Preparation method and application of fluoride sodium-ion battery positive electrode material

Abstract

The invention discloses a preparation method and application of a fluoride sodium-ion battery positive electrode material, and belongs to the technical field of sodium-ion battery positive electrode material preparation, and the preparation method comprises the following steps: (1) dissolving iron salt in deionized water to obtain a solution A; uniformly mixing sodium fluoride and inorganic acid in water to obtain a solution B; (2) dropwise adding the solution A and the solution B into a coprecipitation reaction kettle at the same flow rate, reacting while heating and stirring, monitoring the pH value of a reaction system in real time, and washing and drying to obtain a precursor Na3FeF6; and (3) mixing the precursor with a carbon source, and sintering in an inert atmosphere to obtain the sodium ion battery positive electrode material NaFeF3 (at) C coated with a carbon layer, according to the invention, the problems of capacity fading and cycling stability of the sodium ion battery positive electrode material are effectively solved, and a solution with high performance, low cost and environmental friendliness is provided for development of the sodium ion battery positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of battery material preparation technology, specifically relating to a method for preparing and applying a sodium fluoride battery cathode material. Background Technology

[0002] As the global energy structure accelerates its transition towards cleaner and lower-carbon energy, sodium-ion batteries, with their abundant sodium resources (2.3% of the Earth's crust), wide distribution, and low cost (sodium raw material costs less than 1 / 10 of lithium), have become an ideal alternative for large-scale energy storage. Especially in the fields of frequency regulation in renewable energy power plants, low-speed electric vehicles, and distributed energy storage, sodium-ion batteries demonstrate enormous commercial potential, and are expected to overcome the lithium resource bottleneck, driving the global energy transition.

[0003] However, the industrialization of sodium-ion batteries is still constrained by the key performance characteristics of cathode materials. Currently, mainstream cathode systems all have significant shortcomings: layered oxides (such as NaFeO2), while possessing high energy density (140-160 mAh / g), are prone to slippage or even collapse during repeated sodium ion insertion / extraction, leading to reduced cycle life; polyanionic compounds (such as Na3V2(PO4)3), with their stable three-dimensional framework, can achieve over 2000 cycles, but their theoretical capacity is relatively low (approximately 120 mAh / g) and vanadium-containing raw materials are expensive; Prussian blue materials (such as NaFe(CN)6), while inexpensive and possessing excellent rate performance, suffer from rapid capacity decay due to the difficulty in completely removing water of crystallization. Against this backdrop, fluoride cathode materials (such as FeF3 and NaFeF3) have emerged due to their unique performance advantages—Fe 3+ / Fe 2+ The redox reaction potential is as high as 3.0 V (vs. Na). + The presence of Na significantly improves battery operating voltage. Fluorine is abundant (0.06% of the Earth's crust), and the low cost of iron and sodium (raw material costs are only 1 / 5 to 1 / 10 of cobalt and lithium) makes fluoride systems highly competitive in terms of cost and resource sustainability. Taking NaFeF3 as an example, as a derivative variant of polyanionic materials, it possesses a theoretical capacity of 197 mAh / g and stable three-dimensional ion channels. If its energy density can be adjusted to approach that of layered oxides (≥160 mAh / g) while maintaining the long cycling characteristics of polyanionic materials (>1500 cycles), it will form a significant differentiated competitive advantage.

[0004] Despite the significant theoretical advantages of NaFeF3, its industrialization still faces multiple challenges. On one hand, the intrinsic insulating nature of fluorides results in extremely low electronic conductivity (<10⁻⁶). -8The S / cm ratio severely limits the rate performance of the material (1C capacity is less than 50% of 0.1C capacity), and volume changes during charge and discharge easily lead to particle pulverization, resulting in generally low cycle life. On the other hand, traditional synthesis processes rely on high-temperature solid-state methods (>800℃) or corrosive fluorine sources (such as hydrofluoric acid), which not only have high energy consumption and demanding equipment requirements but also generate fluorine-containing waste gas and wastewater, placing enormous environmental pressure on the industry. Taking the typical hydrothermal method as an example, although it can generate NaFeF3 by reacting FeCl3 with NaF at 180℃, the process window is narrow—the pH needs to be precisely controlled between 1.5 and 2.5 to avoid Fe... 3+ Hydrolysis and fluctuations in fluoride ion concentration easily lead to the formation of NaF impurity phases. Furthermore, the products are coarse-grained with low specific surface area, resulting in sluggish sodium ion diffusion kinetics and an actual capacity of only 80-100 mAh / g. Kisuk Kang et al. from Seoul National University in South Korea used a high-energy ball milling method to ball mill NaF and FeF3 under an argon atmosphere and then coat them with carbon to obtain NaFeF3 / C materials (RA Shakoor, SY Lim, H. Kim, KW Nam, JK Kang, K. Kang, JW Choi, Mechanochemical synthesis and electrochemical behavior of Na3FeF6 in sodium and lithium batteries. SolidState Ionics, 2012, 218:35-40). Under voltage conditions of 0.5-4.25V, the initial discharge specific capacity was 111 mAh / g, and after 20 cycles, the capacity retention was only 50%. The material prepared by this method has large particles and severe agglomeration. High-energy ball milling causes lattice defects in the material, resulting in poor electronic conductivity. The carbon coating is uneven, and it cannot effectively suppress Fe. 3+ Irreversible phase transitions and electrolyte side reactions. Summary of the Invention

[0005] This invention discloses a method for preparing sodium fluoride cathode materials for sodium-ion batteries. By proposing a simple and low-cost two-step synthesis process of liquid-phase precipitation-thermal reduction, the method solves the problem of poor conductivity of traditional fluoride cathode materials and can be directly used as a cathode material for sodium-ion batteries, showing good prospects for industrial application.

[0006] To achieve the above objectives, this application provides the following technical solution.

[0007] A method for preparing a sodium fluoride-based positive electrode material for a battery includes the following steps: (1) Dissolve iron salt in deionized water to obtain solution A; mix sodium fluoride and inorganic acid evenly in water to obtain solution B; (2) A and B solutions were fed into a coprecipitation reactor at the same flow rate and reacted under heating and stirring. The pH value of the reaction system was monitored in real time. The precursor Na3FeF6 was obtained after washing and drying. (3) The aforementioned precursor is mixed with a carbon source and sintered under an inert atmosphere to obtain a sodium fluoride battery cathode material NaFeF3@C coated with a carbon layer.

[0008] Further settings include: In step (1), the iron salt is selected from one or a mixture of two or more of FeCl3, Fe(NO3)3, and Fe2(SO4)3 in any proportion.

[0009] In step (1), ascorbic acid, a reducing agent, is added to the iron salt, and the mass ratio of iron salt to ascorbic acid is 3:1 to 5:1.

[0010] In step (1), the inorganic acid is selected from one or more of hydrochloric acid, sulfuric acid, and nitric acid in any proportion.

[0011] In step (1), the concentration of solution A is 0.2 mol / L-0.4 mol / L, and the concentration of sodium fluoride in solution B is 0.6 mol / L-1.2 mol / L.

[0012] In step (2), the coprecipitation reaction is carried out at a feed rate of 2-10 mL / min, a reaction temperature of 50-80℃, a reaction time of 3-6 h, a pH of 2-4, and a stirring speed of 300-1000 rpm. After the reaction, the washing is performed 1-5 times, the drying temperature is 80-120℃, and the drying time is 8-24 h.

[0013] Particularly preferred, the concentration of solution A is 0.2 mol / L, and the total added volume is 2000 mL; the concentration of solution B is 0.6 mol / L, and the total added volume is 2000 mL. The coprecipitation reaction is carried out at a temperature of 70°C for 4 hours, the pH of the reaction system is 2-3, the stirring speed is 400 rpm, the feed rate of the solution is 2.5 mL / min, the washing is performed 3 times, the drying temperature is 90°C, and the drying time is 12 hours.

[0014] In step (3), the carbon source is selected from one or a mixture of two or more of citric acid, glucose, and ascorbic acid in any proportion.

[0015] In step (3), the mass ratio of the precursor Na3FeF6 to the carbon source is 1:0.05 to 0.2.

[0016] In step (3), the inert atmosphere is at least one of N2 and Ar, the sintering temperature is 300-800℃, and the sintering time is 2-10 hours.

[0017] In step (3), after the sintering is completed, the prepared sodium-ion battery cathode material NaFeF3@C coated with carbon layer is further washed and dried. The washing times are preferably 1 to 5 times, the drying temperature is 80 to 120°C, and the drying time is 8 to 24 hours.

[0018] Particularly preferred is that the mass ratio of the precursor Na3FeF6 to the carbon source is 1:0.05; the sintering atmosphere is N2; the sintering temperature is 700℃; and the sintering time is 8 hours.

[0019] The second objective of this invention is to provide an application of the aforementioned prepared sodium fluoride battery cathode material in the preparation of sodium ion batteries.

[0020] The working principle of this invention is as follows: This invention innovatively proposes a highly efficient and green synthesis route, overcoming the dual bottlenecks of material performance and process cost. The method prepares a high-purity precursor Na3FeF6 through acidic co-precipitation, followed by carbothermic reduction at a relatively low temperature (<700℃) using a carbon source as a reducing agent, generating a carbon-coated fluoride sodium-ion battery cathode material, NaFeF3@C, in two steps. This process eliminates the need for traditional high-temperature sintering and hydrofluoric acid. The introduction of the carbon layer improves electronic conductivity, achieving a capacity of 166 mAh / g at 0.1C rate. Through the synergistic design of morphology control (porous structure, nano-sizing) and in-situ carbon coating, this method overcomes the challenges of capacity decay and cycle stability while retaining the high-voltage characteristics of fluorides, providing a high-performance, low-cost, and environmentally friendly solution for the development of cathode materials for sodium-ion batteries.

[0021] Compared with the prior art, the present invention has the following advantages: (1) This invention innovatively proposes an efficient and green synthesis route. This process eliminates the use of traditional harmful hydrofluoric acid or fluorine gas, and is easy to scale up.

[0022] (2) The present invention uses liquid-phase precipitation to uniformly mix the elements of the precursor. Through the synergistic design of morphology control (porous structure, nano-sizing) and in-situ carbon coating, the high voltage characteristics of fluoride are retained while overcoming the problems of capacity decay and cycle stability. The capacity can reach 166 mAh / g at 0.1C rate.

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0024] Figure 1 The image shows a SEM image of the precursor Na3FeF6 prepared in Example 1.

[0025] Figure 2 The image shows a SEM image of the cathode material NaFeF3@C prepared in Example 1.

[0026] Figure 3 SEM image of Na3FeF6, the precursor prepared in Example 5.

[0027] Figure 4 This is a SEM image of the positive electrode material NaFeF3@C prepared in Example 5.

[0028] Figure 5 XRD patterns of the cathode material NaFeF3@C prepared in Examples 1 and 5 (comparison with JCPDS standard card).

[0029] Figure 6 The charge-discharge curve of the positive electrode material NaFeF3@C prepared in Example 5 is shown.

[0030] Figure 7 The charge-discharge curves of the cathode material prepared in Comparative Example 4 are shown. Detailed Implementation

[0031] The invention will now be explained in more detail through specific embodiments. However, it should be understood that the specific functional details disclosed in this specification should not be construed as limiting, but rather as the basis for the claims and as a representative basis for teaching those skilled in the art to employ the invention in different ways in any suitable detailed embodiment.

[0032] The cathode materials prepared in the following embodiments and comparative examples of this invention were tested for capacity and cycle performance using the following methods: Battery Assembly: The prepared sodium-ion battery positive electrode material was ground through a 200-mesh sieve, then mixed with SP (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 8:1:1. NMP (N-methylpyrrolidone) was added and stirred into a slurry, which was then coated onto aluminum foil. After drying, stamping, and pressing, the sodium-ion battery positive electrode material was formed. Using metallic sodium as the negative electrode, glass fiber (Whatman GF / D brand) as the separator, and a NaPF6 (sodium hexafluorophosphate) PC (propylene carbonate) / EMC (ethyl methyl carbonate) solution as the electrolyte, CR2025 button batteries were assembled in an argon-filled glove box and charge-discharge tests were conducted. Under conditions of a current density of 100 mAh / g and a voltage range of 2-4.5V, the capacity and cycle performance were tested after two cycles at 0.1C, followed by testing at 1C.

[0033] Example 1

[0034] A method for preparing a sodium-ion battery cathode material includes the following steps: (1) Dissolve Fe2(SO4)3 in deionized water to prepare a 0.2 mol / L solution, and label it as solution A; dissolve NaF in deionized water to prepare a 0.6 mol / L solution, add dilute H2SO4 to adjust the pH to 2.0, and label it as solution B.

[0035] (2) A solution and B solution (volume ratio 1:1) were added dropwise into the coprecipitation reactor at the same feed rate of 2.5 mL / min. The pH of the reaction system was controlled to be 2-3. The mixture was heated and stirred (stirring speed was 400 rpm) and reacted at 70℃ for 4 h to generate Na3FeF6 precipitate. The precipitate obtained from the reaction was separated by washing and filtration and placed in a vacuum oven to dry at 90℃ for 12 h to obtain the precursor Na3FeF6.

[0036] (3) The precursor Na3FeF6 prepared above was wet-milled and mixed with citric acid. The mass ratio of precursor Na3FeF6 to citric acid was 1:0.2. The mixture was sintered at 500°C for 6 hours under N2 atmosphere. After washing and drying, sodium-ion battery cathode material NaFeF3@C was obtained.

[0037] Material characterization: Figure 1 Here is a SEM image of the precursor Na3FeF6 prepared in Example 1. Figure 2 This is a SEM image of the positive electrode material NaFeF3@C prepared in Example 1; combined with Figures 1-2 It can be seen that the present invention synthesized a precursor Na3FeF6 with a cubic morphology and a cathode material NaFeF3@C.

[0038] Examples 1-1 to 1-3 The preparation method is the same as in Example 1, except that the molar concentrations of Fe2(SO4)3 solution and NaF solution are adjusted to make the molar ratio of Fe2(SO4)3 to NaF 1:3 to 1:12, and its effect on the performance of the sodium-ion battery cathode material is tested, as shown in Table 1. Table 1 .

[0039] Analysis: As shown in Table 1, the specific capacity and cycle stability of the material exhibit a significant regularity with the change in molar ratio. With the NaF ratio increasing to 1:6, the specific capacity significantly improves, while the capacity retention rate also increases substantially, demonstrating optimal overall performance. Further increasing the NaF ratio slightly decreases both the specific capacity and capacity retention rate. When the molar ratio is further increased to 1:12, the specific capacity further decreases. This result indicates that a molar ratio of Fe2(SO4)3 to NaF of 1:6 can form a crystal structure or morphology more favorable for sodium ion insertion / extraction. Due to the more complete reaction at this ratio, the resulting cathode material has more suitable lattice parameters, fewer structural defects, and a more stable electrode-electrolyte interface, thus achieving higher specific capacity and better cycle stability.

[0040] Example 2

[0041] This embodiment mainly examines the effect of adding the reducing agent ascorbic acid on the performance of the cathode material of sodium-ion batteries.

[0042] A method for preparing a sodium-ion battery cathode material includes the following steps: (1) Dissolve Fe2(SO4)3 in deionized water to prepare a 0.2 mol / L solution, add ascorbic acid as a reducing agent, and the mass ratio of Fe2(SO4)3 to reducing agent is 3:1 to form solution A; dissolve NaF in deionized water to prepare a 1.2 mol / L solution, add dilute H2SO4 to adjust the pH to 2.0 to form solution B.

[0043] (2) A and B solutions (volume ratio 1:1) were added dropwise into the coprecipitation reactor at the same feed rate of 2.5 mL / min. The pH of the reaction system was controlled to be 2-3. The mixture was heated and stirred (stirring speed was 400 rpm) and reacted at 70℃ for 4 h to generate Na3FeF6 precipitate. The precipitate obtained from the reaction was separated by washing and filtration and placed in a vacuum oven to dry at 90℃ for 12 h to obtain the precursor Na3FeF6.

[0044] (3) The precursor Na3FeF6 was wet-milled and mixed with citric acid. The mass ratio of Na3FeF6 to citric acid was 1:0.2. The mixture was sintered at 500°C for 6 hours under N2 atmosphere. After washing and drying, the sodium-ion battery cathode material NaFeF3@C was obtained.

[0045] Examples 2-1 to 2-2 The preparation method is the same as in Example 2, except that the mass ratio of Fe2(SO4)3 to the reducing agent ascorbic acid was adjusted, and its effect on the performance of the sodium-ion battery cathode material was tested, as shown in Table 2. Table 2 .

[0046] Analysis: As shown in Table 2, the addition of the reducing agent ascorbic acid has a certain impact on the specific capacity of the cathode material. As the proportion of the reducing agent decreases, the specific capacity and capacity retention rate decrease significantly. When the mass ratio of Fe2(SO4)3 to the reducing agent is 3:1, the reducing agent is relatively sufficient, which is more conducive to promoting the full reduction reaction of Fe elements, forming a structure or morphology more suitable for sodium ion insertion / extraction, thereby improving the specific capacity of the material. However, during cycling, some structural damage can easily lead to poor cycling stability.

[0047] Example 3

[0048] This embodiment mainly investigates the effect of the mass ratio of Na3FeF6 to citric acid on the performance of the cathode material of sodium-ion batteries.

[0049] (1) Dissolve Fe2(SO4)3 in deionized water to prepare a 0.2 mol / L solution, and label it as solution A; dissolve NaF in deionized water to prepare a 1.2 mol / L solution, add dilute H2SO4 to adjust the pH to 2.0, and label it as solution B.

[0050] (2) A solution and B solution (volume ratio 1:1) were added dropwise into the coprecipitation reactor at the same feed rate of 2.5 mL / min. The pH of the reaction system was controlled to be 2-3. The mixture was heated and stirred (stirring speed was 400 rpm) and reacted at 70℃ for 4 h to generate Na3FeF6 precipitate. The precipitate obtained from the reaction was separated by washing and filtration and placed in a vacuum oven to dry at 90℃ for 12 h to obtain the precursor Na3FeF6.

[0051] (3) The precursor Na3FeF6 prepared above was wet-milled and mixed with citric acid. The mass ratio of precursor Na3FeF6 to citric acid was 1:0.05. The mixture was sintered at 500°C for 6 hours under N2 atmosphere. After washing and drying, sodium-ion battery cathode material NaFeF3@C was obtained.

[0052] Example 3-1 The preparation method is the same as in Example 3, except that the mass ratio of Na3FeF6 to citric acid was adjusted, and its effect on the performance of the sodium-ion battery cathode material was tested, as shown in Table 3. Table 3 .

[0053] Analysis: The amount of citric acid used has a certain impact on the specific capacity and cycle stability of the material. Experiments show that when the mass ratio of Na3FeF6 to citric acid is 1:0.05, the amount of citric acid is moderate, which may be more conducive to its dispersion or modification effect, forming smaller and more uniformly distributed particles, increasing the contact area between the active material and the electrolyte, promoting the rapid intercalation and deintercalation of sodium ions, and may also form a stable protective layer on the material surface, reducing structural damage during cycling, thereby improving the specific capacity and cycle stability of the cathode material.

[0054] Example 4

[0055] This embodiment mainly examines the effect of sintering temperature on the performance of sodium-ion battery cathode materials.

[0056] (1) Dissolve Fe2(SO4)3 in deionized water to prepare a 0.2 mol / L solution, and label it as solution A; dissolve NaF in deionized water to prepare a 1.2 mol / L solution, add dilute H2SO4 to adjust the pH to 2.0, and label it as solution B.

[0057] (2) A solution and B solution (volume ratio 1:1) were added dropwise into the coprecipitation reactor at the same feed rate of 2.5 mL / min. The pH of the reaction system was controlled to be 2-3. The mixture was heated and stirred (stirring speed was 400 rpm) and reacted at 70℃ for 4 h to generate Na3FeF6 precipitate. The precipitate obtained from the reaction was separated by washing and filtration and placed in a vacuum oven to dry at 90℃ for 12 h to obtain the precursor Na3FeF6.

[0058] (3) The precursor Na3FeF6 was wet-milled and mixed with citric acid. The mass ratio of Na3FeF6 to citric acid was 1:0.05. The mixture was sintered at 800°C for 6 hours under N2 atmosphere. After washing and drying, the sodium-ion battery cathode material NaFeF3@C was obtained.

[0059] Examples 4-1 to 4-3 The preparation method is the same as in Example 4, except that the sintering temperature is adjusted to 500–800°C, and its effect on the performance of the sodium-ion battery cathode material is tested, as shown in Table 4. Table 4 .

[0060] Analysis: As shown in Table 4, the specific capacity and cycle stability of the material generally increase with increasing temperature, and the performance varies at different temperatures. Example 4 exhibits better basic electrochemical performance. With increasing temperature, the specific capacity and capacity retention rate significantly improve, and Example 4-3 shows the best overall performance. Sintering temperature has a crucial impact on the crystal structure and microstructure of the cathode material. Lower temperatures may lead to insufficient crystallinity, resulting in more defects or lattice distortions in the crystal structure. Although these temperatures can provide some sodium ion insertion / extraction sites, the structural stability is limited, thus the specific capacity and cycle retention rate are relatively low. In Example 4-3, when the temperature reaches 700℃, the suitable temperature promotes full crystal growth, increases crystallinity, and forms a more complete and stable lattice structure. This increases the number of active sites and enhances the structure's ability to resist volume changes during cycling, making sodium ion migration smoother and significantly improving cycle stability. When the temperature is further increased to 800℃, the cycle retention rate decreases significantly. Excessive temperature may cause NaFeF3 particles to be over-sintered, increasing the grain size. At the same time, the carbon layer derived from citric acid loses its coating effect due to high-temperature decomposition or excessive graphitization, resulting in a longer sodium ion diffusion path. Furthermore, the material is prone to structural collapse during cycling, ultimately leading to a significant decrease in both specific capacity and cycle retention rate.

[0061] Example 5

[0062] This embodiment mainly examines the effect of sintering time on the performance of sodium-ion battery cathode materials.

[0063] (1) Dissolve Fe2(SO4)3 in deionized water to prepare a 0.2 mol / L solution, and label it as solution A; dissolve NaF in deionized water to prepare a 1.2 mol / L solution, add dilute H2SO4 to adjust the pH to 2.0, and label it as solution B.

[0064] (2) A solution and B solution (volume ratio 1:1) were added dropwise into the coprecipitation reactor at the same feed rate of 2.5 mL / min. The pH of the reaction system was controlled to be 2-3. The mixture was heated and stirred (stirring speed was 400 rpm) and reacted at 70℃ for 4 h to generate Na3FeF6 precipitate. The precipitate obtained from the reaction was separated by washing and filtration and placed in a vacuum oven to dry at 90℃ for 12 h to obtain the precursor Na3FeF6.

[0065] (3) The precursor Na3FeF6 was wet-milled and mixed with citric acid. The mass ratio of Na3FeF6 to citric acid was 1:0.05. The mixture was sintered at 700°C for 8 hours under N2 atmosphere. After washing and drying, the sodium-ion battery cathode material NaFeF3@C was obtained.

[0066] Examples 5-1 to 5-2 The preparation method is the same as in Example 5, except that the sintering time is adjusted to 6–10 h, and its effect on the performance of the sodium-ion battery cathode material is tested, as shown in Table 5. Table 5 .

[0067] analyze: As shown in Table 5, the specific capacity and cycle stability of the materials showed a trend of first increasing and then decreasing over time, with Example 5 exhibiting the best overall performance. The shorter sintering time resulted in insufficient crystallinity, with numerous defects or incompletely reacted intermediates in the crystal structure, limiting structural stability and thus leading to relatively low specific capacity and cycle retention. In Example 5, the sufficient reaction time promoted full crystal growth, forming a more complete and ordered crystal lattice structure with a more uniform particle size distribution, significantly improving cycle stability.

[0068] Comparative Example 1 FeCl₂·4H₂O powder and PVP powder were added to anhydrous ethylene glycol solution at a mass ratio of 1:5 to obtain a mixed liquid. A 20 wt% NaOH solution was added to the mixed liquid to obtain an iron-containing precursor solution. The iron-containing precursor solution was placed in a reaction vessel and subjected to a hydrothermal reaction in a vacuum drying oven to obtain a ferric oxide suspension. The ferric oxide suspension was centrifuged and dried to obtain ferric oxide powder. The ferric oxide powder was reduced at 800℃ to obtain elemental iron powder. The elemental iron powder was then subjected to a gas-phase fluorination reaction to obtain micron-sized iron trifluoride.

[0069] Comparative Example 2 Equimolar amounts of NaF and transition metal fluoride MF2 (M = Mn, Fe, Ni) were mixed and ball-milled at 600 rpm under argon atmosphere using 200 zirconia balls with a diameter of 4 mm to obtain perovskite-type sodium metal fluoride NaMF3 (M = Fe, Mn, Ni).

[0070] Comparative Example 3 100g each of sodium carbonate, ferric acetate, and hydrofluoric acid were weighed out in a molar ratio of Na:Fe:F = 3:1:6 and dissolved separately in deionized water at 50℃ to prepare sodium carbonate solution, ferric acetate solution, and hydrofluoric acid solution. The three solutions were simultaneously added to a reactor under mechanical stirring at 500 rpm and reacted for 6 hours. After standing for 12 hours, the mixture was filtered, washed, and vacuum dried at 80℃ for 15 hours to obtain sodium hexafluoroferrate material. Glucose and sodium hexafluoroferrate material were added to a ball mill jar at a mass ratio of 3:7, along with deionized water. The mixture was ball-milled at 700 rpm for 3 hours, and then sintered at 750℃ for 10 hours under an argon atmosphere to obtain amorphous carbon-coated sodium fluoroferrate material.

[0071] Comparative Example 4 0.1 mol of oxalic acid was dissolved in 100 mL of water to obtain an oxalic acid solution; 0.075 mol of sodium hydroxide was dissolved in 20 mL of water to obtain a sodium hydroxide solution. The oxalic acid and sodium hydroxide solutions were mixed and heated to 150 °C with stirring. Simultaneously, 0.3 mol of ferric hydroxide was slowly added. After reacting for 5 hours, the mixture was removed, cooled to room temperature, and filtered to obtain a sodium trioxalatoferrate solution. 0.1 mol of ammonium bifluoride was added to the sodium trioxalatoferrate solution, and the mixture was heated to 90 °C with stirring for 5 hours. The water was then evaporated to obtain a precursor. The precursor was sintered at 800 °C for 10 hours under a nitrogen atmosphere to obtain sodium fluoroferrate, the positive electrode material for the battery.

[0072] Performance testing Table 6 summarizes the capacity, average voltage, and cycle data of the cathode materials prepared in Comparative Examples 1-4 and applied to sodium-ion batteries, tested according to the aforementioned methods.

[0073] Table 6 .

[0074] Summarize: (1) Comparison of Embodiment 1 and Embodiment 5 of the present invention The precursor Na3FeF6 and NaFeF3@C carbon-coated cathode material prepared in Example 5 were subjected to X-ray diffraction and electron microscopy tests, such as... Figures 3-5 As shown, comparing the precursor Na3FeF6 and cathode material NaFeF3@C prepared in Example 5 with those in Example 1, it can be seen that the precursor Na3FeF6 prepared in Example 5 has a crystal structure with better crystallinity, no impurities, and more regular particle morphology than the precursor Na3FeF6 prepared in Example 1.

[0075] Comparing the electrochemical performance data of the cathode material in Example 5 with that in Example 1, it can be seen that the cathode material prepared in Example 5 has significantly higher specific capacity and capacity retention than that in Example 1. This also confirms that Example 5 obtained a crystal structure with better crystallinity, no impurity phases, and more regular particle morphology.

[0076] (2) Comparison of the present invention with Comparative Examples 1-4 As can be seen from the electrochemical performance data of Comparative Example 1, the discharge specific capacity is much lower than that of Example 5 of this invention. This is because the micron-sized particles synthesized by hydrothermal synthesis still maintain their micron-sized dimensions after high-temperature reduction. During gas-phase fluorination, it is difficult for the interior of the large particles to be completely fluorinated, resulting in heterogeneous products. In addition, iron ions dissolve in the electrolyte, migrate to the negative electrode, and destroy the SEI, causing a continuous loss of the active material mass.

[0077] The electrochemical performance data of Comparative Example 2 show that the discharge specific capacity is lower than that of Example 5 of this invention, and the capacity retention after cycling is poor. This is because although mechanical ball milling can achieve raw material mixing, it is difficult to precisely control the formation of the perovskite structure. The ball milling process may introduce lattice distortion or amorphous regions, leading to obstruction of sodium ion transport channels. Furthermore, fluoride cathodes generally suffer from low electronic conductivity; the material synthesized in this comparative example does not incorporate conductive carbon, resulting in extremely low electron transport efficiency and inability to fully utilize its capacity.

[0078] The electrochemical performance data of Comparative Example 3 show that the capacity retention rate is much lower than that of Example 5 of this invention. This is because the precipitation pH of the raw materials varies greatly during the co-precipitation process, and simultaneous feeding easily leads to local concentration fluctuations, generating impurities and hindering the sodium ion transport path. At the same time, uneven mixing during wet ball milling of carbon coating results in discontinuous carbon layer distribution and insufficient conductivity.

[0079] The electrochemical performance data from Comparative Example 4 show that the discharge specific capacity is lower than that of Example 5 of this invention. The 800℃ high-temperature sintering far exceeds the material's thermal stability limit; excessive sintering leads to excessively large particles and a reduced specific surface area, extending the sodium ion diffusion path. Although carbon coating was also performed, the resulting carbon layer has a low degree of graphitization and weak conductivity, which is detrimental to capacity utilization.

[0080] A comparison of the data in Tables 1-6 shows that the battery with the cathode material prepared in Example 5 of this invention has a significantly higher specific capacity than that of Comparative Examples 1-4. This is because the cathode material prepared in this invention is carbon-coated, resulting in more stable performance during battery cycling and thus a higher capacity. Furthermore, replacing oxygen anions with fluoride ions significantly increases the average voltage of the material due to the inductive effect. The sodium fluoride battery cathode material prepared in this invention utilizes a redox-active metal, and combined with the inductive effect of fluoride ions, the material's crystal structure exhibits higher stability. This synergistic effect from multiple aspects contributes to a higher operating voltage, resulting in better electrochemical performance. Therefore, Example 5 demonstrates superior performance in both voltage and capacity compared to Comparative Examples 1-4, leading to a significantly higher specific capacity.

[0081] In summary, this invention provides a two-step process to prepare a high-purity precursor Na3FeF6 through acidic co-precipitation, followed by carbothermic reduction with a carbon source at a relatively low temperature (≤700℃) to generate a carbon-coated NaFeF3@C composite material. This process achieves high capacity, operating voltage, and excellent cycle stability in the cathode material. Furthermore, the synthesis method is green and simple, facilitating large-scale industrial production. When the sodium fluoride cathode material of this invention is used in sodium-ion half-cells, the battery exhibits stable high-current cycling performance and a higher discharge platform compared to sodium-based crystalline oxide cathode materials. It has high practical application value and can be used in energy storage devices such as portable electronic devices, energy storage power supplies, and communication base stations.

[0082] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a sodium fluoride-based positive electrode material for a battery, characterized in that, Includes the following steps: (1) Dissolve iron salt in deionized water to obtain solution A; mix sodium fluoride and inorganic acid evenly in water to obtain solution B; (2) A and B solutions were fed into a coprecipitation reactor at the same flow rate and reacted under heating and stirring. The pH value of the reaction system was monitored in real time. The precursor Na3FeF6 was obtained after washing and drying. (3) The precursor Na3FeF6 prepared in step (2) is mixed with a carbon source and sintered under an inert atmosphere to obtain NaFeF3@C, a sodium fluoride battery cathode material coated with a carbon layer.

2. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (1), the iron salt is selected from one or a mixture of two or more of FeCl3, Fe(NO3)3, and Fe2(SO4)3 in any proportion.

3. A method for preparing a sodium fluoride battery cathode material according to claim 1 or 2, characterized in that: In step (1), ascorbic acid, a reducing agent, is added to the iron salt, and the mass ratio of iron salt to ascorbic acid is 3:1 to 5:

1.

4. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (1), the inorganic acid is selected from one or more of hydrochloric acid, sulfuric acid, and nitric acid in any proportion.

5. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (1), the concentration of solution A is 0.2 mol / L-0.4 mol / L, and the concentration of sodium fluoride in solution B is 0.6 mol / L-1.2 mol / L.

6. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (2), the coprecipitation reaction is carried out at a feed rate of 2-10 mL / min, a reaction temperature of 50-80℃, a reaction time of 3-6 h, a pH of 2-4, and a stirring speed of 300-1000 rpm. After the reaction, the washing is performed 1-5 times, the drying temperature is 80-120℃, and the drying time is 8-24 h.

7. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (3), the carbon source is selected from one or a mixture of two or more of citric acid, glucose, and ascorbic acid in any proportion.

8. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (3), the mass ratio of the precursor Na3FeF6 to the carbon source is 1:0.05 to 0.

2.

9. The method for preparing a sodium fluoride battery cathode material according to claim 1, characterized in that: In step (3), the inert atmosphere is at least one of N2 and Ar, the sintering temperature is 300-800℃, and the sintering time is 2-10 hours.

10. The application of a sodium fluoride cathode material prepared by any one of the methods described in claims 1-9 in the preparation of sodium ion batteries.

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