A porous membrane, its preparation method and use

Abstract

The application provides a porous membrane, a preparation method and application thereof. The porous membrane comprises a first porous membrane layer, a second porous membrane layer and a third porous membrane layer which are sequentially stacked; the first porous membrane layer comprises first through holes, the second porous membrane layer comprises second through holes, and the third porous membrane layer comprises third through holes; the first through holes, the second through holes and the third through holes are coaxially arranged, and the pore diameter of the second through holes is larger than that of the first through holes and the third through holes. The porous membrane has a specific structure, so that the contact probability of the positive and negative electrodes and the self-discharge phenomenon of the battery are reduced, the safety performance of the battery is higher, the structure that the pore diameter of the middle layer of the porous membrane is large and the pore diameters of the upper and lower surface layers are small is beneficial to the storage of the electrolyte, the liquid retention rate of the separator is improved, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of materials and relates to a porous membrane, its preparation method, and its application. Background Technology

[0002] Commercially available lithium-ion batteries currently primarily use organic liquid electrolytes. However, due to the flammability, leakage, and volatility of organic solvents, potential risks such as overcharging and short circuits pose certain safety hazards to liquid organic electrolyte lithium-ion batteries during use. All-solid-state lithium batteries hold the promise of solving the safety issues associated with liquid lithium batteries, including electrolyte leakage, combustion, and short circuits caused by lithium dendrites piercing the separator.

[0003] Currently, the mainstream solid electrolyte membranes are usually obtained by drilling holes in the thin film. Conventional laser drilling uses a high-power-density laser beam to irradiate the material being processed, quickly heating it to its vaporization temperature and causing it to evaporate and form pores. However, the pores formed by direct drilling have high permeability, resulting in porous membranes with poor liquid retention when used as separators, which can easily lead to high battery short-circuit rates and significant self-discharge. Summary of the Invention

[0004] This invention provides a porous membrane with a specific composite structure and pore structure. When applied to an electrolyte membrane, this membrane helps to reduce the contact probability between the positive and negative electrodes and the self-discharge phenomenon of the battery, and can improve the liquid retention rate of the separator, thereby improving the safety performance and cycle performance of the battery.

[0005] The present invention also provides a method for preparing a porous membrane, which uses ultraviolet laser to form a porous membrane by drilling, which can efficiently prepare a porous membrane, and the porous membrane has high surface flatness and mechanical strength while satisfying a specific pore structure.

[0006] The present invention also provides an electrolyte membrane and a battery. Since the electrolyte membrane includes the above-mentioned porous membrane, it has a high liquid retention rate and can avoid the self-discharge phenomenon of the battery and short circuit caused by the contact of positive and negative electrodes, so that the battery has high safety performance and cycle performance.

[0007] This invention provides a porous membrane, comprising a first porous membrane layer, a second porous membrane layer, and a third porous membrane layer stacked sequentially.

[0008] The first porous membrane layer includes a first through-hole, the second porous membrane layer includes a second through-hole, and the third porous membrane layer includes a third through-hole;

[0009] The first through hole, the second through hole, and the third through hole are coaxially arranged, and the diameter of the second through hole is larger than the diameters of the first through hole and the third through hole.

[0010] The porous membrane as described above, wherein the thickness of the second porous membrane layer is 1 to 10 times the thickness of the first porous membrane layer and / or the third porous membrane layer.

[0011] In the porous membrane described above, the ratio of the diameter of the first through-hole and / or the third through-hole to the diameter of the second through-hole is greater than or equal to 0.1 and less than 1.

[0012] The porous membrane as described above, wherein the second porous membrane layer includes an ultraviolet absorber;

[0013] Preferably, the mass content of the ultraviolet absorber in the second porous membrane layer is 0.1% to 2%.

[0014] The porous membrane described above, wherein the materials of the first porous membrane layer, the second porous membrane layer and the third porous membrane layer each independently include one or more of polyethylene terephthalate, polyimide, polybutylene terephthalate, polyethylene naphthalate, polyetheretherketone, polyphenylene sulfide, and polybenzimidazole.

[0015] A second aspect of the present invention provides a method for preparing a porous membrane as described above, comprising the following steps:

[0016] 1) At least three layers of polymer melt are co-extruded, cast, or blow-molded to obtain a composite film; wherein the polymer melt in the middle layer has a stronger UV absorption capacity than the polymer melt in the surface layer;

[0017] 2) The composite membrane is laser-drilled using an ultraviolet laser light source to obtain the porous membrane.

[0018] In the preparation method described above, the content of ultraviolet absorber in the polymer melt located in the intermediate layer is greater than that in the polymer melt located in the surface layer;

[0019] Preferably, the mass content of the ultraviolet absorber in the intermediate layer is 0.1% to 2%.

[0020] In the preparation method described above, the ultraviolet laser source is selected from one or more of rare gas excimer lasers, rare gas halide excimer lasers, and halogen gas excimer lasers;

[0021] And / or, the power of the single light source for the laser drilling process is 0.5W to 20W.

[0022] A third aspect of the present invention provides an electrolyte membrane, including the porous membrane provided in the first aspect of the present invention or a porous membrane prepared by the preparation method provided in the second aspect of the present invention.

[0023] A fourth aspect of the present invention provides a battery comprising the electrolyte membrane provided in the third aspect of the present invention.

[0024] The implementation of this invention has at least the following advantages:

[0025] 1) The porous membrane provided by the present invention has a composite structure of a first porous membrane layer, a second porous membrane layer and a third porous membrane layer stacked in sequence. The pore size of the second through hole in the middle second porous membrane layer is larger than the pore size of the first through hole in the first porous membrane layer and the third through hole in the third porous membrane layer, which are arranged coaxially with it. This allows the pores of the porous membrane to have a certain degree of tortuosity. When applied to an electrolyte membrane, it can reduce the contact probability of the positive and negative electrodes and the self-discharge phenomenon of the battery, thereby giving the battery higher safety performance. Furthermore, the structure of the porous membrane with large pores in the middle layer and small pores in the upper and lower surface layers is beneficial to the storage of electrolyte, which is conducive to improving the liquid retention rate of the separator and improving the cycle performance of the battery.

[0026] 2) The porous membrane preparation method provided by the present invention can efficiently prepare porous membranes by selecting a membrane material with strong ultraviolet absorption capacity as an intermediate layer and then using ultraviolet laser drilling. Furthermore, due to the low thermal effect of ultraviolet laser, melt accumulation on the surface of the porous membrane can be avoided, resulting in a porous membrane with high surface flatness and mechanical strength.

[0027] 3) The electrolyte membrane provided by the present invention includes a porous membrane with a certain porosity, so the membrane can reduce the contact probability of the positive and negative electrodes and the self-discharge phenomenon of the battery, thereby giving the battery higher safety performance. In addition, the middle layer of the porous membrane has a larger pore size than the upper and lower surface layers, which is more conducive to the storage of electrolyte.

[0028] 4) The battery provided by this invention has the advantages of high safety performance and good cycle performance. Attached Figure Description

[0029] Figure 1 This is a 30,000x SEM image of the cross-section of the porous membrane in Embodiment 2 of the present invention;

[0030] Figure 2 This is a 100,000x SEM image of the porous membrane cross-section of Comparative Example 1 of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] The first aspect of the present invention provides a porous membrane, comprising a first porous membrane layer, a second porous membrane layer and a third porous membrane layer stacked sequentially;

[0033] The first porous membrane layer includes a first through-hole, the second porous membrane layer includes a second through-hole, and the third porous membrane layer includes a third through-hole;

[0034] The first through hole, the second through hole, and the third through hole are coaxially arranged, and the diameter of the second through hole is larger than the diameters of the first through hole and the third through hole.

[0035] The porous membrane of the present invention increases the internal tortuosity of the pores in the porous membrane by limiting the pore diameter of the second through-hole in the middle second porous membrane layer to be larger than the pore diameter of the first through-hole and the third through-hole. The larger pore diameter in the second through-hole in the middle layer is beneficial for storing electrolyte, thereby reducing the probability of short circuit in the battery, suppressing the self-discharge phenomenon of the battery, and giving the battery higher safety performance.

[0036] The present invention does not limit the number of the first through hole, the second through hole and the third through hole. They can be selected according to the different requirements of the porous membrane porosity. The higher the porosity requirement, the more first through holes, the second through holes and the third through holes can be set.

[0037] The first, second, and third porous membrane layers of this invention can each be composed of a single polymer film or multiple polymer films. Taking the first porous membrane layer as an example, it can be formed by a single polymer film or by combining multiple polymer films.

[0038] In a preferred embodiment, the thickness of the second porous membrane layer is 1 to 10 times the thickness of the first porous membrane layer and / or the third porous membrane layer.

[0039] Specifically, the thickness of the second porous membrane layer can be 5–20 μm, and the thickness of the first and / or third porous membrane layers can be 0.5–20 μm. For example, the thickness of the second porous membrane layer can be 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, or any two of these values; the thickness of the first and / or third porous membrane layers can be 0.5 μm, 2 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, or any two of these values.

[0040] In a preferred embodiment, the ratio of the diameter of the first through-hole and / or the third through-hole to the diameter of the second through-hole is greater than or equal to 0.1 and less than 1. By setting a specific ratio for the pore diameter, while ensuring the porous membrane has good mechanical strength, the probability of battery short circuit can be further reduced, the self-discharge phenomenon of the battery can be suppressed, and the battery can have higher safety performance.

[0041] Furthermore, the second porous membrane layer includes an ultraviolet absorber. Including an ultraviolet absorber in the second porous membrane layer can enhance its ultraviolet absorption capacity.

[0042] This invention does not specifically limit the type of ultraviolet absorber, which can be selected from ultraviolet absorbers conventionally used in the art, including but not limited to one or more of phenyl benzoate, 2-(2′-hydroxy-3′,5′-di-tert-butylphenyl)-5-chlorobenzotriazole (UV-P), hydroxybenzophenone, and 2-(2'-hydroxy-3',5'-di-tert-phenyl)-5-chlorobenzotriazole (UVP-327). These ultraviolet absorbers not only have strong ultraviolet absorption capacity but also good compatibility with conventional polymer membrane materials, which is beneficial for the processing of porous membranes.

[0043] In addition to including an ultraviolet absorber in the second porous membrane layer, ultraviolet absorbers can also be added to the first and third porous membrane layers, depending on the different absorption capabilities of the membrane material for ultraviolet light sources. For example, when the absorption capabilities of the first and third porous membrane layers for ultraviolet light sources are weak, an ultraviolet absorber can be added to enhance the absorption of ultraviolet light sources by the membrane material, thereby improving the efficiency of perforation.

[0044] In one specific embodiment, the mass content of the ultraviolet absorber in the second porous membrane layer is 0.1% to 2%. For example, the mass content of the ultraviolet absorber in the second porous membrane layer can be 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, or any two of these values. When the content of the ultraviolet absorber is less than 0.1%, it is difficult to effectively improve the ultraviolet absorption capacity of the second porous membrane layer, resulting in no significant difference in pore size between the second porous membrane layer in the middle layer and the surface layer. When the content of the ultraviolet absorber is greater than 2.0%, the ultraviolet absorption capacity of the second porous membrane layer is too strong, resulting in excessively large pore size in the second porous membrane layer in the middle layer, which is detrimental to the mechanical strength of the porous membrane.

[0045] In one specific embodiment, the materials of the first, second, and third porous membrane layers each independently include one or more of polyethylene terephthalate, polyimide, polybutylene terephthalate, polyethylene naphthalate, polyetheretherketone, polyphenylene sulfide, and polybenzimidazole. All of these polymers possess good heat resistance, enabling the prepared porous membrane to have better thermal stability and reducing shrinkage under heating conditions.

[0046] A second aspect of the present invention provides a method for preparing a porous membrane as described above, comprising the following steps:

[0047] 1) At least three layers of polymer melt are co-extruded, cast, or blow-molded to obtain a composite film; wherein the polymer melt in the middle layer has a stronger UV absorption capacity than the polymer melt in the surface layer;

[0048] 2) The composite membrane is laser-drilled using an ultraviolet laser source to obtain a porous membrane.

[0049] Because the polymer melt in the middle layer has a stronger UV absorption capacity than the polymer melt in the surface layer, when using a UV laser source to perform laser perforation on the composite membrane, the polymer melt in the middle layer has better absorption of the UV laser source, and the melting area of ​​the membrane is larger than that of the surface layer. Therefore, the diameter of the second through hole is larger than that of the first and second through holes, resulting in a porous membrane with obvious internal tortuosity.

[0050] Furthermore, compared to the traditional method of drilling using an infrared laser source, the above preparation process uses an ultraviolet laser source for drilling. The ultraviolet laser source has a lower thermal effect, which can avoid the accumulation of molten material on the surface of the porous membrane during the drilling process. This results in the porous membrane having higher flatness and a more uniform internal stress distribution. The more uniform internal stress distribution enables the porous membrane to maintain higher mechanical strength.

[0051] The above preparation method can be used to prepare a porous membrane with a large pore size in the middle layer and a small pore size in the upper and lower layers of the middle layer. The pores of this porous membrane have a certain degree of internal tortuosity and low straightness, which is beneficial to improve the electrolyte retention capacity, reduce the probability of direct contact short circuit between the positive and negative electrodes and the self-discharge phenomenon of the battery, and improve the safety performance and cycle performance of the battery.

[0052] In one specific embodiment, the content of ultraviolet absorber in the polymer melt located in the intermediate layer can be greater than that in the polymer melt located in the surface layer, so that the ultraviolet absorption capacity of the polymer melt located in the intermediate layer is stronger.

[0053] Preferably, the mass content of the ultraviolet absorber in the polymer melt located in the intermediate layer is 0.1% to 2%. For example, the mass content of the ultraviolet absorber in the polymer melt located in the intermediate layer can be 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, and any two of these values.

[0054] This invention does not specifically limit the type of ultraviolet laser source; it can select conventional ultraviolet laser sources, such as one or more of rare gas excimer lasers, rare gas halide excimer lasers, and halogen gas excimer lasers. Specifically, the rare gas excimer laser can be a Xe2 excimer laser or an Ar2 excimer laser, the rare gas halide excimer laser can be an ArF excimer laser, a KrF excimer laser, or a XeCl excimer laser, and the halogen gas excimer laser can be an F2 excimer laser. The appropriate wavelength of ultraviolet laser source can be selected according to the required aperture.

[0055] The present invention does not specifically limit the device for laser drilling, and can be selected from conventional laser drilling devices in the art. For example, an array laser drilling device can be used for drilling.

[0056] The power of the light source is also a crucial factor affecting the quality of the prepared porous membrane. Excessive power can easily generate a strong thermal effect, leading to the accumulation of molten material on the surface of the porous membrane. Conversely, insufficient power makes it difficult to achieve the required processing efficiency. Based on these considerations, the preferred power of a single light source for laser drilling is 0.5W to 20W. Here, single-source power refers to the power of a single excited-state optical element during lattice or dense drilling. For example, the power of a single light source can be 0.5W, 1W, 3W, 5W, 8W, 10W, 13W, 15W, 18W, 20W, or any range of two values ​​thereof.

[0057] A third aspect of the present invention provides an electrolyte membrane, including the porous membrane of the first aspect of the present invention or a porous membrane prepared by the preparation method of the second aspect of the present invention.

[0058] Because the porous membrane has a certain degree of tortuosity, it can reduce the contact probability between the positive and negative electrodes and avoid the self-discharge phenomenon of the battery, thus reducing the probability of short circuit and improving the safety performance of the battery. In addition, since the pore size inside the porous membrane is larger than that on the surface, it is more conducive to the storage of electrolyte. When this membrane is applied to liquid batteries, it can increase the liquid retention capacity of the battery and thus improve the cycle performance of the battery.

[0059] The electrolyte membrane of the present invention can be either the porous membrane described above used directly as a separator, or a functional layer can be coated on its surface to form a composite separator according to different battery functional requirements. For example, for batteries with high safety requirements, a ceramic slurry can be coated on the surface of the porous membrane to form a ceramic layer, and the high heat resistance of ceramics can be used to further improve the safety performance of the battery.

[0060] Furthermore, porous membranes formed by laser drilling have a larger average pore size, making them more suitable for use in solid-state or semi-solid-state batteries. In one specific embodiment, a solid electrolyte can be coated inside the pores and on the surface of the porous membrane to improve the ion conductivity of the electrolyte membrane, thereby improving the rate performance of the battery.

[0061] This invention does not specifically limit the type of solid electrolyte mentioned above, which can be selected from solid electrolytes commonly used in the art, including but not limited to one or more of oxide solid electrolytes, polymer solid electrolytes, sulfide solid electrolytes, and gel electrolytes. Among them, oxide solid electrolytes are preferably titanium aluminum phosphate (LATP) or lithium lanthanum zirconium oxide (LLZO), polymer solid electrolytes are preferably copolymerized modified polyethylene oxide (PEO), and sulfide solid electrolytes are preferably Li6PS5Cl.

[0062] A fourth aspect of the present invention provides a battery comprising the electrolyte membrane provided in the third aspect of the present invention. Because the electrolyte membrane has a certain degree of internal tortuosity, it reduces the probability of direct contact between the positive and negative electrodes and the self-discharge phenomenon of the battery. Therefore, this battery has higher safety performance.

[0063] The porous membrane, its preparation method, and its application provided by the present invention will be further described in detail below through specific embodiments. Obviously, the following embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the technical solutions and embodiments provided in this application, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0064] Example 1

[0065] This embodiment provides a porous membrane, a separator, and a lithium-ion battery. The specific preparation method is as follows:

[0066] 1. Preparation of porous membranes

[0067] 1) The first film material (PET material, 2.5μm thick), the second film material (PET film containing 0.5% o-hydroxybenzoate, 5μm thick), and the third film material (PET material, 2.5μm thick) are placed in sequence and thermally laminated at 120℃ to obtain a composite film;

[0068] 2) The composite membrane is laser-drilled using an array-type laser device to obtain a porous membrane; the light source for the laser drilling process is selected from an ArF excimer laser source with a laser wavelength of 193nm and a single light source power of 5W.

[0069] 2. Preparation of solid electrolyte membranes

[0070] 1) Add 50 parts by weight of solid electrolyte powder LATP, 0.5 parts by weight of thickener CMC, 0.25 parts by weight of dispersant BYK-156, 5 parts by weight of binder Gaorui GR401, 0.1 parts by weight of wetting agent BYK-348, and 50 parts by weight of deionized water to a double planetary slurry reactor, and stir at 2000 rpm / min for 120 min to obtain a coating slurry;

[0071] 2) The above coating slurry is coated onto the porous membrane prepared in step 1 using a dip-coating process, and then dried at a temperature of 60℃~80℃ to obtain a solid electrolyte membrane.

[0072] 3. Battery fabrication and assembly

[0073] 1) The positive electrode active material NCM811, conductive agent SP, conductive agent KS-6 and binder PVDF are mixed in the solvent NMP at a mass ratio of 90:2:1:3 to prepare a positive electrode slurry, which is then coated onto a 10μm aluminum foil current collector; dried in an oven at 95℃, and rolled on a roller press for later use.

[0074] 2) The active material artificial graphite, the conductive agent acetylene black and the binder CMC / SBR (mass ratio of 1:1) are evenly dispersed in water at a mass ratio of 90:5:5 to prepare a negative electrode slurry. The slurry is coated on a copper foil current collector with a thickness of 10μm, dried in an oven at 85℃, and then rolled in a roller press for later use.

[0075] 3) In a drying room with a dew point <-40℃, the solid electrolyte composite membrane of the present invention is stacked and packaged between the positive and negative electrode sheets, and then packaged according to 2.5g / m 2 The electrolyte (Xinzhoubang, LBC3008A) was injected in a certain proportion to obtain the battery.

[0076] Examples 2-7 and Comparative Example 1

[0077] Examples 2-7 and Comparative Example 1 respectively provide a porous membrane, a solid electrolyte membrane, and a lithium-ion battery. The conditions for preparing the porous membrane are basically the same as those in Example 1, and the differences are listed in Table 1. The preparation conditions for the solid electrolyte membrane and the lithium-ion battery are the same as those in Example 1, and will not be repeated here.

[0078] Table 1

[0079]

[0080] SEM tests were performed on the porous membranes prepared in Example 2 and Comparative Example 1. Figure 1 This is a 30,000x SEM image of the porous membrane cross-section from Example 2, as shown below. Figure 1 As shown, the porous membrane has a pore structure that is large in the middle and narrow on both sides, while the protrusions on both sides of the porous membrane are effectively controlled. Figure 2 This is a 100,000x SEM image of the porous membrane cross-section of Comparative Example 1 of the present invention, as shown below. Figure 2 As shown, the porous membrane in Comparative Example 1 has a more direct pore structure with no obvious tortuosity.

[0081] Test case

[0082] I. The following performance tests were performed on the porous membranes of the above embodiments and comparative examples:

[0083] 1. Average heat shrinkage rate at 200℃

[0084] Test method: Refer to GB / T36363-2018, place the porous membrane in an environment of 200℃ for 1 hour, calculate the thermal shrinkage rate in the MD and TD directions, and take the average value of the thermal shrinkage rate in the MD and TD directions as the average thermal shrinkage rate at 200℃.

[0085] 2. MD tensile strength

[0086] Test method: Refer to GB / T36363-2018.

[0087] 3. Puncture strength

[0088] Test method: Refer to GB / T36363-2018.

[0089] 4. Porosity

[0090] Test method: Refer to GB / T36363-2018.

[0091] 5. Diameters of the first through hole, the second through hole, and the third through hole

[0092] Test method: The results can be obtained through calculations using cross-sectional and surface electron microscopy or microscopy combined with image processing software. The first, second, and third through-holes refer to the through-holes in the first, second, and third porous membrane layers, respectively.

[0093] II. The following performance tests were conducted on the solid electrolyte membranes of the above embodiments and comparative examples:

[0094] 1. Insulation strength

[0095] Test method: Place the solid electrolyte membrane between two electrodes, apply a voltage between the electrodes, and increase the voltage until the solid electrolyte membrane is broken down. The voltage value at which the solid electrolyte membrane is broken down is the insulation strength.

[0096] The test results for the above performance are shown in Table 2.

[0097] III. The following performance tests were conducted on the batteries prepared in the above embodiments and comparative examples:

[0098] 1. Energy density

[0099] Test method: The pouch cell is tested within a voltage range of 3.0V to 4.5V. The discharge plateau voltage can be determined by cyclic voltammetry at a rate of 0.5C. Then, the discharge capacity can be measured by discharging at a constant current of 1C under full charge. The volumetric energy density is calculated using the following formula:

[0100]

[0101] 2. Short circuit rate

[0102] Five cells with a capacity of 2Ah were prepared for each type of solid electrolyte membrane. The short circuit of the battery was tested at 100V using a ZC5520 cell short circuit tester in the unfilled state. The short circuit rate was calculated as: number of short-circuited cells / 5 × 100%.

[0103] 3. Capacity retention rate after 200 cycles at 25°C

[0104] The first discharge capacity was measured at 25°C with a charge / discharge rate of 0.5C. At the same time, the ratio of the discharge capacity measured after 200 cycles to the first discharge capacity was calculated under the same conditions, which was used as the capacity retention rate after 200 cycles at 25°C.

[0105] The test results for all the above performance are shown in Table 2.

[0106] Table 2

[0107]

[0108] The following conclusions can be drawn from Table 2:

[0109] 1) By comparing Example 1 and Example 2, it can be seen that PVDF has stronger UV absorption capacity than PET and larger pore size of its second porous membrane layer. Consequently, the average thermal shrinkage rate of the porous membrane at 200°C is higher, the puncture strength and the insulation strength of the solid electrolyte membrane are reduced, but the volumetric energy density and cycle performance of the battery are improved.

[0110] 2) By comparing Examples 1, 3, and 4, it can be seen that when the thickness ratio of the first and third film layers to the second film layer is too small (Example 3), although the puncture strength of the porous membrane and the insulation strength of the solid electrolyte membrane are significantly increased, the volumetric energy density of the battery is significantly reduced, and the short-circuit rate of the cell is increased. When the thickness ratio of the first and third film layers to the second film layer is too large (Example 4), the surface roughness of the porous membrane is significantly increased, the volumetric energy density of the battery is also significantly reduced, and the cell exhibits a certain short-circuit rate. However, the puncture strength of the porous membrane and the insulation strength of the solid electrolyte membrane increase. Furthermore, as the thickness ratio of the second film layer decreases, the internal tortuosity of the porous membrane decreases, and the straightness of the pores increases, thereby increasing the short-circuit rate of the battery.

[0111] 3) By comparing Examples 1 and 5, it can be seen that when too much ultraviolet absorber is added to the second film layer, the thermal effect generated by the second film layer is too strong, which leads to the increased permeability of the porous membrane pores. This not only results in the second film layer forming excessively large permeable pores, but also increases the thermal shrinkage rate of the porous membrane and reduces the puncture strength, and also causes a certain short circuit rate in the battery cell.

[0112] 4) By comparing Examples 1 and 6, it can be seen that the performance of the porous membrane formed is not significantly different regardless of whether an ArF excimer laser or a KrF excimer laser is used as the light source.

[0113] 5) By comparing Examples 1 and 7, it can be seen that whether phenyl phthalate or benzophenone is added to the intermediate membrane layer as an ultraviolet absorber, it is beneficial to form a porous membrane with high tortuosity. Among them, benzophenone has a stronger ultraviolet absorption capacity than phenyl phthalate, and the pore size of the intermediate layer of the formed porous membrane is larger.

[0114] 6) By comparing Example 1 and Comparative Example 1, it can be seen that when the UV absorption capacity of the second film layer is no different from that of the first and third film layers, the through holes of the first, second and third layers of the formed porous film are no different, which leads to a significant increase in the battery short circuit rate.

[0115] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A porous membrane, characterized in that, It includes a first porous membrane layer, a second porous membrane layer and a third porous membrane layer stacked sequentially; The first porous membrane layer includes a first through-hole, the second porous membrane layer includes a second through-hole, and the third porous membrane layer includes a third through-hole; The first through hole, the second through hole, and the third through hole are coaxially arranged, and the diameter of the second through hole is larger than the diameters of the first through hole and the third through hole; The porous membrane is prepared by a method comprising the following steps: 1) At least three layers of polymer melt are co-extruded, cast, or blow-molded to obtain a composite film; wherein the polymer melt in the middle layer has a stronger UV absorption capacity than the polymer melt in the surface layer; 2) The composite membrane is laser-drilled using an ultraviolet laser light source to obtain the porous membrane.

2. The porous membrane according to claim 1, characterized in that, The thickness of the second porous membrane layer is 1 to 10 times the thickness of the first porous membrane layer or the third porous membrane layer.

3. The porous membrane according to claim 1, characterized in that, The ratio of the diameter of the first or third through hole to the diameter of the second through hole is greater than or equal to 0.1 and less than 1.

4. The porous membrane according to any one of claims 1-3, characterized in that, The second porous membrane layer includes an ultraviolet absorber.

5. The porous membrane according to claim 4, characterized in that, The mass content of the ultraviolet absorber in the second porous membrane layer is 0.1% to 2%.

6. The porous membrane according to any one of claims 1-3, characterized in that, The materials of the first porous membrane layer, the second porous membrane layer and the third porous membrane layer each independently include one or more of polyethylene terephthalate, polyimide, polybutylene terephthalate, polyethylene naphthalate, polyetheretherketone, polyphenylene sulfide and polybenzimidazole.

7. The porous membrane according to claim 1, characterized in that, The content of ultraviolet absorber in the polymer melt located in the middle layer is greater than that in the polymer melt located in the surface layer.

8. The porous membrane according to claim 1, characterized in that, The ultraviolet laser source is selected from one or more of rare gas excimer lasers, rare gas halide excimer lasers, and halogen gas excimer lasers; And / or, the power of the single light source for the laser drilling process is 0.5W~20W.

9. An electrolyte membrane, characterized in that, Includes the porous membrane according to any one of claims 1-8.

10. A battery, characterized in that, Includes the electrolyte membrane as described in claim 9.

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