Composite diaphragm and battery using same

Abstract

The invention provides a composite diaphragm. The composite diaphragm comprises a base membrane framework and a first functional layer, and the base membrane framework sequentially comprises a base membrane layer and a nanofiber layer; a nanofiber layer is arranged on at least one side of the base membrane layer in the thickness direction of the composite membrane; at least one surface of the composite diaphragm is provided with a first functional layer; in the thickness direction of the composite diaphragm, the first functional layer comprises a mixed layer and / or an adhesive layer, the mixed layer contains an adhesive compound and ceramic particles which are mixed with each other, and the adhesive layer is formed by the adhesive compound; in the thickness direction of the composite diaphragm, the area of one side surface of the base membrane layer is S1, the projection area of the nanofiber layer on the base membrane layer is S2, and S2 / S1 is equal to 90-100%; the air permeability value of the composite diaphragm at T1 for 1 hour is 10000-20000s / 100mL, and T1 is more than or equal to 200 DEG C. According to the composite diaphragm, the mechanical property, the porosity and the thermal stability of the composite diaphragm can be effectively improved by setting a special layer combination and controlling specific parameters of the composite diaphragm, so that the cycle performance, the safety performance and the like of the lithium battery can be effectively considered.

Description

Technical Field

[0001] The present invention belongs to the technical field of diaphragms and lithium batteries, and particularly relates to a composite diaphragm and a battery using the same. Background Art

[0002] Lithium batteries have many excellent properties such as high energy density, high coulombic efficiency, low self-discharge characteristics and long service life. These excellent properties make them ideal portable devices and are widely used in many fields such as aerospace, national defense and military industry, new energy vehicles and 3C electronic products.

[0003] The positive electrode, negative electrode, electrolyte, and separator are essential components for the proper functioning of lithium-ion batteries. During battery operation, the separator, as a crucial functional component, primarily blocks contact between the positive and negative electrodes and effectively transmits lithium ions. It acts like a solid barrier, separating the positive and negative electrodes and preventing short circuits.

[0004] Traditional separators are primarily made of a single material, such as polyolefins (e.g., polyethylene and polypropylene). These separators are primarily composed of a single polymer film. While these separators offer excellent chemical stability and mechanical strength, they tend to shrink at high temperatures, leading to internal short circuits and potential safety hazards. Furthermore, single-material separators have limited ionic conductivity, making them inadequate for high-performance electrochemical devices.

[0005] To address these issues, composite membrane technology has emerged in recent years. Composite membranes typically consist of two or more layers, most commonly a polyolefin substrate coated with ceramic particles or a polymer coating to improve the membrane's thermal stability and ion conductivity. However, existing composite membranes still have some shortcomings. For example, the ceramic particle coating may reduce the membrane's flexibility, while the polymer coating may not be thermally stable enough for use in high-temperature environments. Summary of the Invention

[0006] In order to solve the problems and shortcomings existing in the prior art, the present invention provides a composite diaphragm and a battery using the same. The composite diaphragm can effectively improve the mechanical properties, porosity and thermal stability of the composite diaphragm by setting a special layer combination and controlling the specific parameters of the composite diaphragm. Therefore, it can effectively take into account the cycle performance, safety performance, etc. of the lithium battery, and can provide basic support for the further development of high-performance lithium batteries.

[0007] According to a first aspect of the present invention, a composite membrane is provided, comprising a base membrane skeleton and a first functional layer, wherein the base membrane skeleton comprises a base membrane layer and a nanofiber layer in sequence; in the thickness direction of the composite membrane, a nanofiber layer is provided on at least one side of the base membrane layer; at least one surface of the composite membrane is provided with a first functional layer; in the thickness direction of the composite membrane, the first functional layer comprises a mixed layer and / or an adhesive layer, the mixed layer contains adhesive compounds and ceramic particles mixed with each other, and the adhesive layer is composed of an adhesive compound; in the thickness direction of the composite membrane, the area of ​​one side surface of the base membrane layer is S1, and the projected area of ​​the nanofiber layer on the base membrane layer is S2, and S2 / S1=90~100%; the air permeability of the composite membrane under T1 for 1h is 10000~20000s / 100mL, and T1≥200℃.

[0008] The polymer film layer mainly provides insulation performance, and the nanofiber layer mainly provides the function of the skeleton, which optimizes the strength of the diaphragm. Therefore, the base film skeleton containing the polymer film layer and the nanofiber layer can take into account the insulation and mechanical strength of the diaphragm. In addition, at least one surface of the composite diaphragm is also provided with a first functional layer. The first functional layer has an adhesive property, which can enhance the adhesiveness and liquid retention of the composite diaphragm, thereby promoting the close adhesion between the positive and negative electrodes and the diaphragm and promoting the infiltration performance of the electrolyte, thereby optimizing the cycle performance of the lithium battery. Under the above-mentioned special multi-layer structure, the layers of the obtained composite diaphragm have good bonding performance, which greatly improves the overall mechanical properties of the composite diaphragm and the performance of the battery. Therefore, the special structural design of the composite diaphragm in the present invention is conducive to improving the comprehensive performance of the diaphragm, thereby improving the battery performance.

[0009] Furthermore, controlling the ratio of the projected area S2 of the nanofiber layer on the polymer film layer to the area S1 of one of the side surfaces of the polymer film layer within a specific range can help to have a nanofiber layer of sufficient size as a skeleton to enhance the mechanical strength of the diaphragm, while effectively improving the overall porosity of the diaphragm, which is beneficial to provide a certain channel for lithium ion transmission and optimize the overall cycle performance of the lithium battery.

[0010] Furthermore, controlling the air permeability of the composite membrane within the above range, that is, the composite membrane having a relatively large air permeability at high temperatures, can promote the thermal closure of the composite membrane at high temperatures (the nanofiber layer has a high porosity, and at high temperatures, the base film or adhesive layer can melt and block the pores of the nanofibers), thereby hindering the normal transmission of lithium ions, thereby avoiding direct contact between the positive and negative electrodes of the battery at high temperatures, and effectively improving the safety performance of the lithium battery. It should be noted here that taking "10,000s / 100mL" as an example, it means that it takes 10,000s for 100mL of gas to pass through the composite membrane, that is, the longer the time required for 100mL of gas to pass through, the worse the air permeability of the membrane. That is, the higher the air permeability value mentioned in the present invention, the worse the air permeability of the composite membrane, which is more conducive to hindering the normal transmission of lithium ions.

[0011] In an optional embodiment, the nanofiber layer includes multiple regions, each of which is spaced apart on at least one side of the basement membrane layer. Even if the nanofiber layer is spaced apart on the surface of the basement membrane layer, as long as S2 / S1 = 90-100%, the nanofiber layer's skeletal properties and the effective increase in the membrane porosity can still be achieved. Preferably, the multiple regions are of equal or different sizes.

[0012] In an optional embodiment, the composite membrane includes an adhesive layer, a basement membrane layer, and a nanofiber layer in sequence, wherein the basement membrane layer and the nanofiber layer in sequence constitute a basement membrane skeleton.

[0013] In an optional embodiment, the composite membrane includes a mixed layer, a basement membrane layer, and a nanofiber layer in sequence, wherein the basement membrane layer and the nanofiber layer in sequence constitute a basement membrane skeleton.

[0014] In an alternative embodiment, the composite membrane includes an adhesive layer, a basement membrane layer, a nanofiber layer, and a mixed layer in sequence, wherein the basement membrane layer and the nanofiber layer sequentially constitute a basement membrane skeleton.

[0015] Preferably, at least one first functional layer is provided on both sides of the base membrane skeleton in the thickness direction of the composite diaphragm. Providing relevant functional layers on both sides of the base membrane skeleton can effectively balance the internal stress of the composite diaphragm under high temperature or external forces, thereby further improving the overall mechanical properties and thermal stability of the diaphragm.

[0016] In an alternative embodiment, the composite membrane comprises, in sequence, a first adhesive layer, a basement membrane layer, a nanofiber layer, and a second adhesive layer. The basement membrane layer and the nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness.

[0017] In an alternative embodiment, the composite membrane comprises, in sequence, a first mixed layer, a basement membrane layer, a nanofiber layer, and a second mixed layer. The basement membrane layer and the nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness.

[0018] Preferably, the composite separator also includes a second functional layer comprising a ceramic layer formed from ceramic particles. Furthermore, the addition of the ceramic layer can further enhance the thermal stability and mechanical properties of the composite separator, effectively suppressing thermal shrinkage of the separator while also assisting the separator's closed-cell function to a certain extent. When the temperature rises to the separator's closed-cell temperature, the ceramic layer can help maintain the stability of the closed-cell structure, preventing the closed-cell separator from reopening, ensuring that the lithium-ion transmission channel is effectively blocked, thereby improving battery safety.

[0019] In an optional embodiment, the composite membrane comprises, in order, a first adhesive layer, a ceramic layer, a basement membrane layer, a nanofiber layer, and a second adhesive layer. The basement membrane layer and the nanofiber layer, in order, form a basement membrane skeleton. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness.

[0020] In an optional embodiment, the composite membrane includes an adhesive layer, a ceramic layer, a base membrane layer, a nanofiber layer, and a mixed layer in sequence, wherein the base membrane layer and the nanofiber layer sequentially constitute a base membrane skeleton.

[0021] In an optional embodiment, the composite membrane includes, in sequence, a first mixed layer, a ceramic layer, a basement membrane layer, a nanofiber layer, and a second mixed layer. The basement membrane layer and the nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness.

[0022] Preferably, in the thickness direction of the composite diaphragm, both sides of the base membrane skeleton are provided with at least one first functional layer and at least one second functional layer.

[0023] In an optional embodiment, the composite diaphragm includes, in order, a first adhesive layer, a first ceramic layer, a basement membrane layer, a nanofiber layer, a second ceramic layer, and a second adhesive layer. The basement membrane layer and the nanofiber layer, in order, form a basement membrane skeleton. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness.

[0024] In an alternative embodiment, the composite diaphragm includes, in order, a first mixed layer, a first ceramic layer, a basement membrane layer, a nanofiber layer, a second ceramic layer, and a second mixed layer. The basement membrane layer and the nanofiber layer, in order, form a basement membrane skeleton. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness.

[0025] Preferably, in the basement membrane skeleton, the thickness of the basement membrane layer is denoted as H1, the thickness of the nanofiber layer is denoted as H2, H1=3~20 μm, and H2=2~25 μm.

[0026] Preferably, when the first functional layer includes an adhesive layer, the thickness of the adhesive layer is H3, and H3=1-2 μm; when the first functional layer includes a mixed layer, the thickness of the mixed layer is H4, and H4=1-5 μm.

[0027] Preferably, the thickness of the second functional layer is H5, where H5=1-5 μm.

[0028] The thickness of the base film layer, nanofiber layer, first functional layer (adhesive layer or mixed layer), and second functional layer are limited to specific ranges, which is conducive to the full performance of the base film layer, nanofiber layer, and each functional layer, and will not restrict each other, thereby being more conducive to obtaining a composite diaphragm with excellent thermal stability and safety performance. At the same time, this will ensure that the total thickness of the final composite diaphragm will not Preferably, at least one nanofiber layer is provided on both sides of the basement membrane skeleton in the thickness direction of the composite diaphragm. That is, the basement membrane skeleton comprises, in sequence, a first nanofiber layer, a basement membrane layer, and a second nanofiber layer. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness.

[0029] In an alternative embodiment, the composite membrane comprises, in sequence, an adhesive layer, a first nanofiber layer, a basement membrane layer, and a second nanofiber layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness.

[0030] In an alternative embodiment, the composite membrane comprises, in sequence, a mixed layer, a first nanofiber layer, a basement membrane layer, and a second nanofiber layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness.

[0031] In an alternative embodiment, the composite membrane comprises, in order, an adhesive layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in order, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness.

[0032] In an alternative embodiment, the composite membrane comprises, in sequence, a first adhesive layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a second adhesive layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness.

[0033] In an alternative embodiment, the composite membrane comprises, in sequence, a first mixed layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a second mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, constitute a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness.

[0034] In an optional embodiment, the composite diaphragm includes, in sequence, a first adhesive layer, a ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a second adhesive layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness.

[0035] In an alternative embodiment, the composite membrane comprises, in order, an adhesive layer, a ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in order, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness.

[0036] In an optional embodiment, the composite diaphragm includes, in sequence, a first mixed layer, a ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, and a second mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness.

[0037] In an optional embodiment, the composite diaphragm includes, in sequence, a first adhesive layer, a first ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, a second ceramic layer, and a second adhesive layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness.

[0038] In an optional embodiment, the composite diaphragm includes, in sequence, a first mixed layer, a first ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, a second ceramic layer, and a second mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer, in sequence, constitute a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first mixed layer and the second mixed adhesive layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness.

[0039] Preferably, the nanofiber material in the nanofiber layer includes at least one of wood fiber (cellulose), PAN (polyacrylonitrile), polyimide (PI), para-aramid (PPTA), polyethylene terephthalate (PET), poly(p-phenylene benzobisoxazole), and polyarylethersulfoneketone (PAESK). Preferably, the adhesive compound in the adhesive layer includes at least one of polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA).

[0040] Preferably, in the mixed layer, the adhesive compound includes at least one of polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA); the ceramic particles include aluminum oxide. , boehmite, zirconium oxide, and titanium oxide.

[0041] Preferably, when the mixed layer includes the adhesive compound and the ceramic particles, the mass ratio of the adhesive compound to the ceramic particles is W1:W2, W1:W2=1:8-15.

[0042] Preferably, in the ceramic layer, the ceramic particles include aluminum oxide , boehmite, zirconium oxide, and titanium oxide.

[0043] Preferably, in the base film layer, the base film material includes at least one of polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), and polyimide (PI).

[0044] Preferably, the composite diaphragm includes a base membrane skeleton, a first functional layer, a second functional layer, and a third functional layer; the first functional layer includes a mixed layer formed by a mixture of an adhesive compound and ceramic particles, or the first functional layer includes an adhesive layer formed by an adhesive compound; the second functional layer includes a ceramic layer formed by ceramic particles; the third functional layer includes a thermosensitive layer formed by a thermosensitive material, and the thermosensitive material includes at least one of a homopolymer or copolymer of a non-fluorinated olefin monomer, an olefin monomer, an unsaturated nitrile monomer, an olefin monomer, and an ester monomer.

[0045] Preferably, the thickness of the third functional layer is H6, where H6=1-5 μm.

[0046] Preferably, the third functional layer is provided in the same manner as the second functional layer, that is, the third functional layer is added to the basement membrane skeleton, the first functional layer, and the second functional layer. The basement membrane skeleton here sequentially includes a basement membrane layer and a nanofiber layer, or sequentially includes a first nanofiber layer, a basement membrane layer, and a second nanofiber layer. The first functional layer, the second functional layer, and the third functional layer are provided on one side or both sides of the basement membrane skeleton, with the basement membrane skeleton serving as the intermediate layer. The first functional layer, the second functional layer, and the third functional layer are provided asymmetrically or symmetrically on both sides of the basement membrane skeleton.

[0047] In an optional embodiment, the composite diaphragm includes, in sequence, a first adhesive layer, a first thermally sensitive layer, a first ceramic layer, a basement membrane layer, a nanofiber layer, a second ceramic layer, a second thermally sensitive layer, and a second adhesive layer. The basement membrane layer and the nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness. Preferably, the first thermally sensitive layer and the second thermally sensitive layer have the same composition and / or thickness.

[0048] In an optional embodiment, the composite diaphragm includes, in sequence, a first mixed layer, a first thermosensitive layer, a first ceramic layer, a basement membrane layer, a nanofiber layer, a second ceramic layer, a second thermosensitive layer, and a second mixed layer. The basement membrane layer and the nanofiber layer, in sequence, form a basement membrane skeleton. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness. Preferably, the first thermosensitive layer and the second thermosensitive layer have the same composition and / or thickness.

[0049] In an optional embodiment, the composite diaphragm includes, in sequence, a first adhesive layer, a first thermosensitive layer, a first ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, a second ceramic layer, a second thermosensitive layer, and a second adhesive layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer sequentially constitute a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first adhesive layer and the second adhesive layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness. Preferably, the first thermosensitive layer and the second thermosensitive layer have the same composition and / or thickness.

[0050] In an optional embodiment, the composite diaphragm includes, in sequence, a first mixed layer, a first thermosensitive layer, a first ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, a second ceramic layer, a second thermosensitive layer, and a second mixed layer. The first nanofiber layer, the basement membrane layer, and the second nanofiber layer sequentially constitute a basement membrane skeleton. Preferably, the first nanofiber layer and the second nanofiber layer have the same composition and / or thickness. Preferably, the first mixed layer and the second mixed layer have the same composition and / or thickness. Preferably, the first ceramic layer and the second ceramic layer have the same composition and / or thickness. Preferably, the first thermosensitive layer and the second thermosensitive layer have the same composition and / or thickness.

[0051] In addition, the composite diaphragm can also be additionally provided with a fourth functional layer, a fifth functional layer, a sixth functional layer, etc. Different functional layers can be provided according to actual performance requirements. These functional layers are symmetrically or asymmetrically provided on both sides of the base membrane skeleton.

[0052] Preferably, a method for preparing a composite diaphragm comprising a mixed layer, a base membrane layer, and a nanofiber layer in sequence is provided, specifically as follows: the mixed layer is coated on one side of the base membrane layer, and then thermally composited with the nanofiber layer on the other side of the base membrane to obtain the above-mentioned composite diaphragm.

[0053] Preferably, a method for preparing a composite diaphragm comprising a first mixed layer, a base membrane layer, a nanofiber layer, and a second mixed layer in sequence is provided, specifically as follows: a mixed layer is first coated on one side of the base membrane layer, denoted as composite membrane A; then a mixed layer is coated on the surface of the nanofiber layer, denoted as composite membrane B; finally, composite membrane A and composite membrane B (without compounding between the mixed layers) are thermally compounded to obtain the above-mentioned composite diaphragm.

[0054] Preferably, a method for preparing a composite diaphragm comprising a first adhesive layer, a ceramic layer, a base film layer, a nanofiber layer, and a second adhesive layer in sequence is provided, specifically as follows: on the same side of the base film surface, a ceramic layer and an adhesive layer are successively coated, which is denoted as composite membrane A; then an adhesive layer is coated on the surface of the nanofiber layer, which is denoted as composite membrane B; finally, composite membrane A and composite membrane B (compounding is performed between the surfaces without adhesive layers) are thermally composited to obtain the above-mentioned composite diaphragm.

[0055] Preferably, a method for preparing a composite diaphragm comprising a first adhesive layer, a first ceramic layer, a first nanofiber layer, a base film layer, a second nanofiber layer, a second ceramic layer, and a second adhesive layer in sequence is provided, specifically as follows: on the same side of the surface of the nanofiber layer, a ceramic layer and an adhesive layer are successively coated, which is recorded as composite membrane A; then, the nanofiber layer is taken, and on the same side of its surface, a ceramic layer and an adhesive layer are successively coated, which is recorded as composite membrane B; then, the composite membrane A, the base film layer, and the composite membrane B are thermally composited in the order (the base film is located in the middle, and the two sides of the base film are composited with the layers without ceramic layer and adhesive layer) to obtain the above-mentioned composite diaphragm.

[0056] Preferably, a method for preparing a composite diaphragm is provided, which sequentially includes a first adhesive layer, a first thermosensitive layer, a first ceramic layer, a first nanofiber layer, a base film layer, a second nanofiber layer, a second ceramic layer, a second thermosensitive layer, and a second adhesive layer, specifically as follows: on the same side of the surface of the nanofiber layer, a ceramic layer, a thermosensitive layer, and an adhesive layer are coated in sequence, which is recorded as a composite membrane A; then, the nanofiber layer is taken, and on the same side of its surface, a ceramic layer, a thermosensitive layer, and an adhesive layer are coated in sequence, which is recorded as a composite membrane B; then, the composite membrane A, the base film layer, and the composite membrane B are thermally composited in the order (the base film is located in the middle, and the two sides of the base film are composited with the layers without ceramic layer, thermosensitive layer, and adhesive layer) to obtain the above-mentioned composite diaphragm.

[0057] Preferably, the thickness of the composite diaphragm is H, H = 8~62μm. Controlling the total thickness of the composite diaphragm within the above-mentioned specific range can effectively improve the lithium ion transmission, mechanical properties, safety performance, energy density and other performance of the lithium battery. For example, if the total thickness of the composite diaphragm is too large, the lithium ion transmission path will be lengthened, the lithium ion transmission resistance will increase, and the charge and discharge efficiency will be reduced; at the same time, if the total thickness of the composite diaphragm is too large, it will occupy more space inside the battery. Under the condition of a certain battery volume, the loading amount of positive and negative active materials will be relatively reduced, thereby reducing the energy density of the battery. If the total thickness of the composite diaphragm is too low, it will cause insufficient mechanical properties, reduce thermal stability and safety, and fail to effectively block some metal impurities or other particulate matter that may exist inside the battery. When impurities pass through the diaphragm and reach the opposite electrode, they may cause problems such as micro-short circuits, affecting the performance and life of the battery.

[0058] Preferably, the thermal shrinkage rate of the composite diaphragm at T2 is ≤3%, and T2 ≥ 200°C.

[0059] Preferably, the liquid absorption rate of the composite membrane is ≥30 mm / 10 min.

[0060] According to another aspect of the present invention, a battery is provided, comprising the composite separator.

[0061] In summary, the composite membrane provided by the present invention has excellent mechanical properties and thermal stability, thus effectively improving the safety performance of lithium batteries. Furthermore, the nanofiber layer in the composite membrane can increase the overall porosity of the composite membrane, facilitating the smooth transmission of lithium ions, thereby further optimizing the cycling performance of lithium batteries under normal operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 Schematic diagram of the composite diaphragm structure in Example 1 of the present invention.

[0063] Figure 2 Schematic diagram of the composite diaphragm structure in Example 2 of the present invention.

[0064] Figure 3 Schematic diagram of the composite diaphragm structure in Example 3 of the present invention.

[0065] Figure 4 Schematic diagram of the composite diaphragm structure in Example 4 of the present invention.

[0066] Figure 5 Schematic diagram of the composite diaphragm structure in Example 5 of the present invention. DETAILED DESCRIPTION

[0067] In order to enable people in this technical field to better understand the solution of this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of this application, not all of the embodiments.

[0068] Example 1 The composite membrane provided in this embodiment includes a mixed layer, a base membrane layer, and a nanofiber layer in sequence, that is, the base membrane skeleton is composed of the base membrane layer and the nanofiber layer, and the mixed layer is the first functional layer; The preparation method is as follows: a mixture directly composed of an adhesive compound and ceramic particles is coated on one side of the base membrane layer to form a mixed layer, and then the nanofiber layer is thermally composited with the other side of the base membrane to obtain the above-mentioned composite diaphragm.

[0069] In the mixed layer, the mass ratio of the adhesive compound to the ceramic particles is 1:12, the adhesive compound is polyvinylidene fluoride (PVDF), and the ceramic particles are alumina.

[0070] The material of the nanofiber layer is polyacrylonitrile (PAN) nanofiber. The preparation of the nanofiber layer can be done by electrospinning as follows: (1) Dissolve PAN nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PAN nanofiber solution; (2) injecting the uniform PAN nanofiber solution into an electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0071] The material of the base film layer is polyethylene (PE).

[0072] In the composite membrane of this embodiment, the thicknesses of the mixed layer, base membrane layer, and nanofiber layer are 2μm, 7μm, and 9μm, respectively. In the thickness direction of the composite membrane, the area of ​​one side of the base membrane layer is S1, and the projected area of ​​the nanofiber layer on the base membrane layer is S2. In this embodiment, S2 / S1=100%. In addition, the composite membrane prepared in this embodiment has an air permeability of 11500s / 100mL at 200°C for 1h. The air permeability of the composite membrane in this embodiment is regulated by adjusting the diameter and / or thickness of the nanofiber layer and / or the thickness of each layer of the composite membrane. In this embodiment, the thickness of the nanofiber layer is 9μm, the diameter of the nanofibers in the nanofiber layer is 50~800nm, the average diameter is 400nm, and the length is 200~2000μm, with an average length of 1200μm.

[0073] The test method for the composite membrane's air permeability at 200°C for 1 hour is as follows: The permeability is evaluated based on the time required for gas to pass through a specific area of ​​the material (volumetric method). Using a membrane air permeability tester, a constant pressure differential is established across the sample at 200°C (the constant pressure differential is set at 0.1-1 MPa). The time required for a certain amount of gas (e.g., 100 ml) to pass through a sample of known area and thickness is recorded. The material's air permeability parameters are calculated using the following formula: Air permeability = Gas volume / (time × area × pressure difference). At least three measurements are performed on the same sample, and the average result is taken as the final result.

[0074] In this embodiment, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0075] Example 2 The composite membrane provided in this embodiment includes a first mixed layer, a base membrane layer, a nanofiber layer, and a second mixed layer in sequence, that is, the base membrane skeleton is composed of the base membrane layer and the nanofiber layer, and the first mixed layer and the second mixed layer are both first functional layers; The preparation method is as follows: on one side of the base membrane layer, a mixture directly consisting of an adhesive compound and ceramic particles is coated to form a first mixed layer, denoted as composite membrane A; then a mixture directly consisting of an adhesive compound and ceramic particles is coated on the surface of the nanofiber layer to form a second mixed layer, denoted as composite membrane B; finally, composite membrane A and composite membrane B (without compounding between the mixed layers) are thermally compounded to obtain the composite diaphragm of this embodiment.

[0076] In the first mixed layer and the second mixed layer, the mass ratio of the adhesive compound to the ceramic particles is 1:12, the adhesive compound is polyvinylidene fluoride (PVDF), and the ceramic particles are aluminum oxide.

[0077] The material of the nanofiber layer is polyethylene terephthalate (PET) nanofiber. The nanofiber layer can be prepared by electrospinning as follows: (1) Dissolving PET nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PET nanofiber solution; (2) injecting the uniform PET nanofiber solution into an electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0078] The material of the base film layer is polyethylene (PE).

[0079] In the composite membrane of this embodiment, the thicknesses of the first mixed layer, base membrane layer, nanofiber layer, and second mixed layer are 2μm, 7μm, 9μm, and 2μm, respectively. In the thickness direction of the composite membrane, the area of ​​one side of the base membrane layer is S1, and the projected area of ​​the nanofiber layer on the base membrane layer is S2. In this embodiment, S2 / S1=100%. In addition, the composite membrane prepared in this embodiment has an air permeability of 13200s / 100mL at 200°C for 1h. The air permeability of the composite membrane in this embodiment is regulated by adjusting the diameter and / or thickness of the nanofiber layer and / or the thickness of the various layers of the composite membrane. In this embodiment, the thickness of the nanofiber layer is 9μm, the diameter of the nanofibers in the nanofiber layer is 50~800nm, the average diameter is 400nm, and the length is 200~2000μm, with an average length of 1200μm.

[0080] The test method for the air permeability of the composite membrane at 200° C. for 1 hour is as described in Example 1.

[0081] In this embodiment, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0082] Example 3 The composite diaphragm provided in this embodiment includes a first adhesive layer, a ceramic layer, a base film layer, a nanofiber layer, and a second adhesive layer in sequence. That is, the base film skeleton is composed of the base film layer and the nanofiber layer. The first adhesive layer and the second adhesive layer are both first functional layers, and the ceramic layer is the second functional layer. The preparation method is as follows: on one side of the base membrane layer, ceramic particles and adhesive compounds are coated in sequence to form a ceramic layer and a first adhesive layer, which is recorded as composite membrane A; then the adhesive compound is coated on the surface of the nanofiber layer to form a second adhesive layer, which is recorded as composite membrane B; finally, composite membrane A and composite membrane B (compounded between the non-adhesive layers) are thermally composited to obtain the composite diaphragm of this embodiment.

[0083] In the first adhesive layer and the second adhesive layer, the adhesive compound is polymethyl methacrylate (PMMA).

[0084] In the ceramic layer, the ceramic particles are boehmite.

[0085] The material of the nanofiber layer is polyimide (PI) nanofiber. The preparation of the nanofiber layer can be done by electrospinning as follows: (1) Dissolve PI nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PI nanofiber solution; (2) injecting the uniform PI nanofiber solution into the electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0086] The material of the base film layer is polyethylene (PE).

[0087] In the composite membrane of this embodiment, the thicknesses of the first adhesive layer, ceramic layer, base film layer, nanofiber layer, and second adhesive layer are 2μm, 2μm, 7μm, 9μm, and 2μm, respectively. In the thickness direction of the composite membrane, the area of ​​one side of the base film layer is S1, and the projected area of ​​the nanofiber layer on the base film layer is S2. In this embodiment, S2 / S1=100%. In addition, the composite membrane prepared in this embodiment has an air permeability of 14700s / 100mL at 200°C for 1h. The air permeability of the composite membrane in this embodiment is regulated by adjusting the diameter and thickness of the nanofiber layer or the thickness of the various layers of the composite membrane. In this embodiment, the thickness of the nanofiber layer is 9μm, and the diameter of the nanofibers in the nanofiber layer is 50-800nm, with an average diameter of 400nm, and the length is 200-2000μm, with an average length of 1200μm.

[0088] The test method for the air permeability of the composite membrane at 200° C. for 1 hour is as described in Example 1.

[0089] In this embodiment, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0090] Example 4 The composite diaphragm provided in this embodiment includes, in sequence, a first adhesive layer, a first ceramic layer, a first nanofiber layer, a basement membrane layer, a second nanofiber layer, a second ceramic layer, and a second adhesive layer. That is, the basement membrane skeleton is composed of the first nanofiber layer, the basement membrane layer, and the second nanofiber layer. The first adhesive layer and the second adhesive layer are both first functional layers, and the first ceramic layer and the second ceramic layer are both second functional layers. The preparation method is as follows: on the same side of the surface of the first nanofiber layer, ceramic particles and adhesive compounds are successively coated to form a first ceramic layer and a first adhesive layer, which are recorded as composite membrane A; on the same side of the surface of the second nanofiber layer, ceramic particles and adhesive compounds are successively coated to form a second ceramic layer and a second adhesive layer, which are recorded as composite membrane B; then, thermally composite membrane A, base membrane layer, and composite membrane B are performed in this order (the base membrane is located in the middle, and the two sides of the base membrane are composited with the non-ceramic layer and the adhesive layer), to obtain the composite diaphragm of this embodiment.

[0091] In the first adhesive layer and the second adhesive layer, the adhesive compound is polyvinylidene fluoride (PVDF).

[0092] In the first ceramic layer and the second ceramic layer, the ceramic particles are aluminum oxide.

[0093] The material of the nanofiber layer is polyacrylonitrile (PAN) nanofiber. The preparation of the nanofiber layer can be done by electrospinning as follows: (1) Dissolve PAN nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PAN nanofiber solution; (2) injecting the uniform PAN nanofiber solution into an electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0094] The material of the base film layer is polyethylene (PE).

[0095] In the composite membrane of this embodiment, the thicknesses of the first adhesive layer, first ceramic layer, first nanofiber layer, basement membrane layer, second nanofiber layer, second ceramic layer, and second adhesive layer are 2 μm, 2 μm, 7 μm, 9 μm, 2 μm, 2 μm, and 2 μm, respectively. In the thickness direction of the composite membrane, the area of ​​one side of the basement membrane layer is denoted as S1, and the projected area of ​​the nanofiber layer on the basement membrane layer is denoted as S2. In this embodiment, S2 / S1=100%. The composite membrane prepared in this embodiment has an air permeability of 17500 s / 100 mL at 200°C for 1 hour. The air permeability of the composite membrane in this embodiment is controlled by adjusting the diameter and thickness of the nanofiber layer or the thickness of each layer of the composite membrane. In this embodiment, the thickness of the nanofiber layer is 9 μm, and the diameter of the nanofibers in the nanofiber layer is 50-800 nm, with an average diameter of 400 nm, and the length is 200-2000 μm, with an average length of 1200 μm.

[0096] The test method for the air permeability of the composite membrane at 200° C. for 1 hour is as described in Example 1.

[0097] In this embodiment, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0098] Example 5 The composite diaphragm provided in this embodiment includes, in sequence, a first adhesive layer, a first thermal-sensitive layer, a first ceramic layer, a first nanofiber layer, a base film layer, a second nanofiber layer, a second ceramic layer, a second thermal-sensitive layer, and a second adhesive layer. That is, the base film skeleton is composed of the first nanofiber layer, the base film layer, and the second nanofiber layer. The first adhesive layer and the second adhesive layer are both first functional layers, the first ceramic layer and the second ceramic layer are both second functional layers, and the first thermal-sensitive layer and the second thermal-sensitive layer are both third functional layers. The preparation method is as follows: on the same side of the surface of the first nanofiber layer, ceramic particles, thermosensitive materials, and adhesive compounds are successively coated to form a first ceramic layer, a first thermosensitive layer, and a first adhesive layer, which are recorded as composite membrane A; on the same side of the surface of the second nanofiber layer, ceramic particles, thermosensitive materials, and adhesive compounds are successively coated to form a second ceramic layer, a second thermosensitive material, and a second adhesive layer, which are recorded as composite membrane B; and then thermally composited in the order of composite membrane A, base membrane layer, and composite membrane B (the base membrane is located in the middle, and the two sides of the base membrane are composited with the non-ceramic layer and the adhesive layer), to obtain the composite diaphragm of this embodiment.

[0099] In the first adhesive layer and the second adhesive layer, the adhesive compound is polyvinylidene fluoride (PVDF).

[0100] In the first ceramic layer and the second ceramic layer, the ceramic particles are aluminum oxide.

[0101] The first heat-sensitive layer and the second heat-sensitive layer are made of heat-sensitive materials of olefin-based monomers.

[0102] The material of the nanofiber layer is polyacrylonitrile (PAN) nanofiber. The preparation of the nanofiber layer can be done by electrospinning as follows: (1) Dissolve PAN nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PAN nanofiber solution; (2) injecting the uniform PAN nanofiber solution into an electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0103] The material of the base film layer is polyethylene (PE).

[0104] In the composite diaphragm of this embodiment, the thicknesses of the first adhesive layer, first ceramic layer, first nanofiber layer, base membrane layer, second nanofiber layer, second ceramic layer, and second adhesive layer are 2 μm, 2 μm, 9 μm, 7 μm, 9 μm, 2 μm, 2 μm, 2 μm, and 2 μm, respectively. In the thickness direction of the composite diaphragm, the area of ​​one side surface of the base membrane layer is denoted as S1, and the projected area of ​​the nanofiber layer on the base membrane layer is denoted as S2. In this embodiment, S2 / S1 equals 100%. In the composite membrane prepared in this embodiment, the air permeability value at 200°C for 1 hour is 18800s / 100mL, and the air permeability value in the composite membrane in this embodiment is regulated by adjusting the diameter and thickness of the nanofiber layer or the thickness of each layer of the composite membrane. In this embodiment, the thickness of the nanofiber layer is 9μm, and the diameter of the nanofibers in the nanofiber layer is 50~800nm, with an average diameter of 400nm, and the length is 200~2000μm, with an average length of 1200μm.

[0105] The test method for the air permeability of the composite membrane at 200° C. for 1 hour is as described in Example 1.

[0106] In this embodiment, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0107] Example 6 The difference between this embodiment and embodiment 4 is that, in the thickness direction of the composite diaphragm, the area of ​​one side surface of the base membrane layer is defined as S1, and the projected area of ​​the nanofiber layer on the base membrane layer is defined as S2. In this embodiment, S2 / S1 is controlled to be 90%. The remaining operations are the same as those in embodiment 4.

[0108] Furthermore, due to the change in S2 / S1, the air permeability value (1 h at 200°C) of the composite membrane in this embodiment is 17600 s / 100 mL.

[0109] Example 7 This embodiment differs from Example 1 in that the diameter and length of the nanofibers in the nanofiber layer are adjusted to control the air permeability of the composite membrane. Specifically, after adjustment, the nanofiber diameter is 100-1000 nm, with an average diameter of 600 nm, and the length is 200-2000 μm, with an average length of 1600 μm. The remaining operations are consistent with Example 1.

[0110] Therefore, the air permeability value of the final composite membrane tested (at 200°C for 1h) is 10,000s / 100mL.

[0111] Example 8 This example differs from Example 4 in that the diameter and length of the nanofibers in the nanofiber layer were adjusted to control the air permeability of the composite membrane. Specifically, the nanofiber diameter was adjusted to 20-200 nm, with an average diameter of 140 nm, and the length was 200-1500 μm, with an average length of 1000 μm. The remaining procedures were consistent with Example 4. As a result, the final composite membrane achieved an air permeability of 20,000 s / 100 mL (at 200°C for 1 hour).

[0112] Example 9 This embodiment differs from embodiment 4 in that the thicknesses of the first adhesive layer, first ceramic layer, first nanofiber layer, base film layer, second nanofiber layer, second ceramic layer, and second adhesive layer are adjusted to 9 μm, 9 μm, 10 μm, 14 μm, 10 μm, 9 μm, and 9 μm, respectively, resulting in a total composite separator thickness of 70 μm. The remaining operations are the same as those in embodiment 4.

[0113] Furthermore, due to the change in the total thickness of the composite membrane, the air permeability value (at 200° C. for 1 h) of the composite membrane in this embodiment is 30,000 s / 100 mL.

[0114] Comparative Example 1 The difference between this comparative example and Example 1 is that, in the thickness direction of the composite diaphragm, the area of ​​one side surface of the base membrane layer is defined as S1, and the projected area of ​​the nanofiber layer on the base membrane layer is defined as S2. In this example, S2 / S1 is controlled to be 80%. The remaining operations are the same as in Example 1.

[0115] Furthermore, due to the change in S2 / S1, the air permeability value (1 h at 200°C) of the composite membrane in this embodiment is 4000 s / 100 mL.

[0116] Comparative Example 2 This comparative example differs from Example 1 in that the diameter and length of the nanofibers in the nanofiber layer were adjusted to control the air permeability of the composite membrane. Specifically, the nanofiber diameter was adjusted to 800–2000 nm, with an average diameter of 1600 nm, and the length was adjusted to 2000–3000 μm, with an average length of 2600 μm. All other procedures were consistent with Example 1. As a result, the final composite membrane achieved an air permeability of 3000 s / 100 mL (at 200°C for 1 hour) in testing.

[0117] Comparative Example 3 This comparative example differs from Example 1 in that the composite diaphragm sequentially comprises a mixed layer and a base membrane layer, i.e., does not include a nanofiber layer. The composite diaphragm in this comparative example is obtained by directly coating the base membrane with a mixture of an adhesive compound and ceramic particles. The remaining components are the same as in Example 1.

[0118] Since the composite membrane does not contain a nanofiber layer, the membrane will break directly at high temperature (200°C for 1 hour), and the air permeability value cannot be tested.

[0119] Comparative Example 4 This comparative example differs from Example 1 in that the composite diaphragm sequentially comprises a base film layer and a nanofiber layer, i.e., does not contain a mixed layer, and the composite diaphragm in this comparative example can be obtained by directly thermally compounding the base film and the nanofiber layer. The rest of the process is the same as Example 1.

[0120] The air permeability value (1h at 200°C) of the final composite membrane tested was 5000s / 100mL.

[0121] Comparative Example 5 The composite diaphragm provided in this comparative example includes a ceramic layer, an adhesive layer, a base membrane layer, and a nanofiber layer in sequence, that is, the base membrane skeleton is composed of the base membrane layer and the nanofiber layer.

[0122] The preparation method is as follows: ceramic particles and adhesive compounds are sequentially coated on one side of the base membrane layer to form a ceramic layer and an adhesive layer, which is recorded as composite membrane A; then the nanofiber layer is thermally composited with the other side of the base membrane to obtain the composite diaphragm of this comparative example.

[0123] In the first adhesive layer and the second adhesive layer, the adhesive compound is polyvinylidene fluoride (PVDF).

[0124] In the ceramic layer, the ceramic particles are aluminum oxide.

[0125] The material of the nanofiber layer is polyacrylonitrile (PAN), and the preparation of the nanofiber layer can be done by electrospinning, as follows: (1) Dissolve PAN nanofibers in a solvent (dimethylformamide (DMF)) at a certain ratio (8%–15%) to obtain a uniformly dispersed PAN nanofiber solution; (2) injecting the uniform PAN nanofiber solution into an electrospinning machine, adjusting the parameters of the electrospinning machine (such as the distance between the needle and the receiving plate), and starting electrospinning; (3) Collecting the above fiber membrane, drying and rolling at 60-80°C, and curing to obtain the desired nanofiber layer.

[0126] The material of the base film layer is polyethylene (PE).

[0127] In the composite diaphragm of this comparative example, the thicknesses of the ceramic layer, adhesive layer, base film layer, and nanofiber layer are 2μm, 2μm, 7μm, and 3μm, respectively. In the thickness direction of the composite diaphragm, the area of ​​one side of the base film layer is S1, and the projected area of ​​the nanofiber layer on the base film layer is S2. In this embodiment, S2 / S1=100%. In addition, the air permeability of the composite diaphragm prepared in this embodiment is 6400s / 100mL at 200°C for 1h, and the air permeability of the composite diaphragm in this comparative example is regulated by adjusting the diameter and thickness of the nanofiber layer or the thickness of each layer of the composite diaphragm. In this comparative example, the thickness of the nanofiber layer is 9μm, the diameter of the nanofibers in the nanofiber layer is 50~800nm, the average diameter is 400nm, and the length is 200~2000μm, with an average length of 1200μm.

[0128] The test method for the air permeability of the composite membrane at 200° C. for 1 hour is as described in Example 1.

[0129] In this comparative example, each layer can be prepared by conventional methods and is not limited to the methods listed above.

[0130] Test Case 1. Experimental Construction Method The composite membranes prepared in Examples 1 to 9 and Comparative Examples 1 to 5 were tested for thermal shrinkage, liquid absorption rate, and safety performance. The specific testing methods are as follows: (1) Thermal shrinkage Cut a piece of diaphragm, approximately 150mm*150mm, draw lines in the length and width directions, and mark the distance of 100mm with a film ruler. Perform a shrinkage test (200℃ for 1h). After the test is completed, use a film ruler to measure the marked distance. The ratio of the reduced distance to the initial distance is the shrinkage rate.

[0131] (2) Liquid absorption rate Prepare the sample: Cut the diaphragm into strips of 25mm (width) * 300mm (length); Prepare electrolyte: prepare standard electrolyte and ensure that the composition and concentration of the electrolyte meet the test requirements; Immerse the sample: Hang one end of the diaphragm on the iron stand; completely immerse the other end in the electrolyte; record the height h1; After 10 minutes, record the height h2 of electrolyte infiltration; Calculate the liquid absorption rate: that is, the climbing speed of the diaphragm: v=(h2-h1) / 10min.

[0132] (3) Safety performance Batteries were prepared using the composite diaphragms prepared in Examples 1 to 9 and Comparative Examples 1 to 5. The preparation of the batteries was as follows: (1) The positive electrode is made of lithium iron phosphate LFP, SP, CNT and PVDF in a certain ratio (mass ratio is (95~98): (0.5~1.0): (0~1.0): (1.0~2.0)) dispersed in NMP solvent, stirred to form a uniform slurry, coated on aluminum foil, dried, rolled and cut to prepare the positive electrode sheet of the required size; (2) The negative electrode is made of graphite Gr, SP, CMC, and SBR in a certain ratio (mass ratio is (95~98): (0.5~1.0): (0.5~1.5): (1.0~2.0)) dispersed in deionized water, stirred to form a uniform slurry, coated on copper foil, dried, rolled, and cut to prepare the negative electrode sheet of the required size; (3) The electrolyte used is LiPF6 electrolyte with a lithium salt concentration of 1M; (4) Assemble the positive electrode sheet, composite diaphragm, and negative electrode sheet in sequence, put them into the shell, inject the electrolyte, and then form and fix the capacity to obtain a battery.

[0133] A. Needle Puncture [10mm steel needle, (80±5) mm / s penetration], refer to YDT2344.2-2015 (stricter). Procedure: Use a 10mm steel needle (smooth surface, free of rust, oxide layer, and oil stains) to pierce the battery cell perpendicular to the plates of a fully charged cell at a speed of 80 mm / s. Record the cell's condition. Safety criteria for routine needle puncture: no fire or explosion.

[0134] B. Thermal Runaway: Refer to GB / T 36276-2023 (stricter). Procedure: Charge fully charged cells at a 1C constant current, activate heating, and continuously heat the test object at its maximum power (800W) (double-sided heating, large surface). When thermal runaway occurs, record the thermal runaway temperature. Conventional thermal runaway criteria include: fire or explosion, or rupture at a location other than the explosion-proof or pressure relief valve.

[0135] 2. Experimental Results The test results of heat shrinkage, liquid absorption rate and safety performance of the composite membranes prepared in Examples 1 to 9 and Comparative Examples 1 to 5 are shown in Table 1.

[0136] Table 1 Test results of thermal shrinkage, liquid absorption rate and safety performance of composite diaphragm

[0137] As shown in Table 1, the composite separator provided by the present invention, due to its unique structure and high high-temperature air permeability, has a low thermal shrinkage rate, thereby effectively improving the battery's needle puncture resistance and thermal runaway resistance, thereby optimizing the battery's safety performance. Furthermore, the composite separator provided by the present invention has a high liquid absorption rate, which can further improve the battery's cycling performance. For details, please refer to Examples 1-9.

[0138] In Comparative Example 1, S2 / S1=80%, and the nanofiber layer accounts for a small proportion, resulting in insufficient fiber layer when heated, and the shrinkage rate of the diaphragm increases, making the air permeability value at 200℃ for 1h 4000s / 100mL, which in turn affects the battery safety performance.

[0139] In Comparative Example 2, the permeability value is 3000s / 100mL, which is relatively low. Its air permeability is large, resulting in a decrease in battery safety performance. It can be seen in the table that due to the low permeability of the diaphragm, the battery cannot completely block the transmission of lithium ions, making the battery thermal runaway temperature higher than that of a normal battery.

[0140] Comparative Example 3 does not contain a nanofiber layer, and the diaphragm breaks when heated to 200°C; this shows that the nanofiber layer can effectively improve the heat resistance of the composite diaphragm.

[0141] Comparative Example 4 does not contain a mixed layer (or adhesive layer), which results in the diaphragm being unable to melt the base film or adhesive layer at high temperatures to block the pores of the nanofibers to form closed pores in the diaphragm, and unable to completely block the transmission of lithium ions, thereby affecting the safety performance of the battery.

[0142] In Comparative Example 5, the adhesive layer is not on the outermost side, which results in the inability to completely fit the positive and negative electrodes, thereby affecting the overall structural stability of the battery and further affecting the safety performance of the battery.

[0143] By observing Examples 1 to 5, it can be found that the composite membrane provided by the present invention can have a multi-layer functional layer structure, and the multi-layer functional layer structure is more conducive to improving the performance of the composite membrane in various aspects, especially the battery safety performance and cycle performance can be effectively improved.

[0144] Comparing Example 4 and Example 6, in Example 6, S2 / S1=90%, that is, the proportion of the nanofiber layer is reduced, resulting in a decrease in the air permeability value of the diaphragm in Example 6, that is, the air permeability is increased, which reduces the safety performance of the battery, that is, the thermal runaway temperature is increased.

[0145] The air permeability values ​​in Examples 7 and 8 are at a critical point. Therefore, compared to Example 1, Example 7 has a larger fiber membrane diameter, a lower air permeability, and reduced safety. Compared to Example 4, Example 8 has a smaller fiber membrane diameter and length, a higher air permeability, and improved safety. However, this higher air permeability value can lead to a decrease in the battery's rate performance, affecting its high-rate performance.

[0146] Compared with Example 4, the total thickness of the composite diaphragm in Example 9 is thicker, and the battery safety performance is improved. However, the high air permeability value will lead to a decrease in the rate performance of the battery, affecting the high-rate performance of the battery.

[0147] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present application can be modified or replaced by equivalents, but these modifications or replacements are all within the scope of protection of the present application.

Claims

1. A composite diaphragm, characterized in that: It comprises a basement membrane skeleton and a first functional layer, wherein the basement membrane skeleton comprises a basement membrane layer and a nanofiber layer in sequence; In the thickness direction of the composite diaphragm, the nanofiber layer is provided on at least one side of the base membrane layer; In the thickness direction of the composite diaphragm, at least one surface of the composite diaphragm is provided with the first functional layer; the first functional layer includes a mixed layer and / or an adhesive layer, the mixed layer contains an adhesive compound and ceramic particles mixed with each other, and the adhesive layer is composed of an adhesive compound; In the thickness direction of the composite diaphragm, the area of ​​one side surface of the base membrane layer is S1, the projected area of ​​the nanofiber layer on the base membrane layer is S2, and S2 / S1=90-100%; The composite membrane has an air permeability of 10,000 to 20,000 s / 100 mL under T1 for 1 hour, and T1 is ≥ 200°C.

2. The composite diaphragm according to claim 1, wherein: In the thickness direction of the composite diaphragm, at least one first functional layer is provided on both sides of the base membrane skeleton.

3. The composite diaphragm according to claim 1, wherein: The composite diaphragm further includes a second functional layer including a ceramic layer formed of ceramic particles.

4. The composite diaphragm according to claim 3, wherein: In the thickness direction of the composite diaphragm, both sides of the base membrane skeleton are provided with at least one layer of the first functional layer and at least one layer of the second functional layer.

5. The composite diaphragm according to claim 1, wherein: In the basement membrane skeleton, the thickness of the basement membrane layer is denoted as H1, the thickness of the nanofiber layer is denoted as H2, H1=3-20 μm, and H2=2-25 μm.

6. The composite diaphragm according to claim 1, wherein: When the first functional layer includes the adhesive layer, the thickness of the adhesive layer is H3, and H3=1-2 μm; When the first functional layer includes the mixed layer, the thickness of the mixed layer is H4, and H4=1-5 μm.

7. The composite diaphragm according to claim 3, wherein: The thickness of the second functional layer is H5, where H5=1-5 μm.

8. The composite diaphragm according to claims 1 to 7, characterized in that: In the thickness direction of the composite diaphragm, at least one nanofiber layer is provided on both sides of the base membrane skeleton.

9. The composite diaphragm according to claim 1, wherein: The thickness of the composite membrane is denoted as H, where H=8-62 μm.

10. A battery, characterized in that: Comprising the composite diaphragm according to any one of claims 1 to 9.

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