Lithium-indigo battery diaphragm, preparation method thereof and lithium-indigo battery energy storage device

Abstract

The invention relates to the technical field of lithium-organic battery energy storage devices, in particular to a lithium-indigo battery diaphragm, a preparation method thereof and a lithium-indigo battery energy storage device. The composite diaphragm takes a polypropylene film as a substrate, and nitrogen-doped iron-cobalt nano-alloy porous biomass conductive carbon is loaded on the surface of the polypropylene film; the nitrogen-doped iron-cobalt nano-alloy porous biomass conductive carbon is of a core-shell structure, the core material is iron-cobalt nano-alloy, and the shell material is nitrogen-doped porous biomass conductive carbon. The composite diaphragm provided by the invention solves the problem of dissolution and diffusion of active substances in a lithium-indigo battery through a triple synergistic mechanism of physical barrier, chemical adsorption and catalytic conversion, and greatly improves the coulombic efficiency and cycling stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-organic battery energy storage device technology, and more particularly to a lithium-indigo battery separator and its preparation method, and a lithium-indigo battery energy storage device. Background Technology

[0002] With the ever-growing global demand for renewable energy and portable electronic devices, the development of high-performance, low-cost, and environmentally friendly energy storage systems has become a top priority. Traditional lithium-ion batteries mainly rely on inorganic cathode materials containing transition metals such as cobalt and nickel. These materials suffer from resource scarcity, high cost, and recycling difficulties. In contrast, organic electrode materials, with their advantages of abundant elements, strong structural designability, environmental friendliness, and high theoretical specific capacity, are considered an ideal choice for next-generation green energy storage systems. Among them, indigo, as a historically significant natural organic dye, possesses a unique redox-active carbonyl structure that can reversibly bind lithium ions, thus showing broad application prospects in the field of lithium-ion battery cathode materials.

[0003] However, applying indigo to lithium-ion batteries still faces significant technical challenges, mainly in the following two aspects: First, as a small organic molecule, indigo has extremely low intrinsic electronic conductivity, severely limiting the battery's rate performance and the utilization rate of active materials. Second, during charge and discharge, indigo and its reduced intermediates readily dissolve in organic electrolytes, resulting in a severe "shuttle effect." The dissolved active molecules diffuse through the separator to the negative electrode side, undergoing irreversible side reactions with lithium metal, leading to the loss of active materials, reduced coulombic efficiency, and rapid decline in battery cycle life.

[0004] As a key component inside the battery that isolates the positive and negative electrodes, prevents short circuits, and provides ion transport channels, the separator plays a decisive role in suppressing the shuttle effect. Currently, commercial lithium-ion batteries generally use polyolefin microporous membranes such as polypropylene (PP) or polyethylene (PE) as separators. For lithium-indigo battery systems, existing polypropylene separators have significant drawbacks:

[0005] First, its physical barrier capability is limited. Due to the relatively large pore size and simple pore structure of polypropylene membranes, they cannot effectively block the free diffusion of organic active molecules or intermediate products such as polysulfides dissolved in the electrolyte, making it difficult to effectively suppress the shuttle effect at the physical level.

[0006] Second, surface chemical inertness. The surface of polypropylene material is non-polar and lacks the ability to chemically adsorb polar indigo molecules and their discharge intermediates. It cannot anchor active substances through chemical interactions, which makes the membrane that relies solely on physical barriers ineffective during long-term cycling.

[0007] Third, insulation leads to the deactivation of active materials. Polypropylene separators are essentially insulators. When dissolved organic active materials migrate and deposit on the separator surface, they lose their electrochemical activity because they cannot gain electrons, forming "dead active materials." This not only causes irreversible capacity loss but may also block the separator pores, hindering the normal transport of lithium ions and increasing the battery's internal resistance.

[0008] To address these issues, existing technologies often employ coating the membrane surface with carbon materials or metal oxide layers. However, while a simple carbon coating can improve conductivity, it is mostly non-polar physical adsorption, resulting in weak binding forces to polar organic molecules. On the other hand, while a single metal oxide coating exhibits some polar adsorption, its conductivity is poor, and it often struggles to achieve efficient catalytic conversion due to limited specific surface area or insufficient active sites.

[0009] Therefore, in order to address the shortcomings of existing polypropylene separators in lithium-indigo battery applications, such as poor conductivity, weak adsorption capacity for organic molecules, and inability to suppress the shuttle effect, it is necessary to develop a composite modified separator that integrates high conductivity, strong chemical adsorption capacity, and excellent catalytic activity. This has significant application value for improving the electrochemical performance of lithium-indigo energy storage batteries. Summary of the Invention

[0010] The purpose of this invention is to provide a lithium-indigo battery separator and its preparation method, as well as a lithium-indigo battery energy storage device, which significantly improves the specific capacity and cycle stability of the battery by utilizing the composite separator.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows: The first aspect of the present invention provides a lithium-indigo battery composite separator, wherein the composite separator is based on a polypropylene film and has nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon loaded on its surface; the nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon has a core-shell structure, wherein the core material is iron-cobalt nanoalloy and the shell material is nitrogen-doped porous biomass conductive carbon.

[0012] A second aspect of the present invention provides a method for preparing the above-mentioned lithium-indigo battery separator, comprising the following steps: S1: Biomass carbon source, nitrogen source, pore-forming agent, iron source and cobalt source are mixed and loaded into a ball mill, ball-milled in ethanol, and dried to obtain a precursor containing FeCo nano-alloys; S2: The precursor obtained in step S1 is calcined, washed, and dried in an inert gas atmosphere to obtain nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon. S3: The nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon obtained in step S2 is placed in an ethanol solution and ultrasonically centrifuged. It is then filtered using a sand core filtration device with a polypropylene film as the substrate to obtain the composite membrane.

[0013] In the above technical solution, further, in step S1, the biomass carbon source includes one of guar gum, creatine, coconut shell, sawdust, rice husk, and corn cob; The nitrogen source includes one of cyanide, amine compounds, and nitrides, preferably one of dicyandiamide, urea, and amino acids; The pore-forming agent includes one of sodium chloride, potassium chloride, calcium chloride, sodium hydroxide, zinc chloride, phosphoric acid, sulfuric acid, calcium hydroxide, sodium carbonate, and silica gel. The iron source includes one of ferric nitrate, ferric oxide, ferric chloride, ferric sulfide, and potassium ferricyanide; The cobalt source includes one of cobalt nitrate, cobalt oxide, cobalt chloride, cobalt sulfide, and cobalt salts.

[0014] In the above technical solution, further, in step S1, the mass ratio of iron in the biomass carbon source, nitrogen source, pore-forming agent, and iron source to cobalt in the cobalt source is 4.0:4.0:5.0:0.2:0.211.

[0015] In the above technical solution, further, in step S2, the inert gas is one or both of nitrogen and argon; The calcination temperature is 800-990℃, the holding time is 1-3h, and the heating rate is 3-6℃ / min.

[0016] A third aspect of the present invention provides a lithium-indigo battery energy storage device, the energy storage device comprising a positive electrode, a positive electrode, a separator, and an electrolyte, wherein the separator is the aforementioned composite separator.

[0017] In the above technical solution, the positive electrode further includes a positive electrode active material, a composite conductive agent, and a binder; the positive electrode active material is an indigo-based compound; the composite conductive agent includes conductive agent A and conductive agent B, wherein conductive agent A is one of carbon nanotubes, graphene, and N-doped porous biomass carbon, and conductive agent B is conductive carbon black.

[0018] In the above technical solution, the electrolyte further includes a solvent, an electrolyte, and mixed additives; The solvent is a mixed solution of ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL), wherein the volume ratio of ethylene glycol dimethyl ether to dioxolane is 1:1. The electrolyte is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); The mixed additives are iron acetylacetone (Fe(acac)3) and lithium nitrate (LiNO3).

[0019] In the above technical solution, the mass fraction of the additive in the electrolyte is 1%-5%.

[0020] In the above technical solution, the negative electrode is further defined as lithium metal.

[0021] Furthermore, in the above technical solution, the energy storage device also includes a positive current collector for loading the positive electrode, wherein the positive current collector is one of copper foil, aluminum foil, or stainless steel foil.

[0022] Furthermore, in the above technical solution, the energy storage device also includes a housing for encapsulating the positive electrode, negative electrode, separator, and electrolyte, wherein the housing is made of polymethyl methacrylate.

[0023] The electrochemical reaction principle of this invention is based on the reversible redox reaction of the carbonyl group (C=O) in the indigo molecule and the insertion / extraction process of lithium ions. The working mechanism is described in detail below: Lithium ions migrate to the positive electrode through the electrolyte and combine with the carbonyl group in the indigo molecule. Simultaneously, electrons flow to the positive electrode through an external circuit, completing the electrochemical reaction. The positive electrode reaction occurs within the indigo molecule, where the carbonyl group (C=O) is reduced to a hydroxyl anion (CO). - ), positive electrode reduction reaction (gains electrons): The negative electrode is oxidized (loses electrons):

[0024] The positive electrode is reduced (gains electrons):

[0025] The reaction equations for the entire discharge process are as follows:

[0026] The beneficial effects of this invention are as follows: 1. The composite separator of this invention solves the problem of active material dissolution and diffusion in lithium-indigo batteries through a triple synergistic mechanism of physical barrier, chemical adsorption, and catalytic conversion. First, porous biomass carbon is used as a physical barrier to effectively prevent dissolved indigo molecules and organic intermediates from migrating to the negative electrode side. Second, the abundant nitrogen-doped sites in the carbon framework exert a strong chemical adsorption effect on polar organic molecules, anchoring the active material to the positive electrode side. Finally, the supported iron-cobalt (FeCo) nanoalloy, as a highly efficient electrocatalyst, can accelerate the redox reaction kinetics of intermediates adsorbed on the separator surface, converting them into insoluble product deposition, thereby significantly suppressing the shuttle effect and greatly improving the coulombic efficiency and cycle stability of the battery.

[0027] 2. To address the issue of poor intrinsic conductivity of indigo materials, the composite separator of this invention introduces a highly conductive carbon-based composite material onto an insulating polypropylene substrate. This not only acts as a secondary current collector, reducing the battery's internal resistance and electrochemical polarization, but also activates the active materials deposited on the separator surface, allowing them to re-participate in electrochemical reactions. Simultaneously, the porous biomass carbon possesses excellent electrolyte wettability, enabling it to absorb and retain electrolyte, providing a smooth channel for the rapid transport of lithium ions, thereby significantly improving the battery's performance under high-rate charge-discharge conditions.

[0028] 3. The composite membrane of this invention uses biomass as both a carbon and nitrogen source. The raw materials are widely available, inexpensive, and renewable, aligning with the principles of green chemistry and sustainable development. Furthermore, the preparation process primarily involves conventional operations such as ball milling, calcination, and filtration. The process is simple, requires minimal equipment, and is easily scalable, demonstrating promising commercial application prospects.

[0029] 4. The present invention effectively prevents dissolved organic molecules from migrating to the negative electrode surface and reacting with metallic lithium through the composite separator, thereby reducing the instability of the passivation layer (SEI film) on the negative electrode surface and indirectly inhibiting the growth of lithium dendrites.

[0030] 5. This invention, by introducing iron acetylacetone as a key additive, enables the in-situ construction of a stable solid electrolyte interface layer with excellent ionic conductivity at the electrode interface, effectively suppressing the decomposition of ether solvents and the growth of lithium dendrites, thereby significantly improving interface stability and chemical compatibility. Thanks to stable interface regulation, this system exhibits excellent long cycle life over a wide temperature range, maintaining a capacity retention of over 85% even after more than 500 cycles at 1C or 2C current densities. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the lithium-indigo battery energy storage device of the present invention; In the diagram: 1. Shell A, 2. Spring, 3. Gasket, 4. Negative electrode, 5. Electrolyte, 6. Diaphragm, 7. Positive electrode, 8. Shell B; Figure 2 The constant current charge-discharge cycle performance of the battery energy storage devices in Examples 1 and 2 at 0.2 C; Figure 3 The constant current charge-discharge cycle performance of the battery energy storage device in Example 3 at 0.2 C; Figure 4 The constant current charge-discharge cycle performance of the battery energy storage device in Comparative Example 1 at 0.2 C is shown. Figure 5 The constant current charge-discharge cycle performance of the battery energy storage device in Comparative Example 2 at 0.2 C is shown. Figure 6The constant current charge-discharge cycle performance of the battery energy storage device in Example 4 at 0.2 C is shown. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention is further described through the following embodiments. Obviously, the following embodiments are only some embodiments of the present invention, and not all embodiments; it should be understood that the embodiments of the present invention are only used to illustrate the technical effects of the present invention, and not to limit the scope of protection of the present invention.

[0033] All raw materials used in the examples are commercially available; unless otherwise specified, the reagents, methods and equipment used in this invention are conventional reagents, methods and equipment in this technical field.

[0034] Example 1 A lithium-indigo battery energy storage device includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein: Preparation of MWCNTs@Indigo cathode: (1) Weigh 5g of indigo powder and 5g of multi-walled carbon nanotube (MWCNT) material powder, place them in a ball mill for thorough ball milling to disperse them evenly, and calcine them at 395℃ for 120min to obtain an indigo mixture for later use. (2) Dry the conductive carbon black Super-C and the above mixture overnight for later use; (3) Weigh 0.1g of adhesive (PVDF) and dissolve it in 0.5ml of NMP and stir thoroughly to obtain an adhesive solution; (4) Weigh 0.2g of dried indigo mixture and 0.7g of super-c and add them to the binder solution. Stir for 180 minutes to obtain the positive electrode slurry. (5) The positive electrode slurry is coated onto aluminum foil using a coating machine, dried in a vacuum oven for 720 minutes, and finally the finished product is cut into pieces. Preparation of N-MPC@FeCo / PP composite membrane: S1: 4g of guar gum, 4g of dicyandiamide, Fe(NO3)3·9H2O containing 0.2g of iron, Co(NO3)2·4H2O containing 0.211g of cobalt, and 5g of NaCl were placed in a ball mill and ball-milled in ethanol (the weight ratio of ethanol to carbon source was 2~5:1) for 12 hours. Then the mixture was placed in an oven and dried overnight to obtain a precursor containing FeCo nano-alloys. S2: The precursor obtained in step S1 is subjected to argon gas at 950℃ for 5 minutes. -1The calcination was carried out at a heating rate of 2 hours, washed and filtered with deionized water until NaCl was completely washed away. The filtrate was tested with silver nitrate solution until no white precipitate was formed. The filtrate was then dried overnight at 70°C in a vacuum oven to obtain nitrogen-doped iron-cobalt nano-alloy porous biomass conductive carbon solid powder. S3: Add 0.6g of the solid powder obtained in step S2 to ethanol, sonicate for 2 hours at 2000 rpm. -1 Centrifuge at a set speed for 15 minutes, take the supernatant, cut a 10×10cm polypropylene film and lay it flat in a sand core filtration flask, filter the supernatant through a filtration device to obtain a diaphragm. Electrolyte preparation: Commercial electrolyte (LiTFSI is dissolved in ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL) at a volume ratio of 1:1, and LiNO3 is added. The concentration of LiTFSI in the electrolyte is 1 mol / L and the mass fraction of LiNO3 is 1%). Battery assembly: Battery structure such as Figure 1 As shown, a lithium metal sheet with a diameter of 12 mm and a thickness of 0.3 mm is used as the negative electrode, and an aluminum foil is used as the positive electrode current collector. The housing includes housing A and housing B, and the material of the housing is polymethyl methacrylate. Housing A is placed on a table, and the lithium metal sheet is placed on housing A. 1.5 μL of electrolyte is measured and suspended in the center of the lithium metal sheet using a pipette. A 16 mm diameter disc is cut, and the disc is covered with electrolyte and placed on the lithium metal sheet. Another 1.5 μL of electrolyte is measured and suspended in the center of the disc using a pipette. The 12 mm aluminum foil, which is used as the positive electrode, is covered with electrolyte and placed on the disc. Finally, a gasket and a spring are added for fixation, and housing B is closed to complete the preparation.

[0035] Example 2 A lithium-indigo battery energy storage device includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein: Preparation of MWCNTs@Indigo cathode: (1) Weigh 5g of indigo powder and 5g of multi-walled carbon nanotube material powder, place them in a ball mill for thorough ball milling to disperse them evenly, and calcine them at 395℃ for 120min to obtain an indigo mixture for later use. (2) Dry the conductive carbon black Super-C and the above mixture overnight for later use; (3) Weigh 0.1g of adhesive (PVDF) and dissolve it in 0.5ml of NMP and stir thoroughly to obtain an adhesive solution; (4) Weigh 0.2g of dried indigo mixture and 0.7g of super-c and add them to the binder solution. Stir for 180 minutes to obtain the positive electrode slurry. (5) The positive electrode slurry is coated onto aluminum foil using a coating machine, dried in a vacuum oven for 720 minutes, and finally the finished product is cut into pieces. Preparation of N-MPC@FeCo / PP composite membrane: S1: 4g of guar gum, 4g of dicyandiamide, Fe(NO3)3·9H2O containing 0.2g of iron, Co(NO3)2·4H2O containing 0.211g of cobalt, and 5g of NaCl were placed in a ball mill and ball-milled in ethanol (the weight ratio of ethanol to carbon source was 2~5:1) for 12 hours. Then the mixture was placed in an oven and dried overnight to obtain a precursor containing FeCo nano-alloys. S2: The precursor obtained in step S1 is subjected to argon gas at 950℃ for 5 minutes. -1 The calcination was carried out at a heating rate of 2 hours, washed and filtered with deionized water until NaCl was completely washed away. The filtrate was tested with silver nitrate solution until no white precipitate was formed. The filtrate was then dried overnight at 70°C in a vacuum oven to obtain nitrogen-doped iron-cobalt nano-alloy porous biomass conductive carbon solid powder. S3: Add 0.6g of the solid powder obtained in step S2 to ethanol, sonicate for 2 hours at 2000 rpm. -1 Centrifuge at a set speed for 15 minutes, collect the supernatant, and cut the polypropylene film into 10 mm pieces. A 10cm piece is laid flat in the sand core filtration flask, and the supernatant is filtered through the filtration device to obtain the diaphragm. Electrolyte preparation: LiTFSI was dissolved in ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL) in a volume ratio of 1:1. LiNO3 and Fe(acac)3 were added. In the electrolyte, the concentration of LiTFSI was 1 mol / L, the mass fraction of LiNO3 was 1%, and the mass fraction of Fe(acac)3 was 2%.

[0036] Battery assembly: Battery structure such as Figure 1 As shown, a lithium metal sheet with a diameter of 12 mm and a thickness of 0.3 mm is used as the negative electrode, and an aluminum foil is used as the positive electrode current collector. The housing includes housing A and housing B, and the material of the housing is polymethyl methacrylate. Housing A is placed on a table, and the lithium metal sheet is placed on housing A. 1.5 μL of electrolyte is measured and suspended in the center of the lithium metal sheet using a pipette. A 16 mm diameter disc is cut, and the disc is covered with electrolyte and placed on the lithium metal sheet. Another 1.5 μL of electrolyte is measured and suspended in the center of the disc using a pipette. The 12 mm aluminum foil, which is used as the positive electrode, is covered with electrolyte and placed on the disc. Finally, a gasket and a spring are added for fixation, and housing B is closed to complete the preparation.

[0037] Example 3 The difference from Example 1 is that the conductive agent A used to prepare the positive electrode is nitrogen-doped porous biomass carbon (denoted as NC, prepared according to the method in Yang X, Zheng X, Yan Z, et al. Construction and preparation of nitrogen-doped porous carbon material based on waste biomass for lithium-ion batteries[J]. International Journal of Hydrogen Energy, 2021, 46(33): 17267-17281). The remaining steps are the same as in Example 1, and the obtained positive electrode is denoted as NC@Indigo.

[0038] Example 4 The difference from Example 2 is that the conductive agent A in the positive electrode is nitrogen-doped porous biomass conductive carbon (denoted as NC, prepared according to the method in Yang X, Zheng X, Yan Z, et al. Construction and preparation of nitrogen-doped porous carbon material based on waste biomass for lithium-ion batteries[J]. International Journal of Hydrogen Energy, 2021, 46(33): 17267-17281), and the remaining steps are the same as in Example 2. The obtained positive electrode is denoted as NC@Indigo.

[0039] Comparative Example 1 The difference from Example 1 is that the diaphragm is a polypropylene film (PP), while the rest of the steps are the same as in Example 1.

[0040] Comparative Example 2 The difference from Example 3 is that the diaphragm is a polypropylene film (PP), while the rest of the steps are the same as in Example 3.

[0041] Constant current charge-discharge tests were conducted at the Shenzhen Xinwei Battery Testing System. Figure 2The specific capacity changes in Examples 1 and 2 are shown. The combination of N-MPC@FeCo / PP composite separator and MWCNTs@Indigo cathode exhibits excellent electrochemical performance, with an initial specific capacity as high as 203.03 mAh / g (the theoretical specific capacity of indigo is 204 mAh / g), and the specific capacity can still be maintained at 160 mAh / g after 300 cycles. In Example 2, the performance is more stable after the addition of acetylacetone iron additive, and the specific capacity can still be maintained at 175 mAh / g after 300 cycles.

[0042] Figure 3 The specific capacity change in Example 3 is shown. The combination of N-MPC@FeCo / PP composite separator and NC@Indigo cathode has an initial specific capacity of 140.12 mAh / g, which is slightly lower than that in Example 1. However, it still maintains a relatively stable specific capacity after long-term cycling tests.

[0043] Figure 4 and Figure 5 The constant current charge-discharge cycle performance of battery energy storage devices assembled using PP separators in Comparative Examples 1 and 2 at 0.2 C is shown. Compared with Examples 1 and 3, the specific capacity at 0.2 C rate shows a significant rapid decay trend. This comparative result indicates that the introduction of N-MPC@FeCo / PP composite separator effectively exerts the dual effects of physical barrier and chemical adsorption, significantly suppressing the shuttle effect of organic active materials during charge and discharge, thereby greatly improving the long-term cycle stability of the battery.

[0044] To further optimize device performance, an acetylacetone iron electrolyte additive was introduced. Figure 6 shows the specific capacity change of Example 4. After 160 cycles, the specific capacity of this device can still be maintained at 150 mAh / g, achieving excellent results. This further greatly reduces production costs and makes it more economical to be applied to the market.

[0045] The foregoing has provided a detailed description of a lithium-indigo battery separator, its preparation method, and a lithium-indigo battery energy storage device. Specific examples have been used to illustrate the principles and implementation methods of the invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of the invention. It should be noted that those skilled in the art can make various improvements and modifications to the invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of the invention.

Claims

1. A lithium-indigo battery composite separator, characterized in that, The composite membrane uses a polypropylene film as a substrate, with nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon loaded on its surface; the nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon has a core-shell structure, with the core material being iron-cobalt nanoalloy and the shell material being nitrogen-doped porous biomass conductive carbon.

2. A method for preparing the lithium-indigo battery separator according to claim 1, characterized in that, Includes the following steps: S1: Biomass carbon source, nitrogen source, pore-forming agent, iron source and cobalt source are mixed and loaded into a ball mill, ball-milled in ethanol, and dried to obtain a precursor containing FeCo nano-alloys; S2: The precursor obtained in step S1 is calcined, washed, and dried in an inert gas atmosphere to obtain nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon. S3: The nitrogen-doped iron-cobalt nanoalloy porous biomass conductive carbon obtained in step S2 is placed in an ethanol solution and subjected to ultrasonication and centrifugation. It is then filtered through a sand core filtration device with a polypropylene film as the substrate to obtain the composite membrane.

3. The preparation method according to claim 2, characterized in that, In step S1, the biomass carbon source includes one of guar gum, creatine, coconut shell, sawdust, rice husk, and corn cob. The nitrogen source includes one of cyanide, amine compounds, and nitrides, preferably one of dicyandiamide, urea, and amino acids; The pore-forming agent includes one of sodium chloride, potassium chloride, calcium chloride, sodium hydroxide, zinc chloride, phosphoric acid, sulfuric acid, calcium hydroxide, sodium carbonate, and silica gel. The iron source includes one of ferric nitrate, ferric oxide, ferric chloride, ferric sulfide, and potassium ferricyanide; The cobalt source includes one of cobalt nitrate, cobalt oxide, cobalt chloride, cobalt sulfide, and cobalt salts.

4. The preparation method according to claim 2, characterized in that, In step S1, the mass ratio of iron in the biomass carbon source, nitrogen source, pore-forming agent, and iron source to cobalt in the cobalt source is 4.0:4.0:5.0:0.2:0.

211.

5. The preparation method according to claim 2, characterized in that, In step S2, the inert gas is one or both of nitrogen and argon. The calcination temperature is 800-990℃, the holding time is 1-3h, and the heating rate is 3-6℃ / min.

6. A lithium-indigo battery energy storage device, characterized in that, The energy storage device includes a positive electrode, a positive electrode, a separator, and an electrolyte, wherein the separator is the composite separator as described in claim 1 or the composite separator prepared by the preparation method described in claims 2-5.

7. The lithium-indigo battery energy storage device according to claim 6, characterized in that, The positive electrode comprises a positive electrode active material, a composite conductive agent, and a binder; the positive electrode active material is an indigo-based compound; the composite conductive agent comprises conductive agent A and conductive agent B, wherein conductive agent A is one of carbon nanotubes, graphene, and N-doped porous biomass carbon, and conductive agent B is conductive carbon black.

8. The lithium-indigo battery energy storage device according to claim 7, characterized in that, The electrolyte includes a solvent, an electrolyte, and a mixture of additives; The solvent is a mixed solution of ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL), wherein the volume ratio of ethylene glycol dimethyl ether to dioxolane is 1:

1. The electrolyte is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); The mixed additives are iron acetylacetone (Fe(acac)3) and lithium nitrate (LiNO3).

9. The lithium-indigo battery energy storage device according to claim 8, characterized in that, The electrolyte contains 1%-5% by mass of additives.

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