Multifunctional composite film, square aluminum shell battery cell and application

Abstract

The invention belongs to the technical field of lithium ion batteries, particularly relates to an ultrathin multifunctional composite film which can be controllably swelled in electrolyte, can be compressed and buffered and can absorb harmful gas in situ, and a square aluminum shell battery cell, and further discloses a preparation method and application of the ultrathin multifunctional composite film. The multifunctional composite membrane comprises an ultrathin three-layer gradient structure of an inner layer membrane, a middle layer membrane and an outer layer membrane which are laminated and compounded, the inner layer is controllable in swelling, the middle layer is closed and compressible and in-situ air suction, and the outer layer is self-lubricating and thermally stable. The total thickness of the three-layer composite film is only 100-200m, and is reduced by more than or equal to 20% compared with the traditional scheme, and the energy density is improved by 5-8%.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to an ultrathin multifunctional composite membrane that can be controlled to swell in an electrolyte, can be compressed and buffered, and can absorb harmful gases in situ, as well as a square aluminum-cased battery cell, and further discloses its preparation method and application. Background Technology

[0002] Lithium-ion batteries are high-energy-density rechargeable batteries with advantages such as long lifespan, no memory effect, and environmental friendliness. Since their commercialization, they have been widely used in various portable electronic devices such as mobile phones and laptops.

[0003] A square aluminum-cased battery cell is a lithium-ion battery with a square aluminum casing. A typical square aluminum-cased battery consists of an aluminum casing, electrolyte, lead-acid, and sealant. Square aluminum-cased batteries are widely used in mobile communications, laptops, electric bicycles, and electric vehicles due to their advantages such as high energy density, light weight, long lifespan, low self-discharge rate, and no memory effect.

[0004] In recent years, with the widespread application of high-nickel cathodes and silicon-carbon anodes, the following problems have occurred in the cycling process of square aluminum-cased cells: (1) the electrode thickness expands by ≥15%, leading to shell bulging and interface contact failure; (2) the electrolyte decomposes to produce gases such as HF and CO2, causing aluminum shell corrosion and increased impedance. Existing technologies attempt to absorb the generated gas through ceramic coating, elastic buffer pads, or independent gas bags, but these are all "single-function" solutions, making it difficult to integrate the three functions of "swelling-compression-gas absorption" within a thickness of <200µm.

[0005] For example, the square aluminum-cased lithium battery insulating film disclosed in Chinese patent CN214411327U has a "bottom support + side film + melamine sponge" sandwich structure at the bottom and sides of the cell, which absorbs the expansion of the electrode through the elasticity of the sponge. The thickness is ≥0.8mm, but it only has the functions of insulation and shock absorption. On the one hand, the structure with a thickness of ≥800µm is prone to causing energy density loss in the cell; and the open structure of the sponge cannot absorb harmful gases such as HF and CO2; in particular, it does not have the function of adaptive swelling of electrolyte.

[0006] For example, the square battery disclosed in Chinese patent CN223230409U uses an "elastic layer" design on the large surface of the square cell casing. This allows the square battery to exert pressure on the internal cells during the entire charging and discharging process, creating a binding effect. This ensures a tight interface between the positive electrode and the separator, and between the negative electrode and the separator, shortening the lithium-ion transport distance and improving the battery's rate performance and cycle performance. However, on the one hand, the thickness d of its elastic layer is relatively large (0.5mm≤d≤2.5mm), resulting in significant energy density loss in the cell, and it lacks electrolyte swelling self-adaptation function. Summary of the Invention

[0007] The first objective of this invention is to provide an ultrathin multifunctional composite membrane that integrates three functions—swelling, compression, and gas absorption—within a thickness of <200µm. This membrane can controllably swell in electrolyte, can be compressed and buffered, and can absorb harmful gases in situ, resulting in superior application performance. The second objective of this invention is to provide a square aluminum-cased battery cell with superior performance. The third objective of this invention is to provide a method for preparing and applying the aforementioned ultrathin multifunctional composite film and square aluminum-cased battery cell.

[0008] To address the aforementioned technical problems, this invention provides a multifunctional composite membrane, comprising an inner layer membrane, a middle layer membrane, and an outer layer membrane stacked sequentially, wherein... The inner membrane is a cross-linked resin membrane, which comprises the following components by mass content: 60-80% cross-linked resin, 15-25% porous SiO2, 5-10% sulfonated lithium salt, and 1-5% sulfonated graphene; The middle layer membrane includes a porous substrate, in which basic magnesium carbonate is grown in situ on the pore walls. The outer membrane includes a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating, or an aramid-PTFE composite membrane.

[0009] Specifically, in the multifunctional composite membrane, the crosslinking resin in the inner layer includes crosslinked PEO-b-PMMA or crosslinked PVDF-HFP.

[0010] Specifically, in the multifunctional composite membrane, the porous matrix of the middle layer membrane includes closed-cell EPDM foam or graphene aerogel; Preferably, the porosity of the porous substrate membrane is 60-80%; Preferably, the pore size D of the porous substrate membrane is... 50 50-100nm; Preferably, the basic magnesium carbonate content based on the porous substrate membrane is 20-40 wt%.

[0011] Specifically, in the multifunctional composite membrane, the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The inorganic particle membrane includes an Al2O3 particle membrane; The self-lubricating coating includes a fluorocarbon self-lubricating coating.

[0012] Specifically, the multifunctional composite membrane: The thickness of the multifunctional composite membrane is 100-200µm; The thickness of the inner layer membrane is 30-50µm; The thickness of the middle layer membrane is 50-120µm; The thickness of the outer membrane is 20-30µm.

[0013] Specifically, in the multifunctional composite membrane, the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The thickness of the PI film is 10-15µm; The thickness of the inorganic particle film is 100-300 nm; The thickness of the self-lubricating coating is 15-20µm.

[0014] The present invention also discloses a method for preparing the aforementioned multifunctional composite membrane, comprising the following steps: (1) Prepare a slurry according to the composition of the inner membrane, and form the desired inner membrane by coating, flash evaporation and UV crosslinking treatment; (2) The porous substrate is vacuum impregnated in basic magnesium carbonate slurry, and then dried and hot-pressed to reduce thickness to obtain the desired middle layer film; (3) Prepare the required outer membrane according to the selected structure; (4) The inner layer film, the middle layer film and the outer layer film are stacked and composited in sequence, and hot-pressed composite treatment is performed under a protective atmosphere to obtain the final product.

[0015] Specifically, the preparation method of the multifunctional composite membrane is as follows: In step (1), the coating step includes slot coating; and / or, In step (1), the temperature of the flash evaporation step is 60-100℃; and / or, In step (1), the irradiation wavelength of the UV crosslinking step is 250-400 nm, and the light intensity is 50 mW / cm². 2 -500mW / cm 2 ; and / or, In step (2), the temperature of the drying step is 100-150℃; and / or, In step (2), the temperature of the hot pressing thickness reduction step is 135±5℃, the pressure is 0.6±0.1MPa, and the time is 60±10s; and / or, In step (3), the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating. Its preparation steps include substrate treatment, nano-ceramic layer deposition, and the coating and curing of the fluorocarbon self-lubricating coating. The substrate processing power is 5-10kW, and the conveyor speed is 10-15m / min; and / or, The deposition temperature of the nano-ceramic layer is 100-150℃, and the number of cycles is 0.1-0.2 nm / cycle; and / or, In the coating and curing steps of the fluorocarbon self-lubricating coating, 1-3 wt% nano-silica is used as a reinforcing agent, and 0.5-1 wt% leveling agent is added to form a slurry with a solid content of 20-30 wt%; and / or, In the coating and curing steps of the fluorocarbon self-lubricating coating, the micro-relief screen cell volume is 8-20 cm³. 3 / m 2 Microgravure rollers, coating speed 5-20 m / min; and / or, In step (4), the temperature of the hot-pressing composite step is 120-150℃, the pressure is 0.4-0.8MPa, and the processing time is 40-80s; Step (4) further includes a roll-to-roll winding of the composite film; preferably, the winding speed is 5-10 m / min and the tension is 20-40 N.

[0016] The present invention also discloses the application of the multifunctional composite film or the multifunctional composite film prepared by the method in the field of square aluminum-cased battery cells.

[0017] The present invention also discloses a square aluminum-cased battery cell, battery module, battery pack, or power device comprising the multifunctional composite membrane or the multifunctional composite membrane prepared by the method.

[0018] The multifunctional composite membrane of this invention comprises an ultrathin three-layer gradient structure consisting of an inner layer membrane, a middle layer membrane, and an outer layer membrane stacked together. The inner layer exhibits controllable swelling, the middle layer is closed-cell and compressible with in-situ gas absorption, and the outer layer is self-lubricating and thermally stable. The total thickness of the three-layer composite membrane is only 100-200µm, representing a reduction of ≥20% compared to traditional solutions, while increasing energy density by 5-8%.

[0019] The multifunctional composite membrane described in this invention solves the problem that traditional battery cell products cannot simultaneously meet the four major shortcomings of "ultra-thin, controllable swelling, compressible buffering, and in-situ HF absorption". The multifunctional composite membrane described in this invention can withstand 1000 cycles at 50% compressive strain with permanent deformation of <5%, significantly delaying shell bulging; and it can absorb HF in situ, reducing the internal pressure of the shell by 60% after 800 cycles, greatly improving product application performance. Attached Figure Description

[0020] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a schematic diagram of the structure of the composite membrane described in this invention; Figure 2This is an exploded view of the composite membrane described in this invention. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Unless otherwise defined, 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 application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0023] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0024] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0025] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0026] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0027] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0028] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0029] In the description of the embodiments of this application, unless otherwise expressly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0030] As attached Figure 1-2 The composite membrane shown in the figure has a structure. In the following embodiment of the present invention, a multifunctional composite membrane is provided, comprising an inner layer membrane, a middle layer membrane, and an outer layer membrane stacked sequentially, wherein... The inner membrane is a cross-linked resin membrane, which comprises the following components by mass content: 60-80% cross-linked resin, 15-25% porous SiO2, 5-10% sulfonated lithium salt, and 1-5% sulfonated graphene; The middle layer membrane includes a porous substrate, in which basic magnesium carbonate is grown in situ on the pore walls. The outer membrane includes a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating, or an aramid-PTFE composite membrane.

[0031] In some specific embodiments, the crosslinked resin in the inner membrane includes crosslinked PEO-b-PMMA or crosslinked PVDF-HFP.

[0032] In some specific embodiments, the porous matrix in the middle layer membrane includes closed-cell EPDM foam or graphene aerogel. In some specific embodiments, the porosity of the porous substrate membrane is 60-80%; In some specific embodiments, the pore size D of the porous substrate membrane 50 50-100nm; In some specific embodiments, the basic magnesium carbonate content based on the porous substrate membrane is 20-40 wt%.

[0033] In some specific embodiments, the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The inorganic particle membrane includes an Al2O3 particle membrane; The self-lubricating coating includes a fluorocarbon self-lubricating coating.

[0034] In some specific embodiments, the thickness of the multifunctional composite membrane is 100-200µm; In some specific embodiments, the thickness of the inner layer film is 30-50µm; In some specific embodiments, the thickness of the intermediate layer film is 50-120µm; In some specific embodiments, the thickness of the outer film is 20-30µm.

[0035] In some specific embodiments, the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The thickness of the PI film is 10-15µm; The thickness of the inorganic particle film is 100-300 nm; The thickness of the self-lubricating coating is 15-20µm.

[0036] Example 1 As attached Figure 1-2 The structure of the composite membrane shown in this embodiment includes an inner layer membrane, a middle layer membrane, and an outer layer membrane that are sequentially stacked.

[0037] The thickness of the inner layer membrane is 30µm, and the inner layer slurry includes 70% PEO-b-PMMA, 20% SiO2, 8% LiTFSI, and 2% sulfonated graphene.

[0038] The thickness of the middle layer membrane is 90µm; the density of the EPDM foam is 0.25g / cm³. 3 The basic magnesium carbonate loading is 25 wt%.

[0039] The outer film thickness is approximately 30µm: a three-layer PI / Al2O3 / fluorocarbon coating, wherein the PI (polyimide) base film layer is 12µm; the Al2O3 (alumina) ceramic coating is 200nm; and the fluorocarbon self-lubricating coating is 18µm.

[0040] In this embodiment, the preparation method of the multifunctional composite membrane includes the following steps.

[0041] Inner coating and crosslinking Under nitrogen protection, the prepared inner layer slurry is applied to the PET release film through a slit coating head at a speed of 8 m / min, resulting in a wet film thickness of 60 µm. After coating, it passes through an 85°C hot air flash evaporation zone to remove most of the solvent. Then, it enters the UV crosslinking zone at a wavelength of 365 nm and a light intensity of 150 mW / cm². 2 Irradiation for 3 seconds (cumulative energy 450 mJ / cm) 2 This forms a cross-linked inner layer with a thickness of 30µm.

[0042] Intermediate layer preparation and functionalization The EPDM foam roll was passed through an impregnation tank filled with basic magnesium carbonate nano-slurry, and a vacuum of -0.09 MPa was applied to the back of the foam to achieve in-situ loading. It was then thoroughly dried through a 120°C hot air drying channel. The foam was then compressed to a fixed thickness of 90 µm using hot press rollers (110°C, 2 MPa), with a basic magnesium carbonate loading of 25 wt%.

[0043] Outer layer preparation and functionalization PI-based membrane: Commercially available 12µm biaxially oriented PI membrane was directly purchased and corona treated (6kW, 12 m / min). Al2O3 deposition: roll-to-roll magnetron sputtering process was used; parameters included: power 3kW, tape speed 0.5m / min, Ar / O2 atmosphere; Fluorocarbon coating and curing: Slurry: PVDF-HFP, solids content 25%; Coating: Microgravure coating, cell volume 12cm³ 3 / m 2 Speed ​​10m / min (wet film thickness ~24µm); Curing: three-stage curing (85℃→140℃→180℃).

[0044] Three-layer hot-pressed composite The inner layer (along with the carrier film), the functionalized intermediate layer, and the outer layer roll are aligned and fed into the laminator. The laminations are held at 135°C and 0.6 MPa for 60 seconds, with the entire process conducted under nitrogen protection.

[0045] Collect After the composite film cools, the PET carrier film is peeled off. The film is then wound up at a speed of 8 m / min under a constant tension of 30 N to obtain the finished composite film.

[0046] The composite membrane described in this embodiment was tested and found to have a swelling rate of 42%, a compression rate of 50%, a residual deformation of 3.2% after 1000 cycles, and an HF absorption of 35 mL / g.

[0047] Example 2 As attached Figure 1-2 The structure of the composite membrane shown in this embodiment includes an inner layer membrane, a middle layer membrane, and an outer layer membrane that are sequentially stacked.

[0048] The thickness of the inner layer membrane is 20µm, and the inner layer slurry includes 70% PEO-b-PMMA, 20% SiO2, 8% LiTFSI, and 2% sulfonated graphene.

[0049] The thickness of the middle layer membrane is 60µm; the density of the EPDM foam is 0.25g / cm³. 3 The basic magnesium carbonate loading is 25 wt%.

[0050] The outer film thickness is 20µm: PI / Al2O3 / fluorocarbon three-layer integrated, wherein the PI (polyimide) base film layer is 8µm; the Al2O3 (alumina) ceramic coating is 150nm; and the fluorocarbon self-lubricating coating is 12µm.

[0051] In this embodiment, the preparation method of the multifunctional composite membrane includes the following steps.

[0052] Inner coating and crosslinking The slot coating speed was increased to 12 m / min, and the wet film thickness was controlled at 40 µm.

[0053] After flash evaporation at 85℃, under a higher light intensity of 300mW / cm 2 Irradiation for 1.5 seconds (cumulative energy 450 mJ / cm²) 2 This forms a cross-linked inner layer with a thickness of 20µm.

[0054] Intermediate layer preparation and functionalization A thinner initial EPDM foam (<120µm) was used. The specific impregnation and drying process was the same as in Example 1. The foam was forcefully compressed to a thickness of 60µm using hot rollers (110°C, 2.5MPa), with a basic magnesium carbonate loading of 25wt%.

[0055] Outer layer preparation and functionalization PI base film: Purchase high-strength 8µm PI film. Because it is thinner, the corona treatment power needs to be reduced to 4kW (speed 15m / min) to prevent breakdown. Al2O3 deposition: High-precision magnetron sputtering parameters: precursor TMA / H2O, temperature 120℃, 1500 cycles; Fluorocarbon coating and curing: Slurry: Solid content increased to 28% to reduce the required wet film thickness; Coating: Microgravure coating, using a gravure roller with a smaller cell volume (8cm). 3 / m 2 ), speed 15m / min (wet film thickness ~16µm); Curing: The curing process is adjusted to 90℃→150℃→185℃. Because the film layer is thinner, the heating rate can be appropriately increased.

[0056] Three-layer hot-pressed composite The composite process parameters are the same as in Example 1.

[0057] Collect Wind up at a speed of 10 m / min under a tension of 25 N.

[0058] Example 3 As attached Figure 1-2 The structure of the composite membrane shown in this embodiment includes an inner layer membrane, a middle layer membrane, and an outer layer membrane that are sequentially stacked.

[0059] The thickness of the inner layer membrane is 50µm, and the inner layer slurry includes 70% PEO-b-PMMA, 20% SiO2, 8% LiTFSI, and 2% sulfonated graphene.

[0060] The thickness of the middle layer membrane is 120µm; the density of the EPDM foam is 0.25g / cm³. 3 The basic magnesium carbonate loading is 25 wt%.

[0061] The outer film thickness is 30µm: PI / Al2O3 / fluorocarbon three-layer integrated, wherein the PI (polyimide) base film layer is 12µm; the Al2O3 (alumina) ceramic coating is 200nm; and the fluorocarbon self-lubricating coating is 18µm.

[0062] In this embodiment, the preparation method of the multifunctional composite membrane includes the following steps.

[0063] Inner coating and crosslinking The slot coating speed was reduced to 5 m / min, and the wet film thickness was controlled at 100 µm. After flash evaporation at 85 °C, the film was coated under light intensity of 100 mW / cm². 2 Irradiation for 6 seconds (cumulative energy 600 mJ / cm²) 2 This ensures that the thick film is fully cross-linked, forming an inner layer with a thickness of 50µm.

[0064] Intermediate layer preparation and functionalization The initial EPDM foam with higher porosity (<250µm) was used. The impregnation and drying processes were the same as in Example 1.

[0065] The material is lightly pressed to a thickness of 120µm using a hot press roller (110℃, 1.5MPa) to retain more of the porous structure, with a basic magnesium carbonate loading of 25wt%.

[0066] Outer layer preparation and functionalization PI-based membrane: Commercially available 12µm biaxially oriented PI membrane was directly purchased and corona treated (6 kW, 12m / min). Al2O3 deposition: roll-to-roll magnetron sputtering process; parameters: power 3kW, tape speed 0.5m / min, Ar / O2 atmosphere; Fluorocarbon coating and curing: Slurry: PVDF-HFP, solids content 25%; Coating: Microgravure coating, cell volume 12cm³ 3 / m 2Speed ​​10m / min (wet film thickness ~24µm); Curing: three-stage curing (85℃→140℃→180℃).

[0067] Three-layer hot-pressed composite The composite process parameters are the same as in Example 1.

[0068] Collect Wind up at a speed of 5 m / min under a tension of 40 N to prevent the thick film from wrinkling.

[0069] Example 4 The structure of the multifunctional composite membrane described in this embodiment is the same as that in Embodiment 1, except that the crosslinking resin in the inner layer membrane is selected as crosslinked PVDF-HFP.

[0070] Example 5 The structure of the multifunctional composite membrane described in this embodiment is the same as that in Embodiment 1, except that the porous matrix in the middle layer membrane is graphene aerogel.

[0071] Example 6 The structure of the multifunctional composite membrane described in this embodiment is the same as that in Embodiment 1, except that the outer membrane is an aramid-PTFE composite membrane.

[0072] Comparative Example 1 This comparative example uses a non-composite film method, i.e., it uses traditional Mylar film.

[0073] Comparative Example 2 The structure of the composite membrane described in this comparative example is the same as that in Example 1, except that only an inner membrane and an outer membrane are provided.

[0074] Comparative Example 3 The structure of the composite membrane described in this comparative example is the same as that in Example 1, except that only an inner membrane and a middle membrane are provided.

[0075] Experimental Example 1. Composite membrane performance The performance of the composite membranes prepared under the above Examples 1-6 and Comparative Examples 2-3 was tested respectively.

[0076] Take the clamping plate of the square battery cell, and then perform a 1C / 1C cycle test for 1000 cycles. The thickness of the cell pack and the battery cell before the test is sampled and tested. Then the thickness of the battery cell and the cell pack after the cycle is tested. The results are shown in Table 1 below.

[0077] Table 1

[0078] As can be seen, in the swelling rate data, both the example schemes and the comparative schemes have good performance. Among them, the data of Experimental Example 2 is the worst, which is because its composite film has the lowest thickness and therefore the effect is poor. Experimental Example 3 has the best effect, but its lead over the other examples is limited.

[0079] 2. Electrochemical performance The composite films prepared under the above-mentioned Examples 1-6 and Comparative Examples 1-3 were placed into aluminum shells of 52148. The aluminum shells were all placed into the same batch of core packs, and the subsequent process parameters were kept consistent. The cells of different schemes were cycled, and the interface was disassembled and confirmed after 1000 cycles. The results are shown in Table 2 below.

[0080] Table 2

[0081] It is evident that the performance of the battery cell described in this invention is improved compared to the comparative example, indicating that the composite film can enhance the performance of the battery cell; at the same time, the expansion rate is also reduced, indicating that the composite film can suppress the expansion of the battery cell during cycling; and as the thickness of the composite film increases, the performance of the battery cell also improves.

[0082] In summary, the multifunctional composite membrane of the present invention has a total thickness of only 100-200µm, which is ≥20% thinner than the traditional solution and increases the energy density by 5-8%. It effectively solves the problem that traditional battery cell products cannot simultaneously meet the four major shortcomings of "ultra-thin, controllable swelling, compressible buffer, and in-situ air intake".

[0083] The embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A multifunctional composite membrane, characterized in that, The composite membrane comprises an inner membrane, a middle membrane, and an outer membrane stacked sequentially, wherein, The inner membrane is a cross-linked resin membrane, which comprises the following components by mass content: 60-80% cross-linked resin, 15-25% porous SiO2, 5-10% sulfonated lithium salt, and 1-5% sulfonated graphene; The middle layer membrane includes a porous substrate, in which basic magnesium carbonate is grown in situ on the pore walls. The outer membrane includes a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating, or an aramid-PTFE composite membrane.

2. The multifunctional composite membrane according to claim 1, characterized in that, In the inner membrane, the crosslinking resin includes crosslinked PEO-b-PMMA or crosslinked PVDF-HFP.

3. The multifunctional composite membrane according to claim 1, characterized in that, In the middle layer membrane, the porous matrix includes closed-cell EPDM foam or graphene aerogel; Preferably, the porosity of the porous substrate membrane is 60-80%; Preferably, the pore size D of the porous substrate membrane is... 50 50-100nm; Preferably, the basic magnesium carbonate content based on the porous substrate membrane is 20-40 wt%.

4. The multifunctional composite membrane according to claim 1, characterized in that, The outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The inorganic particle membrane includes an Al2O3 particle membrane; The self-lubricating coating includes a fluorocarbon self-lubricating coating.

5. The multifunctional composite membrane according to any one of claims 1-4, characterized in that: The thickness of the multifunctional composite membrane is 100-200µm; The thickness of the inner layer membrane is 30-50µm; The thickness of the middle layer membrane is 50-120µm; The thickness of the outer membrane is 20-30µm.

6. The multifunctional composite membrane according to claim 5, characterized in that, The outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating; wherein, The thickness of the PI film is 10-15µm; The thickness of the inorganic particle film is 100-300 nm; The thickness of the self-lubricating coating is 15-20µm.

7. A method for preparing a multifunctional composite membrane as described in any one of claims 1-6, characterized in that, Includes the following steps: (1) Prepare a slurry according to the composition of the inner membrane, and form the desired inner membrane by coating, flash evaporation and UV crosslinking treatment; (2) The porous substrate is vacuum impregnated in basic magnesium carbonate slurry, and then dried and hot-pressed to reduce thickness to obtain the desired middle layer film; (3) Prepare the required outer membrane according to the selected structure; (4) The inner layer film, the middle layer film and the outer layer film are stacked and composited in sequence, and hot-pressed composite treatment is performed under a protective atmosphere to obtain the final product.

8. The method for preparing the multifunctional composite membrane according to claim 7, characterized in that: In step (1), the coating step includes slot coating; and / or, In step (1), the temperature of the flash evaporation step is 60-100℃; and / or, In step (1), the irradiation wavelength of the UV crosslinking step is 250-400 nm, and the light intensity is 50 mW / cm². 2 -500mW / cm 2 ; and / or, In step (2), the temperature of the drying step is 100-150℃; and / or, In step (2), the temperature of the hot pressing thickness reduction step is 135±5℃, the pressure is 0.6±0.1MPa, and the time is 60±10s; and / or, In step (3), the outer membrane is a composite membrane containing a PI membrane, an inorganic particle membrane, and a self-lubricating coating. Its preparation steps include substrate treatment, nano-ceramic layer deposition, and the coating and curing of the fluorocarbon self-lubricating coating. The substrate processing power is 5-10kW, and the conveyor speed is 10-15m / min; and / or, The deposition temperature of the nano-ceramic layer is 100-150℃, and the number of cycles is 0.1-0.2 nm / cycle; and / or, In the coating and curing steps of the fluorocarbon self-lubricating coating, 1-3 wt% nano-silica is used as a reinforcing agent, and 0.5-1 wt% leveling agent is added to form a slurry with a solid content of 20-30 wt%; and / or, In the coating and curing steps of the fluorocarbon self-lubricating coating, the micro-relief screen cell volume is 8-20 cm³. 3 / m 2 Microgravure rollers, coating speed 5-20 m / min; and / or, In step (4), the temperature of the hot-pressing composite step is 120-150℃, the pressure is 0.4-0.8MPa, and the processing time is 40-80s; Step (4) further includes a roll-to-roll winding of the composite film; preferably, the winding speed is 5-10 m / min and the tension is 20-40 N.

9. The application of the multifunctional composite film according to any one of claims 1-6 or the multifunctional composite film prepared by the method according to claim 7 or 8 in the field of square aluminum-cased battery cells.

10. A square aluminum-cased battery cell, battery module, battery pack, or power device comprising the multifunctional composite membrane according to any one of claims 1-6 or the multifunctional composite membrane prepared by the method according to claim 7 or 8.

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