A low-resistivity sodium iron pyrophosphate positive electrode material and its preparation method and application

Abstract

The present invention relates to a low-resistivity sodium iron pyrophosphate cathode material and its preparation method and application, belonging to the technical field of sodium ion batteries. The chemical formula of the low-resistivity sodium iron pyrophosphate cathode material of the present invention is Na 4‑x Fe 3‑y (PO4)2(P2O7) / C, 0≤x≤1, 0≤y≤1, x<y. By adjusting the content of Na vacancies and Fe defects in the low-resistivity sodium iron pyrophosphate cathode material, not only can its band gap width be effectively controlled, but also its conductivity can be controlled. On the basis of regulating Na vacancies and Fe defects, the heating rate, holding temperature and holding time are further regulated to ensure that the phosphate (PO4 3‑ ) and pyrophosphate (P2O7 4‑ ) conversion efficiency, making the material have higher electronegativity, thereby increasing the energy density.

Description

Technical Field

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

[0002] Among lithium batteries currently on the market, sodium (Na)-ion batteries (SIBs) have attracted significant attention due to the abundance of the element Na and its similar physical and chemical properties to lithium, making them an ideal alternative to lithium-ion batteries. The technical challenges currently facing Na-ion batteries primarily focus on cathode materials, which are currently concentrated in vanadium (V), iron (Fe), and manganese (Mn)-based cathode materials. Due to the inherent environmental concerns and instability of V- and Mn-based cathode materials, Fe-based cathode materials are expected to be strong competitors for future Na-ion battery cathodes. Na4Fe3(PO4)2P2O7 is an ideal cathode material for Na-ion batteries due to its low cost, environmental friendliness, excellent structural stability, and long cycle life. However, its poor electrical conductivity results in significant energy loss during use. Therefore, the development of low-resistivity sodium iron pyrophosphate cathode materials offers a promising approach for practical production applications. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides a low-resistivity sodium iron pyrophosphate positive electrode material and its preparation method and application.

[0004] The first object of the present invention is to provide a low resistivity sodium iron pyrophosphate positive electrode material, the chemical formula of the low resistivity sodium iron pyrophosphate positive electrode material is Na 4-x Fe 3-y (PO4)2(P2O7) / C, 0≤x≤1, 0≤y≤1, x<y.

[0005] In one embodiment of the present invention, the band gap Eg of the low-resistivity sodium iron pyrophosphate positive electrode material is less than 2.7 eV.

[0006] In one embodiment of the present invention, the resistivity R of the low-resistivity sodium iron pyrophosphate positive electrode material satisfies the following relationship:

[0007] The second object of the present invention is to provide a method for preparing the low-resistivity sodium ferric pyrophosphate positive electrode material, comprising the following steps: uniformly mixing a sodium source, an iron source, a phosphorus source, a carbon source and water, and obtaining the low-resistivity sodium ferric pyrophosphate positive electrode material by ball milling, spray drying and sintering.

[0008] In one embodiment of the present invention, the sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium sulfate and sodium dihydrogen phosphate; the iron source is selected from one or more of ferric acetate, ferric nitrate, ferrous oxalate, ferric oxide and ferric phosphate; the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate and phosphorus pentoxide; the carbon source is selected from one or more of carbon black, graphite, acetylene black, activated carbon, citric acid, polyethylene glycol, sucrose and glucose.

[0009] In one embodiment of the present invention, the spray drying temperature is 100°C-140°C.

[0010] In one embodiment of the present invention, the sintering is carried out under a protective atmosphere and is divided into two stages; the first stage is to heat the temperature to 300°C-350°C at a rate of 5°C / min-7°C / min and keep it warm for 10min-30min; the second stage is to heat the temperature to 600°C-750°C at a rate of 0.5°C / min-2°C / min and keep it warm for 5h-10h.

[0011] In one embodiment of the present invention, the protective atmosphere is selected from nitrogen atmosphere or argon atmosphere.

[0012] The third object of the present invention is to provide a sodium ion battery positive electrode, comprising the low-resistivity sodium iron pyrophosphate positive electrode material or the low-resistivity sodium iron pyrophosphate positive electrode material prepared by the method described.

[0013] In one embodiment of the present invention, the sodium ion battery positive electrode further includes a conductive agent and a binder.

[0014] In one embodiment of the present invention, the conductive agent is selected from one or more of carbon nanotubes, acetylene black, conductive carbon black, conductive graphite, carbon fiber and graphene.

[0015] In one embodiment of the present invention, the amount of the conductive agent is ≤10wt%. It can be 0.01wt%-10wt%, 1wt%-10wt%, 2wt%-10wt%, 3wt%-10wt%, 4wt%-10wt%, 5wt%-10wt%, 6wt%-10wt%, 7wt%-10wt%, 8wt%-10wt%, 9wt%-10wt%, 0.01wt%, 0.05wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, or any amount between any two values.

[0016] In one embodiment of the present invention, the binder is selected from one or more of polyolefins, fluorine-containing resins, polypropylene resins and rubbers.

[0017] Furthermore, the binder is selected from one or more of polyvinylidene fluoride, styrene rubber, nitrile rubber, styrene-butadiene rubber (SBR), polyacrylamide (PAA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), butadiene rubber, modified butadiene rubber, carboxyl-modified styrene-butadiene rubber and modified polyorganosiloxane polymers.

[0018] In one embodiment of the present invention, the amount of the binder is ≤10wt%. It can be 0.01wt%-10wt%, 1wt%-10wt%, 2wt%-10wt%, 3wt%-10wt%, 4wt%-10wt%, 5wt%-10wt%, 6wt%-10wt%, 7wt%-10wt%, 8wt%-10wt%, 9wt%-10wt%, 0.01wt%, 0.05wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, or any amount between any two values.

[0019] A fourth object of the present invention is to provide a sodium ion battery comprising the sodium ion battery positive electrode.

[0020] The technical solution of the present invention has the following advantages over the prior art:

[0021] (1) The preparation method described in the present invention can effectively control the band gap width and further control the conductivity of the low-resistivity sodium iron pyrophosphate positive electrode material by adjusting the content of Na vacancies and Fe defects.

[0022] (2) The preparation method of the present invention ensures that phosphate (PO4 3- ) and pyrophosphate (P2O7 4- ) conversion efficiency, making the material have higher electronegativity, thereby increasing the energy density.

[0023] (3) The low-resistivity sodium iron pyrophosphate positive electrode material described in the present invention reduces the energy gap between the valence band and the conduction band in the material by adjusting the number of Fe defects to be slightly larger than the number of Na vacancies without introducing environmentally unfriendly and unstable elements such as V and Mn, thereby reducing the Na ion diffusion barrier. A certain defect concentration widens the Na ion diffusion channel, optimizing its conductive properties from the most fundamental material electronic structure layout. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0025] Figure 1 Na4Fe of Example 1 of the present invention 2.9 The first charge / discharge curves of (PO4)2(P2O7) / C as the positive electrode of sodium-ion batteries;

[0026] Figure 2 Na4Fe of Example 1 of the present invention 2.9 UV-visible diffuse reflectance curve of (PO4)2(P2O7) / C;

[0027] Figure 3 is Na in Example 2 of the present invention 3.9 Fe 2.8 The first charge / discharge curves of (PO4)2(P2O7) / C as the positive electrode of sodium-ion batteries;

[0028] Figure 4 is Na in Example 2 of the present invention 3.9 Fe 2.8 UV-visible diffuse reflectance curve of (PO4)2(P2O7) / C;

[0029] Figure 5 is Na in Example 3 of the present invention 3.9 Fe 2.7 The first charge / discharge curves of (PO4)2(P2O7) / C as the positive electrode of sodium-ion batteries;

[0030] Figure 6 is Na in Example 3 of the present invention 3.9 Fe 2.7 UV-visible diffuse reflectance curve of (PO4)2(P2O7) / C;

[0031] Figure 7 The Na of Comparative Example 1 of the present invention 3.9 Fe 2.9 The first charge / discharge curves of (PO4)2(P2O7) / C as the positive electrode of sodium-ion batteries;

[0032] Figure 8 The Na of Comparative Example 2 of the present invention 3.9 First charge / discharge curves of Fe3(PO4)2(P2O7) / C as the positive electrode of sodium-ion battery. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. It should be understood that the specific embodiments are only used to illustrate the present invention and are not intended to limit the present invention.

[0034] In the present invention, unless otherwise defined, technical and scientific terms used herein have the same meanings as commonly understood by one skilled in the art to which the present invention belongs.

[0035] In the present invention, unless stated otherwise, the term "and / or" used in the present invention includes any and all combinations of one or more of the associated listed items.

[0036] In the present invention, unless otherwise stated, the experimental methods used in the examples of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated.

[0037] Example 1

[0038] The low-resistivity sodium iron pyrophosphate positive electrode material of the present invention and its preparation method and application specifically include the following steps:

[0039] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The low resistivity sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0040] Example 2

[0041] The low-resistivity sodium iron pyrophosphate positive electrode material of the present invention and its preparation method and application specifically include the following steps:

[0042] 14.3 g of sodium carbonate, 52.3 g of ferric phosphate dihydrate, 18.7 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 6 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The low resistivity sodium iron pyrophosphate positive electrode material Na 3.9 Fe 2.8 (PO4)2(P2O7) / C.

[0043] Example 3

[0044] The low-resistivity sodium iron pyrophosphate positive electrode material of the present invention and its preparation method and application specifically include the following steps:

[0045] 15.4 g of sodium carbonate, 50.4 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 7 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The low resistivity sodium iron pyrophosphate positive electrode material Na was obtained. 3.9 Fe 2.7 (PO4)2(P2O7) / C.

[0046] Example 4

[0047] The low-resistivity sodium iron pyrophosphate positive electrode material of the present invention and its preparation method and application specifically include the following steps:

[0048] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 300 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The low resistivity sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0049] Example 5

[0050] The low-resistivity sodium iron pyrophosphate positive electrode material of the present invention and its preparation method and application specifically include the following steps:

[0051] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 350 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The low resistivity sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0052] Comparative Example 1 is basically the same as Example 1, except that the ratio of sodium to iron is

[0053] 14.8 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium ferric pyrophosphate positive electrode material Na was obtained. 3.9 Fe 2.9 (PO4)2(P2O7) / C.

[0054] Comparative Example 2 is basically the same as Example 1, except that the ratio of sodium to iron is

[0055] 15.4 g of sodium carbonate, 56.0 g of ferric phosphate dihydrate, 15.6 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium ferric pyrophosphate positive electrode material Na was obtained. 3.9 Fe3(PO4)2(P2O7) / C.

[0056] Comparative Example 3 is basically the same as Example 1, except that the heating rate in the first stage of sintering is

[0057] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 3 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0058] Comparative Example 4 is basically the same as Example 1, except that the heating rate in the first stage of sintering is

[0059] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 8 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0060] Comparative Example 5 is basically the same as Example 1, except that the sintering temperature in the first stage of sintering is

[0061] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 275 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0062] Comparative Example 6 is basically the same as Example 1, except that the sintering temperature in the first stage of sintering is

[0063] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 400 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 20 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0064] Comparative Example 7 is basically the same as Example 1, except that the holding time of the first stage of sintering is

[0065] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 5 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0066] Comparative Example 8 is basically the same as Example 1, except that the holding time of the first stage of sintering is

[0067] 15.4 g of sodium carbonate, 54.2 g of ferric phosphate dihydrate, 17.2 g of sodium dihydrogen phosphate dihydrate, 7.0 g of glucose, 1.7 g of polyethylene glycol and 120.0 g of deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120 ° C. The ground powder was sintered in a tube furnace. The mixture was heated to 325 ° C at a rate of 5 ° C / min in a nitrogen atmosphere and kept warm for 45 min. Then, the mixture was heated to 650 ° C at a rate of 1 ° C / min and kept warm for 8 h. The sodium iron pyrophosphate positive electrode material Na4Fe 2.9 (PO4)2(P2O7) / C.

[0068] Verification Example

[0069] According to the relationship between the resistivity R and x and y of the sodium iron pyrophosphate positive electrode materials of Examples 1-5 and Comparative Examples 1-8, data fitting was performed to obtain the following relationship: The function shows that when x = 0.076 and y = 0.144, the resistivity is the lowest, 1674Ω·cm. This is verified by:

[0070] 14.52g sodium carbonate, 52.3g ferric phosphate dihydrate, 17.46g sodium dihydrogen phosphate dihydrate, 7.0g glucose, 1.7g polyethylene glycol and 120.0g deionized water were mixed together and ball-milled. After ball-milling, the mixture was spray-dried at 120°C. The ground powder was sintered in a tube furnace. The mixture was heated to 325°C at a rate of 5°C / min under a nitrogen atmosphere and kept warm for 20min. The mixture was then heated to 650°C at a rate of 1°C / min and kept warm for 8h. The low resistivity sodium iron pyrophosphate positive electrode material Na was obtained. 3.924 Fe 2.856 (PO4)2(P2O7) / C.

[0071] Test Example: Battery fabrication and performance testing based on the sodium iron pyrophosphate cathode materials of Examples 1-5 and Comparative Examples 1-8

[0072] Production of sodium ion batteries:

[0073] Positive electrode sheet: The sodium iron pyrophosphate positive electrode materials of Examples 1-5 and Comparative Examples 1-8, super P and polyvinylidene fluoride were mixed at a mass ratio of 8:1:1, N-methylpyrrolidone was added and stirred to obtain a positive electrode active slurry, the positive electrode active slurry was evenly coated on the surface of the aluminum foil current collector, and a 200 μm doctor blade was used to prepare a pole sheet. After drying and cold pressing, a positive electrode sheet was obtained. The mass loading of the active material was about 2.5 mg / cm 2 ;

[0074] Negative electrode: sodium metal sheet;

[0075] Separator: PE separator with a thickness of 9 μm;

[0076] Electrolyte: Sodium hexafluorophosphate dissolved in ethylene carbonate at a concentration of 1 mol / L;

[0077] Sodium ion battery assembly: Arrange the positive electrode sheet, separator, negative electrode sheet, and separator in sequence, and use winding as the assembly method.

[0078] Performance testing:

[0079] Charge / discharge test: The battery is charged / discharged at a rate of 0.1C in the voltage range of 1.5V-4.2V.

[0080] Band gap Eg (eV) test: The band gap width (band gap) of the obtained sodium titanium phosphate composite material can be calculated by UV-visible diffuse reflectance test. The band gap is calculated by the intercept method, Eg (eV) = hc / λg = 1240 / λg (nm).

[0081] Resistivity R (Ω·cm) test: The ST2742C automatic powder resistivity tester was used to measure the powder resistivity of the sodium titanium phosphate composite material. Under the condition of the same carbon content, the electronic conductivity of the sodium titanium phosphate composite material can be intuitively reflected. The curve between pressure (MPa) and resistivity (Ω·cm) was recorded, and the resistivity at a pressure of 60 MPa was used as a comparison between different samples.

[0082] Figures 1-8 Table 1 shows the final measured performance parameters:

[0083] Table 1

[0084]

[0085]

[0086] from Figures 1-8 As can be seen from Table 1, by adjusting the number and relationship of Fe defects and Na vacancies, while ensuring that the number of Fe defects is slightly larger than the number of Na vacancies, and combining appropriate heating rate, holding temperature and holding time, the phosphate (PO4 3- ) and pyrophosphate (P2O7 4- ) conversion efficiency, achieving a band gap Eg of less than 2.7 eV and a resistivity of less than 2000 Ω·cm for the sodium iron pyrophosphate cathode material. Under optimal sintering conditions, the relationship between x, y, and powder resistivity in the examples was used to calculate the formula with the lowest resistivity. This was then verified, and the results of the verification example deviated from the predicted value by only 1.25%, demonstrating the accuracy of this prediction method.

[0087] It can be seen from Examples 1-3 and Comparative Examples 1-2 that the chemical formula Na 4-x Fe 3-y When x<y in (PO4)2(P2O7) / C, it can ensure that the material has a certain amount of Fe defects and Na vacancies, and the number of Fe defects is slightly greater than the number of Na vacancies, which is beneficial to reducing the Na ion diffusion barrier and widening the Na ion diffusion channel; the appropriate heating rate makes the phosphate (PO4 3- ) and pyrophosphate (P2O7 4- ) is more completely converted, and the completely converted pyrophosphate (P2O7 4- ) has a higher electronegativity, thereby increasing the energy density. When x ≥ y, a Na-poor structure is formed during the sintering process, and the Fe element is excessive, forming a large amount of dawsonite phase NaFePO4. The dawsonite phase NaFePO4 has a large band gap and low conductivity, resulting in an excessively large overall band gap of the material.

[0088] It can be seen from Example 1 and Comparative Examples 3-4 that even if the material ratio satisfies x<y, when the heating rate of the first stage is lower than 5℃ / min, FePO4 is converted into inactive glass phase FePO4 during the sintering process, and ultimately no active sodium iron pyrophosphate is generated, resulting in a sharp decrease in the specific capacity of the material; when the heating rate of the first stage is higher than 7℃ / min, the phosphate (PO4 3- ) is not able to be converted into pyrophosphate (P2O7 4- ), a large amount of NaFePO4 phase with an olivine structure and no electrochemical activity will be generated, resulting in a decrease in specific capacity.

[0089] It can be seen from Examples 1, 4-5 and Comparative Examples 5-6 that the sintering temperature in the first stage is the thermodynamic basis for determining whether phase formation can occur. When the sintering temperature in the first stage is lower than 300°C, acid radical (PO4 3- ) to pyrophosphate (P2O7 4- ) will generate a NaFePO4 phase with no electrochemical activity, and the specific capacity will decrease; when the sintering temperature of the first stage is greater than 350℃, the pyrophosphate will continue to dehydrate to generate a cyclic polymetaphosphoric acid, which cannot react with Na and Fe elements to generate active substances.

[0090] It can be seen from Example 1 and Comparative Examples 7-8 that the holding time of the first stage is also an important parameter index, which is closely related to the acid radical (PO4 3- ) to pyrophosphate (P2O7 4- ) is closely related to the conversion amount of acid radical (PO4 3- ) to pyrophosphate (P2O7 4- ) conversion, so the specific capacity of Comparative Example 7 is lower; when the holding time is too long, the pyrophosphate will continue to dehydrate to form a cyclic structure of polymetaphosphoric acid, and the cyclic structure of polymetaphosphoric acid cannot react with Na and Fe elements to form active substances.

[0091] Therefore, the ratio of Na and Fe and the sintering conditions in the first stage are two indicators that restrict and influence each other and need to be considered comprehensively to achieve the best performance.

[0092] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A low resistivity sodium iron pyrophosphate positive electrode material, characterized in that The chemical formula of the low resistivity sodium iron pyrophosphate positive electrode material is Na 4-x Fe 3-y (PO4)2(P2O7) / C, 0≤x≤1, 0≤y≤1, x<y; the band gap Eg of the low-resistivity sodium ferric pyrophosphate positive electrode material is less than 2.7 eV; the resistivity R of the low-resistivity sodium ferric pyrophosphate positive electrode material satisfies the following relationship: 0≤x≤1, 0≤y≤1, x<y; The preparation of the low-resistivity sodium iron pyrophosphate positive electrode material comprises the following steps: A sodium source, an iron source, a phosphorus source, a carbon source and water are mixed evenly, and the low-resistivity sodium iron pyrophosphate positive electrode material is obtained by ball milling, spray drying and sintering. The sintering is carried out under a protective atmosphere and is divided into two stages. The first stage is to heat the material to 300-350°C at a rate of 5°C / min-7°C / min and keep it warm for 10-30 minutes. The second stage is to heat the material to 600-750°C at a rate of 0.5-2°C / min and keep it warm for 5-10 hours.

2. The low-resistivity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that The sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium sulfate and sodium dihydrogen phosphate; the iron source is selected from one or more of ferric acetate, ferric nitrate, ferrous oxalate, ferric oxide and ferric phosphate; the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate and phosphorus pentoxide; and the carbon source is selected from one or more of carbon black, graphite, acetylene black, activated carbon, citric acid, polyethylene glycol, sucrose and glucose.

3. The low-resistivity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that The spray drying temperature is 100°C-140°C.

4. The low-resistivity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that The protective atmosphere is selected from nitrogen atmosphere or argon atmosphere.

5. A sodium ion battery positive electrode, characterized in that The invention comprises the low-resistivity sodium iron pyrophosphate positive electrode material according to any one of claims 1 to 4.

6. A sodium ion battery, characterized in that: Including the sodium ion battery positive electrode according to claim 5.

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