Sodium-ion battery positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a sodium ion battery positive electrode material and a preparation method and application thereof. The sodium ion battery positive electrode material comprises carbon composite Na4Fe3 (PO4) 2P2O7, a first coating layer CoMoO4 and a second coating layer SiO2, wherein the first coating layer CoMoO4 is coated on at least part of the surface of the carbon composite Na4Fe3 (PO4) 2P2O7, and the second coating layer SiO2 is coated on at least part of the surface of the first coating layer CoMoO4. According to the sodium ion positive electrode material, a CoMoO4 heterojunction interface is constructed on the surface, and a SiO2 coating layer is further constructed on the outer layer; the CoMoO4 heterojunction interface can utilize an electric field in the interface to accelerate electron migration, so that the reversible specific capacity is improved. In addition, the SiO2 coating layer and the CoMoO4 heterojunction interface can generate a synergistic effect, so that the transmission path of sodium ions in the electrode material is further optimized, the ion diffusion resistance is reduced, and the electron-ion transport operation is accelerated.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a sodium ion battery positive electrode material and a preparation method and application thereof. Background Art

[0002] While lithium-ion batteries (LIBs) are widely used due to their excellent energy storage performance, their application is limited by the scarcity of lithium resources and the high cost of production. In contrast, sodium is abundant, widely distributed, and low-cost, making it a promising alternative to LIBs. In NIB systems, the cathode material is the most critical factor affecting the battery's energy density. Among NIB cathode materials, sodium ferric pyrophosphate (Na₄Fe₃(PO₄)₂P₂Oₐ) (NFPP) possesses a biphosphate group, ensuring stability. Its open sodium ion diffusion channels and moderate operating voltage make it a promising, low-cost, iron-based polyanionic cathode material. However, NFPP suffers from drawbacks such as low electron transfer rate, resulting in poor electrochemical performance. Therefore, further improving NFPP's electron transfer rate is crucial for advancing the development of NIBs. Summary of the Invention

[0003] The present invention aims to provide a sodium ion battery cathode material with a fast electron transfer rate, and a preparation method and application thereof.

[0004] In a first aspect, the present invention provides a positive electrode material for a sodium ion battery, which adopts the following technical solution: A sodium ion battery positive electrode material includes carbon-composite Na4Fe3(PO4)2P2O7, a first coating layer CoMoO4 coated on at least a portion of the surface of the carbon-composite Na4Fe3(PO4)2P2O7, and a second coating layer SiO2 coated on at least a portion of the surface of the first coating layer CoMoO4.

[0005] In a second aspect, the present invention provides a method for preparing a positive electrode material for a sodium ion battery, comprising the following steps: 1) adding a phosphorus source, an iron source, a sodium source, and a carbon source into solvent 1 to obtain a mixed solution; spray drying the mixed solution to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor; 2) dispersing a carbon-composite Na4Fe3(PO4)2P2O7 precursor in an aqueous solution containing a molybdenum source and a cobalt source to obtain a dispersion; subjecting the dispersion to a hydrothermal reaction to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4; 3) Dispersing a carbon composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4 in a mixed solution of a silicon source and solvent 2, stirring to perform a sol-gel hydrolysis reaction, and after the reaction is completed, heating and evaporating the solution to obtain a sodium ion battery positive electrode precursor; 4) The sodium ion battery positive electrode precursor is heat-treated under an inert atmosphere to obtain a sodium ion battery positive electrode material.

[0006] Preferably, in step 1), the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, and sodium phosphate; the iron source is one or more of ferric sulfate, ferric nitrate, ferric acetate, ferrous oxalate, and ferric phosphate; the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium pyrophosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate; and the carbon source is glucose, citric acid, oxalic acid, and ascorbic acid.

[0007] Preferably, in steps 1) and 3), solvent 1 and solvent 2 are one or more of water, ethanol, isopropanol, and acetone.

[0008] Preferably, in step 1), the phosphorus source, iron source and sodium source are added according to the stoichiometric ratio of Na4Fe3(PO4)2P2O7; and the amount of carbon source added is 1-10% of the mass of the theoretically generated Na4Fe3(PO4)2P2O7.

[0009] Preferably, in step 1), the inlet temperature of the spray is 150-200°C.

[0010] Preferably, in step 2), the molybdenum source is one or more of molybdenum acetate, molybdenum oxide, and ammonium molybdate; and the cobalt source is one or more of cobalt nitrate, cobalt sulfate, cobalt carbonate, cobalt oxalate, cobalt chloride, and cobalt hydroxide.

[0011] Preferably, in step 2), the molar ratio of Mo and Co in the molybdenum source and the cobalt source is 1:1; the amount of cobalt source added is 1-8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1).

[0012] Preferably, in step 2), before the hydrothermal reaction of the dispersion, aqueous ammonia is added to adjust the pH of the dispersion to 8-9.

[0013] Preferably, in step 2), the hydrothermal reaction temperature is 160-200° C., and the hydrothermal reaction time is 6-12 h.

[0014] Preferably, in step 3), the silicon source is one or more of butyl orthosilicate, tetraethoxysilane, and tetrapropoxysilane.

[0015] Preferably, in step 3), the amount of silicon source added is 1-8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1).

[0016] Preferably, in step 3), the temperature of the sol-gel hydrolysis reaction is 50-70° C., the time of the sol-gel hydrolysis reaction is 1-2 h, and the temperature of the heating evaporation is 80-100° C.

[0017] Preferably, in step 4), the heat treatment temperature is 500-700° C., and the heat treatment time is 8-10 hours.

[0018] Preferably, in step 4), the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0019] In a third aspect, the present invention provides a sodium ion battery comprising the aforementioned sodium ion battery positive electrode material.

[0020] The above one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: 1) The sodium-ion cathode material of this invention constructs a CoMoO4 heterojunction interface on its surface and further constructs a SiO2 coating on its outer layer. The CoMoO4 heterojunction interface utilizes the electric field within the interface to accelerate electron migration, effectively reducing the electronic resistance of the material, significantly increasing the electron transfer rate, promoting electrochemical reactions, and thus enhancing the reversible specific capacity. The SiO2 coating synergizes with the CoMoO4 heterojunction interface to further optimize the sodium ion transport path within the electrode material, reducing ion diffusion resistance and accelerating electron-ion transport, enabling the sodium-ion cathode material to maintain excellent electrochemical performance under high-rate charge and discharge conditions.

[0021] 2) The outer SiO2 coating of the sodium-ion cathode material in this invention acts as a chemically inert interfacial buffer layer, effectively blocking direct electrolyte corrosion of the cathode material, inhibiting side reactions and the formation of interfacial byproducts, and slowing the growth of the SEI film, thereby improving the long-term stability of the electrode / electrolyte interface and delaying capacity decay. Furthermore, the SiO2 coating mitigates particle volume expansion and mechanical stress changes caused by the Na⁺ insertion and extraction process, preventing particle fracture, pulverization, and interfacial failure. Combined with the excellent electronic conductivity characteristics brought by the CoMoO4 heterostructure, this significantly improves the material's cycling stability and long-life charge-discharge performance.

[0022] 3) In the preparation method of the present invention, the CoMoO4 heterojunction interface is prepared by a hydrothermal method. The CoMoO4 coating prepared by the hydrothermal reaction contains rich hydroxyl groups. When the silicon source is hydrolyzed, silanols are first generated. The silanols are further hydrolyzed to obtain silicon dioxide. When the silanols are hydrolyzed, they react with the hydroxyl groups on the surface of the CoMoO4 to form Si-O-Mo bonds. This improves the bonding strength between the SiO2 coating layer and the CoMoO4 coating layer, avoids excessive interfacial energy between the two coating layers, and thus better improves the overall performance of the sodium ion positive electrode material. Furthermore, the hydrothermal method is used to prepare the CoMoO4 coating layer and the sol-gel hydrolysis method is used to prepare the SiO2 coating layer in this application, which can improve the uniformity of the coating, thereby better improving the effect of the coating layer.

[0023] 4) The preparation method of the sodium ion positive electrode material of the present invention is simple and easy to implement, and can be easily industrialized. In addition, the sodium ion positive electrode material prepared by the preparation method of the present invention has a high battery capacity and excellent cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the SEM image of the sodium ion positive electrode material prepared in Example 1.

[0025] Figure 2 XRD patterns of the ionic positive electrode materials prepared in Example 1 and Comparative Example 1.

[0026] Figure 3 The charge and discharge curves of the button batteries prepared in Examples 1 to 4 at a current density of 0.1C are shown.

[0027] Figure 4 The charge and discharge curves of the button batteries prepared in Comparative Examples 1 to 3 at a current density of 0.1C are shown.

[0028] Figure 5 This is a graph showing the cycling performance of the button batteries prepared in Examples 1 to 4 at a current density of 10C.

[0029] Figure 6 This is a graph showing the cycling performance of button batteries prepared in Comparative Examples 1 to 3 at a current density of 10C. DETAILED DESCRIPTION

[0030] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of the present invention. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0031] As mentioned above, in the first aspect, the present invention provides a positive electrode material for a sodium ion battery, which adopts the following technical solution: A sodium ion battery positive electrode material includes carbon-composite Na4Fe3(PO4)2P2O7, a first coating layer CoMoO4 coated on at least a portion of the surface of the carbon-composite Na4Fe3(PO4)2P2O7, and a second coating layer SiO2 coated on at least a portion of the surface of the first coating layer CoMoO4.

[0032] In the present invention, a layer of CoMoO4 is coated on the surface of the carbon-composite Na4Fe3(PO4)2P2O7. The CoMoO4 heterojunction interface can utilize the electric field within the interface to accelerate electron migration, effectively reducing the electronic resistance of the material, significantly improving the electron transfer rate, promoting electrochemical reactions, and thus increasing the reversible specific capacity. The present invention further coats the CoMoO4 coating with a layer of SiO2. The SiO2 coating synergizes with the CoMoO4 heterojunction interface to further optimize the sodium ion transmission path within the electrode material, reducing ion diffusion resistance, accelerating electron-ion transport behavior, and enabling the sodium ion positive electrode material to maintain excellent electrochemical performance under high-rate charge and discharge conditions.

[0033] In a second aspect, the present invention provides a method for preparing a positive electrode material for a sodium ion battery, comprising the following steps: 1) adding a phosphorus source, an iron source, a sodium source, and a carbon source into solvent 1 to obtain a mixed solution; spray drying the mixed solution to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor; 2) dispersing a carbon-composite Na4Fe3(PO4)2P2O7 precursor in an aqueous solution containing a molybdenum source and a cobalt source to obtain a dispersion; subjecting the dispersion to a hydrothermal reaction to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4; 3) Dispersing a carbon composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4 in a mixed solution of a silicon source and solvent 2, stirring to perform a sol-gel hydrolysis reaction, and after the reaction is completed, heating and evaporating the solution to obtain a sodium ion battery positive electrode precursor; 4) The sodium ion battery positive electrode precursor is heat-treated under an inert atmosphere to obtain a sodium ion battery positive electrode material.

[0034] In the preparation method of the present invention, a spray drying method is used to prepare a carbon-composite Na4Fe3(PO4)2P2O7 precursor. The particles prepared by the spray drying method have better uniformity and dispersion, which is beneficial to the subsequent hydrothermal coating and sol-gel coating steps.

[0035] In the preparation method of the present invention, the hydrothermal method is used to prepare the CoMoO4 heterojunction interface, which can make the binding of CoMoO4 and the carbon-composite Na4Fe3(PO4)2P2O7 precursor more stronger and the coating more uniform, thereby better improving the electrochemical performance of the sodium ion positive electrode material. In addition, the CoMoO4 coating layer prepared by the hydrothermal method in the present invention contains rich hydroxyl groups; when preparing the SiO2 coating layer, the silicon source generates silanols during the hydrolysis process, and the silanols are further hydrolyzed to generate SiO2. When the silanols are further hydrolyzed, they react with the hydroxyl groups on the surface of the CoMoO4 to generate Si-O-Mo bonds; thereby improving the bonding strength of the SiO2 coating layer and the CoMoO4 coating layer, reducing the interfacial energy between the two coating layers, thereby better improving the comprehensive performance of the sodium ion positive electrode material.

[0036] In the preparation method of the present invention, the SiO2 coating layer is prepared by a sol-gel hydrolysis method, which can improve the uniformity of the SiO2 coating layer and the bonding strength of the two coating layers, thereby better playing a buffering role.

[0037] The preparation method of the present invention is finally subjected to heat treatment, which can promote the crystallization of CoMoO4 and the densification of the SiO2 layer, and improve the bonding strength between the SiO2 layer and the CoMoO4 layer, and between the CoMoO4 layer and the core, thereby better improving the comprehensive performance of the sodium ion positive electrode material.

[0038] Preferably, in step 1), the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, and sodium phosphate; the iron source is one or more of ferric sulfate, ferric nitrate, ferric acetate, ferrous oxalate, and ferric phosphate; the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium pyrophosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate; and the carbon source is glucose, citric acid, oxalic acid, and ascorbic acid.

[0039] Preferably, in the step 1), the phosphorus source, iron source and sodium source are added according to the stoichiometric ratio of Na4Fe3(PO4)2P2O7; the amount of the carbon source added is 1-10% of the mass of the theoretically generated Na4Fe3(PO4)2P2O7, including but not limited to 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, etc.

[0040] Preferably, in step 1), the inlet temperature of the spray is 150-200°C, including but not limited to 150°C, 155°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, etc.

[0041] Preferably, in step 2), the molybdenum source is one or more of molybdenum acetate, molybdenum oxide, and ammonium molybdate; and the cobalt source is one or more of cobalt nitrate, cobalt sulfate, cobalt carbonate, cobalt oxalate, cobalt chloride, and cobalt hydroxide.

[0042] Preferably, in the step 2), the molar ratio of Mo and Co in the molybdenum source and the cobalt source is 1:1; the amount of cobalt source added is 1 to 8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1), including but not limited to 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, etc.

[0043] Preferably, in step 2), before the hydrothermal reaction, aqueous ammonia is added to adjust the pH of the dispersion to 8-9, including but not limited to 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, etc.

[0044] In the present invention, by regulating the pH of the dispersion to 8-9, the hydroxyl content on the surface of the CoMoO4 coating layer can be further increased while ensuring the normal progress of the hydrothermal reaction, thereby further improving the bonding strength between the CoMoO4 coating layer and the SiO2 coating layer, reducing the interfacial energy between the coating layers, and improving the comprehensive electrochemical performance of the sodium ion positive electrode material.

[0045] Preferably, in step 2), the hydrothermal reaction temperature is 160-200°C, including but not limited to 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, etc.; the hydrothermal reaction time is 6-12h, including but not limited to 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, 12h, etc.

[0046] Preferably, in step 3), the silicon source is one or more of butyl orthosilicate, tetraethoxysilane, tetrapropoxysilane, and ammonium silicate.

[0047] Preferably, in the step 3), the amount of silicon source added is 1 to 8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1), including but not limited to 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, etc.

[0048] Preferably, in step 3), the temperature of the sol-gel hydrolysis reaction is 50-70°C, including but not limited to 50°C, 60°C, 70°C, etc.; the time of the sol-gel hydrolysis reaction is 1-2 hours, including but not limited to 1 hour, 1.25 hours, 1.5 hours, 1.75 hours, 2 hours, etc.; the temperature of heating and evaporation is 80-100°C, including but not limited to 80°C, 85°C, 90°C, 95°C, 100°C, etc.

[0049] Preferably, in step 4), the heat treatment temperature is 500-700°C, including but not limited to 500°C, 520°C, 550°C, 580°C, 600°C, 620°C, 650°C, 680°C, 700°C, etc. The heat treatment time is 8-10 hours, including but not limited to 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, etc.

[0050] Preferably, in step 4), the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0051] In a third aspect, the present invention provides a sodium ion battery comprising the aforementioned sodium ion battery positive electrode material.

[0052] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0053] Example 1 The sodium ion positive electrode material in this embodiment includes a carbon-composite NFPP core, a first coating layer of CoMoO4, and a second coating layer of SiO2. The specific preparation method is as follows: 1) Adding ammonium dihydrogen phosphate, ferric nitrate, sodium pyrophosphate, and glucose to pure water to obtain a mixed solution; spray drying the mixed solution to obtain a carbon-composite NFPP precursor; wherein: ammonium dihydrogen phosphate, ferric nitrate, and sodium pyrophosphate are added according to the stoichiometric ratio of the NFPP chemical formula, and the amount of glucose added is 5% of the theoretical mass of the generated NFPP; the spray drying inlet temperature is 175°C, and the feed rate is 35 min / L.

[0054] 2) Dispersing a carbon-composite NFPP precursor in an aqueous solution containing cobalt acetate and ammonium molybdate, and adjusting the pH to approximately 8.5 by adding aqueous ammonia to obtain a mixed solution; transferring the mixed solution to a hydrothermal reactor and conducting a hydrothermal reaction at 180° C. for 9 hours to obtain a carbon-composite NFPP precursor coated with CoMoO4; wherein: cobalt acetate and ammonium molybdate are added according to a molar ratio of element Co to Mo of 1:1; the molar number of cobalt acetate is 5% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0055] 3) A carbon composite NFPP precursor coated with CoMoO4 is added to a mixed solution of tetraethoxysilane and water, and a sol-gel hydrolysis reaction is carried out at 60°C with stirring for 1.5 hours. After the reaction, it is evaporated to dryness at 90°C to obtain a sodium ion positive electrode material precursor; the sodium ion positive electrode material precursor is heat-treated in an argon atmosphere at 600°C for 9 hours to obtain a sodium ion positive electrode material; wherein the molar number of tetraethoxysilane is 4% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0056] The micromorphology of the sodium ion positive electrode material prepared in this embodiment is shown in FIG. Figure 1 As shown, the average particle size of the sodium ion positive electrode material particles is about 4 μm, and the particles have good dispersion.

[0057] Comparative Example 1 The sodium ion positive electrode material in this comparative example is obtained by further heat-treating the carbon composite NFPP precursor prepared in step 1) of Example 1; the specific method is as follows: The carbon-composite NFPP precursor was heat-treated in an argon atmosphere at 600°C for 9 hours to obtain a sodium ion positive electrode material.

[0058] The XRD spectra of the sodium ion positive electrode materials prepared in Example 1 and Comparative Example 1 are as follows: Figure 2 As shown, the sodium ion positive electrode material prepared in Example 1 is higher than that prepared in Comparative Example 1 at 2θ=33.5 o There is a new characteristic absorption peak at the position, which is the characteristic absorption peak of CoMoO4; in addition, the intensity of the characteristic peak in Example 1 is enhanced, indicating that the double-coating modification effectively improves the crystallinity of the material.

[0059] Comparative Example 2 The sodium ion positive electrode material in this comparative example includes a carbon-composite NFPP core and a coating layer of SiO2.

[0060] The preparation method is basically the same as that of Example 1, except that: the hydrothermal coating of the CoMoO4 precursor in step 2) is not performed; that is, the carbon-composite NFPP precursor is substituted for the carbon-composite NFPP precursor coated with the CoMoO4 precursor in step 3).

[0061] Comparative Example 3 The sodium ion positive electrode material in this comparative example includes a carbon-composite NFPP core and a coating layer of CoMoO4.

[0062] The method is basically the same as Example 1, except that SiO2 is not coated and the sol-gel reaction is not performed in step 3). Specifically, step 3) is as follows: the carbon composite NFPP precursor coated with the CoMoO4 precursor is heat-treated in an argon atmosphere at 600°C for 10 hours to obtain a sodium ion positive electrode material.

[0063] Example 2 The process is basically the same as Example 1, except that, in step 2), aqueous ammonia is added to adjust the pH to about 6.5 to obtain a mixed solution; and then a hydrothermal reaction is performed.

[0064] Example 3 The sodium ion positive electrode material in this embodiment includes a carbon-composite NFPP core, a first coating layer of CoMoO4, and a second coating layer of SiO2. The specific preparation method is as follows: 1) Ammonium phosphate, ferric acetate, sodium pyrophosphate, and citric acid are added to pure water to obtain a mixed solution; the mixed solution is spray-dried to obtain a carbon-composite NFPP precursor; wherein: ammonium phosphate, ferric acetate, and sodium pyrophosphate are added according to the stoichiometric ratio of the NFPP chemical formula, and the amount of citric acid added is 3% of the theoretical mass of the generated NFPP; the spray drying inlet temperature is 200°C, and the feed rate is 50 min / L.

[0065] 2) Dispersing the carbon-composite NFPP precursor in an aqueous solution containing cobalt sulfate and ammonium molybdate, and adding ammonia water to adjust the pH to about 8 to obtain a mixed solution; transferring the mixed solution to a hydrothermal reactor and conducting a hydrothermal reaction at 160°C for 12 hours to obtain a carbon-composite NFPP precursor coated with CoMoO4; wherein: cobalt sulfate and ammonium molybdate are added according to a molar ratio of element Co to Mo of 1:1; the molar number of cobalt sulfate is 8% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0066] 3) Adding a carbon-composite NFPP precursor coated with CoMoO4 to a mixed solution of butyl orthosilicate and water, carrying out a sol-gel hydrolysis reaction at 50°C with stirring for 2 hours, and after the reaction, evaporating at 80°C to obtain a sodium ion positive electrode material precursor; heat treating the sodium ion positive electrode material precursor in an argon atmosphere at 520°C for 10 hours to obtain a sodium ion positive electrode material; wherein the molar number of butyl orthosilicate is 1% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0067] Example 4 The sodium ion positive electrode material in this embodiment includes a carbon-composite NFPP core, a first coating layer of CoMoO4, and a second coating layer of SiO2. The specific preparation method is as follows: 1) Adding ammonium dihydrogen phosphate, ferric nitrate, sodium pyrophosphate, and ascorbic acid to pure water to obtain a mixed solution; spray drying the mixed solution to obtain a carbon-composite NFPP precursor; wherein: ammonium dihydrogen phosphate, ferric nitrate, and sodium pyrophosphate are added according to the stoichiometric ratio of the NFPP chemical formula, and the amount of ascorbic acid added is 8% of the theoretical mass of the generated NFPP; the spray drying inlet temperature is 150°C, and the feed rate is 25 min / L.

[0068] 2) Dispersing a carbon-composite NFPP precursor in an aqueous solution containing cobalt chloride and ammonium molybdate, and adjusting the pH to approximately 9 by adding aqueous ammonia to obtain a mixed solution; transferring the mixed solution to a hydrothermal reactor and conducting a hydrothermal reaction at 200° C. for 6 hours to obtain a carbon-composite NFPP precursor coated with CoMoO4; wherein: cobalt acetate and ammonium molybdate are added according to a molar ratio of element Co to Mo of 1:1; the molar number of cobalt acetate is 8% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0069] 3) Adding the carbon composite NFPP precursor coated with CoMoO4 to a mixed solution of tetrapropoxysilane and water, carrying out a sol-gel hydrolysis reaction at 70°C with stirring for 1 hour, and after the reaction, evaporating at 100°C to obtain a sodium ion positive electrode material precursor; heat treating the sodium ion positive electrode material precursor in an argon atmosphere at 700°C for 8 hours to obtain a sodium ion positive electrode material; wherein the molar number of tetrapropoxysilane is 1% of the molar number of Fe element in the theoretically generated NFPP in step 1).

[0070] The sodium-ion cathode materials from Examples 1-4 and Comparative Examples 1-3 were assembled into CR2032 button cells using the following assembly method: a sodium metal sheet was used as the counter electrode, a glass fiber separator, and NaPF6 as the electrolyte. The working electrode consisted of 80 wt% sodium-ion cathode material, 10 wt% conductive agent, and 10 wt% polyvinylidene fluoride (PVDF), using N-methyl-2-pyrrolidone (NMP) as the solvent. The material was evenly coated onto aluminum foil using a doctor blade and dried in a vacuum oven at 120°C for 12 hours. The thickness of the electrode sheet was calculated based on the compacted density and then rolled. The electrode diameter was 12 mm. The counter electrode, working electrode, separator, and electrolyte were assembled into a button cell in a vacuum glove box.

[0071] The button battery was tested at 1.5~4.2V and 0.1C or 10C current density. The specific test results can be seen Figures 3 to 6 .

[0072] Figure 3 and Figure 4The charge and discharge curves of Examples 1 to 4 and Comparative Examples 1 to 3 at a current density of 0.1C are shown. It can be seen that the specific capacities of the batteries assembled with the sodium ion positive electrode materials in Examples 1 to 4 reached 126 mAh / g, 123 mAh / g, 119 mAh / g, and 117 mAh / g, respectively. The specific capacities of the batteries assembled with the sodium ion positive electrode materials in Comparative Examples 1 to 3 were 104 mAh / g, 110 mAh / g, and 114 mAh / g, respectively. It can be seen that the battery assembled with the sodium ion positive electrode material without a coating layer in Comparative Example 1 has a lower specific capacity; the battery assembled with the sodium ion positive electrode material having only a SiO2 coating layer in Comparative Example 2 has an improved specific capacity compared with Comparative Example 1, but the improvement is small; the battery assembled with the sodium ion positive electrode material having only a CoMoO4 coating layer in Comparative Example 3 has an improved specific capacity compared with Comparative Example 1, but the improvement is also small; the battery assembled with the sodium ion positive electrode material coated with a double layer of CoMoO4 and SiO2 in Example 1 has a significantly improved specific capacity compared with Comparative Examples 1 to 3, indicating that the CoMoO4 coating layer and the SiO2 coating layer have a synergistic effect. The specific capacity of the battery assembled with the sodium ion positive electrode material prepared in Example 2 is somewhat lower than that in Example 1. This may be because after adjusting the pH of the dispersion to 6.5 in Example 2, the hydroxyl content on the surface of the prepared CoMoO4 coating layer is reduced, the bonding strength between the CoMoO4 coating layer and the SiO2 coating layer is reduced, and the interfacial energy between the coating layers is increased, thereby causing the specific capacity of the sodium ion battery assembled with the sodium ion positive electrode material in Example 2 to decrease. The performance of the sodium ion positive electrode materials in Examples 3 and 4 has a certain degree of variation compared to Example 1, but overall maintains a good specific capacity.

[0073] Figure 5 and Figure 6The charge and discharge curves of Examples 1 to 4 and Comparative Examples 1 to 3 at a current density of 10C are shown. It can be seen that the sodium ion positive electrode materials prepared in Comparative Examples 2 and 3, respectively, using a single SiO2 coating layer and a single CoMoO4 coating layer, have a certain improvement in specific capacity and cycle stability compared to the uncoated sodium ion positive electrode material (Comparative Example 1), but the improvement is not obvious. However, in Example 1, a double coating layer of CoMoO4 coating layer and SiO2 coating layer is used, and its specific capacity and cycle stability are significantly improved, indicating that there is a synergistic effect between the CoMoO4 coating layer and the SiO2 coating layer. The specific capacity and cycle stability of the sodium ion positive electrode material prepared in Example 2 are somewhat lower than those in Example 1. This may be because after adjusting the pH of the dispersion to 6.5 in Example 2, the hydroxyl content on the surface of the prepared CoMoO4 coating layer is reduced, the bonding strength between the CoMoO4 coating layer and the SiO2 coating layer is reduced, and the interfacial energy between the coating layers is increased, thereby causing the specific capacity and cycle stability of the sodium ion battery assembled with the sodium ion positive electrode material in Example 2 to decrease. The initial specific capacity and cycle stability of the sodium ion battery positive electrode materials of Examples 3 and 4 will fluctuate to a certain extent, but overall they maintain good specific capacity and cycle stability.

[0074] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the present invention and implement it accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent variations or additions based on the spirit and substance of the present invention are intended to be encompassed within the scope of protection of the present invention.

Claims

1. A sodium ion battery cathode material, characterized in that It includes carbon-composite Na4Fe3(PO4)2P2O7, a first coating layer CoMoO4 coated on at least a portion of the surface of the carbon-composite Na4Fe3(PO4)2P2O7, and a second coating layer SiO2 coated on at least a portion of the surface of the first coating layer CoMoO4.

2. The method for preparing a positive electrode material for a sodium ion battery according to claim 1, wherein The following steps are involved: 1) adding a phosphorus source, an iron source, a sodium source, and a carbon source into solvent 1 to obtain a mixed solution; spray drying the mixed solution to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor; 2) dispersing a carbon-composite Na4Fe3(PO4)2P2O7 precursor in an aqueous solution containing a molybdenum source and a cobalt source to obtain a dispersion; subjecting the dispersion to a hydrothermal reaction to obtain a carbon-composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4; 3) Dispersing a carbon composite Na4Fe3(PO4)2P2O7 precursor coated with CoMoO4 in a mixed solution of a silicon source and solvent 2, stirring to perform a sol-gel hydrolysis reaction, and after the reaction is completed, heating and evaporating the solution to obtain a sodium ion battery positive electrode precursor; 4) The sodium ion battery positive electrode precursor is heat-treated under an inert atmosphere to obtain a sodium ion battery positive electrode material.

3. The method for preparing the positive electrode material for sodium ion batteries according to claim 2, wherein: In the step 1), the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, and sodium phosphate; The iron source is one or more of ferric sulfate, ferric nitrate, ferric acetate, ferrous oxalate, and ferric phosphate; The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium pyrophosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate; The carbon source is one or more of glucose, citric acid, oxalic acid, and ascorbic acid; Solvent 1 is one or more of pure water, ethanol, isopropanol, and acetone; The phosphorus source, iron source and sodium source are added according to the stoichiometric ratio of Na4Fe3(PO4)2P2O7; the amount of carbon source added is 1~10% of the mass of the theoretically generated Na4Fe3(PO4)2P2O7.

4. The method for preparing the sodium ion battery positive electrode material according to claim 1 or 2, wherein: In step 1), the inlet temperature of the spray is 150-200°C.

5. The method for preparing the positive electrode material for sodium ion batteries according to claim 2, wherein: In step 2), the molybdenum source is one or more of molybdenum acetate, molybdenum oxide, and ammonium molybdate; The cobalt source is one or more of cobalt nitrate, cobalt sulfate, cobalt carbonate, cobalt oxalate, cobalt chloride, and cobalt hydroxide; The molar ratio of Mo and Co in the molybdenum source and the cobalt source is 1:1; the amount of cobalt source added is 1-8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1).

6. The method for preparing the sodium ion battery positive electrode material according to claim 2 or 5, characterized in that: In the step 2), before the dispersion undergoes hydrothermal reaction, aqueous ammonia is added to adjust the pH of the dispersion to 8-9.

7. The method for preparing the positive electrode material for sodium ion batteries according to claim 2 or 5, characterized in that: In the step 2), the hydrothermal reaction temperature is 160-200° C., and the hydrothermal reaction time is 6-12 hours.

8. The method for preparing the sodium ion battery positive electrode material according to claim 2, wherein: In step 3), the silicon source is one or more of butyl orthosilicate, tetraethoxysilane, and tetrapropoxysilane; The amount of silicon source added is 1-8% of the molar amount of Fe element in the theoretically generated Na4Fe3(PO4)2P2O7 in step 1); Solvent 2 is one or more of pure water, ethanol, isopropanol, and acetone.

9. The method for preparing the positive electrode material for sodium ion batteries according to claim 2, wherein: In the step 3), the temperature of the sol-gel hydrolysis reaction is 50-70°C, and the time of the sol-gel hydrolysis reaction is 1-2 hours; the temperature of the heating evaporation is 80-100°C; In the step 4), the inert atmosphere is a nitrogen atmosphere or an argon atmosphere, the heat treatment temperature is 500-700° C., and the heat treatment time is 8-10 hours.

10. A sodium ion battery, characterized in that: The invention comprises the sodium ion battery positive electrode material according to claim 1, or the sodium ion positive electrode material prepared by any preparation method of claims 2 to 9.

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