Composite carbon-coated sodium ferrous sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a composite carbon-coated sodium ferrous sulfate positive electrode material and a preparation method and application thereof, and belongs to the technical field of sodium ion batteries, the composite carbon-coated sodium ferrous sulfate positive electrode material comprises sodium ferrous sulfate and a composite carbon coating layer coating at least part of the surface of the sodium ferrous sulfate; materials for forming the composite carbon coating layer comprise a biomass porous carbon material and an inorganic carbon source. The biomass porous carbon material and the inorganic carbon source are used at the same time to form the composite carbon coating layer, and the composite carbon coating layer is used for coating the sodium ferrous sulfate, so that the compaction density and the cycle performance of the sodium ferrous sulfate positive electrode material are effectively improved, and the resistivity of the sodium ferrous sulfate positive electrode material is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and particularly to a composite carbon-coated sodium iron sulfate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Sodium-ion batteries have received extensive attention from researchers due to their rich raw material sources, low prices, and similar working principles to lithium-ion batteries, and are expected to replace lithium-ion batteries in new energy vehicles and other energy storage fields in the future.

[0003] The cathode material is crucial for the performance of sodium-ion batteries. Currently, transition metal oxides (TMOs) and polyanionic compounds (PACs) are the most promising cathode materials for sodium-ion batteries. Among them, although the theoretical capacity of PACs is slightly lower than that of TMOs, PACs generally have more excellent cycle life than TMOs. Since one of the main potential application fields of sodium-ion batteries is grid-scale energy storage, ultra-long cycle life is one of the most important requirements. Therefore, if the theoretical capacity of PACs can be increased, their application prospects in sodium-ion batteries will be very broad. Among various PACs, iron-based sulfate materials have received attention due to their low cost and non-toxicity, and sodium iron sulfate is one of the iron-based sulfate materials with the highest theoretical capacity, and has a simple synthesis method, excellent cycle performance, and high research value.

[0004] Currently, there are mainly two different process routes for the synthesis of sodium iron sulfate, namely the liquid-phase method and the solid-phase method. Among them, although the sodium iron sulfate cathode material synthesized by the solid-phase method has a high tap density and a low powder resistivity, its requirements for raw materials are more stringent, and the capacity is low, which restricts the application of the sodium iron sulfate cathode material. Compared with the solid-phase method, the liquid-phase method has more relaxed requirements for raw materials when synthesizing the sodium iron sulfate cathode material, and the synthesized material has a higher capacity, which is a more ideal synthesis method. However, when using the liquid-phase method to synthesize the sodium iron sulfate cathode material, problems such as low tap density, high resistivity, and poor cycle performance still need to be further solved. Summary of the Invention

[0005] In view of the technical problems existing in the background art, the present application provides a composite carbon-coated sodium iron sulfate cathode material, a preparation method thereof, and an application thereof, aiming to solve the technical problems of low tap density, high resistivity, and poor cycle performance faced when synthesizing the sodium iron sulfate cathode material by the liquid-phase method.

[0006] In a first aspect, an embodiment of the present application provides a composite carbon-coated sodium iron sulfate cathode material, including sodium iron sulfate and a composite carbon coating layer coated on at least a part of the surface of the sodium iron sulfate; wherein, the material forming the composite carbon coating layer includes a biomass porous carbon material and an inorganic carbon source.

[0007] In the technical solution of the embodiment of the present application, by simultaneously using a biomass porous carbon material and an inorganic carbon source to form a composite carbon coating layer, and using the composite carbon coating layer to coat sodium ferrous sulfate, the tap density and cycle performance of the sodium ferrous sulfate cathode material can be effectively improved, and the resistivity of the sodium ferrous sulfate cathode material can be reduced.

[0008] In some embodiments, the biomass porous carbon material has a flaky structure; and / or, the porosity of the biomass porous carbon material is 20% - 30%, and the pore diameter is 50 - 100 nm.

[0009] In this embodiment, the flaky biomass porous carbon material can serve as a carrier for the growth of sodium ferrous sulfate crystals. Its abundant pores and smaller pore diameter are beneficial to providing a larger buffer space for the volume change of sodium ferrous sulfate during cycling while ensuring the tap density, so as to improve the cycle performance of the cathode material.

[0010] In some embodiments, the mass ratio of the biomass porous carbon material to the inorganic carbon source is 1:(0.3 - 4); and / or, the inorganic carbon source includes graphene oxide and carbon nanotubes, wherein the mass ratio of graphene oxide to carbon nanotubes is 1:(1 - 3).

[0011] In this embodiment, by adjusting the ratio between the biomass porous carbon material and the inorganic carbon source, the composition and structure of the composite carbon coating layer can be regulated, so as to give full play to the synergistic effect of the two, effectively improving the tap density, cycle performance and electrical conductivity of the cathode material. And by simultaneously using graphene oxide and carbon nanotubes as the inorganic carbon source, carbon coating of sodium ferrous sulfate is more effectively carried out, and the resistivity of the cathode material is reduced.

[0012] In some embodiments, the tap density of the composite carbon-coated sodium ferrous sulfate cathode material is not less than 1.9 g / cm 3 , the BET specific surface area is 11 - 22 m 2 / g, and the powder resistivity is less than 180 Ω·cm.

[0013] In this embodiment, the composite carbon-coated sodium ferrous sulfate cathode material has a relatively high tap density and a relatively low powder resistivity, which is beneficial to improving the cycle performance and electrochemical performance of the secondary battery prepared using this composite carbon-coated sodium ferrous sulfate cathode material.

[0014] In a second aspect, the embodiment of the present application provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material, including the following steps:

[0015] Prepare a biomass porous carbon material;

[0016] Mix an iron source, a sodium source, a biomass porous carbon material, an inorganic carbon source, an antioxidant and a solvent to obtain a first slurry;

[0017] Dry, sinter and pulverize the first slurry to obtain a composite carbon-coated sodium ferrous sulfate cathode material.

[0018] In the technical solution of the embodiment of the present application, in the process of preparing sodium ferrous sulfate by the liquid phase method, by simultaneously using a biomass porous carbon material and an inorganic carbon source, the formed composite carbon coating layer coats sodium ferrous sulfate, effectively solving the problems of low tap density, high resistivity and poor cycling performance while maintaining the advantages of the liquid phase method.

[0019] In some embodiments, the preparation method of the biomass porous carbon material includes the following steps:

[0020] Grind the biomass material to obtain a first powder;

[0021] Etch the first powder with an alkali solution, and then wash and dry it to obtain the biomass porous carbon material.

[0022] In this embodiment, by etching the first powder with an alkali solution, micropores can be formed, and in the subsequent sintering process, a more porous morphology with richer pores can be further formed, providing a larger buffer space for the volume change of sodium ferrous sulfate during cycling, thereby effectively improving the cycling performance of the cathode material.

[0023] In some embodiments, the molar ratio of the iron element in the iron source to the sodium element in the sodium source is 1:(1.35 - 1.529); and / or, the total mass of the biomass porous carbon material and the inorganic carbon source accounts for 3% - 5% of the mass of the iron source; and / or, the mass of the antioxidant accounts for 4% - 6% of the mass of the iron source; and / or, the biomass material includes at least one of corn straw, bamboo powder, and rice husk carbon; and / or, the iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, and ferrous oxalate; and / or, the sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, and sodium oxalate; and / or, the antioxidant includes at least one of ascorbic acid, oxalic acid, and citric acid.

[0024] In this embodiment, by adjusting the molar ratio of the iron element in the iron source to the sodium element in the sodium source, the phase of the synthesized sodium ferrous sulfate can be regulated, which is beneficial to improving the electrochemical performance of the cathode material; by adjusting the dosages of the biomass porous carbon material and the inorganic carbon source, the coating rate and thickness of the composite carbon coating layer on sodium ferrous sulfate can be regulated to achieve a better coating effect; by adding an appropriate amount of antioxidant, the oxidation of ferrous ions can be avoided; and in the embodiments of the present application, all raw materials are widely sourced, which is beneficial to industrial mass production.

[0025] In some embodiments, the drying method is spray drying, and the outlet air temperature of the spray drying is 85-95 °C; and / or, the sintering conditions include: maintaining the temperature at 350-400 °C for 8-24 h under an inert atmosphere.

[0026] In this embodiment, by adopting the spray drying method for drying, the precursor can be fully dried and form powders with smaller particle sizes, so as to fully react during the subsequent sintering treatment process; by controlling the sintering conditions, it can promote the reaction raw materials to fully react under these conditions to generate sodium ferrous sulfate, and enable the composite carbon coating layer formed by the biomass porous carbon material and the inorganic carbon source to coat the sodium ferrous sulfate particles.

[0027] In a third aspect, an embodiment of the present application provides a positive electrode sheet, including the composite carbon-coated sodium ferrous sulfate positive electrode material provided in the first aspect of the present application, or the composite carbon-coated sodium ferrous sulfate positive electrode material prepared by the preparation method provided in the second aspect.

[0028] In this embodiment, the positive electrode sheet contains the above-mentioned composite carbon-coated sodium ferrous sulfate positive electrode material, and thus has the advantages of high capacity, good cycle performance, and good electrical conductivity.

[0029] In a fourth aspect, an embodiment of the present application provides a secondary battery, including the positive electrode sheet provided in the third aspect of the present application.

[0030] In this embodiment, the secondary battery contains the above-mentioned positive electrode sheet, and thus has the advantages of high capacity, good cycle performance, and good electrical conductivity.

[0031] The above description is only an overview of the technical solutions of the present application. In order to be able to understand the technical means of the present application more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features, and advantages of the present application more obvious and understandable, the specific embodiments of the present application are specifically listed below. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the following described drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 It is the nitrogen adsorption and desorption curve of the biomass porous carbon material obtained in Example 1;

[0034] Figure 2 It is the SEM image of the biomass porous carbon material obtained in Example 1;

[0035] Figure 3SEM image of the fourth powder obtained in Example 1;

[0036] Figure 4 SEM image of the composite carbon-coated sodium iron sulfate cathode material obtained in Example 1. Detailed implementation manners

[0037] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to illustrate the technical solutions of the present application more clearly, and therefore are only examples and cannot be used to limit the protection scope of the present application.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above drawings are intended to cover non-exclusive inclusion.

[0039] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present application, "a plurality" means more than two unless otherwise specifically defined.

[0040] Referring to "embodiments" herein means that specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0041] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0042] Compared with the solid-phase method for synthesizing the sodium iron sulfate cathode material, when the liquid-phase method is used to synthesize the sodium iron sulfate cathode material, although the requirements for raw materials are relatively loose and the material has a higher capacity after synthesis, it still faces problems such as low tap density, high resistivity, and poor cycle performance.

[0043] In order to solve the technical problems of low tap density, high resistivity, and poor cycle performance of the sodium iron sulfate cathode material synthesized by the liquid-phase method, the present application provides a composite carbon-coated sodium iron sulfate cathode material, its preparation method and application. By preparing a biomass porous carbon material and mixing it with raw materials such as an inorganic carbon source, an iron source, and a sodium source, a composite carbon-coated sodium iron sulfate cathode material is synthesized by the liquid-phase method. The composite carbon coating layer formed by the biomass porous carbon material and the inorganic carbon source effectively improves the tap density and cycle performance of the sodium iron sulfate cathode material and reduces the resistivity of the sodium iron sulfate cathode material.

[0044] In a first aspect, an embodiment of the present application provides a composite carbon-coated sodium iron sulfate cathode material, including sodium iron sulfate and a composite carbon coating layer coated on at least a part of the surface of the sodium iron sulfate; wherein, the materials forming the composite carbon coating layer include a biomass porous carbon material and an inorganic carbon source.

[0045] In the present application, by simultaneously using a biomass porous carbon material and an inorganic carbon source to form a composite carbon coating layer, it is possible to utilize the porous morphology formed by the biomass porous carbon material to provide a larger buffer space for the volume change of sodium iron sulfate during the cycling process, so as to improve the cycle performance of the cathode material; and it is also possible to utilize the good mechanical properties of the inorganic carbon source to better carbon coat sodium iron sulfate and reduce the resistivity of the cathode material; moreover, the biomass porous carbon material and the inorganic carbon source can also jointly fill the pores between sodium iron sulfate particles, effectively improving the tap density of the cathode material.

[0046] Further, in some embodiments, the biomass porous carbon material is in a flake structure; and / or, the porosity of the biomass porous carbon material is 20% - 30%, and the pore diameter is 50 - 100 nm.

[0047] In the present application, there are many small holes irregularly distributed on the biomass porous carbon material in a flake structure. This biomass porous carbon material can further form a porous morphology with richer pores during the sintering process and effectively coat sodium iron sulfate, providing a larger buffer space for the volume change of sodium iron sulfate during the cycling process, so as to improve the cycle performance of the sodium iron sulfate cathode material. Specifically, the porosity of the biomass porous carbon material can be 20%, 22%, 24%, 26%, 28%, 30% or any value within the range of 20% - 30%, and the pore diameter in the biomass porous carbon material can be 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm or any value within the range of 50 - 100 nm.

[0048] Further, in some embodiments, the mass ratio of the biomass porous carbon material to the inorganic carbon source is 1:(0.3 - 4); and / or, the inorganic carbon source includes graphene oxide and carbon nanotubes, wherein the mass ratio of graphene oxide to carbon nanotubes is 1:(1 - 3).

[0049] In the present application, by adjusting the mass ratio of the biomass porous carbon material to the inorganic carbon source within the above range, the synergistic effect of the two can be fully exerted, effectively improving the tap density, cycling performance, and electrical conductivity of the cathode material. Moreover, in the embodiments of the present application, graphene oxide and carbon nanotubes are simultaneously used as the inorganic carbon source. Among them, the multi-layer flaky structure of graphene oxide can provide better ductility, enabling it to jointly serve as the growth carrier of sodium iron sulfate with the biomass porous carbon material, which is also in a flaky structure, for coating sodium iron sulfate. Additionally, there are basically no pores in graphene oxide, which has higher strength and can better coat sodium iron sulfate after sintering. On this basis, by using an appropriate amount of carbon nanotubes with a tubular structure, the sodium ion transport channels can be increased, further shortening the sodium ion transport distance and effectively reducing the resistivity of the cathode material. Specifically, the mass ratio of the biomass porous carbon material to the inorganic carbon source can be 1:0.3, 1:0.5, 1:1, 1:2, 1:3, 1:4, or any value within the range of 1:(0.3 - 4), and the mass ratio of graphene oxide to carbon nanotubes can be 1:1, 1:2, 1:3, or any value within the range of 1:(1 - 3).

[0050] Further, in some embodiments, the tap density of the composite carbon-coated sodium iron sulfate cathode material is not less than 1.9 g / cm 3 , the BET specific surface area is 11 - 22 m 2 / g, and the powder resistivity is less than 180 Ω·cm.

[0051] In the present application, the composite carbon-coated sodium iron sulfate cathode material has a relatively high tap density and a low powder resistivity, which is beneficial to improving the cycling performance and electrical conductivity of the battery.

[0052] Further, in some embodiments, the coating rate of the composite carbon coating layer on the surface of sodium iron sulfate is 90% - 100%, which can achieve effective coating of sodium iron sulfate and is beneficial to improving its electrical conductivity.

[0053] In a second aspect, the embodiments of the present application provide a method for preparing a composite carbon-coated sodium iron sulfate cathode material, including the following steps:

[0054] Prepare a biomass porous carbon material;

[0055] Mix an iron source, a sodium source, a biomass porous carbon material, an inorganic carbon source, an antioxidant, and a solvent to obtain a first slurry;

[0056] The first slurry is dried, sintered, and pulverized to obtain a composite carbon-coated sodium iron sulfate cathode material.

[0057] In this application, by using the liquid-phase method to prepare the composite carbon-coated sodium iron sulfate cathode material, not only are the requirements for raw materials more relaxed, but also the synthesized cathode material can have a higher capacity. Moreover, in the embodiments of this application, a biomass porous carbon material and an inorganic carbon source are used simultaneously when preparing the first slurry, and then the first slurry is sintered. This can enable the biomass porous carbon material to further form a more abundant pore structure during the sintering process to improve the cycling performance of the cathode material; at the same time, the inorganic carbon source can better carbon-coat sodium iron sulfate after sintering; and during the sintering process, the biomass porous carbon material and the inorganic carbon source can also fill the pores between sodium iron sulfate particles to improve the compaction performance of the cathode material. More specifically, after mixing an iron source, a sodium source, a biomass porous carbon material, an inorganic carbon source, an antioxidant, and a solvent, it is preferably dispersed and stirred for 2 to 6 h to obtain a relatively uniform first slurry.

[0058] Furthermore, in some embodiments, the preparation method of the biomass porous carbon material includes the following steps:

[0059] The biomass material is ground to obtain a first powder;

[0060] The first powder is etched with an alkali solution, and then, after washing and drying, a biomass porous carbon material is obtained.

[0061] In this application, by etching the first powder with an alkali solution, micropores can be formed in it, and a porous morphology with more abundant pores can be further formed during the subsequent sintering process, providing a larger buffer space for the volume change of sodium iron sulfate during the cycling process, thereby effectively improving the cycling performance of the cathode material. Moreover, in the embodiments of this application, the preparation process of the biomass porous carbon material is relatively simple and can meet the requirements of industrial production. More specifically, the alkali solution can be a sodium hydroxide solution, a potassium hydroxide solution, etc. The mass fraction of the alkali in the alkali solution is preferably 3% to 8%. The alkali solution etching treatment is preferably carried out under heating conditions, the heating temperature is preferably 70 to 90 °C, and the heating time is preferably 0.5 to 1.5 h. The washing method is preferably multiple times of suction filtration washing with water added, and the washing is stopped when the pH of the suction filtrate is 6 - 7; the drying temperature after washing is preferably 100 to 140 °C.

[0062] Furthermore, in some embodiments, the molar ratio of the iron element in the iron source to the sodium element in the sodium source is 1:(1.35 - 1.529); and / or, the iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, and ferrous oxalate; and / or, the sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, and sodium oxalate.

[0063] In the present application, by adjusting the molar ratio of the iron element in the iron source to the sodium element in the sodium source, the phase of the synthesized sodium ferrous sulfate can be regulated, which is beneficial to improving the electrochemical performance of the cathode material. Moreover, since the sodium ferrous sulfate cathode material is synthesized by the liquid-phase method in the embodiments of the present application, the raw material requirements for the iron source and the sodium source are more relaxed, and the raw materials of the iron source and the sodium source are widely sourced, which is beneficial to industrial mass production. Specifically, the molar ratio of the iron element in the iron source to the sodium element in the sodium source can be 1:1.35, 1:1.529, or any value within the range of 1:(1.35 - 1.529).

[0064] Furthermore, in some embodiments, the total mass of the biomass porous carbon material and the inorganic carbon source accounts for 3% - 5% of the mass of the iron source; and / or, the biomass material includes at least one of corn straw, bamboo powder, and rice husk carbon.

[0065] In the present application, by adjusting the dosages of the biomass porous carbon material and the inorganic carbon source, the coating rate of the composite carbon coating layer on the sodium ferrous sulfate and the thickness of the coating layer can be regulated to achieve a better coating effect, thereby improving the electrical conductivity of the cathode material. Among them, the raw materials of the biomass material for preparing the biomass porous carbon material are widely sourced and have low costs. Among various biomass materials, corn straw is more preferably used, which can significantly improve the tap density and electrical conductivity of the composite carbon-coated sodium ferrous sulfate cathode material compared with other types of biomass materials. Specifically, the percentage of the total mass of the biomass porous carbon material and the inorganic carbon source in the mass of the iron source can be 3%, 3.5%, 4%, 4.5%, 5%, or any value within the range of 3% - 5%.

[0066] Furthermore, in some embodiments, the mass of the antioxidant accounts for 4% - 6% of the mass of the iron source; and / or, the antioxidant includes at least one of ascorbic acid, oxalic acid, and citric acid.

[0067] In the present application, by adding an appropriate amount of antioxidant, the oxidation of ferrous ions can be avoided; among them, the selection range of the raw materials of the antioxidant is relatively wide and can be selected according to actual needs, and the present application is not limited thereto. Specifically, the percentage of the mass of the antioxidant in the mass of the iron source can be 4%, 4.5%, 5%, 5.5%, 6%, or any value within the range of 4% - 6%.

[0068] Furthermore, in some embodiments, the solvent can be selected as needed, and water is generally preferred, specifically it can be pure water, deionized water, or distilled water, and the present application is not limited thereto.

[0069] Further, in some embodiments, the drying method is spray drying, the inlet air temperature of the spray drying is 150-250°C, and the outlet air temperature is 85-95°C; and / or, the sintering conditions include: under an inert atmosphere, heat preservation is carried out at 350-400°C for 8-24 h.

[0070] In the present application, by adopting the spray drying method for drying, the precursor can be fully dried to form powders with smaller particle sizes, so as to fully react in the subsequent sintering treatment process; by controlling the sintering conditions, the reaction raw materials can be promoted to fully react under these conditions to generate sodium ferrous sulfate, and the composite carbon coating layer formed by the biomass porous carbon material and the inorganic carbon source can coat the sodium ferrous sulfate particles. More specifically, the inert atmosphere is preferably argon and / or nitrogen, the sintering temperature can be 350°C, 360°C, 370°C, 380°C, 390°C, 400°C or any value within the range of 350-400°C, and the heat preservation time can be 8 h, 12 h, 16 h, 20 h, 24 h or any value within the range of 8-24 h.

[0071] In the third aspect, an embodiment of the present application provides a positive electrode plate, including the composite carbon-coated sodium ferrous sulfate positive electrode material provided in the first aspect of the present application or the composite carbon-coated sodium ferrous sulfate positive electrode material prepared by the preparation method provided in the second aspect.

[0072] In the present application, the positive electrode plate contains the above-mentioned composite carbon-coated sodium ferrous sulfate positive electrode material, and thus has the advantages of high capacity, good cycle performance and good electrical conductivity.

[0073] In the fourth aspect, an embodiment of the present application provides a secondary battery, including the positive electrode plate provided in the third aspect of the present application.

[0074] In the present application, the secondary battery contains the above-mentioned positive electrode plate, and thus has the advantages of high capacity, good cycle performance and good electrical conductivity. Its initial charge specific capacity is not less than 80 mAh / g, the initial discharge specific capacity is not less than 70 mAh / g, and the capacity retention rate after 500 cycles at a constant rate of 1C is not less than 80%.

[0075] The following lists some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation to the present application. For those technologies or conditions not specified in the embodiments, they are carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchases.

[0076] I. Preparation method

[0077] Example 1

[0078] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material, which includes the following steps:

[0079] S1. Place corn straw in a planetary ball mill for grinding to obtain a first powder.

[0080] S2. Add a sodium hydroxide solution with a mass fraction of 5% to the first powder to completely immerse the first powder. Heat and stir in a water bath at 80 °C for 1 h, then repeat the operations of adding water for washing and suction filtration until the pH value of the suction filtrate is detected to be between 6 and 7 and then stop. Then dry at 120 °C to obtain a second powder, which is the biomass porous carbon material.

[0081] S3. Weigh ferrous sulfate heptahydrate, biomass porous carbon material, inorganic carbon source, and ascorbic acid according to a mass ratio of 1:0.01:0.03:0.05, and weigh anhydrous sodium sulfate according to a molar ratio of iron element to sodium element of 1:1.529. Among them, the inorganic carbon source includes graphene oxide and carbon nanotubes with a mass ratio of 1:2. Add the weighed raw materials to deionized water and disperse and stir for 4 h to obtain a relatively uniform first slurry.

[0082] S4. Spray-dry the first slurry and spray it out at 90 °C to obtain a third powder; place the third powder in a sintering furnace filled with inert gas and keep it at 400 °C for 8 h to obtain a fourth powder; perform comminution treatment on the fourth powder and control the humidity of the comminution environment within 10% to obtain the composite carbon-coated sodium ferrous sulfate cathode material.

[0083] Example 2

[0084] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Example 1, the difference is only that the molar ratio of iron element to sodium element in step S3 is changed to 1:1.35, and the rest of the steps and parameters are basically the same as those in Example 1, and will not be elaborated here.

[0085] Example 3

[0086] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Example 1, the difference is only that the mass ratio of ferrous sulfate heptahydrate, biomass porous carbon material to inorganic carbon source in step S3 is changed to 1:0.02:0.02, and the rest of the steps and parameters are basically the same as those in Example 1, and will not be elaborated here.

[0087] Example 4

[0088] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the only difference is that the mass ratio of the biomass porous carbon material to the inorganic carbon source in step S3 is changed to 1:0.3, and the total mass of the biomass porous carbon material and the inorganic carbon source is the same as that in Embodiment 1, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0089] Embodiment 5

[0090] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the only difference is that the mass ratio of ferrous sulfate heptahydrate, the biomass porous carbon material to the inorganic carbon source in step S3 is changed to 1:0.01:0.04, that is, the mass ratio of the biomass porous carbon material to the inorganic carbon source is 1:4, and the total mass of the biomass porous carbon material and the inorganic carbon source accounts for 5% of the mass of the iron source, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0091] Embodiment 6

[0092] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the only difference is that the mass ratio of ferrous sulfate heptahydrate, the biomass porous carbon material to the inorganic carbon source in step S3 is changed to 1:0.015:0.015, that is, the mass ratio of the biomass porous carbon material to the inorganic carbon source is 1:1, and the total mass of the biomass porous carbon material and the inorganic carbon source accounts for 3% of the mass of the iron source, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0093] Embodiment 7

[0094] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the only difference is that the mass ratio of graphene oxide to carbon nanotubes in the inorganic carbon source in step S3 is changed to 1:1, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0095] Embodiment 8

[0096] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the only difference is that the mass ratio of graphene oxide to carbon nanotubes in the inorganic carbon source in step S3 is changed to 1:3, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0097] Embodiment 9

[0098] This embodiment provides a method for preparing a carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the inorganic carbon source used in Step S3 contains only graphene oxide and no carbon nanotubes, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0099] Embodiment 10

[0100] This embodiment provides a method for preparing a carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the inorganic carbon source used in Step S3 contains only carbon nanotubes and no graphene oxide, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0101] Embodiment 11

[0102] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the corn straw in Step S1 is changed to bamboo powder, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0103] Embodiment 12

[0104] This embodiment provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the corn straw in Step S1 is changed to rice husk carbon, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0105] Comparative Example 1

[0106] This comparative example provides a method for preparing a carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the preparation of the biomass porous carbon material is not carried out, and the biomass porous carbon material is replaced with an equal mass of inorganic carbon source in Step S3, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0107] Comparative Example 2

[0108] This comparative example provides a method for preparing a composite carbon-coated sodium ferrous sulfate cathode material. Compared with Embodiment 1, the difference is only that the preparation of the biomass porous carbon material is not carried out in Step S3, and the biomass porous carbon material is replaced with an equal mass of organic carbon source glucose in Step S3, and the remaining steps and parameters are basically the same as those in Embodiment 1, which will not be elaborated here.

[0109] Comparative Example 3

[0110] This comparative example provides a method for preparing a carbon-coated sodium ferrous sulfate cathode material. Compared with Example 1, the only difference is that in step S3, the inorganic carbon source is replaced with an equal mass of biomass porous carbon material, that is, no inorganic carbon source is added, and the remaining steps and parameters are basically the same as those in Example 1, which will not be elaborated here.

[0111] II. Test Methods

[0112] 1. Property Tests of Biomass Porous Carbon Material

[0113] Porosity Test: The porosity of the biomass porous carbon material prepared in Example 1 was tested with reference to the TCSTM 00553-2022 Method for Determining the Porosity of Lightweight Porous Materials.

[0114] Nitrogen Adsorption Test: The nitrogen adsorption method was used to test the biomass porous carbon material prepared in Example 1, and the results are as Figure 1 shown.

[0115] Morphology Test: The morphology of the biomass porous carbon material prepared in Example 1 was detected by scanning electron microscopy, and the results are as Figure 2 shown.

[0116] 2. Property Tests of Composite Carbon-Coated Sodium Ferrous Sulfate Cathode Material

[0117] Morphology Test: The morphologies of the fourth powder and the composite carbon-coated sodium ferrous sulfate cathode material prepared in Example 1 were detected by scanning electron microscopy, and the results are respectively as Figure 3 , Figure 4 shown.

[0118] Compacted Density Test: Using a compacted density meter, the compacted densities of the composite carbon-coated sodium ferrous sulfate cathode materials prepared in Examples 1-10 and Comparative Examples 1-3 were tested in accordance with "GB / T 24533-2019".

[0119] BET Specific Surface Area Test: Using the gas adsorption BET method, the BET specific surface areas of the composite carbon-coated sodium ferrous sulfate cathode materials prepared in Examples 1-10 and Comparative Examples 1-3 were tested.

[0120] Powder Resistivity Test: Using the four-probe method, the powder resistivities of the composite carbon-coated sodium ferrous sulfate cathode materials prepared in Examples 1-10 and Comparative Examples 1-3 were tested.

[0121] 3. Property Tests of Secondary Batteries

[0122] The composite carbon-coated sodium ferrous sulfate cathode materials prepared in Examples 1-10 and Comparative Examples 1-3 were respectively made into positive electrode sheets and applied to secondary batteries. The battery assembly and test processes are as follows:

[0123] The cathode materials prepared in the above-mentioned examples and comparative examples were mixed with conductive carbon powder and PVDF binder in a ratio of 85:10:5, and then evenly coated on the surface of an aluminum foil current collector. After drying, cold pressing, and slitting, a cathode electrode sheet was obtained. In an inert gas glove box, a button cell was assembled in the order of a negative electrode case, a sodium sheet, an electrolyte, a separator, an electrolyte, a cathode electrode sheet, a gasket, a spring piece, and a positive electrode case.

[0124] The assembled button half-cells were tested for their electrochemical performance using a Wuhan Blue Electric CT2001A battery test system: at a temperature of 20 - 30 °C and under normal pressure, the button cell was charged at a constant rate of 0.1C to 4.5V, and then discharged at a constant rate of 0.1C to 2.0V to measure the initial charge specific capacity and the initial discharge specific capacity; then the button cell was charged at a constant rate of 1C to 4.5V and then discharged at a constant rate of 1C to 2.0V. According to this charge-discharge mode, it was cycled 500 times, and the capacity retention rate after 500 cycles (the percentage of the discharge specific capacity after 500 cycles to the initial discharge specific capacity) was measured.

[0125] III. Analysis of the test results of each example and comparative example

[0126] The porosity of the biomass porous carbon material prepared in Example 1 was about 30%, combined with Figure 1-2 It can be seen that the biomass porous carbon material has a flaky structure, and its nitrogen adsorption and desorption curve conforms to the nitrogen adsorption and desorption characteristics under the macropore aperture. From this, it can be deduced that its pore size range is about 50 - 100 nm. During the sintering process, a porous morphology with richer pores can be further formed, and sodium ferrous sulfate can be effectively coated, so that the volume change of sodium ferrous sulfate during the cycle has a larger buffer space to improve the cycle performance of the sodium ferrous sulfate cathode material.

[0127] From Figure 3 and Figure 4 It can be seen that the fourth powder prepared in Example 1 is spherical, and its surface has irregular small holes; by pulverizing the fourth powder, the finally obtained composite carbon-coated sodium ferrous sulfate cathode material can be made into a flaky shape, and its surface has irregular small holes and irregularly distributed carbon nanotubes, indicating that the composite carbon coating effectively coats the sodium ferrous sulfate.

[0128] The test results of the tap density, BET specific surface area, powder resistivity of the composite carbon-coated sodium ferrous sulfate cathode materials prepared in Examples 1 - 12 and Comparative Examples 1 - 3, and the initial charge specific capacity, initial discharge specific capacity, and capacity retention rate of the button cells assembled from the corresponding cathode materials are shown in Table 1.

[0129] Table 1

[0130]

[0131]

[0132] As can be seen from Table 1, the ratio between various raw materials and the type of biomass material will both affect the performance of the prepared composite carbon-coated sodium ferrous sulfate cathode material and the battery assembled therefrom.

[0133] Specifically, by comparing Examples 1-2, it can be seen that when the molar ratio of iron element to sodium element is 1:1.529, the composite carbon-coated sodium ferrous sulfate cathode material has a higher tap density, lower specific surface area and resistivity. The battery prepared based on this composite carbon-coated sodium ferrous sulfate cathode material has a higher capacity and capacity retention rate, indicating its excellent cycle performance. On this basis, when the molar ratio of iron element to sodium element is adjusted to 1:1.35, the tap density of the composite carbon-coated sodium ferrous sulfate cathode material will decrease slightly, and the resistivity will increase slightly, thereby resulting in a slight decrease in both the capacity and capacity retention rate of the battery.

[0134] By comparing Examples 1, 3-6 and Comparative Examples 1-3, it can be seen that under the condition of using both biomass porous carbon material and inorganic carbon source, as the proportion of the inorganic carbon source increases, the tap density of the prepared composite carbon-coated sodium ferrous sulfate cathode material first increases and then decreases, while the specific surface area and resistivity first decrease and then increase. The capacity retention rate of the corresponding battery also shows a trend of first increasing and then decreasing, but the overall change range is not large. When the mass ratio of the biomass porous carbon material to the inorganic carbon source varies within the range of 1:(0.3-4), the prepared composite carbon-coated sodium ferrous sulfate cathode material can have good performance. When the ratio of the biomass porous carbon material to the inorganic carbon source remains unchanged, increasing the total amount of the two is more conducive to reducing the resistivity. If any one of the biomass porous carbon material and the inorganic carbon source is missing, it will affect their synergistic effect, thereby resulting in a significant decrease in the capacity retention rate of the corresponding battery. Moreover, the biomass porous carbon material prepared in this application can significantly increase the tap density of the composite carbon-coated sodium ferrous sulfate cathode material and reduce the resistivity compared with the conventional organic carbon source, thereby enabling the prepared battery to have a higher capacity and better cycle performance.

[0135] By comparing Example 1 with Examples 7 - 10, it can be seen that under the condition of using graphene oxide and carbon nanotubes as inorganic carbon sources simultaneously, as the proportion of carbon nanotubes increases, the tap density of the prepared composite carbon-coated sodium ferrous sulfate cathode material first increases and then decreases, the specific surface area and resistivity first decrease and then increase, and the capacity retention rate of the corresponding battery also shows a pattern of first increasing and then decreasing, but the overall change range is not large. When the mass ratio of graphene oxide to carbon nanotubes varies within the range of 1:(1 - 3), the prepared composite carbon-coated sodium ferrous sulfate cathode material can have good performance. However, if only graphene oxide is used as the inorganic carbon source, it will lead to a significant increase in resistivity and a substantial decrease in capacity retention rate; if only carbon nanotubes are used as the inorganic carbon source, it will result in a significant decrease in tap density and a substantial decrease in capacity retention rate as well.

[0136] By comparing Example 1 with Examples 11 - 12, it can be seen that the type of biomass material has an important influence on the performance of the prepared composite carbon-coated sodium ferrous sulfate cathode material and its battery. Compared with bamboo powder and rice husk carbon, when using corn straw as the biomass material to prepare the corresponding biomass porous carbon material, it can effectively increase the tap density, significantly reduce the resistivity, and the capacity and capacity retention rate of the battery can also be greatly improved.

[0137] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same function and effect as the technical idea within the technical solution scope of this application are all included in the technical scope of this application. In addition, within the scope of not departing from the main idea of this application, various modifications that those skilled in the art can think of applied to the embodiments, and other ways constructed by combining some constituent elements in the embodiments are also included in the scope of this application.

Claims

1. A composite carbon-coated sodium ferrous sulfate cathode material, characterized in that, It includes sodium ferrous sulfate and a composite carbon coating layer coated on at least a part of the surface of the sodium ferrous sulfate; wherein, the materials forming the composite carbon coating layer include a biomass porous carbon material and an inorganic carbon source.

2. The composite carbon-coated sodium iron sulfate positive electrode material according to claim 1, wherein The biomass porous carbon material is in a sheet structure; and / or, The porosity of the biomass porous carbon material is 20% - 30%, and the pore diameter is 50 - 100 nm.

3. The composite carbon-coated sodium ferrous sulfate cathode material according to claim 1, characterized in that, The mass ratio of the biomass porous carbon material to the inorganic carbon source is 1:(0.3 - 4); and / or, The inorganic carbon source includes graphene oxide and carbon nanotubes, wherein, the mass ratio of graphene oxide to carbon nanotubes is 1:(1 - 3).

4. The composite carbon-coated sodium iron sulfate positive electrode material according to claim 1, wherein The tap density of the composite carbon-coated sodium ferrous sulfate cathode material is not less than 1.9 g / cm 3 , the BET specific surface area is 11 - 22 m 2 / g, and the powder resistivity is less than 180 Ω·cm.

5. A method for preparing the composite carbon-coated sodium ferrous sulfate cathode material according to any one of claims 1-4, characterized in that, It includes the following steps: Prepare a biomass porous carbon material; Mix an iron source, a sodium source, the biomass porous carbon material, an inorganic carbon source, an antioxidant and a solvent to obtain a first slurry; Perform drying, sintering and pulverization treatments on the first slurry to obtain a composite carbon-coated sodium ferrous sulfate cathode material.

6. The preparation method of the composite carbon-coated sodium ferrous sulfate cathode material according to claim 5, characterized in that, The preparation method of the biomass porous carbon material includes the following steps: Grind the biomass material to obtain a first powder; Perform alkali solution etching treatment on the first powder, and then, after washing and drying, obtain the biomass porous carbon material.

7. The preparation method of the composite carbon-coated sodium ferrous sulfate cathode material according to claim 5, characterized in that, The molar ratio of the iron element in the iron source to the sodium element in the sodium source is 1:(1.35 - 1.529); and / or, The total mass of the biomass porous carbon material and the inorganic carbon source accounts for 3% - 5% of the mass of the iron source; and / or, The mass of the antioxidant accounts for 4% - 6% of the mass of the iron source; and / or, The biomass material includes at least one of corn straw, bamboo powder, and rice husk carbon; and / or, The iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, and ferrous oxalate; and / or, The sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, and sodium oxalate; and / or, The antioxidant includes at least one of ascorbic acid, oxalic acid, and citric acid.

8. The preparation method of the composite carbon-coated sodium ferrous sulfate cathode material according to claim 5, characterized in that, The drying method is spray drying, and the outlet air temperature of the spray drying is 85 - 95 °C; and / or, The sintering conditions include: under an inert atmosphere, keep the temperature at 350 - 400 °C for 8 - 24 h.

9. A positive electrode plate, characterized in that, It includes the composite carbon-coated sodium ferrous sulfate cathode material described in any one of claims 1 - 4, or the composite carbon-coated sodium ferrous sulfate cathode material prepared by the preparation method described in any one of claims 5 - 8.

10. A secondary battery, characterized in that, It includes the cathode plate described in claim 9.

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