Positive electrode sheet, preparation method therefor and use thereof

Abstract

A positive electrode sheet, and a preparation method for the positive electrode sheet. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the surface of at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises a dispersion aid, lithium iron phosphate, electrically conductive carbon black and carbon nanotubes. The materials in the positive electrode active material layer satisfy the relational expression: 30≤(B1×C1+B2×C2+B3×C3) / A×0.01≤500, where A is the mass ratio of the dispersion aid in the positive electrode active material layer, B1 is the specific surface area of lithium iron phosphate, C1 is the mass ratio of lithium iron phosphate in the positive electrode active material layer, B2 is the specific surface area of the electrically conductive carbon black, C2 is the mass ratio of the electrically conductive carbon black in the positive electrode active material layer, B3 is the specific surface area of the carbon nanotubes, and C3 is the mass ratio of the carbon nanotubes in the positive electrode active material layer. By rationally matching the specific surface areas of lithium iron phosphate, the carbon black and the carbon nanotubes with the addition amounts of lithium iron phosphate, the carbon black, the carbon nanotubes and the dispersion aid in the positive electrode active material layer, the good distribution of a binder can be ensured in the positive electrode active material layer while a good electrically conductive network is formed, the stripping force of the positive electrode active material layer is improved, and the electrical resistance of the positive electrode active material layer is reduced, thereby reducing the DCR internal resistance of a battery and improving the rate capacity thereof.

Description

A positive electrode sheet and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 14, 2023, with application number 2023117352276. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a positive electrode plate and a preparation method and application thereof. Background Art

[0003] Lithium iron phosphate (LiFePO4) has become the mainstream technology for commercial and even passenger vehicles. With the phasing out of new energy vehicle subsidies at the end of 2022, both battery cell manufacturers and vehicle manufacturers are facing challenges in further reducing battery cell production costs and improving production efficiency.

[0004] CN113328097A discloses an improved conductive adhesive for lithium-ion battery positive electrodes, its preparation method and application. It proposes a conductive adhesive formula for positive electrodes, which mainly involves polyvinyl pyrrolidone as a dispersant, vinyl silicone oil as a suspending agent, YT-9000 polyvinylidene fluoride as a binder, N-methylpyrrolidone as a solvent, and a carbon-based inorganic substance as a conductive agent.

[0005] CN101699642A discloses a method for manufacturing a positive electrode sheet of a lithium iron phosphate battery. The method comprises the following steps: first, lithium iron phosphate, polyvinylidene fluoride, carbon black, conductive graphite and lithium titanate are prepared into a mixture; then, the mixture is baked in an inert gas atmosphere; then, the baked mixture is cooled; then, the cooled mixture is ball-milled in a ball mill, sieved and sealed for storage; then, the sieved mixture powder is poured into an N-methylpyrrolidone solution and stirred under vacuum conditions to fully dissolve it to prepare a slurry; then, aluminum foil is prepared and the slurry is applied to the surface of the aluminum foil; then, the plate coated with the slurry is baked in a vacuum baking oven; and the vacuum-baked plate is rolled into a plate to obtain a positive electrode sheet of the lithium iron phosphate battery. Technical issues

[0006] During the preparation process of the lithium iron phosphate positive electrode sheet described in the above scheme, there are shortcomings such as poor slurry stability, large amount of NMP used, easy cracking of the prepared electrode sheet, and low peeling force, resulting in low energy efficiency and high resistance.

[0007] The present application provides a positive electrode plate, a preparation method thereof, and an application thereof. The present application reasonably matches the proportion and specific surface area of ​​LFP and conductive agent in the positive electrode active material layer and the addition amount of dispersing aid, so that multiple characteristic parameters meet specific relationships, promote the dispersion of multi-materials, ensure a good conductive network and adhesive distribution state, improve the peeling force, reduce the electrode plate resistance, thereby reducing the DCR internal resistance of the battery and improving the rate discharge energy efficiency. Solution

[0008] In the first aspect, the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode active material layer arranged on at least one side of the positive electrode collector, the positive electrode active material layer including a dispersing aid, a lithium iron phosphate positive electrode material (LFP), conductive carbon black and carbon nanotubes (CNTs), and the materials in the positive electrode active material layer satisfy the relationship: 30≤(B1×C1+B2×C2+B3×C3) / A×0.01≤500, wherein A is the mass proportion of the dispersing aid in the positive electrode active material layer, B1 is the specific surface area of ​​the lithium iron phosphate positive electrode material, C1 is the mass proportion of the lithium iron phosphate positive electrode material in the positive electrode active material layer, B2 is the specific surface area of ​​the conductive carbon black, C2 is the mass proportion of the conductive carbon black in the positive electrode active material layer, B3 is the specific surface area of ​​the carbon nanotubes, and C3 is the mass proportion of the carbon nanotubes in the positive electrode active material layer.

[0009] In a second aspect, the present application provides a method for preparing the positive electrode sheet as described in the first aspect, the preparation method comprising the following steps:

[0010] A dispersing agent, lithium iron phosphate positive electrode material, conductive carbon black, carbon nanotubes and a binder are dry-mixed and then a solvent is added to obtain a positive electrode slurry;

[0011] The positive electrode slurry is coated on the surface of the positive electrode current collector, and the positive electrode sheet is obtained through drying, cold pressing and die cutting.

[0012] During the preparation process of the positive electrode sheet described in this application, by ensuring the good dispersion of LFP, conductive carbon black, CNT, and binder in the positive electrode system, the conductive agent and the adhesive are well distributed, thereby improving the sheet resistance and the sheet peeling force, further reducing the DCR internal resistance and improving the rate performance.

[0013] In a third aspect, the present application provides a lithium-ion battery, which comprises the positive electrode sheet, the negative electrode sheet and the electrolyte as described in the first aspect. Beneficial effects

[0014] Compared with the prior art, this application has the following beneficial effects:

[0015] (1) This application reasonably matches the proportion and specific surface area of ​​LFP and conductive agent in the positive electrode active material layer and the amount of dispersing agent added, so that multiple characteristic parameters meet specific relationships, promote the dispersion of multi-materials, ensure a good conductive network and adhesive distribution state, improve the peeling force, reduce the electrode sheet resistance, thereby reducing the DCR internal resistance of the battery and improving the rate discharge energy efficiency.

[0016] (2) During the preparation of the positive electrode sheets described in this application, the solid content of the slurry can reach more than 68.2%, the viscosity of the slurry can be controlled within 13500 mPa.s, the peeling force after cold pressing can reach more than 0.42N / mm, the sheet resistance can reach less than 0.3Ω, the peeling force after cold pressing can reach more than 0.42N / mm, the sheet resistance can reach less than 0.3Ω, the battery can achieve a 50% SOC DCR of less than 38mΩ at 25°C, and a 25°C 3C energy efficiency of more than 80.2%. The positive electrode sheets described in this application do not use additional conductive agents, which greatly shortens the dispersion time of the positive electrode slurry, reduces production costs, and improves production efficiency. The secondary battery has excellent performance in room temperature 3C rate charge and discharge performance, high temperature storage, and high temperature long cycle.

[0017] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is a schematic diagram showing the function of a dispersing aid in one embodiment of the present application. Modes for Carrying Out the Invention

[0019] In a first aspect, the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, the positive electrode active material layer comprising a dispersing aid, a lithium iron phosphate positive electrode material (LFP), conductive carbon black and carbon nanotubes (CNTs), and the materials in the positive electrode active material layer satisfy the relationship: 30≤(B1×1+B2×C2+B3×C3) / A×0.01≤500, wherein A is the mass proportion of the dispersing aid in the positive electrode active material layer, B1 is the specific surface area of ​​the lithium iron phosphate positive electrode material, C1 is the mass proportion of the lithium iron phosphate positive electrode material in the positive electrode active material layer, B2 is the specific surface area of ​​the conductive carbon black, C2 is the mass proportion of the conductive carbon black in the positive electrode active material layer, B3 is the specific surface area of ​​the carbon nanotubes, and C3 is the mass proportion of the carbon nanotubes in the positive electrode active material layer.

[0020] In the calculation formula described in this application, the unit of mass percentage is %, for example, if C1=95%, then in the calculation formula, C1 is calculated as 95.

[0021] The present application adds a dispersing aid to the lithium iron phosphate positive electrode sheet. The actual surface area of ​​the carbon in the active material layer (the sum of the surface area of ​​the LFP (LFP contains carbon and is evenly distributed, so the surface area of ​​the carbon in LFP is the surface area of ​​LFP) and the surface area of ​​the carbon content of conductive agents such as conductive carbon black and CNT) and the amount of dispersing aid added and the structure of the dispersing aid have a synergistic effect on the processing performance and electrical properties of the positive electrode slurry. The present application combines the above-mentioned relationship and the characteristics of the dispersing aid to achieve the goal of improving the solid content of the positive electrode slurry output, improving the slurry stability, significantly reducing the amount of N-methylpyrrolidone (NMP), solving the problem of electrode cracking in the thick positive electrode system, and improving the electrode peeling force, thereby reducing the cost of battery cell preparation and improving the production efficiency of the battery cell. From the perspective of the battery end, it can improve the electrode sheet resistance, reduce the DCR internal resistance, and improve the rate discharge energy efficiency.

[0022] Optionally, the structural formula of the dispersing aid is , wherein R1 includes any one of methyl, ethyl or propyl or a combination of at least two of them, R2 includes any one of methyl, ethyl or propyl or a combination of at least two of them, m is 1 to 3, for example: 1, 2 or 3, and n is 1 to 3, for example: 1, 2 or 3.

[0023] The dispersing aid described in this application is mainly adsorbed or anchored on the surface of the material to form a sufficiently thick adsorption layer, and the solvated chain can present a stretched conformation in NMP. The carbon layer on the surface of LFP, conductive carbon black, and CNT surface all contain a large number of oxygen-containing functional groups such as carboxyl, hydroxyl, and epoxy groups, which form hydrogen bonds with the alkoxy groups in the dispersing aid. The polyester-based solvated chains in the dispersing aid have good compatibility with NMP, which promotes the stretching of the dispersing aid in the solvent, so that the dispersing aid is coated in the middle of each material (LFP, conductive carbon black, CNT), forming a steric hindrance, preventing the agglomeration of each material, and promoting the slip between molecules, thereby reducing internal friction, reducing slurry viscosity, and achieving increased solid content of the discharge, preventing PVDF from moving up, and improving the shortcomings of cracking of the pole pieces in the high-speed coating process.

[0024] Optionally, in the positive electrode active material layer, the mass proportion A of the dispersing aid is 0.05-0.3%, for example, 0.05%, 0.08%, 0.1%, 0.2% or 0.3%.

[0025] Optionally, the specific surface area B1 of the lithium iron phosphate positive electrode material is 10-20 m 2 / g, for example: 10 m 2 / g, 12 m 2 / g, 15 m 2 / g, 18 m 2 / g or 20 m 2 / g, etc.

[0026] Optionally, in the positive electrode active material layer, the mass proportion C1 of the lithium iron phosphate positive electrode material is 95-98%, for example, 95%, 95.5%, 96%, 97% or 98%.

[0027] Optionally, the surface carbon content of the lithium iron phosphate positive electrode material is 1.0-2.5%, for example: 1.0%, 1.2%, 1.5%, 2% or 2.5%, etc.

[0028] Optionally, the specific surface area B2 of the conductive carbon black is 50-150 m 2 / g, for example: 50m 2 / g、80m 2 / g、100m 2 / g, 120m 2 / g or 150m 2 / g, etc.

[0029] Optionally, in the positive electrode active material layer, the mass proportion C2 of the conductive carbon black is 0.5-2%, for example, 0.5%, 0.8%, 1%, 1.5% or 2%.

[0030] Optionally, the specific surface area B3 of the carbon nanotubes is 200-300 m 2 / g, for example: 200m 2 / g, 220m 2 / g, 250m 2 / g, 280m 2 / g or 300m 2 / g, etc.

[0031] Optionally, in the positive electrode active material layer, the mass proportion C3 of the carbon nanotubes is 0.1-1%, for example, 0.1%, 0.2%, 0.5%, 0.8% or 1%.

[0032] Optionally, the positive electrode active material layer further includes a binder.

[0033] Optionally, the binder comprises polyvinylidene fluoride.

[0034] In a second aspect, the present application provides a method for preparing the positive electrode sheet as described in the first aspect, the preparation method comprising the following steps:

[0035] A dispersing agent, lithium iron phosphate positive electrode material, conductive carbon black, carbon nanotubes and a binder are dry-mixed and then a solvent is added to obtain a positive electrode slurry;

[0036] The positive electrode slurry is coated on the surface of the positive electrode current collector, and the positive electrode sheet is obtained through drying, cold pressing and die cutting.

[0037] During the preparation process of the positive electrode sheet described in this application, by ensuring the good dispersion of LFP, conductive carbon black, CNT, and binder in the positive electrode system, the conductive agent and the adhesive are well distributed, thereby improving the sheet resistance and the sheet peeling force, further reducing the DCR internal resistance and improving the rate performance.

[0038] In a third aspect, the present application provides a lithium-ion battery, which comprises the positive electrode sheet, the negative electrode sheet and the electrolyte as described in the first aspect.

[0039] Optionally, the electrolyte injection amount in the lithium-ion battery is 4.2-5.2 g / Ah, for example, 4.2 g / Ah, 4.5 g / Ah, 4.8 g / Ah, 5 g / Ah or 5.2 g / Ah.

[0040] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0041] In the embodiments and comparative examples of the present application, A is the mass proportion of the dispersing aid in the positive electrode active material layer, B1 is the specific surface area of ​​the lithium iron phosphate positive electrode material, C1 is the mass proportion of the lithium iron phosphate positive electrode material in the positive electrode active material layer, B2 is the specific surface area of ​​the conductive carbon black, C2 is the mass proportion of the conductive carbon black in the positive electrode active material layer, B3 is the specific surface area of ​​the carbon nanotubes, and C3 is the mass proportion of the carbon nanotubes in the positive electrode active material layer. Example 1

[0042] This embodiment provides a positive electrode plate, which is prepared by the following method:

[0043] (1) LFP with a surface carbon content of 2.5%: conductive carbon black: CNT: dispersing aid (structural formula: ): PVDF was mixed in a mass percentage of 95:2:1:0.05:1.7 (i.e., C1:C2:C3:A:(100-(C1+C2+C3+A))=95:2:1:0.05:1.7), and NMP was added as a solvent to obtain a positive electrode slurry;

[0044] (2) The positive electrode slurry is evenly coated on aluminum foil, and the positive electrode sheet is obtained after drying, cold pressing, die cutting and other processes, where B1=20m 2 / g,B2=150m 2 / g,B3=280m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=496.

[0045] The schematic diagram of the function of the dispersing aid in the electrode is shown in FIG1 . Example 2

[0046] This embodiment provides a positive electrode plate, which is prepared by the following method:

[0047] (1) LFP with a surface carbon content of 1.0%: conductive carbon black: CNT: dispersing aid (structural formula: ): PVDF was mixed in a mass percentage of 97.35:0.5:0.3:0.3:1.5 (i.e., C1:C2:C3:A:(100-(C1+C2+C3+A))= 97.35:0.5:0.3:0.3:1.5), and NMP was added as a solvent to obtain a positive electrode slurry;

[0048] (2) The positive electrode slurry is evenly coated on aluminum foil, and the positive electrode sheet is obtained after drying, cold pressing, die cutting and other processes, where B1=10m 2 / g,B2=50m 2 / g,B3=200m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=35.

[0049] The schematic diagram of the function of the dispersing aid in the electrode is shown in FIG1 . Example 3

[0050] This embodiment provides a positive electrode plate, which is prepared by the following method:

[0051] (1) LFP with a surface carbon content of 1.8%: conductive carbon black: CNT: dispersing aid (structural formula: ): PVDF is mixed in a mass percentage of 96:1:0.5:0.2:2.3 (i.e., C1:C2:C3:A:(100-(C1+C2+C3+A))= 96:1:0.5:0.2:2.3), and NMP is added as a solvent to obtain a positive electrode slurry;

[0052] (2) The positive electrode slurry is evenly coated on aluminum foil, and the positive electrode sheet is obtained after drying, cold pressing, die cutting and other processes, where B1=15m 2 / g,B2=85m 2 / g,B3=240m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=82.

[0053] The schematic diagram of the function of the dispersing aid in the electrode is shown in FIG1 . Example 4

[0054] The only difference between this embodiment and embodiment 3 is that B1=10m 2 / g,B2=180m 2 / g,B3=320m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=65, and other conditions and parameters are exactly the same as those in Example 3. Example 5

[0055] The only difference between this embodiment and embodiment 3 is that B1=25m 2 / g,B2=40m 2 / g,B3=150m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=125, and other conditions and parameters are exactly the same as those in Example 3. Example 6

[0056] The only difference between this embodiment and embodiment 3 is that B1=15m 2 / g,B2=100m 2 / g,B3=150m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=80, and other conditions and parameters are exactly the same as those in Example 3. Example 7

[0057] The only difference between this embodiment and embodiment 3 is that B1=15m 2 / g,B2=40m 2 / g,B3=250m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=80, and other conditions and parameters are exactly the same as those in Example 3.

[0058] Comparative Example 1

[0059] The only difference between this comparative example and Example 1 is that C1:C2:C3:A:100-(C1+C2+C3+A)=94:2.5:0.5:0.03:2.6, B1=25m 2 / g,B2=180m 2 / g,B3=320m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=987, and other conditions and parameters are exactly the same as those in Example 3.

[0060] Comparative Example 2

[0061] The only difference between this comparative example and Example 1 is that C1:C2:C3:A:100-(C1+C2+C3+A)=98.47:0.3:0.2:0.4:1, B1=6m 2 / g,B2=35m 2 / g,B3=150m 2 / g, (B1×C1+B2×C2+B3×C3) / A×0.01=7, and other conditions and parameters are exactly the same as those in Example 3.

[0062] Comparative Example 3

[0063] The only difference between this comparative example and Example 1 is that no dispersing aid is added, and other conditions and parameters are exactly the same as those of Example 3.

[0064] Performance testing:

[0065] (1) Record the solid content, viscosity, peeling force of the electrode after cold pressing, and resistance of the electrode when preparing the positive electrode sheets in the examples and comparative examples;

[0066] (2) Graphite, CMC, SBR, and conductive carbon black were mixed in a mass ratio of 96:1.5:1.5:1, and deionized water was added as a solvent to mix and stir the materials evenly. The mixture was then coated on a copper foil. After drying, cold pressing, die-cutting, and other processes, a negative electrode sheet was obtained. The positive electrode sheet, diaphragm (Enjie 12μm diaphragm) and negative electrode sheet prepared in the embodiment and comparative example were stacked in order. The diaphragm was placed between the positive and negative electrode sheets to play a role of isolation. Then, a roll core was obtained by lamination or winding. The roll core was placed in an outer packaging shell (such as an aluminum shell or a soft bag). After drying, the electrolyte (Zhonghua Lantian ZP507 type) was injected according to the injection amount of this patent. After vacuum packaging, standing, formation, capacity separation, and other processes, a secondary battery was obtained. The electrical performance of the secondary battery was tested:

[0067] 1) 25℃ HPPC test

[0068] a. At 25±2°C, cycle the battery at 0.5C / 0.5C for 5 weeks;

[0069] b. Place at 25±2℃ for 24 hours;

[0070] c. 1.0C constant current discharge for 10 seconds, then stand for 1 minute;

[0071] d. Adjust the SOC by 0.2C and leave for 1 hour;

[0072] e. 1.0C constant current charging for 10 seconds, then rest for 1 minute;

[0073] f. Discharge DCR calculation: After adjusting the SOC, leave it for 1 hour, and record the terminal voltage as VD0. Discharge at a constant current of 1.0C for 10 seconds.

[0074] The terminal voltage is recorded as VD 10 ; DCR= (VD0- VD 10 ) / I

[0075] g. Test DCR at 50% SOC.

[0076] 2) Discharge performance at 25°C

[0077] a. Under the condition of 25±2℃, place the battery for 10 minutes and cycle at 0.5C / 0.5C for 5 weeks;

[0078] b. The battery is charged at 0.5C constant current and constant voltage;

[0079] c. Leave the battery at 25±2°C for 10 minutes;

[0080] d. Discharge at 3C current until the battery voltage reaches 2.0V;

[0081] e. Record the discharge energy efficiency of the battery cell. The test results are shown in Table 1:

[0082] Table 1

[0083]

[0084] As can be seen from Table 1, from Examples 1-3, in the preparation process of the positive electrode sheet described in the present application, the solid content of the slurry can reach more than 68.2%, the viscosity of the discharge is controlled within 13500 mPa.s, the peeling force after cold pressing can reach more than 0.42N / mm, and the electrode sheet resistance can reach less than 0.3Ω. The peeling force after cold pressing can reach more than 0.42N / mm, and the electrode sheet resistance can reach less than 0.3Ω. The manufactured battery has a 50% SOC DCR of less than 38mΩ at 25°C, and a 3C energy efficiency of more than 80.2% at 25°C. The positive electrode sheet described in the present application does not use an additional conductive agent, which greatly shortens the dispersion time of the positive electrode slurry, reduces production costs, and improves production efficiency. The secondary battery has excellent performance in room temperature 3C rate charge and discharge performance, high temperature storage, and high temperature long cycle.

[0085] From the comparison between Example 3 and Examples 4-7, it can be seen that in the positive electrode sheet of the present application, the specific surface areas B1, B2 and B3 of the lithium iron phosphate positive electrode material, conductive carbon black and carbon nanotubes will affect their performance. 2 / g, B2 is controlled at 50~150m 2 / g, B3 controlled at 200~300m 2 / g, the performance of the positive electrode sheet is better. If it exceeds the above range, the performance of the positive electrode sheet will be significantly and to varying degrees reduced.

[0086] By comparing Example 1 with Comparative Examples 1-2, it can be seen that the present application reasonably matches the LFP ratio, specific surface area, conductive agent ratio, specific surface area and the amount of dispersing agent added, so that multiple characteristic parameters meet the characteristic relationship, promote the dispersion of multi-materials, ensure a good conductive network and adhesive distribution state, improve the peel force, reduce the electrode resistance, thereby reducing the DCR internal resistance and improving the rate discharge energy efficiency.

[0087] A comparison of Example 1 and Comparative Example 3 shows that the dispersing agent described herein is primarily adsorbed or anchored on the material surface, forming a sufficiently thick adsorption layer, allowing the solvated chains to exhibit an extended conformation in NMP. The carbon layer on the surface of LFP, conductive carbon black, and CNT surfaces all contain a large number of oxygen-containing functional groups, such as carboxyl, hydroxyl, and epoxy groups, which form hydrogen bonds with the alkoxy groups in the dispersing agent. The polyester-based solvated chains in the dispersing agent have good compatibility with NMP, promoting the dispersing agent to expand in the solvent and coat the materials (LFP, conductive carbon black, and CNTs) within the solvent, forming a steric hindrance that prevents agglomeration and promotes intermolecular slip. This reduces internal friction, lowers slurry viscosity, increases the solid content of the discharge material, prevents PVDF from shifting upward, and improves electrode cracking during high-speed coating processes.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises a dispersing aid, a lithium iron phosphate positive electrode material, conductive carbon black and carbon nanotubes, and each material in the positive electrode active material layer satisfies the relationship: 30≤(B1×C1+B2×C2+B3×C3) / A×0.01≤500, wherein, A is the mass proportion of the dispersing aid in the positive electrode active material layer, B1 is the specific surface area of ​​the lithium iron phosphate positive electrode material, C1 is the mass proportion of the lithium iron phosphate positive electrode material in the positive electrode active material layer, B2 is the specific surface area of ​​the conductive carbon black, C2 is the mass proportion of the conductive carbon black in the positive electrode active material layer, B3 is the specific surface area of ​​the carbon nanotubes, and C3 is the mass proportion of the carbon nanotubes in the positive electrode active material layer.

2. The positive electrode sheet according to claim 1, wherein: The structural formula of the dispersing aid is , wherein R1 includes any one of methyl, ethyl or propyl or a combination of at least two of them, R2 includes any one of methyl, ethyl or propyl or a combination of at least two of them, m is 1 to 3, and n is 1 to 3.

3. The positive electrode sheet according to claim 1 or 2, wherein: In the positive electrode active material layer, the mass proportion A of the dispersing aid is 0.05-0.3%.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein: The specific surface area B1 of the lithium iron phosphate positive electrode material is 10-20 m 2 / g.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the positive electrode active material layer, the mass proportion C1 of the lithium iron phosphate positive electrode material is 95-98%.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein: The surface carbon content of the lithium iron phosphate positive electrode material is 1.0-2.5%.

7. The positive electrode sheet according to any one of claims 1 to 6, wherein: The specific surface area B2 of the conductive carbon black is 50-150 m 2 / g.

8. The positive electrode sheet according to any one of claims 1 to 7, wherein: In the positive electrode active material layer, the mass proportion C2 of the conductive carbon black is 0.5-2%.

9. The positive electrode sheet according to any one of claims 1 to 8, wherein: The specific surface area B3 of the carbon nanotubes is 200-300 m 2 / g.

10. The positive electrode sheet according to any one of claims 1 to 9, wherein: In the positive electrode active material layer, the mass proportion C3 of the carbon nanotubes is 0.1-1%.

11. The positive electrode sheet according to any one of claims 1 to 10, wherein: The positive electrode active material layer further includes a binder.

12. The positive electrode sheet according to claim 11, wherein: The binder includes polyvinylidene fluoride.

13. A method for preparing a positive electrode sheet according to any one of claims 1 to 12, comprising the following steps: A dispersing agent, a lithium iron phosphate positive electrode material, conductive carbon black, carbon nanotubes and a binder are dry-mixed and then added with a solvent to obtain a positive electrode slurry; The positive electrode slurry is coated on the surface of the positive electrode current collector, and the positive electrode sheet is obtained through drying, cold pressing and die cutting.

14. A lithium ion battery, wherein: The lithium-ion battery comprises the positive electrode sheet, the negative electrode sheet and the electrolyte as described in any one of claims 1 to 12.

15. The lithium ion battery according to claim 14, wherein: The electrolyte injection amount in the lithium ion battery is 4.2-5.2 g / Ah.

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