A method for synthesizing ferric pyrophosphates and preparing sodium ferric pyrophosphate positive electrode materials
The precursor method for synthesizing sodium iron pyrophosphate cathode material solves the problems of difficulty in synthesizing pure phase and low electronic conductivity in existing technologies, achieving high purity, sphericity and high compaction density, thus improving the electrochemical performance of sodium-ion batteries.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CENT SOUTH UNIV
- Filing Date
- 2024-10-29
- Publication Date
- 2026-07-21
AI Technical Summary
Existing technologies struggle to synthesize high-purity sodium iron pyrophosphate cathode materials, resulting in issues such as low electronic conductivity, formation of inactive impurity phases, high burn-off rate, and low tap density, all of which negatively impact the electrochemical performance of sodium-ion batteries.
Iron pyrophosphate was synthesized and sodium iron pyrophosphate cathode material was prepared by using a precursor method. By controlling the pH value, co-precipitation reaction and crystallization process, a precursor with Fe and P elements uniformly arranged at the atomic scale was prepared. Combined with spray drying and sintering processes, spherical carbon-coated Na4Fe3(PO4)2(P2O7) material was formed.
This achievement resulted in high purity, sphericity, and high compaction density of the material, improved its electrochemical performance, solved the problems of inactive impurity phase formation and high burn-off rate, and enhanced the material's energy density and electrochemical performance.
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Figure CN119390031B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, specifically relating to a method for synthesizing iron pyrophosphate and preparing sodium iron pyrophosphate cathode material. Background Technology
[0002] In the development and utilization of renewable and clean energy, electrochemical energy storage technology, using rechargeable batteries (especially lithium-ion batteries), plays a crucial foundational role. However, limited lithium resources are insufficient to meet the ever-increasing demand for energy storage. Sodium-ion batteries, due to the abundant sodium reserves in the Earth's crust and their similar electrochemical behavior to lithium-ion batteries, have become an ideal alternative, demonstrating enormous application potential in electrochemical energy storage systems.
[0003] As a crucial component of sodium-ion batteries, the cathode material largely determines the battery's energy density and cost. Developing cathode materials with high capacity, high voltage, and long cycle stability is currently a research hotspot and cutting-edge issue in sodium-ion batteries. To date, Prussian blue compounds, layered oxides, and polyanionic compounds have become the three main technical routes for sodium-ion battery cathodes. Although Prussian blue compounds and layered oxides have high specific capacities, their respective drawbacks (such as the difficulty in removing the water of crystallization from Prussian blue compounds and the poor air stability of layered oxides) limit their practical applications.
[0004] In comparison, polyanionic compounds possess a stable structural framework and higher operating voltage, exhibiting excellent air stability, thermal stability, and cycle stability, making them a top choice for cathode materials in sodium-ion batteries. Among numerous polyanionic cathode materials (such as phosphates, pyrophosphates, fluorophosphates, sulfates, and silicates), sodium superionic conductor type iron-based mixed phosphate Na4Fe3(PO4)2(P2O7) material (sodium iron pyrophosphate, hereinafter referred to as NFPP) is composed of abundant, low-cost, and non-toxic elements such as Fe and P. It possesses high structural stability (volume strain during sodiumification / desodiumification process <4%), moderate theoretical capacity (128.9 mAh g-1), and operating voltage (3.0 V vs. Na+ / Na), and is attracting widespread attention. However, some problems still need to be overcome in the practical application of NFPP. First, similar to other polyanionic cathode materials, NFPP has inherently low electronic conductivity, resulting in slow electrode reaction kinetics and a significant impact on electrochemical performance. Secondly, it is difficult to synthesize high-purity NFPP. During the synthesis process, an electrochemically inactive phosphogypsum-type NaFePO4 (NFP) impurity phase often appears, hindering the synthesis of Na+. + The diffusion of [something] reduces the reversible capacity of the material.
[0005] Currently, the synthesis processes for NFPP are mostly spray drying-solid-state sintering or ball milling-assisted solid-state sintering. The raw materials used are individual phosphates, sodium salts, and iron salts, and there is no feasible precursor synthesis process. This makes it difficult to achieve uniform mixing of multiple elements at the atomic scale, leading to the formation of inactive impurities during the sintering process. Furthermore, current spray drying processes often use nitrates as raw materials, resulting in extremely high burn-off rates and the generation of nitrogen oxides and other exhaust gases. This easily causes the fragmentation of spherical particles, resulting in low tap and compaction densities of the obtained NFPP materials, limiting the improvement of the material's energy density. Summary of the Invention
[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for synthesizing iron pyrophosphate and preparing sodium iron pyrophosphate cathode material, wherein the prepared cathode material has superior electrochemical performance.
[0007] The technical solution adopted by this invention to solve its technical problem is a method for synthesizing iron pyrophosphate and preparing sodium iron pyrophosphate cathode material, comprising the following steps:
[0008] S1. Prepare iron salt solution, prepare phosphate solution, prepare pyrophosphate solution, prepare ammonia solution, prepare acid solution;
[0009] S2. Prepare the reaction base solution, then add the iron salt solution, phosphate solution, and pyrophosphate solution to the reaction base solution. Use ammonia solution and / or acid solution to control the pH value, and carry out the co-precipitation reaction under stirring to obtain slurry A.
[0010] S3. Filter and wash slurry A, add water and re-slurry to obtain slurry B;
[0011] S4. Heat slurry B to the crystallization temperature and crystallize it under stirring to obtain slurry C;
[0012] S5. Filter the slurry C and dry the precipitate to obtain precipitate D;
[0013] S6. Sinter the precipitate D at a certain temperature to remove the water of crystallization and obtain iron pyrophosphate Fe6(PO4)2(P2O7)3.
[0014] S7. Add the iron pyrophosphate, sodium source, and carbon source obtained in step S6 to water and slurry, then perform sand milling (preferably in a sand mill) to obtain slurry E.
[0015] S8. Spray dry the slurry E to obtain mixed powder F;
[0016] S9. Sinter the mixed powder F in an inert or reducing atmosphere to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
[0017] Further, in step S1, the iron salt is at least one selected from ferric nitrate, ferric chloride, ferric sulfate, ferric dihydrogen phosphate, and ferric tribromide. The concentration of the iron salt solution, expressed as the mass concentration of Fe, is preferably 40–120 g / L.
[0018] Further, in step S1, the phosphate solution is formed by dissolving at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium phosphate, and phosphoric acid in water. The concentration of phosphate ions is preferably 0.05–0.15 mol / L.
[0019] Further, in step S1, the pyrophosphate solution is formed by dissolving at least one of sodium pyrophosphate and potassium pyrophosphate in water. The concentration of pyrophosphate ions is preferably 0.05–0.25 mol / L. The acid is preferably at least one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.
[0020] Furthermore, in step S1, the phosphate solution and pyrophosphate solution can also be prepared by dissolving the solutes together in water.
[0021] Furthermore, in step S2, the reaction substrate is water. Ammonia or an acid solution can be used to adjust the pH of the reaction substrate to a specific value. Specifically, the pH of the substrate is adjusted to the desired pH for the coprecipitation reaction.
[0022] Furthermore, in step S2, the pH value is controlled between 2 and 6. If the pH is too low, a large amount of pyrophosphate will hydrolyze into phosphate, and the target product cannot be obtained; if the pH is too high, the hydrolysis of ferric iron will be violent, producing a large amount of ferric hydroxide, which also prevents the target product from being obtained.
[0023] Furthermore, in step S2, a reaction base solution is prepared, and then the iron salt solution, phosphate solution, and pyrophosphate solution are introduced into the reaction base solution in a parallel flow. This parallel flow facilitates pH control and balances the rates of nucleation and grain growth, thereby controlling the particle size and morphology.
[0024] Furthermore, in step S2, the amount of iron salt solution, phosphate solution, and pyrophosphate solution introduced is based on the molar ratio of each element in the iron pyrophosphate molecular formula, and the amount introduced is Fe:PO4:P2O7 = 5.5-6.0:2-2.2:3-3.2 (preferably 5.7-6.0:2:3, more preferably 6:2:3).
[0025] Furthermore, in step S2, the coprecipitation reaction time is greater than or equal to 0.5 hours. This is mainly to ensure that the precipitation reaction is complete; too long a time is detrimental to the synthesis efficiency, while too short a time will lead to incomplete precipitation.
[0026] Furthermore, in step S3, the slurry A is filtered and washed more than twice, preferably three to five times.
[0027] Furthermore, in step S3, water is added and the pulp is re-pulped. After adding water, the Fe mass concentration is preferably 15-25 g / L, and more preferably 18-20 g / L.
[0028] Furthermore, in step S4, the crystallization temperature is 80–95°C. The purpose of crystallization is twofold: first, to allow the particles to continue growing; and second, to eliminate any small amounts of ferric hydroxide impurities and ionic impurities (such as Na, Mg, Ca, etc.) that may be present between the particles. Excessively high temperatures have little impact, mainly considering the boiling point of water; excessively low temperatures, however, cannot effectively eliminate ferric hydroxide and other impurities.
[0029] Furthermore, in step S4, the crystallization time is more than 2 hours.
[0030] Furthermore, in step S6, the sintering temperature is 300–600℃; the sintering time is 2–4 hours. If the sintering temperature is too high, severe caking will occur, making it difficult to break up; if the sintering temperature is too low, the water of crystallization cannot be fully eliminated; if the sintering time is too long, particles will agglomerate and grow, which is detrimental to particle size; if the sintering time is too short, the water of crystallization cannot be completely removed.
[0031] Furthermore, in step S7, the amounts of iron pyrophosphate and sodium source are determined according to the elemental molar ratio of Na4Fe3(PO4)2(P2O7). The molar amount of Fe can fluctuate between 2.85 and 3.00, and the molar amount of Na can fluctuate between 3.95 and 4.05. The amount of carbon source is 10% to 150% of the theoretical yield of sodium iron pyrophosphate.
[0032] Furthermore, in step S7, the liquid-to-solid ratio of the pulp is 10:1 to 30:1, preferably 20:1.
[0033] Furthermore, in step S7, the output particle size is controlled to be 50–150 nm during the sand milling process. Output particle size that is too large or too small will affect the granulation effect of spray drying.
[0034] Further, in step S7, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium hydroxide, and sodium nitrate; the carbon source is one or more of citric acid, glucose, polyvinyl alcohol, polyethylene glycol, polydopamine, polyvinylpyrrolidone, resorcinol-formaldehyde resin, sucrose, fructose, starch, cellulose, Ketjen black, carbon nanotubes, and graphene.
[0035] Further, in step S9, the inert or reducing atmosphere is one or more of nitrogen, argon, a nitrogen-hydrogen mixture, and an argon-hydrogen mixture. The volume ratio of hydrogen in the nitrogen-hydrogen mixture is 5-10% (preferably 5%). The volume ratio of hydrogen in the argon-hydrogen mixture is 5-10% (preferably 5%).
[0036] Furthermore, in step S9, the sintering temperature is 450–600℃, and the sintering time is 5–20 h. If the sintering temperature is too high or too low, Na4Fe3(PO4)2(P2O7) material cannot be formed; if the sintering time is too long or too short, it mainly affects the particle size.
[0037] Furthermore, in step S9, the mixed powder F can be pre-sintered in an inert or reducing atmosphere before sintering; the pre-sintering temperature is 200-400℃, and the pre-sintering time is 4-8 hours. Pre-sintering allows the sodium and carbon sources to fully melt. During this process, the carbon source typically expands in volume. To improve the contact effect of the raw materials and the powder density, the powder is pre-sintered, then pressed into tablets, and then sintered a second time. The advantages are good carbon coating effect and high powder density.
[0038] Compared with the prior art, the present invention has the following beneficial effects:
[0039] This invention synthesizes Na4Fe3(PO4)2(P2O7) cathode material via a precursor method. A precursor, iron pyrophosphate Fe6(PO4)2(P2O7)3, is pre-synthesized, exhibiting a long-range, uniformly ordered arrangement of Fe and P elements at the atomic scale. Within this structure, Fe and P elements achieve a uniform atomic-level arrangement, effectively suppressing the NaFePO4 impurity phase generated due to uneven ion distribution during subsequent sintering. Furthermore, the precursor synthesis method avoids the problems of generating large amounts of gas and high burn-off rates encountered when using raw materials such as nitrates, acetates, and oxalates for sintering. The resulting Na4Fe3(PO4)2(P2O7) cathode material exhibits good sphericity and high compaction density (up to 2.30 g / cm³). 3 Even 2.35g / cm 3 (Above); the precursor morphology and particle size are controllable, effectively solving the problem of inactive impurity phase NaFePO4 formation during the synthesis of sodium iron pyrophosphate. Furthermore, the raw material utilization rate is high, and the synthesized cathode material exhibits high tap density and compaction density, resulting in superior electrochemical performance. Therefore, the preparation method of this invention has a short process flow and simple technology, effectively improving product purity and the physicochemical properties of the material. Compared with existing technologies, it can significantly improve the electrochemical performance of the prepared cathode material.
[0040] This invention pre-synthesizes Fe6(PO4)2(P2O7)3 precursors with long-range ordered and uniform arrangement of Fe and P elements, achieving uniform mixing at the atomic level. The morphology and particle size of the material are precisely controllable, effectively avoiding the problem of inactive impurity phases caused by uneven ion distribution during sintering, and greatly improving product purity. Attached Figure Description
[0041] Figure 1 The XRD patterns of the iron pyrophosphate (i.e., precursor) and sodium iron pyrophosphate (sodium iron pyrophosphate, abbreviated as NFPP) materials prepared in Example 1 are shown.
[0042] Figure 2 SEM image of the sodium iron pyrophosphate material prepared in Example 1;
[0043] Figure 3 The first charge-discharge curve of the sodium iron pyrophosphate material prepared in Example 1;
[0044] Figure 4 This is a rate performance diagram of the sodium iron pyrophosphate material prepared in Example 1;
[0045] Figure 5 The graph shows the cycling performance of the sodium iron pyrophosphate material prepared in Example 1.
[0046] Figure 6 The XRD pattern of the sodium iron pyrophosphate material prepared in Comparative Example 1;
[0047] Figure 7 SEM image of sodium iron pyrophosphate material prepared in Comparative Example 1;
[0048] Figure 8 The first charge-discharge curve of the sodium iron pyrophosphate material prepared in Comparative Example 1;
[0049] Figure 9 The rate performance diagram is for the sodium iron pyrophosphate material prepared in Comparative Example 1. Detailed Implementation
[0050] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but the present invention is not limited to the following embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of the present invention.
[0051] Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified. All percentages mentioned in this application refer to mass percentages unless otherwise stated.
[0052] Example 1
[0053] A method for synthesizing ferric pyrophosphate and preparing sodium ferric pyrophosphate cathode material includes the following steps:
[0054] (1) Add 17.5 mmol of Fe(NO3)3·9H2O to 20 mL of deionized water and stir to dissolve, obtaining solution A; add 6 mmol of NH4H2PO4 and 9 mmol of Na4P2O7 to 30 mL of deionized water and stir to dissolve, obtaining solution B; use 25% concentrated ammonia as solution C; dilute concentrated sulfuric acid to 4 mol·L⁻¹. -1 Solution D is obtained;
[0055] (2) Add 20 mL of deionized water solution to the reactor, turn on the stirrer, and adjust the pH to 4.0 with acid solution; put solutions A to D into the reactor in parallel, maintain the reaction pH at 4.0 by controlling the flow rate, and stir the reaction at 50℃ and 300 rpm for 2.5 h to obtain slurry A;
[0056] (3) After filtering and washing slurry A twice, add 200 mL of deionized water and re-slurry to obtain slurry B;
[0057] (4) Heat slurry B to 90°C and stir at 100 rpm for 2 hours to crystallize and obtain slurry C;
[0058] (5) The slurry C was filtered, washed, and dried to obtain white powder D;
[0059] (6) Calcine white powder D at 500℃ for 2h to obtain precursor powder;
[0060] (7) Take 2g of precursor powder, 0.6382g of sodium carbonate and 0.3g of anhydrous glucose, add them to 100mL of deionized water, stir and slurry, and then grind them in sand. During the sand grinding process, control the output particle size to be 50-150nm; to obtain slurry E.
[0061] (8) Spray dry the milled slurry E at a spray temperature of 200℃ and a flow rate of 200 mL·min. -1 The resulting mixed powder F is obtained.
[0062] (9) The mixed powder F was pre-calcined at 300°C for 5 hours in an argon-hydrogen atmosphere with a volume percentage of 95% argon and 5% hydrogen. The sintered product was then pressed into sheets and sintered at 550°C for 10 hours to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
[0063] The XRD pattern of the cathode material prepared in this embodiment is as follows:Figure 1 As shown, from Figure 1 It can be seen that the synthesized sodium iron pyrophosphate has an orthorhombic crystal structure with space group Pn21a. Furthermore, Rietveld refinement of the XRD pattern reveals high phase purity and the absence of impurity phases. SEM images of the cathode material are shown below. Figure 2 As shown, from Figure 2 It can be seen that the material particles have good sphericity and are coated with a uniform carbon layer on the surface.
[0064] The obtained positive electrode material was mixed with a conductive agent and PVDF, slurried and coated onto aluminum foil. A sodium sheet was used as the negative electrode, and a 1M NaClO4 EC:PC (1:1 Vol%, containing 5% FEC) solution was used as the electrolyte to assemble a CR2032 coin cell. The electrochemical performance of the positive electrode material was then tested.
[0065] The first charge-discharge curve of the assembled battery is as follows: Figure 3 As shown, the rate performance diagram is as follows: Figure 4 As shown in the figure, the cycle performance graph is as follows: Figure 5 As shown, the compaction density of the obtained Na4Fe3(PO4)2(P2O7) cathode material is 2.35 g / cm³. 3 The cathode material obtained in this embodiment has an initial discharge capacity of greater than 120 mAh / g at 0.1C, a high-rate capacity of greater than 100 mAh / g at 10C, and a capacity retention rate of greater than 99% after 200 cycles at 1C.
[0066] Comparative Example 1
[0067] The preparation of sodium iron pyrophosphate cathode material using a nitrate solution spray drying method includes the following steps:
[0068] (1) Dissolve 15 mmol of Fe(NO3)3·9H2O, 20 mmol of NaH2PO4·H2O, and 20 mmol of citric acid monohydrate in 150 mL of deionized water to obtain solution A;
[0069] (2) Solution A was spray-dried at a temperature of 200℃ and a flow rate of 200 mL·min. -1 The resulting mixed powder B was obtained.
[0070] (3) Place the mixed powder B in a 95% / 5% argon-hydrogen atmosphere and pre-calcine at 300°C for 5 hours. Press the sintered product into a sheet and then sinter at 550°C for 10 hours to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
[0071] The XRD pattern of the cathode material prepared in this comparative example is as follows: Figure 6 As shown, the SEM image is as follows Figure 7As shown. Comparison Figure 1 and Figure 2 It can be seen that, compared with the NFPP cathode material prepared in Example 1, Comparative Example 1 has obvious impurity phases and obvious fragmentation of material particles.
[0072] Using the same method as in Example 1, CR2032 coin cells were assembled, and the electrochemical performance of the cathode material was tested. The first charge-discharge curve of the assembled cell is shown below. Figure 8 As shown, the rate performance diagram is as follows: Figure 9 As shown, the material's initial discharge specific capacity at 0.1C is 91.4 mAh / g, and its capacity at 10C is 76.9 mAh / g, both significantly lower than those in Example 1.
[0073] Example 2
[0074] A method for synthesizing ferric pyrophosphate and preparing sodium ferric pyrophosphate cathode material includes the following steps:
[0075] (1) Add 18.0 mmol of Fe(NO3)3·9H2O to 20 mL of deionized water and stir to dissolve, obtaining solution A; add 6 mmol of NH4H2PO4 and 9 mmol of Na4P2O7 to 30 mL of deionized water and stir to dissolve, obtaining solution B; use 25% concentrated ammonia as solution C; dilute concentrated sulfuric acid to 4 mol·L⁻¹. -1 Solution D is obtained;
[0076] (2) Add 20 mL of deionized water solution to the reactor, turn on the stirrer, and adjust the pH to 6.0 with acid; put solutions A to D into the reactor in parallel, maintain the reaction pH at 6.0 by controlling the flow rate, and stir the reaction at 50℃ and 300 rpm for 2.0 h to obtain slurry A;
[0077] (3) After filtering and washing slurry A twice, add 200 mL of deionized water and re-slurry to obtain slurry B;
[0078] (4) Heat slurry B to 90°C and stir at 100 rpm for 2 hours to crystallize and obtain slurry C;
[0079] (5) The slurry C was filtered, washed, and dried to obtain white powder D;
[0080] (6) The white powder D was calcined at 450℃ for 3h to obtain the precursor powder iron pyrophosphate Fe6(PO4)2(P2O7)3.
[0081] (7) Take 2g of precursor powder, 0.6382g of sodium carbonate, 0.2g of anhydrous glucose and 0.1g of polyethylene glycol, add them to 100mL of deionized water, stir and slurry, and then grind them in sand. During the sand grinding process, control the output particle size to be 50-150nm; to obtain slurry E.
[0082] (8) Spray dry the slurry E at a spray temperature of 220℃ and a flow rate of 400 mL / min. -1 The resulting mixed powder F is obtained.
[0083] (9) The mixed powder F was pre-calcined at 300°C for 5 hours in a high-purity argon atmosphere (volume purity greater than or equal to 99.99%). The sintered product was pressed into sheets and then sintered at 525°C for 12 hours to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
[0084] The compacted density of the Na4Fe3(PO4)2(P2O7) cathode material obtained in this embodiment is 2.36 g / cm³. 3 .
[0085] The electrochemical performance of the cathode material obtained in this example was tested using the same method as in Example 1, and the results are shown in Table 1.
[0086] Example 3
[0087] A method for synthesizing ferric pyrophosphate and preparing sodium ferric pyrophosphate cathode material includes the following steps:
[0088] (1) Add 17.0 mmol of Fe(NO3)3·9H2O to 20 mL of deionized water and stir to dissolve, obtaining solution A; add 6 mmol of NH4H2PO4 and 9 mmol of Na4P2O7 to 30 mL of deionized water and stir to dissolve, obtaining solution B; use 25% concentrated ammonia as solution C; dilute concentrated sulfuric acid to 4 mol·L⁻¹. -1 Solution D is obtained;
[0089] (2) Add 20 mL of deionized water solution to the reactor, turn on the stirrer, and adjust the pH to 3.0 with acid; put solutions A to D into the reactor in parallel, maintain the reaction pH at 3.0 by controlling the flow rate, and stir the reaction at 50°C and 300 rpm for 2.5 h to obtain slurry A;
[0090] (3) After filtering and washing slurry A twice, add 200 mL of deionized water and re-slurry to obtain slurry B;
[0091] (4) Heat slurry B to 90°C and stir at 100 rpm for 2 hours to crystallize and obtain slurry C;
[0092] (5) Then, the slurry C is filtered, washed, and dried to obtain white powder D;
[0093] (6) The white powder D was calcined at 480℃ for 2h to obtain the precursor powder iron pyrophosphate Fe6(PO4)2(P2O7)3.
[0094] (7) Take 2g of precursor powder, add 0.6382g of sodium carbonate, 0.2g of anhydrous glucose and 0.1g of Ketjen black to 100mL of deionized water, stir and slurry, and then grind in sand. During the sand grinding process, control the output particle size to be 50-150nm; to obtain slurry E.
[0095] (8) Spray dry the slurry E at a spray temperature of 180℃ and a flow rate of 300 mL / min. -1 The resulting mixed powder F is obtained.
[0096] (9) The mixed powder F was pre-calcined at 300°C for 5 hours in an argon-hydrogen atmosphere with a volume percentage of 95% argon and 5% hydrogen. The sintered product was then pressed into sheets and sintered at 500°C for 10 hours to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
[0097] The compacted density of the Na4Fe3(PO4)2(P2O7) cathode material obtained in this embodiment is 2.36 g / cm³. 3 .
[0098] The electrochemical performance of the cathode material obtained in this example was tested using the same method as in Example 1, and the results are shown in Table 1.
[0099] Table 1
[0100] Example 1 123.5 103.3 99.3％ Comparative Example 1 91.4 76.9 94.5％ Example 2 118.6 92.3 98.4％ Example 3 115.5 88.7 98.8％
[0101] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A method for synthesizing ferric pyrophosphate, characterized in that, Includes the following steps: S1. Prepare iron salt solution, prepare phosphate solution, prepare pyrophosphate solution, prepare ammonia solution, prepare acid solution; S2. Prepare the reaction base solution, then add the iron salt solution, phosphate solution, and pyrophosphate solution to the reaction base solution. Use ammonia solution and / or acid solution to control the pH value, and carry out the co-precipitation reaction under stirring to obtain slurry A. In step S2, the amounts of iron salt solution, phosphate solution, and pyrophosphate solution introduced are based on the molar ratio of each element in the iron pyrophosphate molecular formula, with the introduction amounts following the formula Fe:PO4:P2O7 = 5.5-6.0:2-2.2:3-3.
2. In step S2, the pH value is 2~6; S3. Filter and wash slurry A, add water and re-slurry to obtain slurry B; S4. Heat slurry B to the crystallization temperature and crystallize it under stirring to obtain slurry C; S5. Filter the slurry C and dry the precipitate to obtain precipitate D; S6. Sinter the precipitate D at a certain temperature to remove the water of crystallization and obtain iron pyrophosphate Fe6(PO4)2(P2O7)3.
2. A method for preparing sodium iron pyrophosphate cathode material, characterized in that, Includes the following steps: S1. Prepare iron salt solution, prepare phosphate solution, prepare pyrophosphate solution, prepare ammonia solution, prepare acid solution; S2. Prepare the reaction base solution, then add the iron salt solution, phosphate solution, and pyrophosphate solution to the reaction base solution. Use ammonia solution and / or acid solution to control the pH value, and carry out the co-precipitation reaction under stirring to obtain slurry A. S3. Filter and wash slurry A, add water and re-slurry to obtain slurry B; S4. Heat slurry B to the crystallization temperature and crystallize it under stirring to obtain slurry C; S5. Filter the slurry C and dry the precipitate to obtain precipitate D; S6. Sinter the precipitate D at a certain temperature to remove the water of crystallization and obtain iron pyrophosphate Fe6(PO4)2(P2O7)3. S7. Add the iron pyrophosphate, sodium source, and carbon source obtained in step S6 to water and slurry, then perform sand milling to obtain slurry E. S8. Spray dry the slurry E to obtain mixed powder F; S9. Sinter the mixed powder F in an inert or reducing atmosphere to obtain the carbon-coated sodium-ion battery cathode material sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) / C.
3. The method for preparing sodium iron pyrophosphate cathode material according to claim 2, characterized in that, In step S1, the iron salt is at least one of ferric nitrate, ferric chloride, ferric sulfate, ferric dihydrogen phosphate, and ferric tribromide; and / or, the phosphate solution is formed by dissolving at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium phosphate, and phosphoric acid in water; and / or, the pyrophosphate solution is formed by dissolving at least one of sodium pyrophosphate and potassium pyrophosphate in water.
4. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S2, the reaction time for the coprecipitation reaction is greater than or equal to 0.5 hours.
5. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S2, the reaction base solution is prepared, and then the iron salt solution, phosphate solution, and pyrophosphate solution are introduced into the reaction base solution in a parallel flow.
6. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S4, the crystallization temperature is 80~95℃; and / or the crystallization time is more than 2 hours.
7. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S6, the sintering temperature is 300~600℃; the sintering time is 2~4 h.
8. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S7, the amounts of iron pyrophosphate and sodium source are determined according to the elemental molar ratio of Na4Fe3(PO4)2(P2O7), with the amount of Fe fluctuating between 2.85 and 3.00, the amount of Na fluctuating between 3.95 and 4.05, and the amount of carbon source being 10% to 150% of the theoretical yield of sodium iron pyrophosphate; and / or, in step S7, the sand milling process controls the output particle size to be 50 to 150 nm.
9. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S7, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium hydroxide, and sodium nitrate; and / or, the carbon source is one or more of citric acid, glucose, polyvinyl alcohol, polyethylene glycol, polydopamine, polyvinylpyrrolidone, resorcinol-formaldehyde resin, sucrose, fructose, starch, cellulose, Ketjen black, carbon nanotubes, and graphene.
10. The method for preparing sodium iron pyrophosphate cathode material according to claim 2 or 3, characterized in that, In step S9, the inert or reducing atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen mixture, and argon-hydrogen mixture; and / or, in step S9, the sintering temperature is 450~600℃ and the sintering time is 5~20 h; and / or, in step S9, the mixed powder F is pre-sintered in an inert or reducing atmosphere before sintering.