A doped modified sodium ferric sulfate positive electrode material and a preparation method and application thereof

Abstract

The application belongs to the field of batteries, and relates to a doped modified sodium iron sulfate positive electrode material and a preparation method and application thereof. 2+2x+z Fe 2‑x‑y M y (SO4)3F z @C, 0<=x<=0.5, 0<=y<=0.15, 0 The anion and the anion group (SO4) in the precursor prepared by the method are combined with transition metal atoms (M) through competitive combination, so that the crystal structure is distorted, the electrode not only keeps the voltage platform, and has excellent specific capacity performance. The doped modified sodium iron sulfate material prepared by the patent has the advantages of rich raw material resources, low cost, high working voltage and the like, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of batteries and relates to cathode materials for sodium-ion batteries. Background Technology

[0002] With the development of the new energy industry, the development and application of energy storage batteries have gradually attracted attention. Among them, lithium-ion batteries have become the mainstream of research and development due to their excellent performance. However, due to the scarcity of lithium resources, the cost is increasing day by day. Therefore, developing new low-cost, high-performance energy storage battery systems has become an urgent problem to be solved by academia and industry. Sodium, due to its similar chemical properties to lithium and its wide resource distribution, has become one of the preferred battery systems for energy storage. Among them, polyanionic sodium iron sulfate cathode materials have attracted widespread attention due to their high operating voltage and low cost. However, its relatively low specific capacity limits the further application of this material. Therefore, the development of high-performance sodium iron sulfate cathode materials is of great significance for the practical application of low-cost sodium-ion batteries.

[0003] Element doping has been widely used in the synthesis and modification of polyanionic material systems. Among these, F atoms and anionic groups (XO) are particularly important. Y Through competitive bonding with transition metal atoms (M), the resulting crystal structure distortion alters the ion transport pathway and diffusion barrier. While the high electronegativity of fluorine (F) often contributes to an increase in discharge voltage plateau, in the sodium ferric sulfate system, SO4 exhibits a higher electron-withdrawing ability. Excessive introduction of F can actually lead to a decrease in voltage plateau, contradicting the initial goal of preparing high-energy-density cathode materials. Therefore, the amount of F introduced warrants further investigation.

[0004] Here, by introducing fluorides and sulfates during the precursor preparation process to facilitate the miscibility between cations and anions, cations enter the sodium ferric sulfate crystal and replace some Fe atoms. Anions, in turn, competitively bind with transition metal atoms (M) to the anionic groups (SO4), causing crystal structure distortion. The resulting lattice expansion promotes the insertion and extraction of sodium ions, ensuring that the electrode not only maintains its voltage plateau but also exhibits excellent specific capacity. The prepared doped sodium ferric sulfate material, as a cathode material for sodium-ion batteries, demonstrates excellent reversible specific capacity, possesses advantages such as abundant raw material resources, low cost, and high operating voltage, and further enhances energy density, showing great promise for industrial applications.

[0005] Application number CN117525314A discloses a doped sodium ferric sulfate compound, a cathode material, its preparation method, and its application (calculated to show that F atoms account for more than 4.76% of the total formula); application number CN117361635A discloses a doped sodium ferric fluorosulfate compound, a cathode material, its preparation method, and a sodium-ion battery (calculated to show that F atoms account for 6.67% of the total formula). Although the above patents use F ion doping to improve the sodium storage performance of the material, the improvement effect is not significant. On the contrary, excessive F atoms and anionic groups (SO4) competitively combine with transition metal atoms (M), which can cause severe crystal structure distortion, resulting in poor cycle stability.

[0006] Application number CN117832453A discloses a sodium ferrous sulfate / carbon composite cathode material, its preparation method, and its application in sodium-ion batteries. Although the patent uses a sol-gel method to construct a highly efficient carbon conductive network, it cannot change the ion transport capacity at the crystal structure level, and still exhibits a low specific capacity.

[0007] Therefore, we propose a doped modified sodium iron sulfate / carbon composite cathode material with good energy density, its preparation method, and a sodium-ion battery. Summary of the Invention

[0008] This invention proposes a doped and modified sodium ferric sulfate cathode material, its preparation method, and its application, which effectively expands the sodium ion transport channels in the bulk phase, giving the electrode excellent specific capacity performance.

[0009] The technical solution of this invention is implemented as follows: A doped and modified sodium ferric sulfate cathode material, with the chemical formula Na 2+2x+z Fe 2-x-y M y (SO4)3F z @C, where 0≤x≤0.5, 0≤y≤0.15, 0<z≤0.6, M is the doping metal element, the total doping amount of element M is 0-8% of the molar mass of iron, the total doping amount of element F is 1-20% of the molar mass of sulfate, and the carbon content accounts for 3-15% of the mass of the cathode material. The doping metal element is selected from one or more elements such as Mg, Zn, Ni, Ca, Ba, and Co.

[0010] The preparation method of the above-mentioned doped sodium ferric sulfate cathode material is as follows: (1) A certain proportion of conductive carbon material powder is dispersed in a solvent, and subjected to ultrasonic and cell pulverization treatment to obtain a dispersion with a concentration of 2-6 g / L. (2) Add a certain proportion of organic carbon source to the above dispersion, stir to dissolve, and then use ammonia water to adjust the pH of the obtained solution to obtain a precursor solution, wherein the concentration of organic carbon source in the precursor solution is 7-14 g / L. (3) Add anhydrous sodium sulfate, iron source, doped metal source and fluorine source to the above precursor solution, stir to dissolve, and then dry the mixed solution under inert gas protection by oil bath heating to obtain precursor 1. (4) The above precursor is dried and then pulverized by ball milling to obtain precursor 2. (5) After grinding the precursor 2 obtained in step (4) evenly, it is placed in a tube furnace under an inert atmosphere for calcination to obtain the doped sodium iron sulfate cathode material.

[0011] In step (1) above, the conductive carbon material powder is one or more of graphene oxide, carbon black particles, and carbon nanotubes; in step (1), the solvent component is one or more of deionized water, anhydrous ethanol, isopropanol, and ethylene glycol.

[0012] In step (2) above, the organic carbon source is one or more of the following: citric acid monohydrate, pyrrole, ascorbic acid, hydroquinone, oleic acid, stearic acid, chitosan quaternary ammonium salt, sodium dodecylbenzenesulfonate, and octadecyltrimethylammonium bromide.

[0013] In step (3) above, the iron source is one or more of FeSO4·7H2O, FeSO4·H2O, and anhydrous FeSO4; the doping metal source in step (3) is one or more of MgSO4, ZnSO4, NiSO4, CaSO4, BaSO4, CoSO4 and their hydrates; and the fluorine source in step (3) is one or more of NaF, ammonium fluoride, and sodium hexafluorophosphate.

[0014] In steps (2) and (3) above, the molar ratio of anhydrous sodium sulfate, iron source, doped metal source, fluorine source and organic carbon source is 1:1-2:0-0.16:0.2-1.2:0.1-0.5; the stirring time in steps (2) and (3) is 0.1-4 h and the rotation speed is 300-800 rpm.

[0015] In step (4) above, the drying method is vacuum heat drying, the temperature of vacuum heat drying is 50-120 ℃, and the drying time is 5-48 h.

[0016] In steps (3) and (5) above, the inert atmosphere is nitrogen, argon, or a mixture of argon and hydrogen. The calcination process in step (5) above involves heating to 300-400℃ at a heating rate of 1-3℃ / min and holding at that temperature for 8-48 h. The carbon content of the doped sodium iron sulfate cathode material obtained in step (5) is 3-15%.

[0017] The aforementioned doped sodium iron sulfate cathode material is the cathode of a sodium-ion battery, the metallic sodium or sodium-ion intercalating / deintercalating active material is the anode of a sodium-ion battery, the separator is a modified cellulose acetate separator, polyethylene, polypropylene microporous membrane, glass fiber separator or a composite separator thereof, and the electrolyte is a soluble sodium salt organic solution.

[0018] The above-mentioned sodium-ion battery positive and negative electrode sheets are obtained by coating the current collector with a slurry obtained by uniformly mixing the positive and negative electrode materials with a conductive agent, a binder and a dispersant. The positive electrode current collector is aluminum foil and the negative electrode current collector is aluminum foil or copper foil.

[0019] The aforementioned sodium-ion battery negative electrode is metallic sodium or an active material capable of intercalating / deintercalating sodium ions, including carbon materials, metal sulfides, metal oxides, alloy compounds, etc.; the sodium-ion battery conductive agent is one or more highly conductive carbon materials such as acetylene black, SuperP, or graphite; the binder is one or more of polytetrafluoroethylene, polyvinylidene fluoride, sodium carboxymethyl cellulose, polyacrylic acid, or styrene-butadiene rubber; the electrolyte soluble sodium salt organic solution is obtained by dissolving sodium salt in an organic solvent, wherein the sodium salt is one or more of sodium hexafluorophosphate, sodium perchlorate, and sodium trifluoromethanesulfonate, and the organic solvent is one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, diethylene glycol dimethyl ether, 1,3-cyclopentanediol, ethylene glycol dimethyl ether, and triethylene glycol dimethyl ether.

[0020] The above-mentioned sodium battery positive electrode sheet, sodium-ion battery positive electrode sheet and negative electrode sheet are all made by coating the slurry obtained by uniformly mixing the above conductive agent, binder and dispersant in a weight ratio of 0.5-0.9:0.05-0.4:0.05-0.4 onto the current collector, and the current collector is aluminum foil or copper foil.

[0021] The outer casing of the aforementioned sodium-ion battery or sodium battery is made of organic plastic, aluminum shell, aluminum-plastic film, stainless steel, or composite materials thereof.

[0022] The sodium-ion battery or sodium battery described above can be in the shape of a button, cylindrical, or square.

[0023] The present invention has the following beneficial effects: Compared with the disclosed preparation methods of sodium iron sulfate cathode materials (a doped sodium iron sulfate compound, cathode material, its preparation method and application, application number: CN117525314A), (sodium fluorosulfate iron compound, cathode material, its preparation method and sodium-ion battery, application number: CN117361635A), (sodium iron sulfate / carbon composite cathode material, preparation method and its application in sodium-ion battery, application number: CN117832453A), the synthesis method disclosed in this invention has the following advantages: 1. Component advantages: ① In the comparative application CN117525314A, the disclosed general formula structure is Na x Fe y M z (SO4) a F, where 0 < x ≤ 3, 0 < y ≤ 2, 0 < z ≤ 0.5, 0 < a ≤ 3. After conversion, the proportion of F atoms in the whole general formula is greater than 4.76%. ② In the comparative application CN117525314A, the disclosed general formula structure is Na3Fe y M z (SO4)2F, where y + z = 1, 0 < z ≤ 0.5. After conversion, the proportion of F atoms in the whole general formula is 6.67%. ③ In the comparative application CN117832453A, the disclosed general formula structure is Na 2+2x Fe 2-x (SO4)3 / C, where 0 ≤ x ≤ 0.7.

[0024] In this patent, the general formula structure is Na 2+2x+z Fe 2-x-y M y (SO4)3F z @C, 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.15, 0 < z ≤ 0.6, and the proportion of F atoms in the whole general formula is less than 3.00%. In the sodium iron sulfate system, the competitive binding of F atoms with the anion group (SO4) to the transition metal atom (M) brings crystal structure distortion, which helps to shorten the ion transport path and reduce the ion diffusion energy barrier. However, compared with F, SO4 has a stronger electron-withdrawing ability, and the excessive introduction of F element will lead to the decline of the voltage plateau, which goes against the original intention of preparing high-energy-density cathode materials. Therefore, this invention strictly controls the introduction of F element, and the prepared F-doped materials do not show the phenomenon of median voltage decline, while it is not mentioned in the comparative patent.

[0025] 2. Process optimization: The solvents used in this patent are one or more of deionized water, anhydrous ethanol, isopropanol, and ethylene glycol. Combinations of different solvents can adjust solubility, thereby controlling the distribution of anions and cations, and ultimately affecting the state of the product. This patent, by adjusting different types of organic carbon sources, on the one hand, prevents subsequent Fe... 2+ The components act as oxidants (e.g., ascorbic acid) and complexes, binding with metal cations to control the uniform formation of precursors (e.g., citric acid monohydrate). Additionally, some components contribute to the uniform distribution of insoluble substances such as carbon materials (e.g., chitosan quaternary ammonium salts). This patent uses ammonia to adjust the pH of the solvent, which not only affects the formation of metal complexes but also influences crystallization habits and controls particle size growth at the macroscopic level. An inert atmosphere is used in this patent to prevent the oxidation of ferrous ions. Vacuum drying is used to remove the solvent involved in the sol-gel. Ball milling is used to pulverize the material to obtain the precursor material. Calcination in a tube furnace under an inert atmosphere yields the doped sodium ferric sulfate cathode material.

[0026] 3. Mechanism and Principle: The selected Mg, Ca, Ba, Zn, Co, and Ni atoms have similar physicochemical properties to Fe atoms, and therefore tend to occupy Fe sites, which helps stabilize the cell phase transition during charge and discharge. F atoms, due to their strong electronegativity, tend to occupy S sites and repel SO4 tetrahedra, leading to cell expansion. This facilitates the insertion and extraction of sodium ions, resulting in excellent specific capacity performance of the electrode. However, excessive introduction of dopants will reduce the sodium storage activity and operating voltage of the material (Mg, Ca, Ba, Zn, Co, and Ni are all inactive / poorly active elements, and F's electron-withdrawing ability is less than that of SO4), which is detrimental to the application of the electrode material. Therefore, the amount of metal dopants introduced is controlled to be 0-8% of the molar amount of iron, and the amount of fluorine dopants introduced is 1-20% of the molar amount of sulfate.

[0027] 4. Performance improvements: ① High specific capacity: Compared with the example in application CN117525314A, the synthesized Na3Fe 0.5 Ni 0.5 (SO4)2F, Na3Fe 0.6 Ni 0.4 (SO4)2F, Na3Fe 0.7 Ni 0.3(SO4)2F material exhibits low specific capacity at 0.1C (less than 100 mAh / g), while the electrode materials synthesized in this invention all exhibit excellent specific capacity (greater than 100 mAh / g at 0.1C). ② Operating voltage: Compared to F, SO4 has a stronger electron-withdrawing ability. Excessive introduction of F can lead to a drop in the voltage plateau, which contradicts the original intention of preparing high-energy-density cathode materials. Therefore, this invention strictly controls the introduction of F, and the prepared F-doped materials do not exhibit a drop in median voltage, a phenomenon not mentioned in the comparative patent. ③ Cycle stability: Compared to the example in application CN117525314A, the synthesized Na3Fe... 0.5 Ni 0.5 (SO4)2F, Na3Fe 0.6 Ni 0.4 (SO4)2F, Na3Fe 0.7 Ni 0.3 The (SO4)2F material exhibited poor cycle stability, with capacity retention of less than 90% after 100 cycles. This is likely due to crystal structure distortion caused by excessive F element introduction. In this invention, the synthesized electrode material achieved a capacity retention of 95.8% after 90 cycles, demonstrating better cycle stability.

[0028] In summary, compared with the prior art, the present invention performs excellently in terms of reversible specific capacity, increasing the capacity by more than 10% without reducing the median voltage and cycle stability, thus meeting the needs of industrial applications.

[0029] The above-mentioned beneficial effects have all been verified through experiments, and detailed comparative results are provided in the specific implementation methods. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is the X-ray diffraction (XRD) pattern of the sodium ferric sulfate cathode material prepared in Example 1.

[0032] Figure 2 This is the EDS image of the 5% fluorine-doped sodium iron sulfate cathode material prepared in Example 5.

[0033] Figure 3 This is a cycle performance diagram of the 5% fluorine-doped sodium iron sulfate cathode material in Example 5.

[0034] Figure 4 This is a charge-discharge curve of a sodium-ion battery using the nickel- and fluorine co-doped sodium iron sulfate cathode material in Example 11. Detailed Implementation

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0037] This application prepares a doped and modified sodium ferric sulfate cathode material with the chemical formula: Na. 2+2x+z Fe 2-x-y M y (SO4)3F z @C, where 0≤x≤0.5, 0≤y≤0.15, 0<z≤0.6, preferably 0.1≤x≤0.35, 0≤y≤0.1; M represents the doping metal element, with the total doping amount of M being 0-8% of the molar mass of iron, the total doping amount of F being 1-20% of the molar mass of sulfate, preferably 3-20% of the molar mass of sulfate, and the carbon content being 3-15% of the mass of the cathode material. The doping metal element is selected from one or more elements such as Mg, Zn, Ni, Ca, Ba, and Co.

[0038] This application also provides a method for preparing the above-mentioned doped and modified sodium ferric sulfate cathode material, the steps of which are as follows: (1) A certain proportion of conductive carbon material powder is dispersed in a solvent, and ultrasonic and cell-pulverizing treatment is performed to obtain a dispersion with a concentration of 2-6 g / L; the conductive carbon material powder is one or more of graphene oxide, carbon black particles, and carbon nanotubes; the solvent is one or more of deionized water, anhydrous ethanol, isopropanol, and ethylene glycol. This application uses deionized water and a mixture of deionized water and ethanol as examples for proof.

[0039] (2) Add a certain proportion of organic carbon source to the above dispersion so that the concentration of organic carbon source in the precursor solution is 7-14 g / L. After stirring and dissolving, adjust the pH of the obtained solution with ammonia water to obtain the precursor solution. The organic carbon source is one or more of citric acid monohydrate, pyrrole, ascorbic acid, hydroquinone, oleic acid, stearic acid, chitosan quaternary ammonium salt, sodium dodecylbenzenesulfonate, and octadecyltrimethylammonium bromide. This application uses citric acid monohydrate, ascorbic acid, sodium dodecylbenzenesulfonate, oleic acid, chitosan quaternary ammonium salt, and octadecyltrimethylammonium bromide as examples for proof.

[0040] (3) Add anhydrous sodium sulfate, iron source, doped metal source and fluorine source to the above precursor solution, stir to dissolve, and then dry the resulting mixed solution by heating in an oil bath under inert gas protection to obtain precursor 1; the iron source is one or more of FeSO4••7H2O, FeSO4••H2O and anhydrous FeSO4; the doped metal source is one or more of MgSO4, ZnSO4, NiSO4, CaSO4, BaSO4, CoSO4 and their hydrates; the fluorine source is one or more of NaF, ammonium fluoride and sodium hexafluorophosphate. The molar ratio of anhydrous sodium sulfate, iron source, doped metal source, fluorine source and organic carbon source in steps (2) and (3) is 1:1-2:0-0.16:0.2-1.2:0.1-0.5; further, this ratio can be 1:1-2:0-0.1:0.2-1.2:0.2-0.4.

[0041] In steps (2) and (3), the stirring time is 0.1-4 h and the rotation speed is 300-800 rpm. In this application, the experiment is carried out at 500 rpm and stirring for 10 min. The purpose of stirring is to make the solution mix evenly and will not affect the reaction product.

[0042] (4) The above precursor is dried. After it is completely dried, the material is pulverized by ball milling to obtain precursor 2. The drying method is vacuum heat drying. The temperature of vacuum heat drying is 50-120 ℃ and the drying time is 5-48 h. The purpose of vacuum drying is to remove the moisture in the precursor without damaging its properties. It can be achieved within the above range.

[0043] (5) After grinding the precursor 2 obtained in step (4) evenly, it is placed in a tube furnace under an inert atmosphere for calcination to obtain the doped sodium iron sulfate cathode material. The calcination process is to heat to 300-400℃ at a heating rate of 1-3℃ / min and hold at that temperature for 8-48h; this application takes a calcination temperature of 350℃ and a holding time of 12h as an example. In fact, when the temperature fluctuates by 50℃, the synthesis temperature of sodium iron sulfate cathode is 300-400℃, and the sodium iron sulfate precursor (Na2Fe(SO4)2·4H2O) is below 300℃. During the heating process, lattice water will still be released, leading to a crystal transformation and the formation of sodium iron sulfate cathode material.

[0044] The carbon content of the doped sodium ferric sulfate cathode material obtained in step (5) is 3-15%.

[0045] The following specific embodiments illustrate the sodium ferric sulfate cathode material and its preparation method of this application.

[0046] Example 1 The undoped sodium ferric sulfate cathode material prepared in this embodiment was used as a control example to analyze its performance. The steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0047] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4) and 0.68 g of anhydrous sodium sulfate (Na2SO4), add them to the above dispersion, and stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain undoped sodium ferric sulfate cathode material Na. 2.4 Fe 1.8 (SO4)3@C.

[0048] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the sodium ferric sulfate cathode material prepared in this embodiment. Figure 1 The prepared cathode material shows good matching with the PDF standard card, indicating the successful synthesis of the material.

[0049] Example 2 This embodiment prepared a 10% fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0050] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.1 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain 10% fluorine-doped sodium iron sulfate cathode material Na. 2.7 Fe 1.8 (SO4)3F 0.3 @C.

[0051] Example 3 This embodiment prepared a 15% fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0052] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.15 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain 15% fluorine-doped sodium iron sulfate cathode material Na. 2.85 Fe 1.8 (SO4)3F 0.45 @C.

[0053] Example 4 This embodiment prepared a 20% fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0054] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.2 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain 20% fluorine-doped sodium iron sulfate cathode material Na3Fe 1.8 (SO4)3F 0.6 @C.

[0055] Example 5 This embodiment prepared a 5% fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0056] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain 5% fluorine-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0057] Figure 2 The image shown is the EDS diagram of the 5% fluorine-doped sodium iron sulfate cathode material prepared in this embodiment. Figure 2 It can be seen that the fluorine element in the prepared cathode material is uniformly distributed.

[0058] Example 6 In this embodiment, a 5% fluorine-doped sodium iron sulfate cathode material 1 was prepared in a mixed solvent system. The steps are as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in a mixture of 20 mL of deionized water and 10 mL of anhydrous ethanol. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0059] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4) and 0.68 g of anhydrous sodium sulfate (Na2SO4), add them to the above dispersion, and stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain a mixed solvent system, 5% fluorine-doped sodium iron sulfate cathode material 1 Na 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0060] Example 7 In this embodiment, a 5% fluorine-doped sodium iron sulfate cathode material 2 was prepared in a mixed solvent system. The steps are as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in a mixture of 15 mL of deionized water and 15 mL of anhydrous ethanol. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0061] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4) and 0.68 g of anhydrous sodium sulfate (Na2SO4), add them to the above dispersion, and stir under an argon atmosphere at 80 ℃ and 450 rpm to obtain a black dry gel. Then transfer it to an 80 ℃ vacuum oven to dry for 12 h. After it is completely dried, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350 ℃ at a heating rate of 1 ℃ / min and hold for 12 h to obtain a mixed solvent system and 5% fluorine-doped sodium iron sulfate cathode material 2Na2SO4. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0062] Example 8 In this embodiment, a 5% fluorine-doped sodium iron sulfate cathode material 3 was prepared in a mixed solvent system. The steps are as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in a mixture of 10 mL of deionized water and 20 mL of anhydrous ethanol. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0063] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4) and 0.68 g of anhydrous sodium sulfate (Na2SO4), add them to the above dispersion, and stir under an argon atmosphere at 80 ℃ and 450 rpm to obtain a black dry gel. Then transfer it to an 80 ℃ vacuum oven to dry for 12 h. After it is completely dried, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350 ℃ at a heating rate of 1 ℃ / min and hold for 12 h to obtain a mixed solvent system and 5% fluorine-doped sodium iron sulfate cathode material 3Na. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0064] Example 9 This embodiment prepared a magnesium and fluorine co-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0065] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), 0.035 g of magnesium sulfate heptahydrate (MgSO4*7H2O), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain magnesium and fluorine co-doped sodium ferric sulfate cathode material Na. 2.55Fe 1.76 Mg 0.04 (SO4)3F 0.15 @C.

[0066] Example 10 This embodiment prepared a nickel- and fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0067] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), 0.038 g of nickel sulfate hexahydrate (NiSO4*6H2O), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain the nickel-fluorine co-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.76 Ni 0.04 (SO4)3F 0.15 @C.

[0068] Figure 4 This is a charge-discharge curve of a sodium-ion battery using nickel and fluorine co-doped sodium iron sulfate cathode material prepared in this embodiment. As can be seen from the figure, the prepared co-doped cathode has good capacity performance, with a discharge specific capacity of 107.3 mAh / g and a median voltage of 3.61 V at a current density of 10 mA / g.

[0069] Example 11 This embodiment prepared a calcium and fluorine co-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0070] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), 0.025 g of calcium sulfate dihydrate (CaSO4*2H2O), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain calcium and fluorine co-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.76 Ca 0.04 (SO4)3F 0.15 @C.

[0071] Example 12 This embodiment prepared a barium and fluorine co-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate and 0.07 g of ascorbic acid and stir at 500 rpm for 10 min. Then add 5 drops of 40% ammonia water to form a precursor solution.

[0072] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), 0.034 g of barium sulfate (BaSO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain the barium and fluorine co-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.76 Ba 0.04 (SO4)3F 0.15 @C.

[0073] Example 13 This embodiment prepared a fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate, 0.07 g of ascorbic acid and 0.1 g of sodium dodecylbenzenesulfonate. Stir at 500 rpm for 10 min and then add 5 drops of 40% ammonia water to form a precursor solution.

[0074] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain fluorine-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0075] Example 14 This embodiment prepared a fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate, 0.07 g of ascorbic acid and 0.11 g of oleic acid. Stir at 500 rpm for 10 min and then add 5 drops of 40% ammonia water to form a precursor solution.

[0076] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain fluorine-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0077] Example 15 This embodiment prepared a fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate, 0.07 g of ascorbic acid and 0.065 g of chitosan quaternary ammonium salt. Stir at 500 rpm for 10 min and then add 5 drops of 40% ammonia water to form a precursor solution.

[0078] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain fluorine-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.8 (SO4)3F0.15 @C.

[0079] Example 16 This embodiment prepared a fluorine-doped sodium ferric sulfate cathode material, and the steps were as follows: (1) Weigh 0.09 g of graphene oxide powder and 0.03 g of CNT and disperse them in 30 mL of deionized water. Stir at 500 rpm for 60 min and sonicate for 60 min. Then add 0.168 g of citric acid monohydrate, 0.07 g of ascorbic acid and 0.157 g of octadecyltrimethylammonium bromide. Stir at 500 rpm for 10 min and then add 5 drops of 40% ammonia water to form a precursor solution.

[0080] (2) Weigh 1.09 g of anhydrous ferrous sulfate (FeSO4), 0.68 g of anhydrous sodium sulfate (Na2SO4), and 0.05 g of NaF and add them to the above dispersion. Stir under an argon atmosphere at 80°C and 450 rpm to obtain a black dry gel. Then transfer it to an 80°C vacuum oven to dry for 12 h. After it is completely dry, transfer it to a ball mill jar and ball mill at 500 r / min for 4 h to obtain the product precursor. After grinding the precursor evenly, transfer it to a magnetic boat and place it in a tube furnace under an argon atmosphere. Heat the furnace to 350°C at a heating rate of 1°C / min and hold for 12 h to obtain fluorine-doped sodium iron sulfate cathode material Na. 2.55 Fe 1.8 (SO4)3F 0.15 @C.

[0081] Application test cases Using sodium ferric sulfate cathode material prepared in Examples 1 and 5-8 as the cathode active material, it was mixed with Super PLi and polyvinylidene fluoride in a mass ratio of 8:1:1, with 1-methyl-2-pyrrolidone as the dispersant. The mixture was thoroughly mixed to form a slurry and coated onto aluminum foil. After vacuum drying at 120 °C, a 12 mm diameter cathode sheet was obtained. A sodium metal sheet (16 mm diameter) was used as the anode, and a glass fiber membrane (Whatman GF / D) was used as the separator. 1 M NaClO4 dissolved in EC:PC (volume ratio 1:1) (5 wt.% FEC additive) was used as the electrolyte. A stainless steel shell was used as the outer casing, and the cells were assembled into CR2025 button batteries. Charge-discharge tests were conducted at room temperature within a potential range of 2.0-4.5 V. The test results are shown in rows 1-5 of Table 1.

[0082] The sodium ferric sulfate cathode material prepared in Examples 1-5 and 9-16 was used as the cathode active material, and mixed with Ketjen black and polyvinylidene fluoride in a mass ratio of 8:1:1. 1-Methyl-2-pyrrolidone was used as a dispersant. The mixture was thoroughly mixed to form a slurry and then coated onto aluminum foil. The battery preparation was the same as in Example 1. Charge-discharge tests were conducted at room temperature within a potential range of 2.0-4.5 V. The test results are shown in rows 6-18 of Table 1. After 90 cycles at a current density of 100 mA / g, the capacity retention was 95.8%. (e.g.) Figure 3 ).

[0083] Table 1: Electrochemical performance tests in the embodiments of this application As shown in Table 1, the sodium iron sulfate cathode materials prepared by adjusting the F / M doping ratio, the type of conductive carbon, the solvent system, and the organic carbon source showed no decrease in median voltage compared with the control samples (using super p Li as the conductive agent, the capacity was 82.7 mAh / g, and the median voltage was 3.53 V; using Ketjen Black as the conductive agent, the capacity was 90.1 mAh / g, and the median voltage was 3.42 V).

[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A doped and modified sodium ferric sulfate cathode material, with the chemical formula Na. 2+2x+z Fe 2-x-y M y (SO4)3F z @C, where 0≤x≤0.5, 0≤y≤0.15, 0<z≤0.6, M is a doped metal element, the total doping amount of element M is 0-8% of the molar mass of iron, the total doping amount of element F is 1-20% of the molar mass of sulfate, and the mass ratio of carbon in sodium iron sulfate cathode material is 3-15%.

2. The doped and modified sodium ferric sulfate cathode material according to claim 1, characterized in that: M is one or more of Mg, Zn, Ni, Ca, Ba, or Co.

3. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 1 or 2, characterized in that, The steps are as follows: (1) Disperse the conductive carbon material in a solvent, and then ultrasonically pulverize it to obtain a dispersion. Then add an organic carbon source, stir to dissolve, and adjust the pH to obtain a precursor solution. (2) Add anhydrous sodium sulfate, iron source, doped metal source and fluorine source to the precursor solution in step (1), stir to dissolve, and then heat in an oil bath under inert gas protection and dry to obtain the precursor. (3) After the precursor in step (2) is dried, crushed and ground evenly, it is calcined in an inert atmosphere to obtain the doped and modified sodium iron sulfate cathode material.

4. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 3, characterized in that: In step (1), the mass ratio of conductive carbon material to organic carbon source is 1:1.5-4, and the pH is 4-5.

5. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 3, characterized in that: The conductive carbon material is one or more of graphene oxide, carbon black, and carbon nanotubes; the solvent is one or more of deionized water, anhydrous ethanol, isopropanol, and ethylene glycol; and the organic carbon source is one or more of citric acid monohydrate, pyrrole, ascorbic acid, hydroquinone, oleic acid, stearic acid, chitosan quaternary ammonium salt, sodium dodecylbenzenesulfonate, and octadecyltrimethylammonium bromide.

6. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 4 or 5, characterized in that: In step (2), the molar ratio of anhydrous sodium sulfate, iron source, doped metal source, fluorine source and organic carbon source is 1:1-2:0-0.16:0.2-1.2:0.1-0.

5.

7. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 6, characterized in that: The iron source is one or more of FeSO4·7H2O, FeSO4·H2O, and anhydrous FeSO4; the doped metal source is one or more of MgSO4, ZnSO4, NiSO4, CaSO4, BaSO4, CoSO4, and their hydrates; and the fluorine source is one or more of NaF, ammonium fluoride, and sodium hexafluorophosphate.

8. The method for preparing the doped and modified sodium ferric sulfate cathode material according to claim 7, characterized in that, The calcination conditions are: temperature 300-400℃, heating rate 1-3℃ / min, and holding time 8-48 h.

9. The application of the sodium ferric sulfate cathode material according to claim 1 or 2 in the preparation of sodium-ion batteries.

10. A sodium-ion battery containing the sodium iron sulfate cathode material as described in claim 1 or 2.

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