Preparation method and application of polytetramethylpentene-based lithium ion battery separator

Abstract

The present invention relates to the field of battery separator technology, and discloses a preparation method and application of a polytetramethylpentene-based lithium-ion battery separator. The preparation method comprises the following steps: in a sealed container, polytetramethylpentene particles are mixed and homogenized 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 the temperature is lowered to a pressure of no more than 5 MPa in the container for phase separation, with an initial cooling rate of ≥15°C / min to obtain a porous base membrane. Using the preparation method of the present invention, the base membrane of the PMP-based separator can have a high porosity and a suitable pore size distribution, thereby making the separator have 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 polytetramethylpentenyl 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) easy 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), it is easy to cause the separator to shrink and the pore closure to fail, causing 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), causing the impedance to soar.

[0003] Polytetramethylpentene (PMP) has a helical molecular chain structure, and its high rigidity provides it with thermodynamic advantages. PMP has a glass transition temperature (Tg) of up to 260°C, and its coefficient of thermal expansion is 50% lower than that of PP. PMP also exhibits good chemical stability and is less susceptible to swelling in ester and ether electrolytes.

[0004] However, due to the high crystallinity of PMP, micropore formation is difficult. Conventional phase separation pore-forming methods tend to result in closed pores, resulting in low porosity (<30%) and difficult-to-control pore size distribution in PMP-based membranes. For example, patent application CN102804450A employs thermally induced phase separation and biaxial stretching to form pores. Despite the inclusion of high levels of polyethylene and polypropylene in the membrane and a PMP content of only 20-21wt%, the membrane still achieves a porosity of only 32-37%. Summary of the Invention

[0005] To address the technical issues of low porosity and difficult-to-control pore size distribution in PMP-based separators, the present invention provides a method for preparing and applying a polytetramethylpentene-based lithium-ion battery separator. This method enables the PMP-based separator to have a high porosity and a suitable pore size distribution, thereby achieving high liquid absorption, puncture resistance, tensile strength, and high-temperature stability.

[0006] The specific technical solutions of the present invention are:

[0007] In a first aspect, the present invention provides a method for preparing a polytetramethylpentene-based lithium-ion battery separator, comprising the following steps: in a closed container, polytetramethylpentene particles and supercritical carbon dioxide are mixed and homogenized at a temperature of 205-215°C and a pressure of 12-18 MPa to form a homogeneous solution, and then the solution is cooled to a pressure in the container not higher than 5 MPa for phase separation, with an initial cooling rate of ≥15°C / min to obtain a porous base membrane.

[0008] In the process of preparing the PMP-based membrane, the present invention adopts a method of dissolving in supercritical CO2 and then reducing pressure to separate the phases to form a base membrane with a stepped pore size distribution. Furthermore, by combining the specific temperature and pressure during the homogenization process of the PMP particles and supercritical CO2, as well as the specific pressure reduction rate during the phase separation process, the resulting porous base membrane can have a high porosity and a suitable pore size distribution, thereby making the PMP-based membrane have high liquid absorption rate, puncture strength, tensile strength and high-temperature stability. Specifically:

[0009] ①Use supercritical CO2 dissolution followed by pressure reduction and phase separation to form a base membrane with a stepped pore size distribution:

[0010] The present invention utilizes the high solubility of supercritical carbon dioxide for PMP to form a homogeneous solution, in which CO2 is uniformly dispersed between the PMP molecular chains. Then, during the pressure reduction process, through pressure-temperature coordinated regulation (by cooling in a closed container to achieve simultaneous pressure reduction), the difference in CO2 desorption rate from the chains is utilized, combined with the rigidity of the PMP helical structure molecular chain (which hinders pore merging during phase separation and improves the stability of small pores on the surface, while the bottom layer forms large pores due to chain relaxation delay), to induce asymmetric phase separation. The surface layer rapidly separates to form smaller pores, and the bottom layer slowly separates to form larger pores, thereby achieving the synergy of high porosity and high strength - utilizing the surface layer with a smaller pore size can inhibit lithium dendrites and improve the puncture strength of the diaphragm. At the same time, it can also reduce the thermal shrinkage of the diaphragm, improve its high-temperature stability, reduce the risk of short circuit due to diaphragm shrinkage, and support the safety of the battery in thermal runaway scenarios; utilizing the bottom layer with a larger pore size, electrolyte can be stored, and the liquid absorption rate of the diaphragm can be improved.

[0011] Moreover, compared with the method of charging supercritical CO2 into a polymer melt, relying on the melt viscosity gradient to form a CO2 concentration gradient in the polymer melt, and then forming a gradient pore structure, the present invention mixes supercritical CO2 and PMP to homogenize to form a homogeneous solution, and uses asymmetric phase separation to form a gradient pore structure. It can make the pore size difference between the surface layer and the bottom layer larger within the limited film thickness of the battery separator, and thus can be better suitable for battery separators to achieve the synergy of high porosity and high strength.

[0012] ②Use a temperature of 205-215°C during the mixing and homogenization process:

[0013] Controlling the temperature of the homogenization of the PMP particles and supercritical carbon dioxide at 205-215°C can make the diaphragm have high liquid absorption rate, tensile strength and puncture strength. When the temperature of the mixing homogenization is too low, it is not conducive to the melting of PMP, and the PMP molecular chain is not fully stretched, which will lead to uneven dissolution of CO2. During phase separation, the number of pore nuclei is small and sparsely distributed, resulting in a decrease in the porosity of the diaphragm and a decrease in liquid absorption rate. At the same time, mechanical weaknesses are formed in the unmelted area, and the fluctuation of tensile strength increases. When the temperature of the mixing homogenization is too high, it will cause excessive disentanglement of the PMP molecular chain, weakening the interchain force. During phase separation, the pore wall becomes thinner and easy to merge, resulting in an excessively 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 crystallization area, resulting in a decrease in tensile strength.

[0014] ③Use 12-18MPa pressure during the mixing and homogenization process:

[0015] When the pressure in the mixing and homogenization process is too low, the supercritical CO2 dissolves insufficiently, the PMP molecular chains are locally aggregated, and the pore cores are unevenly distributed during phase separation, resulting in large fluctuations in the surface pore size and poor connectivity of the bottom pores, which is not conducive to utilizing the surface layer to improve the puncture strength and high-temperature stability of the diaphragm, as well as utilizing the bottom layer to improve the liquid absorption rate of the diaphragm. When the pressure in the mixing and homogenization process is too high, CO2 supersaturation causes excessive growth of the bottom pores, thinning of the pore walls, increased risk of dendrite penetration of the diaphragm, reduced tensile strength, and rapid collapse and closure of the surface pores, resulting in decreased thermal stability of the diaphragm. The present invention can enable the diaphragm to have both high liquid absorption rate, puncture strength and high-temperature stability by controlling the mixing and homogenization pressure at 12-18MPa.

[0016] ④Use an initial cooling rate of ≥15℃ / min for phase separation:

[0017] During the phase separation process, when the initial cooling rate is too slow, the porosity of the diaphragm will be too low, resulting in the application of the diaphragm being limited to low-rate scenarios.

[0018] Preferably, the specific process of cooling the container to a pressure not higher than 5MPa for phase separation includes: cooling the container to a pressure of 8-10MPa at a rate of 25-30°C / min, and then cooling the container to a pressure of 2-5MPa at a rate of 5-10°C / min.

[0019] The two-stage cooling and phase separation approach described above achieves the following effects: Rapid cooling and pressure reduction (25-30°C / min) to 8-10 MPa in the first stage facilitates the nucleation of numerous micropores, forming a high-density of small pores on the surface. Slow cooling and pressure reduction (5-10°C / min) to 2-5 MPa in the second stage promotes directional pore growth, forming macropores in the underlying layer. Cooling rates below 25°C / min in the first stage or above 10°C / min in the second stage are detrimental to achieving these effects. Furthermore, cooling rates above 30°C / min in the first stage disrupt the dynamic equilibrium of the PMP molecular chains, causing pore structure instability. This is primarily due to the rapid solidification of the surface pore walls caused by rapid CO2 desolvation, which forms microcracks. Cooling rates below 5°C / min in the second stage hinder pore growth, resulting in low membrane porosity.

[0020] Preferably, the mass ratio of the polytetramethylpentene particles to supercritical carbon dioxide is 1:5-1:8.

[0021] Preferably, the preparation method further comprises the following steps: coating an aluminum oxide layer on the surface of the porous base membrane, subjecting the surface to plasma treatment, and then coating a polydopamine (PDA) layer; the aluminum oxide layer comprises aluminum oxide and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP).

[0022] The low polarity surface of the PMP diaphragm will form a higher interfacial charge transfer barrier between it and the electrode, resulting in a higher interfacial impedance. To address this problem, the present invention coats an aluminum oxide layer and a polydopamine layer on the surface of the base membrane, which can utilize the aluminum oxide layer and the PDA polar network to form a fast ion channel, thereby reducing the interfacial impedance of the diaphragm. In addition, the present invention performs plasma surface treatment after coating the aluminum oxide layer, which can introduce polar groups to promote Li + conduction, which also helps to reduce the interfacial impedance of the diaphragm.

[0023] In addition, the aluminum oxide layer, polydopamine layer and plasma surface treatment process can also produce the following effects: the PVDF-HFP in the aluminum oxide layer has good electrolyte affinity and can improve the liquid absorption rate of the diaphragm; the aluminum oxide layer and PDA nanofibers can form a synergistic protection of rigid and flexible propyl groups, inhibiting the high-temperature shrinkage of the diaphragm, and blocking dendrite penetration, thereby improving the puncture strength of the diaphragm.

[0024] Preferably, the aluminum oxide layer is coated by electrostatic spraying; the polydopamine layer is coated by ultrasonically induced in-situ polymerization of dopamine monomer; during the plasma surface treatment, the plasma power is 80-100W, the gas atmosphere is nitrogen, and the treatment time is 60-120s.

[0025] Preferably, in the aluminum oxide layer, the average particle size of aluminum oxide 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%.

[0026] Preferably, in the porous base membrane, the surface pore size is 0.1-0.3 μm, and the bottom pore size is 2-5 μm.

[0027] Preferably, the total thickness of the polytetramethylpentene-based lithium-ion battery separator is 15-25 μm; the thicknesses of the aluminum oxide layer and the polydopamine layer are 5-10 μm and 0.5-1 μm, respectively.

[0028] Preferably, the polytetramethylpentene particles are native polytetramethylpentene particles and / or recycled polytetramethylpentene waste particles treated with microwave depolymerization; the number average molecular weight of the recycled polytetramethylpentene waste particles treated with microwave depolymerization is 70,000-120,000 Da, and their mass proportion in the polytetramethylpentene particles is not higher than 30%.

[0029] Preferably, during the microwave depolymerization treatment, the microwave frequency is 2400-2500 MHz and the time is 20-30 min.

[0030] In a second aspect, the present invention provides a lithium-ion battery, comprising a polytetramethylpentenyl lithium-ion battery separator prepared by the preparation method.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] (1) In the process of preparing the PMP-based diaphragm base membrane of the present invention, the comprehensive design of four aspects, namely, ① forming a base membrane with a stepped pore size distribution by adopting a method of supercritical CO2 dissolution followed by pressure reduction phase separation, ② adopting a temperature of 205-215°C during the mixing and homogenization process, ③ adopting a pressure of 12-18 MPa during the mixing and homogenization process, and ④ adopting an initial cooling rate of ≥15°C / min for phase separation, can enable the diaphragm to have high liquid absorption rate, puncture strength, tensile strength and high temperature stability.

[0033] (2) In the process of preparing the PMP-based diaphragm base membrane, the present invention adopts a two-stage cooling and phase separation method, and adopts a specific cooling rate in each stage, which is conducive to better forming a base membrane structure with high-density small pores on the surface and large pores on the bottom layer, thereby improving the liquid absorption rate and puncture strength of the diaphragm to a greater extent.

[0034] (3) The present invention can make the prepared PMP-based diaphragm have lower interfacial impedance, higher liquid absorption rate, high temperature stability and puncture strength by coating an aluminum oxide layer, plasma surface treatment and polydopamine layer on the surface of the porous base membrane in sequence. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to the embodiments.

[0036] A method for preparing a polytetramethylpentene-based lithium-ion battery separator comprises the following steps: in a sealed container, polytetramethylpentene particles are mixed and homogenized with supercritical carbon dioxide at a temperature of 205-215°C and a pressure of 12-18 MPa to form a homogeneous solution; the solution is then cooled to a pressure in the container of no more than 5 MPa for phase separation, with an initial cooling rate of ≥15°C / min, to obtain a porous base membrane.

[0037] In some specific embodiments, the specific process of cooling the container to a pressure not higher than 5 MPa for phase separation includes: cooling the container to a pressure of 8-10 MPa at a rate of 25-30°C / min, and then cooling the container to a pressure of 2-5 MPa at a rate of 5-10°C / min.

[0038] In some specific embodiments, the mass ratio between the polytetramethylpentene particles and supercritical carbon dioxide is 1:5-1:8.

[0039] In some specific embodiments, in the porous base membrane, the surface pore size is 0.1-0.3 μm, and the bottom pore size is 2-5 μm.

[0040] In some embodiments, the polytetramethylpentene particles are virgin polytetramethylpentene particles and / or recycled polytetramethylpentene waste particles subjected to microwave depolymerization; the recycled polytetramethylpentene waste particles subjected to microwave depolymerization have a number average molecular weight of 70,000-120,000 Da and account for no more than 30% of the polytetramethylpentene particles by mass. In this embodiment, optionally or preferably:

[0041] During the microwave depolymerization treatment, the microwave frequency is 2400-2500 MHz and the time is 20-30 min.

[0042] In some specific embodiments, the preparation method further comprises the following steps: coating an aluminum oxide layer on the surface of the porous base membrane, subjecting the surface to plasma treatment, and then coating a polydopamine layer; the aluminum oxide layer comprises aluminum oxide and polyvinylidene fluoride-hexafluoropropylene copolymer. In this specific embodiment, optionally or preferably:

[0043] The aluminum oxide layer is coated by electrostatic spraying; the polydopamine layer is coated by ultrasound-induced in-situ polymerization of dopamine monomers;

[0044] During the plasma surface treatment, the plasma power is 80-100W, the gas atmosphere is nitrogen, and the treatment time is 60-120s;

[0045] In the alumina layer, the average particle size of the alumina is 50-150 nm, and the mass proportion of the alumina is 70-80%; in the comonomer of the polyvinylidene fluoride-hexafluoropropylene copolymer, the mass proportion of hexafluoropropylene is 10-15%;

[0046] The total thickness of the polytetramethylpentene-based lithium-ion battery separator is 15-25 μm; the thicknesses of the aluminum oxide layer and the polydopamine layer are 5-10 μm and 0.5-1 μm respectively.

[0047] A lithium ion battery comprises a polytetramethylpentene-based lithium ion battery separator prepared by the preparation method.

[0048] The present invention is described below by way of specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, any changes and advantages that can be imagined by those skilled in the art are included in the present invention, and the scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

[0049] In the following examples and comparative examples, the meanings of the abbreviations are as follows:

[0050] PMP: polytetramethylpentene;

[0051] PDA: polydopamine;

[0052] PVDF-HFP: polyvinylidene fluoride-hexafluoropropylene copolymer;

[0053] HFP: hexafluoropropylene;

[0054] M n : Number average molecular weight.

[0055] Example 1

[0056] The PMP-based lithium-ion battery separator was prepared by the following steps:

[0057] S1: Porous base film forming

[0058] In a closed container, at a temperature of 210°C and a pressure of 15 MPa, native PMP particles (melt index of 3g / 10min) with a mass ratio of 1:6 were mixed and homogenized with supercritical CO2 to form a homogeneous solution; then the solution was cooled at a rate of 20°C / min to a pressure of 5 MPa in the container to allow the PMP and CO2 to separate into phases, resulting in a porous base membrane.

[0059] S2: Aluminum oxide coating

[0060] Acetone and DMAC were mixed in a volume ratio of 7:3, and then nano-alumina (average particle size 100 nm) and PVDF-HFP (HFP accounted for 12% by weight of the comonomer) were dispersed in a mass ratio of 4:1 to form an alumina coating with an alumina content of 3.8 wt%. The alumina coating was evenly applied to both sides of the porous base membrane using electrostatic spraying at a voltage of 30 kV and a flow rate of 0.5 mL / h. After drying, the alumina / PMP composite membrane was obtained.

[0061] S3: Plasma surface treatment

[0062] The surfaces of both sides of the alumina / PMP composite membrane were subjected to plasma surface treatment in a nitrogen atmosphere with a plasma power of 100 W and a treatment time of 60 s to obtain a surface-activated alumina / PMP composite membrane.

[0063] S4: Coating PDA layer

[0064] The surface-activated alumina / PMP composite membrane was immersed in dopamine / Tris buffer (pH = 8.5, dopamine concentration of 3 mg / mL), the ultrasonic power was set to 40 kHz, and the ultrasound-assisted reaction was carried out for 2 hours. After drying, a PDA layer was formed to obtain a PMP-based lithium-ion battery membrane.

[0065] The average thickness of the PMP-based lithium-ion battery separator prepared in this embodiment is 10 μm, and the average thicknesses of the aluminum oxide layer and the polydopamine layer are 5 μm and 1 μm, respectively.

[0066] Example 2

[0067] The only difference between this embodiment and embodiment 1 is that in step S1, a porous base membrane is prepared by mixing virgin PMP particles and recycled PMP waste particles that have been subjected to microwave depolymerization. The remaining raw materials and steps are the same as those in embodiment 1. Specifically, this embodiment prepares a PMP-based lithium-ion battery separator by the following steps:

[0068] S1: Porous base film forming

[0069] The microwave power was set at 2450 MHz, and the recycled PMP waste (medical waste) particles were subjected to microwave depolymerization for 25 min to obtain M n =85,000Da recycled PMP waste particles were depolymerized and mixed with virgin PMP particles (melt index 3g / 10min) at a mass ratio of 3:7 to produce a PMP particle mixture. In a sealed container, the PMP particle mixture was homogenized with supercritical CO2 at a mass ratio of 1:6 at 210°C and 15MPa to form a homogeneous solution. The solution was then cooled at a rate of 20°C / min to a pressure of 5MPa within the container to allow phase separation between the PMP and CO2, resulting in a porous base membrane.

[0070] S2: Aluminum oxide coating

[0071] Acetone and DMAC were mixed in a volume ratio of 7:3, and then nano-alumina (average particle size 100 nm) and PVDF-HFP (HFP accounted for 12% by weight of the comonomer) were dispersed in a mass ratio of 4:1 to form an alumina coating with an alumina content of 3.8 wt%. The alumina coating was evenly applied to the surface of the porous base membrane using electrostatic spraying at a spray voltage of 30 kV and a coating flow rate of 0.5 mL / h. After drying, the alumina / PMP composite membrane was obtained.

[0072] S3: Plasma surface treatment

[0073] The surfaces of both sides of the alumina / PMP composite membrane were subjected to plasma surface treatment in a nitrogen atmosphere with a plasma power of 100 W and a treatment time of 60 s to obtain a surface-activated alumina / PMP composite membrane.

[0074] S4: Coating PDA layer

[0075] The surface-activated alumina / PMP composite membrane was immersed in dopamine / Tris buffer (pH = 8.5, dopamine concentration of 3 mg / mL), the ultrasonic power was set to 40 kHz, and the ultrasound-assisted reaction was carried out for 2 hours. After drying, a PDA layer was formed to obtain a PMP-based lithium-ion battery membrane.

[0076] The average thickness of the PMP-based lithium-ion battery separator prepared in this embodiment is 12 μm, and the average thicknesses of the aluminum oxide layer and the polydopamine layer are 5 μm and 1 μm, respectively.

[0077] Example 3

[0078] The only difference between this embodiment and embodiment 1 is 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 embodiment 1.

[0079] Example 4

[0080] The only difference between this embodiment and embodiment 1 is 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 embodiment 1.

[0081] Example 5

[0082] The only difference between this embodiment and embodiment 1 is 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 embodiment 1.

[0083] Example 6

[0084] The only difference between this embodiment and embodiment 1 is 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 embodiment 1.

[0085] Example 7

[0086] The only difference between this embodiment and embodiment 1 is that in step S1, the cooling and phase separation process is divided into two stages; the remaining raw materials and steps are the same as those in embodiment 1. Specifically, in this embodiment, step S1 is changed to:

[0087] In a closed container, at a temperature of 210°C and a pressure of 15 MPa, native PMP particles (melt index of 3g / 10min) with a mass ratio of 1:6 were mixed and homogenized with supercritical CO2 to form a homogeneous solution; then the solution was cooled at a rate of 25°C / min to a pressure of 10 MPa in the container, and then cooled at a rate of 5°C / min to a pressure of 5 MPa in the container, to allow the PMP and CO2 to separate into phases and obtain a porous base membrane.

[0088] Example 8

[0089] The only difference between this embodiment and embodiment 1 is that in step S1, the cooling and phase separation process is divided into two stages; the remaining raw materials and steps are the same as those in embodiment 1. Specifically, in this embodiment, step S1 is changed to:

[0090] In a closed container, at a temperature of 210°C and a pressure of 15 MPa, native PMP particles (melt index of 3g / 10min) with a mass ratio of 1:6 were mixed and homogenized with supercritical CO2 to form a homogeneous solution; then the solution was cooled at a rate of 30°C / min to a pressure of 8MPa in the container, and then cooled at a rate of 10°C / min to a pressure of 2MPa in the container to allow the PMP and CO2 to separate into phases, thereby obtaining a porous base membrane.

[0091] Example 9

[0092] The only difference between this embodiment and embodiment 8 is 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 embodiment 1.

[0093] Example 10

[0094] The only difference between this embodiment and embodiment 1 is that steps S2-S4 are not performed, and the porous base membrane prepared in step S1 is used as the PMP-based lithium-ion battery separator; the process for preparing the porous base membrane is the same as that in embodiment 1.

[0095] Comparative Example 1

[0096] The only difference between this comparative example and Example 1 is that the porous base film is prepared by a conventional thermally induced phase separation-biaxial stretching method; the remaining raw materials and steps are the same as those in Example 1. Specifically, in this comparative example, step S1 is changed to:

[0097] A 1:3 mass ratio of virgin PMP particles (melt index 3g / 10min) and paraffin oil were melt-blended at 200°C, then cast and cooled to room temperature for solidification. The film was then stretched longitudinally at 150°C with a draw ratio of 3:1 and transversely at 155°C with a draw ratio of 4:1. After stretching, the film was extracted with n-hexane to remove the paraffin oil, dried, and heat-treated at 160°C for 5 minutes before cooling to room temperature to produce a porous base membrane.

[0098] Comparative Example 2

[0099] The only difference between this comparative example and Example 1 is that in step S1, the method of forming the gradient pore structure is changed. Supercritical CO2 is injected into the polymer melt, and a CO2 concentration gradient is formed in the polymer melt by relying on the melt viscosity gradient to form a gradient pore structure with smaller surface pores and larger bottom pores. The remaining raw materials and steps are the same as those in Example 1. Specifically, step S1 in this comparative example is changed to:

[0100] In a sealed container, the original PMP particles are heated to 210°C to melt, and then supercritical CO2 is filled into the container to a pressure of 15 MPa. After maintaining for 20 minutes, the sealed container is opened and the pressure is released to normal pressure to obtain a porous base membrane.

[0101] Comparative Example 3

[0102] The only difference between this comparative example and Example 1 is 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.

[0103] Comparative Example 4

[0104] The only difference between this comparative example and Example 1 is 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.

[0105] Comparative Example 5

[0106] The only difference between this comparative example and Example 1 is 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.

[0107] Comparative Example 6

[0108] The only difference between this comparative example and Example 1 is 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.

[0109] Comparative Example 7

[0110] The only difference between this comparative example and Example 1 is 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.

[0111] Test Case

[0112] The porous base membranes and PMP-based lithium-ion battery separators prepared according to the methods described in the various Examples and Comparative Examples were assembled into NCM811 / / Si-C soft-pack lithium-ion batteries. The average pore size of the surface layer (upper surface), the average pore size of the bottom layer (lower surface), and the overall porosity of each porous base membrane were measured. The results are shown in Table 1. The puncture strength, tensile strength, thermal shrinkage after heating at 200°C for 1 hour, liquid absorption after immersion in electrolyte at 25°C for 30 minutes, and the interfacial impedance between the separator and the NCM811 positive electrode in the NCM811 / / Si-C soft-pack lithium-ion battery system were also measured. The results are shown in Table 2.

[0113] Table 1 Test results of porous base membrane performance

[0114]

[0115] Table 2 Diaphragm performance test results

[0116]

[0117] From Table 1 and Table 2 we can see that:

[0118] (1) The porous base membranes in Examples 1 to 9 have a structure with a smaller surface pore size and a larger bottom pore size. The surface and bottom pore sizes of the porous base membrane in Comparative Example 1 are the same. Compared with Examples 1 to 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 diaphragm are lower, and the thermal shrinkage rate is higher. The reason for this is that: Examples 1 to 9 adopt the method of pressure reduction and phase separation after supercritical CO2 dissolution, so that the porous base membrane forms a structure with a smaller surface pore size and a larger bottom pore size, which can achieve the synergy of high porosity and high strength - the surface layer with a smaller pore size can suppress lithium dendrites, improve the puncture strength of the diaphragm, and at the same time reduce the thermal shrinkage rate of the diaphragm and improve its high temperature stability; the bottom layer with a larger pore size can store electrolyte and improve the liquid absorption rate of the diaphragm.

[0119] (2) Compared with Examples 1 to 9, the pore size difference between the surface layer and the bottom layer in the porous base membrane of Comparative Example 2 is smaller, and the puncture strength, tensile strength and liquid absorption rate of the membrane are lower, and the thermal shrinkage rate is higher. The reason for this is that compared with the method of Comparative Example 2 in which supercritical CO2 is charged into the polymer melt and a CO2 concentration gradient is formed in the polymer melt by relying on the melt viscosity gradient, and then a gradient pore structure is formed, Examples 1 to 9 mix supercritical CO2 with PMP to form a homogeneous solution, and then in the process of depressurization, through pressure-temperature coordinated regulation, the difference in CO2 desorption rate from the interchain is combined with the rigidity of the PMP helical structure molecular chain to induce asymmetric phase separation. This method can make the pore size difference between the surface layer and the bottom layer larger within the limited membrane thickness of the battery separator, and thus can be better applied to the battery separator, achieving its high porosity and high strength synergy.

[0120] (3) Compared with Example 1, the membrane liquid absorption rate and puncture strength of Examples 7 and 8 are higher; compared with Example 8, the membrane puncture strength and tensile strength of Example 9 are lower. The reason for this is that, compared with Example 1, Examples 7 and 8 adopt a two-stage cooling and phase separation method. The rapid cooling and pressure reduction process in the first stage is conducive to inducing a large number of micropores to nucleate and form high-density small pores on the surface. The slow cooling and pressure reduction process in the second stage can promote the directional growth of pores and form large pores in the bottom layer. Moreover, compared with Example 8, the cooling rate in the first stage of Example 9 is too fast. The rapid phase change of supercritical CO2 will destroy the dynamic equilibrium of the PMP molecular chain and cause the pore structure to become unstable. The main reason is that the rapid CO2 desolvation causes the surface pore wall to solidify rapidly, forming microcracks.

[0121] (4) Compared with Examples 1, 3, and 4, the porous base membrane of Comparative Example 3 has lower porosity, lower membrane puncture strength and liquid absorption rate, and higher thermal shrinkage rate. The reason for this is that when the pressure is too low during the homogenization of the PMP particles and supercritical CO2, the supercritical CO2 is not fully dissolved, the PMP molecular chains are locally aggregated, and the pore cores are unevenly distributed during phase separation, resulting in large fluctuations in the surface pore size and poor connectivity of the bottom pores. This is not conducive to using the surface layer to improve the membrane puncture strength and high-temperature stability, and using the bottom layer to improve the membrane liquid absorption rate.

[0122] (5) Compared with Examples 1, 3, and 4, the porous base membrane of Comparative Example 4 has a larger bottom pore size, lower puncture strength and tensile strength, and a higher thermal shrinkage rate. The reason for this is that when the PMP particles are mixed and homogenized with supercritical CO2, when the pressure is too high, CO2 oversaturation causes excessive growth of the bottom pores and thinning of the pore walls, increasing the risk of dendrite penetration in the membrane and reducing the tensile strength. At the same time, the surface pores quickly collapse and close, resulting in a decrease in the thermal stability of the membrane.

[0123] (6) Compared with Example 1, Example 5, and Example 6, the porous base membrane of Comparative Example 5 has a lower porosity and a lower liquid absorption rate. The reason for this is that when the PMP particles are mixed and homogenized 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 stretched, which leads to uneven dissolution of CO2. During phase separation, the number of pore cores is small and sparsely distributed, resulting in a decrease in the porosity of the membrane and a decrease in the liquid absorption rate.

[0124] (7) Compared with Examples 1, 5 and 6, the puncture strength and tensile strength of the porous base membrane of Comparative Example 6 are lower. The reason for this is that when the temperature is too high during the homogenization of the PMP particles and supercritical CO2, the PMP molecular chains will be excessively disentangled, the interchain forces will be weakened, and the pore walls will become thinner and easy to merge during phase separation, resulting in an excessively high proportion of macropores and a decrease in puncture strength. At the same time, high temperature accelerates the relaxation of chain segments and reduces the crystalline area, resulting in a decrease in tensile strength. In addition, the presence of an appropriate amount of macropores can increase the liquid absorption rate of the membrane, but when the macropore ratio is too high and the pore size is too large, it will have an adverse effect on the liquid storage performance of the membrane. Therefore, although the porosity of Comparative Example 6 is improved, the liquid absorption rate is reduced.

[0125] (8) Compared with Example 1, the porosity of the porous basement membrane of Comparative Example 7 is lower. This indicates that when the pressure reduction rate is too slow during the phase separation process, it is not conducive to pore formation in the basement membrane.

[0126] (9) Compared with Example 1, the puncture strength and liquid absorption rate of the diaphragm of Example 10 are lower, and the interface impedance is higher. The reason is that: Example 1 can form a fast ion channel by coating an aluminum oxide layer and a polydopamine layer on the surface of the base film, thereby reducing the interface impedance of the diaphragm. In addition, after coating the aluminum oxide layer, plasma surface treatment is performed to introduce polar groups, which can promote Li + conduction, which also helps to reduce the interfacial impedance of the diaphragm; in addition, the PVDF-HFP in the alumina layer has good electrolyte affinity, which can improve the liquid absorption rate of the diaphragm; the alumina layer and PDA nanofibers can form a synergistic protection of rigid-flexible propyl, inhibiting the high-temperature shrinkage of the diaphragm, blocking dendrite penetration, and improving the puncture strength of the diaphragm.

[0127] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this disclosure pertains. Unless otherwise specified, the raw materials and equipment used herein are conventional in the art and can be obtained from conventional commercial sources. The methods used herein are conventional in the art, unless otherwise specified.

[0128] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a polytetramethylpentene-based lithium ion battery separator, characterized in that: The following steps are involved: In a closed container, polytetramethylpentene particles and supercritical carbon dioxide are mixed and homogenized at a temperature of 205-215°C and a pressure of 12-18 MPa to form a homogeneous solution, and then the temperature is reduced at a rate of 25-30°C / min to a pressure of 8-10 MPa in the container, and then the temperature is reduced at a rate of 5-10°C / min to a pressure of 2-5 MPa in the container to obtain a porous base membrane.

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

8.

3. The preparation method according to claim 1, characterized in that The following steps are also included: An aluminum oxide layer is coated on the surface of a porous base film, and after the surface is treated by plasma, a polydopamine layer is coated; the aluminum oxide layer comprises aluminum oxide and polyvinylidene fluoride-hexafluoropropylene copolymer.

4. The preparation method according to claim 3, characterized in that The aluminum oxide layer is coated by electrostatic spraying; the polydopamine layer is coated by ultrasonically initiating in-situ polymerization of dopamine monomer; during the plasma surface treatment, the plasma power is 80-100W, the gas atmosphere is nitrogen, and the treatment time is 60-120s.

5. The preparation method according to claim 3 or 4, characterized in that In the aluminum oxide layer, the average particle size of aluminum oxide 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%.

6. The preparation method according to claim 1, characterized in that In the porous base membrane, the surface pore size is 0.1-0.3 μm, and the bottom pore size is 2-5 μm.

7. The preparation method according to claim 3, characterized in that The total thickness of the polytetramethylpentene-based lithium-ion battery separator is 15-25 μm; the thicknesses of the aluminum oxide layer and the polydopamine layer are 5-10 μm and 0.5-1 μm respectively.

8. The preparation method according to claim 1, characterized in that The polytetramethylpentene particles are native 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 70,000-120,000 Da, and the mass proportion of the recycled polytetramethylpentene waste particles in the polytetramethylpentene particles is no more than 30%.

9. A lithium-ion battery, characterized in that: The invention comprises a polytetramethylpentene-based lithium ion battery separator prepared by the preparation method according to any one of claims 1 to 8.

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