Composite cathode sheet and method for manufacturing the same, secondary battery

Abstract

The application provides a composite positive electrode sheet, a preparation method thereof and a secondary battery. The composite positive electrode sheet comprises a current collector and a positive active coating layer stacked on at least one side surface of the current collector, wherein the positive active coating layer comprises a positive active material, a conductive agent and a binder; the positive active material comprises a lithium nickel cobalt manganese oxide material and a lithium manganese iron phosphate material; and the binder comprises a thio-polyurethane elastic polymer. The thio-polyurethane elastic polymer added in the positive active layer can not only solve the problems of active particle peeling and micro-crack generation caused by the large difference in volume expansion rate between the lithium manganese iron phosphate material and the lithium nickel cobalt manganese oxide material during the charging and discharging process, thereby improving the cycle life of the battery, but also effectively inhibit the manganese dissolution, thereby improving the capacity retention rate of the battery.

Description

Technical Field

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

[0002] Secondary batteries are widely used in automobiles and smart grids. One of the bottlenecks currently hindering the rapid development of lithium-ion batteries is the cathode active material, which mainly includes lithium nickel manganese cobalt oxide, lithium iron phosphate, and lithium manganese iron phosphate. However, the intrinsic properties of lithium iron phosphate materials limit the battery's voltage platform, resulting in poor low-temperature performance and low energy density. Furthermore, the limited space available for vehicle installation makes it impossible to meet the demands of power batteries for higher capacity and higher energy density.

[0003] Lithium manganese iron phosphate (LFP) is an upgraded version of lithium iron phosphate. LFP is composed of PO4 tetrahedra, LiO6 (octahedrons), and FeO6 (octahedrons). On one hand, the covalent bonds of PO in the phosphate group have high bond energy, making them difficult to break under high temperature or overcharge conditions; therefore, its structure is stable and its volume change is small. On the other hand, the Mn in LFP... 2+ / Mn 3+ The introduction of a higher voltage platform (approximately 4.1V) brings the advantage of higher energy density. However, like lithium iron phosphate, lithium manganese iron phosphate suffers from a problem: it is a one-dimensional lithium-ion diffusion channel with poor intrinsic conductivity and low ionic conductivity. Furthermore, lithium manganese iron phosphate exhibits the Jahn-Teller effect. In the discharge state, Mnn... 3+ The presence of this form leads to lattice distortion, thereby causing a disproportionation reaction to generate Mn. 2 + And Mn 2+ It will dissolve into the electrolyte, migrate to the negative electrode and damage the SEI film, leading to loss of active lithium and capacity decay.

[0004] Combining lithium manganese iron phosphate (LFP) with lithium nickel manganese cobalt oxide (LiCO) offers several advantages: Compared to pure LFP, 1) pure LFP has a relatively low energy density. Replacing part of LFP with LiCO significantly improves the overall energy density of the composite electrode battery, resulting in longer battery life. 2) Pure LFP has low intrinsic electronic conductivity and ion diffusion rate. Combining it with highly conductive LiCO optimizes the conductive network and improves the electronic conductivity of the composite electrode, thereby enhancing its high-current charge-discharge performance. Compared to pure LiCO, the olivine structure of LFP gives it excellent thermal and chemical stability. It is less prone to decomposition, combustion, or explosion under abusive conditions such as high temperature, overcharge, and needle penetration. Therefore, the overall safety of the composite electrode battery is superior to that of pure LiCO batteries. Furthermore, LFP is abundant and inexpensive. Replacing part of LiCO with LFP is expected to further reduce the cost of battery materials.

[0005] However, composite electrodes also have the following problems: 1) Volume expansion mismatch leading to microcracks: The volume expansion rate of lithium manganese iron phosphate differs significantly from that of lithium nickel cobalt manganese oxide during charge and discharge. After mixing, stress accumulation occurs at the interface due to the expansion difference, leading to active particle peeling and microcrack formation, thereby deteriorating the cycle life of the battery. 2) Manganese dissolution and interfacial side reactions: Mn in lithium manganese iron phosphate... 3+ Disproportionation reactions easily occur during high-pressure cycling, and the dissolved Mn 2+ Migrating to the negative electrode, it damages the SEI film and catalyzes electrolyte decomposition, thereby accelerating battery capacity decay. Therefore, a novel composite positive electrode needs to be developed. Summary of the Invention

[0006] The main objective of this invention is to provide a composite positive electrode sheet and its preparation method, as well as a secondary battery, to solve the problems in the prior art where stress accumulation due to the expansion difference between the two positive electrode active materials leads to the peeling of active particles and the generation of microcracks, as well as manganese dissolution and interfacial side reactions.

[0007] To achieve the above objectives, according to one aspect of the present invention, a composite positive electrode sheet is provided, the composite positive electrode sheet comprising a current collector and a positive active coating stacked on at least one side surface of the current collector, the positive active coating comprising a positive active material, a conductive agent, and a binder; the positive active material comprising lithium nickel cobalt manganese oxide and lithium manganese iron phosphate; the binder comprising a thiopolyurethane elastic polymer.

[0008] The thiopolyurethane elastic polymer added to the positive active layer of the composite positive electrode sheet in this application possesses excellent self-healing capabilities. During charge and discharge, broken bonds can spontaneously recombine, achieving self-repair of cracks and restoring the conductive network, thereby improving the battery's cycle life. The thiopolyurethane elastic polymer exhibits high elasticity and good toughness, effectively buffering the volume changes of the positive active material during lithium insertion / extraction, thus improving the battery's rate performance. The thiopolyurethane elastic polymer forms a stable and flexible coating layer on the outer surface of the positive active material, reducing direct contact between the active material and the electrolyte, minimizing side reactions, and thus inhibiting the dissolution of transition metal ions such as manganese ions and the oxidative decomposition of the electrolyte.

[0009] Furthermore, the mass ratio of the positive electrode active material, conductive agent, and binder is (90~97):(1~5):(2~8).

[0010] Controlling the mass ratio of positive electrode active material, conductive agent, and binder within the above range helps to further improve the energy density, cycle life, and safety of the battery.

[0011] Furthermore, the disulfide bond content in the thiopolyurethane elastic polymer is 2-10% by mass; and / or, the D50 particle size of the thiopolyurethane elastic polymer is 10-40 μm; and / or, the molecular weight of the thiopolyurethane elastic polymer is 80,000-120,000 g / mol; and / or, the glass transition temperature of the thiopolyurethane elastic polymer is -40 to 0 °C; and / or, the elongation at break of the thiopolyurethane elastic polymer is >300%, the tensile strength of the thiopolyurethane elastic polymer is >2.5 MPa; and / or, the thermal decomposition temperature of the thiopolyurethane elastic polymer is >350 °C.

[0012] The preferred use of the aforementioned thiopolyurethane elastic polymer helps to further improve the cycle life of the battery.

[0013] Furthermore, the mass percentage of thiopolyurethane elastic polymer in the positive electrode active coating is 0.4-3%.

[0014] Controlling the mass percentage of thiopolyurethane elastic polymer in the positive electrode active coating within the above range helps to better reduce active particle peeling and microcrack formation, as well as inhibit the dissolution of manganese ions and the oxidative decomposition of the electrolyte, thereby improving the cycle life of the battery.

[0015] Furthermore, the adhesive also includes a first adhesive, which is selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer and polyacrylic acid; the mass ratio of the first adhesive to the thiopolyurethane elastic polymer is (3~7):(3~7).

[0016] Preferably controlling the mass ratio of the first binder and the thiopolyurethane elastic polymer within the above range helps to balance the battery performance improvement effect of the thiopolyurethane elastic polymer and the adhesion performance between the positive electrode active layer and the current collector.

[0017] Furthermore, the chemical formula of lithium nickel cobalt manganese oxide material is LiNi x Co y Mn 1-x-y O2, where 0.5≤x≤0.9, 0<y≤0.2, x+y<1; and / or, the chemical formula of lithium manganese iron phosphate material is LiFe a Mn b PO4, wherein 0.1≤a≤1, 0<b≤0.9; and / or, the mass ratio of lithium nickel cobalt manganese oxide material to lithium manganese iron phosphate material is (20~80):(20~80); and / or, the conductive agent is selected from any one or more of conductive carbon black, carbon nanotubes and graphene; and / or, the current collector is aluminum foil or carbon-coated aluminum foil, and the thickness of the current collector is 8~15μm; and / or, the thickness of the positive electrode active coating is 50~180μm.

[0018] Optimizing the mass ratio of lithium nickel cobalt manganese oxide materials to lithium manganese iron phosphate materials within the above range helps to balance energy density, safety performance, and cost.

[0019] According to another aspect of the present invention, a method for preparing the above-mentioned composite positive electrode sheet is provided, the method comprising: mixing raw materials including a positive active material, a conductive agent and a binder to obtain a positive electrode slurry, and coating the positive electrode slurry on at least one side surface of a current collector to obtain a composite positive electrode sheet.

[0020] The composite positive electrode sheet obtained by the above preparation method can improve the energy density, cycle life, cycle performance and safety of the battery.

[0021] Furthermore, the above preparation method further includes: step S1, mixing raw materials including a binder and a first organic solvent to obtain a first mixture; step S2, mixing raw materials including a conductive agent and the first mixture to obtain a second mixture; step S3, mixing raw materials including a positive electrode active material, the second mixture, and a second organic solvent to obtain a positive electrode slurry; step S4, coating the positive electrode slurry onto at least one side of the current collector, and then drying, rolling, and slitting to obtain a composite positive electrode sheet.

[0022] The above preparation method helps to better prepare composite positive electrode sheets, thereby improving the cycle stability and rate performance of the battery.

[0023] Furthermore, the pressure of the first mixing is -75 to -95 kPa, the mixing time is 3 to 6 hours, and the solid content of the first mixture is 5 to 20%; and / or, the rotation speed of the second mixing is 1000 to 2500 rpm, the mixing time is 1 to 3 hours, and the solid content of the second mixture is 3 to 8%; and / or, the rotation speed of the third mixing is 300 to 800 rpm, the mixing time is 3 to 6 hours, and the solid content of the positive electrode slurry is 50 to 75%; and / or, the coating amount on one side of the current collector is 160 to 200 g / m². 2 .

[0024] The preferred conditions for the first, second, and third mixing processes are within the above-mentioned range, which helps to fully disperse the raw materials, thereby improving the consistency of the composite positive electrode sheet.

[0025] According to another aspect of the present invention, a secondary battery is provided, comprising a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the composite positive electrode described above or prepared by the preparation method described above.

[0026] Secondary batteries including the aforementioned composite positive electrode have high energy density, good rate performance, cycle performance, cycle life, and safety.

[0027] Applying the technical solution of this invention, the thiopolyurethane elastic polymer added to the positive active layer of the composite positive electrode sheet of this application has the following advantages: (1) The thiopolyurethane elastic polymer has good self-healing ability, and its dynamic disulfide bond (-SS-) is a dynamic reversible covalent bond. During the charging and discharging process, the electrode material generates stress microcracks due to volume expansion and contraction, which leads to the breakage of disulfide bonds in the polymer network. Subsequently, when subjected to external stimulation (such as temperature rise during battery charging and discharging), under the action of system entropy drive, the broken bonds can spontaneously recombine, thereby realizing the self-repair of cracks, restoring the conductive network, and suppressing the battery capacity decay caused by binder failure and active material peeling from the source, thereby improving the cycle life of the battery. (2) The thiopolyurethane elastic polymer is composed of soft segments (long-chain polyols provide elasticity) and hard segments (isocyanates provide strength), which have high elasticity and good toughness. It can effectively buffer the volume change of the positive active material during the lithium insertion and extraction process, thereby better wrapping the active particles to maintain the integrity of the electrode structure, thereby reducing polarization and improving the rate performance of the battery. Secondly, the polar groups on its molecular chain (such as urethane groups and thioether groups) can physically combine with the surface of active materials and conductive agents to form intermolecular hydrogen bonds. As reversible physical cross-linking points, they can enhance the integrity of the electrode structure, thereby inhibiting particle shedding. (3) Thiopolyurethane elastic polymers form a stable and flexible coating layer on the outer surface of the positive electrode active material, which can reduce the direct contact between the active material and the electrolyte, reduce side reactions, and thus inhibit the dissolution of transition metal ions such as manganese ions and the oxidative decomposition of the electrolyte. In addition, thioether (-S-) and other groups have a certain weak solvation effect on lithium ions, which can improve the transport of lithium ions, reduce the interfacial impedance, and thus improve the rate performance of the battery. Therefore, thiopolyurethane elastic polymers can enhance interfacial stability and improve ionic conductivity. In summary, the addition of thiopolyurethane elastic polymer to the positive electrode active layer of this application can not only solve the problem of active particle peeling and microcrack generation caused by the large difference in volume expansion rate between lithium manganese iron phosphate and lithium nickel cobalt manganese oxide materials during charging and discharging, thereby improving the cycle life of the battery, but also effectively inhibit manganese dissolution, thereby improving the battery capacity retention rate. Detailed Implementation

[0028] 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.

[0029] As analyzed in the background section of this application, existing composite cathode sheets suffer from stress accumulation due to the expansion difference between the two cathode active materials, leading to active particle peeling and microcrack formation, as well as manganese dissolution and interfacial side reactions. To solve the above problems, this application provides a composite cathode sheet, its preparation method, and a secondary battery.

[0030] In a typical embodiment of this application, a composite positive electrode sheet is provided, which includes a current collector and a positive active coating stacked on at least one side surface of the current collector. The positive active coating includes a positive active material, a conductive agent, and a binder. The positive active material includes lithium nickel cobalt manganese oxide and lithium manganese iron phosphate. The binder includes a thiopolyurethane elastic polymer.

[0031] The thiopolyurethane elastic polymer added to the positive active layer of the composite positive electrode sheet of this application has the following advantages: (1) The thiopolyurethane elastic polymer has good self-healing ability, and its dynamic disulfide bond (-SS-) is a dynamic reversible covalent bond. During the charging and discharging process, the electrode material generates stress microcracks due to volume expansion and contraction, which leads to the breakage of disulfide bonds in the polymer network. Subsequently, when subjected to external stimulation (such as temperature rise during battery charging and discharging), under the action of system entropy drive, the broken bonds can spontaneously recombine, thereby realizing the self-repair of cracks, restoring the conductive network, and suppressing the battery capacity decay caused by binder failure and active material peeling from the source, thereby improving the cycle life of the battery. (2) The thiopolyurethane elastic polymer is composed of soft segments (long-chain polyol segments provide elasticity) and hard segments (isocyanate segments provide strength), which have high elasticity and good toughness, and can effectively buffer the volume change of the positive active material during the lithium insertion and extraction process, thereby better wrapping the active particles to maintain the integrity of the electrode structure, thereby reducing polarization and improving the rate performance of the battery. Secondly, the polar groups on its molecular chain (such as urethane groups and thioether groups) can physically combine with the surface of active materials and conductive agents to form intermolecular hydrogen bonds. As reversible physical cross-linking points, they can enhance the integrity of the electrode structure, thereby inhibiting particle shedding. (3) Thiopolyurethane elastic polymers form a stable and flexible coating layer on the outer surface of the positive electrode active material, which can reduce the direct contact between the active material and the electrolyte, reduce side reactions, and thus inhibit the dissolution of transition metal ions such as manganese ions and the oxidative decomposition of the electrolyte. In addition, thioether (-S-) and other groups have a certain weak solvation effect on lithium ions, which can improve the transport of lithium ions, reduce the interfacial impedance, and thus improve the rate performance of the battery. Therefore, thiopolyurethane elastic polymers can enhance interfacial stability and improve ionic conductivity. In summary, the addition of thiopolyurethane elastic polymer to the positive electrode active layer of this application can not only solve the problem of active particle peeling and microcrack generation caused by the large difference in volume expansion rate between lithium manganese iron phosphate and lithium nickel cobalt manganese oxide materials during charging and discharging, thereby improving the cycle life of the battery, but also effectively inhibit manganese dissolution, thereby improving the battery capacity retention rate.

[0032] In one embodiment of this application, the mass ratio of the positive electrode active material, the conductive agent and the binder is (90~97):(1~5):(2~8).

[0033] Adding positive electrode active materials helps improve the battery's energy density, thermal stability, and safety. Adding conductive agents helps optimize the electrode's electronic conductivity network, thereby improving the electrode's conductivity. Adding binders not only enhances the adhesion between the positive electrode active coating and the current collector, but the thiopolyurethane elastic polymer also helps mitigate the formation of glassy active particles and microcracks caused by the different volume expansion rates of the two positive electrode active materials during battery charging and discharging, thus improving the battery's cycle stability and safety performance. Preferably controlling the mass ratio of positive electrode active material, conductive agent, and binder within the above-mentioned range helps to further improve the battery's energy density, cycle life, and safety.

[0034] In one embodiment of this application, the mass content of disulfide bonds in the thiopolyurethane elastic polymer is 2-10%, preferably 3-8%; and / or, the D50 particle size of the thiopolyurethane elastic polymer is 10-40 μm, preferably 15-30 μm; and / or, the molecular weight of the thiopolyurethane elastic polymer is 80,000-120,000 g / mol, preferably 90,000-110,000 g / mol; and / or, the glass transition temperature of the thiopolyurethane elastic polymer is -40-0℃; and / or, the elongation at break of the thiopolyurethane elastic polymer is >300%, preferably 350-600%, and the tensile strength of the thiopolyurethane elastic polymer is >2.5 MPa, preferably 3-8 MPa; and / or, the thermal decomposition temperature of the thiopolyurethane elastic polymer is >350℃, preferably 380-450℃.

[0035] The preferred disulfide bond content in the thiopolyurethane elastic polymer is within the above range, which helps to promote the full self-healing effect of dynamic disulfide bonds, thereby reducing microcracks caused by volume expansion and contraction of electrode materials during charging and discharging, restoring the conductive network, reducing battery capacity decay caused by binder failure and active material peeling, and thus improving the cycle life of the battery.

[0036] If the particle size of the thiopolyurethane elastic polymer is too small, it is prone to moisture absorption and agglomeration; if the particle size is too large, it dissolves slowly and is prone to settling in the slurry, resulting in uneven coating. Preferably, the particle size of the thiopolyurethane elastic polymer is within the above-mentioned range, which helps the thiopolyurethane elastic polymer to be evenly distributed in the positive electrode active layer, reduces agglomeration, and thus improves the overall performance of the composite positive electrode sheet.

[0037] If the molecular weight of the thiopolyurethane elastic polymer is too low, the film-forming properties and strength of the positive electrode active layer will be poor. If the molecular weight is too high, the dissolution or swelling rate will be slow, resulting in a sharp increase in the viscosity of the positive electrode slurry, making dispersion difficult and affecting the uniformity of coating. Preferably, the molecular weight of the thiopolyurethane elastic polymer is within the above-mentioned range, which helps to maintain the length of the molecular chain of the thiopolyurethane elastic polymer, thereby forming an effective mechanical entanglement and bonding network.

[0038] The glass transition temperature of thiopolyurethane elastic polymers is below room temperature, indicating that the polymer segments are in a highly elastic state with extremely strong mobility at the battery's operating temperature. This mobility facilitates the exchange reaction of dynamic disulfide bonds, allowing for sufficient contact and recombination at the crack interface, thus achieving self-healing. Preferably, the glass transition temperature of the thiopolyurethane elastic polymer is within the aforementioned range, which helps improve its self-healing ability and thereby reduces the formation of microcracks.

[0039] The preferred elongation at break of the thiopolyurethane elastic polymer is within the above range, which helps it to have high ductility, thereby improving its ability to withstand repeated volume expansion and contraction of the positive electrode active material during lithium insertion and extraction, thus buffering stress through its own deformation.

[0040] The preferred thiopolyurethane elastic polymer has a tensile strength within the above range, which helps to provide sufficient strength to resist the pressure and shear force of the electrode rolling process and maintain the structural integrity of the electrode, thereby reducing the breakage of active material particles or separation from the current collector during circulation.

[0041] The preferred thermal decomposition temperature of the thiopolyurethane elastic polymer is within the above range, which helps to improve its stability under high-temperature operation or overcharge conditions of the battery, reduce safety problems caused by thermal decomposition, and thus improve the safety of the battery.

[0042] In one embodiment of this application, the mass percentage of thiopolyurethane elastic polymer in the positive electrode active coating is 0.4-3%.

[0043] Preferably controlling the mass ratio of thiopolyurethane elastic polymer in the positive electrode active coating within the above-mentioned range helps to better reduce the peeling of active particles and the generation of microcracks caused by the large difference in volume expansion rate between lithium manganese iron phosphate and lithium nickel cobalt manganese oxide materials during charging and discharging, thereby improving the cycle life of the battery. It also helps to form a stable and flexible coating layer on the outer surface of the positive electrode active material, thereby reducing the direct contact between the active material and the electrolyte, reducing side reactions, and thus better suppressing the dissolution of manganese ions and the oxidative decomposition of the electrolyte.

[0044] In one embodiment of this application, the adhesive further includes a first adhesive, which is selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer and polyacrylic acid; the mass ratio of the first adhesive to the thiopolyurethane elastic polymer is (3~7):(3~7).

[0045] The binder also includes the first binder of the above types. The first binder has high adhesion, which helps to further improve the adhesion between the positive electrode active layer and the current collector, thereby improving the structural stability of the composite positive electrode sheet.

[0046] Preferably controlling the mass ratio of the first binder and the thiopolyurethane elastic polymer within the above range helps to balance the battery performance improvement effect of the thiopolyurethane elastic polymer and the adhesion performance between the positive electrode active layer and the current collector.

[0047] In one embodiment of this application, the chemical formula of the lithium nickel cobalt manganese oxide material is LiNi. x Co y Mn 1-x-y O2, where 0.5≤x≤0.9, 0<y≤0.2, x+y<1; and / or, the chemical formula of lithium manganese iron phosphate material is LiFe a Mn b PO4, wherein 0.1≤a≤1, 0<b≤0.9; and / or, the mass ratio of lithium nickel cobalt manganese oxide material to lithium manganese iron phosphate material is (20~80):(20~80); and / or, the conductive agent is selected from any one or more of conductive carbon black, carbon nanotubes and graphene; and / or, the current collector is aluminum foil or carbon-coated aluminum foil, and the thickness of the current collector is 8~15μm; and / or, the thickness of the positive electrode active coating is 50~180μm.

[0048] The aforementioned lithium nickel cobalt manganese oxide (LCO) material helps improve its stability and electrochemical performance at high nickel ratios, thereby enhancing the battery's energy density and cycle stability. The aforementioned lithium manganese iron phosphate (LFP) material helps improve the thermal stability and cost advantage of the composite cathode sheet. Furthermore, by adjusting the manganese and iron ratio, it helps optimize the electronic and ionic conductivity of LFP during charge and discharge. Preferably, the mass ratio of LCO to LFP is within the aforementioned range, which helps to balance energy density, safety performance, and cost.

[0049] Preferably controlling the type of conductive agent within the above-mentioned range helps to promote efficient electron transport between positive electrode active materials, thereby reducing the internal resistance of the battery.

[0050] Preferring the type and thickness of the current collector within the above-mentioned range helps to better support the positive electrode active layer while providing sufficient mechanical strength. Preferring the thickness of the positive electrode active coating within the above-mentioned range helps to improve the battery's capacity and energy density.

[0051] In another typical embodiment of this application, a method for preparing the above-mentioned composite positive electrode sheet is provided. The method includes: mixing raw materials including positive active material, conductive agent and binder to obtain positive electrode slurry, and coating the positive electrode slurry on at least one side surface of current collector to obtain composite positive electrode sheet.

[0052] This application obtains a composite positive electrode sheet by mixing a positive electrode active material, a conductive agent, and a binder containing a thiopolyurethane elastic polymer to form a positive electrode slurry, and then coating it onto at least one side of a current collector. The thiopolyurethane elastic polymer in the binder has dynamic disulfide bonds (-SS-), which enable it to self-repair microcracks caused by volume expansion and contraction of the positive electrode active material during charging and discharging, restoring the conductive network and suppressing battery capacity decay caused by binder failure and active material stripping from the source, thereby improving the battery's cycle life. The thiopolyurethane elastic polymer, with its soft segments, also possesses high elasticity and good toughness, effectively buffering the volume changes of the positive electrode active material during lithium insertion / extraction, thus better encapsulating the active particles to maintain the integrity of the electrode structure, thereby reducing polarization and improving the battery's rate performance. Furthermore, the polar groups on its molecular chain can physically bond with the surfaces of the active material and the conductive agent to form intermolecular hydrogen bonds. These bonds, acting as reversible physical cross-linking points, enhance the integrity of the electrode structure, thereby inhibiting particle shedding. Furthermore, the thiopolyurethane elastic polymer forms a stable and flexible coating layer on the outer surface of the positive electrode active material, which can reduce the direct contact between the active material and the electrolyte, reduce side reactions, and thus inhibit the dissolution of transition metal ions such as manganese ions and the oxidative decomposition of the electrolyte. Moreover, thioether and other groups have a certain weak solvation effect on lithium ions, which can improve lithium ion transport, reduce interfacial impedance, and thus improve the rate performance of the battery. Therefore, the thiopolyurethane elastic polymer can enhance interfacial stability and improve ionic conductivity. In summary, the composite positive electrode sheet of this application can improve the energy density, cycle life, cycle performance, and safety of the battery.

[0053] The above preparation method also includes pretreatment of the raw materials to obtain pretreated raw materials; wherein, the pretreatment is drying, the drying temperature is 100~160℃, and the drying time is 8~12h. Preferably, pretreatment of the above raw materials and controlling the drying temperature and time within the above range helps to fully remove moisture.

[0054] In one embodiment of this application, the above preparation method further includes: step S1, mixing raw materials including a binder and a first organic solvent to obtain a first mixture; step S2, mixing raw materials including a conductive agent and the first mixture to obtain a second mixture; step S3, mixing raw materials including a positive electrode active material, the second mixture, and a second organic solvent to obtain a positive electrode slurry; and step S4, coating the positive electrode slurry onto at least one side of the current collector, and then drying, rolling, and slitting to obtain a composite positive electrode sheet.

[0055] Step S1, involving the first mixing of the binder and the first organic solvent, helps to uniformly disperse the thiopolyurethane elastic polymer in the first organic solvent, forming a stable adhesive solution and providing a foundation for subsequent coating preparation. Step S2, involving the second mixing of the conductive agent and the first mixture, helps to fully disperse the conductive agent in the first mixture, thereby improving the electronic conductivity of the positive electrode active coating. Step S3, involving the third mixing of the positive electrode active material, the second mixture, and the second organic solvent, helps to uniformly disperse and fully mix the positive electrode active material, the conductive agent, and the binder, thus facilitating the uniform coating of the positive electrode slurry onto the current collector. Step S4 helps to improve the density of the positive electrode active coating, facilitating its close adhesion to the current collector, thereby improving the battery's cycle stability and rate performance.

[0056] In one embodiment of this application, the pressure of the first mixing is -75 to -95 kPa, the mixing time is 3 to 6 hours, and the solid content of the first mixture is 5 to 20%; and / or, the rotation speed of the second mixing is 1000 to 2500 rpm, the mixing time is 1 to 3 hours, and the solid content of the second mixture is 3 to 8%; and / or, the rotation speed of the third mixing is 300 to 800 rpm, the mixing time is 3 to 6 hours, and the solid content of the positive electrode slurry is 50 to 75%; and / or, the coating amount on one side of the current collector is 160 to 200 g / m². 2 .

[0057] Preferably, the first mixing is carried out in a high negative pressure environment. Controlling the pressure and time of the first mixing within the above-mentioned range helps the binder to be fully dispersed in the first organic solvent and adjusts the solid content of the first mixture within the above-mentioned range.

[0058] Preferably, the rotation speed and time of the second mixing are within the above range, which helps the conductive agent to be fully dispersed in the first mixture and adjusts the solid content of the second mixture to be within the above range.

[0059] Preferably, the rotation speed and time of the third mixing are within the above-mentioned range, which helps the conductive agent, binder and positive electrode active material to fully contact, thereby improving the uniformity of the positive electrode slurry and adjusting the solid content of the positive electrode slurry within the above-mentioned range. The preferred mixing method is stirring.

[0060] In another typical embodiment of this application, a secondary battery is provided, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the composite positive electrode described above or prepared by the preparation method described above.

[0061] Secondary batteries including the aforementioned composite positive electrode have high energy density, good rate performance, cycle performance, cycle life, and safety.

[0062] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0063] Example 1

[0064] Raw materials: The mass ratio of positive electrode active material, conductive agent and binder is 95:2.8:2.2.

[0065] The positive electrode active material is lithium manganese iron phosphate (LiFe) with a mass ratio of 7:3. 0.5 Mn 0.5 PO4 and lithium nickel cobalt manganese oxide materials LiNi 0.6 Co 0.2 Mn 0.2 O2.

[0066] The conductive agent is conductive carbon black.

[0067] The adhesive is a polyvinylidene fluoride and a thiolated polyurethane elastic polymer in a mass ratio of 7:3.

[0068] The disulfide bond content in the thiopolyurethane elastic polymer is 5%, the molecular weight is 100,000 g / mol, the D50 particle size is 25 μm, the glass transition temperature is -28℃, the elongation at break is 360%, the tensile strength is 2.8 MPa, and the thermal decomposition temperature is 370℃.

[0069] The preparation method of the composite positive electrode is as follows:

[0070] The above raw materials are pretreated by baking at 120℃ for 10 hours.

[0071] The binder was dispersed in the first organic solvent N-methylpyrrolidone and mixed for 4 hours under a high negative pressure of -80 kPa to obtain a first mixture with a solid content of 12%.

[0072] The conductive agent was dispersed in the first mixture and then mixed again at 2000 rpm for 2 hours to obtain a second mixture with a solid content of 5%.

[0073] The positive electrode active material was added to the second mixture, and the second organic solvent N-methylpyrrolidone was added. The mixture was then mixed for 5 hours at 500 rpm to obtain a positive electrode slurry with a solid content of 62%.

[0074] After coating the positive electrode slurry onto both surfaces of a 10 μm thick aluminum current collector foil, the composite positive electrode sheet is obtained through drying, rolling, and slitting. The coating amount on each side of the current collector is 175 g / m². 2 The mass percentage of thiopolyurethane elastic polymer in the positive electrode active layer is 0.66%.

[0075] Example 2

[0076] The difference from Example 1 is that the mass ratio of polyvinylidene fluoride and thiopolyurethane elastic polymer is 6:4, and the final composite positive electrode sheet is obtained, in which the mass percentage of thiopolyurethane elastic polymer in the positive electrode active layer is 0.88%.

[0077] Example 3

[0078] The difference from Example 1 is that the mass ratio of polyvinylidene fluoride and thiopolyurethane elastic polymer is 5:5, and the final composite positive electrode sheet is obtained, in which the mass ratio of thiopolyurethane elastic polymer in the positive electrode active layer is 1.1%.

[0079] Example 4

[0080] The difference from Example 1 is that the mass ratio of polyvinylidene fluoride and thiopolyurethane elastic polymer is 4:6, and the final composite positive electrode sheet is obtained, in which the mass ratio of thiopolyurethane elastic polymer in the positive electrode active layer is 1.32%.

[0081] Example 5

[0082] The difference from Example 1 is that the mass ratio of polyvinylidene fluoride and thiopolyurethane elastic polymer is 3:7, and the final composite positive electrode sheet is obtained, in which the mass percentage of thiopolyurethane elastic polymer in the positive electrode active layer is 1.54%.

[0083] Example 6

[0084] The difference from Example 1 is that the mass content of disulfide bonds in the thiopolyurethane elastic polymerization is 3%, the molecular weight is 80000 g / mol, the D50 particle size is 40 μm, the glass transition temperature is -35℃, the elongation at break is 320%, the tensile strength is 3.0 MPa, and the thermal decomposition temperature is 360℃, finally obtaining a composite positive electrode sheet.

[0085] Example 7

[0086] The difference from Example 1 is that the mass content of disulfide bonds in the thiopolyurethane elastic polymerization is 8%, the molecular weight is 120000 g / mol, the D50 particle size is 10 μm, the glass transition temperature is -25℃, the elongation at break is 420%, the tensile strength is 4.2 MPa, and the thermal decomposition temperature is 390℃, finally obtaining a composite positive electrode sheet.

[0087] Example 8

[0088] The difference from Example 1 is that the mass content of disulfide bonds in the thiopolyurethane elastic polymerization is 15%, and a composite positive electrode sheet is finally obtained.

[0089] Example 9

[0090] The difference from Example 1 is that the mass ratio of positive electrode active material, conductive agent and binder is 94:3:3, the mass ratio of lithium nickel cobalt manganese oxide material and lithium manganese iron phosphate material is 5:5, and the mass percentage of thiopolyurethane elastic polymer in the positive electrode active layer is 0.90%, finally obtaining a composite positive electrode sheet with a current collector thickness of 8μm and a positive electrode active coating thickness of 80μm.

[0091] Example 10

[0092] The difference from Example 1 is that the mass ratio of positive electrode active material, conductive agent and binder is 96:2.5:1.5, the mass ratio of lithium nickel cobalt manganese oxide material and lithium manganese iron phosphate material is 3:7, and the mass percentage of thiopolyurethane elastic polymer in the positive electrode active layer is 4%, finally obtaining a composite positive electrode sheet with a current collector thickness of 15μm and a positive electrode active coating thickness of 120μm.

[0093] Comparative Example 1

[0094] The difference from Example 1 is that no thiopolyurethane elastic polymer is added, resulting in a composite positive electrode sheet.

[0095] Comparative Example 2

[0096] The difference from Example 1 is that the thiopolyurethane elastic polymer is replaced with a polyurethane elastic polymer. The polyurethane elastic polymer has an elongation at break of 280%, a tensile strength of 2.0 MPa, and a thermal decomposition temperature of 340°C, ultimately yielding a composite positive electrode sheet.

[0097] Comparative Example 3

[0098] The difference from Example 1 is that the thiopolyurethane elastic polymer is replaced with an acrylate copolymer elastomer (LA133 type, glass transition temperature of -25°C, elongation at break of 310%, tensile strength of 3.5MPa), which does not contain sulfur, and the final composite positive electrode is obtained.

[0099] Test method:

[0100] Preparation of negative electrode sheet:

[0101] Artificial graphite (anode active material), conductive carbon black, styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were dissolved in deionized water at a weight ratio of 95:2:1.5:1.5 and mixed thoroughly to prepare a cathode slurry. The cathode slurry was then coated onto copper foil (cathode current collector), dried, rolled, and slit to obtain the cathode sheet.

[0102] Preparation of lithium-ion batteries:

[0103] The composite positive electrode, separator and negative electrode are stacked in sequence to obtain a bare cell. After shaping, it is vacuum baked for 12 hours to remove water. Then, electrolyte is injected and it is sealed to obtain a non-charged battery. After going through the processes of standing, formation and capacity testing, a lithium-ion stacked battery is obtained.

[0104] Electrochemical testing:

[0105] At 25°C, the battery is charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage of 4.25V to a current of 0.05C. After resting for 30 minutes, it is discharged at 0.33C to 2.5V. The resulting capacity is recorded as the initial capacity C1. The above steps are repeated for the same battery, and the discharge capacity C of the battery after the Xth cycle is recorded. X Then, the battery capacity retention rate A after each cycle X =C X / C1×100%. Record the maximum number of cycles when the battery cycle capacity retention rate is not less than 80%.

[0106] The test results are shown in Table 1.

[0107] Table 1

[0108]

[0109] As can be seen from the above, compared with Comparative Example 1, the addition of thiopolyurethane elastic polymer to the composite positive electrode sheet in the embodiments of this application can effectively improve the cycle life of the battery. This proves that the addition of thiopolyurethane elastic polymer can not only solve the problem of active particle peeling and microcrack generation caused by the large difference in volume expansion rate between lithium manganese iron phosphate material and lithium nickel cobalt manganese oxide material during charging and discharging, thereby improving the cycle life of the battery, but also effectively inhibit manganese dissolution, thereby improving the capacity retention rate of the battery.

[0110] Compared to Comparative Example 3, which uses a sulfur-free acrylate copolymer elastomer, although it possesses a certain degree of flexibility to buffer some volume expansion, its cycle life is far lower than that of the embodiments in this application due to the lack of self-healing function of dynamic disulfide bonds. This further demonstrates that elastic buffering alone, without the ability to repair reversible bonds, is insufficient to fundamentally solve the problem of cycle degradation in lithium manganese iron phosphate / lithium nickel cobalt manganese oxide composite electrodes.

[0111] Compared to Comparative Example 2, which uses a common polyurethane elastic polymer, its low tensile strength makes it prone to fracture and breakage under repeated stress, even inferior to the pure PVDF system (Comparative Example 1). In contrast, the sulfur-containing polyurethane elastic polymer of this application combines high elasticity, appropriate strength, and dynamic disulfide bond self-healing capability, resulting in a significantly improved cycle life, fully demonstrating the crucial role of sulfur-containing dynamic covalent bonds.

[0112] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0113] The thiopolyurethane elastic polymer added to the positive active layer of the composite positive electrode sheet of this application has the following advantages: (1) The thiopolyurethane elastic polymer has good self-healing ability, and its dynamic disulfide bond (-SS-) is a dynamic reversible covalent bond. During the charging and discharging process, the electrode material generates stress microcracks due to volume expansion and contraction, which leads to the breakage of disulfide bonds in the polymer network. Subsequently, when subjected to external stimulation (such as temperature rise during battery charging and discharging), under the action of system entropy drive, the broken bonds can spontaneously recombine, thereby realizing the self-repair of cracks, restoring the conductive network, and suppressing the battery capacity decay caused by binder failure and active material peeling from the source, thereby improving the cycle life of the battery. (2) The thiopolyurethane elastic polymer is composed of soft segments (long-chain polyol provides elasticity) and hard segments (isocyanate provides strength), which have high elasticity and good toughness. It can effectively buffer the volume change of the positive active material during the lithium insertion and extraction process, thereby better wrapping the active particles to maintain the integrity of the electrode structure, thereby reducing polarization and improving the rate performance of the battery. Secondly, the polar groups on its molecular chain (such as urethane groups and thioether groups) can physically combine with the surface of active materials and conductive agents to form intermolecular hydrogen bonds. As reversible physical cross-linking points, they can enhance the integrity of the electrode structure, thereby inhibiting particle shedding. (3) Thiopolyurethane elastic polymers form a stable and flexible coating layer on the outer surface of the positive electrode active material, which can reduce the direct contact between the active material and the electrolyte, reduce side reactions, and thus inhibit the dissolution of transition metal ions such as manganese ions and the oxidative decomposition of the electrolyte. In addition, thioether (-S-) and other groups have a certain weak solvation effect on lithium ions, which can improve the transport of lithium ions, reduce the interfacial impedance, and thus improve the rate performance of the battery. Therefore, thiopolyurethane elastic polymers can enhance interfacial stability and improve ionic conductivity. In summary, the addition of thiopolyurethane elastic polymer to the positive electrode active layer of this application can not only solve the problem of active particle peeling and microcrack generation caused by the large difference in volume expansion rate between lithium manganese iron phosphate and lithium nickel cobalt manganese oxide materials during charging and discharging, thereby improving the cycle life of the battery, but also effectively inhibit manganese dissolution, thereby improving the battery capacity retention rate.

[0114] 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 composite positive electrode sheet, the composite positive electrode sheet comprising a current collector and a positive active coating laminated on at least one surface of the current collector, characterized in that, The positive electrode active coating includes a positive electrode active material, a conductive agent, and a binder; The positive electrode active material includes lithium nickel cobalt manganese oxide and lithium manganese iron phosphate. The adhesive comprises a thiopolyurethane elastic polymer.

2. The composite positive electrode sheet according to claim 1, characterized in that, The mass ratio of the positive electrode active material, the conductive agent and the binder is (90~97):(1~5):(2~8).

3. The composite positive electrode sheet according to claim 1, characterized in that, The mass content of disulfide bonds in the thiopolyurethane elastic polymer is 2-10%; and / or, the D50 particle size of the thiopolyurethane elastic polymer is 10-40 μm; and / or, the molecular weight of the thiopolyurethane elastic polymer is 80,000-120,000 g / mol; and / or, the glass transition temperature of the thiopolyurethane elastic polymer is -40 to 0 °C. And / or, the elongation at break of the thiopolyurethane elastic polymer is >300%, and the tensile strength of the thiopolyurethane elastic polymer is >2.5MPa; and / or, the thermal decomposition temperature of the thiopolyurethane elastic polymer is >350℃.

4. The composite positive electrode sheet according to any one of claims 1 to 3, characterized in that, The mass percentage of the thiopolyurethane elastic polymer in the positive electrode active coating is 0.4-3%.

5. The composite positive electrode sheet according to any one of claims 1 to 3, characterized in that, The adhesive further includes a first adhesive, which is selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer and polyacrylic acid; the mass ratio of the first adhesive to the thiopolyurethane elastic polymer is (3~7):(3~7).

6. The composite positive electrode sheet according to any one of claims 1 to 3, characterized in that, The chemical formula of the lithium nickel cobalt manganese oxide material is LiNi x Co y Mn 1-x-y O2, wherein 0.5≤x≤0.9, 0<y≤0.2, x+y<1; and / or, the chemical formula of the lithium manganese iron phosphate material is LiFe a Mn b PO4, wherein 0.1≤a≤1, 0<b≤0.9; and / or, the mass ratio of the lithium nickel cobalt manganese oxide material to the lithium manganese iron phosphate material is (20~80):(20~80); And / or, the conductive agent is selected from any one or more of conductive carbon black, carbon nanotubes, and graphene; And / or, the current collector is aluminum foil or carbon-coated aluminum foil, and the thickness of the current collector is 8~15μm; and / or, the thickness of the positive electrode active coating is 50~180μm.

7. A method for preparing the composite positive electrode sheet according to any one of claims 1 to 6, characterized in that, The preparation method includes: mixing raw materials including positive electrode active material, conductive agent and binder to obtain positive electrode slurry, coating the positive electrode slurry on at least one side of the current collector to obtain the composite positive electrode sheet.

8. The preparation method according to claim 7, characterized in that, The preparation method further includes: Step S1: The raw materials including the adhesive and the first organic solvent are mixed for the first time to obtain a first mixture; Step S2: The raw materials including the conductive agent and the first mixture are mixed a second time to obtain a second mixture; Step S3: The raw materials including the positive electrode active material, the second mixture, and the second organic solvent are mixed in a third mixture to obtain the positive electrode slurry; Step S4: After coating the positive electrode slurry onto at least one side of the current collector, the composite positive electrode sheet is obtained by drying, rolling and slitting.

9. The preparation method according to claim 8, characterized in that, The pressure of the first mixture is -75 to -95 kPa, the mixing time is 3 to 6 hours, and the solid content of the first mixture is 5 to 20%. And / or, the second mixing speed is 1000~2500 rpm, the second mixing time is 1~3 h, and the solid content of the second mixture is 3~8%; And / or, the rotation speed of the third mixing is 300~800 rpm, the mixing time is 3~6 h, and the solid content of the positive electrode slurry is 50~75%; And / or, the coating amount on one side of the current collector is 160~200 g / m². 2 .

10. A secondary battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The positive electrode sheet is a composite positive electrode sheet according to any one of claims 1 to 6 or is prepared by the preparation method according to any one of claims 7 to 9.

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