Composite separator and secondary battery

Abstract

The application discloses a kind of composite diaphragm and secondary battery, the composite diaphragm includes base film and the coating of at least one side surface of base film, coating includes first polymer, and first polymer is the secondary particle of non-fluorine polymer agglomeration, non-fluorine polymer includes core and the shell layer of covering in the core surface, core includes polymer A, and shell layer includes polymer B, the crosslinking degree of polymer A is X1, and the crosslinking degree of polymer B is X2, 60%≤X1≤70%, 10%≤X2≤20%.The composite diaphragm not only ensures the mechanical strength of the overall structure of battery cell, but also solves the adhesion force attenuation problem caused by the liquid absorption of the adhesive layer in the hot-pressing liquid injection process of traditional composite diaphragm, so as to improve the problems such as battery cell softening, electrode sheet wrinkling and lithium precipitation.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a composite separator and a secondary battery. Background Technology

[0002] With the widespread application of lithium-ion batteries in consumer electronics, electric vehicles, and energy storage systems, higher requirements have been placed on battery safety, cycle life, and mechanical stability. As a key component of lithium-ion batteries, the separator's main function is to isolate the positive and negative electrodes to prevent short circuits, while allowing lithium ions to move freely during charging and discharging. To further improve the separator's thermal stability and adhesion to the electrodes, coated separators are widely used in high-performance battery cells. The coating typically comprises inorganic particles and organic polymer binders, with the organic binder playing a crucial role in achieving effective adhesion between the coating and the electrode sheets.

[0003] In early technological solutions, polyvinylidene fluoride (PVDF) was widely used as a binder material in coated separators due to its good chemical stability, film-forming properties, and certain adhesive capabilities. Through hot-pressing, PVDF could enhance the bonding force between the coated separator and the electrode sheets, thereby improving the overall rigidity and structural stability of the bare battery cell. However, as battery cell designs have evolved towards larger sizes and higher energy densities, the requirements for adhesive strength have significantly increased, and the traditional PVDF system can no longer meet the mechanical support needs of larger-volume cells. Furthermore, the EU's ban on fluorine has restricted the application of fluoropolymers, driving the research and application of non-fluoropolymer alternatives.

[0004] Against this backdrop, non-fluoropolymers such as polymethyl methacrylate (PMMA) are considered ideal alternatives due to their higher initial bond strength. These amorphous organic polymers can effectively improve the interfacial adhesion between the coated separator and the electrode sheets under hot-pressing conditions, resulting in higher mechanical hardness in the assembled bare cell. However, in practical applications, it has been found that these non-fluoropolymers exhibit a significant tendency for electrolyte swelling. After the cell completes electrolyte injection and enters the formation stage, the amorphous polymer continuously absorbs electrolyte and swells, leading to the destruction of its molecular network structure and the gradual failure of adhesive functional sites. This, in turn, causes a significant decrease in the wet adhesion between the coated separator and the electrode sheets. This phenomenon not only weakens the interfacial bonding state but also results in the cell failing to effectively suppress the expansion behavior of the negative electrode material during cycling due to insufficient interfacial constraints, easily causing problems such as electrode wrinkling and lithium plating, severely deteriorating the cell's cycle performance. Simultaneously, as the adhesive performance degrades, the overall mechanical strength of the cell decreases, exhibiting a "softening" phenomenon, posing challenges to subsequent packaging, storage, and transportation.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a composite separator and a secondary battery to improve the above-mentioned technical problems.

[0007] This invention is implemented as follows:

[0008] In a first aspect, the present invention provides a composite membrane comprising a base membrane and a coating located on at least one side surface of the base membrane, the coating comprising a first polymer, the first polymer being secondary particles formed by the agglomeration of a non-fluoropolymer, the non-fluoropolymer comprising a core and a shell coating the surface of the core, the core comprising polymer A, the shell comprising polymer B, the degree of crosslinking of polymer A being X1, the degree of crosslinking of polymer B being X2, 60%≤X1≤70%, 10%≤X2≤20%.

[0009] In an optional embodiment, the polymer A accounts for 60% to 80% of the total mass of the non-fluoropolymer, and the polymer B accounts for 20% to 40% of the total mass of the non-fluoropolymer.

[0010] In an optional embodiment, the glass transition temperature of polymer A is Tg1, the glass transition temperature of polymer B is Tg2, 30℃≤Tg1≤60℃, 30℃≤Tg2≤60℃, and |Tg1-Tg2|≤5℃.

[0011] And / or, the swelling ratio of polymer A is η1, 10%≤η1≤20%, and the overall swelling ratio of the non-fluoropolymer is η2, 50%≤η2≤100%;

[0012] And / or, the particle size of the non-fluoropolymer is 0.1 μm to 0.8 μm;

[0013] And / or, the particle size of the first polymer is 4 μm to 8 μm.

[0014] In an optional embodiment, both polymer A and polymer B are polymerized from monomers comprising a first polymerizing monomer, a second polymerizing monomer, and a crosslinking monomer, wherein the crosslinking monomer accounts for 6% to 8% of the mass of polymer A and 1% to 3% of the mass of polymer B.

[0015] In an optional embodiment, the first polymerizing monomer is selected from at least one of methyl methacrylate, methacrylonitrile, styrene, and acrylonitrile; the second polymerizing monomer is selected from at least one of ethyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, methyl acrylate, n-butyl acrylate, and butyl methacrylate.

[0016] And / or, the crosslinking monomer is selected from at least one of divinylbenzene, diallyl phthalate, diethanol diacrylate, trimethylolpropane trimethacrylate, vinyltriethoxysilane, and vinyltrimethoxysilane.

[0017] In an optional embodiment, the coating further includes a second polymer selected from at least one of polyacrylic acid, polyacrylamide, polyacrylonitrile, and polymethacrylonitrile.

[0018] In an optional embodiment, the glass transition temperature of the second polymer is >80°C;

[0019] And / or, the mass of the second polymer accounts for 2% to 10% of the total mass of the coating.

[0020] In an optional embodiment, the coating is a discontinuous coating; the discontinuous coating has at least one of the following characteristics:

[0021] a. Contains multiple independent units arranged at equal intervals; preferably, the maximum length of the independent unit is L1, 50μm≤L1≤800μm, where L1 is the diameter of the circumscribed circle of the independent unit graphic; the distance between the edges of two equally spaced independent units is L2, 50μm≤L2≤500μm, and L1 and L2 satisfy the following relationship: 0.5≤L1 / L2≤1.6;

[0022] b. The overall coverage of the discontinuous coating is between 6% and 30%;

[0023] c. The basis weight of the discontinuous coating is 0.2 g / m³. 2 ~1.0g / m 2 .

[0024] In an optional embodiment, the hot-pressing adhesion force of the two composite membranes after their coated surfaces are bonded together at 70°C to 85°C is F1, and the wet-pressing adhesion force of the two composite membranes after being bonded together and hot-pressed together and then immersed in lithium salt electrolyte at 60°C for 24 hours is F2. Then the effective adhesion coefficient of the composite membrane is K, K=F2 / F1, and 0.80≤K<1.0;

[0025] And / or, the self-adhesive force of the two composite diaphragms after their coated surfaces are bonded together at room temperature (25°C) is F3, where F3 ≤ 0.5 N / m;

[0026] And / or, the hot-pressing adhesion force between the coating surface of the composite diaphragm and the electrode sheet at 70℃~85℃ is F4, 3N / m≤F4≤10N / m.

[0027] Secondly, the present invention also provides a secondary battery comprising the aforementioned composite separator.

[0028] This invention offers the following advantages: by employing a core-shell structure design for the non-fluoropolymer, the core exhibits a significantly higher degree of crosslinking than the shell. This structural design provides a dual advantage: firstly, the highly crosslinked core ensures the polymer possesses an extremely low swelling rate, maintaining its structural integrity even after absorbing electrolyte, thus providing effective support without affecting its adhesive properties; secondly, the lower degree of crosslinking in the shell guarantees good adhesion between the polymer and the electrode sheets. When this non-fluoropolymer is applied to a coated separator and assembled with electrode sheets into a battery cell, after electrolyte injection, hot pressing, and formation processes, the wet adhesion performance between the coated separator and the electrode sheets does not show a significant decrease compared to the dry adhesion performance, effectively meeting the battery cell's overall hardness requirements. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the dot matrix distribution of the discontinuous coating of the composite diaphragm in Embodiment 1 of the present invention.

[0031] Icons: 1 - Independent unit of discontinuous coating; 2 - Base film; L2 - Distance between independent units. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0033] The present invention provides a specific description of a composite separator and a secondary battery.

[0034] Some embodiments of the present invention provide a composite membrane comprising a base membrane and a coating on at least one side of the base membrane. The coating comprises a first polymer, which is a secondary particle formed by the agglomeration of a non-fluoropolymer. The non-fluoropolymer comprises a core and a shell covering the surface of the core. The core comprises polymer A and the shell comprises polymer B. The degree of crosslinking of polymer A is X1 and the degree of crosslinking of polymer B is X2, where 60%≤X1≤70% and 10%≤X2≤20%.

[0035] The non-fluoropolymer constituting the first polymer employs a highly cross-linked core and a low-cross-linked shell design, combining excellent adhesive properties and structural stability. Specifically, the highly cross-linked core maintains an extremely low swelling rate when absorbing electrolyte, effectively preserving the integrity of the polymer structure and preventing adhesion site failure. While the low-cross-linked shell can swell moderately, its cross-linked structure prevents unlimited expansion, ensuring good thermo-pressing adhesion to the electrode sheet. This unique structural design not only guarantees a strong bond between the non-fluoropolymer and the electrode sheet but also effectively solves problems such as structural collapse and deterioration of adhesive properties caused by electrolyte absorption.

[0036] It should be noted that the core component is polymer A, and the shell component is polymer B.

[0037] In some embodiments, polymer A accounts for 60% to 80% of the total mass of the non-fluoropolymer. Exemplarily, it can be selected as 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, or 80%, etc., while polymer B accounts for 20% to 40% of the total mass of the non-fluoropolymer. Exemplarily, it can be selected as 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%, etc. By limiting the main components in the core and shell, a better balance can be achieved between the swelling ratio and adhesive properties of the non-fluoropolymer.

[0038] In some embodiments, the glass transition temperature of polymer A is Tg1, the glass transition temperature of polymer B is Tg2, 30℃≤Tg1≤60℃, 30℃≤Tg2≤60℃, and |Tg1-Tg2|≤5℃.

[0039] The glass transition temperature (Tg) of the non-fluoropolymer core and shell is designed within the range of 30–60°C. This design offers two advantages: firstly, it effectively prevents self-adhesion of the coated diaphragm at room temperature; secondly, it ensures good adhesion between the diaphragm and the electrode sheet during the hot-pressing process. It is important to note that a Tg design value that is too low may lead to self-adhesion of the coated diaphragm; while a Tg design value that is too high will significantly reduce the hot-pressing adhesion between the diaphragm and the electrode sheet. In the core-shell structure design, the Tg settings of the core and shell layers can be differentiated (the core Tg can be lower or higher than the shell Tg), but the temperature difference must be controlled within a small range. This design principle aims to maintain material compatibility between the core and shell layers. When the Tg difference between the core and shell layers is too large, interfacial compatibility will significantly deteriorate, especially after the non-fluoropolymer absorbs the electrolyte, easily leading to separation between the shell and core layers, thereby impairing the adhesion performance between the diaphragm and the electrode sheet. Based on this, the Tg design prioritizes setting the Tg of the core and shell layers to the same level.

[0040] Specifically, based on the above design, in some embodiments, the swelling ratio of polymer A is η1, 10%≤η1≤20%, and the comprehensive swelling ratio of the non-fluoropolymer is η2, 50%≤η2≤100%. This swelling ratio ensures that the structure can still maintain its integrity after absorbing the electrolyte, and will not cause its bonding sites to fail.

[0041] In some embodiments, the particle size of the non-fluoropolymer is 0.1 μm to 0.8 μm. This particle size range can meet the combined requirements of adhesive strength, ionic conductivity, and cycle stability.

[0042] Furthermore, in some embodiments, both polymer A and polymer B contain a first polymerization unit, a second polymerization unit, and a crosslinking monomer in their molecular chains. The crosslinking monomer accounts for 6% to 8% of the mass of polymer A and 1% to 3% of the mass of polymer B. By precisely controlling the addition ratio of the crosslinking monomer, a differentiated crosslinking degree distribution can be established between polymer A and polymer B. This gradient crosslinking structure provides an effective way to achieve performance regulation of core-shell non-fluoropolymers.

[0043] For reference, the first polymerization unit is selected from at least one of methyl methacrylate, methacrylonitrile, styrene and acrylonitrile; the second polymerization unit is selected from at least one of ethyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, methyl acrylate, n-butyl acrylate and butyl methacrylate.

[0044] For example, the crosslinking monomer is selected from at least one of divinylbenzene, diallyl phthalate, diethanol diacrylate, trimethylolpropane trimethacrylate, vinyltriethoxysilane, and vinyltrimethoxysilane.

[0045] Furthermore, some embodiments of the present invention also provide polymer particles comprising secondary particles agglomerated from the non-fluoropolymer described in any of the foregoing embodiments, with a particle size of 4 μm to 8 μm. The particle size of these polymer particles is larger than the pores of the membrane substrate, thus not blocking ion channels, and the interparticle gaps and internal micropores accelerate electrolyte wetting, ensuring ion conduction efficiency.

[0046] In some embodiments, the coating further includes a second polymer selected from at least one of polyacrylic acid, polyacrylamide, polyacrylonitrile, and polymethacrylonitrile. The second polymer's molecular chain contains strongly polar groups such as carboxyl groups (-COOH) and amide groups (-CONH2), which can form hydrogen bonds with the hydroxyl groups and active sites on the surface of the membrane substrate (PE / PP), and exhibit strong adsorption with electrode active materials (such as silicon carbide and lithium iron phosphate), significantly improving the interfacial adhesion between the coating and the membrane / electrode, preventing coating peeling or secondary particle shedding during cycling. Furthermore, the second polymer has a linear or weakly cross-linked structure, which can fill the gaps between non-fluorinated secondary particles, forming a porous network of "secondary particles-linear polymer," retaining ion transport channels while enhancing the electrolyte's adsorption and retention capacity. The nitrile groups (-CN) of polyacrylonitrile and polymethacrylonitrile can also enhance compatibility with the electrolyte, accelerate electrolyte penetration, and reduce interfacial resistance.

[0047] Specifically, the glass transition temperature of the second polymer is >80℃; the mass of the second polymer accounts for 2% to 10% of the total mass of the coating, for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%. The second polymer with a glass transition temperature >80℃ has a rigid molecular chain, exhibiting a glassy state at room temperature, and its chain segment movement remains restricted at high temperatures (>80℃), which can suppress the thermal softening and creep of non-fluorinated polymers. This 2% to 10% proportion can form a "rigid skeleton" in the coating, working synergistically with the microporous structure of the non-fluorinated secondary particles to significantly reduce the thermal shrinkage rate of the separator at high temperatures, preventing coating collapse and pore blockage at high temperatures, and improving battery thermal safety.

[0048] In some embodiments, the base film is selected from PE base film, PP base film, PP and PE composite base film, and has a thickness of 4μm to 16μm, such as 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm or 16μm.

[0049] Furthermore, in some embodiments, the coating is a discontinuous coating.

[0050] After the electrolyte filling, hot pressing, and formation processes are completed, the coated separator and electrode sheets maintain good interfacial adhesion. However, the electrode sheets expand and generate internal stress after absorbing electrolyte. If this stress cannot be effectively released, wrinkling may occur on the electrode sheets. Therefore, a discontinuous coating process design is adopted, with advantages in two aspects: first, ensuring excellent wet adhesion strength between the coated separator and the electrode sheets; second, providing sufficient stress release space for the electrode sheets through a reasonable coating interval design. This design effectively prevents wrinkling of the electrode sheets due to stress concentration during the electrolyte filling, hot pressing, and formation stages, as well as during subsequent long-cycle processes, thus avoiding lithium plating problems caused by this.

[0051] Specifically, in order to ensure that the discontinuous coating has better adhesion and maximizes the preservation of the membrane's air permeability, ion conduction efficiency, and flexibility, in some embodiments, the discontinuous coating has at least one of the following characteristics:

[0052] a. Contains multiple equally spaced independent units. In some embodiments, the maximum length of an independent unit is L1, 50μm≤L1≤800μm, where L1 is the diameter of the circumscribed circle of the independent unit pattern; the distance between the edges of two equally spaced independent units is L2, 50μm≤L2≤500μm, and L1 and L2 satisfy the following relationship: 0.5≤L1 / L2≤1.6;

[0053] b. The overall coverage of discontinuous coatings ranges from 6% to 30%, for example, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%. This low coverage of 6% to 30% avoids the "excessively high interfacial impedance" problem of continuous coatings: coating units only bond at key contact points, while the gaps retain electrolyte transport channels, ensuring unimpeded ion conduction efficiency in a wet state. This coverage also ensures that the compressive stress of the electrode during hot pressing can be released through the gaps, preventing microcracks at the coating-electrode interface due to stress concentration; the volume expansion of the SEI film formed on the electrode surface during the formation stage can also be dispersed through the gaps, preventing wrinkling and deformation of the electrode sheet. If the coverage is less than 6%, the coating units are sparse, the bonding sites are insufficient, the wet bonding strength decreases, and it is impossible to suppress the shedding during the electrode cycling process; if the coverage is greater than 30%, the coating spacing is too small, the stress release space is insufficient, the electrode sheet is prone to wrinkles due to stress concentration during the hot pressing / formation stage, and the membrane permeability increases significantly, hindering ion conduction.

[0054] c. The basis weight of the discontinuous coating is 0.2 g / m². 2 ~1.0g / m 2For example, 0.2g / m 2 0.3g / m 2 0.4g / m 2 0.5g / m 2 0.6g / m 2 0.7g / m 2 0.8g / m 2 0.9g / m 2 or 1g / m 2 This weight range falls within the category of ultra-thin coatings, far lower than traditional continuous coatings. The coating is distributed only as discrete points / islands on the separator surface, without increasing the overall thickness of the separator. The flexural modulus and flexibility of the separator are basically the same as the original substrate, perfectly adapting to the electrode assembly process of battery winding / stacking, avoiding the risk of core wrinkling and short circuits caused by separator hardening. The extremely low coating amount will not block the micropore channels of the separator, and the ion conduction efficiency is almost unaffected, ensuring the battery charge and discharge performance at high rates. Moreover, this weight range can form uniform discrete coating sites, meeting the basic requirements for wet adhesion strength, while providing sufficient stress release intervals for the electrodes; if it is below 0.2 g / m², the coating sites are sparse, the adhesion strength is insufficient, and the coating is prone to peeling off during cycling. Furthermore, the coating site density within this weight range is moderate, and the spacing will not shrink due to excessive coating amount, thus maintaining sufficient stress buffer space and effectively preventing wrinkles on the electrode during hot pressing and formation stages. If it exceeds 1.0 g / m², the coating is prone to change from "discrete" to "semi-continuous", resulting in insufficient stress release space and a significant increase in air permeability.

[0055] Through the structural design and selection of the membrane coating polymer and the related design of the discontinuous coating in the above embodiments, the performance of the composite membrane can meet the following requirements:

[0056] The hot-pressing adhesion force of the two composite membranes after their coated surfaces are bonded together at 70℃~85℃ is F1. The wet-pressing adhesion force of the two composite membranes after hot-pressing together and immersing them in lithium salt electrolyte at 60℃ for 24 hours is F2. The effective adhesion coefficient of the composite membrane is K, K=F2 / F1, and 0.80≤K<1.0.

[0057] The self-adhesive force of the two composite diaphragms after their coated surfaces are bonded together at room temperature (25°C) is F3, where F3 ≤ 0.5 N / m.

[0058] The hot-pressing adhesion force between the coating surface of the composite diaphragm and the electrode sheet at 70℃~85℃ is F4, 3N / m≤F4≤10N / m.

[0059] Some embodiments of the present invention also provide a secondary battery comprising a composite separator as described in any of the foregoing embodiments.

[0060] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0061] Example 1

[0062] This embodiment provides a method for preparing a composite membrane, the specific steps of which are as follows:

[0063] (1) Synthesis of the core of the non-fluorinated polymer: 7.0 parts of sodium dodecylbenzenesulfonate and 3000 parts of deionized water were placed in a reactor, nitrogen gas was introduced to purge the air in the reactor, 446.9 parts of acrylonitrile, 191.5 parts of 2-ethylhexyl acrylate and 49.0 parts of vinyltriethoxysilane were added, and 0.6 mol / L ammonia water was used to adjust the pH to about 7~8. Then the temperature and pressure were increased to 80℃ and 3.0 MPa. After the temperature and pressure stabilized, the stirring speed was adjusted to 200 rpm / min, and 5.6 parts of sodium persulfate were added. The polymerization reaction was carried out at constant temperature and pressure for 8 hours. The temperature and pressure were then reduced to room temperature and normal pressure. The swelling rate, Tg and crosslinking degree of the non-fluorinated polymer primary particle core layer polymer A were tested.

[0064] (2) Synthesis of non-fluorinated polymer (primary particles): 204.3 parts acrylonitrile, 87.6 parts 2-ethylhexyl acrylate and 6.0 parts vinyltriethoxysilane were added to the reactor. The temperature and pressure were increased to 70℃ and 4.0MPa. When the temperature and pressure stabilized, the stirring speed was adjusted to 80rpm / min. 2.1 parts sodium persulfate were added. The polymerization reaction was carried out at constant temperature and pressure for 12h. Then the temperature and pressure were reduced to room temperature and pressure to obtain a core-shell structured non-fluorinated polymer primary particle emulsion. The particle size, swelling ratio, Tg and crosslinking degree of the non-fluorinated polymer primary particles were tested.

[0065] (3) Preparation of emulsion of first polymer particles: Take 1000 parts of the emulsion synthesized in step (2) and put it into the reaction vessel. Use ultraviolet light with a wavelength of 280nm for UV irradiation for 30min. Add 2.5 parts of 0.5Mol / L potassium nitrate, adjust the rotation speed to 100rpm / min, and stir for 4h to break up and aggregate the non-fluorinated polymer primary particle emulsion, and finally obtain the emulsion of first polymer particles. Test the particle size of the first polymer particles.

[0066] (4) Preparation of the composite membrane: The emulsion of the first polymer particles was diluted with deionized water to a solid content of 20%. Then, 500 parts of the diluted emulsion of the first polymer particles were added, along with 6.0 parts of polyacrylamide with a Tg of 85℃ and 0.5 parts of alkylphenol polyoxyethylene ether. The mixture was stirred evenly to obtain a mixed slurry. Using microgravure dot coating technology, the microgravure dot matrix specifications and coating speed ratio were adjusted, and the mixed slurry was coated onto one side of the Jieli 7μm base film. After drying, the circular particles with a diameter of 200μm, a particle spacing of 200μm, a coverage of 19.6%, and a surface density of 0.5g / m³ were obtained. 2lattice composite membrane (lattice distribution as follows) Figure 1 As shown in the figure, the air permeability, effective adhesion coefficient K, self-adhesion force F3 of the diaphragm, and hot-press adhesion force F4 between the composite diaphragm and the electrode sheet were tested.

[0067] The composite separator and positive and negative electrodes were used to prepare the battery cell. The hardness decay, DC resistance degradation, electrode state and lithium plating state after cycling were tested before and after cycling, as well as the cycle performance of the battery cell.

[0068] Example 2

[0069] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 451.8 parts, 193.6 parts and 42.0 parts respectively, while the rest is the same as in Example 1.

[0070] Example 3

[0071] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 442.0 parts, 189.4 parts and 56.0 parts respectively, and the rest is the same as in Example 1.

[0072] Example 4

[0073] The only difference between this embodiment and Example 1 is that in step (2), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 206.4 parts, 88.5 parts and 3.0 parts respectively, while the rest is the same as in Example 1.

[0074] Example 5

[0075] The only difference between this embodiment and Example 1 is that in step (2), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 202.2 parts, 86.7 parts and 9.0 parts respectively, while the rest is the same as in Example 1.

[0076] Example 6

[0077] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 421.3 parts, 217.1 parts, and 49.0 parts, respectively; and in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 192.7 parts, 99.2 parts, and 6.0 parts, respectively. The rest is the same as in Example 1.

[0078] Example 7

[0079] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 472.4 parts, 166.0 parts, and 49.0 parts, respectively; and in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 216.0 parts, 75.9 parts, and 6.0 parts, respectively. The rest is the same as in Example 1.

[0080] Example 8

[0081] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 510.7 parts, 218.9 parts, and 56.0 parts, respectively, and sodium dodecylbenzenesulfonate and sodium persulfate are 8.0 parts and 6.4 parts, respectively; in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 136.2 parts, 58.4 parts, and 4.0 parts, respectively, and sodium persulfate is 1.4 parts, and the rest is the same as in Example 1.

[0082] Example 9

[0083] The only difference between this embodiment and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 383.0 parts, 164.2 parts, and 42.0 parts, respectively, and sodium dodecylbenzenesulfonate and sodium persulfate are 6.0 parts and 4.8 parts, respectively; in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 272.4 parts, 116.8 parts, and 8.0 parts, respectively, and sodium persulfate is 2.8 parts, and the rest is the same as in Example 1.

[0084] Comparative Example 1

[0085] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 456.7 parts, 195.7 parts and 35.0 parts respectively, and the rest is the same as in Example 1.

[0086] Comparative Example 2

[0087] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 437.1 parts, 187.3 parts and 63.0 parts respectively, and the rest is the same as in Example 1.

[0088] Comparative Example 3

[0089] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 207.3 parts, 88.8 parts and 1.8 parts respectively, and the rest is the same as in Example 1.

[0090] Comparative Example 4

[0091] The only difference between this comparative example and Example 1 is that in step (2), acrylonitrile, 2-ethylhexyl acrylate and vinyltriethoxysilane are 200.1 parts, 85.8 parts and 12.0 parts respectively, and the rest is the same as in Example 1.

[0092] Comparative Example 5

[0093] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 542.6 parts, 232.6 parts, and 59.5 parts, respectively, and sodium dodecylbenzenesulfonate and sodium persulfate are 8.5 parts and 6.8 parts, respectively; in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 102.2 parts, 43.8 parts, and 3.0 parts, respectively, and sodium persulfate is 1.1 parts, and the rest is the same as in Example 1.

[0094] Comparative Example 6

[0095] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 351.1 parts, 150.5 parts, and 38.5 parts, respectively, and sodium dodecylbenzenesulfonate and sodium persulfate are 4.5 parts and 4.4 parts, respectively; in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 306.5 parts, 131.4 parts, and 9.0 parts, respectively, and sodium persulfate is 3.2 parts, and the rest is the same as in Example 1.

[0096] Comparative Example 7

[0097] The only difference between this comparative example and Example 1 is that, in step (4), the microgravure dot matrix specifications and coating speed ratio are adjusted to obtain circular particles with a diameter of 90 μm, a particle spacing of 300 μm, a coverage rate of 4.2%, and a surface density of 0.5 g / m³. 2 The lattice composite diaphragm is the same as in Example 1.

[0098] Comparative Example 8

[0099] The only difference between this comparative example and Example 1 is that, in step (4), the microgravure dot matrix specifications and coating speed ratio are adjusted to obtain circular particles with a diameter of 600 μm, a particle spacing of 300 μm, a coverage of 34.9%, and a surface density of 0.5 g / m³. 2 The lattice composite diaphragm is the same as in Example 1.

[0100] Comparative Example 9

[0101] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 395.8 parts, 242.6 parts, and 49.0 parts, respectively; and in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 181.0 parts, 110.9 parts, and 6.0 parts, respectively. The rest is the same as in Example 1.

[0102] Comparative Example 10

[0103] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 491.6 parts, 146.8 parts, and 49.0 parts, respectively; and in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 224.8 parts, 67.1 parts, and 6.0 parts, respectively. The rest is the same as in Example 1.

[0104] Comparative Example 11

[0105] The only difference between this comparative example and Example 1 is that in step (1), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 446.9 parts, 191.5 parts, and 49.0 parts, respectively; and in step (2), acrylonitrile, 2-ethylhexyl acrylate, and vinyltriethoxysilane are 213.1 parts, 78.8 parts, and 6.0 parts, respectively. The rest is the same as in Example 1.

[0106] Performance testing

[0107] 1. Polymer performance testing

[0108] 1.1 Particle size testing of non-fluoropolymer and first polymer particles: Non-fluoropolymer (primary particle) emulsion and first polymer particle emulsion were taken, and their particle sizes were tested using a Malvern 3000. The test results showed a refractive index of 1.52, an absorptivity of 0.1, and an opacity of 8%–16%. The particle sizes D1 of the core-shell structured non-fluoropolymer and D2 of the first polymer particles were then obtained.

[0109] 1.2 Swelling rate test of polymer A and core-shell structured non-fluoropolymer: The emulsion of core polymer A was dried to obtain a film. The weight of the film was M0. The film was immersed in lithium salt electrolyte (EC:EMC:DEC=3:5:2, 1.5mol / L LiPF6) at 60℃ for 48h. The film was removed, the surface electrolyte was wiped off, and the weight of the film after immersion was M1. The swelling rate of core polymer A is η1=(M1-M0) / M0. Similarly, the swelling rate η2 of core-shell structured non-fluoropolymer can be measured.

[0110] 1.3. Tg Test of Core Polymer A and Shell Polymer B: Emulsions of core polymer A and the core-shell non-fluoropolymer were dried, and 5-10 mg of each sample was weighed. Differential scanning calorimetry (DSC) was used to test the Tg. The testing equipment was a METTLER DSC3, with a temperature range of -80 to 120℃ and a heating rate of 1℃ / min. During the test, the first heating and cooling was performed to eliminate thermal history before testing the Tg of the polymers. After the test, the test curves were integrated to obtain the Tg1 of core polymer A and the Tg of the core-shell non-fluoropolymer. 2-1 and Tg 2-2 (Tg) 2-1 and Tg 2-2 There is one Tg1 for the core polymer A and another Tg2 for the shell polymer B.

[0111] 1.4 Crosslinking Degree Test:

[0112] Crosslinking degree test of core polymer A: Dry the emulsion of core polymer A, weigh a certain mass of film M0, immerse the film in tetrahydrofuran reagent at room temperature for 48 hours, remove the film, wash the film three times with alcohol, then dry the film and weigh the mass of the dried film M1. The crosslinking degree of core polymer A X1 = M1 / M0

[0113] Crosslinking degree test of shell polymer B: Take a non-fluoropolymer primary particle emulsion, dry it, and weigh a certain mass of film m1 (assuming the shell ratio is P, then the shell mass is P). m1, kernel quality is m1-P m1, where P can be calculated from the total mass of the core monomers and the total mass of the shell monomers added during the synthesis of the core-shell non-fluorinated polymer), the film is soaked in tetrahydrofuran reagent for 48 hours, the film is taken out, the film is washed with alcohol three times, and then the film is dried. The mass of the dried film is weighed m2. Then the degree of crosslinking of the shell polymer X2 = (m2 - (1 - P)). m1 X1) / (P m1).

[0114] 2. Composite diaphragm performance testing

[0115] 2.1 Composite membrane coating air permeability growth rate test: Air permeability refers to the time required for 100ml of gas to pass through a fixed area membrane. Air permeability growth rate = (composite membrane air permeability - base membrane air permeability) / base membrane air permeability 100%.

[0116] 2.2 Effective Adhesion Coefficient K Test: Take the composite diaphragm and cut it into 2cm pieces. For a 10cm specification, take two composite membranes, overlap them with the coated surfaces facing each other, and place the stacked membranes in a hot press. Adjust the hot press temperature to 80℃, pressure to 3MPa, and time to 1min. After hot pressing, cut the composite membranes into 2cm pieces. Two 5cm samples were used. One sample was used to test the hot-press adhesion F1 between the composite separator and the composite separator. The other sample was immersed in a lithium salt electrolyte (EC:EMC:DEC=3:5:2, 1.5mol / L LiPF6) at 60℃ for 24h. After immersion, the sample was removed, the electrolyte on the surface was wiped off, and the wet-press adhesion F2 between the composite separators was tested. Then K=F2 / F1. Ten parallel tests were performed, and the final average value was taken.

[0117] 2.3. Composite diaphragm self-adhesion strength F3 test: Take the composite diaphragm and cut it into 2cm pieces. For a 10cm specification, take two composite membranes, stack them with the coated surfaces facing each other, and put the stacked membranes into a hot press. Adjust the hot press temperature to 25℃, the pressure to 1MPa, and the time to 5min. After pressing, test the self-adhesion force F3 between the composite membranes.

[0118] 2.4. Hot-pressing adhesion test (F4) between composite diaphragm and electrode sheet: Take the composite diaphragm and cut it into 2cm pieces. For a 10cm specification, take an electrode sheet (aluminum foil + 95.5 parts lithium iron phosphate + 3.5 parts PVDF + 1 part conductive agent SP), and cut the electrode sheet into 2cm pieces. For a 10cm specification, the coated surfaces of the composite diaphragm and the coated surfaces of the electrode sheet are stacked together and then placed in a hot press. The temperature of the hot press is adjusted to 80℃, the pressure to 3MPa, and the time to 1min. After pressing, the hot pressing adhesion force F4 between the composite diaphragm and the electrode sheet is tested.

[0119] 2.5. Test of maximum length L1 of independent unit: Take an electron microscope with a coated diaphragm and measure at 2.56mm. Under a field of view of 1.92mm, select five independent units, draw the circumcircle of each of the five independent units, measure the diameter of the circumcircle, and take the average of the diameters of the five independent units as the maximum length of the independent unit of the sample. In the same way, test 10 samples in parallel and take the final average value as the maximum length L1 of the independent unit.

[0120] 2.6 Test of distance L2 between independent units: Take an electron microscope with a diaphragm-coated lens and measure the distance at 2.56mm. Under a field of view of 1.92mm, the circumscribed circle of each independent unit is drawn. Along the reverse direction of the diaphragm MD or TD, the shortest distance between the circumscribed circles of two adjacent independent units is measured. Five groups are randomly tested, and the average value is recorded as the distance between independent units of the sample. The same method is used to test 10 samples in parallel, and the final average value is taken as the distance L2 between independent units.

[0121] 2.7 Coverage Test: Coated diaphragm coverage = 3.14 (L1) L1) / (4 (L1+L2)).

[0122] 3. Cell performance testing

[0123] 3.1 Hardness Attenuation Test Before and After Cell Cycling: Bare cells were fabricated using a stacking process with the composite separators from the examples and comparative examples, along with lithium iron phosphate positive and graphite negative electrodes. The lithium iron phosphate positive electrode consisted of 95.1 wt% LFP + 2.0 wt% PVDF + 0.8 wt% SP + 2 wt% CNT + 0.1 wt% plasticizer; the graphite negative electrode consisted of 96.0 wt% graphite + 1.4 wt% conductive carbon + 1.4 wt% CMC + 1.2 wt% styrene-butadiene rubber. The bare cells were charged at a constant current of 0.5C to 3.6V, then charged at a constant voltage of 3.6V to a current of 0.02C, charging was terminated, and the cells were allowed to stand for 30 minutes. Discharge at a constant current of 0.5C to 2.0V, stop the discharge, let stand for 30 minutes, and then test the hardness R1 of the bare cell. Cycle charge and discharge for 300 cycles according to the parameters of the first cycle, and record the hardness R2 of the bare cell after the 300th cycle. Then the hardness decay rate of the cell before and after the cycle is (R1-R2) / R1.

[0124] 3.2. Cell Cycling Tests: The composite separators of the examples and comparative examples were used to form cells with lithium iron phosphate positive electrode sheets and graphite negative electrode sheets using a winding process. The cells were charged at 3C constant current to 3.6V, and then charged at 3.6V constant voltage to 0.02C. The charging was then terminated and the cells were left to stand for 30 minutes. Discharged at a constant current of 3C to 2.0V, then discharged and allowed to stand for 30 minutes. The first discharge capacity was recorded. The cell was then cycled 300 times using the same parameters as the first cycle. The discharge capacity of the 300th cycle was recorded. The capacity retention rate after 300 cycles of high-temperature treatment was calculated by dividing the 300-cycle discharge capacity by the first cycle capacity. The cell was then disassembled to observe the wrinkling and lithium plating on the electrodes. Slight wrinkles were characterized by shallow, smooth wavy lines or slight creases without sharp edges, narrow width, and a protrusion height generally less than 10μm. Severe wrinkles were characterized by deep and sharp creases, dead creases, or large-area wavy lines with a larger width and a protrusion height generally greater than 10μm. Slight lithium plating was characterized by sporadic, dotted grayish-white or pale gold spots on the negative electrode surface without obvious dendritic protrusions. Severe lithium plating was characterized by large, continuous areas of silvery-white metallic luster on the negative electrode surface, or visible moss-like or dendritic protrusions.

[0125] The test results are shown in Tables 1, 2 and 3.

[0126] Table 1 Polymer performance parameters

[0127]

[0128] Table 2 Performance parameters of composite membrane

[0129]

[0130] Table 3 Cell Performance Parameters

[0131]

[0132] Analysis of the test results in Tables 1, 2, and 3 shows that, within the technical parameter range defined in this invention, the core-shell structured non-fluoropolymer and its application in coated separators exhibit excellent comprehensive performance. Specifically, this core-shell structured non-fluoropolymer possesses good electrolyte stability and low overall swelling. When used as the first polymer particle in a lithium battery coated separator, the resulting coated separator has a high effective adhesion coefficient. After hot-pressing and bonding with the electrode sheet and completing the cell electrolyte injection process, the first polymer particles are less prone to structural damage due to electrolyte absorption, maintaining the integrity of the initial bonding points. This effectively avoids the decrease in overall cell hardness caused by adhesion attenuation during cycling, significantly improving the cell's cycle life and structural stability.

[0133] Furthermore, by rationally controlling the coverage of the coated separator, a sufficient number of bonding sites can be ensured while avoiding adverse effects on the separator's permeability, thus ensuring unobstructed ion transport channels and maintaining good ion conductivity. This design creates a strong interfacial bond between the coated separator and the electrode plates, enhancing the mechanical strength of the battery cell while effectively suppressing the deformation behavior of the electrode plates during liquid absorption and expansion. This prevents localized current concentration and lithium dendrite deposition (i.e., lithium plating) caused by electrode wrinkles, further optimizing the cycle performance and reliability of the battery cell.

[0134] However, if the cross-linking degree of the core-shell structure of the non-fluoropolymer is too low, it will lead to an increase in the core swelling rate, reducing the overall structural stability of the first polymer particles, and thus weakening their adhesion performance with the electrode sheet under hot pressing and liquid injection conditions, ultimately negatively impacting the cell performance. Conversely, when the core cross-linking degree is too high, although it can slightly inhibit core swelling, excessive cross-linking will make the core more rigid, reducing the compatibility and synergy between the core and shell, easily inducing shell peeling, thereby destroying the integrity of the adhesive structure under liquid absorption and swelling conditions, affecting the cell's performance in long-term cycling. These defects have been verified in Comparative Examples 1 and 2. Similarly, the shell cross-linking degree also needs to be within an appropriate range. If the shell cross-linking degree is too low, it is difficult to effectively constrain the swelling behavior of the core, leading to increased deformation of the overall particle structure, which in turn worsens the adhesion performance after liquid absorption; while when the shell cross-linking degree is too high, it will significantly reduce the fluidity and interfacial fusion ability of the first polymer particles during hot pressing, resulting in weakened adhesion between the coated separator and the electrode sheet, and a decrease in the overall hardness of the cell. In this case, the electrode sheet lacks sufficient external constraint during the liquid absorption and expansion process, which makes it prone to wrinkling and deformation, increasing the risk of lithium plating and thus damaging the cell's cycle performance. This problem has been demonstrated in Comparative Examples 3 and 4.

[0135] Regarding the mass ratio of the shell layer in the core-shell structure, there is also an optimal range. When the shell layer ratio is too low, the adhesive functional components provided are insufficient, resulting in insufficient hot-pressing adhesion, low cell hardness, and inability to effectively suppress wrinkles and lithium plating caused by electrode sheet expansion. On the other hand, when the shell layer ratio is too high, although it is beneficial to improve the initial adhesion strength, it will lead to an increased tendency of the shell layer to swell, which will deteriorate the adhesion stability under liquid absorption conditions and affect the overall performance of the cell, as shown in Comparative Examples 5 and 6.

[0136] The size and spacing of the individual cells on the coated separator together determine the coating coverage, which in turn affects the adhesion density and ion transport characteristics. When the size of the individual cells is too small, if their spacing is too large, it will lead to a decrease in coverage, fewer adhesion sites, weakened hot-pressing adhesion, and reduced cell hardness, which is insufficient to restrain the electrode sheet from absorbing liquid and expanding, easily causing wrinkles and lithium plating. Conversely, when the size of the individual cells is too large and the spacing is too small, although it can improve the coverage and enhance the adhesion effect, it may cause blockage of the separator pores, deterioration of the permeability, and hindering lithium ion migration, also inducing the risk of lithium plating. In addition, excessively high coverage will also compress the stress release space in the composite structure. During long-term cell cycling, the electrode sheet continuously absorbs liquid and expands without a place to release internal stress, which can easily lead to electrode sheet buckling and wrinkling, exacerbating lithium plating behavior and ultimately causing cycle performance degradation. The above problems are clearly reflected in Comparative Examples 7 and 8.

[0137] The glass transition temperature (Tg) of core-shell non-fluorinated polymers is also a key control parameter. If Tg is too low, although it helps to improve interfacial wettability and adhesion activity during hot pressing, it will exacerbate the swelling of the first polymer particles in the electrolyte environment, leading to instability of the adhesive structure. At the same time, an excessively low Tg may also cause self-adhesion of the coated separator during winding or storage, increasing the difficulty of subsequent processing, affecting production efficiency, and raising manufacturing costs. When Tg is too high, the material rigidity increases, and the ability of molecular chain segments to move during hot pressing is restricted, which severely weakens the adhesion strength between the coated separator and the electrode sheet, resulting in insufficient cell hardness and inability to effectively restrain the expansion of the electrode sheet. This can also easily lead to wrinkles and lithium plating, thereby reducing cycle performance. Related defects are seen in Comparative Examples 9 and 10.

[0138] Furthermore, when the Tg difference between the core and the shell is too large, their thermodynamic compatibility decreases significantly. Under dynamic conditions such as liquid absorption and expansion, interfacial stress concentration is easily generated, causing the shell to detach or peel off from the core surface, destroying the continuity and effectiveness of the bonding structure, thereby reducing the effective bonding coefficient and deteriorating the overall electrochemical performance and mechanical stability of the cell. This problem has been confirmed in Comparative Example 11.

[0139] In summary, the embodiments of the present invention achieve a multi-objective synergistic balance between bonding performance, electrolyte stability, ion conductivity and stress buffering mechanism by systematically optimizing the crosslinking degree, shell ratio, Tg and its distribution of the non-fluoropolymer in the core-shell structure, as well as the microstructure parameters of the coated diaphragm. This effectively solves the problems of cell cycle performance degradation and safety hazards caused by bonding failure, uncontrolled swelling or unreasonable structural design in the prior art, demonstrating significant technological progress and practical value.

[0140] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite diaphragm, characterized in that, It includes a base film and a coating located on at least one side of the base film. The coating includes a first polymer, which is a secondary particle formed by the agglomeration of a non-fluoropolymer. The non-fluoropolymer includes a core and a shell covering the surface of the core. The core contains polymer A, and the shell contains polymer B. The degree of crosslinking of polymer A is X1, and the degree of crosslinking of polymer B is X2, with 60%≤X1≤70% and 10%≤X2≤20%. The glass transition temperature of polymer A is Tg1, and the glass transition temperature of polymer B is Tg2, with 30℃≤Tg1≤60℃ and 30℃≤Tg2≤60℃, and |Tg1-Tg2|≤5℃.

2. The composite diaphragm according to claim 1, characterized in that, The polymer A accounts for 60% to 80% of the total mass of the non-fluoropolymer, and the polymer B accounts for 20% to 40% of the total mass of the non-fluoropolymer.

3. The composite diaphragm according to claim 1, characterized in that, The swelling rate of polymer A is η1, 10%≤η1≤20%, and the overall swelling rate of the non-fluoropolymer is η2, 50%≤η2≤100%. And / or, the particle size of the non-fluoropolymer is 0.1 μm to 0.8 μm; And / or, the particle size of the first polymer is 4 μm to 8 μm.

4. The composite diaphragm according to any one of claims 1 to 3, characterized in that, Both polymer A and polymer B are polymerized from monomers comprising a first polymerizing monomer, a second polymerizing monomer, and a crosslinking monomer. The crosslinking monomer accounts for 6% to 8% of the mass of polymer A and 1% to 3% of the mass of polymer B.

5. The composite diaphragm according to claim 4, characterized in that, The first polymerization monomer is selected from at least one of methyl methacrylate, methacrylonitrile, styrene, and acrylonitrile; the second polymerization monomer is selected from at least one of ethyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, methyl acrylate, n-butyl acrylate, and butyl methacrylate. And / or, the crosslinking monomer is selected from at least one of divinylbenzene, diallyl phthalate, diethanol diacrylate, trimethylolpropane trimethacrylate, vinyltriethoxysilane, and vinyltrimethoxysilane.

6. The composite diaphragm according to claim 1, characterized in that, The coating further includes a second polymer selected from at least one of polyacrylic acid, polyacrylamide, polyacrylonitrile, and polymethacrylonitrile.

7. The composite diaphragm according to claim 6, characterized in that, The glass transition temperature of the second polymer is >80°C; And / or, the mass of the second polymer accounts for 2% to 10% of the total mass of the coating.

8. The composite diaphragm according to claim 1, characterized in that, The coating is a discontinuous coating; the discontinuous coating has at least one of the following characteristics: a. Contains multiple independent units arranged at equal intervals; the maximum length of each independent unit is L1, 50μm≤L1≤800μm, where L1 is the diameter of the circumscribed circle of the independent unit graphic; the distance between the edges of two equally spaced independent units is L2, 50μm≤L2≤500μm, and L1 and L2 satisfy the following relationship: 0.5≤L1 / L2≤1.6; b. The overall coverage of the discontinuous coating is between 6% and 30%; c. The basis weight of the discontinuous coating is 0.2 g / m³. 2 ~1.0g / m 2 .

9. The composite diaphragm according to claim 1, characterized in that, The hot-pressing adhesion force of the two composite membranes after their coated surfaces are bonded together at 70℃~85℃ is F1. The wet-pressing adhesion force of the two composite membranes after hot-pressing together and immersing them in lithium salt electrolyte at 60℃ for 24 hours is F2. The effective adhesion coefficient of the composite membrane is K, K=F2 / F1, and 0.80≤K<1.

0. And / or, the self-adhesive force of the two composite diaphragms after their coated surfaces are bonded together at room temperature (25°C) is F3, where F3 ≤ 0.5 N / m; And / or, the hot-pressing adhesion force between the coating surface of the composite diaphragm and the electrode sheet at 70℃~85℃ is F4, 3N / m≤F4≤10N / m.

10. A secondary battery, characterized in that, It includes the composite membrane as described in any one of claims 1 to 9.

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