Positive electrode sheet and preparation method therefor, and lithium-ion battery

Abstract

A positive electrode sheet and a preparation method therefor, and a lithium-ion battery. The positive electrode sheet comprises a positive current collector and a composite coating, wherein the composite coating comprises a first membrane layer and a second membrane layer that are arranged in a stacked manner, the first membrane layer is arranged close to the positive current collector, the material of the first membrane layer comprises a first binder, the material of the second membrane layer comprises a second binder, and the mass percentage of the first binder in the first membrane layer is greater than the mass percentage of the second binder in the second membrane layer.

Description

Positive electrode sheet and preparation method thereof and lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on October 31, 2023, with application number 202311451390.X. The entire contents of the above application are incorporated by reference into this application. Technical Field

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

[0003] Cathode materials significantly impact the performance of lithium-ion batteries and are a key constraint to improving battery performance. Lithium iron manganese phosphate (LiMnFePO4) is a new-generation cathode material for lithium-ion batteries. Its olivine-like structure is structurally stable during charge and discharge, and it offers high operating voltage, high energy density, and excellent safety.

[0004] In the related art, since lithium manganese iron phosphate has smaller particles and a larger specific surface area, it is difficult to obtain a high coating surface density when using lithium manganese iron phosphate to prepare the coating of the positive electrode plate. Once the coating thickness is too high, problems such as material falling off and surface cracking are likely to occur, thereby limiting the improvement of the battery energy density. Technical issues

[0005] The present application provides a positive electrode plate and a preparation method thereof, and a lithium-ion battery to solve the above technical problems. Technical Solutions

[0006] In a first aspect, an embodiment of the present application provides a positive electrode plate, comprising a positive electrode current collector and a composite coating disposed on one side of the positive electrode current collector, wherein the composite coating comprises a first film layer and a second film layer stacked together, and the first film layer is disposed adjacent to the positive electrode current collector, wherein:

[0007] The material of the first film layer includes a first positive electrode active material and a first binder, and the material of the second film layer includes a second positive electrode active material and a second binder. The first positive electrode active material and the second positive electrode active material are each independently selected from one or more lithium manganese iron phosphate materials, and the mass percentage of the first binder in the first film layer is greater than the mass percentage of the second binder in the second film layer.

[0008] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode sheet, comprising the following steps:

[0009] Providing a positive electrode current collector, a first slurry, and a second slurry, wherein the first slurry includes a first positive electrode active material and a first binder, the second slurry includes a second positive electrode active material and a second binder, and the content of the first binder in the first slurry is greater than the content of the second binder in the second slurry;

[0010] coating the first slurry on the positive electrode current collector and drying the slurry to form a first film layer;

[0011] The second slurry is coated on the first film layer and dried to form a second film layer.

[0012] In a third aspect, an embodiment of the present application provides a lithium-ion battery, comprising the positive electrode sheet as described above, or comprising the positive electrode sheet prepared by the preparation method described above. Beneficial effects

[0013] Beneficial effects of this application:

[0014] In the embodiments of the present application, by designing the coating on the positive electrode plate as a double-layer film layer, and designing the first binder content in the first film layer to be higher than the second binder content in the second film layer, while constructing a stable current collector-coating interface, the proportion of active material in the coating is maintained, thereby effectively achieving a high coating surface density and improving the energy density of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG1 is a schematic structural diagram of a positive electrode sheet provided in an embodiment of the present application;

[0016] FIG2 is a schematic flow chart of a method for preparing a positive electrode sheet according to an embodiment of the present application;

[0017] FIG3 is a schematic structural diagram of a lithium-ion battery provided in an embodiment of the present application;

[0018] FIG4 is a schematic structural diagram of the core package in FIG3 ;

[0019] Explanation of the accompanying symbols: 100-positive electrode sheet; 10-positive electrode current collector; 20-composite coating; 21-first film layer; 22-second film layer; 200-lithium-ion battery; 1-housing; 11-shell; 12-shell cover; 2-core pack; 3-negative electrode sheet; 4-diaphragm; 51-positive electrode ear; 52-positive electrode column; 53-first connecting piece; 61-negative electrode ear; 62-negative electrode column; 63-second connecting piece. Modes for Carrying Out the Invention

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art, and the materials and reagents used in the Examples and Comparative Examples of this application are commercially available. In addition, any methods and materials similar or equivalent to those described herein can be applied to this application. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0021] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments. The various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0022] In the description of this application, the term "including" means "including but not limited to".

[0023] The terms "multiple", "multiple times" or similar expressions refer to two (times) or more than two (times), for example, it can be two (times), three (times), four (times), five (times), six (times), etc.

[0024] The selection scope of the term "and / or" includes any one of two or more related listed items, and also includes any and all combinations of the related listed items, and the said any and all combinations include any two related listed items, any more related listed items, or the combination of all related listed items. For example, "A and / or B" includes three parallel solutions of A, B and A+B. For another example, the technical solution of "A, and / or, B, and / or, C, and / or, D" includes any one of A, B, C, and D (that is, the technical solution connected by "logical OR"), and also includes any and all combinations of A, B, C, and D, that is, the combination of any two or any three of A, B, C, and D, and also includes the four-item combination of A, B, C, and D (that is, the technical solution connected by "logical AND").

[0025] The term "solid content" refers to the ratio of the mass of solid matter in the slurry to the total mass of the slurry.

[0026] An embodiment of the present application provides a positive electrode plate 100. Referring to FIG. 1 , the positive electrode plate 100 includes a positive current collector 10 and a composite coating 20 disposed on one side of the positive current collector 10. The composite coating 20 includes a first film layer 21 and a second film layer 22, which are stacked together. The first film layer 21 is disposed adjacent to the positive current collector 10, and the second film layer 22 is disposed on a side of the first film layer 21 facing away from the positive current collector 10. The materials of the first film layer 21 include a first positive electrode active material and a first binder, while the materials of the second film layer 22 include a second positive electrode active material and a second binder. The first positive electrode active material is selected from one or more lithium iron manganese phosphate materials, and the second positive electrode active material is selected from one or more lithium iron manganese phosphate materials. The mass percentage of the first binder in the first film layer 21 is greater than the mass percentage of the second binder in the second film layer 22.

[0027] In some embodiments, the material of the positive electrode current collector 10 can be any common conductive metal material in the art, including but not limited to aluminum foil, platinum foil or palladium foil, and this application does not impose any limitation on this.

[0028] In some embodiments, the lithium manganese iron phosphate material includes a chemical formula of Li a M b (PO4) c One or more materials, wherein M is Fe 1-x-z Mn x D z. Wherein, a, b, c represent the molar amounts of Li, M and PO4 in the material respectively, and 1-xz, x, z represent the molar amounts of Fe, Mn and metal element D in M ​​respectively. In lithium manganese iron phosphate material: the ratio of the molar amount a of Li to the molar amount c of P is 0.95-1.10, for example, it can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10 and values ​​between any two of the above values; the ratio of the molar amount b of M to the molar amount c of P is 0.90-1.15, for example, it can be 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, and values ​​between any two of the above values; the ratio of the molar amount a of Li to the molar amount b of M is 1.01-1.10, for example, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, and values ​​between any two of the above values. In some embodiments of the present application, in M, D is selected from one or more of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, and Zr. The lithium manganese iron phosphate can be purchased on the market, or can be prepared by referring to the preparation method of lithium manganese iron phosphate in the art. Specifically, the corresponding ion sources (lithium source, iron source, manganese source, D source, phosphorus source) can be weighed according to the molar ratio of each element, and the mixture can be mixed and calcined to obtain the lithium manganese iron phosphate.

[0029] The first positive electrode active material and the second positive electrode active material may be the same or different, which is not limited in this application.

[0030] The interfaces in a pole piece can be divided into two types: the interface between the current collector and the coating, and the interface between the coating particles. Research has found that the coating's surface density is more susceptible to the interfacial bond strength between the current collector and the coating. Increasing the binder content in the coating can help increase the coating's surface density, but this also reduces the proportion of lithium manganese iron phosphate in the coating, resulting in a smaller increase in battery energy density. In view of this, in the positive electrode plate 100 proposed in the embodiment of the present application, by designing the coating on the positive electrode plate 100 as a double-layer film layer, and designing the first binder content contained in the first film layer 21 to be higher than the second binder content contained in the second film layer 22, in this way, the first film layer 21 contains more first binder, and the second film layer 22 contains more lithium manganese iron phosphate. On the one hand, a stable current collector-coating interface can be constructed while effectively improving the coating surface density, thereby reducing the number of layers of the positive electrode plate 100 in the battery and the number of current collectors therein, thereby effectively reducing the weight of the battery and improving the energy density. On the other hand, the proportion of active materials in the coating can be maintained to avoid reducing the energy density due to the reduction in the active material content, thereby effectively improving the energy density of the battery as a whole.

[0031] In one embodiment, the first binder includes polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); in another embodiment, the second binder includes polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The first binder and the second binder can be the same or different.

[0032] In some embodiments, the first binder is PVDF and the second binder is PVDF.

[0033] In some embodiments, the mass percentage of the first binder in the first film layer 21 is 1.7% to 2.3%; for example, it can be 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, or any value between the above two values. Controlling the first binder content within this range helps to increase the coating surface density and the battery energy density, while preventing agglomeration between the component particles in the slurry. In other embodiments, the mass percentage of the first binder is 1.9% to 2.1%; controlling the first binder content within this range helps to further increase the surface density.

[0034] In some embodiments, the mass percentage of lithium manganese iron phosphate in the first film layer 21 is 96.6% to 97.2%; for example, it can be 96.6%, 96.8%, 96.9%, 97.0%, 97.1%, 97.2% and values ​​between any two of the above values, which helps to maximize the surface density while maintaining the lithium manganese iron phosphate content.

[0035] In some embodiments, the weight percentage of the second binder in the second film layer 22 is 1.1% to 1.7%, for example, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, or any value between the above two values. In other embodiments, the weight percentage of the second binder in the second film layer 22 is 1.3% to 1.5%.

[0036] The second film layer 22 can be a single film layer or a composite film layer composed of multiple film layers stacked in sequence. When the second film layer 22 includes two or more sub-film layers, the two or more sub-film layers are stacked in a direction away from the positive electrode current collector 10. The second binder content in each of the multiple sub-film layers can be the same or different. However, regardless of whether the second film layer is a single film layer or a composite film layer, the total weight percentage of the second binder in the second film layer 22 is less than the weight percentage of the first binder in the first film layer 21. In some embodiments, the total weight percentage of the second binder in the second film layer 22 is 1.1% to 1.7%.

[0037] In some embodiments, the mass percentage of lithium manganese iron phosphate in the second film layer 22 is 97.2% to 97.8%; for example, it can be 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8% and values ​​between any two of the above values, which helps to increase the content of lithium manganese iron phosphate and improve the energy density.

[0038] In some embodiments, in the first film layer, the mass percentage of the first binder is 1.7% to 2.3%, and the mass percentage of the first positive electrode active material is 96.6% to 97.2%; in the second film layer, the mass percentage of the second binder is 1.1% to 1.7%, and the mass percentage of the second positive electrode active material is 97.2% to 97.8%. In this way, the first binder content in the first film layer can be precisely controlled, increasing the coating surface density and thus the battery energy density, while simultaneously regulating the content of the active material in the composite coating to increase the battery energy density.

[0039] In one embodiment, the material of the first film layer 21 further includes a first conductive agent. That is, the material of the first film layer 21 includes a first positive electrode active material, a first conductive agent, and a first binder. The first conductive agent includes one or more of conductive carbon black (SP), carbon nanotubes (CNTs), and graphene. In one specific embodiment, the first conductive agent includes a mixture of conductive carbon black (SP) and carbon nanotubes (CNTs).

[0040] In some embodiments, the material of the first film layer 21 includes 96.6 wt % to 97.2 wt % of lithium manganese iron phosphate, 1.7 wt % to 2.3 wt % of the first binder, and the remainder is the first conductive agent.

[0041] In one embodiment, the material of the second film layer 22 further includes a second conductive agent, that is, the material of the second film layer 22 includes a second positive electrode active material, a second conductive agent, and a second binder. The second conductive agent includes one or more of conductive carbon black (SP), carbon nanotubes (CNT), and graphene. In one specific embodiment, the second conductive agent includes a mixture of conductive carbon black (SP) and carbon nanotubes (CNT).

[0042] In some embodiments, the material of the second film layer 22 includes 97.2 wt % to 97.8 wt % of lithium manganese iron phosphate, 1.1 wt % to 1.7 wt % of the second binder, and the remainder is a second conductive agent.

[0043] The first conductive agent and the second conductive agent may be the same or different.

[0044] As the thickness of the electrode increases, the battery resistance increases, which can easily affect the liquid-phase lithium ion transport rate, affecting the battery's rate performance and cycle capacity. In view of this, in one embodiment, the material of the first film layer 21 also includes a first pore-forming agent. That is, the material of the first film layer 21 includes a first positive electrode active material, a first conductive agent, a first binder, and a first pore-forming agent. The addition of the first pore-forming agent helps to increase the surface density and energy density while effectively improving the liquid-phase lithium ion transport rate of the electrode, thereby maintaining its rate performance and cycle performance.

[0045] In another embodiment, the material of the second film layer 22 also includes a second pore-forming agent, that is, the material of the second film layer 22 includes a second positive electrode active material, a second conductive agent, a second binder and a second pore-forming agent; by adding the second pore-forming agent, it helps to increase the surface density and energy density while effectively improving the lithium ion liquid phase transmission rate of the electrode, so that its rate performance and cycle performance are not affected.

[0046] In some embodiments, the first pore-forming agent includes one or more of ammonium carbonate, ammonium bicarbonate, urea, and sodium chloride; in other embodiments, the second pore-forming agent includes one or more of ammonium carbonate, ammonium bicarbonate, urea, and sodium chloride. The first and second pore-forming agents may be the same or different.

[0047] In some embodiments, the material of the first film layer 21 includes, by mass percentage, 96.6% to 97.2% lithium manganese iron phosphate, 0.5% to 1.3% first conductive agent, 1.7% to 2.3% first binder, and 0.1% to 0.4% first pore-forming agent.

[0048] In some embodiments, the material of the second film layer 22 includes, by mass percentage, 97.2% to 97.8% lithium manganese iron phosphate, 0.5% to 1.3% second conductive agent, 1.1% to 1.7% second binder, and 0.1% to 0.4% second pore-forming agent.

[0049] In a second aspect, the present application proposes a method for preparing the positive electrode sheet 100. Referring to FIG. 2 , the method includes:

[0050] S10, providing a positive electrode current collector 10, a first slurry, and a second slurry, wherein the first slurry includes a first positive electrode active material and a first binder, and the second slurry includes a second positive electrode active material and a second binder, and the content of the first binder in the first slurry is greater than the content of the second binder in the second slurry;

[0051] S20, coating the first slurry on the positive electrode current collector 10, and drying to form a first film layer 21;

[0052] S30 , coating the second slurry on the first film layer 21 and drying to form the second film layer 22 .

[0053] The first slurry further includes a first solvent, wherein the first solvent includes one or more of N-methylpyrrolidone, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and acetone. The second slurry further includes a second solvent, wherein the second solvent includes one or more of N-methylpyrrolidone, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and acetone.

[0054] In a third aspect, embodiments of the present application provide a lithium-ion battery 200, including but not limited to button cells, soft-pack cells, prismatic lithium-ion batteries, cylindrical lithium-ion batteries, and the like. The lithium-ion battery 200 includes a positive electrode sheet 100 as described in any of the preceding embodiments, the positive electrode sheet 100 including a positive current collector 10 and a composite coating 20 disposed on one side of the positive current collector 10, the composite coating 20 including a first film layer 21 and a second film layer 22 stacked together, with the first film layer 21 disposed adjacent to the positive current collector 10, wherein: the material of the first film layer 21 includes a first positive electrode active material and a first binder, the material of the second film layer 22 includes a second positive electrode active material and a second binder, the first positive electrode active material and the second positive electrode active material are each independently selected from one or more lithium manganese iron phosphate materials, and the mass percentage of the first binder in the first film layer 21 is greater than the mass percentage of the second binder in the second film layer 22. Specifically, referring to Figures 3 and 4, in some embodiments, the lithium-ion battery 200 includes an outer shell 1 and a core pack 2. The outer shell 1 is composed of a shell 11 and a shell cover 12. The shell 11 defines a storage space, and the core pack 2 is installed in the storage space. The core pack 2 includes the positive electrode sheet 100, the separator 4, and the negative electrode sheet 3 stacked in sequence. In the positive electrode sheet 100, the positive current collector 10 is disposed adjacent to the separator 4, and the composite coating 20 is disposed on the side of the positive current collector 10 facing away from the separator 4. In addition, in some embodiments, the core pack 2 also includes a positive electrode tab 51 connected to the positive electrode sheet 100 and a negative electrode tab 61 connected to the negative electrode sheet 3. The core pack 2 is filled with electrolyte. In addition, the lithium-ion battery 200 also includes a positive electrode column 52 and a negative electrode column 62 provided on the outer shell 1, wherein the positive electrode column 52 is provided corresponding to the positive electrode ear 51, and passes through the outer shell 1 and is connected to the positive electrode ear 51 through the first connecting piece 53, and the negative electrode column 62 is provided corresponding to the negative electrode ear 61, and passes through the outer shell 1 and is connected to the negative electrode ear 61 through the second connecting piece 63.

[0055] Example 1

[0056] (1) According to the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder and the first pore-forming agent of 96.9:0.9:2:0.2, lithium manganese iron phosphate, the first conductive agent, the first binder and the first pore-forming agent are weighed, wherein the chemical formula of the lithium manganese iron phosphate material is LiFe 0.35 Mn 0.55 Mg 0.1 PO4; the first conductive agent is a mixture of SP and CNTs, with a mass ratio of SP to CNTs of 6:3; the first binder is PVDF, and the first pore-forming agent is ammonium bicarbonate. Lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent are mixed and dispersed in N-methylpyrrolidone and stirred to form a first slurry.

[0057] (2) According to the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder and the second pore-forming agent of 97.5:0.9:1.4:0.2, lithium manganese iron phosphate, the second conductive agent, the second binder and the second pore-forming agent are weighed, wherein the chemical formula of the lithium manganese iron phosphate material is LiFe 0.35 Mn 0.55 Mg 0.1 PO4; the second conductive agent is a mixture of SP and CNTs with a mass ratio of 6:3; the second binder is PVDF, and the second pore-forming agent is ammonium bicarbonate. The lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent are mixed and dispersed in N-methylpyrrolidone and stirred to form a second slurry.

[0058] (3) Coating a first slurry on an aluminum foil and drying to obtain a first film layer, wherein the contents of the first film layer are 96.9 wt% of lithium manganese iron phosphate, 0.9 wt% of a first conductive agent, 2 wt% of a first binder, and 0.2 wt% of a first pore-forming agent; then coating a second slurry on the first film layer and drying to obtain a second film layer, wherein the contents of the second film layer are 97.5 wt% of lithium manganese iron phosphate, 0.9 wt% of a second conductive agent, 1.4 wt% of a second binder, and 0.2 wt% of a second pore-forming agent, to obtain a positive electrode sheet.

[0059] Example 2

[0060] This embodiment is basically the same as the embodiment 1, except that the first and second film layers of the positive electrode sheet in this embodiment do not contain a pore-forming agent. Accordingly, steps (1) to (3) are adjusted as follows:

[0061] In step (1), the first pore-forming agent was omitted, and the mass ratio of lithium manganese iron phosphate, the first conductive agent, and the first binder was adjusted to 96.9:1.1:2. Other than that, the other parameters and steps remained unchanged;

[0062] In step (2), the second pore-forming agent was omitted, and the mass ratio of lithium manganese iron phosphate, the second conductive agent, and the second binder was adjusted to 97.5:1.1:1.4. Other than that, the other parameters and steps remained unchanged;

[0063] In the positive electrode sheet prepared in step (3), the contents of the components in the first film layer are 96.9wt% of lithium manganese iron phosphate, 1.1wt% of the first conductive agent, and 2wt% of the first binder; the contents of the components in the second film layer are 97.5wt% of lithium manganese iron phosphate, 1.1wt% of the second conductive agent, and 1.4wt% of the second binder.

[0064] Example 3

[0065] This example is essentially the same as Example 1, differing only in that, in this example, the first and second pore-forming agents are replaced by a mixture of urea and sodium chloride instead of ammonium bicarbonate, with the mass ratio of urea to sodium chloride being 1:1. Other than this, all other parameters and steps remain unchanged.

[0066] Example 4

[0067] This embodiment is substantially the same as embodiment 1, except that in this embodiment, the first pore-forming agent and the second pore-forming agent are both changed from ammonium bicarbonate to ammonium carbonate. Other parameters and steps remain unchanged.

[0068] Example 5

[0069] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the first binder in the first film layer of the positive electrode sheet of this embodiment is changed from 2 wt % to 1.7 wt %, and the mass percentage of the first conductive agent is correspondingly adjusted from 0.9 wt % to 1.2 wt %.

[0070] Accordingly, in step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent was changed from 96.9:0.9:2:0.2 to 96.9:1.2:1.7:0.2. Apart from this, the other parameters and steps remained unchanged.

[0071] Example 6

[0072] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the first binder in the first film layer of the positive electrode sheet of this embodiment is changed from 2 wt % to 1.9 wt %, and the mass percentage of the first conductive agent is correspondingly adjusted from 0.9 wt % to 1.0 wt %.

[0073] Accordingly, in step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent was changed from 96.9:0.9:2:0.2 to 96.9:1.0:1.9:0.2. Apart from this, the other parameters and steps remained unchanged.

[0074] Example 7

[0075] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the first binder in the first film layer of the positive electrode sheet of this embodiment is changed from 2 wt % to 2.1 wt %, and the mass percentage of the first conductive agent is correspondingly adjusted from 0.9 wt % to 0.8 wt %.

[0076] Accordingly, in step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent was changed from 96.9:0.9:2:0.2 to 96.9:0.8:2.1:0.2. Apart from this, the other parameters and steps remained unchanged.

[0077] Example 8

[0078] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the first binder in the first film layer of the positive electrode sheet of this embodiment is changed from 2 wt % to 2.3 wt %, and the mass percentage of the first conductive agent is correspondingly adjusted from 0.9 wt % to 0.6 wt %.

[0079] Accordingly, in step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent was changed from 96.9:0.9:2:0.2 to 96.9:0.6:2.3:0.2. Apart from this, the other parameters and steps remained unchanged.

[0080] Example 9

[0081] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the first binder in the first film layer of the positive electrode sheet of this embodiment is changed from 2 wt % to 2.5 wt %, and the mass percentage of the first conductive agent is correspondingly adjusted from 0.9 wt % to 0.4 wt %.

[0082] Accordingly, in step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent was changed from 96.9:0.9:2:0.2 to 96.9:0.4:2.5:0.2. Apart from this, the other parameters and steps remained unchanged.

[0083] Example 10

[0084] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the second binder in the second film layer of the positive electrode plate of this embodiment is changed from 1.4 wt % to 1.1 wt %, and the mass percentage of the second conductive agent is correspondingly adjusted from 0.9 wt % to 1.2 wt %.

[0085] Accordingly, in step (2), the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent was changed from 97.5:0.9:1.4:0.2 to 97.5:1.2:1.1:0.2. Apart from this, the other parameters and steps remained unchanged.

[0086] Example 11

[0087] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the second binder in the second film layer of the positive electrode plate of this embodiment is changed from 1.4 wt % to 1.3 wt %, and the mass percentage of the second conductive agent is correspondingly adjusted from 0.9 wt % to 1.0 wt %.

[0088] Accordingly, in step (2), the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent was changed from 97.5:0.9:1.4:0.2 to 97.5:1.0:1.3:0.2. Apart from this, the other parameters and steps remained unchanged.

[0089] Example 12

[0090] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the second binder in the second film layer of the positive electrode plate of this embodiment is changed from 1.4 wt % to 1.5 wt %, and the mass percentage of the second conductive agent is correspondingly adjusted from 0.9 wt % to 0.8 wt %.

[0091] Accordingly, in step (2), the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent was changed from 97.5:0.9:1.4:0.2 to 97.5:0.8:1.5:0.2. Apart from this, the other parameters and steps remained unchanged.

[0092] Example 13

[0093] This embodiment is substantially the same as embodiment 1, except that the mass percentage of the second binder in the second film layer of the positive electrode plate of this embodiment is changed from 1.4 wt % to 1.7 wt %, and the mass percentage of the second conductive agent is correspondingly adjusted from 0.9 wt % to 0.6 wt %.

[0094] Example 14

[0095] This embodiment is substantially the same as the first embodiment, except that the mass percentage of the second binder in the second film layer of the positive electrode plate of this embodiment is changed from 1.4 wt % to 0.9 wt %, and the mass percentage of the second conductive agent is correspondingly adjusted from 0.9 wt % to 1.4 wt %.

[0096] Accordingly, in step (2), the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent was changed from 97.5:0.9:1.4:0.2 to 97.5:1.4:0.9:0.2. Apart from this, other parameters and steps remained unchanged.

[0097] Comparative Example 1

[0098] This comparative example is essentially the same as Example 1, except that the materials for the first and second film layers of the positive electrode sheet in this comparative example are changed to 96.9 wt% lithium manganese iron phosphate, 0.9 wt% conductive agent, 2 wt% PVDF, and 0.2 wt% ammonium bicarbonate. Accordingly, the positive electrode sheet of this comparative example is prepared as follows:

[0099] (1) According to the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder and the first pore-forming agent of 96.9:0.9:2:0.2, lithium manganese iron phosphate, the first conductive agent, the first binder and the first pore-forming agent are weighed, wherein the chemical formula of the lithium manganese iron phosphate material is LiFe 0.35 Mn 0.55 Mg 0.1 PO4; the first conductive agent is a mixture of SP and CNTs, with a mass ratio of SP to CNTs of 6:3; the first binder is PVDF, and the first pore-forming agent is ammonium bicarbonate. Lithium manganese iron phosphate, the first conductive agent, the first binder, and the first pore-forming agent are mixed and dispersed in N-methylpyrrolidone and stirred to form a first slurry.

[0100] (2) According to the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder and the second pore-forming agent of 96.9:0.9:2:0.2, lithium manganese iron phosphate, the second conductive agent, the second binder and the second pore-forming agent are weighed, wherein the chemical formula of the lithium manganese iron phosphate material is LiFe 0.35 Mn 0.55 Mg 0.1 PO4; the second conductive agent is a mixture of SP and CNTs with a mass ratio of 6:3; the second binder is PVDF, and the second pore-forming agent is ammonium bicarbonate. The lithium manganese iron phosphate, the second conductive agent, the second binder, and the second pore-forming agent are mixed and dispersed in N-methylpyrrolidone and stirred to form a second slurry.

[0101] (3) Coating a first slurry on an aluminum foil and drying to obtain a first film layer, wherein the contents of the first film layer are 96.9 wt% of lithium manganese iron phosphate, 0.9 wt% of a first conductive agent, 2 wt% of a first binder, and 0.2 wt% of a first pore-forming agent; then coating a second slurry on the first film layer and drying to obtain a second film layer, wherein the contents of the second film layer are 96.9 wt% of lithium manganese iron phosphate, 0.9 wt% of a second conductive agent, 2 wt% of a second binder, and 0.2 wt% of a second pore-forming agent, to obtain a positive electrode sheet.

[0102] Comparative Example 2

[0103] This comparative example is basically the same as comparative example 1, except that the materials of the first and second film layers of the positive electrode sheet of this comparative example are changed to 96.9wt% lithium manganese iron phosphate, 1.1wt% conductive agent, and 2wt% PVDF; correspondingly:

[0104] In step (1), the first pore-forming agent was omitted, and the mass ratio of lithium manganese iron phosphate, the first conductive agent, and the first binder was adjusted to 96.9:1.1:2. Other than that, the other parameters and steps remained unchanged;

[0105] In step (2), the second pore-forming agent was omitted, and the mass ratio of lithium manganese iron phosphate, the second conductive agent, and the second binder was adjusted to 96.9:1.1:2. Apart from this, the other parameters and steps remained unchanged.

[0106] Comparative Example 3

[0107] This comparative example is basically the same as comparative example 1, except that the materials of the first and second film layers of the positive electrode sheet of this comparative example are changed to 97.2wt% lithium manganese iron phosphate, 0.9wt% conductive agent, 1.7wt% PVDF, and 0.2wt% ammonium bicarbonate. Correspondingly:

[0108] In step (1), the mass ratio of lithium manganese iron phosphate, the first conductive agent, the first binder and the first pore-forming agent is changed from 96.9:0.9:2:0.2 to 97.2:0.9:1.7:0.2;

[0109] In step (1), the mass ratio of lithium manganese iron phosphate, the second conductive agent, the second binder and the second pore-forming agent is changed from 96.9:0.9:2:0.2 to 97.2:0.9:1.7:0.2.

[0110] Comparative Example 4

[0111] This comparative example is basically the same as Example 1, except that the first film layer of the positive electrode sheet in this comparative example is prepared using the second slurry, and the second film layer is prepared using the first slurry. That is, step (3) is changed to:

[0112] A second slurry was coated on the aluminum foil and dried to obtain a first film layer, wherein the first film layer contained 97.5 wt% of lithium manganese iron phosphate, 0.9 wt% of the second conductive agent, 1.4 wt% of the second binder, and 0.2 wt% of the second pore-forming agent. The first slurry was then coated on the first film layer and dried to obtain a second film layer, wherein the second film layer contained 96.9 wt% of lithium manganese iron phosphate, 0.9 wt% of the first conductive agent, 2 wt% of the first binder, and 0.2 wt% of the first pore-forming agent. A positive electrode sheet was obtained.

[0113] Experimental example

[0114] The positive electrode sheets prepared in the above embodiments and comparative examples were used as the positive electrode, artificial graphite was used as the negative electrode, porous polypropylene film PP was used as the separator, and LiPF6 solution with a concentration of 1 mol / L was used as the electrolyte to assemble into a square aluminum shell battery.

[0115] The 3C capacity retention rate is tested as follows: at a constant temperature of 25°C, the battery capacity at 0.33C and 3C is tested respectively. The ratio of the 3C capacity to the 0.33C capacity is the battery capacity retention rate at 3C.

[0116] The cycle number detection method is: in a constant temperature environment of 25°C, the battery cycle charge and discharge life test is carried out under 1C-1C charge and discharge conditions, and the cycle number that the battery can achieve is tested when the battery capacity drops to 80% of the rated capacity;

[0117] The energy density test method is as follows: under a constant temperature of 25°C, the above-mentioned battery is fully charged at a 0.33C rate and fully discharged at a 0.33C rate, and the actual discharge energy at this time is recorded; the ratio of the actual discharge energy of the lithium-ion battery at 0.33C to the weight of the lithium-ion battery is the actual energy density of the lithium-ion battery.

[0118] The results are recorded in Table 1.

[0119] Table 1

[0120] From the table above we can see that:

[0121] The batteries made from the positive electrode sheets of Examples 1 to 14 all have high energy density, and Example 1 has an energy density significantly better than Comparative Examples 1, 3, and 4, and Example 2 has an energy density significantly better than Comparative Example 2. This indicates that the double-coating design of the present application and controlling the PVDF content in the first film layer to be higher than the PVDF content in the second film layer contribute to improving the energy density of the battery;

[0122] In addition, by comparing Example 1, Example 2 and Comparative Example 2, it can be seen that, relative to Comparative Example 2, Example 1 not only has a much higher energy density improvement than Example 2, but also has a significant improvement in capacity retention and cycle number, indicating that the addition of a pore-forming agent helps to maintain or even improve rate performance and cycle performance while increasing the surface density.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a composite coating disposed on one side of the positive electrode current collector, wherein the composite coating comprises a first film layer and a second film layer stacked, and the first film layer is disposed close to the positive electrode current collector, wherein: The material of the first film layer includes a first positive electrode active material and a first binder, and the material of the second film layer includes a second positive electrode active material and a second binder. The first positive electrode active material and the second positive electrode active material are each independently selected from one or more of lithium manganese iron phosphate materials, and the mass percentage of the first binder in the first film layer is greater than the mass percentage of the second binder in the second film layer.

2. The positive electrode sheet according to claim 1, wherein: The first binder includes polyvinylidene fluoride or polytetrafluoroethylene.

3. The positive electrode sheet according to claim 1, wherein: The second binder includes polyvinylidene fluoride or polytetrafluoroethylene.

4. The positive electrode sheet according to claim 1, wherein: The second film layer includes at least one sub-film layer. When the second film layer includes two or more sub-film layers, the two or more sub-film layers are stacked in a direction away from the positive electrode current collector.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the first film layer, the mass percentage of the first binder is 1.7% to 2.3%.

6. The positive electrode sheet according to claim 5, wherein: In the first film layer, the mass percentage of the first binder is 1.9% to 2.1%.

7. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the second film layer, the mass percentage of the second binder is 1.1% to 1.7%.

8. The positive electrode sheet according to claim 7, wherein: In the second film layer, the mass percentage of the second binder is 1.3% to 1.5%.

9. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the first film layer, the mass percentage of the first positive electrode active material is 96.6% to 97.2%.

10. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the second film layer, the mass percentage of the second positive electrode active material is 97.2% to 97.8%.

11. The positive electrode sheet according to any one of claims 1 to 4, wherein: In the first film layer, the mass percentage of the first binder is 1.7% to 2.3%, and the mass percentage of the first positive electrode active material is 96.6% to 97.2%; In the second film layer, the mass percentage of the second binder is 1.1% to 1.7%, and in the second film layer, the mass percentage of the second positive electrode active material is 97.2% to 97.8%.

12. The positive electrode sheet according to claim 1, wherein: The material of the first film layer also includes a first conductive agent, and the first conductive agent includes one or more of conductive carbon black, carbon nanotubes, and graphene.

13. The positive electrode sheet according to claim 12, wherein: The material of the first membrane layer also includes a first pore-forming agent, and the first pore-forming agent includes one or more of ammonium carbonate, ammonium bicarbonate, urea and sodium chloride.

14. The positive electrode sheet according to claim 13, wherein: Calculated by mass percentage, the material of the first film layer includes 96.6% to 97.2% of lithium manganese iron phosphate, 0.5% to 1.3% of the first conductive agent, 1.7% to 2.3% of the first binder and 0.1% to 0.4% of the first pore-forming agent.

15. The positive electrode sheet according to claim 1, wherein: The material of the second film layer also includes a second conductive agent, and the second conductive agent includes one or more of conductive carbon black, carbon nanotubes, and graphene.

16. The positive electrode sheet according to claim 15, wherein: The material of the second membrane layer further includes a second pore-forming agent, and the second pore-forming agent includes one or more of ammonium carbonate, ammonium bicarbonate, urea and sodium chloride.

17. The positive electrode sheet according to claim 16, wherein: Calculated by mass percentage, the material of the second film layer includes 97.2% to 97.8% of lithium manganese iron phosphate, 0.5% to 1.3% of the second conductive agent, 1.1% to 1.7% of the second binder and 0.1% to 0.4% of the second pore-forming agent.

18. A method for preparing a positive electrode sheet, comprising the following steps: Providing a positive electrode current collector, a first slurry and a second slurry, wherein the first slurry includes a first positive electrode active material and a first binder, the second slurry includes a second positive electrode active material and a second binder, and the content of the first binder in the first slurry is greater than the content of the second binder in the second slurry; coating the first slurry on the positive electrode current collector and drying to form a first film layer; The second slurry is coated on the first film layer and dried to form a second film layer.

19. The positive electrode sheet according to claim 18, wherein: The first slurry further includes a first solvent, wherein the first solvent includes one or more of N-methylpyrrolidone, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and acetone; The second slurry further includes a second solvent, and the second solvent includes one or more of N-methylpyrrolidone, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and acetone.

20. A lithium ion battery, comprising the positive electrode sheet according to any one of claims 1 to 17, or comprising the positive electrode sheet prepared by the preparation method according to claim 18 or 19.

Images