Composite diaphragm, preparation method thereof and battery

Abstract

The invention provides a composite diaphragm which comprises a base membrane and a composite coating located on at least one side of the base membrane, the composite coating comprises lithium-intercalated oxide, gas-phase ceramic and carboxylated cellulose lithium, and the particle size of the carboxylated cellulose lithium is larger than that of the gas-phase ceramic. The invention also provides a battery which comprises the composite diaphragm. The invention further provides a preparation method of the composite diaphragm, which comprises the following steps: sequentially adding the solvent, the gas-phase ceramic, the lithium-intercalated oxide and the carboxylated cellulose lithium, and mixing to form dispersion liquid; adding an adhesive to the dispersion liquid to form a coating slurry; and coating at least one side of a base membrane with the coating slurry, and drying to obtain the composite diaphragm comprising the composite coating.

Description

Technical Field

[0001] The present invention relates to the field of battery separators, and particularly to a high heat-resistant and lithium dendrite-resistant composite separator, a preparation method thereof, and a battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, high power density, long cycle life, and no memory effect, and have become the preferred power sources in the fields of electric vehicles, wind and solar power generation, energy storage, 3C, etc. A lithium-ion battery mainly consists of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the separator acts as a safety fuse and plays a key role in separating the positive and negative electrodes, insulating electrons, and transporting lithium ions.

[0003] During the charging and discharging cycles, lithium reacts with the liquid electrolyte to continuously form a SEI film (Solid Electrolyte Interphase), consuming lithium ions and reducing the charge and discharge efficiency of the electrode material. During the growth process of lithium dendrites, the electrolyte is continuously consumed, and irreversible deposition of metallic lithium is caused, reducing the Coulomb efficiency and damaging the formed solid electrolyte interface (SEI) film. The rapid development of high battery energy density batteries means that the battery can store more electrical energy under the same weight or volume. At the same time, the increase in energy density will exacerbate the formation of lithium dendrites, and the continuously growing lithium dendrites will penetrate the separator, leading to battery short-circuiting and posing a great safety hazard.

[0004] As an indispensable and important part of the battery, the properties of the separator will also affect the generation of lithium dendrites. At present, most traditional separators have the following problems: (1) poor wettability with the electrolyte, resulting in "dry areas" in the battery, which in turn causes excessive impedance or lithium plating; (2) poor mechanical properties (mechanical strength, Young's modulus) are insufficient to inhibit the growth of lithium dendrites; (3) poor thermal stability, etc. The above adverse factors limit the application of the separator in lithium metal batteries.

[0005] Based on this, there is an urgent need to develop a stable high-performance composite separator, its preparation technology, and a battery, which can fundamentally solve the problem of lithium dendrite growth, that is, inhibit the growth of lithium dendrites inside the battery to improve the battery's stability, electrochemical performance, and cycle life. Summary of the Invention

[0006] Based on the above deficiencies of the prior art, the main object of the present invention is to provide a composite separator, a preparation method thereof, and a battery to improve the thermomechanical properties, mechanical strength, and cycle performance of the composite separator.

[0007] In a first aspect, in some embodiments of the present invention, a composite separator is provided, which includes a base film and a composite coating located on at least one side of the base film. The composite coating includes a lithium-inserting oxide, a vapor-phase ceramic, and lithium carboxymethyl cellulose, and the particle size of the lithium carboxymethyl cellulose is larger than that of the vapor-phase ceramic.

[0008] In some embodiments of the present invention, the material of the base film includes at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyimide.

[0009] In some embodiments of the present invention, the composite coating is composed of 1 wt% - 30 wt% lithium-inserted oxide, 55 wt% - 70 wt% gas-phase ceramic, and 1 wt% - 10 wt% lithium carboxylated cellulose.

[0010] In some embodiments of the present invention, the composite coating includes lithium-inserted oxide, gas-phase ceramic, lithium carboxylated cellulose, and an adhesive.

[0011] In some embodiments of the present invention, the composite coating is composed of 1 wt% - 30 wt% lithium-inserted oxide, 55 wt% - 70 wt% gas-phase ceramic, 1 wt% - 10 wt% lithium carboxylated cellulose, and 1 wt% - 5 wt% adhesive.

[0012] In some embodiments of the present invention, the lithium-inserted oxide includes at least one of SnO2, Fe2O3, Co3O4, NiO, CuO, Sb2O3, and CaO.

[0013] In some embodiments of the present invention, the gas-phase ceramic includes at least one of inorganic gas-phase alumina, gas-phase silica, gas-phase zirconia, and gas-phase titanium dioxide.

[0014] In some embodiments of the present invention, the lithium carboxylated cellulose includes at least one of nano-cellulose lithium carboxylate modification, cellulose nanocrystal whisker lithium carboxylate modification, cellulose nanofiber lithium carboxylate modification, and microfibrillated cellulose lithium carboxylate modification.

[0015] In some embodiments of the present invention, the thermal decomposition temperature of the lithium carboxylated cellulose is > 280 °C.

[0016] In some embodiments of the present invention, the adhesive includes at least one of polyacrylamide, polybutyl methacrylate, poly(2-hydroxyethyl methacrylate), styrene / acrylates, polyvinylidene fluoride, and polyvinyl alcohol.

[0017] In some embodiments of the present invention, the particle size of the lithium-inserted oxide is D 50 ≤ 0.5 μm; the particle size of the gas-phase ceramic is D 50 ≤ 0.1 μm; and / or the particle size of the lithium carboxylated cellulose is D 50 ≤ 0.5 μm.

[0018] Second, embodiments of the present invention provide a battery, including the composite separator provided in any of the embodiments of the first aspect.

[0019] In a third aspect, an embodiment of the present invention provides a method for preparing a composite separator, including: sequentially adding a solvent, a dispersant, a vapor-phase ceramic, a lithium-inserted oxide, and lithium carboxylated cellulose, and mixing to form a dispersion; adding an adhesive to the dispersion to form a coating slurry; and coating the coating slurry on at least one side of a base film, and drying to obtain a composite separator including a composite coating.

[0020] In some embodiments of the present invention, the particle size of the dispersion is D 50 < 1 μm, and the maximum particle size of the dispersion is D max < 3 μm. The particle size of the dispersion is based on the particle size of the materials therein.

[0021] In some embodiments of the present invention, the particle size of the lithium-inserted oxide is D 50 ≤ 0.5 μm; the particle size of the vapor-phase ceramic is D 50 ≤ 0.1 μm; and / or the particle size of the lithium carboxylated cellulose is D 50 ≤ 0.5 μm.

[0022] In some embodiments of the present invention, the addition amount of the solvent accounts for 50 wt% - 95 wt% of the coating slurry, and the solvent includes at least one of water, ethanol, acetone, NMP, and isopropanol.

[0023] In some embodiments of the present invention, the addition amount of the dispersant accounts for 0.0001 wt% - 1 wt% of the coating slurry, and the dispersant includes at least one of polyacrylic acids, polyoxyethylene ethers, and sodium alkylbenzene sulfonates.

[0024] In some embodiments of the present invention, the addition amount of the lithium-inserted oxide accounts for 10 wt% - 35 wt% of the coating slurry, and the lithium-inserted oxide includes at least one of SnO2, Fe2O3, Co3O4, NiO, CuO, Sb2O3, and CaO.

[0025] In some embodiments of the present invention, the addition amount of the vapor-phase ceramic accounts for 50 wt% - 75 wt% of the coating slurry, and the vapor-phase ceramic includes at least one of inorganic vapor-phase alumina, vapor-phase silica, vapor-phase zirconia, and vapor-phase titanium dioxide.

[0026] In some embodiments of the present invention, the addition amount of the lithium carboxylated cellulose accounts for 5 wt% - 15 wt% of the coating slurry, and the lithium carboxylated cellulose includes at least one of nano-cellulose lithium carboxylate modification, cellulose nanocrystal whisker lithium carboxylate modification, cellulose nanofibril lithium carboxylate modification, and microfibrillated cellulose lithium carboxylate modification.

[0027] In some embodiments of the present invention, in the step of coating the slurry on at least one side of the substrate film, the coating method includes at least one of roll coating, spin coating, dip coating, casting coating, and spray coating.

[0028] In some embodiments of the present invention, in the step of coating the slurry on at least one side of the substrate film, the coating thickness range of the composite coating is 0.3μm to 3μm.

[0029] In some embodiments of the present invention, the substrate film includes a low-closed pore and high-breakage film substrate film, and the thickness range of the substrate film is 3μm to 20μm.

[0030] The composite separator provided by the present invention is coated on at least one side of the substrate by adding a lithium intercalation oxide, a gas-phase ceramic, and lithium carboxymethyl cellulose to the composite coating, and the co-addition of the aforementioned lithium intercalation oxide, gas-phase ceramic, and lithium carboxymethyl cellulose produces a synergistic effect on the performance of the composite separator, effectively improving the thermomechanical stability, heat resistance, mechanical strength of the composite separator, inhibiting the generation of lithium dendrites, reducing the direct piercing of the lithium dendrites through the separator, and improving the electrochemical performance, capacity retention rate, and cycle life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0032] Figure 1 It is a schematic structural diagram of the composite separator provided by the embodiment of the present invention; Figure 2 It is a process flow diagram of the preparation method of the composite separator provided by the embodiment of the present invention; Figure 3 It is a scanning electron micrograph of the composite separator provided by Embodiment 1 of the present invention; Figure 4 It is a schematic diagram of the size of the test needle used in the nail penetration coefficient detection method.

[0033] Reference numerals: 100 - composite separator; 110 - substrate film; 120 - lithium intercalation oxide; 130 - gas-phase ceramic; 140 - lithium carboxymethyl cellulose; 150 - binder; S210 - step; S220 - step; S230 - step. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] To make the above and / or other objectives, effects, and features of the present invention more obvious and understandable, the following specific preferred embodiments are given for detailed description: AsFigure 1 As shown in Figure 1 , the main object of the present invention is to provide a composite separator 100, which includes a base film 110 and a composite coating located on at least one side of the base film 110. The composite coating includes a lithium-inserting oxide 120, a gas-phase ceramic 130, and a lithium carboxymethyl cellulose 140. In some embodiments, the composite coating further includes an adhesive 150.

[0035] In some embodiments, the material of the base film 110 includes at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyimide. Optionally, the material of the base film includes polyethylene or a composite of polyethylene and polypropylene.

[0036] Exemplarily, the proportion of the lithium-inserting oxide 120 added to the composite coating is 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, or 40 wt%. Optionally, the proportion of the lithium-inserting oxide 120 added to the composite coating is 1 wt% - 30 wt%.

[0037] Exemplarily, the proportion of the gas-phase ceramic 130 added to the composite coating is 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, or 80 wt%. Optionally, the proportion of the gas-phase ceramic 130 added to the composite coating is 55 wt% - 70 wt%.

[0038] Exemplarily, the proportion of the lithium carboxymethyl cellulose 140 added to the composite coating is 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, or 30 wt%. Optionally, the proportion of the lithium carboxymethyl cellulose 140 added to the composite coating is 1 wt% - 10 wt%.

[0039] Exemplarily, the proportion of the adhesive 150 added to the composite coating is 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 15 wt%, or 20 wt%. Optionally, the proportion of the adhesive 150 added to the composite coating is 1 wt% - 5 wt%.

[0040] In some embodiments, the lithium intercalated oxide 120 has poor heat resistance and is mainly responsible for capturing excess lithium ions, preventing the formation of lithium dendrites and inhibiting the continued formation of the formed lithium dendrites. The gas-phase ceramic 130 serves as the structural main body of the composite coating, where the heat resistance and liquid retention requirements of the composite coating are considered. Lithium carboxylated cellulose 140 provides a skeletal support force for the aforementioned lithium intercalated oxide 120 and gas-phase ceramic 130, enabling the composite coating to have better mechanical properties.

[0041] In some embodiments, the lithium intercalated oxide 120 includes at least one of SnO2, Fe2O3, Co3O4, NiO, CuO, Sb2O3, and CaO. In some embodiments, the lithium intercalation mechanism of the lithium intercalated oxide 120 is to undergo a decomposition reduction reaction with active lithium ions to reduce the formation of lithium dendrites. Among them, the lithium intercalated oxide 120 reacts with active lithium ions to form a metal alloy LixZ (Z = Sn, Si, Ge, Fe, Co, Ni, Cu, Sb, etc.) and its corresponding oxide Li2O, which is beneficial to reducing the generation of lithium dendrites during battery cycling and inhibiting the formation of the SEI film. This metal alloy LixZ has volume expansibility and forms a uniform and dense interphase layer on the negative electrode. This interphase layer has good puncture resistance to lithium dendrites, avoids internal short circuits in the battery, reduces battery heat generation, and improves the overall electrochemical efficiency and battery cycle life of the battery.

[0042] In some embodiments, taking tin dioxide (SnO2) as an example, the lithium intercalated oxide 120 has a theoretical specific capacity of 782 mAh g-1, a lithium storage potential of 0.6 V, and is non-toxic and easily available. Its lithium intercalation mechanism is that SnO2 first undergoes a conversion reaction with lithium ions, and then Sn continues to react with lithium ions to form a metal alloy. The reaction formula is as follows.

[0043]

[0044]

[0045] In some embodiments, the lithium carboxylated cellulose 140 includes at least one of nano-cellulose lithium carboxylate modification, cellulose nanocrystal whisker lithium carboxylate modification, cellulose nanofiber lithium carboxylate modification, and microfibrillated cellulose lithium carboxylate modification. Exemplarily, the thermal decomposition temperature of the lithium carboxylated cellulose 140 is 281 °C, 282 °C, 283 °C, 284 °C, 285 °C, 286 °C, 287 °C, 288 °C, 289 °C, 290 °C, 291 °C, 295 °C, 300 °C, or 400 °C. Optionally, the thermal decomposition temperature of the lithium carboxylated cellulose 140 is >280 °C.

[0046] In some embodiments, lithium carboxylated cellulose 140 is obtained by carboxylating conventional cellulose, where lithium carboxylate replaces hydroxyl groups to improve the processability of cellulose, reduce water absorption, and mitigate the adverse factors caused by the water content in the battery cell. Additionally, lithium carboxylated cellulose 140 provides a lithium supplement during the battery cycling process, reducing the formation of lithium dendrites.

[0047] In some embodiments, lithium carboxylated cellulose 140 maintains a stable lithium concentration by providing the lithium salt it carries, reducing the concentration gradient of lithium ions and decreasing the formation of lithium dendrites. In some embodiments, lithium carboxylated cellulose 140 guides the uniform deposition of lithium ions through its grid-like framework structure, reducing the formation of spiky lithium dendrites. Additionally, in some embodiments, the fibrous structure of lithium carboxylated cellulose 140 facilitates the formation of a network framework structure in the coating, enabling gas-phase ceramic 130 particles and lithium-inserted oxide 120 particles to be embedded in the modified fiber pores, enhancing the porosity and structural strength of the separator. When the separator is under stress, the force applied to a single point in the network structure of the separator can be evenly spread from the point to the surface. If lithium dendrites form, the network structure in the separator can guide their uniform deposition, reducing the formation of spiky lithium dendrites and effectively preventing the separator from being pierced by lithium dendrites, thereby improving the battery's cycle life. Moreover, lithium carboxylated cellulose 140 has the advantages of high heat resistance and low density, which are conducive to achieving a high heat resistance and a thin and light structure for the composite coating, and leaving more space for battery materials.

[0048] In some embodiments, the gas-phase ceramic 130 includes at least one of inorganic gas-phase alumina, gas-phase silica, gas-phase zirconia, and gas-phase titanium dioxide. As an example, in the preparation process of synthesizing the gas-phase ceramic 130 by the gas-phase method, the primary single particles of the ceramic fuse into an aggregate structure. The aforementioned aggregate has a porous structure, and the particle size D of the aggregate 50 is 80 nm, 85 nm, 90 nm, 95 nm, 96 nm, 97 nm, 98 nm, or 99 nm. Optionally, the particle size of the aggregate is D 50 < 100 nm.

[0049] In some embodiments, the vapor-phase ceramic 130 has the property of high heat resistance. During the vapor-phase synthesis process, the primary single particles of the ceramic are fused into an aggregate structure, and the porous structure of the aggregate has good liquid retention performance. Supported by lithium carboxymethyl cellulose 140, the vapor-phase ceramic 130 is uniformly dispersed in the fiber pore structure to resist the shrinkage of the composite separator 100 due to temperature changes and enhance the mechanical strength of the separator. The vapor-phase ceramic 130 has a large specific surface area and a particle size much smaller than that of conventional ceramics, which is the key to a thin and light coating. The vapor-phase ceramic 130 has good heat resistance. At the same thickness, the ceramic particles are stacked in multiple layers to provide a stronger heat-resistant layer. There are pores between the primary particles of the vapor-phase ceramic 130 in the heat-resistant layer, which improves the efficiency of the liquid retention rate. Additionally, in some embodiments, to improve the influence of the high water absorption of the vapor-phase ceramic, the vapor-phase ceramic 130 is combined with a hydrophobic lithium-inserted oxide 120 to improve the aforementioned influence.

[0050] In some embodiments, the binder 150 includes at least one of polyacrylamide, polybutyl methacrylate, poly(2-hydroxyethyl methacrylate), styrene / acrylates, polyvinylidene fluoride, and polyvinyl alcohol. In some embodiments, the binder promotes the adhesion between materials and between the materials and the base film 110.

[0051] As an example, the particle size D of the lithium-inserted oxide 120 50 is 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, or 0.5 μm. Optionally, the particle size of the lithium-inserted oxide 120 is D 50 ≤0.5 μm.

[0052] As an example, the particle size D of the vapor-phase ceramic 130 50 is 0.001 μm, 0.01 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, or 0.1 μm. Optionally, the particle size of the vapor-phase ceramic 130 is D 50 ≤0.1 μm.

[0053] As an example, the particle size D of the lithium carboxymethyl cellulose 140 50 is 0.01 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, or 0.5 μm. Optionally, the particle size of the lithium carboxymethyl cellulose 140 is D 50 ≤0.5 μm.

[0054] In some embodiments, the particle size of the lithium intercalated oxide 120 is larger than that of the gas-phase ceramic 130 particles. The purpose is that the lithium intercalated oxide 120 can preferentially capture the excess and active lithium ions, react with them to undergo a decomposition reduction reaction, reduce the formation of lithium dendrites, and inhibit the SEI film. If lithium dendrites have formed, the lithium intercalated oxide 120 reacts with the formed lithium dendrites to form a deposition layer with volume expansion properties (for example: the deposition layer after volume expansion forms a uniform interphase layer, which is uniformly distributed on the negative electrode of the battery and forms a protective layer), adding an extra protective wall on the separator to preferentially resist the piercing of lithium dendrites. In addition, the particle size of the lithium carboxymethyl cellulose 140 is larger than that of the gas-phase ceramic 130 particles because the lithium carboxymethyl cellulose 140 needs to provide a large number of embedding and attachment sites for the gas-phase ceramic 130, and the lithium carboxymethyl cellulose 140 forms a high-strength network structure to facilitate the filling of the gas-phase ceramic 130 particles in the aforementioned grid to resist thermal shrinkage and puncture, achieving the purpose that the composite separator 100 has high heat resistance and high mechanical strength.

[0055] Another object of the present invention is to provide a battery, which includes the composite separator 100 as described above.

[0056] As Figure 2 shown, another object of the present invention is to provide a preparation method of the composite separator 100 as described above, including: S210, successively adding a solvent, a dispersant, the gas-phase ceramic 130, the lithium intercalated oxide 120, and the lithium carboxymethyl cellulose 140, and mixing to form a dispersion; S220, adding the binder 150 to the dispersion to form a coating slurry; and S230, coating the coating slurry on at least one side of a base film 110, and drying to obtain the composite separator 100 including a composite coating.

[0057] In some embodiments, in the aforementioned step S210, the materials therein are uniformly dispersed by methods such as stirring / milling to form a dispersion with good stability and a good particle size distribution state. In some embodiments, the preparation process is in the forms of stirring, grinding, ultrasonic waves, etc. commonly used in the industry, and the mixing is uniform.

[0058] In some embodiments, in the aforementioned step S220, other additives can be selectively added to the dispersion to form a coating slurry. For example: a wetting agent. Optionally, the wetting agent includes at least one of siloxane, alkylphenol polyoxyethylene ether, and succinic acid.

[0059] Exemplarily, the particle size D of the dispersion 50 is 0.1 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, or 0.9 μm. Optionally, the particle size of the dispersion is D 50 <1 μm. The maximum particle size D of the dispersionmax is 1 μm, 2 μm or 2.5 μm. Optionally, the maximum particle size of the dispersion is D max < 3 μm, and the particle size of the dispersion is based on the particle size of the material therein.

[0060] Exemplarily, the addition amount of the solvent accounts for 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt% or 95 wt% of the dispersion. Optionally, the addition amount of the solvent accounts for 50 wt% - 95 wt% of the dispersion, and the solvent includes at least one of water, ethanol, acetone, NMP, and isopropanol.

[0061] Exemplarily, the addition amount of the dispersant accounts for 0.0001 wt%, 0.0002 wt%, 0.0003 wt%, 0.0004 wt%, 0.0005 wt%, 0.0006 wt%, 0.0007 wt%, 0.0008 wt%, 0.0009 wt%, 0.001 wt%, 0.01 wt%, 0.1 wt% or 1 wt% of the dispersion. Optionally, the addition amount of the dispersant accounts for 0.0001 wt% - 1 wt% of the dispersion, and the dispersant includes at least one of polyacrylic acids, polyoxyethylene ethers, and sodium alkylbenzene sulfonates.

[0062] Exemplarily, the addition amount of the lithium intercalated oxide 120 accounts for 5 wt%, 10 wt%, 20 wt%, 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt% or 40 wt% of the dispersion. Optionally, the addition amount of the lithium intercalated oxide 120 accounts for 10 wt% - 35 wt% of the dispersion.

[0063] Exemplarily, the addition amount of the gas-phase ceramic 130 accounts for 40 wt%, 50 wt%, 60 wt%, 70 wt%, 75 wt% or 80 wt% of the dispersion. Optionally, the addition amount of the gas-phase ceramic 130 accounts for 50 wt% - 75 wt% of the dispersion.

[0064] Exemplarily, the addition amount of the lithium carboxylated cellulose 140 accounts for 5 wt%, 10 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt% or 20 wt% of the dispersion. Optionally, the addition amount of the lithium carboxylated cellulose 140 accounts for 5 wt% - 15 wt% of the dispersion.

[0065] In some embodiments, in step S230, the coating method includes at least one of roll coating, spin coating, dip coating, cast coating, and spray coating.

[0066] Exemplarily, in step S230, the coating thickness range of the composite coating is 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm or 4μm. Optionally, the coating thickness range of the composite coating is 0.3μm to 3μm.

[0067] In some embodiments, the base film 110 includes a low-closed pore and high-breakage film base film 110. Exemplarily, the thickness range of the base film 110 is 1μm, 2μm, 3μm, 4μm, 5μm, 10μm, 20μm or 30μm. Optionally, the thickness range of the base film 110 is 3μm to 20μm. In some embodiments, the separator has a structure with high porosity, high mechanical strength, good mechanical properties and Young's modulus. The large pore structure can enable the separator to have better liquid absorption and retention properties and ionic conductivity, which is beneficial for the battery to have a higher capacity retention rate.

[0068] In some embodiments, the above coating material is prepared into a coating slurry and coated on the base film 110 to obtain the composite separator 100. Among them, the aforementioned composite separator 100 has the advantages of good thermomechanical properties, heat resistance stability, mechanical strength, high porosity, ionic conductivity and good liquid retention rate. In addition, during the battery cycle and lithium ion transmission process, the composite separator 100 proposed by the present invention can make lithium ions deposit uniformly, reduce the consumption of electrolyte, have a lithium supplementing effect and inhibit the growth of lithium dendrites. If lithium dendrites are generated, the lithium-embedded oxide 120 reacts with them to form a deposition layer with volume expansion, reducing the risk of lithium dendrites piercing the separator, which is beneficial for improving the cycle life and energy density of the battery.

[0069] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0070] The above embodiments of the present invention will be illustrated by the following examples: Table 1. Coating composition, coating orientation and particle size of the constituent materials of the examples and comparative examples

[0071] Example 1 For the separator of this embodiment, the thickness of the base film 110 (including PE and PP materials) is 7μm, and the coating thickness is 1.5μm. The separator still meets the requirements of high heat resistance and high strength, and has excellent liquid absorption and retention properties. It improves the battery safety performance and cycle performance. At the same time, it reserves space for battery materials and improves the energy density of the battery.

[0072] First, a water solvent, 60 wt% of fumed alumina, 30 wt% of tin dioxide, and 5 wt% of lithium carboxylated cellulose nanofiber (CNF) are sequentially added and mixed to form a dispersion. Then, 5 wt% of binder 150 (polyacrylamide) is added to the dispersion and mixed to form a coating slurry (also known as a composite coating). Herein, the aforementioned volume percentages are relative to the coating slurry. Among them, the particle size D of the fumed alumina 50 is 0.1 μm, the particle size D of the tin dioxide 50 is 0.4 μm, and the particle size D of the lithium carboxylated cellulose nanofiber CNF 50 is 0.5 μm.

[0073] Next, the coating slurry is roll-coated on at least one side of the base film 110 and then dried to obtain the composite separator 100.

[0074] The aforementioned composite separator 100 is used to prepare a battery, and the separator coating in the battery faces the side where the negative electrode is located.

[0075] In addition, as Figure 3 shown, it is the scanning electron microscope image of the composite separator provided in Example 1. It can be seen from the figure that in the coating of the composite separator, there are particles and fibers, and the fumed ceramic 130 particles and the lithium-inserted oxide 120 particles are filled in the network structure of the modified fibers.

[0076] Example 2 The preparation method of this example is substantially the same as that of Example 1, except that in the coating, the lithium-inserted oxide 120, tin dioxide, fumed aluminum oxide, and lithium carboxylated cellulose nanofiber CNF account for 25 wt%, 60 wt%, and 10 wt% respectively.

[0077] Example 3 The preparation method of this example is substantially the same as that of Example 1, except that in the coating, the lithium-inserted oxide 120, tin dioxide, fumed alumina, and lithium carboxylated cellulose nanofiber CNF account for 35 wt%, 50 wt%, and 15 wt% respectively.

[0078] Example 4 The preparation method of this example is substantially the same as that of Example 1, except that in the coating, the lithium-inserted oxide 120, tin dioxide, fumed alumina, and lithium carboxylated cellulose nanofiber CNF account for 15 wt%, 70 wt%, and 10 wt% respectively.

[0079] Example 5 This example is prepared in substantially the same manner as Example 1, except that in the coating, the lithium-inserting oxide 120, tin dioxide, fumed alumina, and lithium carboxylated cellulose CNF each account for 10 wt%, 75 wt%, and 10 wt%.

[0080] Example 6 This example is prepared in substantially the same manner as Example 1, except that in the coating, the lithium-inserting oxide 120, tin dioxide, fumed silica, and lithium carboxylated cellulose CNC (Lithium Carboxylated Cellulose Nanocrystals) each account for 35 wt%, 50 wt%, and 10 wt%.

[0081] Example 7 This example is prepared in exactly the same manner and has the same coating as Example 2. The difference is that after the battery is made, the side of the separator coating faces the positive electrode.

[0082] Comparative Example 1 This example is prepared in substantially the same manner as Example 2, except that the addition ratio of fumed alumina in the coating is 95 wt%, and it contains no lithium storage oxide and no lithium carboxylated cellulose 140.

[0083] Comparative Example 2 This example is prepared in substantially the same manner as Comparative Example 2, except that in the coating, the lithium-inserting oxide 120, tin dioxide, and fumed alumina each account for 35 wt% and 60 wt%, and it contains no lithium carboxylated cellulose 140.

[0084] Comparative Example 3 This example is prepared in substantially the same manner as Comparative Example 2, except that the cellulose is unmodified (e.g., CNC) and contains no lithium carboxyl groups.

[0085] Comparative Example 4 This example is prepared in the same manner and uses the same coating materials as Example 2, except that the particle size D of tin dioxide 50 is 0.3 m, the particle size D of fumed alumina 50 is 0.3 m, and the particle size D50 of lithium carboxylated cellulose 140 is 0.5 μm.

[0086] Comparative Example 5 This example is prepared in the same manner and uses the same coating materials as Example 2, except that the particle size D of tin dioxide 50 is 0.4 μm, the D of fumed alumina 50 is 0.3 μm, and the particle size D of lithium carboxylated cellulose 140 50 is 0.2 μm.

[0087] Testing method: 1. Raw materials and slurry test items (1) Particle size: Test using a Mastersizer 3000 laser particle size analyzer. Take a sample and add it to the test equipment to make the obscuration reach the test range. Test the particle size of the sample three times and take the average value.

[0088] 2. Diaphragm test items (2) Thickness: The measuring tool used is a Mahr Millimar thickness gauge. First, keep the surface of the composite diaphragm 100 sample flat, and then randomly measure the thickness at 5 to 10 points on the composite diaphragm 100 sample along the longitudinal (MD) direction. Then take the average value of the obtained measurement results to measure the thickness of the composite diaphragm 100 sample.

[0089] (3) Needle punching strength: Cut 3 test samples from the composite diaphragm 100 sample, where the length and width of each test sample are not less than 5 cm * 5 cm. Fix the test sample on the sample stage and use a puncture needle with a diameter (φ) of 1.65 mm and a needle tip (SR) of 0.5 mm, and an electronic puncture strength testing machine for measurement. After the test is completed, take the average value of the 3 groups of test results to measure the needle punching strength of the composite diaphragm 100 sample.

[0090] (4) Liquid absorption rate and liquid retention rate: Cut 3 test samples with a length and width of 100 mm * 100 mm from the composite diaphragm 100 sample, and weigh the initial weight (m0) of each test sample. Place the test sample in the electrolyte and soak it sealed for 1 hour, then take it out and wipe the electrolyte on the surface of the test sample with a dust-free cloth, and then weigh the liquid absorption weight (m1) of the test sample. Lay the weighed test sample flat and let it stand at room temperature for 1 hour, then weigh the liquid retention weight (m2) of the test sample. Among them: The liquid absorption rate of the test sample = ((m1 - m0)) / m0 * 100%; and the liquid retention rate = ((m2 - m0)) / m0 * 100%. Finally, take the average value of the above 3 groups of experimental data to measure the liquid absorption rate and liquid retention rate of the composite diaphragm 100 sample.

[0091] (5) Moisture: Weigh 3 portions of diaphragm samples of about 0.1 g, seal them in sample bottles, place the sealed sample bottles in a heating furnace, heat at a temperature of 150 °C for 300 s, and use a Karl Fischer moisture analyzer for testing. After the test is completed, take the average value of the measurement values of 3 specimens.

[0092] (6) Blistering temperature: Using the internal resistance method during heating, cut a circular diaphragm sample with a diameter of φ50 mm, seal it in a mold filled with conductive liquid, place the mold with the sample in an oven, heat it at a rate of 5 °C / min, and record the change in internal resistance in the mold with temperature during heating. Closing temperature Tc: The starting temperature when the internal resistance value starts to increase significantly; Blistering temperature Tb: The temperature when the internal resistance value starts to decrease significantly.

[0093] (7) Thermal shrinkage: Cut the sample to be tested into a size of 100 mm × 100 mm, mark the MD and TD directions, sandwich it between two thick glass plates, place it in an oven for baking, bake it at 180 °C for 1 h, take it out and use an optical projector to measure the length after shrinkage in the MD / TD directions. The specific shrinkage rate calculation formula is as follows: Transverse shrinkage rate MD: ΔM = (M1 - M2) / M1 × 100%; Longitudinal shrinkage rate TD: ΔT = (T1 - T2) / T1 × 100%; Wherein, in the above calculation formula, M1, T1 are the initial lengths, in mm; M2, T2 are the final lengths, in mm. The larger the thermal shrinkage rate, the poorer the heat resistance of the diaphragm.

[0094] (8) Nail puncture coefficient: Use a thermal nail puncture tester (model: EL-EQ-048), cut the sample larger than the test mold (diameter 40 mm), fix it on the test mold through a fixed clamp, and try to keep the surface flat and the stress uniform. Test conditions: Needle tip temperature: 250 °C, needle tip size: 0.2 mm, distance: -0.1 mm, dwell time: 10 S.

[0095] Use a digital microscope to measure the blistering diameter and mark it as X, in mm.

[0096] The test needle tip size is as Figure 4 shown, where the top diameter of the test needle tip is 5 mm and the needle tip diameter is 0.2 mm. The nail puncture coefficient is X / 215, and the calculation formula is as follows:

[0097]

[0098]

[0099] (9) Scanning electron microscope (SEM): Cut a test sample with a length and width of 0.5 cm * 0.5 cm from the composite diaphragm 100 sample, paste the test sample on the sample stage with conductive glue, then place it in an ion sputtering instrument for gold spraying, and finally use a scanning electron microscope to scan and test the gold-sprayed test sample to take pictures of the coating morphology on the test sample.

[0100] 3. Battery Performance Test (1) Cycling Performance: A full battery is made by configuring the separator with the positive electrode and the negative electrode, and an electrochemical performance test is carried out. The capacity retention rate is recorded after 800 cycles.

[0101] All test results are shown in the following table: Table 2. Test Results

[0102] First of all, it can be seen from Table 1 and Table 2 that Examples 1-7 are generally superior to the comparative examples in terms of thermal shrinkage characteristics, membrane breakage temperature, needle punching strength, liquid absorption rate, liquid retention rate and battery capacity retention characteristics in the MD direction and the TD direction. In particular, after 800 cycles of the battery in Examples 1, 2, 3 and 6, the battery capacity retention rate is maintained above 92% or even higher, indicating that the batteries in the example group perform better in terms of electrochemical performance and cycle life.

[0103] Next, the following describes the examples (using lithium carboxylated cellulose 140, lithium intercalated oxide 120 and gas-phase ceramic 130 added to the coating together), and a synergistic effect is observed, that is, the membrane breakage temperature, needle punching strength, liquid absorption rate, liquid retention rate and battery capacity retention characteristics of the composite separator 100 are significantly better than those of the comparative examples. Specifically, it can be known from Comparative Examples 1-3 that in Comparative Example 1, the composite separator 100 containing gas-phase alumina and without lithium intercalated oxide 120 and without lithium carboxylated cellulose 140 has a membrane breakage temperature of 151 °C. In Comparative Example 2, the composite separator 100 containing gas-phase alumina and tin dioxide and without lithium carboxylated cellulose 140 has a membrane breakage temperature of 146 °C. In Comparative Example 3, the composite separator 100 containing gas-phase alumina, tin dioxide and unmodified CNC has a membrane breakage temperature of 182 °C. Theoretically, when lithium carboxylated cellulose 140, lithium intercalated oxide 120 and gas-phase ceramic 130 are added to the composite coating at the same time, the theoretically observable membrane breakage temperature is between 146 °C and 182 °C. However, it can be seen from Examples 2-7 that the membrane breakage temperature is above 185 °C or even higher, indicating that lithium carboxylated cellulose 140 (lithium carboxyl-modified cellulose), lithium intercalated oxide 120 and gas-phase ceramic 130 have a synergistic effect on the membrane breakage temperature of the composite separator 100.

[0104] Next, as can be seen from Comparative Examples 1 to 3, the puncture strength of the composite separator 100 was observed to be 490 (gf) in Comparative Example 1, 484 (gf) in Comparative Example 2, and 551 (gf) in Comparative Example 3. Theoretically, when lithium carboxylated cellulose 140, lithium-inserted oxide 120, and gas-phase ceramic 130 were added to the composite coating simultaneously, the observed puncture strength was between 484 (gf) and 551 (gf). However, as can be seen from Examples 2, 3, 4, 5, and 7, the puncture strength was above 556 (gf) or even higher, indicating that lithium carboxylated cellulose 140 (lithium carboxyl-modified cellulose), lithium-inserted oxide 120, and gas-phase ceramic 130 had a synergistic effect on the puncture strength of the composite separator 100.

[0105] Next, as can be seen from Comparative Examples 1 to 3, the liquid absorption rate of the composite separator 100 was observed to be 92% in Comparative Example 1, 90% in Comparative Example 2, and 92% in Comparative Example 3. Theoretically, when lithium carboxylated cellulose 140, lithium-inserted oxide 120, and gas-phase ceramic 130 were added to the composite coating simultaneously, the observed liquid absorption rate was between 90% and 92%. However, as can be seen from Examples 2, 4, 5, and 7, the liquid absorption rate was above 93% or even higher, indicating that lithium carboxylated cellulose 140 (lithium carboxyl-modified cellulose), lithium-inserted oxide 120, and gas-phase ceramic 130 had a synergistic effect on the liquid absorption rate of the composite separator 100.

[0106] Next, as can be seen from Comparative Examples 1 to 3, the liquid retention rate of the composite separator 100 was observed to be 91% in Comparative Example 1, 88% in Comparative Example 2, and 92% in Comparative Example 3. Theoretically, when lithium carboxylated cellulose 140, lithium-inserted oxide 120, and gas-phase ceramic 130 were added to the composite coating simultaneously, the observed liquid retention rate was between 88% and 92%. However, as can be seen from Examples 4 and 5, the liquid retention rate was above 96% or even higher, indicating that lithium carboxylated cellulose 140 (lithium carboxyl-modified cellulose), lithium-inserted oxide 120, and gas-phase ceramic 130 had a synergistic effect on the liquid retention rate of the composite separator 100.

[0107] Next, as can be seen from Comparative Examples 1 to 3, the battery capacity retention rate of the composite separator 100 was observed to be 85% in Comparative Example 1, 90% in Comparative Example 2, and 88% in Comparative Example 3. Theoretically, when lithium carboxylated cellulose 140, lithium-inserted oxide 120, and gas-phase ceramic 130 were added to the composite coating simultaneously, the observed battery capacity retention rate was between 85% and 90%. However, as can be seen from Examples 1, 2, 3, and 6, the battery capacity retention rates were all above 92% or even higher, indicating that lithium carboxylated cellulose 140 (lithium carboxyl-modified cellulose), lithium-inserted oxide 120, and gas-phase ceramic 130 had a synergistic effect on the battery capacity retention rate of the composite separator 100.

[0108] In addition, the following describes the examples (using lithium carboxylated cellulose 140, lithium-inserted oxide 120, and gas-phase ceramic 130 added to the coating together, and their particle sizes under specific conditions), and the thermal shrinkage characteristics, film-breaking temperature, needle-punching strength, liquid absorption rate, liquid retention rate, and battery capacity retention characteristics of the composite separator 100 were significantly better than those of the comparative examples. Specifically, as can be seen from Comparative Examples 4 and 5, even though the preparation methods and coating materials of Comparative Examples 4 and 5 were roughly the same as those of Example 2, the difference was that the particle size of the lithium-inserted oxide 120 that did not meet the requirement was larger than that of the gas-phase ceramic 130 and / or the particle size of the lithium carboxylated cellulose 140 was not larger than that of the gas-phase ceramic 130. As a result, it was observed that the thermal shrinkage rate of the composite separator 100 was significantly higher than that of the example, and the heat resistance was poor. In detail, when the coating thickness was the same, an increase in the particle size of the gas-phase ceramic 130 would reduce the superposition of the heat-resistant layer, affecting the comprehensive physical properties of the composite separator 100, making the heat resistance, mechanical strength, liquid absorption, and liquid retention of the separator worse, and at the same time affecting the electrochemical performance of the battery, the capacity retention rate after a certain number of battery cycles, and the cycle life.

[0109] Continuing from the measurement results of Comparative Examples 4 and 5, that is to say, if the particle size of the lithium intercalated oxide 120 is not larger than that of the gas-phase ceramic 130 (i.e., when the particle size condition of the coating material of the present invention is not satisfied), the lithium intercalated oxide 120 cannot preferentially capture the free lithium ions upstream of the negative electrode and react with them to form a metal alloy, reducing the formation of the metal alloy with the aforementioned specific volume expansion property. When the formation of the metal alloy as the protective layer is reduced, when lithium dendrites are generated, the separator lacking the aforementioned metal alloy as the protective layer will be directly stressed and there is a risk of lithium dendrites piercing the separator, affecting the battery capacity retention rate and the cycle stability of the battery. In addition, if the particle size of the lithium carboxymethyl cellulose 140 is not larger than the particle size of the gas-phase ceramic 130 (i.e., when the particle size condition of the coating material of the present invention is not satisfied), the small-sized lithium carboxymethyl cellulose 140 is dispersed and buried in the gas-phase ceramic 130, which is not conducive to the formation of a grid structure for the gas-phase ceramic 130 particles to fill by the lithium carboxymethyl cellulose 140, and then there is a lack of a stable supporting effect in the coating, affecting the mechanical strength of the composite separator 100.

[0110] Furthermore, as can be seen from Example 2 and Comparative Example 4 in the same way, the small difference in particle size between the lithium intercalated oxide 120 and the gas-phase alumina will affect the capacity retention rate of the battery after a certain number of cycles.

[0111] In addition, it should be supplemented and explained that, as can be seen from Examples 2 to 5, the gas-phase ceramic 130 provides the functions of heat resistance and liquid retention for the composite separator 100. Due to the small particle size of the gas-phase ceramic 130, the superposition of multiple heat-resistant particles is realized in the same coating thickness, filled in the modified fiber skeleton, and its original particle size is smaller. After forming aggregates, there are still a large number of pores between the particles, providing the characteristics of air permeability, liquid absorption and liquid retention for the coating. As the proportion of the gas-phase ceramic 130 increases, the heat resistance, membrane rupture temperature, liquid absorption rate and liquid retention rate of the separator are better. The combination of the gas-phase ceramic 130 and the hydrophobic lithium intercalated oxide 120 significantly reduces the moisture content, reducing the risk of battery energy attenuation.

[0112] It should be supplemented and explained that, as can be seen from Examples 1 to 6, a high proportion of the lithium intercalated oxide 120 will reduce the heat resistance, while the battery capacity retention rate is significantly improved.

[0113] As can be seen from Example 2 and Example 7, for the same separator coating on the negative electrode, there is a good capacity retention rate. Relatively, after the separator coating is arranged facing the positive electrode, it is observed that the capacity retention rate of the battery decreases significantly after a certain number of cycles. It is explained that during the battery discharge process, the lithium intercalation oxide 120 alleviates the problem of insufficient lithium intercalation sites at the negative electrode during the discharge process. As the lithium ion concentration increases, the lithium intercalation oxide 120 reacts with the free lithium ions upstream of the negative electrode to form a metal alloy and its corresponding oxide, and the metal alloy forms a protective layer on the surface of the negative electrode due to its specific volume expansion property, inhibiting the continued formation of lithium dendrites and inhibiting the SEI film. The metal alloy (LixZ) has a certain volume expansion property, forming a uniform and dense interphase layer at the negative electrode. This interphase layer has good puncture resistance, avoiding internal short circuit of the battery, reducing battery heat generation, and improving the use safety performance, electrochemical properties, battery cycle stability and cycle life of the battery.

[0114] As can be seen from Example 1 and Example 3, although the proportion of the gas-phase ceramic 130 decreases, affecting the liquid absorption rate and liquid retention rate of the composite separator 100, with the increase in the proportion of lithium carboxymethyl cellulose 140, the battery still has an advantage in capacity retention rate. Lithium carboxymethyl cellulose 140 reduces the concentration difference of lithium ions by lithium supplementation and reduces the generation of lithium dendrites. During the charging process, lithium carboxymethyl cellulose 140 adjusts the lithium ion concentration gradient difference by providing its lithium ions, avoiding the risk of continuously increasing lithium ion concentration at the positive electrode and gradually decreasing lithium ion concentration at the negative electrode, which may form lithium dendrites.

[0115] Increasing the addition ratio of lithium carboxymethyl cellulose 140 has an enhancing effect on the mechanical strength of the composite separator 100. It provides a fibrous grid support. In addition, the gas-phase ceramic 130 particles are filled in the aforementioned fibrous network structure like nails, enhancing the overall mechanical strength of the composite separator 100. When the composite separator 100 is stressed, the aforementioned fibrous network structure is similar to a dynamic functional layer, realizing the conversion of the stress point from point to surface, weakening the local stress, and strengthening the mechanical strength of the composite separator 100. When the composite separator 100 is heated, the gas-phase ceramic 130 particles filled in the aforementioned fibrous network structure exhibit excellent heat resistance, inhibiting the thermal shrinkage of the fiber mesh surface and the entire coating. Therefore, reducing the proportion of the gas-phase ceramic 130 and lithium carboxymethyl cellulose 140 in the coating will affect the mechanical strength, heat resistance and liquid retention property of the composite separator 100.

[0116] As can be seen from the examples and Comparative Example 1, the gas-phase ceramic 130 has obvious advantages in heat resistance and liquid retention, but when used alone, it has the disadvantage of high moisture content, which affects the battery performance. As can be seen from Comparative Example 2, after the co-addition of the lithium intercalation oxide 120, the high moisture content is improved and the battery capacity retention rate is increased. As can be seen from Comparative Example 3, even though the particle size of the coating material meets the foregoing conditions, however, cellulose that has not been modified with lithium carboxylate is used, and a large number of hydroxyl groups in the cellulose that has not been modified with lithium carboxylate cause high moisture content, and moreover, the lithium supplementation effect of the carboxylated cellulose lithium 140 is lacking, which affects the battery cycle performance and still cannot achieve the desired effect of the present invention.

[0117] The above content related to common general knowledge will not be described in detail, and those skilled in the art can understand it.

[0118] The above-described embodiments merely exemplarily illustrate the principles and effects of the present invention, and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A composite diaphragm, characterized in that: The invention comprises a base film and a composite coating located on at least one side of the base film, wherein the composite coating comprises lithium-embedded oxide, gas-phase ceramics and carboxylated cellulose lithium, and the particle size of the carboxylated cellulose lithium is larger than that of the gas-phase ceramics.

2. The composite diaphragm according to claim 1, characterized in that: The material of the base film includes at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET) and polyimide.

3. The composite diaphragm according to claim 1, characterized in that: The composite coating is composed of 1 wt% to 30 wt% of lithium-embedded oxide, 55 wt% to 70 wt% of gas-phase ceramics and 1 wt% to 10 wt% of carboxylated lithium cellulose.

4. The composite diaphragm according to claim 1, characterized in that: The composite coating further includes an adhesive.

5. The composite diaphragm according to claim 4, characterized in that: The composite coating is composed of 1 wt% to 30 wt% of lithium-embedded oxide, 55 wt% to 70 wt% of gas-phase ceramics, 1 wt% to 10 wt% of carboxylated lithium cellulose and 1 wt% to 5 wt% of an adhesive.

6. The composite diaphragm according to claim 1, characterized in that: The lithium-intercalated oxide includes at least one of SnO2, Fe2O3, Co3O4, NiO, CuO, Sb2O3, and CaO.

7. The composite diaphragm according to claim 1, characterized in that: The gas phase ceramic includes at least one of inorganic gas phase alumina, gas phase silicon dioxide, gas phase zirconium oxide and gas phase titanium dioxide.

8. The composite diaphragm according to claim 1, characterized in that: The carboxylated lithium cellulose includes at least one of nanocellulose carboxylated lithium modified, cellulose nano whisker carboxylated lithium modified, cellulose nanofibril carboxylated lithium modified, and microfibrillated cellulose carboxylated lithium modified.

9. The composite diaphragm according to claim 1, characterized in that: The thermal decomposition temperature of the carboxylated lithium cellulose is greater than 280°C.

10. The composite diaphragm according to claim 4, characterized in that: The adhesive includes at least one of polyacrylamide, polybutyl methacrylate, polyhydroxyethyl methacrylate, styrene / acrylate, polyvinylidene fluoride and polyvinyl alcohol.

11. The composite diaphragm according to claim 1, characterized in that: The particle size of the lithium-intercalated oxide is D 50 ≤0.5μm; the particle size of the gas phase ceramic is D 50 ≤0.1μm; and / or the particle size of carboxylated lithium cellulose is D 50 ≤0.5μm.

12. The composite diaphragm according to claim 1, characterized in that: The particle size of the lithium-intercalated oxide is larger than that of the gas-phase ceramic.

13. A battery, characterized in that: Comprising a composite diaphragm as described in any one of claims 1 to 12.

14. A method for preparing a composite diaphragm according to any one of claims 1 to 12, characterized in that: include: Adding a solvent, a dispersant, a vapor-phase ceramic, a lithium-intercalated oxide and carboxylated lithium cellulose in sequence, and mixing to form a dispersion; adding a binder to the dispersion to form a coating slurry; and The coating slurry is coated on at least one side of the base film, and after drying, a composite diaphragm including a composite coating is obtained.

15. The preparation method according to claim 14, characterized in that: The particle size of the dispersion is D 50 <1μm, the maximum particle size of the dispersion is D max <3μm, the particle size of the dispersion is based on the particle size of the material therein.

16. The preparation method according to claim 14, characterized in that: The particle size of the lithium-intercalated oxide is D 50 ≤0.5μm; the particle size of the gas phase ceramic is D 50 ≤0.1μm; and / or the particle size of carboxylated lithium cellulose is D 50 ≤0.5μm.

17. The preparation method according to claim 14, characterized in that: The particle size of the lithium-intercalated oxide is larger than that of the gas-phase ceramic.

18. The preparation method according to claim 14, characterized in that: The added amount of the solvent accounts for 50 wt% to 95 wt% of the coating slurry, and the solvent includes at least one of water, ethanol, acetone, NMP, and isopropanol.

19. The preparation method according to claim 14, characterized in that: The added amount of the dispersant accounts for 0.0001 wt% to 1 wt% of the coating slurry, and the dispersant includes at least one of polyacrylic acid, polyoxyethylene ether, and sodium alkylbenzene sulfonate.

20. The preparation method according to claim 14, characterized in that: The added amount of the lithium intercalation oxide accounts for 10 wt% to 35 wt% of the coating slurry, and the lithium intercalation oxide includes at least one of SnO2, Fe2O3, Co3O4, NiO, CuO, Sb2O3, and CaO.

21. The preparation method according to claim 14, characterized in that: The added amount of the fumed ceramic accounts for 50 wt% to 75 wt% of the coating slurry, and the fumed ceramic includes at least one of inorganic fumed alumina, fumed silica, fumed zirconia, and fumed titania.

22. The preparation method according to claim 14, characterized in that: The added amount of the carboxylated lithium cellulose accounts for 5 wt% to 15 wt% of the coating slurry, and the carboxylated lithium cellulose includes at least one of nanocellulose carboxylated lithium modified, cellulose nanowhisker carboxylated lithium modified, cellulose nanofibril carboxylated lithium modified, and microfibrillated cellulose carboxylated lithium modified.

23. The preparation method according to claim 14, characterized in that: In the step of coating the slurry on at least one side of the resin base film, the coating method includes at least one of roller coating, spin coating, dip coating, cast coating and spray coating.

24. The preparation method according to claim 14, characterized in that: In the step of coating the slurry on at least one side of the resin base film, the coating thickness of the composite coating is in the range of 0.3 μm to 3 μm.

25. The preparation method according to claim 14, characterized in that: The base film comprises a low closed-pore and high broken film base film, and the thickness of the base film ranges from 3 μm to 20 μm.

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