Double-fluorine-doped carbon composite sodium ferric sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention relates to a double-fluorine-doped carbon composite sodium ferric sulfate positive electrode material and a preparation method and application thereof, the positive electrode material is hollow microsphere particles formed by using fluorine-doped inorganic carbon composite inorganic conductive carbon as a coating layer and coating a fluorine-doped sodium ferric sulfate bulk phase, and the inorganic conductive carbon is uniformly distributed in the positive electrode material. The invention also discloses a preparation method and application of the double-fluorine-doped carbon composite sodium ferric sulfate positive electrode material. A battery assembled by a positive electrode plate prepared from the positive electrode material is high in median voltage, good in electronic conductivity, high in median voltage and energy density, long in cycle and high in specific energy. The method is simple in process, short in production period, accurate in control, low in raw material cost and suitable for large-scale continuous production.

Description

Technical Field

[0001] This invention relates to a sodium ferric sulfate cathode material, its preparation method, and its application, specifically to a fluorine-doped carbon composite sodium ferric sulfate cathode material, its preparation method, and its application. Background Technology

[0002] With the implementation of the "dual-carbon" policy and the rapid development of the new energy storage industry, sodium-ion battery polyanionic cathode materials, with their ultra-low cost advantage, can achieve comprehensive coverage of the low-speed electric vehicle and power tool, backup power, and large-scale energy storage system markets. Polyanionic materials Na... x M y (X a O b ) z Z w In this formula, M represents one or more elements such as Co, Ni, Mn, Fe, and Ti; X represents elements such as Si, S, and P; and Z represents elements such as F and OH. Sodium iron sulfate, with its advantages of wide availability of raw materials, high operating voltage, and stable chemical structure, has become the mainstream choice for the commercial application of cathode materials in sodium-ion batteries.

[0003] Sodium iron sulfate (FeS) is a key sodium-ion battery cathode material driving the rapid development of energy storage batteries and the lead-to-sodium conversion market. Its advantages, such as low raw material cost and high operating voltage, have attracted widespread attention from secondary battery researchers. Similar to lithium-ion batteries, sodium-ion batteries utilize sodium... + The conversion of electrical energy into chemical energy is achieved through the extraction and insertion between positive and negative electrode materials. However, sodium ferric sulfate cathode materials face two major challenges: poor intrinsic electronic conductivity and low theoretical capacity, resulting in low energy density and thus limiting their widespread application. To meet the needs of the energy storage and low-speed power markets, research is needed to modify the energy density and electronic conductivity of sodium ferric sulfate materials.

[0004] CN117525314A discloses a doped sodium iron sulfate compound, a cathode material, its preparation method, and its application. CN117476858A discloses a preparation method for a modified sodium iron sulfate cathode material. Both methods utilize transition metal ion doping and conductive metal particle composite rare earth metal quantum dots to dope and modify the surface carbon layer. Although this improves the electronic conductivity of the material, it also greatly increases the BOM cost of the material.

[0005] CN113511692A discloses a method for synthesizing lithium-rich manganese-based cathode materials using a short-time rapid thermal shock method. This method utilizes a sol-gel method to prepare precursor materials, followed by multi-stage thermal shock to prepare the lithium-rich manganese-based cathode material for lithium-ion batteries. However, the precursor materials prepared by the sol-gel method have relatively poor uniformity and consistency; simultaneously, the use of multi-stage high-temperature thermal shock results in poor batch stability of the prepared cathode materials and increases production costs.

[0006] CN116239094A discloses a method for rapidly synthesizing Na3V2(PO4)2F3 cathode material, and CN115650199A discloses a method for rapidly synthesizing sodium vanadium phosphate / carbon cathode composite material via high-temperature thermal shock. Both methods disclose a method for preparing sodium-phosphate battery materials via rapid high-temperature thermal shock. However, the precursor material prepared by mechanical activation will cause elemental segregation during the accelerated crystal transformation process using thermal shock, which will amplify the drawbacks in kilogram-scale experiments. At the same time, simple carbon layer coating cannot sufficiently improve the electronic conductivity of the composite material.

[0007] In summary, there is an urgent need to develop a double-fluorine-doped carbon composite sodium iron sulfate cathode material and its applications, which produces batteries with high median voltage, good electronic conductivity, high median voltage and energy density, long cycle life, and high specific energy. Furthermore, there is a need for a simple, short-cycle, precisely controlled, and low-cost preparation method for this double-fluorine-doped carbon composite sodium iron sulfate cathode material suitable for large-scale continuous production. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a double fluorine-doped carbon composite sodium iron sulfate positive electrode material and its application, which has high median voltage, good electronic conductivity, high median voltage and energy density, long cycle life and high specific energy, and is used to assemble batteries with positive electrode sheets.

[0009] The further technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for preparing a double-fluorine-doped carbon composite sodium iron sulfate cathode material that is simple in process, short in production cycle, precise in control, low in raw material cost, and suitable for large-scale continuous production.

[0010] The technical solution adopted by the present invention to solve its technical problem is as follows: a double fluorine-doped carbon composite sodium iron sulfate cathode material is a hollow microsphere particle formed by coating a fluorine-doped inorganic carbon composite inorganic conductive carbon as a coating layer, and the inorganic conductive carbon is uniformly distributed in the cathode material. Introducing fluorine atoms into the carbon coating layer to replace carbon atoms forms CF bonds, altering the relative content of sp2 and sp3 carbon atoms, thereby changing the degree of carbon layer defects and improving the electronic conductivity of the modified layer material. Simultaneously, the introduction of fluorine atoms into the carbon layer changes the degree of carbon layer defects and strengthens the atomic interaction with the transition metal iron element in the bulk sodium ferric sulfate, forming TM-F bonds. This increases the binding energy between the carbon coating layer and the bulk material, constructing an electronic conductivity bridge, thus improving the electronic conductivity and electrical conductivity of the composite material, while significantly reducing the BOM cost. Secondly, fluorine ions enter the crystal lattice of the sodium ferric sulfate material, replacing some of the O atom positions in the Fe-O octahedrons, causing Fe-O octahedron distortion and forming Fe-F bonds. This alters the Fe-O bond length, changes the electron cloud density distribution, and strengthens the inductive effect of anionic groups. Furthermore, fluorine ion doping weakens the electron-rich nucleophilic effect on the surface of the sodium ferric sulfate material, suppressing surface side reactions, thereby increasing the median voltage and energy density of the composite material.

[0011] Preferably, the mass fraction of inorganic carbon in the fluorine-doped inorganic carbon composite sodium iron sulfate cathode material is 0.1–20.0 wt% (more preferably 1–12 wt%, and even more preferably 2–8 wt%). If the inorganic carbon content is too high, the proportion of active material in the material will be too low, resulting in lower conductivity and increased electrochemical polarization during charge and discharge. Since the fluorine in the fluorine-doped inorganic carbon originates from the carbonization of an organic fluorine source, if the inorganic carbon content is too low, it means that the amount of organic fluorine source used is too low, which in turn leads to a low fluorine doping amount.

[0012] Preferably, the amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 0.05–10.00 wt% (more preferably 0.5–8.0 wt%, and even more preferably 2–6 wt%) of the mass of the double-fluorine-doped carbon composite sodium iron sulfate cathode material. When fluorine-doped inorganic carbon is used as a coating layer, if the fluorine content in the coating layer is too high, the carbon content in the coating layer decreases, the degree of defects in the coated carbon layer increases, and the degree of graphitization formed after carbonization decreases, which leads to a decrease in the conductivity of the composite material. If the fluorine content in the coating layer is too low, the binding energy between F in the carbon layer and Fe in the material decreases, resulting in a decrease in electronic conductivity.

[0013] Preferably, the fluoride ion doping amount in the fluoride-doped sodium ferric sulfate is equivalent to 0.01–40.00 at% (more preferably 0.6–15.0 at%, and even more preferably 1–8 at%) of the number of sulfate atomic groups in the bulk sodium ferric sulfate phase. Too much or too little fluoride ion doping will affect the material structure and electrochemical performance. If the fluoride ion doping amount is too high, due to the small atomic radius of F (0.71 Å, close to the radius of O being 0.74 Å), it will occupy too many O sites on the Fe-O tetrahedra, thus affecting SO42-. 2- With F - The combined effect of strong electronegativity leads to a stronger inductive effect, causing Fe... 2+ 3D orbit and O 2- The 2p orbital electron shift, the increased distance between the vacuum orbital and the antibonding orbital, and the shift in electron cloud distribution further affect Fe. 3+ / Fe 2+ The increased redox potential of this increases the requirements for high-voltage electrolytes in full batteries, making it impractical for actual production; if the fluoride ion doping level is too low, F... - The redox couple has little effect on the induction effect, making it difficult to increase the median voltage of the material and improve the energy density of sodium ferric sulfate.

[0014] Preferably, the inorganic conductive carbon in the difluorinated carbon composite sodium ferric sulfate cathode material has a mass fraction of 0.1–4.5 wt% (more preferably 1.0–3.5 wt%, and even more preferably 1.5–2.5 wt%). The inorganic carbon is uniformly distributed and coated in the difluorinated carbon composite sodium ferric sulfate cathode material. Excessive inorganic carbon content will hinder the exchange of Na+ between the material and the electrolyte interface. + The migration of inorganic carbon leads to a decrease in electrochemical performance; if the inorganic carbon content is too low, it is difficult to improve the electronic conductivity of the material, resulting in a decrease in the overall electrochemical performance of the material.

[0015] Preferably, the chemical formula of the fluorine-doped sodium ferric sulfate is Na. 2.6 Fe 1.7 (SO4) 3-x F 2x Where 0 < x ≤ 0.5 (more preferably 0.01 ≤ x ≤ 0.20, and even more preferably 0.02 ≤ x ≤ 0.10), and the law of conservation of charge is satisfied.

[0016] Preferably, the inorganic conductive carbon includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P, or graphene.

[0017] Preferably, the average particle size of the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 5–30 μm (more preferably 10–28 μm).

[0018] The technical solution adopted by the present invention to further solve its technical problem is as follows: A method for preparing a double-fluorine-doped carbon composite sodium iron sulfate cathode material, comprising the following steps: (1) Sodium source, ferrous source, inorganic fluorine source, sulfate source, organic fluorine source and organic acid antioxidant are added to an organic aqueous solution, stirred and dissolved, and then inorganic conductive carbon source slurry is added. After stirring and mixing, spray drying is performed to obtain fluorine-carbon composite sodium iron sulfate cathode material precursor. (2) The fluorine-carbon composite sodium iron sulfate cathode material precursor obtained in step (1) is subjected to thermal shock treatment under a protective atmosphere to obtain a fluorine-doped carbon composite sodium iron sulfate cathode material precursor. (3) The precursor of the double-fluorine-doped carbon composite sodium iron sulfate cathode material obtained in step (2) is sintered under a protective atmosphere to obtain the double-fluorine-doped carbon composite sodium iron sulfate cathode material.

[0019] The inventive concept of the method of this invention is as follows: In step (1), the added organic solvent, such as ethanol, can react with Fe. 2+ This forms water-soluble [Fe(C2H5OH)6] 2+ Complexes, thereby retaining Fe 2+ The stable existence of Fe 2+ + 6CH3CH2OH → [Fe(CH3CH2OH)6] 2+ In addition, organic solvents can also be used as dispersion media. Based on the principle of "like dissolves like," organic fluorine sources that are poorly soluble in water can be effectively dispersed. Combined with organic acid antioxidants, a suitable acidic pH environment is provided to ensure the Fe... 2+ Stable existence; From a thermodynamic perspective, under a certain humidity (RH) environment, sodium ferric sulfate material combines with water molecules at a rate of ΔG < 0, spontaneously forming a hydrated crystal compound; from a theoretical chemical perspective, SO4 2- The Lewis basicity of O in H2O is close to that of O in H2O, making it inherently susceptible to moisture erosion, forming hydrated compounds and causing the material to lose its electrochemical activity. Therefore, the method of this invention achieves atomic-level in-situ self-assembly of raw materials through spray drying and reduces the amount of precursor intermediate phase material Na. 2+2x Fe 2-x The water content of crystallization in (SO4)3·nH2O (n=2,4); In step (2), due to the intermediate phase Na 2+2x Fe 2-xThe decomposition of (SO4)3 is sensitive to drastic energy fluctuations. Thermal shock-assisted preparation of sodium ferric sulfate material utilizes rapid heating within a short timeframe to provide intense energy, removing water of crystallization and significantly enhancing the kinetics of adsorbed and crystalline water separation. This promotes the transformation of the high Na / Fe ratio Na6Fe(SO4)4 impurity phase and the FeSO4 phase, effectively improving the conversion degree of the non-stoichiometric transition state sodium ferric sulfate crystal form. Finally, extremely short-duration low-temperature crystal growth enhances the crystallinity of the material, resulting in better elemental distribution both theoretically and practically compared to mechanical activation, and the directional generation of more consistent targeted precursor products. In step (3), the composite material after thermal shock treatment is subjected to short-term low-temperature sintering in an inert environment to stably improve the crystallinity of the material; the spray drying-thermal shock combined technology enables the synthesized material to have a more consistent crystal structure and a shorter production cycle.

[0020] Preferably, in step (1), the molar ratio of sodium in the sodium source, ferrous element in the ferrous source, sulfate in the sulfate source, and fluoride ion in the inorganic fluorine source is the same as that of the chemical formula Na. 2.6 Fe 1.7 (SO4) 3-x F 2x Among them, the molar ratios of the corresponding substances in 0 < x ≤ 0.5 are matched.

[0021] Preferably, in step (1), the amount of the organic fluorine source is equivalent to 0.2 to 30.0 wt% (more preferably 5 to 25 wt%, and even more preferably 8 to 20 wt%) of the theoretical mass of fluorine-doped sodium ferric sulfate. Since organic fluorine will cause some loss of fluorine element when the sintering temperature is close to 400°C, the added organic fluorine source should be in excess to ensure the stability of fluorine element content after sintering.

[0022] Preferably, in step (1), the carbonized mass of the organic acid antioxidant is equivalent to 0.1–1.5 wt% (more preferably 0.5–1.2 wt%) of the theoretical mass of fluorine-doped sodium iron sulfate. The organic acid antioxidant can control the pH of the solution environment to be maintained between 2 and 7, ensuring that Fe... 2+The organic acid antioxidant remains stable, preventing oxidation or hydrolysis. Simultaneously, during sintering, the organic acid antioxidant carbonizes and decomposes, forming a conductive carbon coating layer. If the amount of organic acid antioxidant is too small, it is difficult to ensure the stable presence of ferrous ions throughout the reaction. If the amount of organic acid antioxidant is too large, the carbonization quality of the inorganic carbon after carbonization will be too high, while the proportion of active substances in the material will be too low, resulting in low conductivity and increased electrochemical polarization during charge and discharge. Different organic acid antioxidants yield different carbonization qualities at different sintering temperatures. The carbonization quality is the percentage of the organic acid antioxidant's mass remaining after carbonization at the corresponding temperature, which is obtained from thermogravimetric analysis.

[0023] Preferably, in step (1), the molar volume ratio of ferrous ions to the organic aqueous solution in the ferrous source is 1.0–1.6 mol / L (more preferably 1.0–1.4 mol / L). The spray drying parameters determine the size of the atomized droplets. If the material concentration is too low, the solvent mass ratio increases, leading to increased pore volume in the resulting hollow microspheres of the precursor and reduced compaction density. If the material concentration is too high, thicker hollow microspheres will form, which is detrimental to Na+. + When materials undergo insertion / extraction migration within the bulk phase, their electrochemical properties cannot be fully utilized.

[0024] Preferably, in step (1), the organic aqueous solution is a mixed solution with a volume ratio of organic solvent to water of 5–25:100 (more preferably 8–20:100). Based on the principle of like dissolves like, organic solvents such as ethanol can be used to fully dissolve and disperse the organic fluorine source. Excessive use of organic solvent will increase the raw material production cost; insufficient use of organic solvent will make it difficult to fully disperse the organic fluorine source, resulting in uneven coating of the organic fluorine source on the surface of the composite material precursor.

[0025] Preferably, the organic solvent includes one or more of methanol, ethanol, or ethylene glycol.

[0026] Preferably, in step (1), the stirring and dissolving temperature is 25–30°C, the stirring speed is 300–800 rpm, and the time is 20–30 min. First, inorganic salts are added and uniformly mixed, dispersed, and dissolved in water and ethanol solutions. Fe 2+ It forms a complex with ethanol to prevent Fe 2+ Oxidation is performed after the inorganic substances have dissolved, followed by the addition of organic substances for uniform mixing and dissolution. If an organic fluorine source and organic acid antioxidant are added first, the antioxidant reacts with Fe... 2+ It will form complexes that are difficult to dissolve, turning the mixture into a suspension, resulting in insufficient dissolution of inorganic salts.

[0027] Preferably, in step (1), the solid content of the inorganic conductive carbon source slurry is 3-8% (more preferably 4-6%). If the solid content is too high, the consistency and stability of the slurry will be poor, and it will easily agglomerate into a gel, which is not conducive to long-term use, and the cost of raw materials will increase. If the solid content is too low, the solid content of the entire system will be reduced, affecting the particle size and conductivity of the material, and reducing the electrochemical performance of the material.

[0028] Preferably, in step (1), the inorganic conductive carbon in the inorganic conductive carbon source slurry is equivalent to 0.1–6.0 wt% (more preferably 1.0–4.0 wt%) of the theoretical mass of fluorine-doped sodium iron sulfate. Sodium iron sulfate cathode material has low intrinsic electronic conductivity, and the addition of an inorganic carbon source can improve the electronic conductivity of the material. If the amount of inorganic carbon source is too high, it will reduce the proportion of active material in the material system and increase the material production cost. If the amount of inorganic carbon source is too low, the material conductivity will be low, and the electrochemical performance will be difficult to effectively utilize.

[0029] Preferably, in step (1), the stirring temperature is 25-30°C, the stirring speed is 600-800 rpm, and the stirring time is 20-30 min.

[0030] Preferably, in step (1), the feed solution is continuously stirred at 700-900 rpm while the spray drying feed is being fed.

[0031] Preferably, in step (1), the process parameters for spray drying are: the frequency of the induced draft fan is 20-40 Hz (more preferably 25-35 Hz), the inlet air temperature is 200-230℃ (more preferably 210-220℃), and the outlet air temperature is 100-130℃ (more preferably 120-130℃). According to the principle of spray drying, the atomization process is the evaporation of the solvent phase of the raw material in a very short time, and the homogeneous precipitation of the solute. Matching the inlet air temperature with the frequency of the induced draft fan can control the size of the atomized droplets and the content of water of crystallization in the material; in principle, the outlet air temperature is ≥ the solvent evaporation temperature to reduce the generation of water of crystallization. This invention prepares precursors through spray drying technology, which theoretically and practically has a better elemental distribution than mechanical activation, and can directionally generate targeted precursor products with better consistency. It can also assist the surface carbon layer F atom doping to form TM-F bonds with the bulk TM elements, enhance the bonding energy between the coating layer and the bulk material, and improve the electronic conductivity of the composite material.

[0032] Preferably, in step (1), the sodium source includes one or more of sodium sulfate, sodium acetate, sodium formate, or sodium nitrate, and their hydrates. When the sodium source is sodium sulfate, it can be used as a partial source of sulfuric acid.

[0033] Preferably, in step (1), the ferrous source includes one or more of ferrous sulfate and / or ferrous nitrate, and their hydrates. When the ferrous source is ferrous sulfate, it can be partially used as a sulfuric acid source.

[0034] Preferably, in step (1), the sulfate source includes sulfuric acid, and one or more of ammonium sulfate, sodium sulfate or ferrous sulfate, and their hydrates. When the sulfate source is sodium sulfate or ferrous sulfate, it can be used as a sodium source or an iron source.

[0035] Preferably, in step (1), the inorganic fluorine source includes one or more of hydrofluoric acid, sodium fluoride, ferrous fluoride, or ammonium fluoride, and their hydrates. More preferably, the inorganic fluorine source is sodium fluoride, which can be used as a partial sodium source.

[0036] Preferably, in step (1), the organic fluorine source includes one or more of polyvinylidene fluoride, tetrabutylammonium fluoride, or nitrogen- or sulfur-containing polyvinylidene fluoride derivatives.

[0037] Preferably, in step (1), the organic acid antioxidant includes one or more of citric acid, tannic acid, or ascorbic acid.

[0038] Preferably, in step (1), the inorganic conductive carbon source includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P, or graphene.

[0039] Preferably, in step (2), the thermal shock treatment refers to: placing the fluorine-carbon composite sodium iron sulfate cathode material precursor on a carrier, passing a direct current of 0-45A (more preferably 0-40A), and heating at a rate of 200-300K / s (more preferably 250-280 K / s) from room temperature to 350-380℃, with each heating lasting 5-30s (more preferably 10-20s), and repeating the thermal shock 1-15 times (more preferably 5-13 times). The thermal shock treatment achieves a rapid temperature rise and fall within a short time through changes in current, thereby realizing the thermal shock effect. The rate of thermal shock heating directly affects the non-stoichiometric transition state crystalline form Na in the composite material. 2+2x Fe 2-x The transformation of (SO4)3 is hindered by a low heating rate, leading to the accumulation of Na / Fe-rich Na6Fe(SO4)4 and FeSO4, making it difficult to achieve a rapid and complete transformation to the target crystal form. The number of thermal shocks determines the crystallinity of the sodium ferric sulfate composite material; fewer thermal shocks are insufficient to completely transform the raw material phase into the non-stoichiometric transition state Na. 2+ 2x Fe 2-xThe (SO4)3 crystal form leads to the presence of heterogeneous phases or Na / Fe-rich transition phases in the product. However, a high number of thermal shocks has no significant effect on the crystal form transformation and increases energy consumption during preparation. Furthermore, combining intermittent, one-stage low-temperature thermal shock technology can avoid the loss of excess F during high-temperature sintering and achieve rapid crystal form transformation, resulting in cathode materials with good batch stability, uniform material structure, and better consistency. This also optimizes the material's electronic conductivity and energy density. If the thermal shock current is too large or the duration is too long, the energy generated by the thermal shock is too high, which is not conducive to the formation of the intermediate targeted crystal form and leads to the formation of iron-rich phase byproducts. If the thermal shock current is too small or the duration is too short, the energy generated by the thermal shock is too low, easily leading to the formation of sodium-rich phase byproducts. If the thermal shock temperature is too high, it easily leads to the appearance of the sulfate transition state, damaging the crystal structure of the sodium iron sulfate composite material. If the thermal shock temperature is too low, the crystal form conversion degree of the non-stoichiometric transition state sodium iron sulfate material decreases, making it difficult to generate the targeted precursor product.

[0040] Preferably, in step (2), the protective atmosphere includes nitrogen, argon, a hydrogen-argon mixture, or a hydrogen-nitrogen mixture.

[0041] Preferably, in step (3), before sintering, the system is ventilated with a protective atmosphere at room temperature for 1.0 to 1.5 hours to ensure that the sintering system is filled with a protective atmosphere and to remove other gases.

[0042] Preferably, in step (3), the sintering refers to: heating from room temperature to 350-400℃ (more preferably 350-380℃) at a rate of 1-5℃ / min, and sintering for 3-8 hours (more preferably 3-6 hours). One-step sintering can further improve and perfect the crystallinity of the sodium ferric sulfate precursor, and reduce the water of crystallization content to a minimum. If the sintering temperature is too high, the sulfate ions in the material will decompose into SO2, making it difficult to obtain sodium ferric sulfate material; if the sintering temperature is too low, it will be difficult to obtain a good degree of crystallinity in the material.

[0043] Preferably, in step (3), the protective atmosphere includes nitrogen, argon, a hydrogen-argon mixture, or a hydrogen-nitrogen mixture.

[0044] The nitrogen, argon, or atmosphere used in the preparation of the mixed gas in this invention are all high-purity atmospheres with a purity of ≥99.999%.

[0045] The technical solution adopted by the present invention to further solve its technical problem is as follows: the application of a difluorine-doped carbon composite sodium iron sulfate cathode material, wherein the cathode electrode sheet made of the difluorine-doped carbon composite sodium iron sulfate cathode material is used to assemble a sodium-ion battery.

[0046] The beneficial effects of the present invention are as follows: (1) The battery assembled from the positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material of the present invention has a reversible discharge specific capacity of up to 91.79 mAh / g, an initial coulombic efficiency of up to 90.75%, a median voltage of 3.75 V, an energy density of 339 Wh / kg, and a conductivity of up to 9.16 S / m at a 0.1C rate. At a 1C rate, the initial discharge specific capacity is up to 89.50 mAh / g, and the capacity retention rate after 100 cycles is up to 95.79%. This shows that the present invention comprehensively improves the median voltage, electronic conductivity, median voltage and energy density of the material through the synergistic effect of surface carbon layer structure modification, interfacial element interaction and bulk anion doping, and successfully prepares sodium iron sulfate positive electrode material for sodium-ion batteries with long cycle and high specific energy. (2) The method of the present invention is simple, has a short production cycle, precise control, and low raw material cost, and is suitable for large-scale continuous production. Attached Figure Description

[0047] Figure 1 This is a SEM image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 of the present invention; Figure 2 This is the SEM-EDS image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 of the present invention; Figure 3 This is a SEM image of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in Comparative Example 1 of this invention; Figure 4 These are the XRD patterns of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1. Figure 5 These are Raman diagrams of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1. Figure 6 This is the XPS image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 of the present invention; Figure 7 This is the XPS image of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in Comparative Example 1 of this invention; Figure 8 This is a three-dimensional comparison diagram of the conductivity of the double-fluorine-doped carbon composite sodium iron sulfate cathode materials in Examples 1 and 2 of the present invention and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1. Figure 9 This is a bar chart comparing the discharge specific capacity, energy density, and median voltage of the dual-fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1, the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1, and the monofluorine-doped carbon composite sodium iron sulfate cathode material obtained in Comparative Example 2. Figure 10This is the first charge-discharge curve of the battery assembled from the double fluorine-doped carbon composite sodium iron sulfate cathode material of Example 1 and the cathode electrode sheet made in Comparative Example 1, in the voltage range of 2.0 to 4.5V and the current rate of 0.1 C (11mA / g). Figure 11 This is a cycle curve of a battery assembled from a positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material of Example 1 and the microsphere structure carbon composite sodium iron sulfate positive electrode material obtained in Comparative Example 1, in a voltage range of 2.0 to 4.5V and a current rate of 1C (1C=110mAh / g). Detailed Implementation

[0048] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0049] The aqueous carbon nanotube conductive paste used in the embodiments and comparative examples of this invention has a solid content of 5% and was purchased from Xiamen Kaina Graphene Technology Co., Ltd.; the nitrogen gas used in the embodiments and comparative examples of this invention is a high-purity atmosphere with a purity ≥99.999%; in the embodiments and comparative examples of this invention, thermogravimetric analysis showed that the residual percentage of ascorbic acid after carbonization at 350℃ and 380℃ was 44.76% and 40.11%, respectively; the purity of the chemical reagents used in the embodiments and comparative examples of this invention is: anhydrous sodium sulfate 98%, ferrous sulfate heptahydrate 99.95%, sodium fluoride 99.00%, ascorbic acid 99.7%, ammonium fluoride 99.00%, and sodium acetate 99.00%; unless otherwise specified, the raw materials or chemical reagents used in the embodiments and comparative examples of this invention were obtained through conventional commercial channels.

[0050] In this embodiment of the invention, the mass fraction of inorganic carbon in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is analyzed by sulfur-carbon analysis; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is analyzed to be equivalent to the mass fraction of the double-fluorine-doped carbon composite sodium iron sulfate cathode material by ion chromatography; and the atomic percentage of sulfate groups in the bulk phase of sodium iron sulfate is detected by ICP.

[0051] Example 1 of a dual-fluorine-doped carbon composite sodium ferric sulfate cathode material The aforementioned fluorine-doped carbon composite sodium ferric sulfate cathode material consists of fluorine-doped inorganic carbon composite conductive carbon nanotubes as a coating layer, encapsulating the bulk sodium ferric sulfate phase Na. 2.6 Fe 1.7 (SO4) 2.98 F 0.04Hollow microspheres are formed, and conductive carbon nanotubes are uniformly distributed in the cathode material; the mass fraction of inorganic carbon in the fluorine-doped inorganic carbon in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 2.78 wt%; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 2.02 wt% of the mass of the double-fluorine-doped carbon composite sodium iron sulfate cathode material; the doping amount of fluorine ions in the fluorine-doped sodium iron sulfate is equivalent to 1.34 at the number of sulfate atomic groups in the bulk phase of sodium iron sulfate; the mass fraction of conductive carbon nanotubes in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 1.91 wt%; the average particle size of the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 25 μm.

[0052] like Figure 1 As shown, the average particle size of the double fluorine-doped carbon composite sodium iron sulfate cathode material in this embodiment of the invention is 25 μm, and its morphology has an obvious microspherical structure, which to some extent proves that the precursor is produced by spray drying technology.

[0053] like Figure 2 As shown in the material cross-section diagram of the double fluorine-doped carbon composite sodium iron sulfate cathode material in the embodiment of the present invention, EDS revealed that not only has the F element been successfully doped into the material lattice, but the Na, Fe, F, and S elements are also uniformly distributed.

[0054] like Figure 4 As shown, the crystal plane diffraction peaks of the fluorine-doped carbon composite sodium iron sulfate cathode material in the embodiments of the present invention can all correspond to the standard card PDF#97-025-2403. Among them, the main crystal plane reflection peaks (200), (130), (-112) and (240) at 15.52°, 22.74°, 28.71° and 32.12° respectively, prove that the sodium iron sulfate material has been successfully synthesized. At the same time, according to the relative diffraction intensity of (200) and (240) in the XRD diffraction pattern, that is, the intensity ratio of I(240):I(200) is less than 1.0, it indicates that the relative content of Na6Fe(SO4)4 in the bulk phase of the material is low, reflecting that thermal shock and short-time sintering have a good coupling effect on the crystallinity and phase composition of the material.

[0055] like Figure 5 As shown, the fluorine-doped carbon composite sodium iron sulfate cathode material of this embodiment exhibits a D / G peak intensity ratio of approximately 1.00 according to Raman spectroscopy, indicating that the material coating layer has a good degree of graphitization; at a wavenumber of 641.8 cm⁻¹... -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1828.8 cm⁻¹... -1 A weak CF Raman vibration peak was found at the carbon layer in the composite material, indicating the presence of trace amounts of F.

[0056] like Figure 6 As shown, XPS analysis of the difluorozac thermal shock sodium ferric sulfate material in this embodiment of the invention revealed the presence of Na1s, Fe2p, S2p, O1s, C1s, N1s, and F1s elemental orbitals in the full spectrum. Specifically, the F1s orbital exhibits a peak within the region where the binding energy exceeds 687 eV, with a full width at half maximum (FWHM) exceeding 2.2 eV, indicating the presence of organic F bonds (CF bonds) on the material surface, suggesting the presence of F elements in the carbon layer. Simultaneously, a doublet of inorganic F bonds exists between binding energies of 682.5 eV and 687.5 eV, corresponding to the FO and Fe-F bonds in the sodium ferric sulfate material. XPS peak fitting of the Fe2p orbital revealed that Fe2p... 3 / 2 The binding energy exceeds 711.2 eV, Fe2p 1 / 2 The binding energy exceeds 724.9 eV, and surface TM-F bonding exists at the 688 eV peak, proving that F doping in the bulk phase and on the surface of sodium ferric sulfate enhances the binding energy of Fe.

[0057] like Figure 8 As shown, the initial powder conductivity of the fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention was tested by a powder resistance meter and was 8.47 S / m. This indicates that the electronic conductivity of the composite material after the surface carbon layer was modified by F-doping was improved compared with the carbon composite sodium iron sulfate cathode material obtained by direct sintering without fluorine doping in Comparative Example 1.

[0058] Example 1: Preparation method of a double-fluorine-doped carbon composite sodium iron sulfate cathode material (1) 7.42 g (0.0512 mol) anhydrous sodium sulfate, 18.91 g (0.068 mol) ferrous sulfate heptahydrate, 0.0679 g (0.0016 mol) sodium fluoride, 2.13 g polyvinylidene fluoride and 0.46 g ascorbic acid were added to a mixture of 50 mL deionized water and 10 mL ethanol. The mixture was stirred and dissolved for 20 min at 25 °C and 700 rpm. Then, 7.3 g of aqueous carbon nanotube conductive slurry was added. The mixture was stirred and mixed for 20 min at 25 °C and 800 rpm. The resulting feed solution was continuously stirred at 800 rpm. Spray drying was carried out at a blower frequency of 30 Hz, an inlet air temperature of 220 °C and an outlet air temperature of 120 °C to obtain a fluorine-carbon composite sodium ferric sulfate cathode material precursor. (2) The fluorine-carbon composite sodium iron sulfate cathode material precursor obtained in step (1) is placed on the carrier of the Joule heating device. Under the protection atmosphere of high-purity nitrogen, a DC current of 0-30A is passed through, and the temperature is raised from room temperature to 380℃ at a heating rate of 250K / s. Each heating lasts for 15s. After 13 heat shock treatments, the fluorine-doped carbon composite sodium iron sulfate cathode material precursor is obtained. (3) The precursor of the difluorinated carbon composite sodium iron sulfate cathode material obtained in step (2) is placed in a tube furnace. First, it is purged with a protective atmosphere at room temperature for 1.5 h to ensure that the sintering system is filled with a protective atmosphere and other gases are removed. Then, the temperature is increased from room temperature to 380℃ at a rate of 5℃ / min and sintered for 5 h to obtain the difluorinated carbon composite sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4) 2.98 F 0.04 / CF.

[0059] Application Example 1 of a fluorine-doped carbon composite sodium ferric sulfate cathode material The positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material described in the embodiments of the present invention is used to assemble sodium-ion batteries.

[0060] Battery Assembly: Under conditions where the dew point is below -30°C, weigh 0.0800g of the double-fluorine-doped carbon composite sodium iron sulfate positive electrode material described in this embodiment of the invention, add 0.0100g of conductive carbon black as a conductive agent and 0.0100g of PVDF (polyvinylidene fluoride) as a binder, mix evenly, and coat it on aluminum foil to form a positive electrode sheet. In a vacuum glove box, use a metallic sodium sheet as the negative electrode, a Whatman GF / D glass fiber separator as the battery separator, and a 1mol / L NaClO4 electrolyte (EC:DMC=1:1 (vol%) + 5%FEC) to assemble a CR2032 coin cell.

[0061] Battery performance testing: Electrochemical performance and cycle performance tests were conducted on the assembled coin cells in the range of 2 to 4.5V: the first charge-discharge specific capacity and first coulombic efficiency at 0.1C, the discharge specific capacity at 1C current rate and the capacity retention rate after 100 cycles. The test results are shown in Table 1.

[0062] like Figure 9 As shown, within the 2–4.5V range, the assembled coin cell exhibits a discharge specific capacity of 91.79 mAh / g, an energy density of 339 Wh / kg, and a median voltage of 3.754V at a 0.1C rate.

[0063] like Figure 10As shown, the sodium-ion battery assembled from the cathode electrode sheet made of the fluorine-doped carbon composite sodium iron sulfate cathode material of this embodiment of the invention exhibits a first-charge specific capacity of 101.15 mAh / g, a first-discharge specific capacity of 91.79 mAh / g, and a first-charge-discharge coulombic efficiency of 90.75% at 0.1C. These are all superior to the microsphere structure carbon composite sodium iron sulfate cathode material obtained by direct sintering without fluorine doping in Comparative Example 1. This indicates that inorganic F ion doping weakens the nucleophilicity of electron-rich oxygen atoms in the bulk phase, effectively reduces the electron supply capacity of the NFS material interface, suppresses surface side reactions, reduces CEI film formation, and still has a high first-cycle coulombic efficiency at high voltage.

[0064] like Figure 11 As shown, the sodium-ion battery assembled from the cathode electrode sheet made of the fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention has a discharge specific capacity of 89.50 mAh / g at a 1C rate (starting from the 4th cycle) and a discharge specific capacity of 85.74 mAh / g after 100 cycles, with a capacity retention rate of 95.80%. Both are superior to the microsphere structure carbon composite sodium iron sulfate cathode material obtained by direct sintering without fluorine doping in Comparative Example 1. The fluorine-doped carbon layer on the surface inhibits the side reactions that deteriorate the affinity reaction on the material surface and forms a stable CEI film.

[0065] Example 2 of a dual-fluorine-doped carbon composite sodium iron sulfate cathode material The aforementioned fluorine-doped carbon composite sodium ferric sulfate cathode material consists of fluorine-doped inorganic carbon composite conductive carbon nanotubes as a coating layer, encapsulating the bulk sodium ferric sulfate phase Na. 2.6 Fe 1.7 (SO4) 2.98 F 0.04 Hollow microspheres are formed, and conductive carbon nanotubes are uniformly distributed in the cathode material; the mass fraction of inorganic carbon in the fluorine-doped inorganic carbon in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 3.85 wt%; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 2.97 wt% of the mass of the double-fluorine-doped carbon composite sodium iron sulfate cathode material; the doping amount of fluorine ions in the fluorine-doped sodium iron sulfate is equivalent to 1.34 at the number of sulfate atomic groups in the bulk phase of sodium iron sulfate; the mass fraction of conductive carbon nanotubes in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 1.87 wt%; the average particle size of the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 25 μm.

[0066] Testing revealed that the average particle size of the fluorine-doped carbon composite sodium iron sulfate cathode material in this embodiment of the invention is 25 μm, and its morphology exhibits a distinct microspherical structure, which to some extent proves that the precursor was prepared using spray drying technology.

[0067] Upon examination, in the cross-sectional image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention, EDS analysis revealed that not only has the F element been successfully doped into the interior of the material lattice, but the Na, Fe, F, and S elements are also uniformly distributed.

[0068] Upon testing, the crystal plane diffraction peaks of the fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention all correspond to the standard card PDF#97-025-2403, proving that the sodium iron sulfate material was successfully synthesized. Furthermore, the intensity ratio of I(240):I(200) is less than 1.0, indicating that the relative content of Na6Fe(SO4)4 in the bulk phase of the material is low, reflecting that thermal shock and short-time sintering have a good coupling effect on the crystallinity and phase composition of the material.

[0069] Testing revealed that the fluorine-doped carbon composite sodium iron sulfate cathode material of this invention exhibited a D-peak / G-peak intensity ratio of approximately 1.02 using Raman spectroscopy, indicating a good degree of graphitization in the material's coating layer; at a wavenumber of 641.5 cm⁻¹... -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1801.5 cm⁻¹... -1 A weak CF Raman vibration peak was found at the carbon layer in the composite material, indicating the presence of trace amounts of F.

[0070] XPS analysis of the sodium ferric sulfate material subjected to thermal shock according to this invention revealed the presence of a peak with a full width at half maximum (FWHM) exceeding 2.3 eV in the F1s region where the binding energy exceeds 687 eV. This indicates the presence of organic F bonds, specifically CF bonds, on the material surface, suggesting the presence of F elements in the carbon layer. Simultaneously, a bimodal inorganic F bond was observed between binding energies of 682.4 eV and 687.4 eV, corresponding to the FO and Fe-F bonds in the sodium ferric sulfate material. Peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy exceeds 711.4 eV, and the presence of surface TM-F bonding at the 688.2 eV peak proves that F doping in the bulk phase and on the surface of sodium ferric sulfate enhances the binding energy of Fe.

[0071] like Figure 8 As shown, the fluorine-doped carbon composite sodium iron sulfate cathode material of this embodiment of the invention was tested with a powder resistivity meter, and its initial powder conductivity was 9.16 S / m, indicating that the surface carbon layer was treated with F... - The electronic conductivity of the doped and modified composite material was improved compared with that of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained by direct sintering without fluorine doping in Comparative Example 1.

[0072] Example 2: Preparation method of a double-fluorine-doped carbon composite sodium iron sulfate cathode material The difference between this embodiment and Method Embodiment 1 is only that: in step (1), 2.13g of polyvinylidene fluoride is replaced with 3.04g of polyvinylidene fluoride, and the fluorine-carbon composite sodium iron sulfate cathode material precursor is finally obtained; in step (3), the difluorine-doped carbon composite sodium iron sulfate cathode material Na is obtained. 2.6 Fe 1.7 (SO4) 2.98 F 0.04 / CF. Same as Example 1.

[0073] Application Example 2 of a Dual-Fluoride-Doped Carbon Composite Sodium Ferric Sulfate Cathode Material The positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material described in the embodiments of the present invention is used to assemble sodium-ion batteries.

[0074] Battery assembly: Same as in Application Example 1.

[0075] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0076] Example 3 of a dual-fluorine-doped carbon composite sodium iron sulfate cathode material The aforementioned fluorine-doped carbon composite sodium ferric sulfate cathode material consists of fluorine-doped inorganic carbon composite conductive carbon nanotubes as a coating layer, encapsulating the bulk sodium ferric sulfate phase Na. 2.6 Fe 1.7 (SO4) 2.96 F 0.08 Hollow microspheres are formed, and conductive carbon nanotubes are uniformly distributed in the cathode material; the mass fraction of inorganic carbon in the fluorine-doped inorganic carbon in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 2.99 wt%; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 2.09 wt% of the mass of the double-fluorine-doped carbon composite sodium iron sulfate cathode material; the doping amount of fluorine ions in the fluorine-doped sodium iron sulfate is equivalent to 2.68 at the number of sulfate atomic groups in the bulk phase of sodium iron sulfate; the mass fraction of conductive carbon nanotubes in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 1.90 wt%; the average particle size of the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 25 μm.

[0077] Testing revealed that the average particle size of the fluorine-doped carbon composite sodium iron sulfate cathode material in this embodiment of the invention is 25 μm, and its morphology exhibits a distinct microspherical structure, which to some extent proves that the precursor was prepared using spray drying technology.

[0078] Upon examination, in the cross-sectional image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention, EDS analysis revealed that not only has the F element been successfully doped into the interior of the material lattice, but the Na, Fe, F, and S elements are also uniformly distributed.

[0079] Upon testing, the crystal plane diffraction peaks of the fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention all correspond to the standard card PDF#97-025-2403, proving that the sodium iron sulfate material was successfully synthesized. Furthermore, the intensity ratio of I(240):I(200) is less than 1.0, indicating that the relative content of Na6Fe(SO4)4 in the bulk phase of the material is low, reflecting that thermal shock and short-time sintering have a good coupling effect on the crystallinity and phase composition of the material.

[0080] Testing revealed that the fluorine-doped carbon composite sodium iron sulfate cathode material of this invention exhibited a Raman spectral density (D / G peak intensity ratio) of approximately 0.98, indicating a good degree of graphitization in the coating layer. At a wavenumber of 642.4 cm⁻¹, the material showed a similar intensity. -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1799.6 cm⁻¹... -1 A weak CF Raman vibration peak was found at the carbon layer in the composite material, indicating the presence of trace amounts of F.

[0081] XPS analysis of the sodium ferric sulfate material subjected to thermal shock according to this invention revealed that in the F1s region, there were peaks with a full width at half maximum (FWHM) exceeding 2.1 eV in the region where the binding energy exceeded 687 eV, indicating the presence of organic F bonds, i.e., CF bonds, on the material surface; this also indicates the presence of F element in the carbon layer. Simultaneously, a bimodal inorganic F bond was observed between binding energies of 682.6 eV and 687.7 eV, corresponding to FO and Fe-F bonds in the sodium ferric sulfate material. Peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy exceeds 711.3 eV, and the presence of surface TM-F bonding at the 688.1 eV peak proves that F doping in the bulk phase and on the surface of sodium ferric sulfate enhances the binding energy of Fe.

[0082] Example 3: Preparation method of a double-fluorine-doped carbon composite sodium iron sulfate cathode material The difference between this embodiment and Method Embodiment 1 is only that: in step (1), 7.42g (0.0512mol) of anhydrous sodium sulfate is replaced with 7.30g (0.0504mol) of anhydrous sodium sulfate, and 0.0679g (0.0016mol) of sodium fluoride is replaced with 0.1197g (0.0032mol) of ammonium fluoride and 0.27g (0.0032mol) of sodium acetate, finally obtaining the fluorine-carbon composite sodium iron sulfate cathode material precursor; step (3) obtains the double fluorine-doped carbon composite sodium iron sulfate cathode material Na 2.6 Fe 1.7 (SO4) 2.96 F 0.08 / CF. Same as Example 1.

[0083] Application Example 3 of a Dual-Fluoride-Doped Carbon Composite Sodium Ferric Sulfate Cathode Material The positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material described in the embodiments of the present invention is used to assemble sodium-ion batteries.

[0084] Battery assembly: Same as in Application Example 1.

[0085] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0086] Example 4 of a dual-fluorine-doped carbon composite sodium iron sulfate cathode material The aforementioned fluorine-doped carbon composite sodium ferric sulfate cathode material consists of fluorine-doped inorganic carbon composite conductive carbon nanotubes as a coating layer, encapsulating the bulk sodium ferric sulfate phase Na. 2.6 Fe 1.7 (SO4) 2.98 F 0.04 Hollow microspheres are formed, and conductive carbon nanotubes are uniformly distributed in the cathode material; the mass fraction of inorganic carbon in the fluorine-doped inorganic carbon in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 2.93 wt%; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 2.86 wt% of the mass of the double-fluorine-doped carbon composite sodium iron sulfate cathode material; the doping amount of fluorine ions in the fluorine-doped sodium iron sulfate is equivalent to 1.34 at the number of sulfate atomic groups in the bulk phase of sodium iron sulfate; the mass fraction of conductive carbon nanotubes in the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 1.88 wt%; the average particle size of the double-fluorine-doped carbon composite sodium iron sulfate cathode material is 25 μm.

[0087] Testing revealed that the average particle size of the fluorine-doped carbon composite sodium iron sulfate cathode material in this embodiment of the invention is 25 μm, and its morphology exhibits a distinct microspherical structure, which to some extent proves that the precursor was prepared using spray drying technology.

[0088] Upon examination, in the cross-sectional image of the double-fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention, EDS analysis revealed that not only has the F element been successfully doped into the interior of the material lattice, but the Na, Fe, F, and S elements are also uniformly distributed.

[0089] Upon testing, the crystal plane diffraction peaks of the fluorine-doped carbon composite sodium iron sulfate cathode material of the present invention all correspond to the standard card PDF#97-025-2403, proving that the sodium iron sulfate material was successfully synthesized. Furthermore, the intensity ratio of I(240):I(200) is less than 1.0, indicating that the relative content of Na6Fe(SO4)4 in the bulk phase of the material is low, reflecting that thermal shock and short-time sintering have a good coupling effect on the crystallinity and phase composition of the material.

[0090] Testing revealed that the fluorine-doped carbon composite sodium iron sulfate cathode material of this invention exhibited a D-peak / G-peak intensity ratio of approximately 0.97 using Raman spectroscopy, indicating a good degree of graphitization in the material's coating layer; at a wavenumber of 640.3 cm⁻¹... -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1802.1 cm⁻¹... -1 A weak CF Raman vibration peak was found at the carbon layer in the composite material, indicating the presence of trace amounts of F.

[0091] XPS analysis of the sodium ferric sulfate material subjected to thermal shock according to this invention revealed the following: In the F1s region, there are peaks with a full width at half maximum (FWHM) exceeding 2.1 eV in the region where the binding energy exceeds 687 eV, indicating the presence of organic F bonds, i.e., CF bonds, on the material surface; this indicates the presence of F element in the carbon layer. Simultaneously, a double-peaked inorganic F bond exists between the binding energies of 682.5 eV and 687.5 eV, corresponding to the FO and Fe-F bonds in the sodium ferric sulfate material. Peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy exceeds 711.1 eV, and the presence of surface TM-F bonding at the 679.8 eV peak proves that F doping in the bulk phase and on the surface of sodium ferric sulfate enhances the binding energy of Fe.

[0092] Example 4: Preparation method of a double-fluorine-doped carbon composite sodium iron sulfate cathode material The difference between this embodiment and method embodiment 1 is only that: in step (1), spray drying is performed at a fan frequency of 35Hz, an inlet air temperature of 210℃, and an outlet air temperature of 130℃; in step (2), the carbon composite sodium iron sulfate cathode material precursor obtained in step (1) is placed on the carrier of a Joule heating device, and under a high-purity nitrogen protective atmosphere, a DC current of 0-25A is passed through, and the temperature is raised from room temperature to 360℃ at a heating rate of 250K / s, with each heating lasting 15s, for a total of 10 thermal shock treatments; in step (3), the temperature is raised from room temperature to 350℃ at a rate of 5℃ / min, and sintered for 3h, finally obtaining the difluorine-doped carbon composite sodium iron sulfate cathode material Na 2.6 Fe 1.7 (SO4) 2.98 F 0.04 / CF. Same as Example 1.

[0093] Application Example 4 of a Dual-Fluoride-Doped Carbon Composite Sodium Ferric Sulfate Cathode Material The positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material described in the embodiments of the present invention is used to assemble sodium-ion batteries.

[0094] Battery assembly: Same as in Application Example 1.

[0095] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0096] Comparative Example 1 The only difference between this comparative example and method example 1 is that in step (1), 0.0679 g (0.0016 mol) of sodium fluoride and 2.13 g of polyvinylidene fluoride are not added, and the mixture of 50 mL of deionized water and 10 mL of ethanol is replaced with 50 mL of deionized water; step (2) is deleted, and the final product is a microsphere-structured carbon composite sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4)3 / C. Same as Example 1.

[0097] like Figure 3 As shown, the average particle size of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention is 25 μm, and its morphology has obvious microsphere structure, which to some extent proves that the precursor is produced by spray drying technology.

[0098] Upon examination, in the material cross-section diagram of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention, EDS analysis revealed that the elements Na, Fe, S, O, and C in the material are uniformly distributed.

[0099] like Figure 4 As shown, the crystal plane diffraction peaks of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention can be compared with the standard card PDF#97-025-2403. Among them, the main crystal plane reflection peaks (200), (130), (-112) and (240) are corresponding to 15.52°, 22.74°, 28.71° and 32.12° respectively. However, the crystal form of pure sodium iron sulfate material is difficult to be completely transformed under a short sintering time. Through XRD refinement, it was found that there are electrochemically inactive FeSO4 impurity phase diffraction peaks in the material. According to the relative diffraction intensity of (200) and (240) in the XRD diffraction pattern, that is, the intensity ratio of I(240):I(200) is close to 1.0, indicating that the relative content of Na6Fe(SO4)4 impurity phase in the bulk phase of the material is relatively high.

[0100] like Figure 5 As shown, the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention exhibits a wavenumber of 665 cm⁻¹. -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material.

[0101] like Figure 7As shown, XPS analysis of the microsphere-structured carbon composite sodium iron sulfate material obtained in the comparative example of this invention revealed the presence of Na1s, Fe2p, S2p, O1s, C1s, and N1s elemental orbitals in the full spectrum, but no F1s orbital, indicating no F element doping; the N element originates from the surface-coated carbon source. Peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy is 710.8 eV, Fe2p 1 / 2 The binding energy is 724.8 eV, indicating that even without internal or external fluorine doping, sodium ferric sulfate materials have almost no impact on the change of Fe-O bond energy.

[0102] like Figure 8 As shown, the microsphere-structured carbon composite sodium iron sulfate material obtained in the comparative example of this invention was tested with a powder resistivity meter, and its initial powder conductivity was only 5.04 S / m, indicating that the electronic conductivity of the material modified only by inorganic carbon source is significantly different from that of the material modified by fluorocarbon.

[0103] The positive electrode sheet made from the microsphere-structured carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention is used to assemble sodium-ion batteries.

[0104] Battery assembly: Same as in Application Example 1.

[0105] Battery performance test: Same as application example 1.

[0106] like Figure 9 As shown, within the 2–4.5V range, the assembled coin cell exhibits a discharge specific capacity of 85.39 mAh / g, an energy density of 324.77 Wh / kg, and a median voltage of 3.702V at a 0.1C rate.

[0107] like Figure 10 As shown, the sodium-ion battery assembled from the positive electrode sheet made of the microsphere-structured carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention has an initial discharge specific capacity of 88.17 mAh / g and an initial charge-discharge coulombic efficiency of 89.07% at 0.1C.

[0108] like Figure 11 As shown, the sodium-ion battery assembled from the positive electrode sheet made of the microsphere-structured carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention has a discharge specific capacity of 85.91 mAh / g at a 1C rate (starting from the 4th cycle). After 100 cycles, the discharge specific capacity is only 79.5 mAh / g, and the capacity retention rate is only 92.54%.

[0109] Comparative Example 2 The only difference between this comparative example and Method Example 1 is that in step (1), 0.0679 g (0.0016 mol) of sodium fluoride and 2.13 g of polyvinylidene fluoride are replaced with 0.204 g (0.0048 mol) of sodium fluoride; in step (3), the final product is a monofluorinated carbon composite sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4) 2.94 F 0.12 / C. Same as Example 1.

[0110] Testing revealed that the average particle size of the monofluorinated carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention was 23 μm, exhibiting a distinct microspherical structure.

[0111] Upon examination, in the material cross-section diagram of the monofluorinated carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention, EDS analysis revealed that not only has the fluorine element been successfully doped into the material lattice, but the Na, Fe, S, O, C, and F elements are also uniformly distributed.

[0112] Testing revealed that the crystal plane diffraction peaks of the monofluorinated carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention all corresponded to the standard card PDF#97-025-2403, proving that the sodium iron sulfate material was successfully synthesized, and no other crystal plane reflection peaks were found in the XRD diffraction pattern.

[0113] Testing showed that the monofluorinated carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention exhibited a wavenumber of 665 cm⁻¹. -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1800 cm⁻¹... -1 The absence of CF Raman vibration peaks nearby indicates that inorganic fluorine source doping cannot penetrate the inorganic carbon layer and cannot form CF bonds.

[0114] XPS analysis of the monofluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention revealed a double-peaked inorganic F bond in F1s with binding energies of 682.6 eV and 687.8 eV, corresponding to the FO and Fe-F bonds in the sodium iron sulfate material; peak fitting of Fe2p showed that Fe2p... 3 / 2 The binding energy exceeds 711.6 eV, proving that F doping in the bulk phase of sodium ferric sulfate enhances the binding energy of Fe.

[0115] The positive electrode sheet made from the monofluorine-doped carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention was used to assemble a sodium-ion battery.

[0116] Battery assembly: Same as in Application Example 1.

[0117] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0118] like Figure 9 As shown, within the 2–4.5V range, the assembled coin cell exhibits a discharge specific capacity of 88.41 mAh / g, an energy density of 329.15 Wh / kg, and a median voltage of 3.748V at a 0.1C rate.

[0119] Comparative Example 3 The only difference between this comparative example and Method Example 1 is that step (2) is omitted. Everything else is the same as Method Example 1.

[0120] Testing revealed that the average particle size of the fluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention was 23 μm, and its morphology exhibited a distinct microspherical structure.

[0121] Upon testing, in the material cross-section diagram of the dual-fluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention, EDS analysis revealed that Na, Fe, S, O, C, and F elements were uniformly distributed in the material, and no other elements were detected.

[0122] Upon testing, the crystal plane diffraction peaks of the fluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention at 15.52°, 22.74°, 28.71° and 32.12°, namely (200), (130), (-112) and (240), correspond to the standard card PDF#97-025-2403. However, the precursor material, which has not undergone thermal shock treatment but only sintered, has very low crystallinity and contains crystal plane diffraction peaks of other Na2SO4 and FeSO4 impurity phases.

[0123] Testing revealed that the dual-fluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention exhibited a wavenumber of 665 cm⁻¹. -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material; at a wavenumber of 1801.4 cm⁻¹... -1 A weak CF Raman vibration peak was found at [location missing], proving the presence of trace amounts of F element in the carbon layer of the composite material; in the low-frequency region 220–350 cm⁻¹... -1 There is a distinct Fe-O vibration peak, which belongs to the FeSO4-related vibration.

[0124] XPS analysis of the fluorine-doped carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention revealed the presence of peaks in the F1s region where the binding energy exceeds 687 eV, with a full width at half maximum (FWHM) exceeding 2.0 eV. This indicates the presence of organic F bonds, i.e., CF bonds, on the material surface, suggesting the presence of F element in the carbon layer. Simultaneously, a bimodal inorganic F bond exists between binding energies of 682.6 eV and 687.6 eV, corresponding to the FO and Fe-F bonds in the sodium iron sulfate material. XPS peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy exceeds 711.3 eV, and the presence of surface TM-F bonding at the 688.1 eV peak proves that F doping in the bulk phase and on the surface of sodium ferric sulfate enhances the binding energy of Fe.

[0125] The positive electrode sheet made from the dual-fluorine-doped carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention was used to assemble a sodium-ion battery.

[0126] Battery assembly: Same as in Application Example 1.

[0127] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0128] Comparative Example 4 The only difference between this comparative example and method example 1 is that in step (1), 0.0679 g (0.0016 mol) of sodium fluoride and 2.13 g of polyvinylidene fluoride are not added, and the mixture of 50 mL of deionized water and 10 mL of ethanol is replaced with 50 mL of deionized water; step (3) is deleted, and the final product is a microsphere-structured carbon composite sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4)3 / C. Same as Example 1.

[0129] Testing revealed that the average particle size of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention was 23 μm, and its morphology exhibited a distinct microsphere structure.

[0130] Upon testing, in the material cross-section diagram of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention, EDS analysis revealed that Na, Fe, S, O, and C elements were uniformly distributed in the material, and no other elements were detected.

[0131] Testing revealed that the crystal plane diffraction peaks of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention all corresponded to the standard card PDF#97-025-2403. Among them, multiple crystal plane reflection peaks (200), (130), (-112), and (240) were corresponding to 15.52°, 22.74°, 28.71°, and 32.12°, respectively, proving that the sodium ferric sulfate material was successfully synthesized. Furthermore, no other crystal plane reflection peaks were found in the XRD diffraction pattern, but the relative intensities of the diffraction characteristic peaks were all relatively weak. This indicates that the material, which underwent thermal shock treatment for only a short time without long-term sintering, had poor crystallinity.

[0132] Testing showed that the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention exhibited a wavenumber of 665 cm⁻¹. -1 The presence of an S=O asymmetric stretching reflection peak indicates the successful preparation of sodium ferric sulfate material.

[0133] XPS analysis of the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in this invention revealed the presence of Na1s, Fe2p, S2p, O1s, C1s, and N1s elemental orbitals in the full spectrum, but no F1s orbital, indicating no F element doping. The N element originates from the surface-coated carbon source. Peak fitting of Fe2p revealed that Fe2p... 3 / 2 The binding energy is 710.8 eV, Fe2p 1 / 2 The binding energy is 724.8 eV, indicating that thermal shock does not change the Fe-O binding energy in the bulk phase of the material.

[0134] The positive electrode sheet made from the microsphere-structured carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention is used to assemble sodium-ion batteries.

[0135] Battery assembly: Same as in Application Example 1.

[0136] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0137] Table 1. Comparison of test results of cycle performance and electrical performance of batteries assembled from positive electrode sheets made of the positive electrode materials obtained in Examples 1-4 and Comparative Examples 1-4 of the present invention.

[0138] Note: In the table, the median voltage was measured using a button cell testing system, and the conductivity was measured using a four-probe powder resistance meter.

[0139] As shown in Table 1, the fluorine-doped thermal shock sodium iron sulfate composite materials of Examples 1-4 of this invention incorporate fluorine ions into the surface-modified carbon layer and the bulk sodium iron sulfate phase, respectively. The introduction of fluorine into the carbon layer improves the electronic conductivity of the surface-modified material and the binding energy between the carbon layer and the bulk material, thus enhancing the conductivity of the composite material. The presence of fluorine ions within the sodium iron sulfate lattice increases the median voltage of the composite material, improves the energy density of the cathode material, and weakens the nucleophilicity at the material interface, effectively suppressing interfacial side reactions. Testing revealed that the battery assembled from the cathode electrode sheet made from the obtained material exhibits a reversible discharge specific capacity of 91.79 mAh / g at 0.1C rate, an initial coulombic efficiency of 90.75%, and a capacity retention rate of 95.80% after 100 cycles at 1C rate. Figure 9 As shown, at a 0.1C rate, the median voltage is 3.754V, the energy density is 339Wh / kg, and the conductivity is as high as 8.78S / m. Due to the increased doping amount of fluorine in inorganic carbon in Example 2 of this invention, the conductivity is increased to 9.16S / m. This indicates that this invention, through the synergistic effect of surface carbon layer structure modification, interfacial element interaction, and bulk anion doping, comprehensively improves the electronic conductivity, median voltage, and energy density of the cathode material, and successfully prepares sodium iron sulfate material for high-energy-density sodium-ion batteries.

[0140] From Table 1, Figure 9 It can be seen that the carbon modification layer on the surface of the cathode material in Comparative Example 1 is undoped with fluorine, the bulk phase is not doped with fluorine, and the composite material is obtained directly through low-temperature sintering without thermal shock treatment. Firstly, because the surface-modified carbon layer in Comparative Example 1 does not introduce F ions to change the composition of sp2 and sp3 and the degree of structural defects in the material, the binding energy between the surface-modified carbon layer and the bulk material weakens during the first charge-discharge process, resulting in a significant decrease in electronic conductivity. Secondly, due to the lack of synergistic effect from bulk F ion doping, the anion-induced effect in the material is weakened, and the redox couple (i.e., Fe) is reduced. 3+ / Fe 2+ The voltage is relatively low, resulting in a decrease in material energy density. Furthermore, without thermal shock treatment, not only is it unable to significantly reduce the material preparation and production cycle, but it is also difficult to precisely control the conversion of the non-stoichiometric transition state sodium ferric sulfate crystal form. In summary, Comparative Example 1 is inferior to Example 1 of the present invention in terms of electrochemical performance, including cycle performance, electronic conductivity, median voltage, and energy density.

[0141] From Table 1, Figure 9As can be seen, in Comparative Example 2, fluorine ion doping was performed inside the bulk phase compared to Comparative Example 1, and the material underwent thermal shock and low-temperature sintering treatments to prepare a single fluorine ion-doped thermally shockable sodium iron sulfate material. The thermally shock-treated bulk fluorinated material exhibits a relatively higher median voltage, thus improving the energy density of the sodium iron sulfate cathode material. The thermal shock treatment provides intense energy through rapid heating in a short time, removing the water of crystallization in the material and promoting the conversion of the high Na / Fe ratio Na6Fe(SO4)4 impurity phase. This effectively improves the conversion degree of the non-stoichiometric transition state sodium iron sulfate crystal form and enhances the cycling performance of the material. However, due to the low intrinsic electronic conductivity of the material, the kinetic polarization intensifies at higher median voltages.

[0142] As shown in Table 1, in Comparative Example 3, the surface carbon layer fluorine modification and bulk fluorine doping improved the electronic conductivity and the inductive effect of anions in the bulk material, thereby increasing the redox couple voltage and changing the median voltage and energy density of the material. However, the lack of thermal shock to induce crystal transformation, and the reliance on low-temperature sintering, resulted in a relatively low degree of crystal transformation and a relatively decreased cycle performance of the composite material.

[0143] As shown in Table 1, in Comparative Example 4, the targeted non-stoichiometric transition state crystal structure obtained by simply subjecting the pure phase material to thermal shock treatment without sintering exhibits good electrochemical performance under initial charge and discharge conditions. However, during long-cycle reaction, due to the lack of long-term high-temperature sintering, the material has low crystallinity and unstable crystal structure, making it prone to collapse and distortion, which leads to a decrease in the electrochemical performance of the material under long-cycle conditions.

Claims

1. A double-fluorine-doped carbon composite sodium ferric sulfate cathode material, characterized in that: Hollow microspheres are formed by coating fluorine-doped inorganic carbon composite inorganic conductive carbon as a coating layer, which is then coated with fluorine-doped sodium iron sulfate bulk phase, and the inorganic conductive carbon is uniformly distributed in the cathode material.

2. The double-fluorine-doped carbon composite sodium iron sulfate cathode material according to claim 1, characterized in that: The mass fraction of inorganic carbon in the fluorine-doped inorganic carbon composite sodium iron sulfate cathode material is 0.1–20.0 wt%; the doping amount of fluorine atoms in the fluorine-doped inorganic carbon is equivalent to 0.05–10.00 wt% of the mass of the sodium iron sulfate cathode material; the doping amount of fluorine ions in the fluorine-doped sodium iron sulfate is equivalent to 0.01–40.00 at the number of sulfate groups in the bulk sodium iron sulfate phase; the mass fraction of inorganic conductive carbon in the sodium iron sulfate cathode material is 0.1–4.5 wt%; the chemical formula of the fluorine-doped sodium iron sulfate is Na. 2.6 Fe 1.7 (SO4) 3-x F 2x Where 0 < x ≤ 0.5 and the law of conservation of charge is satisfied; the inorganic conductive carbon includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P or graphene; the average particle size of the double fluorine-doped carbon composite sodium iron sulfate cathode material is 5 to 30 μm.

3. A method for preparing the double-fluorine-doped carbon composite sodium iron sulfate cathode material as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Sodium source, ferrous source, inorganic fluorine source, sulfate source, organic fluorine source and organic acid antioxidant are added to an organic aqueous solution, stirred and dissolved, and then inorganic conductive carbon source slurry is added. After stirring and mixing, spray drying is performed to obtain fluorine-carbon composite sodium iron sulfate cathode material precursor. (2) The fluorine-carbon composite sodium iron sulfate cathode material precursor obtained in step (1) is subjected to thermal shock treatment under a protective atmosphere to obtain a fluorine-doped carbon composite sodium iron sulfate cathode material precursor. (3) The precursor of the double-fluorine-doped carbon composite sodium iron sulfate cathode material obtained in step (2) is sintered under a protective atmosphere to obtain the double-fluorine-doped carbon composite sodium iron sulfate cathode material.

4. The preparation method of the double-fluorine-doped carbon composite sodium iron sulfate cathode material according to claim 3, characterized in that: In step (1), the molar ratio of sodium in the sodium source, ferrous element in the ferrous source, sulfate in the sulfate source, and fluoride ion in the inorganic fluorine source is the same as that of the chemical formula Na. 2.6 Fe 1.7 (SO4) 3-x F 2x Wherein, the molar ratios of the corresponding substances in 0 < x ≤ 0.5 are matched; the amount of the organic fluorine source is equivalent to 0.2 to 30.0 wt% of the theoretical mass of fluorine-doped sodium ferric sulfate; the carbonized mass of the organic acid antioxidant is equivalent to 0.1 to 1.5 wt% of the theoretical mass of fluorine-doped sodium ferric sulfate; the molar volume ratio of ferrous ions to the organic aqueous solution in the ferrous source is 1.0 to 1.6 mol / L; the organic aqueous solution is a mixed solution of organic solvent and water in a volume ratio of 5 to 25:100; the organic solvent includes one or more of methanol, ethanol, or ethylene glycol; the stirring and dissolving temperature is 25 to 30°C, the stirring speed is 300 to 800 rpm, and the time is 20 to 30 min.

5. The method for preparing the double-fluorine-doped carbon composite sodium iron sulfate cathode material according to claim 3 or 4, characterized in that: In step (1), the solid content of the inorganic conductive carbon source slurry is 3-8%; the inorganic conductive carbon in the inorganic conductive carbon source slurry is equivalent to 0.1-6.0 wt% of the theoretical mass of fluorine-doped sodium iron sulfate; the stirring temperature is 25-30℃, the rotation speed is 600-800 rpm, and the stirring time is 20-30 min; while the feed is being fed into the spray dryer, the feed solution is continuously stirred at 700-900 rpm; the process parameters for the spray dryer are: the frequency of the induced draft fan is 20-40 Hz, the inlet air temperature is 200-230℃, and the outlet air temperature is 100-130℃.

6. The method for preparing the double-fluorine-doped carbon composite sodium iron sulfate cathode material according to any one of claims 3 to 5, characterized in that: In step (1), the sodium source includes sodium sulfate, sodium acetate, sodium formate, or sodium nitrate, and one or more of their hydrates; the ferrous source includes ferrous sulfate and / or ferrous nitrate, and one or more of their hydrates; the sulfate source includes sulfuric acid, and ammonium sulfate, sodium sulfate, or ferrous sulfate, and one or more of their hydrates; the inorganic fluorine source includes hydrofluoric acid, sodium fluoride, ferrous fluoride, or ammonium fluoride, and one or more of their hydrates; the organic fluorine source includes polyvinylidene fluoride, tetrabutylammonium fluoride, or nitrogen- and sulfur-containing polyvinylidene fluoride derivatives, and one or more; the organic acid antioxidant includes citric acid, tannic acid, or ascorbic acid, and one or more; the inorganic conductive carbon source includes carbon quantum dots, carbon nanotubes, Ketjen Black, Super P, or graphene, and one or more.

7. The method for preparing the double-fluorine-doped carbon composite sodium iron sulfate cathode material according to any one of claims 3 to 6, characterized in that: In step (2), the thermal shock treatment refers to: placing the fluorine-carbon composite sodium iron sulfate cathode material precursor on a carrier, passing a DC current of 0-45A, heating from room temperature to 350-380℃ at a heating rate of 200-300K / s, with each heat shock lasting 5-30s, and repeating the thermal shock 1-15 times; the protective atmosphere includes nitrogen, argon, a hydrogen-argon mixture, or a hydrogen-nitrogen mixture.

8. The method for preparing the double-fluorine-doped carbon composite sodium iron sulfate cathode material according to any one of claims 3 to 7, characterized in that: In step (3), before sintering, the system is ventilated with a protective atmosphere at room temperature for 1.0 to 1.5 hours to ensure that the sintering system is filled with a protective atmosphere and to remove other gases. The sintering refers to heating from room temperature to 350 to 400°C at a rate of 1 to 5°C / min and sintering for 3 to 8 hours. The protective atmosphere includes nitrogen, argon, a hydrogen-argon mixture, or a hydrogen-nitrogen mixture.

9. An application of the double-fluorine-doped carbon composite sodium iron sulfate cathode material as described in claim 1 or 2, characterized in that: The positive electrode sheet made of the double fluorine-doped carbon composite sodium iron sulfate positive electrode material as described in claim 1 or 2 is used to assemble a sodium-ion battery.

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