A lithium-rich positive electrode sheet, a preparation method thereof, and a secondary battery

Abstract

The application provides a lithium-rich positive electrode sheet, a preparation method thereof and a secondary battery. The lithium-rich positive electrode sheet comprises a current collector and a lithium-rich material layer arranged on the surface of the current collector, and a protective layer is arranged on the surface of the lithium-rich material layer, wherein the protective layer is formed by cross-linking reaction of a film-forming agent. The protective film on the surface of the lithium-rich positive electrode sheet can effectively isolate water and carbon dioxide in the air, inhibit deterioration of the lithium-rich material, improve the stability of the electrode sheet, and be beneficial to production, storage and transportation of the positive electrode sheet.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a lithium-rich positive electrode sheet, its preparation method, and a secondary battery. Background Technology

[0002] Lithium-ion batteries have a wide range of applications due to their outstanding advantages such as high operating voltage, light weight, low self-discharge, and no environmental pollution. The electrochemical performance of lithium-ion batteries largely depends on the electrode materials. Lithium-rich materials have the advantages of low cost and high capacity, which is beneficial to improving the energy density of batteries. However, lithium-rich materials are prone to reacting with substances such as water and carbon dioxide in the air, leading to the failure of active components and a reduction in battery capacity and cycle life. To prevent the deterioration of lithium-rich materials, existing positive electrode sheets have extremely stringent environmental requirements during use and storage, which increases the storage cost of lithium-rich material electrodes and limits their application. Therefore, it is necessary to provide a new lithium-rich positive electrode sheet and its preparation method to solve the problems of poor stability and high storage cost of existing positive electrode sheets. Summary of the Invention

[0003] In view of this, this application provides a lithium-rich positive electrode sheet with a protective film of a specific composition on its surface. This protective film can effectively isolate water and carbon dioxide in the air, inhibit the deterioration of lithium-rich materials, improve the stability of the electrode sheet, and facilitate the production, storage, and transportation of the positive electrode sheet. This application also provides a method for preparing the lithium-rich positive electrode sheet.

[0004] The first aspect of this application provides a lithium-rich positive electrode sheet, the lithium-rich positive electrode sheet comprising a current collector and a lithium-rich material layer disposed on the surface of the current collector, the surface of the lithium-rich material layer being provided with a protective layer, the protective layer being formed by cross-linking through a reaction of the film-forming agent.

[0005] Optionally, the film-forming agent includes one or more of a first film-forming agent and a second film-forming agent; the first film-forming agent reacts with water to crosslink and form the protective layer; the second film-forming agent reacts with oxygen to crosslink and form the protective layer.

[0006] Optionally, the first film-forming agent includes one or more of isocyanates, siloxanes, and silyl esters, and the second film-forming agent includes unsaturated glycerides.

[0007] Optionally, the first film-forming agent comprises isocyanate, and the protective layer comprises polyurea.

[0008] Optionally, the first film-forming agent comprises a siloxane, and the protective layer comprises a polysiloxane.

[0009] Optionally, the first film-forming agent comprises silyl ester, and the protective layer comprises a hydrolysis product of silyl ester.

[0010] Optionally, the second film-forming agent comprises an unsaturated glycerol ester, and the protective layer comprises a saturated glycerol ester.

[0011] Optionally, the isocyanate includes one or more of hexamethylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, and triallyl isocyanurate.

[0012] Optionally, the siloxane includes one or more of polydimethylsiloxane, trimethylmethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane.

[0013] Optionally, the silyl ester includes one or more of the following: silyl carboxylate, silyl sulfate, silyl sulfonate, silyl phosphite, silyl phosphate compounds, and silyl borate.

[0014] Optionally, the unsaturated glycerides include one or more of glyceryl linoleate, glyceryl oleate, ethyl 2,4-decadienoate, glyceryl tung oil, and methyl tung oil.

[0015] Optionally, the thickness of the protective layer is 1 μm to 30 μm.

[0016] Optionally, the porosity of the protective layer is 1% to 30%.

[0017] Optionally, the protective layer is a discontinuous film layer.

[0018] Optionally, the lithium-rich material layer includes a lithium-rich material, which includes one or more of binary lithium-containing compounds and multi-component lithium-containing compounds.

[0019] Optionally, the binary lithium-containing compound includes compounds with the chemical formula Li. a X b The compound; wherein 1≤a≤3, 1≤b≤3, and X is selected from any one of F, S, N, B, P, O, and Se; the multi-element lithium-containing compound includes the compound with the chemical formula Li x M y O z The compound wherein 0.1 < x / y < 10, 0 < y < 5, 2 ≤ z < 10; wherein M includes one or more of Mn, Fe, Cr, Co, Ni, Cu, Zn, Mg, Ti, Si, Sn, Ce, Re, Ru, Mo or Zr.

[0020] Optionally, the lithium-rich material layer further includes a binder and a conductive agent.

[0021] Optionally, the adhesive includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0022] Optionally, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes.

[0023] A second aspect of this application provides a method for preparing a positive electrode sheet, comprising:

[0024] A lithium-rich material is mixed with a film-forming agent and coated onto the surface of a current collector to form an active material layer. The film-forming agent reacts with water and / or oxygen to crosslink and form a protective layer, thus obtaining a lithium-rich positive electrode sheet.

[0025] Alternatively, a lithium-rich material can be coated onto the surface of the current collector to form an active material layer, and a film-forming agent can be coated onto the surface of the active material layer. The film-forming agent reacts with water and / or oxygen to crosslink and form a protective layer, thereby obtaining a lithium-rich positive electrode sheet.

[0026] The lithium-rich positive electrode includes a current collector and a lithium-rich material layer disposed on the surface of the current collector. The surface of the lithium-rich material layer is provided with a protective layer, which is formed by the cross-linking of a film-forming agent.

[0027] Optionally, the film-forming agent includes one or more of a first film-forming agent and a second film-forming agent; the first film-forming agent includes one or more of isocyanate, siloxane and silyl ester, and the second film-forming agent includes unsaturated glycerol ester.

[0028] Optionally, the mass ratio of the first film-forming agent to the second film-forming agent is 1:(0.01 to 0.1).

[0029] Optionally, the active material layer comprises a lithium-rich material, and the mass ratio of the film-forming agent to the lithium-rich material is 0.01% to 15%.

[0030] A third aspect of this application provides a secondary battery, including a positive electrode as described in the first aspect of this application.

[0031] In the lithium-rich positive electrode provided in this application, the film-forming agent can react with water and / or oxygen in the air to cross-link and form a protective layer. This protective layer can prevent the intrusion of corrosive media, protect the electrode from the influence of the external environment, and thus extend the storage life of the electrode. During charging and discharging, the protective layer can also inhibit the side reactions between the electrolyte and the active material in the electrode, thereby improving the cycle performance of the lithium secondary battery. Attached Figure Description

[0032] Figure 1This is a schematic diagram of the structure of the positive electrode sheet provided in one embodiment of this application. Detailed Implementation

[0033] 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.

[0034] This application provides a positive electrode sheet that exhibits good stability in air, which is beneficial for its application in batteries. Please refer to... Figure 1 , Figure 1 This is a schematic diagram of the structure of a positive electrode sheet provided in one embodiment of the present application. The positive electrode sheet 100 includes a current collector 30, a lithium-rich material layer 20 and a protective layer 10 stacked sequentially, with the lithium-rich material layer 20 located between the current collector 30 and the protective layer 10.

[0035] In this application, the lithium-rich material layer includes a lithium-rich material, which can be used as a positive electrode active material or as a lithium replenishment material. In some embodiments of this application, the lithium-rich material includes one or more of binary lithium-containing compounds and multi-component lithium-containing compounds, wherein the binary lithium-containing compound includes the chemical formula Li... a X b Where 1≤a≤3, 1≤b≤3, X is selected from any one of F, S, N, B, P, O, and Se, and the multi-element lithium-containing compounds include those with the chemical formula Li. x M y O z The compound, wherein 0.1 < x / y < 10, 0 < y < 5, 2 ≤ z < 10; wherein M includes one or more of Mn, Fe, Cr, Co, Ni, Cu, Zn, Mg, Ti, Si, Sn, Ce, Re, Ru, Mo, or Zr. The above-mentioned lithium-rich materials have high capacity and can greatly improve the energy density of batteries. However, lithium-rich materials also suffer from high activity, readily reacting with substances such as water or carbon dioxide in the air, causing the lithium-rich materials to deteriorate and fail. Furthermore, the weight of the electrode increases after absorbing water, leading to a significant reduction in the battery's energy density and cycle performance. In this application, the surface of the lithium-rich material layer has a protective layer that prevents the intrusion of corrosive media in the air, protecting the electrode from external environmental influences, thereby improving the stability of the electrode and extending its service life.

[0036] In this embodiment, the protective layer on the surface of the lithium-rich material layer is formed by the cross-linking reaction of a film-forming agent. The film-forming agent can react with water or oxygen to form the protective layer, wherein the water or oxygen can originate from the air. This protective layer can effectively prevent the penetration of substances such as moisture, oxygen, and carbon dioxide from the air, isolating the lithium-rich material layer from the external environment, improving the stability of the positive electrode in air, ensuring that the lithium-rich positive electrode has good electrochemical performance, and reducing the production and storage costs of the lithium-rich positive electrode. In addition, during the charging and discharging process of the battery, this protective layer can inhibit the reaction between the electrolyte and the active material in the electrode, thereby improving the cycle performance of the battery.

[0037] In the lithium-rich positive electrode of this application, the lithium-rich material layer may or may not contain a film-forming agent. During the preparation of the lithium-rich positive electrode, the film-forming agent may partially cross-link to form a protective layer covering the surface of the lithium-rich material layer, or it may completely react and cross-link to form a protective layer covering the surface of the lithium-rich material layer. When the film-forming agent partially cross-links to form a protective layer, the film-forming agent in the lithium-rich material layer may be distributed inside, on the surface, or at the bottom of the lithium-rich material layer.

[0038] In this application embodiment, the film-forming agent includes one or more of a first film-forming agent and a second film-forming agent, wherein the first film-forming agent includes one or more of isocyanate, siloxane and silyl ester, and the second film-forming agent includes unsaturated glycerol ester.

[0039] In this application, the first film-forming agent can react with water to generate a first protective film. The first protective film includes one or more of the hydrolysis products of polyurea, polysiloxane, and silyl ester. Specifically, isocyanate hydrolyzes to generate polyurea, siloxane hydrolyzes to generate polysiloxane, and silyl ester hydrolyzes to generate the hydrolysis product of silyl ester. After the first film-forming agent comes into contact with water in the air, it hydrolyzes to generate a large molecular network structure, thereby forming a film. In some embodiments of this application, the isocyanate includes one or more of hexamethylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, and triallyl isocyanurate; the siloxane includes one or more of polydimethylsiloxane, trimethylmethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane; and the silyl ester includes one or more of carboxylic acid silyl ester, silyl sulfate silyl ester, sulfonate silyl ester, phosphite silyl ester, phosphate silyl ester compound, and borate silyl ester.

[0040] In this application, the second film-forming agent can react with oxygen to crosslink and form a second protective film. The second protective film includes saturated glycerides. Unsaturated glycerides react with oxygen to form saturated glycerides, which then form a compound film. In some embodiments of this application, the unsaturated glycerides include one or more of linoleic acid glycerides, oleic acid glycerides, ethyl 2,4-decadienoate, tung oil glycerides, and methyl tung oil esters. These unsaturated glycerides have long-chain fatty acids, which makes the second protective film formed by the reaction highly hydrophobic. In some embodiments of this application, the second film-forming agent includes one or more of linseed oil, tung oil, and tung oil, which are rich in unsaturated glycerides.

[0041] In some embodiments of this application, the film-forming agent includes both a first film-forming agent and a second film-forming agent, and the protective layer includes both a first protective film and a second protective film. The first and second protective films can be cross-linked to form a protective layer or can form a layered structure. The first protective film is a highly cross-linked film layer, which can effectively prevent the penetration of substances such as moisture, oxygen, and carbon dioxide from the air. The second protective film not only has strong hydrophobicity to inhibit moisture penetration, but also has good compatibility with the first protective film. The second protective film can further fill the voids formed by the first protective film, improving the density of the protective layer. The protective layer formed by the first and second protective films can effectively isolate the lithium-rich material from the external environment, improve the stability of the positive electrode in air, ensure good electrochemical performance of the positive electrode, and reduce the production and storage costs of the positive electrode. In addition, during the charging and discharging process of the battery, this protective layer can inhibit the reaction between the electrolyte and the active material in the electrode, thereby improving the cycle performance of the battery.

[0042] In some embodiments of this application, the thickness of the protective layer is 1 μm to 30 μm. The specific thickness of the protective layer can be, but is not limited to, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm. In some embodiments, the thickness of the protective layer is 1 μm to 20 μm. A protective layer of a certain thickness can fully cover the lithium-rich material layer, achieving effective protection for the lithium-rich material, and the protective layer has little impact on the electrochemical performance of the electrode, which is beneficial to the utilization of the capacity of the lithium-rich material.

[0043] In some embodiments of this application, the porosity of the protective layer is 1% to 30%. The porosity of the protective layer can specifically be, but is not limited to, 1%, 3%, 5%, 10%, 15%, 20%, or 30%. In some embodiments, the porosity of the protective layer is 5% to 13%. Because the protective layer itself is hydrophobic, a porosity of 1% to 30% is sufficient to effectively isolate air, and lithium ions can freely escape from the electrode during battery cycling, ensuring the electrode's capacity is utilized.

[0044] In some embodiments of this application, the protective layer is a discontinuous film layer, meaning it has a discontinuous structure and is dispersedly covering the surface of the lithium-rich material layer. A protective layer with a discontinuous structure is beneficial for improving the conductivity and lithium-ion mobility of the lithium-rich cathode. In some embodiments of this application, the protective layer is a continuous film layer, meaning it has a continuous structure and completely covers the surface of the lithium-rich material layer. A protective layer with a continuous structure is beneficial for preventing the intrusion of corrosive media and improving the stability of the lithium-rich cathode.

[0045] In some embodiments of this application, the lithium-rich material layer includes an active material, a lithium-supplementing material, a binder, and a conductive agent. In some embodiments of this application, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. In some embodiments of this application, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes. In some embodiments of this application, the active material and / or the lithium-supplementing material is a lithium-rich material. In some embodiments of this application, the active material includes one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium fluorinated vanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. In some embodiments of this application, the active material accounts for 90%–99% of the mass percentage of the lithium-rich material layer, and the lithium-supplementing material accounts for 1%–10% of the mass percentage of the lithium-rich material layer. In some embodiments of this application, the current collector of the positive electrode includes any one of copper foil and aluminum foil.

[0046] In the positive electrode provided in this application, the protective layer on the surface of the lithium-rich material layer can improve the surface and interface stability of the lithium-rich material and extend the storage life of the lithium-rich material. Moreover, the protective layer is thin and dense, and has a low weight ratio in the electrode, which makes the positive electrode have a high specific capacity. The interface of the positive electrode has hydrophobic and stable characteristics, and its production and storage do not require a harsh operating environment, which is conducive to large-scale production.

[0047] In some embodiments of this application, the preparation process of the lithium-rich positive electrode sheet is as follows: lithium-rich material is mixed with a film-forming agent to form an electrode slurry; the electrode slurry is coated onto the surface of a current collector; during the drying process, the film-forming agent floats to the surface of the active material layer and undergoes a hydrolysis / oxidation reaction to form a protective layer; the active material layer is transformed into a lithium-rich material layer; the protective layer covers the surface of the lithium-rich material layer; and the electrode sheet is then rolled to obtain the lithium-rich positive electrode sheet. In some embodiments of this application, the drying temperature of the electrode slurry is 80℃~120℃, and the drying time is 6h~12h.

[0048] In some embodiments of this application, the preparation process of the lithium-rich positive electrode sheet is as follows: a lithium-rich material is coated on the surface of the current collector, dried to obtain a lithium-rich material layer, a film-forming agent is coated on the surface of the lithium-rich material layer, dried, and then rolled to obtain the lithium-rich positive electrode sheet.

[0049] In some embodiments of this application, the preparation process of the lithium-rich positive electrode sheet is as follows: the lithium-rich material is mixed with a film-forming agent to form an electrode slurry, the electrode slurry is coated on the surface of the current collector, and after drying, a lithium-rich material layer is obtained. A film-forming agent is then coated on the surface of the lithium-rich material layer, and after drying, the material is rolled to obtain the lithium-rich positive electrode sheet.

[0050] In the preparation method of this application, the added film-forming agent can react with water and / or oxygen in whole or in part to form a protective layer. When the film-forming agent is partially cross-linked to form a film, the lithium-rich positive electrode will also contain the film-forming agent. The film-forming agent can be distributed inside, on the surface or at the bottom of the lithium-rich material layer. When the film-forming agent is completely cross-linked to form a film, the lithium-rich positive electrode will not contain the film-forming agent.

[0051] In this embodiment, the film-forming agent includes one or more of a first film-forming agent and a second film-forming agent. The first film-forming agent includes one or more of isocyanate, polysiloxane, and silyl ester; the second film-forming agent includes unsaturated glycerol ester. During the preparation of the electrode, the first film-forming agent hydrolyzes to form a first protective film, and the second film-forming agent oxidizes to form a second protective film. The first and second protective films form a protective layer on the surface of the lithium-rich material layer. In some embodiments, the first film-forming agent is isocyanate. Because the surface of the lithium-rich material has a certain alkalinity, isocyanate can react rapidly with water under high pH conditions to form polyurea, thereby achieving effective protection of the lithium-rich material layer.

[0052] In some embodiments of this application, the mass ratio of the first film-forming agent to the second film-forming agent is 1:(0.01 to 0.1). Specifically, but not limited to, the mass ratio of the first film-forming agent to the second film-forming agent may be 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.08, or 1:0.1. The first protective layer formed by the first film-forming agent has a certain structural strength and can serve as a supporting matrix for the protective layer, enabling the protective layer to stably cover the surface of the lithium-rich material and form effective protection. The addition of the second film-forming agent can further improve the stability and density of the protective layer and inhibit the penetration of moisture and gas from the air. Controlling the mass ratio of the first film-forming agent to the second film-forming agent is beneficial for forming a structurally stable protective layer and achieving effective protection of the lithium-rich material layer.

[0053] In some embodiments of this application, the active material layer includes a positive electrode active material and a lithium-rich material, wherein the positive electrode active material and the lithium-rich material are lithium-rich materials, and the mass ratio of the film-forming agent to the lithium-rich material is 0.01% to 15%. In some embodiments, the mass ratio of the film-forming agent to the lithium-rich material is 1% to 10%. The specific mass ratio of the film-forming agent to the lithium-rich material may be, but is not limited to, 0.01%, 0.05%, 0.1%, 0.5%, 0.8%, 1%, 2%, 5%, 10%, 12%, or 15%. In some embodiments of this application, the active material layer includes a positive electrode active material, wherein the positive electrode active material is lithium-rich material, and the mass ratio of the film-forming agent to the lithium-rich material is 0.01% to 15%. In some embodiments, the mass ratio of the film-forming agent to the lithium-rich material is 0.01% to 10%. The mass ratio of the film-forming agent to the lithium-rich material can be, but is not limited to, 0.01%, 0.05%, 0.1%, 0.5%, 0.8%, 1%, 2%, 5%, 10%, 12%, or 15%. Controlling the relative content of the film-forming agent to the lithium-rich material helps to adjust the thickness of the protective layer, prevents the lithium-rich material layer from contacting air, and ensures that the positive electrode has a high effective capacity. Furthermore, the lithium ions in the core of the lithium-rich material have a moderate extraction rate, thereby enabling effective lithium replenishment. In addition, the low mass ratio of the protective layer also ensures that the battery has a high energy density.

[0054] This application describes a method to improve the stability of the positive electrode by adding a film-forming agent during the electrode preparation process. This agent exhibits good compatibility with lithium-rich materials, does not consume the active lithium in the electrode, and does not affect the battery's initial efficiency. The protective layer formed by the agent is thin and dense, hydrophobic, and resistant to electrolyte corrosion, demonstrating excellent corrosion resistance and stability, thus improving the battery's cycle performance. This preparation method is simple, convenient, and environmentally friendly, yielding a stable positive electrode and reducing storage and transportation costs, making it suitable for large-scale production.

[0055] This application also provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The separator is stacked between the positive and negative electrodes, and the positive electrode includes the positive electrode sheet provided in this application. Because the secondary battery provided in this application uses the positive electrode sheet of this application, it has better cycle performance and safety performance, which is beneficial for the application of secondary batteries in various fields.

[0056] The implementation of this application will be further described below through several embodiments.

[0057] Example 1

[0058] 1) Preparation of lithium-rich positive electrode sheets

[0059] LiFePO4 positive electrode active material, Li6MnO4 lithium supplement material, PVDF binder, and SP conductive agent were mixed with an appropriate amount of NMP solvent in a mass ratio of 93:2:3:2. Then, hexamethylene diisocyanate, a film-forming agent, was added. The mass ratio of hexamethylene diisocyanate to the sum of the masses of positive electrode active material LiFePO4 and lithium supplement material Li6MnO4 was 0.01%. After being mixed evenly, the mixture was coated on the surface of the current collector, dried at 90°C, and then rolled to obtain a lithium-rich positive electrode sheet.

[0060] 2) Preparation of lithium secondary batteries

[0061] Ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7, and LiPF6 was added to form an electrolyte with a concentration of 1 mol / L. The positive electrode, polypropylene microporous separator, lithium sheet, and electrolyte were then assembled to obtain a lithium secondary battery.

[0062] Example 2

[0063] The difference between Example 2 and Example 1 is that the ratio of the film-forming agent hexamethylene diisocyanate to the sum of the masses of the positive electrode active material LiFePO4 and the lithium supplementation material Li6MnO4 in the electrode slurry is 4%.

[0064] A lithium secondary battery was prepared using the same method as in Example 1.

[0065] Example 3

[0066] The difference between Example 3 and Example 1 is that the ratio of the film-forming agent hexamethylene diisocyanate to the sum of the masses of the positive electrode active material LiFePO4 and the lithium supplementation material Li6MnO4 in the electrode slurry is 6%.

[0067] A lithium secondary battery was prepared using the same method as in Example 1.

[0068] Example 4

[0069] The difference between Example 4 and Example 1 is that the ratio of the film-forming agent hexamethylene diisocyanate to the sum of the masses of the positive electrode active material LiFePO4 and the lithium supplementation material Li6MnO4 in the electrode slurry is 10%.

[0070] A lithium secondary battery was prepared using the same method as in Example 1.

[0071] Example 5

[0072] The difference between Example 5 and Example 1 is that the ratio of the film-forming agent hexamethylene diisocyanate to the sum of the masses of the positive electrode active material LiFePO4 and the lithium supplement material Li6MnO4 in the electrode slurry is 0.001%.

[0073] A lithium secondary battery was prepared using the same method as in Example 1.

[0074] Example 6

[0075] The difference between Example 6 and Example 1 is that the ratio of the film-forming agent hexamethylene diisocyanate to the sum of the masses of the positive electrode active material LiFePO4 and the lithium supplementation material Li6MnO4 in the electrode slurry is 17%.

[0076] A lithium secondary battery was prepared using the same method as in Example 1.

[0077] Example 7

[0078] The difference between Example 7 and Example 2 is that the film-forming agent contains two film-forming agents: the first film-forming agent is diisocyanate, and the second film-forming agent is glyceryl linoleate, with a mass ratio of diisocyanate to glyceryl linoleate of 1:0.03.

[0079] A lithium secondary battery was prepared using the same method as in Example 2.

[0080] Example 8

[0081] The difference between Example 8 and Example 2 is that the film-forming agent contains two film-forming agents, with the mass ratio of the first film-forming agent, diisocyanate, and the second film-forming agent, glyceryl linoleate, being 1:0.12.

[0082] A lithium secondary battery was prepared using the same method as in Example 2.

[0083] Example 9

[0084] The difference between Example 9 and Example 7 is that the first film-forming agent is polydimethylsiloxane and the second film-forming agent is glyceryl oleate.

[0085] A lithium secondary battery was prepared using the same method as in Example 7.

[0086] Example 10

[0087] The difference between Example 10 and Example 2 is that the film-forming agent contains two film-forming agents, and the mass ratio of the first film-forming agent, silyl carboxylate, to the second film-forming agent, ethyl 2,4-decadienoate, is 1:0.05.

[0088] A lithium secondary battery was prepared using the same method as in Example 2.

[0089] Example 11

[0090] LiFePO4 positive electrode active material, Li6MnO4 lithium supplementary material, PVDF binder, and SP conductive agent were mixed uniformly in a mass ratio of 93:2:3:2 to form a slurry, which was then coated onto a current collector. After drying, a lithium-rich material layer was formed. A film-forming agent was then coated on the surface of the lithium-rich material layer. The mass ratio of the first film-forming agent (silyl carboxylate) and the second film-forming agent (ethyl 2,4-decadienoate) in the film-forming agent was 1:0.05, and the film-forming agent accounted for 4 wt% of the lithium-rich material. After drying, the material was rolled to obtain a lithium-rich positive electrode sheet.

[0091] A lithium secondary battery was prepared using the same method as in Example 1.

[0092] To demonstrate the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0093] Comparative Example 1

[0094] The difference between Comparative Example 1 and Example 1 is that the electrode slurry does not contain a film-forming agent, and the mass ratio of LiFePO4 positive electrode active material, Li6MnO4 lithium supplementary material, PVDF binder, and SP conductive agent is 93:2:3:2. The electrode slurry is coated on the current collector, and after drying and rolling, a lithium-rich positive electrode sheet is obtained.

[0095] The battery was prepared using the same method as in Example 1.

[0096] Effect Example

[0097] 1) The morphology of the lithium-rich positive electrode sheet of Example 1 was characterized by scanning electron microscopy, and the structural parameters of the electrode sheets in Examples 1-11 were obtained. The specific parameters are shown in Table 1.

[0098] Table 1. Structural parameters of the electrode plates in Examples 1-11

[0099] experimental group Protective layer thickness (μm) Example 1 1.20 Example 2 2.10 Example 3 2.40 Example 4 2.69 Example 5 0.78 Example 6 5.28 Example 7 2.45 Example 8 2.44 Example 9 2.38 Example 10 2.43 Example 11 2.45

[0100] 2) The porosity of the electrode protective layer in Examples 1-11 was characterized by mercury porosimetry. The characterization results are shown in Table 2.

[0101] Table 2. Structural parameters of the electrode protective layer in Examples 1-11

[0102]

[0103]

[0104] 3) The stability of the lithium-rich positive electrode sheets of Examples 1-11 and Comparative Example 1 was tested. The test conditions were as follows: the lithium-rich positive electrode sheets of each example and comparative example were placed in an environment with a humidity of 35% and left for 24 hours. The decay rate of the first charge specific capacity of the electrode sheets was then measured. The test results of the lithium-rich positive electrode sheets of Examples 1-11 and Comparative Example 1 are shown in Table 3.

[0105] Table 3 shows the stability test results of the positive electrode sheets in Examples 1-11 and Comparative Example 1.

[0106] experimental group Experimental results (attenuation rate %) Example 1 2.94 Example 2 1.56 Example 3 1.46 Example 4 1.30 Example 5 4.85 Example 6 7.34 Example 7 0.46 Example 8 1.01 Example 9 0.52 Example 10 0.25 Example 11 0.34 Comparative Example 1 28.89

[0107] As can be seen from Table 3, the capacity decay rate of the lithium-rich cathode sheet of this invention is much lower than that of the lithium-rich cathode sheet of Comparative Example 1 under a humidity of 35%, indicating that adding a film-forming agent to the lithium-rich cathode sheet can greatly improve the stability of the lithium-rich cathode sheet.

[0108] 4) The electrochemical performance of the lithium secondary batteries of Examples 1-11 and Comparative Example 1 was tested under the following conditions:

[0109] The coin cell battery was charged at a constant current and constant voltage rate of 0.05C to 4.3V, with a cutoff current of 0.01C. After resting for 5 minutes, it was discharged at a rate of 0.05C to 2.5V, and then charged at a constant current and constant voltage rate of 1C to 3.65V, with a cutoff current of 0.01C. After resting for 5 minutes, it was discharged at a rate of 0.05C to 2.5V for 100 cycles. The test results are shown in Table 4.

[0110] Table 4 Performance parameters of the batteries in Examples 1-11 and Comparative Example 1

[0111]

[0112] As can be seen from Table 4, the lithium-ion batteries of Examples 1-11 of this application have superior electrochemical performance compared to the lithium-ion battery of Comparative Example 1. Specifically, in Example 5, the content of the film-forming agent is relatively low, resulting in a thinner protective film and slightly poorer battery performance. In Example 6, the content of the film-forming agent is relatively high, resulting in a thicker protective film and slightly poorer battery performance. In Example 7, two film-forming agents are used, and the battery performance is better. In Example 8, the mass ratio of the two film-forming agents is not within the preferred range, and the battery performance is slightly worse than that of Example 7.

[0113] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A lithium-rich positive electrode sheet, characterized by, The lithium-rich positive electrode plate comprises a current collector and a lithium-rich material layer arranged on the surface of the current collector, and a protective layer is arranged on the surface of the lithium-rich material layer, the protective layer is formed by cross-linking reaction of a film-forming agent; the film-forming agent comprises a first film-forming agent and a second film-forming agent; the protective layer comprises a first protective film and a second protective film, the first film-forming agent is cross-linked with water to form the first protective film; the second film-forming agent is cross-linked with oxygen to form the second protective film; the porosity of the protective layer is 1% to 30%; the first film-forming agent comprises one or more of isocyanate, siloxane and silyl ester, and the second film-forming agent comprises unsaturated glyceride; the mass ratio of the first film-forming agent to the second film-forming agent is 1:(0.01 to 0.1).

2. The lithium-rich cathode sheet of claim 1, wherein The first film-forming agent comprises isocyanate, and the protective layer comprises polyurea. And / or the first film-forming agent comprises siloxane, and the protective layer comprises polysiloxane. And / or the first film-forming agent comprises silyl ester, and the protective layer comprises a hydrolysis product of silyl ester. And / or the second film-forming agent comprises unsaturated glyceride, and the protective layer comprises saturated glyceride.

3. The lithium-rich cathode sheet of claim 1, wherein The thickness of the protective layer is 1 μm to 30 μm.

4. The lithium-rich cathode sheet of claim 1, wherein The protective layer is a discontinuous film layer.

5. The lithium-rich cathode sheet of claim 1, wherein, The lithium-rich material layer comprises a lithium-rich material, and the lithium-rich material comprises one or more of binary lithium-containing compounds and multi-component lithium-containing compounds.

6. A method of making a lithium-rich cathode electrode sheet, characterized by, Comprise: Mixing the lithium-rich material with a film-forming agent to form an active material layer on the surface of the current collector, the film-forming agent is cross-linked with water and oxygen to form a protective layer, thereby obtaining a lithium-rich positive electrode plate; Or coating the lithium-rich material on the surface of the current collector to form an active material layer, and coating a film-forming agent on the surface of the active material layer, the film-forming agent is cross-linked with water and oxygen to form a protective layer, thereby obtaining a lithium-rich positive electrode plate; The film-forming agent comprises a first film-forming agent and a second film-forming agent; the first film-forming agent comprises one or more of isocyanate, siloxane and silyl ester, and the second film-forming agent comprises unsaturated glyceride; the protective layer comprises a first protective film and a second protective film, the first film-forming agent is cross-linked with water to form the first protective film; the second film-forming agent is cross-linked with oxygen to form the second protective film; the mass ratio of the first film-forming agent to the second film-forming agent is 1:(0.01 to 0.1). The lithium-rich positive electrode plate comprises a current collector and a lithium-rich material layer arranged on the surface of the current collector, and a protective layer is arranged on the surface of the lithium-rich material layer, the protective layer is formed by cross-linking reaction of a film-forming agent.

7. The production method according to claim 6, wherein The mass ratio of the first film-forming agent to the second film-forming agent is 1:(0.01 to 0.1).

8. The production method according to any one of claims 6 to 7, wherein The mass ratio of the film-forming agent to the lithium-rich material is 0.01% to 15%.

9. A secondary battery characterized by comprising: Comprise the lithium-rich positive electrode plate according to any one of claims 1 to 5.

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