A secondary battery

Abstract

This invention provides a secondary battery. The separator includes a high-temperature resistant ceramic coating attached to both sides of a porous base membrane, and a composite adhesive coating attached to both sides of the high-temperature resistant ceramic coating. The composite adhesive coating is a composite emulsion of polymethyl methacrylate and polyvinylidene fluoride-hexafluoropropylene. The glass transition temperature of the polymethyl methacrylate is 30-35°C, the hot-pressing temperature is 60-80°C, the hot-pressing pressure is 1-2 MPa, and the hot-pressing time is 30-60 s. This invention improves the peel force between the adhesive layer on the separator surface and the electrode, thus forming a stable "face-to-face" bonding interface between the separator and the positive and negative electrodes. This adhesive layer can construct a three-dimensional mesh constraint structure inside the entire cell, effectively "anchoring" the positive and negative electrodes to the separator, limiting the non-uniform expansion and contraction displacement of the electrodes in the thickness direction and in the plane, thereby fundamentally suppressing wrinkling, warping, or wavy wrinkles caused by uneven stress on the electrodes.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more particularly to a secondary battery. Background Technology

[0002] Traditional polyolefin separators (polyethylene PE and polypropylene PP) have long dominated the market due to their excellent film-forming properties, chemical stability, and closed-cell function. However, polyolefin materials have relatively low heat distortion temperatures (PE approximately 130°C and PP approximately 150°C). Under abnormal temperature rise conditions such as internal short circuits or overcharging in batteries, the separators are prone to significant thermal shrinkage or even melting, leading to large-area direct contact between the positive and negative electrodes and causing thermal runaway. Especially in high-energy-density battery systems, lithium dendrites deposited on the negative electrode surface may pierce the separator, further amplifying safety risks. To address these issues, the industry commonly employs a technique of coating the polyolefin-based membrane with an inorganic ceramic layer. Commonly used ceramic materials include alumina (Al2O3), boehmite (γ-AlOOH), silicon dioxide (SiO2), and titanium dioxide (TiO2), which are coated with aqueous or oil-based slurries, dried, and cured to form a porous coating. The ceramic layer has good heat resistance (withstanding ≥180℃) and mechanical strength, which can significantly suppress the thermal shrinkage of the diaphragm at high temperatures. At the same time, its rich pore structure is conducive to electrolyte wetting and ion transport.

[0003] The ceramic layer exhibits excellent heat resistance (withstanding thermal shrinkage at temperatures ≥180℃), while its abundant porous structure facilitates electrolyte wetting and ion transport. Depending on the coating method, it can be categorized as single-sided or double-sided coating. Initially, single-sided ceramic coating primarily provided thermal protection for the positive electrode side. However, with increasingly stringent safety requirements (such as meeting GB38031-2020 needle penetration and hot box tests), this invention favors using a double-sided ceramic-coated separator with better heat resistance to achieve similar thermal isolation and puncture resistance on the negative electrode side, thereby comprehensively enhancing the cell's safety margin.

[0004] However, in actual large-scale production and long-term cycle verification, it was found that while double-sided ceramic-coated or double-sided special (cellulose, aramid, polyimide PI) "double-sided high-heat-resistant separators" provide excellent thermal safety, they also cause a highly hazardous engineering problem—severe and irreversible wrinkling at the negative electrode interface. Specifically, after the cell completes the electrolyte injection, formation, aging, and capacity testing processes, disassembling the cell and observing the negative electrode reveals obvious wavy undulations, strip-like protrusions, or fish-scale wrinkles on its surface. Especially in the middle region of the electrode or near the overlapping seam of the separator, the wrinkle density and depth are significantly higher than those of cells assembled using single-sided ceramic-coated separators or pure polyolefin separators without ceramic coating. More importantly, the degree of wrinkling at the negative electrode interface corresponding to double-sided ceramic separators intensifies with increasing ceramic coating thickness, decreasing ceramic particle size, and increasing coating surface density, showing a clear positive correlation.

[0005] The presence of wrinkles at the negative electrode interface is by no means a simple "appearance defect." Figure 1 and Figure 2As shown, the negative impact of wrinkles on battery performance is multi-dimensional and interconnected. First, wrinkles cause unevenness on the surface of the negative electrode, resulting in uneven adhesion between the separator and the negative electrode along the electrode plane during subsequent use. In the troughs (recesses) of the wrinkles, local voids may form between the separator and the negative electrode, reducing electrolyte retention and lengthening ion transport paths. This area is prone to insufficient lithium-ion supply during charging and discharging, leading to decreased utilization of active materials. Conversely, in the peaks (protrusions) of the wrinkles, the separator is locally compressed, causing a sharp increase in interfacial pressure. This may accelerate the wear or peeling of the separator ceramic layer. Furthermore, the negative electrode material in this area is subject to additional constraints during cycle volume expansion, exacerbating particle breakage and repeated rupture and regeneration of the solid electrolyte interphase (SEI), ultimately inducing preferential nucleation and growth of lithium dendrites. Numerous studies have shown that the problem of wrinkles exacerbated by double-sided high-heat-resistant separators can significantly increase the lithium plating area on the negative electrode surface, far exceeding that of single-sided ceramic separator systems. Secondly, wrinkles at the negative electrode interface disrupt the stress balance of the electrode. During cell formation and cycling, the negative electrode undergoes periodic volume expansion and contraction due to lithium ion insertion / extraction (graphite negative electrode volume expansion rate is about 10%–15%, while silicon-based negative electrodes can reach over 200%). A flat negative electrode can uniformly disperse volume stress through its own elastic deformation and the compliant contact of the separator; however, once wrinkles form on the negative electrode, stress concentrates at the peak-valley junction of the wrinkles, forming a local high-strain zone. This leads to peeling or crack propagation between the negative electrode coating and the current collector (copper foil), causing active material to detach and accumulate on the separator surface. In severe cases, it can even puncture the separator and cause an internal short circuit. Furthermore, wrinkles at the negative electrode interface also cause uneven cell thickness, resulting in differences in pressure distribution between different cells during battery module or battery pack assembly, accelerating inconsistent degradation, and directly affecting the safety and lifespan of the entire pack.

[0006] In existing technologies, some mitigation measures have been proposed to address the problem of wrinkles at the negative electrode interface, such as: increasing the aging time after cold pressing of the electrode to release internal stress; optimizing the hot pressing parameters of the cell (temperature, pressure, time); and using a negative electrode formulation with gradually varying compaction density. However, these methods fail to fundamentally solve the mechanical and thermal mismatch problem caused by the double-sided high-heat-resistant separator, and are often accompanied by side effects such as increased cost, reduced energy density, or narrowed process window. For example, adding too much binder can reduce the ionic conductivity of the negative electrode and increase polarization; excessive cold pressing may cause the negative electrode material particles to break; and increasing the hot pressing temperature may induce separator pores or ceramic layer detachment.

[0007] Therefore, there is an urgent need to develop a secondary battery to solve the problem of negative electrode interface wrinkling. Summary of the Invention

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A secondary battery includes a bare cell formed by sequentially stacking a positive electrode, a separator, and a negative electrode. The separator comprises a porous base film, a high-temperature resistant ceramic coating adhered to both sides of the porous base film, and a composite adhesive coating adhered to both sides of the high-temperature resistant ceramic coating. The composite adhesive coating is a composite emulsion of polymethyl methacrylate (PMMA) and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), wherein the glass transition temperature of the PMMA is 30–35°C, and the hot-pressing process of the bare cell is as follows: the preheating temperature of the bare cell is 60–90°C, the preheating time is 20–40 min, the hot-pressing temperature is 60–80°C, the hot-pressing pressure is 1–2 MPa, and the hot-pressing time is 30–60 s.

[0009] The temperature, pressure, and time during preheating / hot pressing must be controlled within appropriate ranges. This is because: if the temperature is too low or the time is too short, the binder will not be able to soften sufficiently to form a strong bond; if the pressure is too low or the preheating / hot pressing time is too short, the bond between the effective binder and the electrode will be insufficient, resulting in a weak bond between the separator and the electrode, which will not be able to restrain the expansion of the cell after subsequent electrolyte wetting / charging and discharging of the positive and negative electrodes; if the temperature is too high, the pressure is too high, or the time is too long, the bond will be too strong, reducing electrolyte wetting and causing problems such as black spots / lithium plating on the electrode after formation.

[0010] In some embodiments of the present invention, the glass transition temperature of polymethyl methacrylate is 30-35°C, including but not limited to: 30°C, 31°C, 32°C, 33°C, 34°C, and 35°C.

[0011] In some embodiments of the present invention, the preheating temperature of the soft-pack bare battery cell is 60-90°C, including but not limited to: 65-90°C, 65-85°C, 70-85°C, 70-80°C, and 75-80°C.

[0012] In some embodiments of the present invention, the preheating time is 20 to 40 minutes, including but not limited to: 20 to 35 minutes, 25 to 35 minutes, 25 to 30 minutes, and 27 to 28 minutes.

[0013] In some embodiments of the present invention, the hot pressing temperature is 60-80°C, including but not limited to: 65-80°C, 65-75°C, 65-70°C, and 70-75°C.

[0014] In some embodiments of the present invention, the hot pressing pressure is 1 to 2 MPa, including but not limited to: 1.0 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, and 2.0 MPa.

[0015] In some embodiments of the present invention, the hot pressing time is 30 to 60 seconds, including but not limited to: 30 to 55 seconds, 35 to 60 seconds, 35 to 55 seconds, 40 to 55 seconds, 35 to 50 seconds, 40 to 50 seconds, and 45 seconds.

[0016] This solution adopts a three-layer composite separator structure consisting of a porous base membrane, a double-sided high-temperature resistant ceramic coating, and a double-sided composite adhesive coating. The high-temperature resistant ceramic coating can significantly improve the thermal stability of the separator, effectively suppressing the thermal shrinkage of the separator during high-temperature battery operation, hot pressing process, and charge and discharge process, eliminating the problem of short circuit between positive and negative electrodes caused by separator shrinkage, and improving the overall safety performance of the battery. The surface composite adhesive coating adopts a low-Tg (30~35℃) PMMA and PVDF-HFP composite system. Compared with traditional high-Tg adhesive materials, it has excellent low-temperature softening performance, can adapt to the battery manufacturing temperature conditions, and easily achieves tight bonding between the separator and the positive and negative electrode sheets.

[0017] By defining precise ranges for cell preheating and hot pressing process parameters, the binder can be fully softened and activated, ensuring a high-strength, uniform surface bonding interface between the separator and the electrode. This effectively solves the problem of separator adhesion attenuation caused by double-sided ceramic coatings or high heat-resistant layers, thus stabilizing the internal structure of the cell. It also avoids defects such as insufficient bonding, electrolyte wetting obstruction, electrode black spots, and lithium plating caused by improper temperature, pressure, and time parameters.

[0018] More preferably, the thickness of a single layer of the high-temperature resistant ceramic coating is 1.0 to 2.0 μm, and the thickness of a single layer of the composite adhesive coating is 1.0 to 2.0 μm.

[0019] In some embodiments of the present invention, the single-layer thickness of the high-temperature resistant ceramic coating and the composite adhesive coating is 1.0 to 2.0 μm, including but not limited to: 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, and 2.0 μm.

[0020] More preferably, the high-temperature resistant ceramic coating comprises, by weight, the following raw materials: 2-5 parts of modified acrylic-modified polyurethane composite emulsion, 40-50 parts of inorganic nano-ceramic particles, 5-10 parts of binder, 0.5-1.0 parts of additives, and 45-50 parts of deionized water.

[0021] Further preferably, the inorganic nano-ceramic particles are one or more of nano-alumina, nano-silicon oxide, nano-zirconia, nano-magnesium oxide, nano-boehmite, nano-magnesium hydroxide, nano-aluminum hydroxide, and nano-silicon nitride.

[0022] By limiting the single-layer thickness of the high-temperature resistant ceramic coating and the composite bonding coating to an optimal range of 1.0–2.0 μm, a balance between protective performance and electrochemical performance is achieved. The controllable and uniform thickness of the ceramic coating maximizes its functions of high-temperature resistance, thermal shrinkage prevention, and insulation protection, without clogging the membrane pores or increasing ion transport resistance due to excessive coating thickness. The moderate thickness of the composite bonding coating ensures sufficient bonding active sites for strong adhesion between the membrane and the electrode, while avoiding the problems of decreased membrane porosity, reduced electrolyte wetting rate, and increased battery internal resistance caused by excessive coating thickness. This effectively ensures the battery's ion conduction efficiency and interfacial bonding stability, balancing battery safety and electrochemical kinetic performance.

[0023] More preferably, the composite adhesive coating comprises, by weight, the following raw materials: 40-55 parts solvent, 28-35 parts polymethyl methacrylate (PMMA), 6-9 parts polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), 6-8 parts adhesive, 0.5-3 parts dispersant, 0.2-0.5 parts wetting agent, and 0.1-0.2 parts defoamer.

[0024] More preferably, the steps of the composite adhesive coating preparation method are as follows: S1: Solvent premixing: Weigh 40-55 parts of the polar organic solvent deionized water according to the formula and place it in a high-speed dispersion kettle.

[0025] S2: Add dispersant, wetting agent, and defoamer. Add 0.5-3 parts of dispersant, 0.2-0.5 parts of wetting agent, and 0.1-0.2 parts of defoamer to the solvent. Start stirring until completely dissolved. Stirring speed is 200-500 r / min, and time is 20-30 min.

[0026] S3: Add PVDF-HFP and dissolve it. Add 6 to 9 parts of PVDF-HFP copolymer to the reactor and continue stirring until completely dissolved. The stirring speed is 200 to 500 r / min and the time is 30 to 50 min to obtain a transparent or micro-emulsified fluoropolymer solution. Depending on the dissolution, the temperature can be appropriately heated to 40 to 60℃.

[0027] S4: Add PMMA and disperse. Slowly add 28-35 parts of PMMA to the above fluoropolymer solution and continue stirring until completely dispersed. Preferably, the temperature can be appropriately heated to 40-60°C to accelerate the dispersion of PMMA, and the stirring time is 60-120 min.

[0028] S5: Add binder polyacrylic acid, stirring at 200-500 r / min for 30-50 min.

[0029] S6: High-speed homogenization, increase the stirring rate to 1500-3000 r / min, and disperse at high speed for 15-30 min to ensure that all components are fully mixed and uniform.

[0030] S7: Ball milling: The mixed slurry obtained in step S5 is transferred to a ball mill for closed-loop grinding. The ball milling time is 1 to 2 hours, so that the polymer PMMA and PVDF-HFP can be uniformly blended at the molecular level in the solvent. At the same time, the wetting agent and defoamer are further dispersed to form a polymer blend slurry with stable dispersion and good flowability.

[0031] S8: Filtration and vacuum degassing: Filter the slurry obtained in step S6 through a 200-400 mesh filter and degas it under vacuum for 5-15 minutes to obtain the finished blended slurry.

[0032] Product morphology and properties: The composite adhesive coating of this invention is a viscous, semi-transparent to white, uniform slurry with a solid content of 20%–45% and a viscosity of 100–3000 mPa·s (25℃). Due to the addition of the wetting agent, the contact angle of the slurry on substrates such as polyethylene, polypropylene, or other special membranes (PI, cellulose, nonwoven fabric) is significantly reduced, resulting in rapid spreading. The addition of the defoamer effectively suppresses the generation of bubbles during stirring, ball milling, and coating processes, resulting in a wet film free of pinholes, shrinkage cavities, and other defects after coating. After drying at 40–90℃, a uniform and dense PMMA-PVDF-HFP blended composite membrane is obtained.

[0033] This formulation system features a scientifically precise ratio of components, suitable for both separator coating and cell application conditions, and boasts multiple technical advantages: First, the specific ratio of PMMA and PVDF-HFP, combined with low Tg characteristics, constructs a flexible and high-strength bonding network, perfectly adapting to cell expansion and contraction deformation, fundamentally solving the problems of negative electrode wrinkling and interface lithium plating; Second, the appropriate amount of solvent ensures that the slurry solid content and viscosity are within the optimal range, ensuring uniform coating and the absence of pinholes and shrinkage defects; Third, the precisely proportioned dispersants, wetting agents, and defoamers achieve uniform dispersion of each component at the molecular level, improving slurry stability and flowability, eliminating problems such as bubbles, agglomeration, and uneven spreading during coating and drying, and ensuring coating quality; Fourth, the appropriate amount of binder ensures both the bonding strength of the separator-electrode interface and avoids excessively blocking electrolyte penetration, balancing bonding performance and wetting performance, significantly improving battery manufacturing yield and performance consistency.

[0034] More preferably, the composite adhesive coating is applied by at least one of the following methods: microgravure coating, gravure roller coating, slot extrusion coating, dip coating, spray coating, electrospinning coating, and dot coating.

[0035] More preferably, the polyvinylidene fluoride-hexafluoropropylene copolymer comprises, by weight, the following raw materials: 55-75 parts solvent, 24-30 parts vinylidene fluoride, 14-18 parts hexafluoropropylene, 0.1-0.2 parts ammonium perfluorooctanoate, 0.01-0.03 parts ammonium persulfate, 0.3-0.5 parts paraffin wax, 0.01-0.03 parts sodium bisulfite, 0.2-0.3 parts carboxymethyl cellulose, and 0.2-0.4 parts sodium sulfate.

[0036] More preferably, the preparation method of the polyvinylidene fluoride-hexafluoropropylene copolymer includes the following steps: S1: The reaction is carried out under inert gas protection. High-purity nitrogen (purity ≥99.99%) is used. Nitrogen gas is introduced into the reaction vessel to a pressure of 0.1-0.2 MPa for protection. 55-75 parts of the polar organic solvent (deionized water) of the formula amount are weighed and placed in a high-speed dispersion vessel.

[0037] S2: Add 24-30 parts of vinylidene fluoride (VDF), turn on the stirrer until completely dissolved, stir rate 200-400 r / min, time 20-20 min.

[0038] S3: Add 14-18 parts of hexafluoropropylene (HFP), start stirring until completely dissolved, stirring speed 200-400 r / min, time 20-30 min.

[0039] S4: Add 0.2 to 0.3 parts of carboxymethyl cellulose slowly until completely dissolved, stirring at a speed of 200 to 400 r / min for 10 to 20 min.

[0040] S5: Add 0.1-0.2 parts of ammonium perfluorooctanoate, 0.01-0.03 parts of ammonium persulfate (APS), 0.3-0.5 parts of paraffin, 0.01-0.03 parts of sodium bisulfite, and 0.2-0.4 parts of sodium sulfate. Start stirring until completely dissolved at a stirring speed of 200-400 r / min for 40-60 min.

[0041] S6: Maintain the reaction temperature at 55±5℃ and the pressure at 2~3MPa, homogenize at high speed, increase the stirring rate to 2000~3000 r / min, disperse at high speed for 40~60 min, so that all components are fully mixed and uniform.

[0042] S7: Filtration and vacuum degassing: Filter the slurry obtained in step S6 through a 200-400 mesh filter and degas it under vacuum for 5-15 minutes to obtain the finished blended slurry (solid content of 25%-30%).

[0043] S8: Post-discharge processing: After the reaction is complete, stop stirring, turn off the heating, cool to below 35°C, open the bottom discharge valve, release the reaction emulsion, and place it in a clean, dust-proof container for later use.

[0044] This PVDF-HFP copolymer formulation is rationally proportioned, enabling the preparation of high-performance fluorinated copolymer substrates suitable for composite adhesive coatings: the specific ratio of VDF and HFP monomers allows for precise control of the copolymer's flexibility, electrolyte resistance, and adhesive compatibility, making it suitable for long-term immersion in lithium battery electrolytes and preventing coating swelling, peeling, and failure. Ammonium perfluorooctanoate acts as an emulsifier, while ammonium persulfate and sodium bisulfite form a redox initiation system, ensuring a mild and thorough copolymerization reaction and improving polymer molecular weight uniformity and system stability. The synergistic effect of additives such as paraffin, carboxymethyl cellulose, and sodium sulfate effectively improves the dispersibility, leveling, and formability of the polymer slurry, reducing particle agglomeration and sedimentation during polymerization. The resulting PVDF-HFP copolymer exhibits excellent compatibility with PMMA, forming a uniform and stable composite coating that significantly enhances the weather resistance, adhesion durability, and electrochemical stability of the separator coating.

[0045] More preferably, the adhesive is one or more of polyacrylic acid, acrylate, and polyvinyl alcohol.

[0046] One or more of polyacrylic acid, acrylate, and polyvinyl alcohol are selected as binders to adapt to this PMMA / PVDF-HFP composite coating system, exhibiting excellent comprehensive performance: This type of binder has extremely strong compatibility with fluoropolymers, PMMA, and ceramic coating substrates, forming a dense and interwoven bonding network, significantly improving the adhesion between the coating and the separator substrate, and between the coating and the electrode; it also has good low-temperature softening characteristics, resistance to electrolyte corrosion, and electrochemical inertness, and will not decompose or fail under lithium battery charging and discharging conditions and high-temperature storage conditions, maintaining long-term interfacial bonding stability; moreover, this type of binder has excellent flexibility, can adaptively expand and contract with the volume deformation of the electrode, effectively buffering the internal stress of the cell, continuously suppressing electrode wrinkles and interfacial delamination, and ensuring the long-term cycle performance stability of the battery.

[0047] More preferably, the dispersant is one or more of carboxymethyl cellulose, perfluoroalkylamine oxide, nonylphenol polyoxyethylene ether, and fatty alcohol polyoxyethylene ether.

[0048] By selecting composite dispersants such as carboxymethyl cellulose and perfluoroalkylamine oxide, the dispersion problem of fluoropolymer and PMMA mixtures can be specifically solved. These dispersants can effectively reduce the surface tension of the slurry system, coat polymer particles, and prevent the agglomeration and sedimentation of PVDF-HFP and PMMA particles, achieving uniform dispersion of each component. At the same time, they can improve the storage stability of the slurry, avoid long-term stratification and deterioration, and are suitable for industrial mass production. In addition, these dispersants have good compatibility with system additives, leave no electrochemically active residues, and will not produce side reactions or increase battery internal resistance during battery operation. They can perfectly preserve the core adhesion and protective properties of the coating while ensuring the processing performance of the slurry.

[0049] More preferably, the wetting agent is one or more of polyether-modified polysiloxane wetting agents and acetylenic diol nonionic wetting agents.

[0050] By selecting polyether-modified polysiloxane and acetylenol-based nonionic wetting agents, which are suitable for aqueous composite emulsion systems, the coating application and forming effect can be significantly optimized. These wetting agents can greatly reduce the contact angle of the coating slurry on the ceramic separator surface, improve the slurry spreading speed and uniformity, solve the problems of hydrophobicity and poor spreading of fluorinated coatings, and avoid defects such as pinholes, missed coatings, and uneven thickness during the coating process. At the same time, these wetting agents have the characteristics of good temperature resistance, no residue, and strong electrochemical inertness. After drying and film formation, no active impurities remain, and they do not affect electrolyte wetting and ion transport, which can ensure that the coating is dense and uniform, improve the overall performance consistency of the separator and the yield of batteries.

[0051] More preferably, the defoamer is one or more of silicone defoamers and polyether defoamers.

[0052] Organosilicon and polyether defoamers are selected to precisely adapt to the entire process of slurry mixing, ball milling, and coating. These defoamers have high defoaming efficiency and long-lasting foam suppression effect, which can quickly eliminate microbubbles generated during slurry preparation and inhibit the regeneration of bubbles in subsequent processing steps, thus completely solving molding defects such as pinholes, voids, and pitting in the coating. Moreover, these defoamers require small amounts and have excellent compatibility with the system. They will not damage the dispersion stability and adhesion performance of the slurry. After film formation, there are no residual impurities and no electrochemical side reactions, which can ensure the density and integrity of the composite coating and prevent problems such as local short circuits, uneven current distribution, and lithium plating caused by coating defects, thereby improving battery safety and cycle stability.

[0053] More preferably, the porous base membrane has a porosity of 30-70% and a thickness of 2μm-16μm; the porous base membrane is one or more of polyethylene, polypropylene, nonwoven fabric, polymethylpentene, and polyimide.

[0054] The porous base membrane is limited to a thickness of 2μm to 16μm and a porosity range of 30% to 70%. A variety of high-performance substrates are selected to achieve an optimal balance between membrane permeability, mechanical strength, and safety. Appropriate porosity ensures rapid and complete electrolyte wetting and efficient lithium-ion shuttle transport, reducing battery polarization and internal resistance, and improving rate performance. Suitable base membrane thickness guarantees excellent mechanical strength and puncture resistance, effectively resisting the risk of lithium dendrite puncture, while avoiding a decrease in cell volumetric energy density due to excessive base membrane thickness. Multiple substrate types are available to meet different operating conditions, combining advantages such as high temperature resistance, oxidation resistance, and high flexibility. These substrates are suitable for various secondary battery systems, including form-shell and pouch cells, ensuring structural stability and long-term performance during cell charging and discharging.

[0055] More preferably, the areal density of the composite adhesive coating is 0.2 to 0.8 g / m².

[0056] The optimal range of composite adhesive coating areal density (0.2–0.8 g / m²) is defined to achieve a precise balance between adhesive performance and electrochemical performance. The lower limit of areal density ensures full coating coverage without any gaps, providing sufficient active adhesive material to ensure a stable bonding interface between the separator and the electrode, effectively restraining electrode deformation and suppressing interface defects. The upper limit of areal density avoids problems such as separator pore blockage, electrolyte wetting obstruction, and increased ion transport resistance caused by excessive coating thickness and accumulation, preventing increased battery internal resistance, decreased rate performance, and accelerated cycle degradation. This parameter range stably achieves the characteristics of a thin, uniform, and highly adhesive coating, maximizing the preservation of battery electrochemical performance while improving cell structural stability, thus meeting the needs of industrial precision coating production.

[0057] More preferably, the thermal shrinkage performance of the high-temperature resistant ceramic layer meets the following requirements: thermal shrinkage greater than 180°C, thermal shrinkage rate in the TD direction < 4%, and thermal shrinkage rate in the MD direction < 5%.

[0058] This high-temperature resistant ceramic layer possesses excellent dimensional stability and heat resistance: the thermal shrinkage rate in the TD direction is less than 4%, and the thermal shrinkage rate in the MD direction is less than 5%. The deformation is minimal under high-temperature conditions, which can effectively avoid problems such as layer warping, cracking, and delamination. At the same time, the heat resistance temperature can reach above 180℃, enabling it to operate stably under high-temperature conditions for a long time and ensuring that the overall structure and function are not affected by high temperatures.

[0059] This invention introduces a polymer layer with strong adhesion by modifying the surface of the separator substrate with a functional coating, significantly improving the interfacial adhesion between the separator and the positive and negative electrodes. Based on this improvement, this invention achieves the following beneficial effects: 1. Effectively binds the positive and negative electrodes, suppressing electrode deformation and interface slippage during charging and discharging. During the immersion of the battery cell in electrolyte and the charging and discharging process, the positive and negative electrode active materials undergo inherent volume expansion and contraction, leading to stress accumulation and micron-level dimensional changes in the electrode sheets. Traditional separators rely solely on weak physical contact or lack any adhesive bonding, making it difficult to resist the interfacial shear stress caused by these volume changes. This easily results in relative slippage, delamination, or even wavy buckling between the electrode sheets and the separator. This invention addresses the issue of reduced heat penetration during the winding process of the battery cell due to high-temperature resistant layers (aramid, cellulose, etc.) or ceramic layers, thereby decreasing the peel force of the adhesive layer and exacerbating wrinkles at the negative electrode interface. This invention improves the peel force between the adhesive layer on the separator surface and the electrode sheets, creating a stable "face-to-face" adhesive interface between the separator and the positive and negative electrodes. This adhesive layer can build a three-dimensional mesh constraint structure inside the entire cell, effectively "anchoring" the positive and negative electrodes to the separator, limiting the non-uniform expansion and contraction displacement of the electrodes in the thickness direction and in the plane, thereby fundamentally suppressing wrinkling, warping or wavy wrinkles caused by uneven stress in the electrodes.

[0060] Second: Significantly improves cell interface flatness, reduces interface impedance, and enhances cycle stability. Due to the effective binding of the separator to the positive and negative electrodes, the internal electrode interface of the battery cell can still maintain a highly flat, dense, and wrinkle-free ideal state after experiencing electrolyte immersion, multiple cycles, high-temperature storage, or high-rate charge-discharge. A flat interface means that the separator and electrodes maintain uniform and stable contact pressure, avoiding obstruction of ion transport paths or uneven current distribution caused by local wrinkles. Experiments have shown that applying the separator coating of this invention can reduce the interface contact impedance by 15%-25%, and after 1000 cycles, the interface flatness still maintains more than 90% of its initial state. This directly leads to improved cell rate performance, reduced polarization, and a significant extension of cycle life, while effectively reducing the safety risks of lithium plating or micro-short circuits caused by local wrinkles.

[0061] Third: Improve the yield and consistency of battery cell manufacturing. In post-processing steps such as electrolyte injection, formation, and hot and cold pressing of battery cells, traditional designs often suffer from insufficient binding force between the electrode and the separator, leading to random wrinkles or bubbles at the interface, affecting the appearance and performance consistency of the finished product. This invention, through a pre-set strong bonding interface, ensures a stable bond between the electrode and the separator after assembly, effectively preventing interface misalignment and wrinkles caused by external forces or thermal stress in subsequent processes, significantly improving the manufacturing yield and product consistency of the battery cells.

[0062] In summary, this invention effectively binds the positive and negative electrode sheets by enhancing the adhesion of the separator. It not only mechanically solves the technical problem of reduced adhesive peeling force caused by double-sided heat-resistant coating, resulting in wrinkles at the negative electrode interface, but also has a significant synergistic effect on the electrochemical performance, safety, and manufacturing process control of the battery cell. It has outstanding substantive features and significant progress. Attached Figure Description

[0063] Figure 1 Schematic diagram of a wrinkled battery cell in the background art Figure 1 ; Figure 2 Schematic diagram of a wrinkled battery cell in the background art Figure 2 ; Figure 3 Test data on the heat shrinkage of the diaphragm in the examples and comparative examples; Figure 4 The test data for the cycle life of the cells in the examples and comparative examples are as follows. Detailed Implementation

[0064] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Example 1

[0065] A secondary battery includes a positive electrode, a negative electrode, an electrolyte, a separator, and a square / aluminum shell. It has a rated capacity of 200 Ah. The positive electrode is made of lithium iron phosphate, the negative electrode is made of graphite, and the electrolyte is lithium hexafluorophosphate. The wet-process polyethylene separator base film is 7 μm thick. Both sides of the base film are coated with a 1.5 μm high-temperature resistant (heat shrinkage > 180℃, TD < 4%, MD < 5%) ceramic layer. The composite adhesive coating of this invention is applied by double-sided spraying, with a coating thickness of 2 μm on each side.

[0066] The high-temperature resistant ceramic layer is prepared from the following raw materials by weight fraction: 3 parts modified acrylic-modified polyurethane composite emulsion, 44 parts inorganic nano-ceramic particles, 5.5 parts binder, 0.7 parts additives, and 46.8 parts deionized water. The high-temperature resistant ceramic layer is coated onto both sides of the porous base membrane using a microgravure coating method at a temperature of 40°C, resulting in a coating thickness of 1.5 μm. It is then dried at 48°C for 1 minute.

[0067] The PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) is prepared from the following raw materials in the indicated weight fractions: 59.16 parts solvent (deionized water), 25 parts vinylidene fluoride, 15 parts hexafluoropropylene, 0.1 parts ammonium perfluorooctanoate, 0.02 parts ammonium persulfate, 0.3 parts paraffin wax, 0.02 parts sodium bisulfite, 0.2 parts carboxymethyl cellulose, and 0.2 parts sodium sulfate.

[0068] More preferably, the preparation method of the polyvinylidene fluoride-hexafluoropropylene copolymer includes the following steps: S1: The reaction is carried out under inert gas protection. High-purity nitrogen (purity ≥99.99%) is used. Nitrogen gas is introduced into the reaction vessel to a pressure of 0.1-0.2 MPa for protection. 59.16 parts of the polar organic solvent (deionized water) of the formula amount are weighed and placed in a high-speed dispersion vessel.

[0069] S2: Add 25 parts of vinylidene fluoride (VDF), start stirring until completely dissolved, stirring speed 300 r / min, time 30min.

[0070] S3: Add 15 parts of hexafluoropropylene (HFP), start stirring until completely dissolved, stirring speed 200 r / min, time 30min.

[0071] S4: Add 0.2 parts of carboxymethyl cellulose slowly until completely dissolved, stirring at 200 r / min for 20 min.

[0072] S5: Add 0.1 parts of ammonium perfluorooctanoate, 0.02 parts of ammonium persulfate (APS), 0.3 parts of paraffin, 0.02 parts of sodium bisulfite, and 0.2 parts of sodium sulfate. Start stirring until completely dissolved at a stirring speed of 300 r / min for 50 min.

[0073] S6: Maintain the reaction temperature at 55±5℃ and the pressure at 2MPa, homogenize at high speed, increase the stirring rate to 2000r / min, disperse at high speed for 60 min, so that all components are fully mixed and uniform.

[0074] S7: Filtration and vacuum degassing: Filter the slurry obtained in step S6 through a 200-mesh filter and degas it under vacuum for 15 minutes to obtain the finished blended slurry.

[0075] S8: Post-discharge processing: After the reaction is complete, stop stirring, turn off the heating, cool to below 35°C, open the bottom discharge valve, release the reaction emulsion, and place it in a clean, dust-proof container for later use.

[0076] The composite adhesive coating comprises: solvent (deionized water): 45 parts; main ingredient 1 PMMA emulsion with a solid content of approximately 25% (polymethyl methacrylate Tg 30-35℃): 35 parts; main ingredient 2 PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer): 9 parts; binder (polyacrylic acid): 8 parts; dispersant (carboxymethyl cellulose): 2.6 parts; wetting agent (silicone): 0.2 parts; defoamer (polyether / silicone): 0.2 parts; and a composite adhesive coating thickness of 2 μm.

[0077] The composite adhesive coating is prepared using the following synthesis process: S1: Solvent premixing: Weigh 45 parts of the polar organic solvent deionized water according to the formula and place it in a high-speed dispersion vessel.

[0078] S2: Add dispersant, wetting agent and defoamer. Add 2.6 parts of dispersant, 0.2 parts of wetting agent and 0.2 parts of defoamer to the solvent, and start stirring until completely dissolved. Stirring speed is 500 r / min and time is 30 min.

[0079] S3: Add PVDF-HFP and dissolve it. Add 9 parts of PVDF-HFP copolymer to the reactor and continue stirring until completely dissolved. The stirring speed is 300 r / min and the time is 50 min to obtain a transparent or microemulsified fluoropolymer solution. Depending on the dissolution, the temperature can be appropriately heated to 60℃.

[0080] S4: Add PMMA and disperse. Slowly add 35 parts of PMMA to the above fluoropolymer solution and continue stirring until completely dispersed. Preferably, the temperature can be appropriately heated to 60°C to accelerate the dispersion of PMMA, and the stirring time is 80 min.

[0081] S5: Add 8 parts of binder polyacrylic acid, stir at 400 r / min for 50 min.

[0082] S6: High-speed homogenization, increasing the stirring rate to 3000 r / min, high-speed dispersion for 30 min, so that all components are fully mixed and uniform.

[0083] S7: Ball milling: The mixed slurry obtained in step S5 is transferred to a ball mill for closed-loop grinding. The ball milling time is 1 hour, so that the polymer PMMA and PVDF-HFP can be uniformly blended at the molecular level in the solvent. At the same time, the wetting agent and defoamer are further dispersed to form a polymer blend slurry with stable dispersion and good flowability.

[0084] S8: Filtration and vacuum degassing: Filter the slurry obtained in step S6 through a 400-mesh filter and degas it under vacuum for 10 minutes to obtain the finished blended slurry.

[0085] Coating: Sprayed onto the double-sided high-temperature resistant ceramic layer using a dispersion disc method, at a coating temperature of 40℃, with a coating thickness of 2μm.

[0086] Cell fabrication: The positive and negative electrodes of the lithium-ion battery are wound and hot-pressed using a separator with adhesive. The preheating parameters are: temperature 80℃, time 20min; hot pressing parameters are: temperature 70℃, pressure 1.5MPa, time 40s. Example 2

[0087] The method in Example 2 is basically the same as that in Example 1, except that the differences are as follows: Composite adhesive coating raw material ratio (weight fraction): Solvent (deionized water): 50 parts; Main material 1 PMMA emulsion solid content about 25% (polymethyl methacrylate Tg 30~35℃): 32 parts; Main material 2 PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer): 7 parts; The dosage of other additives and binders, preparation process, coating parameters, and hot pressing parameters are the same as in Example 1. Example 3

[0088] The method in Example 3 is basically the same as that in Example 1, except that the difference between Example 3 and Example 1 is as follows: Composite adhesive coating raw material ratio (weight fraction): Solvent (deionized water): 55 parts, main material 1 PMMA emulsion solid content about 25% (polymethyl methacrylate Tg 30~35℃): 28 parts, main material 2 PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer): 6 parts; the dosage of other additives and binders, preparation process, coating parameters, and hot pressing parameters are the same as in Example 1. Example 4

[0089] The method in Example 4 is basically the same as that in Example 1, except that the differences are as follows: Cell fabrication: The preheating parameters are 70℃ for 20 min; the hot pressing parameters are 60℃ for 1.5 MPa for 40 s. Example 5

[0090] The method in Example 5 is basically the same as that in Example 1, except that the difference between Example 5 and Example 1 is as follows: Cell preparation: The preheating parameters are 85℃ for 25 minutes; the hot pressing parameters are 75℃ for 2 MPa for 60 seconds.

[0091] Comparative Example 1: The method of Comparative Example 1 is basically the same as that of Example 1, except that the difference between Comparative Example 1 and Example 1 is as follows: The composite adhesive coating raw material ratio (by weight) is as follows: 10 parts polyvinylidene fluoride, 4.8 parts adhesive, 0.2 parts additives, and 85 parts deionized water. The adhesive layer is applied to both sides of the high-temperature resistant ceramic layer by spraying at a temperature of 45°C. The coating surface density is 0.6 g / m², followed by drying at a temperature of 45°C.

[0092] The diaphragms produced in the examples and comparative examples were subjected to heat shrinkage tests. Test data are shown below. Figure 3 ;from Figure 3The test data shows that, compared with Comparative Example 1 (traditional PVDF), the dry adhesion of the inner layer of the example is 3-4 times higher than that of the comparative example traditional PVDF material, among which the dry adhesion of Example 1 is the optimal one.

[0093] The battery cells produced in the examples and comparative examples were subjected to cyclic testing at room temperature (25°C), voltage (2.5–4.2V), and current density (1C / 1C). The test data are shown below. Figure 4 ;from Figure 4 The test data shows that The lithium-ion battery in Example 1 exhibits superior retention rate after 1500 cycles. Other examples show slightly lower adhesion. As the number of cycles increases, the wrinkling effect gradually amplifies, affecting normal lithium intercalation after wrinkling of the negative electrode. However, Comparative Example 1 shows significant rapid capacity decay, and this effect gradually amplifies with increasing cycle count.

[0094] The diaphragm of this invention exhibits low high-temperature thermal shrinkage and excellent dimensional stability; the dry adhesion of the inner layer and the wet adhesion after electrolyte impregnation are significantly better than those of the traditional PVDF comparative ratio, with an adhesion performance improvement of 3 to 4 times, and no problem of abrupt attenuation in wet adhesion; the capacity retention rate after 1500 cycles can reach up to 92.84%, which is significantly better than the traditional solution.

[0095] This invention effectively solves the industry pain points of insufficient adhesion of double-sided heat-resistant coating separator, negative electrode wrinkling, and rapid cycle decay in large-capacity square-shell wound batteries by modifying composite polymers, precise additive ratios, and optimizing hot pressing process. It takes into account battery safety, interface stability, and long cycle performance, and has strong industrial adaptability.

[0096] The modified adhesive of this invention can solve the problem of decreased adhesive strength of the separator caused by high heat-resistant separators, thereby increasing the binding force of the negative electrode, reducing or eliminating the problem of negative electrode interface wrinkles, and improving the safety and cycle performance of lithium batteries.

Claims

1. A secondary battery, characterized in that, The invention comprises a bare battery cell formed by sequentially stacking a positive electrode, a separator, and a negative electrode, characterized in that: the separator comprises a porous base film, a high-temperature resistant ceramic coating attached to both sides of the porous base film, and a composite adhesive coating attached to both sides of the high-temperature resistant ceramic coating; the composite adhesive coating is a composite emulsion of polymethyl methacrylate and polyvinylidene fluoride-hexafluoropropylene, wherein the glass transition temperature of the polymethyl methacrylate is 30-35°C, and the hot-pressing process of the bare battery cell is as follows: preheating temperature is 60-90°C, preheating time is 20-40 min, hot-pressing temperature is 60-80°C, hot-pressing pressure is 1-2 MPa, and hot-pressing time is 30-60 s.

2. A secondary battery according to claim 1, characterized in that, The thickness of a single layer of the high-temperature resistant ceramic coating is 1.0–2.0 μm, and the thickness of a single layer of the composite adhesive coating is 1.0–2.0 μm.

3. A secondary battery according to claim 1, characterized in that, The composite adhesive coating comprises, by weight, the following raw materials: 40-50 parts solvent, 28-35 parts polymethyl methacrylate, 6-9 parts polyvinylidene fluoride-hexafluoropropylene copolymer, 6-8 parts binder, 0.5-3 parts dispersant, 0.2-0.5 parts wetting agent, and 0.1-0.2 parts defoamer.

4. A secondary battery according to claim 3, characterized in that, The polyvinylidene fluoride-hexafluoropropylene copolymer comprises, by weight, the following raw materials: 55-75 parts solvent, 24-30 parts vinylidene fluoride, 14-18 parts hexafluoropropylene, 0.1-0.2 parts ammonium perfluorooctanoate, 0.01-0.03 parts ammonium persulfate, 0.3-0.5 parts paraffin wax, 0.01-0.03 parts sodium bisulfite, 0.2-0.3 parts carboxymethyl cellulose, and 0.2-0.4 parts sodium sulfate.

5. A secondary battery according to claim 3, characterized in that, The adhesive is one or more of polyacrylic acid, acrylate, and polyvinyl alcohol.

6. A secondary battery according to claim 3, characterized in that, The dispersant is one or more of carboxymethyl cellulose, perfluoroalkylamine oxide, nonylphenol polyoxyethylene ether, and fatty alcohol polyoxyethylene ether.

7. A secondary battery according to claim 3, characterized in that, The wetting agent is one or more of polyether-modified polysiloxane wetting agents and acetylenic diol nonionic wetting agents.

8. A secondary battery according to claim 3, characterized in that, The defoamer is one or more of the following: silicone defoamers and polyether defoamers.

9. A secondary battery according to claim 1, characterized in that, The porous base membrane has a porosity of 30-70% and a thickness of 2μm-16μm; the porous base membrane is one or more of polyethylene, polypropylene, nonwoven fabric, polymethylpentene, and polyimide.

10. A secondary battery according to claim 1, characterized in that, The areal density of the composite adhesive coating is 0.2 to 0.8 g / m².

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