Positive electrode sheet and manufacturing method therefor, and battery cell

Abstract

The present application provides a positive electrode sheet and a manufacturing method therefor, and a battery cell. The positive electrode sheet comprises a current collector, and a first film layer and a second film layer which are sequentially arranged on the current collector. The material of the first film layer comprises a lithium-rich manganese-based material, and the material of the second film layer comprises lithium transition metal phosphate.

Description

Positive electrode sheet and its preparation method and battery cell

[0001] This application claims priority to Chinese Patent Application No. 202411529190.6, filed on October 29, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, specifically to a positive electrode sheet, its preparation method, and a battery cell. Background Technology

[0003] Lithium transition metal phosphates with an olivine structure possess advantages such as being non-toxic, pollution-free, highly safe, and low-cost, making them highly promising cathode materials for lithium-ion batteries. However, the rapid capacity decay of lithium transition metal phosphate materials during cycling severely hinders their application. To address this issue, one improvement method in related technologies is to directly dope lithium-rich manganese-based materials to compensate for the loss of active lithium in lithium transition metal phosphate materials, thereby enhancing the long-cycle performance of the lithium transition metal phosphate system. Invention Overview

[0004] However, due to the poor intrinsic electron and ion conduction of lithium transition metal phosphates, their polarization is high during charging and discharging. Direct mixing with lithium-rich manganese-based materials can easily lead to overcharging of the lithium-rich manganese-based materials, resulting in a large and irreversible phase transition of Li2MnO3, rapid material structure decay, and significant loss of active lithium, which is not conducive to long-cycle lithium replenishment.

[0005] This application provides a positive electrode sheet, including a current collector and a first film layer and a second film layer sequentially disposed on the current collector. The material of the first film layer includes a lithium-rich manganese-based material, and the material of the second film layer includes a lithium transition metal phosphate.

[0006] This application also provides a method for preparing a positive electrode sheet, comprising the following steps: providing a current collector and a first film layer slurry, disposing the first film layer slurry on at least one side of the current collector to obtain a first film layer disposed on the current collector, wherein the first film layer slurry comprises a lithium-rich manganese-based material; providing a second film layer slurry, disposing the second film layer slurry on the side of the first film layer opposite to the current collector to obtain a positive electrode sheet, wherein the second film layer slurry comprises a lithium transition metal phosphate.

[0007] This application also provides a battery cell, including the above-mentioned positive electrode sheet, or a positive electrode sheet prepared by the above-mentioned method for preparing the positive electrode sheet. Beneficial effects

[0008] The positive electrode provided in this application, through layered arrangement, allows lithium transition metal phosphate and lithium-rich manganese-based material to be arranged in layers, thereby reducing the probability of lattice cracks caused by the difference in expansion and contraction between the two during direct mixing and maintaining the stability of the conductive network. By placing lithium transition metal phosphate in the second film layer and lithium-rich manganese-based material in the first film layer, overcharging of lithium-rich manganese-based material can be effectively alleviated, the structural stability of lithium-rich manganese-based material can be maintained, and lithium can be slowly released from the Li2MnO3 phase, thereby stably and persistently replenishing lithium for lithium transition metal phosphate. Attached Figure Description

[0009] Figure 1 is a scanning electron microscope (SEM) image (3000x magnification) of a cross section of the positive electrode sheet provided in Embodiment 1 of this application.

[0010] Figure 2 is a scanning electron microscope (SEM) image (5000x magnification) of a cross section of the positive electrode sheet provided in Embodiment 1 of this application. Embodiments of the present invention

[0011] In the description of this embodiment, the terms "upper," "lower," "left," "right," "front," and "rear," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first" and "second" are used for distinction in description and have no special meaning.

[0012] The technical solution of this application is as follows:

[0013] In a first aspect, embodiments of this application provide a positive electrode sheet, including a current collector and a first film layer and a second film layer sequentially disposed on the current collector. The material of the first film layer includes a lithium-rich manganese-based material, and the material of the second film layer includes a lithium transition metal phosphate.

[0014] In this application, the lithium transition metal phosphate and the layered oxide (lithium-rich manganese-based material) particles are of different sizes and have significantly different degrees of expansion and contraction. By layering them, the particles can be oriented, reducing the probability of lattice cracks caused by the difference in expansion and contraction between the two when directly mixed, thus maintaining the stability of the conductive network. By placing the lithium transition metal phosphate in the second film layer and the lithium-rich manganese-based material in the first film layer, the overcharging of the lithium-rich manganese-based material can be effectively alleviated, the structural stability of the lithium-rich manganese-based material can be maintained, and the lithium can be slowly released from the Li2MnO3 phase, thereby stably and persistently replenishing the lithium for the phosphate.

[0015] In some embodiments, the material of the first film layer further includes lithium manganese oxide.

[0016] In this application, since lithium transition metal phosphates have poor electron and ion conductivity, while lithium manganese oxide has far superior ion conductivity, the low-temperature performance of lithium transition metal phosphates in the second film layer can be effectively improved by incorporating lithium manganese oxide in the first film layer. Simultaneously, lithium manganese oxide and lithium transition metal phosphate particles differ in size and have significantly different degrees of expansion and contraction. Layering these particles allows for directional alignment, reducing the probability of lattice cracks caused by the difference in expansion and contraction between the two particles during direct mixing, thus maintaining the stability of the conductive network.

[0017] In some embodiments, the material of the first film layer further includes lithium transition metal phosphate, and the mass ratio of lithium transition metal phosphate to lithium-rich manganese-based material in the first film layer is A:C, where 0 < A ≤ 40 and 20 < C < 99.

[0018] In this application, the type of lithium transition metal phosphate in the first film layer can be the same as or different from the type of lithium transition metal phosphate in the second film layer. Lithium manganese oxide and lithium-rich manganese-based materials have larger particles than lithium transition metal phosphates, and mixing in a small amount of lithium transition metal phosphate can increase the electrode compaction density.

[0019] In some embodiments, the lithium transition metal phosphate is granular, and the ratio of the minimum to the maximum Martin diameter of the lithium transition metal phosphate is (0.1~0.6):1, for example, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, etc. Within this range, a combination of large and small particles can be achieved in the first film layer, thereby improving the electrode compaction density.

[0020] In some embodiments, the lithium transition metal phosphate is in particulate form, and the particle size D50 of the lithium transition metal phosphate is 0.5 μm to 2 μm. Within this range, a combination of particles of different sizes can be achieved in the first film layer, thereby increasing the electrode compaction density.

[0021] In some embodiments, the lithium-rich manganese-based material is granular, and the ratio of the minimum to the maximum diameter of the Martin diameter of the lithium-rich manganese-based material is 0.2 to 0.8, for example, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or 0.8. Within this range, a combination of large and small particles can be achieved in the first film layer, thereby increasing the electrode compaction density.

[0022] In some embodiments, the primary particle D50 of the lithium-rich manganese-based material is 100nm~800nm, for example, it can be 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, etc. The secondary particle D50 of the lithium-rich manganese-based material is 4μm~9μm, for example, it can be 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, etc. In this application, the secondary particles are formed by the agglomeration of primary particles. Within this range, the particle size distribution in the first film layer can be achieved, thereby improving the electrode compaction density.

[0023] In some embodiments, the material of the second film layer further includes a lithium replenishing agent.

[0024] In this application, the lithium replenishment agent can compensate for the active lithium lost during the initial cycle due to the formation of the solid electrolyte interphase (SEI) film, thereby improving the battery's initial efficiency and the system's capacity retention.

[0025] In some embodiments, the lithium supplement includes one or more of Li2O and Li2O2.

[0026] In some embodiments, the material of the first film layer further includes a first conductive agent and a first binder, wherein the mass percentage of lithium transition metal phosphate, lithium manganese oxide, lithium-rich manganese-based material, first conductive agent and first binder in the first film layer is A:B:C:D:(1-ABCD), where 0<A≤40, 0≤B≤40, 20<C<99, 0<D≤7, and 0<1-ABCD≤7.

[0027] In some embodiments, the material of the second film layer further includes a second conductive agent and a second binder, and the mass percentage ratio of lithium transition metal phosphate, lithium replenishing agent, second conductive agent and second binder in the second film layer is X:Y:Z:(1-XYZ), where 90≤X≤98, 0<Y≤1, 0<Z≤3, and 0<1-XYZ≤4.

[0028] In this application, the types of the first conductive agent and the second conductive agent can be the same or different. Similarly, the types of the first adhesive and the second adhesive can be the same or different.

[0029] As an example, the conductive agents in the materials of the first film layer and the second film layer each independently include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0030] As an example, the binders in the materials of the first and second membrane layers each independently include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0031] In some embodiments, the mass ratio of lithium transition metal phosphate, lithium replenishing agent, conductive agent, and binder in the second film layer is 96.8:0.7:1:1.5.

[0032] In some embodiments, the mass ratio of the second film layer to the first film layer is (3~18):2. As an example, the mass ratio of the second film layer to the first film layer is 3:2, 4:2, 5:2, 6:2, 7:2, 8:2, 9:2, 10:2, 11:2, 12:2, 13:2, 14:2, 15:2, 16:2, 17:2, or 18:2.

[0033] In some embodiments, the mass ratio of the second film layer to the first film layer is 7:3.

[0034] In some embodiments, the ratio of the sum of the thicknesses of the first and second films to the thickness of the current collector is (10~80):1, for example, it can be 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, etc.

[0035] In some embodiments, the ratio of the sum of the thicknesses of the first and second films to the thickness of the current collector is 25:1.

[0036] In some embodiments, the areal density of the first film layer is 20 g / m³. 2 ~120g / m 2 For example, it can be 20g / m 2 30g / m 2 40g / m 2 50g / m 2 60g / m 2 70g / m 2 80g / m 2 90g / m 2 100g / m 2 110g / m 2 120g / m 2 The areal density of the second film layer is 90 g / m³. 2 ~250g / m 2 For example, it can be 90g / m 2 110g / m 2 130g / m 2 150g / m2 170g / m 2 190g / m 2 210g / m 2 230g / m 2 250g / m 2 wait.

[0037] In some embodiments, the chemical formula of the lithium transition metal phosphate is LiMn. (1-x-y) Fe x M y PO4, wherein M includes one or more metallic elements, 0.8≥1-xy≥0.2, 0.8≥x≥0.2, y≤0.1.

[0038] In some embodiments, M includes, but is not limited to, at least one of Ti, Zr, Al, K, and Mg.

[0039] In some embodiments, the chemical formula of the lithium-rich manganese-based material is Li d Ni a Co b Mn c A (1-a-b-c) O2, where A includes one or more metallic elements, 1.5 > d > 0, 1 ≥ a ≥ 0, 0.3 ≥ b ≥ 0, 1 ≥ c ≥ 0, 0.1 ≥ 1 - abc.

[0040] In some embodiments, A includes, but is not limited to, at least one of Ti, W, Al, K, and Mg.

[0041] In some embodiments, the primary particles of the lithium-rich manganese-based material are in the shape of strips. This can reduce side reactions and improve the kinetic performance of the battery.

[0042] In this application, the primary particle shape of the lithium-rich manganese-based material can be a regular cuboid or an irregular strip.

[0043] In some embodiments, the chemical formula of lithium manganese oxide is LiMn. (2-y) D y O4, where D includes one or more metallic elements, and y≤0.1.

[0044] In some embodiments, D includes, but is not limited to, at least one of Co, Cr, Ti, Al, and V.

[0045] In some embodiments, the proportion of particles with a diameter of 0.5 μm to 2 μm in the first film layer is 30% to 70%, for example, it can be 30%, 40%, 50%, 60%, 70%, etc.

[0046] In some embodiments, the proportion of particles with a diameter of 0.1 μm to 1 μm in the second film layer is 30% to 70%, for example, it can be 30%, 40%, 50%, 60%, 70%, etc.

[0047] In some embodiments, the open porosity of the positive electrode is 15% to 40%, for example, it can be 15%, 20%, 25%, 30%, 35%, 40%, etc., and the ratio of the open porosity of the second film layer to the first film layer is 0.8 to 1.5, for example, it can be 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, etc. Within this range, the porosity of the electrode is moderate, which allows the electrode to have suitable impedance and a relatively low energy density.

[0048] In some embodiments, the specific adhesive strength of the positive electrode sheet is 0.05~40, for example, it can be 0.05, 5, 10, 15, 20, 25, 30, 35, 40, etc. The specific adhesive strength of the positive electrode sheet = the weight percentage of the adhesive in the electrode sheet A (%) / the peel force of the electrode sheet B (N).

[0049] In some embodiments, the resistivity of the powder after scraping the positive electrode sheet is ≤300000Ω*cm, for example, it can be 250000Ω*cm, 200000Ω*cm, 150000Ω*cm, 100000Ω*cm, 50000Ω*cm, etc.

[0050] Secondly, embodiments of this application provide a method for preparing a positive electrode sheet, comprising the following steps:

[0051] S1. Provide a current collector and a first membrane slurry, and dispose of the first membrane slurry on at least one side of the current collector to obtain a first membrane layer disposed on the current collector. The first membrane slurry includes a lithium-rich manganese-based material.

[0052] S2. Provide a second film layer slurry and place the second film layer slurry on the side of the first film layer away from the current collector to obtain a positive electrode sheet. The second film layer slurry includes lithium transition metal phosphate.

[0053] In step S1,

[0054] In some embodiments, the material of the first film layer further includes lithium manganese oxide.

[0055] In some embodiments, the material of the first film layer further includes lithium transition metal phosphate.

[0056] In some embodiments, the first film slurry further includes a conductive agent and a binder.

[0057] In some embodiments, the method for preparing the first film layer slurry includes the following steps:

[0058] A lithium-rich manganese-based material, lithium manganese oxide, lithium transition metal phosphate, conductive agent, binder and solvent are provided and mixed to obtain a first film layer slurry.

[0059] In some embodiments, a first membrane slurry is disposed on at least one side of the current collector to obtain a first membrane layer disposed on the current collector, comprising:

[0060] The first membrane slurry is coated on at least one side of the current collector and dried to obtain the first membrane layer disposed on the current collector.

[0061] In some embodiments, the drying temperature is 80℃-140℃, for example, it can be 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, etc., and the drying time is 8h-24h, for example, it can be 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, etc.

[0062] In step S2,

[0063] In some embodiments, the material of the second film layer further includes a lithium replenishing agent.

[0064] In some embodiments, the second film slurry further includes a conductive agent and a binder.

[0065] In some embodiments, the method for preparing the second film layer slurry includes the following steps:

[0066] A lithium transition metal phosphate, a lithium supplement, a conductive agent, a binder, and a solvent are provided and mixed to obtain a second film slurry.

[0067] In some embodiments, a second film slurry is disposed on the side of the first film layer away from the current collector to obtain a positive electrode sheet, comprising:

[0068] The second film slurry is coated onto the side of the first film layer away from the current collector, dried, and pressed to obtain the positive electrode sheet.

[0069] In some embodiments, the drying temperature is 80℃-140℃, for example, it can be 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, etc., and the drying time is 8h-24h, for example, it can be 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, etc.

[0070] The lithium-rich manganese-based material, lithium manganese oxide, lithium transition metal phosphate, conductive agent and binder used in the first film layer and the lithium transition metal phosphate, lithium replenishing agent, conductive agent and binder used in the second film layer are as described above.

[0071] Thirdly, embodiments of this application provide a battery cell including the aforementioned positive electrode sheet.

[0072] Example 1

[0073] A method for preparing a positive electrode sheet includes the following steps:

[0074] (1) Preparation of the first film layer slurry: LiMn transition metal phosphate material (with a minimum diameter to maximum diameter ratio of 0.4 and a particle size D50 of 1.5 μm) is used. 0.6 Fe 0.4 A slurry was prepared by uniformly dispersing lithium transition metal phosphate material (LiMn2O4), lithium manganese oxide material (LiMn2O4), lithium-rich manganese-based material (chemical formula 0.15Li2MnO3•0.85LiMnO2, minimum diameter to maximum diameter ratio of Martin diameter is 0.6, primary particle D50 is 500nm, secondary particle D50 is 6μm), conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) solvent. The mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) was 10:21:67:0.8:1.2.

[0075] (2) Preparation of the second film layer slurry: LiMn transition metal phosphate material (with a minimum diameter to maximum diameter ratio of 0.4 and a particle size D50 of 1.5 μm) is used. 0.6 Fe 0.4 A slurry was prepared by uniformly dispersing lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP), wherein the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) was 96.8:0.7:1:1.5.

[0076] (3) Take the first film layer slurry, coat it evenly on the surface of the aluminum foil, and dry it at 110°C for 16 hours to form the first film layer;

[0077] (4) Take the second film slurry (the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry to that in the first film slurry is 7:3), uniformly coat it on the surface of the first film layer, dry it at 110℃ for 16 hours, and then roll it to form a positive electrode sheet (the ratio of the sum of the thicknesses of the first and second film layers to the thickness of the current collector is 45:1, and the surface density of the first film layer is 70 g / m³). 2 The areal density of the second film layer is 170 g / m³. 2 ).

[0078] A method for manufacturing a battery cell includes the following steps:

[0079] (1) Preparation of negative electrode sheet: Graphite, conductive agent acetylene black, binder carboxymethyl cellulose (CMC) and SBR (styrene-butadiene rubber) are mixed in a mass ratio of 96:1:1.2:1.8, and then deionized water is added as solvent and stirred evenly to obtain slurry;

[0080] (2) The slurry is evenly coated on the surface of the copper foil, and then dried and rolled to form a negative electrode sheet. The drying temperature is 90℃ and the drying time is 10h.

[0081] (3) Assembly: The above positive electrode, negative electrode and separator are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and shaping, the cell is obtained. The electrolyte is prepared by dissolving LiPF6 in an organic solvent in a volume ratio of 1:1:1 of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC).

[0082] Example 2

[0083] This embodiment is basically the same as Embodiment 1, except that the first film layer slurry in this embodiment does not contain lithium manganese oxide and lithium transition metal phosphate, and the mass ratio of lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) is 98:0.8:1.2.

[0084] Example 3

[0085] This embodiment is basically the same as Embodiment 1, except that the first film layer slurry in this embodiment does not contain lithium manganese oxide, and the mass ratio of lithium transition metal phosphate material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) is 10:88:0.8:1.2.

[0086] Example 4

[0087] This embodiment is basically the same as Embodiment 1, except that the first film layer slurry in this embodiment does not contain lithium transition metal phosphate, and the mass ratio of lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) is 21:77:0.8:1.2.

[0088] Example 5

[0089] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) in the first film layer slurry in this embodiment is 37:40:21:0.8:1.2.

[0090] Example 6

[0091] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) in the first film layer slurry in this embodiment is 29:2:67:0.8:1.2.

[0092] Example 7

[0093] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) in the first film layer slurry in this embodiment is 20:21:57:0.8:1.2.

[0094] Example 8

[0095] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) in the first film layer slurry in this embodiment is 2:21:75:0.8:1.2.

[0096] Example 9

[0097] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry in this embodiment is 90:0.7:1:1.5.

[0098] Example 10

[0099] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry in this embodiment is 98:0.7:1:1.5.

[0100] Example 11

[0101] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry in this embodiment is 96.8:0.1:1:1.5.

[0102] Example 12

[0103] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry in this embodiment is 96.8:1:1:1.5.

[0104] Example 13

[0105] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry used in this embodiment to that in the first film slurry used is 9:1.

[0106] Example 14

[0107] This embodiment is basically the same as Embodiment 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry used in this embodiment to that in the first film slurry used is 3:2.

[0108] Example 15

[0109] This comparative example is basically the same as Example 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp) and polyvinylidene fluoride (PVDF) in the first film layer slurry in this comparative example is 80:0:20:0.8:1.2.

[0110] Example 16

[0111] This comparative example is basically the same as Example 1, except that the mass ratio of lithium transition metal phosphate material, Li2O, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry in this comparative example is 99:0:0.5:0.5.

[0112] Example 17

[0113] This comparative example is basically the same as Example 1, except that the mass ratio of lithium transition metal phosphate material, lithium manganese oxide material, lithium-rich manganese-based material, conductive carbon black (sp), and polyvinylidene fluoride (PVDF) in the second film slurry used in this comparative example to that in the first film slurry used is 3:8.

[0114] Comparative Example 1

[0115] This comparative example is basically the same as Example 1, except that the coating material of the positive electrode in this comparative example does not contain lithium manganese oxide material and lithium-rich manganese-based material, and contains only one coating layer (i.e., the first film layer slurry and the second film layer slurry are mixed together and coated).

[0116] Comparative Example 2

[0117] This comparative example is basically the same as Example 1, except that the first film layer slurry and the second film layer slurry are mixed together and coated in this comparative example.

[0118] Comparative Example 3

[0119] This comparative example is basically the same as Example 1, except that the positions of the first film layer and the second film layer are interchanged.

[0120] Test example:

[0121] The positive electrode sheets obtained in the examples and comparative examples were subjected to roll peeling force tests. The weight percentage of the adhesive in the electrode sheet was defined as A (%), the electrode peeling force was defined as B (N), and the specific adhesive force of the electrode sheet was defined as C, C=A / B. The test results of the specific adhesive force C of the electrode sheet are shown in Table 1.

[0122] The battery cells obtained in the examples and comparative examples were placed in a 45°C incubator and charged at a constant current and constant voltage of 1C to 4.25V, with a cutoff voltage of 0.05C; then discharged at 1C to 2.5V. The discharge capacity in the first cycle is A, and the discharge capacity after 1000 cycles is B. The capacity retention rate is B / A*100%, as shown in Table 1.

[0123] Table 1

[0124] Adhesive strength at 45°C / 1C for 1000 cycles; Capacity retention: Example 1: 0.322100.7%; Example 2: 0.3297.3%; Example 3: 0.32497.7%; Example 4: 0.32298.1%; Example 5: 0.36599.7%; Example 6: 0.31998.9%; Example 7: 0.38799.1%; Example 8: 0.32199.4%; Example 9: 0.33399.3%; Example 10 0.32598.7% Example 11 0.33397.7% Example 12 0.33198.5% Example 13 0.4199.1% Example 14 0.34198.6% Example 15 0.40295.1% Example 16 0.3294.7% Example 17 0.26896.8% Comparative Example 1 0.493.3% Comparative Example 2 0.31394.3% Comparative Example 3 0.31693.7%

[0125] As can be seen from Table 1:

[0126] Compared with Examples 2-4, the battery in Example 1 has a higher capacity retention rate. It can be seen that by adding lithium manganese oxide and lithium transition metal phosphate to the first film layer, the capacity retention rate of the battery can be improved.

[0127] Compared with Example 15, the batteries of Examples 1 and 5-8 have higher capacity retention rates. It can be seen that the capacity retention rate of the battery can be adjusted by controlling the mass ratio of lithium transition metal phosphate, lithium manganese oxide, lithium-rich manganese-based material, first conductive agent and first binder in the first film layer.

[0128] Compared with Example 16, the batteries of Example 1 and Examples 9-12 have higher capacity retention rates. It can be seen that the capacity retention rate of the battery can be adjusted by controlling the mass ratio of lithium transition metal phosphate material, lithium replenishing agent, second conductive agent and second binder in the second film layer.

[0129] Compared with Example 17, the batteries of Examples 1, 13 and 14 have higher capacity retention rates. It can be seen that the capacity retention rate of the battery can be adjusted by controlling the mass ratio of the second film layer to the first film layer.

[0130] Compared with Comparative Example 1, the battery in this embodiment has a higher capacity retention rate. It can be seen that by controlling the composition of the first and second film layers and setting them on the current collector in a layered manner, the capacity retention rate of the battery can be improved.

[0131] Compared with Comparative Example 2, the battery in this embodiment has a higher capacity retention rate. It can be seen that by disposing the first film layer and the second film layer on the current collector in a layered manner, the capacity retention rate of the battery can be improved.

[0132] Compared with Comparative Example 3, the battery in this embodiment has a higher capacity retention rate. It can be seen that by sequentially disposing the first film layer and the second film layer on the current collector in a layered manner, the capacity retention rate of the battery can be improved.

Claims

1. A positive electrode sheet, comprising a current collector and a first film layer and a second film layer sequentially disposed on the current collector, wherein the material of the first film layer comprises a lithium-rich manganese-based material, and the material of the second film layer comprises a lithium transition metal phosphate.

2. The positive electrode sheet according to claim 1, wherein, The material of the first film layer also includes lithium manganese oxide; and / or The material of the first film layer also includes the lithium transition metal phosphate, and the mass ratio of the lithium transition metal phosphate to the lithium-rich manganese-based material in the first film layer is A:C, where 0<A≤40 and 20<C<99.

3. The positive electrode sheet according to claim 2, wherein, The chemical formula of the lithium transition metal phosphate is LiMn. (1-x-y) Fe x M y PO4, wherein M includes one or more metallic elements, 0.8 ≥ 1 - xy ≥ 0.2, 0.8 ≥ x ≥ 0.2, y ≤ 0.1; and / or The chemical formula of the lithium-rich manganese-based material is Li d Ni a Co b Mn c A (1-a-b-c) O2, wherein A includes one or more metallic elements, 1.5 > d > 0, 1 ≥ a ≥ 0, 0.3 ≥ b ≥ 0, 1 ≥ c ≥ 0, 0.1 ≥ 1 - abc; and / or The chemical formula of the lithium manganese oxide is LiMn. (2-y) D y O4, where D includes one or more metallic elements, and y≤0.

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4. The positive electrode sheet according to claim 1, wherein, The lithium transition metal phosphate is granular, and the ratio of its minimum to maximum Martin diameter is (0.1~0.6):1; and / or The lithium transition metal phosphate is in particulate form, and the particle size D50 of the lithium transition metal phosphate is 0.5 μm to 2 μm; and / or The lithium-rich manganese-based material is granular, and the ratio of its minimum to maximum diameter (Martin diameter) is 0.2 to 0.8; and / or The primary particle size (D50) of the lithium-rich manganese-based material is 100 nm to 800 nm, and the secondary particle size (D50) is 4 μm to 9 μm.

5. The positive electrode sheet according to claim 1, wherein the material of the second film layer further includes a lithium replenishing agent.

6. The positive electrode sheet according to claim 5, wherein, The material of the first film layer further includes lithium manganese oxide, the lithium transition metal phosphate, a first conductive agent, and a first binder. The mass percentage ratio of the lithium transition metal phosphate, the lithium manganese oxide, the lithium-rich manganese-based material, the first conductive agent, and the first binder in the first film layer is A:B:C:D:(1-ABCD), where 0 < A ≤ 40, 0 ≤ B ≤ 40, 20 < C < 99, 0 < D ≤ 7, 0 < 1-ABCD ≤ 7; and / or The material of the second film layer also includes a second conductive agent and a second binder. The mass percentage ratio of the lithium transition metal phosphate material, the lithium replenishing agent, the second conductive agent and the second binder in the second film layer is X:Y:Z:(1-XYZ), where 90≤X≤98, 0<Y≤1, 0<Z≤3, and 0<1-XYZ≤4.

7. The positive electrode sheet according to claim 1, wherein, The mass ratio of the second film layer to the first film layer is (3~18):2; and / or The ratio of the sum of the thicknesses of the first and second films to the thickness of the current collector is (10~80):1; and / or The areal density of the first film layer is 20 g / m³. 2 ~120g / m 2 The areal density of the second film layer is 90 g / m³. 2 ~250g / m 2 .

8. A method for preparing a positive electrode sheet, comprising the following steps: A current collector and a first membrane slurry are provided, and the first membrane slurry is disposed on at least one side of the current collector to obtain a first membrane layer disposed on the current collector. The first membrane slurry comprises a lithium-rich manganese-based material. A second film slurry is provided and disposed on the side of the first film layer away from the current collector to obtain a positive electrode sheet. The second film slurry includes lithium transition metal phosphate.

9. The method for preparing the positive electrode sheet according to claim 8, wherein, The method for preparing the first film layer slurry includes the following steps: The lithium-rich manganese-based material, lithium manganese oxide, lithium transition metal phosphate, a first conductive agent, a first binder, and a first solvent are mixed to obtain the first film slurry; and / or The method for preparing the second film slurry includes the following steps: The lithium transition metal phosphate, lithium supplementer, second conductive agent, second binder and second solvent are mixed to obtain the second film slurry.

10. A battery cell comprising a positive electrode sheet as described in any one of claims 1-7, or a positive electrode sheet prepared by the method described in claim 8 or 9.

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