Separator and preparation method therefor, and lithium-ion battery

Abstract

A separator and a preparation method therefor, and a lithium-ion battery. The separator comprises the following raw material components: polyimide, pentafluorocyclotriphosphazene and a fluorophosphite. Based on the total mass of the polyimide, the pentafluorocyclotriphosphazene and the fluorophosphite, the mass fraction of the polyimide is greater than or equal to 70%; the mass fraction of the pentafluorocyclotriphosphazene is 5-15%; and the mass fraction of the fluorophosphite is 10-15%.

Description

A diaphragm and its preparation method and lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on March 1, 2024, with application number 2024102415258. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of lithium-ion battery separators, and in particular to a separator and a preparation method thereof, and a lithium-ion battery. Background Art

[0003] As one of the key components of lithium-ion batteries, the lithium-ion battery separator has the main function of isolating the positive and negative electrodes and preventing electrons from passing through, while allowing lithium ions to be transmitted, thereby completing the rapid transmission of lithium ions between the positive and negative electrodes during the charge and discharge process; the quality of the separator performance directly affects the internal resistance, discharge capacity, cycle life and quality of the lithium-ion battery; the improvement of the high porosity, pore uniformity, mechanical and electrical properties and safety performance of the lithium-ion battery separator is not only conducive to preventing short circuits and thermal runaway in lithium-ion batteries and improving safety; it can also greatly improve the rate and cyclability of lithium-ion batteries. Technical issues

[0004] At present, commercial polyolefin separators and coating films have poor heat resistance and are prone to shrinkage when heated. Lithium dendrites can easily cause the separator to be punctured, which in turn can easily lead to direct contact between the positive and negative electrodes, causing a short circuit, thereby affecting the safety, life and performance of lithium-ion batteries. Technical Solutions

[0005] In a first aspect, the present application provides a diaphragm, the raw material components of which include the following: polyimide, pentafluorocyclotriphosphazene and fluorophosphite; based on the total mass of polyimide, pentafluorocyclotriphosphazene and fluorophosphite, the mass fraction of polyimide is ≥70%, the mass fraction of pentafluorocyclotriphosphazene is 5%~15%, and the mass fraction of fluorophosphite is 10%~15%.

[0006] In a second aspect, the present application provides a method for preparing a diaphragm, comprising the following steps:

[0007] S1. Dissolving polyimide, pentafluorocyclotriphosphazene, and fluorophosphite in a solvent to prepare a homogeneous spinning solution, wherein the mass fraction of polyimide is ≥70%, the mass fraction of pentafluorocyclotriphosphazene is 5% to 15%, and the mass fraction of fluorophosphite is 10% to 15%, based on the total mass of polyimide, pentafluorocyclotriphosphazene, and fluorophosphite;

[0008] S2. Electrospinning the homogeneous spinning solution obtained in step S1 to obtain the diaphragm.

[0009] In a third aspect, the present application provides a lithium-ion battery comprising the separator. Beneficial effects

[0010] (1) The raw material components of the diaphragm of the present application include polyimide, pentafluorocyclotriphosphazene and fluorophosphite. Among them, the polyimide as the matrix of the diaphragm not only has the beneficial effects of crystallization, corrosion resistance, toughness and auxiliary lithium ion transmission, but also has the beneficial properties of high strength and high temperature resistance. Furthermore, the present application further adds flame retardants - pentafluorocyclotriphosphazene and fluorophosphite. The pentafluorocyclotriphosphazene and fluorophosphite are compounded and used as flame retardants in the polyimide diaphragm of the present application, which has high electrolyte wettability and lithium ion transmission ability, improves the high temperature resistance and mechanical strength of the diaphragm of the present application, and reduces the thermal shrinkage of the diaphragm, thereby achieving the improvement of the rate performance, cycle performance and safety performance of lithium-ion batteries. In this application, based on the total mass of polyimide, pentafluorocyclotriphosphazene and fluorophosphite, the mass fraction of polyimide is ≥70%, which can ensure that the prepared lithium-ion battery separator has sufficient strength, high temperature resistance and the ability to assist lithium ion transmission, thereby further improving the rate performance, cycle performance and safety performance of the lithium-ion battery. At the same time, based on the total mass of polyimide, pentafluorocyclotriphosphazene and fluorophosphite, the mass fraction of polyimide is ≥70%, the mass fraction of pentafluorocyclotriphosphazene is 5%~15%, and the mass fraction of fluorophosphite is 10%~15%. The mass fractions of the above three raw materials are within the above ranges, which can not only improve ionic conductivity and reduce impedance, thereby optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery, but also fully play a flame retardant effect, thereby helping to improve the high temperature resistance and mechanical strength of the lithium-ion battery separator.

[0011] (2) The preparation method of the diaphragm of the present application is simple and suitable for large-scale industrial production. Modes for Carrying Out the Invention

[0012] In this application, based on the total mass of polyimide, pentafluorocyclotriphosphazene and fluorophosphite, the mass fraction of polyimide is 70% to 85%, for example, it can be 70%, 72%, 75%, 78%, 80%, 82%, 85%, but is not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable; the mass fraction of pentafluorocyclotriphosphazene is 5% to 15%, for example, it can be 5%, 8%, 10%, 12%, 15%, but is not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable; the mass fraction of fluorophosphite is 10% to 15%, for example, it can be 10%, 11%, 12%, 13%, 14%, 15%, but is not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable.

[0013] In some embodiments, the fluorophosphite comprises tris(2,2,2-trifluoroethyl)phosphite.

[0014] In some embodiments, the thickness of the diaphragm is 5 to 12 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, or 12 μm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0015] The thickness of the diaphragm in this solution is 5~12μm. The diaphragm thickness within this range can not only ensure sufficient thickness of the diaphragm, improve the mechanical strength of the diaphragm, and improve the safety of the lithium-ion battery and the processing performance of the diaphragm stacking; it can also reduce the thickness of the diaphragm and degrade the thickness consistency, ensuring that the diaphragm thickness maintains appropriate permeability and impedance, thereby optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery.

[0016] In some embodiments, the porosity of the diaphragm is 45% to 65%, for example, it can be 45%, 48%, 50%, 55%, 58%, 60%, 62%, or 65%, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0017] The porosity of the diaphragm in this scheme is 45%~65%. The porosity within this range can not only maintain good lithium ion permeability, ensure low resistance and high ion conductivity, thereby optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery; it can also reduce the probability of contact between the positive and negative electrodes of the battery, thereby not easily causing a short circuit, and thus play the role of a diaphragm.

[0018] In some embodiments, the air permeability of the diaphragm is 40~170s / 100mL, for example, it can be 40s / 100mL, 50s / 100mL, 60s / 100mL, 70s / 100mL, 80s / 100mL, 90s / 100mL, 100s / 100mL, 110s / 100mL, 120s / 100mL, 130s / 100mL, 140s / 100mL, 150s / 100mL, 160s / 100mL, or 170s / 100mL, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0019] The air permeability of the diaphragm of this solution is 40~170s / 100mL. Within this range, the air permeability can maintain good lithium ion permeability, ensure low resistance and high ion conductivity, and thus optimize the rate performance, cycle performance and safety performance of lithium-ion batteries.

[0020] In some embodiments, the polyimide has a thermal decomposition temperature (Td) of 500-600°C and a heat-resistant temperature of 400-450°C.

[0021] In some embodiments, in step S1, the solvent includes N,N-dimethylacetamide.

[0022] In step S1 of this solution, the solvent includes N,N-dimethylacetamide. N,N-dimethylacetamide is used as the solvent for preparing the homogeneous spinning solution to make the prepared homogeneous spinning solution have higher wetting effect, uniformity and stability, and avoid the occurrence of agglomeration.

[0023] In some embodiments, in step S2, the applied voltage for electrospinning is 2-5 kV, for example, 2 kV, 3 kV, 4 kV, or 5 kV, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0024] In step S2 of this scheme, the applied voltage of the electrospinning is 2~5kV. The applied voltage of the electrospinning is within this range, which can ensure that a high-porosity diaphragm is obtained, thereby reducing impedance and optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery; it can also improve the consistency (uniformity) of the pore size and the consistency (uniformity) of the mechanical strength of the prepared lithium-ion diaphragm, and at the same time obtain a porosity with good mechanical strength, thereby also optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery.

[0025] In some embodiments, in step S2, the flow rate of the homogeneous spinning solution in the electrospinning is 0.01~0.02 mL / min, for example, it can be 0.01 mL / min, 0.012 mL / min, 0.014 mL / min, 0.016 mL / min, 0.018 mL / min, 0.02 mL / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0026] In step S2 of this scheme, the flow rate of the homogeneous spinning solution in the electrospinning is 0.01~0.02mL / min. The flow rate of the homogeneous spinning solution is within this range, which can not only ensure sufficient thickness of the diaphragm, improve the mechanical strength of the diaphragm, and improve the safety of the lithium-ion battery and the processing performance of the diaphragm stacking; but also reduce the thickness of the diaphragm and degrade the thickness consistency, ensure that the thickness of the diaphragm maintains appropriate permeability and impedance, and thus optimize the rate performance, cycle performance and safety performance of the lithium-ion battery.

[0027] In some embodiments, in step S2, the pore size of the electrospinning spinneret is 0.5~3 μm, for example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0028] In step S2 of this solution, the aperture of the electrospinning spinneret is 0.5~3μm. The aperture of the electrospinning spinneret within this range can ensure sufficient thickness of the diaphragm, improve the mechanical strength of the diaphragm, improve the safety of the lithium-ion battery and the processing performance of the diaphragm stack; it can also ensure that the thickness of the diaphragm maintains appropriate permeability and impedance, thereby optimizing the rate performance, cycle performance and safety performance of the lithium-ion battery.

[0029] In some embodiments, in step S2, the electrospinning temperature is 40-60°C, for example, 40°C, 45°C, 50°C, 55°C, or 60°C, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0030] In step S2 of this solution, the electrospinning temperature is 40-60° C. The electrospinning temperature within this range can further improve the mechanical properties of the lithium-ion battery separator, thereby optimizing the safety performance of the lithium-ion battery.

[0031] In some embodiments, in step S2, the viscosity of the melt of the homogeneous spinning solution after being subjected to the electrospinning temperature is 200-800 mPa·s.

[0032] In the preparation method of this scheme, in step S2, the viscosity of the melt of the homogeneous spinning solution after the electrospinning temperature is 200-800mPa·s. The viscosity within this range can easily obtain a dry electrospinning membrane, thereby reducing processing costs, and can also increase the speed of electrospinning to obtain the membrane and the yield of the finished membrane, thereby also reducing processing costs.

[0033] When the mass ratios of the raw materials (polyimide, pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite) are used in the following Examples 1 to 3, the electrospinning process for preparing the best high-temperature resistant diaphragm is adopted.

[0034] The polyimide used in the following examples and comparative examples has a thermal decomposition temperature (Td) of 550°C and a heat-resistant temperature of 420°C.

[0035] Example 1

[0036] S1. Dissolve 80 g of polyimide, 10 g of pentafluorocyclotriphosphazene, and 10 g of tris(2,2,2-trifluoroethyl)phosphite in 200 mL of N,N-dimethylacetamide (DMAc) to prepare a homogeneous spinning solution.

[0037] S2. The homogeneous spinning solution obtained in step S1 was electrospun. The specific electrospinning process was as follows: spinning voltage 3 kV, homogeneous spinning solution flow rate 0.015 mL / min, spinneret aperture 1 μm, temperature 50° C., to prepare a diaphragm.

[0038] Example 2

[0039] S1. Dissolve 70 g of polyimide, 15 g of pentafluorocyclotriphosphazene, and 15 g of tris(2,2,2-trifluoroethyl)phosphite in 200 mL of N,N-dimethylacetamide (DMAc) to prepare a homogeneous spinning solution.

[0040] S2. The homogeneous spinning solution obtained in step S1 was electrospun. The specific electrospinning process was as follows: spinning voltage 5 kV, homogeneous spinning solution flow rate 0.01 mL / min, spinneret aperture 3 μm, temperature 60° C., to prepare a diaphragm.

[0041] Example 3

[0042] S1. Dissolve 85 g of polyimide, 5 g of pentafluorocyclotriphosphazene, and 10 g of tris(2,2,2-trifluoroethyl)phosphite in 200 mL of N,N-dimethylacetamide (DMAc) to prepare a homogeneous spinning solution.

[0043] S2. The homogeneous spinning solution obtained in step S1 was electrospun. The specific electrospinning process was as follows: spinning voltage 2 kV, homogeneous spinning solution flow rate 0.02 mL / min, spinneret aperture 0.5 μm, temperature 40° C., to prepare a diaphragm.

[0044] Example 4

[0045] The spinning voltage in step S2 of this embodiment is 1.5 kV, and the rest is the same as in embodiment 1.

[0046] Example 5

[0047] The spinning voltage in step S2 of this embodiment is 6 kV, and the rest is the same as in embodiment 1.

[0048] Example 6

[0049] The temperature of electrospinning in step S2 of this embodiment is 35° C., and the rest is the same as in embodiment 1.

[0050] Example 7

[0051] The temperature of electrospinning in step S2 of this embodiment is 65° C., and the rest is the same as in embodiment 1.

[0052] Comparative Example 1

[0053] In this comparative example, step S1 uses meta-aramid to replace polyimide, and the rest is the same as in Example 1.

[0054] Comparative Example 2

[0055] In this comparative example, step S1 used 10 g of trimethyl phosphate (TMP) and 10 g of perfluoro(2-methyl-3-pentanone) (PFMP) to replace 10 g of pentafluorocyclotriphosphazene and 10 g of tris(2,2,2-trifluoroethyl)phosphite, and the rest was the same as in Example 1.

[0056] Comparative Example 3

[0057] In this comparative example, step S1 uses 20 g of pentafluorocyclotriphosphazene to replace 10 g of pentafluorocyclotriphosphazene and 10 g of tris(2,2,2-trifluoroethyl)phosphite, and the rest is the same as in Example 1.

[0058] Comparative Example 4

[0059] In this comparative example, step S1 uses 20 g of tris(2,2,2-trifluoroethyl)phosphite to replace 10 g of pentafluorocyclotriphosphazene and 10 g of tris(2,2,2-trifluoroethyl)phosphite, and the rest is the same as in Example 1.

[0060] Comparative Example 5

[0061] In this comparative example, step S1 uses 50g of polyimide, 25g of pentafluorocyclotriphosphazene and 25g of tris(2,2,2-trifluoroethyl)phosphite to replace 80g of polyimide, 10g of pentafluorocyclotriphosphazene and 10g of tris(2,2,2-trifluoroethyl)phosphite, and the rest is the same as in Example 1.

[0062] The thickness, porosity and air permeability of the diaphragms prepared in all the above examples and comparative examples are shown in Table 1 below:

[0063] Table 1

[0064] Group Thickness (μm) Porosity (%) Air permeability (s / 100mL) Example 1 125189 Example 2 1249150 Example 3 1247.8170 Example 4 1246.9185 Example 5 125680 Example 6 1248.1160 Example 7 1248.2162 Comparative Example 1 1240250 Comparative Example 2 1239.9271 Comparative Example 3 1238.6292 Comparative Example 4 1237.2303 Comparative Example 5 1237.5311

[0065] Performance Testing

[0066] Preparation of lithium-ion batteries

[0067] (1) Preparation of positive electrode

[0068] The ternary material NCM811 positive electrode active material, binder PVDF (polyvinylidene fluoride), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly in a mass ratio of 97:1.6:1.4 to obtain a positive electrode slurry. The positive electrode slurry is then coated on aluminum foil through a coating process, and the positive electrode sheet is obtained after drying and cold pressing processes.

[0069] (2) Preparation of negative electrode sheet

[0070] The silicon-carbon negative electrode composite material (600mAh / g), graphite, conductive agent SP (conductive carbon black Super-P) and binder PAA (polyacrylic acid) are mixed and stirred in a mass ratio of 10:84:3:3 to obtain a negative electrode slurry. The solid content is controlled at 48%. The negative electrode slurry is then coated on the current collector on the copper foil through a coating process, and the negative electrode sheet is obtained after vacuum drying and cold pressing processes.

[0071] (3) Selection of electrolyte

[0072] A 1 mol / L LiPF6 / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl methyl carbonate) (the volume ratio of EC, DMC and EMC = 1:1:1) electrolyte was selected to prepare lithium-ion batteries.

[0073] (4) Selection of diaphragm

[0074] The diaphragms prepared in the above examples and comparative examples are used as lithium-ion battery diaphragms.

[0075] (5) Preparation of lithium-ion batteries

[0076] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to serve as an isolation, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0077] (1) Hot box test

[0078] After fully charging the prepared lithium-ion battery, place it in a temperature chamber and heat it from room temperature to 150±2°C at a rate of 5°C / min. Maintain this temperature for 30 minutes before stopping the heating and observing for 1 hour. Repeat the hot chamber test five times using five identical lithium-ion batteries in parallel. The fire rate is calculated as the number of lithium-ion batteries that caught fire / 5 × 100%.

[0079] (2) Acupuncture test

[0080] After a lithium-ion battery is fully charged, a high-temperature resistant steel needle with a diameter of 3 mm (tip angle of 45-60°, smooth surface, free of rust, oxide layer, and oil) is used to penetrate the battery plate perpendicularly at a speed of (25±5) mm / s. The penetration point should be close to the geometric center of the surface being penetrated. The needle remains in the battery and is observed for 1 hour. The needle penetration test is repeated five times using five identical lithium-ion batteries in parallel. The fire rate is calculated as the number of lithium-ion batteries that caught fire / 5 × 100%.

[0081] The results of the hot box test and the needle penetration test are shown in Table 2 below.

[0082] Table 2

[0083] Group Hot box test fire rate (%) Needle puncture test fire rate (%) Example 1 0% 0% Example 2 20% 20% Example 3 40% 40% Example 4 60% 60% Example 5 60% 60% Example 6 60% 60% Example 7 60% 60% Comparative Example 1 100% 100% Comparative Example 2 100% 100% Comparative Example 3 100% 100% Comparative Example 4 100% 100% Comparative Example 5 100% 100%

[0084] As shown in Table 2, the hot box and needle penetration test fire rates of lithium-ion batteries prepared from the separators of Examples 1-3 were all ≤40%, with the hot box and needle penetration test fire rates of Example 1 reaching 0%. Furthermore, the hot box and needle penetration test fire rates of lithium-ion batteries prepared from the separators of Examples 4-7 were all 60%. Compared with Example 1, the spinning voltage of Example 4 was 1.5 kV, which was relatively low, and the hot box and needle penetration test fire rates of Example 4 were slightly higher. Compared with Example 1, the spinning voltage of Example 5 was 6 kV, which was relatively high, which reduced the consistency (uniformity) of the pore size and mechanical strength of the separator, increasing the risk of puncture by lithium dendrites, and the hot box and needle penetration test fire rates of Example 5 were slightly higher. Compared with Example 1, the electrospinning temperature in Example 6 was 35°C, which was relatively low, while the electrospinning temperature in Example 7 was 60°C, which was relatively high. Both relatively low and relatively high electrospinning temperatures degraded the mechanical properties of the separator, and the ignition rates in the hot box test and needle penetration test of Examples 6 and 7 increased slightly. Compared with Example 1, Comparative Example 1 used meta-aramid instead of polyimide as the separator matrix. The thermal decomposition temperature (Td) and heat resistance temperature of meta-aramid were significantly lower than those of polyimide. The ignition rates in the hot box test and needle penetration test of Comparative Example 1 increased significantly, reaching 100% for both the hot box test and the needle penetration test. Compared with Example 1, Comparative Example 2 uses flame retardants trimethyl phosphate (TMP) and perfluoro(2-methyl-3-pentanone) (PFMP) to replace pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in Example 1. When other flame retardants are used for compounding, the ignition rate of the hot box test and the needle penetration test of Comparative Example 2 is significantly increased, which fully demonstrates that the composite flame retardant of the present application - pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in conjunction with polyimide can significantly improve the high temperature resistance and mechanical properties of the diaphragm. Compared with Example 1, Comparative Example 3 uses pentafluorocyclotriphosphazene alone (i.e., tris(2,2,2-trifluoroethyl)phosphite is not used), and Comparative Example 4 uses tris(2,2,2-trifluoroethyl)phosphite alone (i.e., pentafluorocyclotriphosphazene is not used), and the amount of flame retardant used in Comparative Example 3 and Comparative Example 4 is the same as that in Example 1, but the ignition rate of Comparative Example 3 and Comparative Example 4 in the hot box test and the needle penetration test is significantly increased, which fully demonstrates that the use of pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite alone in the present application cannot significantly improve the high temperature resistance and mechanical properties of the diaphragm, and the beneficial effects of the present application cannot be achieved. Compared with Example 1, the ratio of polyimide, pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in Comparative Example 5 is not within the scope of the present application. The ignition rate of the hot box test and the needle penetration test in Comparative Example 5 is significantly increased, which fully demonstrates that the dosage range of polyimide, pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in the technical scheme of the present application can significantly improve the high temperature resistance and mechanical properties of the diaphragm.

[0085] (3) Electrical performance test

[0086] The rate performance and cycle performance of the lithium-ion battery test cell prepared above were tested on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. at room temperature (25°C). The charge and discharge voltage was limited to 3.0V~4.2V. The rate performance and cycle performance are shown in Table 3 below:

[0087] Table 3

[0088]

[0089] The 1C constant current charge ratio in Table 3 indicates the ratio of the charge capacity corresponding to a stable voltage of 4.2V at a rate of 1C to the total capacity charged at the rate; the 2C constant current charge ratio indicates the ratio of the charge capacity corresponding to a stable voltage of 4.2V at a rate of 2C to the total capacity charged at the rate; the 3C constant current charge ratio indicates the ratio of the charge capacity corresponding to a stable voltage of 4.2V at a rate of 3C to the total capacity charged at the rate; the 4C constant current charge ratio indicates the ratio of the charge capacity corresponding to a stable voltage of 4.2V at a rate of 4C to the total capacity charged at the rate.

[0090] As can be seen from Table 3, the lithium-ion batteries prepared by the diaphragms of Examples 1 to 3 have a 1C / 1C cycle capacity retention rate of ≥76% after 1000 cycles, of which the capacity retention rate of Example 1 is as high as 85%. The lithium-ion batteries prepared by the diaphragms of Examples 1 to 3 have a 1C constant current injection ratio of ≥94.2%, a 2C constant current injection ratio of ≥89.2%, a 3C constant current injection ratio of ≥84.8, and a 4C constant current injection ratio of ≥72.1%, all of which have excellent rate performance. Compared with Example 1, the spinning voltage of Example 4 is 1.5kV, the spinning voltage is relatively low, the diaphragm porosity is relatively low, and the impedance increases. The capacity retention rate and the double charge performance of Example 4 are both reduced compared with Example 1. Compared with Example 1, the spinning voltage of Example 5 is 6kV, the spinning voltage is relatively high, the consistency (uniformity) of the pore size of the diaphragm and the consistency (uniformity) of the mechanical strength are reduced, and the capacity retention rate and the double charge performance of Example 5 are both reduced compared with Example 1. Compared with Example 1, the electrospinning temperature of Example 6 is 35°C, which is a lower electrospinning temperature, and the electrospinning temperature of Example 7 is 60°C, which is a higher electrospinning temperature. The lower and higher electrospinning temperatures, the capacity retention rate and the double charge performance of Examples 6 and 7 are both reduced compared with Example 1. Compared with Example 1, Comparative Example 1 uses meta-aramid to replace polyimide as the separator matrix. Since the effect of meta-aramid in assisting lithium ion transport is significantly lower than that of polyimide, and the thermal decomposition temperature (Td) and heat resistance temperature of meta-aramid are significantly lower than those of polyimide, the high temperature resistance and mechanical properties are significantly lower than those of Example 1. Therefore, the capacity retention rate and double charge performance of Comparative Example 1 are both reduced compared with Example 1. Compared with Example 1, Comparative Example 2 uses flame retardants trimethyl phosphate (TMP) and perfluoro(2-methyl-3-pentanone) (PFMP) to replace pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in Example 1. When compounded with other flame retardants, the high temperature resistance and mechanical properties of Comparative Example 2 are significantly lower than those of Example 1, and the capacity retention rate and double charge performance of Comparative Example 2 are both reduced compared with those of Example 1. Compared with Example 1, Comparative Example 3 uses pentafluorocyclotriphosphazene alone (i.e., tris(2,2,2-trifluoroethyl)phosphite is not used), and Comparative Example 4 uses tris(2,2,2-trifluoroethyl)phosphite alone (i.e., pentafluorocyclotriphosphazene is not used). The amount of flame retardant used in Comparative Examples 3 and 4 is the same as that in Example 1. The thermal resistance and mechanical properties of Comparative Examples 3 and 4 are significantly lower than those of Example 1, and the capacity retention rate and double charge performance of Comparative Examples 3 and 4 are both reduced compared with those of Example 1. Compared with Example 1, the ratio of polyimide, pentafluorocyclotriphosphazene and tris(2,2,2-trifluoroethyl)phosphite in Comparative Example 5 is not within the scope of this application. The thermal and high-temperature resistance and mechanical properties of Comparative Example 5 are significantly lower than those of Example 1. The capacity retention rate and double charge performance of Comparative Example 5 are both lower than those of Example 1.

Claims

1. A diaphragm, wherein the raw material components of the diaphragm include the following: polyimide, pentafluorocyclotriphosphazene and fluorophosphite; Based on the total mass of polyimide, pentafluorocyclotriphosphazene and fluorophosphite, the mass fraction of polyimide is ≥70%, the mass fraction of pentafluorocyclotriphosphazene is 5%-15%, and the mass fraction of fluorophosphite is 10%-15%.

2. The diaphragm according to claim 1, wherein: The air permeability of the diaphragm is 40-170s / 100mL.

3. The diaphragm according to claim 2, wherein: The thickness of the separator is 5-12 μm.

4. The diaphragm according to claim 2, wherein: The porosity of the diaphragm is 45% to 65%.

5. A method for preparing the diaphragm according to any one of claims 1 to 4, comprising the following steps: S1. The polyimide, pentafluorocyclotriphosphazene and fluorophosphite are dissolved in a solvent to prepare a homogeneous spinning solution; S2. Electrospinning the homogeneous spinning solution obtained in step S1 to obtain the diaphragm.

6. The method for preparing the diaphragm according to claim 5, wherein: In step S2, the applied voltage of the electrospinning is 2-5 kV.

7. The method for preparing a diaphragm according to claim 5, wherein: In step S2, the flow rate of the homogeneous spinning solution in the electrospinning is 0.01-0.02 mL / min.

8. The method for preparing a diaphragm according to claim 5, wherein: In step S2, the pore size of the electrospinning spinneret is 0.5-3 μm.

9. The method for preparing a diaphragm according to claim 5, wherein: In step S2, the temperature of the electrospinning is 40-60°C.

10. A lithium ion battery comprising the separator according to any one of claims 1 to 4.

Images