Positive electrode and preparation method therefor, and lithium battery

Abstract

A positive electrode and a preparation method therefor, and a lithium battery. The positive electrode comprises a current collector and a positive electrode material layer coated on the surface of the current collector; the positive electrode material layer comprises a positive electrode material; the carbon-coated current collector comprises a current collector substrate and a conductive coating arranged on the current collector substrate; the conductive coating comprises a first conductive agent and a lithium compensation agent; and at least one positive electrode material and the lithium compensation agent have the same crystal structure. By introducing a specific type of lithium compensation agent into a carbon coating layer and using the lithium compensation agent in combination with the first conductive agent, the performance of the positive electrode can be improved. The lithium battery assembled by using the positive electrode has high initial coulombic efficiency, long cycle life, and high energy density during operation.

Description

A positive electrode, its preparation method, and a lithium battery Technical Field

[0001] This application relates to the field of battery technology, such as a positive electrode and its preparation method and a lithium battery. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, energy storage, and automotive power due to their advantages such as good cycle performance, high energy density, high output voltage, and low self-discharge. To meet the ever-increasing energy demand, developing lithium-ion batteries with high energy density and long cycle life has become a key objective. However, due to unavoidable parasitic reactions, lithium-ion batteries always suffer from the loss of active lithium, which is the main reason for the performance degradation of lithium-ion batteries.

[0003] Lithium-ion batteries can balance these issues by providing sufficient lithium at the anode. However, the instability and fragility of lithium metal in air mean that adding excessive lithium to the positive electrode coating is not a solution to the reduced energy density and cycle life of lithium-ion batteries.

[0004] CN109713367B discloses a scheme to improve the energy density and cycle life of lithium batteries, which provides a silicon-containing positive electrode coating material. This strategy does not fundamentally solve the problem of lithium deficiency during the initial operation of lithium batteries; it only addresses the surface issue.

[0005] The working principle of a lithium-ion battery refers to the migration of lithium ions from the positive electrode material (such as lithium iron phosphate or lithium cobalt oxide) to the negative electrode material (such as graphite) during charging. Specifically, when a lithium-ion battery is charged, lithium ions are released from the positive electrode material and transported to the negative electrode via the electrolyte (usually a mixture of organic solvent and lithium salt), where they are then embedded into the structure of the negative electrode material. During discharge, lithium ions are deintercalated from the negative electrode material, transported to the positive electrode via the electrolyte, and recombined with the positive electrode material. This process is reversible, allowing lithium-ion batteries to be charged and discharged multiple times. The lithium replenishment mechanism is key to the charge-discharge cycle capability of lithium-ion batteries and is one of the reasons for their high energy density and long cycle life.

[0006] Current research often places the lithium replenishment process within the cathode slurry. By adding lithium replenishment materials to the cathode slurry, sufficient sacrificial lithium ions are provided for the first charge and discharge cycle of the battery, thus achieving the purpose of lithium replenishment. However, this strategy has the following problems: 1. The main material in the cathode slurry is the cathode material (such as lithium iron phosphate), which has poor conductivity, while lithium replenishment materials are often insulating, which will have a greater impact on the battery's electrical performance; 2. Adding lithium replenishment materials to the cathode material will reduce the mechanical strength of the coating, thereby affecting subsequent processes in battery manufacturing.

[0007] Therefore, it is necessary to provide an effective lithium replenishment strategy to enable batteries to have high energy density and cycle life, which is a technical problem that urgently needs to be solved. Summary of the Invention

[0008] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0009] This application proposes a positive electrode, a method for preparing the same, and a lithium battery.

[0010] In a first aspect, this application provides a positive electrode, the positive electrode comprising a current collector and a positive electrode material layer coated on the surface of the current collector, the positive electrode material layer comprising a positive electrode material, the current collector comprising a current collector substrate and a conductive coating disposed on the current collector substrate, the conductive coating comprising a first conductive agent and a lithium replenishing agent, wherein at least one of the positive electrode material and the lithium replenishing agent has the same crystal structure.

[0011] In this application, the same crystal structure can refer to the same substance or different substances, as long as the crystal structure is the same. For example, substances with the same crystal structure can be phosphate-based cathode materials, such as lithium manganese iron phosphate and lithium iron phosphate, both of which belong to the olivine crystal structure. They can also be ternary cathode materials, such as lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide, both of which belong to the layered crystal structure.

[0012] It should be noted that for a given substance, the substance obtained after elemental doping has the same crystal structure as the original substance, for example, LiNi. 0.8 Co 0.1 Mn 0.1 O2 and LiNi 0.8 Co 0.1 Mn 0.05 Zr 0.05 O2 belongs to the same crystal structure.

[0013] This application does not specifically limit the type of current collector substrate. For example, it can be an aluminum current collector or other composite current collectors. Those skilled in the art can choose according to their needs.

[0014] This application introduces a specific type of lithium replenishing agent into the carbon coating layer and uses it in conjunction with a first conductive agent. This allows the carbon coating layer to not only improve the conductivity of the positive electrode but also replenish lithium. This method has virtually no impact on the battery's electrical performance while providing excellent lithium replenishment. It replenishes the lithium ions lost during the initial operation and subsequent cycles of the lithium battery, preventing lithium ion loss and enabling the lithium battery to have a high initial efficiency. It also effectively increases the battery's cycle life and energy density during operation.

[0015] In this application, the lithium replenishing agent and at least one positive electrode material in the positive electrode material layer need to have the same crystal structure. This is because: typically, lithium replenishing agents are inorganic metal compounds such as anhydrous LiCl, LiOH, lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), lithium iron oxide (LiFe5O8), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium cobalt nickel oxide (LiCoNiO2), and lithium iron nickel oxide (LiFeNiO2). These materials all contain Li, and during the first charge and discharge of the lithium battery, the Li in the carbon coating layer can replenish the Li missing in the positive electrode, thus playing a role in lithium replenishment. However, this type of material is not the optimal choice because, during battery operation, the difference between the carbon coating layer material and the positive electrode material can affect the electron transport rate. Meanwhile, the purpose of the carbon coating layer is to provide conductivity to the positive electrode. However, most aqueous solutions of lithium salts are alkaline or weakly alkaline, affecting the microstructure morphology of the conductive agent in the carbon coating layer, thus affecting the sample structure of the carbon coating layer and increasing the interfacial resistance between the carbon coating layer and the electrode material layer. Therefore, the addition of inorganic metal compound lithium replenishers will change the internal elemental structure of the carbon coating layer, thereby affecting battery efficiency, which is counterproductive. This application uses a material with the same crystal structure as at least one of the positive electrode materials in the positive electrode material layer for lithium replenishment. Substances with the same crystal structure have the principle of similar compatibility, and the high anisotropy consistency between substances with the same crystal structure is more conducive to increasing the lithium-ion transport speed and reducing the repulsion of the electron output process. Therefore, the method of this application can replenish the lithium elements lost during the first operation of the battery and during subsequent cycles without reducing the battery's electrical performance.

[0016] In one embodiment, the crystal structure is selected from any one of layered structure, spinel structure, or olivine structure.

[0017] The following are optional technical solutions for this application, but are not intended to limit the technical solutions provided in this application. The technical objectives and beneficial effects of this application can be better achieved through the following optional technical solutions.

[0018] In one embodiment, the particle size of the lithium supplement meets the following requirements: D50 is 13μm-17μm, for example, it can be 13μm, 13.5μm, 14μm, 14.5μm, 15μm, 15.5μm, 16μm, 16.5μm or 17μm; D90 is 23μm-27μm, for example, it can be 23μm, 23.5μm, 24μm, 24.5μm, 25μm, 25.5μm, 26μm, 26.5μm or 27μm.

[0019] In one embodiment, the particle size of the positive electrode material satisfies the following: D50 is 8μm-14μm, for example, it can be 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 13μm or 14μm, etc.; D90 is 18μm-22μm, for example, it can be 18μm, 18.5μm, 19μm, 19.5μm, 20μm, 20.5μm, 21μm or 22μm, etc.

[0020] In one embodiment, the ratio of the lithium replenishing agent to the particle size D50 of the cathode material is 0.8-1.2, for example, it can be 0.8, 0.85, 0.9, 1.0, 1.1 or 1.2, etc., with 0.9-1.1 being optional. It should be noted that the "cathode material" here refers to a cathode material with the same crystal structure as the lithium replenishing agent.

[0021] If the particle size difference between the lithium supplement and the cathode material is large, it may lead to a decrease in coating adhesion, affecting the product's service life and reliability, and consequently increasing coating corrosion, thus affecting product safety.

[0022] In one embodiment, based on the total mass of the first conductive agent and the lithium replenishing agent as 100%, the mass percentage of the lithium replenishing agent is 0%-60% and does not include 0%, for example, it can be 0.5%, 1%, 3%, 5%, 6%, 8%, 10%, 13%, 14%, 16%, 18%, 20%, 22%, 25%, 27%, 30%, 33%, 36%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 56%, 58%, or 60%, etc., and can be selected as 5%-15%.

[0023] Since lithium replenishing agents have poor conductivity, a strategy of co-doping with conductive agents is required. By optimizing the ratio of lithium replenishing agents to conductive agents, the electrochemical performance of the cathode can be improved.

[0024] In one embodiment, the conductive coating is a carbon coating layer.

[0025] In one embodiment, the first conductive agent includes at least one of carbon black, Ketjen black, and acetylene black.

[0026] In one embodiment, the conductive coating further includes a first adhesive.

[0027] In one embodiment, the first adhesive comprises at least one of acrylate, polyacrylic acid (PAA), modified polyacrylic acid, or waterborne polyurethane, optionally modified polyacrylic acid.

[0028] In this application, modified polyacrylic acid refers to a class of materials whose molecular structure is modified through chemical or physical modification to improve its performance and application value. For example, modification can increase the conductivity, thermal stability, and so on of PAA adhesives.

[0029] In one embodiment, the first adhesive has a mass content of 6%-15% in the conductive coating, for example, it can be 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 11%, 11.5%, 12%, 12.5%, 13%, 14%, 14.5% or 15%, etc.

[0030] In one embodiment, the thickness of the carbon coating layer is 0.3μm-2μm, for example, it can be 0.3μm, 0.5μm, 0.8μm, 1μm, 1.2μm, 1.5μm, 1.7μm, 1.8μm or 2μm.

[0031] In this application, the conductive coating is relatively thin, and the addition of lithium supplementation material will not affect the mechanical stability of the positive electrode.

[0032] In one embodiment, the positive electrode material layer includes a positive electrode material, a second conductive agent, and a second binder.

[0033] This application does not specifically limit the types of the second conductive agent and the second binder, nor the mass ratio of the positive electrode material, the second conductive agent, and the second binder. Those skilled in the art can refer to the relevant technologies for preparing the positive electrode material layer in related technologies for selection. This application does not make specific limitations.

[0034] Secondly, this application provides a method for preparing a positive electrode as described in the first aspect, the method comprising the following steps:

[0035] (1) Disperse the raw materials of the conductive coating in a solvent to obtain a conductive slurry;

[0036] (2) The conductive paste is coated onto the surface of the current collector substrate, and after drying, a conductive coating is formed on the surface of the current collector substrate to obtain a current collector;

[0037] (3) An active slurry is coated on the conductive coating of the current collector, and after drying, a positive electrode material layer is formed on the surface of the conductive coating to obtain a positive electrode;

[0038] The conductive coating is made from a first conductive agent and a lithium supplement agent.

[0039] The method described in this application has simple preparation steps, requires few pieces of equipment, and has low maintenance and production costs, which is conducive to its widespread application.

[0040] In one embodiment, the raw materials of the conductive coating in step (1) further include a first binder.

[0041] In one embodiment, the first adhesive comprises at least one of acrylate, polyacrylic acid, modified polyacrylic acid, or waterborne polyurethane, optionally modified polyacrylic acid.

[0042] In one embodiment, the first adhesive is a modified polyacrylic acid liquid with a solid content of 15%-30%, for example, it can be 15%, 16%, 17%, 18%, 20%, 22%, 24%, 25%, 26%, 28%, or 30%, etc.; the viscosity of the modified polyacrylic acid is 200 mPa·s-1500 mPa·s, for example, it can be 200 mPa·s, 300 mPa·s, 400 mPa·s, 500 mPa·s, etc. Pa.s, 550mPa.s, 600mPa.s, 650mPa.s, 700mPa.s, 750mPa.s, 800mPa.s, 850mPa.s, 900mPa.s, 9 50mPa.s, 1000mPa.s, 1050mPa.s, 1100mPa.s, 1200mPa.s, 1300mPa.s, 1400mPa.s or 1500mPa.s, etc.

[0043] In one embodiment, the raw material of the carbon coating layer in step (1) also includes a wetting agent.

[0044] In one embodiment, the wetting agent includes at least one of polyether siloxane, modified polyether siloxane, and alcohol, with modified polyether siloxane being optional.

[0045] In this application, modified polyether siloxane refers to a class of materials whose molecular structure is modified through chemical or physical modification to improve their performance and application value. Generally, modification is used to improve the oleophilicity, hydrophilicity, and thermal stability of polyether siloxane materials.

[0046] In one embodiment, the amount of the wetting agent is 5%-20% of the total mass of the reaction system, for example, it can be 5%, 6%, 8%, 9%, 10%, 12%, 14%, 15%, 16%, 18%, or 20%. Here, the total mass of the reaction system refers to the mass of all liquids after the addition of the wetting agent.

[0047] In one embodiment, the solvent in step (1) is water, which may be deionized water.

[0048] As an optional technical solution for the preparation method described in this application, step (1) includes:

[0049] S1 adds a first conductive agent and a lithium supplement agent to the adhesive solution, disperses them, and obtains the first material;

[0050] S2 adds a wetting agent to the first material, stirs it, and then grinds it to obtain a conductive paste.

[0051] In one embodiment, the solid content of the adhesive liquid in step S1 is 15%-25%, for example, it can be 15%, 17%, 18%, 20%, 22%, 23%, 24% or 25%, etc.

[0052] In one embodiment, step S1 includes: first adding a portion of the first conductive agent and a portion of the lithium supplement agent to the adhesive solution for a first high-speed dispersion, and then adding the remaining first conductive agent and lithium supplement agent to the obtained material for a second high-speed stirring.

[0053] In this application, by adding the conductive agent in two stages, the problem of excessively high slurry viscosity due to the large amount of conductive agent (e.g., carbon black) being added all at once, which would prevent stirring, can be avoided. Adding the lithium supplementer in two stages can increase the uniformity of the lithium supplementer in the slurry.

[0054] In one embodiment, the portion of the first conductive agent accounts for 40%-60% of the total conductive agent, for example, it can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 53%, 55%, 57% or 60%, etc.

[0055] In one embodiment, the portion of the lithium replenishing agent accounts for 40%-60% of the total lithium replenishing agent, for example, it can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 53%, 55%, 57% or 60%, etc.

[0056] In one embodiment, the rotation speed of the first high-speed stirring is 2000rpm-2600rpm, for example, it can be 2000rpm, 2100rpm, 2150rpm, 2200rpm, 2250rpm, 2300rpm, 2350rpm, 2400rpm, 2500rpm, 2550rpm or 2600rpm, etc.

[0057] In one embodiment, the time for the first high-speed stirring is 30-60 minutes, for example, it can be 20 minutes, 22 minutes, 25 minutes, 27 minutes, 30 minutes, 35 minutes, 38 minutes, 40 minutes, 43 minutes, 46 minutes, 48 ​​minutes, 50 minutes, 53 minutes, 56 minutes or 60 minutes.

[0058] In one embodiment, the rotation speed of the second high-speed stirring is 2000rpm-2600rpm, for example, it can be 2000rpm, 2100rpm, 2150rpm, 2200rpm, 2250rpm, 2300rpm, 2350rpm, 2400rpm, 2500rpm, 2550rpm or 2600rpm, etc.

[0059] In one embodiment, the second high-speed stirring time is 30-60 minutes, for example, it can be 20 minutes, 22 minutes, 25 minutes, 27 minutes, 30 minutes, 35 minutes, 38 minutes, 40 minutes, 43 minutes, 46 minutes, 48 ​​minutes, 50 minutes, 53 minutes, 56 minutes, or 60 minutes.

[0060] In one embodiment, the stirring speed in step S2 is 10 rpm to 15 rpm, for example, it can be 10 rpm, 11 rpm, 12 rpm, 13 rpm or 15 rpm.

[0061] In one embodiment, the stirring time in step S2 is 30 min to 45 min, for example, it can be 30 min, 35 min, 38 min, 40 min, 43 min or 45 min, etc.

[0062] In one embodiment, the number of milling operations in step S2 is 4 to 8 times, for example, 4, 5, 6, 7, or 8 times. Those skilled in the art can adjust the number of milling operations according to the particle size requirements of the slurry.

[0063] In one embodiment, the parameters for sand milling in step S2 are as follows: the sand milling speed is 800 r / min-1200 r / min, for example, it can be 800 r / min, 850 r / min, 900 r / min, 950 r / min, 1000 r / min, 1050 r / min, 1100 r / min, 1150 r / min or 1200 r / min, etc.; the sand milling time is 5 min-15 min, for example, it can be 5 min, 8 min, 10 min, 12 min or 15 min, etc.

[0064] Thirdly, this application provides a lithium battery, the lithium battery including the positive electrode described in the first aspect.

[0065] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this application will not exhaustively list the specific point values ​​included in the range.

[0066] Compared with related technologies, this application has the following advantages:

[0067] This application introduces a specific type of lithium replenishing agent into the carbon coating layer and uses it in conjunction with a first conductive agent. This allows the carbon coating layer to not only improve the conductivity of the positive electrode but also replenish lithium. This method has virtually no impact on the battery's electrical performance while providing excellent lithium replenishment. It replenishes the lithium ions lost during the initial operation and subsequent cycles of the lithium battery, preventing lithium ion loss and enabling the lithium battery to have a high initial efficiency. It also effectively increases the battery's cycle life and energy density during operation.

[0068] After reading and understanding the detailed description, other aspects can be understood. Detailed Implementation

[0069] The technical solution of this application will be further described below through specific implementation methods.

[0070] The embodiments of this application are described in detail below so that the advantages and features of the application can be more easily understood by those skilled in the art, thereby making a clearer and more explicit definition of the scope of protection of this application.

[0071] In this embodiment, the modified polyacrylic acid adhesive was purchased from Sinopharm Chemical Reagent Co., Ltd., with an average molecular weight of MW~5000 and a solid content of 20%.

[0072] The areal capacity of the positive electrode prepared in Examples 1-12 and Comparative Examples 1-3 of this application is 4 mAh / 100cm². 2 .

[0073] Example 1

[0074] This embodiment provides a positive electrode sheet, including a carbon-coated current collector and a positive electrode material layer coated on the surface of the carbon-coated current collector. The carbon-coated current collector includes a current collector substrate and a carbon coating layer (0.5 μm thick) disposed on the current collector substrate. The carbon coating layer includes a first conductive agent, a first binder, and a lithium replenishing agent. The positive electrode material layer includes a positive electrode material, a second conductive agent, and a second binder. The lithium replenishing agent and the positive electrode material are substances with the same crystal structure.

[0075] The current collector substrate is aluminum foil, the first conductive agent is carbon black, the first binder is modified polyacrylic acid, the lithium supplementer is lithium iron phosphate (D50 is 130μm, D90 is 25μm), the mass ratio of the first conductive agent to the lithium supplementer is 90:10, the mass content of the first binder in the carbon coating layer is 10%, the positive electrode material is lithium iron phosphate (D50 is 12μm, D90 is 20μm), the second conductive agent is graphite, the second binder is PVDF, the mass ratio of the positive electrode material, the second conductive agent and the second binder is 96:2:2, and the ratio of the particle size D50 of the lithium supplementer to the positive electrode material is 1.08.

[0076] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0077] (1) Provide a first adhesive solution (modified polyacrylic acid solution, solid content 20%, viscosity 1000 mPa.s);

[0078] (2) Add half of the first conductive agent and half of the lithium supplement to the first adhesive liquid in step (1), disperse it at high speed of 2500 rpm for 40 min in a 200L double star stirring tank, then add the remaining conductive agent and lithium supplement, and continue to disperse at high speed of 2500 rpm for 40 min to obtain the first material.

[0079] (3) Add a wetting agent (modified polyether siloxane) to the first material. The wetting agent accounts for 10% of the total mass of the reaction system. Stir at a low speed of 10 rpm for 40 min to obtain a slurry.

[0080] (4) Transfer the slurry into a sand mill and grind it 6 times;

[0081] (5) The slurry after sand milling is evenly coated onto aluminum foil and dried to obtain a carbon-coated current collector;

[0082] (6) Dissolve the positive electrode material, the second conductive agent and the second binder in NMP to obtain the positive electrode slurry;

[0083] (7) The positive electrode slurry is uniformly coated onto the surface of the carbon-coated current collector and dried to obtain the positive electrode sheet.

[0084] By obtaining microscopic images of the carbon coating layer using a scanning electron microscope and then performing energy dispersive spectroscopy on the images, the lithium incorporation at the microscopic level was observed. It was found that lithium elements can be uniformly dispersed on the spherical carbon black particles. This close interaction promotes the efficient transport of electrons and lithium ions and provides sufficient sacrificial lithium ions for the first charge and discharge of the lithium battery, thereby increasing the conductivity and electrical performance of the lithium battery.

[0085] Example 2

[0086] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 80:20.

[0087] Example 3

[0088] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 70:30.

[0089] Example 4

[0090] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 60:40.

[0091] Example 5

[0092] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 50:50.

[0093] Example 6

[0094] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 40:60.

[0095] Example 7

[0096] The difference between this embodiment and Embodiment 1 is that the lithium supplement is lithium manganese iron phosphate (chemical formula LiFe). 0.5 Mn 0.5 PO4), and the cathode material is lithium iron phosphate.

[0097] Example 8

[0098] The difference between this embodiment and Embodiment 1 is that the lithium supplement is a ternary lithium (chemical formula LiNi). 0.8 Co 0.15 Mn 0.05 O2), the cathode material is ternary lithium (chemical formula LiNi). 0.8 Co 0.15 Mn 0.05 O2).

[0099] Example 9

[0100] This embodiment provides a positive electrode sheet, including a carbon-coated current collector and a positive electrode material layer coated on the surface of the carbon-coated current collector. The carbon-coated current collector includes a current collector substrate and a carbon coating layer (1.5 μm thick) disposed on the current collector substrate. The carbon coating layer includes a first conductive agent, a first binder, and a lithium replenishing agent. The positive electrode material layer includes a positive electrode material, a second conductive agent, and a second binder. The lithium replenishing agent and the positive electrode material are substances with the same crystal structure.

[0101] The current collector substrate is aluminum foil, the first conductive agent is carbon black, the first binder is modified polyacrylic acid, the lithium supplementer is lithium iron phosphate (D50 is 15μm, D90 is 27μm), the mass ratio of the first conductive agent to the lithium supplementer is 80:20, the mass content of the first binder in the carbon coating layer is 6%, the positive electrode material is lithium iron phosphate (D50 is 10μm, D90 is 22μm), the second conductive agent is graphite, the second binder is PVDF, the mass ratio of the positive electrode material, the second conductive agent and the second binder is 96:2:2, and the ratio of the particle size D50 of the lithium supplementer to the positive electrode material is 1.5.

[0102] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0103] (1) Provide a first adhesive solution (modified polyacrylic acid solution, solid content 20%, viscosity 1000 mPa.s);

[0104] (2) Add half of the first conductive agent and half of the lithium supplement to the first adhesive solution in step (1), disperse it at high speed of 2000 rpm for 60 min in a 200L double star stirring tank, then add the remaining conductive agent and lithium supplement, and continue to disperse at high speed of 2600 rpm for 30 min to obtain the first material.

[0105] (3) Add a wetting agent (modified polyether siloxane) to the first material. The wetting agent accounts for 5% of the total mass of the reaction system. Stir at a low speed of 15 rpm for 30 min to obtain a slurry.

[0106] (4) Transfer the slurry into a sand mill and grind it 7 times;

[0107] (5) The slurry after sand milling is evenly coated onto aluminum foil and dried to obtain a carbon-coated current collector;

[0108] (6) Dissolve the positive electrode material, the second conductive agent and the second binder in NMP to obtain the positive electrode slurry;

[0109] (7) The positive electrode slurry is uniformly coated onto the surface of the carbon-coated current collector and dried to obtain the positive electrode sheet.

[0110] Example 10

[0111] This embodiment provides a positive electrode sheet, including a carbon-coated current collector and a positive electrode material layer coated on the surface of the carbon-coated current collector. The carbon-coated current collector includes a current collector substrate and a carbon coating layer (2 μm thick) disposed on the current collector substrate. The carbon coating layer includes a first conductive agent, a first binder, and a lithium replenishing agent. The positive electrode material layer includes a positive electrode material, a second conductive agent, and a second binder. The lithium replenishing agent and the positive electrode material are substances with the same crystal structure.

[0112] The current collector substrate is aluminum foil, the first conductive agent is carbon black, the first binder is modified polyacrylic acid, the lithium supplementer is lithium iron phosphate (D50 is 17μm, D90 is 25μm), the mass ratio of the first conductive agent to the lithium supplementer is 97:3, the mass content of the first binder in the carbon coating layer is 6%, the positive electrode material is lithium iron phosphate (D50 is 8μm, D90 is 18μm), the second conductive agent is graphite, the second binder is PVDF, the mass ratio of the positive electrode material, the second conductive agent and the second binder is 96:2:2, and the ratio of the particle size D50 of the lithium supplementer to the positive electrode material is 2.13.

[0113] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0114] (1) Provide a first adhesive solution (modified polyacrylic acid solution, solid content 20%, viscosity 1000 mPa.s);

[0115] (2) Add half of the first conductive agent and half of the lithium supplement agent to the first adhesive solution in step (1), disperse it at high speed of 2300 rpm for 50 min in a 200L double star stirring tank, then add the remaining conductive agent and lithium supplement agent, and continue to disperse it at high speed of 2200 rpm for 40 min to obtain the first material.

[0116] (3) Add a wetting agent (modified polyether siloxane) to the first material. The wetting agent accounts for 20% of the total mass of the reaction system. Stir at a low speed of 10 rpm for 45 min to obtain a slurry.

[0117] (4) Transfer the slurry into a sand mill and grind it 6 times;

[0118] (5) The slurry after sand milling is evenly coated onto aluminum foil and dried to obtain a carbon-coated current collector;

[0119] (6) Dissolve the positive electrode material, the second conductive agent and the second binder in NMP to obtain the positive electrode slurry;

[0120] (7) The positive electrode slurry is uniformly coated onto the surface of the carbon-coated current collector and dried to obtain the positive electrode sheet.

[0121] Example 11

[0122] The difference between this embodiment and embodiment 1 is that in the preparation method of the positive electrode, step (2) is: add all the first conductive agent and all the lithium supplement agent to the binder solution in step (1), and disperse it at high speed of 2500 rpm for 80 min in a 200L double star stirring tank to obtain the first material.

[0123] Example 12

[0124] The lithium replenisher has a particle size D50 of 15 μm and a D90 of 27 μm, while the cathode material has a particle size D50 of 9 μm and a D90 of 19 μm. Therefore, the ratio of the particle size D50 of the lithium replenisher to that of the cathode material is 1.66.

[0125] Comparative Example 1

[0126] The difference between this comparative example and Example 1 is that the mass ratio of the first conductive agent to the lithium replenishing agent is 100:0.

[0127] Comparative Example 2

[0128] The difference between this comparative example and Example 1 is that the lithium supplement is replaced with LiCl.

[0129] Comparative Example 3

[0130] The difference between this comparative example and Example 1 is that the lithium supplement is replaced with LiOH.

[0131] Full battery assembly:

[0132] Preparation of negative electrode:

[0133] A negative electrode slurry was prepared by mixing the negative electrode active material (graphite), conductive carbon black Super P, and binder CMC at a mass ratio of 90:7:3 and then uniformly mixing with pure water. The negative electrode slurry was then uniformly coated onto both surfaces of the negative electrode current collector (coated with carbon copper foil) to form a negative electrode active coating. The coating was dried at 100°C under vacuum for 1 hour, followed by cold pressing and slitting to obtain a surface capacity of 4 mAh / 100 cm³. 2 The negative electrode plate.

[0134] Preparation of the diaphragm:

[0135] A PP membrane with a thickness of 15μm and a ceramic layer coated on one side was used as the separator.

[0136] Electrolyte preparation:

[0137] A solvent was prepared by mixing ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1. LiPF6 was then dissolved in the solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0138] Battery assembly

[0139] The positive electrode sheets of Examples 1-12 and Comparative Examples 1-3 were stacked with the separator and negative electrode sheet in sequence, and the electrolyte was injected. After vacuum sealing, standing, formation and shaping processes, the battery was obtained.

[0140] Button-type symmetrical battery assembly:

[0141] Except for replacing the negative electrode with a lithium metal negative electrode, the other methods and conditions are the same as for assembling a full cell.

[0142] Performance testing: First-efficiency test of the full battery: At 25°C, the lithium-ion batteries prepared using the positive electrode sheets of Examples 1-12 and Comparative Examples 1-3 were charged and discharged for the first time at a current of 0.5C (i.e., the current value at which the theoretical capacity is completely discharged within 2 hours). The charging was constant current and constant voltage charging, with a termination voltage of 4.2V, a cutoff current of 0.05C, and a discharge termination voltage of 2.8V. After the battery was left to stand for 24 hours, it was charged to 3.85V at a constant current and constant voltage of 0.5C. The first-efficiency was then calculated and recorded as follows: First-efficiency (η) = (charging capacity / charging time) / (battery set capacity / charging time).

[0143] Cycle capacity retention test of coin cell batteries: The coin cell batteries were tested at room temperature (25℃). They were charged at a 3C rate with constant current and constant voltage to 4.2V, with a cutoff current of 3C. Then, they were discharged at a 3C rate with constant current to 0V. This constitutes one charge-discharge cycle. The coin cell batteries were cycled 100 times in the same manner. The discharge capacity of the coin cell batteries before and after the cycle was tested. The capacity retention rate of the coin cell batteries after 100 cycles was calculated using the following formula: Capacity retention rate (%) after 100 cycles = (Discharge capacity of the 100th cycle / Initial discharge capacity) × 100%.

[0144] The results are shown in Table 1.

[0145] Table 1

[0146] As shown in Table 1, this application can improve the performance of the positive electrode by introducing a specific type of lithium replenishing agent into the carbon coating layer and using it in combination with the first conductive agent. The lithium battery assembled using this positive electrode has higher initial efficiency, cycle life and energy density during operation.

[0147] A comparison of Examples 1-6 shows that when the mass ratio of the first conductive agent to the lithium replenisher is (50%-70%):(30%-50%), the lithium battery exhibits better initial efficiency and cycle performance. When the mass ratio of the first conductive agent to the lithium replenisher is 60:40, the lithium battery possesses the best initial efficiency and the longest cycle stability, demonstrating the best battery electrical performance. Comparative Example 1, without the addition of a lithium replenisher, shows poor electrical performance, with a performance significantly different from that of the lithium-replenished cathodes in Examples 1-12.

[0148] A comparison between Example 1 and Example 11 shows that the slurry needs to be fully and evenly dispersed. If the slurry is not evenly dispersed, the battery performance will not meet the standards.

[0149] A comparison between Example 1 and Example 12 shows that if the particle size difference between the cathode material and the lithium replenishing agent is too large, it will lead to uneven coating due to inconsistent particle size at the microscopic level.

[0150] The applicant declares that this application illustrates the detailed method of this application through the above embodiments, but this application is not limited to the above detailed method, that is, it does not mean that this application must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to this application, equivalent substitutions of the raw materials of this application's product, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of this application.

Claims

1. A positive electrode, comprising a current collector and a positive electrode material layer coated on the surface of the current collector, wherein the positive electrode material layer comprises a positive electrode material, the current collector comprises a current collector substrate and a conductive coating layer disposed on the current collector substrate, the conductive coating layer comprises a first conductive agent and a lithium supplement agent, and at least one of the positive electrode material and the lithium supplement agent has the same crystal structure.

2. The positive electrode according to claim 1, wherein The crystal structure is selected from any one of a layered structure, a spinel structure or an olivine structure.

3. The positive electrode according to claim 1 or 2, wherein The particle size of the lithium supplement agent satisfies: D50 is 13 μm-17 μm, and D90 is 23 μm-27 μm.

4. The positive electrode according to any one of claims 1 to 3, wherein The particle size of the positive electrode material satisfies: D50 is 8 μm-14 μm, and D90 is 18 μm-22 μm.

5. The positive electrode according to any one of claims 1 to 4, wherein The ratio of the particle size D50 of the lithium supplement agent to the positive electrode material is 0.8-1.2, and is optionally 0.9-1.

1. Optionally, the mass fraction of the lithium supplement agent in the total mass of the first conductive agent and the lithium supplement agent is 0%-60% and does not contain 0%, and is optionally 5%-15%. Optionally, the first conductive agent comprises at least one of carbon black, ketjen black and acetylene black.

6. The positive electrode according to any one of claims 1 to 5, wherein The conductive coating layer is a carbon coating layer. Optionally, the conductive coating layer further comprises a first binder. Optionally, the first binder comprises at least one of an acrylate, a polyacrylic acid, a modified polyacrylic acid or a water-based polyurethane, and is optionally a modified polyacrylic acid. Optionally, the mass content of the first binder in the carbon coating layer is 6%-15%. Optionally, the thickness of the conductive coating layer is 0.3 μm-2 μm. 7.A method for preparing the positive electrode according to any one of claims 1-6, comprising the following steps: (1) dispersing raw materials of a conductive coating layer in a solvent to obtain a conductive slurry; (2) coating the conductive slurry on the surface of a current collector substrate, and forming a conductive coating layer on the surface of the current collector substrate after drying to obtain a current collector; (3) coating an active slurry on the conductive coating layer of the current collector, and forming a positive electrode material layer on the surface of the conductive coating layer after drying to obtain a positive electrode; wherein the raw materials of the conductive coating layer comprise a first conductive agent and a lithium supplement agent.

8. The production method according to claim 7, wherein The raw materials of the conductive coating layer in step (1) further comprise a first binder, and the first binder is a binder glue.

9. The production method according to claim 8, wherein The first binder comprises at least one of an acrylate, a polyacrylic acid, a polyacrylic acid or a water-based polyurethane, and is optionally a modified polyacrylate.

10. The production method according to claim 8 or 9, wherein The first binder is a glue solution of a modified polyacrylic acid, and the solid content is 15%-30%, and the viscosity of the modified polyacrylic acid is 200 mPa.s-1500 mPa.s.

11. The method of making according to any one of claims 7-10, wherein, The raw materials of the conductive coating layer in step (1) further comprise a wetting agent. Optionally, the wetting agent comprises at least one of a polyether siloxane, a modified polyether siloxane and an alcohol, and is optionally a modified polyether siloxane. Optionally, the amount of the wetting agent accounts for 5%-20% of the total mass of the reaction system.

12. The method of making according to any one of claims 7-11, wherein, Step (1) comprises: S1 adding a first conductive agent and a lithium supplement agent to a binder glue solution, and dispersing to obtain a first material; S2 adding a wetting agent to the first material, stirring and sanding to obtain a conductive slurry.

13. The method of making according to claim 12, wherein, The solid content of the adhesive solution in step S1 is 15%-25%; Optionally, step S1 includes: first adding a portion of the first conductive agent and a portion of the lithium supplement to the adhesive solution for a first high-speed dispersion, and then adding the remaining first conductive agent and lithium supplement to the obtained material for a second high-speed stirring. Optionally, the first conductive agent accounts for 40%-60% of the total conductive agent; Optionally, the portion of the lithium replenishing agent accounts for 40%-60% of the total lithium replenishing agent; Optionally, the rotation speed of the first high-speed stirring is 2000 r / min-2600 r / min; Optionally, the first high-speed stirring time is 30-60 minutes; Optionally, the rotation speed of the second high-speed stirring is 2000 r / min-2600 r / min; Optionally, the second high-speed stirring time is 30-60 minutes.

14. The production method according to claim 12 or 13, wherein The stirring speed described in step S2 is 10 rpm-15 rpm; Optionally, the stirring time in step S2 is 30 min to 45 min; Optionally, the number of times the sanding is performed in step S2 is 4 to 8 times; Optionally, the parameters for sand milling in step S2 are: a sand milling speed of 800 r / min-1200 r / min and a sand milling time of 5 min-15 min.

15. A lithium battery, wherein, The lithium battery includes the positive electrode as described in any one of claims 1-6.

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