WO2024174760A1

WO2024174760A1 - Positive electrode plate, energy storage device, and method for manufacturing positive electrode plate

Info

Publication number: WO2024174760A1
Application number: PCT/CN2024/071355
Authority: WO
Inventors: 谢炎崇
Original assignee: 厦门海辰储能科技股份有限公司
Priority date: 2023-02-24
Filing date: 2024-01-09
Publication date: 2024-08-29
Also published as: TW202435476A; CN116565142A

Abstract

A positive electrode plate, an energy storage device, and a method for manufacturing a positive electrode plate. The positive electrode plate comprises a current collector, an electrically conductive layer, a plurality of functional layers and a plurality of protective layers, wherein the electrically conductive layer is stacked on a surface of the current collector; the plurality of functional layers and the plurality of protective layers are alternately stacked on the surface of the electrically conductive layer that faces away from the current collector; the innermost functional layer is in contact with the electrically conductive layer; the outermost layer of the positive electrode plate is a protective layer; the materials of each functional layer comprise LiMn_xFe_1-xPO₄ and xLi₂MnO₃(1-x)LiMO₂, where 0 < x < 1, and M represents Ni or Mn; and the material of each protective layer comprises LiNi_0.5Mn_0.3Co_0.2O₂.

Description

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

This application claims the priority of the Chinese patent application filed with the China Patent Office on February 24, 2023, with application number 202310164900.9 and application name “Positive electrode sheet, energy storage device and method for manufacturing positive electrode sheet”, all contents of which are incorporated by reference in this application.

Technical Field

The present application relates to the field of battery technology, and in particular to a positive electrode sheet, an energy storage device, and a method for manufacturing the positive electrode sheet.

Background Art

With the development of the times, high energy density energy storage devices have begun to become a new development direction. In existing lithium-ion batteries, lithium manganese oxide, lithium iron manganese phosphate and lithium-rich manganese base are commonly used positive electrode active materials. However, due to the existence of the Jahn-Teller effect, manganese ions will dissolve during the battery charge and discharge cycle. 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 decay of the cycle capacity of the energy storage device, and in severe cases, it may also cause a short circuit.

Summary of the invention

The present application provides a positive electrode sheet, an energy storage device and a method for preparing the positive electrode sheet, which are used to prevent the dissolution of manganese ions, improve the cycle capacity of lithium-ion batteries, and ensure the reliability of lithium-ion batteries.

In a first aspect, the present application provides a positive electrode sheet, comprising a current collector, a conductive layer, a plurality of functional layers and a plurality of protective layers, wherein the conductive layer is stacked on the surface of the current collector, a plurality of functional layers and a plurality of protective layers are alternately stacked on the surface of the conductive layer 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;

The material of each functional layer includes LiMn _x Fe _1-x PO ₄ and xLi ₂ MnO ₃ (1-x)LiMO ₂ , 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 O ₂ .

In a second aspect, the present application also provides an energy storage device, comprising a negative electrode sheet, a separator and any one of the above-mentioned positive electrode sheets.

In a third aspect, the present application also provides a method for manufacturing a positive electrode sheet, comprising:

providing a fluid collector;

forming a conductive layer on the surface of the current collector;

A plurality of functional layers and a plurality of protective layers are alternately formed on a surface of the conductive layer away from the current collector to obtain a positive electrode sheet, wherein 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, and the material of each of the functional layers includes LiMn _x Fe _1-x PO ₄ and xLi ₂ MnO ₃ (1-x)LiMO ₂ , 0<x<1, M is Ni or Mn, and the material of each of the protective layers includes LiNi _0.5 Mn _0.3 Co _0.2 O ₂ .

In summary, the present application alternately arranges multiple functional layers and multiple protective layers on the surface of the current collector. Under the premise of ensuring the wetting effect of the electrolyte, each protective layer plays a role in blocking the precipitation of manganese ions in the functional layer, thereby effectively avoiding the dissolution of manganese ions, which can significantly improve the cycle performance of lithium-ion batteries and ensure the reliability of lithium-ion batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the implementation manner will be briefly introduced below. Obviously, the drawings described below are only some implementation manners of the present application. For ordinary technicians in this field, other drawings can be obtained like these drawings without paying any creative work.

FIG1 is a schematic diagram of the cross-sectional structure of a positive electrode sheet provided in an embodiment of the present application;

FIG2 is a scanning electron microscope image of a cross section of a positive electrode sheet provided in an embodiment of the present application.

DETAILED DESCRIPTION

The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

The present application provides a lithium-ion battery, which includes a shell, a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The positive electrode sheet, the negative electrode sheet, the separator and the electrolyte are all contained on the inner side of the shell. The positive electrode sheet and the negative electrode sheet are stacked, the separator is located between the positive electrode sheet and the negative electrode sheet, and the electrolyte infiltrates the positive electrode sheet, the negative electrode sheet and the separator. In some other embodiments, the lithium-ion battery may also include a plurality of positive electrode sheets, a plurality of negative electrode sheets and a plurality of separators, the plurality of positive electrode sheets and the plurality of negative electrode sheets are alternately stacked with each other, and each separator is located between a positive electrode sheet and a negative electrode sheet.

Please refer to FIG. 1 , which is a schematic diagram of the cross-sectional structure of a positive electrode sheet 100 provided in an embodiment of the present application.

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.

It should be noted that the directional terms such as “inside” and “outside” involved in the embodiments of the present application are described with reference to the orientation shown in FIG. 1 , with the orientation toward the current collector 10 being “inside” and the orientation away from the current collector 10 being “outside”. They do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

The current collector 10 may be an aluminum foil made of metal aluminum. In this embodiment, the thickness of the current collector 10 may be between 15 μm and 20 μm. In some other embodiments, the current collector 10 may also be made of other conductive materials, and the present application does not specifically limit the structure of the current collector 10.

The material of the conductive layer 20 includes acetylene black and polyacrylate. In the conductive layer 20, the mass proportion of acetylene black is 90% to 95%, and the mass proportion of polyacrylate is 5% to 10%. Because the main component of the conductive layer 20, acetylene black, has excellent conductivity, the conductive layer can improve the 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.

The materials of each functional layer 30 include lithium iron manganese phosphate, lithium-rich manganese base, conductive agent and adhesive. In the functional layer 30, the mass proportion of lithium iron manganese phosphate is 60% to 80%, the mass proportion of lithium-rich manganese base is 10% to 30%, the mass proportion of conductive agent is 2% to 5%, and the mass proportion of adhesive is 2% to 5%. The chemical formula of lithium iron manganese phosphate is LiMn _x Fe _1-x PO ₄ , where 0<x<1; the chemical formula of lithium-rich manganese base is xLi ₂ MnO ₃ (1-x)LiMO ₂ , and M can be Ni or Mn. Because the main components of the functional layer 30 are lithium iron manganese phosphate and lithium-rich manganese base, the functional layer 30 can be used as a lithium source for lithium-ion batteries, so the functional layer 30 can determine the performance of lithium-ion batteries 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 adhesive can be polyvinylidene fluoride.

In this embodiment, the thickness of each functional layer 30 is between 30 μm and 100 μm. The compaction density of the multi-layer functional layer 30 gradually decreases from the inner layer to the outer layer of the positive electrode sheet 100. In other words, among the multi-layer functional layer 30, the compaction density of the functional layer 30 located in the inner layer is greater than the compaction density of the functional layer 30 located in the outer layer, which is beneficial for the electrolyte in the lithium ion battery to infiltrate the functional layer 30 located in the inner layer, thereby improving the electrical performance of the lithium ion battery.

The materials of each protective layer 40 include a ternary material, a conductive agent and an adhesive. In the protective layer 40, the mass proportion of the ternary material is 90% to 95%, the mass proportion of the conductive agent is A1, 2%≤A1≤5%, and the mass proportion of the adhesive is A2, 2%≤A2≤5%. In one embodiment, the mass ratio of the conductive agent to the adhesive is A1:A2=0.65~0.85, and 2%≤(A1+A2)<10%, so as to ensure that the protective layer 40 has good conductivity while having sufficient adhesion. The chemical formula of the ternary material is LiNi _0.5 Mn _0.3 Co _0.2 O ₂ . Because the main component of the protective layer 40 is the ternary material LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , the ternary material can well match the voltage range of the lithium iron manganese phosphate and the lithium-rich manganese base in the functional layer 30, and can prevent the dissolution of manganese ions in the lithium iron manganese phosphate and the lithium-rich manganese base, so the protective layer 40 can reduce the dissolution of manganese ions and improve the cycle performance of the lithium-ion battery. It is understandable that due to the poor conductivity of the ternary material, it will not increase the internal resistance of the secondary battery too much, which helps to ensure the electrical performance of the lithium-ion battery. Exemplarily, in the protective layer 40, the conductive agent can be conductive carbon black, carbon nanotubes or graphene, and the adhesive can be polyvinylidene fluoride.

In this embodiment, the thickness of each protective layer 40 is between 15 μm and 30 μm, which can not only ensure that the thickness of the protective layer 40 is not too large to increase the internal resistance, but also ensure that the thickness of the protective layer 40 is not too small, avoiding uneven coating and causing holes in the protective layer 40. In the direction from the outer layer to the inner layer of the positive electrode sheet 100, the thickness of the multi-layer protective layer 40 gradually decreases, and the compaction density of the multi-layer protective layer 40 gradually increases. It can be understood that the thickness of the protective layer 40 located in the inner layer is reduced, and the inside of the positive electrode sheet 100 can be equivalent to a sieve. Each layer of the protective layer 40 can play a role in blocking the precipitation of manganese ions, thereby helping to improve the cycle performance of the lithium-ion battery. Moreover, the compaction density of the protective layer 40 located in the outer layer is reduced, which is beneficial for the electrolyte in the lithium-ion battery to infiltrate the protective layer 40 located in the inner layer, thereby helping to improve the electrical performance of the lithium-ion battery.

In addition, the mass proportion of the conductive agent in the multi-layer protective layer 40 gradually decreases from the outer layer to the inner layer of the positive electrode sheet 100. It can be understood that the outermost protective layer 40 is best wetted by the electrolyte, but the farther away from the current collector 10, the more conductive agent is needed to improve the conductivity, while the inner protective layer 40 is poorly wetted by the electrolyte, but the closer to the current collector 10, the less conductive agent is needed.

In addition, the mass proportion of the adhesive in the multi-layer protective layer 40 gradually increases from the outer layer to the inner layer of the positive electrode sheet 100. It can be understood that since the compaction density of the outermost protective layer 40 is high, only a small amount of adhesive is required, and the compaction density of the protective layer 40 located in the inner layer is low, so a larger amount of adhesive is required to prevent the electrode material from peeling off.

Next, taking the positive electrode sheet 100 including two functional layers 30 and two protective layers 40 as an example, the structure of the positive electrode sheet 100 is specifically described.

Please refer to FIG. 2 , which is a scanning electron microscope image of a cross section of a positive electrode sheet 100 provided in an embodiment of the present application.

In the positive electrode sheet 100, the two functional layers 30 are respectively the first functional layer 31 and the second functional layer 32, and the two protective layers 40 are respectively the first protective layer 41 and the second protective layer 42. It can be understood that the two functional layers 30 and the two protective layers 40 are sequentially stacked 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.

The compaction density of the first functional layer 31 is greater than the compaction density of the second functional layer 32, and the ratio of the compaction density of the first functional layer 31 to the compaction density of the second functional layer 32 is about 1.2:1. The compaction density of the first protective layer 41 is greater than the compaction density of the second protective layer 42, and the ratio of the compaction density of the first protective layer 41 to the compaction density of the second protective layer 42 is about 1.2:1. Exemplarily, the compaction density of the first protective layer 41 is 3.20 g/cm ³ , and the compaction density of the second protective layer 42 is 2.66 g/cm ³ . In addition, 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 about 1:1.5.

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

When the positive electrode sheet 100 is scanned by electron microscope, EDS element analysis is performed on each layer at the same time, and Table 1 is obtained.

Table 1: EDS element analysis results of each layer in the positive electrode sheet 100 shown in FIG. 2

Combining FIG. 2 and Table 1, it can be seen that in the positive electrode sheet 100 provided in the embodiment of the present application, the protective layer 40, the functional layer 30 and the current collector 10 have uniform thickness, clear boundaries, and no obvious impurity interference between the layers. It should be noted that the conductive layer 20 is too thin and accounts for too little mass in the positive electrode sheet 100, so the composition information of the conductive layer 20 cannot be analyzed in the EDS element analysis.

The present embodiment also provides a method for preparing a positive electrode sheet 100, comprising:

Step S1, providing a current collector.

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

Step S21 , preparing conductive layer slurry, wherein the material of the conductive layer slurry includes acetylene black and polyacrylate.

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.

Step S3, alternately forming multiple functional layers and multiple protective layers on the surface of the conductive layer away from the current collector to obtain a positive electrode sheet, wherein 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, and the material of each functional layer includes LiMn _x Fe _1-x PO ₄ and xLi ₂ MnO ₃ (1-x)LiMO ₂ , 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 O ₂ . This embodiment includes steps 1 to 4.

Step 1: prepare functional layer slurry and protective layer slurry. The materials of the functional layer slurry include LiMn _x Fe _1-x PO ₄ , xLi ₂ MnO ₃ (1-x)LiMO ₂ , a conductive agent and an adhesive, 0<x<1, M is Ni or Mn. The materials of the protective layer slurry include LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , a conductive agent and an adhesive. Exemplarily, the conductive agent may be conductive carbon black, carbon nanotubes or graphene, and the adhesive may be polyvinylidene fluoride.

Step 2: coating the functional layer slurry on the surface of the conductive layer away from the current collector to form the functional layer. Specifically, after coating the functional layer slurry on the surface of the conductive layer away from the current collector, the functional layer slurry is vacuum dried to form the functional layer.

Step three: coating the protective layer slurry on the surface of the functional layer away from the conductive layer to form a protective layer. Specifically, after coating the protective layer slurry on the surface of the functional layer away from the conductive layer, the protective layer slurry is vacuum dried to form a protective layer.

Step 4: coating the functional layer slurry on the surface of the protective layer away from the conductive layer to form the functional layer. Specifically, after coating the functional layer slurry on the surface of the protective layer away from the conductive layer, the functional layer slurry is vacuum dried to form the functional layer.

Repeat steps 3 and 4 1 to 5 times until a positive electrode sheet is obtained.

Next, the electrical properties of the positive electrode sheets prepared in multiple embodiments and multiple comparative examples are compared and analyzed.

Example 1

The positive electrode is prepared according to the following steps:

Step S1, providing a current collector, wherein the thickness of the current collector may be 15 μm.

Step S21, preparing conductive layer slurry. Specifically, acetylene black and polyacrylate are placed in a stirring tank, deionized water is added, and mechanical stirring is performed for 2 hours to prepare conductive layer slurry. The mass proportion of acetylene black is 95%, and the mass proportion of polyacrylate is 5%.

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, place it in a vacuum oven at 150° C. and dry it for 10 hours to form a conductive layer.

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. This embodiment includes steps 1 to 4.

Step 1: prepare functional layer slurry and protective layer slurry. Specifically, take conductive carbon black (conductive agent), polyvinylidene fluoride (binder), LiMn _x Fe _1-x PO ₄ and xLi ₂ MnO ₃ (1-x)LiMO ₂ , 0<x<1, M is Ni or Mn, place them in a stirring tank, add N-methylpyrrolidone and stir for 6 hours to prepare a functional layer slurry, wherein the mass proportion of LiMn _x Fe _1-x PO ₄ is 85%, the mass proportion of xLi ₂ MnO ₃ (1-x)LiMO ₂ is 10%, the mass proportion of conductive carbon black is 2%, and the mass proportion of polyvinylidene fluoride is 3%. Conductive carbon black (conductive agent), polyvinylidene fluoride (binder), and LiNi _0.5 Mn _0.3 Co _0.2 O ₂ are placed in a stirring tank, and N-methylpyrrolidone is added and stirred for 6 hours to prepare a protective layer slurry, in which the mass proportion of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ is 90%, the mass proportion of conductive carbon black is about 5%, and the mass proportion of polyvinylidene fluoride is about 5%.

Step 2: Apply the functional layer slurry on the surface of the conductive layer away from the current collector to form the functional layer. Specifically, after applying the functional layer slurry on the surface of the conductive layer away from the current collector, place the slurry in a vacuum oven at 150° C. and dry it for 10 hours to form the functional layer.

Step 3: Apply protective layer slurry on the surface of the functional layer away from the conductive layer to form a protective layer. Specifically, after applying protective layer slurry on the surface of the functional layer away from the conductive layer, place the surface in a vacuum oven at 150° C. and dry for 10 hours to form a protective layer.

Step 4: Apply the functional layer slurry on the surface of the protective layer away from the conductive layer to form the functional layer. Specifically, after applying the functional layer slurry on the surface of the functional layer away from the protective layer, place it in a vacuum oven at 150° C. and dry it for 10 hours to form the functional layer.

Repeat steps 3 and 4 1 to 5 times until a positive electrode sheet is obtained.

Example 2

In Example 2, the preparation steps of the positive electrode sheet are the same as those in Example 1, except that in the functional layer slurry, the mass proportion of LiMn _x Fe _1-x PO ₄ is 75%, the mass proportion of xLi ₂ MnO ₃ (1-x)LiMO ₂ is 20%, the mass proportion of conductive carbon black is 2%, and the mass proportion of polyvinylidene fluoride is 3%.

Example 3

In Example 3, the preparation steps of the positive electrode sheet are the same as those in Example 1, except that in the functional layer slurry, the mass proportion of LiMn _x Fe _1-x PO ₄ is 90%, the mass proportion of xLi ₂ MnO ₃ (1-x)LiMO ₂ is 5%, the mass proportion of conductive carbon black is 2%, and the mass proportion of polyvinylidene fluoride is 3%.

Example 4

In Example 4, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 10 μm.

Example 5

In Example 5, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 15 μm.

Example 6

In Example 6, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 25 μm.

Example 7

In Example 7, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 30 μm.

Comparative Example 1

In Comparative Example 1, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 35 μm.

Comparative Example 2

In Comparative Example 2, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 40 μm.

Comparative Example 3

In Comparative Example 3, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 50 μm.

Comparative Example 4

In Comparative Example 4, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 5 μm.

Comparative Example 5

In Comparative Example 5, the steps for preparing the positive electrode sheet are the same as those in Example 1, except that the thickness of the protective layer is 0 μm, that is, only the functional layer is provided on the current collector without the protective layer.

In order to test the electrochemical performance of the positive electrode sheets prepared in Examples 1 to 7 and Comparative Examples 1 to 5, the positive electrode sheets prepared in Examples 1 to 7 and Comparative Examples 1 to 5 were respectively assembled into energy storage devices for electrochemical testing. The steps for preparing the energy storage device are as follows:

a. Take artificial graphite, conductive carbon black and sodium carboxymethyl cellulose in a stirring tank, add deionized water and stir for 5 hours to prepare a negative electrode coating slurry, wherein the mass proportion of artificial graphite is 95%, the mass proportion of conductive carbon black is 2.5%, and the mass proportion of sodium carboxymethyl cellulose is 2.5%; then apply the negative electrode coating slurry on a copper foil (negative electrode current collector) with a thickness of 10 μm, put it in a vacuum oven, and dry it at 150°C for 15 hours to obtain a negative electrode sheet.

b. Place the positive and negative electrode sheets into a press for pressing, and then use a puncher to cut out Φ15mm positive electrode discs and Φ18mm negative electrode discs respectively.

c. Place the positive and negative electrode discs in a glove box filled with argon protective atmosphere for battery assembly. The electrolyte is a mixed solvent of 1 mol/L lithium hexafluorophosphate dissolved in ethylene carbonate and diethyl carbonate with a molar ratio of 1:1. The positive electrode disc, polyethylene diaphragm, negative electrode disc and other components are stacked and assembled in sequence, and then the electrolyte is injected to finally obtain a button-type lithium-ion battery.

The energy storage devices made from the positive electrodes of Examples 1 to 7 and Comparative Examples 1 to 5 were subjected to electrochemical performance tests using a battery tester (Neware CT4000, Neware Electronics Co., Ltd.), and the internal resistance, 1C discharge capacity, and capacity retention rate of each energy storage device were measured. Among them, the capacity retention rate refers to the percentage of the battery capacity after 300 cycles to the battery capacity after the first cycle in the 1C charge and discharge cycle test. The data in Table 2 are the results of the performance test under the above test conditions.

Table 2: Electrochemical test results of energy storage devices made using the positive electrode sheets of the embodiments and comparative examples

Combined with Table 2 above, it can be seen that the mass ratios of lithium manganese iron phosphate and lithium-rich manganese base in the functional layer are different between Examples 1 to 3, and the parameters of other layers remain the same. Relative to Example 1, the mass ratio of lithium manganese iron phosphate in the functional layer of the positive electrode sheet of Example 2 is reduced from 85% to 75%, and the mass ratio of lithium-rich manganese base is increased from 10% to 20%. The energy storage device prepared using the positive electrode sheet of Example 2 has an internal resistance increased from 43mΩ to 49mΩ, and a 1C discharge capacity increased from 174.5mAh/g to 182.4mAh/g.

It is understandable that because the lithium-rich manganese base is not conductive, when the mass ratio of the lithium-rich manganese base in the functional layer decreases, the internal resistance of the energy storage device also decreases. At the same time, the lithium-rich manganese base can provide lithium ions as a lithium source, so when the mass ratio of the lithium-rich manganese base in the functional layer decreases, the 1C discharge capacity of the battery also decreases.

Relative to Example 1, the mass ratio of lithium iron manganese phosphate in the functional layer of the positive electrode sheet of Example 3 is reduced from 85% to 75%, and the mass ratio of the lithium-rich manganese base is increased from 10% to 20%. The energy storage device prepared using the positive electrode sheet of Example 3 has an internal resistance increased from 43mΩ to 49mΩ, and a 1C discharge capacity increased from 174.5mAh/g to 182.4mAh/g. This further shows the significant effect of the lithium-rich manganese base on the two performance indicators of battery internal resistance and 1C discharge capacity. Specifically, the battery internal resistance and the battery 1C discharge capacity are positively correlated with the mass ratio of the lithium-rich manganese base in the functional layer. An energy storage device with excellent performance should have a small battery internal resistance and a large 1C discharge capacity. By comparing Examples 1 to 3, it can be found that Example 1 better balances the two performance indicators of battery internal resistance and battery 1C discharge capacity. Therefore, when the mass proportion of lithium iron manganese phosphate in the functional layer of the positive electrode sheet is 85% and the mass proportion of the lithium-rich manganese base is 10%, the electrical performance of the energy storage device is better.

Examples 1, 4 to 7 and comparative examples 1 to 5 differ only in the parameter of the thickness of the protective layer, and the other parameters remain the same. By comparing Examples 1, 4 to 7, it can be found that when the thickness of the protective layer varies in the range of 10 to 30 μm, the parameters such as the battery internal resistance, 1C discharge capacity and capacity retention rate change little, and are all maintained within a preferred range. Specifically, when the thickness of the protective layer 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 rate is maintained at around 85.5%. From the above, it can be seen that when the thickness of the protective layer varies in the range of 10 μm to 30 μm, the battery internal resistance is positively correlated with the thickness of the protective layer, the 1C discharge capacity is negatively correlated with the thickness of the protective layer, and the capacity retention rate remains basically unchanged. When the thickness of the protective layer exceeds the range of 10 to 30 μm, some parameters such as the battery internal resistance, 1C discharge capacity and capacity retention rate change greatly, causing the electrical performance of the lithium-ion battery to deteriorate rapidly.

Specifically, it can be seen from Comparative Examples 1 to 3 that when the thickness of the protective layer is greater than 30 μm, as the thickness of the protective layer increases to 50 μm, The 1C discharge capacity of the energy storage device quickly dropped to 169.7mAh/g, and the capacity retention rate also quickly dropped to 82.3%. This is because a protective layer that is too thick will make it difficult for the electrolyte to infiltrate the positive electrode sheet, greatly reducing the 1C discharge capacity and capacity retention rate of the battery, that is, reducing the discharge performance and cycle performance of the battery. As can be seen from Comparative Examples 4 and 5, when the thickness of the protective layer is less than 10μm, as the thickness of the protective layer is reduced to 5μm or even 0μm, the 1C discharge capacity of the energy storage device rises to 183.4mAh/g, but the capacity retention rate quickly drops to below 80%. This is because a protective layer that is too thin will cause the protective layer to be unable to form a complete membrane structure to prevent the dissolution of manganese ions in the functional layer, thereby significantly reducing the capacity retention rate of the energy storage device and significantly weakening the cycle performance. From the above discussion, it can be seen that 10μm to 30μm is the optimal range of variation for the thickness of the protective layer, and the energy storage device with a protective layer thickness in the range of 10 to 30μm has excellent discharge performance and cycle performance.

The embodiments of the present application are introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. At the same time, for general technical personnel in this field, according to the idea of the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

A positive electrode sheet, characterized in that it comprises a current collector, a conductive layer, a plurality of functional layers and a plurality of protective layers, wherein the conductive layer is stacked on the surface of the current collector, a plurality of functional layers and a plurality of protective layers are alternately stacked on the surface of the conductive layer 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;

The material of each functional layer includes LiMn x Fe 1-x PO 4 and xLi 2 (1-x)LiMO 2 , 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 O 2 .
The positive electrode sheet according to claim 1 is characterized in that the thickness of each of the protective layers is between 15 μm and 30 μm.
The positive electrode sheet according to claim 1 or 2 is characterized in that the thickness of the multiple protective layers gradually decreases from the outer layer to the inner layer of the positive electrode sheet.
The positive electrode sheet according to claim 1 or 2 is characterized in that, along the direction from the inner layer to the outer layer of the positive electrode sheet, the compaction density of the multiple layers of the protective layer gradually decreases.
The positive electrode sheet according to claim 1, characterized in that, in each layer of the protective layer, the mass proportion of the LiNi 0.5 Mn 0.3 Co 0.2 O 2 is between 90% and 95%.
The positive electrode sheet according to claim 5 is characterized in that the material of each protective layer also includes a conductive agent, and the content of the conductive agent in the multiple protective layers gradually decreases in the direction from the outer layer to the inner layer of the positive electrode sheet.
The positive electrode sheet according to claim 6 is characterized in that, in each of the protective layers, the mass proportion of the conductive agent is A1, 2%≤A1≤5%.
The positive electrode sheet according to claim 5 is characterized in that the material of each protective layer also includes an adhesive, and the content of the adhesive in the multiple protective layers gradually increases in the direction from the outer layer to the inner layer of the positive electrode sheet.
The positive electrode sheet according to claim 8 is characterized in that, in each of the protective layers, the mass proportion of the adhesive is A2, 2%≤A2≤5%.
The positive electrode sheet according to claim 5 is characterized in that the material of each protective layer also includes a conductive agent and an adhesive, and the mass proportion A1 of the conductive agent and the mass proportion A2 of the adhesive satisfy the formula, A1:A2=0.65~0.85, and 2%≤(A1+A2)<10%.
The positive electrode sheet according to claim 1 is characterized in that, 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.
The positive electrode sheet according to claim 1 or 11, characterized in that the thickness of each of the functional layers is between 30 μm and 100 μm.
The positive electrode sheet according to claim 1 or 11, characterized in that, in each of the functional layers, the mass proportion of the LiMn x Fe 1-x PO 4 is between 60% and 80%, and the mass proportion of the xLi 2 MnO 3 (1-x)LiMO 2 is between 10% and 30%.
The positive electrode sheet according to claim 1 is characterized in that the thickness of the conductive layer is between 1 μm and 3 μm.
The positive electrode sheet according to claim 1 or 14 is characterized in that the material of the conductive layer includes acetylene black and polyacrylate, and in the conductive layer, the mass proportion of the acetylene black is between 90% and 95%, and the mass proportion of the polyacrylate is between 5% and 10%.
An energy storage device, characterized by comprising a negative electrode sheet, a separator and a positive electrode sheet as claimed in any one of claims 1 to 15.
A method for manufacturing a positive electrode sheet, characterized by comprising:

providing a fluid collector;

forming a conductive layer on the surface of the current collector;

A plurality of functional layers and a plurality of protective layers are alternately formed on a surface of the conductive layer away from the current collector to obtain a positive electrode sheet, wherein 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, and the material of each of the functional layers includes LiMn x Fe 1-x PO 4 and xLi 2 MnO 3 (1-x)LiMO 2 , 0<x<1, M is Ni or Mn, and the material of each of the protective layers includes LiNi 0.5 Mn 0.3 Co 0.2 O 2 .
The method for manufacturing a positive electrode sheet according to claim 17 is characterized in that, in the step of "alternatingly forming a plurality of functional layers and a plurality of protective layers on a surface of the conductive layer away from the current collector to obtain a positive electrode sheet", it comprises:

Step 1, preparing a functional layer slurry and a protective layer slurry, wherein the material of the functional layer slurry includes the LiMn x Fe 1-x PO 4 and the xLi 2 MnO 3 (1-x)LiMO 2 , 0<x<1, M is Ni or Mn, and the material of the protective layer slurry includes the LiNi 0.5 Mn 0.3 Co 0.2 O 2 ;

Step 2: coating the functional layer slurry on the surface of the conductive layer away from the current collector to form the functional layer;

Step three, coating the protective layer slurry on the surface of the functional layer away from the conductive layer to form the protective layer;

Step 4, coating the functional layer slurry on the surface of the protective layer away from the conductive layer to form the functional layer;

Repeat step 3 and step 4 1 to 5 times until the positive electrode sheet is obtained.
The method for manufacturing a positive electrode sheet according to claim 17 or 18, characterized in that in the step of "forming a conductive layer on the surface of the current collector", it includes:

Preparing a conductive layer slurry, wherein the conductive layer slurry comprises acetylene black and polyacrylate;

The conductive layer slurry is coated on the surface of the current collector to form the conductive layer.