Positive electrode sheet and preparation method therefor, and battery

Abstract

The present application provides a positive electrode sheet and a preparation method therefor, and a battery. The positive electrode sheet comprises a positive electrode current collector, a first positive electrode active coating and a second positive electrode active coating, wherein in the first positive electrode active coating, the mass ratio of a first lithium manganese iron phosphate active material to a nickel cobalt manganese ternary active material is 80-90:8-15; and the second positive electrode active coating comprises a second lithium manganese iron phosphate active material. The areal density ρ1 of the first positive electrode active coating and the areal density ρ2 of the second positive electrode active coating satisfy: ρ1≥240 g / m2; ρ2≥240 g / m2; and |ρ1-ρ2|≤5 g / m2.

Description

A positive electrode sheet, a preparation method thereof, and a battery

[0001] This application claims priority to the Chinese patent application No. 202411259258.3, filed on September 9, 2024, with the Chinese Patent Office, the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of lithium ion batteries, and specifically relates to a positive electrode sheet, a preparation method thereof, and a battery. BACKGROUND

[0003] In recent years, energy storage technology has developed rapidly, among which lithium ion batteries are the most representative, and the mainstream representatives are lithium iron phosphate (LFP) and ternary (NCM) batteries. Lithium iron phosphate has the advantages of low cost, long cycle life, high safety, etc. However, its energy density is low, and its low-temperature performance is also unsatisfactory; while the energy density of ternary material is high, and the rate performance is good, but the cost is also higher and there is a certain safety risk.

[0004] Lithium manganese iron phosphate (LMFP) has a similar structure to lithium iron phosphate (LFP), both of which are olivine structures, and also have a higher voltage platform (4.2V), which can provide a relatively higher energy density compared to LFP. And LMFP has higher low-temperature performance, and can provide a capacity retention rate of 80% at-20 o C, which is higher than that of LFP (about 15%). Lithium manganese iron phosphate (LMFP) is developed on the basis of lithium iron phosphate (LFP) and lithium manganese phosphate (LMP), and combines the advantages of LFP and LMP, so it has the characteristics of low cost, safety and stability, etc., and has become a new emerging lithium ion battery cathode material with great market prospect. However, LMFP also has the inherent defects of olivine-type cathode materials, such as low intrinsic electronic conductivity (10 -9 -10 -10 S·cm -1 ) and lithium ion diffusion rate (10 -14 -10 -16 cm2·s -1 ), which greatly affects its energy storage capacity and rate performance, hindering its large-scale application in high-performance lithium ion batteries.

[0005] Therefore, based on theoretical research, people have proposed various modification methods to improve the electronic / ion conductivity of LMFP, mainly including surface coating, morphology control and ion doping. Such modification methods have higher requirements for material synthesis, and generally it is difficult to achieve large production, so they also limit the large-scale application of LMFP to some extent.

[0006] Therefore, the modified method capable of simply and effectively improving the performance of the LMFP has important practical significance for further promoting the large-scale application of the LMFP in lithium batteries. TECHNICAL PROBLEM

[0007] In the related art, the modification method of the LMFP has high requirements for material synthesis, and generally it is difficult to achieve large production, thus to some extent, the large-scale application of the LMFP is limited. TECHNICAL SOLUTION

[0008] According to a first aspect of the present application, a positive electrode sheet is provided, comprising a positive electrode current collector and a first positive electrode active coating and a second positive electrode active coating arranged on at least one side of the positive electrode current collector in sequence; the first positive electrode active coating is close to one side of the positive electrode current collector, and the first positive electrode active coating comprises a first lithium iron manganese phosphate active material and a nickel-cobalt-manganese ternary active material; in the first positive electrode active coating, the mass ratio of the first lithium iron manganese phosphate active material and the nickel-cobalt-manganese ternary active material is 80-90:8-15; the second positive electrode active coating is away from one side of the positive electrode current collector, and the second positive electrode active coating comprises a second lithium iron manganese phosphate active material; the area density p1 of the first positive electrode active coating and the area density p2 of the second positive electrode active coating satisfy: p1≥240 g / m 2 , p2≥240 g / m 2 ; and |p1-p2|≤5 g / m 2 .

[0009] According to a second aspect of the present application, a preparation method of the positive electrode sheet in the first aspect of the present application is provided, comprising the following steps: S1. preparing a lithium iron manganese phosphate positive electrode slurry by using a second lithium iron manganese phosphate active material, a second conductive agent and a second binder; and preparing a lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry by using a first slurry, a nickel-cobalt-manganese ternary active material, a first conductive agent and a first binder; S2. controlling the viscosity difference between the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry to be not higher than 4000 mPa·s, simultaneously coating the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry on the positive electrode current collector, and drying to obtain the positive electrode sheet; the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry is coated on the side close to the positive electrode current collector, and the lithium iron manganese phosphate positive electrode slurry is coated on the side away from the positive electrode current collector.

[0010] According to a third aspect of the present application, a battery is provided, comprising the positive electrode sheet in the first aspect of the present application. ADVANTAGEOUS EFFECTS

[0011] The application provides a positive plate and a preparation method thereof and a battery. The positive plate designed by the application contains a double-layer positive active material layer. By controlling the positive active material layer close to the current collector to have higher conductivity and the two layers of positive active material layers to have very close surface densities, the designed positive plate has higher electron and ion conductivity, effectively reduces the polarization of the positive plate, and greatly improves the cycle performance and thermal stability of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a schematic diagram of the structure of a positive plate in some embodiments of the application. Embodiments of the application

[0013] Referring to FIG. 1, the application provides a positive plate, which includes a positive current collector 10 and a first positive active coating layer 20 and a second positive active coating layer 30 arranged on at least one side of the positive current collector 10 in sequence. The first positive active coating layer 20 is close to one side of the positive current collector 10. The first positive active coating layer 20 includes a first lithium iron manganese phosphate active material and a nickel-cobalt-manganese ternary active material. In the first positive active coating layer 20, the mass ratio of the first lithium iron manganese phosphate active material to the nickel-cobalt-manganese ternary active material is 80-90:8-15. The second positive active coating layer 30 is away from one side of the positive current collector 10. The second positive active coating layer 30 includes a second lithium iron manganese phosphate active material. The surface density p1 of the first positive active coating layer 20 and the surface density p2 of the second positive active coating layer 30 satisfy: p1≥240 g / m 2 , p2≥240 g / m 2 ; and |p1-p2|≤5 g / m 2 .

[0014] For the modification of LMFP, compared with other modification methods, directly using ready-made materials to physically mix and dope LMFP is more simple and convenient and has high operability. The nickel-cobalt-manganese (NCM) ternary active material has the advantages of high conductivity, high lithium ion diffusion performance, high energy density, etc., and is often used as a modified LMFP material. At present, most of the products on the market are to mix LMFP with NCM as active materials, and then use conductive agents and binders according to the electrode formula ratio to make electrodes. The electrode plates made by this method have inconsistent ion and electron transmission rates of the active materials on the two sides close to the current collector and away from the current collector, which easily amplifies the polarization phenomenon, resulting in an unsatisfactory cycle life. A small part of the products are to make double-layer electrodes by separating LMFP and NCM, but the pure NCM layer has poor thermal stability and safety risks.

[0015] This solution, through a special double-layer positive electrode active material design, effectively improves the conductivity and lithium-ion transfer efficiency of the LMFP-based positive electrode, thus effectively reducing the battery DCR growth rate and improving the cycle performance of the LMFP-based positive electrode. Specifically, in the positive electrode design of this application, the first positive electrode active material layer, namely the first positive electrode active coating 20 (the side closer to the positive electrode current collector 10), adopts an LMFP+NCM hybrid layer, while the second positive electrode active material layer, namely the second positive electrode active coating 30 (the side away from the positive electrode current collector 10), adopts only an LMFP layer. With this design, the first layer (lower layer) utilizes the high conductivity of NCM to enhance the electron transfer rate on the positive electrode current collector 10 side, which can better coordinate with the ion transfer on the side away from the positive electrode current collector 10, accelerating electron transport. Therefore, it can combine with ions in the electrolyte more quickly, improving electrochemical reaction activity, thereby reducing polarization and improving cycle performance. The second (top) layer uses only an LMFP layer because LMFP has higher structural stability and better electrolyte resistance. As an outer layer, it effectively acts as an interfacial barrier to the inner LMFP+NCM layer, reducing the contact between the electrolyte and NCM, thereby reducing side reactions on the positive electrode side, lowering positive electrode heat generation, improving battery thermal stability, and reducing the rate of change rate (DCR), thus further optimizing the battery's cycle performance and safety. Simultaneously, the outer LMFP layer also provides some protection against heat transfer and diffusion, further enhancing the battery's thermal stability and safety.

[0016] This application also defines the areal density and the difference in areal density of the first positive electrode active material layer and the second positive electrode active material layer. The purpose of controlling both is twofold: firstly, keeping the areal density of these two positive electrode active material layers within a wide range ensures that the LMFP-based positive electrode has a high energy density, thereby significantly improving the charge-discharge performance of the LMFP-based battery; secondly, controlling the difference in areal density between these two positive electrode active material layers to a less than a certain value ensures that the two active material layers have almost identical areal densities, thus achieving a better balance between the electron and ion transport efficiencies of these two layers. This effectively reduces battery polarization during charge-discharge cycles, reduces the increase in battery DCR, and optimizes battery cycle performance.

[0017] This application also specifies the mass ratio of the first lithium manganese iron phosphate active material to the nickel cobalt manganese ternary active material in the first positive electrode active material layer. Simultaneously controlling the layer densities of the first and second positive electrode active materials, as well as the mass ratio of the first lithium manganese iron phosphate active material to the nickel cobalt manganese ternary active material, is more conducive to ensuring the balance of lithium-ion transport. This allows for full utilization of the advantages of LMFP itself—long cycle life and high safety—reducing the degree of battery side reactions, lowering battery heat generation, thereby reducing battery polarization and optimizing battery thermal stability and cycle performance.

[0018] In some embodiments, ρ1, ρ2 satisfy: ρ1 = ρ2. Controlling the area density of the first positive electrode active coating 20 to be consistent with the area density of the second positive electrode active coating 30 can make the electron and ion transmission efficiency of the two layers reach a better balance, further promote the synergistic effect, and further optimize the battery cycle performance.

[0019] In some embodiments, ρ1, ρ2 satisfy: 240 g / m 2 ≤ ρ1 ≤ 260 g / m 2 , 240 g / m 2 ≤ ρ2 ≤ 260 g / m 2 Generally speaking, the greater the area density of the positive electrode sheet, the higher the energy density of the battery. However, if the area density of the positive electrode sheet is too high, the migration path of lithium ions becomes longer, the internal resistance of the battery is larger, and congestion is likely to occur on the migration path of lithium ions, which can cause lithium ions to not be completely deintercalated in a short time, resulting in loss of specific capacity, and also causing the working temperature of the battery to rise, thereby affecting the service life and safety of the battery. Therefore, controlling the area densities of the first positive electrode active coating 20 and the second positive electrode active coating 30 within a certain numerical range, i.e., controlling the area density of the positive electrode sheet within a certain numerical range, is beneficial to balancing the energy density and cycle performance and safety performance of the battery.

[0020] In some embodiments, the thickness d1 of the first positive electrode active coating 20 and the thickness d2 of the second positive electrode active coating 30 satisfy: d1 ≥ 105 μm, d2 ≥ 105 μm, and |d1 - d2| ≤ 3 μm. Generally speaking, the thicker the coating thickness of the positive electrode active material layer, the greater the capacity of the battery. However, excessive coating of the material can increase the internal resistance of the battery, affecting the output power and cycle life of the battery. Therefore, a balance needs to be considered in the selection of the coating thickness. The present application controls d1 and d2 within a certain numerical range, and controls the thickness difference between the two within a certain range, which is beneficial to achieving optimal battery performance and thereby optimizing the cycle stability of the battery.

[0021] In some embodiments, the thickness D of the positive electrode sheet is ≥ 220 μm. Ensuring that the sheet has a certain thickness is beneficial to ensuring that the battery has a high energy density, and at the same time, ensuring that both layers of the positive electrode active material layer have a certain thickness, so as to ensure that the performance of the two layers of the positive electrode active material layer is fully utilized, thereby optimizing the performance of the battery.

[0022] In some embodiments, the thickness D of the positive electrode sheet satisfies: 220 μm ≤ D ≤ 230 μm. If the positive electrode sheet is too thick, the electron and ion transmission path is longer, the sheet resistance increases, and this is not conducive to the cycle performance and thermal stability of the battery.

[0023] In some embodiments, the thickness of the positive electrode current collector 10 is 10-15 μm.

[0024] In some embodiments, the positive electrode current collector 10 comprises an aluminum foil. Specifically, in the first positive electrode active coating layer 20, a first conductive agent and a first binder are further included; the mass ratio of the first lithium iron manganese phosphate active material, the nickel-cobalt-manganese ternary active material, the first conductive agent, and the first binder is 80-90:8-15:1.2-1.8:1.5-3.

[0025] In some embodiments, in the second positive electrode active coating layer 30, a second conductive agent and a second binder are further included; the mass ratio of the second lithium iron manganese phosphate active material, the second conductive agent, and the second binder is 92-98:1.2-1.8:1.5-3.

[0026] In some embodiments, the first conductive agent and the second conductive agent independently comprise conductive carbon black (SP) and carbon nanotubes (CNT).

[0027] In some embodiments, the first binder and the second binder independently comprise polyvinylidene fluoride (PVDF).

[0028] According to a second aspect of the present application, a preparation method of the positive electrode sheet described in the above embodiments is provided, comprising the following steps: S1. preparing a lithium iron manganese phosphate positive electrode slurry by using a second lithium iron manganese phosphate active material, a second conductive agent, and a second binder; and preparing a lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry by using the first slurry, a nickel-cobalt-manganese ternary active material, a first conductive agent, and a first binder; S2. controlling the viscosity difference between the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry to be not higher than 4000 mPa·s, simultaneously coating the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry on the positive electrode current collector, drying, and obtaining the positive electrode sheet; the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry is coated on the side close to the positive electrode current collector, and the lithium iron manganese phosphate positive electrode slurry is coated on the side away from the positive electrode current collector. The viscosity can be understood as the fluidity of the slurry. When the viscosity difference between the upper and lower layers (the second layer and the first layer) is too large, the surface density is not easy to control during the coating process, which can easily lead to a large difference in surface density between the upper and lower layers, and a large difference in the transmission efficiency of electrons and ions between the upper and lower layers, thereby increasing the polarization and reducing the performance of the battery.

[0029] In some embodiments, in S1 or S2, the viscosity of the lithium iron manganese phosphate positive electrode slurry is controlled to be 8000-16000 mPa·s, and the viscosity of the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry is controlled to be 8000-16000 mPa·s.

[0030] According to a third aspect of the present application, a battery is provided, comprising the positive electrode sheet described in the above embodiments.

[0031] In some embodiments, the battery further comprises a negative electrode sheet, and the surface density of the negative electrode sheet is 100 g / m2 -105g / m 2 The thickness of the negative electrode sheet is 127-135 pm. Ensuring that the area density and thickness of the negative electrode sheet are within the above ranges can have a better matching effect with the positive electrode sheet provided in the present application, so that the battery has better cycle performance and thermal stability, and the DCR growth rate is lower.

[0032] In some embodiments, in the negative electrode sheet, the negative electrode active material layer comprises a negative electrode active material, a third conductive agent, and a third binder, and the mass ratio of the negative electrode active material, the third binder, and the third conductive agent is 92-97:1.5-3:1.5-3.5.

[0033] In some embodiments, the negative electrode active material comprises graphite; the third conductive agent comprises conductive carbon black (SP); and the third binder comprises carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR).

[0034] In some embodiments, in the negative electrode sheet, the negative electrode current collector comprises a copper foil.

[0035] In summary, the double-layer positive electrode active material layer positive electrode sheet provided in the present application can effectively improve the electrical conductivity and ion conductivity of the LMFP material, reduce the polarization and DCR growth of the battery, and thus effectively improve the cycle performance and thermal stability of the battery.

[0036] Embodiment 1

[0037] 1. Preparation of the positive electrode sheet

[0038] The positive electrode sheet of the present embodiment was prepared according to the following steps:

[0039] S1. LMFP (LiMnPO4), SP, PVDF, and CNT were mixed and stirred in NMP (N-methyl pyrrolidone) as a solvent at a mass ratio of 96.5:0.9:2:0.6 to obtain an LMFP positive electrode slurry, and the slurry discharge viscosity was controlled to be 25 0.6 Fe 0.4 PO4), SP, PVDF, and CNT were mixed and stirred in NMP (N-methyl pyrrolidone) as a solvent at a mass ratio of 96.5:0.9:2:0.6 to obtain an LMFP positive electrode slurry, and the slurry discharge viscosity was controlled to be 25 o C 8000-16000 mPa·s;

[0040] At the same time, the obtained LMFP positive electrode slurry was mixed with NCM (NCM811), SP, PVDF, and CNT to obtain an LMFP mixed NCM positive electrode slurry, and the slurry discharge viscosity was controlled to be 25 oC 8000-16000 mPa s; wherein, the LMFP positive electrode slurry is added in an amount calculated based on the mass ratio of LMFP being 90% and the mass ratio of NCM (NCM811) being 10% in the final first positive electrode active coating, and the mass ratio of NCM (NCM811), SP, PVDF, and CNT being 96.5:0.9:2:0.6,

[0041] S2. The above two kinds of positive electrode slurries are respectively placed in buffer tanks, and the viscosity difference of the two kinds of positive electrode slurries is controlled to be not higher than 4000 mPa s, a double-layer coating die is used to simultaneously and uniformly coat the two kinds of positive electrode slurries on both sides of a 12 μm aluminum foil (the aluminum foil contains the first positive electrode active coating and the second positive electrode active coating on each side), and the coating thickness of the two layers is respectively controlled, wherein the LMFP mixed NCM positive electrode slurry is coated on the side close to the aluminum foil, and the LMFP positive electrode slurry is coated on the side away from the aluminum foil, and the positive electrode slurry is dried at 90 o C to obtain a positive electrode sheet; finally, the areal density of the first positive electrode active coating and the areal density of the second positive electrode active material layer in the positive electrode sheet prepared in this embodiment are both 250 g / m 2 , and the thickness of the first positive electrode active coating is 107 μm, and the thickness of the second positive electrode active coating is 107 μm.

[0042] 2. Preparation of a negative electrode sheet and a battery

[0043] (1) Preparation of a negative electrode sheet

[0044] Deionized water is used as a solvent, and artificial graphite, SP, CMC, and SBR are mixed and stirred in a mass ratio of 95.5:2:0.5:2 to obtain a negative electrode slurry, and then the negative electrode slurry is coated on a 8 μm copper foil and vacuum dried at 100 o C to obtain a negative electrode sheet. The areal density of the negative electrode sheet prepared in this embodiment is 100 g / m 2 , and the thickness is 125 μm.

[0045] (2) Preparation of a battery

[0046] The vacuum-dried positive and negative electrode sheets and the separator are cut, die-cut, and striped, and then electrolyte is injected under high temperature and negative pressure to assemble a square aluminum shell battery; the electrolyte used is 1 M LiPF6, 1:1.1 (mass ratio) dimethyl carbonate (DMC) and ethylene carbonate (EC).

[0047] Example 2

[0048] 1. Preparation of a positive electrode sheet

[0049] In the preparation of the positive electrode sheet of the present example, the difference from Example 1 is that the gap of the coating pad in the coating equipment is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 251.5 g / m2, and the area density of the second positive electrode active material layer is 248.5 g / m2. 2 The area density of the second positive electrode active material layer is 246.5 g / m2. 2 The rest of the operations are consistent with Example 1.

[0050] 2. Preparation of negative electrode sheet and battery

[0051] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0052] Example 3

[0053] 1. Preparation of positive electrode sheet

[0054] In the preparation of the positive electrode sheet of the present example, the difference from Example 1 is that the gap of the coating pad in the coating equipment is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 253.5 g / m2, and the area density of the second positive electrode active material layer is 246.5 g / m2. 2 The area density of the second positive electrode active material layer is 246.5 g / m2. 2 The rest of the operations are consistent with Example 1.

[0055] 2. Preparation of negative electrode sheet and battery

[0056] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0057] Example 4

[0058] 1. Preparation of positive electrode sheet

[0059] In the preparation of the positive electrode sheet of the present example, the difference from Example 1 is that the gap of the coating pad in the coating equipment is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 253.5 g / m2, and the area density of the second positive electrode active material layer is 246.5 g / m2. 2 The rest of the operations are consistent with Example 1.

[0060] 2. Preparation of negative electrode sheet and battery

[0061] The preparation of the negative electrode sheet and battery in the present example is consistent with Example 1.

[0062] Example 5

[0063] 1. Preparation of positive electrode sheet

[0064] In the preparation of the positive electrode sheet of the present example, different from Example 1 is that by controlling the coating pad control in the coating equipment, the gap of the pad is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer and the area density of the second positive electrode active material layer in the finally prepared positive electrode sheet are both 260 g / m2, and the thicknesses of the first positive electrode active coating layer and the second positive electrode active material layer are 107.5 μm, 107.5 μm respectively. The rest of the operation is consistent with Example 1. 2 , and the thicknesses of the first positive electrode active coating layer and the second positive electrode active material layer are 107.5 μm, 107.5 μm respectively. The rest of the operation is consistent with Example 1.

[0065] 2. Preparation of negative electrode sheet and battery

[0066] The preparation of the negative electrode sheet and the battery in the present example is consistent with Example 1.

[0067] Example 6

[0068] 1. Preparation of positive electrode sheet

[0069] In the preparation of the positive electrode sheet of the present example, different from Example 1 is that by controlling the coating thickness of the NCM positive electrode slurry mixed with LMFP in S2, so that the thickness of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 103 μm, and thus the thickness difference between the first and second positive electrode active material layers is 4 μm. The rest of the operation is consistent with Example 1.

[0070] 2. Preparation of negative electrode sheet and battery

[0071] The preparation of the negative electrode sheet and the battery in the present example is consistent with Example 1.

[0072] 3. Preparation of negative electrode sheet and battery

[0073] The preparation of the negative electrode sheet and the battery in the present example is consistent with Example 1.

[0074] Example 7

[0075] 1. Preparation of positive electrode sheet

[0076] In the preparation of the positive electrode sheet of the present example, different from Example 1 is that by controlling the coating thickness of the two positive electrode slurries in S2, so that the thickness of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 102 μm, the thickness of the second positive electrode active coating layer is 102 μm, and the total thickness of the positive electrode sheet is 214 μm. The rest of the operation is consistent with Example 1.

[0077] 2. Preparation of negative electrode sheet and battery

[0078] The preparation of the negative electrode sheet and the battery in the present example is consistent with Example 1.

[0079] Example 8

[0080] 1. Preparation of positive electrode sheet

[0081] In the preparation of the positive electrode sheet in this embodiment, the difference from that in Example 1 is that, by adjusting the coating thickness of the two positive electrode slurries in S2, the thickness of the first positive electrode active coating and the second positive electrode active coating in the final prepared positive electrode sheet are both 112 μm, and the total thickness of the positive electrode sheet is 234 μm. The remaining operations are the same as in Example 1.

[0082] 2. Preparation of negative electrode and battery

[0083] The preparation of the negative electrode and the battery in this embodiment is the same as in Example 1.

[0084] Example 9

[0085] 1. Preparation of positive electrode sheet

[0086] The positive electrode in this embodiment is the same as that in Embodiment 1.

[0087] 2. Preparation of negative electrode and battery

[0088] In the preparation of the negative electrode sheet in this embodiment, the areal density of the prepared negative electrode sheet was controlled to be 107 g / m³. 2 .

[0089] The battery preparation in this embodiment is the same as in Example 1.

[0090] Example 10

[0091] 1. Preparation of positive electrode sheet

[0092] The positive electrode in this embodiment is the same as that in Embodiment 1.

[0093] 2. Preparation of negative electrode and battery

[0094] In the preparation of the negative electrode sheet in this embodiment, the areal density of the prepared negative electrode sheet was controlled to be 98 g / m². 2 .

[0095] The battery preparation in this embodiment is the same as in Example 1.

[0096] Example 11

[0097] 1. Preparation of positive electrode sheet

[0098] The positive electrode in this embodiment is the same as that in Embodiment 1.

[0099] 2. Preparation of negative electrode and battery

[0100] In the preparation of the negative electrode sheet in this embodiment, the thickness of the prepared negative electrode sheet is controlled to be 124 μm.

[0101] The preparation of the battery in this example is consistent with Example 1.

[0102] Comparative Example 1

[0103] 1. Preparation of the positive electrode sheet

[0104] In the process of preparing the positive electrode sheet of this comparative example, the difference from Example 1 is that by controlling the coating pad in the coating equipment, the gap of the pad is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 250.5 g / m 2 , and the area density of the second positive electrode active material layer is 241.5 g / m 2 . The rest of the operations are consistent with Example 1.

[0105] 2. Preparation of the negative electrode sheet and the battery

[0106] The preparation of the negative electrode sheet and the battery in this comparative example is consistent with Example 1.

[0107] Comparative Example 2

[0108] 1. Preparation of the positive electrode sheet

[0109] In the process of preparing the positive electrode sheet of this comparative example, the difference from Example 1 is that by controlling the coating pad in the coating equipment, the gap of the pad is controlled to adjust the size of the area density, so that the area density of the first positive electrode active coating layer in the finally prepared positive electrode sheet is 250.5 g / m 2 , and the area density of the second positive electrode active material layer is 241.5 g / m 2 . The rest of the operations are consistent with Example 1.

[0110] 2. Preparation of the negative electrode sheet and the battery

[0111] The preparation of the negative electrode sheet and the battery in this comparative example is consistent with Example 1.

[0112] Comparative Example 3

[0113] 1. Preparation of the positive electrode sheet

[0114] In the process of preparing the positive electrode sheet of this comparative example, the difference from Example 1 is that when coating the positive electrode slurry, both layers are coated with LMFP mixed NCM positive electrode slurry, i.e. the active material of the first positive electrode active coating layer and the second positive electrode active coating layer is LMFP+NCM. The rest of the operations are consistent with Example 1.

[0115] 2. Preparation of the negative electrode sheet and the battery

[0116] The preparation of the negative electrode sheet and the battery in this comparative example is consistent with Example 1.

[0117] Comparative Example 4

[0118] 1. Preparation of the positive electrode sheet

[0119] In the preparation of the positive electrode sheet of the present comparative example, the difference from Example 1 is that, when coating the positive electrode slurry, the first layer (the side close to the aluminum foil) is coated with the LMFP positive electrode slurry, and the second layer (the side away from the aluminum foil) is coated with the NCM positive electrode slurry, that is, the active material layers of the first and second positive electrode active material layers are LMFP and NCM respectively; wherein the preparation of the LMFP positive electrode slurry is consistent with Example 1; the preparation of the NCM positive electrode slurry is that, taking water as the solvent, NCM (NCM811), SP, PVDF, and CNT are mixed and stirred at a mass ratio of 96.5:0.9:2:0.6 to obtain the NCM positive electrode slurry, and the slurry discharge viscosity also needs to be controlled at 25 o 8000-16000 mPa·s at 80 °C. The remaining operations are consistent with Example 1.

[0120] 2. Preparation of the negative electrode sheet and the battery

[0121] The preparation of the negative electrode sheet and the battery in the present comparative example is consistent with Example 1.

[0122] Comparative Example 5

[0123] 1. Preparation of the positive electrode sheet

[0124] In the preparation of the positive electrode sheet of the present comparative example, the difference from Example 1 is that, by adjusting the mass ratio of LMFP to NCM (NCM811) when mixing the NCM positive electrode slurry in S1, so that the mass ratio of the first lithium manganese iron phosphate active material to the nickel-cobalt-manganese ternary active material in the first positive electrode active material layer of the final positive electrode sheet is 90.5% to 6%. The remaining operations are consistent with Example 1.

[0125] 2. Preparation of the negative electrode sheet and the battery

[0126] The preparation of the negative electrode sheet and the battery in the present comparative example is consistent with Example 1.

[0127] Comparative Example 6

[0128] 1. Preparation of the positive electrode sheet

[0129] In the preparation of the positive electrode sheet of the present comparative example, the difference from Example 1 is that, by adjusting the mass ratio of LMFP to NCM (NCM811) when mixing the NCM positive electrode slurry in S1, so that the mass ratio of the first lithium manganese iron phosphate active material to the nickel-cobalt-manganese ternary active material in the first positive electrode active material layer of the final positive electrode sheet is 80% to 16.5%. The remaining operations are consistent with Example 1.

[0130] 2. Preparation of the negative electrode sheet and the battery

[0131] The negative electrode sheet and the battery in the present comparative example were prepared in accordance with Example 1.

[0132] Test Example

[0133] 1. Experimental construction

[0134] (1) DCIR test:

[0135] The batteries obtained from each of the examples and each of the comparative examples were subjected to capacity calibration at 25 o C at 1C, and the SOC was adjusted to 50%, and then all the batteries were discharged at 1C for 30s at 50% SOC, the discharge DCIR (DCR) was calculated, DCIR = (cut-off voltage after discharge - cut-off voltage before discharge) / current; and the DCR increase rate after 50 cycles was tested, DCIR increase rate = (DCIR at the 50th cycle - initial DCIR) / initial DCIR, and the test results are shown in Table 1.

[0136] (2) Cycle performance test:

[0137] The batteries obtained from each of the examples and each of the comparative examples were subjected to cycle test at 25°C after standing for 1h at 1C / 1C, and the charge and discharge voltage range was 2.5V-4.25V, the capacity retention rate of the batteries after 500 cycles was tested, and the cycle test results are shown in Table 1.

[0138] (3) Thermal runaway test:

[0139] The batteries obtained from each of the examples and each of the comparative examples were charged to 100% SOC, and then overcharged with 1C current to trigger thermal runaway, and the charging was stopped, and after 1h, the temperature of the large surface (the surface of the shell) of the battery was tested, and the results are shown in Table 1.

[0140] 2. Experimental results

[0141] The relevant electrochemical performance test results of the batteries obtained from Examples 1-11 and Comparative Examples 1-6 are shown in Table 1.

[0142] Table 1. Relevant electrochemical performance test results of the batteries obtained from Examples 1-11 and Comparative Examples 1-6

[0143]

[0144] As can be seen from Table 1, the battery prepared by using the positive electrode sheet provided in the present application has a lower DICR increase rate, and has a higher cycle capacity retention rate after a certain number of cycles, and more importantly, the battery has very good thermal stability, and has a lower thermal runaway temperature, for specific reference, see Examples 1-11.

[0145] In Comparative Example 1, the difference in the area densities of the first and second positive electrode active material layers is 9 g / m2 2 , which is too large, and the efficiencies of electron and ion transmission cannot reach a good balance, resulting in an increased DICR, a decreased cycle retention rate, and an increased thermal runaway temperature.

[0146] In Comparative Example 2, the area densities of the first and second positive electrode active material layers are both too small, the energy density of the battery is decreased, and the DICR, the cycle retention rate, and the thermal runaway temperature of the battery are also affected.

[0147] In Comparative Example 3, the first and second positive electrode active material layers are both coated with a LMFP + NCM positive electrode slurry, that is, the active materials of the first and second positive electrode active material layers are both LMFP + NCM, and the combination of the two active materials is poor, resulting in a decrease in the ion and electron transmission performance during the cycle process, an increased DICR, a decreased cycle retention rate, and an increased thermal runaway temperature.

[0148] In Comparative Example 4, the first layer (close to the aluminum foil) is coated with a LMFP positive electrode slurry, and the second layer (far from the aluminum foil) is coated with a NCM positive electrode slurry, that is, the active materials of the first and second positive electrode active material layers are LMFP and NCM, respectively, and such a combination will result in a decrease in the ion and electron transmission performance during the cycle process, an increased DICR, a decreased cycle retention rate, and an increased thermal runaway temperature, similar to Comparative Example 3.

[0149] In Comparative Examples 5 and 6, the proportion of LMFP in the first positive electrode active material layer is too large or too small, which will result in a decreased energy density or a decreased thermal stability of the battery, and affect the performance of the battery.

[0150] Further comparison between Example 1 and Examples 2-3 shows that, when the difference in the area densities of the first and second positive electrode active material layers is not higher than 7 g / m2 2 , the battery prepared from the positive electrode sheet has good cycle performance and thermal stability, and the DICR is also low. It can also be seen that, when the difference in the area densities of the first and second positive electrode active material layers is 0 (the area densities of the first and second positive electrode active material layers are equal, and in actual situations, the difference in the area densities of the two is very small, close to 0 or equal to 0), the performance of the battery is better, and the battery has better comprehensive performance.

[0151] Comparison between Example 1 and Examples 4-5 shows that, compared with Example 1, the area densities in Examples 4 and 5 are relatively small and large, respectively, which will also affect the performance of the battery in Examples 4 and 5, because too small area density will affect the energy density of the battery, and too large area density will affect the transmission performance of lithium ions and electrons.

[0152] Comparing Example 1 and Examples 6-8, compared with Example 1, the thickness d1 of the first positive electrode active coating in Example 6 is too small, and the thickness difference between the first and second positive electrode active coatings is too large, which affects the lithium ion transmission or electron transmission performance during battery cycling, and thus the battery performance is reduced. In Example 8, both the thickness d1 of the first positive electrode active coating and the thickness d2 of the first positive electrode active coating are too small, and the total thickness of the positive electrode sheet is too small, which reduces the energy density of the battery and also affects the cycle performance and other properties of the battery to some extent. In Example 9, both the thickness d1 of the first positive electrode active coating and the thickness d2 of the first positive electrode active coating are too large, and the total thickness of the positive electrode sheet is too large, which makes the lithium ion transmission or electron transmission path longer, and thus affects the overall cycle performance, resistance, thermal stability and other properties of the battery.

[0153] Comparing Example 1 and Examples 9-11, compared with Example 1, the negative electrode sheet surface density in Example 9 is too large, the negative electrode sheet surface density in Example 10 is too small, and the negative electrode sheet thickness in Example 11 is too small, which will all cause the battery cycle performance to decline. This shows that the negative electrode sheet with a certain surface density and thickness needs to be controlled in order to be more suitable for the positive electrode sheet and to optimize the related performance of the battery together.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a first positive electrode active coating and a second positive electrode active coating sequentially arranged on at least one side of the positive electrode current collector; the first positive electrode active coating is close to one side of the positive electrode current collector, and the first positive electrode active coating comprises a first lithium iron manganese phosphate active material and a nickel-cobalt-manganese ternary active material; the mass percentage of the first lithium iron manganese phosphate active material in the first positive electrode active coating is 82-88%, and the mass percentage of the nickel-cobalt-manganese ternary active material is 7-15%; the second positive electrode active coating is away from one side of the positive electrode current collector, and the second positive electrode active coating comprises a second lithium iron manganese phosphate active material; The face density p1 of the first positive electrode active coating, the face density p2 of the second positive electrode active coating satisfy: p1≥240 g / m 2 , p2≥240 g / m 2 ; and |p1-p2|≤7 g / m 2 .

2. The positive electrode sheet according to claim 1, wherein ρ1 and ρ2 satisfy: ρ1 = ρ2.

3. The positive electrode sheet according to any one of claims 1 to 2, wherein ρ1, ρ2 satisfy: 240 g / m 2 240 g / m < ρ1 < 260 g / m 2 240 g / m 2 240 g / m < ρ2 < 260 g / m 2 .

4. The positive electrode sheet according to any one of claims 1 to 3, wherein The thickness d1 of the first positive electrode active coating and the thickness d2 of the second positive electrode active coating satisfy: d1 ≥ 105 μm, d2 ≥ 105 μm, and |d1-d2| ≤ 3 μm.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein The thickness D of the positive electrode sheet is ≥ 220 μm.

6. The positive electrode sheet according to claim 5, wherein The thickness D of the positive electrode sheet satisfies: 220 μm ≤ D ≤ 230 μm. 7.A method for preparing the positive electrode sheet according to any one of claims 1-6, comprising the following steps: S1. preparing a lithium iron manganese phosphate positive electrode slurry using the second lithium iron manganese phosphate active material, a second conductive agent and a second binder; preparing a lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry using the first slurry, the nickel-cobalt-manganese ternary active material, a first conductive agent and a first binder; S2. controlling the viscosity difference between the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry to be not higher than 4000 mPa·s, simultaneously coating the lithium iron manganese phosphate positive electrode slurry and the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry on the positive electrode current collector, and drying to obtain the positive electrode sheet; the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry is coated on the side close to the positive electrode current collector, and the lithium iron manganese phosphate positive electrode slurry is coated on the side away from the positive electrode current collector.

8. The method of producing a positive electrode sheet according to claim 7, wherein In the S1 or the S2, the viscosity of the lithium iron manganese phosphate positive electrode slurry is controlled to be 8000-16000 mPa·s, and the viscosity of the lithium iron manganese phosphate mixed nickel-cobalt-manganese ternary positive electrode slurry is controlled to be 8000-16000 mPa·s. 9.A battery comprising the positive electrode sheet according to any one of claims 1-6.

10. The battery of claim 9, further comprising a negative electrode sheet having an areal density of 100-105 g / m2. 2 ; The thickness of the negative electrode sheet is 127-135 μm.

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