A high temperature resistant heat conductive flame retardant composite diaphragm and preparation method thereof, and lithium sulfur battery

Abstract

The present invention belongs to the technical field of lithium-sulfur batteries, and in particular, relates to a high-temperature resistant, heat-conductive, flame-retardant composite diaphragm, a preparation method thereof, and a lithium-sulfur battery. The high-temperature resistant, heat-conductive, flame-retardant composite diaphragm consists of an electrospinning composite matrix membrane and a coating; the electrospinning composite matrix membrane is prepared by modifying heat-conductive nanoparticles and organic flame retardants, respectively, and then mixing them into a high-temperature resistant polymer and then drying them by electrospinning; the coating is coated on the surface of the electrospinning composite matrix membrane, and the material of the coating is titanium carbide and PVDF doped with nitrogen and sulfur atoms. When the high-temperature resistant, heat-conductive, flame-retardant composite diaphragm is used to assemble a lithium-sulfur battery, the coating faces the sulfur positive electrode to achieve the inhibition of the shuttling of polysulfides. The high-temperature resistant, heat-conductive, flame-retardant properties of the composite diaphragm itself can further improve the thermal stability, flame retardant properties, and electrolyte wettability of the lithium-sulfur battery diaphragm, thereby achieving a comprehensive performance improvement in the battery's electrochemical and safety performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and in particular relates to a high-temperature resistant, heat-conductive, flame-retardant composite diaphragm and a preparation method thereof, and a lithium-sulfur battery. Background Art

[0002] Lithium-sulfur batteries (LSBs) are a new type of high-energy-density battery that has attracted much attention in the field of energy storage. Compared with ester solvents in lithium-ion battery (LIBs) electrolytes, ether solvents in LSBs electrolytes have lower flash points and higher vapor pressures. However, the flammable and explosive properties of elemental sulfur, the high reactivity of metallic lithium, and the strong oxidizing properties of the additive LiNO3 can lead to safety hazards in LSBs. LSBs have a higher risk of thermal runaway. Therefore, the separator plays a vital role in LSBs, which can not only realize the Li + At the same time, direct contact between the sulfur positive electrode and the lithium negative electrode should be prevented to avoid battery short circuit.

[0003] At present, polyolefin separators are widely used in high energy density lithium batteries, but this type of separator has a low melting point and poor thermal stability. It is very easy to shrink at high temperature or be pierced by lithium dendrites, causing short circuits in the battery. Continuous accumulation of heat will cause the battery temperature to be too high and cause thermal runaway, which will in turn cause a series of safety problems such as fire or even explosion. The separator can be modified by coating with heat-resistant materials, but the low thermal stability of the polyolefin matrix itself makes the heat resistance of the modified separator limited. The heat resistance of the separator can be effectively improved by replacing the polyolefin matrix with a polymer matrix with higher thermal stability.

[0004] In the Chinese invention application with application number CN114204208A, a preparation method of a PVDF-CTFE-based lithium-sulfur battery composite diaphragm is disclosed, which includes: firstly preparing oxide @ covalent organic framework powder, and mixing it with PVDF-CTFE in proportion, aging, stirring, and degassing to obtain a spinning precursor solution; spinning the spinning precursor solution under certain spinning conditions to obtain a fiber membrane, and then coating the surface of the dried fiber membrane with a carbon material / metal compound coating to obtain a modified PVDF-CTFE composite diaphragm. The diaphragm porosity in this patent is only 80%-85%, and the liquid absorption rate is 233%-289%.

[0005] In the Chinese invention application with application number CN114204209A, a method for preparing a secondary functionalized double-coated modified polyethersulfone lithium-sulfur battery separator is disclosed. First, sulfonated polyethersulfone is prepared, and the sulfonated polyethersulfone and the high molecular polymer are mixed in proportion, matured, stirred, and degassed to obtain a spinning precursor solution, which is then spun to obtain a fiber membrane; the surface of the dried fiber membrane is grafted with a secondary monomer and immersed in modified dopamine to obtain a secondary functionalized double-coated modified polyethersulfone lithium-sulfur battery separator. The membrane has a porosity of 70%-80%, a liquid absorption rate of 322%-345%, and a first-cycle discharge capacity of a battery equipped with the separator is 1280 mAh g -1 .

[0006] The two lithium-sulfur battery separators disclosed above have effectively improved the puncture strength of the separator through the coating, and have a certain degree of limiting effect on the migration of polysulfides; however, the two lithium-sulfur battery separators disclosed above have not shown significant improvements in liquid absorption rate and safety performance. Summary of the invention

[0007] The purpose of the present invention is to overcome the above-mentioned shortcomings of the lithium-sulfur battery separator in the prior art, mainly based on the existing technical solutions, to further improve the thermal stability, flame retardancy and electrolyte wettability of the lithium-sulfur battery separator, thereby achieving a comprehensive performance improvement in the battery's electrochemical and safety performance.

[0008] In order to achieve the above object, the present invention adopts the following technical solution:

[0009] In the first aspect, the invention provides a high temperature resistant, thermally conductive, flame retardant composite diaphragm, which consists of an electrospun composite matrix membrane and a coating; the electrospun composite matrix membrane is prepared by modifying thermally conductive nanoparticles and organic flame retardants respectively, mixing them into a high temperature resistant polymer, and then drying them by electrospinning; the coating is coated on the surface of the electrospun composite matrix membrane, and when the high temperature resistant, thermally conductive, flame retardant composite diaphragm is used to assemble a lithium-sulfur battery, the coating faces the sulfur positive electrode to achieve the inhibition of the shuttling of polysulfides; the material of the coating is nitrogen and sulfur atom doped titanium carbide and PVDF, and the nitrogen and sulfur atom doped titanium carbide and PVDF are mixed in a mass ratio of (1:1) to (1:5) and then coated, wherein the nitrogen and sulfur atom doped titanium carbide is obtained by calcining thiourea and MXenes in a tubular furnace after ultrasonic and freeze drying.

[0010] The thermal runaway of lithium-sulfur batteries is mainly caused by internal heat accumulation. Therefore, the present invention introduces thermally conductive nanoparticles and flame retardants into a heat-resistant polymer matrix to form a continuous heat-conducting path and a flame-retardant network to promote heat conduction and evacuation, which is used to inhibit the thermal runaway chain reaction inside the lithium battery, thereby improving the electrochemical and safety performance of the battery. The introduction of thermally conductive nanoparticles and flame retardants also needs to consider the compatibility of both with the polymer base material. Therefore, in the present invention, thermally conductive nanoparticles containing hydroxyl functional groups and an organic flame retardant DOPO-HQ containing hydroxyl functional groups are selected, and the modifier containing an active phosphorus-chlorine bond is used to graft the modifier on the surface of the thermally conductive nanoparticles and DOPO-HQ, respectively, by using a nucleophilic substitution reaction between the hydroxyl functional group and the modifier containing an active phosphorus-chlorine bond, and then the organic amine is further grafted on the surface by using a nucleophilic substitution reaction between the remaining active phosphorus-chlorine bond and the amine group of the organic amine molecule, thereby completing the modification treatment.

[0011] The high temperature resistant polymer is one of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyurethane, and poly(p-phenylene terephthalamide);

[0012] The thermally conductive nanoparticles have a thermal conductivity greater than 0.4 W m -1 k -1 Inorganic nanoparticles, preferably one of boron nitride, aluminum nitride, silicon nitride, aluminum oxide, and magnesium oxide;

[0013] The modifier is a nano-molecular compound containing an active phosphorus-chlorine bond, and is preferably one of phenylphosphoryl dichloride, diphenylphosphine chloride, diphenyl chlorophosphate, and cyanuric chloride;

[0014] The organic amine is any one of 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, piperazine, diethyleneimine, ethylenediamine, o-phenylenediamine, m-phenylenediamine, m-phenylenediamine or triethylamine, or a mixture of two thereof.

[0015] Preferably, the step of modifying the thermally conductive nanoparticles comprises: placing the thermally conductive nanoparticles in acetonitrile and ultrasonically dispersing them for 1 hour; then adding a modifier and continuing ultrasonic stirring; then dissolving an organic amine in acetonitrile and slowly dropping it into the above-mentioned mixed solution; after the dropwise addition is completed, heating to 60 degrees and reacting for 12 hours to obtain modified thermally conductive nanoparticles; wherein the mass ratio of the thermally conductive nanoparticles to the modifier is 1:(1-3.5), and the mass ratio of the thermally conductive nanoparticles to the organic amine is 1:(1-3.5).

[0016] Preferably, the step of modifying the organic flame retardant comprises: taking an appropriate amount of DOPO-HQ and an organic amine and placing them in acetone to dissolve them completely; taking the modifier and dissolving it in acetone, and then slowly dropping it into the above solution; after the dropwise addition is completed, heating the reaction to 70 degrees, condensing and refluxing the reaction for 10 hours to obtain a crude product; wherein the mass ratio of DOPO-HQ to the modifier is (1:1) to (5:1), and the mass ratio of the organic amine to the modifier is (1:1) to (5:1); after washing the crude product with acetone 3 times and water 3 times, drying it at 70 degrees to obtain the modified flame retardant product P-DOPO.

[0017] In a second aspect, the present invention also provides a preferred preparation method for the above-mentioned high-temperature resistant, thermally conductive, flame-retardant composite diaphragm, wherein the preparation method of the electrospinning composite matrix membrane comprises the steps of: first, adding PVDF to a mixture of N,N-dimethylformamide and acetone, stirring and dissolving to form a spinning solution, then adding the above-mentioned modified thermally conductive nanoparticles and modified organic flame retardant P-DOPO to the spinning solution, magnetically stirring for 12 hours, and obtaining a spinning solution after uniform dispersion; using a disposable syringe to absorb an appropriate amount of spinning solution, using a No. 18 needle, and the ambient temperature and humidity are 25±5°C, 25±5%. The voltage is 20-30KV, the injection speed is 0.8-2.5mL / h, the needle tip is 15-22 cm away from the receiver, and the stroke is 20-60 mm; wherein the mass concentration of the spinning solution is 10 wt%-18 wt%; the mass ratio of N,N-dimethylformamide to acetone is (1:1)-(4:1); the mass ratio of the thermal conductive nanoparticles to P-DOPO is (1:1)-(1:6); the mass ratio of the thermal conductive nanoparticles to PVDF is (1:15)-(1:25);

[0018] The method for preparing the titanium carbide doped with nitrogen and sulfur atoms in the coating material comprises the steps of dissolving LiF in a 1 to 10 M HCl solution, then adding Ti3AlC2 to the mixed solution, reacting for at least 24 hours, and then washing the resulting solution with deionized water and centrifuging until the pH value is neutralized. The collected Ti3C2T x The product was further exfoliated by ultrasound in an ice bath under argon flow, and centrifuged at 3500 rpm to obtain a layered MXenes (abbreviated as MX) nanosheet supernatant, i.e., MX suspension; the MX suspension was sealed and refrigerated; the pre-synthesized MXene suspension was diluted with deionized water, and then thiourea (TA) was added to the above suspension, and a uniform suspension was formed by ultrasound treatment, and freeze-dried to obtain TA@MX. Finally, TA@MX was transferred to a tube furnace to obtain the final product (NSMX); wherein the mass ratio of LiF, HCl and Ti3AlC2 was 1:(5-40):1; and the volume mass ratio of deionized water and thiourea was (500-100) ml:1g.

[0019] In a third aspect, the present invention also provides a lithium-sulfur battery assembled using the above-mentioned high-temperature thermally conductive flame-retardant composite diaphragm, and as a preferred embodiment, the assembly method comprises the steps of: the positive electrode of the battery is a sulfur positive electrode, the negative electrode is a lithium sheet, the electrolyte is LiPF6 and EMC / EC with a volume ratio of 1:1 and a concentration of 1 M, and the assembly order of the battery is: negative electrode shell-lithium sheet-diaphragm-electrolyte-positive electrode sheet-stainless steel gasket-spring sheet-positive electrode shell, and finally it is placed in a packaging machine for packaging to complete the battery assembly.

[0020] The present invention has the following technical effects:

[0021] The present invention grafts phosphorus-based flame-retardant organic components on the surface of heat-conducting nanoparticles, and further grafts organic amines on the surface through nucleophilic substitution reactions between active phosphorus-chlorine bonds and the amine groups of organic amine molecules to achieve modification. Not only does it improve compatibility, it can also impart flame retardancy, mainly playing a gas-phase flame retardant mechanism, and has a higher flame retardant efficiency. The high-temperature resistant, heat-conductive, flame-retardant composite diaphragm made by the present invention has been tested to be able to burn in an open flame for 10 seconds without burning out, and the porosity and liquid absorption rate of the diaphragm are as high as 374.2% and 774.6%. Compared with commercial diaphragms, the diaphragm prepared by the present invention has better flame retardancy, thermal conductivity, thermal stability, fire resistance, and electrolyte wettability.

[0022] Compared with the existing invention with application number CN114204208A, the composite diaphragm prepared by electrospinning in the present invention has better heat resistance and thermal stability, and can burn in an open flame for 10 seconds without burning out. The high temperature resistant diaphragm is modified by using modified thermal conductive nanoparticles and organic flame retardants, and the obtained composite diaphragm effectively improves the electrochemical and safety performance of the battery.

[0023] Compared with the existing invention with application number CN114204209A, the novel composite membrane of the present invention has higher porosity (374.2%) and liquid absorption rate (774.6%). At the same time, the composite membrane prepared by the present invention exhibits higher specific capacity and good cycle performance in electrochemical tests.

[0024] Compared with the inventions with application numbers CN114204208A and CN114204209A, the present invention studies the effect of the diaphragm on thermal runaway behavior and finds that the thermal runaway trigger time of the battery using the diaphragm of the present invention is increased from 201.1 min to 306.9 min compared with the battery equipped with the commercial diaphragm. In addition, the thermal runaway activation energy is increased from 0.521 eV to 0.813 eV, indicating the excellent safety performance of the battery.

[0025] In summary, the high temperature resistant, heat conductive and flame retardant composite diaphragm prepared in the present invention can significantly improve the electrochemical performance of the battery and inhibit the occurrence of thermal runaway of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a comparison chart of the porosity of Celgard2325 commercial membrane and PVDF, FM-PVDF-1 and FM-PVDF-2 membranes;

[0027] Figure 2 This is a comparison chart of the liquid absorption rate of Celgard2325 commercial membrane and PVDF, FM-PVDF-1 and FM-PVDF-2 membranes;

[0028] Figure 3a The combustion test of Celgard2325 commercial diaphragm;

[0029] Figure 3b For the combustion test of PVDF diaphragm;

[0030] Figure 3c For the combustion test of M-Celgard commercial diaphragm;

[0031] Figure 3d It is the combustion test of M-PVDF diaphragm;

[0032] Figure 3e It is the combustion test of FM-PVDF-1 diaphragm;

[0033] Figure 4a This is a photo of the residue left after the Celgard 2325 commercial diaphragm combustion test;

[0034] Figure 4b This is a photo of the residue left after the PVDF diaphragm combustion test;

[0035] Figure 4c This is a photo of the residue left after the M-Celgard commercial diaphragm combustion test;

[0036] Figure 4d This is a photo of the residue left after the combustion test of the M-PVDF diaphragm;

[0037] Figure 4e This is a photo of the residue left after the combustion test of FM-PVDF-1 diaphragm;

[0038] Figure 5 The thermal conductivity test results of Celgard2325 commercial membrane, PVDF, FM-PVDF-1 and FM-PVDF-2 membranes at different temperatures are shown in the figure;

[0039] Figure 6Cycling performance of batteries equipped with Celgard2325 commercial separator, FM-PVDF-1 and FM-PVDF-2 separators;

[0040] Figure 7a The result of the thermal runaway temperature parameter of the ARC test in Test Example 6;

[0041] Figure 7b The result of the thermal runaway time parameter of the ARC test in Test Example 6;

[0042] Figure 7c The result of the temperature rise rate of the ARC test in Test Example 6;

[0043] Figure 7d This is the activation energy bar graph of the ARC test in Test Example 6. DETAILED DESCRIPTION

[0044] In order to further illustrate the technical scheme of the present invention, the preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. At the same time, the raw materials or reagents mentioned below that are not described in detail are all commercially available products, and the process steps or methods not mentioned in detail are all process steps or methods known to those skilled in the art.

[0045] The sources of some raw materials and reagents involved in the following examples, comparative examples and test examples are as follows:

[0046] Raw material name Where to buy purity PVDF McLean Biotech AR, 99wt% Boron Nitride (BN) McLean Biotech AR, 98.5wt% DOPO-HQ Hubei Meiba New Materials Co., Ltd. AR, 99wt% Phenylphosphoryl dichloride McLean Biotech AR, 99wt% Triethylamine McLean Biotech AR, 99wt% 4,4'-Diaminodiphenylmethane Myrel Biotech AR, 99wt% <![CDATA[Ti3AlC2]]> Jilin Yiyi Technology Co., Ltd. AR, 98wt% TA Myrel Biotech AR, 99wt% Concentrated hydrochloric acid (HCl, 37%) Sinopharm Chemical Reagent Co., Ltd. (China) AR, 99wt% Lithium Fluoride (LiF) Aladdin Reagent Co., Ltd. (China) AR, 98wt% Celgard Commercial Diaphragms Dongguan Kelude New Energy Technology Co., Ltd. AR, 99wt% PVDF membrane Dongguan Kelude New Energy Technology Co., Ltd. AR, 99wt%

[0047] Example 1 Modified membrane (FM-PVDF-1)

[0048] (1) Preparation of modified boron nitride (f-BN)

[0049] Take 1 g of BN and place it in 100 mL of acetonitrile, and disperse it ultrasonically for 1 hour. Then add 3 g of phenylphosphoryl dichloride and 3.2 g of triethylamine solution, and continue ultrasonic stirring. Then take 3 g of 4,4'-diaminodiphenylmethane and dissolve it in acetonitrile, and slowly add it to the BN mixture. After the addition is completed, heat it to 60 ° C and react for 12 h to obtain modified boron nitride;

[0050] (2) Preparation of organic flame retardant (P-DOPO)

[0051] 3.24 g DOPO-HQ, 2.2 g triethylamine were dissolved in 80 ml acetone. 2 g phenylphosphoryl dichloride was dissolved in 20 mL acetone and slowly added to the above solution. After the addition was completed, the reaction temperature was raised to 70°C, and the product was obtained after condensation and reflux reaction for 10 hours. The product was washed with acetone 3 times and water 3 times, and then dried at 70 degrees to obtain a flame retardant product;

[0052] (3) Preparation of modified substrate membrane (F-PVDF-1)

[0053] 1.11 g PVDF was added to a mixture of 8 g N,N-dimethylformamide solvent and 2 g acetone to form a spinning solution, and then a mixture of 0.055 g f-BN and 0.055 g P-DOPO was added to the spinning solution. After magnetic stirring for 12 hours, a spinning solution was obtained. A disposable needle was used to draw an appropriate amount of spinning solution, and a No. 18 needle was used. The voltage was 20 KV, the push speed was 0.8 mL / h, the needle was 15 cm away from the receiver, and the stroke was 20 mm; the ambient temperature and humidity were 30°C and 30%. A spinning membrane was obtained.

[0054] (4) Preparation of modified membrane (FM-PVDF-1)

[0055] In a polypropylene container, 1 g of LiF was dissolved in 20 mL of 9 M HCl solution after preparation; then, 1 g of Ti3AlC2 was added to the mixed solution, and the reaction was maintained at 35 °C for 24 h. The resulting solution was then washed with deionized water and centrifuged until the pH was neutralized. The collected Ti3C2Tx product was further exfoliated by ultrasound in an ice bath under an argon flow and centrifuged at 3500 rpm to obtain a layered MXenes (abbreviated as MX) nanosheet supernatant. Finally, the dark green MX suspension (3.0 mg / mL) was sealed and refrigerated. The pre-synthesized MX suspension was diluted with 7.5 mL of deionized water. Then, 30 mg of thiourea (TA) was added to the above suspension, and a uniform suspension was formed by ultrasound treatment for 10 min, and freeze-dried for 12 h to obtain TA@MX; finally, TA@MX was transferred to a tube furnace and heated at 5 °C min -1 The mixture was heated to 400°C at a heating rate of 1:2 and maintained for 2 hours to obtain the final product (NSMX); the obtained NSMX and PVDF were mixed in a mass ratio of 1:2, and then the mixture was evenly sprayed onto the spinning membrane (with a diameter of 16 mm) with a spray gun, and then moved into a vacuum drying oven and dried at 60°C for 10 hours.

[0056] Example 2 Modified membrane (FM-PVDF-2)

[0057] The preparation process of f-BN and P-DOPO in Example 2 is exactly the same as that in Example 1. The coating material and coating process are also the same as those in Example 1. The only difference is that: (3) Preparation of modified substrate membrane (F-PVDF-2): A solution formed by adding 2.22 g PVDF to a mixture of 5 g N,N-dimethylformamide solvent and 5 g acetone is the spinning solution. A mixture of 0.11 g f-BN and 0.11 g P-DOPO is added and magnetically stirred for 12 hours. An appropriate amount of spinning solution is drawn with a disposable syringe and an 18-gauge needle is used. The voltage is 20 KV, the push speed is 0.8 mL / h, and the needle is 15 cm away from the receiver with a stroke of 20 mm.

[0058] Referring to the above method, two high temperature resistant, thermally conductive and flame retardant composite membranes were prepared. When preparing the spinning solution, the specific addition amounts of PVDF, BN and organic flame retardant were shown in Table 1. According to the different masses of added BN and organic flame retardant, FM-PVDF-1 and FM-PVDF-2 membranes were prepared at the same time.

[0059] Table 1 Composition of different composite diaphragms

[0060] Diaphragm type PVDF(g) f-BN(g) P-DOPO(g) FM-PVDF-1 1.11 0.055 0.055 FM-PVDF-2 2.22 0.11 0.11

[0061] The diaphragm prepared in the present invention was compared with the Celgard commercial diaphragm by conducting a combustion test, a porosity test, a liquid absorption test, a thermal conductivity test, an electrochemical performance test and an ARC test.

[0062] Comparative Example 1 Coated Commercial Separator (M-Celgard)

[0063] The only difference between Comparative Example 1 and Example 1 is that the base membrane in Comparative Example 1 is a Celgard 2325 commercial diaphragm, and the Celgard 2325 commercial diaphragm is coated with the same coating as in Example 1 to prepare the coated commercial diaphragm of Comparative Example 1.

[0064] Comparative Example 2 Coated PVDF diaphragm (M-PVDF)

[0065] The only difference between Comparative Example 2 and Example 1 is that the base membrane in Comparative Example 2 is a PVDF membrane, and the PVDF membrane is coated with the same coating as in Example 1 to prepare the coated PVDF diaphragm of Comparative Example 2.

[0066] Test Example 1: Porosity Test

[0067] Porosity is a key factor affecting the performance of microporous polymer electrolyte membranes. It accelerates the conduction of lithium ions in the polymer electrolyte by infiltrating sufficient electrolyte in the pores, thereby improving the ion conductivity and effectively improving the interface problem between the electrode / electrolyte, which is of great help in improving the performance of lithium-ion batteries. By measuring the weight difference between the dry film and the sealed n-butanol in the membrane pores, the porosity P% of the membrane is obtained by the weight method, and then the porosity of the membrane is calculated according to the following formula.

[0068]

[0069] Among them, w d is the dry weight of the membrane, w w is the wet weight of the membrane, ρ b is the density of n-butanol and V m is the volume of the membrane. Figure 1 As shown, Porosity represents porosity. The porosity of the FM-PVDF-1 membrane prepared in Example 1 is 374.2%, and the porosity of the FM-PVDF-2 membrane prepared in Example 2 is 297.6%. The porosity of the two modified PVDF membranes prepared in the present invention is much higher than 49.6% of the pure Celgard commercial membrane; it is also much higher than 80%-85% in Comparative Document 1 and 70%-80% in Comparative Document 2.

[0070] Test Example 2: Liquid Absorption Test

[0071] The commercial and modified separators were weighed and immersed in the electrolyte for 30 s. The separators were then taken out and weighed again. The mass after immersion was divided by the mass before immersion to obtain the liquid absorption rate of the separator. Figure 2 As shown, electrolyte uptake represents the liquid absorption rate, among which the liquid absorption rate of FM-PVDF-1 membrane is 774.6%, the liquid absorption rate of FM-PVDF-2 membrane is 564.3%, and the ordinary pure commercial membrane Celgard is only 77.9%.

[0072] Test Example 3: Combustion Test

[0073] like Figure 3a As shown in Figure 1, when the commercial diaphragm comes into contact with an open flame, it ignites instantly. Figure 3b As shown, PVDF diaphragms will also ignite instantly after contact with an open flame, but the burning duration is longer. Figure 3c and Figure 3d The combustion test of M-Celgard and M-PVDF is a comparative example. The test shows that the diaphragm with only coating but no modification does not show excellent flame retardant performance in the combustion test. Figure 3eThis is a combustion test of the modified PVDF membrane. The modified PVDF membrane is not easy to ignite. Among them, the FM-PVDF-1 membrane did not burn out after burning in an open flame for 10 seconds. This shows that the flame retardant properties of the modified PVDF membrane are greatly improved.

[0074] like Figure 4a As shown in Figure 2, there is almost no residue left after the Celgard commercial diaphragm is burned; Figure 4b As shown in Figure 2, pure PVDF membranes also leave almost no residue after combustion. Figure 4c and Figure 4d After the combustion of comparative examples 1 and 2, almost no residue is left; Figure 4e As shown, the modified PVDF membrane prepared in Example 1 has more residue after combustion.

[0075] The combustion test shows that the flame retardant property of the modified PVDF diaphragm prepared by the present invention is far superior to that of the pure Celgard commercial diaphragm, and an open flame cannot directly ignite it.

[0076] Test Example 4: Thermal conductivity test of diaphragm at different temperatures

[0077] The thermal conductivity of pure Celgard membrane, pure PVDF membrane, FM-PVDF-1 and FM-PVDF-2 prepared in Examples 1 to 2 was tested at 20°C, 40°C, 60°C, 80°C and 100°C respectively. Figure 5 As shown in the figure, Thermal conductivity represents thermal conductivity. The thermal conductivity of traditional pure commercial separators at different temperatures is 0.07, 0.07, 0.08, 0.08 and 0.075 W m -1 k -1 The thermal conductivity of FM-PVDF-1 membrane and FM-PVDF-2 membrane at different temperatures are 0.45, 0.4, 0.46, 0.49, 0.47 and 0.52, 0.53, 0.51, 0.55, 0.55 W m -1 k -1 , which shows that the modified PVDF membrane of the present invention has excellent thermal conductivity.

[0078] Test Example 5: Cycle test after battery assembly

[0079] The battery was assembled using Celgard diaphragm, FM-PVDF-1 diaphragm and FM-PVDF-2 diaphragm respectively. The positive electrode of the battery was SPS, the negative electrode was a lithium sheet, the electrolyte was 1 M LiPF6 and EMC / EC (v / v=1 / 1), and the assembly order of the battery was: negative electrode shell-lithium sheet-diaphragm-electrolyte 80μL-positive electrode sheet-stainless steel gasket-spring sheet-positive electrode shell. Finally, it was placed in a packaging machine, and the battery was assembled after a pressure of 5 MPa was applied to the battery for 5 s. Its cycle performance at 0.5 C was tested. Figure 6 As shown in the figure, the coulombic efficiency on the right represents the coulombic efficiency, and the specific capacity on the left represents the specific capacity. It can be seen that the battery equipped with FM-PVDF-1 membrane shows good cycle performance, and the initial discharge specific capacity of the battery is as high as 1401.5 mAh g -1 After 50 cycles, it still has 565.4 mAh g -1 Similarly, the battery equipped with FM-PVDF-2 membrane also showed excellent electrochemical performance and high Coulomb efficiency. The initial discharge capacity of the battery equipped with Celgard commercial membrane was 473.3 mAh g -1 The modified PVDF diaphragm prepared by the present invention can give the battery better cycle performance.

[0080] Test Example 6: ARC test after battery assembly

[0081] The thermal runaway behavior of batteries equipped with Celgard, PVDf, M-Celgard, M-PVDF and FM-PVDF-1 membranes was studied by ARC testing. The relevant parameters are shown in the figure, where T0, T1 and T2 represent the self-heating starting temperature, trigger temperature and maximum temperature respectively. Figure 7aAs shown, the T0 and T1 of the battery equipped with Celgard membrane at 125.2 and 201.1 min were 146.6 and 337.8 ° C, respectively; the T0 and T1 of the battery equipped with PVDF membrane at 150.9 and 221.8 min were 175.7 and 328.1 ° C, respectively; the T0 and T1 of the battery equipped with M-Celgard membrane at 165.2 and 218.8 min were 156.6 and 340.8 ° C, respectively; the T0 and T1 of the battery equipped with M-PVDF membrane at 195.8 and 281.5 min were 198.9 and 350.2 ° C, respectively; the T0 and T1 of the battery equipped with FM-PVDF-1 membrane at 245.1 and 306.9 min were 250.5 and 385.5 ° C, respectively. Specifically, the T1 value of the battery is the highest after using the FM-PVDF-1 diaphragm, especially compared with the commercialized Celgard diaphragm, the T1 of the battery is increased by 47.7℃ after using the FM-PVDF-1 diaphragm. Figure 7b As shown, the battery equipped with Celgard membrane has t0, t1, t2 of 125.2, 201.1, 212.7 min, the battery equipped with PVDF membrane has t0, t1, t2 of 150.9, 221.8, 238.4 min, the battery equipped with M-Celgard membrane has t0, t1, t2 of 165.2, 218.8, 269.5 min, the battery equipped with M-PVDF membrane has t0, t1, t2 of 195.8, 281.5, 293.4 min, and the battery equipped with FM-PVDF-1 membrane has t0, t1, t2 of 245.1, 306.9, 326.2 min. The above data show that after using FM-PVDF-1 membrane, the battery's t0, t1, t2 are all improved. Figure 7c As shown in the figure, the temperature rise rates of the batteries equipped with Celgard, PVDF, M-Celgard, M-PVDF and FM-PVDF-1 are 148.6, 108.6, 89.7, 85.6 and 28.5°C / min, respectively. It can be seen that the temperature rise rate of the battery using the FM-PVDF-1 separator is significantly reduced. Figure 7d As shown in the figure, the thermal runaway reaction activation energy (E a ) is 0.521 eV, while the E a The result is 0.813 eV, an increase of 56.1%. These results mean that the thermal safety of the battery has been effectively improved.

Claims

1. A high temperature resistant, heat conductive, flame retardant composite diaphragm, characterized in that: The high temperature resistant, heat conductive and flame retardant composite membrane is composed of an electrospun composite matrix membrane and a coating. The electrospun composite matrix membrane is prepared by modifying heat conductive nanoparticles and organic flame retardants respectively, mixing them into a high temperature resistant polymer and then drying them by electrospinning. The coating is coated on the surface of the electrospun composite matrix membrane. The coating is made of nitrogen and sulfur atom doped titanium carbide and PVDF, which are mixed in a mass ratio of (1:1) to (1:5) and then coated. The nitrogen and sulfur atom doped titanium carbide is obtained by calcining thiourea and MXenes in a tubular furnace after ultrasonic and freeze drying. The high temperature resistant polymer is polyvinylidene fluoride PVDF; The thermally conductive nanoparticles have a thermal conductivity greater than 0.4 W m -1 k -1 The invention relates to an inorganic nanoparticle containing a hydroxyl functional group; the organic flame retardant is 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxa-10-phosphaphenanthrene-10-oxide DOPO-HQ; the modifier is a nano-molecular compound containing an active phosphorus-chlorine bond; the modification treatment is to utilize a nucleophilic substitution reaction between a hydroxyl functional group and the modifier containing an active phosphorus-chlorine bond to graft the modifier onto the surface of the thermal conductive nanoparticle and DOPO-HQ respectively, and then utilize a nucleophilic substitution reaction between the remaining active phosphorus-chlorine bond and the amine group of the organic amine molecule to graft the organic amine onto the surface to complete the modification treatment; the organic amine is any one of 4, 4'-diaminodiphenylmethane, 4, 4'-diaminodiphenyl ether, piperazine, diethyleneimine, ethylenediamine, o-phenylenediamine, m-phenylenediamine, m-phenylenediamine or triethylamine, or a mixture of two or more thereof.

2. A high temperature resistant, heat conductive, flame retardant composite diaphragm according to claim 1, characterized in that: The thermally conductive nanoparticles are one of boron nitride, aluminum nitride, silicon nitride, aluminum oxide, and magnesium oxide.

3. The high temperature resistant, heat conductive and flame retardant composite diaphragm according to claim 1, characterized in that: The modifier is one of phenylphosphoryl dichloride, diphenylphosphinyl chloride, diphenyl chlorophosphate and cyanuric chloride.

4. The high temperature resistant, heat conductive and flame retardant composite diaphragm according to claim 1, characterized in that: The steps of modifying the thermal conductive nanoparticles include: placing the thermal conductive nanoparticles in acetonitrile and ultrasonically dispersing them for 1 hour; then adding a modifier and continuing ultrasonic stirring; then dissolving an organic amine in acetonitrile and slowly dropping it into the above-mentioned mixed solution; after the dropwise addition is completed, heating to 60 degrees and reacting for 12 hours to obtain modified thermal conductive nanoparticles; wherein the mass ratio of the thermal conductive nanoparticles to the modifier is 1: (1~3.5), and the mass ratio of the thermal conductive nanoparticles to the organic amine is 1: (1~3.5).

5. The high temperature resistant, heat conductive and flame retardant composite diaphragm according to claim 1, characterized in that: The steps of modifying the organic flame retardant include: taking an appropriate amount of DOPO-HQ and an organic amine and placing them in acetone to dissolve them completely; taking a modifier and dissolving it in acetone, and then slowly dropping it into the above solution; after the dropping is completed, heating the reaction to 70 degrees, condensing and refluxing the reaction for 10 hours to obtain a crude product; wherein the mass ratio of DOPO-HQ to the modifier is (1:1) to (5:1), and the mass ratio of the organic amine to the modifier is (1:1) to (5:1); after washing the crude product with acetone for 3 times and water for 3 times, drying it at 70 degrees to obtain the modified flame retardant product P-DOPO.

6. The method for preparing a high temperature resistant, heat conductive, flame retardant composite diaphragm according to claim 1, characterized in that: The preparation method of the electrospinning composite matrix membrane comprises the following steps: firstly, adding PVDF to a mixture of N, N-dimethylformamide and acetone, stirring and dissolving to form a spinning solution, then adding modified thermal conductive nanoparticles and modified organic flame retardant P-DOPO to the spinning solution, magnetically stirring for 12 hours, and obtaining a spinning solution after uniform dispersion; using a disposable syringe to absorb an appropriate amount of spinning solution, using a No. 18 needle, the ambient temperature and humidity are 25±5°C, 25±5%; the voltage is 20-30 KV, the injection speed is 0.8-2.5 mL / h, the needle is 15-22 cm away from the receiver, and the stroke is 20-60 mm; wherein the mass concentration of the spinning solution is 10 wt%-18 wt%; N, The mass ratio of N-dimethylformamide and acetone is (1:1)~(4:1); the mass ratio of thermal conductive nanoparticles and P-DOPO is (1:1)~(1:6); the mass ratio of thermal conductive nanoparticles and PVDF is (1:15)~(1:25).

7. The preparation method according to claim 6, characterized in that: The preparation method of the titanium carbide doped with nitrogen and sulfur atoms in the coating material comprises the following steps: dissolving LiF in a 1-10 M HCl solution, then adding carbon aluminum titanium Ti3AlC2 to the mixed solution, reacting for at least 24 hours, then washing the obtained solution with deionized water, and centrifuging until the pH value is neutralized; collecting the Ti3C2T x The product is further exfoliated by ultrasound in an ice bath under argon flow, and centrifuged at 3500 rpm to obtain a layered MXenes, i.e., MX nanosheet supernatant, i.e., suspension; the MX suspension is sealed and refrigerated; the pre-synthesized MXene suspension is diluted with deionized water, and then thiourea TA is added to the above suspension, and a uniform suspension is formed by ultrasonic treatment, which is freeze-dried to obtain TA@MX; finally, TA@MX is transferred to a tubular furnace to obtain the final product; wherein the mass ratio of LiF, HCl and Ti3AlC2 is 1:(5~40):1; the mass ratio of deionized water and thiourea is (500~100) ml:1g.

8. A lithium-sulfur battery assembled with the high-temperature resistant, heat-conductive, flame-retardant composite diaphragm as described in any one of claims 1 to 5.

9. The lithium-sulfur battery according to claim 8, characterized in that The positive electrode of the lithium-sulfur battery is a sulfur positive electrode, the negative electrode is a lithium sheet, and the electrolyte is LiPF6 and EMC / EC with a volume ratio of 1:1 and a concentration of 1 M. The assembly order of the lithium-sulfur battery is: negative electrode shell-lithium sheet-diaphragm-electrolyte-positive electrode sheet-stainless steel gasket-spring sheet-positive electrode shell, and finally put it into the packaging machine for packaging to complete the battery assembly.

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