Sodium iron sulfate positive electrode material and preparation method thereof, positive electrode sheet, sodium ion battery and electrical equipment

Abstract

The present invention provides a sodium ferric sulfate positive electrode material and a preparation method thereof, a positive electrode sheet, a sodium ion battery and an electrical device, and relates to the field of new energy technology. The sodium ferric sulfate positive electrode material utilizes elements such as Al, Mn, Ca, and Co to achieve doping modification of the sodium ferric sulfate positive electrode material, thereby improving ionic conductivity and electronic conductivity, and thus improving cycle stability. At the same time, Ti2O3 is utilized to coat the material, which is beneficial to reducing its water absorption and improving its air stability. The carbon coating layer can achieve complete coating of the material, improve the electronic conductivity of the material, alleviate phase change, and improve the stability of the material. The preparation method uses hematite as a raw material to directly synthesize and prepare the sodium ferric sulfate positive electrode material, effectively utilizing the associated Al, Mn, Ca, Co, and Ti elements in the ore, eliminating the ore impurity removal process, shortening the process flow, and effectively reducing production costs.

Description

Technical Field

[0001] The present application relates to the field of new energy technology, and in particular to a sodium iron sulfate positive electrode material and a preparation method thereof, a positive electrode sheet, a sodium ion battery and electrical equipment. Background Art

[0002] Currently, the most studied cathode materials for sodium-ion batteries include transition metal layered oxides, Prussian blue analogs, and polyanionic compounds. Among them, polyanionic materials have advantages over other types of materials, such as good cycling stability and excellent thermal stability.

[0003] High-energy-density iron-based polyanionic cathode materials have attracted widespread attention in recent years. As an excellent, low-cost iron-based cathode material, Na2Fe2(SO4)3 exhibits a reversible plateau of up to 3.8V, a rare occurrence among polyanionic materials, and a theoretical specific capacity of 121 mAh / g, laying a solid foundation for high energy density. From an electrochemical performance perspective, Na2Fe2(SO4)3 is a promising cathode material with a long cycle life.

[0004] However, the existing Na2Fe2(SO4)3 material has poor material stability and will absorb water and dissociate when exposed to air. In addition, its inherent poor electrical conductivity makes its industrial application difficult. In addition, the existing methods for preparing Na2Fe2(SO4)3 are based on mixing finished sodium sources, sulfur sources, and iron sources in a certain way and then sintering them into the required positive electrode material. The procurement of raw materials such as sodium sources, iron sources, and carbon sources for synthesis and preparation has increased costs to a certain extent. Currently, there are no reports on the synthesis and preparation of sodium iron sulfate positive electrode materials from ore raw materials. Summary of the Invention

[0005] The purpose of this application is to provide a sodium iron sulfate positive electrode material and a preparation method thereof, a positive electrode sheet, a sodium ion battery, and an electrical device to solve the above problems.

[0006] To achieve the above objectives, this application adopts the following technical solutions:

[0007] The first aspect of the present application provides a sodium ferric sulfate positive electrode material, the chemical formula of which is Na2Fe 2-x-y-z-n Co x Al y Mn z Ca n (SO4)3@Ti2O3@C, where, 0<x+y+z+n<0.5, 0.0005<x<0.485, 0.0005<y<0.485, 0.0005<z<0.485, 0.0005<n<0.485, Na2Fe 2-x-y-z-nCo x Al y Mn z Ca n (SO4)3 is the core, Ti2O3 is the first coating layer covering the core, and C is the second coating layer covering the first coating layer.

[0008] The second aspect of the present application further provides a method for preparing a sodium iron sulfate positive electrode material, comprising:

[0009] dissolving hematite with an acid solution to obtain a first solution;

[0010] mixing the first solution with a reducing carbon source, a sodium source, and a sulfur source to obtain a second solution;

[0011] drying the second solution to obtain a precursor;

[0012] The precursor is calcined to obtain a sodium iron sulfate positive electrode material.

[0013] The third aspect of the present application further provides a sodium ferric sulfate positive electrode material, which is prepared by the preparation method of the sodium ferric sulfate positive electrode material according to the second aspect.

[0014] The fourth aspect of the present application further provides a positive electrode sheet comprising the sodium iron sulfate positive electrode material of the first aspect or the third aspect.

[0015] The fifth aspect of the present application further provides a sodium ion battery, comprising the positive electrode sheet according to the fourth aspect.

[0016] The sixth aspect of the present application also provides an electrical device, including the sodium ion battery of the fifth aspect mentioned above.

[0017] Compared with the prior art, the advantages of this application include:

[0018] The sodium ferric sulfate positive electrode material of the present application utilizes elements such as Al, Mn, Ca, and Co to achieve doping modification of the sodium ferric sulfate positive electrode material, forming a multi-entropy structure, thereby improving the ionic conductivity and electronic conductivity of the sodium ferric sulfate positive electrode material, and thus improving the cycle stability of the sodium ferric sulfate positive electrode material. At the same time, Ti2O3 is used to coat the material, which is beneficial to reducing its water absorption and improving its air stability. The carbon coating layer of the sodium ferric sulfate positive electrode material of the present application can achieve complete coating of the material, improve the electronic conductivity of the material, alleviate phase change, and improve the stability of the material.

[0019] The preparation method of the sodium ferric sulfate positive electrode material provided in the present application uses hematite as raw material to directly synthesize the sodium ferric sulfate positive electrode material. After a one-step acid dissolution, the material is doped and modified by trace impurity elements in the ore, effectively utilizing the associated Al, Mn, Ca, and Co elements in the ore to dope and modify it. At the same time, Ti2O3 is coated on the material, improving the environmental stability of the material, eliminating the ore impurity removal process, shortening the process flow, and effectively reducing production costs. This preparation method introduces a reducing carbon source during the acid dissolution process. On the one hand, it can reduce the metal elements to the corresponding valence state. On the other hand, it coats the material during the subsequent drying and sintering process, avoiding direct contact between the electrolyte and the material, and improving the electrochemical performance of the material.

[0020] The positive electrode sheet and sodium ion battery prepared by the sodium iron sulfate positive electrode material of the present application have good cycle performance and high capacity retention rate; the electrical performance of the electrical equipment is excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0022] Figure 1 This is a process flow chart of the preparation method of the sodium iron sulfate positive electrode material of Example 1;

[0023] Figure 2 This is a charge and discharge curve of a sodium ion battery prepared using the sodium iron sulfate cathode material of Example 1;

[0024] Figure 3 This is a cycle diagram of a sodium ion battery prepared from the sodium iron sulfate positive electrode material of Example 1;

[0025] Figure 4 This is an SEM image of the sodium iron sulfate positive electrode material of Example 2;

[0026] Figure 5 This is an SEM image of the sodium iron sulfate positive electrode material of Comparative Example 3;

[0027] Figure 6 This is the XRD pattern of the sodium iron sulfate positive electrode material of Example 3. DETAILED DESCRIPTION

[0028] As used herein:

[0029] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0030] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0031] The first aspect of the present application provides a sodium ferric sulfate positive electrode material, the chemical formula of which is Na2Fe 2-x-y-z-n Co x Al y Mn z Ca n (SO4)3@Ti2O3@C, where, 0<x+y+z+n<0.5, 0.0005<x<0.485, 0.0005<y<0.485, 0.0005<z<0.485, 0.0005<n<0.485, Na2Fe 2-x-y-z-n Co x Al y Mn z Ca n (SO4)3 is the core, Ti2O3 is the first coating layer covering the core, and C is the second coating layer covering the first coating layer.

[0032] wherein x, y, z, and n may be, for example, 0.0007, 0.001, 0.003, 0.005, 0.007, 0.01, 0.012, 0.013, 0.014, 0.02, 0.05, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or 0.45, respectively.

[0033] In some embodiments, the average thickness of the second coating layer is 0.5 to 5 nm, for example, it can be 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm or 5 nm; the mass of the Ti element in the first coating layer relative to the sodium ferric sulfate positive electrode material is 500 to 4000 ppm, for example, it can be 500 ppm, 700 ppm, 900 ppm, 1000 ppm, 1200 ppm, 1500 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2500 ppm, 2700 ppm, 2800 ppm, 3000 ppm, 3500 ppm, 4000 ppm or any value between 500 and 4000 ppm.

[0034] The second aspect of the present application further provides a method for preparing a sodium iron sulfate positive electrode material, comprising:

[0035] dissolving hematite with an acid solution to obtain a first solution;

[0036] mixing the first solution with a reducing carbon source, a sodium source, and a sulfur source to obtain a second solution;

[0037] drying the second solution to obtain a precursor;

[0038] The precursor is calcined to obtain a sodium iron sulfate positive electrode material.

[0039] The preparation method of sodium ferric sulfate cathode material provided in this application uses hematite as a raw material to directly synthesize the sodium ferric sulfate cathode material, shortening the process flow and effectively reducing production costs. This preparation method introduces a reducing carbon source during the acid dissolution process, which can not only reduce the metal elements to their corresponding valence states, but also achieves coating of the material during the subsequent drying and sintering processes, avoiding direct contact between the electrolyte and the material.

[0040] In some embodiments, the acid solution comprises a mixture of one or more of phosphoric acid, perchloric acid, dichromic acid, iodic acid, hydroiodic acid, hydrosulfuric acid, boric acid, hydrochloric acid, sulfuric acid, sulfurous acid, nitric acid, hydrofluoric acid, and acetic acid, wherein nitric acid and sulfuric acid are not used simultaneously.

[0041] The concentration of sulfuric acid is 0.01 to 6 mol / L, for example, 0.01 mol / L, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L or 6 mol / L; the concentration of hydrofluoric acid is 0.01 to 0.4 mol / L, for example, 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L or 0.4 mol / L.

[0042] In some embodiments, the reaction temperature for dissolving the hematite with an acid solution is 0°C to 40°C, for example, the reaction time for dissolving the hematite with an acid solution is 6 to 12 hours, for example, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, and the pH of the first solution is 4 to 11, for example, the pH of the first solution can be 4, 5, 6, 7, 8, 9, 10 or 11.

[0043] In some embodiments, the reducing carbon source includes any one or more of citric acid, oxalic acid, oleic acid, pyromellitic acid, glucose, ethylene glycol, polyethylene glycol, formaldehyde, ascorbic acid, and pyrrole. The reducing carbon source of the present application not only reduces the metal elements of the hematite, but also later achieves complete coating of the sodium ferric sulfate positive electrode material, thereby improving the electronic conductivity of the material, alleviating phase transitions, and improving the stability of the material.

[0044] In some embodiments, the sodium source includes any one or more of sodium carbonate, sodium sulfate, sodium oxalate, sodium acetate, sodium alginate, and sodium citrate.

[0045] In some embodiments, the sulfur source includes any one or more of sulfuric acid, sulfur dioxide, sulfur trioxide, sodium sulfate, sodium sulfite, ammonium sulfate, and sodium thiosulfate.

[0046] The sodium source and the sulfur source can be the same substance. For example, if both the sodium source and the sulfur source are sodium sulfate, then only sodium sulfate needs to be added. If the sodium source and the sulfur source are different substances, then the sodium source and the sulfur source can be added separately.

[0047] Wherein, when the sulfur source is sulfuric acid, if the acid solution used in the dissolving step includes sulfuric acid, sulfuric acid may not be added in the step of adding the sulfur source.

[0048] In some embodiments, the first solution is mixed with the reducing carbon source, sodium source, and sulfur source at a speed of 100 to 1600 rpm, preferably 500 to 1600 rpm, and more preferably 1000 to 1500 rpm; the mixing time is 20 to 600 min, preferably 100 to 500 min, and more preferably 200 to 400 min.

[0049] The mixing method includes any one or more of mechanical stirring, double-screw stirring, and sand milling. Sand milling can not only play a role in mixing, but also can control the particle size of the material.

[0050] In some embodiments, the solid content of the second solution is 30%-70%, for example, 30%, 40%, 50%, 60% or 70%.

[0051] In some embodiments, the drying comprises any one of vacuum drying, freeze drying, and spray drying.

[0052] The vacuum drying temperature is 100-120° C., for example, 100° C., 105° C., 110° C., 115° C. or 120° C., and the drying time is 8-12 h, for example, 8 h, 9 h, 10 h, 11 h or 12 h.

[0053] The freeze-drying temperature is -40 to -60°C, for example, -40°C, -45°C, -50°C, -55°C or -60°C, and the freeze-drying time is 5 to 10 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours.

[0054] The inlet air temperature of the spray drying is 200-300°C, and the inlet air temperature of the spray drying can be, for example, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C or 300°C; the outlet air temperature of the spray drying is 90-150°C, and the outlet air temperature of the spray drying can be, for example, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C or 150°C; the fan power of the spray drying is 30-50HZ, for example, 30HZ, 35HZ, 40HZ, 45HZ or 50HZ; the peristaltic speed of the spray drying is 60-150rpm, for example, 60rpm, 70rpm, 80rpm, 90rpm, 100rpm, 110rpm, 120rpm, 130rpm, 140rpm or 150rpm.

[0055] In a preferred embodiment, the calcination includes a first stage calcination and a second stage calcination.

[0056] In some embodiments, the temperature of the one-stage calcination is 10-200°C, for example, it can be 10°C, 50°C, 80°C, 100°C, 120°C, 150°C, 170°C or 200°C; the time of one-stage calcination is 1-24h, for example, it can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 12h, 15h, 16h, 18h, 20h, 22h, 23h or 24h; the heating rate is 1-10°C / min, for example, it can be 1°C / min, 3°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min or 10°C / min.

[0057] In some embodiments, the temperature of the second-stage calcination is 200-500°C, for example, it can be 200°C, 250°C, 300°C, 350°C, 400°C, 450°C or 500°C; the time of the second-stage calcination is 4-24h, for example, it can be 4h, 5h, 6h, 8h, 9h, 10h, 12h, 15h, 18h, 20h, 22h or 24h; the heating rate is 1-8°C / min, for example, it can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min or 8°C / min.

[0058] In some embodiments, the calcination is carried out in a protective gas, wherein the protective gas comprises any one or more of argon, nitrogen, hydrogen, ammonia, and methane. The flow rate of the protective gas is 0-30 m / s. 3 / h.

[0059] The oxygen content in the second-stage calcination process is 10-100 ppm, for example, 10 ppm, 20 ppm, 30 ppm, 40 ppm, 50 ppm, 60 ppm, 70 ppm, 80 ppm, 90 ppm or 100 ppm.

[0060] The third aspect of the present application further provides a sodium ferric sulfate positive electrode material, which is prepared by the preparation method of the sodium ferric sulfate positive electrode material according to the second aspect.

[0061] The sodium ferric sulfate positive electrode material obtained by this preparation method directly utilizes elements such as Al, Mn, Ca, and Co present in the ore to achieve doping modification of the sodium ferric sulfate positive electrode material, forming a multi-entropy structure, thereby improving the ionic conductivity and electronic conductivity of the sodium ferric sulfate positive electrode material, and thus improving the cycle stability of the sodium ferric sulfate positive electrode material.

[0062] The fourth aspect of the present application further provides a positive electrode sheet comprising the sodium iron sulfate positive electrode material of the first aspect or the third aspect.

[0063] The fifth aspect of the present application further provides a sodium ion battery, comprising the positive electrode sheet according to the fourth aspect.

[0064] The sodium ion battery prepared from the sodium ferric sulfate positive electrode material obtained by the preparation method of the sodium ferric sulfate positive electrode material of the present application has good cycle performance and high capacity retention rate.

[0065] The sixth aspect of the present application also provides an electrical device, including the sodium ion battery of the fifth aspect mentioned above.

[0066] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0067] The hematite used in the following examples and comparative examples was purchased from 1688 online, and the content of each element is shown in Table 1.

[0068] Table 1 Element content of hematite

[0069] Iron content (wt%) 58.76％ Manganese content (wt%) 4.25％ Calcium content (wt%) 2.04％ Cobalt content (wt%) 4.17％ Aluminum content (wt%) 1.55％ Titanium content (wt%) 0.28％

[0070] Example 1

[0071] Example 1 provides a sodium iron sulfate positive electrode material and a preparation method thereof, please refer to Figure 1 , Figure 1 The process flow chart of the preparation method of the sodium iron sulfate positive electrode material of the present application includes the following steps:

[0072] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain a mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 15L of 0.2mol / L dilute sulfuric acid solution, heat to 40°C, react for 8h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0073] Step 2: Add 0.25 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 50%, control the sand milling time to 400 min, and the sand milling speed to 1500 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0074] Step 3: Control the spray peristaltic speed to 80 rpm, the spray drying air inlet temperature to 210° C., and the air outlet temperature to 105° C. to obtain a spray-dried material, which is a calcined precursor.

[0075] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 100°C at a heating rate of 5°C / min for 3 hours, then heated to 350°C at a heating rate of 2°C / min for 8 hours, naturally cooled to room temperature, and sieved through a 300-mesh sieve after air flow crushing to obtain the sodium iron sulfate positive electrode material of Example 1.

[0076] The chemical formula of the sodium ferric sulfate positive electrode material obtained in Example 1 is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 (SO4)3@Ti2O3@C, the carbon layer thickness is 2nm, the mass of Ti element in the Ti2O3 coating layer is 900ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 10.8*10 -5 S / cm, and the ion migration coefficient is 6.7*10 -7 cm 2 / s, as shown in Table 2. The titanium element in hematite is insoluble in acid and cannot form subsequent sulfate. Therefore, a Ti2O3 coating forms on the surface of the sodium ferric sulfate cathode material during the sintering process. The thickness of the carbon coating can be measured by TEM, and the amount of Ti2O3 coating can be determined by XRF testing.

[0077] The resulting positive electrode material was then prepared into a positive electrode slurry, which was then prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet were then assembled into a button cell. The button cell was assembled according to the following steps: 1) The slurry was stirred for 6 hours in NMP solvent at a ratio of active material: conductive agent: binder = 8:1:1; 2) The slurry was coated onto 16u aluminum foil to a thickness of 120μm and baked in a vacuum oven at 105°C for 4 hours; 3) The electrode sheet was rolled and cut into small discs; 4) The battery was assembled in the following order: positive electrode shell - positive electrode sheet - separator - sodium sheet - negative electrode shell, and 5 drops of sodium electrolyte were added; 5) The battery was allowed to stand for 12 hours before being tested in a cabinet.

[0078] Figure 2 This is a charge and discharge curve of a sodium ion battery prepared from the sodium iron sulfate positive electrode material of Example 1. The platform voltage of the material can reach 3.75V; Figure 3 This is a cycle diagram of the sodium ion battery prepared from the sodium iron sulfate positive electrode material of Example 1. At a rate of 1C, the discharge capacity reaches 90 mAh / g after 400 cycles, and the capacity retention rate reaches more than 94%.

[0079] Example 2

[0080] Example 2 provides a sodium iron sulfate positive electrode material and a preparation method thereof, please refer to Figure 1 , including the following steps:

[0081] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 30L of 0.2mol / L nitric acid solution, heat to 25°C, react for 6h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0082] Step 2: Add 0.3 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate, 1 mol of ammonium sulfate, and deionized water at the same time, control the solid content to 60%, control the sand milling time to 600 min, and the sand milling speed to 600 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0083] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0084] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 150°C at a heating rate of 5°C / min for 3 hours, then heated to 400°C at a heating rate of 2°C / min for 8 hours, cooled naturally to room temperature, and crushed by vibration and passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Example 2. Figure 4 This is the SEM image of the sodium iron sulfate positive electrode material of Example 2.

[0085] The chemical formula of the sodium ferric sulfate positive electrode material obtained in Example 2 is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 2.4nm, the mass of Ti element in the Ti2O3 coating layer is 925ppm relative to the sodium ferric sulfate positive electrode material, and the electronic conductivity is 11.4*10 -5 S / cm, and the ion migration coefficient is 7.3*10 -7 cm 2 / s, as shown in Table 2.

[0086] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is based on Example 1. The platform voltage of the material can reach 3.77V; at a 1C rate, the discharge capacity after 400 cycles reaches 88mAh / g, and the capacity retention rate reaches more than 88%.

[0087] Example 3

[0088] Example 3 provides a sodium iron sulfate positive electrode material and its preparation method, please refer to Figure 1, including the following steps:

[0089] Step 1: Take 152g of hematite, crush it in air, and sieve to obtain mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 15L of 0.4mol / L acetic acid solution, heat to 30°C, react for 12h, adjust the solution pH to ~7, filter out insoluble impurities, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0090] Step 2: Add 0.45 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate and 1 mol of ammonium sulfate, control the sand milling time to 200 min, the sand milling speed to 1100 r, and the particle size of the material after sand milling is controlled to be <200 nm.

[0091] Step 3: Control the vacuum drying temperature to 120° C. and the vacuum drying time to 10 h to obtain a vacuum dried material, which is the calcined precursor.

[0092] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 200°C at a heating rate of 5°C / min for 3 hours, then heated to 500°C at a heating rate of 2°C / min for 8 hours, cooled naturally to room temperature, and passed through a 150-mesh sieve after air flow crushing to obtain the sodium ferric sulfate positive electrode material of Example 3. The chemical formula of the obtained sodium ferric sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 3nm, the mass of Ti element in the Ti2O3 coating layer is 938ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 10.5*10 -5 S / cm, and the ion migration coefficient is 5.8*10 -7 cm 2 / s, as shown in Table 2, XRD pattern Figure 6 shown.

[0093] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is based on Example 1. The platform voltage of the material can reach 3.74V; at a rate of 0.2C, the discharge capacity after 100 cycles reaches 95mAh / g, and the capacity retention rate reaches more than 88%.

[0094] Example 4

[0095] Example 4 provides a sodium iron sulfate positive electrode material and its preparation method, please refer to Figure 1 , including the following steps:

[0096] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 7.5L of 0.4mol / L dilute sulfuric acid solution, heat to 20°C, react for 10h, adjust the solution pH to ~4, filter out insoluble impurities, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0097] Step 2: Add 0.4 mol of citric acid to the solution until the solution turns light green, then add sodium carbonate and adjust the pH to 7. Control the sand milling time to 100 min and the sand milling speed to 900 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0098] Step 3: Control the vacuum drying temperature to 120° C. and the vacuum drying time to 10 h to obtain a vacuum dried material, which is the calcined precursor.

[0099] Step 4: The precursor was calcined in a methane atmosphere, first heated to 100°C at a heating rate of 5°C / min for 3 hours, then heated to 350°C at a heating rate of 2°C / min for 8 hours, cooled naturally to room temperature, and sieved through a 300-mesh sieve after air flow crushing to obtain the sodium ferric sulfate positive electrode material of Example 4. The chemical formula of the obtained sodium ferric sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 2.9nm, the mass of Ti element in the Ti2O3 coating layer is 915ppm relative to the sodium ferric sulfate positive electrode material, and the electronic conductivity is 11.1*10 -5 S / cm, and the ion migration coefficient is 6.0*10 - 7 cm 2 / s, as shown in Table 2.

[0100] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is based on Example 1. The platform voltage of the material can reach 3.74V; at a 1C rate, the discharge specific capacity after 100 cycles reaches 84mAh / g, and the capacity retention rate reaches more than 85%.

[0101] Example 5

[0102] The sodium ferric sulfate positive electrode material and its preparation method of Example 5 are different from those of Example 1 in that they are synthesized using purchased raw materials such as a sulfur source, a sodium source, a carbon source, and a dopant. The preparation method of the sodium ferric sulfate positive electrode material of Example 5 includes the following steps:

[0103] Step 1: Take 1.6 mol of ferrous sulfate heptahydrate, 0.11 mol of cobalt sulfate, 0.045 mol of aluminum sulfate, 0.12 mol of manganese sulfate, 0.08 mol of calcium sulfate, 0.0045 mol of Ti2O3, 0.25 mol of oxalic acid, and then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 60%, control the sand milling time to 150 minutes, control the sand milling speed to 1000 rpm, and control the particle size of the material after sand milling to be less than 200 nm.

[0104] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0105] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 150°C at a heating rate of 5°C / min for 3 hours, then heated to 400°C at a heating rate of 2°C / min for 8 hours, naturally cooled to room temperature, vibrated and crushed, and then passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Example 5.

[0106] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 (SO4)3@Ti2O3@C, the electrochemical performance of the material is comparable to that of Example 1, but the cost is 30% higher.

[0107] Example 6

[0108] The difference between the sodium ferric sulfate positive electrode material and the preparation method thereof of Example 6 and Example 5 is that the amount of Ti2O3 added is 0.0023 mol. The preparation method of the sodium ferric sulfate positive electrode material of Example 6 includes the following steps:

[0109] Step 1: Take 1.6 mol of ferrous sulfate heptahydrate, 0.11 mol of cobalt sulfate, 0.045 mol of aluminum sulfate, 0.12 mol of manganese sulfate, 0.08 mol of calcium sulfate, 0.0023 mol of Ti2O3, 0.25 mol of oxalic acid, and then add 1 mol of sodium sulfate and deionized water. Control the solid content to 60%, control the sand milling time to 150 min, and control the sand milling speed to 1000 rpm. The particle size of the material after sand milling is controlled to be less than 200 nm.

[0110] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0111] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first reacted at 150°C for 3 hours with a heating rate of 5°C / min, then reacted at 400°C for 8 hours with a heating rate of 2°C / min, naturally cooled to room temperature, vibrated and crushed, and then passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Example 6.

[0112] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 2.1nm, the mass of Ti element in the Ti2O3 coating layer is 505ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 10.1*10 -5 S / cm, and the ion migration coefficient is 6.3*10 -7 cm 2 / s, at a rate of 1C, the discharge capacity after 150 cycles reaches 89mAh / g, and the capacity retention rate is over 95%.

[0113] Example 7

[0114] The difference between the sodium ferric sulfate positive electrode material and the preparation method thereof of Example 7 and Example 5 is that the amount of Ti2O3 added is 0.0188 mol. The preparation method of the sodium ferric sulfate positive electrode material of Example 7 includes the following steps:

[0115] Step 1: Take 1.6 mol of ferrous sulfate heptahydrate, 0.11 mol of cobalt sulfate, 0.045 mol of aluminum sulfate, 0.12 mol of manganese sulfate, 0.08 mol of calcium sulfate, 0.0188 mol of Ti2O3, 0.25 mol of oxalic acid, and then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 60%, control the sand milling time to 150 minutes, control the sand milling speed to 1000 rpm, and control the particle size of the material after sand milling to be less than 200 nm.

[0116] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0117] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first reacted at 150°C for 3 hours with a heating rate of 5°C / min, then reacted at 400°C for 8 hours with a heating rate of 2°C / min, naturally cooled to room temperature, vibrated and crushed, and then passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Example 7.

[0118] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11Al 0.09 Mn 0.12 Ca 0.08 (SO4)3@Ti2O3@C, the carbon layer thickness is 1.8nm, the mass of Ti element in the Ti2O3 coating layer is 4000ppm relative to the sodium ferric sulfate positive electrode material, and the electronic conductivity is 10.4*10 -5 S / cm, and the ion migration coefficient is 6.5*10 -7 cm 2 / s, at a rate of 1C, the discharge capacity after 120 cycles reaches 93mAh / g, and the capacity retention rate is over 90%.

[0119] Comparative Example 1

[0120] The sodium ferric sulfate positive electrode material and its preparation method of Comparative Example 1 are different from those of Example 1 in that the first-stage calcination and the second-stage calcination temperatures exceed the set range. The preparation method of the sodium ferric sulfate positive electrode material of Comparative Example 1 includes the following steps:

[0121] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain a mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 15L of 0.2mol / L dilute sulfuric acid solution, heat to 40°C, react for 8h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0122] Step 2: Add 0.25 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 50%, control the sand milling time to 400 min, and the sand milling speed to 1500 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0123] Step 3: Control the spray peristaltic speed to 80 rpm, the spray drying air inlet temperature to 210° C., and the air outlet temperature to 105° C. to obtain a spray-dried material, which is a calcined precursor.

[0124] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 300°C at a heating rate of 5°C / min for 3 hours, then heated to 600°C at a heating rate of 2°C / min for 8 hours, naturally cooled to room temperature, and passed through a 300-mesh sieve after air flow crushing to obtain the sodium iron sulfate positive electrode material of Comparative Example 1.

[0125] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08The carbon layer thickness of (SO4)3@Ti2O3@C is 1.7nm, the mass of Ti element in the Ti2O3 coating layer is 934ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 8.3*10 -5 S / cm, and the ion migration coefficient is 3.4*10 -7 cm 2 / s, as shown in Table 2.

[0126] The resulting positive electrode material was then prepared into a positive electrode slurry, which was then prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet were then assembled into a button cell. The button cell was assembled according to the following steps: 1) The slurry was stirred for 6 hours in NMP solvent at a ratio of active material: conductive agent: binder = 8:1:1; 2) The slurry was coated onto 16u aluminum foil to a thickness of 120μm and baked in a vacuum oven at 105°C for 4 hours; 3) The electrode sheet was rolled and cut into small discs; 4) The battery was assembled in the following order: positive electrode shell - positive electrode sheet - separator - sodium sheet - negative electrode shell, and 5 drops of sodium electrolyte were added; 5) The battery was allowed to stand for 12 hours before being tested in a cabinet.

[0127] The platform voltage of the material can reach 3.25V; at a rate of 1C, the discharge capacity reaches 50mAh / g after 100 cycles, and the capacity retention rate reaches more than 60%.

[0128] Comparative Example 2

[0129] The sodium ferric sulfate positive electrode material and its preparation method of Comparative Example 2 differ from those of Example 1 in that only a two-stage calcination process is used without a single-stage calcination process. The preparation method of the sodium ferric sulfate positive electrode material of Comparative Example 2 includes the following steps:

[0130] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain a mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 15L of 0.2mol / L dilute sulfuric acid solution, heat to 40°C, react for 8h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0131] Step 2: Add 0.25 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 50%, control the sand milling time to 400 min, and the sand milling speed to 1500 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0132] Step 3: Control the spray peristaltic speed to 80 rpm, the spray drying air inlet temperature to 210° C., and the air outlet temperature to 105° C. to obtain a spray-dried material, which is a calcined precursor.

[0133] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, heated to 350°C at a heating rate of 5°C / min for 8 hours, naturally cooled to room temperature, and passed through a 300-mesh sieve after air flow crushing to obtain the sodium iron sulfate positive electrode material of Comparative Example 2.

[0134] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 1.4nm, the mass of Ti element in the Ti2O3 coating layer is 929ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 7.6*10 -5 S / cm, and the ion migration coefficient is 4.5*10 -7 cm 2 / s, as shown in Table 2.

[0135] The resulting positive electrode material is then prepared into a positive electrode slurry, which is then prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are then assembled into a button cell. The button cell is assembled according to the following steps: 1) The slurry is stirred for 6 hours in NMP solvent at a ratio of active material: conductive agent: binder = 8:1:1; 2) The slurry is coated onto 16u aluminum foil, controlling the coating thickness to 120μm, and then baked in a vacuum oven at 105°C for 4 hours; 3) The electrode sheet is rolled and cut into small discs; 4) The battery is assembled in a positive electrode shell-positive electrode sheet-diaphragm-sodium sheet-negative electrode shell, and 5 drops of sodium electrolyte are added; 5) The battery is allowed to stand for 12 hours and then tested in a cabinet. The material has a platform voltage of 3.15V. At a 1C rate, the discharge capacity reaches 60mAh / g after 100 cycles, with a capacity retention rate of over 70%.

[0136] Comparative Example 3

[0137] The difference between the sodium ferric sulfate positive electrode material and the preparation method thereof of Comparative Example 3 and Example 2 is that the reducing carbon source is adjusted to flake graphite. The preparation method of the sodium ferric sulfate positive electrode material of Comparative Example 3 includes the following steps:

[0138] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 30L of 0.2mol / L nitric acid solution, heat to 25°C, react for 6h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0139] Step 2: Add 0.3 mol of flake graphite, 1 mol of sodium sulfate, 1 mol of ammonium sulfate, and deionized water to the solution, control the solid content to 60%, control the sand milling time to 600 min, and the sand milling speed to 600 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0140] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0141] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 150°C at a heating rate of 5°C / min for 3 hours, then heated to 400°C at a heating rate of 2°C / min for 8 hours, cooled naturally to room temperature, and vibrated and crushed, and then passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Comparative Example 3. The SEM image of the sodium iron sulfate positive electrode material of Comparative Example 3 is as follows: Figure 5 In Example 2, citric acid is used as the carbon source, which not only plays a reducing role but also coats the material. It can be seen that the material particles are more rounded and have obvious traces of amorphous carbon coating. When flake graphite is used to coat the material, a clear flake shape can still be seen, with sharp edges and corners.

[0142] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 2.2nm, the mass of Ti element in the Ti2O3 coating layer is 917ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 7.4*10 -5 S / cm, and the ion migration coefficient is 3.2*10 -7 cm 2 / s, as shown in Table 2.

[0143] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is the same as that in Example 1. The platform voltage of the material can reach 3.73V; at a 1C rate, the discharge capacity after 200 cycles reaches 84mAh / g, and the capacity retention rate reaches more than 66%.

[0144] Comparative Example 4

[0145] The sodium ferric sulfate positive electrode material and its preparation method of Comparative Example 4 differ from those of Example 2 in that no reducing carbon source is added. The preparation method of the sodium ferric sulfate positive electrode material of Comparative Example 4 comprises the following steps:

[0146] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 30L of 0.2mol / L nitric acid solution, heat to 25°C, react for 6h, adjust the solution pH to ~7, and obtain an iron / aluminum / cobalt / manganese / calcium / titanium solution.

[0147] Step 2: Add 1 mol of sodium sulfate, 1 mol of ammonium sulfate, and deionized water at the same time, control the solid content to 60%, control the sand milling time to 600 min, and the sand milling speed to 600 r. The particle size of the material after sand milling is controlled to be less than 200 nm.

[0148] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0149] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 150°C at a heating rate of 5°C / min for 3 hours, then heated to 400°C at a heating rate of 2°C / min for 8 hours, cooled naturally to room temperature, and crushed by vibration and passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Comparative Example 4.

[0150] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3 is 0nm, the mass of Ti element in the Ti2O3 coating layer is 903ppm relative to the sodium iron sulfate positive electrode material, and the electronic conductivity is 1.2*10 -5 S / cm, and the ion migration coefficient is 4.1*10 -7 cm 2 / s, as shown in Table 2.

[0151] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is the same as that in Example 1. The platform voltage of the material can reach 3.0V; at a rate of 1C, the discharge capacity after 50 cycles reaches 50mAh / g, and the capacity retention rate reaches more than 40%.

[0152] Comparative Example 5

[0153] The main difference between the positive electrode material preparation method of Comparative Example 5 and Example 1 is that an additional filtration step is added when dissolving hematite to remove acid-insoluble Ti2O3. The preparation method of the sodium iron sulfate positive electrode material of Comparative Example 5 includes the following steps:

[0154] Step 1: Take 152g of hematite, crush it in air, and sieve it to obtain a mineral powder with a particle size of less than 5μm. Place the mineral powder in a container, add 15L of 0.2mol / L dilute sulfuric acid solution, heat to 40°C, react for 8h, adjust the solution pH to ~7, filter out insoluble impurities, and obtain an iron / aluminum / cobalt / manganese / calcium solution.

[0155] Step 2: Add 0.25 mol of citric acid to the solution until the solution turns light green, then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 50%, control the sand milling time to 400 min, and the sand milling speed to 1500 r. The particle size of the material after sand milling is controlled to be <200 nm.

[0156] Step 3: Control the spray peristaltic speed to 80 rpm, the spray drying air inlet temperature to 210° C., and the air outlet temperature to 105° C. to obtain a spray-dried material, which is a calcined precursor.

[0157] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 100°C at a heating rate of 5°C / min and reacted for 3 hours, then heated to 350°C at a heating rate of 2°C / min and reacted for 8 hours, naturally cooled to room temperature, and passed through a 300-mesh sieve after air flow crushing to obtain the sodium iron sulfate positive electrode material of Comparative Example 5.

[0158] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.6 Co 0.11 Al 0.09 Mn 0.12 Ca 0.08 (SO4)3@C, the carbon layer thickness is 2nm, the mass of Ti element in the Ti2O3 coating layer is 0 relative to the sodium ferric sulfate positive electrode material, and the electronic conductivity is 8.8*10 -5 S / cm, and the ion migration coefficient is 3.5*10 -7 cm 2 / s, as shown in Table 2.

[0159] The resulting positive electrode material is then prepared into a positive electrode slurry, which is then prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are then assembled into a button cell. The button cell is assembled according to the following steps: 1) The slurry is stirred for 6 hours in NMP solvent at a ratio of active material: conductive agent: binder = 8:1:1; 2) The slurry is coated onto 16u aluminum foil to a thickness of 120μm and baked in a vacuum oven at 105°C for 4 hours; 3) The electrode sheet is rolled and cut into small discs; 4) The battery is assembled in a positive electrode shell-positive electrode sheet-diaphragm-sodium sheet-negative electrode shell, into which 5 drops of sodium electrolyte are added; 5) The battery is allowed to stand for 12 hours and then tested in a cabinet. The material has a platform voltage of 3.69V. At a 1C rate, the discharge capacity reaches 80mAh / g after 100 cycles, with a capacity retention rate of over 74%.

[0160] Comparative Example 6

[0161] The sodium ferric sulfate positive electrode material and its preparation method of Comparative Example 6 are different from those of Example 5 in that the doping elements Co and Al are absent. The preparation method of the sodium ferric sulfate positive electrode material of Comparative Example 6 comprises the following steps:

[0162] Step 1: Take 1.8 mol of ferrous sulfate heptahydrate, 0.12 mol of manganese sulfate, 0.08 mol of calcium sulfate, 0.0045 mol of Ti2O3, 0.25 mol of oxalic acid, and then add 1 mol of sodium sulfate and deionized water at the same time, control the solid content to 60%, control the sand milling time to 150 minutes, control the sand milling speed to 1000r, and control the particle size of the material after sand milling to be less than 200nm.

[0163] Step 3: Control the freeze-drying temperature to -50°C and the freeze-drying time to 8 hours to obtain a freeze-dried material, which is the calcined precursor.

[0164] Step 4: The precursor was calcined in a H2 / Ar (volume ratio 5% / 95%) atmosphere, first heated to 150°C at a heating rate of 5°C / min for 3 hours, then heated to 400°C at a heating rate of 2°C / min for 8 hours, naturally cooled to room temperature, vibrated and crushed, and then passed through a 200-mesh sieve to obtain the sodium iron sulfate positive electrode material of Example 5.

[0165] The chemical formula of the obtained sodium iron sulfate positive electrode material is Na2Fe 1.8 Mn 0.12 Ca 0.08 The carbon layer thickness of (SO4)3@Ti2O3@C is 1.9 nm, the mass of Ti element in the Ti2O3 coating layer is 950 ppm relative to the sodium ferric sulfate positive electrode material, and the electronic conductivity is 6.9*10 -5 S / cm, and the ion migration coefficient is 2.9*10 -7 cm 2 / s, as shown in Table 2.

[0166] The obtained positive electrode material is then prepared into a positive electrode slurry, and then the slurry is prepared into a positive electrode sheet. The positive electrode sheet and the sodium sheet are assembled into a button battery. The assembly method is the same as that in Example 1. The platform voltage of the material can reach 3.4V; at a 1C rate, the discharge capacity after 50 cycles reaches 60mAh / g, and the capacity retention rate reaches more than 60%.

[0167] Table 2 Related parameters of sodium iron sulfate positive electrode materials in various embodiments and comparative examples

[0168]

[0169] In summary, the sodium ferric sulfate positive electrode material of the present application utilizes elements such as Al, Mn, Ca, and Co to achieve doping modification of the sodium ferric sulfate positive electrode material, forming a multi-entropy structure, thereby improving the ionic conductivity and electronic conductivity of the sodium ferric sulfate positive electrode material, thereby improving the cycle stability of the sodium ferric sulfate positive electrode material. At the same time, Ti2O3 is utilized to coat the material, which is beneficial to reducing its water absorption and improving its air stability. The carbon coating layer of the sodium ferric sulfate positive electrode material of the present application can achieve complete coating of the material, improve the electronic conductivity of the material, alleviate phase change, and improve the stability of the material.

[0170] The preparation method of the sodium ferric sulfate positive electrode material provided in the present application uses hematite as raw material to directly synthesize the sodium ferric sulfate positive electrode material. After a one-step acid dissolution, the material is doped and modified by trace impurity elements in the ore, effectively utilizing the associated Al, Mn, Ca, and Co elements in the ore to dope and modify it. At the same time, Ti2O3 is coated on the material, improving the environmental stability of the material, eliminating the ore impurity removal process, shortening the process flow, and effectively reducing production costs. This preparation method introduces a reducing carbon source during the acid dissolution process. On the one hand, it can reduce the metal elements to the corresponding valence state. On the other hand, it coats the material during the subsequent drying and sintering process, avoiding direct contact between the electrolyte and the material, and improving the electrochemical performance of the material.

[0171] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0172] Furthermore, those skilled in the art will appreciate that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and to form distinct embodiments. For example, in the claims above, any of the claimed embodiments may be used in any combination. The information disclosed in this background section is intended solely to enhance understanding of the overall background of this application and should not be construed as an admission or any implication that such information constitutes prior art known to those skilled in the art.

Claims

1. A sodium iron sulfate positive electrode material, characterized in that The chemical formula of the sodium iron sulfate positive electrode material is Na2Fe 2-x-y-z-n Co x Al y Mn z Ca n (SO4)3@Ti2O3@C, where, 0<x+y+z+n<0.5, 0.0005<x<0.485, 0.0005<y<0.485, 0.0005<z<0.485, 0.0005<n<0.485, Na2Fe 2-x-y-z-n Co x Al y Mn z Ca n (SO4)3 is the core, Ti2O3 is the first coating layer covering the core, and C is the second coating layer covering the first coating layer; The sodium iron sulfate positive electrode material is directly synthesized using hematite as a raw material.

2. The sodium iron sulfate positive electrode material according to claim 1, characterized in that The average thickness of the second coating layer is 0.5-5 nm, and the mass of the Ti element in the first coating layer is 500-4000 ppm relative to the sodium ferric sulfate positive electrode material.

3. A method for preparing the sodium ferric sulfate positive electrode material according to claim 1 or 2, characterized in that: include: dissolving hematite with an acid solution to obtain a first solution; mixing the first solution with a reducing carbon source, a sodium source, and a sulfur source to obtain a second solution; drying the second solution to obtain a precursor; The precursor is calcined to obtain the sodium ferric sulfate positive electrode material.

4. The preparation method according to claim 3, characterized in that At least one of the following conditions is met: A. The acid solution includes one or a mixture of phosphoric acid, perchloric acid, dichromic acid, iodic acid, hydroiodic acid, hydrosulfuric acid, boric acid, hydrochloric acid, sulfuric acid, sulfurous acid, nitric acid, hydrofluoric acid, and acetic acid; B. When condition A is met, the concentration of the sulfuric acid is 0.01 to 6 mol / L, and the concentration of the hydrofluoric acid is 0.01 to 0.4 mol / L; C. When condition A is met, the nitric acid and the sulfuric acid are not used at the same time; D. the reaction temperature of dissolving the hematite with an acid solution is 0° C. to 40° C., and the reaction time is 6 to 12 hours; E. The pH of the first solution is 4-11.

5. The preparation method according to claim 3, characterized in that At least one of the following conditions is met: a. The reducing carbon source comprises any one or more of citric acid, oxalic acid, oleic acid, pyromellitic acid, glucose, ethylene glycol, polyethylene glycol, formaldehyde, ascorbic acid, and pyrrole; b. The sodium source includes any one or more of sodium carbonate, sodium sulfate, sodium oxalate, sodium acetate, sodium alginate, and sodium citrate; c. The sulfur source includes any one or more of sulfuric acid, sulfur dioxide, sulfur trioxide, sodium sulfate, sodium sulfite, ammonium sulfate, and sodium thiosulfate; d. The mixing speed is 100 to 1600 rpm, and the mixing time is 20 to 600 min; e. The mixing method includes mechanical stirring, double-screw stirring, sand milling in any one or more; f. The solid content of the second solution is 30%-70%.

6. The preparation method according to any one of claims 3 to 5, characterized in that The calcination includes a first stage calcination and a second stage calcination; and / or, The drying includes any one of vacuum drying, freeze drying, and spray drying; The preparation method satisfies at least one of the following conditions: (1) The temperature of the first stage calcination is 10-200°C, the time is 1-24h, and the heating rate is 1-10°C / min; (2) The temperature of the second stage calcination is 200-500°C, the time is 4-24h, and the heating rate is 1-8°C / min; (3) The calcination is carried out in a protective gas, which includes any one or more of argon, nitrogen, hydrogen, ammonia, and methane; the flow rate of the protective gas is not higher than 30m 3 / h; (4) The oxygen content during the second calcination process is 10-100 ppm; (5) The vacuum drying temperature is 100-120°C and the time is 8-12 hours; (6) The freeze-drying temperature is -40 to -60°C and the time is 5 to 10 hours; (7) The inlet air temperature of the spray drying is 200-300°C, the outlet air temperature is 90-150°C, the fan power is 30-50HZ, and the peristaltic speed is 60-150rpm.

7. A positive electrode sheet, characterized in that: The invention comprises the sodium iron sulfate positive electrode material according to claim 1 or 2.

8. A sodium ion battery, characterized in that: Including the positive electrode sheet according to claim 7.

9. An electrical device, characterized in that: Including the sodium ion battery according to claim 8.

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