Composite phosphate positive pole piece, preparation method thereof and battery

Abstract

The invention provides a composite phosphate positive pole piece, a preparation method thereof and a battery, the composite phosphate positive pole piece comprises a current collector, a buffer layer and a positive pole material layer, the buffer layer and the positive pole material layer are respectively laminated on two opposite side surfaces of the current collector, and the buffer layer is arranged between the current collector and the positive pole material layer; active materials in the positive electrode material layer comprise lithium manganese iron phosphate and nickel cobalt lithium manganate. According to the invention, lithium manganese iron phosphate with relatively high specific surface area, relatively small particle size and relatively low tap density is adopted as a first active material, nickel cobalt lithium manganate with relatively low specific surface area, relatively large particle size and relatively high tap density is adopted as a second active material, and the first active material and the second active material are compounded according to a specific mass ratio; the small-particle-size lithium manganese iron phosphate particles can be filled in the stacking gaps among the large-particle-size lithium nickel cobalt manganese oxide particles, so that the compaction density of the composite phosphate positive electrode plate is greatly improved, the high safety performance is kept, and meanwhile, the electrical performance is also remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion secondary battery technology, and more specifically, to a composite phosphate-based positive electrode sheet, its preparation method, and a battery. Background Technology

[0002] Lithium manganese iron phosphate (LMP), as a novel cathode material for lithium-ion rechargeable batteries, introduces metallic manganese ions into its structure, resulting in a higher voltage platform and a significantly improved energy density compared to lithium iron phosphate. Furthermore, LMP shares a similar olivine structure with lithium iron phosphate, exhibiting good thermal stability. Compared to ternary materials, it offers superior safety performance, overall energy efficiency, and overall performance, thus making LMP a promising third-generation cathode material for lithium-ion rechargeable batteries.

[0003] However, due to the addition of manganese, the ionic and electronic conductivity of lithium manganese iron phosphate is at least an order of magnitude lower than that of lithium iron phosphate. Material layers typically compensate for this lower ionic and electronic conductivity by reducing the particle size of lithium manganese iron phosphate; therefore, the specific surface area of ​​lithium manganese iron phosphate is usually >15 m². 2 / g, however, this will affect the material's processing performance and specific capacity. Insufficient manufacturing maturity and electrical properties hinder the commercialization of the material.

[0004] In view of the above, this application is hereby submitted. Summary of the Invention

[0005] The main objective of this invention is to provide a composite phosphate-based positive electrode sheet, its preparation method, and a battery, in order to solve the problem that in the prior art, lithium manganese iron phosphate materials in lithium-ion battery applications usually compensate for ionic and electronic conductivity by reducing the particle size of the material, but this leads to poor processing performance and electrical performance of the material.

[0006] To achieve the above objectives, according to one aspect of the present invention, a composite phosphate-based positive electrode sheet is provided, comprising a current collector, and a buffer layer and a positive electrode material layer respectively stacked on opposite sides of the current collector, wherein the buffer layer is disposed between the current collector and the positive electrode material layer; wherein the positive electrode material layer comprises an active material, a first conductive agent, and a first binder; the active material is composed of a first active material and a second active material, and the mass ratio of the two is (60-72):(25-38); the first active material is lithium manganese iron phosphate, wherein the D50 particle size of lithium manganese iron phosphate is 0.4-1.5 μm and the specific surface area is 10-23 m². 2 / g; the second active material is lithium nickel cobalt manganese oxide, with a D50 particle size of 2.0–5.0 μm and a specific surface area of ​​0.3–2.0 m². 2 / g.

[0007] Furthermore, by weight percentage, the positive electrode material layer comprises: 60% to 72% of a first active material, 25% to 38% of a second active material, 0.5% to 2.5% of a first conductive agent, and 1% to 3% of a first binder.

[0008] Furthermore, the general chemical formula of lithium manganese iron phosphate is LiFe. x Mn y PO4, 0.35≤x≤0.55, 0.45≤y≤0.65.

[0009] Furthermore, the general chemical formula of lithium nickel cobalt manganese oxide is LiNi a Co b Mn 1-a-b O2, a = 0 or 0.55 ≤ a ≤ 0.9, 0 ≤ b ≤ 0.2, 0 ≤ 1 - ab ≤ 1.

[0010] Furthermore, the first conductive agent includes the active ingredients in solid conductive powder and / or liquid conductive slurry.

[0011] Furthermore, the solid conductive powder is conductive carbon black;

[0012] Furthermore, the effective components in the liquid conductive paste include at least one of carbon nanotubes, graphene, and conductive carbon black.

[0013] Furthermore, the first adhesive is polyvinylidene fluoride.

[0014] Furthermore, the current collector is made of aluminum alloy.

[0015] Furthermore, the material of the buffer layer includes a second conductive agent, a second binder, and a dispersant.

[0016] Furthermore, the mass ratio of the second conductive agent, the second binder, and the dispersant is (80-90):(8-12):(2-8).

[0017] Furthermore, the second conductive agent includes at least one of amorphous carbon, carbon black, carbon nanotubes, graphite, graphene, etc.

[0018] Furthermore, the second adhesive is polyvinylidene fluoride.

[0019] Furthermore, the dispersant includes at least one of CMC, PVP, and PEG.

[0020] Furthermore, the thickness of the current collector is 10–15 μm.

[0021] Furthermore, the thickness of the buffer layer on one side is 0.5–5 μm.

[0022] Furthermore, the thickness of the composite phosphate-based positive electrode sheet is 120–190 μm.

[0023] Furthermore, the areal loading of the cathode material layer is 320–460 g / m². 2 .

[0024] Furthermore, the preparation method of the composite phosphate-based positive electrode sheet includes the following steps: Step S1, mixing the first active material, the second active material, the first conductive agent, the first binder and the first solvent to obtain a positive electrode slurry; Step S2, providing a buffer slurry, coating the buffer slurry onto the opposite surfaces of the current collector, and drying to obtain a buffer layer electrode sheet; Step S3, coating the positive electrode slurry onto the opposite surfaces of the buffer layer electrode sheet, and sequentially drying and rolling to obtain the composite phosphate-based positive electrode sheet.

[0025] According to a third aspect of the present invention, a battery is provided, the battery including a positive electrode sheet, the positive electrode sheet being a composite phosphate-based positive electrode sheet provided in the first aspect or a composite phosphate-based positive electrode sheet obtained by the preparation method provided in the second aspect.

[0026] Applying the technical solution of this invention, this application uses lithium manganese iron phosphate, which has a high specific surface area, small particle size, and low tap density, as the first active material, and lithium nickel cobalt manganese oxide, which has a low specific surface area, large particle size, and high tap density, as the second active material. The active material obtained by compounding the first and second active materials in a specific mass ratio has physical properties such as specific surface area, particle size, and tap density that are between the two, greatly optimizing the processing performance of the active material. Simultaneously, by utilizing the synergistic effect of particles of different sizes in a specific mass ratio, the small-diameter lithium manganese iron phosphate particles can fill the stacking gaps between the large-diameter lithium nickel cobalt manganese oxide particles, thereby greatly improving the compaction density of the composite phosphate-based positive electrode sheet while retaining the high safety performance of the composite phosphate-based positive electrode sheet, with no significant reduction in safety compared to conventional lithium iron phosphate electrodes.

[0027] Furthermore, by utilizing the high specific capacity and low specific surface area of ​​lithium nickel cobalt manganese oxide material, its combination with lithium manganese iron phosphate at a specific mass ratio significantly improves the electrical performance of composite phosphate-based cathode sheets, including energy density, cycle stability, and power characteristics. Detailed Implementation

[0028] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0029] As described in the background section of this application, lithium manganese iron phosphate materials are currently used in lithium-ion battery applications by reducing the particle size to compensate for ionic and electronic conductivity. However, this results in poor processing performance and electrical performance. To address this issue, this application provides a composite phosphate-based positive electrode sheet, its preparation method, and a battery thereof.

[0030] In a first typical embodiment of this application, a composite phosphate-based positive electrode sheet is provided. This composite phosphate-based positive electrode sheet includes a current collector, and a buffer layer and a positive electrode material layer respectively stacked on opposite sides of the current collector, with the buffer layer disposed between the current collector and the positive electrode material layer. The positive electrode material layer includes an active material, a first conductive agent, and a first binder. The active material is composed of a first active material and a second active material, with a mass ratio of (60–72):(25–38). The first active material is lithium manganese iron phosphate, with a D50 particle size of 0.4–1.5 μm and a specific surface area of ​​10–23 m². 2 / g; the second active material is lithium nickel cobalt manganese oxide, with a D50 particle size of 2–5 μm and a specific surface area of ​​0.3–2.0 m². 2 / g.

[0031] Lithium manganese iron phosphate (LMP) is formed by adding manganese to lithium iron phosphate (LFP) active material. Compared to LFP, LMP has a higher voltage plateau and energy density, and it shares a similar crystal structure with LFP, resulting in better safety performance. However, because the conductivity of manganese ions is lower than that of iron ions, the conductivity and ion kinetics performance of LMP are weaker than those of LFP. Reducing the particle size of LFP can improve ionic and electronic conductivity, but it also increases the specific surface area of ​​LMP, thus affecting its processing properties such as slurry solids content and compaction density.

[0032] For the reasons mentioned above, the composite phosphate-based positive electrode sheet provided by the present invention first attaches a buffer layer to both sides of the current collector that are arranged opposite to each other, and then attaches a positive electrode material layer to the surface of the buffer layer, forming a structure of positive electrode material layer-buffer layer-current collector-buffer layer-positive electrode material layer. The setting of the buffer layer effectively improves the adhesion between the positive electrode material layer and the current collector, and also reduces the contact resistance.

[0033] Meanwhile, lithium manganese iron phosphate, with its high specific surface area, small particle size, and low tap density, is used as the first active material, while lithium nickel cobalt manganese oxide, with its low specific surface area, large particle size, and high tap density, is used as the second active material. The active material obtained by blending the first and second active materials in a specific mass ratio has physical properties such as specific surface area, particle size, and tap density that fall between the two, greatly optimizing the processing performance of the active material. Furthermore, by utilizing the synergistic effect of the specific mass ratio of large and small particles, the small-diameter lithium manganese iron phosphate particles can fill the stacking gaps between the large-diameter lithium nickel cobalt manganese oxide particles, thereby significantly improving the compaction density of the composite phosphate-based positive electrode sheet while maintaining its high safety performance. Compared to conventional lithium iron phosphate electrodes, there is no significant reduction in safety.

[0034] Furthermore, by utilizing the high specific capacity and low specific surface area of ​​lithium nickel cobalt manganese oxide material, its combination with lithium manganese iron phosphate at a specific mass ratio significantly improves the electrical performance of composite phosphate-based cathode sheets, including energy density, cycle stability, and power characteristics.

[0035] Typical, but not limiting, the mass ratios of the first and second active materials in the active materials provided in this application are, for example, 60:38, 62:36, 64:34, 66:32, 68:30, 70:28, 72:25, or any range of two such ratios. The D50 particle size of lithium manganese iron phosphate is, for example, 0.4 μm, 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, or any range of two such ratios, and the specific surface area of ​​lithium manganese iron phosphate is, for example, 10 m². 2 / g、12m 2 / g、14m 2 / g, 16m 2 / g、18m 2 / g、21m 2 / g、23m 2 / g or a range of any two values; the D50 particle size of lithium nickel cobalt manganese oxide is 2.0μm, 3.0μm, 4.0μm, 5.0μm or a range of any two values; the specific surface area of ​​lithium nickel cobalt manganese oxide is 0.3m². 2 / g, 0.5m 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.3m 2 / g, 1.7m 2 / g, 2.0m 2 / g or a range of values ​​consisting of any two numbers.

[0036] In some embodiments, the positive electrode material layer comprises, by weight percentage: 60% to 72% of a first active material, 25% to 38% of a second active material, 0.5% to 2.5% of a first conductive agent, and 1% to 3% of a first binder. The combination of each group within the above range further improves the processability of the positive electrode sheet and the electrochemical performance of the battery.

[0037] Typically, but not limitingly, in the positive electrode material layer provided in this application, the content of the first active material, by mass percentage, is 60%, 62%, 64%, 66%, 68%, 70%, 72%, or any combination of two values; the content of the second active material is 25%, 28%, 32%, 34%, 36%, 38%, or any combination of two values; the content of the first conductive agent is 0.5%, 1%, 1.5%, 2%, 2.5%, or any combination of two values; and the content of the first binder is 1%, 1.5%, 2%, 2.5%, 3%, or any combination of two values.

[0038] In some embodiments, the general chemical formula of lithium manganese iron phosphate is LiFe. x Mn y With PO4, 0.35≤x≤0.55, 0.45≤y≤0.65, the lithium iron phosphate material with this preferred ratio has high specific capacity, high voltage plateau, high stability, and excellent processing performance.

[0039] Typically, but not limitingly, in the lithium manganese iron phosphate provided in this application, x is a range of values ​​such as 0.35, 0.38, 0.40, 0.42, 0.45, 0.50, 0.52, 0.55, or any two of these values; and y is a range of values ​​such as 0.45, 0.48, 0.50, 0.55, 0.58, 0.60, 0.62, 0.65, or any two of these values.

[0040] In some embodiments, due to the layered structure of lithium nickel cobalt manganese oxide (LCO), oxygen release occurs at high temperatures. Therefore, LCO cells exhibit a combustion-supporting effect during thermal runaway, resulting in significantly lower safety performance compared to active materials with an olivine structure, such as lithium iron phosphate (LFP). The use of LCO as a secondary active material in combination with LFP has a significant impact on the safety performance of the composite cell. Therefore, to ensure the safety performance of the composite system, the selection of LCO, including its nickel content, and the proportion of LCO added to the composite system are optimized. Preferably, the general chemical formula of LCO is LiNi. a Co b Mn 1-a-bO2, a = 0 or 0.55 ≤ a ≤ 0.9, 0 ≤ b ≤ 0.2, 0 ≤ 1 - ab ≤ 1, to further ensure the safety performance of the positive electrode, and there is no significant attenuation compared to lithium iron phosphate. In addition, it can further improve electrical performance and processing performance.

[0041] Typically, but not limitingly, in the lithium nickel cobalt manganese oxide provided in this application, a is 0, or is a range of values ​​such as 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, or any two of these values; b is a range of values ​​such as 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, or any two of these values.

[0042] To adapt to the system of mixed particles of different sizes, the first conductive agent preferably includes the effective components in solid conductive powder and / or liquid conductive slurry. In order to further meet the system compatibility requirements of high specific surface area lithium manganese iron phosphate, the solid conductive powder is preferably conductive carbon black. At the same time, liquid conductive slurry is used in combination with different amounts of lithium nickel cobalt manganese oxide. The effective components in the liquid conductive slurry preferably include any one or more of carbon nanotubes, graphene, and conductive carbon black. By constructing a conductive network connecting particles of different sizes, a reliable conductive framework is provided for the entire cathode material layer.

[0043] In some embodiments, the liquid conductive paste is composed of an active ingredient and a solvent, preferably including NMP (N-methylpyrrolidone) and / or PVP (polyvinylpyrrolidone), and preferably the mass content of the active ingredient in the liquid conductive paste is 3-6%.

[0044] To further improve the bonding performance, polyvinylidene fluoride is preferred as the first adhesive.

[0045] To further improve the electrical performance of the battery, the current collector is preferably made of aluminum alloy, and the thickness of the current collector is preferably 10-15 μm.

[0046] In some embodiments, the material of the buffer layer includes a second conductive agent, a second binder, and a dispersant; in order to further improve the adhesion between the positive electrode material and the current collector, and at the same time reduce the internal resistance, the mass ratio of the second conductive agent, the second binder, and the dispersant is preferably (80-90):(8-12):(2-8).

[0047] Typical, but not limiting, the mass ratios of the second conductive agent, the second binder, and the dispersant are, for example, 80:12:8, 85:10:5, 87:9:4, 90:8:2, or any range of two values.

[0048] To further reduce internal resistance, the second conductive agent preferably includes any one or more of amorphous carbon, carbon black, carbon nanotubes, graphite, graphene, etc.

[0049] To further improve the bonding performance, polyvinylidene fluoride is preferred as the second adhesive.

[0050] To promote uniform particle dispersion and prevent agglomeration, the preferred dispersant includes at least one of CMC (sodium carboxymethyl cellulose), PVP (polyvinylpyrrolidone), and PEG (polyethylene glycol).

[0051] To ensure better processing performance of the cathode material, the thickness of the buffer layer on either side of the current collector is 0.5–5 μm (e.g., 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or any combination of two values); preferably, the thickness of the composite phosphate-based cathode sheet is 120–190 μm (e.g., 120 μm, 140 μm, 160 μm, 180 μm, 190 μm or any combination of two values).

[0052] In some embodiments, the preparation method of the positive electrode sheet includes the following steps: Step S1, mixing a first active material, a second active material, a first conductive agent, a first binder, and a first solvent to obtain a positive electrode slurry; Step S2, providing a buffer slurry, coating the buffer slurry onto the oppositely disposed surfaces of the current collector, and drying to obtain a buffer layer electrode sheet; Step S3, coating the positive electrode slurry onto the oppositely disposed surfaces of the buffer layer electrode sheet, and sequentially drying and rolling to obtain the positive electrode sheet. The preparation method provided in this application is simple to operate and suitable for industrial production.

[0053] To simplify the preparation method, it is preferable to mix the second conductive agent, the second binder, the dispersant and the second solvent to obtain a buffer slurry.

[0054] To further promote the dissolution and dispersion of the components, the first solvent is preferably N-methylpyrrolidone, and the second solvent is preferably deionized water.

[0055] In some embodiments, the drying temperature in step S3 is 80–110°C.

[0056] To fully utilize the electrochemical performance of the cathode material, the preferred areal loading of the cathode material layer is 320–460 g / m². 2 (e.g., 320g / m) 2 340g / m 2 360g / m 2 380g / m 2 400g / m 2 420g / m 2 440g / m2 460g / m 2 (or any range of two values), the areal load of the positive electrode material layer in this application refers to the total areal load of all positive electrode material layers in the positive electrode sheet.

[0057] In a third typical embodiment of this application, a battery is provided, which includes a positive electrode sheet, which is a composite phosphate-based positive electrode sheet provided in the first typical embodiment above, or a composite phosphate-based positive electrode sheet obtained by the preparation method provided in the second typical embodiment above.

[0058] Because the battery includes the aforementioned composite phosphate-based positive electrode, it exhibits excellent electrochemical performance.

[0059] The beneficial effects of this application will be further illustrated below with reference to embodiments and comparative examples.

[0060] Example 1

[0061] This embodiment provides a method for preparing a composite phosphate-based positive electrode sheet, the specific steps of which are as follows:

[0062] (1) The first active material LiFe 0.4 Mn 0.6 PO4, second active material LiNi 0.7 Co 0.08 Mn 0.22 O2, a first conductive agent, and polyvinylidene fluoride were mixed with N-methylpyrrolidone in a mass ratio of 72:25:2:1 and thoroughly dispersed to obtain a stable positive electrode slurry; wherein, the D50 particle size of the first active material was 0.57 μm and the specific surface area was 21.8 m². 2 / g, the D50 particle size of the second active material is 2.26μm, and the specific surface area is 1.83m². 2 / g; The first conductive agent includes conductive carbon black (solid conductive powder) and carbon nanotubes (stored in liquid conductive slurry, the solvent is NMP), and the mass ratio of conductive carbon black to carbon nanotubes is 1:1;

[0063] (2) Graphene, polyvinylidene fluoride, and dispersant CMC (sodium carboxymethyl cellulose) are mixed with deionized water at a mass ratio of 85:10:5 and fully dispersed to obtain a buffer slurry;

[0064] (3) Select aluminum alloy foil material with a thickness of 13μm, coat the two opposite sides of the foil with buffer slurry, dry at 120℃ to form a buffer layer, and obtain a buffer layer electrode, wherein the thickness of the buffer layer on one side is 1μm.

[0065] (4) Subsequently, positive electrode slurry was coated on both sides of the buffer layer electrode sheet that were facing each other. The total areal loading of the positive electrode slurry was 360 g / m².2 The material is dried at varying temperatures of 80–120°C to form a positive electrode material layer, which is then rolled to obtain a composite phosphate-based positive electrode sheet; the thickness of the composite phosphate-based positive electrode sheet is 160 μm.

[0066] Example 2

[0067] This embodiment provides a method for preparing a composite phosphate-based positive electrode sheet, the specific steps of which are as follows:

[0068] (1) The first active material LiFe 0.35 Mn 0.65 PO4, second active material LiNi 0.9 Co 0.05 Mn 0.05 O2, conductive carbon black, and polyvinylidene fluoride were mixed with N-methylpyrrolidone in a mass ratio of 60:38:1:1 and thoroughly dispersed to obtain a stable positive electrode slurry; wherein, the D50 particle size of the first active material was 0.72 μm and the specific surface area was 18.9 m². 2 / g, the D50 particle size of the second active material is 3.95μm, and the specific surface area is 0.62m². 2 / g; The first conductive agent includes conductive carbon black (solid conductive powder) and carbon nanotubes (stored in liquid conductive slurry, the solvent is NMP), and the mass ratio of conductive carbon black to carbon nanotubes is 1:1;

[0069] (2) Graphene, polyvinylidene fluoride, and dispersant CMC (sodium carboxymethyl cellulose) are mixed with deionized water at a mass ratio of 87:9:4 and fully dispersed to obtain a buffer slurry;

[0070] (3) Select aluminum alloy foil material with a thickness of 13μm, coat the two opposite sides of the foil with buffer slurry, and dry at 125℃ to obtain buffer layer electrode, wherein the thickness of the buffer layer on one side is 1μm;

[0071] (4) Subsequently, positive electrode slurry was coated on both sides of the buffer layer electrode sheet that were facing each other. The total areal loading of the positive electrode slurry was 460 g / m². 2 The material is dried at varying temperatures of 80–120°C to form a positive electrode material layer, which is then rolled to obtain a composite phosphate-based positive electrode sheet; the thickness of the composite phosphate-based positive electrode sheet is 190 μm.

[0072] Example 3

[0073] The difference from Example 1 is that in step (4) of this example, the total areal loading of the positive electrode slurry is adjusted to 320 g / m². 2 After further rolling, a composite phosphate-based positive electrode sheet with a thickness of 130 μm is obtained.

[0074] Example 4

[0075] The difference from Example 1 is that in this example, the mass ratio of the first active material, the second active material, the conductive agent, and the binder is adjusted to 60:36:2.5:1.5 in step (1).

[0076] Example 5

[0077] The difference from Example 1 is that in this example, the mass ratio of the first active material, the second active material, the conductive agent, and the binder is adjusted to 65:30:2:3 in step (1).

[0078] Example 6

[0079] The difference from Example 5 is that in step (1) of this example, the chemical formula of the first active material is adjusted to LiFe. 0.55 Mn 0.45 PO4, adjust the chemical formula of the second active material to LiNi 0.55 Co 0.1 Mn 0.35 O2.

[0080] Example 7

[0081] The difference from Example 5 is that in step (1) of this example, the D50 particle size of the first active material is adjusted to 0.4 μm and the specific surface area is 23 m². 2 / g, the D50 particle size of the second active material is 5.0μm, and the specific surface area is 0.5m². 2 / g.

[0082] Example 8

[0083] The difference from Example 5 is that in step (1) of this example, the D50 particle size of the first active material is adjusted to 1.5 μm and the specific surface area is 10 m². 2 / g, the D50 particle size of the second active material is 2.0μm, and the specific surface area is 2.0m². 2 / g.

[0084] Example 9

[0085] The difference from Example 5 is that in step (1) of this example, the chemical formula of the second active material is adjusted to LiNi. 0.45 Co 0.35 Mn 0.2 O2.

[0086] Comparative Example 1

[0087] The difference from Example 5 is that, in this comparative example, the second active material LiNi was not added to the positive electrode slurry in step (1). 0.7 Co0.08 Mn 0.22 O2, and adjust the mass ratio of the first active material, conductive agent, and binder to 95:2:3.

[0088] Comparative Example 2

[0089] The difference from Example 5 is that in step (1) of this comparative example, the mass ratio of the first active material, the second active material, the conductive agent, and the binder is adjusted to 80:18:0.5:1.5.

[0090] Comparative Example 3

[0091] The difference from Example 5 is that in step (1) of this comparative example, the mass ratio of the first active material, the second active material, the conductive agent, and the binder is adjusted to 50:45:2:3.

[0092] Comparative Example 4

[0093] The difference from Example 5 is that, in step (1), the D50 particle size of the first active material was adjusted to 2.0 μm and the specific surface area to 7.8 m². 2 / g, the D50 particle size of the second active material is 5.9μm, and the specific surface area is 0.36m². 2 / g.

[0094] Comparative Example 5

[0095] The difference from Example 5 is that, in step (1), the D50 particle size of the first active material was adjusted to 0.28 μm and the specific surface area to 24.9 m². 2 / g, the D50 particle size of the second active material is 1.4μm, and the specific surface area is 6.9m². 2 / g.

[0096] Test case

[0097] The composite phosphate-based positive electrode, negative electrode, separator, and electrolyte provided in the above embodiments and comparative examples were used to assemble 110Ah square batteries; wherein,

[0098] Preparation method of negative electrode sheet: add artificial graphite, conductive agent and binder to deionized water at a mass ratio of 97:1.2:1.8 and disperse evenly to obtain a stable negative electrode slurry, which is then coated on the surface of copper foil. At the same time, the coating surface loading satisfies: negative electrode specific capacity * negative electrode surface loading / positive electrode specific capacity / positive electrode surface loading = 1.12.

[0099] Electrolyte: The solvent system is EC (ethylene carbonate):DEC (dimethyl carbonate):EMC (ethyl methyl carbonate) = 1:1:1, and LiPF6 is the lithium salt (concentration of 1.1M). Film-forming additives such as VC (ethylene carbonate), PS (propylene sulfite), DTD (ethylene sulfate), and LiODFB (lithium difluorooxalatoborate) can be selected.

[0100] Battery assembly: The positive and negative electrode sheets are wound and assembled, with the negative electrode covering the positive electrode by ≥1mm and the separator covering the negative electrode by ≥1mm, to form a 110Ah square battery. After the battery is sorted and tested, the basic performance data of the cell is obtained.

[0101] Formation steps: (1) Charge to 3.0V at 0.1C; (2) Charge to 3.3V at 0.15C; (3) Charge to a total charging capacity of 33Ah at 0.33C; (4) Let stand for 5 minutes to complete the formation.

[0102] Capacity testing steps: (1) Charge to 4.3V at 0.33C, then charge at constant current and constant voltage to the cutoff voltage of 0.05C; (2) Discharge to 2.5V at 0.33C; (3) Let stand for 5 minutes to complete the capacity testing.

[0103] Electrochemical tests were performed on the batteries assembled in the above embodiments and comparative examples, such as energy density, cycle capacity retention, and hybrid pulse power characteristic test (HPPC test). The results are shown in Table 1.

[0104] 1. Test method for energy density: (1) Discharge to 2.5V at 0.33C and let stand for 30min; (2) Discharge to 2.5V at 0.05C and let stand for 30min; (3) Charge to 4.3V at 0.33C constant current and constant voltage, cut off current at 0.05C and let stand for 30min; (4) Discharge to 2.5V at 0.33C; Repeat steps 3)-4) a total of 3 times, take the last test discharge result, and calculate the energy density based on energy / cell mass.

[0105] 2. Test method for cycle capacity retention rate: (1) Charge to 4.2V with 1C, charge to 4.3V with 0.5C constant current and constant voltage, cut off current 0.05C, and let stand for 30min; (2) Discharge to 2.5V with 1C, let stand for 30min, the discharge capacity is the discharge capacity of the first cycle; repeat steps (1)-(2) a total of n times, each repetition can obtain the corresponding nth cycle discharge capacity Cn, the capacity retention rate calculation formula = Cn / [(C1+C2+C3+...+C10) / 10].

[0106] 3. HPPC power test method: (1) Obtain the cell discharge capacity C0 according to the energy density test method; (2) Charge to 4.3V with constant current and constant voltage at 0.33C, cut-off current 0.05C, and let stand for 30min; (3) Discharge to cut-off capacity = 0.5C0 at 1C, and let stand for 30min; (4) Test the voltage V0 before testing, discharge at 5C0 rate for 10s, and the voltage V1 at the end of the 10s discharge. Cell DCR(R) = (V0-V1) / 5C0; HPPC power calculation formula = V0(V0-2.5) / R1.

[0107] Table 1

[0108]

[0109]

[0110]

[0111] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0112] This application employs lithium manganese iron phosphate (LMFP), which has a high specific surface area and a small particle size, as the first active material, and lithium nickel cobalt manganese oxide (LCO), which has a low specific surface area and a large particle size, as the second active material. By utilizing the synergistic effect of particles of different sizes in a specific mass ratio, the smaller LFP particles can fill the stacking gaps between the larger LCO particles, thereby significantly improving the compaction density of the active material. Simultaneously, the specific surface area, particle size, and tap density of the active material provided in this application fall between the two, greatly optimizing the processing performance of LFP. Furthermore, the high specific capacity and low specific surface area of ​​LCO material significantly improve the electrochemical performance of the battery cell, including energy density, cycle stability, and power characteristics.

[0113] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite phosphate-based positive electrode, characterized in that, It includes a current collector, and a buffer layer and a positive electrode material layer respectively stacked on two opposite surfaces of the current collector, wherein the buffer layer is disposed between the current collector and the positive electrode material layer; The positive electrode material layer includes an active material, a first conductive agent, and a first binder; the active material is composed of a first active material and a second active material, and the mass ratio of the two is (60-72):(25-38); The first active material is lithium manganese iron phosphate, wherein the D50 particle size of the lithium manganese iron phosphate is 0.4–1.5 μm and the specific surface area is 10–23 m². 2 / g; the second active material is lithium nickel cobalt manganese oxide, wherein the D50 particle size of the lithium nickel cobalt manganese oxide is 2.0–5.0 μm and the specific surface area is 0.3–2.0 m². 2 / g.

2. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The positive electrode material layer comprises, by weight percentage: 60% to 72% of the first active material, 25% to 38% of the second active material, 0.5% to 2.5% of the first conductive agent, and 1% to 3% of the first binder.

3. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The general chemical formula of the lithium manganese iron phosphate is LiFe. x Mn y PO4, 0.35≤x≤0.55, 0.45≤y≤0.

65.

4. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The general chemical formula of the lithium nickel cobalt manganese oxide is LiNi a Co b Mn 1-a-b O2, a = 0 or 0.55 ≤ a ≤ 0.9, 0 ≤ b ≤ 0.2, 0 ≤ 1 - ab ≤ 1.

5. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The first conductive agent includes the effective components in solid conductive powder and / or liquid conductive slurry; Preferably, the solid conductive powder is conductive carbon black; Preferably, the effective components in the liquid conductive slurry include at least one of carbon nanotubes, graphene, and conductive carbon black.

6. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The first adhesive is polyvinylidene fluoride; And / or, the material of the current collector is an aluminum alloy.

7. The composite phosphate-based positive electrode sheet according to claim 1, characterized in that, The material of the buffer layer includes a second conductive agent, a second binder, and a dispersant; Preferably, the mass ratio of the second conductive agent, the second binder, and the dispersant is (80-90): (8～12)：(2～8)； Preferably, the second conductive agent includes at least one of amorphous carbon, carbon black, carbon nanotubes, graphite, graphene, etc. Preferably, the second adhesive is polyvinylidene fluoride; Preferably, the dispersant includes at least one of CMC, PVP, and PEG.

8. The composite phosphate-based positive electrode sheet according to any one of claims 1 to 7, characterized in that, The thickness of the current collector is 10–15 μm; And / or, the thickness of the buffer layer on one side is 0.5 to 5 μm; And / or, the thickness of the composite phosphate-based positive electrode sheet is 120–190 μm; And / or, the areal loading of the positive electrode material layer is 320–460 g / m². 2 .

9. A method for preparing a composite phosphate-based positive electrode sheet according to any one of claims 1 to 8, characterized in that, The preparation method includes the following steps: Step S1: Mix the first active material, the second active material, the first conductive agent, the first binder and the first solvent to obtain a positive electrode slurry; Step S2: Provide a buffer slurry, coat the buffer slurry onto the two opposite surfaces of the current collector, and dry to obtain a buffer layer electrode. Step S3: The positive electrode slurry is coated onto the two opposite surfaces of the buffer layer electrode, and then dried and rolled to obtain the composite phosphate-based positive electrode.

10. A battery, characterized in that, The battery includes a positive electrode sheet, which is a composite phosphate-based positive electrode sheet according to any one of claims 1 to 8, or a composite phosphate-based positive electrode sheet obtained by the preparation method described in claim 9.

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