Diaphragm, preparation method thereof and battery

Abstract

The invention belongs to the field of batteries, and particularly relates to a diaphragm, a preparation method thereof and a battery. The preparation method of the diaphragm comprises the following steps: 1) preparing the PAA-BMIMCl ionic elastomer; 2) performing low-temperature curing, and removing residual liquid to obtain an ionic elastomer-based pre-separation membrane; 3) adding the ionic elastomer-based pre-diaphragm into an organic solvent for infiltration; and 4) calendaring the infiltrated ionic elastomer-based pre-diaphragm to a standard thickness to obtain the diaphragm. When the diaphragm is applied to a lithium iron phosphate battery and a ternary lithium battery, the self-extinguishing time can be remarkably shortened, the flame retardant property is improved, and the safety of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and in particular relates to a diaphragm, a preparation method thereof, and a battery. Background Art

[0002] Secondary lithium-ion batteries have the advantages of high energy density, long cycle life, low self-discharge, low cost, and environmental friendliness, and are widely used in fields such as electric vehicles. With the vigorous development of new energy vehicles, the market has put forward higher and higher requirements for secondary lithium-ion battery technology. However, increasing the energy density of the battery will threaten the safety of the battery, thereby endangering personal safety. The main safety hazard of lithium-ion batteries is the perforation of the diaphragm caused by the growth of lithium dendrites. The local fracture of the diaphragm causes direct contact with the positive electrode, which may cause serious fire or explosion. Therefore, designing a flame-retardant diaphragm with high thermal stability to avoid the overall failure of the battery at abnormally high temperatures is the key to improving battery safety.

[0003] As one of the four main materials in lithium-ion batteries, the separator possesses inherent ion conductivity and is a crucial material for separating the positive and negative electrodes, preventing contact between the two electrodes and causing a short circuit. The performance of the separator determines the battery's interface structure and internal resistance, directly impacting its capacity and safety. A separator with excellent performance plays a crucial role in improving the battery's overall performance. Therefore, battery safety depends largely on the separator's high strength and thermal stability. Currently, commercial lithium-ion battery separators are primarily polyolefin separators, primarily polyethylene and polypropylene. While these separators have high porosity and strength, they suffer from poor liquid absorption, low thermal stability, and a long self-extinguishing time, making them susceptible to thermal runaway.

[0004] There are many methods for developing and preparing flame-retardant lithium-ion battery separators. CN 115224435 B provides a method of modifying the base membrane, such as coating a flame-retardant organic / inorganic material on the surface of the base membrane to increase the flame retardancy of the modified separator. However, this method will inevitably increase the thickness of the separator, block the pores of the separator and reduce its porosity, while also reducing the mechanical properties of the separator. CN 112133868 B provides a new type of flammable separator, which blends a nanocomposite material rich in flame retardant properties with a separator raw material (such as PP), sintering and annealing processes. This method increases the mechanical properties while improving the flame retardancy of the separator, but it is necessary to consider the solubility and fusion of the blended raw materials, and has the disadvantages of complex process, high cost, and environmental unfriendliness. Therefore, the research and development of a high-strength, high-thermal stability, flame-retardant separator with simple process, low cost, and suitable for market production is of great significance for improving the safety performance of lithium-ion batteries. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a separator, a preparation method thereof, and a battery.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a diaphragm comprises the following steps: 1) preparing a PAA-BMIMCl ionoelastomer; 2) performing low-temperature curing and removing residual liquid to obtain an ionoelastomer-based pre-diaphragm; 3) adding an organic solvent to the ionoelastomer-based pre-diaphragm for impregnation; and 4) calendering the impregnated ionoelastomer-based pre-diaphragm to a standard thickness to obtain a diaphragm.

[0008] The specific steps of step 1) are: using acrylic acid AA and 1-butyl-3-methylimidazole chloride BMIMCl as monomer raw materials, adding an initiator to carry out polymerization to obtain PAA-BMIMCl ion elastomer.

[0009] The initiator is a photoinitiator; preferably photoinitiator 2959, and the content of the photoinitiator is 0.5 to 2 mol%, preferably 1 mol%, based on the monomer raw material.

[0010] The molar ratio of AA to BMIMCl is (1-2): (1-2); preferably 1:1.

[0011] The specific steps of step 2) are: after using liquid nitrogen to low-temperature solidify the ion elastomer, transfer it to a vacuum low-temperature freeze dryer for vacuum freeze drying to obtain the ion elastomer-based pre-diaphragm.

[0012] The temperature range of the vacuum low-temperature freeze dryer is -60℃~-35℃, the pressure range is 1~10Pa, and the time range is 12~36h.

[0013] The specific steps of step 3) are: in a vacuum environment, the ion elastomer-based pre-diaphragm is immersed in an organic solvent, the organic solvent is fully filled into its pores, and the pre-diaphragm matrix is ​​reinforced with the organic solvent to obtain a PAA-BMIMCl membrane; preferably, the organic solvent is dimethyl sulfoxide (DMSO); preferably, the vacuum environment temperature range is 60 to 90 ° C, and the organic solvent infiltration time range is 1 to 3 hours.

[0014] The present invention also includes a diaphragm obtained by the preparation method.

[0015] The thickness of the diaphragm is 20-40 μm, preferably 25-30 μm.

[0016] The present invention also includes a battery comprising the separator.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention combines acrylic acid (AA) with 1-butyl-3-methylimidazolium chloride (BMIMCl) to prepare a three-dimensional network structure of the polymer elastomer. By adjusting the ratio of AA to BMIMCl and the content of the photoinitiator, a high-strength, high-thermal-stability and flame-retardant PAA-BMIMCl membrane is synthesized. The method is simple and efficient. AA polymerizes to form a PAA chain network. The imidazolium cations in BMIMCl generate electrostatic interactions with the AA carboxyl groups, making the cationic BMIMCl + and anion Cl - Free migration, giving the elastomer excellent ionic conductivity. - The high negative charge density and strong hydrogen bonding ability of the membrane form strong electrostatic and hydrogen bonding interactions with imidazolium ions, creating an elastomer with high Young's modulus, high strength, and high tensile properties. Its surface is rich in hydrogen bonding sites, further enhancing its tear resistance. The porosity is increased through a low-temperature freeze-drying process, and the membrane thickness is controlled to 20-40μm through a calendering process. When used in lithium iron phosphate batteries and ternary lithium batteries, this membrane can significantly shorten the self-extinguishing time, increase flame retardancy, and improve battery safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Graph showing the self-extinguishing time test results of the embodiments of the present invention and the comparative example;

[0020] Figure 2 The figure shows the puncture strength test results of the embodiments of the present invention and the comparative example. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and the best embodiments.

[0022] Example 1

[0023] The preparation method of the diaphragm comprises the following steps:

[0024] 1) Preparation of PAA-BMIMCl ionoelastomer: First, AA and BMIMCl (n(AA):n(BMIMCl)=1:1) were mixed and stirred for 30 minutes to uniformly disperse. 1 mol% of photoinitiator 2959 was added and mixed and stirred for 30 minutes. The mixed solution was placed on a 10*10 cm polytetrafluoroethylene plate and placed under a UV lamp (365 nm, 22.4 mW / cm 2 ) were irradiated with UV light for 2 h.

[0025] 2) low-temperature curing to obtain an ion elastomer-based pre-diaphragm; specifically, after 2 hours, the polymerized ion elastomer is removed and placed in a thermos, 5 g of liquid nitrogen is poured into it, and after 1 minute, it is transferred to a vacuum low-temperature freeze dryer, and vacuum freeze-dried at -45°C and 5 Pa for 24 hours to obtain the ion elastomer-based pre-diaphragm.

[0026] 3) An organic solvent was added to the ion elastomer-based pre-diaphragm for infiltration; the ion elastomer-based pre-diaphragm was placed in a vacuum dry environment (133 Pa, 80° C.) and immersed in 5 g of DMSO solvent for 2 h to allow the DMSO to fully fill the pores of the membrane to obtain a PAA-BMIMCl membrane.

[0027] 4) The impregnated ionoelastomer-based pre-diaphragm was calendered to a standard thickness to obtain a diaphragm; finally, the PAA-BMIMCl diaphragm was calendered to a thickness of 25 μm using a calender to obtain Material 1. The performance parameters of the diaphragm of Material 1 are shown in Table 1.

[0028] Material 1 is a pre-prepared PAA-BMIMCl separator. A lithium-ion battery electrolyte was prepared using a ratio of 0.5M NaPF6 / EC:DEC = 1:1 vol%. The positive electrode material was LiFePO4 or NCM811 (LiFePO4 is used as an example in this application). The negative electrode material was graphite or silicon carbon (graphite is used as an example in this application). A button cell was assembled in a dry environment using the PAA-BMIMCl separator, electrolyte, positive electrode, and negative electrode. This cell is labeled Battery 1.

[0029] Battery 1 was allowed to rest at 25°C for 10 minutes. Then, it was charged at a constant current of 1.0C to 3.65V. Then, it was charged at a constant voltage until the current dropped to 0.05C. Charging was stopped. After resting for 5 minutes, it was discharged at a constant current of 1.0C to 2.5V. This charge and discharge procedure was repeated five times. The discharge capacity of the first cycle was recorded as the initial discharge capacity of the battery.

[0030] Battery 1 was allowed to rest at 25°C for 10 minutes, then charged at a constant current of 1.0C to 3.65V. Then, it was charged at a constant voltage until the current dropped to 0.05C, and then charging was stopped. The battery was allowed to rest for 5 minutes, and then discharged at a constant current of 1.0C to 2.5V. This charge and discharge procedure was repeated 100 times. The charge and discharge capacities at the 100th cycle were recorded, and the capacity retention rate was calculated as the battery's 100-cycle capacity retention rate.

[0031] Battery 1 was discharged to 50% SOC at 25°C and stored at room temperature. The voltage change was measured after 30 days of storage. The voltage drop at 25°C for 30 days is shown in Table 2.

[0032] Example 2

[0033] The difference between Example 2 and Example 1 is that in step 1), the molar ratio of AA and BMIMCl is 1:2, and a material 2 separator is obtained, which is assembled to obtain a battery 2.

[0034] Example 3

[0035] The difference between Example 3 and Example 1 is that in step 1), the molar ratio of AA and BMIMCl is 2:1, and a material 3 separator is obtained, which is assembled to obtain battery 3.

[0036] Example 4

[0037] The difference between Example 4 and Example 1 is that in step 1), 0.5 mol % of photoinitiator 2959 is added and mixed and stirred for 30 minutes to obtain a separator of Material 4, which is then assembled to obtain Battery 4.

[0038] Example 5

[0039] The difference between Example 4 and Example 1 is that in step 1), 2 mol % of photoinitiator 2959 is added and mixed and stirred for 30 minutes to obtain a material 5 separator, which is assembled to obtain a battery 5.

[0040] Comparative Example 1

[0041] A purchased PP separator was used. The performance parameters of the PP separator of Material 6 are shown in Table 1. Battery 6 was assembled.

[0042] Comparative Example 2

[0043] A purchased PE separator was used. The performance parameters of the PE separator of Material 7 are shown in Table 1, and battery 7 was assembled.

[0044] Comparative Example 3

[0045] A commercial PP / PE / PP separator Celgard 2340 was purchased. The performance parameters of material 8 PP / PE / PP separator Celgard 2340 are shown in Table 1, and battery 8 was assembled.

[0046] Table 1. Performance parameters of the diaphragm of the present invention

[0047]

[0048]

[0049] Table 2. 1C battery cycle data of the embodiments of the present invention and the comparative examples

[0050]

[0051] Figure 1 、 Figure 2The puncture strength and self-extinguishing time of materials 1-8 are shown respectively. The above results show that materials 1-5 have shorter self-extinguishing time and better flame retardancy than the comparative examples. The thickness meets the requirements of lithium-ion batteries, and has good porosity and puncture strength. In particular, material 1 has both flame retardancy and puncture strength. This is because Cl - The high negative charge density and strong hydrogen bonding ability of the imidazolium ions form strong electrostatic and hydrogen bonding interactions with the imidazolium ions, creating an elastomer with high Young's modulus, high strength, and high tensile properties. The rich hydrogen bonding sites on its surface further enhance its tear resistance.

[0052] The results in Table 2 show that Battery 1 exhibits superior overall electrochemical performance compared to the comparative example. The excellent initial discharge capacity and 100-cycle capacity retention are due to the formation of a PAA chain network through AA polymerization. The electrostatic interaction between the imidazolium cations in BMIMCl and the AA carboxyl groups allows for the free migration of cations BMIM+ and anions Cl-, imparting excellent ionic conductivity to the elastomer. The low voltage drop is attributed to the perfect diaphragm preparation process and the absence of diaphragm defects, eliminating the risk of micro-short circuits and preventing battery self-discharge.

[0053] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a diaphragm, characterized in that: The method comprises the following steps: 1) preparing a PAA-BMIMCl ionoelastomer; 2) performing low-temperature curing and removing residual liquid to obtain an ionoelastomer-based pre-diaphragm; 3) adding an organic solvent to the ionoelastomer-based pre-diaphragm for impregnation; and 4) rolling the impregnated ionoelastomer-based pre-diaphragm to a standard thickness to obtain a diaphragm.

2. The method for preparing a diaphragm according to claim 1, wherein: The specific steps of step 1) are: using acrylic acid AA and 1-butyl-3-methylimidazole chloride BMIMCl as monomer raw materials, adding an initiator to carry out polymerization to obtain PAA-BMIMCl ion elastomer.

3. The method for preparing a diaphragm according to claim 2, wherein: The initiator is a photoinitiator; preferably photoinitiator 2959, and the content of the photoinitiator is 0.5 to 2 mol% based on the monomer raw material.

4. The method for preparing a diaphragm according to claim 2, wherein: The molar ratio of AA and BMIMCl is (1-2): (1-2).

5. The method for preparing a diaphragm according to claim 1, wherein: The specific steps of step 2) are: after using liquid nitrogen to low-temperature solidify the ion elastomer, transfer it to a vacuum low-temperature freeze dryer for vacuum freeze drying to obtain the ion elastomer-based pre-diaphragm.

6. The method for preparing a diaphragm according to claim 5, wherein: The temperature range of the vacuum low-temperature freeze dryer is -60℃~-35℃, the pressure range is 1~10Pa, and the time range is 12~36h.

7. The method for preparing a diaphragm according to claim 1, wherein: The specific steps of step 3) are: in a vacuum environment, the ion elastomer-based pre-diaphragm is immersed in an organic solvent, the organic solvent is fully filled into its pores, and the pre-diaphragm matrix is ​​reinforced with the organic solvent to obtain a PAA-BMIMCl membrane; preferably, the organic solvent is dimethyl sulfoxide (DMSO); preferably, the vacuum environment temperature range is 60 to 90 ° C, and the organic solvent infiltration time range is 1 to 3 hours.

8. A diaphragm obtained by the preparation method according to any one of claims 1 to 7.

9. The diaphragm according to claim 8, characterized in that The thickness of the diaphragm is 20-40 μm, preferably 25-30 μm.

10. A battery, characterized in that: The diaphragm according to claim 8 or 9 is included.

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