Polyisophthaloyl metaphenylene diamine-based electrostatic spinning flame-retardant diaphragm and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and discloses a poly (isophthaloyl metaphenylene diamine)-based electrostatic spinning flame-retardant diaphragm and application thereof.The poly (isophthaloyl metaphenylene diamine)-based electrostatic spinning diaphragm, namely a PMIA-based electrostatic spinning diaphragm, is prepared by mixing modified PZS microspheres, PMIA and an organic solvent and then adopting an electrostatic spinning method, the modified PZS microspheres are prepared by in-situ growth of an inorganic layer SiO2 on the surfaces of polyphosphazene PZS microspheres with flame retardant property; the polyphosphazene PZS microspheres are prepared by utilizing a polymerization reaction between tannic acid and phosphonitrilic chloride trimer. The prepared spinning diaphragm has very excellent porosity, liquid absorption rate and ionic conductivity and good thermal stability and flame retardance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a poly(m-phenylene isophthalamide)-based electrospun flame-retardant separator and its application. Background Art

[0002] Lithium-ion batteries (LIBs) have significant advantages such as high energy density, long life, fast charging, environmental protection and no pollution, and are widely used in scenarios such as portable devices, electric vehicles, and energy storage power stations. However, once in an abusive working condition, the chemical energy stored in LIBs may suddenly be released in the form of fire or explosion, triggering catastrophic accidents. The battery separator is a thin film that separates the positive and negative electrodes and allows the rapid transmission of ionic charge carriers. It is considered one of the most important components affecting the safety of LIBs.

[0003] Currently, the most widely used separators in lithium-ion secondary batteries are mainly composed of polyethylene (PE), polypropylene (PP), and composites of the two. Although these separators provide relatively high mechanical strength and thermal shutdown ability, there are still problems such as low porosity, poor electrolyte wettability, and low thermal stability. When the internal temperature of the battery rises and approaches the melting point, the size of the commercial separator will shrink until it melts. Once the separator melts, the positive and negative electrodes of the battery will come into contact, resulting in an internal short circuit, triggering thermal runaway or fire accidents. In addition, the heat release of traditional polyolefin separators in the combustion state will exacerbate the fire hazard. The poor thermal stability and flammability of commercial polyolefin separators pose a serious threat to the safety performance of lithium-ion batteries.

[0004] The molecular chain of poly(m-phenylene isophthalamide) (PMIA) appears as regularly arranged zigzags in appearance, and its molecular main chain is composed of a large number of benzene ring amide bonds. Since there is no conjugation effect in the covalent bonds within this molecular structure, the cohesive behavior is relatively low, and the molecular chain has good flexibility, making the fibers prepared therefrom have excellent physical and chemical properties. The thermal decomposition temperature of PMIA is between 400 and 430 °C, so it is expected to replace traditional polyolefin materials and become an ideal separator material.

[0005] Although PMIA has certain advantages in high-temperature resistance as a separator base material, during thermal runaway, the flame retardancy of the base material itself is far from sufficient. Therefore, to further improve the flame retardancy of PMIA-based separator materials, adding flame retardants or flame-retardant coatings is a commonly used modification method. For example, in the patent with publication number CN115911752A, a method for coating sodium alginate on a meta-aramid lithium-ion battery separator is disclosed. In this method, the sodium alginate coating solution is dropped on the dried PMIA porous membrane, and the coating thickness is 20 - 200 μm. After standing and drying, the Na-Alg / PMIA composite separator is obtained. However, the porosity of the Na-Alg / PMIA composite separator prepared by this invention is only 40 - 60%, and the specific capacity of the first discharge is 168.8 mAh / g. Another example is in the patent with publication number CN115241606A, which discloses an aramid resin-based composite coating lithium battery separator and its preparation method. This invention selects PMIA as the high-temperature-resistant base material, and a mixture of polymer and inorganic ceramic is coated on the surface of the separator. The porosity of this separator is only 25% - 85%. Although these modification methods improve the flame retardancy of PMIA-based separator materials to a certain extent, they have an adverse impact on the electrochemical performance of the separator. Summary of the Invention

[0006] The main object of the present invention is to provide a poly(m-phenylene isophthalamide)-based separator material that can maintain good electrochemical performance while improving its flame retardancy and safety.

[0007] To achieve the above object, in the first aspect, the present application provides a poly(m-phenylene isophthalamide)-based electrospun flame-retardant separator, hereinafter referred to as PMIA-based electrospun separator. The PMIA-based electrospun separator is prepared by mixing modified PZS microspheres, PMIA, and an organic solvent and then using the electrospinning method. The modified PZS microspheres are obtained by in-situ growth of an inorganic layer SiO 2 on the surface of polyphosphazene PZS microspheres with flame retardant properties; the inorganic layer SiO 2 has the function of inhibiting the growth of lithium dendrites, and the polyphosphazene PZS microspheres are obtained by the polymerization reaction between tannic acid and hexachlorocyclotriphosphazene.

[0008] Preferably, the preparation process of the polyphosphazene PZS microspheres includes: dissolving tannic acid in acetonitrile to obtain a first solution after complete dissolution, wherein the mass ratio of tannic acid to acetonitrile is 1:(250 - 300); then adding hexachlorocyclotriphosphazene, and the mass ratio of the added hexachlorocyclotriphosphazene to the above first solution is 1:(300 - 350), and continuing to stir ultrasonically for 30 - 60 min to obtain a second solution; then slowly dropping a pyridine solution into the above second solution within 1 h, and the mass ratio of the added pyridine solution to the above second solution is 1:(20 - 30), reacting at room temperature for 12 h, finally collecting the product by centrifuging at 8000 revolutions, washing with acetonitrile and deionized water, and finally freeze-drying.

[0009] Further preferably, the preparation process of the modified PZS microspheres includes: ultrasonically dispersing the polyphosphazene PZS microspheres in ethanol to obtain a third solution, and the mass ratio of the polyphosphazene PZS microspheres to ethanol is 1:(180 - 240), ultrasonically for 30 - 45 min, then adding the precursor sodium metasilicate, and the mass ratio of the added sodium metasilicate to the third solution is 1:(15 - 20), keeping stirring and ultrasonically for 30 - 45 min to obtain a fourth solution, then adding a catalyst, the catalyst is one of hydrofluoric acid, nitric acid, hydrochloric acid, ammonia water, triethylamine, ethylenediamine, and the mass ratio of the added catalyst to the fourth solution is 1:(200 - 250), after ultrasonically for 10 - 15 min, transferring to a hydrothermal reaction kettle, reacting at 180 °C for 24 h, finally collecting the product by centrifuging at 8000 revolutions, and drying at 80 °C to obtain the product modified PZS microspheres SiO 2 @PZS.

[0010] Further preferably, the preparation process of the spunbonded separator obtained by mixing the modified PZS microspheres, PMIA and an organic solvent using the electrospinning method includes: taking the modified PZS microsphere powder and dispersing it in N,N-dimethylacetamide as the fifth solution. The mass ratio of the modified PZS microsphere powder to N,N-dimethylacetamide is 1:(70 - 75), and more preferably, the mass ratio is 1:72. After ultrasonic stirring and dispersing for 1 h, PMIA is added. The mass ratio of the fifth solution to PMIA is 1:(1.3 - 1.5), and more preferably, the mass ratio is 73:100. Continuing ultrasonic stirring and dispersing for 10 h, the obtained mixture is the spinning solution. Appropriate amount of the spinning solution is sucked with a disposable syringe, using an 18-gauge needle, with a positive voltage of 20 - 30 kV, a negative voltage of -1.5 - -2 kV, a pushing speed of 1.2 - 1.5 mL / h, the needle is 15 - 20 cm away from the receiver, and the stroke is 10 - 12 mm. More preferably, the electrospinning parameters are: positive voltage 20 kV, negative voltage -1.5 kV, pushing speed 1.2 mL / h, the distance between the needle and the receiver is 20 cm, and the needle stroke is 10 mm. Finally, the PMIA-based spunbonded separator is obtained. After electrospinning, the PMIA-based spunbonded separator is placed in a vacuum oven for vacuum drying for 24 h, and thus the poly(m-phenylene isophthalamide)-based electrospun flame-retardant separator SiO 2 @PZS@PMIA.

[0011] In a second aspect, the present application provides the application of the electrospun separator in the production of lithium-ion battery separators, and further provides its application in the production of lithium-ion batteries. The PMIA-based spunbonded separator has an obvious porous fiber structure, so the PMIA-based spunbonded separator itself has very excellent porosity, liquid absorption rate and ionic conductivity. After adding the modified flame-retardant material SiO 2 @PZS@PMIA separator has better thermal stability and flame retardancy and is suitable for assembling lithium-ion batteries.

[0012] The technical principle is as follows:

[0013] The electrospun separator SiO 2 @PZS@PMIA provided by the present application is formed by the decomposition of the precursor to generate silica, which then wraps around the pre-prepared polyphosphazene PZS microspheres to form SiO 2 @PZS microspheres, and then they are incorporated into the PMIA separator by electrospinning to obtain the PMIA-based separator.

[0014] In the polyphosphazene PZS microsphere structure, nitrogen and phosphorus elements can release non-combustible gases such as ammonia and phosphine during pyrolysis. These gases can dilute the concentration of combustible gases, thereby reducing the flammability of the flame. At high temperatures, PZS can decompose to form phosphoric acid or polyphosphoric acid, which can promote the formation of a glassy or viscous protective layer on the material surface, effectively isolating heat and oxygen. During combustion, phosphorus elements will form phosphorus oxides, which can capture free radicals generated during combustion, thus interrupting the combustion chain reaction and inhibiting the spread of the flame. PZS can promote the formation of a carbon layer during combustion. The carbon layer can form a protective film on the material surface, isolate air, prevent further combustion, and reduce heat transfer.

[0015] PMIA, as a high-temperature resistant fiber material, after adding SiO 2 @PZS, the two play a synergistic role and inhibit combustion through various mechanisms such as the covering layer effect, gas dilution effect, endothermic effect, etc., effectively improving the flame retardancy of the PMIA-based electrospun separator. When a thermal runaway reaction occurs inside the battery, they play a synergistic flame retardant role and interrupt the inhibition of the chain reaction; due to its micro-nano structure, traditional electrospun fibers usually have low mechanical properties and are easily damaged during operations such as bending and folding, so they are easily punctured, leading to internal short circuits in the battery. In-situ growth of an inorganic layer SiO 2 that can inhibit the growth of lithium dendrites on the surface of PZS can endow the separator with a better effect of inhibiting the growth of lithium dendrites and further improve the safety performance of the battery.

[0016] Advantages of the present invention:

[0017] (1) In the patent with the publication number CN115911752A, the initial specific capacity of the lithium-ion battery assembled with the disclosed Na-Alg / PMIA composite separator is 168 mAh / g, and the specific capacity drops to 129 mAh / g after 50 charge-discharge cycles; the initial specific capacity of the lithium-ion battery assembled with the SiO 2 @PZS@PMIA separator prepared by the present invention is 229 mAh / g, and the specific capacity is 220 mAh / g after 50 charge-discharge cycles. From the difference in specific capacity after multiple charge-discharge cycles, it can be seen that the Na-Alg / PMIA separator does not have the function of inhibiting the growth of lithium dendrites. And the SiO 2 @PZS microspheres in the novel separator prepared in this application have excellent functions of inhibiting the growth of lithium dendrites. Therefore, the separator prepared in this application can effectively inhibit the growth of lithium dendrites, reduce the loss of active substances, and extend the service life and performance of the battery.

[0018] (2) In the patent with the publication number CN115241606A, the porosity of a disclosed aramid resin-based composite coating lithium battery separator is only 25% - 85%. The SiO2 @PZS@The liquid absorption rate of the PMIA separator is 322.01%. The novel separator prepared in this application can provide more transmission channels for lithium ions, significantly improving the electrochemical performance of the battery.

[0019] (3) In the patent with the publication number CN118909296A, the initial specific capacities of a lithium-ion battery assembled with a PDA-BN and PMIA composite porous membrane at different rates of 0.1C, 0.2C, 0.5C, 1C, and 2C are 150, 148, 139, 128, and 109 mAh / g respectively. The SiO of the present invention 2 @PZS@The discharge specific capacities of the batteries assembled with the PMIA separator are 229, 219, 210, 199, 177, and 161 mAh g -1 , indicating that the novel separator can well adapt to the cycling test under high-density current, inhibit the growth of lithium dendrites, reduce the degree of battery polarization, and reduce capacity attenuation.

[0020] (4) In the patent with the publication number CN118496498A, the melting point of a 9-phenoxy-10-phenylanthracene structure-modified PMIA separator disclosed therein is 350 °C. The melting points of the novel separators prepared in the present invention are all above 550 °C, so the battery can be given higher thermal stability and safety.

[0021] (5) After testing, the battery assembled with the commercial separator undergoes an SEI film decomposition reaction at 174 °C, the maximum thermal runaway temperature reaches 220 °C, the thermal runaway start time is 300 min, and the activation energy is 0.64 eV. However, for the batteries assembled with the PMIA separator and SiO 2 @PZS@PMIA separators, the SEI film decomposition temperatures are 182 °C and 189 °C respectively, the maximum thermal runaway temperatures are 452 °C and 509 °C respectively, and the activation energies are 0.77 eV and 0.83 eV respectively. The novel separator of the present invention can effectively delay the thermal runaway of the battery and increase the activation energy of the thermal runaway reaction. Description of the Drawings

[0022] Figure 1 It is the SEM photograph of the PZS microspheres prepared in Example 1 of this application;

[0023] Figure 2 It is the SEM photograph of the SiO 2 @PZS material prepared in Example 1 of this application;

[0024] Figure 3 It is the scanning electron microscope (SEM) photograph of the SiO 2 @PZS@PMIA spun separator prepared in Example 1 of this application;

[0025] Figure 4SiO prepared for commercial separator and Example 1 2 @PZS@ Comparison chart of residues after PMIA combustion test;

[0026] Figure 5 SiO prepared for commercial separator and Example 1 2 @PZS@ Thermal shrinkage test of PMIA separator;

[0027] Figure 6 SiO prepared for commercial separator and Example 1 2 @PZS@ Porosity comparison chart of PMIA separator;

[0028] Figure 7 SiO prepared for commercial separator and Example 1 2 @PZS@ Liquid absorption rate comparison chart of PMIA separator;

[0029] Figure 8 Result chart of the battery assembled with the separator cycled 50 times at a current density of 0.1C;

[0030] Figure 9 Result chart of the battery assembled with the separator cycled 50 times at a current density of 0.5C;

[0031] Figure 10 Result chart of the rate test of the battery assembled with the separator;

[0032] Figure 11 Cycling chart of the battery assembled with the separator at different operating voltages;

[0033] Figure 12 SEM image of the lithium foil after cycling of the battery assembled with the separator at different operating voltages;

[0034] Figure 13 Cycling chart of the battery assembled with the separator at high voltage;

[0035] Figure 14 SEM image of the lithium foil after cycling of the battery assembled with the separator at high voltage;

[0036] Figure 15 Adiabatic test result of the battery assembled with the separator;

[0037] Figure 16 Activation energy chart of the battery assembled with the separator;

[0038] Figure 17 Safety assessment radar chart of the battery assembled with the separator. Detailed implementation mode

[0040] To further illustrate the technical solution of the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0041] Meanwhile, for the raw materials or reagents not specifically described below, they are all commercially available products, and the process steps or methods not specifically mentioned are all process steps or methods known to those skilled in the art.

[0042] The sources of some raw materials and reagents involved in the following examples, comparative examples and test examples are as follows:

[0043] PMIA is manufactured by Ningbo Taihe Company, with a molecular weight of 200,000 and a solid content of 20%;

[0044] The solute of the pyridine solution is pyridine, the solution is water, and the concentration is 0.5 mol / L;

[0045] The traditional commercial separator Celgard is purchased from Celgard Company of the United States, and the model is Celgard 2325;

[0046] Hexachlorocyclotriphosphazene is purchased from Macklin Biochemical Technology Co., Ltd., with the model of AR and a purity of 99 wt%.

[0047] Example 1

[0048] Step S1: Weigh 0.6 g of tannic acid and dissolve it in 210 ml (i.e., 165 g) of acetonitrile until completely dissolved to form a uniform and stable solution. Subsequently, accurately add 0.51 g of hexachlorocyclotriphosphazene to this solution and stir it ultrasonically to make them fully mixed. Immediately afterwards, within 1 h, slowly and uniformly add 7 ml (i.e., 7 g) of the pyridine solution dropwise to the above solution, strictly controlling the dropping rate to ensure a stable reaction. The reaction system is left standing at room temperature for 12 h. After the reaction is completed, the product is collected by centrifugation at a speed of 8000, washed with acetonitrile and deionized water to remove impurities, and finally freeze-dried to obtain polyphosphazene microspheres PZS with a diameter of about 120 nm.

[0049] Step S2: Measure 0.2 g of PZS and place it in 50 ml (i.e., 39.5 g) of ethanol, and disperse it ultrasonically for 30 min to form a stable system. Subsequently, add 1 ml (i.e., 2.4 g) of sodium metasilicate, stir and ultrasonicate for 30 min to make it fully mixed and preliminarily react with PZS. Then, add 0.2 ml (i.e., 0.18 g) of ammonia water and ultrasonicate for 1 h to promote the reaction and fusion. Then transfer the system to a hydrothermal reaction kettle and react at 180 °C for 24 h to generate the target product. After the reaction is completed, the product is collected by centrifugation at 8000 r / min and dried at 80 °C to obtain SiO 2@PZS.

[0050] Step S3, measure 0.1 g of SiO 2 @PZS powder and place it in 7.2 g of DMAC. Ultrasonically stir for 1 h until a uniform and stable dispersion system is formed. Subsequently, slowly add 10 g of PMIA and ultrasonically stir for 10 h to obtain a uniform mixture, completing the preparation before spinning. After that, use a disposable syringe equipped with an 18-gauge needle to aspirate the spinning solution. Set the parameters of the spinning equipment: positive voltage 20 kV, negative voltage -1.5 kV, pushing speed 1.2 mL / h, distance between the needle and the receiver 20 cm, needle stroke 10 mm, and successfully prepare the SiO 2 @PZS@PMIA separator.

[0051] Characterize the materials prepared in Example 1, such as Figure 1 is the SEM image of the PZS microspheres prepared in Example 1 at different magnifications; Figure 2 is the SEM image of the SiO 2 @PZS microspheres; as Figure 3 is the SEM image of the SiO 2 @PZS@PMIA separator. By taking a transmission electron microscope image of the separator, it can be seen that such a separator has dense pores after electrospinning.

[0052] Hereinafter, the traditional commercial separator Celgard and the PMIA separator are used as Comparative Example 1 and Comparative Example 2 respectively, and performance tests and comparisons are carried out with the separator prepared in Example 1 of the present invention. Specifically, combustion tests, thermal shrinkage tests, porosity tests, and liquid absorption rate tests are carried out respectively.

[0053] Test Example 1: Combustion test

[0054] As Figure 4 shown, the combustion time of the Celgard separator is very short, and the entire combustion time lasts for less than 2 seconds. The combustion time of the PMIA separator is significantly extended. The SiO 2 @PZS@PMIA separator with the added modified flame retardant material has a further extended combustion time, and there will be obvious residues after extinguishing, which indicates that the addition of SiO 2 @PZS does improve the flame retardant effect of the PMIA-based electrospun separator.

[0055] Test Example 2: Thermal shrinkage test

[0056] As Figure 5 shown, the traditional commercial separators Celgard, PMIA separator and SiO 2The @PZS@ PMIA separator was subjected to a heat shrinkage experiment under the same temperature gradient. The Celgard separator started to undergo severe deformation at only 140 °C, while the PMIA and SiO 2 The @PZS@ PMIA separator showed almost no change at the same temperature. Subsequently, at 160 °C, the Celgard separator had already pyrolyzed, while the PMIA and SiO 2 The @PZS@ PMIA separator could still maintain a good shape. At 180 °C - 200 °C, the Celgard separator continued to pyrolyze and had completely lost its original shape, while the PMIA separator also showed obvious deformation at 200 °C, and SiO 2 The @PZS@ PMIA separator still maintained a relatively complete shape as a whole at 200 °C and did not show obvious deformation. This indicates that the spun separator prepared by the present invention has good heat resistance and strong thermal stability, far exceeding ordinary commercial separators.

[0057] Test Example 3: Porosity Test

[0058] Porosity is the volume ratio of the pores in the separator to the separator. The pores in the separator are the paths for ion flow during the charge and discharge of lithium-ion batteries. A high porosity of the separator can improve ionic conductivity, and at the same time, the separator also needs a sufficiently high porosity to improve the liquid absorption rate of the separator. The separator was cut into circular pieces with a diameter of 17 mm, the weight and thickness of the separator were measured, and then the separator was immersed in a beaker containing a n-butanol solution for 1 hour, and then taken out and the excess n-butanol on the surface was wiped off with filter paper, and then the weight of the separator at this time was measured. The porosity was calculated by dividing the mass difference of the measured separator before and after infiltration by the mass of n-butanol.

[0059] As Figure 6 shown, the porosity of the Celgard separator is only 73.51%, while the porosities of the PMIA and SiO 2 @PZS@ PMIA separators are 318.74% and 322.04% respectively, far higher than the porosity of the Celgard separator.

[0060] Test Example 4: Liquid Absorption Rate Test

[0061] The liquid absorption rate of the separator can illustrate the ability of the separator to store the electrolyte from the side. The process of the liquid absorption rate test of the separator is as follows: The separator was cut into circular pieces with a diameter of 17 mm and the weight of the separator was measured, and then the separator was immersed in a beaker containing an electrolyte solution for one hour, and then the excess electrolyte was wiped off with filter paper, and the weight after infiltration was weighed. The porosity was calculated by dividing the mass difference of the measured separator before and after infiltration by the mass of the measured separator before infiltration.

[0062] As Figure 7As shown in Figure 2, the Celgard membrane has a liquid absorption rate of only 128.53%, while PMIA and SiO 2 The liquid absorption rates of @PZS@PMIA are 1132.11% and 1071.34%, respectively. Therefore, in this experiment, even if a modified flame retardant material is added to the polyimide (PMIA)-based diaphragm with a spinning structure, its liquid absorption rate does not change significantly. This result shows that the special structure of the diaphragm given by electrospinning in this application largely determines its excellent liquid absorption level. Therefore, for this application, the liquid absorption rate of the spinning diaphragm is mainly determined by its unique structure, rather than the addition of flame retardants.

[0063] Example 2: Battery Assembly

[0064] The prepared diaphragms were assembled into batteries in a glove box: Celgard diaphragm, PMIA diaphragm and diaphragm prepared in Example 1 were first cut into circular shapes with a diameter of 16 mm, and then assembled with NCM811 battery positive electrode and lithium sheet negative electrode respectively, and 1 M LiPF was added. 6 and EMC / EC (v / v=1 / 1) electrolyte. The battery assembly order is: negative electrode shell - lithium sheet - diaphragm - electrolyte 80μL - positive electrode sheet - stainless steel gasket - spring sheet - positive electrode shell. Finally, put it into the packaging machine, and 5MPa pressure acts on the battery for 5s to complete the battery assembly. Test Example 5: Cycle test after battery assembly

[0065] The batteries assembled with different separators in Example 2 were subjected to cycle tests respectively. The discharge specific capacity change curves of the cycle tests are shown in FIG. Figure 8 and Figure 9 As shown, Figure 8 This is the discharge capacity change curve of the battery at a current density of 0.1C, equipped with Celgard diaphragm, PMIA diaphragm and SiO 2 The first cycle discharge specific capacities of the batteries with @PZS@PMIA separator are 220.7, 211.5 and 229.8 mAh g -1 The capacity of the battery using Celgard separator began to decay after 30 cycles. The discharge specific capacities at the 30th, 40th, and 50th cycles were 205.1, 196.8, and 189.1 mAh g, respectively. -1 , while using PMIA diaphragm and SiO 2 The discharge capacity of the battery with @PZS@PMIA separator is significantly increased at the same number of cycles. 2 The discharge specific capacities of @PZS@PMIA membrane battery at the 30th, 40th and 50th cycles are 221.2, 214.7 and 220.0 mAh g -1 .

[0066] Figure 9Shows the discharge specific capacity of batteries equipped with different diaphragms at 0.5C. After the first five cycles of activation, the current density becomes 0.5C starting from the sixth cycle, so the discharge specific capacity of the battery shows an obvious downward trend. The discharge specific capacity of the Celgard diaphragm battery is 158.9 mAh g when the current density reaches 0.5C at the sixth cycle -1 , and the capacity briefly rises to 170 mAh g at the 20th cycle -1 , but the capacity continues to decline in subsequent cycles, and the discharge specific capacity is 165.4 mAh g at the 50th cycle -1 . While the batteries using PMIA diaphragm and SiO 2 @PZS@PMIA diaphragm show higher discharge specific capacities. The discharge specific capacities at the sixth cycle are 199.3 and 210.7 mAh g respectively -1 , which are much higher than those of the Celgard diaphragm battery. The discharge specific capacities of the PMIA diaphragm battery at the 10th, 30th, and 50th cycles are 197.6, 189.9, and 184.5 mAh g respectively -1 , while the discharge specific capacities of the batteries using SiO 2 @PZS@PMIA diaphragm at the 10th, 30th, and 50th cycles are 208.2, 198.8, and 197.8 mAh g respectively -1 , with extremely strong stability. Thus, it can be seen that the SiO 2 @PZS@PMIA diaphragm battery has higher charge-discharge capacities than the PMIA diaphragm battery both at a current density of 0.1C and 0.5C

[0067] Test Example 6: Rate Test after Battery Assembly

[0068] Assemble the battery in the same way as the cycle test steps, and the results of the battery rate performance test are Figure 10 shown. Due to the polarization phenomenon, the discharge specific capacity of the batteries using different diaphragms decreases as the rate increases. The charge-discharge capacity attenuation of the Celgard diaphragm battery is more obvious, and the discharge specific capacities at 0.1, 0.2, 0.5, 1, 2, and 5C are 192.7, 180.1, 166.6, 151.1, 127.9, and 104.3 mAh g respectively -1 . While the discharge specific capacities of the batteries using PMIA diaphragm and SiO 2 @PZS@PMIA diaphragm are significantly improved at 0.1, 0.2, 0.5, 1, 2, and 5C. Especially at high rates, the discharge specific capacity of the SiO 2 @PZS@PMIA battery is 229.8, 219.3, 210.7, 199.3, 177.3, and 161.3 mAh g at the same current density -1, are far higher than those of Celgard batteries and PMIA batteries. Moreover, when the current density is restored to 0.1C, the discharge specific capacity of the SiO 2 @PZS@PMIA battery can also recover to a relatively high level, indicating that these two diaphragms can reduce the polarization degree of the battery and reduce the attenuation of the battery capacity.

[0069] Test Example 7: Lithium dendrite growth test

[0070] Assemble the battery in the same way as the cycle test, replace the positive and negative electrodes of the battery with lithium sheets and divide the battery into two groups. One group studies the changes of lithium dendrites in the battery under different working condition voltages (1mAh / cm 2 1mA / cm 2 ; 2mAh / cm 2 2mA / cm 2 ; 3mAh / cm 2 3mA / cm 2 ; 4mAh / cm 2 4mA / cm 2 ; 5mAh / cm 2 5mA / cm 2 ), and the other group studies the changes of lithium dendrites in the battery at a high voltage of 5mAh / cm 2 5mA / cm 2 .

[0071] Figure 11 is the cycle diagram of the battery under different working condition voltages. It can be seen that as the voltage increases, the battery voltage of the SiO 2 @PZS@PMIA diaphragm rises steadily and the voltage fluctuation range is stable, while the voltage fluctuation range of the Celgard diaphragm battery is large, and the voltage drops suddenly at 180h. The voltage of the PMIA diaphragm battery is relatively stable in the first 180h and starts to fluctuate greatly at 240h.

[0072] Figure 12 is the SEM diagram of the lithium sheet taken after cycling. It can be seen that the lithium sheet of the SiO 2 @PZS@PMIA diaphragm battery shows a relatively smooth surface morphology at magnification of 50μm and 10μm, and the particles on the surface are relatively small and evenly distributed.

[0073] Figure 13 is the cycle diagram of the battery under high voltage working conditions. It can be seen that the SiO 2 @PZS@PMIA diaphragm battery can cycle stably for 550h under high voltage working conditions, while the voltage of the Celgard diaphragm is unstable at the beginning of the cycle, and the voltage of the PMIA diaphragm battery is relatively stable before 300h and then becomes unstable.

[0074] Figure 14 SEM images of the lithium foils taken after cycling, where the lithium dendrites of Celgard separator and PMIA separator Figure 12 have a rougher structure than those of the commercial separator in 2 . At magnification of 50 μm and 10 μm, the surface presents an irregular granular structure with different particle sizes and relatively random distribution. While for the lithium foil surface of the SiO 2 @PZS@PMIA separator battery, it still shows a relatively smooth morphology, with finer and evenly distributed particles on the surface. The addition of flame-retardant particles improves the surface uniformity of the electrode, reduces the growth of lithium dendrites, helps the uniform diffusion and deposition of lithium ions, and improves the performance and stability of the battery.

[0075] Test Example 8: Thermal Runaway Test The batteries with three separators were tested using an adiabatic accelerating calorimeter THT-ARC. During the test, THT-ARC was in the "heating - waiting - searching" mode to heat the battery. When the battery showed self-heating and the temperature rise rate exceeded 0.02 °C / min, the heating of the battery would stop. According to the Arrhenius equation, the activation energy of the battery was calculated from the data obtained in the ARC experiment to evaluate the thermal runaway risk of the battery. Figure 15 The ARC experimental diagrams of the three separators are shown below. Table 1 below shows the temperature characteristic points of the batteries: Table 1 Temperature Characteristic Points of the Batteries

[0076] Separator <![CDATA[T 0 (℃)]]> <![CDATA[T 1 (℃)]]> <![CDATA[T 2 (℃)]]> Celgard 174.9 220.4 505.1 PMIA 182.4 291.0 452.6 <![CDATA[SiO 2 @PZS@PMIA]]> 189.7 240.8 509.9

[0077] From Figure 15 and Table 1, it can be seen that the T 0 of the Celgard separator battery is 174.9 °C, the T 0 of the PMIA separator battery is 182.41 °C, and the SiO 2 @PZS@PMIA separator battery is 189.7 °C. The reason for the appearance of T 0 is that the SEI film formed during the cycling of the battery starts to decompose when heated, resulting in the battery starting to release heat. The T 0 of the Celgard separator is significantly lower than that of other separators, indicating that the new separator has excellent thermal stability. In terms of time, during the test, the self-heating time of the batteries using PMIA and SiO 2 @PZS@PMIA separators is significantly later than that of the Celgard separator battery, indicating that the new spun separators can make the battery have higher safety. The T 1 of the Celgard separator battery is 220.4 °C, that of the PMIA separator battery is 291.0 °C, and the T 2 of the SiO 2 @PZS@PMIA separator battery 1is 240.8 °C, and the corresponding times are 300.5 min, 400.2 min, and 573.9 min respectively. It can be seen that SiO 2 @PZS@PMIA separator significantly improves the heat resistance and flame retardancy of the battery. When the battery is heated, the separator remains stable without deformation or collapse, greatly delaying the time of battery thermal runaway. Using Celgard separator, PMIA separator and SiO 2 @PZS@PMIA separator, the times when the maximum temperature of battery thermal runaway reaches are 573.4 min, 442.8 min, and 680.7 min respectively. Through the comparison in time, it can also be known that PMIA-based separator reduces the risk of the battery at high temperature.

[0078] Figure 16 is the activation energy diagram of the three separators. It can be seen from it that the activation energies of Celgard separator, PMIA separator and SiO 2 @PZS@PMIA separator are 0.64, 0.77, and 0.83 ev respectively. It can be seen that the new PMIA-based separator increases the activation energy of battery thermal runaway.

[0079] Figure 17 is the radar chart of the thermal runaway safety of the three separators. It can be seen that SiO 2 @PZS@PMIA separator has a larger area than the commercial separator and PMIA separator. It can be seen that the new PMIA-based separator reduces the risk of the battery at high temperature and also improves the safety of the battery.

Claims

1. A poly(m-phenylene isophthalamide)-based electrospun flame-retardant membrane, i.e., a PMIA-based electrospun membrane, wherein the PMIA-based electrospun membrane is prepared by mixing modified PZS microspheres with PMIA and an organic solvent and then adopting an electrospinning method; the modified PZS microspheres are prepared by in-situ growing an inorganic layer of SiO2 on the surface of polyphosphazene PZS microspheres having flame retardant properties; and the polyphosphazene PZS microspheres are prepared by utilizing a polymerization reaction between tannic acid and hexachlorocyclotriphosphazene.

2. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 1, characterized in that: The preparation process of the polyphosphazene PZS microspheres comprises: dissolving tannic acid in acetonitrile to obtain a first solution after complete dissolution, wherein the mass ratio of the tannic acid to the acetonitrile is 1:(250-300); then adding hexachlorocyclotriphosphazene, wherein the mass ratio of the added hexachlorocyclotriphosphazene to the first solution is 1:(300-350), and continuing ultrasonic stirring for 30-60 minutes to obtain a second solution; then slowly dropping a pyridine solution into the second solution within 1 hour, wherein the mass ratio of the added pyridine solution to the second solution is 1:(20-30), the reaction is maintained at room temperature for 12 hours, and finally the product is collected by centrifugation at 8000 revolutions of a centrifuge, washed with acetonitrile and deionized water, and finally freeze-dried.

3. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 2, characterized in that: The preparation process of the modified PZS microspheres comprises: ultrasonically dispersing polyphosphazene PZS microspheres in ethanol as a third solution, wherein the mass ratio of the polyphosphazene PZS microspheres to ethanol is 1:(180-240), ultrasonicating for 30-45 minutes, then adding a precursor sodium metasilicate, wherein the mass ratio of the added sodium metasilicate to the third solution is 1:(15-20), maintaining stirring and ultrasonicating for 30-45 minutes as a fourth solution, then adding a catalyst, wherein the catalyst is one of hydrofluoric acid, nitric acid, hydrochloric acid, ammonia water, triethylamine, and ethylenediamine, and the mass ratio of the added catalyst to the fourth solution is 1:(200-250), ultrasonicating for 10-15 minutes, transferring to a hydrothermal reactor, maintaining 180° C. for reaction for 24 hours, finally collecting the product by centrifugation at 8000 revolutions per minute in a centrifuge, and drying at 80° C. to obtain the product modified PZS microspheres SiO2@PZS.

4. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 3, characterized in that: The preparation process of the modified PZS microspheres mixed with PMIA and an organic solvent to obtain a spinning membrane by electrospinning includes: taking modified PZS microsphere powder, dispersing it in N,N-dimethylacetamide as the fifth solution, the mass ratio of the modified PZS microsphere powder to N,N-dimethylacetamide is 1: (70-75), after ultrasonic stirring and dispersing for 1 hour, adding PMIA, the configuration mass ratio of the fifth solution to PMIA is 1: (1.3-1.5), continuing ultrasonic stirring and dispersing for 10 hours, the obtained mixture is the spinning solution, using a disposable syringe to absorb an appropriate amount of the spinning solution for electrospinning to obtain a PMIA-based spinning membrane; after the electrospinning is completed, the PMIA-based spinning membrane is placed in a vacuum oven for vacuum drying for 24 hours, and the polyisophthalamide-based electrospun flame-retardant membrane SiO2@PZS@PMIA is prepared.

5. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 4, characterized in that: The electrospinning step includes: using a No. 18 needle, a positive voltage of 20 to 30 kV, a negative voltage of -1.5 to -2 kV, a push speed of 1.2 to 1.5 mL / h, a needle distance of 15 to 20 cm from a receiver, and a stroke of 10 to 12 mm.

6. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 4, characterized in that: The electrospinning process includes: positive voltage 20 kV, negative voltage -1.5 kV, push speed 1.2 mL / h, distance between needle and receiver 20 cm, and needle stroke 10 mm.

7. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 4, characterized in that: The mass ratio of the modified PZS microsphere powder to N,N-dimethylacetamide is 1:

72.

8. The poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 4, characterized in that: The configuration mass ratio of the fifth solution to PMIA is 73:

100.

9. Use of the poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 1 in the preparation of lithium-ion battery diaphragms.

10. A lithium-ion battery, comprising the poly(m-phenylene isophthalamide)-based electrospun flame-retardant diaphragm according to claim 1.

Images