Positive electrode sheet, energy storage device, and method for manufacturing positive electrode sheet

Abstract

The application discloses a positive electrode sheet, an energy storage device and a manufacturing method of the positive electrode sheet. The positive electrode sheet comprises a current collector, a conductive layer, a plurality of functional layers and a plurality of protective layers. The conductive layer is laminated on the surface of the current collector. The plurality of functional layers and the plurality of protective layers are alternately laminated on the surface of the conductive layer away from the current collector. The innermost functional layer is in contact with the conductive layer. The outermost layer of the positive electrode sheet is the protective layer. The material of each functional layer comprises LiMn x Fe 1‑x PO4 and xLi2MnO3(1-x)LiMO2 (0 0.5 Mn 0.3 Co 0.2 O2.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to a positive electrode sheet, an energy storage device, and a method for manufacturing the positive electrode sheet. Background Art

[0002] With the development of the era, energy storage devices with high energy density have begun to become a new development direction. In existing lithium-ion batteries, lithium manganate, lithium manganese iron phosphate, and lithium-rich manganese-based are commonly used positive electrode active materials. However, due to the Jahn-Teller effect of manganese, manganese ions will dissolve during the charge and discharge cycles of the battery. The dissolved manganese ions will diffuse through the electrolyte and precipitate on the surface of the negative electrode, increasing the impedance of the negative electrode interface, resulting in the attenuation of the cycle capacity of the energy storage device, and may even cause a short circuit in severe cases. Summary of the Invention

[0003] This application provides a positive electrode sheet, an energy storage device, and a method for preparing the positive electrode sheet, which are used to block the dissolution of manganese ions, improve the cycle capacity of lithium-ion batteries, and ensure the reliability of the use of lithium-ion batteries.

[0004] In a first aspect, this application provides a positive electrode sheet, which includes a current collector, a conductive layer, multiple functional layers, and multiple protective layers. The conductive layer is laminated on the surface of the current collector, and the multiple functional layers and the multiple protective layers are alternately laminated on the surface of the conductive layer facing away from the current collector. The innermost functional layer is in contact with the conductive layer, and the outermost layer of the positive electrode sheet is the protective layer;

[0005] Among them, the material of each functional layer includes LiMn x Fe 1-x PO4 and xLi2MnO3(1 - x)LiMO2, 0 < x < 1, M is Ni or Mn, and the material of each protective layer includes LiNi 0.5 Mn 0.3 Co 0.2 O2.

[0006] In one embodiment, the thickness of each protective layer is between 15 μm and 30 μm.

[0007] In one embodiment, along the direction from the outer layer to the inner layer of the positive electrode sheet, the thickness of the multiple protective layers gradually decreases.

[0008] In one embodiment, along the direction from the inner layer to the outer layer of the positive electrode sheet, the compaction density of the multiple protective layers gradually decreases.

[0009] In one embodiment, in each protective layer, the LiNi 0.5 Mn 0.3 Co 0.2The mass percentage of O2 is between 90% and 95%.

[0010] In one embodiment, the material of each of the protective layers further includes a conductive agent, and the content of the conductive agent in the multiple protective layers gradually decreases along the direction from the outer layer to the inner layer of the positive electrode sheet.

[0011] In one embodiment, in each of the protective layers, the mass percentage A1 of the conductive agent is 2% ≤ A1 ≤ 5%.

[0012] In one embodiment, the material of each of the protective layers further includes an adhesive, and the content of the adhesive gradually increases in the multiple protective layers along the direction from the outer layer to the inner layer of the positive electrode sheet.

[0013] In one embodiment, in each of the protective layers, the mass percentage A2 of the adhesive is 2% ≤ A2 ≤ 5%.

[0014] In one embodiment, the material of each of the protective layers further includes a conductive agent and an adhesive, wherein the mass ratio A1 of the conductive agent and the mass ratio A2 of the adhesive satisfy the formula, A1:A2 = 0.65 to 0.85, and 2% ≤ (A1 + A2) < 10%.

[0015] In one embodiment, the compaction density of the multiple functional layers gradually decreases along the direction from the inner layer to the outer layer of the positive electrode sheet.

[0016] In one embodiment, the thickness of each of the functional layers is between 30 μm and 100 μm.

[0017] In one embodiment, in each of the functional layers, the LiMn x Fe 1-x The mass percentage of PO4 is between 60% and 80%, and the mass percentage of xLi2MnO3(1-x)LiMO2 is between 10% and 30%.

[0018] In one embodiment, the thickness of the conductive layer is between 1 μm and 3 μm.

[0019] In one embodiment, the conductive layer is made of acetylene black and polyacrylate, wherein the mass percentage of acetylene black in the conductive layer is between 90% and 95%, and the mass percentage of polyacrylate is between 5% and 10%.

[0020] Secondly, this application also provides an energy storage device, including a negative electrode, a separator, and any of the above-mentioned positive electrode.

[0021] Thirdly, this application also provides a method for manufacturing a positive electrode sheet, comprising:

[0022] Provide a fluid collection;

[0023] Form a conductive layer on the surface of the current collector;

[0024] Form multiple functional layers and multiple protective layers alternately on the surface of the conductive layer facing away from the current collector to obtain a positive electrode sheet, wherein the innermost functional layer contacts the conductive layer, and the outermost protective layer is the outermost layer of the positive electrode sheet. The material of each functional layer includes LiMn x Fe 1-x PO4 and xLi2MnO3(1 - x)LiMO2, 0 < x < 1, M is Ni or Mn. The material of each protective layer includes LiNi 0.5 Mn 0.3 Co 0.2 O2.

[0025] In one embodiment, in the step of "forming multiple functional layers and multiple protective layers alternately on the surface of the conductive layer facing away from the current collector to obtain a positive electrode sheet", it includes:

[0026] Step 1, prepare a functional layer slurry and a protective layer slurry. The material of the functional layer slurry includes the LiMn x Fe 1-x PO4 and the xLi2MnO3(1 - x)LiMO2, 0 < x < 1, M is Ni or Mn. The material of the protective layer slurry includes the LiNi 0.5 Mn 0.3 Co 0.2 O2;

[0027] Step 2, coat the functional layer slurry on the surface of the conductive layer facing away from the current collector to form the functional layer;

[0028] Step 3, coat the protective layer slurry on the surface of the functional layer facing away from the conductive layer to form the protective layer;

[0029] Step 4, coat the functional layer slurry on the surface of the protective layer facing away from the conductive layer to form the functional layer;

[0030] Repeat Step 3 and Step 4 1 - 5 times until the positive electrode sheet is obtained.

[0031] In one embodiment, in the step of "forming a conductive layer on the surface of the current collector", it includes:

[0032] Prepare a conductive layer slurry, where the conductive layer slurry includes acetylene black and polyacrylate;

[0033] Coat the conductive layer slurry on the surface of the current collector to form the conductive layer.

[0034] In summary, this application alternately sets multiple functional layers and multiple protective layers on the surface of the current collector. While ensuring the electrolyte wetting effect, each protective layer plays a role in preventing the precipitation of manganese ions from the functional layers, thus effectively avoiding the phenomenon of manganese ion dissolution. This can significantly improve the cycle performance of lithium-ion batteries and ensure the reliability of lithium-ion batteries. Attached Figure Description

[0035] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the cross-sectional structure of the positive electrode sheet provided in the embodiments of this application;

[0037] Figure 2 This is a scanning electron microscope image of a cross-section of a positive electrode provided in an embodiment of this application. Detailed Implementation

[0038] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0039] This application provides a lithium-ion battery, which includes a casing, a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode, negative electrode, separator, and electrolyte are all housed inside the casing. The positive and negative electrodes are stacked, with the separator located between them, and the electrolyte wets the positive electrode, negative electrode, and separator. In other embodiments, the lithium-ion battery may also include multiple positive electrodes, multiple negative electrodes, and multiple separators, with the multiple positive and multiple negative electrodes stacked alternately, and each separator located between one positive electrode and one negative electrode.

[0040] Please refer to Figure 1 , Figure 1 This is a schematic diagram of the cross-sectional structure of the positive electrode 100 provided in the embodiments of this application.

[0041] The positive electrode sheet 100 includes a current collector 10, a conductive layer 20, a multi-layer functional layer 30, and a multi-layer protective layer 40. The conductive layer 20 is stacked on the surface of the current collector 10. Along the thickness direction of the positive electrode sheet 100, the multi-layer functional layer 30 and the multi-layer protective layer 40 are alternately stacked on the surface of the conductive layer 20 away from the current collector 10. Among them, the innermost functional layer 30 is in contact with the surface of the conductive layer 20 away from the current collector 10. The outermost protective layer 40 serves as the outermost layer of the positive electrode sheet 100.

[0042] It should be noted that the orientation terms such as "inner" and "outer" involved in the embodiments of the present application are described based on the orientation shown in the drawings. Taking the direction towards the current collector 10 as "inner" and the direction away from the current collector 10 as "outer", it does not indicate or imply that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application. Figure 1 Taking the direction towards the current collector 10 as "inner" and the direction away from the current collector 10 as "outer", it does not indicate or imply that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application. <P

[0043] The current collector 10 can be an aluminum foil made of metallic aluminum. In this embodiment, the thickness of the current collector 10 can be between 15 μm and 20 μm. In some other embodiments, the current collector 10 can also be made of other conductive materials, and the present application does not specifically limit the structure of the current collector 10. <<P

[0044] The material of the conductive layer 20 includes acetylene black and polyacrylate. In the conductive layer 20, the mass ratio of acetylene black is 90% to 95%, and the mass ratio of polyacrylate is 5% to 10%. Because the main component acetylene black in the conductive layer 20 has excellent electrical conductivity, the conductive layer can improve the electrical conductivity of the positive electrode sheet 100. In this embodiment, the thickness of the conductive layer 20 can be between 1 μm and 3 μm.

[0045] The material of each functional layer 30 includes lithium iron phosphate manganese, lithium-rich manganese-based, conductive agent, and binder. In the functional layer 30, the mass ratio of lithium iron phosphate manganese is 60% to 80%, the mass ratio of lithium-rich manganese-based is 10% to 30%, the mass ratio of the conductive agent is 2% to 5%, and the mass ratio of the binder is 2% to 5%. The chemical formula of lithium iron phosphate manganese is LiMn x Fe 1-x PO4, where 0 < x < 1; the chemical formula of the lithium-rich manganese-based is xLi2MnO3(1 - x)LiMO2, and M can be Ni or Mn. Because the main components of the functional layer 30 are lithium iron phosphate manganese and lithium-rich manganese-based, the functional layer 30 can be used as a lithium source for a lithium-ion battery, so the functional layer 30 can determine the performance of the lithium-ion battery such as voltage and energy density. Exemplarily, in the functional layer 30, the conductive agent can be conductive carbon black, carbon nanotubes, or graphene, and the binder can be polyvinylidene fluoride.

[0046] In this embodiment, the thickness of each functional layer 30 is between 30 μm and 100 μm. Along the direction from the inner layer to the outer layer of the positive electrode 100, the compaction density of the multilayer functional layers 30 gradually decreases. In other words, in the multilayer functional layers 30, the compaction density of the inner functional layers 30 is greater than that of the outer functional layers 30. This is beneficial for the electrolyte to wet the inner functional layers 30 in the lithium-ion battery, thereby improving the electrical performance of the lithium-ion battery.

[0047] Each protective layer 40 comprises a ternary material, a conductive agent, and an adhesive. In the protective layer 40, the ternary material accounts for 90% to 95% of the mass, the conductive agent accounts for A1 (2% ≤ A1 ≤ 5%), and the adhesive accounts for A2 (2% ≤ A2 ≤ 5%). In one embodiment, the mass ratio of conductive agent to adhesive, A1:A2, is 0.65 to 0.85, and 2% ≤ (A1 + A2) < 10%, to ensure that the protective layer 40 has sufficient adhesion while also possessing good conductivity. The chemical formula of the ternary material is LiNi. 0.5 Mn 0.3 Co 0.2 O2. Because the main component of protective layer 40 is the ternary material LiNi. 0.5 Mn 0.3 Co 0.2 O2, a ternary material, can well match the voltage range of lithium manganese iron phosphate and lithium-rich manganese-based materials in functional layer 30, and can prevent the dissolution of manganese ions from these materials. Therefore, protective layer 40 can reduce manganese ion dissolution and improve the cycle performance of lithium-ion batteries. It is understandable that because ternary materials have poor conductivity, they will not significantly increase the internal resistance of the secondary battery, helping to ensure the electrical performance of the lithium-ion battery. For example, in protective layer 40, the conductive agent can be conductive carbon black, carbon nanotubes, or graphene, and the binder can be polyvinylidene fluoride.

[0048] In this embodiment, the thickness of each protective layer 40 is between 15μm and 30μm. This ensures that the thickness of the protective layer 40 is not too large, which would increase the internal resistance, and that the thickness is not too small, which would prevent uneven coating and voids in the protective layer 40. Along the direction from the outer layer to the inner layer of the positive electrode 100, the thickness of the multiple protective layers 40 gradually decreases, while the compaction density of the multiple protective layers 40 gradually increases. It can be understood that the reduced thickness of the inner protective layers 40 acts like a sieve inside the positive electrode 100, with each protective layer 40 effectively preventing manganese ion deposition, thus contributing to improved cycle performance of the lithium-ion battery. Furthermore, the reduced compaction density of the outer protective layers 40 facilitates the wetting of the inner protective layers 40 by the electrolyte in the lithium-ion battery, thereby improving the electrical performance of the lithium-ion battery.

[0049] Furthermore, along the direction from the outermost layer to the innermost layer of the positive electrode 100, the mass percentage of the conductive agent in the multilayer protective layers 40 gradually decreases. It is understandable that the outermost protective layer 40 has the best wetting effect from the electrolyte, but the farther it is from the current collector 10, the more conductive agent is needed to improve conductivity. Conversely, the inner protective layers 40 have a poorer wetting effect from the electrolyte, but the closer they are to the current collector 10, the less conductive agent is required.

[0050] Furthermore, along the direction from the outermost layer to the innermost layer of the positive electrode 100, the mass percentage of the binder in the multilayer protective layers 40 gradually increases. It is understandable that, since the outermost protective layer 40 has a high compaction density, only a small amount of binder is needed, while the inner protective layers 40 have a low compaction density, thus requiring a larger amount of binder to prevent electrode material peeling.

[0051] Next, taking the positive electrode 100, which includes two functional layers 30 and two protective layers 40, as an example, the structure of the positive electrode 100 will be explained in detail.

[0052] Please refer to Figure 2 , Figure 2 This is a scanning electron microscope image of a cross-section of a positive electrode sheet 100 provided in an embodiment of this application.

[0053] In the positive electrode 100, the two functional layers 30 are the first functional layer 31 and the second functional layer 32, respectively, and the two protective layers 40 are the first protective layer 41 and the second protective layer 42, respectively. It can be understood that the two functional layers 30 and the two protective layers 40 are stacked sequentially on the surface of the conductive layer 20 away from the current collector 10 in the order of the first functional layer 31, the first protective layer 41, the second functional layer 32, and the second protective layer 42.

[0054] The compaction density of the first functional layer 31 is greater than that of the second functional layer 32, and the ratio of their compaction densities is approximately 1.2:1. Similarly, the compaction density of the first protective layer 41 is greater than that of the second protective layer 42, and the ratio of their compaction densities is also approximately 1.2:1. For example, the compaction density of the first protective layer 41 is 3.20 g / cm³. 3 The compaction density of the second protective layer 42 is 2.66 g / cm³. 3 Furthermore, the thickness of the first protective layer 41 is less than the thickness of the second protective layer 42, and the ratio of the thickness of the first protective layer 41 to the thickness of the second protective layer 42 is approximately 1:1.5.

[0055] from Figure 2As can be seen, the successively stacked current collector 10, first functional layer 31, first protective layer 41, second functional layer 32, and second protective layer 42 are all layered structures with uniform thickness, and the boundaries between them are distinct. It should be noted that compared with the current collector 10, functional layer 30, and protective layer 40, the thickness of the conductive layer 20 is too small, so it is difficult to be shown in the scanning electron microscope image.

[0056] When the positive electrode sheet 100 is scanned by an electron microscope, EDS element analysis is simultaneously performed on each layer to obtain Table 1.

[0057] Table 1: Figure 2 EDS element analysis results of each layer in the shown positive electrode sheet 100

[0058]

[0059] Combined Figure 2 With Table 1, it can be seen that in the positive electrode sheet 100 provided by the embodiment of the present application, each layer of the protective layer 40, functional layer 30, and current collector 10 has uniform thickness, distinct boundaries, and there is no obvious impurity interference between layers. It should be noted that due to the too small thickness of the conductive layer 20, the mass ratio of the conductive layer 20 in the positive electrode sheet 100 is too small, so the component information of the conductive layer 20 cannot be analyzed in the EDS element analysis.

[0060] The embodiment of the present application also provides a preparation method for a positive electrode sheet 100, including:

[0061] Step S1, providing a current collector.

[0062] Step S2, forming a conductive layer on the surface of the current collector. In this embodiment, step S2 includes steps S21 to S22.

[0063] Step S21, preparing a conductive layer slurry, wherein the materials of the conductive layer slurry include acetylene black and polyacrylate.

[0064] Step S22, coating the conductive layer slurry on the surface of the current collector to form a conductive layer. Specifically, after coating the conductive layer slurry on the surface of the current collector, the conductive layer slurry is vacuum dried to form a conductive layer.

[0065] Step S3, alternately forming multiple functional layers and multiple protective layers on the surface of the conductive layer facing away from the current collector to obtain a positive electrode sheet, wherein the innermost functional layer is in contact with the conductive layer, the outermost protective layer is the outermost layer of the positive electrode sheet, and the material of each functional layer includes LiMn x Fe 1-x PO4 and xLi2MnO3(1 - x)LiMO2, 0 < x < 1, M is Ni or Mn, and the material of each protective layer includes LiNi 0.5 Mn 0.3 Co0.2 O2. This embodiment includes steps one to four.

[0066] Step one, prepare the functional layer slurry and the protective layer slurry. Among them, the materials of the functional layer slurry include LiMn x Fe 1- x PO4, xLi2MnO3(1 - x)LiMO2, a conductive agent, and a binder, where 0 < x < 1 and M is Ni or Mn. The materials of the protective layer slurry include LiNi 0.5 Mn 0.3 Co 0.2 O2, a conductive agent, and a binder. Exemplarily, the conductive agent can be conductive carbon black, carbon nanotubes, or graphene, and the binder can be polyvinylidene fluoride.

[0067] Step two, coat the functional layer slurry on the surface of the conductive layer facing away from the current collector to form a functional layer. Specifically, after coating the functional layer slurry on the surface of the conductive layer facing away from the current collector, the functional layer slurry is dried in vacuum to form a functional layer.

[0068] Step three, coat the protective layer slurry on the surface of the functional layer facing away from the conductive layer to form a protective layer. Specifically, after coating the protective layer slurry on the surface of the functional layer facing away from the conductive layer, the protective layer slurry is dried in vacuum to form a protective layer.

[0069] Step four, coat the functional layer slurry on the surface of the protective layer facing away from the conductive layer to form a functional layer. Specifically, after coating the functional layer slurry on the surface of the protective layer facing away from the conductive layer, the functional layer slurry is dried in vacuum to form a functional layer.

[0070] Repeat steps three and four 1 to 5 times until the positive electrode sheet is obtained.

[0071] Next, compare and analyze the electrical properties of the positive electrode sheets prepared in multiple embodiments and multiple comparative examples.

[0072] Example 1

[0073] The positive electrode sheet is prepared according to the following steps:

[0074] Step S1, provide a current collector. Among them, the thickness of the current collector can be 15 μm.

[0075] Step S2, form a conductive layer on the surface of the current collector. In this embodiment, step S2 includes steps S21 to S22.

[0076] Step S21, prepare the conductive layer slurry. Specifically, take acetylene black and polyacrylate in a stirring tank, add deionized water, and mechanically stir for 2 h to prepare the conductive layer slurry. Among them, the mass ratio of acetylene black is 95%, and the mass ratio of polyacrylate is 5%.

[0077] Step S22: Coat the conductive layer slurry on the surface of the current collector to form a conductive layer. Specifically, after coating the conductive layer slurry on the surface of the current collector, place it in a vacuum oven and dry it at 150°C for 10 hours to form a conductive layer.

[0078] Step S3: Alternately form multiple functional layers and multiple protective layers on the surface of the conductive layer facing away from the current collector to obtain a positive electrode sheet. This embodiment includes steps one to four.

[0079] Step one: Prepare the functional layer slurry and the protective layer slurry. Specifically, take conductive carbon black (conductive agent), polyvinylidene fluoride (adhesive), LiMn x Fe 1-x PO4 and xLi2MnO3(1-x)LiMO2, where 0 < x < 1 and M is Ni or Mn, place them in a stirring tank, and then add N-methylpyrrolidone and stir for 6 hours to prepare the functional layer slurry. Among them, the mass ratio of LiMn x Fe 1-x PO4 is 85%, the mass ratio of xLi2MnO3(1-x)LiMO2 is 10%, the mass ratio of conductive carbon black is 2%, and the mass ratio of polyvinylidene fluoride is 3%. Take conductive carbon black (conductive agent), polyvinylidene fluoride (adhesive), and LiNi 0.5 Mn 0.3 Co 0.2 O2 in a stirring tank, and then add N-methylpyrrolidone and stir for 6 hours to prepare the protective layer slurry. Among them, the mass ratio of LiNi 0.5 Mn 0.3 Co 0.2 O2 is 90%, the mass ratio of conductive carbon black is approximately 5%, and the mass ratio of polyvinylidene fluoride is approximately 5%.

[0080] Step two: Coat the functional layer slurry on the surface of the conductive layer facing away from the current collector to form a functional layer. Specifically, after coating the functional layer slurry on the surface of the conductive layer facing away from the current collector, place it in a vacuum oven and dry it at 150°C for 10 hours to form a functional layer.

[0081] Step three: Coat the protective layer slurry on the surface of the functional layer facing away from the conductive layer to form a protective layer. Specifically, after coating the protective layer slurry on the surface of the functional layer facing away from the conductive layer, place it in a vacuum oven and dry it at 150°C for 10 hours to form a protective layer.

[0082] Step four: Coat the functional layer slurry on the surface of the protective layer facing away from the conductive layer to form a functional layer. Specifically, after coating the functional layer slurry on the surface of the functional layer facing away from the protective layer, place it in a vacuum oven and dry it at 150°C for 10 hours to form a functional layer.

[0083] Repeat steps three and four 1 to 5 times until a positive electrode is obtained.

[0084] Example 2

[0085] In Example 2, the preparation steps of the positive electrode are the same as in Example 1, the difference being that in the functional layer slurry, LiMn... x Fe 1-x The mass percentage of PO4 is 75%, the mass percentage of xLi2MnO3(1-x)LiMO2 is 20%, the mass percentage of conductive carbon black is 2%, and the mass percentage of polyvinylidene fluoride is 3%.

[0086] Example 3

[0087] In Example 3, the preparation steps of the positive electrode are the same as in Example 1, the difference being that in the functional layer slurry, LiMn... x Fe 1-x The mass percentage of PO4 is 90%, the mass percentage of xLi2MnO3(1-x)LiMO2 is 5%, the mass percentage of conductive carbon black is 2%, and the mass percentage of polyvinylidene fluoride is 3%.

[0088] Example 4

[0089] In Example 4, the preparation steps of the positive electrode are the same as in Example 1, except that the thickness of the protective layer is 10 μm.

[0090] Example 5

[0091] In Example 5, the preparation steps of the positive electrode are the same as in Example 1, except that the thickness of the protective layer is 15 μm.

[0092] Example 6

[0093] In Example 6, the preparation steps of the positive electrode are the same as in Example 1, except that the thickness of the protective layer is 25 μm.

[0094] Example 7

[0095] In Example 7, the preparation steps of the positive electrode are the same as in Example 1, except that the thickness of the protective layer is 30 μm.

[0096] Comparative Example 1

[0097] In Comparative Example 1, the preparation steps of the positive electrode were the same as in Example 1, except that the thickness of the protective layer was 35 μm.

[0098] Comparative Example 2

[0099] In Comparative Example 2, the preparation steps of the positive electrode were the same as in Example 1, except that the thickness of the protective layer was 40 μm.

[0100] Comparative Example 3

[0101] In Comparative Example 3, the preparation steps of the positive electrode were the same as in Example 1, except that the thickness of the protective layer was 50 μm.

[0102] Comparative Example 4

[0103] In Comparative Example 4, the preparation steps of the positive electrode were the same as in Example 1, except that the thickness of the protective layer was 5 μm.

[0104] Comparative Example 5

[0105] In Comparative Example 5, the preparation steps of the positive electrode were the same as in Example 1, except that the thickness of the protective layer was 0 μm, that is, only the functional layer was provided on the current collector, and there was no protective layer.

[0106] To test the electrochemical performance of the positive electrode sheets prepared in Examples 1-7 and Comparative Examples 1-5, the positive electrode sheets prepared in Examples 1-7 and Comparative Examples 1-5 were assembled into energy storage devices for electrochemical testing. The specific steps for preparing the energy storage devices are as follows:

[0107] a. Artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose were placed in a mixing tank, and deionized water was added and stirred for 5 hours to prepare a negative electrode coating slurry. The artificial graphite accounted for 95% of the mass, the conductive carbon black accounted for 2.5% of the mass, and the sodium carboxymethyl cellulose accounted for 2.5% of the mass. The negative electrode coating slurry was then coated onto a copper foil (negative electrode current collector) with a thickness of 10 μm and placed in a vacuum oven. After drying at 150°C for 15 hours, a negative electrode sheet was obtained.

[0108] b. Place the positive and negative electrode sheets into a press for pressing, and then use a punch to cut Φ15mm positive electrode discs and Φ18mm negative electrode discs respectively.

[0109] c. The positive and negative electrode discs are placed in a glove box filled with an argon protective atmosphere for battery assembly. The electrolyte is a mixture of 1 mol / L lithium hexafluorophosphate dissolved in a 1:1 molar ratio of ethylene carbonate and diethyl carbonate. The positive electrode disc, polyethylene separator, negative electrode disc, and other components are sequentially stacked and assembled, then the electrolyte is injected, finally yielding a coin cell lithium-ion battery.

[0110] The energy storage devices fabricated using the positive electrode sheets of Examples 1-7 and Comparative Examples 1-5 were subjected to electrochemical performance tests using a battery tester (Neware CT4000, Neware Electronics Co., Ltd.). Parameters such as internal resistance, 1C discharge capacity, and capacity retention rate were measured for each energy storage device. The capacity retention rate refers to the percentage of battery capacity after 300 cycles in a 1C charge-discharge cycle test compared to the battery capacity after the first cycle. The data in Table 2 are the results of performance tests conducted under the aforementioned test conditions.

[0111] Table 2: Electrochemical test results of energy storage devices prepared using the positive electrode sheets of the examples and comparative examples

[0112]

[0113] As shown in Table 2 above, the mass ratios of lithium manganese iron phosphate and lithium-rich manganese-based functional layers differ between Examples 1 and 3, while the parameters of other layers remain consistent. Compared to Example 1, in Example 2, the mass ratio of lithium manganese iron phosphate in the functional layer of the positive electrode decreased from 85% to 75%, while the mass ratio of lithium-rich manganese-based functional layers increased from 10% to 20%. The energy storage device prepared using the positive electrode of Example 2 showed an increase in internal resistance from 43mΩ to 49mΩ and an increase in 1C discharge capacity from 174.5mAh / g to 182.4mAh / g.

[0114] Understandably, because lithium-rich manganese-based materials are non-conductive, the internal resistance of the energy storage device decreases when the mass ratio of lithium-rich manganese-based materials in the functional layer decreases. Simultaneously, since lithium-rich manganese-based materials serve as a lithium source, they can provide lithium ions; therefore, the 1C discharge capacity of the battery also decreases when the mass ratio of lithium-rich manganese-based materials in the functional layer decreases.

[0115] Compared to Example 1, in Example 3, the mass ratio of lithium manganese iron phosphate in the functional layer of the positive electrode decreased from 85% to 75%, while the mass ratio of lithium-rich manganese-based material increased from 10% to 20%. The energy storage device prepared using the positive electrode of Example 3 showed an increase in internal resistance from 43 mΩ to 49 mΩ and an increase in 1C discharge capacity from 174.5 mAh / g to 182.4 mAh / g. This further demonstrates the significant impact of lithium-rich manganese-based material on the battery's internal resistance and 1C discharge capacity. Specifically, the battery's internal resistance and 1C discharge capacity are positively correlated with the mass ratio of lithium-rich manganese-based material in the functional layer. A high-performance energy storage device should possess both low internal resistance and high 1C discharge capacity. Comparing Examples 1 to 3 reveals that Example 1 better balances the battery's internal resistance and 1C discharge capacity. Therefore, when the mass ratio of lithium manganese iron phosphate in the functional layer of the positive electrode is 85% and the mass ratio of lithium-rich manganese-based material is 10%, the energy storage device exhibits superior electrical performance.

[0116] Examples 1, 4-7, and Comparative Examples 1-5 differ only in the thickness of the protective layer; all other parameters remain consistent. Comparing Examples 1, 4-7 reveals that when the protective layer thickness varies within the range of 10-30 μm, parameters such as battery internal resistance, 1C discharge capacity, and capacity retention show minimal changes and remain within optimal ranges. Specifically, when the protective layer thickness increases from 10 μm to 30 μm, the battery internal resistance gradually increases from 42 mΩ to 47 mΩ, the 1C discharge capacity gradually decreases from 177.3 mAh / g to 172.4 mAh / g, and the capacity retention remains around 85.5%. Therefore, when the protective layer thickness varies within the range of 10 μm-30 μm, the battery internal resistance is positively correlated with the protective layer thickness, the 1C discharge capacity is negatively correlated with the protective layer thickness, and the capacity retention remains essentially unchanged. When the thickness of the protective layer varies beyond the range of 10 to 30 μm, some parameters such as the battery internal resistance, 1C discharge capacity, and capacity retention rate change significantly, causing the electrical performance of lithium-ion batteries to deteriorate rapidly.

[0117] Specifically, as shown in Comparative Examples 1-3, when the protective layer thickness is greater than 30 μm, as the thickness increases to 50 μm, the 1C discharge capacity of the energy storage device rapidly decreases to 169.7 mAh / g, and the capacity retention rate also rapidly decreases to 82.3%. This is because an excessively thick protective layer makes it difficult for the electrolyte to wet the positive electrode, significantly reducing the battery's 1C discharge capacity and capacity retention rate, thus reducing the battery's discharge performance and cycle performance. However, as shown in Comparative Examples 4 and 5, when the protective layer thickness is less than 10 μm, as the thickness decreases to 5 μm or even 0 μm, the 1C discharge capacity of the energy storage device increases to 183.4 mAh / g, but the capacity retention rate rapidly drops below 80%. This is because an excessively thin protective layer prevents the formation of a complete film structure to prevent the dissolution of manganese ions within the functional layer, thus significantly reducing the capacity retention rate and weakening the cycle performance of the energy storage device. As discussed above, 10μm to 30μm is the optimal range for the thickness of the protective layer. Energy storage devices with a protective layer thickness in the range of 10 to 30μm have superior discharge and cycle performance.

[0118] The embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes a current collector, a conductive layer, multiple functional layers, and multiple protective layers. The conductive layer is stacked on the surface of the current collector. Multiple functional layers and multiple protective layers are alternately stacked on the conductive layer away from the surface of the current collector. The innermost functional layer is in contact with the conductive layer. The outermost layer of the positive electrode is the protective layer. Among them, the material of each of the functional layers includes LiMn x Fe 1-x PO4 and xLi2MnO3(1-x)LiMO2, where 0 < x < 1 and M is Ni or Mn. In each of the functional layers, the mass ratio of the LiMn x Fe 1-x PO4 is between 60% and 80%, and the mass ratio of the xLi2MnO3(1-x)LiMO2 is between 10% and 30%. The material of each of the protective layers includes LiNi 0.5 Mn 0.3 Co 0.2 O2. The thickness of each of the protective layers is between 10 μm and 30 μm. Along the direction from the inner layer to the outer layer of the positive electrode sheet, the compaction density of the multiple functional layers gradually decreases, and the compaction density of the multiple protective layers gradually decreases.

2. The positive electrode sheet according to claim 1, characterized in that, The thickness of each of the protective layers is between 15 μm and 30 μm.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, Along the direction from the outer layer to the inner layer of the positive electrode, the thickness of the multiple protective layers gradually decreases.

4. The positive electrode sheet according to claim 1, characterized in that, In each of the protective layers, the LiNi 0.5 Mn 0.3 Co 0.2 The mass percentage of O2 is between 90% and 95%.

5. The positive electrode sheet according to claim 1, characterized in that, The material of each of the protective layers also includes a conductive agent, and the content of the conductive agent gradually decreases in the multiple protective layers along the direction from the outer layer to the inner layer of the positive electrode sheet.

6. The positive electrode sheet according to claim 5, characterized in that, In each of the protective layers, the mass percentage A1 of the conductive agent is 2% ≤ A1 ≤ 5%.

7. The positive electrode sheet according to claim 4, characterized in that, The material of each of the protective layers also includes an adhesive, and the content of the adhesive gradually increases in the multiple protective layers along the direction from the outer layer to the inner layer of the positive electrode sheet.

8. The positive electrode sheet according to claim 7, characterized in that, In each of the protective layers, the adhesive accounts for a mass percentage A2 of 2% ≤ A2 ≤ 5%.

9. The positive electrode sheet according to claim 4, characterized in that, Each of the protective layers further includes a conductive agent and an adhesive, wherein the mass ratio A1 of the conductive agent and the mass ratio A2 of the adhesive satisfy the formula, A1:A2=0.65~0.85, and 2%≤(A1+A2)<10%.

10. The positive electrode sheet according to claim 1, characterized in that, The thickness of each of the functional layers is between 30 μm and 100 μm.

11. The positive electrode sheet according to claim 1, characterized in that, The thickness of the conductive layer is between 1 μm and 3 μm.

12. The positive electrode sheet according to claim 1 or 11, characterized in that, The conductive layer is made of acetylene black and polyacrylate, wherein the mass percentage of acetylene black in the conductive layer is between 90% and 95%, and the mass percentage of polyacrylate is between 5% and 10%.

13. An energy storage device, characterized in that, It includes a negative electrode, a separator, and a positive electrode as described in any one of claims 1 to 12.

14. A method for manufacturing a positive electrode sheet, characterized in that, include: Provide a fluid collection; A conductive layer is formed on the surface of the current collector; On the surface of the conductive layer facing away from the current collector, multiple layers of functional layers and multiple layers of protective layers are alternately formed to obtain a positive electrode sheet. Among them, the innermost functional layer is in contact with the conductive layer, and the outermost protective layer is the outermost layer of the positive electrode sheet. The material of each functional layer includes LiMn x Fe 1-x PO4 and xLi2MnO3(1 - x)LiMO2, where 0 < x < 1, and M is Ni or Mn. In each functional layer, the mass ratio of the LiMn x Fe 1-x PO4 is between 60% and 80%, and the mass ratio of the xLi2MnO3(1 - x)LiMO2 is between 10% and 30%. The material of each protective layer includes LiNi 0.5 Mn 0.3 Co 0.2 O2. The thickness of each protective layer is between 10 μm and 30 μm. Along the direction from the inner layer to the outer layer of the positive electrode sheet, the compaction density of the multiple functional layers gradually decreases, and the compaction density of the multiple protective layers gradually decreases.

15. The method for manufacturing a positive electrode sheet according to claim 14, characterized in that, The step of "alternatingly forming multiple functional layers and multiple protective layers on the surface of the conductive layer opposite to the current collector to obtain a positive electrode sheet" includes: Step 1, prepare the functional layer slurry and the protective layer slurry. Among them, the materials of the functional layer slurry include the LiMn x Fe 1-x PO4 and the xLi2MnO3(1-x)LiMO2, 0 < x < 1, M is Ni or Mn. The materials of the protective layer slurry include the LiNi 0.5 Mn 0.3 Co 0.2 O2; Step 2: Apply the functional layer paste to the surface of the conductive layer opposite to the current collector to form the functional layer; Step 3: Apply the protective layer paste to the surface of the functional layer opposite to the conductive layer to form the protective layer; Step 4: Apply the functional layer paste to the surface of the protective layer opposite to the conductive layer to form the functional layer; Repeat steps three and four 1 to 5 times until the positive electrode is obtained.

16. The method for manufacturing a positive electrode sheet according to claim 14 or 15, characterized in that, The step of "forming a conductive layer on the surface of the current collector" includes: A conductive layer paste is prepared, wherein the materials of the conductive layer paste include acetylene black and polyacrylate; The conductive layer paste is coated on the surface of the current collector to form the conductive layer.

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