A carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F, its preparation and application

Abstract

This invention relates to a method for preparing carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F and its application in sodium-ion batteries. By using a low-temperature solvothermal method to in-situ coat the surface of sodium iron phosphate pyrophosphate material with a layer of tetragonal sodium vanadium monofluorophosphate material with high air stability, direct contact between the sodium iron phosphate pyrophosphate material and air can be avoided, thereby improving the Fe... 2+ Storage stability in air.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, and relates to sodium-ion battery electrode materials, particularly to the preparation method of Na4Fe3(PO4)2P2O7@NaVPO4F and its application in sodium-ion batteries. Background Technology

[0002] In recent years, sodium-ion batteries have become a research hotspot, attracting widespread research both domestically and internationally and achieving rapid development. Given the current limitations of high cost and resource scarcity of lithium-ion batteries, sodium-ion batteries are expected to find widespread application in low-speed electric vehicles, electric bicycles, electric ships, distributed energy storage, and large-scale energy storage due to their abundant resources, low cost, and high cost-effectiveness. Currently reported sodium-ion battery products mainly fall into three categories: layered oxides, Prussian blue compounds, and polyanionic compounds. Compared to the first two systems, phosphate-based polyanionic compounds have become the preferred cathode material for high-energy-density, high-power-density, and high-stability sodium-ion batteries due to their structural stability, rapid sodium diffusion, and high safety. Sodium iron pyrophosphate has received widespread attention in recent years due to its readily available constituent elements, low price, and high cost-effectiveness. However, prolonged storage in air can lead to partial oxidation of Fe2+ to Fe3+, causing irreversible capacity loss, resulting in lower initial efficiency and poorer battery performance. Therefore, improving the air storage stability of sodium iron pyrophosphate is crucial for its practical and large-scale application. Summary of the Invention

[0003] To address this issue, this invention proposes an in-situ method using a low-temperature solvothermal process to grow a layer of highly air-stable tetragonal sodium vanadium fluorophosphate material on sodium iron pyrophosphate. This avoids direct contact between the sodium iron pyrophosphate material and air, thereby improving the storage stability of Fe2+ in air. Sodium-ion batteries assembled from this material exhibit stable air storage performance, and by controlling the precursor particle size, nanoscale particles with short sodium diffusion paths can be obtained, which is beneficial for improving the electrochemical performance of the material.

[0004] A method for preparing carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F, the preparation steps of which include:

[0005] A carbon-coated square NaVPO4F crystal coating layer was grown in situ on the surface of carbon-coated Na4Fe3(PO4)2P2O7.

[0006] The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:9 to 1:250, preferably 1:40 to 1:160.

[0007] The carbon content in carbon-coated Na4Fe3(PO4)2P2O7 is 2-7% by mass.

[0008] 1) Mix vanadium source, sodium salt, fluoride salt, phosphate, reducing agent, carbon source, carbon-coated Na4Fe3(PO4)2P2O7 and surfactant into an organic solvent to obtain mixture a;

[0009] 2) Place mixture a in a sealed reaction vessel and carry out a solvothermal reaction to obtain mixture b; the solvothermal reaction temperature is 150-250℃ and the reaction pressure is 1-3MPa;

[0010] 3) Centrifuge the mixture b obtained in step 2), and after washing and drying the solid with ethanol, obtain the final product carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F;

[0011] The molar ratio of V to Na4Fe3(PO4)2P2O7 in the vanadium source is 1:9 to 1:250;

[0012] The molar ratio of V, Na, F, and P elements in the vanadium source, sodium salt, fluoride salt, and phosphate is 1:1:1:1;

[0013] If the vanadium source has a +5 valence, the molar ratio of the vanadium source to the reducing agent is e, where e is 1:1.05~1.1;

[0014] If the vanadium source has a +3 valence, the molar ratio of the vanadium source to the reducing agent is f, where f is 1:0.05~0.1;

[0015] The surfactant constitutes 1-5% of the mass fraction of the carbon-coated Na4Fe3(PO4)2P2O7;

[0016] The carbon source accounts for 10-30% of the total mass of the vanadium source, sodium salt, fluoride salt, phosphate, and reducing agent.

[0017] The vanadium source is one or more of ammonium metavanadate, vanadium pentoxide, vanadium trioxide, and vanadium phosphate; preferably, the vanadium source is ammonium metavanadate.

[0018] The sodium salt is one or more of sodium hydroxide, sodium oxalate, sodium sulfate, sodium citrate, sodium nitrate, sodium fluoride, sodium bicarbonate, and sodium carbonate; preferably, the sodium source is sodium hydroxide.

[0019] The phosphate is one or more of the following: ammonium dihydrogen phosphate, sodium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; preferably, the phosphate is ammonium dihydrogen phosphate.

[0020] The fluoride salt is one or more of ammonium fluoride and sodium fluoride; preferably, both the sodium source and the fluoride source are sodium fluoride.

[0021] The reducing agent is one or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, citric acid, sucrose, malic acid, oxalic acid, adipic acid, and starch; preferably, citric acid is the reducing agent.

[0022] The carbon source is one or more of sucrose, glucose, starch, and oxalic acid;

[0023] The organic solvent is one or more selected from methanol, ethanol, propanol, isopropanol, acetone, methyl butyl ketone, methyl isobutyl ketone, diethyl ether, and ethylene glycol dimethyl ether.

[0024] The surfactant is one or more of sodium dodecyl sulfate, polyethylene glycol, alkyl glycoside, cocoyl diethanolamide, and dodecylbenzene sulfonic acid.

[0025] The solvothermal reaction time in step 2) is 12-36 hours, preferably 15-20 hours;

[0026] In step 1), the solid content / solvent concentration is 15-35 mg / ml, preferably 20-30 mg / ml;

[0027] After centrifugation, mixture b in step 3) is washed with ethanol 2-5 times and dried at 80-150℃ for 8-18 hours.

[0028] Carbon-coated Na4Fe3(PO4)2P2O7 electrode materials were prepared by solid-phase method, sol-gel method or wet-phase ball milling method; the carbon content of the carbon-coated Na4Fe3(PO4)2P2O7 was 2-7% by mass.

[0029] The carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F was prepared by the method described above.

[0030] A carbon-coated square NaVPO4F crystal coating layer was grown in situ on the surface of carbon-coated Na4Fe3(PO4)2P2O7.

[0031] The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:9 to 1:250, preferably 1:40 to 1:160.

[0032] The carbon content in carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F is 2-7% by mass.

[0033] The application of carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F as a positive electrode active material in sodium-ion batteries.

[0034] Beneficial effects

[0035] This invention utilizes a low-temperature solvothermal method to in-situ coat the surface of sodium iron pyrophosphate material with a layer of tetragonal sodium vanadium monofluorophosphate material with high air stability. This avoids direct contact between the sodium iron pyrophosphate material and air, thereby improving the storage stability of Fe2+ in air.

[0036] Firstly, the in-situ coating method is a solvothermal preparation method with high activity but low temperature. This is because the conventional high-temperature calcination method (>600 degrees) will cause the crystal phase of sodium iron pyrophosphate to decompose above 550 degrees, producing impurities such as sodium iron phosphate.

[0037] In addition, the in-situ coating material can maximize its capacity within the same charge-discharge voltage range (1.5-4.0V, with a median full-cell voltage of approximately 3.0V) as sodium iron pyrophosphate, thereby reducing inactive components in the electrode material. Based on this, square sodium vanadium monofluorophosphate with a full-cell voltage plateau of 3.25V and 3.85V was selected as the coating material. Other materials, such as sodium vanadium trifluorophosphate, have a high-voltage plateau of 4.2V. Furthermore, sodium vanadium phosphate cannot be synthesized by low-temperature solvothermal methods.

[0038] In addition, during the solvothermal preparation process, a solvent insoluble in sodium ferric pyrophosphate should be selected. Since sodium ferric pyrophosphate is soluble in water but insoluble in organic solvents, an organic solvent is selected as the solvent for the reaction of sodium vanadium monofluorophosphate.

[0039] Finally, since the precursor contains insoluble particulate raw materials such as sodium iron pyrophosphate, in order to obtain a nano-sized and uniformly distributed final product, the precursor needs to be ball-milled or sand-milled, and a suitable dispersant needs to be added to disperse the insoluble particles evenly.

[0040] Based on the above-mentioned invention, the prepared Na4Fe3(PO4)2P2O7@NaVPO4F exhibits a nanoparticle morphology and good dispersion uniformity. Due to the in-situ coating of highly stable sodium vanadium monofluorophosphate on the electrode surface, the oxidation of Fe2+ to Fe3+ in Na4Fe3(PO4)2P2O7@NaVPO4F is effectively suppressed, resulting in significantly improved air storage stability of the material. Furthermore, the sodium-ion battery assembled from it exhibits excellent rate performance due to the shorter sodium diffusion path. Detailed Implementation

[0041] Examples 1-2: The molar ratio of carbon-coated NaVPO4F and carbon-coated Na4Fe3(PO4)2P2O7 was adjusted to 1:9-1:250; the carbon source mass fraction was 10-30%. Comparative Examples 1-2: The amount of NVPF coating was adjusted to be too much or too little.

[0042] Example 1: The molar ratio of carbon-coated NaVPO4F and carbon-coated Na4Fe3(PO4)2P2O7 was 1:40.

[0043] Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3·9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3·9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0044] The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0045] 3) Add 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g) and 0.3276g of polyethylene glycol (2.5%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0046] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0047] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 4.5%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, designated as 1#.

[0048] Example 2: The molar ratio of carbon-coated NaVPO4F and carbon-coated Na4Fe3(PO4)2P2O7 was 1:160.

[0049] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0050] 2) The carbon content of the carbon-coated Na4Fe3(PO4)2P2O7 is 4.5%;

[0051] 3) Mix 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), 0.08 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, 52.166 g), and 1.304 g polyethylene glycol (2.5%) into 2.15 L of ethanol solvent (25 mg / ml) to form mixture a;

[0052] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0053] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 4.2%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, designated as 2#.

[0054] Comparative examples 1-2, adjust the NVPF coverage to see if it is too much or too little.

[0055] Comparative Example 1: The ratio was 1:7 (excessive coating).

[0056] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0057] 2) The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0058] 3) Add 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.0035 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 2.2932g and 0.0573g of polyethylene glycol (2.5%) were mixed and added to 0.106L of ethanol solvent (25mg / ml) to form mixture a;

[0059] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0060] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 4.3%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 1*.

[0061] Comparative Example 2: The ratio was 1:300 (the coating amount was too small).

[0062] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0063] 2) The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0064] 3) Mix 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (10% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), 0.15 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, 97.68 g), and 2.442 g polyethylene glycol (2.5%) into 4.016 L of ethanol solvent (25 mg / ml) to form mixture a;

[0065] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0066] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 4.4%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 2*. XPS analysis revealed the presence of some Fe3+ impurities in the product.

[0067] Examples 3-4: Comparative Examples 3-4, with the solvothermal reaction temperature adjusted to 150-250 degrees Celsius, corresponding to a pressure of 1-3 MPa.

[0068] Example 3: The reaction heat temperature was 150 degrees Celsius, and the pressure was 1 MPa.

[0069] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0070] 2) The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0071] 3) Add 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g) and 0.3276g of polyethylene glycol (2.5%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0072] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 150°C and 1 MPa for 18 hours to obtain mixture b.

[0073] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 4.6%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, designated as 3#.

[0074] Example 4: The reaction heat temperature was 250 degrees Celsius, and the pressure was 3 MPa.

[0075] Example 4 follows the same steps as Example 3, except that:

[0076] In step 4), mixed solution a undergoes a solvothermal reaction at 250°C and a reaction pressure of 3 MPa for 18 hours to obtain mixture b.

[0077] The final product obtained in step 5) is denoted as 4#.

[0078] Comparative Example 3: Reaction heat temperature is 100 degrees Celsius, pressure is <1 MPa

[0079] Comparative Example 3 follows the same steps as Example 3, except that:

[0080] In step 4), mixed solution a undergoes a solvothermal reaction at 100℃ and a reaction pressure of <1MPa for 18 hours to obtain mixture b;

[0081] The final product obtained in step 5) is denoted as 3*. Thermogravimetric analysis showed that the carbon content in the final product was 4.4%. XRD analysis indicated that the final product was mainly a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, but contained some unreacted impurities.

[0082] Comparative Example 4: Reaction heat temperature 300 degrees Celsius, pressure > 3 MPa

[0083] Comparative Example 4 follows the same steps as Example 3, except that:

[0084] In step 4), mixed solution a undergoes a solvothermal reaction at 300℃ and a reaction pressure >3MPa for 18 hours to obtain mixture b;

[0085] The final product obtained in step 5) is denoted as 4*. Thermogravimetric analysis showed that the carbon content in the final product was 4.6%. XRD analysis indicated that the final product was mainly a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F phases.

[0086] Example 1: Comparative Examples 5-6, with the molar ratio of reducing agent adjusted to 1:1.05~1.1.

[0087] Comparative Example 5: The molar ratio of vanadium source and reducing agent is 1:1. When the reducing agent is insufficient;

[0088] Comparative Example 5 follows the same steps as Example 1, except that:

[0089] In step 3), 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.0005 mol citric acid C6H10O8 (210.14, 0.105 g), 0.0484 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, 13.104g) and 0.3276g of polyethylene glycol (2.5%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0090] The carbon content of the final product obtained in step 5) is 4.3%. XRD analysis identified the final product as 5*, with its main components being a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F. XPS analysis showed that, due to insufficient reducing agent, V3+ and V4+ coexisted in the product.

[0091] Comparative Example 6: The molar ratio of vanadium source and reducing agent is 1:1.4, with excess reducing agent;

[0092] Comparative Example 6 follows the same steps as Example 1, except that:

[0093] In step 3), 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.0007 mol citric acid C6H10O8 (210.14, 0.147 g), 0.0568 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, 13.104g) and 0.3276g of polyethylene glycol (2.5%) were mixed and added to 0.551L of ethanol solvent (25mg / ml) to form mixture a;

[0094] The carbon content of the final product obtained in step 5) is 7.5%. According to XRD analysis, the final product is designated as 6*, and its main components are a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F.

[0095] Examples 5-6: Comparative Examples 7-8, adjusting the surfactant content to 1-5% of the mass fraction of carbon-coated Na4Fe3(PO4)2P2O7.

[0096] Example 5: The surfactant accounted for 1% of the mass fraction of carbon-coated Na4Fe3(PO4)2P2O7.

[0097] Example 5 follows the same steps as Example 1, except that:

[0098] In step 3), 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g) and 0.131g of polyethylene glycol (1%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0099] The carbon content of the final product obtained in step 5) is 4.4%. XRD analysis shows that the final product is a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 5#.

[0100] Example 6: The surfactant accounted for 5% of the mass fraction of carbon-coated Na4Fe3(PO4)2P2O7.

[0101] Example 6 follows the same steps as Example 1, except that:

[0102] In step 3), 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g) and 0.655g of polyethylene glycol (5%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0103] The carbon content of the final product obtained in step 5) is 4.6%. XRD analysis shows that the final product is a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 6#.

[0104] Comparative Example 7: No surfactant added

[0105] Comparative Example 7 follows the same steps as Example 1, except that:

[0106] No surfactant is added in step 3);

[0107] The carbon content of the final product obtained in step 5) is 4.2%. XRD analysis identified the final product as 7*, whose main components are a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, along with unreacted impurities. The final product exhibits uneven particle distribution and severe agglomeration.

[0108] Comparative Example 8: The surfactant accounted for 10% of the mass fraction of carbon-coated Na4Fe3(PO4)2P2O7.

[0109] Comparative Example 8 follows the same steps as Example 1, except that:

[0110] In step 3), 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g and 1.31g of polyethylene glycol (10%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0111] The carbon content of the final product obtained in step 5) is 5.2%. According to XRD analysis, the final product is designated as 8*, and its main components are a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F phases, and there are also unreacted impurity phases.

[0112] Examples 7-8: Comparative Examples 9-10, adjusting the carbon source to 10-30% of the total mass of vanadium source, sodium salt, fluoride salt, phosphate, and reducing agent.

[0113] Example 7: Carbon source mass fraction is 10%

[0114] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0115] 2) The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0116] 3) Add 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0252 g sucrose (10% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.02 mol carbon-coated Na4Fe3(PO4)2P2O7 (624, ... 13.104g) and 0.3276g of polyethylene glycol (2.5%) were mixed and added to 0.549L of ethanol solvent (25mg / ml) to form mixture a;

[0117] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0118] 5) The mixture b obtained in step 4) was centrifuged, washed three times with ethanol, and dried to obtain the final product: carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F. Thermogravimetric analysis showed that the carbon content in the final product was 3.5%. XRD analysis confirmed that the final product was a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, designated as 7#.

[0119] Example 8: Carbon source mass fraction is 30%

[0120] Example 8 is the same as Example 7 in terms of steps, except that:

[0121] In step 3), the mass of the carbon source sucrose is 0.0756g;

[0122] Step 5) The carbon content of the final product obtained is 7.5%. XRD analysis shows that the final product is a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 8#.

[0123] Comparative Example 9: Carbon source mass fraction is 2%

[0124] Comparative Example 9 follows the same steps as Example 7, except that:

[0125] In step 3), the mass of the carbon source sucrose is 0.00504 g.

[0126] Step 5) The carbon content of the final product obtained is 0.3%. XRD analysis shows that the final product is a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 9*.

[0127] Comparative Example 10: Carbon source mass fraction is 50%

[0128] Comparative Example 10 follows the same steps as Example 7, except that:

[0129] In step 3), the mass of the carbon source sucrose is 0.126g;

[0130] Step 5) The carbon content of the final product obtained is 12.5%. XRD analysis shows that the final product is a composite phase of Na4Fe3(PO4)2P2O7 and NaVPO4F, denoted as 10*.

[0131] Comparative Example 11: Differences in Physical Coating

[0132] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3.9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3.9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material.

[0133] The carbon content of carbon-coated Na4Fe3(PO4)2P2O7 is ~5%;

[0134] 3) Mix 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.3276 g polyethylene glycol (2.5%) into 0.024 L of ethanol solvent (25 mg / ml) to form mixture a;

[0135] 4) Place mixture a in a sealed reaction vessel, sonicate the mixture for 2 hours, and then carry out a solvothermal reaction at 200℃ and 2MPa for 18 hours to obtain mixture b;

[0136] 5) Centrifuge the mixture b obtained in step 4), wash it three times with ethanol, and dry it to obtain carbon-coated NaVPO4F.

[0137] 6) The carbon-coated NaVPO4F obtained in step 5) was physically mixed with 0.02 mol of carbon-coated Na4Fe3(PO4)2P2O7 and ball-milled to form the final product, denoted as 11*. Thermogravimetric analysis showed that the carbon content in the final product was 4.3%. XRD analysis confirmed that the final product was a mixed phase of Na4Fe3(PO4)2P2O7 and NaVPO4F.

[0138] Comparative Example 12, Coating Material Method (High-Temperature Calcination Method)

[0139] 1) Carbon-coated Na4Fe3(PO4)2P2O7 electrode material was synthesized using the spray drying method described in Example 3 of Patent 202110872471.1: 0.03 mol of ferric nitrate nonahydrate Fe(NO3)3·9H2O, 0.04 mol of sodium dihydrogen phosphate Fe(NO3)3·9H2O, and 0.03 mol of citric acid monohydrate C6H10O8 were mixed evenly in an aqueous solution according to the stoichiometric ratio and then spray-dried. The obtained precursor was calcined at 550°C for 8 hours in an argon atmosphere to obtain 1 mol of in-situ carbon-coated Na4Fe3(PO4)2P2O7 electrode material, with a carbon content of ~5%.

[0140] 2) Mix 0.0005 mol ammonium metavanadate NH4VO3 (116.978, 0.0585 g), 0.0005 mol sodium fluoride NaF (41.988, 0.021 g), 0.0005 mol NH4H2PO4 (115.026, 0.0575 g), 0.00055 mol citric acid C6H10O8 (210.14, 0.1156 g), 0.0506 g sucrose (20% of the total mass of NH4VO3, NaF, NH4H2PO4, and C6H10O8), and 0.3276 g polyethylene glycol (2.5%) into 0.024 L of ethanol solvent (25 mg / ml) to form mixture a;

[0141] 3) After thoroughly drying the mixture a, it was placed in a high-temperature tube furnace and pre-calcined at 350°C for 5 hours, followed by high-temperature calcination at 700°C for 5 hours; the final product was carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F, denoted as 12*. Thermogravimetric analysis showed that the carbon content in the final product was 5.5%. XRD analysis showed that the impurities of sodium iron phosphate and sodium iron pyrophosphate in the final product exceeded 30%.

[0142] Test Example 1

[0143] Sample 1# prepared in Example 1 was used as the positive electrode active material for sodium-ion batteries. It was mixed uniformly with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone was added and mixed thoroughly to form a paste, which was then coated onto an aluminum current collector. After drying, it was cut into circular pieces with a diameter of 14 mm. The areal density of the active material was 1.1~3.2 mg / cm² (2.1 mg / cm² in this case), and this was used as the positive electrode of the battery. A sodium metal sheet was used as the negative electrode. A 1M NaClO₄ / EC / DEC electrolyte (EC / DEC V / V = 1:1) was used, and a glass fiber membrane was used as the separator. The assembled battery was subjected to charge-discharge tests at a voltage range of 1.8-3.6V, and the discharge specific capacity was tested under 0.2C / 5.0C conditions. The results are recorded in Table 1.

[0144] Test Example 2

[0145] Examples 2#-8# and Comparative Examples 1*-12* were tested according to the test method of Test Example 1, and the discharge specific capacity under 0.2C / 5.0C conditions was recorded in Table 1.

[0146]

[0147] Comparison of effects:

[0148] Examples 1-8 (1#-8#) involve adjusting the molar ratio of carbon-coated NaVPO4F and carbon-coated Na4Fe3(PO4)2P2O7 to 1:9-1:250, the solvothermal reaction temperature to 150-250℃, the molar ratio of vanadium source to reducing agent (e = 1:1.05~1.1), the surfactant content in the carbon-coated Na4Fe3(PO4)2P2O7 mass fraction to 1-5%, and the carbon source content in the total mass of vanadium source, sodium salt, fluoride salt, phosphate, and reducing agent to 10-30%. Based on experimental results, within the parameter range protected by this invention, good initial specific capacity and rate performance can be obtained. The initial capacity range is 117-127 mAh / g. The specific capacity at 5C rate range is 106-116 mAh / g.

[0149] When the amount of carbon-coated NaVPO4F is too much or too little, the resulting coating layer is either too large or too small. If it is too large, it will mask the performance of iron and sodium, and at the same time, the particle size will be large, resulting in a decrease in rate performance (1*). If it is too small, there will be more exposed NFPP, and some Fe2+ will oxidize Fe3+, resulting in a decrease in effective specific capacity (2*).

[0150] When the temperature of the hydrothermal reactor is too low (100 degrees), the reaction is incomplete, impurities appear in the final product, resulting in poor battery capacity and rate performance (3*); when the temperature of the hydrothermal reactor is too high (300 degrees), although the battery capacity and rate performance meet the requirements, there are safety hazards in the reaction (4*), which is not a reasonable reaction temperature range.

[0151] When there is insufficient reducing agent, V3+ is prone to partial oxidation of V4+ in subsequent reactions, leading to a decrease in material performance (5*); when there is excessive reducing agent, the carbon coating in the final product is too large, which limits the capacity of the active material (6*).

[0152] When the amount of surfactant is too small, the raw material dispersion effect is poor, the final product contains more impurities and the performance is low (7*); when the amount of additive is too large, it is easy to cover the surface of the raw material, resulting in incomplete reaction between the raw materials, which also leads to impurities in the product, both of which will reduce the specific capacity and rate performance (8*).

[0153] When the carbon source mass is too low (9*), the total carbon content of the product will be insufficient, and the product reduction will be incomplete, accompanied by the appearance of impurities. Ultimately, this will affect the material performance, especially the rate performance. When the carbon source mass is too high (10*), the carbon content of the final product will be too high, affecting the capacity of the active material.

[0154] When carbon-coated NaVPO4F and carbon-coated Na4Fe3(PO4)2P2O7 are physically and mechanically mixed, it cannot be guaranteed that NFPP is uniformly coated with NVPF, and Fe3+ still exists, resulting in a lower actual specific capacity (11*).

[0155] When the coating method does not employ a solvothermal method but instead uses a high-temperature calcination method, the calcination temperature of NaVPO4F is around 700 degrees Celsius, far exceeding the calcination temperature of Na4Fe3(PO4)2P2O7 (550 degrees Celsius). Therefore, the final product formed after the reaction contains Na4Fe3(PO4)2P2O7.

[0156] High-temperature decomposition results in a significant amount of impurities, such as NaFePO4, with the final product containing over 30% sodium iron phosphate and sodium iron pyrophosphate impurities. This drastically reduces the material's specific capacity (12*).

Claims

1. A method for preparing carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F, the preparation steps of which include: A carbon-coated square NaVPO4F crystal coating layer was grown in situ on the surface of carbon-coated Na4Fe3(PO4)2P2O7. The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:9 to 1:

250. The carbon content in carbon-coated Na4Fe3(PO4)2P2O7 is 2-7% by mass. 1) Mix vanadium source, sodium salt, fluoride salt, phosphate, reducing agent, carbon source, carbon-coated Na4Fe3(PO4)2P2O7 and surfactant into an organic solvent to obtain mixture a; 2) Place mixture a in a sealed reaction vessel and carry out a solvothermal reaction to obtain mixture b; the solvothermal reaction temperature is 150-250℃ and the reaction pressure is 1-3MPa; 3) Centrifuge the mixture b obtained in step 2), and after washing and drying the solid with ethanol, obtain the final product carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F.

2. The preparation method according to claim 1, characterized in that: The molar ratio of V to Na4Fe3(PO4)2P2O7 in the vanadium source is 1:9 to 1:250; The molar ratio of V, Na, F, and P elements in the vanadium source, sodium salt, fluoride salt, and phosphate is 1:1:1:1; If the vanadium source has a +5 valence, the molar ratio of the vanadium source to the reducing agent is e, where e is 1:1.05~1.1; If the vanadium source has a +3 valence, the molar ratio of the vanadium source to the reducing agent is f, where f is 1:0.05~0.1; The surfactant constitutes 1-5% of the mass fraction of the carbon-coated Na4Fe3(PO4)2P2O7; The carbon source accounts for 10-30% of the total mass of the vanadium source, sodium salt, fluoride salt, phosphate, and reducing agent.

3. The preparation method according to claim 1, characterized in that: The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:40 to 1:

160.

4. The preparation method according to claim 1, characterized in that: The vanadium source is one or more of ammonium metavanadate, vanadium pentoxide, vanadium trioxide, and vanadium phosphate. The sodium salt is one or more of sodium hydroxide, sodium oxalate, sodium sulfate, sodium citrate, sodium nitrate, sodium fluoride, sodium bicarbonate, and sodium carbonate. The phosphate is one or more of the following: ammonium dihydrogen phosphate, sodium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. The fluoride salt is one or more of ammonium fluoride and sodium fluoride; the reducing agent is one or more of oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, citric acid, sucrose, malic acid, oxalic acid, adipic acid, and starch; the carbon source is one or more of sucrose, glucose, starch, and oxalic acid. The organic solvent is one or more selected from methanol, ethanol, propanol, isopropanol, acetone, methyl butyl ketone, methyl isobutyl ketone, diethyl ether, and ethylene glycol dimethyl ether. The surfactant is one or more of sodium dodecyl sulfate, polyethylene glycol, alkyl glycoside, cocoyl diethanolamide, and dodecylbenzene sulfonic acid.

5. The preparation method according to claim 1, characterized in that: The solvothermal reaction time in step 2) is 12-36 hours; The solid content / solvent concentration in step 1) is 15-35 mg / ml; After centrifugation, mixture b in step 3) is washed with ethanol 2-5 times and dried at 80-150℃ for 8-18 hours.

6. The preparation method according to claim 5, characterized in that: The solvothermal reaction time in step 2) is 15-20 hours; The solid content / solvent concentration in step 1) is 20-30 mg / ml.

7. The preparation method according to claim 1, characterized in that: Carbon-coated Na4Fe3(PO4)2P2O7 electrode materials were prepared by solid-phase method, sol-gel method or wet-phase ball milling method; the carbon content of the carbon-coated Na4Fe3(PO4)2P2O7 was 2-7% by mass.

8. A carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F prepared by any of the preparation methods described in claims 1-7.

9. The carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F according to claim 8, characterized in that: A carbon-coated square NaVPO4F crystal coating layer was grown in situ on the surface of carbon-coated Na4Fe3(PO4)2P2O7. The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:9 to 1:

250. The carbon content in carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F is 2-7% by mass.

10. The carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F according to claim 9, characterized in that: The molar ratio of V to Na4Fe3(PO4)2P2O7 in the carbon-coated square NaVPO4F crystal is 1:40 to 1:

160.

11. The application of the carbon-coated Na4Fe3(PO4)2P2O7@carbon-coated NaVPO4F as a positive electrode active material for sodium-ion batteries according to claim 8.

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