Positive plate, preparation method thereof and lithium ion battery

Abstract

The invention provides a positive plate, a preparation method thereof and a lithium ion battery. The positive plate comprises a current collector and a positive active layer arranged on the surface of the current collector, wherein the positive active layer comprises a positive active material, a conductive agent, a binder and a modified solid electrolyte; wherein the modified solid electrolyte comprises a solid electrolyte core and a coating layer coating the outer surface of the solid electrolyte core, the coating layer comprises a pore-forming agent, and the pore-forming agent comprises ammonium salt; the positive electrode active layer has a three-dimensional through pore channel. The modified solid electrolyte is added into the positive active layer of the positive plate, so that the Zeta potential of positive slurry can be improved, and the agglomeration of electrolyte particles is inhibited. The three-dimensional through porous network improves the wettability of the electrolyte, and the inner wall of the three-dimensional through porous network enables the active surface of the modified solid electrolyte to be exposed and forms an efficient ion transmission interface with the electrolyte, so that the ionic conductivity is improved. In conclusion, the positive plate comprising the positive active layer has good electrochemical performance and safety.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a positive electrode sheet and its preparation method, and a lithium-ion battery. Background Technology

[0002] In lithium-ion batteries, solid electrolytes have attracted much attention due to their high safety and potential high energy density. Furthermore, the introduction of nanoscale solid electrolytes can improve the ionic conductivity of the positive electrode. However, their application still faces the following challenges: 1) Poor dispersibility: Nanoparticle solid electrolytes have high surface energy and are prone to aggregation, leading to localized enrichment during coating and significantly increasing the electrode resistance; 2) Wetting barriers: The electrolyte penetration time for traditional dense electrode structures is as long as 120-300 seconds, resulting in low battery activation efficiency; 3) High interfacial impedance: Insufficient solid-liquid interface contact between the solid electrolyte and the liquid electrolyte leads to charge transfer impedance exceeding 250 Ω·cm².

[0003] Chinese patent CN114300649B applies a safety coating to the surface of an electrode and then calcines it at high temperature. It utilizes the decomposition of a pore-forming agent to construct a porous structure, but it has the following problems: 1) The process is complex and requires secondary coating, making it difficult to control the coating thickness; 2) The electrode containing the safety coating needs to undergo a secondary high-temperature treatment, which is detrimental to the positive electrode itself and increases the difficulty of the process.

[0004] Therefore, there is an urgent need to develop a method for preparing a positive electrode that incorporates a solid electrolyte. Summary of the Invention

[0005] The main objective of this invention is to provide a positive electrode sheet and its preparation method, as well as a lithium-ion battery, to solve the problems of easy agglomeration, low electrolyte wetting efficiency, and solid-liquid interface charge transport barrier in the prior art for nanoscale solid electrolytes in positive electrode slurry.

[0006] To achieve the above objectives, according to one aspect of the present invention, a positive electrode sheet is provided, comprising a current collector and a positive electrode active layer disposed on the surface of the current collector. The positive electrode active layer is prepared from a positive electrode slurry and comprises: a positive electrode active material, a conductive agent, a binder, and a modified solid electrolyte; wherein the modified solid electrolyte comprises a solid electrolyte core and a coating layer covering the outer surface of the solid electrolyte core, the coating layer comprising a pore-forming agent, the pore-forming agent comprising an ammonium salt; the positive electrode active layer has three-dimensional through-holes.

[0007] Furthermore, the mass ratio of the positive electrode active material, conductive agent, binder and modified solid electrolyte is 80~95:1~10:2~8:0.5~15.

[0008] Furthermore, the coating layer has a thickness of 2~50 nm, a coverage rate of ≥85%, and a mass percentage of 2~23% in the modified electrolyte; and / or, the pore-forming agent is selected from any one or more of ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium nitrate.

[0009] Furthermore, the solid electrolyte core is selected from any one or more of LATP solid electrolyte, LAGP solid electrolyte, LLZO solid electrolyte and LLTO solid electrolyte; the D50 particle size of the solid electrolyte core is 20~1000nm; and / or, the zeta potential of the positive electrode slurry is 30~60mV; and / or, the equivalent pore size of the three-dimensional through-hole is 10~500nm, and the porosity of the positive electrode active layer is 40~65%.

[0010] Furthermore, the positive electrode active material is selected from LiCoO2 and LiNi. 1-x-y Co x M y O2, LiFePO4, lithium-rich manganese-based materials, any one or more of them; wherein M is selected from any one or more of Mn, Al, Zr, x is 0~1, y is 0~1; and / or, the binder is selected from any one or more of polyvinylidene fluoride, polyacrylate and styrene-butadiene rubber; and / or, the conductive agent is selected from any one or more of conductive carbon black, graphene and carbon nanotubes.

[0011] According to another aspect of the present invention, a method for preparing the above-mentioned positive electrode sheet is provided, the method comprising: step S1, coating a pore-forming agent onto the outer surface of a solid electrolyte core by chemical coating or physical coating to obtain a modified solid electrolyte; the chemical coating process comprising: sequentially subjecting a raw material including a first raw material corresponding to the pore-forming agent, a solid electrolyte, and a first solvent to a first reaction and heat treatment; the physical coating process comprising: sequentially subjecting a raw material including a pore-forming agent and a second solvent to a first mixing to obtain a first mixture, and sequentially subjecting a raw material including the first mixture and the solid electrolyte to a second mixing and a first drying; step S2, mixing a raw material including a positive electrode active material, a conductive agent, a binder, a modified solid electrolyte, and a third solvent to obtain a positive electrode slurry; and step S3, coating the positive electrode slurry onto the surface of a current collector and then subjecting it to a second drying to obtain a positive electrode sheet; wherein the temperature of the second drying is ≥80°C.

[0012] Further, in step S1 above, the mass concentration of the pore-forming agent in the first mixture during the physical coating process is 10-50%; the first solvent and the second solvent are each independently selected from any one or more of ethanol, tetrahydrofuran, and water; the temperatures of the first mixing and the second mixing are each independently 50-80°C, and the times of the first mixing and the second mixing are each independently 1-6 h; and / or, the temperature of the first drying is 25-60°C, and the time of the first drying is 30-90 min; and / or, during the chemical coating process, the first raw material corresponding to the pore-forming agent includes an ammonium source and an acid radical ion source, the ammonium source being ammonium chloride and / or ammonia water, and the acid radical ion source being selected from any one or more of sodium bicarbonate, sodium carbonate, sodium oxalate, sodium nitrate, potassium bicarbonate, potassium carbonate, potassium oxalate, and potassium nitrate; the temperature of the first reaction is 25-50°C, and the time of the first reaction is 60-120 min; the temperature of the heat treatment is 20-150°C, and the time of the heat treatment is 60-120 min; and / or, the mass ratio of the pore-forming agent to the solid electrolyte core is 1-3:10-50.

[0013] Further, in step S2 above, the third solvent is N-methylpyrrolidone; the solid content of the positive electrode slurry is 68~74%, and the viscosity of the positive electrode slurry is 4500~10000 mPa·s; and / or, in step S3 above, the second drying is baking, the temperature of the second drying is 80~150℃, the time of the second drying is 10~120 min, and the heating rate of the second drying is 2~5℃ / min.

[0014] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is the positive electrode described above or prepared by the preparation method described above.

[0015] Further, the electrolyte is either a first electrolyte or a second electrolyte; the first electrolyte includes a first lithium salt and a fourth solvent; the molar concentration of the first lithium salt in the first electrolyte is 1~1.2 mol / L, the first lithium salt is lithium hexafluorophosphate, and the fourth solvent is selected from any one or more of ethylene carbonate, dimethyl carbonate, and diethyl carbonate; the second electrolyte includes a gel polymer electrolyte containing a second lithium salt, the second lithium salt is lithium bis(fluorosulfonyl)imide, and the molar concentration of the second lithium salt in the gel polymer electrolyte is 0.5~2 mol / L; the viscosity of the electrolyte at 25℃ is 1~15 mPa·s, the penetration time of the electrolyte in the positive electrode active layer is ≤30s, and the penetration depth of the electrolyte in the positive electrode active layer is ≥80μm.

[0016] By applying the technical solution of this invention, adding modified solid electrolyte to the positive electrode active layer enables the positive electrode active layer to form a three-dimensional interconnected porous network. This structural design achieves triple optimization: 1) The aforementioned pore-forming agents modify the outer surface of the solid electrolyte core, which can increase the Zeta potential of the positive electrode slurry, thereby inhibiting the agglomeration of modified solid electrolyte particles; 2) The aforementioned pore-forming agents decompose during the drying process after the positive electrode slurry is coated, forming three-dimensional interconnected channels in the positive electrode active layer. These three-dimensional interconnected channels can significantly shorten the electrolyte penetration time and increase the penetration depth, thereby improving the wettability of the electrolyte; 3) The inner wall of the three-dimensional interconnected channels allows the modified solid electrolyte to expose its active surface, forming a highly efficient ion transport interface with the electrolyte, thereby improving the ionic conductivity of the positive electrode and reducing the interface impedance; simultaneously, the active sites (Li) exposed by the modified solid electrolyte... + The migration channels can form an ultrathin continuous film with the binder on the inner wall of the channels, thereby enabling rapid charge transport at the solid-liquid interface. Furthermore, the porous structure of the positive electrode active layer can increase the thermal runaway trigger temperature, thus improving battery safety. In summary, the positive electrode sheet including the above-mentioned positive electrode active layer exhibits excellent electrochemical performance and safety. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0018] As analyzed in the background section of this application, the prior art has problems such as easy agglomeration of nanoscale solid electrolytes in positive electrode slurry, low electrolyte wetting efficiency, and solid-liquid interface charge transport barrier. In order to solve the above problems, this application provides a positive electrode sheet and its preparation method, as well as a lithium-ion battery.

[0019] In a typical embodiment of this application, a positive electrode sheet is provided, including a current collector and a positive electrode active layer disposed on the surface of the current collector. The positive electrode active layer is prepared from a positive electrode slurry and includes: a positive electrode active material, a conductive agent, a binder, and a modified solid electrolyte. The modified solid electrolyte includes a solid electrolyte core and a coating layer covering the outer surface of the solid electrolyte core. The coating layer includes a pore-forming agent, which includes an ammonium salt. The positive electrode active layer has three-dimensional through-holes.

[0020] The addition of modified solid electrolyte to the positive electrode active layer in this application enables the formation of a three-dimensional interconnected porous network. This structural design achieves triple optimization: 1) The aforementioned pore-forming agents modify the outer surface of the solid electrolyte core, increasing the Zeta potential of the positive electrode slurry and thus inhibiting the agglomeration of modified solid electrolyte particles; 2) The aforementioned pore-forming agents decompose during the drying process after coating the positive electrode slurry, forming three-dimensional interconnected channels in the positive electrode active layer. These three-dimensional interconnected channels significantly shorten the electrolyte penetration time and increase the penetration depth, thereby improving the wettability of the electrolyte; 3) The inner wall of the three-dimensional interconnected channels exposes the active surface of the modified solid electrolyte, forming a highly efficient ion transport interface with the electrolyte, thereby increasing the ionic conductivity of the positive electrode and reducing the interfacial impedance; simultaneously, the active sites (Li) exposed by the modified solid electrolyte... + The migration channels can form an ultrathin continuous film with the binder on the inner wall of the channels, thereby enabling rapid charge transport at the solid-liquid interface. Furthermore, the porous structure of the positive electrode active layer can increase the thermal runaway trigger temperature, thus improving battery safety. In summary, the positive electrode sheet including the above-mentioned positive electrode active layer exhibits excellent electrochemical performance and safety.

[0021] Active sites exposed in modified solid electrolytes (Li + The migration channel can form an ultrathin continuous film with the binder on the inner wall of the channel, with a film thickness of ≤10nm, thereby further improving the charge transport speed at the solid-liquid interface.

[0022] In one embodiment of this application, the mass ratio of the positive electrode active material, conductive agent, binder and modified solid electrolyte is 80~95:1~10:2~8:0.5~15.

[0023] Positive electrode active materials contribute to improved battery capacity and energy density. Conductive agents help improve the electronic conductivity of the positive electrode sheet and reduce charge transport resistance, thereby improving the battery's power density and cycle performance. Binders help maintain the structural stability of the positive electrode active materials and conductive agents, thus improving the integrity of the electrode during cycling. Preferably, controlling the mass ratio of positive electrode active materials, conductive agents, binders, and modified solid electrolytes within the above-mentioned range helps to further improve the battery's electrochemical performance and safety. Furthermore, the mass ratio of positive electrode active materials, conductive agents, binders, and modified solid electrolytes can be 80:10:5:5, 82:1:2:15, 85:2:5:8, 90:5:2:3, or 95:1:2:2; of course, the mass ratio of positive electrode active materials, conductive agents, binders, and modified solid electrolytes can be any mass ratio within the above range.

[0024] In one embodiment of this application, the thickness of the coating layer is 2~50nm, the coverage of the coating layer is ≥85%, and the mass percentage of the coating layer in the modified electrolyte is 2~23%; and / or, the pore-forming agent is selected from any one or more of ammonium bicarbonate, ammonium carbonate, ammonium oxalate and ammonium nitrate.

[0025] Preferably having the mass percentage, thickness, and coverage of the coating layer within the aforementioned range in the modified electrolyte helps to further improve the dispersion of the modified solid electrolyte particles in the cathode slurry. It also promotes good contact between the exposed active crystal faces of the modified solid electrolyte after pore formation and the electrolyte, thereby reducing interfacial resistance. Preferably having the type of pore-forming agent within the aforementioned range helps to optimize the pore structure and control the porosity of the cathode active layer, thereby promoting heat dissipation within the battery and reducing the risk of thermal runaway. Furthermore, the coating layer thickness can be 2nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, or 50nm, and the coating layer coverage can be 85%, 88%, 90%, 92%, or 95%. Of course, the coating layer thickness and coverage can be any value within the aforementioned range. The mass percentage of the coating layer in the modified electrolyte can be 2%, 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or 23%. Of course, the mass percentage of the coating layer in the modified electrolyte can be any value within the above range.

[0026] The preferred pore-forming agent is a combination of ammonium oxalate and ammonium nitrate, with a mass ratio of ammonium oxalate to ammonium nitrate of 2 to 5:1. This helps to better coat the outer surface of the solid electrolyte core, thereby improving the dispersibility of the modified electrolyte and better controlling the pore size and porosity of the channels in the positive electrode active layer.

[0027] In one embodiment of this application, the solid electrolyte core is selected from any one or more of LATP solid electrolyte, LAGP solid electrolyte, LLZO solid electrolyte and LLTO solid electrolyte; the D50 particle size of the solid electrolyte core is 20~1000nm; and / or, the zeta potential of the positive electrode slurry is 30~60mV; and / or, the equivalent pore size of the three-dimensional through-hole is 10~500nm, and the porosity of the positive electrode active layer is 40~65%.

[0028] Preferring a solid electrolyte core type within the above-mentioned range helps improve the lithium-ion conductivity of the cathode. Preferring a solid electrolyte core particle size within the above-mentioned range helps improve dispersibility. Preferring a cathode slurry zeta potential within the above-mentioned range helps further suppress the agglomeration of modified solid electrolyte particles. Furthermore, the zeta potential of the cathode slurry can be 30mV, 35mV, 40mV, 45mV, 50mV, 55mV, or 60mV; of course, the zeta potential of the cathode slurry can be any value within the above-mentioned range.

[0029] The optimal equivalent pore size and porosity of the three-dimensional through-hole channels within the aforementioned range not only facilitates rapid and uniform penetration of the electrolyte into the positive electrode active layer, thereby improving the electrolyte wetting efficiency, but also provides a good migration channel for lithium ions, reducing resistance during ion transport, thus improving battery conductivity and reducing interfacial impedance, and consequently improving battery kinetic and rate performance. Furthermore, it helps maintain the structural integrity of the positive electrode material. The optimal porosity of the positive electrode active layer within the aforementioned range helps provide sufficient space for electrolyte wetting, improving the utilization rate of the electrode material, reducing the proportion of inactive materials, and thus increasing the battery's energy density. Here, the equivalent pore size is the average size of the pores. Moreover, the equivalent pore size of the three-dimensional through-hole channels can be 10nm, 30nm, 50nm, 70nm, 100nm, 150nm, 200nm, 250nm, 350nm, 400nm, 450nm, or 500nm; of course, the equivalent pore size of the three-dimensional through-hole channels can be any value within the aforementioned range. The porosity of the positive electrode active layer can be 40%, 42%, 45%, 47%, 50%, 52%, 55%, 57%, 60%, 62%, or 65%. Of course, the porosity of the positive electrode active layer can be any value within the above range.

[0030] To further improve the electrochemical performance of the positive electrode, in one embodiment of this application, the positive electrode active material is selected from LiCoO2 and LiNi. 1-x-y Co x M y O2, LiFePO4, lithium-rich manganese-based materials, any one or more of them; wherein M is selected from any one or more of Mn, Al, Zr, x is 0~1, y is 0~1; and / or, the binder is selected from any one or more of polyvinylidene fluoride, polyacrylate and styrene-butadiene rubber; and / or, the conductive agent is selected from any one or more of conductive carbon black, graphene and carbon nanotubes.

[0031] In another typical embodiment of this application, a method for preparing the above-mentioned positive electrode sheet is provided. The method includes: step S1, coating a pore-forming agent onto the outer surface of a solid electrolyte core by chemical coating or physical coating to obtain a modified solid electrolyte; the chemical coating process includes: sequentially performing a first reaction and heat treatment on raw materials including a first raw material corresponding to the pore-forming agent, a solid electrolyte, and a first solvent; the physical coating process includes: performing a first mixing on raw materials including a pore-forming agent and a second solvent to obtain a first mixture, and sequentially performing a second mixing and a first drying on raw materials including the first mixture and the solid electrolyte; step S2, mixing raw materials including a positive electrode active material, a conductive agent, a binder, a modified solid electrolyte, and a third solvent to obtain a positive electrode slurry; and step S3, coating the positive electrode slurry onto the surface of a current collector and then performing a second drying to obtain a positive electrode sheet; wherein the temperature of the second drying is ≥80°C.

[0032] The positive electrode obtained by the above preparation method exhibits high ionic conductivity, good electrochemical performance, and safety. Specifically, in step S1, the pore-forming agent is coated onto the outer surface of the solid electrolyte core through the aforementioned chemical and physical coating methods. This adjusts the Zeta potential of the positive electrode slurry, thereby inhibiting the agglomeration of modified solid electrolyte particles. The thermal decomposition temperature range of the pore-forming agent is 60~150℃. After coating the positive electrode slurry, drying at the above temperature allows the pore-forming agent to decompose, forming three-dimensional interconnected channels in the positive electrode active layer through in-situ pore formation. These three-dimensional interconnected channels not only significantly shorten the electrolyte penetration time and increase the penetration depth, thereby improving the wettability of the electrolyte, but also reduce interfacial impedance and increase ion transport speed, thus improving the ionic conductivity of the positive electrode. In summary, this application directly composites the pore-forming agent with the solid electrolyte and adds it to the positive electrode slurry, simultaneously combining the slurry and coating it together, eliminating the need for secondary coating. Furthermore, since the decomposition temperature of the pore-forming agent is lower than the coating oven temperature, it can be effectively and completely decomposed, eliminating the need for secondary processing. Furthermore, the preparation method of this application has good process compatibility and does not require modification of existing coating production lines.

[0033] For example, chemical coating refers to the first raw material corresponding to the pore-forming agent undergoing a first reaction in a solvent. For instance, ammonium bicarbonate reacts according to the equation NH4Cl + NaHCO3 = NH4HCO3 + NaCl, and the ammonium bicarbonate is adsorbed onto the outer surface of the solid electrolyte core in solid form, followed by heat treatment to form crystals. Physical coating refers to the finished pore-forming agent material, such as ammonium bicarbonate powder, dissolved in a solvent, physically precipitating and adsorbing onto the outer surface of the solid electrolyte core, followed by low-temperature drying.

[0034] In one embodiment of this application, in step S1 above, the mass concentration of the pore-forming agent in the first mixture during the physical coating process is 10-50%; the first solvent and the second solvent are each independently selected from any one or more of ethanol, tetrahydrofuran, and water; the temperatures of the first mixing and the second mixing are each independently 50-80°C, and the times of the first mixing and the second mixing are each independently 1-6 hours; and / or, the temperature of the first drying is 25-60°C, the time of the first drying is 30-90 minutes, and the first drying is oven drying; and / or, chemical... During the coating process, the first raw material corresponding to the pore-forming agent includes an ammonium source and an acid radical ion source. The ammonium source is ammonium chloride and / or ammonia water, and the acid radical ion source is selected from any one or more of sodium bicarbonate, sodium carbonate, sodium oxalate, sodium nitrate, potassium bicarbonate, potassium carbonate, potassium oxalate, and potassium nitrate. The temperature of the first reaction is 25~50℃, and the time of the first reaction is 60~120min. The temperature of the heat treatment is 20~150℃, and the time of the heat treatment is 60~120min. And / or, the mass ratio of the pore-forming agent to the solid electrolyte core is 1~3:10~50.

[0035] In the preferred physical coating process, controlling the mass concentration of the pore-forming agent in the first mixture, and the temperature and time of the first and second mixing within the aforementioned ranges, helps the pore-forming agent to physically precipitate and adsorb onto the outer surface of the solid electrolyte core. Preferably, the temperature and time of the first drying process are within the aforementioned ranges, which helps the pore-forming agent to more tightly coat the outer surface of the solid electrolyte core.

[0036] In the preferred chemical coating process, the type of the first raw material corresponding to the pore-forming agent, the temperature of the first reaction, and the time are preferably within the above-mentioned ranges, which helps the first raw material to undergo the first reaction, thereby forming the pore-forming agent adsorbed on the outer surface of the solid electrolyte core. The preferred heat treatment temperature and time are also preferably within the above-mentioned ranges, which helps the pore-forming agent to be more tightly coated on the outer surface of the solid electrolyte core.

[0037] The preferred mass ratio of pore-forming agent to solid electrolyte core within the above-mentioned range helps the pore-forming agent to more effectively modify the outer surface of the solid electrolyte core, thereby increasing the zeta potential of the positive electrode slurry and inhibiting the agglomeration of modified solid electrolyte particles. Furthermore, the mass ratio of pore-forming agent to solid electrolyte core can be 1:10, 1.5:25, 2:30, 2.5:35, or 3:50; of course, the mass ratio of pore-forming agent to solid electrolyte core can be any mass ratio within the above-mentioned range.

[0038] In one embodiment of this application, in step S2 above, the third solvent is N-methylpyrrolidone; the solid content of the positive electrode slurry is 68~74%, and the viscosity of the positive electrode slurry is 4500~10000 mPa·s; and / or, in step S3 above, the second drying is baking, the temperature of the second drying is 80~150℃, the time of the second drying is 10~120 min, and the heating rate of the second drying is 2~5℃ / min.

[0039] The preferred solid content and viscosity of the positive electrode slurry are within the above-mentioned range, which helps to improve the uniformity of coating, thereby obtaining a positive electrode active layer with uniform composition. The second drying is preferably oven drying, and the temperature, time, and heating rate of the second drying are preferably within the above-mentioned range, which helps to decompose the pore-forming agent and obtain a positive electrode active layer with the porosity and pore size within the above-mentioned range. The temperature of the second drying can be 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, or 150°C; of course, the temperature of the second drying can be any value within the above-mentioned range.

[0040] In another typical embodiment of this application, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is the positive electrode described above or prepared by the preparation method described above.

[0041] Lithium-ion batteries, including the aforementioned positive electrode, exhibit excellent electrochemical performance and safety. Under 1C charge-discharge conditions, the interface impedance of the lithium-ion battery is ≤150Ω·cm², the lithium-ion transference number is ≥0.6, and the capacity retention rate is ≥90% after 500 cycles at 45℃.

[0042] In one embodiment of this application, the electrolyte is a first electrolyte or a second electrolyte; the first electrolyte includes a first lithium salt and a fourth solvent; the molar concentration of the first lithium salt in the first electrolyte is 1~1.2 mol / L, the first lithium salt is lithium hexafluorophosphate, and the fourth solvent is selected from any one or more of ethylene carbonate, dimethyl carbonate, and diethyl carbonate; the second electrolyte includes a gel polymer electrolyte containing a second lithium salt, the second lithium salt is lithium bis(fluorosulfonyl)imide, and the molar concentration of the second lithium salt in the gel polymer electrolyte is 0.5~2 mol / L; the viscosity of the electrolyte at 25°C is 1~15 mPa·s, the penetration time of the electrolyte in the positive electrode active layer is ≤30s, and the penetration depth of the electrolyte in the positive electrode active layer is ≥80μm, preferably 100~120μm.

[0043] Using the above-mentioned types of electrolytes facilitates lithium-ion migration, thereby improving the battery's rate performance and fast-charging capability. Gel polymer electrolytes offer higher safety; lithium bis(fluorosulfonyl)imide exhibits good high-temperature stability and forms a gel electrolyte with the polymer matrix in the electrolyte through a crosslinking agent. This helps reduce the risk of thermal runaway under high-temperature or overcharge conditions, thus improving battery safety. The polymer matrix is ​​selected from any one or more of polyethylene oxide, polyacrylonitrile, and polyvinylidene fluoride. Preferably, the electrolyte viscosity at 25°C, the electrolyte penetration time in the positive electrode active layer, and the penetration depth are within the above-mentioned ranges, which helps improve the electrolyte's wetting performance. Preferably, using the above-mentioned types of separators and controlling the areal density and porosity of the solid electrolyte coating within the above-mentioned ranges helps provide sufficient ion migration channels while reducing resistance to lithium-ion transport, thereby improving the battery's conductivity and charge / discharge efficiency.

[0044] The preferred diaphragm is selected from any one or more of polyolefin porous membranes, cellulose membranes, and ceramic composite diaphragms.

[0045] The negative electrode sheet includes a negative electrode active layer, and preferably the negative electrode active material in the negative electrode active layer is selected from any one or more of graphite, silicon-carbon composite material or metallic lithium.

[0046] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0047] Example 1

[0048] Preparation of modified electrolyte: 1g of pore-forming agent ammonium bicarbonate and second solvent ethanol solution were mixed at 50℃ for 1h to obtain a first mixture, in which the mass concentration of pore-forming agent was 10%. 10g of solid electrolyte core LATP (D50 particle size of 100nm) and the first mixture were mixed at 50℃ for 4h, followed by a first drying (baking) at 35℃ for 60min to obtain a modified solid electrolyte. The modified solid electrolyte was coated with a coating layer, which was a pore-forming agent. The mass percentage of the coating layer in the modified solid electrolyte was 9%, the thickness of the coating layer was 13nm, and the coverage rate of the coating layer was 92%.

[0049] Preparation of the positive electrode: The positive electrode active material LiFePO4, conductive carbon black SP, binder polyvinylidene fluoride, and modified solid electrolyte were mixed in a mass ratio of 90:2:4:2. The mixture was then stirred under vacuum for 4 hours with the addition of a third solvent, N-methylpyrrolidone (NMP), to obtain the positive electrode slurry. The solid content of the positive electrode slurry was 68%, and the viscosity was 4500 mPa. The positive electrode slurry is coated on the surface of an aluminum foil with a thickness of 12 μm and a coating surface density of 23 mg / cm². It is then subjected to a second drying (baking) at 120°C for 10 min (heating rate of 5°C / min) to form a positive electrode active layer with three-dimensional through-holes, thus obtaining the positive electrode sheet.

[0050] Preparation of the negative electrode: Graphite (anode active material), conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber latex were added to water in a mass ratio of 86:2:1:1 to obtain a negative electrode slurry. The negative electrode slurry was coated onto the surface of copper foil and then dried, achieving a coating areal density of 12 mg / cm³. 2 This yields the negative electrode.

[0051] Electrolyte preparation: Ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1:1 to obtain a fourth solvent. Lithium hexafluorophosphate was added to the fourth solvent to obtain the electrolyte, with a molar concentration of 1 mol / L for lithium hexafluorophosphate. The viscosity of the electrolyte at 25 °C was 6 mPa·s.

[0052] Preparation of lithium-ion batteries: A lithium-ion battery is obtained by assembling a positive electrode, a negative electrode, a polyolefin porous membrane separator, and an electrolyte.

[0053] Example 2

[0054] The difference from Example 1 lies in the preparation of the modified electrolyte: 0.05 mol of ammonium chloride and 0.05 mol of sodium bicarbonate, the raw materials corresponding to the pore-forming agent, were reacted in the first solvent, water, at 30°C for 60 min to obtain 3 g of solid ammonium bicarbonate as the pore-forming agent. The ammonium bicarbonate was adsorbed onto the outer surface of 50 g of the solid electrolyte core LATP in solid form. Subsequently, the mixture was heat-treated at 30°C for 60 min to obtain the modified solid electrolyte. The mass percentage of the coating layer in the modified solid electrolyte was 5.7%, the thickness of the coating layer was 10 nm, and the coverage rate of the coating layer was 89%.

[0055] Preparation of the positive electrode: The positive electrode active material LiNi... 0.8 Co 0.1 Mn 0.1 O2, conductive carbon nanotubes, binder polyvinylidene fluoride, and modified solid electrolyte are mixed in a mass ratio of 85:2:3:10. N-methylpyrrolidone (NMP) is added and the mixture is stirred under vacuum for 4 hours to obtain a positive electrode slurry. Finally, a positive electrode sheet and a lithium-ion battery are obtained.

[0056] Example 3

[0057] The difference from Example 1 is that the mass ratio of the positive electrode active material LiFePO4, the conductive agent conductive carbon black SP, the binder polyvinylidene fluoride, and the modified solid electrolyte is 80:1:4:15, ultimately yielding a positive electrode sheet and a lithium-ion battery.

[0058] Example 4

[0059] The difference from Example 1 is that the mass ratio of the positive electrode active material LiFePO4, the conductive agent conductive carbon black SP, the binder polyvinylidene fluoride and the modified solid electrolyte is 95:2.5:2:0.5, which ultimately yields the positive electrode sheet and the lithium-ion battery.

[0060] Example 5

[0061] The difference from Example 1 is that the mass ratio of the positive electrode active material LiFePO4, the conductive agent conductive carbon black SP, the binder polyvinylidene fluoride and the modified solid electrolyte is 90:6:3.9:0.1, and the positive electrode sheet and lithium-ion battery are finally obtained.

[0062] Example 6

[0063] The difference from Example 1 is that the mass of the pore-forming agent is 3g, the mass of the solid electrolyte core is 10g, the mass ratio of the pore-forming agent to the solid electrolyte core is 3:10, the mass ratio of the coating layer in the modified solid electrolyte is 23%, the thickness of the coating layer is 50nm, and the coverage of the coating layer is 93%, finally obtaining the positive electrode sheet and the lithium-ion battery.

[0064] Example 7

[0065] The difference from Example 1 is that the mass of the pore-forming agent is 0.5g, the mass of the solid electrolyte core is 50g, the mass ratio of the pore-forming agent to the solid electrolyte core is 0.5:50, the thickness of the coating layer is 0.5nm, the coverage of the coating layer is 80%, and the mass ratio of the coating layer in the modified solid electrolyte is 1%, finally obtaining the positive electrode and the lithium-ion battery.

[0066] Example 8

[0067] The difference from Example 1 is that the second drying temperature is 80°C, the heating rate is 2°C / min, the time is 120min, the equivalent pore size of the three-dimensional through-hole is 500nm, the porosity of the positive electrode active layer is 40%, and finally a lithium-ion battery is obtained.

[0068] Example 9

[0069] The difference from Example 1 is that the second drying temperature is 160°C, the heating rate is 6°C / min, the time is 5min, the equivalent pore size of the three-dimensional through-hole is 8nm, the porosity of the positive electrode active layer is 35%, and finally a lithium-ion battery is obtained.

[0070] Example 10

[0071] The difference from Example 1 is that the pore-forming agent is a combination of ammonium oxalate and ammonium nitrate, with a mass ratio of ammonium oxalate to ammonium nitrate of 2:1, ultimately yielding a positive electrode sheet and a lithium-ion battery.

[0072] Example 11

[0073] The difference from Example 1 is that the pore-forming agent is a combination of ammonium oxalate and ammonium nitrate, with a mass ratio of ammonium oxalate to ammonium nitrate of 6:1, ultimately yielding a positive electrode sheet and a lithium-ion battery.

[0074] Comparative Example 1

[0075] The difference from Example 1 is that the outer surface of the solid electrolyte core is not coated with a pore-forming agent, and the positive electrode and lithium-ion battery are finally obtained.

[0076] Comparative Example 2

[0077] The difference from Example 1 is that the second drying temperature is 55°C, and the positive electrode and lithium-ion battery are finally obtained.

[0078] Test method:

[0079] Zeta potential test:

[0080] 1) Sample Preparation: Diluting the Slurry: Take a small amount of positive electrode slurry (1 mL) and gradually dilute it with NMP, the solvent of the original slurry, until the solution appears "slightly turbid with no obvious particle precipitation". Dispersing Particles: Place the diluted solution in an ultrasonic instrument and sonicate for 1-2 minutes. Filtering Impurities: Draw the solution into a disposable syringe, filter it through a filter membrane, and keep the filtered clear liquid for later use. Sample Loading: Take a clean sample cell, slowly inject the filtered clear liquid with a pipette until the liquid level is just up to the scale line of the sample cell, and gently tap the cell wall to remove small air bubbles. 3) Starting the Measurement: Place the sample cell into the sample chamber of the zeta potential meter, close the lid, press the "Measure" button on the instrument, and wait for the instrument to complete the automatic run (about 3-5 minutes). 4) Reading the Results: The instrument screen will display a "zeta potential" value (with a positive or negative sign), and record the value.

[0081] Equivalent pore size test of three-dimensional through-holes in the positive electrode active layer:

[0082] Cut a small piece of dry positive electrode active layer (ensure no aluminum foil is present) and place it in an oven. Dry it at 100°C for 2 hours. Place the sample in a nitrogen adsorption instrument and introduce nitrogen gas. The instrument will automatically record the amount of gas adsorbed and directly output the "average pore size" (i.e., equivalent pore size).

[0083] Porosity test: Take a positive electrode active layer sample and weigh it using a balance (recorded as ). The sample was completely immersed in anhydrous ethanol for 30 minutes, then dried and weighed (recorded as ). Hang the ethanol-soaked sample on a balance, fully immerse it in water, and weigh it (record the weight as follows). Calculate porosity = [(m2-m1) / (m2-m3)] × 100%

[0084] Electrolyte penetration time and penetration depth tests in the positive electrode active layer:

[0085] 1) Sample Preparation: Weigh 10g of positive electrode slurry and pour it into the coating tank. Use a 100μm wire rod coater to coat the slurry evenly on a clean aluminum foil (5cm×5cm), ensuring a coating width of 3cm and uniform thickness (error ≤5μm). Place the coated aluminum foil in a vacuum oven and dry at 60℃ for 12 hours. After drying, cut the electrode sheets into 2cm×2cm test samples, label and number them, and prepare for later use (prepare at least 5 samples, 3 for testing and 2 for backup).

[0086] 2) Penetration Time Test: Set the constant temperature and humidity chamber to 25℃ and RH 50%, and place the electrode sample inside the chamber for equilibration for 30 minutes. Remove the electrode and place it horizontally on the stage. Use a microsyringe to draw 10 μL of electrolyte, keeping the needle 1 cm away from the electrode surface, and drop it vertically into the center area of ​​the electrode (avoiding the edge by 1 mm). Start the stopwatch immediately after dropping and observe the electrolyte wetting process: initially, it is a droplet, gradually spreading until there is no obvious droplet residue on the surface and the wetting area has a uniform color (no difference in reflectivity compared to the unwetting area). Stop the stopwatch when complete wetting occurs and record the time (accurate to 0.1 s), which is the penetration time. Repeat the test 3 times for the same sample and take the average value as the final penetration time.

[0087] 3) Penetration depth test: Immediately after the penetration time test, cut along the cross-section of the electrode with a blade (perpendicular to the coating direction), place the slice in epoxy resin embedding agent, and cure for 2 hours. Observe the cross-section of the slice under an optical microscope (magnification 200×) to find the vertical distance between the "active layer surface" and the "electrolyte wetting front" (i.e., penetration depth), and measure it using the microscope's built-in scale (accurate to 1μm).

[0088] Electrochemical testing: 1C / 1C 25℃ cycle regime. The battery is charged at a rate of 1C until it is fully charged under constant temperature conditions of 25℃, and then discharged at a rate of 1C until the battery has cycled 1000 times.

[0089] The test results are shown in Table 1.

[0090] Table 1

[0091]

[0092] As can be seen from the above, in Comparative Example 1, the outer surface of the solid electrolyte core was not coated with a pore-forming agent, resulting in a low Zeta potential of the positive electrode slurry, which easily led to agglomeration and thus lower battery performance. In Comparative Example 2, the second drying temperature was below 60°C, which prevented the pore-forming agent from decomposing and forming three-dimensional interconnected channels in the positive electrode active layer, resulting in a longer electrolyte penetration time and a shallower penetration depth.

[0093] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0094] The addition of modified solid electrolyte to the positive electrode active layer in this application enables the formation of a three-dimensional interconnected porous network. This structural design achieves triple optimization: 1) The aforementioned pore-forming agents modify the outer surface of the solid electrolyte core, increasing the Zeta potential of the positive electrode slurry and thus inhibiting the agglomeration of modified solid electrolyte particles; 2) The aforementioned pore-forming agents decompose during the drying process after coating the positive electrode slurry, forming three-dimensional interconnected channels in the positive electrode active layer. These three-dimensional interconnected channels significantly shorten the electrolyte penetration time and increase the penetration depth, thereby improving the wettability of the electrolyte; 3) The inner wall of the three-dimensional interconnected channels exposes the active surface of the modified solid electrolyte, forming a highly efficient ion transport interface with the electrolyte, thereby increasing the ionic conductivity of the positive electrode and reducing the interfacial impedance; simultaneously, the active sites (Li) exposed by the modified solid electrolyte... + The migration channels can form an ultrathin continuous film with the binder on the inner wall of the channels, thereby enabling rapid charge transport at the solid-liquid interface. Furthermore, the porous structure of the positive electrode active layer can increase the thermal runaway trigger temperature, thus improving battery safety. In summary, the positive electrode sheet including the above-mentioned positive electrode active layer exhibits excellent electrochemical performance and safety.

[0095] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A positive electrode sheet, comprising a current collector and a positive electrode active layer disposed on the surface of the current collector, wherein the positive electrode active layer is prepared from a positive electrode slurry, characterized in that, The positive electrode active layer comprises: a positive electrode active material, a conductive agent, a binder, and a modified solid electrolyte; wherein, the modified solid electrolyte comprises a solid electrolyte core and a coating layer covering the outer surface of the solid electrolyte core, the coating layer comprising a pore-forming agent, the pore-forming agent comprising an ammonium salt; the positive electrode active layer has three-dimensional through-pores.

2. The positive electrode sheet according to claim 1, characterized in that, The mass ratio of the positive electrode active material, the conductive agent, the binder, and the modified solid electrolyte is 80~95:1~10:2~8:0.5~15.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, The coating layer has a thickness of 2-50 nm, a coverage rate of ≥85%, and a mass percentage of 2-23% in the modified electrolyte; and / or, the pore-forming agent is selected from any one or more of ammonium bicarbonate, ammonium carbonate, ammonium oxalate, and ammonium nitrate.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The solid electrolyte core is selected from any one or more of LATP solid electrolyte, LAGP solid electrolyte, LLZO solid electrolyte and LLTO solid electrolyte; the D50 particle size of the solid electrolyte core is 20~1000nm; And / or, the zeta potential of the positive electrode slurry is 30~60mV; And / or, the equivalent pore size of the three-dimensional through-hole is 10~500nm, and the porosity of the positive electrode active layer is 40~65%.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The positive electrode active material is selected from LiCoO2 and LiNi. 1-x-y Co x M y O2, LiFePO4, lithium-rich manganese-based materials, or any one or more thereof; wherein M is selected from any one or more of Mn, Al, and Zr, x is 0~1, and y is 0~1; and / or, the binder is selected from any one or more of polyvinylidene fluoride, polyacrylate, and styrene-butadiene rubber; and / or, the conductive agent is selected from any one or more of conductive carbon black, graphene, and carbon nanotubes.

6. A method for preparing a positive electrode sheet according to any one of claims 1 to 5, characterized in that, The preparation method includes: Step S1: The pore-forming agent is coated onto the outer surface of the solid electrolyte core by chemical or physical coating to obtain a modified solid electrolyte. The chemical coating process includes: sequentially subjecting the raw material, which includes the first raw material corresponding to the pore-forming agent, the solid electrolyte, and the first solvent, to a first reaction and a heat treatment; The physical coating process includes: first mixing raw materials including the pore-forming agent and the second solvent to obtain a first mixture; and second mixing and first drying of raw materials including the first mixture and the solid electrolyte in sequence. Step S2 involves mixing raw materials including positive electrode active material, conductive agent, binder, the modified solid electrolyte, and a third solvent to obtain a positive electrode slurry; and Step S3: After coating the positive electrode slurry onto the surface of the current collector, perform a second drying to obtain the positive electrode sheet; The temperature of the second drying process is ≥80℃.

7. The preparation method according to claim 6, characterized in that, In step S1, during the physical coating process, the mass concentration of the pore-forming agent in the first mixture is 10-50%; the first solvent and the second solvent are each independently selected from any one or more of ethanol, tetrahydrofuran, and water; the temperatures of the first mixing and the second mixing are each independently 50-80°C, and the times of the first mixing and the second mixing are each independently 1-6 hours; and / or, the temperature of the first drying is 25-60°C, and the time of the first drying is 30-90 minutes. And / or, in the chemical coating process, the first raw material corresponding to the pore-forming agent includes an ammonium source and an acid radical ion source, wherein the ammonium source is ammonium chloride and / or ammonia water, and the acid radical ion source is selected from any one or more of sodium bicarbonate, sodium carbonate, sodium oxalate, sodium nitrate, potassium bicarbonate, potassium carbonate, potassium oxalate, and potassium nitrate; the temperature of the first reaction is 25~50℃, and the time of the first reaction is 60~120min; the temperature of the heat treatment is 20~150℃, and the time of the heat treatment is 60~120min; And / or, the mass ratio of the pore-forming agent to the solid electrolyte core is 1~3:10~50.

8. The preparation method according to claim 6 or 7, characterized in that, In step S2, the third solvent is N-methylpyrrolidone; the solid content of the positive electrode slurry is 68-74%, and the viscosity of the positive electrode slurry is 4500-10000 mPa·s. And / or, in step S3, the second drying is baking, the temperature of the second drying is 80~150℃, the time of the second drying is 10~120min, and the heating rate of the second drying is 2~5℃ / min.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode sheet is any one of claims 1 to 6 or is prepared by the preparation method described in claim 7 or 8.

10. The lithium-ion battery according to claim 9, characterized in that, The electrolyte is either a first electrolyte or a second electrolyte; the first electrolyte comprises a first lithium salt and a fourth solvent; the molar concentration of the first lithium salt in the first electrolyte is 1~1.2 mol / L, the first lithium salt is lithium hexafluorophosphate, and the fourth solvent is selected from any one or more of ethylene carbonate, dimethyl carbonate, and diethyl carbonate; the second electrolyte comprises a gel polymer electrolyte containing a second lithium salt, the second lithium salt is lithium bis(fluorosulfonyl)imide, and the molar concentration of the second lithium salt in the gel polymer electrolyte is 0.5~2 mol / L; the viscosity of the electrolyte at 25°C is 1~15 mPa·s, the penetration time of the electrolyte in the positive electrode active layer is ≤30s, and the penetration depth of the electrolyte in the positive electrode active layer is ≥80μm.

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