Preparation method and application of polytetramethyl pentenyl lithium ion battery diaphragm

Abstract

The invention relates to the technical field of battery diaphragms, and discloses a preparation method and application of a polytetramethyl pentenyl lithium ion battery diaphragm. The preparation method comprises the following steps: in a closed container, mixing and homogenizing polytetramethylpentene particles and supercritical carbon dioxide at the temperature of 205-215 DEG C and the pressure of 12-18 MPa to form a homogeneous solution, then cooling until the pressure in the container is not higher than 5 MPa, and carrying out phase splitting at the initial cooling rate of not less than 15 DEG C / min to obtain the porous base membrane. By adopting the preparation method disclosed by the invention, the base membrane of the PMP-based diaphragm has relatively high porosity and proper pore size distribution, so that the diaphragm has relatively high liquid absorption rate, puncture strength, tensile strength and high-temperature stability.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, and in particular to a preparation method and application of a polytetramethylpentene-based lithium-ion battery separator. Background Art

[0002] The inherent shortcomings of traditional polyethylene (PE) and polypropylene (PP) battery separators are becoming increasingly prominent in new battery systems, including: (1) Prone to thermal failure: The melting temperature of PE is about 135°C (about 140°C for high-density polyethylene), and the melting temperature of PP is about 165°C. When the battery is locally overheated (such as fast charging), the separator is prone to shrinkage and pore closure failure, leading to thermal runaway; (2) Poor high-voltage compatibility: PE and PP will oxidize in electrolytes above 4.5V, and the by-products accelerate the thickening of the interfacial passivation layer (CEI), resulting in a soaring impedance.

[0003] The molecular chain of polytetramethylpentene (PMP) has a helical structure, and its high rigidity can endow it with thermodynamic advantages. The glass transition temperature (Tg) of PMP is as high as 260°C, and the thermal expansion coefficient is 50% lower than that of PP. At the same time, PMP also has good chemical stability and is not prone to swelling in ester and ether electrolytes.

[0004] However, due to the high crystallinity of PMP, it is difficult to form micropores. When using the conventional phase separation pore-forming method, closed pores are easily formed, resulting in a low porosity (porosity < 30%) of the PMP-based separator and difficult control of the pore size distribution. For example, in the patent application CN102804450A, the thermally induced phase separation-biaxial stretching method is used to form pores. Although a large amount of polyethylene and polypropylene are added to the separator and the PMP content is only 20-21 wt%, the porosity of the separator can only reach 32-37%. Summary of the Invention

[0005] In order to solve the technical problems of low porosity and difficult control of pore size distribution of the PMP-based separator, the present invention provides a preparation method and application of a polytetramethylpentene-based lithium-ion battery separator. By using the preparation method of the present invention, the base film of the PMP-based separator can have a high porosity and a suitable pore size distribution, and further make the separator have high liquid absorption rate, puncture strength, tensile strength and high-temperature stability.

[0006] The specific technical solution of the present invention is as follows: In the first aspect, the present invention provides a preparation method of a polytetramethylpentene-based lithium-ion battery separator, including the following steps: In a closed container, mix and homogenize polytetramethylpentene particles with supercritical carbon dioxide at a temperature of 205-215°C and a pressure of 12-18 MPa to form a homogeneous solution, and then cool down to a pressure in the container not higher than 5 MPa for phase separation, with an initial cooling rate ≥ 15°C / min to obtain a porous base film.

[0007] In the process of preparing the PMP-based separator, the method of depressurizing and phase separation after dissolving with supercritical CO2 is adopted to form a base membrane with a stepped pore size distribution. By coordinating the specific temperature and pressure during the mixing and homogenization process of PMP particles and supercritical CO2, and the specific pressure reduction rate during the phase separation process, the obtained porous base membrane can have a high porosity and a suitable pore size distribution, so that the PMP-based separator has both a high liquid absorption rate, puncture strength, tensile strength and high-temperature stability. Specifically: ① The method of depressurizing and phase separation after dissolving with supercritical CO2 is adopted to form a base membrane with a stepped pore size distribution: The present invention utilizes the high solubility of supercritical carbon dioxide in PMP to form a homogeneous solution, in which CO2 is uniformly dispersed between the PMP molecular chains. Then, during the depressurization process, through the coordinated regulation of pressure and temperature (by cooling in a closed container to synchronously reduce the pressure), using the difference in the desorption rate of CO2 from between the chains, and coordinating with the rigidity of the PMP helical structure molecular chains (hindering pore coalescence during phase separation, improving the stability of small pores on the surface layer, while the bottom layer forms large pores due to chain relaxation delay), asymmetric phase separation can be induced. The surface layer rapidly phase-separates to form smaller pores, and the bottom layer slowly phase-separates to form larger pores, thus achieving the coordination of high porosity and high strength - using the surface layer with smaller pore sizes can inhibit lithium dendrites, improve the puncture strength of the separator, and at the same time reduce the thermal shrinkage rate of the separator, improve its high-temperature stability, reduce the risk of short circuit caused by separator shrinkage, and support the safety of the battery in the thermal runaway scenario; using the bottom layer with larger pore sizes can store the electrolyte and improve the liquid absorption rate of the separator.

[0008] Moreover, compared with the method of charging supercritical CO2 into the polymer melt and relying on the melt viscosity gradient to form a CO2 concentration gradient in the polymer melt and then form a gradient pore structure, the present invention mixes supercritical CO2 with PMP to form a homogeneous solution and uses asymmetric phase separation to form a gradient pore structure, which can make the pore size difference between the surface layer and the bottom layer larger within the limited film thickness of the battery separator, so it can be better applied to the battery separator and achieve the coordination of high porosity and high strength.

[0009] ② The temperature of 205 - 215 °C is adopted during the mixing and homogenization process: Controlling the temperature of mixing and homogenizing PMP particles with supercritical carbon dioxide at 205 - 215 °C can endow the separator with high liquid absorption rate, tensile strength, and puncture strength. When the temperature of mixing and homogenizing is too low, it is not conducive to the melting of PMP, the PMP molecular chains are not fully extended, resulting in uneven dissolution of CO2, fewer pore nuclei and sparse distribution during phase separation, reducing the porosity of the separator and the liquid absorption rate. At the same time, mechanical weaknesses are formed in the unmelted area, increasing the fluctuation of tensile strength. When the temperature of mixing and homogenizing is too high, the PMP molecular chains are overly untangled, the intermolecular force is weakened, the pore walls become thinner and are prone to merging during phase separation, resulting in a too high proportion of large pores and a decrease in puncture strength. At the same time, high temperature accelerates the relaxation of chain segments and reduces the crystalline region, leading to a decrease in tensile strength.

[0010] ③ Adopting a pressure of 12 - 18 MPa during the mixing and homogenizing process: When the pressure during the mixing and homogenizing process is too low, the supercritical CO2 is not fully dissolved, the PMP molecular chains locally aggregate, and the pore nuclei are unevenly distributed during phase separation, resulting in large fluctuations in the surface pore diameter and poor pore connectivity at the bottom layer, which is not conducive to using the surface layer to improve the puncture strength and high-temperature stability of the separator, and using the bottom layer to improve the liquid absorption rate of the separator. When the pressure during the mixing and homogenizing process is too high, the oversaturation of CO2 causes excessive growth of the bottom layer pores and thinner pore walls, increasing the risk of dendrite penetration of the separator and reducing the tensile strength. At the same time, the surface layer pores quickly collapse and close, resulting in a decrease in the thermal stability of the separator. By controlling the mixing and homogenizing pressure within 12 - 18 MPa, the separator of the present invention can have high liquid absorption rate, puncture strength, and high-temperature stability.

[0011] ④ Using an initial cooling rate of ≥15 °C / min for phase separation: During the phase separation process, when the initial cooling rate is too slow, the porosity of the separator will be too low, resulting in the application of the separator being limited to low-rate scenarios.

[0012] Preferably, the specific process of cooling to a pressure in the container not higher than 5 MPa for phase separation includes: cooling at a rate of 25 - 30 °C / min until the pressure in the container is 8 - 10 MPa, and then cooling at a rate of 5 - 10 °C / min until the pressure in the container is 2 - 5 MPa.

[0013] By adopting the above-mentioned two-stage cooling and phase separation method, the following effects can be achieved: in the first stage, the temperature and pressure are rapidly reduced (25-30 °C / min) to 8-10 MPa, which is conducive to inducing a large number of micropore nucleations and forming high-density small holes on the surface layer; in the second stage, the temperature and pressure are slowly reduced (5-10 °C / min) to 2-5 MPa, which can promote the directional growth of pores and form large holes at the bottom layer. When the cooling rate in the first stage is lower than 25 °C / min or the cooling rate in the second stage is higher than 10 °C / min, it will be unfavorable for the realization of the above effects; in addition, when the cooling rate in the first stage is higher than 30 °C / min, the rapid phase change of supercritical CO2 will destroy the dynamic balance of PMP molecular chains and cause the instability of the pore structure. The main reason is that the rapid CO2 desolvation leads to the rapid solidification of the surface pore wall and the formation of microcracks; when the cooling rate in the second stage is lower than 5 °C / min, it will be unfavorable for the growth of pores and result in a lower porosity of the separator.

[0014] Preferably, the mass ratio between the poly(tetramethylpentene) particles and supercritical carbon dioxide is 1:5-1:8.

[0015] Preferably, the preparation method further includes the following steps: coating an alumina layer on the surface of the porous base membrane, and after plasma surface treatment, coating a polydopamine (PDA) layer; the alumina layer includes alumina and poly(vinylidene fluoride-hexafluoropropylene) copolymer (PVDF-HFP).

[0016] The low-polarity surface of the PMP separator will form a relatively high interfacial charge transfer barrier between it and the electrode, resulting in a relatively high interfacial impedance. To solve this problem, in the present invention, by coating an alumina layer and a polydopamine layer on the surface of the base membrane, a fast ion channel can be formed by utilizing the polar networks of the alumina layer and PDA, thereby reducing the interfacial impedance of the separator. And, in the present invention, plasma surface treatment is carried out after coating the alumina layer, which can introduce polar groups and promote the conduction of Li + . This also helps to reduce the interfacial impedance of the separator.

[0017] In addition, the alumina layer, the polydopamine layer, and the plasma surface treatment process can also have the following effects: PVDF-HFP in the alumina layer has good electrolyte affinity and can improve the liquid absorption rate of the separator; the alumina layer and PDA nanofibers can form a rigid-flexible synergistic protection to inhibit the high-temperature shrinkage of the separator and block the penetration of dendrites, thereby improving the puncture strength of the separator.

[0018] Preferably, the method of coating the alumina layer is electrostatic spraying; the method of coating the polydopamine layer is ultrasonic-induced in-situ polymerization of dopamine monomers; during the plasma surface treatment process, the plasma power is 80-100 W, the gas atmosphere is nitrogen, and the treatment time is 60-120 s.

[0019] Preferably, in the alumina layer, the average particle size of alumina is 50 - 150 nm, and the mass ratio is 70 - 80%; in the comonomer of the polyvinylidene fluoride - hexafluoropropylene copolymer, the mass ratio of hexafluoropropylene is 10 - 15%.

[0020] Preferably, in the porous base film, the surface layer pore size is 0.1 - 0.3 μm, and the bottom layer pore size is 2 - 5 μm.

[0021] Preferably, the total thickness of the poly - 4 - methyl - 1 - pentene - based lithium - ion battery separator is 15 - 25 μm; the thicknesses of the alumina layer and the polydopamine layer are 5 - 10 μm and 0.5 - 1 μm, respectively.

[0022] Preferably, the poly - 4 - methyl - 1 - pentene particles are virgin poly - 4 - methyl - 1 - pentene particles and / or recycled poly - 4 - methyl - 1 - pentene waste particles treated by microwave depolymerization; the number - average molecular weight of the recycled poly - 4 - methyl - 1 - pentene waste particles treated by microwave depolymerization is 70000 - 120000 Da, and the mass ratio in the poly - 4 - methyl - 1 - pentene particles is not higher than 30%.

[0023] Preferably, during the microwave depolymerization process, the microwave frequency is 2400 - 2500 MHz, and the time is 20 - 30 min.

[0024] In a second aspect, the present invention provides a lithium - ion battery, including the poly - 4 - methyl - 1 - pentene - based lithium - ion battery separator prepared by the above - mentioned preparation method.

[0025] Compared with the prior art, the present invention has the following advantages: (1) During the preparation of the PMP - based separator base film of the present invention, through the comprehensive design of the following four aspects: ① using the method of supercritical CO2 dissolution followed by pressure reduction and phase separation to form a base film with a stepped pore size distribution, ② using a temperature of 205 - 215 °C during the mixing and homogenization process, ③ using a pressure of 12 - 18 MPa during the mixing and homogenization process, and ④ using an initial cooling rate of ≥15 °C / min for phase separation, the separator can have both high liquid absorption rate, puncture strength, tensile strength, and high - temperature stability.

[0026] (2) During the preparation of the PMP - based separator base film of the present invention, by adopting the method of two - stage cooling and phase separation, and using a specific cooling rate in each stage, it is beneficial to better form a base film structure with high - density small pores on the surface layer and large pores on the bottom layer, thereby improving the liquid absorption rate and puncture strength of the separator to a greater extent.

[0027] (3) By sequentially coating an alumina layer, performing plasma surface treatment, and coating a polydopamine layer on the surface of the porous base film, the prepared PMP - based separator of the present invention can have lower interfacial impedance, as well as higher liquid absorption rate, high - temperature stability, and puncture strength. Detailed implementation manners

[0028] The present invention will be further described below in conjunction with embodiments.

[0029] A preparation method of a polytetramethylpentenyl lithium ion battery separator includes the following steps: in a closed container, mixing and homogenizing polytetramethylpentene particles with supercritical carbon dioxide at a temperature of 205 - 215°C and a pressure of 12 - 18 MPa to form a homogeneous solution, and then cooling to a pressure in the container not higher than 5 MPa for phase separation, with an initial cooling rate ≥ 15°C / min to obtain a porous base film.

[0030] In some specific implementation manners, the specific process of cooling to a pressure in the container not higher than 5 MPa for phase separation includes: cooling at a rate of 25 - 30°C / min to a pressure in the container of 8 - 10 MPa, and then cooling at a rate of 5 - 10°C / min to a pressure in the container of 2 - 5 MPa.

[0031] In some specific implementation manners, the mass ratio between the polytetramethylpentene particles and supercritical carbon dioxide is 1:5 - 1:8.

[0032] In some specific implementation manners, in the porous base film, the surface layer pore size is 0.1 - 0.3 μm, and the bottom layer pore size is 2 - 5 μm.

[0033] In some specific implementation manners, the polytetramethylpentene particles are virgin polytetramethylpentene particles and / or recycled polytetramethylpentene waste particles treated by microwave depolymerization; the number average molecular weight of the recycled polytetramethylpentene waste particles treated by microwave depolymerization is 70000 - 120000 Da, and the mass proportion in the polytetramethylpentene particles is not higher than 30%. In this specific implementation manner, optionally or preferably: During the microwave depolymerization treatment, the microwave frequency is 2400 - 2500 MHz, and the time is 20 - 30 min.

[0034] In some specific implementation manners, the preparation method further includes the following steps: also includes the following steps: coating an alumina layer on the surface of the porous base film, after plasma surface treatment, and then coating a polydopamine layer; the alumina layer includes alumina and a polyvinylidene fluoride - hexafluoropropylene copolymer. In this specific implementation manner, optionally or preferably: The method of coating the alumina layer is electrostatic spraying; the method of coating the polydopamine layer is ultrasonic - induced in - situ polymerization of dopamine monomers; During the plasma surface treatment, the plasma power is 80 - 100 W, the gas atmosphere is nitrogen, and the treatment time is 60 - 120 s; In the alumina layer, the average particle size of alumina is 50 - 150 nm, and the mass proportion is 70 - 80%; in the comonomer of the polyvinylidene fluoride - hexafluoropropylene copolymer, the mass proportion of hexafluoropropylene is 10 - 15%. The total thickness of the poly(4 - methyl - 1 - pentene) - based lithium - ion battery separator is 15 - 25 μm; the thicknesses of the alumina layer and the polydopamine layer are 5 - 10 μm and 0.5 - 1 μm respectively.

[0035] A lithium - ion battery includes a poly(4 - methyl - 1 - pentene) - based lithium - ion battery separator prepared by the above - mentioned preparation method.

[0036] The present invention will be described below through specific examples. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, the changes and advantages that can be conceived by those skilled in the art are included in the present invention, and the protection scope of the present invention is defined by the appended claims and any equivalents thereof.

[0037] In the following examples and comparative examples, the meanings of the following abbreviations are as follows: PMP: Poly(4 - methyl - 1 - pentene); PDA: Polydopamine; PVDF - HFP: Polyvinylidene fluoride - hexafluoropropylene copolymer; HFP: Hexafluoropropylene; M n : Number - average molecular weight.

[0038] Example 1 A PMP - based lithium - ion battery separator was prepared through the following steps: S1: Porous substrate film forming In a closed container, at a temperature of 210 °C and a pressure of 15 MPa, virgin PMP particles (melt index of 3 g / 10 min) and supercritical CO2 were mixed and homogenized at a mass ratio of 1:6 to form a homogeneous solution; then the temperature was decreased at a rate of 20 °C / min until the pressure in the container was 5 MPa, causing PMP and CO2 to phase - separate, and a porous substrate film was obtained.

[0039] S2: Coating the alumina layer After mixing acetone and DMAC at a volume ratio of 7:3, nano - alumina (average particle size of 100 nm) and PVDF - HFP (in the comonomer, the mass proportion of HFP is 12%) with a mass ratio of 4:1 were dispersed into it to form an alumina layer coating with an alumina content of 3.8 wt%. By means of electrostatic spraying, with a spraying voltage of 30 kV and a coating flow rate of 0.5 mL / h, the alumina layer coating was evenly sprayed on both sides of the porous substrate film, and after drying, an alumina / PMP composite separator was obtained.

[0040] S3: Plasma surface treatment Perform plasma surface treatment on both surfaces of the alumina / PMP composite diaphragm in a nitrogen atmosphere. The plasma power is 100 W and the treatment time is 60 s to obtain a surface-activated alumina / PMP composite diaphragm.

[0041] S4: Coating the PDA layer Immerse the surface-activated alumina / PMP composite diaphragm in dopamine / Tris buffer solution (pH = 8.5, dopamine concentration is 3 mg / mL), set the ultrasonic power to 40 kHz, perform ultrasonic-assisted reaction for 2 h, and after drying, form a PDA layer to obtain a PMP-based lithium-ion battery diaphragm.

[0042] The average thickness of the PMP-based lithium-ion battery diaphragm prepared in this example is 10 μm, and the average thicknesses of the alumina layer and the polydopamine layer are 5 μm and 1 μm respectively.

[0043] Example 2 The difference between this example and Example 1 is only that: in step S1, a porous base film is prepared by compounding virgin PMP particles and recycled PMP waste particles treated by microwave depolymerization; the other raw materials and steps are the same as those in Example 1. Specifically, the PMP-based lithium-ion battery diaphragm is prepared through the following steps: S1: Porous base film forming Set the microwave power to 2450 MHz, perform microwave depolymerization on the recycled PMP waste (medical waste) particles for 25 min to obtain a depolymerization product of recycled PMP waste particles with M n = 85000 Da. Mix it with virgin PMP particles (melt index is 3 g / 10 min) according to a mass ratio of 3:7 to obtain a PMP particle mixture. In a closed container, under the conditions of a temperature of 210 °C and a pressure of 15 MPa, mix and homogenize the PMP particle mixture and supercritical CO2 with a mass ratio of 1:6 to form a homogeneous solution; then cool it at a rate of 20 °C / min until the pressure in the container is 5 MPa to separate PMP and CO2 phases to obtain a porous base film.

[0044] S2: Coating the alumina layer After mixing acetone and DMAC according to a volume ratio of 7:3, disperse nano-alumina (average particle size is 100 nm) and PVDF-HFP (in the copolymer monomer, the mass fraction of HFP is 12%) with a mass ratio of 4:1 into it to form an alumina layer coating with an alumina content of 3.8 wt%. By means of electrostatic spraying, set the spraying voltage to 30 kV and the coating flow rate to 0.5 mL / h, and evenly spray the alumina layer coating on the surface of the porous base film. After drying, obtain an alumina / PMP composite diaphragm.

[0045] S3: Plasma surface treatment Perform plasma surface treatment on both sides of the alumina / PMP composite separator in a nitrogen atmosphere. The plasma power is 100 W and the treatment time is 60 s to obtain a surface-activated alumina / PMP composite separator.

[0046] S4: Coating the PDA layer Immerse the surface-activated alumina / PMP composite separator in dopamine / Tris buffer solution (pH = 8.5, dopamine concentration is 3 mg / mL), set the ultrasonic power to 40 kHz, perform ultrasonic-assisted reaction for 2 h, and form a PDA layer after drying to obtain a PMP-based lithium-ion battery separator.

[0047] The average thickness of the PMP-based lithium-ion battery separator prepared in this example is 12 μm, and the average thicknesses of the alumina layer and the polydopamine layer are 5 μm and 1 μm respectively.

[0048] Example 3 The difference between this example and Example 1 is only that: in step S1, the pressure during the mixing and homogenization process is changed from 15 MPa to 12 MPa; the remaining raw materials and steps are the same as those in Example 1.

[0049] Example 4 The difference between this example and Example 1 is only that: in step S1, the pressure during the mixing and homogenization process is changed from 15 MPa to 18 MPa; the remaining raw materials and steps are the same as those in Example 1.

[0050] Example 5 The difference between this example and Example 1 is only that: in step S1, the temperature during the mixing and homogenization process is changed from 210 °C to 205 °C; the remaining raw materials and steps are the same as those in Example 1.

[0051] Example 6 The difference between this example and Example 1 is only that: in step S1, the temperature during the mixing and homogenization process is changed from 210 °C to 215 °C; the remaining raw materials and steps are the same as those in Example 1.

[0052] Example 7 The difference between this example and Example 1 is only that: in step S1, the process of cooling and phase separation is divided into two stages; the remaining raw materials and steps are the same as those in Example 1. Specifically, this example changes step S1 to: In a closed container, under the conditions of a temperature of 210 °C and a pressure of 15 MPa, the virgin PMP particles (melt index of 3 g / 10 min) with a mass ratio of 1:6 are mixed and homogenized with supercritical CO2 to form a homogeneous solution; then the temperature is decreased at a rate of 25 °C / min until the pressure in the container is 10 MPa, and then the temperature is further decreased at a rate of 5 °C / min until the pressure in the container is 5 MPa, so that the PMP and CO2 are phase-separated to obtain a porous base membrane.

[0053] Example 8 The difference between this example and Example 1 is only that in step S1, the process of cooling and phase separation is divided into two stages; the remaining raw materials and steps are the same as those in Example 1. Specifically, this example changes step S1 to: In a closed container, under the conditions of a temperature of 210 °C and a pressure of 15 MPa, the virgin PMP particles (melt index of 3 g / 10 min) with a mass ratio of 1:6 are mixed and homogenized with supercritical CO2 to form a homogeneous solution; then the temperature is decreased at a rate of 30 °C / min until the pressure in the container is 8 MPa, and then the temperature is further decreased at a rate of 10 °C / min until the pressure in the container is 2 MPa, so that the PMP and CO2 are phase-separated to obtain a porous base membrane.

[0054] Example 9 The difference between this example and Example 8 is only that in step S1, the cooling rate in the first-stage cooling and phase separation process is changed from 30 °C / min to 35 °C / min; the remaining raw materials and steps are the same as those in Example 1.

[0055] Example 10 The difference between this example and Example 1 is only that: steps S2 - S4 are not carried out, and the porous base membrane prepared by step S1 is used as the PMP-based lithium-ion battery separator; the process of preparing the porous base membrane is the same as that in Example 1.

[0056] Comparative Example 1 The difference between this comparative example and Example 1 is only that: a conventional thermally induced phase separation - biaxial stretching method is used to prepare the porous base membrane; the remaining raw materials and steps are the same as those in Example 1. Specifically, this comparative example changes step S1 to: The virgin PMP particles (melt index of 3 g / 10 min) with a mass ratio of 1:3 are melt-blended with paraffin oil at 200 °C, then cast into a film, cooled to room temperature for curing, and then longitudinally stretched at 150 °C with a draw ratio of 3:1, and then transversely stretched at 155 °C with a draw ratio of 4:1. After the stretching is completed, the paraffin oil is removed by extraction with n-hexane, dried, and then heat-treated at 160 °C for 5 min, cooled to room temperature, to obtain a porous base membrane.

[0057] Comparative Example 2 The difference between this comparative example and Example 1 is only that: in step S1, the method of forming the gradient pore structure is changed. Instead of the previous method, supercritical CO2 is charged into the polymer melt, and a CO2 concentration gradient is formed in the polymer melt depending on the melt viscosity gradient, thereby forming a gradient pore structure with smaller pore size on the surface layer and larger pore size on the bottom layer; the remaining raw materials and steps are the same as those in Example 1. Specifically, this comparative example changes step S1 to: In a closed container, the virgin PMP particles are heated to 210 °C until melted, then supercritical CO2 is charged into the container until the air pressure reaches 15 MPa. After maintaining for 20 min, the closed container is opened to release pressure to atmospheric pressure, and a porous base membrane is obtained.

[0058] Comparative Example 3 The difference between this comparative example and Example 1 is only that: in step S1, the pressure during the mixing and homogenization process is changed from 15 MPa to 10 MPa; the remaining raw materials and steps are the same as those in Example 1.

[0059] Comparative Example 4 The difference between this comparative example and Example 1 is only that: in step S1, the pressure during the mixing and homogenization process is changed from 15 MPa to 20 MPa; the remaining raw materials and steps are the same as those in Example 1.

[0060] Comparative Example 5 The difference between this comparative example and Example 1 is only that: in step S1, the temperature during the mixing and homogenization process is changed from 210 °C to 190 °C; the remaining raw materials and steps are the same as those in Example 1.

[0061] Comparative Example 6 The difference between this comparative example and Example 1 is only that: in step S1, the temperature during the mixing and homogenization process is changed from 210 °C to 230 °C; the remaining raw materials and steps are the same as those in Example 1.

[0062] Comparative Example 7 The difference between this comparative example and Example 1 is only that: in step S1, the pressure reduction rate during the phase separation process is changed from 20 °C / min to 10 °C / min; the remaining raw materials and steps are the same as those in Example 1.

[0063] Test Example Take the porous base membranes and PMP-based lithium-ion battery separators prepared by the methods in each example and comparative example, and assemble the separators into NCM811 / / Si-C soft-pack lithium-ion batteries. Detect the average pore size of the surface layer (upper surface layer) and the bottom layer (lower surface layer) of each porous base membrane and the porosity of the whole porous base membrane. The results are shown in Table 1. Detect the puncture strength, tensile strength, thermal shrinkage rate after heating at 200 °C for 1 h, liquid absorption rate after soaking in the electrolyte at 25 °C for 30 min, and the interfacial impedance between the separator and the NCM811 positive electrode in the NCM811 / / Si-C soft-pack lithium-ion battery system of each PMP-based lithium-ion battery separator. The results are shown in Table 2.

[0064] Table 1 Detection Results of Porous Base Membrane Properties

[0065] Table 2 Detection Results of Separator Properties

[0066] It can be seen from Table 1 and Table 2 that: (1) The porous base membranes in Examples 1-9 have a structure with a smaller pore size on the surface layer and a larger pore size on the bottom layer. The surface layer and the bottom layer of the porous base membrane in Comparative Example 1 have the same pore size. Compared with Examples 1-9, the porous base membrane in Comparative Example 1 has a lower porosity, and the puncture strength, tensile strength, and liquid absorption rate of the separator are lower, while the thermal shrinkage rate is higher. The reason for the analysis is that in Examples 1-9, the method of depressurizing and phase separation after supercritical CO2 dissolution is adopted, so that a structure with a smaller pore size on the surface layer and a larger pore size on the bottom layer is formed in the porous base membrane, which can achieve the coordination of high porosity and high strength - by using the surface layer with a smaller pore size, lithium dendrites can be inhibited, the puncture strength of the separator can be improved, and at the same time, the thermal shrinkage rate of the separator can be reduced, and its high-temperature stability can be improved; by using the bottom layer with a larger pore size, the electrolyte can be stored, and the liquid absorption rate of the separator can be increased.

[0067] (2) Compared with Examples 1-9, the pore size difference between the surface layer and the bottom layer of the porous base membrane in Comparative Example 2 is smaller, and the puncture strength, tensile strength, and liquid absorption rate of the separator are lower, while the thermal shrinkage rate is higher. The reason for the analysis is that compared with the method in Comparative Example 2 of charging supercritical CO2 into the polymer melt and relying on the viscosity gradient of the melt to form a CO2 concentration gradient in the polymer melt and then form a gradient pore structure, in Examples 1-9, supercritical CO2 and PMP are mixed and homogenized to form a homogeneous solution, and then during the depressurization process, through the coordinated regulation of pressure and temperature, using the difference in the desorption rate of CO2 from between the chains and the rigidity of the molecular chains of the PMP helical structure, asymmetric phase separation is induced. This method can make the pore size difference between the surface layer and the bottom layer larger within the limited film thickness of the battery separator, so it can be better applied to the battery separator and achieve the coordination of its high porosity and high strength.

[0068] (3) Compared with Example 1, the diaphragm liquid absorption rate and puncture strength of Examples 7 - 8 are higher; compared with Example 8, the diaphragm puncture strength and tensile strength of Example 9 are lower. The reasons are as follows: compared with Example 1, in Examples 7 - 8, a two-stage cooling and phase separation method is adopted. The process of rapid cooling and pressure reduction in the first stage is conducive to inducing a large number of micropore nucleations, forming high-density small holes on the surface layer. The process of slow cooling and pressure reduction in the second stage can promote the directional growth of holes, forming large holes at the bottom layer; moreover, compared with Example 8, the cooling rate in the first stage of Example 9 is too fast, and the rapid phase change of supercritical CO2 will disrupt the dynamic balance of PMP molecular chains, leading to the instability of the pore structure. The main reason is that the too-fast CO2 desolvation causes the rapid solidification of the surface pore walls, forming microcracks.

[0069] (4) Compared with Examples 1, 3, and 4, the porosity of the porous base membrane of Comparative Example 3 is lower, the diaphragm puncture strength and liquid absorption rate are lower, and the thermal shrinkage rate is higher. The reasons are as follows: during the process of mixing and homogenizing PMP particles with supercritical CO2, when the pressure is too low, the supercritical CO2 is not fully dissolved, local aggregation of PMP molecular chains occurs, and the pore nucleus distribution is uneven during phase separation, resulting in large fluctuations in surface pore diameters and poor connectivity of bottom-layer pores, which is not conducive to using the surface layer to improve the diaphragm puncture strength and high-temperature stability, and using the bottom layer to improve the diaphragm liquid absorption rate.

[0070] (5) Compared with Examples 1, 3, and 4, the bottom-layer pore diameters of the porous base membrane of Comparative Example 4 are larger, the diaphragm puncture strength and tensile strength are lower, and the thermal shrinkage rate is higher. The reasons are as follows: during the process of mixing and homogenizing PMP particles with supercritical CO2, when the pressure is too high, CO2 supersaturation causes excessive growth of bottom-layer pores and thinning of pore walls, increasing the risk of dendrite penetration of the diaphragm and reducing the tensile strength. At the same time, the surface pores rapidly collapse and close, resulting in a decrease in the thermal stability of the diaphragm.

[0071] (6) Compared with Examples 1, 5, and 6, the porosity of the porous base membrane of Comparative Example 5 is lower, and the diaphragm liquid absorption rate is lower. The reasons are as follows: during the process of mixing and homogenizing PMP particles with supercritical CO2, when the temperature is too low, it is not conducive to the melting of PMP, the PMP molecular chains are not fully extended, which will lead to uneven CO2 dissolution, few pore nuclei and sparse distribution during phase separation, resulting in a decrease in the porosity of the diaphragm and a decrease in the liquid absorption rate.

[0072] (7) Compared with Example 1, Example 5 and Example 6, the puncture strength and tensile strength of the porous base film of Comparative Example 6 are lower. The reason is as follows: During the process of mixing and homogenizing PMP particles with supercritical CO2, when the temperature is too high, the PMP molecular chains will be overly untangled, the intermolecular force will weaken, the pore walls will become thinner and prone to merging during phase separation, resulting in an overly high proportion of large pores and a decrease in puncture strength. At the same time, high temperature accelerates the relaxation of chain segments and reduces the crystalline region, leading to a decrease in tensile strength. In addition, the presence of an appropriate amount of large pores can improve the liquid absorption rate of the separator, but when the proportion of large pores is too high and the pore size is too large, it will instead have an adverse effect on the liquid storage performance of the separator. Therefore, although the porosity of Comparative Example 6 has increased, the liquid absorption rate has decreased.

[0073] (8) Compared with Example 1, the porosity of the porous base film of Comparative Example 7 is lower. This indicates that during the phase separation process, when the pressure reduction rate is too slow, it is not conducive to forming pores in the base film.

[0074] (9) Compared with Example 1, the puncture strength and liquid absorption rate of the separator of Example 10 are lower, and the interfacial impedance is higher. The reason is as follows: In Example 1, by coating an alumina layer and a polydopamine layer on the surface of the base film, a fast ion channel can be formed by utilizing the alumina layer and the PDA polar network, thereby reducing the interfacial impedance of the separator. Moreover, after coating the alumina layer, plasma surface treatment was carried out, which can introduce polar groups and promote the conduction of Li + . This also helps to reduce the interfacial impedance of the separator. In addition, PVDF-HFP in the alumina layer has good electrolyte affinity, which can improve the liquid absorption rate of the separator. The alumina layer and PDA nanofibers can form a synergistic protection of rigidity and flexibility, inhibit the high-temperature shrinkage of the separator, and block the penetration of dendrites, improving the puncture strength of the separator.

[0075] Unless otherwise defined, all technical terms and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which the present disclosure belongs. The raw materials and equipment used in the present invention are conventional raw materials and equipment in the art and can be obtained from conventional commercial channels without special instructions; the methods used in the present invention are conventional methods in the art without special instructions.

[0076] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A preparation method of a polytetramethylpentene-based lithium-ion battery separator, characterized in that, Comprising the following steps: In a closed container, mix and homogenize polytetramethylpentene particles with supercritical carbon dioxide at a temperature of 205 - 215 °C and a pressure of 12 - 18 MPa to form a homogeneous solution, and then cool down to a pressure in the container not higher than 5 MPa for phase separation. The initial cooling rate is ≥15 °C / min to obtain a porous base membrane.

2. The preparation method according to claim 1, characterized in that, The specific process of cooling down to a pressure in the container not higher than 5 MPa for phase separation includes: cooling at a rate of 25 - 30 °C / min until the pressure in the container is 8 - 10 MPa, and then cooling at a rate of 5 - 10 °C / min until the pressure in the container is 2 - 5 MPa.

3. The preparation method according to claim 1 or 2, characterized in that, The mass ratio between the polytetramethylpentene particles and supercritical carbon dioxide is 1:5 - 1:

8.

4. The preparation method according to claim 1, characterized in that, Also comprising the following steps: Coat an alumina layer on the surface of the porous base membrane, and after plasma surface treatment, coat a polydopamine layer; the alumina layer includes alumina and a polyvinylidene fluoride - hexafluoropropylene copolymer.

5. The preparation method according to claim 4, characterized in that, The way to coat the alumina layer is electrostatic spraying; the way to coat the polydopamine layer is ultrasonic - induced in - situ polymerization of dopamine monomers; during the plasma surface treatment, the plasma power is 80 - 100 W, the gas atmosphere is nitrogen, and the treatment time is 60 - 120 s.

6. The preparation method according to claim 4 or 5, characterized in that, In the alumina layer, the average particle size of alumina is 50 - 150 nm, and the mass fraction is 70 - 80%; in the copolymer monomers of the polyvinylidene fluoride - hexafluoropropylene copolymer, the mass fraction of hexafluoropropylene is 10 - 15%.

7. The preparation method according to claim 1, wherein In the porous base membrane, the surface layer pore size is 0.1 - 0.3 μm, and the bottom layer pore size is 2 - 5 μm.

8. The preparation method according to claim 4, characterized in that, The total thickness of the polytetramethylpentene - based lithium - ion battery separator is 15 - 25 μm; the thicknesses of the alumina layer and the polydopamine layer are 5 - 10 μm and 0.5 - 1 μm respectively.

9. The preparation method according to claim 1, wherein The polytetramethylpentene particles are virgin polytetramethylpentene particles and / or recycled polytetramethylpentene waste particles treated by microwave depolymerization; the number - average molecular weight of the recycled polytetramethylpentene waste particles treated by microwave depolymerization is 70000 - 120000 Da, and the mass fraction in the polytetramethylpentene particles is not higher than 30%.

10. A lithium-ion battery, characterized in that, Comprising a polytetramethylpentene - based lithium - ion battery separator prepared by using the preparation method according to any one of claims 1 - 9.

Images