A method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate.

Abstract

This invention relates to the technical field of sodium-ion battery cathode materials, and discloses a method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate, comprising the following steps: Step 1: Adding iron phosphate, a chelating agent, a sodium source, a phosphorus source, and a carbon source to water to prepare a mixed solution; Step 2: Heating and stirring the mixed solution until the solution becomes clear; Step 3: Spray drying the clear solution to obtain a precursor powder; Step 4: Sintering the precursor powder under an inert atmosphere to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C. This invention utilizes an ionization equilibrium mechanism, allowing iron phosphate and the chelating agent to exist in ionic form in the aqueous phase, effectively shortening the synthesis time, solving the problem of easily generated inert phases during material synthesis, and simultaneously improving the uniformity of the generated sodium iron pyrophosphate.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, and more specifically relates to a method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate. Background Technology

[0002] Sodium-ion batteries are considered the most promising alternative to lithium-ion batteries due to abundant sodium resources, low cost, and relatively high safety performance. In recent years, significant efforts have been made to develop key technologies for sodium-ion batteries, including cathode materials, anode materials, and electrolytes. However, the development of cathode materials for sodium-ion batteries is constrained by unavoidable drawbacks such as the large radius and low standard electrochemical potential of sodium ions.

[0003] To date, the main cathode materials studied include transition metal oxides, Prussian blue compounds, and polyanionic compounds. Among polyanionic cathode materials, iron-based polyanionic cathode materials have promising application prospects due to their strong structural stability, excellent cycle performance, low cost, and good safety. However, NaFePO4 synthesized by common solid-phase or liquid-phase methods is an electrochemically inert sodium ferrophosphate structure, lacking sodium ion diffusion channels and therefore unsuitable for battery cathode materials. While NaFePO4 with an olivine structure can be synthesized by electrochemically delithitoylating and sodium-intercalating LiFePO4, its complex synthesis process limits its future development and application. In recent years, sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)) has shown promise due to its high theoretical specific capacity (128.9 mAh g / L). -1 Sodium iron phosphate (Na4Fe3(PO4)2(P2O7)) is a proven electrochemically active iron-based polyanionic cathode material due to its inexpensive and readily available raw materials, simple synthesis process, and good cycle performance, making it suitable for large-scale energy storage systems. However, during its synthesis, sodium iron phosphate readily generates electrochemically inert sodium phosphate ore-type NaFePO4, the content of which directly affects the cycle life, rate capability, polarization, and other electrochemical performance of the sodium iron phosphate cathode material. Therefore, developing high-purity, low-cost sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2(P2O7)) has become one of the key research areas for sodium-ion battery cathode materials.

[0004] Commercial lithium iron phosphate (LFP) batteries, as a key precursor in the synthesis of lithium iron phosphate (LFP), possess advantages such as structural stability, affordability, high atom utilization, and large-scale production. With the rapid development of the new energy vehicle industry, the demand for LFP batteries will grow rapidly, making the recycling of spent LFP batteries a crucial issue. Typically, recycling 1 ton of spent LFP produces over 1.5 tons of wet LFP residue. Although numerous studies have reported on the regeneration of LFP from LFP residue, the electrochemical performance of the regenerated LFP is significantly inferior to that of commercial LFP. Therefore, achieving the sustainable recycling and reuse of spent LFP batteries by directly regenerating valuable materials from LFP residue is of great significance.

[0005] Patent CN 115230923 A discloses a carbon-coated sodium iron pyrophosphate cathode material, its preparation method, and its applications. Using ferric phosphate slag from commercial lithium iron phosphate or lithium iron phosphate cathode waste powder as raw material, the material is obtained by ball milling, mixing, and high-temperature calcination. While this method is simple, the sodium iron pyrophosphate nanopowder prepared by this solid-state method is prone to agglomeration, resulting in a relatively low specific surface area and electrode material utilization rate.

[0006] Patent CN 113060714A discloses a method for preparing Na4Fe3(PO4)2P2O7 from FePO4 in the liquid phase. Using FePO4 as a raw material, spherical sodium iron pyrophosphate is prepared by mixing, wet milling, spray drying, and high-temperature calcination. This method is characterized by its greenness, simplicity, and ease of scale-up. However, while prolonged milling can reduce the grain size of FePO4, it also increases the preparation time. Furthermore, FePO4 is not completely dissolved, therefore it is not a strictly liquid-phase preparation.

[0007] As mentioned earlier, there are two main methods for synthesizing sodium ferric pyrophosphate from iron phosphate: solid-phase and liquid-phase methods. The sodium ferric pyrophosphate nanopowder obtained by the existing solid-phase method is prone to agglomeration, while the existing liquid-phase method results in a longer synthesis time. Furthermore, the electrochemically inert phase sodium ferric phosphate (NaFePO4) is inevitably generated during the synthesis process. Summary of the Invention

[0008] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a method for synthesizing sodium iron phosphate pyrophosphate cathode material using iron phosphate, so that the synthesis method can synthesize sodium iron phosphate pyrophosphate cathode material using iron phosphate as raw material, solve the problem of easy generation of associated electrochemical inert phase during the material synthesis process, shorten the synthesis time, and provide a new technical route for the recycling and reuse of waste lithium iron phosphate batteries.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: A method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate includes the following steps:

[0010] Step 1: Add ferric phosphate, chelating agent, sodium source, phosphorus source and carbon source to water to prepare a mixed solution;

[0011] Step 2: Heat and stir the mixed solution until the solution becomes clear;

[0012] Step 3: Spray dry the clarified solution to obtain precursor powder;

[0013] Step 4: Sinter the precursor powder under an inert atmosphere to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0014] More preferably, the chelating agent in step 1 is at least one of oxalic acid, citric acid, and ethylenediaminetetraacetic acid.

[0015] More preferably, in step 1, the molar ratio of ferric phosphate to the chelating agent is (1-5):1; the molar ratio of ferric phosphate to the carbon source is (4-7):1; and sodium and phosphorus sources are supplemented according to the molar ratio of Na:Fe:P = 4:3:4 in the mixed solution.

[0016] Chelating agents are mainly used to chelate iron ions in ferric phosphate, making them dissolve in water. This improves solubility and promotes dispersion, thereby shortening the reaction time. Furthermore, the chelating effect of chelating agents on ions promotes the forward reaction, further reducing the formation of the inert NaFePO4 phase. Different chelating agents have different chelation ratios with iron ions. If the amount of chelating agent added is too small, the chelation will be incomplete, and ferric phosphate will not exist in the aqueous phase in ionic form, affecting the purity of the final product. In addition, since chelating agents generate additional carbon during carbonization, adding too much chelating agent will correspondingly reduce the relative content of active substances, thus affecting the performance of the final product.

[0017] More preferably, the iron phosphate in step 1 includes at least one of commercial iron phosphate or iron phosphate residue after lithium extraction from waste lithium iron phosphate battery cathode powder.

[0018] More preferably, the sodium source in step 1 is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium citrate, sodium dihydrogen citrate, disodium hydrogen citrate, and sodium hydroxide.

[0019] More preferably, the phosphorus source in step 1 is at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, and monosodium trihydrogen pyrophosphate.

[0020] More preferably, the carbon source in step 1 is at least one of polyvinylpyrrolidone, dopamine hydrochloride, ascorbic acid, tartaric acid, glucose, sucrose, starch, maltose, and dextrin.

[0021] The introduction of a carbon source can form a carbon coating layer on the surface of sodium iron pyrophosphate material, which can increase the conductivity between particles and provide an electron transport channel for sodium iron pyrophosphate material to compensate for Na+. + The dynamic charge balance during the sodium insertion / extraction process improves the electrochemical performance of sodium iron pyrophosphate materials. However, excessive carbon source introduction leads to a relative decrease in the content of active substances, thus affecting the performance of the final product. Since chelating agents can also act as partial carbon sources, the amounts of carbon source, chelating agent, and iron phosphate added need to be coordinated to obtain better electrochemical performance of the product.

[0022] More preferably, the heating temperature for heating and stirring in step 2 is 60-80°C, and the heating and stirring time is 0.5-3 hours.

[0023] Heating and stirring can disperse ferric phosphate in the aqueous phase in ionic form. Especially with the addition of a chelating agent, the heating and dissolving step helps to form a chelating effect, which promotes the smooth progress of subsequent reactions and solves the problems of ferric phosphate being slightly soluble in water and easily generating associated inert phases.

[0024] More preferably, the spray drying temperature in step 3 is 130–220°C, and the feed rate is 0.5%–20%.

[0025] More preferably, the inert atmosphere in step 4 is either argon or nitrogen; the sintering temperature is 450–600°C, and the time is 5–12 hours.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) By using the ionization equilibrium mechanism, heating and stirring are used to make iron phosphate and chelating agent exist in the aqueous phase in ionic form, which effectively shortens the synthesis time, solves the problem of easy generation of associated inert phases during material synthesis, and improves the uniformity of sodium iron pyrophosphate generated by the reaction.

[0028] (2) The raw materials used include iron phosphate, sodium source, phosphorus source, chelating agent, and carbon source. They are inexpensive, readily available, and widely distributed.

[0029] (3) The preparation method of this application is simple and low in cost, and has the potential for commercialization and large-scale production, providing a new technical route for the resource recycling and high-value reuse of waste lithium iron phosphate batteries. Attached Figure Description

[0030] The invention will now be further described with reference to the accompanying drawings.

[0031] Figure 1 The XRD pattern of the sodium iron pyrophosphate cathode material prepared in Example 1 of this invention;

[0032] Figure 2 The image shows the SEM image of the sodium iron pyrophosphate cathode material prepared in Example 1 of this invention.

[0033] Figure 3 The charge-discharge curve of the sodium-ion battery prepared in Example 1 of this invention at a current density of 0.1C is shown.

[0034] Figure 4 The charge-discharge curve of the sodium-ion battery prepared in Example 2 of this invention at a current density of 0.1C is shown.

[0035] Figure 5 The charge-discharge curve of the sodium-ion battery prepared in Example 3 of this invention at a current density of 0.1C is shown.

[0036] Figure 6 This is a charge-discharge cycle curve of the sodium-ion battery prepared in Example 1 of the present invention at a current density of 1C. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the embodiments, and the technical content and effects thereof are not limited thereto.

[0038] Example 1

[0039] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0040] (1) Sodium pyrophosphate, ferric phosphate, sodium acetate, ethylenediaminetetraacetic acid and glucose are used as raw materials; among them, sodium pyrophosphate is both a sodium source and a phosphorus source, ferric phosphate is both an iron source and a phosphorus source, sodium acetate is a sodium source, ethylenediaminetetraacetic acid is both a chelating agent and a carbon source, and glucose is a supplementary carbon source.

[0041] (2) Add 4.46g sodium pyrophosphate, 9.06g ferric phosphate, 3.28g sodium acetate, 17.53g ethylenediaminetetraacetic acid and 1.80g glucose to 500mL of water to prepare a mixed solution;

[0042] (3) Heat and stir the mixed solution at 80℃ for 1 hour until the solution becomes clear.

[0043] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0044] (5) The precursor was placed in an argon atmosphere and calcined at 600℃ for 6h to obtain Na4Fe3(PO4)2(P2O7)@C.

[0045] Example 2

[0046] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0047] (1) Using ammonium dihydrogen phosphate, lithium iron phosphate residue after lithium extraction from waste lithium iron phosphate battery cathode powder, sodium dihydrogen citrate, oxalic acid and sucrose as raw materials; among them, ammonium dihydrogen phosphate is the phosphorus source, lithium iron phosphate residue after lithium extraction from waste lithium iron phosphate battery cathode powder is both an iron source and a phosphorus source, sodium dihydrogen citrate is the sodium source, oxalic acid is the chelating agent and sucrose is the supplementary carbon source.

[0048] (2) Add 2.30g ammonium dihydrogen phosphate, 9.06g lithium iron phosphate residue after lithium extraction from lithium iron phosphate battery cathode waste powder, 4.18g sodium dihydrogen citrate, 4.20g oxalic acid, and 3.42g sucrose to 500mL of water to prepare a mixed solution;

[0049] (3) Heat and stir the mixed solution at 80℃ for 1 hour until the solution becomes clear.

[0050] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0051] (5) The precursor was placed in an argon atmosphere and calcined at 550°C for 8 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0052] Example 3

[0053] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0054] (1) Using ammonium dihydrogen phosphate, iron phosphate, sodium carbonate, citric acid and tartaric acid as raw materials; among them, ammonium dihydrogen phosphate is the phosphorus source, iron phosphate is both an iron source and a phosphorus source, sodium carbonate is the sodium source, citric acid is both a chelating agent and a carbon source, and tartaric acid is a supplementary carbon source.

[0055] (2) Add 2.30g ammonium dihydrogen phosphate, 9.06g ferric phosphate, 6.36g sodium oxalate, 2.52g citric acid and 1.50g tartaric acid to 500mL of water to prepare a mixed solution;

[0056] (3) Heat and stir the mixed solution at 80°C for 1 hour until the solution becomes clear.

[0057] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0058] (5) The precursor was placed in an argon atmosphere and calcined at 500°C for 12 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0059] Example 4 (using different heating temperatures and times)

[0060] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0061] (1) Using ammonium dihydrogen phosphate, iron phosphate, sodium carbonate, citric acid and tartaric acid as raw materials; among them, ammonium dihydrogen phosphate is the phosphorus source, iron phosphate is both an iron source and a phosphorus source, sodium carbonate is the sodium source, citric acid is both a chelating agent and a carbon source, and tartaric acid is a supplementary carbon source.

[0062] (2) Add 2.30g ammonium dihydrogen phosphate, 9.06g ferric phosphate, 6.36g sodium carbonate, 2.52g citric acid and 1.50g tartaric acid to 500mL of water to prepare a mixed solution;

[0063] (3) Heat and stir the mixed solution at 60°C for 2 hours until the solution becomes clear.

[0064] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0065] (5) The precursor was placed in an argon atmosphere and calcined at 500°C for 12 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0066] Comparative Example 1 (different from Example 1 in that no chelating agent was added)

[0067] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0068] (1) Sodium pyrophosphate, ferric phosphate, sodium acetate and glucose are used as raw materials; among them, sodium pyrophosphate is both a sodium source and a phosphorus source, ferric phosphate is both an iron source and a phosphorus source, sodium acetate is a sodium source and glucose is a carbon source.

[0069] (2) Add 4.46g sodium pyrophosphate, 9.06g ferric phosphate, 3.28g sodium acetate and 1.80g glucose to 500mL of water to prepare a mixed solution;

[0070] (3) Heat and stir the mixed solution at 80°C for 3 hours until the solution becomes clear.

[0071] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0072] (5) The precursor was placed in an argon atmosphere and calcined at 600℃ for 6h to obtain Na4Fe3(PO4)2(P2O7)@C.

[0073] Comparative Example 2 (The difference from Example 1 is that an excessive amount of chelating agent was added)

[0074] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0075] (1) Sodium pyrophosphate, ferric phosphate, sodium acetate, ethylenediaminetetraacetic acid and glucose are used as raw materials; among them, sodium pyrophosphate is both a sodium source and a phosphorus source, ferric phosphate is both an iron source and a phosphorus source, sodium acetate is a sodium source, ethylenediaminetetraacetic acid is both a chelating agent and a carbon source, and glucose is a supplementary carbon source.

[0076] (2) Add 4.46g sodium pyrophosphate, 9.06g ferric phosphate, 3.28g sodium acetate, 35.07g ethylenediaminetetraacetic acid and 1.80g glucose to 500mL of water to prepare a mixed solution;

[0077] (3) Heat and stir the mixed solution at 80°C for 1 hour until the solution becomes clear.

[0078] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0079] (5) The precursor was placed in an argon atmosphere and calcined at 600℃ for 6h to obtain Na4Fe3(PO4)2(P2O7)@C.

[0080] Comparative Example 3 (different from Example 2 in that no carbon source was added)

[0081] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0082] (1) Using ammonium dihydrogen phosphate, lithium iron phosphate residue after lithium extraction from waste lithium iron phosphate battery cathode powder, sodium dihydrogen citrate, and oxalic acid as raw materials; wherein, ammonium dihydrogen phosphate is the phosphorus source, the lithium iron phosphate residue after lithium extraction from waste lithium iron phosphate battery cathode powder is both an iron source and a phosphorus source, sodium dihydrogen citrate is the sodium source, and oxalic acid is the chelating agent.

[0083] (2) Add 2.30g ammonium dihydrogen phosphate, 9.06g lithium iron phosphate residue after lithium extraction from lithium iron phosphate battery cathode waste powder, 4.18g sodium dihydrogen citrate, and 4.20g oxalic acid to 500mL of water to prepare a mixed solution;

[0084] (3) Heat and stir the mixed solution at 80°C for 1 hour until the solution becomes clear.

[0085] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0086] (5) The precursor was placed in an argon atmosphere and calcined at 550°C for 8 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0087] Comparative Example 4 (different from Example 3 in that no carbon source was added)

[0088] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0089] (1) Using ammonium dihydrogen phosphate, ferric phosphate, sodium carbonate and citric acid as raw materials; among them, ammonium dihydrogen phosphate is the phosphorus source, ferric phosphate is both the iron source and the phosphorus source, sodium carbonate is the sodium source and oxalic acid is the chelating agent.

[0090] (2) Add 2.30g ammonium dihydrogen phosphate, 11.21g ferric phosphate, 6.36g sodium carbonate and 2.52g citric acid to 500mL of water to prepare a mixed solution;

[0091] (3) Heat and stir the mixed solution at 80°C for 1 hour until the solution becomes clear.

[0092] (4) The obtained clarified solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0093] (5) The precursor was placed in an argon atmosphere and calcined at 500°C for 12 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0094] Comparative Example 5 (different from Example 3 in that no heating or stirring was performed)

[0095] A sodium iron pyrophosphate cathode material synthesized using iron phosphate, with the general formula Na4Fe3(PO4)2(P2O7)@C, is prepared by the following method:

[0096] (1) Using ammonium dihydrogen phosphate, ferric phosphate, sodium carbonate, citric acid and tartaric acid as raw materials; among them, ammonium dihydrogen phosphate is the phosphorus source, ferric phosphate is both the iron source and the phosphorus source, sodium carbonate is the sodium source, oxalic acid is the chelating agent and tartaric acid is the carbon source.

[0097] (2) Add 2.30g ammonium dihydrogen phosphate, 9.06g ferric phosphate, 6.36g sodium carbonate, 2.52g citric acid and 1.50g tartaric acid to 500mL of water to prepare a mixed solution;

[0098] (3) The obtained mixed solution was spray-dried at an air inlet rate of 100%, an air inlet temperature of 220°C, and a feed rate of 10% to obtain the precursor.

[0099] (4) The precursor was placed in an argon atmosphere and calcined at 500°C for 12 hours to obtain Na4Fe3(PO4)2(P2O7)@C.

[0100] Electrode pastes were prepared using the Na4Fe3(PO4)2(P2O7)@C materials prepared in Examples 1-4 and Comparative Examples 1-5, with a mass ratio of Na4Fe3(PO4)2(P2O7)@C: conductive carbon black: binder of 7:2:1. These pastes were then coated onto aluminum foil using a doctor blade coating method. The aluminum foil was dried in an oven at 80°C for 8 hours. The dried electrode sheets were then pressed using a tablet press at 12 kg·cm³. 2 The tablets were compressed under pressure and then cut into 12 mm diameter discs using a slicer. A sodium metal disc was used as the counter electrode, and a glass fiber membrane was used as the diaphragm. (1 mol·L⁻¹) -1 A sodium perchlorate (NaClO4) solution containing ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte (EC to DEC molar volume ratio of 1:1). CR2025 coin cells were assembled in a glove box with an oxygen content below 0.01 ppm. The electrochemical performance of the assembled coin cells was then tested.

[0101] like Figure 1 The image shown is the XRD pattern of the Na4Fe3(PO4)2(P2O7)@C material from Example 1. Figure 1As can be seen from the data, the diffraction peaks of the sodium pyrophosphate sodium Na4Fe3(PO4)2(P2O7)@C cathode material synthesized using iron phosphate completely overlap with the standard card of Na4Fe3(PO4)2(P2O7), with no impurity peaks, indicating that the pure phase material was successfully prepared.

[0102] like Figure 2 The image shown is a SEM image of the Na4Fe3(PO4)2(P2O7)@C material from Example 1. Figure 2 As can be seen from the data, the morphology of the positive electrode material using sodium iron phosphate pyrophosphate Na4Fe3(PO4)2(P2O7)@C is quasi-spherical, and the particle size of the product is controllable.

[0103] like Figure 3 The figure shows the charge-discharge curves of the Na4Fe3(PO4)2(P2O7)@C material of Example 1 at a current density of 0.1C. Figure 3 It can be seen that the coin cell assembled using Na4Fe3(PO4)2(P2O7)@C prepared in this embodiment as the positive electrode active material achieves a reversible capacity of 110 mAh·g at 0.1C. -1 .

[0104] like Figure 6 The figure shows the cycling performance of the Na4Fe3(PO4)2(P2O7)@C material of Example 1 at a current density of 1C. Figure 6 It can be seen that the coin cell assembled using Na4Fe3(PO4)2(P2O7)@C prepared in this embodiment as the positive electrode active material achieves a reversible capacity of 87 mAh·g at 1C. -1 After 200 cycles, the capacity retention rate is 97%, demonstrating excellent cycling performance.

[0105] like Figure 4 The figure shows the charge-discharge curves of the Na4Fe3(PO4)2(P2O7)@C material in Example 2 at a current density of 0.1C. Figure 4 It can be seen that the coin cell assembled using Na4Fe3(PO4)2(P2O7)@C prepared in this embodiment as the positive electrode active material achieves a reversible capacity of 102 mAh·g at 0.1C. -1 .

[0106] like Figure 5 The figure shows the charge-discharge curves of the Na4Fe3(PO4)2(P2O7)@C material in Example 3 at a current density of 0.1C. Figure 5 It can be seen that the coin cell assembled using Na4Fe3(PO4)2(P2O7)@C prepared in this embodiment as the positive electrode active material achieves a reversible capacity of 94 mAh·g at 0.1C. -1 .

[0107] Table 1

[0108]

[0109]

[0110] As shown in Table 1, comparing Example 1 and Comparative Example 1, it can be seen that the addition of chelating agent is beneficial to the synthesis of sodium iron pyrophosphate cathode material without inert phase material, and the assembled battery has excellent electrochemical performance.

[0111] Comparing Example 1 and Comparative Example 2, it can be seen that the amount of chelating agent added should be appropriate. Excessive addition of chelating agent will lead to an increase in the amorphous carbon content in the sodium iron pyrophosphate material, and a corresponding decrease in the active material of sodium iron pyrophosphate, resulting in a decline in the electrochemical performance of the battery.

[0112] Comparing Examples 2 and 3 with Comparative Examples 3 and 4, it can be seen that adding an appropriate amount of carbon source can form a carbon coating layer on the surface of sodium iron pyrophosphate material, increasing the conductivity between particles and providing an electron transport channel for the sodium iron pyrophosphate material to compensate for Na+. + The dynamic charge balance during the sodium insertion / extraction process improves the electrochemical performance of the battery.

[0113] Comparing Examples 3 and 4, it can be seen that by appropriately adjusting the heating and stirring temperature and time, iron phosphate and chelating agent can also exist in the aqueous phase in ionic form. The synthesized sodium iron pyrophosphate does not produce an associated inert phase, and the assembled battery has excellent electrochemical performance.

[0114] Comparing Example 3 and Comparative Example 5, it can be seen that when heating and stirring are not performed, iron phosphate cannot exist completely in the aqueous phase in ionic form. The synthesized sodium iron pyrophosphate produces an associated inert phase, which reduces the electrochemical performance of the battery.

[0115] The above embodiments of the present invention are not merely illustrative examples, but rather intended to limit the implementation of the invention. Those skilled in the art will recognize that various variations and modifications can be made based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of this invention are still within the scope of protection of this invention.

Claims

1. A method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate, characterized in that, Includes the following steps: Step 1: Add ferric phosphate, chelating agent, sodium source, phosphorus source and carbon source to water to prepare a mixed solution; the chelating agent is ethylenediaminetetraacetic acid; the molar ratio of ferric phosphate to chelating agent is (1~5):1; the molar ratio of ferric phosphate to carbon source is (4~7):1; add sodium source and phosphorus source according to the molar ratio of Na:Fe:P = 4:3:4 in the mixed solution; Step 2: Heat and stir the mixed solution at a temperature of 60-80 ℃ for 0.5-1 h until the solution becomes clear; Step 3: Spray dry the clarified solution to obtain precursor powder; Step 4: Sinter the precursor powder under an inert atmosphere to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

2. The method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate as described in claim 1, characterized in that, The carbon source mentioned in step 1 is at least one of polyvinylpyrrolidone, dopamine hydrochloride, tartaric acid, glucose, sucrose, starch, maltose, and dextrin.

3. The method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate as described in claim 1, characterized in that, The sodium source mentioned in step 1 is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium citrate, sodium dihydrogen citrate, disodium hydrogen citrate, and sodium hydroxide.

4. The method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate as described in claim 1 or 2, characterized in that, The phosphorus source mentioned in step 1 is at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, and monosodium trihydrogen pyrophosphate.

5. The method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate as described in claim 1, characterized in that, The spray drying temperature in step 3 is 130~220 ℃, and the feed rate is 0.5%~20%.

6. The method for synthesizing sodium iron pyrophosphate cathode material using iron phosphate as described in claim 1 or 4, characterized in that, The inert atmosphere mentioned in step 4 is either argon or nitrogen; the sintering temperature is 450~600 ℃ and the time is 5~12 h.

7. A sodium iron pyrophosphate cathode material synthesized by the method according to any one of claims 1-6, characterized in that, The sodium iron pyrophosphate cathode material is used in the field of sodium-ion batteries.

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