A thermosensitive composite diaphragm and its preparation method and application

Abstract

This invention relates to the field of lithium-ion battery technology, and in particular to a thermosensitive composite separator, its preparation method, and its application. The thermosensitive composite separator includes a base film and a coating. The coating is applied to one or both surfaces of the base film. The coating includes a polymer resin and a ceramic. The polymer resin is a polyimide-type bismaleimide resin, and the ceramic is inorganic ceramic particles. The thermosensitive composite separator prepared by this invention exhibits high thermal stability and can achieve thermal shutdown function at relatively low temperatures, effectively ensuring the safety of the battery cell.

Description

Technical Field

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

[0002] As one of the four main materials of lithium-ion batteries, the separator not only isolates electrons and transports lithium ions, but also has a thermal pore-closing function. In the event of thermal runaway, it instantly closes the pores, preventing the battery from continuing to operate. However, existing separators suffer from high thermal pore-closing temperatures, leading to problems with timely pore closure. For example, the thermal pore-closing temperature of polyolefin separators is around 135℃, which is relatively high. This high temperature prevents timely thermal pore closure when the lithium-ion battery experiences temperature increases due to short circuits or overcharging, hindering lithium ions from passing through the separator and potentially causing further thermal runaway or even fire and explosion. Therefore, developing a lithium-ion battery separator with high thermal stability and the ability to achieve thermal pore-closing at lower temperatures has become an increasingly popular research topic.

[0003] To address the above issues, existing patent CN205723722U discloses a ceramic-coated lithium-ion battery separator, comprising a surface layer and a separator layer, with a polymer layer between the surface layer and the separator layer. The polymer layer includes fibrous filaments and ceramic particles. The fibrous filaments form a porous, sponge-like structure that wraps around the ceramic particles, effectively fixing them and preventing powder shedding. The surface layer has nanoscale micropores, making it dense and preventing coating peeling at high temperatures. However, when a short circuit occurs in the battery, the separator cannot immediately and rapidly thermally shut off at high temperatures, and its low rupture temperature can cause a short circuit between the positive and negative electrodes at high temperatures, posing a safety hazard during use.

[0004] CN113969006A discloses a method for preparing a polyimide-coated modified polyolefin separator composite membrane. Microspherical polyimide is prepared using electrospinning technology and coated onto the separator surface. The high adhesion strength between the polyimide coating and the polyolefin separator layer significantly improves the thermal dimensional stability and electrolyte wettability of the separator. In addition to its function as a separator at normal temperatures, the polyolefin separator can also act as a pore-closing agent at high temperatures, preventing internal short circuits and further improving battery safety. However, although this polyimide structure exhibits high heat resistance, its numerous rigid groups make film formation difficult, hindering rapid melting and film formation to prevent thermal runaway. Furthermore, the complex synthesis method and poor solubility limit its application in separators. Summary of the Invention

[0005] The purpose of this invention is to provide a thermosensitive composite separator, its preparation method and application. The prepared thermosensitive composite separator has high heat resistance and stability, and can achieve thermal shutdown function at a low temperature, effectively ensuring the safety of battery cell use.

[0006] To achieve the above objectives, the present invention provides a thermosensitive composite membrane, which includes a base membrane and a coating. The coating is applied to one or both sides of the base membrane. The coating includes a polymer resin and a ceramic. The polymer resin is a polyimide-type bismaleimide resin (BMI resin), and the ceramic is inorganic ceramic particles.

[0007] Preferably, the base membrane is a porous thin film.

[0008] Preferably, the porous film is a polyolefin film.

[0009] This invention coats a base membrane with a mixture of BMI resin and ceramic, resulting in a membrane with high heat resistance, high membrane breakage rate, and low closed-cell characteristics, enabling it to melt and form a film at high temperatures to achieve thermal shut-off.

[0010] Preferably, the polyimide-type bismaleimide resin is a maleic anhydride-terminated long-chain aliphatic polyimide-type bismaleimide resin with the structural formula shown in formula (I) below, wherein the number of carbons in the long-chain hydrocarbon structure of the polyimide-type bismaleimide resin is 25 to 45.

[0011]

[0012] R represents a long-chain aliphatic hydrocarbon.

[0013] Preferably, the inorganic ceramic particles have a particle size of 0.01 to 5 μm, and the inorganic ceramic particles contain one or more of the following: magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, boehmite, aluminum oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, or barium sulfate.

[0014] The above-mentioned method for preparing a thermosensitive composite diaphragm includes the following steps:

[0015] S1. Preparation of coating slurry: Polyimide-type bismaleimide resin is added to a solvent to dissolve and form a polymer solution. Inorganic ceramic particles are added to the polymer solution and stirred evenly to obtain the coating slurry.

[0016] S2. Coating: The coating slurry is coated on at least one side of the base film, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0017] Preferably, the solid content of the coating slurry in S1 is 20-40 wt%, and the inorganic ceramic particles account for 5-95 wt% of the total solid content in the coating slurry. By controlling the solid content of the coating slurry within the above range, the present invention facilitates the improvement of coating quality. A solid content below 20% results in excessively low viscosity of the coating slurry, while a solid content above 40% results in excessively high viscosity, making coating difficult and hindering uniform coating.

[0018] Preferably, the polyimide-type bismaleimide resin in S1 is prepared using the following steps:

[0019] 1) Dissolve the long-chain aliphatic diamine in toluene for later use. Add pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drip the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel. Control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0020] 2) Slowly add maleic anhydride to the reaction vessel in step 1), stir for 10 minutes, and keep the temperature at 25°C throughout the process until the addition is complete;

[0021] 3) Add p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing the product, polyimide-type bismaleimide resin is obtained.

[0022] This invention controls the temperature of the reactor at 25°C to avoid the reaction exothermicly causing the reactor temperature to rise too high and generate more byproducts.

[0023] Preferably, in step 1), the molar ratio of long-chain aliphatic diamine to pyromellitic dianhydride is 1.2 to 1.

[0024] In this invention, by controlling the amounts of long-chain aliphatic diamine and pyromellitic dianhydride within the above-mentioned range, an excess of long-chain aliphatic diamine is made, which is beneficial for the full reaction between the long-chain aliphatic diamine and pyromellitic dianhydride.

[0025] Preferably, in step 2), the molar ratio of maleic anhydride to long-chain aliphatic diamine is 5 to 5.1.

[0026] Preferably, in step 3), the molar ratio of p-toluenesulfonic acid to long-chain aliphatic diamine is 2 to 3.

[0027] By controlling the amount of p-toluenesulfonic acid in the above-mentioned proportion, the present invention achieves a higher content of p-toluenesulfonic acid as a catalyst, which is beneficial to promoting the reaction.

[0028] The above-mentioned application of a thermosensitive composite separator is applied to a lithium-ion battery pack. The lithium-ion battery pack includes a battery module, the battery module includes a lithium-ion battery, the lithium-ion battery includes electrodes, a separator and an electrolyte, and the separator is the above-mentioned thermosensitive composite separator.

[0029] Mechanism of the invention:

[0030] The BMI resin in the coating slurry of this invention possesses a long-chain aliphatic polyimide structure and double bonds, giving it the characteristic of easy film formation at lower temperatures; high film-breaking capacity of polyimide resin; and the characteristic of double bonds polymerizing into a high-heat-resistant polymer with a high molecular network structure. Therefore, the resulting thermosensitive composite separator exhibits the ability to melt and form a film quickly at lower temperatures, and the thermally closed-cell characteristics of the polyolefin separator, preventing lithium-ion battery operation and thus preventing safety issues such as fires and explosions caused by thermal runaway. Furthermore, the thermosensitive composite separator containing BMI resin possesses the high film-breaking capacity and high heat resistance of polyimide resin, ensuring that the positive and negative electrodes cannot contact at high temperatures, thereby preventing further thermal runaway.

[0031] The beneficial effects of this invention are:

[0032] This invention uses a composite coating of BMI resin and inorganic materials to give the separator high heat resistance and stability, while also enabling thermal shutdown at lower temperatures, effectively ensuring the safety of the battery cell.

[0033] In this invention, the coating is composed of BMI resin and inorganic ceramics. When the battery temperature reaches above 100°C, the BMI resin melts upon heating and quickly fills the micropores of the base membrane, cutting off the lithium-ion transport channels and achieving thermal shutdown of the separator. At the same time, the rigid skeleton of the inorganic ceramic particles provides a certain support for the separator, preventing the separator from shrinking severely due to the melting of the BMI resin.

[0034] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of a thermosensitive composite diaphragm according to the present invention;

[0036] Figure 2 The BMI resin in the thermosensitive composite diaphragm coating of Embodiment 7 of the present invention 1 H-NMR spectrum;

[0037] Figure 3 This is the IR spectrum of BMI resin in the thermosensitive composite diaphragm coating of Embodiment 7 of the present invention;

[0038] Figure 4 This is a film formation diagram of BMI resin in the thermosensitive composite diaphragm coating in Embodiment 7 of the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0040] Example 1

[0041] Figure 1 This is a schematic diagram of the structure of a thermosensitive composite diaphragm according to the present invention, as shown below. Figure 1 As shown, the present invention provides a thermosensitive composite diaphragm, which includes a base film and a coating. The base film is a polyethylene diaphragm, and the coating is applied to one or both sides of the base film. The coating includes polyimide-type bismaleimide resin (BMI resin) and boehmite.

[0042] Example 2

[0043] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0044] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0045] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0046] The preparation of BMI resin in S1 is as follows:

[0047] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (36 carbons in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0048] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0049] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0050] Example 3

[0051] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0052] S1. Preparation of coating slurry: 16.5g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 13.5g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 55wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0053] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0054] The preparation of BMI resin in S1 is as follows:

[0055] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 40 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, controlling the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0056] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0057] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0058] Example 4

[0059] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0060] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0061] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0062] The preparation of BMI resin in S1 is as follows:

[0063] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 40 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, controlling the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0064] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0065] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0066] Example 5

[0067] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0068] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0069] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0070] The preparation of BMI resin in S1 is as follows:

[0071] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 45 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drip the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0072] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0073] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0074] Example 6

[0075] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0076] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of alumina particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0077] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0078] The preparation of BMI resin in S1 is as follows:

[0079] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 50 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, controlling the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0080] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0081] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0082] Example 7

[0083] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0084] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0085] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0086] The preparation of BMI resin in S1 is as follows:

[0087] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 50 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, controlling the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0088] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0089] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0090] Example 8

[0091] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0092] S1. Preparation of coating slurry: 18g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 12g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 60wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0093] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0094] The preparation of BMI resin in S1 is as follows:

[0095] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 45 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drip the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0096] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0097] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0098] Figure 2 The BMI resin in the thermosensitive composite diaphragm coating of Embodiment 8 of the present invention 1 H-NMR spectrum; as shown Figure 2 As shown, the aliphatic diamine, pyromellitic dianhydride, and maleic anhydride undergo dehydration and ring closure. The NH bond in the raw material disappears, and H combines with the O in the pyromellitic dianhydride and maleic anhydride to form H2O, which is released to form an O=CN closed ring. The product contains only the benzene ring in the pyromellitic dianhydride, the double bond in the maleic anhydride, the CH bond near the N atom in the aliphatic diamine, and the CH bond on the long chain in the aliphatic diamine, proving that the BMI resin was successfully synthesized.

[0099] Figure 3 This is the IR spectrum of the BMI resin in the thermosensitive composite diaphragm coating of Embodiment 8 of the present invention; as shown. Figure 3 As shown, the product was characterized by infrared spectral analysis in the range of 4000–60 cm⁻¹. -1 The number of scans was 32. The infrared spectrum shows a value of 1698 cm⁻¹. -1The characteristic peak of C=O is at 1340 cm⁻¹. -1 The characteristic peak of C=N is 3463 cm⁻¹. -1 and 3101cm -1 The peaks are characteristic of the CH ring of the benzene ring in pyromellitic dianhydride (PMDA), which proves that the maleic anhydride-terminated aliphatic polyimide bismaleimide resin has been successfully synthesized.

[0100] Figure 4 This is a film formation diagram of BMI resin in the thermosensitive composite diaphragm coating of Embodiment 8 of the present invention, as shown below. Figure 4 As shown, BMI resin was dissolved in toluene, and the solution was directly coated and dried to obtain a film with a thickness of 0.06 mm, indicating that the long-chain aliphatic polyimide BMI resin has film-forming properties.

[0101] Comparative Example 1

[0102] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0103] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0104] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0105] The preparation of BMI resin in S1 is as follows:

[0106] 1) Dissolve 0.4 mol of long-chain aliphatic diamine (36 carbons in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0107] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0108] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0109] Comparative Example 2

[0110] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0111] S1. Preparation of coating slurry: 15g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 15g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 50wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0112] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0113] The preparation of BMI resin in S1 is as follows:

[0114] 1) Dissolve 0.6 mol of long-chain aliphatic diamine (36 carbons in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drop the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0115] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0116] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0117] Comparative Example 3

[0118] This invention provides a method for preparing a thermosensitive composite separator, comprising the following steps:

[0119] S1. Preparation of coating slurry: 19.5g of BMI resin is added to 70g of N-methylpyrrolidone solvent to dissolve and form a polymer solution. 10.5g of boehmite particles are added to the polymer solution and stirred evenly to obtain the coating slurry. The solid content of the coating slurry is 30wt%, and BMI resin accounts for 65wt% of the total solids in the coating slurry (total amount of BMI resin and ceramic particles).

[0120] S2. Coating: Coating slurry is applied to one side surface of a polyethylene diaphragm with a thickness of 12μm, and after curing and drying, a thermosensitive composite diaphragm is obtained.

[0121] The preparation of BMI resin in S1 is as follows:

[0122] 1) Dissolve 0.5 mol of long-chain aliphatic diamine (with 45 carbon atoms in the long-chain hydrocarbon structure) in toluene for later use. Add 0.45 mol of pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drip the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel, and control the temperature of the reactor at 25°C throughout the process until the addition is complete.

[0123] 2) Slowly add 2.5 mol of maleic anhydride to the reaction vessel in step 1), stir for 10 min, and keep the temperature at 25℃ throughout the process until the addition is complete;

[0124] 3) Add 1 mol of p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing, the product is obtained as polyimide-type bismaleimide resin.

[0125] Performance testing

[0126] The thermistor composite separators prepared in Examples 2-8 and Comparative Examples 1-3 were combined with electrodes and electrolytes to form lithium-ion batteries, and overcharge tests were performed on the assembled lithium-ion batteries. The positive electrode used ternary 523, the negative electrode used graphite, and the electrolyte was 1 mol of lithium hexafluorophosphate dissolved in EC:DMC = 1:1. The positive and negative electrodes, the thermistor composite separator, and the electrolyte were assembled into a 2.6 Ah pouch cell. The testing method was to discharge the pouch cell at 1C to 2.75V, then charge it at 1C to 8.4V, and observe whether the cell caught fire. The test structure is shown in Table 1.

[0127] Table 1 Overcharge Test Data

[0128]

[0129]

[0130]

[0131] As shown in Table 1, the lithium-ion battery prepared using the thermistor composite separator provided by this invention exhibits excellent safety performance. A comparison of Example 2 and Comparative Examples 1-2 reveals that this invention, by increasing the proportion of long-chain aliphatic diamines, effectively reduces the thermal pore-closing temperature and increases the membrane rupture temperature of the composite separator, significantly improving the safety performance of the lithium-ion battery. However, the optimal amount of long-chain aliphatic diamine is 0.5 mol; when the amount exceeds 0.5 mol, the performance of the resulting lithium-ion battery decreases.

[0132] As can be seen from the comparison of Examples 2, 4, 5, and 7, the safety performance of the lithium-ion battery improves with the increase of the number of carbons in the chain hydrocarbon structure. The performance is best when the number of carbons is 45. If the number of carbons is too large, the rigid groups will decrease, resulting in poor heat resistance of the separator.

[0133] As can be seen from the comparison of Examples 6 and 7, the type of ceramic in the coating slurry has little effect on the thermal pore-closing temperature of the diaphragm and the increase in the rupture temperature.

[0134] As can be seen from the comparison of Examples 5 and 8 and Comparative Example 3, the effect is best when the proportion of BMI resin in the coating slurry is 60wt%. Too much BMI resin can easily cause pore blockage, resulting in too high air permeability of the separator and thus too high cell resistance. Too little BMI resin can easily lead to failure to melt and form a film to isolate lithium ions when the separator is thermally runaway.

[0135] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A thermosensitive composite diaphragm, characterized in that: The thermosensitive composite diaphragm includes a base membrane and a coating. The coating is applied to one or both sides of the base membrane. The coating includes a polymer resin and a ceramic. The polymer resin is a polyimide-type bismaleimide resin, and the ceramic is inorganic ceramic particles. The polyimide-type bismaleimide resin is a maleic anhydride-terminated long-chain aliphatic polyimide-type bismaleimide resin with the following structural formula (I). The number of carbons in the long-chain hydrocarbon structure of the polyimide-type bismaleimide resin is 25 to 45. （I） R represents a long-chain aliphatic hydrocarbon.

2. The thermosensitive composite diaphragm according to claim 1, characterized in that: The inorganic ceramic particles have a particle size of 0.01~5μm and contain one or more of the following: magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, boehmite, aluminum oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, or barium sulfate.

3. A method for preparing a thermosensitive composite diaphragm as described in any one of claims 1-2, characterized in that: Includes the following steps, S1. Preparation of coating slurry: Polyimide-type bismaleimide resin is added to a solvent to dissolve and form a polymer solution. Inorganic ceramic particles are added to the polymer solution and stirred evenly to obtain the coating slurry. S2. Coating: The coating slurry is coated on at least one side of the base film, and after curing and drying, a thermosensitive composite diaphragm is obtained.

4. The method for preparing a thermosensitive composite diaphragm according to claim 3, characterized in that: The solid content of the coating slurry in S1 is 20-40 wt%, and inorganic ceramic particles account for 5-95 wt% of the total solid content in the coating slurry.

5. The method for preparing a thermosensitive composite diaphragm according to claim 3, characterized in that: The polyimide-type bismaleimide resin in S1 is prepared using the following steps. 1) Dissolve the long-chain aliphatic diamine in toluene for later use. Add pyromellitic dianhydride and toluene to a reactor equipped with a cooling and stirring device, stir to dissolve, and when the temperature in the reactor drops to 25°C, slowly drip the dissolved long-chain aliphatic diamine into the reactor through a constant pressure funnel. Control the temperature of the reactor at 25°C throughout the process until the addition is complete. 2) Slowly add maleic anhydride to the reaction vessel in step 1), stir for 10 minutes, and keep the temperature at 25°C throughout the process until the addition is complete; 3) Add p-toluenesulfonic acid to the reactor in step 2), raise the temperature to 220°C and reflux to remove water until no water is removed. Maintain the temperature at 220°C and reflux for 5 hours. After washing and neutralizing the product, polyimide-type bismaleimide resin is obtained.

6. The method for preparing a thermosensitive composite diaphragm according to claim 3, characterized in that: In step 1), the molar ratio of the medium-chain aliphatic diamine to pyromellitic dianhydride is 1 to 1.

2.

7. The method for preparing a thermosensitive composite diaphragm according to claim 3, characterized in that: In step 2), the molar ratio of maleic anhydride to long-chain aliphatic diamine is 5 to 5.

1.

8. The method for preparing a thermosensitive composite diaphragm according to claim 3, characterized in that: Step 3) The molar ratio of p-toluenesulfonic acid to long-chain aliphatic diamine is 2-3.

9. An application of the thermosensitive composite diaphragm as described in any one of claims 1-2, characterized in that: The invention is applied to a lithium-ion battery pack, which includes a battery module, a lithium-ion battery, and an electrolyte, wherein the separator is the thermosensitive composite separator as described in any one of claims 1-2.

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