A lithium battery cell with a composite separator and a method of making the same

Abstract

The application discloses a lithium battery cell with a composite diaphragm and a preparation method thereof, and relates to the technical field of lithium batteries. The base film is prepared by blending melamine formaldehyde resin prepolymer and cellulose and simultaneously introducing functional nanoparticles through spinning. The adsorption capacity of the diaphragm to electrolyte is improved by increasing the density of the polar functional groups of the base film. Then, the base film is laminated with a flexible graphene oxide film to improve the electrolyte retention rate and safety of the battery. The porous structure of the carbon base film screens lithium ions, and its electronic insulation avoids electronic leakage. In addition, the physical barrier of the nanoparticles and the chelation of nitrogen elements in melamine further improve the cycle performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a lithium battery cell with a composite separator and its preparation method. Background Technology

[0002] In today's rapidly developing technological landscape, lithium batteries, with their high energy density, long cycle life, and low self-discharge rate, have become indispensable energy storage devices in modern society, finding widespread application in consumer electronics, electric vehicles, and energy storage systems. From everyday devices like mobile phones and laptops to electric vehicles leading the green travel trend, and energy storage power stations supporting a stable supply of renewable energy, lithium batteries are ubiquitous, providing stable and efficient power support for various devices and systems. The battery cell, as the core component of a lithium battery, is like the heart of the human body, playing a crucial role in the overall performance of the lithium battery.

[0003] Cycle life refers to the number of complete charge-discharge cycles a battery can complete under certain charge-discharge conditions. When a battery's cycle life ends, its capacity significantly decreases, failing to meet the normal operating requirements of devices. For example, some traditional lithium-ion cells may experience a capacity drop to less than 80% of their initial capacity after several hundred to over a thousand charge-discharge cycles. This poses a significant challenge for devices that require long-term, stable battery operation. Summary of the Invention

[0004] The purpose of this invention is to provide a lithium battery cell with a composite separator and a method for preparing the same, so as to solve the problems existing in the prior art.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing a lithium battery cell with a composite separator, comprising the following steps: (1) Adjust the pH of formaldehyde to 9-10 with 10wt% sodium hydroxide solution, then add melamine, heat to 70-90℃, carry out prepolymerization reaction for 20-70 min, then add cellulose solution, adjust the pH of solution to 3-5 with 5wt% oxalic acid solution, stir at 300 rpm for 1 h, add functional nanoparticle dispersion, sonicate at 21 kHz for 30 min, and obtain blended spinning solution; (2) Electrospin the blended spinning solution, then immerse it in a 1wt% sodium hydroxide solution at a bath ratio of 1:5~10, then wash it 5 times with deionized water, and finally dry it with hot air at a temperature of 60℃ for 1~4h to obtain the base film. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at a temperature of 60℃ for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane. (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 80~100℃, a pressure of 8MPa, and a time of 5~10min to obtain a composite membrane; (5) Place the composite separator between the positive electrode and the negative electrode, then insert it into an aluminum-plastic film with a thickness of 100 μm, inject electrolyte, and heat seal it at a temperature of 180°C for 10 s to obtain a lithium battery cell with a composite separator.

[0006] Furthermore, the molar ratio of formaldehyde to melamine in step (1) is 2~4:1.

[0007] Furthermore, the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion in step (1) is 10:5~10:2~5.

[0008] Further, the preparation method of the cellulose solution in step (1) is as follows: cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid are mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain the solution.

[0009] Furthermore, the functional nanoparticle dispersion in step (1) is composed of functional nanoparticles, ethanol, and deionized water in a mass ratio of 0.3~0.6:7:3.

[0010] Furthermore, the functional nanoparticles are at least one of silicon dioxide, aluminum oxide, titanium dioxide, and zinc oxide.

[0011] Furthermore, the electrospinning process parameters in step (2) are: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, and speed of 0.5 mL / h.

[0012] Furthermore, the thickness of the base film in step (2) is 20~30μm.

[0013] Furthermore, the filter membrane in step (3) is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm.

[0014] Furthermore, the thickness of the carbon-based film in step (3) is 5~10 μm.

[0015] Compared with the prior art, the beneficial effects achieved by the present invention are: This invention involves blending melamine-formaldehyde resin prepolymer with cellulose and introducing functional nanoparticles to form a base membrane via spinning. Under acidic conditions, the hydroxyl groups of cellulose and the hydroxymethyl groups of melamine-formaldehyde resin undergo a condensation reaction to generate ether bonds. This increases the polar functional group density of the base membrane, enhancing its electrolyte adsorption capacity. Simultaneously, the nanoscale capillary channels of cellulose utilize capillary action to strengthen liquid absorption and shorten the lithium-ion diffusion path. The nanoparticles embed into the matrix network, filling pore defects and maintaining fiber uniformity, thus reducing ion transport resistance. The base membrane is then laminated with a flexible graphene oxide membrane. The nanoparticles in the base membrane and the carbon-based membrane form a physical interlock, enhancing interfacial bonding strength and improving electrolyte retention and safety. The porous structure of the carbon-based membrane filters lithium ions, while its electronic insulation prevents electron leakage. Combined with the physical barrier of the nanoparticles, this inhibits dendrite growth. Furthermore, the chelating effect of nitrogen in melamine reduces side reactions, further improving the battery's cycle performance. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0017] To more clearly illustrate the method provided by the present invention, the following embodiments are provided in detail. The test methods for various indicators of the lithium battery cell with composite separator produced in the following embodiments are as follows: Cyclic performance: Examples and comparative examples of the same size were tested for cyclic performance at 45°C. Example 1

[0018] (1) Cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid were mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain a cellulose solution; the pH of formaldehyde was adjusted to 9 with 10wt% sodium hydroxide solution, and then melamine was added, heated to 70℃, and a prepolymerization reaction was carried out for 20min. Then the cellulose solution was added, the pH of the solution was adjusted to 3 with 5wt% oxalic acid solution, stirred at 300rpm for 1h, and functional nanoparticle dispersion was added. The mixture was sonicated at 21kHz for 30min to obtain a blended spinning solution; the molar ratio of formaldehyde to melamine was 2:1; the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion was 10:5:2; the functional nanoparticle dispersion was composed of silica, ethanol, and deionized water, with a mass ratio of 0.3:7:3; the particle size of silica was 40nm. (2) The blended spinning solution was electrospun with the following process parameters: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, speed of 0.5 mL / h, and then immersed in 1 wt% sodium hydroxide solution at a bath ratio of 1:5. Next, it was washed 5 times with deionized water and finally dried with hot air at a temperature of 60 °C for 1 h to obtain a base film with a thickness of 20 μm. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at 60°C for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane with a thickness of 5 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 80℃, a pressure of 8MPa and a time of 5min to obtain a composite membrane; (5) Mix the positive electrode material NCM811, conductive agent Super P, and PVDF binder at a mass ratio of 90:5:5, add N-methylpyrrolidone at a mass ratio of 7 times that of the PVDF binder, and then coat it onto aluminum foil with a wet film thickness of 180 μm. Dry at 80°C for 12 h to obtain the positive electrode sheet; mix the graphite negative electrode material and conductive agent Super P. P and SBR-CMC composite binder are mixed at a mass ratio of 95:2:3, and 2.5 times the mass of deionized water is added. The mixture is then coated onto copper foil with a wet film thickness of 120 μm. After drying at 100°C for 12 hours, a negative electrode sheet is obtained. A composite separator is placed between the positive and negative electrode sheets and then encased in a 100 μm thick aluminum-plastic film. Electrolyte is injected and heat-sealed at 180°C for 10 seconds to obtain a lithium battery cell with a composite separator. The electrolyte is composed of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a mass ratio of 1:4.3:3.5:0.09. Example 2

[0019] (1) Cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid were mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain a cellulose solution; the pH of formaldehyde was adjusted to 9.5 with 10wt% sodium hydroxide solution, and then melamine was added. The mixture was heated to 80℃ for a prepolymerization reaction for 50min. Then, the cellulose solution was added, and the pH of the solution was adjusted to 4 with 5wt% oxalic acid solution. The mixture was stirred at 300rpm for 1h, and then a functional nanoparticle dispersion was added. The mixture was sonicated at 21kHz for 30min to obtain a blended spinning solution; the molar ratio of formaldehyde to melamine was 3:1; the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion was 10:8:3.5; the functional nanoparticle dispersion was composed of silica, ethanol, and deionized water in a mass ratio of 0.5:7:3; and the particle size of silica was 40nm. (2) The blended spinning solution was electrospun with the following process parameters: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, speed of 0.5 mL / h, and then immersed in 1 wt% sodium hydroxide solution at a bath ratio of 1:8. Next, it was washed 5 times with deionized water and finally dried with hot air at a temperature of 60 °C for 2.5 h to obtain a base film with a thickness of 25 μm. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at 60°C for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane with a thickness of 8 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 90°C, a pressure of 8MPa and a time of 8min to obtain a composite membrane; (5) Mix the positive electrode material NCM811, conductive agent Super P, and PVDF binder at a mass ratio of 90:5:5, add N-methylpyrrolidone at a mass ratio of 7 times that of the PVDF binder, and then coat it onto aluminum foil with a wet film thickness of 180 μm. Dry at 80°C for 12 h to obtain the positive electrode sheet; mix the graphite negative electrode material and conductive agent Super P. P and SBR-CMC composite binder are mixed at a mass ratio of 95:2:3, and 2.5 times the mass of deionized water is added. The mixture is then coated onto copper foil with a wet film thickness of 120 μm. After drying at 100°C for 12 hours, a negative electrode sheet is obtained. A composite separator is placed between the positive and negative electrode sheets and then encased in a 100 μm thick aluminum-plastic film. Electrolyte is injected and heat-sealed at 180°C for 10 seconds to obtain a lithium battery cell with a composite separator. The electrolyte is composed of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a mass ratio of 1:4.3:3.5:0.09. Example 3

[0020] (1) Cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid were mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain a cellulose solution; the pH of formaldehyde was adjusted to 10 with 10wt% sodium hydroxide solution, and then melamine was added. The temperature was raised to 90℃ for a prepolymerization reaction for 70min. Then the cellulose solution was added, and the pH of the solution was adjusted to 5 with 5wt% oxalic acid solution. The mixture was stirred at 300rpm for 1h, and then a functional nanoparticle dispersion was added. The mixture was sonicated at 21kHz for 30min to obtain a blended spinning solution; the molar ratio of formaldehyde to melamine was 4:1; the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion was 10:10:5; the functional nanoparticle dispersion was composed of silica, ethanol, and deionized water in a mass ratio of 0.6:7:3; and the particle size of silica was 40nm. (2) The blended spinning solution was electrospun with the following process parameters: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, speed of 0.5 mL / h, and then immersed in 1 wt% sodium hydroxide solution at a bath ratio of 1:10. Next, it was washed 5 times with deionized water and finally dried with hot air at a temperature of 60 °C for 4 h to obtain a base film with a thickness of 30 μm. (3) Pour a 1 mg / mL graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at a temperature of 60°C for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane with a thickness of 10 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 100℃, a pressure of 8MPa and a time of 10min to obtain a composite membrane; (5) Mix the positive electrode material NCM811, conductive agent Super P, and PVDF binder at a mass ratio of 90:5:5, add N-methylpyrrolidone at a mass ratio of 7 times that of the PVDF binder, and then coat it onto aluminum foil with a wet film thickness of 180 μm. Dry at 80°C for 12 h to obtain the positive electrode sheet; mix the graphite negative electrode material and conductive agent Super P. P and SBR-CMC composite binder are mixed at a mass ratio of 95:2:3, and 2.5 times the mass of deionized water is added. The mixture is then coated onto copper foil with a wet film thickness of 120 μm. After drying at 100°C for 12 hours, a negative electrode sheet is obtained. A composite separator is placed between the positive and negative electrode sheets and then encased in a 100 μm thick aluminum-plastic film. Electrolyte is injected and heat-sealed at 180°C for 10 seconds to obtain a lithium battery cell with a composite separator. The electrolyte is composed of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a mass ratio of 1:4.3:3.5:0.09. Example 4

[0021] (1) Cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid were mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain a cellulose solution; the pH of formaldehyde was adjusted to 9.5 with 10wt% sodium hydroxide solution, and then melamine was added. The mixture was heated to 80℃ for a prepolymerization reaction for 50min. Then the cellulose solution was added, and the pH of the solution was adjusted to 4 with 5wt% oxalic acid solution. The mixture was stirred at 300rpm for 1h, and then a functional nanoparticle dispersion was added. The mixture was sonicated at 21kHz for 30min to obtain a blended spinning solution; the molar ratio of formaldehyde to melamine was 3:1; the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion was 10:2:3.5; the functional nanoparticle dispersion was composed of silica, ethanol, and deionized water in a mass ratio of 0.5:7:3; and the particle size of silica was 40nm. (2) The blended spinning solution was electrospun with the following process parameters: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, speed of 0.5 mL / h, and then immersed in 1 wt% sodium hydroxide solution at a bath ratio of 1:8. Next, it was washed 5 times with deionized water and finally dried with hot air at a temperature of 60 °C for 2.5 h to obtain a base film with a thickness of 25 μm. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at 60°C for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane with a thickness of 8 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 90°C, a pressure of 8MPa and a time of 8min to obtain a composite membrane; (5) Mix the positive electrode material NCM811, conductive agent Super P, and PVDF binder at a mass ratio of 90:5:5, add N-methylpyrrolidone at a mass ratio of 7 times that of the PVDF binder, and then coat it onto aluminum foil with a wet film thickness of 180 μm. Dry at 80°C for 12 h to obtain the positive electrode sheet; mix the graphite negative electrode material and conductive agent Super P. P and SBR-CMC composite binder are mixed at a mass ratio of 95:2:3, and 2.5 times the mass of deionized water is added. The mixture is then coated onto copper foil with a wet film thickness of 120 μm. After drying at 100°C for 12 hours, a negative electrode sheet is obtained. A composite separator is placed between the positive and negative electrode sheets and then encased in a 100 μm thick aluminum-plastic film. Electrolyte is injected and heat-sealed at 180°C for 10 seconds to obtain a lithium battery cell with a composite separator. The electrolyte is composed of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a mass ratio of 1:4.3:3.5:0.09. Example 5

[0022] (1) Cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid were mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain a cellulose solution; the pH of formaldehyde was adjusted to 9.5 with 10wt% sodium hydroxide solution, and then melamine was added. The temperature was raised to 80℃ for a prepolymerization reaction for 50min. Then the cellulose solution was added, and the pH of the solution was adjusted to 4 with 5wt% oxalic acid solution. The mixture was stirred at 300rpm for 1h, and then a functional nanoparticle dispersion was added. The mixture was sonicated at 21kHz for 30min to obtain a blended spinning solution; the molar ratio of formaldehyde to melamine was 3:1; the mass ratio of melamine, cellulose solution, and functional nanoparticle dispersion was 10:8:3.5; the functional nanoparticle dispersion was composed of silica, ethanol, and deionized water in a mass ratio of 0.5:7:1; the particle size of silica was 40nm. (2) The blended spinning solution was electrospun with the following process parameters: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, speed of 0.5 mL / h, and then immersed in 1 wt% sodium hydroxide solution at a bath ratio of 1:8. Next, it was washed 5 times with deionized water and finally dried with hot air at a temperature of 60 °C for 2.5 h to obtain a base film with a thickness of 25 μm. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane at 60°C for 12 hours, and then peel off the filter membrane to obtain a carbon-based membrane with a thickness of 8 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed at a temperature of 90°C, a pressure of 8MPa and a time of 8min to obtain a composite membrane; (5) Mix the positive electrode material NCM811, conductive agent Super P, and PVDF binder at a mass ratio of 90:5:5, add N-methylpyrrolidone at a mass ratio of 7 times that of the PVDF binder, and then coat it onto aluminum foil with a wet film thickness of 180 μm. Dry at 80°C for 12 h to obtain the positive electrode sheet; mix the graphite negative electrode material and conductive agent Super P. P and SBR-CMC composite binder are mixed at a mass ratio of 95:2:3, and 2.5 times the mass of deionized water is added. The mixture is then coated onto copper foil with a wet film thickness of 120 μm. After drying at 100°C for 12 hours, a negative electrode sheet is obtained. A composite separator is placed between the positive and negative electrode sheets and then encased in a 100 μm thick aluminum-plastic film. Electrolyte is injected and heat-sealed at 180°C for 10 seconds to obtain a lithium battery cell with a composite separator. The electrolyte is composed of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a mass ratio of 1:4.3:3.5:0.09.

[0023] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that no cellulose solution is added; the remaining steps are the same as in Example 2.

[0024] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that no functional nanoparticle dispersion was added; the remaining steps are the same as in Example 2.

[0025] Comparative Example 3 The difference between Comparative Example 3 and Example 2 is that step (3) is different. Step (3) is changed to: pouring 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filtering it to obtain a graphene oxide wet membrane, drying the graphene oxide wet membrane together with the filter membrane at a temperature of 60°C for 12 hours, and then peeling off the filter membrane to obtain a carbon-based membrane with a thickness of 3 μm; the filter membrane is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm; the remaining steps are the same as in Example 2.

[0026] Example of effect Table 1 below presents the performance analysis results of lithium battery cells with composite separators using Examples 1 to 5 and Comparative Examples 1 to 3 of the present invention. Table 1

[0027] A comparison of the experimental data from the examples and comparative examples in Table 1 reveals that the present invention, through blending melamine-formaldehyde resin prepolymer with cellulose and introducing functional nanoparticles, produces a base membrane via spinning. By increasing the density of polar functional groups in the base membrane, the membrane's adsorption capacity for electrolyte is enhanced. Simultaneously, the nanoscale capillary channels of cellulose can shorten the lithium-ion diffusion path through capillary action, and the nanoparticles embedded in the matrix network fill pore defects, reducing ion transport resistance. Subsequently, the base membrane is laminated with a flexible graphene oxide membrane, forming a physical interlock between the nanoparticles and the carbon-based membrane, enhancing the interfacial bonding strength, thereby improving the electrolyte retention rate and safety of the battery. The porous structure of the carbon-based membrane filters lithium ions, while its electronic insulation prevents electron leakage. Combined with the physical barrier of the nanoparticles, this further improves the battery's cycle performance.

[0028] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for preparing a lithium battery cell with a composite separator, characterized in that, Includes the following steps: (1) Adjust the pH of formaldehyde to 9-10 with 10wt% sodium hydroxide solution, then add melamine for prepolymerization reaction, then add cellulose solution, adjust the pH of solution to 3-5 with 5wt% oxalic acid solution, stir, add functional nanoparticle dispersion, and sonicate to obtain blended spinning solution. (2) The blended spinning solution was electrospun, then immersed in a 1wt% sodium hydroxide solution, then washed with deionized water, and finally dried with hot air to obtain a base film. (3) Pour 1 mg / mL of graphene oxide aqueous dispersion onto the filter membrane, filter it to obtain a graphene oxide wet membrane, dry the graphene oxide wet membrane together with the filter membrane, and then peel off the filter membrane to obtain a carbon-based membrane. (4) The carbon-based membrane is laid flat on the base membrane and hot-pressed to obtain a composite membrane; (5) Place the composite separator between the positive electrode and the negative electrode, then insert it into the aluminum-plastic film, inject the electrolyte, and heat seal it to obtain a lithium battery cell with a composite separator.

2. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The molar ratio of formaldehyde and melamine in step (1) is 2~4:

1.

3. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The mass ratio of melamine, cellulose solution and functional nanoparticle dispersion in step (1) is 10:5~10:2~5.

4. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The preparation method of the cellulose solution in step (1) is as follows: cotton pulp and 50wt% 1-ethyl-3-methylimidazolium acetate ionic liquid are mixed at a mass ratio of 1:10, heated to 80℃, and stirred at 300rpm for 2h to obtain the solution.

5. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The functional nanoparticle dispersion in step (1) is composed of functional nanoparticles, ethanol, and deionized water in a mass ratio of 0.3~0.6:7:

3.

6. A method for preparing a lithium battery cell with a composite separator according to claim 5, characterized in that, The functional nanoparticles are at least one of silicon dioxide, aluminum oxide, titanium dioxide, and zinc oxide.

7. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The electrospinning process parameters in step (2) are: needle diameter of 1 mm, voltage of 15 kV, receiving distance of 10 cm, and speed of 0.5 mL / h.

8. The method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The thickness of the base film in step (2) is 20~30μm.

9. A method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The filter membrane mentioned in step (3) is specifically a polytetrafluoroethylene filter membrane with a pore size of 0.22 μm.

10. A method for preparing a lithium battery cell with a composite separator according to claim 1, characterized in that, The thickness of the carbon-based film in step (3) is 5~10μm.

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