Flame-retardant separator, method for preparing the same, and secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a flame-retardant separator, its preparation method, and a secondary battery. The flame-retardant separator includes a base film and a coating disposed on at least one surface of the base film; the coating includes a modified MCA filler, a binder polymer, and a wetting agent; the modified MCA filler includes MCA and a phenolic resin prepolymer bonded to the MCA. This invention introduces phenolic resin prepolymer-modified MCA into the coating. Compared to pure MCA, the modified MCA has a significantly lower melting and decomposition temperature, allowing time for early flame retardancy; and after melting, it can rapidly fill the separator pores, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes, cutting off the thermal runaway trigger path from the source; it also improves the tensile strength of the separator at high temperatures, preventing separator rupture and coating peeling at high temperatures, thus improving the mechanical and safety performance of the battery cell; furthermore, the modified MCA has good dispersibility and solubility in solvents, contributing to the formation of a uniform and stable coating structure.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a flame-retardant separator, its preparation method, and a secondary battery. Background Technology

[0002] Polyolefin separators are prone to thermal shrinkage and melt collapse under thermal abuse conditions (such as overcharging and internal short circuits), leading to direct contact between the positive and negative electrodes and inducing thermal runaway. Although traditional ceramic-coated separators can improve thermal stability, the bonding force between their inorganic coating and polymer base film is weak, making them prone to cracking and peeling at high temperatures. Furthermore, ceramics themselves do not have active flame-retardant properties, only providing a passive thermal insulation barrier, and cannot suppress thermal runaway.

[0003] To enhance the flame retardancy of the diaphragm, existing technologies attempt to add melamine cyanurate (MCA) as a flame retardant additive to the coating. When heated, MCA undergoes a pyrolysis reaction, decomposing into solid products of melamine and cyanuric acid, while simultaneously releasing non-flammable inert gases such as ammonia and carbon dioxide. This can dilute the concentration of flammable volatile components in the electrolyte, and the decomposition products gradually construct a dense, heat-insulating carbon layer in the condensed phase, blocking oxygen and heat conduction, thereby improving the high-temperature thermal safety performance and basic flame retardant effect of the diaphragm to a certain extent.

[0004] However, due to the strong hydrogen bonds and π-π stacking interactions between MCA molecules, the crystals have high regularity, making them prone to aggregation and poor dispersibility in solvents, thus hindering the formation of a uniform separator coating. Furthermore, the high melting / decomposition temperature of MCA results in a slow flame-retardant response when the battery experiences abnormal temperature rise, making it difficult to exert its effect promptly and effectively suppress early thermal runaway.

[0005] Therefore, there is an urgent need for an MCA-based flame-retardant coating system that combines low-temperature rapid response with high dispersion process adaptability, enabling it to initiate decomposition and melting pore sealing in the early stages of thermal runaway, and maintain the integrity of the coating structure at high temperatures, thereby achieving efficient flame retardancy of the diaphragm.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a flame-retardant separator, its preparation method, and a secondary battery. The invention introduces phenolic resin prepolymer-modified MCA into the separator coating. Compared to traditional pure MCA, the modified MCA melts and decomposes at a lower temperature, allowing time for early flame retardancy. Furthermore, after melting, it rapidly fills the separator pores, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes, thus cutting off the thermal runaway trigger path at its source. It also improves the tensile strength of the separator at high temperatures, preventing separator rupture and coating peeling at high temperatures, thereby enhancing the mechanical and safety performance of the battery cell. Simultaneously, the modified MCA exhibits good dispersibility and solubility in solvents, contributing to the formation of a uniform and stable coating structure.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A flame-retardant membrane includes a base membrane and a coating disposed on at least one surface of the base membrane; the coating includes a modified MCA filler, a binder polymer, and a wetting agent; the modified MCA filler includes MCA and a phenolic resin prepolymer bonded to the MCA.

[0009] Preferably, in the modified MCA filler, the content of the phenolic resin prepolymer is 7% to 15% of the mass of the MCA.

[0010] Preferably, the number-average molecular weight Mn of the phenolic resin prepolymer is 460~600 g / mol, and the degree of polymerization Xn of the phenolic resin prepolymer is 1.9~2.5.

[0011] Preferably, the thermal decomposition temperature of the phenolic resin prepolymer is 150~250℃.

[0012] Preferably, the adhesive polymer includes at least one of styrene-butadiene rubber, polyacrylate, waterborne polyurethane, and polyvinyl alcohol.

[0013] Preferably, the wetting agent includes at least one of isooctanol polyoxyethylene ether, polyoxyethylene fatty alcohol ether, and polyoxyethylene polyoxypropylene block copolymer.

[0014] Preferably, the phenolic resin prepolymer is a polycondensation product of para-substituted long-chain alkylphenols and formaldehyde, wherein the para-substituted long-chain alkylphenols refer to phenols having a monosubstituted alkyl group at the para position of the phenolic hydroxyl group, and the total number of carbon atoms in the monosubstituted alkyl group is 8 to 10.

[0015] Preferably, the para-substituted long-chain alkylphenol includes at least one of p-tert-octylphenol, p-nonylphenol, and p-decylphenol.

[0016] Preferably, the method for preparing the phenolic resin prepolymer includes: mixing the para-substituted long-chain alkylphenol, the formaldehyde, and the catalyst to carry out a polycondensation reaction to obtain the phenolic resin prepolymer.

[0017] Preferably, the molar ratio of the para-substituted long-chain alkylphenol to the formaldehyde is 1:(1.65-2.0).

[0018] Preferably, the catalyst is an alkaline catalyst, including at least one of sodium hydroxide, barium hydroxide, and ammonia water.

[0019] Preferably, the amount of catalyst used is 2% to 5% of the mass of the para-substituted long-chain alkylphenol.

[0020] Preferably, the temperature of the polycondensation reaction is 60~80℃, and the reaction time after reaching the reaction temperature is 1~4h.

[0021] Preferably, the preparation method of the modified MCA filler includes: A first solution containing phenolic resin prepolymer and a second solution containing MCA powder are mixed at 0-25°C, and the mixture is then subjected to vacuum distillation and drying to obtain the modified MCA.

[0022] Preferably, the solvent in the first solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone.

[0023] Preferably, the solvent in the second solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone.

[0024] Preferably, the mixing method of the first solution and the second solution includes stirring and / or ultrasonic mixing.

[0025] Preferably, the mixing time of the first solution and the second solution is 1 to 2 hours.

[0026] The method for preparing the flame-retardant diaphragm according to any one of the foregoing embodiments includes the following steps: A slurry containing modified MCA filler, adhesive polymer and wetting agent is prepared, the slurry is coated on at least one side surface of the base film and dried to obtain the flame-retardant diaphragm.

[0027] A secondary battery comprising the flame-retardant separator described in any of the foregoing embodiments.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention introduces phenolic resin prepolymer-modified MCA filler into the flame-retardant separator coating. Compared with traditional unmodified MCA, the modified MCA exhibits better dispersibility and solubility in solvents, contributing to the formation of a uniform and stable coating structure. Furthermore, the modified MCA melts and decomposes at lower temperatures, allowing time for early flame retardancy. The molten modified MCA rapidly fills the separator pores, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes, thus cutting off the thermal runaway trigger path at its source. It also solves problems such as separator rupture at high temperatures and the easy peeling off of traditional flame-retardant coatings. While maintaining the flame-retardant effect, it also improves the compatibility of the coating with the separator and electrolyte, and enhances the tensile strength of the separator after high-temperature (150℃-180℃) drying. After drying at 180℃ for 1 hour, the tensile strength retention rate of the separator is as high as 65%, improving the mechanical and safety performance of the battery cell. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a comparison curve of TG between the modified MCA and the unmodified pure MCA in Example 1 of the present invention; Figure 2 These are product images of the diaphragms after drying at 180°C for 1 hour in Examples 1-4, Example 9, and Comparative Example 4 of the present invention. Figure 3 This is a SEM image of the modified MCA in Example 1 of the present invention after drying at 180°C for 1 hour; Figure 4 These are state diagrams of the diaphragm coating slurry in Embodiment 1 and Comparative Example 4 of the present invention; wherein, the left diagram is Comparative Example 4 and the right diagram is Embodiment 1. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0032] A first aspect of the present invention provides a flame-retardant membrane, comprising a base membrane and a coating disposed on at least one surface of the base membrane; the coating comprises a modified MCA filler, a binder polymer, and a wetting agent; the modified MCA filler comprises MCA and a phenolic resin prepolymer bonded to MCA; the phenolic resin prepolymer forms a uniformly embedded distribution structure through interfacial wetting and hydrogen bonding adsorption.

[0033] In this invention, the modification of phenolic resin prepolymer weakens the hydrogen bonds and π-π stacking interactions between MCA molecules, thereby disrupting its crystal integrity. This not only enables the modified MCA to have better dispersibility and solubility in solvents, which helps to form a uniform and stable coating structure, but also reduces the melting and decomposition temperature of the modified MCA, allowing time for early flame retardancy. Furthermore, the molten modified MCA can quickly fill the membrane pores, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes, thus cutting off the thermal runaway triggering path at the source.

[0034] Modified MCA can achieve temperature-graded response. The para-long-chain alkylphenol resin prepolymer synthesized under low-temperature weak alkaline conditions is mainly composed of methylene bonds, and also contains a certain amount of methylene ether bonds. At 150-180℃, the methylene ether bonds in the long-chain alkylphenol resin begin to break and recombine, undergoing a self-crosslinking reaction. At the same time, a small amount of unreacted active phenolic hydroxyl groups remaining in the resin system condense with the amino groups on the MCA surface. At 180-200℃, the modified MCA begins to melt and initiates preliminary decomposition. The MCA molecules with increased fluidity interpenetrate with the resin crosslinking network, forming an interpenetrating structure in which the resin network encapsulates the molten MCA, solving problems such as diaphragm rupture at high temperatures and easy peeling off of traditional flame-retardant coatings.

[0035] This invention introduces phenolic resin prepolymer-modified MCA into the coating, which not only facilitates the formation of a uniform coating structure but also suppresses thermal runaway from an early stage, preventing the separator from rupturing and the coating from peeling off at high temperatures, thus significantly improving battery safety.

[0036] In some specific embodiments of the present invention, the content of phenolic resin prepolymer in the modified MCA filler is 7% to 15% of the mass of MCA. For example, it can be any value from 7%, 10%, 12%, and 15%, or a range of any two values. If the content of phenolic resin prepolymer is too high, it will lead to excessive densification of the network structure, causing premature cross-linking at room temperature and affecting electrochemical performance. If the content of phenolic resin prepolymer is too low, the prepolymer cannot form a continuous bonding skeleton at high temperatures, reducing the pore sealing rate and failing to effectively block the spread of thermal runaway.

[0037] This invention adjusts the content of phenolic resin prepolymer to form a uniform resin network structure, which improves the compatibility of the coating with the diaphragm and electrolyte while maintaining the flame retardant effect, and enhances the tensile strength of the diaphragm after high temperature (150℃-180℃). After drying at 180℃ for 1 hour, the tensile strength retention rate of the diaphragm is as high as 65% or more, thus improving the mechanical and safety performance of the battery cell.

[0038] In some specific embodiments of the present invention, the number-average molecular weight Mn of the phenolic resin prepolymer is 460~600 g / mol, for example, it can be any one value or a range of any two values ​​among 460 g / mol, 480 g / mol, 500 g / mol, 520 g / mol, 550 g / mol, 580 g / mol, and 600 g / mol. Within this molecular weight range, the phenolic resin prepolymer has sufficient reactivity and molecular chain length to effectively weaken the hydrogen bonding and π-π stacking of MCA molecules, and also has good solubility, which can form a stable dispersion system in the solvent. The degree of polymerization Xn of the phenolic resin prepolymer is 1.9 to 2.5. For example, it can be any value among 1.9, 2.1, 2.3, and 2.5, or a range of any two values. This degree of polymerization range matches the above molecular weight range, which can ensure that the prepolymer molecular chain has appropriate length and flexibility, which is conducive to forming a uniform mosaic structure on the surface and inside of MCA particles, thereby improving the dispersibility, solubility, and melting characteristics of the modified MCA particles.

[0039] In some specific embodiments of the present invention, the thermal decomposition temperature of the phenolic resin prepolymer is 150~250℃, for example, it can be any single value or a range of any two values ​​among 150℃, 180℃, 200℃, and 250℃. Compared with unmodified MCA, the thermal decomposition temperature is significantly reduced, solving the problem that traditional MCA cannot exert its flame-retardant effect in a timely manner due to its excessively high decomposition temperature (far exceeding the initial temperature of thermal runaway).

[0040] In some specific embodiments of the present invention, the adhesive polymer used includes at least one of styrene-butadiene rubber, polyacrylate, waterborne polyurethane, and polyvinyl alcohol.

[0041] In some specific embodiments of the present invention, the wetting agent used includes at least one of isooctanol polyoxyethylene ether, polyoxyethylene fatty alcohol ether, and polyoxyethylene polyoxypropylene block copolymer.

[0042] In some specific embodiments of the present invention, the phenolic resin prepolymer is a polycondensation product of para-substituted long-chain alkylphenols and formaldehyde, wherein the para-substituted long-chain alkylphenols refer to phenols having a monosubstituted alkyl group at the para position of the phenolic hydroxyl group, and the total number of carbon atoms in the monosubstituted alkyl group is 8 to 10.

[0043] In some specific embodiments of the present invention, the para-substituted long-chain alkylphenols include at least one of p-tert-octylphenol, p-nonylphenol, and p-decylphenol.

[0044] In some specific embodiments of the present invention, the preparation method of phenolic resin prepolymer includes: mixing para-substituted long-chain alkylphenol, formaldehyde and catalyst to carry out polycondensation reaction to obtain phenolic resin prepolymer.

[0045] In some specific embodiments of the present invention, the molar ratio of the para-substituted long-chain alkylphenol to formaldehyde is 1:(1.65-2.0), for example, it can be any single value or a range of any two values ​​from 1:1.65, 1:1.75, 1:1.90, 1:2.0. Excess formaldehyde promotes polyhydroxymethylation, ensuring the crosslinking activity of the prepolymer at high temperatures.

[0046] In some specific embodiments of the present invention, the catalyst used is an alkaline catalyst, including at least one of sodium hydroxide, barium hydroxide, and ammonia water.

[0047] In some specific embodiments of the present invention, the amount of catalyst used is 2% to 5% of the mass of the para-substituted long-chain alkylphenol, for example, it can be any one value or a range of any two values ​​among 2%, 3%, 4%, and 5%.

[0048] In some specific embodiments of the present invention, the temperature of the polycondensation reaction is 60~80°C, for example, it can be any one value or a range of any two values ​​among 60°C, 65°C, 70°C, 75°C, and 80°C; the reaction time after reaching the reaction temperature is 1~4h, for example, it can be any one value or a range of any two values ​​among 1h, 2h, 3h, and 4h.

[0049] In some specific embodiments of the present invention, the preparation method of the modified MCA filler includes: A first solution containing phenolic resin prepolymer and a second solution containing MCA powder are mixed at 0-25°C, and then subjected to vacuum distillation and drying to obtain modified MCA.

[0050] This invention uses MCA powder as a substrate and phenolic resin prepolymer as a modifier. A composite modified structure is formed through the intermolecular interaction between the phenolic resin prepolymer and the MCA powder. The phenolic resin prepolymer is uniformly dispersed inside and on the surface of the MCA particles, weakening the hydrogen bonding and π-π stacking between MCA molecules and destroying the original complete crystal structure of MCA. The mixing reaction rate is slow and controllable at 0-25℃.

[0051] In some specific embodiments of the present invention, the solvent in the first solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone.

[0052] In some specific embodiments of the present invention, the solvent in the second solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone.

[0053] In some specific embodiments of the present invention, the mixing method of the first solution and the second solution includes stirring and / or ultrasonic mixing.

[0054] In some specific embodiments of the present invention, the mixing time of the first solution and the second solution is 1 to 2 hours. For example, it can be any one value or a range of any two values ​​from 1 hour, 1.2 hours, 1.5 hours, 1.8 hours, and 2 hours.

[0055] In some specific embodiments of the present invention, the power of the ultrasonic mixing is 500-700W, for example, it can be any one value or a range of any two values ​​among 500W, 550W, 600W, 650W, and 700W.

[0056] A second aspect of the present invention provides a method for preparing a flame-retardant diaphragm according to any one of the foregoing embodiments, comprising the following steps: A slurry containing modified MCA filler, adhesive polymer and wetting agent is prepared, the slurry is coated on at least one side surface of the base membrane and dried to obtain a flame-retardant diaphragm.

[0057] In some specific embodiments of the present invention, a slurry is coated on both sides of the base film, and the total coating amount of the slurry on both sides is 2.0~4.0 g / m². 2 For example, it can be 2.0 g / m 2 2.5g / m 2 3.0g / m 2 3.5g / m 2 4.0g / m 2 The range of values ​​consisting of any one point value or any two point values.

[0058] A third aspect of the present invention provides a secondary battery comprising the flame-retardant separator described in any of the foregoing embodiments.

[0059] The following detailed description of some embodiments of the present invention is provided in conjunction with specific application examples. Unless otherwise specified, all raw materials used in the embodiments can be obtained commercially available.

[0060] Example 1 (1) Synthesis of reactive phenolic resin prepolymer Nonylphenol and a 37% formaldehyde aqueous solution were mixed at a molar ratio of 1:1.75. A 10% sodium hydroxide aqueous solution was added as a catalyst, with sodium hydroxide accounting for 4% of the mass of nonylphenol. The reaction was carried out in a water bath at 60°C with high-efficiency stirring for 1 hour. When the reaction solution was dropped into water, a slight milky white turbidity appeared and did not disappear after stirring. The reaction was then stopped. The reaction vessel was quickly placed in a cold water bath to lower the temperature of the reaction solution to below 35°C and the pH value was adjusted to 7.0 to obtain a reactive phenolic resin prepolymer with a number-average molecular weight Mn = 500 Da and a degree of polymerization Xn = 2.13.

[0061] (2) Preparation of modified MCA filler The reactive phenolic resin prepolymer was dissolved in an appropriate amount of anhydrous ethanol to prepare a clear solution with a mass concentration of 30%, which is the first solution. The MCA powder was dispersed in anhydrous ethanol to obtain the second solution. The first solution and the second solution were mixed and stirred in an ice-water bath for 1 hour according to the ratio of phenolic resin prepolymer to MCA mass of 10%. Then, the mixture was ultrasonically mixed at 600 W for 30 min. The ultrasonically mixed suspension was then distilled under reduced pressure at below 60 °C using a rotary evaporator to remove the solvent and obtain a viscous composite. The composite was then dried, pulverized into a fine powder, and sieved to obtain the final composite powder.

[0062] (3) Preparation of flame-retardant membrane Take 90 parts of modified MCA powder, dilute with deionized water, then add 5 parts of adhesive polymer (styrene-butadiene rubber) and 5 parts of wetting agent (isooctyl alcohol polyoxyethylene ether), stir evenly, and disperse at 25℃ for 30 min to obtain a slurry; use a roller coating method to coat the slurry onto both sides of a 7μm thick PE base film, with a total coating weight of 3.2g / m² on both sides. 2 The coated diaphragm is obtained by drying.

[0063] Example 2 Example 2 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.65, sodium hydroxide accounts for 2% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 460 Da, and the degree of polymerization Xn is 1.96; in step (2), the phenolic resin prepolymer accounts for 7% of the mass of MCA; the other conditions are the same as in Example 1.

[0064] Example 3 Example 3 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.70, sodium hydroxide accounts for 3% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 470 Da, and the degree of polymerization Xn is 2.00; in step (2), the phenolic resin prepolymer accounts for 8% of the mass of MCA; the other conditions are the same as in Example 1.

[0065] Example 4 Example 4 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.75, sodium hydroxide accounts for 3% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 480 Da, and the degree of polymerization Xn is 2.04; in step (2), the phenolic resin prepolymer accounts for 9% of the mass of MCA; the other conditions are the same as in Example 1.

[0066] Example 5 Example 5 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.80, sodium hydroxide accounts for 4% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 520 Da, and the degree of polymerization Xn is 2.21; in step (2), the phenolic resin prepolymer accounts for 11% of the mass of MCA; the other conditions are the same as in Example 1.

[0067] Example 6 Example 6 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.85, sodium hydroxide accounts for 4% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 540 Da, and the degree of polymerization Xn is 2.30; in step (2), the phenolic resin prepolymer accounts for 12% of the mass of MCA; the other conditions are the same as in Example 1.

[0068] Example 7 Example 7 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.90, sodium hydroxide accounts for 5% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 560 Da, and the degree of polymerization Xn is 2.38; in step (2), the phenolic resin prepolymer accounts for 13% of the mass of MCA; the other conditions are the same as in Example 1.

[0069] Example 8 Example 8 is similar to Example 1, except that: in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:2.0, sodium hydroxide accounts for 5% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 580 Da, and the degree of polymerization Xn is 2.47; in step (2), the phenolic resin prepolymer accounts for 15% of the mass of MCA; the other conditions are the same as in Example 1.

[0070] Example 9 Example 9 is similar to Example 1, except that in step (1), p-tert-octylphenol is used to prepare phenolic resin prepolymer, and the molar ratio of p-octylphenol to formaldehyde is 1:1.75. Sodium hydroxide accounts for 4% of the mass of p-octylphenol. The number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 480 Da, and the degree of polymerization Xn is 2.10. In step (2), the phenolic resin prepolymer accounts for 10% of the mass of MCA. All other conditions are the same as in Example 1.

[0071] Example 10 Example 10 is similar to Example 1, except that: in step (1), p-decylphenol is used to prepare phenolic resin prepolymer, and the molar ratio of p-decylphenol to formaldehyde is 1:1.75, sodium hydroxide accounts for 4% of the mass of p-decylphenol, and the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 520 Da, and the degree of polymerization Xn is 2.15; in step (2), the phenolic resin prepolymer accounts for 10% of the mass of MCA; the other conditions are the same as in Example 1.

[0072] Comparative Example 1 Comparative Example 1 is similar to Example 1, except that in step (1), p-nonylphenol is replaced with an equimolar amount of p-tert-butylphenol, and the number-average molecular weight Mn of the resulting reactive phenolic resin prepolymer is 320 Da, and the degree of polymerization Xn is 1.36; the other conditions are the same as those in Example 1.

[0073] Comparative Example 2 Comparative Example 2 is similar to Example 1, except that in step (1), p-nonylphenol is replaced with an equimolar amount of p-dodecylphenol, and the number-average molecular weight of the resulting reactive phenolic resin prepolymer is Mn=720Da and the degree of polymerization is Xn=3.06; the other conditions are the same as those in Example 1.

[0074] Comparative Example 3 Comparative Example 3 is similar to Example 1, except that formaldehyde was not added in step (1); all other conditions are the same as in Example 1.

[0075] Comparative Example 4 Comparative Example 4 is similar to Example 1, except that steps (1) and (2) were not performed, and an equal amount of unmodified MCA was used instead of modified MCA in step (3); all other conditions are the same as in Example 1.

[0076] Comparative Example 5 It uses a pure PE-based membrane with no coating.

[0077] Comparative Example 6 Comparative Example 6 is similar to Example 1, except that in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.50, sodium hydroxide accounts for 1% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 420 Da, and the degree of polymerization Xn is 1.79; in step (2), the phenolic resin prepolymer accounts for 5% of the mass of MCA; the other conditions are the same as in Example 1.

[0078] Comparative Example 7 Comparative Example 7 is similar to Example 1, except that in step (1), the molar ratio of p-nonylphenol to formaldehyde is 1:1.60, sodium hydroxide accounts for 2% of the mass of p-nonylphenol, the number average molecular weight Mn of the obtained reactive phenolic resin prepolymer is 440 Da, and the degree of polymerization Xn is 1.87; in step (2), the phenolic resin prepolymer accounts for 6% of the mass of MCA; the other conditions are the same as in Example 1.

[0079] Test case 1. TG test Approximately 20 mg of modified MCA powder was tested for TG using differential scanning calorimetry. The testing equipment was a METTLER DSC3, with a testing temperature range of 30~400℃ and a heating rate of 10℃ / min. During the test, the curve of sample mass change with temperature was recorded in real time, and the initial decomposition temperature, the maximum thermal weight loss rate temperature, and the final char residue were calculated. Depend on Figure 1 It can be seen that, compared with unmodified pure MCA, the thermal decomposition temperature of modified MCA is significantly lower, and it can decompose and release non-flammable gases at a lower temperature, solving the problem that traditional MCA cannot play a flame-retardant role in time due to excessively high decomposition temperature.

[0080] 2. Tensile strength retention rate after 180℃ / h Using the Siker Test pneumatic tensile strength tester, the original coated diaphragm samples in each embodiment and comparative example were tested. Five sets of tests were conducted, and the average value of the five sets was taken to calculate the average initial tensile strength (σ1). The coated diaphragm was then placed in a 180℃ oven and baked for 1 hour, and then cooled to room temperature. The tensile strength of the coated diaphragm after high-temperature curing was tested using the Siker Test pneumatic tensile strength tester. Five sets of tests were conducted, and the average value of the five sets was taken to calculate the average tensile strength (σ2) of the diaphragm after high-temperature curing. The tensile strength retention rate is then calculated as (σ2 / σ1) × 100%.

[0081] 3. Ionic conductivity Inside an argon-filled glove box, a symmetrical "stainless steel / diaphragm / stainless steel" battery was assembled. An appropriate amount of electrolyte (EC:EMC:DEC = 3:5:2, LiPF6, 1 mol / L) was added. Using an electrochemical workstation, the AC impedance was measured, yielding σ = L / (R). b ×A), where σ is the ionic conductivity (S cm). 1 L is the thickness of the diaphragm (cm); R b A is the intrinsic resistance of the diaphragm (Ω); A is the effective area (cm²). 2 ).

[0082] 4. Peel strength The non-test side of the coated diaphragm was adhered to a stainless steel plate with double-sided tape, and then 3M peel adhesive was used to firmly attach it to the coating. A 2kg pressure roller was used to press back and forth three times to peel off half of the coating. Then, a tensile testing machine was used to test the peel force of the coating after peeling off 180°. The average peel force during the steady phase was collected to characterize the interfacial bonding strength between the coating and the base film.

[0083] 5. Pore plugging rate after 180℃ / h The coated diaphragm was placed in a 180℃ oven and baked for 1 hour, then cooled to room temperature. The air permeability of the diaphragm before and after heat treatment was measured using a Gurley air permeability meter. The pore blockage rate of the diaphragm after high-temperature heat treatment was calculated based on the air permeability difference. Pore blockage rate (%) = ×100%,T2: Air permeability of the diaphragm at room temperature without heat treatment (s / 100cc);T2: Air permeability of the diaphragm after heat treatment at 180℃ / h (s / 100cc).

[0084] 6. UL94 Flame Retardant Rating Test The coated diaphragm was cut into standard specimens with dimensions of 125 mm in length and 13 mm in width. Five specimens were prepared in parallel for each formulation. The specimens were vertically clamped onto the test fixture, with the lower end of the specimen at a standard distance from the burner flame nozzle. The burner was turned on, and the flame was adjusted to the standard height. The lower free end of the specimen was ignited for the first time for 10 seconds. After ignition, the flame was removed, and the duration of continuous flaming combustion was recorded. After the flame extinguished itself, a second 10-second ignition was immediately performed. After the second ignition, the flame was removed, and the afterflame combustion time and smoldering time of the specimen were recorded again. The combustion state of the specimen was observed and recorded throughout the process: whether open flame spread, whether molten drips appeared, whether the drips ignited the degreased cotton below, and whether the specimen was completely burned. The average combustion time and combustion phenomena of each group were statistically analyzed to determine the flame retardant rating.

[0085] 7. Limiting Oxygen Index (LOI) Test The coated diaphragm was cut into standard samples with dimensions of 150 mm in length and 6.5 mm in width. Using a limiting oxygen index (LOI) instrument (FTT0077 from the UK), the samples were placed vertically in a combustion chamber, and oxygen-nitrogen mixtures with different ratios were introduced. The samples were ignited using a standard ignition source, and the oxygen content of the gas was gradually adjusted. The minimum oxygen volume fraction required for the sample to maintain stable and continuous combustion was determined, which is the LOI value. The higher the value, the better the flame retardant performance of the material. This value can effectively evaluate the diaphragm's ability to provide early flame retardancy and inhibit the initiation of thermal runaway in the low-heat, weak-ignition environment of the battery cell.

[0086] The test results of the diaphragm's ionic conductivity, peel strength, tensile strength retention rate after 180℃ / h, pore blockage rate after 180℃ / h, UL94 flame retardant rating, and limiting oxygen index (LOI) are shown in Table 1.

[0087] Table 1

[0088] As shown in Table 1, the modified MCA introduced into the coating in this embodiment of the invention can improve the peel strength, tensile strength retention rate after 180℃ / h, and pore blockage rate of the separator while maintaining high ionic conductivity. The pore blockage rate after drying at 180℃ for 1 hour is greater than 70%, reaching a maximum of 89.5%, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes, thus cutting off the thermal runaway triggering path at the source. The tensile strength retention rate of the separator after drying at 180℃ for 1 hour is above 65%, improving the mechanical and safety performance of the battery cell. Increasing the content of phenolic resin prepolymer can improve mechanical properties and thermal stability, but it will sacrifice some electrochemical performance. The flame retardant performance of the separator in this embodiment of the invention is better than that of the comparative example, especially the limiting oxygen index, which is significantly higher in the embodiment than in the comparative example, indicating that the separator provided by this invention can suppress the occurrence of thermal runaway from an early stage. Figure 2 It can be seen that in Examples 1 and 9, the diaphragm morphology was intact after high-temperature treatment, with no cracks and no coating peeling; in Example 3, although the coating of the diaphragm had cracks after high-temperature treatment, the morphology was still basically intact and did not peel off; in Example 2, the coating of the diaphragm had cracks and slight peeling after high-temperature treatment; in Example 4, the diaphragm shrank after high-temperature treatment, but the overall integrity of the diaphragm was good, and the toughness was better than that of Examples 2 and 3; while in Comparative Example 4, the coating of the diaphragm was crushed and peeled off after high-temperature treatment. These results show that modifying MCA with phenolic resin prepolymer and introducing it into the coating can ensure the stability of the diaphragm at 180°C, which can effectively solve the problems of diaphragm rupture at high temperature and easy peeling off of traditional flame-retardant coatings.

[0089] 8. SEM after 180℃ / h A 5mm × 5mm sample of the high-temperature cured diaphragm treated at 180℃ for 1 hour in Example 1 was taken for SEM testing to observe its microstructure.

[0090] like Figure 3 As shown, after high temperature, the modified MCA begins to melt. The increased fluidity of the MCA molecules and the resin cross-linking network interpenetrate with each other, forming an interpenetrating structure in which the resin network encapsulates the molten MCA. Moreover, the pore blocking rate reaches more than 80% at high temperature, effectively preventing lithium dendrite penetration and direct contact between the positive and negative electrodes.

[0091] 9. Comparison of the dispersion properties of MCA in solvents before and after modification Depend on Figure 4 As can be seen, in Comparative Example 4, the slurry prepared using unmodified MCA was paste-like (left figure); while in Example 1, the slurry prepared using modified MCA was fluid-like (right figure). This result shows that the modified MCA has better dispersibility and solubility in solvents, which helps to form a uniform and stable coating structure.

[0092] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A flame-retardant separator, characterized by, The coating comprises a base film and a coating disposed on at least one surface of the base film; the coating comprises a modified MCA filler, a binder polymer, and a wetting agent; the modified MCA filler comprises MCA and a phenolic resin prepolymer bonded to the MCA.

2. The flame retardant separator of claim 1, wherein It meets at least one of the following characteristics: (1) In the modified MCA filler, the content of the phenolic resin prepolymer is 7% to 15% of the mass of the MCA; (2) The number-average molecular weight Mn of the phenolic resin prepolymer is 460~600 g / mol, and the degree of polymerization Xn of the phenolic resin prepolymer is 1.9~2.5; (3) The thermal decomposition temperature of the phenolic resin prepolymer is 150~250℃; (4) The adhesive polymer includes at least one of styrene-butadiene rubber, polyacrylate, waterborne polyurethane, and polyvinyl alcohol; (5) The wetting agent includes at least one of isooctanol polyoxyethylene ether, polyoxyethylene fatty alcohol ether, and polyoxyethylene polyoxypropylene block copolymer.

3. The flame retardant separator of claim 1, wherein The phenolic resin prepolymer is a polycondensation product of para-substituted long-chain alkylphenols and formaldehyde. The para-substituted long-chain alkylphenols refer to phenols with a monosubstituted alkyl group at the para position of the phenolic hydroxyl group, and the total number of carbon atoms in the monosubstituted alkyl group is 8 to 10.

4. The flame retardant separator of claim 3, wherein The para-substituted long-chain alkylphenols include at least one of p-tert-octylphenol, p-nonylphenol, and p-decylphenol.

5. The flame retardant separator according to claim 3 or 4, characterized in that, The method for preparing the phenolic resin prepolymer includes: mixing the para-substituted long-chain alkylphenol, the formaldehyde, and the catalyst to carry out a polycondensation reaction to obtain the phenolic resin prepolymer.

6. The flame retardant separator of claim 5, wherein It meets at least one of the following characteristics: (1) The molar ratio of the para-substituted long-chain alkylphenol to the formaldehyde is 1:(1.65-2.0). (2) The catalyst is an alkaline catalyst, including at least one of sodium hydroxide, barium hydroxide, and ammonia water; (3) The amount of the catalyst used is 2% to 5% of the mass of the para-substituted long-chain alkylphenol; (4) The temperature of the polycondensation reaction is 60~80℃, and the reaction time after reaching the reaction temperature is 1~4h.

7. The flame retardant separator of claim 1, wherein The preparation method of the modified MCA filler includes: A first solution containing phenolic resin prepolymer and a second solution containing MCA powder are mixed at 0-25°C, and the mixture is then subjected to vacuum distillation and drying to obtain the modified MCA.

8. The flame retardant separator of claim 7, wherein It meets at least one of the following characteristics: (1) The solvent in the first solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone; (2) The solvent in the second solution includes at least one of methanol, ethanol, isopropanol, ethylene glycol monomethyl ether, ethyl acetate, and acetone; (3) The mixing methods of the first solution and the second solution include stirring and / or ultrasonic mixing; (4) The mixing time of the first solution and the second solution is 1~2h.

9. The method for preparing the flame-retardant diaphragm according to any one of claims 1 to 8, characterized in that, Includes the following steps: A slurry containing modified MCA filler, adhesive polymer and wetting agent is prepared, the slurry is coated on at least one side surface of the base film and dried to obtain the flame-retardant diaphragm.

10. A secondary battery characterized by comprising: Includes the flame-retardant diaphragm as described in any one of claims 1 to 8.

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