Positive plate and preparation method thereof, battery cell and battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive plate and a preparation method thereof, a battery cell and a battery, the preparation method of the positive plate comprises the following steps: S1, stirring and mixing a binder, a positive active material, a conductive agent and a solvent by a dry method to obtain positive slurry; s2, coating at least one side of a current collector with the positive electrode slurry to obtain a pole piece wet blank, and putting the pole piece wet blank into a multi-section oven for baking; wherein the baking temperature in the multi-section oven is sequentially divided into a first high-temperature section, a first low-temperature section, a second high-temperature section and a second low-temperature section. The positive plate provided by the invention can effectively improve baking cracking and improve the product percent of pass.

Description

Technical Field

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

[0002] In the positive electrode preparation process, when the solvent evaporates rapidly, the slurry layer shrinks unevenly inside and outside, generating stress. This will cause the electrode to crack to varying degrees during coating and baking, seriously restricting the qualification rate of the production line.

[0003] Existing methods for improving cracking usually involve designing the positive electrode slurry. For example, the invention with announcement number CN111029581A discloses a positive electrode slurry and a preparation method thereof, a positive electrode sheet and a preparation method thereof, a lithium-ion battery and its application. The positive electrode slurry is prepared by adding special additives, which can effectively improve the cracking of the electrode sheet and the appearance of black spots during the battery positive electrode preparation and coating process.

[0004] However, relying solely on slurry formulation improvements still has limitations: the introduction of additives may cause interfacial side reactions, affecting the battery's electrochemical performance, and it is difficult to fully adapt to different active material systems. Therefore, it is urgent to find a more universal solution to the baking cracking problem through baking process optimization. Summary of the Invention

[0005] To solve the above problems, the present invention provides a positive electrode sheet and a preparation method thereof, a battery cell, and a battery. The positive electrode sheet obtained by the preparation method can effectively improve baking cracking and improve the product qualification rate.

[0006] To achieve the above-mentioned purpose, the present invention is implemented through the following technical solutions: In a first aspect, a method for preparing a positive electrode sheet comprises the following steps: S1. A binder, a positive electrode active material, a conductive agent and a solvent are mixed by dry stirring to obtain a positive electrode slurry; S2. Coat the positive electrode slurry on at least one side of the current collector to obtain a wet electrode blank, and place the wet electrode blank into a multi-section oven for baking; wherein the baking temperature in the multi-section oven is divided into a first high temperature section, a first low temperature section, a second high temperature section, and a second low temperature section in sequence.

[0007] The present invention prepares the positive electrode slurry by dry stirring, and combines it with the design of a "high-low-high-low" temperature gradient in the baking oven to achieve the effect of improving the cracking of the electrode. Specifically, since the occurrence of cracking is due to the stress generated by the uneven volatilization of the solvent inside the coating exceeding the strength limit of the coating, the positive electrode slurry prepared by the dry stirring method of the present invention has a smaller total amount of solvent and has better stability and leveling properties than the slurry prepared by the wet method, providing a basis for subsequent temperature gradient changes; the first high-temperature section can quickly remove most of the surface and shallow solvents, and the second low-temperature section balances the evaporation rate with the binder migration rate through temperature callback, reducing thermal stress accumulation. The second high-temperature section can further remove the deep residual solvent and the bound solvent, and finally relax and homogenize the stress through the second low-temperature section, ensuring the complete removal of trace solvents and making the internal structure of the coating more uniform and stable. Therefore, the high-low-high-low temperature gradient can optimize the solvent volatilization path, avoid the formation of a surface crust, and promote internal diffusion. The two synergistically significantly reduce the internal stress level and stress concentration risk of the coating, enhance the toughness of the coating structure, and effectively inhibit cracking.

[0008] Preferably, the multi-section oven is divided into 10 sections, sections 1 to 2 are the first high temperature section, sections 3 to 5 are the first low temperature section, sections 6 to 8 are the second high temperature section, and sections 9 to 10 are the second low temperature section.

[0009] Preferably, the temperatures of ovens in sections 1 to 10 are respectively: 130-135°C, 130-135°C, 110-120°C, 110-120°C, 110-120°C, 130-135°C, 130-135°C, 130-135°C, 110-120°C, and 110-120°C.

[0010] Preferably, the wind frequency in the multi-section oven is divided into a first low frequency band, a high frequency band and a second low frequency band in sequence.

[0011] The present invention further proposes a "low-high-low" wind frequency gradient design. Since the surface structure has just begun to form at this time, the first low-frequency band with small air volume can significantly reduce the wind shear stress and excessive convective heat transfer on the coating surface, thereby promoting uniform heating of the surface and uniform volatilization of the solvent; after the coating has undergone initial baking, the surface structure has been initially strengthened, and the high-frequency band can efficiently remove residual solvents that have migrated to the surface or deeper layers; in the later stage of baking, most of the solvent has been removed and the coating is close to a dry state. The design of the second low-frequency band avoids destroying the formed structure, thereby achieving more uniform and slow cooling and final drying.

[0012] Preferably, sections 1 to 5 are the first low frequency band, sections 6 to 9 are the high frequency band, and section 10 is the second low frequency band.

[0013] The lower wind frequency of sections 1 to 5 is used to protect the first high-temperature section and the first low-temperature section from the mechanical impact of strong winds during the initial stage and critical stress relaxation period of baking respectively; sections 6 to 8 combine high-frequency wind and high-temperature baking to overcome difficult-to-remove residual solvents with the highest efficiency and ensure the depth of drying; the low-frequency wind of section 10 (the end of the second low-temperature section) eliminates the risk of mechanical stress in the final stage when the structure is finalized and vulnerable to damage, ensuring the integrity and uniformity of the final structure.

[0014] Preferably, the fan frequencies of ovens in sections 1 to 10 are: 15~20Hz, 15~20Hz, 15~20Hz, 15~20Hz, 22~25Hz, 22~25Hz, 22~25Hz, 15~20Hz respectively.

[0015] Preferably, the positive electrode slurry is obtained by mixing a binder, a positive electrode active material, a conductive agent and a solvent by dry stirring, comprising: Add the binder, conductive agent and positive electrode active material to a double planetary stirring tank in sequence, stir and disperse to obtain powder; add the powder to a solvent, stir to obtain a first slurry; scrape the slurry, stir and disperse, and scrape the slurry again to obtain a second slurry; add the solvent to the second slurry to adjust the viscosity, stir and disperse, and obtain a positive electrode slurry.

[0016] Preferably, the binder comprises polyvinylidene fluoride (PVDF).

[0017] Preferably, the conductive agent includes superconducting carbon black (SP).

[0018] Preferably, the positive electrode active material includes lithium iron phosphate.

[0019] Preferably, the solvent comprises N-methylpyrrolidone (NMP).

[0020] In a second aspect, the present invention further provides a positive electrode sheet prepared by the positive electrode sheet preparation method described above.

[0021] In a third aspect, the present invention further provides a battery cell comprising the positive electrode sheet as described above.

[0022] In a fourth aspect, the present invention further provides a battery comprising the battery cell described above.

[0023] The beneficial effects of the present invention are: 1. The present invention adopts dry stirring to prepare the positive electrode slurry, reducing the total amount of solvent and building a stable initial network, laying the foundation for stress reduction; combined with the "high-low-high-low" temperature gradient design, it optimizes the solvent volatilization path, avoids the formation of surface hard shells, promotes internal diffusion, and provides a critical stress relaxation period. The two synergistically significantly reduce the internal stress level and stress concentration risk of the coating, enhance the toughness of the coating structure, and thus effectively inhibit cracking.

[0024] 2. The present invention further proposes a "low-high-low" wind frequency design. By dynamically adjusting the air volume at different baking stages, on the one hand, it protects the coating in the fragile period from mechanical damage and local over-drying, and on the other hand, it enhances mass transfer and improves efficiency at the appropriate stage, further improving the cracking phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the cracked electrode in Comparative Example 1. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only a portion of the embodiments of the present invention, rather than all of the embodiments. Therefore, all other embodiments derived by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0027] Example 1 S1. Dry preparation of positive electrode slurry: 21.12 kg PVDF, 8.89 kg SP, 540.9 kg lithium iron phosphate, and 540.9 kg lithium iron phosphate were added to a double planetary mixing tank in sequence, stirred at 30 rpm and dispersed at 500 rpm for 60 min to obtain a powder; the powder was added to 342 kg NMP, stirred at 15 rpm for 30 min, and then stirred at 25 rpm for 30 min to obtain a first slurry; scraped, stirred at 30 rpm and dispersed at 500 rpm for 60 min, and scraped again to obtain a second slurry; 284 kg NPM was added to the second slurry to adjust the viscosity, stirred at 30 rpm and dispersed at 1200 rpm for 180 min to obtain a positive electrode slurry.

[0028] S2. Coating and baking: Coat the positive electrode slurry on both sides of the current collector to obtain a wet electrode embryo; place the wet electrode embryo into a multi-section oven for baking. The multi-section oven is divided into 10 sections, and the temperature and wind frequency settings of the ovens in sections 1-10 are shown in Table 1.

[0029] Table 1 Temperature parameters and wind frequency parameters of ovens in sections 1-10 of Example 1 Preparation of battery cells The positive electrode sheet obtained in Example 1 was cut and stacked with the negative electrode sheet and the separator, and then wound to form a battery cell; wherein the negative electrode sheet was formed by coating the negative electrode active material artificial graphite, the binder sodium carboxymethyl cellulose (CMC) and the conductive polymer PEDOT:PSS on copper foil and baking it, the mass ratio of artificial graphite, CMC and PEDOT:PSS was 97:2:1, and the separator was a 12μm thick PE-based ceramic coating separator.

[0030] Battery preparation The encapsulated and injected cells are placed in a formation machine and first charged at a constant current of 0.05C to 3.65V. The cells are then switched to a constant voltage of 3.65V until the current decays to ≤0.01C. After 10 minutes of stabilization, they are discharged at a constant current of 0.1C to 2.5V to complete the formation process. The cells are then placed in a 45°C aging environment for 48 hours. The cells are then charged at a constant current of 0.5C to 3.65V and then maintained at a constant voltage until the current is ≤0.01C. The cells are then immediately discharged at a constant current of 0.5C to 2.5V for capacity grading. Finally, the cells are graded according to the capacity grading data and, after venting and resealing, are finished batteries.

[0031] Comparative Example 1 It is basically the same as Example 1, except that: S1. Wet preparation of positive electrode slurry: Under the condition of stirring rate of 30 rpm, 21.12 kg PVDF was added to 411.8 kg NMP, and dispersed at 500 rpm for 30 min to obtain a mixed glue; 8.89 kg SP was added to the mixed glue, stirred and dispersed at 1200 rpm for 45 min to form a conductive slurry; 540.9 kg and 540.9 kg of lithium iron phosphate were added to the conductive slurry in two batches, stirred and dispersed at 500 rpm for 30 min, and then 159.9 kg NMP was added, stirred and dispersed at 1200 rpm for 10 min to obtain the first slurry; scraped, stirred and dispersed at 1200 rpm for 180 min to obtain the second slurry; 130 kg NPM was added to the second slurry to adjust the viscosity, stirred and dispersed at 1200 rpm for 30 min to obtain the positive electrode slurry.

[0032] Comparative Example 2 It is basically the same as Example 1, except that: S2. Coating and baking: Coat the positive electrode slurry on both sides of the current collector to obtain a wet electrode embryo; place the wet electrode embryo into a multi-section oven for baking. The multi-section oven is divided into 10 sections, and the temperature and wind frequency settings of the ovens in sections 1-10 are shown in Table 2.

[0033] Table 2 Temperature parameters and wind frequency parameters of ovens 1-10 in comparative example 2 Comparative Example 3 It is basically the same as Example 1, except that: S2. Coating and baking: Coat the positive electrode slurry on both sides of the current collector to obtain a wet electrode embryo; place the wet electrode embryo into a multi-section oven for baking. The multi-section oven is divided into 10 sections, and the temperature and wind frequency settings of the ovens in sections 1-10 are shown in Table 3.

[0034] Table 3 Temperature parameters and wind frequency parameters of ovens 1-10 in Comparative Example 3 Performance Testing The electrode quality inspection was performed on Example 1 and Comparative Examples 1-3. The electrode quality inspection included visual inspection and weight measurement. The results are shown in Table 4.

[0035] Table 4 Electrode quality test results of Example 1 and Comparative Examples 1-3 Figure 1 The schematic diagram of the electrode in Comparative Example 1 is shown in Table 4 and Figure 1 As can be seen, Comparative Example 1, which used a wet process, Comparative Example 2, which used a "low-high-low" temperature gradient design, and Comparative Example 3, which used a "high-low-high" wind frequency gradient design, all experienced cracking and had high failure rates. Furthermore, the higher areal densities of Comparative Examples 2 and 3 indicate a large number of residual bubbles (the solvent was unable to escape, forming pores).

Claims

1. A method for preparing a positive electrode sheet, characterized in that: The steps include: S1. A binder, a positive electrode active material, a conductive agent and a solvent are mixed by dry stirring to obtain a positive electrode slurry; S2. Coat the positive electrode slurry on at least one side of the current collector to obtain a wet electrode blank, and place the wet electrode blank into a multi-section oven for baking; wherein the baking temperature in the multi-section oven is divided into a first high temperature section, a first low temperature section, a second high temperature section, and a second low temperature section in sequence.

2. The method for preparing a positive electrode sheet according to claim 1, wherein: The multi-section oven is divided into 10 sections, sections 1 to 2 are the first high temperature section, sections 3 to 5 are the first low temperature section, sections 6 to 8 are the second high temperature section, and sections 9 to 10 are the second low temperature section.

3. The method for preparing a positive electrode sheet according to claim 2, wherein: The temperatures of the ovens in sections 1 to 10 are: 130~135℃, 130~135℃, 110~120℃, 110~120℃, 110~120℃, 130~135℃, 130~135℃, 130~135℃, 110~120℃, 110~120℃.

4. The method for preparing a positive electrode sheet according to claim 1, wherein: The wind frequency in the multi-section oven is divided into a first low frequency band, a high frequency band and a second low frequency band in sequence.

5. The method for preparing a positive electrode sheet according to claim 5, wherein: The multi-section oven is divided into 10 sections, sections 1 to 5 are the first low frequency section, sections 6 to 9 are the high frequency section, and section 10 is the second low frequency section.

6. The method for preparing a positive electrode sheet according to claim 6, wherein: The fan frequencies of ovens in sections 1 to 10 are: 15~20Hz, 15~20Hz, 15~20Hz, 15~20Hz, 22~25Hz, 22~25Hz, 22~25Hz, 15~20Hz.

7. The method for preparing a positive electrode sheet according to claim 1, wherein: The positive electrode slurry is obtained by mixing a binder, a positive electrode active material, a conductive agent and a solvent by dry stirring, including: Add the binder, conductive agent and positive electrode active material to a double planetary stirring tank in sequence, stir and disperse to obtain powder; add the powder to a solvent, stir to obtain a first slurry; scrape the slurry, stir and disperse, and scrape the slurry again to obtain a second slurry; add the solvent to the second slurry to adjust the viscosity, stir and disperse, and obtain a positive electrode slurry.

8. A positive electrode sheet, characterized in that: The positive electrode sheet is prepared by the method for preparing the positive electrode sheet according to any one of claims 1 to 7.

9. A battery cell, characterized in that: Comprising the positive electrode sheet as claimed in claim 8.

10. A battery, characterized in that: Comprising the battery cell as claimed in claim 9.

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