A sodium ferric pyrophosphate positive electrode material, a preparation method and application thereof

Abstract

The application belongs to the technical field of batteries, and particularly relates to a sodium pyrophosphate ferric phosphate positive electrode material, a preparation method and application thereof. The preparation method of the sodium pyrophosphate ferric phosphate positive electrode material comprises the following steps: (1) dissolving sulfuric acid, sodium pyrophosphate and a soluble vanadium salt in water to obtain solution A, dissolving a soluble ferrous source and a first sodium source containing a phosphate group in water to obtain solution B, then mixing solution A and solution B to obtain a mixed solution, and performing a co-precipitation reaction after heating the mixed solution and adding an oxidizing agent, performing solid-liquid separation, drying the obtained solid to obtain a precursor powder; (2) dissolving the precursor powder obtained in step (1), a second sodium source and an organic weak acid carbon source in water, grinding into two slurries with different particle sizes, respectively, spray drying the two slurries to obtain two kinds of granular powders with different particle size ranges, mixing the two kinds of granular powders, and sintering to obtain the sodium pyrophosphate ferric phosphate positive electrode material.

Description

Technical Field

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

[0002] Sodium-ion batteries share similar charging and discharging principles with lithium-ion batteries, and sodium is abundant globally, making it an important direction for green energy storage and a potential complement or alternative to lithium-ion batteries. Sodium-ion battery cathode materials mainly include three categories: layered transition metal oxides, Prussian blue compounds, and polyanionic compounds. Layered transition metal oxides suffer from low reversible capacity and contain precious metals such as cobalt, nickel, and copper, resulting in high costs. Prussian blue compounds exhibit poor thermal stability due to lattice defects and coordination water, and also contain toxic CN. Issues such as functional groups exist. Polyanionic compounds, however, possess advantages such as high operating voltage, structural stability, and minimal volume change during cycling, thus attracting widespread attention.

[0003] Sodium iron phosphate pyrophosphate cathode material, with the chemical formula Na4Fe3(PO4)2P2O7 (abbreviated as NFPP), combines the advantages of all iron-based phosphates. Its core raw materials include iron, sodium, and phosphorus sources, which are abundant and inexpensive. NFPP also possesses a high theoretical specific capacity (129 mAh / g) and a high average operating voltage (3.1 V / g). + NaFePO4 (NaFePO4) possesses advantages such as good cycle stability and a simple, environmentally friendly synthesis process, demonstrating enormous commercial application potential in large-scale energy storage. However, existing synthesis methods involving the mixing, milling, and spraying of conventional iron, sodium, and phosphorus sources into a single sintering process often result in the formation of electrochemically inactive NaFePO4 impurities, along with numerous surface pores and low compaction, leading to low energy density. Furthermore, it suffers from inherently low electronic and ionic conductivity, severely impacting its rate capability and cycle performance. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in related technologies. To this end, this invention proposes a sodium iron phosphate pyrophosphate cathode material, its preparation method, and its applications. This preparation method can significantly improve the energy density, rate capability, and cycle performance of the sodium iron phosphate pyrophosphate cathode material.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: (1) Dissolve sulfuric acid, sodium pyrophosphate and soluble vanadium salt in water to obtain solution A, dissolve soluble ferrous source and sodium source containing phosphate in water to obtain solution B, then mix solution A and solution B to obtain mixed solution, heat the mixed solution and add oxidant to cause coprecipitation reaction, separate solid and liquid, dry the obtained solid to obtain precursor powder; (2) Dissolve the precursor powder obtained in step (1) with the second sodium source and the organic weak acid carbon source in water, grind them into two slurries with different particle sizes, spray dry the two slurries respectively to obtain two granular powders with different particle size ranges, mix the two granular powders and sinter them to obtain the final product.

[0006] In one embodiment, in step (1), the chemical formula of the precursor powder is: Na2[Fe 3-x V x [(OH)(PO4)2P2O7], where 0 < x ≤ 0.1.

[0007] In one embodiment, in step (1), the concentration of sulfuric acid is 3wt%-8wt%.

[0008] In one embodiment, in step (1), the soluble vanadium salt includes at least one of vanadium oxalate and ammonium metavanadate.

[0009] In one embodiment, in step (1), the soluble ferrous source includes at least one of ferrous sulfate, ferrous acetate, and ferrous chloride.

[0010] In one embodiment, in step (1), the first sodium source containing phosphate ions includes at least one of sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0011] In one embodiment, in step (1), the pH of the coprecipitation reaction is 3-5.

[0012] In one embodiment, in step (1), the coprecipitation reaction takes 4-8 hours.

[0013] In one embodiment, in step (1), the heating refers to heating to 80-120°C.

[0014] In one embodiment, in step (1), the oxidant is hydrogen peroxide with a concentration of 20wt%-50wt%.

[0015] In one embodiment, in step (1), the molar ratio of ferric iron, phosphate, and pyrophosphate ions in the mixed solution is controlled to be (3-x):2:1, where 0 < x ≤ 0.1. The ratio of phosphate to pyrophosphate is tested using the method described in the literature “Qiu Xingchu, He Weiming, Deng Qianmin, Qiu Shan. Photometric determination of phosphate and pyrophosphate in copper pyrophosphate plating solution. Materials Protection. 2000, (12)”.

[0016] In one embodiment, in step (2), the second sodium source includes sodium carbonate.

[0017] In one embodiment, in step (2), the organic weak acid carbon source includes at least one of oxalic acid, citric acid, ascorbic acid and benzoic acid, and the amount of the organic weak acid carbon source added is 10%-15% of the theoretical mass of sodium iron phosphate pyrophosphate cathode material.

[0018] In one embodiment, in step (2), D is obtained after grinding. V 50 represents two different slurries with particle sizes of 0.30-0.32μm and 0.40-0.50μm, respectively.

[0019] In one embodiment, in step (2), the spray drying method is two-fluid spray drying, the air source pressure of the spray drying is 0.3-0.6MPa, the inlet air temperature is 210℃-240℃, and the outlet air temperature is 105℃-120℃.

[0020] In one embodiment, in step (2), the particle size D V 50 refers to the particle size D of the granular powder obtained by spray drying a slurry with a particle size of 0.30-0.32 μm. V 50 represents 10-20 μm, and the particle size D is... V 50 refers to the particle size D of the granular powder obtained by spray drying a slurry with a particle size of 0.40-0.50 μm. V 50 is 2-8μm.

[0021] In one embodiment, in step (2), mixing the two types of particulate powder involves adjusting the particle size D. V 50 refers to 10-20μm particulate powder and D V 50 granular powders of 2-8 μm were mixed at a weight ratio of (7-9):(1-3).

[0022] In one embodiment, in step (2), the mixing speed is 200-400 rpm / min and the mixing time is 1-3 h.

[0023] In one embodiment, step (2) includes sintering at low temperature and sintering at high temperature in sequence.

[0024] In one embodiment, in step (2), the low-temperature preheating is carried out by heating to 360℃-420℃ at a heating rate of 1-3℃ / min and holding for 2-4 hours.

[0025] In one embodiment, in step (2), the high-temperature sintering is carried out at a heating rate of 3-5℃ / min to 480℃-550℃ and held for 6-10h.

[0026] A sodium iron phosphate pyrophosphate cathode material is prepared by the preparation method described above.

[0027] A sodium-ion battery comprising the sodium iron phosphate pyrophosphate cathode material as described above.

[0028] The beneficial effects of this invention are: (1) In the preparation method of sodium iron phosphate pyrophosphate cathode material of the present invention, dilute sulfuric acid is used to inhibit the hydrolysis of pyrophosphate, and a soluble high-valence metal vanadium dopant is introduced. Then, ferrous salt and sodium salt containing phosphate are added. First, ferrous ions react with pyrophosphate ions to generate Fe2P2O7. Then, under the action of an oxidant, it undergoes a co-precipitation reaction with sodium salt containing phosphate, and ferrous ions are oxidized to trivalent ferric ions. By controlling the ratio of iron ions, vanadium ions, phosphate ions and pyrophosphate ions, pH value and reaction temperature, Na2[Fe] is synthesized within a suitable range. 3-x V x [(OH)(PO4)2P2O7] precursor (0<x≤0.1), the ratio of phosphate to pyrophosphate in the precursor is 2:1, which is beneficial to the synthesis of pure phase sodium iron pyrophosphate, thereby significantly improving the capacity of sodium iron pyrophosphate cathode material. (2) The preparation method of sodium iron phosphate pyrophosphate cathode material of the present invention introduces soluble high-valence metal vanadium ions for in-situ doping into the synthesis precursor. On the one hand, this generates iron defects, reducing the generation of impurity phases during sintering. On the other hand, during the sintering reduction process, Fe... 3+ and V 5+ All ions were reduced to divalent, Fe 2+ and V 2+ The ionic radii are approximately 78 pm and 79 pm, respectively. Vanadium ions with similar radii are more likely to be incorporated into the lattice of sodium iron phosphate pyrophosphate to replace iron sites, which is beneficial to improving the ionic conductivity of the material and enhancing its rate capability and cycle performance. The additional sodium ions can be incorporated into the precursor during the sintering process to obtain pure-phase sodium iron phosphate pyrophosphate cathode material. (3) In the preparation method of sodium iron phosphate pyrophosphate cathode material of the present invention, the slurry with large particle size is sprayed into small balls by adjusting the spray parameters, and the slurry with small particle size is sprayed into large balls by adjusting the spray parameters. On the one hand, the particle size distribution is formed during the sintering process to improve compaction. On the other hand, the material inside the large secondary sphere has a smaller primary particle size and higher reactivity, which solves the problem that impurities are easily generated due to uneven sintering inside the large particles. (4) In the preparation method of sodium iron phosphate pyrophosphate cathode material of the present invention, an organic weak acid carbon source is used, which has a reducing effect and fully reduces the trivalent iron to divalent iron during the sintering process, which is conducive to the synthesis of pure phase sodium iron phosphate pyrophosphate and improves its capacity; the sintering process adopts segmented sintering, and the carbon source slowly decomposes at low temperature, resulting in fewer pores and uniform carbon coating, which improves the electronic conductivity of the material; the grains grow and fuse to fill the pores during high temperature sintering, which further improves the compaction density of the material. (5) The synthesis method of the present invention is simple, the whole preparation process is green and environmentally friendly, and it is conducive to large-scale industrial preparation. It has very good industrial applicability. The sodium iron phosphate / C composite cathode material prepared by in-situ doping precursor has good physical and chemical properties. Its compaction density can reach 2.32 g / cc, the 0.1C discharge capacity can reach 118.6 mAh / g, the 1C discharge capacity can reach 108.3 mAh / g, and the capacity retention rate after 500 cycles at 1C can reach 98.8%. Attached Figure Description

[0029] Figure 1 This is a 5Kx SEM image of the cathode material precursor prepared in Example 1 of the present invention; Figure 2 This is a 5W SEM image of the cathode material precursor prepared in Example 1 of the present invention; Figure 3 The XRD pattern of the cathode material precursor prepared in Example 1 of this invention; Figure 4 This is a 5Kx SEM image of the positive electrode material mixed spray prepared in Example 1 of the present invention; Figure 5 This is a 5W SEM image of the positive electrode material mixed spray prepared in Example 1 of the present invention; Figure 6 This is a 5Kx SEM image of the positive electrode material prepared in Example 1 of the present invention; Figure 7 This is a 5W magnified SEM image of the cathode material prepared in Example 1 of the present invention; Figure 8 The XRD pattern of the cathode material prepared in Example 1 of this invention; Figure 9 The XRD pattern of the cathode material prepared in Comparative Example 7 of this invention; Figure 10 This is a 5Kx SEM image of the cathode material prepared in Comparative Example 7 of this invention; Figure 11 This is a 5W SEM image of the cathode material prepared in Comparative Example 7 of this invention; Figure 12 This is the first charge-discharge curve of the composite cathode material prepared in Example 1 of the present invention; Figure 13 The capacity retention rate of the composite cathode material prepared in Example 1 of this invention after 500 cycles at 1C. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments.

[0031] Example 1: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: (1) Dissolve dilute sulfuric acid, sodium pyrophosphate, and vanadium oxalate in deionized water to obtain solution A. Dissolve ferrous sulfate and sodium dihydrogen phosphate in deionized water to obtain solution B. Then mix solution A and solution B to obtain a mixed solution. Heat the mixed solution to 100℃, and add 30wt% hydrogen peroxide to oxidize ferrous iron while stirring. Control the pH value of the mixed solution to 4, and the molar ratio of ferric iron, phosphate, and pyrophosphate ions in the mixed solution to 2.92:2:1. After reacting for 6 hours, perform solid-liquid separation. The obtained solid is washed, dried, and crushed to obtain Na2[Fe 2.92 V 0.08 [OH)(PO4)2P2O7] precursor powder, its SEM image is as follows Figure 1 and Figure 2 As shown, by Figure 1 and Figure 2 It can be seen that the precursor is composed of spherical particles formed by the aggregation of primary particles. The particle size of the primary particles is <300nm, and they have high reactivity. Its XRD pattern is shown in the figure. Figure 3 As shown, by Figure 3 It can be known that it has an amorphous, amorphous structure.

[0032] (2) The obtained sodium iron phosphate pyrophosphate precursor was mixed with sodium carbonate in an elemental ratio of Na:Fe:V:P = 4:2.92:0.08:4. Then, 15% citric acid was added based on the theoretical mass of sodium iron phosphate pyrophosphate. The mixed raw materials were dispersed with deionized water. The grinding time of the sand mill was controlled to grind the mixed slurry to obtain two slurries with different particle sizes: Dv50 of 0.50 μm and Dv50 of 0.30 μm. Different slurries were sprayed using a two-fluid spray drying equipment (inlet air temperature 220℃, outlet air temperature 110℃). By adjusting different air source pressures, spherical spray materials of different particle sizes were obtained. When the air source pressure was 0.4MPa and the slurry particle size Dv50 was 0.30μm, the resulting spherical spray material had a particle size Dv50 of 15μm. When the air source pressure was 0.6MPa and the slurry particle size Dv50 was 0.50μm, the resulting spherical spray material had a particle size Dv50 of 2μm.

[0033] (3) Add spherical spray powders of different particle sizes to a high-speed mixer and mix for 2 hours at a speed of 200 rpm / min. The mixing ratio of spherical spray powders with a particle size Dv50 of 15 μm to spherical spray powders with a particle size Dv50 of 2 μm is 9:1. The SEM image of the resulting spray powder is shown in the figure. Figure 4 and Figure 5 As shown, by Figure 4 and Figure 5 It is known that the sprayed material has high sphericity and density, and a high gradation of large and small spheres. The mixed sprayed material is then placed in a tube furnace for sintering under a nitrogen atmosphere. The temperature is increased to 400℃ at a rate of 2℃ / min and held for 3 hours, then increased to 520℃ at a rate of 4℃ / min and held for 8 hours. Finally, after cooling, the sodium iron phosphate / C composite cathode material is obtained, with the chemical formula Na₄Fe₂O₃. 2.92 V 0.08 (PO4)2P2O7, its SEM image is as follows Figure 6 and Figure 7 As shown, by Figure 6 and Figure 7 It can be seen that the NFPP / C finished secondary granules have high sphericity and uniform particle size distribution with good gradation. Its XRD pattern is shown below. Figure 8 As shown, by Figure 8 It can be seen that the NFPP / C finished product has a high degree of crystallinity, and vanadium ion doping did not destroy the crystal structure of NFPP, nor were there any impurity peaks of NaFePO4 and Na2FeP2O7. Example 2: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: (1) Dissolve dilute sulfuric acid, sodium pyrophosphate, and ammonium metavanadate in deionized water to obtain solution A. Dissolve ferrous sulfate and sodium monohydrogen phosphate in deionized water to obtain solution B. Then mix solution A and solution B to obtain a mixed solution. Heat the mixed solution to 80°C and add 30wt% hydrogen peroxide to oxidize ferrous iron while stirring. Control the pH value of the mixed solution to 3, and the molar ratio of ferric iron, phosphate, and pyrophosphate ions in the mixed solution to 2.96:2:1. After reacting for 6 hours, perform solid-liquid separation. The obtained solid is washed, dried, and crushed to obtain Na2[Fe 2.96 V 0.04 [OH)(PO4)2P2O7] precursor powder; (2) The obtained sodium iron phosphate pyrophosphate precursor was mixed with sodium carbonate in an elemental ratio of Na:Fe:V:P = 4:2.96:0.04:4. Then, 15% oxidized acid based on the theoretical mass of sodium iron phosphate pyrophosphate was added. The mixed raw materials were dispersed with deionized water. The grinding time of the sand mill was controlled to grind the mixed slurry to obtain two slurries with different particle sizes: Dv50 of 0.43 μm and Dv50 of 0.31 μm. These two slurries with different particle sizes were then processed. Different slurries were sprayed using a two-fluid spray drying device (inlet air temperature 225℃, outlet air temperature 105℃). By adjusting different air source pressures, spherical spray particles of different sizes were obtained. When the air source pressure was 0.40MPa and the slurry particle size Dv50 was 0.31μm, the resulting spherical spray particles had a Dv50 of 13μm. When the air source pressure was 0.55MPa and the slurry particle size Dv50 was 0.43μm, the resulting spherical spray particles had a Dv50 of 3μm. (3) Add spherical spray powders of different particle sizes to a high-speed mixer and mix for 3 hours at a speed of 200 rpm / min. The mixing ratio of spherical spray powders with a particle size Dv50 of 13 μm to spherical spray powders with a particle size Dv50 of 3 μm is 7:3. Then, place the mixed spray powder in a tube furnace and sinter it under a nitrogen atmosphere. Heat the material to 360℃ at a heating rate of 2℃ / min and hold for 4 hours. Then heat the material to 550℃ at a heating rate of 4℃ / min and hold for 6 hours. Finally, after cooling, the sodium iron phosphate / C composite cathode material is obtained, with the chemical formula Na4Fe 2.96 V 0.04 (PO4)2P2O7.

[0034] Example 3: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: (1) Dissolve dilute sulfuric acid, sodium pyrophosphate, and ammonium metavanadate in deionized water to obtain solution A. Dissolve ferrous chloride and sodium phosphate in deionized water to obtain solution B. Then mix solution A and solution B to obtain a mixed solution. Heat the mixed solution to 90°C and add 30wt% hydrogen peroxide to oxidize ferrous iron while stirring. Control the pH value of the mixed solution to 5. The molar ratio of ferric iron, phosphate, and pyrophosphate ions in the mixed solution is 2.96:2:1. After reacting for 7 hours, perform solid-liquid separation. The obtained solid is washed, dried, and crushed to obtain Na2[Fe 2.96 V 0.04 [(OH)(PO4)2P2O7] precursor powder.

[0035] (2) The obtained sodium iron phosphate pyrophosphate precursor was mixed with sodium carbonate in an elemental ratio of Na:Fe:V:P = 4:2.96:0.04:4. Then, 11% benzoic acid was added based on the theoretical mass of sodium iron phosphate pyrophosphate. The mixed raw materials were dispersed with deionized water. The grinding time of the sand mill was controlled to grind the mixed slurry to obtain two slurries with different particle sizes: Dv50 of 0.48 μm and Dv50 of 0.31 μm. These two slurries with different particle sizes were then... Different slurries were sprayed using a two-fluid spray drying equipment (inlet air temperature 230℃, outlet air temperature 110℃). By adjusting different air source pressures, spherical spray materials of different particle sizes were obtained. When the air source pressure was 0.35MPa and the slurry particle size Dv50 was 0.31μm, the resulting spherical spray material had a particle size Dv50 of 16μm. When the air source pressure was 0.60MPa and the slurry particle size Dv50 was 0.48μm, the resulting spherical spray material had a particle size Dv50 of 2μm.

[0036] (3) Add spherical spray powders of different particle sizes to a high-speed mixer and mix for 3 hours at a speed of 300 rpm / min. The mixing ratio of spherical spray powders with a particle size Dv50 of 16 μm to spherical spray powders with a particle size Dv50 of 2 μm is 8:2. Then, place the mixed spray powder in a tube furnace and sinter it under a nitrogen atmosphere. Heat the material to 400℃ at a heating rate of 2℃ / min and hold for 3 hours. Then heat the material to 500℃ at a heating rate of 4℃ / min and hold for 10 hours. Finally, after cooling, the sodium iron phosphate / C composite cathode material is obtained, with the chemical formula Na4Fe 2.96 V 0.04 (PO4)2P2O7.

[0037] Example 4: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: (1) Dissolve dilute sulfuric acid, sodium pyrophosphate, and vanadium oxalate in deionized water to obtain solution A. Dissolve ferrous chloride and sodium dihydrogen phosphate in deionized water to obtain solution B. Then mix solution A and solution B to obtain a mixed solution. Heat the mixed solution to 90°C and add 30wt% hydrogen peroxide to oxidize ferrous iron while stirring. Control the pH value of the mixed solution to 5. The molar ratio of ferric iron, phosphate, and pyrophosphate ions in the mixed solution is 2.90:2:1. After reacting for 4 hours, perform solid-liquid separation. The obtained solid is washed, dried, and crushed to obtain Na2[Fe 2.90 V 0.10 [(OH)(PO4)2P2O7] precursor powder.

[0038] (2) The obtained sodium iron phosphate pyrophosphate precursor was mixed with sodium carbonate in an elemental ratio of Na:Fe:V:P = 4:2.90:0.10:4. Then, 14% citric acid was added based on the theoretical mass of sodium iron phosphate pyrophosphate. The mixed raw materials were dispersed with deionized water. The grinding time of the sand mill was controlled to grind the mixed slurry to obtain two slurries with different particle sizes: Dv50 of 0.50 μm and Dv50 of 0.32 μm. Different slurries were sprayed using a two-fluid spray drying equipment (inlet air temperature 230℃, outlet air temperature 110℃). By adjusting different air source pressures, spherical spray materials of different particle sizes were obtained. When the air source pressure was 0.35MPa and the slurry particle size Dv50 was 0.32μm, the resulting spherical spray material had a particle size Dv50 of 18μm. When the air source pressure was 0.55MPa and the slurry particle size Dv50 was 0.50μm, the resulting spherical spray material had a particle size Dv50 of 5μm.

[0039] (3) Add spherical spray powders of different particle sizes to a high-speed mixer and mix for 2 hours at a speed of 400 rpm / min. The mixing ratio of spherical spray powders with a particle size Dv50 of 18 μm to spherical spray powders with a particle size Dv50 of 5 μm is 8:2. Then, place the mixed spray powder in a tube furnace and sinter it under a nitrogen atmosphere. Heat the material to 360℃ at a heating rate of 2℃ / min and hold for 3 hours. Then heat the material to 550℃ at a heating rate of 4℃ / min and hold for 6 hours. Finally, after cooling, the sodium iron phosphate / C composite cathode material is obtained, with the chemical formula Na4Fe 2.90 V 0.10 (PO4)2P2O7.

[0040] Comparative Example 1: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that the molar ratio of trivalent iron, phosphate and pyrophosphate ions in the mixed solution in step (1) is 2.85:2:1.

[0041] Comparative Example 2: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that: vanadium ions are not doped in step (1), and the molar ratio of trivalent iron, phosphate and pyrophosphate ions in the mixed solution is 3:2:1.

[0042] Comparative Example 3: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that: in step (2), the particle size of the sand-milled slurry is only D. V 50 represents 0.50 μm.

[0043] Comparative Example 4: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that the carbon source used in step (2) is glucose.

[0044] Comparative Example 5: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that step (3) does not involve mixing and sintering of sprayed materials of different sizes.

[0045] Comparative Example 6: A method for preparing sodium iron phosphate pyrophosphate cathode material differs from Example 1 only in that: the sintering heating rate in step (3) is 3℃ / min, the holding temperature is 550℃, and the holding time is 10h; Comparative Example 7: A method for preparing sodium iron phosphate pyrophosphate cathode material includes the following steps: Ferrous oxalate was used as the iron source, sodium pyrophosphate as both the sodium and phosphorus source, and phosphoric acid as a supplementary phosphorus source. The raw materials were formulated according to an elemental molar ratio of Na / Fe / P = 4:3:4, and glucose (12% of the theoretical mass of sodium iron phosphate pyrophosphate) was added. The mixture was dispersed with deionized water and ground into a slurry with a sand mill to a D50 of 0.5 μm. The slurry was then dried using a two-fluid spray dryer (inlet air temperature 220℃, outlet air temperature 110℃) to obtain spherical dried powder. This spherical dried powder was then placed in a nitrogen tube atmosphere furnace and heated from room temperature to 400℃ at a rate of 2℃ / min, holding for 4 hours to completely decompose the ferrous oxalate. The temperature was then increased to 520℃ at a rate of 3℃ / min and held for 10 hours. After natural cooling, sodium iron phosphate pyrophosphate / C cathode material was obtained, and its XRD pattern is shown below. Figure 9 As shown, by Figure 9 It can be seen that characteristic diffraction peaks of inactive NaFePO4 exist at diffraction angles of 32.92° and 33.18°, as shown in the SEM images below. Figure 10 and Figure 11 As shown, by Figure 10 and Figure 11 It can be seen that secondary particles have more pores.

[0046] Test example: The NFPP / C composite cathode material prepared in the examples and comparative examples was dissolved in NMP with acetylene black and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1. The mixture was then stirred into a slurry, coated onto aluminum foil, and dried and pressed to form the cathode material electrode sheet. Using metallic sodium as the counter electrode and 1 mol / L NaClO4 / (EC+DMC+EMC) as the electrolyte, which also contained 5% fluoroethylene carbonate (FEC), and Cellgard 2035 as the separator, button batteries (model CR2016) were assembled in a glove box. Electrochemical tests were performed on the assembled batteries using a Land battery tester, with test voltages ranging from 1.5 to 3.65 V. The initial charge-discharge curve of the composite cathode material prepared in Example 1 is shown below. Figure 12 As shown, by Figure 12 It can be seen that it has high charge / discharge capacity, with a 0.1C discharge capacity of 118.6 mAh / g and a 1C discharge capacity of 108.3 mAh / g. The capacity retention rate of the composite cathode material prepared in Example 1 after 500 cycles at 1C is as follows: Figure 13 As shown, by Figure 13 It can be seen that it has good cycle performance. After 500 cycles at 1C, the discharge capacity is 107.0mAh / g, and the capacity retention rate is as high as 98.8%. The final test results are shown in Table 1.

[0047] Table 1. Performance test results of the cathode materials prepared in the examples and comparative examples.

[0048] As shown in Table 1, the compaction density of the sodium iron phosphate pyrophosphate cathode material prepared by this invention is above 2.26 g / cc, the specific capacity at 0.1C discharge is above 117.7 mAh / g, the specific capacity at 1C discharge is above 107.4 mAh / g, and the capacity retention rate after 500 cycles at 1C can reach above 98.3%.

[0049] Comparing the data of Example 1 and Comparative Examples 1-6, it can be seen that the use of an in-situ doped vanadium ion precursor and the proportion of doping elements in the precursor, the size of the milling particles, the type of carbon source, the proportion of the particle size distribution of the sprayed material, and the sintering process parameters all have a significant impact on the product performance. Compared with the conventional preparation method, the compaction density and discharge capacity of the cathode material prepared by the present invention are significantly improved, and the compaction, capacity, and cycle retention rate of Example 1 are maintained at a very high level.

Claims

1. A method for preparing a sodium iron phosphate pyrophosphate cathode material, characterized in that: Includes the following steps: (1) Dissolve sulfuric acid, sodium pyrophosphate and soluble vanadium salt in water to obtain solution A, dissolve soluble ferrous source and sodium source containing phosphate in water to obtain solution B, then mix solution A and solution B to obtain mixed solution, heat the mixed solution and add oxidant to cause coprecipitation reaction, separate solid and liquid, dry the obtained solid to obtain precursor powder; (2) Dissolve the precursor powder obtained in step (1) with the second sodium source and the organic weak acid carbon source in water, grind them into two slurries with different particle sizes, spray dry the two slurries respectively to obtain two granular powders with different particle size ranges, mix the two granular powders and sinter them to obtain the final product.

2. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that: In step (1), the chemical formula of the precursor powder is: Na2[Fe 3-x V x [(OH)(PO4)2P2O7], where 0 < x ≤ 0.

1.

3. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 2, characterized in that: In step (1), the molar ratio of ferric iron, phosphate and pyrophosphate ions in the mixed solution is controlled to be (3-x):2:1, where 0<x≤0.

1.

4. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that: In step (2), the organic weak acid carbon source includes at least one of oxalic acid, citric acid, ascorbic acid and benzoic acid, and the amount of organic weak acid carbon source added is 10%-15% of the theoretical mass of sodium iron phosphate pyrophosphate cathode material.

5. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that: In step (2), D is obtained after grinding. V 50 represents two different slurries with particle sizes of 0.30-0.32μm and 0.40-0.50μm, respectively.

6. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 5, characterized in that: In step (2), the particle size D V 50 refers to the particle size D of the granular powder obtained by spray drying a slurry with a particle size of 0.30-0.32 μm. V 50 represents 10-20 μm, and the particle size D is... V 50 refers to the particle size D of the granular powder obtained by spray drying a slurry with a particle size of 0.40-0.50 μm. V 50 is 2-8μm.

7. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 6, characterized in that: In step (2), mixing the two types of particulate powder involves adjusting the particle size D. V 50 refers to 10-20μm particulate powder and D V 50 granular powders of 2-8 μm were mixed at a weight ratio of (7-9):(1-3).

8. The method for preparing a sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that: In step (2), the sintering includes low-temperature pre-firing and high-temperature sintering performed sequentially.

9. A sodium iron phosphate pyrophosphate cathode material, characterized in that: It is prepared by the preparation method according to any one of claims 1-8.

10. A sodium-ion battery, characterized in that: Includes the sodium iron phosphate pyrophosphate cathode material as described in claim 9.

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