Composite diaphragm and preparation method and application thereof

Abstract

The application provides a composite diaphragm, comprising a diaphragm base and a gel layer which is compounded on the surface of the diaphragm base; the gel layer is obtained through cross-linking polymerization reaction of acrylamide, N, N'-methylenebisacrylamide, amino acid salt and an acid absorbent. The gel layer prepared by the application has the properties of absorbing CO2, absorbing acid and absorbing water at the same time through hydroxylation reaction of Acr, amino acid salt and acid absorbent on the macromolecular chain, so that the gel can absorb or eliminate HF, trace water molecules and CO2 generated in the battery cycle, and the gel layer can play a flame-retardant effect when the battery short-circuits and catches fire, thereby improving the safety of the battery, and a lithium ion battery with excellent electrochemical performance can be prepared.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a composite diaphragm and a preparation method and application thereof. BACKGROUND

[0002] The rapid development of human society relies on the large consumption of fossil energy. Various machines using coal, oil and other fuels are continuously developed, especially after the industrial revolution, the large consumption of energy makes people feel the energy crisis. Lithium ion batteries are a good choice for large energy storage due to their high energy density and high power density. In recent years, they have also been widely used in portable electronic devices and new energy vehicles.

[0003] With the increase of the use time of lithium batteries, the side reactions between electrolyte and lithium metal cause a certain amount of damage to the solid electrolyte interface film (SEI film) of the positive and negative electrodes and the problems of gas production and other side reactions, which leads to the trend of capacity decline and safety problems. In order to solve these problems, researchers add various additives to the electrolyte and increase the kinetic energy of the diaphragm to enhance the firmness and stability of the SEI film and avoid the harmful reaction between the electrolyte and the electrode surface. However, the electrolyte additives and functional diaphragms they use are too single, and most of them only solve one or two byproducts generated by electrolyte reaction.

[0004] Therefore, it is necessary to find a method to solve the HF, trace amount of water molecules and gas CO2 generated in the cycle process, to adsorb or eliminate the three harmful substances, so as to further promote the commercial development of lithium batteries. SUMMARY

[0005] Therefore, the technical problem to be solved by the present application is to provide a composite diaphragm and a preparation method and application thereof. The composite diaphragm provided by the present application can adsorb or eliminate HF, trace amount of water molecules and gas CO2 generated in the battery cycle, and the gel layer can play a flame-retardant effect when the battery short-circuits and catches fire, thereby improving the safety of the battery, so as to prepare a lithium ion battery with excellent electrochemical performance.

[0006] The present application provides a composite diaphragm, comprising a diaphragm substrate and a gel layer compounded on the surface of the diaphragm substrate.

[0007] The gel layer is obtained by cross-linking polymerization reaction of acrylamide, N, N'-methylene bisacrylamide, amino acid salt and acid absorbent.

[0008] Preferably, the gel layer is prepared from raw materials including the following mass parts:

[0009] acrylamide 20-272 mass parts;

[0010] N,N'-methylenebisacrylamide 0.5-11.5 parts by mass;

[0011] amino acid salt 5-32 parts by mass;

[0012] acid-adsorbing agent 3-20 parts by mass.

[0013] Preferably, the gel layer is prepared from raw materials including the following parts by mass:

[0014] acrylamide 52-137 parts by mass;

[0015] N,N'-methylenebisacrylamide 2-6 parts by mass;

[0016] amino acid salt 7-20 parts by mass;

[0017] acid-adsorbing agent 7-20 parts by mass.

[0018] Preferably, the amino acid salt is selected from one or more of sodium / potassium sarcosinate, sodium / potassium glutamate, sodium / potassium histidine, sodium / potassium lysine, sodium / potassium arginine, sodium / potassium aspartate, sodium / potassium tryptophan, sodium / potassium serine, sodium / potassium threonine, sodium / potassium cysteine, sodium / potassium methionine, sodium / potassium glutamine, and sodium / potassium asparagine.

[0019] Preferably, the acid-adsorbing agent is selected from one or more of calcium stearate, hydrotalcite, zinc oxide, magnesium oxide, and divinylbenzene.

[0020] Preferably, the raw materials for preparing the composite separator further include one or more of a pH adjuster, a catalyst, and an oxidizing agent;

[0021] the pH adjuster is selected from boric acid;

[0022] the catalyst is selected from ammonium persulfate;

[0023] the oxidizing agent is selected from tetramethylethylenediamine.

[0024] Preferably, the thickness of the gel layer is 8-12 μm.

[0025] Preferably, the separator substrate is selected from a polyethylene separator or a polypropylene separator.

[0026] The present application also provides a method for preparing the above composite separator, comprising the following steps:

[0027] A) mixing acrylamide, N,N'-methylenebisacrylamide, an amino acid salt, an acid-adsorbing agent, a pH adjuster, and a solvent to perform an addition reaction to obtain an addition product;

[0028] B) placing the addition product on the surface of the separator substrate with a mixed solution of catalyst and oxidant to perform a polymerization reaction, and after drying, obtaining a composite separator.

[0029] Preferably, the solvent is selected from methanol, ethanol or ethylene glycol.

[0030] The temperature of the addition reaction is room temperature, and the time is 30-60 min.

[0031] The temperature of the polymerization reaction is room temperature, and the time is 1-3 h.

[0032] The temperature of the drying is 40-50°C, and the time is 12-24 h.

[0033] The application also provides a lithium ion battery comprising the composite separator or the composite separator prepared by the preparation method.

[0034] Compared with the prior art, the application provides a composite separator comprising a separator substrate and a gel layer compounded on the surface of the separator substrate; the gel layer is obtained by cross-linking polymerization reaction of acrylamide, N, N'-methylenebisacrylamide, amino acid salt and acid absorbent. The gel layer prepared by the application has the properties of absorbing CO2, acid and water at the same time through hydroxylation reaction of Acr, amino acid salt and acid absorbent on the macromolecular chain, and can absorb or eliminate HF, trace water molecules and CO2 generated in the battery cycle, and can play a flame-retardant effect in the case of battery short circuit and fire, thereby improving the safety of the battery, so as to prepare a lithium ion battery with excellent electrochemical performance. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 The gas composition determination results of the lithium ion battery prepared by the separator of Example 4 and the comparative example after cycle. DETAILED DESCRIPTION

[0036] The application provides a composite separator comprising a separator substrate and a gel layer compounded on the surface of the separator substrate.

[0037] The gel layer is obtained by cross-linking polymerization reaction of acrylamide, N, N'-methylenebisacrylamide, amino acid salt and acid absorbent.

[0038] The composite separator provided by the application comprises a separator substrate, and in the application, the separator substrate is selected from a polyethylene separator or a polypropylene separator.

[0039] The composite separator provided by the application further comprises a gel layer compounded on the surface of the separator substrate.

[0040] The gel layer is obtained by cross-linking polymerization of acrylamide, N, N'-methylenebisacrylamide, amino acid salt and acid absorbent;

[0041] In some embodiments of the present application, the gel layer is prepared from raw materials including the following mass parts:

[0042] acrylamide 20-272 mass parts;

[0043] N, N'-methylenebisacrylamide 0.5-11.5 mass parts;

[0044] amino acid salt 5-32 mass parts;

[0045] acid absorbent 3-20 mass parts.

[0046] The preparation raw materials of the gel layer provided by the present application include 80-273 mass parts of acrylamide (Acr), preferably 20, 40, 80, 100, 125, 150, 175, 200, 225, 250, 272, or any value between 20-272 mass parts, and further preferably any value between 52-137 mass parts.

[0047] The preparation raw materials of the gel layer provided by the present application further include 0.5-11.5 mass parts of N, N'-methylenebisacrylamide (Bis), preferably 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 11.5, or any value between 2-12 mass parts, and further preferably any value between 2-6 mass parts.

[0048] The preparation raw materials of the gel layer provided by the present application further include 5-32 mass parts of amino acid salt, preferably 5, 7, 10, 12, 15, 16, 18, 20, 22, 24, 26, 28, 30, 32, or any value between 5-32 mass parts, and further preferably any value between 7-20 mass parts. In the present application, the amino acid salt is selected from one or more of sodium / potassium sarcosinate, sodium / potassium glutamate, sodium / potassium histidine, sodium / potassium lysine, sodium / potassium arginine, sodium / potassium aspartate, sodium / potassium tryptophan, sodium / potassium serine, sodium / potassium threonine, sodium / potassium cysteine, sodium / potassium methionine, sodium / potassium glutamine and sodium / potassium asparagine, and preferably potassium histidine.

[0049] The preparation raw materials of the gel layer provided by the present application further include 3-20 mass parts of acid absorbent, preferably 3, 5, 7, 10, 11, 13, 15, 17, 20, or any value between 3-20 mass parts, and further preferably any value between 7-20 mass parts. In the present application, the acid absorbent is selected from one or more of calcium stearate, hydrotalcite, zinc oxide, magnesium oxide and divinylbenzene, and preferably calcium stearate.

[0050] In some embodiments of the present application, the preparation of the gel layer further comprises one or more of a pH adjuster, a catalyst, and an oxidizing agent.

[0051] In the present application, the pH adjuster is selected from boric acid, and the pH adjuster is added in an amount of 2-14 parts by mass, preferably 2, 4, 6, 8, 10, 12, 14, or any value between 2-14 parts by mass, and further preferably any value between 5-9 parts by mass.

[0052] The catalyst is selected from ammonium persulfate (AP).

[0053] The oxidizing agent is selected from tetramethyl ethylenediamine.

[0054] In the present application, the thickness of the gel layer is 8-12 μm, preferably 8, 9, 10, 11, 12, or any value between 8-12. If the thickness of the gel layer is too small, the adsorption effect of HF, water molecules, and CO2 and the flame retardant effect of the gel layer are limited. If the thickness of the gel layer is too large, the energy density of the battery is reduced.

[0055] The present application also provides a preparation method of the composite separator described above, comprising the following steps:

[0056] A) mixing acrylamide, N, N'-methylenebisacrylamide, an amino acid salt, an acid absorbent, a pH adjuster, and a solvent to perform an addition reaction to obtain an addition product;

[0057] B) placing a mixed solution of the addition product, a catalyst, and an oxidizing agent on the surface of the separator substrate to perform a polymerization reaction, and drying to obtain a composite separator.

[0058] Specifically, the present application dissolves the amino acid salt in a solvent, then adds an acid absorbent to perform heating, mixing, and stirring, fully dissolves, then adds a pH adjuster to adjust the pH of the solution to obtain a mixed solution of the amino acid salt and the acid absorbent.

[0059] The solvent is selected from methanol, ethanol, or ethylene glycol, and preferably methanol.

[0060] The temperature of the heating, mixing, and stirring is 70-80 °C, preferably 70, 72, 74, 75, 76, 78, 80, or any value between 70-80 °C, and the time of the heating, mixing, and stirring is 10-14 h, preferably 10, 11, 12, 13, 14, or any value between 10-14 h.

[0061] The pH of the solution is adjusted to 8-10 by adding boric acid, preferably 8, 8.5, 9, 9.5, 10, or any value between 8-10.

[0062] The acrylamide (Acr) and N, N'-methylene bisacrylamide are dissolved in a solvent to obtain a mixed solution of a reaction monomer and a crosslinking agent.

[0063] Then, the mixed solution of the reaction monomer and the crosslinking agent is mixed with a mixed solution of an amino acid salt and an acid absorbent to perform an addition reaction, so that the Acr molecular chain contains calcium stearate (acid absorbent) and an amino acid salt to obtain an addition product. The addition reaction is performed at room temperature for 30-60 min, preferably 30, 40, 50, 60, or any value between 30-60 min.

[0064] Then, a catalyst and an oxidizing agent are added to the addition product and uniformly mixed to obtain a mixed solution, and the obtained mixed solution is placed on the surface of the separator substrate to perform a polymerization reaction.

[0065] The separator substrate can be coated with a certain amount of methanol on the surface in advance, and attached to the glass groove to facilitate the preparation of the gel layer.

[0066] The polymerization reaction is performed at room temperature for 12-24 h, preferably 12, 18, 24, or any value between 12-24 h.

[0067] After the polymerization reaction is completed, drying is performed to obtain a composite separator. The drying temperature is 40-50℃, preferably 40, 42, 45, 48, 50, or any value between 40-50℃, and the drying time is 12-24 h, preferably 12, 18, 24, or any value between 12-24 h.

[0068] The application also provides a lithium ion battery comprising the composite separator or the composite separator prepared by the preparation method.

[0069] The gel layer on the surface of the composite separator optimizes the functionality of the separator, and can adsorb or eliminate HF, trace water molecules and gaseous CO2 generated during battery cycling. This avoids harmful chemical reactions between HF, water molecules and lithium metal, reduces the consumption of lithium metal, and reduces the risk of battery explosion by absorbing CO2. Moreover, the gel layer can play a flame-retardant effect when the battery short-circuits and catches fire, thereby improving the safety performance of the lithium ion battery.

[0070] In addition, the preparation process provided by the application is simple and efficient, and is easy to realize industrial production.

[0071] In order to further understand the application, the composite separator and the preparation method and application thereof provided by the application will be described below in conjunction with examples, and the protection scope of the application is not limited by the following examples.

[0072] Comparative Example 1

[0073] The commercial PP separator does not contain a gel layer.

[0074] Example 1

[0075] Preparation of the composite separator:

[0076] Solution A (500 mL) was prepared by weighing acrylamide (Acr) 145 g and N, N'-methylenebisacrylamide (Bis) 5 g in methanol solvent.

[0077] Solution B (500 mL) was prepared by weighing potassium histidine 45 g in methanol solvent, adding calcium stearate 45 g and heating to 70 °C, stirring for 12 h to fully dissolve; then adding boric acid 20 g, adjusting the pH to 8, and stirring with a glass rod until the substance was completely dissolved.

[0078] The volume ratio of solution A to solution B was 5:4, and the total volume of solution A and solution B was 500 mL. Methanol solution 5 g, 20 wt% ammonium persulfate (AP) 100 μL, and tetramethyl ethylenediamine solution 10 μL were added. The solution was transferred to the surface of the prepared PP separator, cross-linked and polymerized to form a gel layer, and dried in an oven at 40 °C for 12 h. A composite separator with a gel layer thickness of 8 μm was obtained.

[0079] Example 2

[0080] Preparation of the composite separator:

[0081] Solution A (1000 mL) was prepared by weighing acrylamide (Acr) 290 g and N, N'-methylenebisacrylamide (Bis) 10 g in methanol solvent.

[0082] Solution B (1000 mL) was prepared by weighing sodium histidine 55 g in methanol solvent, adding calcium stearate 55 g and heating to 70 °C, stirring for 12 h to fully dissolve; then adding boric acid 24 g, adjusting the pH to 8, and stirring with a glass rod until the substance was completely dissolved.

[0083] The volume ratio of solution A to solution B was 5:4, and the total volume of solution A and solution B was 500 mL. Methanol solution 5 g, 20 wt% ammonium persulfate (AP) 100 μL, and tetramethyl ethylenediamine solution 10 μL were added. The solution was transferred to the surface of the prepared PP separator, cross-linked and polymerized to form a gel layer, and dried in an oven at 40 °C for 12 h. A composite separator with a gel layer thickness of 8 μm was obtained.

[0084] Example 3

[0085] Preparation of the composite separator:

[0086] Configuration A solution (2000 mL), acrylamide (Acr) 380 g, N, N'- methylene bisacrylamide (Bis) 18 g were weighed and dissolved in methanol solvent;

[0087] Configuration B solution (2000 mL), potassium arginine 65 g was dissolved in methanol solution, calcium stearate 65 g was added and heated to 70 °C, stirred for 12 h to make it fully dissolved; then boric acid 65 g was added, the PH was adjusted to 8, and the glass rod was stirred until the substance was completely dissolved;

[0088] The volume ratio of A solution to B solution was 5:4, and the total volume of A solution and B solution was 500 mL. Methanol solution 8 g, 20 wt% ammonium persulfate (AP) 260 μL, and tetramethyl ethylenediamine solution 26 μL were added. The solution was transferred to the surface of the prepared PP separator, cross-linked to form a gel layer, and dried in an oven at 50 °C for 24 h to obtain a composite separator with a gel layer thickness of 8 μm;

[0089] Example 4

[0090] Preparation of composite separator:

[0091] Configuration A solution (1000 mL), acrylamide (Acr) 490 g, N, N'- methylene bisacrylamide (Bis) 20 g were weighed and dissolved in methanol solvent;

[0092] Configuration B solution (1000 mL), lysine sodium 70 g was dissolved in methanol solution, calcium stearate 70 g was added and heated to 70 °C, stirred for 12 h to make it fully dissolved; then boric acid 30 g was added, the PH was adjusted to 8, and the glass rod was stirred until the substance was completely dissolved;

[0093] The volume ratio of A solution to B solution was 5:4, and the total volume of A solution and B solution was 500 mL. Methanol solution 8 g, 20 wt% ammonium persulfate (AP) 260 μL, and tetramethyl ethylenediamine solution 26 μL were added. The solution was transferred to the surface of the prepared PP separator, cross-linked to form a gel layer, and dried in an oven at 50 °C for 24 h to obtain a composite separator with a gel layer thickness of 8 μm;

[0094] Example 5

[0095] Preparation of composite separator:

[0096] Configuration A solution (1000 mL), acrylamide (Acr) 490 g, N, N'- methylene bisacrylamide (Bis) 20 g were weighed and dissolved in methanol solvent;

[0097] Configuration B solution (1000 mL), weigh 58 g of sodium lysine into a methanol solution, add 58 g of zinc oxide and heat to 70°C, stir for 12 h to fully dissolve; then add 30 g of boric acid, adjust the pH to 8, and stir with a glass rod until the material is completely dissolved;

[0098] Take A solution and B solution with a volume ratio of 5:4, stir for 60 min, the total volume of A solution and B solution is 500 mL; add 10 g of methanol solution, 300 μL of 20 wt% ammonium persulfate (AP), and 30 μL of tetramethyl ethylenediamine solution, transfer the solution to the surface of the prepared PP separator, crosslink polymerization to form a gel layer, dry in an oven at 40°C for 12 h, and obtain a composite separator with a gel layer thickness of 8 μm.

[0099] Battery preparation:

[0100] 1. Positive electrode sheet preparation:

[0101] (1) Mix lithium iron phosphate, conductive agent, and binder according to the mass ratio of 8:1:1, then add NMP solvent for uniform mixing to prepare a positive electrode slurry, and uniformly coat the positive electrode slurry on an aluminum foil coated with a conductive carbon layer, and vacuum dry at 80-120°C to obtain a positive electrode material coated with an active material layer.

[0102] The binder is PVDF;

[0103] The conductive agent is SP;

[0104] (2) After cold pressing and die cutting, the positive electrode sheet in step 1 is obtained;

[0105] 2. Negative electrode sheet preparation: Mix the negative electrode active material graphite, conductive carbon black, and negative electrode binder according to the ratio of 8:1:1 with deionized water to prepare a negative electrode slurry. Uniformly coat the negative electrode slurry on a copper foil according to a certain proportion, and vacuum dry at 70-90°C, and die cut to obtain a negative electrode sheet. The negative electrode binder is composed of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) with a mass ratio of 4:6.

[0106] 3. Battery preparation: Assemble the positive electrode sheet, separator, and negative electrode sheet together to make a battery, wherein the separator needs to completely wrap the positive electrode sheet and negative electrode sheet. Inject electrolyte (1M lithium hexafluorophosphate dissolved in a solvent with a volume ratio of EC:THF=1:1+5% DENE) into the battery. Finally, after standing, formation, and capacity, a soft package lithium iron phosphate battery is prepared.

[0107] 4. Soft package battery gas volume and content test:

[0108] The measuring instrument is selected as GC / GC-MS, the soft package battery is completed three weeks of charge-discharge cycle at 1C current density, and then the gas volume after production is obtained by subtracting the gas volume before production from the gas volume after production; then the composition of the gas is analyzed.

[0109] 5. After the cycle, the electrolyte in the battery is taken out to test the HF content

[0110] The content of HF in the electrolyte is determined by using the potentiometric titration method. The titrant is selected as sodium methoxide methanol solution 0.02 mol / L and benzoic acid methanol solution 400 mg / L; the titrant is calibrated by using the sodium methoxide methanol solution titrant to titrate in 50.00 mL of the benzoic acid methanol solution, and the titration degree is calculated; then 30.00 mL of the electrolyte is taken out and titrated with the titrant in the beaker, and the content of HF in the sample is calculated.

[0111] Reference Example Figure 1 , Figure 1 The gas composition determination results of the lithium ion battery after cycle prepared by the separator provided in Example 4 and the comparative example.

[0112] Table 1. Parameters and results of examples and comparative examples

[0113] Examples / Comparative Examples HF content in electrolyte after cycling (ppm) CO2 gas content (%) Example 1 32.55 14.63％ Example 2 26.63 12.87％ Example 3 18.62 8.29％ Example 4 11.89 5.56％ Example 5 14.66 5.88％ Comparative Example 1 143.81 40.12％

[0114] As can be seen from Table 1, by comparing the composite separators of Examples 1-5 with Comparative Example 1, it can be seen that the battery prepared by using the separator with the gel layer has a much smaller percentage of CO2 gas generation and HF content than that without the gel layer, which indicates that the gel layer has a good adsorption effect on the CO2 and HF gases generated during the cycle process, and the specific type of the acid absorbent has a lower influence on the final adsorption effect. As can be seen from Examples 1-4, the CO2 gas content is related to the content of the functional additive and the amount of the gel raw material. The increase of the content of the functional additive is beneficial to reducing the CO2 gas content and the HF content within a certain range, which indicates that the functional gel layer has a certain adsorption effect on the CO2 and HF gases generated during the cycle process; however, when the ratio of the functional additive to the gel is too large, the CO2 gas content and the HF content relatively increase, and the amount of the functional additive and the gel needs to be reasonably matched. In summary, the functional gel layer has an adsorption effect on the HF and CO2 gases generated during the cycle process.

[0115] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A composite separator, characterized by, The composite diaphragm comprises a diaphragm substrate and a gel layer compounded on the surface of the diaphragm substrate. The gel layer is obtained by cross-linking polymerization of acrylamide, N, N'-methylene bisacrylamide, amino acid salt and acid absorbent; The preparation method of the composite diaphragm comprises the following steps: A) mixing acrylamide, N, N'-methylene bisacrylamide, amino acid salt, acid absorbent, pH regulator and solvent to obtain an addition product by addition reaction; B) placing the addition product and a mixed solution of catalyst and oxidant on the surface of the diaphragm substrate to perform polymerization reaction, and drying to obtain the composite diaphragm.

2. The composite separator of claim 1, wherein The gel layer is prepared from raw materials comprising the following mass fractions: acrylamide 20-272 mass parts; N, N'-methylene bisacrylamide 0.5-11.5 mass parts; amino acid salt 5-32 mass parts; acid absorbent 3-20 mass parts.

3. The composite separator of claim 2, wherein, The gel layer is prepared from raw materials comprising the following mass fractions: acrylamide 52-137 mass parts; N, N'-methylene bisacrylamide 2-6 mass parts; amino acid salt 7-20 mass parts; acid absorbent 7-20 mass parts.

4. The composite separator of claim 1, wherein The amino acid salt is selected from one or more of sodium / potassium sarcosinate, sodium / potassium glutamate, sodium / potassium histidine, sodium / potassium lysine, sodium / potassium arginine, sodium / potassium aspartate, sodium / potassium tryptophan, sodium / potassium serine, sodium / potassium threonine, sodium / potassium cysteine, sodium / potassium methionine, sodium / potassium glutamine and sodium / potassium asparagine.

5. The composite separator of claim 1, wherein The acid absorbent is selected from one or more of calcium stearate, hydrotalcite, zinc oxide, magnesium oxide and divinylbenzene.

6. The composite separator of claim 1, wherein The raw materials for preparing the composite diaphragm further comprise one or more of pH regulator, catalyst and oxidant; The pH regulator is selected from boric acid; The catalyst is selected from ammonium persulfate; The oxidant is selected from tetramethyl ethylenediamine.

7. The composite separator of claim 1, wherein The thickness of the gel layer is 8-12 μm.

8. A method of producing the composite separator according to any one of claims 1 to 7, characterized by, The preparation method comprises the following steps: A) mixing acrylamide, N, N'-methylene bisacrylamide, amino acid salt, acid absorbent, pH regulator and solvent to obtain an addition product by addition reaction; B) placing the addition product and a mixed solution of catalyst and oxidant on the surface of the diaphragm substrate to perform polymerization reaction, and drying to obtain the composite diaphragm.

9. The production method according to claim 8, characterized by, The solvent is selected from methanol, ethanol or ethylene glycol; The temperature of the addition reaction is room temperature, and the time is 30-60 min; The temperature of the polymerization reaction is room temperature, and the time is 1-3 h; The temperature of the drying is 40-50 °C, and the time is 12-24 h.

10. A lithium-ion battery, characterized by, The composite diaphragm is prepared by the preparation method of any one of claims 1-7 or 8 or 9.

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