CN120413985B - Preparation method and application of polytetramethylpentenyl lithium ion battery diaphragm - Google Patents
Preparation method and application of polytetramethylpentenyl lithium ion battery diaphragmInfo
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- CN120413985B CN120413985B CN202510919794.XA CN202510919794A CN120413985B CN 120413985 B CN120413985 B CN 120413985B CN 202510919794 A CN202510919794 A CN 202510919794A CN 120413985 B CN120413985 B CN 120413985B
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Abstract
本发明涉及电池隔膜技术领域，公开了一种聚四甲基戊烯基锂离子电池隔膜的制备方法和应用。该制备方法包括以下步骤：在密闭容器内，将聚四甲基戊烯颗粒与超临界二氧化碳在205‑215℃的温度和12‑18MPa的压力下进行混合均质化，形成均相溶液，而后降温至容器内压力不高于5MPa进行分相，起始降温速率≥15℃/min，得到多孔基膜。采用本发明的制备方法，能够使PMP基隔膜的基膜中，具有较高的孔隙率和合适的孔径分布，进而使隔膜兼具较高的吸液率、穿刺强度、拉伸强度和高温稳定性。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
The invention relates to the technical field of battery diaphragms, in particular to a preparation method and application of a polytetramethylpentenyl lithium ion battery diaphragm.
Background
The inherent short plates of traditional Polyethylene (PE) and polypropylene (PP) battery separators are increasingly prominent in novel battery systems, and comprise (1) the PE is easy to thermally fail, the PE has a melting temperature of about 135 ℃ (high-density polyethylene is about 140 ℃), the PP has a melting temperature of about 165 ℃, the separator is easy to shrink and the pore is closed to fail when the battery is locally overheated (such as fast charge), thermal runaway is caused, and (2) the PE and the PP have poor high-voltage compatibility, oxidation can occur in electrolyte with the voltage of more than 4.5V, and a by-product accelerates the thickening of an interface passivation layer (CEI), so that impedance is increased.
The molecular chain of polytetramethylpentene (PMP) has a helical structure, and higher rigidity can give thermodynamic advantage, and the PMP has a glass transition temperature (Tg) as high as 260 ℃ and a thermal expansion coefficient 50% lower than that of PP. Meanwhile, PMP has better chemical stability, and is not easy to swell in ester and ether electrolyte.
However, since PMP has high crystallinity, which makes it difficult to form micropores, closed pores are easily formed when a conventional phase separation pore-forming method is employed, resulting in a PMP-based separator having a low porosity (porosity < 30%) and a pore size distribution which is not easily controlled. For example, in patent application CN102804450a, pores are formed by thermally induced phase separation-biaxially stretching, and although more polyethylene and polypropylene are added to the separator, the PMP content is only 20-21wt%, the porosity of the separator is still only 32-37%.
Disclosure of Invention
The invention provides a preparation method and application of a polytetramethylene pentenyl lithium ion battery diaphragm, aiming at solving the technical problems that the PMP-based diaphragm has low porosity and the pore size distribution is not easy to control. By adopting the preparation method, the PMP-based membrane has higher porosity and proper pore size distribution in the base membrane, so that the membrane has higher liquid absorption rate, puncture strength, tensile strength and high-temperature stability.
The specific technical scheme of the invention is as follows:
In the first aspect, the invention provides a preparation method of a polytetramethylene pentenyl lithium ion battery diaphragm, which comprises the following steps of mixing and homogenizing polytetramethylene pentenyl particles and supercritical carbon dioxide in a closed container at a temperature of 205-215 ℃ and a pressure of 12-18MPa to form a homogeneous solution, cooling to a pressure of not higher than 5MPa in the container, carrying out phase separation, and obtaining a porous base film at an initial cooling rate of not less than 15 ℃.
In the process of preparing the PMP-based diaphragm, the method of reducing pressure and phase separation after supercritical CO 2 is adopted to form the base diaphragm with the pore diameter in a step distribution mode, and the specific temperature and pressure in the process of mixing and homogenizing PMP particles and supercritical CO 2 and the specific pressure reducing rate in the process of phase separation are matched, so that the prepared porous base diaphragm has higher porosity and proper pore diameter distribution, and the PMP-based diaphragm has higher liquid absorption rate, puncture strength, tensile strength and high-temperature stability. Specifically:
① The method of reducing pressure and phase separation after supercritical CO 2 is adopted to form a base film with aperture step distribution:
The invention utilizes supercritical carbon dioxide to form a homogeneous solution with high solubility of PMP, wherein CO 2 is uniformly dispersed among PMP molecular chains, then in the depressurization process, the pressure-temperature cooperative regulation (by cooling in a closed container, depressurization is synchronously realized), the difference of desorption rate of CO 2 from chains is utilized, the rigidity of PMP spiral structure molecular chains (blocking pore combination during phase separation, improving the stability of surface pores, and the bottom layer forms macropores due to chain relaxation delay) is matched, 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, thereby realizing the cooperation of high porosity and high strength, namely, utilizing the surface layer with smaller pore diameter, inhibiting lithium dendrites, improving the puncture strength of a diaphragm, simultaneously reducing the thermal shrinkage rate of the diaphragm, improving the high temperature stability, reducing the risk of short circuit caused by diaphragm shrinkage, supporting the safety of a battery under a thermal runaway condition, and utilizing the bottom layer with larger pore diameter, the electrolyte can be stored, and the liquid absorption rate of the diaphragm is improved.
Compared with the method that supercritical CO 2 is filled into a polymer melt, the method forms a CO 2 concentration gradient in the polymer melt depending on the melt viscosity gradient, and then forms a gradient pore structure, in the method, supercritical CO 2 and PMP are mixed and homogenized to form a homogeneous solution, and the gradient pore structure is formed by utilizing asymmetric phase separation, so that the pore diameter difference between a surface layer and a bottom layer is larger in the limited thickness of a membrane layer of a battery membrane, and the method can be better applied to the battery membrane, and the high porosity and high strength synergy of the battery membrane is realized.
② Temperatures of 205-215 ℃ were used during the mixing homogenization process:
The temperature for mixing and homogenizing PMP particles and supercritical carbon dioxide is controlled at 205-215 ℃, so that the diaphragm has higher liquid absorption rate, tensile strength and puncture strength. When the temperature of mixing homogenization is too low, the melting of PMP is not facilitated, the PMP molecular chain is not fully stretched, the CO 2 is not uniformly dissolved, the number of pore cores is small and the distribution is sparse during phase separation, the porosity of a diaphragm is reduced, the liquid absorption rate is reduced, meanwhile, a mechanical weak point is formed in an unmelted area, and the fluctuation of tensile strength is increased. When the temperature of mixing homogenization is too high, PMP molecular chains can be excessively unwound, the action force between the chains is weakened, the pore walls are thinned and are easy to combine during phase separation, the proportion of macropores is too high, the puncture strength is reduced, meanwhile, the chain segment is accelerated to relax at high temperature, the crystallization area is reduced, and the tensile strength is reduced.
③ The pressure of 12-18MPa is adopted in the mixing and homogenizing process:
When the pressure in the mixing homogenization process is too low, supercritical CO 2 is insufficiently dissolved, PMP molecular chains are locally aggregated, pore nuclei are unevenly distributed in phase separation, so that the surface layer has large pore diameter fluctuation, the bottom layer has poor pore connectivity, the membrane puncture strength and high-temperature stability are not facilitated to be improved by using the surface layer, and the membrane liquid absorption rate is improved by using the bottom layer. When the pressure in the mixing homogenization process is too high, the CO 2 is supersaturated to cause overgrowth of bottom layer holes, the hole wall is thinned, the risk of diaphragm dendrite penetration is increased, the tensile strength is reduced, meanwhile, the surface layer holes collapse and close rapidly, and the thermal stability of the diaphragm is reduced. The invention can ensure that the diaphragm has higher liquid absorption rate, puncture strength and high-temperature stability by controlling the mixing homogenization pressure to be 12-18 MPa.
④ The initial cooling rate of more than or equal to 15 ℃ per minute is adopted for phase separation:
In the phase separation process, when the initial cooling rate is too slow, the porosity of the diaphragm is too low, so that the application of the diaphragm can only be limited to a low-rate scene.
Preferably, the specific process of cooling to the pressure not higher than 5MPa in the container for phase separation comprises cooling to the pressure of 8-10MPa in the container at the speed of 25-30 ℃ per minute, and cooling to the pressure of 2-5MPa in the container at the speed of 5-10 ℃ per minute.
The two-stage cooling and phase splitting mode has the advantages that the first stage is fast in cooling and pressure reduction (25-30 ℃ per min) to 8-10MPa, which is favorable for inducing a large number of micropores to nucleate, high-density micropores are formed in the surface layer, and the second stage is slow in cooling and pressure reduction (5-10 ℃ per min) to 2-5MPa, which can promote directional growth of the holes and form macropores in the bottom layer. When the first stage cooling rate is lower than 25 ℃ per minute or the second stage cooling rate is higher than 10 ℃ per minute, the realization of the effects is not facilitated, in addition, when the first stage cooling rate is higher than 30 ℃ per minute, the rapid phase transition of supercritical CO 2 can damage the dynamic balance of PMP molecular chains, the destabilization of a pore structure is initiated, the main reasons are that the too fast CO 2 desolventizing leads to the rapid solidification of the pore wall of the surface layer to form micro cracks, and when the second stage cooling rate is lower than 5 ℃ per minute, the growth of the pores is not facilitated, and the porosity of the membrane is lower.
Preferably, the mass ratio of the polytetramethylpentene particles to the supercritical carbon dioxide is 1:5-1:8.
Preferably, the preparation method further comprises the steps of coating an aluminum oxide layer on the surface of the porous base film, carrying out plasma surface treatment, and then coating a Polydopamine (PDA) layer, wherein the aluminum oxide layer comprises aluminum oxide and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP).
The low polarity surface of the PMP separator creates a higher interfacial charge transport barrier between it and the electrode, resulting in a higher interfacial impedance. According to the invention, the aluminum oxide layer and the polydopamine layer are coated on the surface of the base film, so that a rapid ion channel can be formed by utilizing the aluminum oxide layer and a PDA polar network, and the interface impedance of the diaphragm is reduced. In addition, the invention is subjected to plasma surface treatment after the aluminum oxide layer is coated, so that polar groups can be introduced, the conduction of Li + can be promoted, and the interface impedance of the diaphragm can be reduced.
In addition, the PVDF-HFP in the alumina layer has better electrolyte affinity, can improve the liquid absorption rate of the diaphragm, and the alumina layer and the PDA nanofiber can form a synergistic protection of rigid flexible propyl, inhibit the high-temperature shrinkage of the diaphragm, block dendrite penetration and improve the puncture strength of the diaphragm.
Preferably, the aluminum oxide layer is coated by electrostatic spraying, the polydopamine layer is coated by ultrasonic initiation of in-situ polymerization of dopamine monomers, and in the plasma surface treatment process, the plasma power is 80-100W, the gas atmosphere is nitrogen, and the treatment time is 60-120s.
Preferably, in the alumina layer, the average particle diameter of the alumina is 50-150nm, the mass ratio is 70-80%, and in the comonomer of the polyvinylidene fluoride-hexafluoropropylene copolymer, the mass ratio of hexafluoropropylene is 10-15%.
Preferably, in the porous base film, the pore diameter of the surface layer is 0.1-0.3 μm, and the pore diameter of the bottom layer is 2-5 μm.
Preferably, the total thickness of the polytetramethylene pentenyl lithium ion battery diaphragm is 15-25 mu m, and the thicknesses of the aluminum oxide layer and the polydopamine layer are 5-10 mu m and 0.5-1 mu m respectively.
Preferably, the polytetramethylene pentene particles are virgin polytetramethylene pentene particles and/or recycled polytetramethylene pentene waste particles subjected to microwave depolymerization treatment, wherein the number average molecular weight of the recycled polytetramethylene pentene waste particles subjected to microwave depolymerization treatment is 70000-120000Da, and the mass ratio of the polytetramethylene pentene particles is not higher than 30%.
Preferably, in the process of the microwave depolymerization treatment, the microwave frequency is 2400-2500MHz, and the time is 20-30min.
In a second aspect, the invention provides a lithium ion battery, comprising a polytetramethylpentaenyl lithium ion battery diaphragm prepared by the preparation method.
Compared with the prior art, the invention has the following advantages:
(1) In the process of preparing the PMP-based diaphragm base film, the method of depressurization and phase separation after ① is adopted for dissolution of supercritical CO 2 is adopted to form the base film + ② with pore diameter in step distribution, the temperature of 205-215 ℃ is adopted in the mixing homogenization process + ③, the pressure of 12-18MPa is adopted in the mixing homogenization process + ④, and the initial cooling rate of more than or equal to 15 ℃ per minute is adopted for phase separation, so that the diaphragm has higher liquid absorption rate, puncture strength, tensile strength and high-temperature stability.
(2) In the process of preparing the PMP-based diaphragm base film, the invention adopts a two-stage cooling and phase-splitting mode and adopts a specific cooling rate at each stage, thereby being beneficial to better forming a base film structure with high-density small holes on the surface layer and large holes on the bottom layer, and further improving the liquid absorption rate and puncture strength of the diaphragm to a greater extent.
(3) According to the invention, the alumina layer, the plasma surface treatment and the polydopamine layer are sequentially coated on the surface of the porous base membrane, so that the prepared PMP-based membrane has lower interface impedance, higher liquid absorption rate, higher high-temperature stability and higher puncture strength.
Detailed Description
The invention is further described below with reference to examples.
A preparation method of a polytetramethylene pentenyl lithium ion battery diaphragm comprises the following steps of mixing and homogenizing polytetramethylene pentenyl particles and supercritical carbon dioxide in a closed container at a temperature of 205-215 ℃ and a pressure of 12-18MPa to form a homogeneous solution, cooling to a pressure of not higher than 5MPa in the container for phase separation, and obtaining a porous base film, wherein the initial cooling rate is not less than 15 ℃ per minute.
In some specific embodiments, the cooling to the pressure not higher than 5MPa in the container for phase separation comprises cooling to the pressure of 8-10MPa in the container at a rate of 25-30 ℃ per min, and cooling to the pressure of 2-5MPa in the container at a rate of 5-10 ℃ per min.
In some embodiments, the mass ratio between the polytetramethylpentene particles and the supercritical carbon dioxide is in the range of 1:5 to 1:8.
In some embodiments, the pore size of the top layer in the porous base membrane is 0.1-0.3 μm and the pore size of the bottom layer is 2-5 μm.
In some specific embodiments, the polytetramethylene pentene particles are virgin polytetramethylene pentene particles and/or recycled polytetramethylene pentene waste particles subjected to microwave depolymerization treatment, wherein the recycled polytetramethylene pentene waste particles subjected to microwave depolymerization treatment have a number average molecular weight of 70000-120000Da and a mass ratio in the polytetramethylene pentene particles of not more than 30%. In this particular embodiment, optionally or preferably:
in the microwave depolymerization treatment process, the microwave frequency is 2400-2500MHz, and the time is 20-30min.
In some specific embodiments, the preparation method further comprises the step of coating an aluminum oxide layer on the surface of the porous base film, and coating a polydopamine layer after plasma surface treatment, wherein the aluminum oxide layer comprises aluminum oxide and polyvinylidene fluoride-hexafluoropropylene copolymer. In this particular embodiment, optionally or preferably:
the method for coating the polydopamine layer is that ultrasonic initiates in-situ polymerization of dopamine monomers;
In the plasma surface treatment process, the plasma power is 80-100W, the gas atmosphere is nitrogen, and the treatment time is 60-120s;
In the alumina layer, the average grain diameter of the alumina is 50-150nm, the mass ratio is 70-80%, and in the comonomer of the polyvinylidene fluoride-hexafluoropropylene copolymer, the mass ratio of hexafluoropropylene is 10-15%;
the total thickness of the polytetramethylene pentenyl lithium ion battery diaphragm is 15-25 mu m, and the thicknesses of the aluminum oxide layer and the polydopamine layer are 5-10 mu m and 0.5-1 mu m respectively.
A lithium ion battery comprises the polytetramethylpentanenyl lithium ion battery diaphragm prepared by the preparation method.
The invention is illustrated by the following specific examples. It is to be understood that these embodiments are merely for illustrating the present invention and are not to be construed as limiting the scope of the present invention, and that variations and advantages which can be conceived by those skilled in the art are included therein without departing from the spirit and scope of the inventive concept, and the appended claims and any equivalents thereof are intended to be protected by the present invention.
In the following examples and comparative examples, the meanings of the abbreviations are as follows:
PMP is polytetramethylpentene;
PDA is polydopamine;
PVDF-HFP, polyvinylidene fluoride-hexafluoropropylene copolymer;
HFP is hexafluoropropylene;
m n number average molecular weight.
Example 1
The PMP-based lithium ion battery separator is prepared by the following steps:
S1, forming a porous base film
Mixing and homogenizing primary PMP particles (with a melt index of 3g/10 min) with supercritical CO 2 in a mass ratio of 1:6 in a closed container at a temperature of 210 ℃ and a pressure of 15MPa to form a homogeneous solution, and then cooling to a pressure of 5MPa in the container at a rate of 20 ℃ per min to separate phases of PMP and CO 2 to obtain the porous base membrane.
S2 coating an alumina layer
After acetone and DMAC were mixed in a volume ratio of 7:3, nano alumina (average particle diameter of 100 nm) and PVDF-HFP (HFP mass ratio of 12% in comonomer) in a mass ratio of 4:1 were dispersed therein to form an alumina layer coating having an alumina content of 3.8 wt%. Setting the spraying voltage to be 30kV and the flow rate of the coating to be 0.5mL/h in an electrostatic spraying mode, uniformly spraying the coating of the alumina layer on two sides of the porous base film, and drying to obtain the alumina/PMP composite diaphragm.
S3 plasma surface treatment
And (3) carrying out plasma surface treatment on the surfaces of both sides of the aluminum oxide/PMP composite membrane in a nitrogen atmosphere, wherein the plasma power is 100W, and the treatment time is 60s, so as to obtain the surface-activated aluminum oxide/PMP composite membrane.
S4, coating PDA layer
Immersing the surface-activated alumina/PMP composite membrane in a dopamine/Tris buffer solution (pH=8.5, and the concentration of dopamine is 3 mg/mL), setting the ultrasonic power to be 40kHz, performing ultrasonic auxiliary reaction for 2 hours, and drying to form a PDA layer to obtain the PMP-based lithium ion battery membrane.
The average thickness of the PMP-based lithium ion battery diaphragm prepared in the embodiment is 10 μm, and the average thickness of the alumina layer and the polydopamine layer is 5 μm and 1 μm respectively.
Example 2
This example differs from example 1 only in that in step S1, a porous base film was prepared by compounding virgin PMP particles and recovered PMP waste particles subjected to microwave depolymerization treatment, and the remaining raw materials and steps were the same as in example 1. Specifically, this example prepared a PMP-based lithium ion battery separator by the steps of:
S1, forming a porous base film
Setting the microwave power to 2450MHz, carrying out microwave depolymerization on the recovered PMP waste (medical waste) particles for 25min to obtain a recovered PMP waste particle depolymerization product with the M n =85000 Da, and mixing the recovered PMP waste particle depolymerization product with the raw PMP particles (with the melting point of 3g/10 min) according to the mass ratio of 3:7 to obtain the PMP particle mixture. And (2) mixing and homogenizing PMP particle mixture with the mass ratio of 1:6 with supercritical CO 2 in a closed container at the temperature of 210 ℃ and the pressure of 15MPa to form a homogeneous solution, and then cooling to the pressure of 5MPa in the container at the speed of 20 ℃ per minute to separate the PMP from the CO 2 to obtain the porous base film.
S2 coating an alumina layer
After acetone and DMAC were mixed in a volume ratio of 7:3, nano alumina (average particle diameter of 100 nm) and PVDF-HFP (HFP mass ratio of 12% in comonomer) in a mass ratio of 4:1 were dispersed therein to form an alumina layer coating having an alumina content of 3.8 wt%. Setting the spraying voltage to be 30kV and the flow rate of the coating to be 0.5mL/h in an electrostatic spraying mode, uniformly spraying the alumina layer coating on the surface of the porous base film, and drying to obtain the alumina/PMP composite membrane.
S3 plasma surface treatment
And (3) carrying out plasma surface treatment on the surfaces of both sides of the aluminum oxide/PMP composite membrane in a nitrogen atmosphere, wherein the plasma power is 100W, and the treatment time is 60s, so as to obtain the surface-activated aluminum oxide/PMP composite membrane.
S4, coating PDA layer
Immersing the surface-activated alumina/PMP composite membrane in a dopamine/Tris buffer solution (pH=8.5, and the concentration of dopamine is 3 mg/mL), setting the ultrasonic power to be 40kHz, performing ultrasonic auxiliary reaction for 2 hours, and drying to form a PDA layer to obtain the PMP-based lithium ion battery membrane.
The average thickness of the PMP-based lithium ion battery diaphragm prepared in the embodiment is 12 μm, and the average thickness of the alumina layer and the polydopamine layer is 5 μm and 1 μm respectively.
Example 3
The present example differs from example 1 only in that in step S1, the pressure during the mixing and homogenizing process was changed from 15MPa to 12MPa, and the remaining raw materials and steps were the same as in example 1.
Example 4
The present example differs from example 1 only in that in step S1, the pressure during the mixing and homogenizing process was changed from 15MPa to 18MPa, and the remaining raw materials and steps were the same as in example 1.
Example 5
The present example differs from example 1 only in that in step S1, the temperature during the mixing and homogenizing process was changed from 210 ℃ to 205 ℃, and the remaining raw materials and steps were the same as in example 1.
Example 6
The present example differs from example 1 only in that in step S1, the temperature during the mixing and homogenizing process was changed from 210 ℃ to 215 ℃, and the remaining raw materials and steps were the same as in example 1.
Example 7
The difference between this example and example 1 is that in step S1, the process of cooling and phase separation is divided into two stages, and the rest of the raw materials and steps are the same as in example 1. Specifically, the present embodiment changes step S1 to:
Mixing and homogenizing primary PMP particles (with a melt index of 3g/10 min) with supercritical CO 2 at a mass ratio of 1:6 in a closed container at a temperature of 210 ℃ and a pressure of 15MPa to form a homogeneous solution, cooling to 10MPa at a rate of 25 ℃ per min, cooling to 5MPa at a rate of 5 ℃ per min, and separating phases of PMP and CO 2 to obtain the porous base film.
Example 8
The difference between this example and example 1 is that in step S1, the process of cooling and phase separation is divided into two stages, and the remaining materials and steps are the same as those in example 1. Specifically, the present embodiment changes step S1 to:
Mixing and homogenizing primary PMP particles (with a melt index of 3g/10 min) with supercritical CO 2 at a mass ratio of 1:6 in a closed container at a temperature of 210 ℃ and a pressure of 15MPa to form a homogeneous solution, cooling to 8MPa at a rate of 30 ℃ per min, cooling to 2MPa at a rate of 10 ℃ per min, and separating phases of PMP and CO 2 to obtain the porous base film.
Example 9
The difference between this example and example 8 is that in step S1, the cooling rate in the cooling and phase-splitting process in the first stage is changed from 30 ℃ to 35 ℃ and the rest of the raw materials and steps are the same as in example 1.
Example 10
This example differs from example 1 only in that steps S2 to S4 are not performed, the porous base film produced in step S1 is used as a PMP-based lithium ion battery separator, and the process for producing the porous base film is the same as that of example 1.
Comparative example 1
This comparative example differs from example 1 only in that a porous base film was produced by a conventional thermally induced phase separation-biaxially oriented method, and the remaining raw materials and steps were the same as in example 1. Specifically, this comparative example changes step S1 to:
The raw PMP particles (with a melting finger of 3g/10 min) with a mass ratio of 1:3 are melted and blended with paraffin oil at 200 ℃, then cast and molded, cooled to room temperature for solidification, then longitudinally stretched at 150 ℃, the stretching ratio is 3:1, and then transversely stretched at 155 ℃, and the stretching ratio is 4:1. After the stretching is completed, paraffin oil is removed by extraction with normal hexane, drying is carried out, heat treatment is carried out for 5min at 160 ℃, and the porous base film is obtained after cooling to room temperature.
Comparative example 2
The comparative example differs from example 1 only in that in step S1, the manner of forming the gradient pore structure is changed, and in that supercritical CO 2 is charged into the polymer melt, the gradient pore structure with smaller surface layer pore diameter and larger bottom layer pore diameter is formed by forming the concentration gradient of CO 2 in the polymer melt depending on the melt viscosity gradient, and the other raw materials and steps are the same as those of example 1. Specifically, this comparative example changes step S1 to:
heating the primary PMP particles to 210 ℃ in a closed container, melting, filling supercritical CO 2 into the container until the air pressure is 15MPa, maintaining for 20min, and opening the closed container to release pressure to normal pressure to obtain the porous base membrane.
Comparative example 3
The comparative example differs from example 1 only in that the pressure during the mixing and homogenizing process was changed from 15MPa to 10MPa in step S1, and the remaining raw materials and steps were the same as in example 1.
Comparative example 4
The comparative example differs from example 1 only in that the pressure during the mixing and homogenizing process was changed from 15MPa to 20MPa in step S1, and the remaining raw materials and steps were the same as in example 1.
Comparative example 5
The comparative example differs from example 1 only in that the temperature during the mixing and homogenizing process was changed from 210 to 190 ℃ in step S1, and the remaining raw materials and steps were the same as in example 1.
Comparative example 6
The comparative example differs from example 1 only in that the temperature during the mixing and homogenizing process was changed from 210 to 230 ℃ in step S1, and the remaining raw materials and steps were the same as in example 1.
Comparative example 7
The comparative example differs from example 1 only in that the depressurization rate during the phase separation was changed from 20℃/min to 10℃/min in step S1, and the remaining raw materials and steps were the same as those in example 1.
Test case
The porous base film and the PMP-based lithium ion battery separator prepared according to the methods in each example and comparative example were taken, and the separator was assembled into NCM811// Si-C soft-pack lithium ion battery. The average pore size of the surface layer (upper surface layer), the average pore size of the bottom layer (lower surface layer) and the porosity of the porous base film as a whole were examined, and the results are shown in table 1. The puncture strength, tensile strength, heat shrinkage after heating at 200 ℃ for 1 hour, liquid absorption at 25 ℃ after soaking in an electrolyte for 30min, and interface impedance between the separator and the positive electrode of NCM811 in the NCM811// Si-C soft-pack lithium ion battery system were measured for each PMP-based lithium ion battery separator, and the results are shown in Table 2.
TABLE 1 porous base film Performance test results
TABLE 2 diaphragm Performance test results
As can be seen from tables 1 and 2:
(1) The porous base film in example 1-example 9 had a structure in which the pore diameter of the surface layer was smaller and the pore diameter of the bottom layer was larger, and the porous base film in comparative example 1 had the same pore diameters of the surface layer and the bottom layer, and the porous base film in comparative example 1 had a lower porosity, and a lower puncture strength, tensile strength and liquid absorption of the separator, and a higher heat shrinkage than those in example 1-example 9. The analysis is that the method of reducing pressure and splitting phase after dissolving supercritical CO 2 is adopted in the embodiment 1-the embodiment 9, so that a structure with smaller surface layer aperture and larger bottom layer aperture is formed in the porous base film, high-porosity and high-strength synergy can be realized, lithium dendrite can be restrained by using the surface layer with smaller aperture, the puncture strength of the diaphragm can be improved, the thermal shrinkage rate of the diaphragm can be reduced, the high-temperature stability of the diaphragm can be improved, and electrolyte can be stored by using the bottom layer with larger aperture, and the liquid absorption rate of the diaphragm can be improved.
(2) The porous base film of comparative example 2 has a smaller difference in pore diameter between the surface layer and the bottom layer, and has a lower puncture strength, tensile strength and liquid absorption, and a higher heat shrinkage, than those of examples 1 to 9. The analysis is that compared with comparative example 2, in the way that supercritical CO 2 is filled into a polymer melt, a concentration gradient of CO 2 is formed in the polymer melt depending on a melt viscosity gradient, and then a gradient pore structure is formed, in the way that in the examples 1-9, supercritical CO 2 is mixed and homogenized with PMP to form a homogeneous solution, then in the depressurization process, the asymmetric phase separation is induced by utilizing the difference of desorption rate of CO 2 from chains and the rigidity of PMP helical structure molecular chains through pressure-temperature cooperative regulation, and the way can lead the pore diameter difference of a surface layer and a bottom layer to be larger in the limited film thickness of a battery diaphragm, so that the method can be better suitable for the battery diaphragm, and the synergy of high porosity and high strength of the battery diaphragm is realized.
(3) The barrier films of examples 7-8 had higher absorption and puncture strength than example 1, and the barrier film of example 9 had lower puncture strength and tensile strength than example 8. The analysis is that compared with the embodiment 1, the embodiment 7-8 adopts a two-stage cooling and phase splitting mode, the rapid cooling and depressurization process in the first stage is favorable for inducing a large number of micropores to nucleate, high-density small holes are formed on the surface layer, the rapid cooling and depressurization process in the second stage can promote directional growth of the holes to form bottom macropores, and compared with the embodiment 8, the rapid cooling rate in the first stage in the embodiment 9 is too rapid, the rapid phase transition of supercritical CO 2 can damage the PMP molecular chain dynamic balance, the instability of the hole structure is caused, and the main reason is that the wall of the surface layer is rapidly solidified due to the too rapid desolventizing of CO 2 to form microcracks.
(4) The porous base film of comparative example 3 has lower porosity, lower puncture strength and liquid absorption, and higher heat shrinkage than those of examples 1, 3 and 4. The analysis is that in the process of mixing and homogenizing PMP particles and supercritical CO 2, when the pressure is too low, supercritical CO 2 is insufficiently dissolved, PMP molecular chains are locally aggregated, pore cores are unevenly distributed in phase separation, so that the surface layer pore diameter fluctuation is large, the bottom layer pore connectivity is poor, the membrane puncture strength and high-temperature stability are not improved by utilizing the surface layer, and the membrane liquid absorption rate is improved by utilizing the bottom layer.
(5) The porous base film of comparative example 4 has a larger pore size, lower puncture strength and tensile strength of the separator, and higher heat shrinkage than those of examples 1, 3, and 4. The analysis is that during the process of mixing and homogenizing PMP particles and supercritical CO 2, when the pressure is too high, the CO 2 is supersaturated to cause overgrowth of bottom layer holes, thinning of hole walls, increasing the risk of diaphragm dendrite penetration, reducing the tensile strength, and simultaneously rapidly collapsing and closing surface layer holes, resulting in reduction of the thermal stability of the diaphragm.
(6) The porous base film of comparative example 5 has lower porosity and lower separator wicking than those of examples 1, 5 and 6. The analysis is that in the process of mixing and homogenizing PMP particles and supercritical CO 2, when the temperature is too low, the PMP is not easy to melt, the PMP molecular chain is not fully stretched, the CO 2 is not uniformly dissolved, the number of pore cores is small and the distribution is sparse in the phase separation process, the porosity of the diaphragm is reduced, and the liquid absorption rate is reduced.
(7) The porous base film of comparative example 6 was lower in puncture strength and tensile strength than those of examples 1, 5 and 6. The reason for analysis is that in the process of mixing and homogenizing PMP particles and supercritical CO 2, when the temperature is too high, excessive disentanglement of PMP molecular chains is caused, the action force between chains is weakened, the pore walls are thinned and are easy to combine in the phase separation process, so that the proportion of macropores is too high, the puncture strength is reduced, meanwhile, the high-temperature accelerating chain segments are relaxed, the crystallization area is reduced, the tensile strength is reduced, in addition, the existence of a proper amount of macropores can improve the liquid absorption rate of the diaphragm, but when the proportion of macropores is too high and the pore diameter is too large, the liquid storage performance of the diaphragm is adversely affected, so that the porosity of comparative example 6 is improved, but the liquid absorption rate is reduced.
(8) The porous base film of comparative example 7 has a lower porosity than that of example 1. It is shown that pore formation in the base film is not favored when the depressurization rate is too slow during phase separation.
(9) The membrane penetration strength and wicking rate were lower and the interfacial resistance was higher in example 10 than in example 1. The analysis is that the method is characterized in that in the embodiment 1, the alumina layer and the polydopamine layer are coated on the surface of the base film, a rapid ion channel can be formed by utilizing the alumina layer and a PDA polar network, so that the interface impedance of the diaphragm is reduced, a plasma surface treatment is carried out after the alumina layer is coated, polar groups can be introduced, li + conduction can be promoted, the interface impedance of the diaphragm is reduced, in addition, PVDF-HFP in the alumina layer has better electrolyte affinity, the liquid absorption rate of the diaphragm can be improved, and the alumina layer and PDA nanofiber can form a synergistic protection of rigid-flexible propyl, inhibit the high-temperature shrinkage of the diaphragm, block dendrite penetration and improve the puncture strength of the diaphragm.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The raw materials and equipment used in the invention are conventional in the art, and can be obtained from conventional commercial sources unless otherwise specified, and the methods used in the invention are conventional in the art.
The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and any simple modification, variation and equivalent transformation of the above embodiment according to the technical substance of the present invention still fall within the scope of the technical solution of the present invention.
Claims (9)
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