Composite temperature-resistant coating lithium battery diaphragm and preparation method thereof

Abstract

The application discloses a kind of composite temperature-resistant coating lithium electric diaphragm and preparation method thereof, belong to lithium ion battery diaphragm technical field.The diaphragm is coated with ternary composite coating on the surface of polypropylene base material, which includes polymethyl methacrylate, surface phosphate-modified lithium zirconium phosphate (modified LZP) and surface boric acid functionalized polystyrene microspheres (PS / BA).Boric acid groups on the surface of PS / BA microspheres and phosphate groups on the surface of modified LZP form dynamic borate ester bond through reversible reaction, thereby constructing "self-adapting interface phase" in the coating.The structure simultaneously significantly enhances the interface bonding strength of coating and the dimensional stability at high temperature.The prepared diaphragm has a heat shrinkage rate of less than 4% at 150℃, the peeling strength is significantly improved, and good ionic conductivity is maintained.The application has low raw material cost, good process compatibility, and provides an effective solution for improving the safety and comprehensive performance of lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a composite heat-resistant coated lithium battery separator and its preparation method. Background Technology

[0002] The separator is a key component of lithium-ion batteries, its function being to isolate the positive and negative electrodes to prevent short circuits, while also providing a transport channel for lithium ions. Polypropylene (PP) separators are widely used in power batteries and other fields due to their high melting point and good mechanical strength. However, PP separators are prone to thermal shrinkage at high temperatures, posing a risk of thermal runaway, and their low surface energy results in poor wettability to the electrolyte, limiting further improvements in battery performance.

[0003] Currently, the mainstream modification technology involves coating the surface of PP separators. This method is simple and easy to industrialize. There are three main approaches: The first is organic polymer coatings, commonly using polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA). PVDF is chemically stable but expensive, has poor adhesion to the PP substrate, and swells significantly in the electrolyte. PMMA is cheaper and has good adhesion to PP, but its pure coating softens easily at high temperatures, and cross-linking introduces brittleness and cracking. The second is inorganic ceramic filler coatings, typically using materials such as alumina and silica to improve the separator's heat resistance. However, these insulating fillers hinder lithium-ion transport and have weak interfacial bonding with the polymer matrix, making them prone to detachment. These traditional methods struggle to achieve a good balance between thermal stability, ionic conductivity, and interfacial bonding.

[0004] To simultaneously improve the heat resistance and ion conductivity of membranes, NASICON-type fast ion conductors (such as LAGP) are used as functional fillers. However, LAGP, a representative material in this category, has significant drawbacks: firstly, the germanium (Ge) it contains is expensive and toxic, which is detrimental to environmental protection and large-scale application; secondly, its high particle hardness easily abrades coating equipment, and its bonding with the substrate still relies on a large amount of coupling agent, failing to fundamentally improve the problem of easy peeling at the coating interface. Therefore, developing novel composite coatings that combine high performance, low cost, and a stable interface remains an important technological direction. Summary of the Invention

[0005] In view of this, the present invention proposes a composite high-temperature resistant coated lithium battery separator and its preparation method. This separator constructs a "PMMA-modified LZP-PS / BA" ternary composite system and utilizes the dynamic covalent bonding between boric acid and phosphate ester groups to build an adaptive interface phase between the filler and the matrix, thereby synergistically and significantly improving the high-temperature dimensional stability, interfacial bonding strength, and electrochemical performance of the separator.

[0006] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides a composite heat-resistant coated lithium battery separator, which uses a polypropylene separator as a substrate and coats a ternary composite coating on one or both sides of the substrate. The ternary composite coating comprises polymethyl methacrylate, surface-modified lithium zirconium phosphate, and surface-borate-functionalized polystyrene microspheres.

[0007] Based on the above technical solutions, preferably, the surface-modified lithium zirconium phosphate is obtained by surface modification of lithium zirconium phosphate powder using a phosphate ester-based coupling agent; the phosphate ester-based coupling agent includes bis(2-methacryloyloxyethyl) phosphate, and its amount is 1.0-5.0% of the mass of lithium zirconium phosphate (LZP) powder. This phosphate ester coupling agent forms a strong PO-Zr bond with the LZP surface through its phosphate groups, and at the same time, the polymerizable groups at the ends of its molecules introduce reactive active sites on the LZP surface, laying the foundation for subsequent interfacial reactions.

[0008] More preferably, the matrix material of the modified LZP is lithium zirconium phosphate, a NASICON-type fast ion conductor, with the molecular formula LiZr2(PO4)3, a particle size of 50-200 nm, and a room temperature ionic conductivity of 10. -6 -10 -4 S / cm, melting point ≥1000℃.

[0009] More preferably, the surface-boronic acid-functionalized polystyrene (PS / BA) microspheres have a particle size of 100-500 nm, and their surface is covalently modified with phenylboronic acid groups. The boronic acid groups on the surface of the PS / BA microspheres and the phosphate ester groups of the modified LZP form dynamic borate ester bonds (BOP) through a reversible transesterification reaction. This dynamic covalent bonding makes it a crosslinking point for constructing an adaptive interface network, thereby simultaneously enhancing interface stability and optimizing interfacial ion transport.

[0010] More preferably, the mass ratio of polymethyl methacrylate (PMMA), surface-modified lithium zirconium phosphate (LZP), and surface-boronic acid-functionalized polystyrene microspheres is (65-80):(15-30):(2-8). This specific ratio aims to achieve deep functional coupling: PMMA acts as a continuous phase, providing adhesion and film-forming properties; modified LZP acts as a fast ion conductor, constructing a lithium-ion transport framework; and PS / BA microspheres act as key functional interface modifiers, with their boric acid groups and modified LZP phosphate groups forming a reversible and reconfigurable "adaptive crosslinking network" between the filler and the matrix through dynamic covalent bonding.

[0011] More preferably, the thickness of the ternary composite coating is 2-10 μm. The coating uniformly covers the surface and pore walls of the PP substrate without clogging the micropores of the substrate.

[0012] Secondly, the present invention also provides a method for preparing the above-mentioned composite heat-resistant coated lithium battery separator, comprising the following steps: S1. Surface modification of lithium zirconium phosphate powder was performed using a phosphate ester-based coupling agent to obtain surface-modified lithium zirconium phosphate; surface boric acid-functionalized polystyrene microspheres were prepared and dispersed in a solvent to form a microsphere dispersion; polymethyl methacrylate was dried. S2. Dissolve the dried polymethyl methacrylate in a mixed solvent to obtain a polymer solution; under stirring, add the surface-modified lithium zirconium phosphate and the microsphere dispersion to the polymer solution in sequence, and after dispersion, ultrasonication and filtration, obtain a uniform ternary composite slurry; S3. The ternary composite slurry obtained in step S2 is coated on one or both sides of the polypropylene diaphragm substrate and dried in a segmented process to obtain the coated diaphragm. S4. The coated separator obtained in step S3 is subjected to heat setting treatment at a temperature of 100-120℃ for 5-15 minutes to obtain the composite heat-resistant coated lithium battery separator.

[0013] Based on the above technical solutions, preferably, in step S1, the preparation conditions of the surface-modified lithium zirconium phosphate are as follows: at 60-80℃ and 300-500 rpm, lithium zirconium phosphate powder and phosphate ester coupling agent are mixed and reacted in anhydrous ethanol for 4-8 hours.

[0014] In a further preferred embodiment, LZP powder is dispersed in anhydrous ethanol to prepare a suspension with a mass concentration of 5-10%, and the phosphate ester coupling agent accounts for 1.0-5.0% of the mass of LZP powder. After the reaction is completed, the powder is centrifuged, washed with ethanol, vacuum dried at 80°C for 6-12 hours, and finally ground and passed through a 200-mesh sieve to obtain modified LZP powder with polymerizable groups grafted onto its surface.

[0015] More preferably, in step S1, the method for preparing the surface boric acid functionalized polystyrene microspheres includes the following steps: first preparing polystyrene microspheres, then reacting them with 3-(acryloyloxy)propylboronic acid pinacol ester, and then hydrolyzing them by acid treatment.

[0016] More specifically: First, using styrene as a monomer, polyvinylpyrrolidone as a dispersant, and azobisisobutyronitrile (AIB) as an initiator, a dispersion polymerization reaction was carried out at 70°C for 24 hours in an ethanol / water mixture. After washing and drying, monodisperse polystyrene (PS) microspheres with a particle size of approximately 500 nm were obtained. Subsequently, using 3-(acryloyloxy)propylboronic acid pinacol ester as a functionalizing agent, this agent was grafted onto the surface of the PS microspheres via a free radical reaction in toluene solvent. The pinacol protecting groups were then removed by acid treatment and hydrolysis, ultimately yielding PS / BA microspheres with exposed boric acid groups (-B(OH)2) on the surface. The PS / BA microspheres were added to a portion of the mixed solvent to be used subsequently, preparing a suspension with a mass concentration of 2-5%. At room temperature, the suspension was subjected to high-speed shearing and ultrasonic treatment to obtain a uniform dispersion for later use. This step aims to break up the soft agglomeration of the microspheres and ensure their uniform distribution in the coating.

[0017] More preferably, in step S2, the mixed solvent includes a toluene-ethyl acetate mixed solvent or a butyl acetate-toluene mixed solvent; the total solid mass concentration in the ternary composite slurry is 10-20%. The volume ratio of toluene-ethyl acetate or butyl acetate-toluene in the mixed solvent is 1:1-3:1.

[0018] More preferably, in step S3, the segmented drying process includes: first drying at 40-60°C for 10-20 minutes to gently remove most of the solvent and prevent the surface from forming a skin too quickly, which would cause internal solvent retention and generate bubbles; then drying at 70-90°C for 20-40 minutes to completely remove the residual solvent and form a dense coating; and the drying process is carried out under nitrogen protection.

[0019] More preferably, in step S4, the heat setting treatment helps to eliminate internal stress in the coating, promotes the relaxation and rearrangement of PMMA molecular chains, and provides activation energy for the dynamic covalent bonding (BOP) between the boric acid groups on the surface of PS / BA microspheres and the phosphate ester groups on the surface of modified LZP powder, thereby forming a stable adaptive crosslinking network at the interface, ultimately ensuring that the composite coating has excellent interfacial bonding strength and structural integrity.

[0020] The present invention has the following advantages over the prior art: (1) Active sites are introduced on the surface of LZP by phosphate ester modification and dynamic covalent bonds (BOP) are formed with the boric acid groups of PS / BA, thus constructing an "adaptive interface phase" between the inorganic filler and the organic matrix. This interface can effectively dissipate heat and mechanical stress, inhibit coating cracking and peeling, and greatly improve the interfacial bonding strength and cycle durability.

[0021] (2) The chemically bonded modified LZP and the PS / BA microspheres, which act as crosslinking nodes, together form a rigid three-dimensional network that can effectively support the membrane even after the glass transition temperature of PMMA is exceeded, and significantly inhibit the thermal shrinkage of the PP substrate.

[0022] (3) Modified LZP provides fast ion conduction channels, and its surface phosphate groups become anchor points for interfacial reactions; PS / BA microspheres, as dynamic cross-linking nodes, enhance interfacial bonding and may optimize the interfacial ion transport microenvironment. The three components, in the specific ratio of this invention, produce a synergistic effect of "1+1+1>3".

[0023] (4) Using environmentally friendly LZP, which does not contain rare metals, to replace the expensive LAGP, and using low-cost PMMA as the substrate, the above-mentioned performance is achieved while effectively controlling the material cost. Moreover, the preparation process is compatible with traditional coating technology and has significant potential for industrial application. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] All materials used in this invention were purchased from the market, specifically: lithium zirconium phosphate (LZP) powder was purchased from Fujian Ruisen New Materials Co., Ltd.; bis(2-methacryloyloxyethyl) phosphate was purchased from Shanghai McLean Biochemical Technology Co., Ltd.; 3-(acryloyloxy)propylborate pinacol ester was purchased from Shanghai McLean Biochemical Technology Co., Ltd.; and PMMA particles were purchased from Aladdin Reagent (Shanghai) Co., Ltd.

[0026] Example 1: S1. Preparation of modified LZP: 10g of lithium zirconium phosphate (LZP) powder (particle size approximately 150 nm) was dispersed in 200mL of anhydrous ethanol to prepare a 5% (w / w) suspension. 0.3g of bis(2-methacryloyloxyethyl) phosphate (3.0% by weight of LZP) was added. The reaction was carried out at 70℃ and 400 rpm for 6 hours with stirring. After the reaction was complete, the powder was centrifuged, washed with ethanol, vacuum dried at 80℃ for 8 hours, and finally ground and passed through a 200-mesh sieve to obtain modified LZP powder with polymerizable groups grafted onto its surface.

[0027] Preparation and dispersion of PS / BA microspheres: First, 6g of styrene, 2g of polyvinylpyrrolidone, and 0.06g of azobisisobutyronitrile were mixed and reacted at 70℃ for 24 hours in an 80mL ethanol / water (7:3, v / v) mixture. After washing and drying, approximately 5.2g of monodisperse polystyrene (PS) microspheres with a particle size of approximately 500 nm were obtained. 5g of these PS microspheres were dispersed in toluene, and 0.5g of 3-(acryloyloxy)propylboronic acid pinacol ester and a small amount of initiator were added. The mixture was reacted at 80℃ for 12 hours, followed by acid treatment and hydrolysis to obtain PS / BA microspheres with exposed boric acid groups on the surface. Take 1g of PS / BA microspheres and add them to 19g of toluene-ethyl acetate (2:1, v / v) mixed solvent to prepare a 5 wt% suspension. After high-speed shearing (2500 rpm, 30 min) and ultrasonic treatment (250W, 20 min), a uniform PS / BA microsphere dispersion is obtained for later use.

[0028] PMMA particles were vacuum dried at 60°C for 6 hours and then set aside for later use.

[0029] S2. Weigh 7.5g of dried PMMA and dissolve it in 42.5g of a toluene-ethyl acetate (2:1, v / v) mixed solvent. Stir at 700rpm for 1 hour until completely dissolved. While stirring continuously, slowly add 2.5g of the modified LZP powder and 12g of the PS / BA microsphere dispersion (containing 0.6g of PS / BA solids) sequentially. Transfer the mixture to a high-speed disperser and disperse at 2000rpm for 2 hours, followed by ultrasonic treatment (400W, 40 min). Finally, filter through a 1μm filter membrane to obtain a uniform ternary composite slurry with a total solids concentration of 16.4%. The mass ratio of PMMA, modified LZP, and PS / BA is 75:25:6.

[0030] S3. A doctor blade coater is used to coat both sides of a 20 μm thick PP diaphragm substrate, with the wet film thickness controlled at 6 μm. After coating, the diaphragm is first dried at 50℃ for 15 minutes, then dried at 80℃ for 30 minutes. The entire drying process is carried out at a flow rate of 0.8 m³ / min. 3 The procedure is carried out under nitrogen protection at a rate of / h.

[0031] S4. Place the completely dried separator in a 110℃ oven for 10 minutes for heat treatment, and then let it cool naturally to room temperature to obtain the PMMA-modified LZP-PS / BA composite heat-resistant coating lithium battery separator.

[0032] Example 2: S1. Preparation of modified LZP: 10g of lithium zirconium phosphate (LZP) powder (particle size approximately 50 nm) was dispersed in 200 mL of anhydrous ethanol to prepare a 5% (w / w) suspension. 0.1g of bis(2-methacryloyloxyethyl) phosphate (1.0% of LZP mass) was added. The reaction was carried out at 60℃ and 300 rpm for 4 hours with stirring. After the reaction was complete, the powder was centrifuged, washed with ethanol, vacuum dried at 80℃ for 6 hours, and finally ground and passed through a 200-mesh sieve to obtain modified LZP powder with polymerizable groups grafted onto its surface, for later use.

[0033] Preparation and dispersion of PS / BA microspheres: The preparation method is the same as described in Example 1, yielding PS / BA microspheres (approximately 100 nm in diameter) with exposed boric acid groups on the surface. 1.0 g of PS / BA microspheres was added to 49.0 g of a toluene-ethyl acetate (volume ratio 1:1) mixed solvent, and subjected to high-speed shearing (2000 rpm, 20 min) and ultrasonic treatment (200 W, 15 min) to prepare a uniform dispersion with a mass concentration of 2%, which was then set aside.

[0034] PMMA particles were vacuum dried at 60°C for 6 hours and then set aside for later use.

[0035] S2. Weigh 6.5 g of dried PMMA and dissolve it in an appropriate amount of toluene-ethyl acetate (volume ratio 1:1) mixed solvent. While stirring, slowly add 1.5 g of the modified LZP powder and 10 g of the PS / BA microsphere dispersion (corresponding to 0.2 g of PS / BA solid mass). After dispersion, ultrasonication, and filtration, a uniform ternary composite slurry with a total solid mass concentration of 10% is obtained. In this slurry, the mass ratio of PMMA, modified LZP, and PS / BA is 65:15:2.

[0036] S3. A doctor blade coater is used to coat one side of the PP diaphragm substrate, with the wet film thickness controlled at 2 μm. After coating, the diaphragm is first dried at 40°C for 10 minutes, and then dried at 70°C for 20 minutes. The entire drying process is carried out under nitrogen protection.

[0037] S4. Place the completely dried separator in a 100°C oven for 5 minutes for heat treatment, and then let it cool naturally to room temperature to obtain the composite heat-resistant coated lithium battery separator.

[0038] Example 3: S1. Preparation of modified LZP: 10g of lithium zirconium phosphate (LZP) powder (particle size approximately 200 nm) was dispersed in 100 mL of anhydrous ethanol to prepare a 10% (w / w) suspension. 0.5g of bis(2-methacryloyloxyethyl) phosphate (5.0% by weight of LZP) was added. The reaction was carried out at 80℃ and 500 rpm for 8 hours with stirring. After the reaction was complete, the powder was centrifuged, washed with ethanol, vacuum dried at 80℃ for 12 hours, and finally ground and passed through a 200-mesh sieve to obtain modified LZP powder with polymerizable groups grafted onto its surface, for later use.

[0039] Preparation and dispersion of PS / BA microspheres: The preparation method is the same as described in Example 1, yielding PS / BA microspheres (approximately 500 nm in diameter) with exposed boric acid groups on the surface. 1.0 g of PS / BA microspheres were added to 19.0 g of a butyl acetate-toluene (volume ratio 1:2) mixed solvent, and subjected to high-speed shearing (3000 rpm, 40 min) and ultrasonic treatment (300 W, 30 min) to prepare a uniform dispersion with a mass concentration of 5%, which was then set aside.

[0040] PMMA particles were vacuum dried at 60°C for 6 hours and then set aside for later use.

[0041] S2. Weigh 8.0 g of dried PMMA and dissolve it in an appropriate amount of butyl acetate-toluene (volume ratio 1:2) mixed solvent. While stirring, slowly add 3.0 g of the modified LZP powder and 16 g of the PS / BA microsphere dispersion (corresponding to 0.8 g of PS / BA solid mass). After dispersion, ultrasonication, and filtration, a uniform ternary composite slurry with a total solid mass concentration of 20% is obtained. In this slurry, the mass ratio of PMMA, modified LZP, and PS / BA is 80:30:8.

[0042] S3. A doctor blade coater is used to coat both sides of the PP diaphragm substrate, with the wet film thickness controlled at 10 μm. After coating, the diaphragm is first dried at 60°C for 20 minutes, and then dried at 90°C for 40 minutes. The entire drying process is carried out under nitrogen protection.

[0043] S4. Place the completely dried separator in a 120°C oven for 15 minutes for heat treatment, and then allow it to cool naturally to room temperature to obtain the composite heat-resistant coated lithium battery separator.

[0044] Comparative Example 1: Unlike Example 1, only 10.0g of dried PMMA was weighed and dissolved in 40.0g of a toluene-ethyl acetate (volume ratio 2:1) mixed solvent, stirred and dissolved to prepare a pure PMMA slurry with a solid mass concentration of 20%. The remaining steps were the same as in Example 1, and will not be repeated here.

[0045] Comparative Example 2: Unlike Example 1, 7.5g of dried PMMA was weighed and dissolved in 42.5g of a toluene-ethyl acetate (volume ratio 2:1) mixed solvent, and stirred until dissolved. 2.5g of the above-mentioned unmodified LZP powder was added while stirring, and the mixture was dispersed at high speed, ultrasonicated, and filtered to obtain a binary composite slurry (solid concentration 20%) with a PMMA to unmodified LZP mass ratio of 75:25. The remaining steps were the same as in Example 1 and will not be repeated here.

[0046] Comparative Example 3: Unlike Example 1, 7.5g of dried PMMA was weighed and dissolved in 42.5g of a toluene-ethyl acetate (volume ratio 2:1) mixed solvent, and stirred until dissolved. 2.5g of modified LZP powder was added while stirring, and the mixture was then dispersed at high speed, sonicated, and filtered to obtain a binary composite slurry (solid concentration 20%) with a PMMA to modified LZP mass ratio of 75:25. No PS / BA microspheres or their dispersion were added. The remaining steps were the same as in Example 1 and will not be repeated here.

[0047] Comparative Example 4: Unlike Example 1, 7.0 g of dried PMMA was weighed and dissolved in an appropriate amount of toluene-ethyl acetate (volume ratio 2:1) mixed solvent, and stirred until dissolved. Then, 2.5 g of modified LZP powder and 20.0 g of the above PS / BA microsphere dispersion (corresponding to 1.5 g of PS / BA solid mass) were added sequentially. After dispersion, ultrasonication, and filtration, a slurry was obtained. In this slurry, the mass ratio of PMMA, modified LZP, and PS / BA was 70:25:15 (PS / BA accounted for 10%). The remaining steps were the same as in Example 1 and will not be repeated here.

[0048] Comparative Example 5: Unlike Example 1, 8.0 g of dried PMMA, 1.9 g of modified LZP powder, and a microsphere dispersion containing only 0.1 g of PS / BA solid (corresponding to 2.0 g of a 5% concentration dispersion) were weighed to prepare a ternary composite slurry. The mass ratio of PMMA, modified LZP, and PS / BA was 80:19:1. The remaining steps were the same as in Example 1 and will not be repeated here.

[0049] Comparative Example 6: Unlike Example 1, 6.5g of dried PMMA, 2.0g of modified LZP powder, and a microsphere dispersion containing 1.5g of PS / BA solids (corresponding to 30.0g of a 5% concentration dispersion) were weighed to prepare a ternary composite slurry. The mass ratio of PMMA, modified LZP, and PS / BA was 65:20:15. The remaining steps were the same as in Example 1 and will not be repeated here.

[0050] To evaluate the interfacial bonding strength and durability, the above-described examples and comparative samples were subjected to 180° peel strength tests and -20°C / 150°C thermal cycling tests. The results are shown in Table 1. The results indicate that the ternary composite system exhibits excellent interfacial integrity.

[0051] Table 1: Table 1 shows that the peel strength of the ternary coating is approximately 80% higher than that of the binary system, and it exhibits the highest retention rate after thermal cycling. This confirms that the adaptive crosslinking network constructed by dynamic borate ester bonds can significantly enhance interfacial toughness and effectively dissipate heat stress.

[0052] Comparing Comparative Example 4 with Example 1, it is evident that when the PS / BA content increases to 15% (exceeding the preferred range of this invention), both the peel strength and interface retention rate show a significant decrease. The number of phosphate ester active sites on the modified LZP surface is limited. When PS / BA is excessive, some boric acid groups cannot form effective BOP dynamic covalent bonds, becoming unbonded "free" microspheres. These free microspheres mainly function as physical fillers in the coating, not only failing to contribute additional interfacial bonding force but also potentially disrupting the continuity of the PMMA matrix, leading to a decrease in coating cohesion and thus weakening the overall peel strength.

[0053] As can be seen from the comparison between Comparative Example 5 and Example 1, when the PS / BA content is reduced to 1% (below the lower limit of the preferred range of the present invention, 2%), its peel strength drops significantly from 94.3 N / cm to 58.1 N / cm, and the interface integrity retention rate after thermal cycling also drops from 93.4% to 90%. This is because PS / BA, as the core donor of dynamic borate bonds, cannot form a continuous and complete adaptive crosslinking network when its content is too low. Most of the phosphate ester active sites on the modified LZP surface cannot react with borate groups, and can only form scattered dynamic crosslinking points. At this time, the interfacial bonding of the coating mainly depends on the physical entanglement between the phosphate ester coupling agent and the PMMA matrix, and the dynamic stress dissipation capacity is extremely limited. Therefore, the performance is only slightly improved compared with the binary modified LZP system (Comparative Example 3), and the synergistic advantages of the ternary composite system cannot be fully utilized.

[0054] Comparing Comparative Example 6 with Example 1, it can be seen that when the PS / BA content increases to 15% and the modified LZP content decreases to 20%, the peel strength further decreases to 67.5 N / cm, and the thermal cycling interface retention rate decreases to 85.2%, which is inferior to Comparative Example 4 with only excessive PS / BA. This result reveals a dual negative impact: on the one hand, the reduction in modified LZP content directly leads to a decrease in the skeletal strength of the rigid support network, while the total amount of phosphate ester active sites that can be used to form dynamic bonds decreases, causing more PS / BA microspheres to become free; on the other hand, excessive free PS / BA microspheres not only disrupt the continuity of the PMMA matrix but also form a large number of interface defects inside the coating. During thermal cycling, these defects become stress concentration points, accelerating the cracking and peeling of the coating, ultimately leading to a significant decrease in interface durability.

[0055] Comparing Comparative Example 2 with Example 1, it is evident that the binary coating prepared using unmodified LZP exhibits a peel strength of only 22.8 N / cm, less than a quarter of that in Example 1, and an interface retention rate of only 82.3% after thermal cycling. This fully demonstrates the necessity of phosphate ester-based coupling agent modification: the unmodified LZP surface contains only a small number of hydroxyl groups, relying solely on weak van der Waals forces and physical adsorption to bond with the PMMA matrix, resulting in extremely poor interfacial adhesion. Under the repeated thermal stress generated by thermal cycling, the filler-matrix interface is prone to debonding, leading to the overall detachment of the coating. However, after phosphate ester modification, reactive methacrylate groups are introduced onto the LZP surface through strong PO-Zr covalent bonds, enhancing compatibility with the PMMA matrix and providing necessary active sites for the subsequent formation of dynamic borate ester bonds.

[0056] Comparing Comparative Example 3 with Example 1, it can be seen that the binary coating with only modified LZP has a peel strength of 52.3 N / cm, which is 44.5% lower than that of Example 1, and a thermal cycling interface retention rate of 89.7%. This comparison intuitively demonstrates the core role of PS / BA microspheres and dynamic borate ester bonds: in the binary system, modified LZP and PMMA are only bonded by physical entanglement and a small number of covalent bonds, resulting in insufficient interfacial toughness and an inability to effectively dissipate thermal stress. However, after introducing PS / BA microspheres, the borate groups on their surface form reversible dynamic borate ester bonds with the phosphate ester groups on the surface of modified LZP, constructing an adaptive interfacial phase between the filler and the matrix. This interfacial phase can dissipate stress through the breaking and reconstruction of dynamic bonds while maintaining the integrity of the interface, thereby achieving a significant improvement in both interfacial bonding strength and cycle durability.

[0057] Comparing Comparative Example 1 with Example 1, it can be seen that the peel strength of the pure PMMA coating is only 1.85 N / cm, and the interface retention rate after thermal cycling is only 78.1%, which is far lower than all systems containing inorganic fillers. This is because the pure PMMA coating and the PP substrate are only bonded by van der Waals forces, resulting in extremely weak interfacial adhesion. Furthermore, PMMA itself has a low glass transition temperature (approximately 105°C), making it prone to softening and deformation at high temperatures. In contrast, the ternary composite system of this invention constructs a rigid three-dimensional network through chemically bonded modified LZP and PS / BA microspheres. This not only significantly enhances the interfacial adhesion between the coating and the substrate but also provides effective mechanical support for the diaphragm at high temperatures, fundamentally solving the problem of easy softening and detachment of the pure PMMA coating at high temperatures. After heat-treating the diaphragm sample in a 150°C oven for 1 hour, its thermal shrinkage rate was measured and its morphology was observed. The results are shown in Table 2.

[0058] Table 2: Table 2 shows that the ternary coated membrane exhibits excellent high-temperature dimensional stability with a thermal shrinkage rate of less than 4%. This is attributed to the rigid three-dimensional network framework composed of chemically bonded modified LZP and PS / BA microspheres, which can still provide effective support after exceeding the glass transition temperature of PMMA, while the dynamic covalent network efficiently dissipates thermal stress.

[0059] However, when the PS / BA content increases to 15%, the thermal shrinkage rate increases significantly, and the morphological uniformity also decreases. Excessive PS / BA causes some microspheres to fail to participate in the construction of an effective dynamic covalent cross-linked network, resulting in poorer overall network structure uniformity and integrity of the coating. At high temperatures, these structural weak points are more likely to become the breakthrough points for thermal stress release, leading to increased local shrinkage.

[0060] Comparing Comparative Example 5 with Example 1, it can be seen that when the PS / BA content decreases to 1%, the longitudinal thermal shrinkage rate increases from 3.7% to 6.3%, and the transverse thermal shrinkage rate increases from 3.6% to 6.1%, with a small number of dispersed microcracks appearing in the coating. This is because the boric acid groups on the surface of the PS / BA microspheres are the core components for constructing dynamic borate bonds. When the content is too low, the phosphate ester active sites on the modified LZP surface cannot be fully utilized, and only sporadic dynamic crosslinking points can be formed, failing to construct a complete adaptive stress dissipation network. At this time, although the coating still has the rigid skeleton support provided by the modified LZP, the thermal stress cannot be effectively released through the reversible breakage and reconstruction of dynamic bonds, and can only dissipate energy by generating microcracks. Therefore, the thermal shrinkage rate increases significantly and crack defects appear.

[0061] Comparing Comparative Example 2 with Example 1, it is evident that the binary coating prepared using unmodified LZP exhibits a longitudinal thermal shrinkage rate as high as 8.7% and a transverse thermal shrinkage rate of 8.2%, with numerous microcracks and localized edge detachment. This fully demonstrates the decisive role of phosphate ester-based coupling agent modification in constructing a continuous rigid network: unmodified LZP has a low surface hydroxyl content and poor compatibility with the PMMA matrix, making it prone to agglomeration during slurry preparation and unable to form a uniform and continuous three-dimensional rigid framework in the coating. Thermal stress concentrates at the interface between the LZP agglomerates and the PMMA matrix, leading to interfacial debonding and coating cracking. Furthermore, the discontinuous framework cannot effectively suppress the thermal shrinkage of the PP substrate, resulting in a significant decrease in thermal stability.

[0062] Comparing Comparative Example 3 with Example 1, it can be seen that the binary coating with only modified LZP has a longitudinal thermal shrinkage rate of 6.6% and a transverse thermal shrinkage rate of 6.4%. The coating is basically intact, but a few microcracks are visible at the edges. This comparison clearly reveals the key role of the dynamic covalent network in high-temperature stress dissipation: In the binary system, although modified LZP can form a continuous rigid skeleton and effectively suppress most of the thermal shrinkage of the PP substrate, the filler and the matrix are only bonded by physical entanglement and a few covalent bonds, resulting in insufficient interfacial toughness. At high temperatures, the thermal stress generated by the shrinkage of the PP substrate cannot be effectively dissipated and will accumulate in stress concentration areas such as the coating edges, eventually leading to the formation of microcracks. In contrast, the dynamic borate ester bonds in the ternary system can uniformly disperse the concentrated stress throughout the entire coating network through reversible bonding and dissociation, thereby avoiding the formation of cracks.

[0063] Comparing Comparative Example 6 with Example 1, it can be seen that when the PS / BA content increases to 15% and the modified LZP content decreases to 20%, the longitudinal thermal shrinkage rate increases to 5.8%, and the transverse thermal shrinkage rate increases to 5.3%. Slight shrinkage deformation and a small number of fine cracks appear locally in the coating. This result reflects the synergistic dependence between the rigid skeleton and the dynamic cross-linking network: on the one hand, the decrease in modified LZP content from 25% to 20% directly leads to a decrease in the overall strength of the rigid three-dimensional skeleton, weakening its ability to suppress the thermal shrinkage of the PP substrate; on the other hand, excessive PS / BA causes a large number of microspheres to be unable to participate in dynamic bonding, existing in a free state in the coating. This not only disrupts the continuity of the PMMA matrix but also forms a large number of structural defects internally. Under high-temperature thermal stress, these defects will first undergo local shrinkage deformation, leading to fine cracks and ultimately a decrease in overall thermal stability.

[0064] As can be seen from the comparison between Comparative Example 1 and Example 1, the longitudinal thermal shrinkage rate of the pure PMMA coating is as high as 13.8%, and the transverse thermal shrinkage rate is as high as 13.7%, and the coating exhibits slight flow and local cracking. This is because the glass transition temperature of pure PMMA is only about 105°C, and it has completely softened under the test conditions of 150°C, losing its mechanical support capacity and unable to suppress the severe thermal shrinkage of the PP substrate. The coating will undergo large-scale deformation or even flow along with the substrate. In contrast, in the ternary composite coating of this invention, the modified LZP has a melting point of over 1000°C, which can maintain a stable rigid skeleton at high temperatures. At the same time, the dynamic borate bond network can effectively dissipate heat stress. Even if the PMMA matrix softens to a certain extent, the entire coating can still maintain structural integrity, thereby achieving excellent high-temperature dimensional stability.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite heat-resistant coated lithium battery separator, characterized in that, A ternary composite coating is applied to one or both sides of a polypropylene diaphragm as a substrate. The ternary composite coating comprises polymethyl methacrylate, surface-modified lithium zirconium phosphate, and surface-borate-functionalized polystyrene microspheres.

2. The composite heat-resistant coated lithium battery separator as described in claim 1, characterized in that, The surface-modified lithium zirconium phosphate is obtained by surface modification of lithium zirconium phosphate powder using a phosphate ester-based coupling agent.

3. The composite heat-resistant coated lithium battery separator as described in claim 2, characterized in that, The phosphate-based coupling agent includes bis(2-methacryloyloxyethyl) phosphate, which is used in an amount of 1.0-5.0% of the mass of lithium zirconium phosphate powder.

4. The composite heat-resistant coated lithium battery separator as described in claim 1, characterized in that, The surface boric acid functionalized polystyrene microspheres have a particle size of 100-500 nm, and their surface is covalently modified with phenylboronic acid groups.

5. The composite heat-resistant coated lithium battery separator as described in claim 1, characterized in that, The mass ratio of polymethyl methacrylate, surface-modified lithium zirconium phosphate, and surface borate-functionalized polystyrene microspheres is (65-80):(15-30):(2-8).

6. A method for preparing a composite heat-resistant coated lithium battery separator as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Surface modification of lithium zirconium phosphate powder was performed using a phosphate ester-based coupling agent to obtain surface-modified lithium zirconium phosphate; surface boric acid-functionalized polystyrene microspheres were prepared and dispersed in a solvent to form a microsphere dispersion; polymethyl methacrylate was dried. S2. Dissolve the dried polymethyl methacrylate in a mixed solvent to obtain a polymer solution; under stirring, add the surface-modified lithium zirconium phosphate and the microsphere dispersion to the polymer solution in sequence, and after dispersion, ultrasonication and filtration, obtain a uniform ternary composite slurry; S3. The ternary composite slurry obtained in step S2 is coated on one or both sides of the polypropylene diaphragm substrate and dried in a segmented process to obtain the coated diaphragm. S4. The coated separator obtained in step S3 is subjected to heat setting treatment at a temperature of 100-120℃ for 5-15 minutes to obtain the composite heat-resistant coated lithium battery separator.

7. The preparation method according to claim 6, characterized in that, In step S1, the preparation conditions for the surface-modified lithium zirconium phosphate are as follows: lithium zirconium phosphate powder and phosphate ester coupling agent are mixed at 60-80℃ and 300-500 rpm and reacted in anhydrous ethanol for 4-8 hours.

8. The preparation method according to claim 6, characterized in that, In step S1, the preparation method of the surface boric acid functionalized polystyrene microspheres includes the following steps: first, preparing polystyrene microspheres, then reacting them with 3-(acryloyloxy)propylboronic acid pinacol ester, and then hydrolyzing them by acid treatment.

9. The preparation method according to claim 6, characterized in that, In step S2, the mixed solvent includes a toluene-ethyl acetate mixed solvent or a butyl acetate-toluene mixed solvent; the total solid mass concentration in the ternary composite slurry is 10-20%.

10. The preparation method according to claim 6, characterized in that, In step S3, the segmented drying process includes: first drying at 40-60℃ for 10-20 minutes, and then drying at 70-90℃ for 20-40 minutes, and the drying process is carried out under nitrogen protection.

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