Preparation method of lithium iron manganese phosphate composite material positive plate and soft package battery

Abstract

The invention discloses a preparation method of a lithium iron manganese phosphate composite material positive plate and a soft package battery. Lithium manganese iron phosphate and a ternary material are used as main materials, slurries are respectively prepared and then mixed according to a certain proportion to obtain three different positive electrode slurries of lithium manganese iron phosphate, lithium manganese iron phosphate-ternary and ternary, and then the three positive electrode slurries are sequentially coated on an aluminum foil according to a composition structure of ternary / lithium manganese iron phosphate-ternary / lithium manganese iron phosphate. And obtaining the lithium iron manganese phosphate composite material positive plate. Compared with a manganese iron lithium-ternary composite material positive plate prepared by a single mixing mode, the composite material positive plate prepared by the method has the advantages of high compaction, high capacity, long cycle life and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a method for preparing a lithium manganese iron phosphate composite material positive electrode sheet and a soft-pack battery. Background Art

[0002] Compared to lithium iron phosphate (LFP), lithium manganese iron phosphate (LMP) boasts a 10-20% increase in energy density while also exhibiting excellent low-temperature and safety performance, making it a next-generation, high-energy-density cathode material. Currently, LMP preparation technology is immature, and the material still faces challenges such as low compacted density, large specific surface area, and low electrical conductivity, which hinder its commercial application. Industry experts generally believe that LMP applications can be categorized into two approaches: mixed use and pure use. In the short term, mixed use will be the first to be implemented, and ultimately, pure use will replace LFP as the technology develops and improves.

[0003] In the field of mixed use, since the discharge average voltage of lithium manganese iron phosphate is around 3.6V, which is close to the average discharge voltage of ternary materials, the two can be mixed without voltage mutation. Different ternary materials can be mixed with lithium manganese iron phosphate to achieve different effects. Lithium manganese iron phosphate and medium nickel ternary materials are close in gram capacity, and the battery cell price is lower. Mixing can achieve cost reduction while maintaining quality. Lithium manganese iron phosphate and high nickel ternary materials are close in battery cell price, and there is a certain gap in gram capacity, but they have good safety performance, and mixing can achieve improved safety performance.

[0004] The technology of mixing lithium manganese iron phosphate and ternary materials can be divided into material composites, electrode composites and battery cell / module composites according to different types. Material composites are the mixing of two materials, and there are several mixing methods: ① Physical mixing of the two materials by dry method ② Material 1 is coated on the surface of material 2 ③ The two materials are prepared into slurries separately and then the slurries are mixed; Electrode composites are the coating of the slurries of the two materials on the foil, and there are several mixing methods: ① Alternate coating of the two slurries ② Coating of the two slurries in different areas ③ Coating of the mixed slurry; Battery cell / module composites are the use of battery cells of the two materials in combination to perform different functions. The more common mixing technology is the composite of materials and electrodes. A single composite method has limitations and cannot solve problems such as uniformity and cycle stability at the same time. Summary of the Invention

[0005] In response to the above technical problems, the present invention provides a method for preparing a positive electrode sheet of a lithium iron manganese phosphate composite material and a soft-pack battery, which can make the mixing of lithium iron manganese phosphate and ternary materials more uniform. The surface layer of the prepared positive electrode sheet has a protective function, can reduce the occurrence of side reactions, and improve the cycle stability of the soft-pack battery.

[0006] In order to achieve the above object, the present invention provides a lithium iron manganese phosphate composite material positive electrode sheet, which is composed of a ternary layer, a lithium iron manganese phosphate-ternary mixed layer and a lithium iron manganese phosphate layer from the inside to the outside; The ternary layer is composed of a ternary material, which is a positive electrode material of lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide, wherein the molar ratio of nickel is 0.5-0.95, i.e. 50-95%.

[0007] The lithium iron manganese phosphate-ternary mixed layer is composed of lithium iron manganese phosphate and ternary materials, wherein the proportion of lithium iron manganese phosphate is at least 10wt%.

[0008] The lithium iron manganese phosphate layer is composed of lithium iron manganese phosphate material, wherein the molar ratio of manganese is 0.6-0.8, that is, 60-80%.

[0009] The lithium manganese iron phosphate composite material positive electrode sheet is prepared by the following method, which includes the following steps: (1) The lithium iron manganese phosphate material and the ternary material are homogenized separately to obtain lithium iron manganese phosphate positive electrode slurry A and ternary positive electrode slurry B, and then a portion of A and B are taken and mixed in a certain proportion to obtain lithium iron manganese phosphate-ternary mixed positive electrode slurry C.

[0010] (2) Slurry B is evenly coated on aluminum foil, and after drying, a layer of slurry C is evenly coated on the foil, and after drying again, a layer of slurry A is evenly coated on the foil, and finally drying is performed to obtain a lithium manganese iron phosphate composite material positive electrode sheet.

[0011] Preferably, in step (1), the particle size D50 of the lithium manganese iron phosphate material is 0.8-1.3 μm, and the specific surface area is ≤17m 2 / g, the powder compaction density is 2.25-2.40g / cm 3 .

[0012] Preferably, in step (1), the ternary material is one or more of a single crystal material and a polycrystalline material, and the particle size D50 of the ternary material is 3-5 μm.

[0013] Preferably, in step (1), the solid content of the slurry A is 50-60 wt%, the slurry viscosity is less than 10000 cp, and the slurry A contains a dispersant and the proportion of the dispersant is 0.1-1.0 wt%.

[0014] Dispersants include polyvinyl pyrrolidone, polyacrylic acid, sodium lauryl sulfate, oleic acid, and polyacrylamide.

[0015] Preferably, in step (1), the solid content of the slurry B is 50-60 wt %, and the slurry viscosity is less than 10000 cp.

[0016] Preferably, in step (1), the mixing ratio of slurry A (calculated by the mass of lithium manganese iron phosphate) and slurry B (calculated by the mass of the ternary material) in the slurry C is 1:9-5:5.

[0017] Preferably, in step (1), the slurry C is prepared by mixing slurry A and slurry B and then continuing to disperse at high speed for 1-3 hours.

[0018] Preferably, in step (1), the solid content of the slurry C is 50-60 wt%, and the slurry viscosity is less than 10000 cp; Preferably, in step (2), the surface density of the three slurries during coating is such that the double-sided surface density of slurry A and slurry B is 1.0-3.0 mg / cm 2 The double-sided density of slurry C is 20-30 mg / cm 2 .

[0019] 1. Slurry layer A reduces side reactions and improves stability, while slurry layer B improves conductivity and adhesion between slurry layer C and the foil (similar to the role of the carbon coating in carbon-coated aluminum foil). Both layers A and B need to be very thin. If layer A is too thick, conductivity decreases (pure lithium manganese iron phosphate has poor conductivity), and if layer B is too thick, stability decreases (pure ternary cycle stability is poor).

[0020] 2. The slurry C layer is the main layer. The thickness of this layer is relatively thick. It is a manganese iron lithium-ternary mixed layer that takes into account energy density, cycle stability and safety. If it is too thin, it will not play the above role.

[0021] Preferably, in step (4), the additive is one of lithium carbonate, titanium dioxide, vanadium pentoxide and magnesium oxide, accounting for 0.05%-0.2% of the total mass.

[0022] The present invention also provides an application of a lithium manganese iron phosphate composite material positive electrode in a soft-pack battery.

[0023] Preferably, the positive electrode of the soft-pack battery is a lithium manganese iron phosphate composite material, and the negative electrode is a graphite material. The preparation method of the soft-pack battery includes the following steps: (1) rolling, slitting and die-cutting the above-mentioned lithium manganese iron phosphate composite material positive electrode sheet to obtain the required positive electrode sheet; (2) The graphite material is homogenized, coated, rolled, slit and die-cut to obtain the required negative electrode sheet; (3) The positive electrode sheet, separator, and negative electrode sheet are stacked in order, and then the required soft-pack battery is obtained through liquid injection, primary packaging, pre-forming, secondary packaging, subsequent formation, and capacity division.

[0024] The beneficial effects of the present invention are: The present invention adopts a wet method to mix lithium manganese iron phosphate and ternary materials to improve the uniformity of the composite material, and then coats different types of slurries on the surface of the foil to form a sandwich structure with thin inner and outer layers and a thick middle layer. The outer layer of lithium manganese iron phosphate has good stability, which can reduce the occurrence of side reactions and improve the stability of the electrode. The inner layer of ternary has good conductivity and can improve the adhesion of the lithium manganese iron phosphate composite material slurry in the middle layer during coating to make the coating more uniform. Finally, since the lithium manganese iron phosphate composite material slurry in the middle layer occupies the main body of the electrode, compared with pure ternary, cost reduction, quality assurance and safety performance improvement can be achieved. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention are further explained below with reference to specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention shall be determined by the claims. Any modifications or substitutions made to the technical solutions of the present invention by those skilled in the art without creative effort shall fall within the scope of protection of the present invention.

[0026] Example 1 9.69g of lithium manganese iron phosphate, 0.01g of dispersant (converted to solid, polyvinyl pyrrolidone), 0.1g of SuperP, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 3000-7000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.25g / cm 3 .

[0027] 9.7 g of ternary material (5-series NCM), 1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 3000-6000 cp to obtain slurry B; the ternary material had the following characteristics: the molar proportion of nickel was 50%, and the average particle size was 4.97 μm.

[0028] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 1:9, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 3000-8000cp to obtain slurry C.

[0029] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 2 mg / cm 2 , slurry B is 3mg / cm 2 , slurry C is 20mg / cm2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0030] Example 2 9.67g of lithium manganese iron phosphate, 0.03g of dispersant (polyvinyl pyrrolidone), 0.1g of Super P, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 3000-7000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.30g / cm 3 .

[0031] 9.7 g of ternary material (5-series NCM), 1 g of Super P, 0.2 g of PVDF, and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was adjusted to 3000-6000 spc to obtain slurry B. The ternary material has the following characteristics: the molar proportion of nickel is 50%, and the average particle size is 4.97 μm.

[0032] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 2:8, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 3000-7000cp to obtain slurry C.

[0033] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 1 mg / cm 2 , slurry B is 2mg / cm 2 , slurry C is 30mg / cm 2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0034] Example 3 9.6g of lithium manganese iron phosphate, 0.1g of dispersant (sodium lauryl sulfate), 0.1g of Super P, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 6000-10000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.35g / cm 3 .

[0035] 9.7 g of ternary material (5-series NCM), 1 g of Super P, 0.2 g of PVDF, and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was adjusted to 5000-8000 spc to obtain slurry B. The ternary material has the following characteristics: the molar proportion of nickel is 50%, and the average particle size is 3.04 μm.

[0036] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 3:7, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 5000-10000cp to obtain slurry C.

[0037] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 1 mg / cm 2 , slurry B is 1mg / cm 2 , slurry C is 25mg / cm 2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0038] Example 4 9.65g of lithium manganese iron phosphate, 0.5g of dispersant (converted to solid, sodium lauryl sulfate), 0.1g of Super P, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 6000-10000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 7:3, average particle size of 1.27μm, specific surface area of ​​15.7m 2 / g, compacted density is 2.40g / cm 3 .

[0039] 9.7 g of ternary material (8-series NCM), 1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 3000-6000 cp to obtain slurry B; the ternary material had the following characteristics: the molar proportion of nickel was 80%, and the average particle size was 3.46 μm.

[0040] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 2:8, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 3000-10000cp to obtain slurry C.

[0041] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 3mg / cm 2 , slurry B is 3mg / cm 2, slurry C is 23mg / cm 2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0042] Example 5 9.62g of lithium manganese iron phosphate, 0.8g of dispersant (converted to solid, polyacrylic acid), 0.1g of Super P, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 6000-10000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 8:2, average particle size of 1.19μm, specific surface area of ​​16.8m 2 / g, compacted density is 2.33g / cm 3 .

[0043] 9.7 g of ternary material (8-series NCM), 1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 5000-8000 cp to obtain slurry B; the ternary material had the following characteristics: the molar proportion of nickel was 80%, and the average particle size was 3.46 μm.

[0044] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 5:5, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 6000-10000cp to obtain slurry C.

[0045] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 1 mg / cm 2 , slurry B is 3mg / cm 2 , slurry C is 26mg / cm 2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0046] Example 6 9.67g of lithium manganese iron phosphate, 0.3g of dispersant (converted to solid, polyacrylic acid), 0.1g of Super P, 0.2g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 3000-7000cp to obtain slurry A; wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.28g / cm 3 .

[0047] 9.7 g of ternary material (8-series NCM), 1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was 5000-8000 cp to obtain slurry B; the ternary material had the following characteristics: the molar proportion of nickel was 80%, and the average particle size was 3.46 μm.

[0048] Slurry A (calculated as lithium manganese iron phosphate) and slurry B (calculated as ternary material) are mixed in a mass ratio of 3:7, and then stirred at high speed for 3 hours. The solid content is controlled at 50-60%, and the discharge viscosity is 3000-8000cp to obtain slurry C.

[0049] Coat the aluminum foil surface with slurry B, dry it, then coat it with slurry C, dry it, and finally coat it with slurry A, dry it. Control the coating double-sided density: slurry A is 1 mg / cm 2 , slurry B is 2mg / cm 2 , slurry C is 28mg / cm 2 , and obtain a lithium manganese iron phosphate composite positive electrode sheet.

[0050] Comparative Example 1 9.7g of ternary material (8-series NCM), 1g of Super P, 0.2g of PVDF, and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was adjusted to 5000-8000cp to obtain a ternary slurry. The ternary material has the following characteristics: the molar ratio of nickel is 80%, and the average particle size is 3.46μm. The ternary slurry was coated on the surface of aluminum foil and dried, and the coating double-sided density was controlled to 36mg / cm 2 , and obtain the ternary material positive electrode sheet.

[0051] Comparative Example 2 9.7g of ternary material (5-series NCM), 1g of Super P, 0.2g of PVDF, and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the discharge viscosity was adjusted to 5000-8000cp to obtain a ternary slurry. The ternary material had the following characteristics: a nickel molar ratio of 50% and an average particle size of 3.04μm. The ternary slurry was coated on the surface of aluminum foil and dried, and the coating double-sided density was controlled to 36mg / cm 2 , and obtain the ternary material positive electrode sheet.

[0052] Comparative Example 3 3g of lithium manganese iron phosphate and 7g of ternary material (8 series NCM) were mechanically stirred and uniformly mixed for 3h to obtain a mixed positive electrode material, wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.35g / cm 3The ternary material has the following characteristics: the molar proportion of nickel is 80% and the average particle size is 3.46μm.

[0053] Then, 9.7 g of the mixed cathode material, 0.1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the viscosity of the discharge material was adjusted to 3000-10000 cp to obtain a mixed cathode material slurry.

[0054] Coat the mixed cathode material slurry on the aluminum foil surface and dry it. Control the coating density on both sides to be 30 mg / cm 2 , and obtain a mixed positive electrode material positive electrode sheet.

[0055] Comparative Example 4 3g of lithium manganese iron phosphate and 7g of ternary material (5 series NCM) were mechanically stirred and uniformly mixed for 3h to obtain a mixed positive electrode material, wherein the lithium manganese iron phosphate has the following characteristics: manganese iron ratio of 6:4, average particle size of 0.82μm, specific surface area of ​​13.5m 2 / g, compacted density is 2.33g / cm 3 The ternary material has the following characteristics: the molar proportion of nickel is 50% and the average particle size is 3.04μm.

[0056] Then, 9.7 g of the mixed cathode material, 0.1 g of Super P, 0.2 g of PVDF and an appropriate amount of solvent NMP were uniformly mixed, and the solid content was adjusted to 50-60%, and the viscosity of the discharge material was adjusted to 3000-10000 cp to obtain a mixed cathode material slurry.

[0057] Coat the mixed cathode material slurry on the aluminum foil surface and dry it. Control the coating density on both sides to be 30 mg / cm 2 , and obtain a mixed positive electrode material positive electrode sheet.

[0058] Comparative Example 5 The preparation process is the same as that of Example 3, except that the compacted density of lithium manganese iron phosphate is 2.15 g / cm 3 .

[0059] Comparative Example 6 The preparation process is the same as that of Example 3, except that the compacted density of lithium manganese iron phosphate is 2.45 g / cm 3 .

[0060] Comparative Example 7 The preparation process is the same as that of Example 3, except that the coating double-sided density is controlled: slurry A is 8 mg / cm 2 , slurry B is 8mg / cm 2 , slurry C is 20mg / cm 2 .

[0061] Comparative Example 8 The preparation process is the same as that of Example 3, except that the coating double-sided density is controlled: slurry A is 8 mg / cm 2 , slurry B is 8mg / cm 2 , slurry C is 10mg / cm 2 .

[0062] The positive electrode sheet is rolled, slit, and die-cut to obtain the desired positive electrode sheet. The graphite material is homogenized, coated, rolled, slit, and die-cut to obtain the desired negative electrode sheet. The positive electrode sheet, separator, and negative electrode sheet are then stacked in sequence. The desired soft-pack battery is obtained through liquid injection, primary packaging, pre-formation, secondary packaging, subsequent formation, and capacity division. The soft-pack battery is then tested for charge and discharge in the voltage range of 2.5 to 4.3 V.

[0063] The prepared positive electrode sheets were assembled into soft-pack batteries for performance testing. The data in the table show that when the lithium manganese iron phosphate composite material positive electrode sheets prepared by the present invention are assembled into soft-pack batteries, the composite material has improved initial efficiency and cycle stability and reduced cost compared to a single ternary material; compared to a mixed material obtained by dry mixing, the composite material has improved compaction, initial efficiency, capacity, and cycle stability, indicating that the lithium manganese iron phosphate composite material can achieve the complementary advantages of lithium manganese iron phosphate and ternary materials, and that the combination of multiple composite methods has better performance than a single composite method.

[0064] Table 1 Test results of soft-pack batteries prepared with lithium manganese iron phosphate composite cathode sheets

[0065] The above embodiments are merely examples for illustrative purposes only and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all implementation methods here. Therefore, any obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A lithium manganese iron phosphate composite material positive electrode sheet, characterized in that: The ternary positive electrode slurry B is evenly coated on the aluminum foil. After drying, a layer of slurry C formed by mixing lithium iron manganese phosphate material and ternary material is evenly coated. After drying again, a layer of lithium iron manganese phosphate positive electrode slurry A is evenly coated. Finally, drying is performed to obtain a lithium iron manganese phosphate composite material positive electrode sheet composed of a ternary layer, a lithium iron manganese phosphate-ternary mixed layer and a lithium iron manganese phosphate layer from the inside to the outside.

2. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: The ternary layer is a positive electrode material of lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide, in which the molar ratio of nickel is 0.5-0.95; The lithium iron manganese phosphate-ternary mixed layer is composed of lithium iron manganese phosphate and ternary materials, wherein the mass proportion of lithium iron manganese phosphate is at least 10wt%; The lithium iron manganese phosphate layer is composed of lithium iron manganese phosphate material, wherein the molar ratio of manganese is 0.6-0.

8.

3. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: The particle size D50 of the lithium manganese iron phosphate material is 0.8-1.3 μm, and the specific surface area is ≤17m 2 / g, the powder compaction density is 2.25-2.40g / cm 3 ; The ternary material is one or more of a single crystal material and a polycrystalline material, and the particle size D50 of the ternary material is 3-5 μm.

4. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: In step (1), the solid content of the lithium manganese iron phosphate positive electrode slurry A is 50-60wt%, the slurry viscosity is less than 10000cp, and the slurry A contains a dispersant and the proportion of the dispersant is 0.1-1.0wt%.

5. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: In step (1), the solid content of the ternary positive electrode slurry B is 50-60wt%, and the slurry viscosity is less than 10000cp.

6. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: In step (1), the mass proportion of lithium manganese iron phosphate in the slurry C is 10-50wt%.

7. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 6, characterized in that: In step (1), the slurry C is prepared by mixing slurry A and slurry B and then continuing to disperse at high speed for 1-3 hours.

8. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: In step (1), the solid content of the slurry C is 50-60wt%, and the slurry viscosity is less than 10000cp.

9. The method for preparing a lithium manganese iron phosphate composite material positive electrode sheet according to claim 1, characterized in that: In step (2), the surface density of the three slurries during coating is 1.0-3.0 mg / cm2 for the lithium manganese iron phosphate positive electrode slurry A and the ternary positive electrode slurry B. 2 The double-sided density of slurry C is 20-30 mg / cm 2 .

10. A soft-pack lithium-ion battery, wherein: The soft-pack lithium-ion battery comprises the lithium manganese iron phosphate composite material positive electrode sheet according to any one of claims 1 to 9 as a positive electrode.

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