry is 15-25 m / min. The spinning process can be controlled by adjusting the belt speed to allow the spun fibers to fall onto the foil surface before entering the oven for setting.

[0014] Preferably, an electrospinning device is used for electrospinning, with the receiving distance controlled at 1-3 cm, the spinning voltage controlled at 15-20 KV, and the feeding speed controlled at 50-70 mL / h.

[0015] Preferably, at the spinning feed outlet, a mixed solution consisting of a second conductive agent, a second binder, and a second organic solvent is sprayed onto the spinning surface.

[0016] Preferably, the second conductive agent is selected from one or more of acetylene black, carbon black, metal fiber, vapor-grown carbon fiber, carbon nanotube, graphene and its mixed conductive slurry, and conductive graphite; the second binder is selected from one or more of polyvinylidene fluoride, polyacrylic acid, sodium carboxymethyl cellulose and styrene-butadiene rubber; and the second organic solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and ethylene glycol dimethyl ether.

[0017] More preferably, the second binder is the same as the first binder in the positive electrode slurry, and the second organic solvent is the same as the first organic solvent in the positive electrode slurry.

[0018] More preferably, the mass ratio of the second conductive agent, the second binder, and the second organic solvent is (85-95):(2-7):(3-8).

[0019] Preferably, the first half of the oven uses a heating range of 80-100℃, and the second half of the oven uses a cooling range of 100-80℃.

[0020] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0021] This invention makes reasonable use of electrospinning technology. During the coating process, spinning is added to form a double electric layer structure, so that the spinning is embedded in the middle of the positive electrode sheet, which improves the overall structural connectivity of the electrode sheet, prevents the electrode sheet from shedding powder, and improves the electrode sheet conductivity.

[0022] This invention involves uniformly spraying a solution composed of a second conductive agent, a second binder, and a second organic solvent onto the surface of an electrospun fiber. This improves the conductivity and connectivity of the fiber while allowing it to better integrate with the positive electrode components under gravity. In particular, when the second binder and the first binder are the same, and the second organic solvent and the first organic solvent are the same, the degree of integration between the electrospun fiber and the positive electrode components is even higher. Attached Figure Description

[0023] Figure 1 The images show cross-sectional views of the spinning electrodes prepared in Examples 1-3 of this invention.

[0024] Explanation of reference numerals in the attached diagram: 1-feeding outlet; 2-positive electrode slurry; 3-spinning; 4-foil. Detailed Implementation

[0025] 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.

[0026] 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.

[0027] Example 1

[0028] like Figure 1 As shown, this embodiment provides a coating method for a lithium-ion battery positive electrode sheet, including:

[0029] (1) The positive electrode material (ternary nickel-cobalt-manganese), the first conductive agent (acetylene black), and the first binder (polyvinylidene fluoride) are mixed evenly in a mass ratio of 95:3:2, and then dispersed in the first organic solvent (N-methylpyrrolidone) solution and stirred evenly. The overall solid content of the slurry is 64%.

[0030] (2) The prepared positive electrode slurry 1 is uniformly coated onto a foil 4 (aluminum foil) with a thickness of 12 μm at a coating speed of 20 m / min. At the same time, electrospinning 3 is performed on the rollers of the electrospinning device at a receiving distance of 2 cm, a voltage of 17 KV, and a feeding speed of 60 mL / h. At the feeding outlet 1, a mixed solution of a second organic solvent (N-methylpyrrolidone), a second binder (polyvinylidene fluoride), and a second conductive agent (carbon black) in a mass ratio of 90:5:5 is sprayed onto the spinning surface. The spinning surface falls from the outlet and enters the interior of the positive electrode slurry under the action of gravity.

[0031] (3) Before the spinning falls onto the foil surface, the foil with spinning and positive electrode slurry is dried so that the spinning is suspended and fixed between the upper and lower surfaces of the positive electrode slurry. The length of the oven is 30m. The heating range in the first half of the oven is 80-100℃, and the cooling range in the second half of the oven is 100-80℃.

[0032] Example 2

[0033] like Figure 1 As shown, this embodiment provides a coating method for a lithium-ion battery positive electrode sheet, including:

[0034] (1) The positive electrode material (lithium iron phosphate), the first conductive agent (acetylene black), and the first binder (polyacrylic acid) are mixed evenly in a mass ratio of 90:5:5, and then dispersed in the first organic solvent (N,N-dimethylformamide) solution and stirred evenly. The overall solid content of the slurry is 61%.

[0035] (2) The prepared positive electrode slurry is uniformly coated onto a 11µm thick copper foil at a coating speed of 15m / min. Simultaneously, electrospinning is performed on the rollers of the electrospinning device at a receiving distance of 1cm, a voltage of 15KV, and a feeding speed of 50mL / h. At the feeding outlet, a mixed solution of a second organic solvent (N,N-dimethylformamide), a second binder (sodium carboxymethyl cellulose), and a second conductive agent (vapor-grown carbon fiber) in a mass ratio of 85:7:8 is sprayed onto the spinning surface. The spinning fiber falls from the outlet and enters the interior of the positive electrode slurry under gravity.

[0036] (3) Before the spinning falls onto the foil surface, the foil with spinning and positive electrode slurry is dried so that the spinning is suspended and fixed between the upper and lower surfaces of the positive electrode slurry. The length of the oven is 30m. The heating range in the first half of the oven is 80-100℃, and the cooling range in the second half of the oven is 100-80℃.

[0037] Example 3

[0038] like Figure 1As shown, this embodiment provides a coating method for a lithium-ion battery positive electrode sheet, including:

[0039] (1) The positive electrode material (lithium cobalt oxide), the first conductive agent (carbon nanotubes), and the first binder (polyvinylidene fluoride) are mixed evenly in a mass ratio of 98:1:1, and then dispersed in the first organic solvent (ethylene glycol dimethyl ether) solution and stirred evenly. The overall solid content of the slurry is 67%.

[0040] (2) The prepared positive electrode slurry is uniformly coated onto a 13µm thick foil (aluminum foil) at a coating speed of 25m / min. Simultaneously, electrospinning is performed on the rollers of the electrospinning device at a receiving distance of 3cm, a voltage of 20KV, and a feeding speed of 70mL / h. At the feeding outlet, a mixed solution of a second organic solvent (ethylene glycol dimethyl ether), a second binder (styrene-butadiene rubber), and a second conductive agent (carbon black) in a mass ratio of 95:2:3 is sprayed onto the spinning surface. The spinning fibers fall from the outlet and enter the interior of the positive electrode slurry under gravity.

[0041] (3) Before the spinning falls onto the foil surface, the foil with spinning and positive electrode slurry is dried so that the spinning is suspended and fixed between the upper and lower surfaces of the positive electrode slurry. The length of the oven is 30m. The heating range in the first half of the oven is 80-100℃, and the cooling range in the second half of the oven is 100-80℃.

[0042] Experiment Example 4

[0043] This experimental example provides a method for applying a positive electrode sheet in a lithium-ion battery, including:

[0044] Take the positive electrode sheet of the lithium-ion battery prepared in Example 1, mix 5 kg of graphite (the main negative electrode material) and 0.03 kg of conductive agent SP evenly in a stirring tank, add 3.5 kg of water and 0.055 kg of thickener CMC to the stirring tank, pre-knead at a dispersion speed of 8 m / s for 30 min, and then disperse at a linear speed of 12 m / s for 60 min. At this time, add 0.23 kg of binder SBR and disperse at a linear speed of 8 m / s for 20 min. The slurry is ready to be discharged, coated, dried, and rolled to form the negative electrode sheet. Taking the total mass of the precursor solution as 100%, mix lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalate borate, and lithium hexafluorophosphate to obtain the precursor solution, wherein the concentrations of lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalate borate, and lithium hexafluorophosphate are 0.08 mol / L, 0.2 mol / L, and 0.1 mol / L, respectively. Ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate in a molar ratio of 1:1.2:1:1.13:0.9 were added to the precursor solution. The prepared positive electrode-separator-negative electrode were assembled into a battery using a traditional stacking process. Then, the electrolyte precursor was injected and the battery was sealed to prepare the battery.

[0045] Comparative Example 1

[0046] The only difference between this example and Example 1 is that there is no electrospinning process. The positive electrode sheet prepared in this example is used to make a battery, and the assembly method is the same as in Example 4.

[0047] Comparative Example 2

[0048] The only difference between this example and Example 1 is that the spraying of the mixed solution of the second organic solvent (NMP), the second binder (PVDF), and the second conductive agent (SP) onto the spinning surface at the feed outlet is omitted. The positive electrode sheet prepared in this example is used to fabricate a battery, and the assembly method is the same as in Example 4.

[0049] Comparative Example 3

[0050] The only difference between this example and Comparative Example 2 is that the spinning is fixed to the upper surface of the positive electrode slurry. The positive electrode sheet prepared in this example is used to make a battery, and the assembly method is the same as in Example 4.

[0051] Comparative Example 4

[0052] The only difference between this example and Comparative Example 2 is that the spinning is fixed to the lower surface of the positive electrode slurry. The positive electrode sheet prepared in this example is used to make a battery, and the assembly method is the same as in Example 4.

[0053] The present invention uses the batteries prepared in Experimental Example 4 and Comparative Examples 1-4 as examples to test the actual application effect of the cycle performance of each battery. The test results are shown in Table 1. The test method for cycle performance is as follows: charge at a constant current and constant voltage of 1C to 4.2V, cut off current of 0.05C, let stand for 5 minutes, then discharge at a constant current of 1C to 3.0V, let stand for 5 minutes after discharge, and record the discharge capacity. This is one cycle. Then repeat the above charge and discharge steps and record the capacity data for each cycle.

[0054] Table 1

[0055] Cyclic performance (%) Experiment Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 100-week capacity retention 95％ 90％ 93％ 93％ 93％ 200-week capacity retention 93％ 88％ 90％ 89％ 88％ 300-week capacity retention 91％ 85％ 88％ 85％ 86％

[0056] The present invention uses the positive electrode sheets prepared in Experimental Example 1 and Comparative Examples 1-4 as examples to test the adhesion force and film resistance of the positive electrode sheets. The test results are shown in Tables 2 and 3. The test method for adhesion force and film resistance is as follows: A 3cm wide and 20cm long positive electrode sheet is pasted onto a standard steel plate. The positive electrode powder on the surface of the foil is peeled off at a uniform speed of 1m / min using a tensile testing instrument. The force applied by the instrument is recorded, which is the adhesion force of the positive electrode sheet. The rolled film is cut into a rectangle of about 5cm × 10cm and placed between the two electrodes of the film resistance meter. The test pressure is set to 0.5T and the holding time is 5s in the software. The software automatically reads and records the resistance data.

[0057] Table 2

[0058] Adhesion force (N) Experiment Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Sample 1 0.60 0.48 0.55 0.52 0.53 Sample 2 0.59 0.52 0.54 0.51 0.52 Sample 3 0.59 0.51 0.55 0.51 0.53

[0059] Table 3

[0060]

[0061] Adding electrospinning to the central region of the positive electrode creates a "reinforced concrete structure," connecting the various positive electrode particles and increasing the electrode's adhesion. This strong connection prevents the positive electrode active material from detaching from the foil during the later stages of deep charge / discharge, thus preventing coating powder shedding. Furthermore, the "electric double layer structure" formed between the electrospinning and the positive electrode active material accelerates electron flow. Adding conductive and binder components to the electrospinning process further enhances its advantages. These benefits are evident in electrode adhesion testing, cycle life testing, and resistance performance testing.

[0062] The test data from Comparative Examples 1-4 show that, compared to suspending and fixing the spinning fibers in the middle of the positive electrode slurry, the present invention improves the cycle performance, adhesion performance, and membrane resistance performance by suspending and fixing the fibers in the middle of the positive electrode slurry. The test data from Comparative Examples 2 and Example 4 show that, by spraying a mixed solution composed of a second organic solvent, a second binder, and a second conductive agent onto the spinning surface, the present invention can significantly improve the cycle performance, adhesion performance of the positive electrode sheet, and membrane resistance performance of the battery.

[0063] 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 coating method for a positive electrode sheet of a lithium-ion battery, characterized in that, Includes the following steps: S1. Preparation of positive electrode slurry (2): Mix the positive electrode material, the first conductive agent and the first binder evenly, and then disperse them in the first organic solvent and stir evenly; S2. The prepared positive electrode slurry (2) is coated on the surface of the foil (4), and electrospinning is performed on the foil (4) coated with the positive electrode slurry (2). At the spinning feed outlet (1), a mixed solution composed of a second conductive agent, a second binder and a second organic solvent is sprayed and adhered to the surface of the spinning (3). The second binder is the same as the first binder, and the second organic solvent is the same as the first organic solvent. S3. Before the spinning (3) falls onto the surface of the foil (4), the foil (4) with the spinning (3) and the positive electrode slurry (2) is dried so that the spinning (3) is suspended and fixed between the upper and lower surfaces of the positive electrode slurry (2).

2. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, The mass ratio of the positive electrode material, the first conductive agent and the first binder is (90-98):(1-5):(1-5).

3. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 2, characterized in that, The positive electrode material is selected from any one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, ternary nickel cobalt manganese, or ternary nickel cobalt aluminum; the first conductive agent is selected from one or more of acetylene black, carbon black, metal fiber, vapor-grown carbon fiber, carbon nanotube, graphene and its mixed conductive slurry, and conductive graphite; the first binder is selected from one or more of polyvinylidene fluoride, polyacrylic acid, sodium carboxymethyl cellulose, and styrene-butadiene rubber; and the first organic solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and ethylene glycol dimethyl ether.

4. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, The solid content of the positive electrode slurry (2) is 64±3%.

5. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, The coating speed of the positive electrode slurry (2) is 15-25 m / min.

6. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, Electrospinning was performed using an electrospinning device, with the receiving distance controlled at 1-3 cm, the spinning voltage controlled at 15-20 KV, and the feeding speed controlled at 50-70 mL / h.

7. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, The second conductive agent is selected from one or more of acetylene black, carbon black, metal fiber, vapor-grown carbon fiber, carbon nanotube, graphene and its mixed conductive paste, and conductive graphite; the second binder is selected from one or more of polyvinylidene fluoride, polyacrylic acid, sodium carboxymethyl cellulose and styrene-butadiene rubber; the second organic solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, and ethylene glycol dimethyl ether.

8. The coating method for the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, The mass ratio of the second conductive agent, the second binder, and the second organic solvent is (85-95):(2-7):(3-8).

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