Ferric sodium pyrophosphate phosphate positive electrode material, preparation method and application

Abstract

The application discloses a sodium iron pyrophosphosphate positive electrode material, a preparation method and application, and the sodium iron pyrophosphosphate positive electrode material comprises an inner core and a carbon coating layer coated on the surface of the inner core, and the inner core is vanadium and fluorine doped sodium iron pyrophosphosphate. The sodium iron pyrophosphosphate positive electrode material in the application realizes the improvement of the performance of the sodium iron pyrophosphosphate positive electrode material through the synergistic effect of vanadium and fluorine co-doping and carbon coating. The double doping is responsible for atomic scale local activation, the carbon coating is responsible for macroscopic electron transmission and interface protection, and the common action realizes the improvement of specific capacity, rate performance and low-temperature performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, and more specifically, to sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, due to their abundant resources, low cost, and relatively high safety, show broad application prospects in large-scale energy storage and other fields. Polyanionic compounds, especially phosphate systems (such as NaFePO4, Na...), are also promising candidates. 4-x Fe3(PO4)2P2O7, etc., have become mainstream cathode material candidates due to their stable three-dimensional framework structure.

[0003] However, the practical application of this type of material is limited by the fact that its actual reversible capacity is significantly lower than the theoretical value. For a long time, the industry has generally attributed this to sodium ions (Na+). + The slow diffusion kinetics caused by the large radius have been addressed through strategies such as material nano-sizing, carbon coating, and bulk heterovalent element doping. While some progress has been made, the bottleneck has not been fundamentally overcome.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide sodium iron pyrophosphate cathode material, preparation method and application, which has high specific capacity and rate performance.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a sodium iron pyrophosphate cathode material, comprising a core and a carbon coating layer covering the surface of the core, wherein the core is vanadium and fluorine-doped sodium iron pyrophosphate.

[0007] In an optional implementation, the molar ratio of Na, Fe, V, P, O and F in the kernel is (3.3~3.5):(2~3):(0.4~1):4:(14~15):(0.04~0.1); And / or, the thickness of the carbon coating layer is 2~5 nm; And / or, the sodium iron pyrophosphate has a D10 ≥ 600 and a D90 ≤ 900 nm; And / or, the D50 of the sodium iron pyrophosphate is 700-800 nm; And / or, the general formula of the kernel is Na z Fe 3-x V x (PO4)2P2O 7-y F yWhere 0.4≤x≤1, 0.04≤y≤0.1, 3.3≤z≤3.5; And / or, the core includes trivalent vanadium; And / or, vanadium is doped into the iron sites of the sodium iron pyrophosphate material.

[0008] Secondly, the present invention provides a method for preparing the sodium iron pyrophosphate cathode material according to any one of the foregoing embodiments, comprising: The raw material mixture containing sodium, iron, vanadium, phosphorus and fluorine sources is mixed with fuel for the first time to obtain a mixture. The mixture is pre-calcined to obtain a pre-calcined product; The pre-calcined product was mixed with the carbon source solution for a second time and then dried to obtain the dried product. The dried product is calcined to obtain the sodium iron pyrophosphate cathode material.

[0009] In an optional embodiment, the sodium source is selected from at least one of sodium chloride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, and sodium nitrate; And / or, the iron source is selected from at least one of ferric phosphate, ferric oxide, ferrous oxalate, ferric nitrate, ferric oxalate, ferrous acetate, ferrous sulfate, ferrous ammonium sulfate, and ferrous chloride; And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate; And / or, the carbon source is selected from at least one of glucose, citric acid, sucrose, fructooligosaccharide (PEG) and polyvinylpyrrolidone (PVP); And / or, the vanadium source is selected from at least one of ammonium metavanadate, vanadium oxalate, and vanadium trioxide; And / or, the fluorine source is selected from at least one of polyvinylidene fluoride, hydrofluoric acid, ammonium fluoride and sodium fluoride; And / or, the first mixing method is grinding and mixing, and the grinding time is 1~2 hours.

[0010] In an optional embodiment, the fuel is selected from citric acid; And / or, the ratio of the amount of the fuel to the amount of the metal element in the raw material mixture is 40% to 80%.

[0011] In an optional embodiment, the pre-firing includes first heating the mixture to 110~130℃ and holding it at that temperature for 0.5~2h, then heating it to 750~950℃ to burn the mixture and holding it at that temperature for 4~7h.

[0012] In an optional embodiment, the mass ratio of the carbon source in the carbon source solution to the pre-calcined product is 1-4%. And / or, the second mixing method is ball milling, wherein the revolution speed of the ball milling is 200~330 r / min, the rotation speed is 500~650 r / min, and the ball milling time is 10-15 h; And / or, the drying temperature is 80~110℃; And / or, the calcination atmosphere is air, the calcination temperature is 400~550℃, and the calcination time is 10~14h.

[0013] Thirdly, the present invention provides a positive electrode sheet, comprising the sodium iron pyrophosphate positive electrode material described in the foregoing embodiments or the sodium iron pyrophosphate positive electrode material prepared by the preparation method described in the foregoing embodiments.

[0014] In an optional embodiment, the positive electrode sheet further includes a conductive agent, graphene, and a binder, and the mass ratio of the sodium iron pyrophosphate positive electrode material to the conductive agent, graphene, and binder is 80~86:6~8:6~8:6~8.

[0015] Fourthly, the present invention provides a sodium-ion battery, including the positive electrode sheet described in the foregoing embodiments.

[0016] The present invention has the following beneficial effects: The sodium iron pyrophosphate cathode material in this application achieves performance enhancement through the synergistic effect of vanadium and fluorine co-doping and carbon coating. The dual doping is responsible for atomic-scale local activation, while the carbon coating is responsible for macroscopic electron transport and interface protection; both are indispensable. Among them, vanadium (V³... + Selective substitution of Fe sites, due to their unique electronic structure and coordination ability, can precisely control the local oxygen coordination environment adjacent to sodium sites, lower the sodium ion insertion / extraction energy barrier, and activate the originally electrochemically inert sodium sites; fluorine (F - The carbon coating partially replaces lattice oxygen, enhancing the P–O / F bond strength, stabilizing the anionic framework, and suppressing structural distortions and side reactions. Both components work synergistically to optimize sodium ion migration channels and improve structural stability; the carbon coating provides a continuous electron conduction pathway, improves interfacial contact, and suppresses electrolyte side reactions. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0018] The applicant's research found that the actual reversible capacity of phosphate system materials is significantly lower than the theoretical value, possibly because in phosphate crystal structures such as olivine, some thermodynamically existing sodium sites are in an "electrochemically inert" state due to their unfavorable local coordination environment. The anions (mainly O²⁺) surrounding these inert sodium sites... - The arrangement of sodium ions (i.e., the geometry and bonding strength of the coordination polyhedra) results in an extremely high sodium ion migration barrier, preventing these sites from participating in reversible insertion / extraction reactions under normal operating voltages, thus creating a significant "capacity gap." In other words, the key limiting factor is not an absolute deficiency of sodium content in the material, but rather the excessively low "utilization" of sodium ions under the existing structure.

[0019] Therefore, the key to improving the specific capacity and rate performance of phosphate cathode materials lies in precisely controlling the local coordination chemical environment of sodium ions at the atomic scale, breaking the original coordination restrictions, activating inert sodium sites into active sites, and simultaneously optimizing the migration channel network of sodium ions.

[0020] Therefore, embodiments of the present invention provide a sodium iron pyrophosphate cathode material, comprising a core and a carbon coating layer covering the surface of the core, wherein the core is vanadium and fluorine-doped sodium iron pyrophosphate.

[0021] The sodium iron pyrophosphate cathode material in this application achieves performance enhancement through the synergistic effect of vanadium and fluorine co-doping and carbon coating. The dual doping is responsible for atomic-scale local activation, while the carbon coating is responsible for macroscopic electron transport and interface protection; both are indispensable. Among them, vanadium (V³... + Selective substitution of Fe sites, due to their unique electronic structure and coordination ability, can precisely control the local oxygen coordination environment adjacent to sodium sites, lower the sodium ion insertion / extraction energy barrier, and activate the originally electrochemically inert sodium sites; fluorine (F - The carbon coating partially replaces lattice oxygen, enhancing the P–O / F bond strength, stabilizing the anionic framework, and suppressing structural distortions and side reactions. Both components work synergistically to optimize sodium ion migration channels and improve structural stability; the carbon coating provides a continuous electron conduction pathway, improves interfacial contact, and suppresses electrolyte side reactions.

[0022] In an optional embodiment, the molar ratio of Na, Fe, V, P, O and F in the core is (3.3~3.5):(2~3):(0.4~1):4:(14~15):(0.04~0.1); limiting the amount of V and F doping is beneficial to improving the "utilization rate" of sodium ions while taking into account stability.

[0023] And / or, the thickness of the carbon coating layer is 2~5 nm, for example 2 nm, 2.3 nm, 2.6 nm, 2.9 nm, 3.2 nm, 3.5 nm, 3.8 nm, 4.1 nm, 4.4 nm, 4.7 nm, 5 nm; to ensure the formation of a continuous conductive network and not hinder the transport of sodium ions at the interface; if it is too thin, the conductivity will be insufficient, and if it is too thick, the impedance will increase and the proportion of active material will decrease.

[0024] And / or, the sodium iron pyrophosphate has a D10 ≥ 600, for example 600 nm, 611 nm, 622 nm, 633 nm, 644 nm, 656 nm, 667 nm, 678 nm, 689 nm, 700 nm; and a D90 ≤ 900 nm, for example 800 nm, 811 nm, 822 nm, 833 nm, 844 nm, 856 nm, 867 nm, 878 nm, 889 nm, 900 nm; And / or, the D50 of the sodium iron pyrophosphate is 700-800 nm, for example 700 nm, 711 nm, 722 nm, 733 nm, 744 nm, 756 nm, 767 nm, 778 nm, 789 nm, 800 nm; Sodium iron pyrophosphate has a narrow particle size distribution and relatively uniform nanoscale particles, which is beneficial to ensuring electrode homogeneity. If the particles of sodium iron pyrophosphate are too fine, side reactions will be aggravated; if they are too coarse, the diffusion path of sodium ions in the bulk phase will be prolonged. Within the above range, it is beneficial to balance rate performance, low temperature performance and cycle life.

[0025] And / or, the general formula of the kernel is Na z Fe 3-x V x (PO4)2P2O 7-y F y Where 0.4≤x≤1, 0.04≤y≤0.1, 3.3≤z≤3.5; And / or, the core includes trivalent vanadium, which is a prerequisite for the electronic structure to achieve local activation.

[0026] And / or, vanadium doping into the iron sites of the sodium iron pyrophosphate material facilitates its direct action on the transition metal coordination layer surrounding the sodium migration channel, thereby directionally regulating the local coordination environment of the sodium sites.

[0027] The present invention also provides a method for preparing the sodium iron pyrophosphate cathode material according to any one of the foregoing embodiments, comprising: The raw material mixture containing sodium, iron, vanadium, phosphorus and fluorine sources is mixed with fuel for the first time to obtain a mixture. The mixture is pre-calcined to obtain a pre-calcined product; The pre-calcined product was mixed with the carbon source solution for a second time and then dried to obtain the dried product. The dried product is calcined to obtain the sodium iron pyrophosphate cathode material.

[0028] In the preparation method of sodium iron pyrophosphate cathode material of this application, citric acid is used as fuel in the first mixing stage. Its chelating effect achieves atomic-level uniform dispersion of Na, Fe, V, P, and F, ensuring V³ + Selective occupation of Fe sites, F - Entering the oxygen site provides a structural basis for local sodium coordination regulation; the pre-calcination stage promotes the pure formation of sodium iron pyrophosphate phase; the second mixing loads the carbon source in solution onto the surface of the crystallized core to ensure uniform carbon layer thickness, followed by calcination to form a carbon coating layer, thus obtaining the aforementioned sodium iron pyrophosphate cathode material with vanadium and fluorine doped core and carbon coating on the outside.

[0029] In an optional embodiment, the sodium source is selected from at least one of sodium chloride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, and sodium nitrate; And / or, the iron source is selected from at least one of ferric phosphate, ferric oxide, ferrous oxalate, ferric nitrate, ferric oxalate, ferrous acetate, ferrous sulfate, ferrous ammonium sulfate, and ferrous chloride; And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate; And / or, the carbon source is selected from at least one of glucose, citric acid, sucrose, fructooligosaccharide (PEG) and polyvinylpyrrolidone (PVP); And / or, the vanadium source is selected from at least one of ammonium metavanadate, vanadium oxalate, and vanadium trioxide; And / or, the fluorine source is selected from at least one of polyvinylidene fluoride, hydrofluoric acid, ammonium fluoride and sodium fluoride; And / or, the first mixing method is grinding mixing, and the grinding time is 1~2h, for example 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2.0h.

[0030] In an optional embodiment, the fuel is selected from citric acid; its multi-carboxyl structure can chelate a variety of metal ions, achieving atomic-level uniform mixing while reducing the introduction of impurities.

[0031] And / or, the ratio of the amount of the fuel to the amount of the metal element in the raw material mixture is 40% to 80%, for example 40%, 44%, 48%, 52%, 56%, 60%, 64%, 68%, 72%, 76%, 80%, to provide sufficient reduction potential and exothermic density, ensuring combustion self-sustaining and phase formation driving force; if the amount of fuel is too low, the reaction will be incomplete, and if the amount of fuel is too high, it will easily lead to excessive carbon residue or local overheating that damages the crystal lattice.

[0032] In an optional embodiment, the pre-combustion includes first heating the mixture to 110-130°C, for example, 110°C, 112°C, 114°C, 116°C, 118°C, 120°C, 122°C, 124°C, 126°C, 128°C, or 130°C and holding it at that temperature for 0.5-2 hours, for example, 0.5 hours, 0.7 hours, 0.9 hours, 1.1 hours, 1.3 hours, 1.5 hours, 1.7 hours, 1.9 hours, or 2.0 hours; then heating it to 750-950°C, for example, 750°C, 772°C, 794°C, 817°C, 839°C, 861°C, 883°C, 906°C, 928°C, or 950°C, to combust the mixture and holding it at that temperature for 4-7 hours, for example, 4.0 hours, 4.3 hours, 4.6 hours, 4.9 hours, 5.2 hours, 5.5 hours, or 5.8 hours. h, 6.1 h, 6.4 h, 6.7 h, 7.0 h. In the low-temperature section, the raw materials are dehydrated and pre-nucleated to avoid violent vaporization of moisture in the high-temperature section, which would cause material splashing and structural breakage; the instantaneous combustion in the high-temperature section provides activation energy to promote phase purification, and then heat preservation completes the process to homogenize the composition and grow the grains appropriately, taking into account both crystallinity and doping stability.

[0033] In an optional embodiment, the mass ratio of the carbon source in the carbon source solution to the pre-calcined product is 1-4%, for example, 1%, 1.3%, 1.6%, 1.9%, 2.2%, 2.5%, 2.8%, 3.1%, 3.4%, 3.7%, or 4%; and the carbon layer thickness is controlled to be 2–5 nm.

[0034] And / or, the second mixing method is ball milling, wherein the revolution speed of the ball mill is 200~330 r / min, for example 200 r / min, 214 r / min, 229 r / min, 243 r / min, 257 r / min, 271 r / min, 286 r / min, 300 r / min, 314 r / min, 329 r / min, 330 r / min; the rotation speed is 500~650 r / min, for example 500 r / min, 517 r / min, 533 r / min, 550 r / min, 567 r / min, 583 r / min, 600 r / min, 617 r / min, 633 r / min, 650 r / min; and the ball milling time is 10-15 h, for example 10.0 h, 10.6 h, 11.1 h, 11.7 h, 12.2 h. h, 12.8h, 13.3h, 13.9h, 14.4h, 15.0h; are conducive to uniform mixing of carbon source and pre-calcined product.

[0035] And / or, the drying temperature is 80~110℃, for example 80℃, 83℃, 86℃, 89℃, 92℃, 95℃, 98℃, 101℃, 104℃, 107℃, 110℃; to remove the solvent from the carbon source solution, for example, the solvent in the carbon source solution is ethanol.

[0036] And / or, the calcination atmosphere is air, and the calcination temperature is 400~550℃, for example 400℃, 417℃, 433℃, 450℃, 467℃, 483℃, 500℃, 517℃, 533℃, 550℃; the calcination time is 10~14h, for example 10.0 h, 10.4 h, 10.8h, 11.2 h, 11.6 h, 12.0 h, 12.4 h, 12.8 h, 13.2 h, 13.6 h, 14.0 h. This provides a moderate oxidizing environment to promote the complete pyrolysis of the carbon source into a conductive amorphous carbon layer, avoiding excessive carbon residue under an inert atmosphere or fluorine loss and metal valence state fluctuations under a reducing atmosphere.

[0037] The present invention also provides a positive electrode sheet, comprising the sodium iron pyrophosphate positive electrode material described in the foregoing embodiments or the sodium iron pyrophosphate positive electrode material prepared by the preparation method described in the foregoing embodiments.

[0038] In an optional embodiment, the positive electrode sheet further includes a conductive agent, graphene, and a binder, and the mass ratio of the sodium iron pyrophosphate positive electrode material to the conductive agent, graphene, and binder is 80~86:6~8:6~8:6~8.

[0039] This invention also provides a sodium-ion battery, including the positive electrode sheet described in the foregoing embodiments.

[0040] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0041] Example 1 This embodiment provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na : 2.6 mol Fe : 0.4 mol V : 4 mol P : 0.05 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120 °C for 1 h. Then, the muffle furnace was heated to 800 °C, and the mixture immediately began to burn. It was held at 800 °C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 4 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. Then, it was dried at 90 °C. The dried mixture was placed in a high-temperature furnace and calcined at 480 °C for 12 h in air atmosphere to obtain carbon-coated, V and F-doped sodium iron pyrophosphate cathode material.

[0042] Example 2: This embodiment provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 2.3 mol Fe: 0.7 mol V: 4 mol P: 0.08 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120 °C for 1 h. Then, the muffle furnace was heated to 800 °C, and the mixture immediately began to burn. It was held at 800 °C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 6 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. The mixture was dried at 90 °C and then placed in a high-temperature furnace and calcined at 480 °C for 12 h in air atmosphere to obtain carbon-coated, V and F-doped sodium iron pyrophosphate cathode material.

[0043] Example 3: This embodiment provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 2 mol Fe: 1 mol V: 4 mol P: 0.1 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120°C for 1 h. Then, the muffle furnace was heated to 800°C, and the mixture immediately began to burn. It was held at 800°C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 8 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. The mixture was dried at 90 °C and then placed in a high-temperature furnace and calcined at 480 °C for 12 h in air atmosphere to obtain carbon-coated, V and F-doped sodium iron pyrophosphate cathode material.

[0044] Comparative Example 1: This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 3 mol Fe: 0 mol V: 4 mol P: 0 mol F. Citric acid, comprising 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120°C for 1 h. Then, the muffle furnace was heated to 800°C, and the mixture immediately began to burn. It was held at 800°C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 4 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. The mixture was dried at 90 °C and then placed in a high-temperature furnace and calcined at 480 °C for 12 h in air atmosphere to obtain carbon-coated sodium iron pyrophosphate cathode material.

[0045] Comparative Example 2: This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 2.6 mol Fe: 0.4 mol V: 4 mol P: 0 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120°C for 1 h. Then, the muffle furnace was heated to 800°C, and the mixture immediately began to burn. It was held at 800°C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 4 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. The mixture was then dried at 80-110 °C. The dried mixture was placed in a high-temperature furnace and calcined at 400-550 °C for 12 h in air atmosphere to obtain carbon-coated, V-doped sodium iron pyrophosphate cathode material.

[0046] Comparative Example 3 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 3 mol Fe: 0 mol V: 4 mol P: 0.05 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120°C for 1 h. Then, the muffle furnace was heated to 800°C, and the mixture immediately began to burn. It was held at 800°C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. Separately, 4 g of glucose was dissolved in anhydrous ethanol (AR) and ball-milled with the pre-calcined product at a speed of 300 r / min revolution and 600 r / min rotation for 12 h. The mixture was dried at 90 °C and then placed in a high-temperature furnace and calcined at 480 °C for 12 h in air atmosphere to obtain carbon-coated, F-doped sodium iron pyrophosphate cathode material.

[0047] Comparative Example 4 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, specifically including the following steps: Using NaC₂H₃O₂·3H₂O (AR), Fe(NO₃)₃·9H₂O (AR), V₂O₃ (AR), H₃PO₄ (AR), and NH₄F (AR) as raw materials, they were mixed in a ratio of 3.4 mol Na: 2.6 mol Fe: 0.4 mol V: 4 mol P: 0.05 mol F. Citric acid, accounting for 50% of the total metal ions in the raw materials, was used as fuel. The raw materials and fuel were mixed and ground for 1 h. The ground mixture was placed in a muffle furnace and held at 120 °C for 1 h. Then, the muffle furnace was heated to 800 °C, and the mixture immediately ignited. It was held at 800 °C for 5 h. After the muffle furnace cooled naturally, a pre-calcined product was obtained, with a total weight of 200 g. The pre-calcined product was calcined in air at 480 °C for 12 h to obtain V and F-doped sodium iron pyrophosphate cathode material.

[0048] Test Example 1 The particle size of the sodium iron pyrophosphate cathode materials prepared in the above embodiments and comparative examples was statistically analyzed using a Malvern laser particle size analyzer, and the results are shown in Table 1.

[0049] Test Example 2 The sodium iron pyrophosphate cathode materials prepared in the above embodiments and comparative examples were stirred and mixed with a conductive agent, graphene, PVDF, and NMP to obtain a cathode slurry, which was then coated onto a current collector. The mass ratio of the sodium iron pyrophosphate cathode material, conductive agent, graphene, and binder was 80:6:6:8. Next, the cathode material coated onto the current collector was dried under vacuum, and the dried cathode material was pressed into a sheet with a thickness of 0.3 mm. Then, the sheet-like cathode material, sodium metal sheet, separator, and electrolyte were assembled into a CR2032 coin cell to test the electrochemical performance of the material. The specific capacity obtained by constant current charge-discharge test at 25℃ and the further calculated discharge efficiency are shown in Table 1.

[0050] The assembled CR2032 battery was first cycled 3 times at a constant current of 0.5C at 25℃, and then placed at 0℃, -20℃, and -40℃ for 24 hours respectively. The charge / discharge cutoff voltage was 2.0~3.9V (vs. Na / Na). + Then, the battery material was charged and discharged at 0.5 C to obtain its specific capacity and complete the low-temperature performance test. The results are shown in Table 2.

[0051] The assembled CR2032 battery was subjected to constant current charge and discharge at 0.5C, 5C, 10C and 30C at 25°C to perform rate performance tests. The results are shown in Table 2.

[0052] Table 1

[0053] Table 2

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A sodium iron pyrophosphate cathode material, characterized in that, It includes a core and a carbon coating layer covering the surface of the core, wherein the core is vanadium and fluorine-doped sodium iron pyrophosphate.

2. The sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The molar ratio of Na, Fe, V, P, O and F in the core is (3.3~3.5):(2~3):(0.4~1):4:(14~15):(0.04~0.1); And / or, the thickness of the carbon coating layer is 2~5 nm; And / or, the sodium iron pyrophosphate has a D10 ≥ 600 and a D90 ≤ 900 nm; And / or, the D50 of the sodium iron pyrophosphate is 700-800 nm; And / or, the general formula of the kernel is Na z Fe 3-x V x (PO4)2P2O 7-y F y Where 0.4≤x≤1, 0.04≤y≤0.1, 3.3≤z≤3.5; And / or, the core includes trivalent vanadium; And / or, vanadium is doped into the iron sites of the sodium iron pyrophosphate material.

3. A method for preparing the sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, include: The raw material mixture containing sodium, iron, vanadium, phosphorus and fluorine sources is mixed with fuel for the first time to obtain a mixture. The mixture is pre-calcined to obtain a pre-calcined product; The pre-calcined product was mixed with the carbon source solution for a second time and then dried to obtain the dried product. The dried product is calcined to obtain the sodium iron pyrophosphate cathode material.

4. The method for preparing the sodium iron pyrophosphate cathode material according to claim 3, characterized in that, The sodium source is selected from at least one of sodium chloride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, and sodium nitrate. And / or, the iron source is selected from at least one of ferric phosphate, ferric oxide, ferrous oxalate, ferric nitrate, ferric oxalate, ferrous acetate, ferrous sulfate, ferrous ammonium sulfate, and ferrous chloride; And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate; And / or, the carbon source is selected from at least one of glucose, citric acid, sucrose, fructooligosaccharide (PEG) and polyvinylpyrrolidone (PVP); And / or, the vanadium source is selected from at least one of ammonium metavanadate, vanadium oxalate, and vanadium trioxide; And / or, the fluorine source is selected from at least one of polyvinylidene fluoride, hydrofluoric acid, ammonium fluoride and sodium fluoride; And / or, the first mixing method is grinding and mixing, and the grinding time is 1~2 hours.

5. The method for preparing the sodium iron pyrophosphate cathode material according to claim 3, characterized in that, The fuel is selected from citric acid; And / or, the ratio of the amount of the fuel to the amount of the metal element in the raw material mixture is 40% to 80%.

6. The method for preparing the sodium iron pyrophosphate cathode material according to claim 3, characterized in that, The pre-firing process involves first heating the mixture to 110-130°C and holding it at that temperature for 0.5-2 hours, then heating it to 750-950°C to ignite the mixture and holding it at that temperature for 4-7 hours.

7. The method for preparing the sodium iron pyrophosphate cathode material according to claim 3, characterized in that, The mass ratio of carbon source in the carbon source solution to the pre-calcined product is 1-4%. And / or, the second mixing method is ball milling, wherein the revolution speed of the ball milling is 200~330 r / min, the rotation speed is 500~650 r / min, and the ball milling time is 10-15 h; And / or, the drying temperature is 80~110℃; And / or, the calcination atmosphere is air, the calcination temperature is 400~550℃, and the calcination time is 10~14h.

8. A positive electrode sheet, characterized in that, The sodium iron pyrophosphate cathode material according to claim 1 or 2, or the sodium iron pyrophosphate cathode material prepared by the preparation method according to any one of claims 3-7.

9. The positive electrode sheet according to claim 8, characterized in that, The positive electrode sheet also includes a conductive agent, graphene, and a binder, and the mass ratio of the sodium iron pyrophosphate positive electrode material to the conductive agent, graphene, and binder is 80~86:6~8:6~8:6~8.

10. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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