Composite membrane and preparation method thereof, solid electrolyte, battery cell, electrochemical device and electronic equipment

Abstract

The invention discloses a composite membrane and a preparation method thereof, a solid electrolyte, a battery cell, an electrochemical device and electronic equipment. The composite membrane comprises a nuclear track membrane substrate and polyvinylidene fluoride, the polyvinylidene fluoride is distributed on the surface and in pore channels of the nuclear pore membrane substrate; the cross section S of the composite membrane comprises a surface area of the nuclear track membrane substrate and a pore channel area S1, A1 is the percentage of the mass of a fluorine element contained in the pore channel area S1 in the total mass of all elements in the pore channel area, and A is the percentage of the mass of a fluorine element contained in the cross section S in the total mass of all elements in the cross section S; the ratio of A1 to A is (0.52-0.9): 1. The composite membrane provided by the invention has high porosity and high tensile strength, and has excellent performance of high lithium ion migration rate and low short circuit risk when being used in the battery diaphragm.

Description

Technical Field

[0001] This invention specifically relates to composite membranes and their preparation methods, solid electrolytes, battery cells, electrochemical devices, and electronic devices. Background Technology

[0002] With ever-increasing energy demands, there is a need to develop secondary batteries with higher energy density to extend the driving range of electric vehicles. Lithium metal is widely recognized as one of the ideal anode materials due to its ultra-high theoretical capacity and extremely low electrochemical potential. However, lithium metal as an anode material typically exhibits low coulombic efficiency and cycle life, caused by lithium dendrite formation during battery operation. Lithium dendrites can penetrate the separator, causing short circuits and leading to serious safety accidents. Therefore, before lithium metal anodes can be practically applied, the key issue of lithium dendrite growth needs to be addressed. A diffusion-limited mechanism model proposes that the space charge region formed near the anode can generate a strong electric field on the anode surface, inducing rapid lithium dendrite growth. Therefore, the formation time of the space charge region can be delayed by immobilizing anions within the electrolyte, thereby delaying dendrite nucleation. This model provides new insights for the development of dendrite-free lithium anodes.

[0003] Lithium-ion battery separators are porous, high-performance materials that not only transport lithium ions through their pores but also isolate the positive and negative electrodes, preventing short circuits caused by direct contact. The separator plays a crucial role in lithium-ion transport within the battery cell, significantly impacting the transport and diffusion rates of lithium ions. The performance of the separator is critical to the characteristics of the battery cell; a separator with good wettability improves ionic conductivity, facilitating lithium-ion transport and increasing ion conduction efficiency, thereby promoting rapid charging and discharging of the cell. However, conventionally used polyolefin separators, non-woven fabric separators, and electrospun separators in batteries suffer from the following drawbacks: a low number of through-pores, high tortuosity, long lithium-ion transport paths, slow migration speeds, and low effective ionic conductivity.

[0004] Therefore, the use of nucleoporous membranes as lithium-ion battery separators was explored to address the aforementioned problems. Nucleoporous membranes are produced by irradiating a homogeneous polymer membrane with high-energy rays, such as 4–5 MeV alpha particles, causing the chemical bonds in the polymer to break, followed by etching to obtain cylindrical pores. The thickness of nucleoporous membranes varies from 6 μm to 50 μm. Pore diameters range from 0.01 μm to 30 μm, with porosity reaching up to 50%. They possess a through-hole structure and offer advantages such as consistent pore size, controllable size, easily adjustable pore density, high mechanical strength, and good flexibility. The more through-holes in the separator, the shorter the lithium-ion transport path and the faster the migration speed. However, increasing the number of through-holes reduces the separator's strength and increases the risk of short circuits. Summary of the Invention

[0005] This invention primarily aims to overcome the shortcomings of existing technologies using nucleoporous membranes as battery separators, which struggle to simultaneously achieve high porosity and high tensile strength, leading to poor battery cycle performance and susceptibility to short circuits. It provides a composite membrane, its preparation method, a solid electrolyte, a battery cell, an electrochemical device, and an electronic device. The composite membrane provided by this invention possesses excellent properties, including low tortuosity, high porosity, good tensile strength, low interfacial resistance, and, when used as a battery separator, exhibits rapid lithium-ion migration, low short-circuit risk, and good cycle performance.

[0006] The present invention overcomes the above-mentioned technical problems through the following technical solution.

[0007] In a first aspect, the present invention provides a composite membrane comprising a nuclear pore membrane substrate and polyvinylidene fluoride (PVDF); the PVDF is distributed on the surface and in the pores of the nuclear pore membrane substrate, wherein the fluorine content in the pore region is higher than the fluorine content in the surface region; the cross section S of the composite membrane includes the surface region of the nuclear pore membrane substrate and the pore region S1, where A1 is the percentage of the mass of fluorine contained in the pore region S1 relative to the total mass of all elements in the pore region, and A is the percentage of the mass of fluorine contained in the cross section S relative to the total mass of all elements in the cross section S;

[0008] The ratio of A1 to A is (0.52-0.9):1.

[0009] The inventors discovered that by distributing or filling polyvinylidene fluoride (PVDF) on the surface and within the pores of a core-pore membrane, the PVDF adheres to the inner wall of the pores, maintaining the straight pore path of the membrane. Compared to traditional membranes, this effectively shortens the lithium-ion transport distance and increases the lithium-ion migration rate at the same thickness. Furthermore, the addition of PVDF enhances the strength of the core-pore membrane matrix, resulting in good tensile strength. Simultaneously, the presence of PVDF within the pores maximizes the contact area between PVDF and the electrolyte, facilitating anion capture and delaying the formation time of the space charge region. This avoids the short-circuit risk associated with core-pore membranes with multiple straight pores, mitigates the growth of lithium dendrites in the negative electrode, and increases the number of cycles at room temperature. However, excessively high PVDF content in the pores can clog the pores, reducing the electrolyte wettability of the membrane. For example, when the ratio of Al to A exceeds 0.9, the battery's electrical performance deteriorates.

[0010] Secondly, the present invention provides a method for preparing the composite membrane as described above, wherein a polyvinylidene fluoride slurry is coated on one or both sides of the nuclear pore membrane substrate, and after casting, excess polyvinylidene fluoride slurry on the surface is removed to obtain the composite membrane; the concentration of the polyvinylidene fluoride slurry is 1-10 wt%, and the coating amount of the polyvinylidene fluoride slurry is 2-15 g / m³. 2 .

[0011] Thirdly, the present invention provides a solid electrolyte membrane comprising the composite membrane as described above.

[0012] Fourthly, the present invention provides a battery cell comprising the composite film as described above.

[0013] Fifthly, the present invention provides an electrochemical device comprising the composite membrane as described above.

[0014] In a sixth aspect, the present invention provides an electronic device comprising the electrochemical device as described above.

[0015] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0016] The reagents and raw materials used in this invention are all commercially available.

[0017] The positive and progressive effects of this invention are as follows:

[0018] The composite membrane provided by this invention has the advantages of high porosity, high tensile strength, and excellent performance when used in battery separators, including fast lithium-ion migration rate and low short-circuit risk.

[0019] The method for preparing composite membranes provided by this invention can adjust and control the distribution of polyvinylidene fluoride on the surface or in the pores of the core-pore membrane by adjusting the concentration and coating amount of the polyvinylidene fluoride slurry during the preparation process, thereby obtaining different composite membrane structures and achieving good performance. Attached Figure Description

[0020] Figure 1 This is a cross-sectional schematic diagram of the composite membrane prepared in Example 1. Detailed Implementation

[0021] Composite membrane

[0022] The first aspect of the present invention provides a composite membrane comprising a nuclear pore membrane substrate and polyvinylidene fluoride (PVDF); the PVDF is distributed in the surface region and the pore region of the nuclear pore membrane substrate; the cross section S of the composite membrane includes the surface region and the pore region S1 of the nuclear pore membrane substrate; the fluorine content in the pore region is higher than the fluorine content in the surface region; the cross section S of the composite membrane includes the surface region and the pore region S1 of the nuclear pore membrane substrate; A1 is the percentage of the mass of fluorine contained in the pore region S1 relative to the total mass of all elements in the pore region; and A is the percentage of the mass of fluorine contained in the cross section S relative to the total mass of all elements in the cross section S.

[0023] The ratio of A1 to A is (0.52-0.9):1.

[0024] In this invention, both A1 and A are obtained through EDS scanning. An EDS surface scan is performed on any cross-section S1 of the composite membrane, which includes only the pore region inside the composite membrane. The percentage of the mass of element F (denoted as a1) relative to the total mass of all elements in cross-section S1 (denoted as b1: element F + other elements) is measured and calculated as A1: a1 / b1*100%. Then, an EDS surface scan is performed on any cross-section S, which includes both the surface region and the pore region S1 of the composite membrane, to obtain a percentage of the mass of element F (denoted as a) relative to the total mass of all elements in cross-section S (denoted as b: element F + other elements) as A: a / b*100%.

[0025] In this invention, the cross section refers to the interface conventionally understood by those skilled in the art, that is, a plane obtained by cutting along the thickness direction of the composite film, which allows observation of the pore structure, thickness, tortuosity, and coating of the composite film.

[0026] In this invention, the ratio of fluorine content A1 in the channel region to fluorine content A in the cross-sectional region can be (0.57-0.87):1, preferably (0.6-0.87):1, and more preferably (0.65-0.87):1.

[0027] In some specific implementations, the ratio of fluorine content A1 in the channel region S1 to fluorine content A in the cross section S is 0.52:1, 0.57:1, 0.65:1, 0.74:1, 0.87:1, or 0.9:1.

[0028] In this invention, the thickness of polyvinylidene fluoride in the channel region can be 0.1μm-0.6μm, preferably 0.3μm-0.6μm.

[0029] In some specific embodiments, the thickness of the polyvinylidene fluoride in the channel region is 0.1 μm, 0.2 μm, 0.3 μm, 0.5 μm or 0.6 μm.

[0030] In this invention, the porosity of the composite membrane can be 30%-50%, preferably 35%-45%.

[0031] In some specific embodiments, the porosity of the composite membrane may be 35%, 37%, 40%, 41%, or 43%.

[0032] In this invention, the average pore size of the composite membrane can be 1μm-5μm, preferably 2.4μm-2.9μm, for example 2.4μm, 2.5μm, 2.7μm, 2.8μm or 2.9μm.

[0033] In this invention, the tortuosity of the composite membrane can be 1.08-1.13, for example, 1.08, 1.09, 1.1, 1.11, 1.12, or 1.13. Tortuosity primarily reflects the permeability of lithium ions through the membrane. An increase in membrane pore tortuosity will cause a quadratic decrease in permeability. Pore tortuosity is defined as the ratio of the actual path traveled by gas or liquid through the membrane to the membrane thickness.

[0034] In this invention, the thickness ratio of the surface region to the nuclear pore membrane substrate is 1:(30-60), preferably 1:(40-45), and more preferably 1:(40-42).

[0035] In some specific embodiments, the thickness ratio of the surface region to the nuclear pore membrane substrate is 1:40, 1:41, 1:42, 1:43, 1:44, and 1:45.

[0036] In this invention, the material of the nuclear pore membrane substrate can be one or more of the following materials commonly used in the art: polycarbonate, polyester, polyimide, polyvinylidene fluoride, and polypropylene.

[0037] In some specific embodiments, the material of the nuclear pore membrane substrate is polyester.

[0038] In some specific embodiments, the material of the nuclear pore membrane substrate is polycarbonate.

[0039] In this invention, the average pore size of the nuclear pore membrane substrate is 2.5 μm-5.5 μm, for example, 3 μm.

[0040] In this invention, the porosity of the nuclear pore membrane substrate is 35%-55%, for example 45%.

[0041] Preparation method of composite membrane

[0042] The second aspect of the present invention provides a method for preparing a composite membrane, comprising the following steps: coating a polyvinylidene fluoride slurry onto one or both sides of the nuclear pore membrane substrate; after casting, removing excess polyvinylidene fluoride slurry from the surface to obtain the composite membrane; wherein the concentration of the polyvinylidene fluoride slurry is 1-10 wt%, and the coating amount of the polyvinylidene fluoride slurry is 2-15 g / m³. 2 .

[0043] In this invention, the concentration of polyvinylidene fluoride in the polyvinylidene fluoride slurry can be 4-6 wt%.

[0044] In some specific embodiments, the concentration of polyvinylidene fluoride in the polyvinylidene fluoride slurry may be 1 wt%, 5 wt%, or 10 wt%.

[0045] In this invention, the coating amount of the polyvinylidene fluoride slurry can be 8-12 g / m².2 .

[0046] In some specific embodiments, the coating amount of the polyvinylidene fluoride slurry is 2 g / m². 2、 8g / m 2 10g / m 2 12g / m 2 Or 15g / m 2 .

[0047] In this invention, the polyvinylidene fluoride slurry can be prepared using conventional methods in the art, for example: adding polyvinylidene fluoride powder to an organic solvent, heating and stirring to obtain PVDF slurry;

[0048] In some embodiments, the organic solvent is selected from one or more of dimethylacetamide, N,N-dimethylformamide, N,N-dimethylethylamine, N-methylpyrrolidone, tetrahydrofuran, toluene, and dimethyl sulfoxide.

[0049] In some embodiments, the heating temperature may be 25-80°C, for example 60°C.

[0050] In some implementations, the heating time may be 12-48 hours, for example, 24 hours.

[0051] In this invention, after removing the surface polyvinylidene fluoride, the composite film is generally dried.

[0052] In some embodiments, the drying temperature may be 25-120°C, for example 80°C.

[0053] In some implementations, the drying time may be 1-48 hours, for example 24 hours.

[0054] solid electrolyte membrane

[0055] The solid electrolyte membrane provided in the third aspect of the present invention includes the composite membrane as described above.

[0056] battery cells

[0057] The battery cell provided in the fourth aspect of the present invention includes the composite film as described above.

[0058] In this invention, the battery cell can be a lithium metal system soft-pack battery cell, which includes a positive electrode, a negative electrode, an electrolyte, and a composite membrane as described above as a separator.

[0059] The positive electrode includes a positive electrode material layer and a composite current collector.

[0060] The positive electrode material layer may include positive electrode active material, and binders and conductive agents may be further added as needed.

[0061] The positive electrode active material may be a positive electrode active material conventionally used in the field for lithium-ion battery positive electrodes, and may include one or more of lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), and lithium-rich manganese-based oxide (LRMO).

[0062] The negative electrode sheet includes a negative electrode material layer and a composite current collector. The negative electrode material layer may include a negative electrode active material, which is preferably lithium metal.

[0063] The electrolyte contains a small molecule plasticizer and a lithium salt.

[0064] The preferred mass ratio of the small molecule plasticizer to the lithium salt is (8-9):(1-2);

[0065] The small molecule plasticizer preferably includes one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, fluorinated ethylene carbonate, dipropyl carbonate, dimethyl sulfoxide dimethoxyethane, N-methyl-2-pyrrolidone, γ-butyrolactone, and polyethylene glycol dimethyl ether.

[0066] The lithium salt preferably includes one or more of LiBF4, LiBF6, LiAsF6, LiPF6, LiClO4, LiFSI, LiTFSI, LiB(C6H5)4, LiAlCl4, LiBr, LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SOSO2)3.

[0067] In this invention, the method for preparing the battery cell can be conventional in the art, and preferably includes the following steps:

[0068] S1. Preparation of dry cell: The positive electrode, negative electrode and separator are assembled into the shell to make a dry cell;

[0069] S2. Prepare the electrolyte: The electrolyte contains small molecule plasticizers and lithium salts;

[0070] S3. Electrolyte injection: Inject the electrolyte into the dry cell;

[0071] S4, formation, degassing and aging, to obtain the lithium metal system soft-pack cell.

[0072] The conditions for formation, venting, and aging are as follows: charging to the cutoff voltage at a rate of 0.1 to 0.33C, venting vacuum of -40 to -98 kPa, and venting time of more than 10 seconds; aging temperature of 40 to 50°C and aging time of 24 to 72 hours.

[0073] Electrochemical device

[0074] The electrochemical device provided in the fifth aspect of the present invention includes the composite membrane as described above.

[0075] In this invention, the electrochemical device is preferably a lithium-ion battery, which includes the composite membrane, positive electrode, negative electrode and electrolyte as described above.

[0076] In this invention, the method for preparing the lithium-ion battery can be a conventional method in the art, which can be to wind the positive electrode, the negative electrode and the separator to obtain a battery cell, then package it in a packaging shell and inject the electrolyte; or it can be to stack the negative electrode, the separator, the positive electrode and the separator in sequence to obtain a battery cell, then package it in a packaging shell and inject the electrolyte.

[0077] electronic devices

[0078] The electronic device provided in the sixth aspect of the present invention includes the electrochemical device as described above.

[0079] Exemplary examples include, but are not limited to, mobile devices (e.g., mobile phones, tablets, laptops, video recorders, portable printers / copiers, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, and backup power supplies. Based on common knowledge in the field, the above optional conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0080] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0081] Sources of raw materials in the examples and comparative examples:

[0082] PVDF is sourced from Arkema, model number HSK-900;

[0083] The PET core-pore membrane is derived from it4ip. The PET core-pore membrane has a porosity of 45%, a pore size of 3μm, and a thickness of 20μm. The degree of polymerization of PET is 104, and the average relative molecular mass is 20000.

[0084] The polycarbonate core-porous membrane is from it4ip, model number 1000M50 / 451P401, with a porosity of 45%, a pore size of 3μm, and a thickness of 20μm.

[0085] LiNi 0.8 Co 0.1 Mn 0.1 O2 is sourced from Xiamen Tungsten Co., Ltd., model number M821A;

[0086] Diethyl carbonate, fluoroethylene carbonate, difluoroethyl acetate, dimethyl carbonate, and LiPF6 are all sourced from Guangzhou Tinci Advanced Materials Co., Ltd.

[0087] LNMO is a single-crystal LiNi coated with Li₂ZrO₃. 0.5 Mn 1.5 O4; sourced from Xiamen Tungsten Co., Ltd., model number XW46;

[0088] The PEO powder is sourced from Dow Chemical in the United States, with a molecular weight of 600,000 and a model number of POLYOXWSR205.

[0089] LiTFSI originates from Guangzhou Tinci Advanced Materials Co., Ltd.

[0090] Example 1

[0091] S1. Add 5g of PVDF powder to 95g of N,N-dimethylformamide, heat and stir at 60℃ for 24h to mix thoroughly and evenly, to obtain a PVDF slurry with a concentration of 5wt%.

[0092] S2. Coat the PVDF slurry obtained in step S1 onto one side of the PET core pore membrane substrate, so that a layer of PVDF slurry is cast into the micropores of the core pore membrane substrate, with a coating amount of 10 g / m. 2 Remove excess PVDF slurry from the surface of the nuclear pore membrane substrate;

[0093] S3. The treated nuclear pore membrane is placed in an 80℃ forced-air oven and dried for 24 hours to obtain a composite membrane.

[0094] Example 2

[0095] The only difference from Example 1 is that the concentration of the PVDF slurry is 1 wt%.

[0096] Example 3

[0097] The only difference from Example 1 is that the concentration of the PVDF slurry is 10 wt%.

[0098] Example 4

[0099] The difference from Example 1 is that the coating amount is 15 g / m². 2 .

[0100] Example 5

[0101] The difference from Example 1 is that the coating amount is 2 g / m². 2 .

[0102] Example 6

[0103] The difference from Example 1 is that the material of the nuclear pore membrane is polycarbonate.

[0104] Example 7

[0105] The difference from Example 1 is that the average pore size of the nuclear pore membrane is 2.5 μm.

[0106] Example 8

[0107] The difference from Example 1 is that the average pore size of the nuclear pore membrane is 5.5 μm.

[0108] Example 9

[0109] The difference from Example 1 is that the porosity of the nuclear pore membrane is 35%.

[0110] Example 10

[0111] The difference from Example 1 is that the porosity of the nuclear pore membrane is 55%.

[0112] Application Example 1

[0113] S1. Preparation of dry battery cells: LiNi 0.8 Co 0.1 Mn 0.1 O2 positive electrode, lithium metal negative electrode and composite membrane prepared in Example 1 are assembled into the shell as separator to form dry cell.

[0114] S2. Prepare the electrolyte: The electrolyte contains 60g of diethyl carbonate, 10g of fluoroethylene carbonate, 10g of difluoroethyl acetate, 6g of dimethyl carbonate and 14g of LiPF6, which are mixed evenly.

[0115] S3. Electrolyte injection: Inject 3.4g of electrolyte into the dry cell. The dew point is -60℃ and the vacuum degree is -98kPa. After injection, let it stand at room temperature for 48 hours to soak.

[0116] S4, Formation, Degassing and Aging: Charged to 4.3V at a charging rate of 0.1C, degassing time was 20s, and the degassing vacuum degree was -98kPa; the aging temperature was 45℃ and the aging time was 24h to obtain the soft-pack battery cell.

[0117] Application Example 2

[0118] The difference from Application Example 1 is that the composite membrane prepared in Example 2 is used as the separator.

[0119] Application Example 3

[0120] The difference from Application Example 1 is that the composite membrane prepared in Example 3 is used as the separator.

[0121] Application Example 4

[0122] The difference from Application Example 1 is that the composite membrane prepared in Example 4 is used as the separator.

[0123] Application Example 5

[0124] The difference from Application Example 1 is that the composite membrane prepared in Example 5 is used as the separator.

[0125] Application Example 6

[0126] The difference from Application Example 1 is that the composite membrane prepared in Example 6 is used as the separator.

[0127] Application Example 7

[0128] The difference from Application Example 1 is that the composite membrane prepared in Example 7 is used as the separator.

[0129] Application Example 8

[0130] The difference from Application Example 1 is that the composite membrane prepared in Example 8 is used as the separator.

[0131] Application Example 9

[0132] The difference from Application Example 1 is that the composite membrane prepared in Example 9 is used as the separator.

[0133] Application Example 10

[0134] The difference from Application Example 1 is that the composite membrane prepared in Example 10 is used as the separator.

[0135] Application Example 11

[0136] The only difference from Application Example 1 is that LNMO cathode sheet is used as the cathode sheet.

[0137] Application Example 12

[0138] S1. Preparation of PEO-LiTFSI slurry: Add 5g PEO powder and 2g LiTFSI to 45g acetonitrile, heat and stir at 45℃ for 24h to fully mix and obtain PEO-LiTFSI slurry;

[0139] S2. Solid electrolyte membrane preparation: PEO-LiTFSI slurry is coated on both sides of the composite membrane prepared in Example 1, so that the micropores of the composite membrane are filled with PEO-LiTFSI slurry.

[0140] S3. The wet membrane obtained in S2 is placed in an 80°C forced-air drying oven and dried for 24 hours to obtain the solid electrolyte membrane with a thickness of 20 μm.

[0141] Comparative Example 1

[0142] The only difference from Example 1 is that the concentration of the PVDF slurry is 15 wt%.

[0143] Comparative Example 2

[0144] The difference from Example 1 is that excess PVDF slurry is retained on the surface of the nuclear pore membrane substrate.

[0145] Comparative Example 3

[0146] The difference from Example 1 is that the coating amount is 1 g / m². 2 .

[0147] Comparative Example 4

[0148] The difference from Example 1 is that the coating amount is 20 g / m². 2 .

[0149] Comparative Example 5

[0150] An alumina-coated polyethylene diaphragm.

[0151] Comparative Example 6

[0152] A PET nuclear pore membrane.

[0153] Comparative Example 7

[0154] A polyethylene diaphragm

[0155] Comparative Example 8

[0156] A polyimide diaphragm made by electrospinning.

[0157] Comparative Example 9

[0158] S1. Preparation of PVDF slurry: Add 5g of PVDF powder to 45g of N,N-dimethylformamide, heat and stir at 60℃ for 24h to mix thoroughly and evenly, to obtain a PVDF slurry with a concentration of 10wt%.

[0159] S2. Preparation of PVDF membrane: PVDF slurry was coated onto a glass plate using a doctor blade, and then immersed in deionized water for phase inversion. After standing for 2 hours, the PVDF membrane was taken out and dried in an 80℃ forced-air oven for 24 hours to obtain a PVDF membrane. The thickness of the PVDF dry membrane was 20 μm and the average pore size was 0.3 μm.

[0160] Application Comparative Example 1

[0161] The difference from Application Example 1 is that the composite membrane prepared in Comparative Example 1 is used as the cell separator.

[0162] Application Comparative Example 2

[0163] The difference from Application Example 1 is that the composite membrane prepared in Comparative Example 2 is used as the cell separator.

[0164] Application Comparative Example 3

[0165] The difference from Application Example 1 is that the composite membrane prepared in Comparative Example 3 is used as the cell separator.

[0166] Application Comparative Example 4

[0167] The difference from Application Example 1 is that the composite membrane prepared in Comparative Example 4 is used as the cell separator.

[0168] Application Comparative Example 5

[0169] The difference from Application Example 1 is that the separator of Comparative Example 5 is used as the cell separator.

[0170] Application Comparative Example 6

[0171] The difference from Application Example 1 is that the separator of Comparative Example 6 is used as the cell separator.

[0172] Application Comparative Example 7

[0173] The difference from Application Example 1 is that the separator of Comparative Example 7 is used as the cell separator.

[0174] Application Comparative Example 8

[0175] The difference from Application Example 1 is that the separator of Comparative Example 8 is used as the cell separator.

[0176] Application Comparison Example 9

[0177] S1. Preparation of dry battery cells: LiNi 0.8 Co 0.1 Mn 0.1 O2 positive electrode, lithium metal negative electrode and PVDF film prepared in Comparative Example 9 are used as separators and assembled into the shell to make dry cell.

[0178] S2. Prepare the electrolyte: The electrolyte contains 60g of diethyl carbonate, 10g of fluoroethylene carbonate, 10g of difluoroethyl acetate, 6g of dimethyl carbonate and 14g of LiPF6, which are mixed evenly.

[0179] S3, electrolyte injection: Take 3.4g of the electrolyte prepared in step S4 and inject it into the dry cell. The dew point is -60℃ and the vacuum degree is -98kPa. After the electrolyte is injected, let it stand at room temperature for 48h to soak.

[0180] S4, Formation, Degassing and Aging: Charged to 4.3V at a charging rate of 0.1C, degassing time was 20s, and the degassing vacuum degree was -98kPa; the aging temperature was 45℃ and the aging time was 24h to obtain a lithium metal system soft-pack cell.

[0181] Application Comparison Example 10

[0182] S1. Preparation of PEO-LiTFSI slurry: Add 5g of PEO powder and 2g of LiTFSI to 45g of acetonitrile, heat and stir at 45℃ for 24h to fully mix and obtain PEO-LiTFSI slurry;

[0183] S2. Preparation of solid electrolyte membrane: PEO-LiTFSI slurry is coated on a glass plate;

[0184] S3. Place the above wet film in an 80°C forced-air drying oven and dry for 24 hours to obtain a solid electrolyte membrane with a thickness of 100 μm.

[0185] Example 1: The thickness of polyvinylidene fluoride in the channels and the ratio of the thickness of the surface area to the thickness of the core pore membrane substrate.

[0186] The thickness of the PVDF in the channels and the thickness ratio of the surface area to the core pore membrane substrate were determined by scanning the cross-section of the composite membrane using scanning electron microscopy (SEM). The composite membrane was cut into pieces of approximately 1.5 mm × 2.0 mm and stained with ruthenium. The stained samples and ethanol were placed in gelatin capsules, frozen with liquid nitrogen, and then the samples were cut with a hammer. The samples were vapor-deposited using vapor deposition and observed at an accelerating voltage of 1.0 kV and 30,000 cycles to calculate the thickness of the PVDF coating and the thickness ratio of the surface area to the core pore membrane substrate. The calculation results are recorded in Table 1.

[0187] Example 2: Average pore size

[0188] The average pore size (μm) was determined using a Perm Porometer (Porous Materials, Inc.: CFP-1500AE) according to the semi-dry method. The impregnation solution used was a perfluoropolyester manufactured by the same company (surface tension 15.6 dyn / cm). For the drying and wetting curves, the applied pressure and air permeability were measured. The average pore size dHD (μm) was calculated from the pressure PHD (Pa) at the intersection of the half-curve of the drying curve and the wetting curve using the following formula: dHD = 2860 × γ / PHD. The calculation results are recorded in Table 1.

[0189] Example 3: Porosity

[0190] (1) Permeate tanks are set up on both sides of the diaphragm, and the permeate is placed in the tanks on both sides respectively;

[0191] (2) Apply a certain pressure in the permeate tank to allow the permeate to permeate through the membrane pores to the other side;

[0192] (3) Measure the pressure change and flow rate of the permeate in the liquid pool before and after permeation.

[0193] (4) Calculate the membrane porosity for: Where Q is the permeate flow rate, Δt is the permeation time, A is the membrane area, and ΔP is the pressure difference. The calculation results are recorded in Table 1.

[0194] Example 4: Fluorine content (A1 / A)

[0195] EDS surface scanning was used to calculate the ratio of the fluorine content A1 in the inner wall of the pores of the composite membranes in the examples and comparative examples to the fluorine content A in the cross-section; the specific steps are as follows:

[0196] Figure 1 The cross-sectional diagram of the composite membrane is shown. Using EDS surface scanning, any cross-section S1 parallel to the thickness direction of the composite membrane (the cross-section S1 does not include the polyvinylidene fluoride layer on the surface of the composite membrane) is tested and calculated to obtain the percentage A1: a1 / b1*100% of the mass of element F (denoted as a1) to the total mass of all elements in cross-section S1 (denoted as b1: element F + other elements).

[0197] Then, control the EDS surface scan to scan any section S parallel to the thickness direction of the composite film, and obtain the percentage A of the mass of element F (denoted as a) to the total mass of all elements in section S (denoted as b: element F + other elements): a / b*100%. The calculation results are recorded in Table 1.

[0198] Example 5: Tensile Strength

[0199] The composite membranes and solid electrolyte membranes of the examples and comparative examples were cut into strips with a width of 20 mm and a length of 100 mm. The tensile strength was tested using a tensile testing machine at a speed of 100 ± 10 mm / min. The test results are recorded in Table 1.

[0200] Example 6: Ionic Conductivity

[0201] At 25°C, the solid electrolyte membranes prepared by Application Example 12 and Application Comparative Example 10 were subjected to AC impedance spectroscopy tests. The ionic conductivity of the solid electrolyte membranes was calculated based on the impedance values, and the calculation results are recorded in Table 3.

[0202] Example 7: Lithium-ion transference number

[0203] At 25°C, the composite membranes of the examples and comparative examples, as well as the solid electrolyte membrane, were assembled with two lithium metal sheets to form a lithium symmetric battery. The battery was subjected to comprehensive tests of DC polarization and AC impedance to determine the lithium-ion transference number of the solid electrolyte membrane. The test results are recorded in Table 1.

[0204] Example 8: Interface Resistance

[0205] S1: A testing device is provided, having an upper electrode plate and a lower electrode plate arranged opposite each other in the vertical direction; the projection area of ​​the upper electrode plate on the lower electrode plate is a clamping area, and the area of ​​the clamping area is S;

[0206] S2: Obtain the blank resistance R0 of the electrolyte between the upper and lower electrode plates;

[0207] S3: The diaphragm to be tested is placed between the upper electrode plate and the lower electrode plate. The diaphragm to be tested covers the clamping area, and only the part of the diaphragm to be tested located in the clamping area is immersed in the electrolyte.

[0208] S4: Adjust the upper electrode plate so that the pressure applied by the upper electrode plate to the diaphragm under test and the lower electrode plate is P; measure the resistance RS1 of the diaphragm under test between the upper and lower electrode plates at 100kHz using the AC impedance method.

[0209] S5: Calculate the interface resistance R1 of the diaphragm under test using R1 = (RS1R0) × S.

[0210] Example 9: Cyclic Performance

[0211] At 25°C, long-cycle charge-discharge tests were conducted on the lithium metal system soft-pack cells prepared in Examples 1-11 and Comparative Examples 1-9, and the number of cycles at room temperature was measured. The state at which the capacity decayed to 80% SOH was recorded. If no short circuit occurred when 80% SOH was reached, the actual number of cycles was recorded as the room temperature cycle number, and whether a short circuit occurred was recorded as "no". If a short circuit occurred before the capacity decayed to 80% SOH, the actual number of cycles at the time of the short circuit was recorded as the room temperature cycle number, and whether a short circuit occurred was recorded as "yes". The operating voltage range was 2.8–4.3V. The room temperature cycle test consisted of 0.1C constant current and constant voltage charging, followed by 1C constant current discharging. For Example 5, the specific testing method was the same as other examples and comparative examples, except that the operating voltage range was 3.4–4.85V.

[0212] Table 1

[0213]

[0214] As shown in Table 1, the average pore size, porosity, and thickness of the PVDF coating on the inner wall of the core pore membrane can be adjusted, at least by controlling the application of different concentrations of PVDF slurry into the pores of the composite membrane. PVDF penetrates into the pores of the core pore membrane and adheres to the inner wall, thereby maximizing the contact area between PVDF and the electrolyte, and thus maximizing the effectiveness of PVDF. Specifically, the strong binding force between PVDF and anions significantly reduces the migration efficiency of anions, thereby extending the nucleation time of lithium dendrites. Appropriately concentrated PVDF slurry penetrates into the pores of the core pore membrane and adheres to the inner wall of the pores as much as possible. If the concentration is too low or the coating amount is too small, the PVDF thickness on the inner wall of the core pore membrane will be insufficient, reducing the PVDF effectiveness; if the concentration is too high or the coating amount is too large, the PVDF thickness on the inner wall of the core pore membrane will be too thick, reducing the pore size and porosity, and affecting the ion transport effect.

[0215] Table 2

[0216] Implementation Corresponding diaphragm Number of cycles at room temperature Has a short circuit occurred? Application Example 1 Example 1 402 no Application Example 2 Example 2 389 no Application Example 3 Example 3 363 no Application Example 4 Example 4 368 no Application Example 5 Example 5 390 no Application Example 6 Example 6 398 no Application Example 7 Example 7 386 no Application Example 8 Example 8 378 no Application Example 9 Example 9 392 no Application Example 10 Example 10 373 no Application Example 11 Example 1 328 no Application Comparative Example 1 Comparative Example 1 331 no Application Comparative Example 2 Comparative Example 2 364 no Application Comparative Example 3 Comparative Example 3 285 yes Application Comparative Example 4 Comparative Example 4 324 no Application Comparative Example 5 Alumina-coated polyethylene diaphragm 277 no Application Comparative Example 6 PET nuclear pore membrane 204 yes Application Comparative Example 7 Polyethylene diaphragm 241 no Application Comparative Example 8 polyimide membrane 253 no Application Comparison Example 9 PVDF membrane 340 no

[0217] As can be seen from Table 2, when the composite membrane of the present invention is used as the separator of the lithium metal system soft-pack battery cell, the lithium ion transport path is greatly shortened by the core pore membrane. By utilizing the strong binding of PVDF and anions, high porosity and average pore size are ensured, while avoiding the short circuit risk caused by using a core pore membrane with multiple through holes. This can effectively improve the number of room temperature cycles of the lithium metal system soft-pack battery cell.

[0218] Based on the performance data from Application Examples 1-3 and Comparative Example 1, it can be seen that when the concentration of PVDF slurry is 1-10 wt%, the corresponding lithium metal system soft-pack battery cell exhibits superior performance. Conversely, when the concentration of PVDF slurry is outside this range, the corresponding lithium metal system soft-pack battery cell exhibits poor performance. For example, in Comparative Example 1, the PVDF slurry concentration of the separator is 15 wt%. When the PVDF slurry concentration is too high, it will clog the pores of the core-pore membrane, reducing the porosity and pore size of the composite membrane, thereby affecting the electrolyte wetting efficiency, increasing the interfacial impedance between the electrolyte and the electrode, and thus deteriorating the battery cell performance. As shown in Application Example 11, the composite membrane of the present invention also has good adaptability to high-voltage positive electrode LNMO.

[0219] Table 3 Performance of Solid Electrolyte Membranes

[0220]

[0221] For Application Example 12 and Comparative Example 10, it can be seen that when the composite membrane of the present invention is used as the matrix of the solid electrolyte, the mechanical properties, ionic conductivity, and lithium-ion transference number of the solid electrolyte membrane can be effectively improved. The use of a high-strength core-pore membrane greatly enhances the mechanical properties of the PEO-based solid electrolyte and improves its processability; simultaneously, the core-pore membrane significantly shortens the lithium-ion transport path and accelerates the lithium-ion migration process; PVDF molecules contain abundant fluorine atoms, which carry a negative charge, thus exhibiting cation selectivity based on electrostatic interactions. By utilizing the strong binding force between PVDF and anions, the migration efficiency of anions is significantly reduced, and the decomposition of LiTFSI can be promoted, releasing more free lithium ions, thereby improving the ionic conductivity and lithium-ion transference number of the solid electrolyte.

[0222] The embodiments described above are merely some embodiments of the present invention, designed to enable those skilled in the art to understand and use the invention. Obviously, anyone skilled in the art can make slight modifications or variations to these embodiments and apply them to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any equivalent changes, simple modifications, and alterations made within the scope of the present invention still fall within its coverage.

Claims

1. A composite membrane, characterized in that, The composite membrane includes a nuclear pore membrane substrate and polyvinylidene fluoride (PVDF). The PVDF is distributed in the surface region and the pore region of the nuclear pore membrane substrate. The cross section S of the composite membrane includes the surface region and the pore region S1 of the nuclear pore membrane substrate. A1 is the percentage of the mass of fluorine element contained in the pore region S1 relative to the total mass of all elements in the pore region. A is the percentage of the mass of fluorine element contained in the cross section S relative to the total mass of all elements in the cross section S. The ratio of A1 to A is (0.52-0.9):

1.

2. The composite membrane as described in claim 1, characterized in that, The composite membrane satisfies one or more of the following conditions a: a. The ratio of the fluorine content A1 in the channel region S1 to the fluorine content A in the cross section S is (0.57-0.87):1; b. The thickness of the polyvinylidene fluoride in the channel region is 0.1 μm-0.6 μm; c. The average pore size of the composite membrane is 1μm-5μm; d. The porosity of the composite membrane is 30%-50%; e. The thickness ratio of the surface region to the nuclear pore membrane substrate is 1:(40-42); and, f. The fluorine content in the channel region is higher than the fluorine content in the surface region.

3. The composite membrane as described in claim 1, characterized in that, The composite membrane satisfies one or two of the following conditions: g. The ratio of the fluorine content A1 in the channel region S1 to the fluorine content A in the cross section S is (0.65-0.87):1; and, h. The thickness of the polyvinylidene fluoride in the channel region is 0.3μm-0.6μm.

4. The composite membrane as described in claim 1, characterized in that, The nuclear pore membrane substrate satisfies one or more of the following conditions: i. The material of the nuclear pore membrane substrate is one or more of polycarbonate, polyester, polyimide, polyvinylidene fluoride, and polypropylene; j. The average pore size of the nuclear pore membrane substrate is 2.5 μm-5.5 μm; and, k. The porosity of the nuclear pore membrane substrate is 35%-55%.

5. A method for preparing a composite membrane as described in any one of claims 1-4, characterized in that, Polyvinylidene fluoride (PVDF) slurry is coated onto one or both sides of the nuclear pore membrane substrate. After casting, excess PVDF slurry is removed from the surface to obtain the composite membrane. The concentration of PVDF in the PVDF slurry is 1-10 wt%, and the coating amount of the PVDF slurry is 2-15 g / m³. 2 .

6. The method for preparing the composite membrane as described in claim 5, characterized in that, The preparation method satisfies one or two of the following conditions: l. The concentration of polyvinylidene fluoride in the polyvinylidene fluoride slurry is 4-6 wt%; and, m. The coating amount of the polyvinylidene fluoride slurry is 8-12 g / m. 2 .

7. A solid electrolyte membrane, characterized in that, It includes the composite membrane as described in any one of claims 1-4.

8. A battery cell, characterized in that, It includes the composite membrane as described in any one of claims 1-4.

9. An electrochemical device, characterized in that, It includes the composite membrane as described in any one of claims 1-4.

10. An electronic device, characterized in that, It includes the electrochemical device as described in claim 9.

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