Method for improving initial capacity of battery cell pole piece and battery cell pole piece

Abstract

The application provides a method for improving initial capacity of an electrode sheet of a battery cell and the electrode sheet, and comprises the following steps: rolling the electrode sheet by using a first pressure value, so that the electrode sheet reaches a limit compaction state; scanning the electrode sheet in the limit compaction state, and selecting the electrode sheet containing large-grained lithium iron phosphate; adjusting the pressure of the rolling, so that the pressure of the rolling is adjusted from the first pressure value to a second pressure value; rolling the electrode sheet containing the large-grained lithium iron phosphate by using the second pressure value, so that the large-grained lithium iron phosphate is crushed, and the initial capacity of the electrode sheet is improved; the large-grained lithium iron phosphate in the electrode sheet is crushed by increasing the pressure of the rolling of the electrode sheet, so that the diffusion distance between lithium ions is reduced, the initial capacity of the electrode sheet is increased, and the capacity climbing phenomenon in the subsequent production process of the electrode sheet is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a method for improving initial capacity of electrode sheet of battery cell and electrode sheet of battery cell. BACKGROUND

[0002] Ferrous lithium phosphate (LFP) as the most important lithium battery positive electrode material, is more and more widely used in electric vehicles, in order to further improve the energy density of LFP, the development of high density of LFP material becomes the main development direction of everyone. Now the powder compaction density of LFP can be > 2.50g / cc, even some manufacturers have reached 2.70g / cc. People usually adopt the method of mixing different particle sizes to realize high compaction density. This mixing can be naturally formed by temperature gradient in the sintering process, or LFP with different particle sizes is prepared and mixed, and the compaction density of the material is improved by using the dense packing of different particle sizes in space, and the typical results are as shown in Figure 1

[0003] Although the current high compaction density of LFP can improve the energy density of the battery, because it introduces a larger particle size, the capacity and rate will be greatly reduced, especially the initial capacity will be much lower, and the capacity will gradually climb in the subsequent cycle. The low capacity phenomenon in the initial state, capacity climbing, as shown in Figure 2

[0004] At present, the methods for improving the capacity climbing phenomenon generally mainly have two kinds, the first is to dope LFP particle size to improve its rate performance, and the second is to accurately control the size of LFP material for mixing. These two methods will have the shortcomings of complex preparation method, high equipment requirement and high cost. In order to solve the above problems, the present application provides a method for improving the initial capacity of the electrode sheet of the battery cell and the electrode sheet of the battery cell. SUMMARY

[0005] The present application aims to provide a method for improving the initial capacity of the electrode sheet of the battery cell and the electrode sheet of the battery cell, which is used to improve the problem of capacity climbing in the production and preparation process of the electrode sheet of the battery cell.

[0006] In order to achieve the above purpose, the present application provides a method for improving the initial capacity of the electrode sheet of the battery cell, which comprises the following steps:

[0007] The first pressure value is used to roll the electrode sheet of the battery cell, so that the electrode sheet of the battery cell reaches the limit compaction state;

[0008] The electrode sheet of the battery cell in the limit compaction state is scanned, and the electrode sheet of the battery cell containing large particle ferrous lithium phosphate is selected;​​

[0009] adjusting the pressure of the rolling, such that the pressure of the rolling is increased from the first pressure value to a second pressure value;

[0010] applying the rolling to the electrode sheet containing the large particle lithium iron phosphate at the second pressure value to break the large particle lithium iron phosphate, such that the initial capacity of the electrode sheet is increased;

[0011] wherein the electrode sheet in the limit compaction state has a compaction density between 2.65 and 2.70 g / cc; and the large particle in the high compaction LFP has a particle size between 1 and 2 μm.

[0012] Optionally, the first pressure value is between 6.0 and 8.0 tons.

[0013] Optionally, in the limit compaction state, the electrode sheet has an electrical resistance between 40 and 200 mΩ.

[0014] Optionally, the second pressure value is between 8.0 and 9.0 tons, and the second pressure value includes 8 tons.

[0015] Optionally, in the large particle lithium iron phosphate broken state, the electrode sheet has a compaction density between 2.70 and 2.71 g / cc, and the electrode sheet has an electrical resistance between 67 and 350 mΩ.

[0016] Optionally, the electrode sheet includes lithium iron phosphate, a binder, and a conductive agent, wherein the lithium iron phosphate and the conductive agent are aggregated together by the binder.

[0017] after the applying the rolling to the electrode sheet containing the large particle lithium iron phosphate at the second pressure value;

[0018] continuing to increase the pressure of the rolling, such that the pressure of the rolling is changed from the second pressure value to a third pressure value;

[0019] applying the rolling to the electrode sheet after the large particle lithium iron phosphate is broken at the third pressure value, such that small particle lithium iron phosphate is broken, and the binder and the conductive agent are both detached from the electrode sheet.

[0020] wherein the small particle in the lithium iron phosphate has a particle size less than 1 μm.

[0021] Optionally, the third pressure value is greater than 9 tons, and the third pressure value includes 9 tons.

[0022] Optionally, the compacted density of the electrode sheet is between 2.70 and 2.71 g / cc when the small particle lithium iron phosphate is broken, and the resistance of the electrode sheet is more than 350 mΩ.

[0023] Optionally, an electrode sheet is prepared by the method for improving the initial capacity of the electrode sheet.

[0024] The method for improving the initial capacity of the electrode sheet and the electrode sheet provided by the application have the following beneficial effects:

[0025] By increasing the pressure of the electrode sheet rolling, the large particle lithium iron phosphate in the electrode sheet is broken, thereby reducing the diffusion distance between lithium ions and reducing the contact resistance, so as to increase the initial capacity of the electrode sheet, and further improve the capacity climbing phenomenon in the subsequent production process of the electrode sheet; and the method is simple and easy to operate, which can not only improve the initial capacity and rate performance of the electrode sheet, but also improve the cycle performance of the electrode sheet. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The laser particle size distribution and the scanning electron microscope diagram of the high compacted lithium iron phosphate material in the application;

[0027] Figure 2 The capacity climbing diagram of the high compacted lithium iron phosphate material in the application;

[0028] Figure 3 The flowchart of the embodiment of the application;

[0029] Figure 4 The compacting window diagram of the electrode sheet rolling in the application;

[0030] Figure 5 The scanning electron microscope diagram of the electrode sheet in the non-limiting compacted state in the application;

[0031] Figure 6 The scanning electron microscope diagram of the large particle lithium iron phosphate in the broken state in the application;

[0032] Figure 7 The capacity climbing diagram under different rolling pressures in the application. DETAILED DESCRIPTION

[0033] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application. Unless otherwise defined, the technical terms or scientific terms used herein should be understood as the common meanings by those skilled in the art. The similar words such as "comprise" used herein mean that the elements or objects before the words cover the elements or objects listed after the words and their equivalents, and do not exclude other elements or objects.

[0034] In view of the problems in the prior art, one of the embodiments of the present application provides a method for improving the initial capacity of an electrode sheet of a battery cell, as shown in Figure 3 The method comprises the following steps:

[0035] S01: The electrode sheet of the battery cell is rolled at a first pressure value, so that the electrode sheet reaches a limit compaction state.

[0036] During the production and preparation process of the electrode sheet of the battery cell, the rolling needs to be performed on the electrode sheet, so that the compaction density of the electrode sheet is continuously increased. In this step, the rolling is performed on the electrode sheet at the first pressure value, so that the electrode sheet can quickly reach the limit compaction state. In the limit compaction state, the compaction density of the electrode sheet is between 2.65 and 2.70 g / cc. It should be noted that the limit compaction state of the electrode sheet of the battery cell is the maximum compaction density that the electrode sheet can reach, and at this density, the large particle lithium iron phosphate does not break.

[0037] S02: Scanning the electrode sheet in the limit compaction state to select the electrode sheet containing large particle lithium iron phosphate.

[0038] Specifically, the cross section of the electrode sheet in the limit compaction state is scanned by a scanning electron microscope, and the electrode sheet with a good cross section state and the large particle lithium iron phosphate not broken is selected. In this step, the particle size of the large particle lithium iron phosphate is between 1 and 2 μm.

[0039] S03: Adjusting the pressure of the rolling, so that the pressure of the rolling is adjusted from the first pressure value to a second pressure value.

[0040] At the first pressure value, the electrode tab reaches a maximum limit, at which the large particle lithium iron phosphate is not broken. The first pressure value is raised to the second pressure value. The pressure of the second pressure value exceeds the maximum pressure that the electrode tab can bear, and the electrode tab is broken when the second pressure value is applied to the electrode tab. Correspondingly, the large particle lithium iron phosphate in the electrode tab is also broken.

[0041] S04: The electrode tab containing the large particle lithium iron phosphate is rolled at the second pressure value to break the large particle lithium iron phosphate, so that the initial capacity of the electrode tab is improved.

[0042] As can be seen from S03, after the electrode tab is rolled at the second pressure value, the electrode tab is broken, and the large particle lithium iron phosphate in the electrode tab is also broken. After the large particle lithium iron phosphate is broken, the diffusion distance of lithium ions is shortened, the contact resistance is reduced, and the capacity of the charged electrode tab is increased. Compared with the electrode tab not prepared by the second pressure value, the initial capacity of the electrode tab prepared by the second pressure value is significantly improved, so that the capacity climbing phenomenon of the electrode tab prepared by the second pressure value can be effectively improved during subsequent production and preparation of the electrode tab. Further, during the crushing of the large particle lithium iron phosphate, the small particle lithium iron phosphate remains intact due to its high pressure resistance.

[0043] In this embodiment, the first pressure value is between 6.0 and 8.0 tons. In the limit compaction state, the resistance of the electrode tab is between 40 and 200 mΩ.

[0044] In the compaction experiment of the electrode tab, the compaction density of the electrode tab becomes larger and larger as the pressure applied to the electrode tab becomes larger, as shown in Figure 4 From Figure 4 it can be seen that when the provided rolling is between 6.0 and 8.0 tons, the compaction density of the electrode tab reaches between 2.65 and 2.70 g / cc, at which the electrode tab reaches the limit compaction state.

[0045] And when the first pressure value is between 6.0 and 8.0 tons, the large particle lithium iron phosphate is not broken, and the binder and the conductive agent are separated from the electrode tab, as shown in Figure 5 .

[0046] Further, in the embodiment, the first pressure value is selected to be 7 tons. When the first pressure value is selected to be 7 tons, the compaction density of the electrode sheet is 2.70 g / cc, and at this time, the resistance of the electrode sheet is 120 mΩ, as shown in Table 1:

[0047]

[0048] Table 1: Resistance of electrode sheet with different roll pressures

[0049] In the embodiment, the second pressure value is between 8.0 and 9.0 tons, and the second pressure value includes 8 tons. In the broken state of the large particle lithium iron phosphate, the compaction density of the electrode sheet is between 2.70 and 2.71 g / cc, and at this time, the resistance of the electrode sheet is between 67 and 350 mΩ, as shown in Table 1.

[0050] When the second pressure value is between 8.0 and 9.0 tons, the electrode sheet is subjected to the roll pressure, at this time, the large particle lithium iron phosphate is broken under the roll pressure, the small particle lithium iron phosphate is not broken under the roll pressure, and the binder and the conductive agent are detached from the electrode sheet, so that the electrode sheet resistance is increased, as shown in Table 1. Figure 7 As shown in Table 1, it can be seen that after the large particle lithium iron phosphate is broken, the diffusion distance of lithium ions is shortened, the internal resistance of the electrode sheet is reduced, so that the initial capacity of the electrode sheet is improved, and the capacity climbing phenomenon can be effectively improved in the subsequent preparation process of the electrode sheet. Figure 6

[0051] Further, the second pressure value is selected to be 8 tons, and at this pressure, the large particle lithium iron phosphate is broken, and as shown in Table 1, at this time, the compaction density of the electrode sheet is 2.71 g / cc, and at this time, the resistance of the electrode sheet is 68 mΩ.

[0052] In the embodiment, the electrode sheet includes lithium iron phosphate, a binder, and a conductive agent, wherein the lithium iron phosphate and the conductive agent are aggregated together by the binder. After the electrode sheet containing the large particle lithium iron phosphate is subjected to the roll pressure with the second pressure value, the roll pressure is continuously increased to change the roll pressure from the second pressure value to a third pressure value. The electrode sheet after the large particle lithium iron phosphate is broken is subjected to the roll pressure again with the third pressure value, so that the small particle lithium iron phosphate is broken, and the binder and the conductive agent are detached from the electrode sheet. The small particle size of the lithium iron phosphate is less than 1 μm.

[0053] ​To better verify that the initial capacity value of the electrode plate reaches the best when the second pressure value is applied to the electrode plate. In this embodiment, the electrode plate is also provided with an experiment of rolling at the third pressure value.

[0054] Specifically, the third pressure value is greater than 9 tons, and the third pressure value includes 9 tons. In the broken state of the small particle lithium iron phosphate, the electrode plate has a compaction density of 2.70-2.71 g / cc, and the electrode plate has a resistance of more than 350 mΩ.

[0055] At the third pressure value, the large particle lithium iron phosphate and the small particle lithium iron phosphate are broken in the rolling state, and the binder and the conductive agent are separated from the electrode plate.

[0056] Further, the third pressure value is selected as 9 tons, at which the large particle lithium iron phosphate and the small particle lithium iron phosphate are broken in the rolling state, and the binder and the conductive agent are separated from the electrode plate. As can be seen from Table 1, at this time, the electrode plate has a compaction density of 2.71 g / cc, and the electrode plate has a resistance of 350 mΩ.

[0057] In summary, when the first pressure value is, the large particle lithium iron phosphate and the small particle lithium iron phosphate are not broken in the rolling state, and the binder and the conductive agent are separated from the electrode plate; when the second pressure value is, the large particle lithium iron phosphate is broken in the rolling state, and the small particle lithium iron phosphate is not broken in the rolling state, and the binder and the conductive agent are separated from the electrode plate; when the third pressure value is, the large particle lithium iron phosphate and the small particle lithium iron phosphate are not broken in the rolling state, and the binder and the conductive agent are separated from the electrode plate. By comparing the resistances of the electrode plates at the above three different pressure values, it can be seen that the resistance value at the second pressure value is the smallest, that is, the capacity of the electrode plate is the largest at the second pressure value. By improving the initial capacity of the electrode plate at the second pressure value, the subsequent capacity climbing phenomenon can be effectively improved, as shown in Figure 7

[0058] The present application provides another embodiment, specifically provides an electrode plate prepared by the method for improving the initial capacity of the electrode plate.

[0059] ​The present invention also provides a comparative embodiment in which all other processes are the same as the method for increasing the initial capacity of the battery cell electrode, except that the 7-ton rolling mill is used to prepare the battery cell electrode. Hereinafter, the battery cell electrode prepared using the method for increasing the initial capacity of the battery cell electrode will be referred to as battery cell electrode A; the battery cell electrode prepared in the comparative embodiment will be referred to as battery cell electrode B.

[0060] The electrical performance of the battery cell electrodes obtained by the two embodiments described above was tested, see reference. Figure 7 As shown, the 2C capacity retention rate of cell electrode A is 92.2%, and that of cell electrode B is 88.3%. The initial capacity of cell electrode A is significantly higher than that of Comparative Example 1, reaching its maximum after 6 cycles with a capacity ramp-up rate of 2.4%, and a capacity retention rate of 94.8% after 1000 cycles. In contrast, cell electrode B only reaches 5.08 Ah after 30 cycles, with a capacity ramp-up rate of 5.4% and a capacity retention rate of 90.2% after 1000 cycles. See Table 2 for details.

[0061]

[0062] Table 2: Results of Electrical Performance Tests for Cell Electrodes

[0063] The above results indicate that the battery electrode prepared by the method described above not only improves the initial capacity and rate performance, but also enhances the cycle performance. Specifically, this is because the micro-cracks in the lithium iron phosphate particles increase the specific surface area and shorten the diffusion path, making lithium insertion / extraction easier. This improvement on the capacity ramp-up problem of lithium iron phosphate batteries from the battery electrode end effectively increases the battery yield and reduces manufacturing costs.

[0064] In this invention, by increasing the pressure of the cell electrode rolling process, the large lithium iron phosphate particles in the cell electrode are broken, thereby reducing the diffusion distance between lithium ions and the contact resistance, which increases the initial capacity of the cell electrode and improves the capacity ramp-up phenomenon in the subsequent cell electrode production process. Moreover, this method is simple and easy to operate, and can not only improve the initial capacity and rate performance of the cell electrode, but also improve the cycle performance of the cell electrode.

[0065] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A method for increasing the initial capacity of battery cell electrodes, characterized in that, Includes the following steps: The cell electrode sheet is rolled using a first pressure value to bring the cell electrode sheet to the ultimate compaction state. The cell electrode sheets under extreme compaction state are scanned, and the cell electrode sheets containing large-particle lithium iron phosphate are selected; The pressure of the roller is adjusted to increase from a first pressure value to a second pressure value; the first pressure value is between 6.0 and 8.0 tons; the second pressure value is between 8.0 and 9.0 tons, and the second pressure value includes 8 tons; The second pressure value is used to roll the cell electrode containing the large lithium iron phosphate particles to break the large lithium iron phosphate particles into small lithium iron phosphate particles, thereby increasing the initial capacity of the cell electrode. The compaction density of the battery cell electrode sheet under extreme compaction state is between 2.65 and 2.70 g / cc; the particle size of the large-particle lithium iron phosphate is between 1 and 2 μm.

2. The method for increasing the initial capacity of battery cell electrodes according to claim 1, characterized in that, Under the extreme compaction state, the resistance of the cell electrode is between 40 and 200 mΩ.

3. The method for increasing the initial capacity of battery cell electrodes according to claim 1, characterized in that, When the large-particle lithium iron phosphate is in a crushed state, the compaction density of the cell electrode is between 2.70 and 2.71 g / cc, and the resistance of the cell electrode is between 67 and 350 mΩ.

4. The method for increasing the initial capacity of battery cell electrodes according to claim 1, characterized in that, The battery cell electrode includes lithium iron phosphate, a binder, and a conductive agent, wherein the lithium iron phosphate and the conductive agent are polymerized together by the binder; After the cell electrode containing the large lithium iron phosphate particles is rolled using the second pressure value; Continue to increase the pressure of the roller press, so that the pressure of the roller press changes from the second pressure value to the third pressure value; The third pressure value is used to roll the battery cell electrode sheet after the large lithium iron phosphate particles are crushed again, so that the small lithium iron phosphate particles are crushed and the binder and the conductive agent are detached from the battery cell electrode sheet. The smaller particles in the lithium iron phosphate compacted using the third pressure value have a particle size of less than 1 μm.

5. The method for increasing the initial capacity of battery cell electrodes according to claim 4, characterized in that, The third pressure value is greater than 9 tons, and the third pressure value includes 9 tons.

6. The method for increasing the initial capacity of battery cell electrodes according to claim 5, characterized in that, When the smaller lithium iron phosphate particles are broken, the compaction density of the cell electrode is between 2.70 and 2.71 g / cc, and the resistance of the cell electrode exceeds 350 mΩ.

7. A battery cell electrode, characterized in that, The battery cell electrode sheet is prepared using the method for increasing the initial capacity of the battery cell electrode sheet as described in any one of claims 1-6.

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