Method for preparing composite sodium iron pyrophosphate by sulfur cycle

Abstract

The application discloses a method for preparing composite pyrophosphate sodium ferric phosphate by sulfur circulation, and comprises the following steps: S1, preparation of composite pyrophosphate sodium ferric phosphate: after wet mixing of ferrous sulfate, a sodium source, a phosphorus source and a dispersing agent, precursor is obtained by spray drying, and the precursor is calcined at high temperature under a protective atmosphere to obtain composite pyrophosphate sodium ferric phosphate; S2, tail gas recovery and conversion: the sulfur-containing tail gas generated in the high-temperature calcination process of the precursor and air pass through a multilayer vanadium pentoxide catalyst, and temperature control reaction is performed to realize conversion of SO2 into SO3, thereby obtaining SO3 tail gas; S3, SO3 tail gas collection and conversion: the SO3 tail gas is transferred into a sulfuric acid collector by temperature control transfer through a heat exchanger, and SO3 and H2O are cooled and converted into a sulfuric acid solution; S4, preparation of ferrous sulfate: an iron source is added into the sulfuric acid solution, heating reaction is performed, solid-liquid separation is performed, and a reducing agent is added after cooling to perform reaction, thereby obtaining ferrous sulfate used in the step S1. The application has the characteristics of low cost, high reversible specific capacity and green environmental protection.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a method for preparing composite pyrometaphosphoric sodium iron phosphate through sulfur circulation. BACKGROUND

[0002] Lithium ion batteries are a new type of secondary batteries that have attracted much attention. Due to its excellent theoretical capacity, reliable cycle performance, environmental protection, high safety and other advantages, lithium ion batteries have become the core components of electric vehicles, electronic products and other important commercial fields. However, with the increasing demand for lithium ion batteries, the cost of lithium resources has also increased year by year, and in addition, the geographical distribution of lithium resources is uneven, so scientists have invested in the research of sodium which is similar to lithium in nature and rich in content. And the sodium ion battery with excellent performance needs a positive electrode material with excellent electrochemical performance.

[0003] Among the positive electrode materials of sodium ion batteries, polyanion-type sodium ion batteries have attracted market attention due to their good structural stability and thermal stability, low cost and excellent cycle stability. At present, the polyanion material that is studied more is composite sodium iron phosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP), which has an open three-dimensional framework structure and contains a three-dimensional sodium ion diffusion path composed of PO4 3− and P2O7 4- The preparation process of the material inevitably generates sodium iron phosphate (NaFePO4) impurities, resulting in low actual capacity and difficulty in industrialization. At present, the main iron source for preparing NFPP is ferric nitrate, but ferric nitrate has high cost, its decomposition products are harmful to the environment, it is easy to explode, and mixtures with combustible materials are easy to ignite and will burn violently, which has high safety hazards. Therefore, selecting a low-cost, more environmentally friendly and safer iron source to prepare NFPP has become an important topic for the practical application of NFPP. SUMMARY

[0004] The purpose of the present application is to provide a method for preparing composite pyrometaphosphoric sodium iron phosphate through sulfur circulation, which has the characteristics of low cost, high reversible specific capacity and green environmental protection.

[0005] The present application can be realized by the following technical solutions:

[0006] The present application discloses a method for preparing composite pyrometaphosphoric sodium iron phosphate through sulfur circulation, comprising the following steps:

[0007] S1, preparation of composite pyrometaphosphoric sodium iron phosphate: mixing ferrous sulfate, a sodium source, a phosphorus source and a dispersant by wet method, then spray drying to obtain a precursor, and calcining the precursor under a protective atmosphere to obtain composite pyrometaphosphoric sodium iron phosphate;

[0008] S2, tail gas recovery conversion: the sulfur-containing tail gas generated in the high-temperature calcination process of the precursor and air pass through the multilayer vanadium pentoxide catalyst at the same time, and SO2 is converted into SO3 to obtain SO3 tail gas by temperature control reaction;

[0009] S3, SO3 tail gas collection conversion: the SO3 tail gas is transferred into a sulfuric acid collector by temperature control through a heat exchanger to realize the cooling conversion of SO3 and H2O into a sulfuric acid solution;

[0010] S4, preparation of ferrous sulfate: an iron source is added into the sulfuric acid solution, heated for reaction, solid-liquid separation is performed, and a reducing agent is added after cooling to perform reaction to obtain a ferrous sulfate solution used in step S1.

[0011] After solid-liquid separation, the unsolved solid substances are removed to obtain a recovery solution. Since the iron source may contain high-valence iron, and the oxygen in the air can oxidize the ferrous ions in the recovery solution, a reducing agent is added into the recovery solution after cooling to room temperature, solid-liquid separation is performed to remove the impurities of the solid products after the oxidation-reduction reaction, and a ferrous sulfate solution is obtained. Specifically, the solid-liquid separation can be performed by one or more than two of filtration, suction filtration, pressure filtration and centrifugation.

[0012] In the present application, the safe and low-cost ferrous sulfate prepared by the sulfur cycle is used to replace ferric nitrate as the iron source to synthesize the Na4Fe3(PO4)2P2O7 precursor by the spray drying method, and the composite sodium iron phosphate (NFPP) is obtained by high-temperature calcination under a protective atmosphere. The Fe 2+ The Fe 3+ The carbon generated by high-temperature decomposition is not used for reduction, the process of carbon thermal reduction is reduced, the Na4Fe3(PO4)2P2O7 has smaller crystal grains and a more stable lattice, and thus the Na4Fe3(PO4)2P2O7 material synthesized by ferrous sulfate exhibits more excellent performance than ferric nitrate.

[0013] Further, in step S1, the molar ratio of sodium in the sodium source, iron in the ferrous sulfate and phosphorus in the phosphorus source is 4: (2.91-3): 4; when the molar amount of iron in the iron source is too low, sodium iron pyrophosphate (NaFe2P2O7) with low average working voltage and low theoretical capacity is derived, the sodium iron pyrophosphate accounts for a certain weight in the Na4Fe3(PO4)2P2O7 material, and the release of the capacity of the Na4Fe3(PO4)2P2O7 is affected; and when the molar amount of iron in the iron source is too high, a large amount of sodium iron phosphate (NaFePO4) impurities without electrochemical activity are derived, the sodium iron phosphate accounts for a certain weight in the Na4Fe3(PO4)2P2O7 material, and thus the synthesized Na4Fe3(PO4)2P2O7 material exhibits a lower actual capacity.

[0014] Further, in step S2, the flow rate ratio of the sulfur-containing tail gas to the air introduced is 1:5-10 to ensure that the sulfur dioxide and oxygen in the air are fully mixed and reacted under the action of the catalyst, while avoiding too high air flow rate resulting in low sulfur-containing gas proportion and low yield; the vanadium pentoxide catalyst is in a plate structure, and the number of layers of the plate structure is 3-5; if the number of layers of the plate structure is less than 3, the sulfur dioxide and air cannot fully contact and react with the catalyst; if the number of layers of the plate structure is more than 5, the cost will be high; the temperature of the temperature-controlled reaction is 400-600°C, and the reaction rate is low below 400°C; and the activity of the catalyst is low above 600°C.

[0015] Further, in step S3, the temperature range of the SO3 tail passing through the heat exchanger for temperature control is 100-150°C; the cost of heat dissipation is high below 100°C, and the gas temperature is too high above 150°C, which can cause a large amount of water to evaporate, affecting the yield of sulfuric acid; the molar ratio of SO3 to water in the sulfuric acid collector is 1:2-5; if the molar amount of water is too low, sulfur trioxide cannot fully contact and react with water to generate sulfuric acid; and if the molar amount of water is too high, the sulfuric acid content in the solution is low, which can increase the cost of subsequent evaporation of the solution.

[0016] Further, in step S4, the molar ratio of sulfuric acid to iron in the iron source is 1:1-3, and the reaction time is 4-12 h to ensure that the sulfuric acid can fully react while saving the cost and time cost of the iron source; the heating temperature is 60-70°C, and the reaction rate of sulfuric acid and the iron source is slow and insufficient below 60°C, and the cost of heating is high above 70°C.

[0017] Further, in step S4, the molar ratio of ferrous sulfate to the added reducing agent is 1:0.01-1; if the molar ratio of the added reducing agent is too low, the trivalent iron and the reducing agent cannot fully react; if the molar ratio of the added reducing agent is too high, the cost of the reducing agent is high and not economical; and the reducing agent is one or more of iron wire, iron powder, iron block, hydrogen sulfide gas, and hydrogen sulfide solution.

[0018] Further, in step S4, the ferrous sulfate solution is evaporated and concentrated under a protective atmosphere to precipitate ferrous sulfate crystals, and the ferrous sulfate crystals are directly added to step S1 for preparing composite sodium iron phosphate pyrophosphate; the protective atmosphere is one or more of nitrogen, argon, and helium; the solution is evaporated and concentrated to 0.5-0.8 of the total volume to save costs while ensuring as much ferrous sulfate crystals as possible to precipitate; and the cooling temperature is 10-30°C; if the cooling temperature is below 10°C, a large amount of ice water needs to be used, the cost is high, and if the cooling temperature is above 30°C, the ferrous sulfate crystals cannot be fully precipitated.

[0019] Further, in step S1, the high-temperature calcination temperature is 500-600 DEG C, and the holding time is 5-15 h, so as to ensure the full crystallization growth of the material and save the cost required for heating; the protective atmosphere is one or more than two of nitrogen, argon and helium.

[0020] Further, in step S1, a carbon source is also added, and the addition amount of the carbon source is 2%-10% of the total solid content; the carbon source is one or more than two of citric acid, starch, maltose, sucrose, glucose, L-ascorbic acid (vitamin C, VC for short), carbon nanotube, graphene and graphite.

[0021] Further, the ferrous sulfate is added in the form of one or more than two of ferrous sulfate heptahydrate, ferrous sulfate pentahydrate, ferrous sulfate monohydrate and anhydrous ferrous sulfate.

[0022] Further, the sodium source is one or more than two of sodium carbonate, sodium acetate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, sodium nitrate, sodium sulfate, sodium oxalate, sodium citrate, sodium pyrophosphate and disodium pyrophosphate.

[0023] Further, the phosphorus source is one or more than two of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0024] Further, the dispersant is one or more than two of polyethylene glycol series, polyvinyl alcohol series, polyvinylpyrrolidone series, polyacrylic acid series and carboxymethyl cellulose series.

[0025] The application discloses a method for preparing composite sodium iron phosphate pyrophosphate by sulfur circulation.

[0026] First, the reversible specific capacity is high, ferrous sulfate is used as an iron source, and the sodium source, the phosphorus source, the carbon source, the dispersant and water are uniformly mixed, and then the Na4Fe3 (PO4) 2P2O7 (NFPP) powder material is obtained by spray drying and calcination, Fe 2+ Fe 3+ The carbon generated by high-temperature decomposition of the carbon source is not reduced, the process of carbon thermal reduction is reduced, therefore, the NFPP carbon-coated composite positive electrode material with smaller crystal grains and more stable crystal lattices is generated, the capacity can be fully exerted, and the actual discharge specific capacity is greatly improved. Meanwhile, the seven water ferrous sulfate prepared by the sulfur circulation method is mixed with the sodium source, the phosphorus source, the carbon source and the dispersant in a wet manner, then spray drying is carried out to obtain a precursor, and then the precursor is calcined in a protective atmosphere, so that the NFPP is successfully obtained, and the constant-current charge / discharge performance of the synthesized NFPP is more excellent than that of the NFPP synthesized by using ferric nitrate, which is beneficial to industrial production.

[0027] Second, low cost, using ferrous sulfate instead of ferric nitrate as iron source to synthesize Na4Fe3(PO4)2P2O7(NFPP) with excellent constant current charge / discharge performance, reduces the cost. Moreover, the toxic gas SO2, SO3 generated by high temperature calcination in the process of preparing NFPP from ferrous sulfate is recycled to generate industrial important raw material sulfuric acid, and the sulfuric acid is reacted with the iron source to synthesize ferrous sulfate again, which can be used as raw material, mixed with sodium source, phosphorus source, carbon source and dispersant, and then spray dried to obtain a precursor, which is calcined under a protective atmosphere to prepare NFPP with more excellent charge / discharge performance than ferric nitrate, realizing material circulation and greatly saving cost, which is conducive to the commercial production of NFPP.

[0028] Third, green and environmental protection. Ferric nitrate is easy to explode, and its decomposition products are harmful to the environment. The mixture with combustible material is easy to ignite and can burn violently, with high safety hazard. Compared with ferric nitrate, ferrous sulfate is not only cheaper and lower in cost, but also safer and more environmentally friendly, and does not belong to explosive dangerous chemicals, which is more conducive to industrial production. Moreover, the present application utilizes sulfur cycle to recycle the toxic gases SO2, SO3 generated by high temperature calcination in the process of preparing NFPP from ferrous sulfate to generate sulfuric acid, and the ferrous sulfate synthesized again by the reaction of sulfuric acid and iron source can be used as raw material, mixed with sodium source, phosphorus source, carbon source and dispersant, and then spray dried, and calcined under a protective atmosphere to prepare NFPP, realizing sulfur element circulation and being more environmentally friendly and green. DETAILED DESCRIPTION

[0029] In order to enable the personnel in the technical field to better understand the technical solutions of the present application, the product of the present application will be further described in detail below in combination with embodiments.

[0030] The present application discloses a method for preparing composite pyrophosphate sodium iron phosphate by sulfur cycle, comprising the following steps:

[0031] S1, preparation of composite pyrophosphate sodium iron phosphate: ferrous sulfate, sodium source, phosphorus source and dispersant are mixed by wet method, and then spray dried to obtain a precursor, which is calcined at high temperature under a protective atmosphere to obtain composite pyrophosphate sodium iron phosphate;

[0032] S2, tail gas recovery and conversion: the sulfur-containing tail gas generated in the high temperature calcination process of the precursor and air are simultaneously passed through a plurality of vanadium pentoxide catalysts to control the temperature and realize the conversion of SO2 to SO3 to obtain SO3 tail gas;

[0033] S3, SO3 tail gas collection and conversion: the SO3 tail gas is transferred into a sulfuric acid collector by controlling the temperature of a heat exchanger to realize the cooling conversion of SO3 and H2O into a sulfuric acid solution;

[0034] S4, preparation of ferrous sulfate: the iron source is added to the sulfuric acid solution, heated for reaction, solid-liquid separation, and then cooled to add a reducing agent for reaction to obtain the ferrous sulfate solution used in step S1.

[0035] Further, in step S1, the molar ratio of sodium element in the sodium source, iron element in the ferrous sulfate, and phosphorus element in the phosphorus source is 4: (2.91-3): 4.

[0036] Further, in step S2, the flow rate ratio of the sulfur-containing tail gas to the air introduced is 1:5-10; the vanadium pentoxide catalyst is in a plate structure, and the number of layers of the plate is 3-5 layers; and the temperature of the temperature-controlled reaction is 400-600℃.

[0037] Further, in step S3, the temperature range of the SO3 tail controlled by the heat exchanger is 100-150℃, and the molar ratio of SO3 to water in the sulfuric acid collector is 1:2-5.

[0038] Further, in step S4, the molar ratio of sulfuric acid to iron element in the iron source is 1:1-3, the reaction time is 4-12h, and the heating temperature is 60-70℃.

[0039] Further, in step S4, the molar ratio of ferrous sulfate to the added reducing agent is 1:0.01-1; the reducing agent is one or two or more of iron wire, iron powder, iron block, hydrogen sulfide gas, and hydrogen sulfide solution.

[0040] Further, in step S4, the ferrous sulfate solution is concentrated by evaporation under a protective atmosphere, and the ferrous sulfate crystals are precipitated by cooling, and the ferrous sulfate crystals are directly added to step S1 for preparation of the composite sodium iron phosphate pyrophosphate; the protective atmosphere is one or two or more of nitrogen, argon, and helium; the solution is concentrated by evaporation to 0.5-0.8 of the total volume; and the cooling temperature is 10-30℃.

[0041] Further, in step S1, the high-temperature calcination temperature is 500-600℃, and the holding time is 5-15h; and the protective atmosphere is one or two or more of nitrogen, argon, and helium.

[0042] Further, in step S1, a carbon source is also added, and the amount of the carbon source added is 2%-10% of the total solid content; and the carbon source is one or two or more of citric acid, starch, maltose, sucrose, glucose, L-ascorbic acid (vitamin C, abbreviated as VC), carbon nanotubes, graphene, and graphite.

[0043] Further, the ferrous sulfate is added in the form of one or two or more of ferrous sulfate heptahydrate, ferrous sulfate pentahydrate, ferrous sulfate monohydrate, and anhydrous ferrous sulfate.

[0044] Further, the sodium source is one or two or more of sodium carbonate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium sulfate, sodium oxalate, and sodium citrate.

[0045] Further, the phosphorus source is one or two or more of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, and disodium hydrogen pyrophosphate.

[0046] Further, the dispersant is one or two or more of a polyethylene glycol series, a polyvinyl alcohol series, a polyvinylpyrrolidone series, a polyacrylic acid series, and a carboxymethyl cellulose series.

[0047] Example 1

[0048] This embodiment relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur recycling, and specifically includes the following steps:

[0049] (1) FeSO4·7H2O and sodium dihydrogen phosphate dihydrate are uniformly mixed with citric acid (added in an amount of 2% of the total solid content), polyethylene glycol (added in an amount of 2% of the total solid content), and pure water in a molar ratio of 2.91:4 by magnetic stirring. The above solution is spray dried with an inlet temperature of 240°C and an outlet temperature of 100°C to achieve solid-liquid separation and obtain a dry precursor powder. Then, the precursor powder is calcined at 600°C for 5h under a nitrogen atmosphere, and NFPP is obtained after natural cooling. The NFPP, acetylene black, and 4% PVDF are uniformly mixed in a mass ratio of 8:1:1. Then, the black slurry is coated on an aluminum foil using a 120um four-side coater, and dried in a 100°C oven for 2h. The electrode film is punched into a circular sheet with a radius of 6mm using a sheet puncher, and a CR2032 type button cell is assembled in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0050] (2) The sulfur-containing tail gas generated during the high-temperature calcination of the precursor and air are introduced into a 3-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:5, and SO2 and oxygen in the air are converted into SO3 at 400°C.

[0051] (3) The mixed gas is passed through a heat exchanger to 100°C and enters a sulfuric acid collector, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector is 1:2, and sulfuric acid is directly generated by the reaction.

[0052] (4) Iron powder was added into sulfuric acid solution (molar ratio of sulfuric acid to iron powder was 1:1), heated to 60 °C for 12 h, and then the solution was recovered by filtration. Hydrogen sulfide gas was introduced into the recovered solution cooled to room temperature (molar ratio of ferrous sulfate to hydrogen sulfide was 1:0.01), and the solid product impurities after redox reaction were removed by filtration to obtain a ferrous sulfate solution. The ferrous sulfate solution was concentrated to 0.5 of the total volume under argon atmosphere, and cooled to 10 °C to precipitate FeSO4·7H2O crystals.

[0053] (5) The FeSO4·7H2O crystals obtained in step (4) were repeated in step (1) to synthesize NFPP, and assembled into CR2032 type button cells for testing.

[0054] Example 2

[0055] This example relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur cycle, specifically comprising the following steps:

[0056] (1) FeSO4·5H2O, sodium acetate, and ammonium dihydrogen phosphate were uniformly mixed by ultrasonic at a molar ratio of 3:4:4, citric acid (added amount was 10% of the total solid content), polyvinylpyrrolidone (added amount was 10% of the total solid content), and pure water. The above solution was spray dried, with an inlet temperature of 230 °C and an outlet temperature of 95 °C to achieve solid-liquid separation and obtain dry precursor powder. Then, the NFPP was calcined at 500 °C for 15 h under argon atmosphere, and naturally cooled to obtain NFPP. The NFPP, acetylene black, and 4% PVDF were uniformly mixed at a mass ratio of 8:1:1. Then, the black slurry was coated on an aluminum foil using a 120 um four-side coater, and dried in a 100 °C oven for 2 h. The electrode film was punched into a 6 mm radius disc using a sheet punching machine, and assembled into a CR2032 type button cell in a glove box using metallic sodium as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0057] (2) The sulfur-containing tail gas generated during the high-temperature calcination of the precursor and air were introduced into a 5-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:10, and reacted at 600 °C to convert SO2 and oxygen in the air into SO3.

[0058] (3) The gas passed through a heat exchanger to 150 °C and entered a sulfuric acid collector, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector was 1:5, and the reaction directly generated sulfuric acid.

[0059] (4) Add ferrous oxide to sulfuric acid solution (molar ratio of sulfuric acid to ferrous oxide is 1:3), heat to 70°C for 4h, then recover the solution by pressure filtration. Add iron powder (molar ratio of ferrous sulfate to iron powder is 1:1) to the recovered solution cooled to room temperature, remove the solid product impurities after redox reaction by pressure filtration to obtain a ferrous sulfate solution. Evaporate the ferrous sulfate solution under a helium atmosphere to a total volume of 0.8, cool to 30°C, and precipitate FeS04·7H20 crystals.

[0060] (5) Repeat step (1) to synthesize NFPP from the FeS04·7H20 crystals obtained in step (4) and assemble into CR2032 type button cells for testing.

[0061] Example 3

[0062] This example relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur cycle, specifically comprising the following steps:

[0063] (1) Mix FeS04·H20, sodium carbonate, ammonium dihydrogen phosphate, and glucose (added amount is 4% of total solid content), L-ascorbic acid (vitamin C, added amount is 2% of total solid content), polyvinyl alcohol (added amount is 6% of total solid content), and pure water in a molar ratio of 2.95:4:4:4 by ultrasonic mixing. Spray dry the above solution at an inlet temperature of 250°C and an outlet temperature of 105°C to achieve solid-liquid separation and obtain dry precursor powder. Then calcine the NFPP at 550°C for 10h under a helium atmosphere, and obtain NFPP after natural cooling. Mix the NFPP, acetylene black, and 4% PVDF in a mass ratio of 8:1:1. Then use a 120um four-side coater to coat the black slurry on an aluminum foil, and dry in a 100°C oven for 2h. Use a sheet punching machine to punch the electrode film into a 6mm radius circle, use metallic sodium as the counter electrode, 1mol / L NaCl04EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and PP / PE / PP three-layer separator, and assemble into CR2032 type button cells for testing in a glove box.

[0064] (2) Pass the sulfur-containing tail gas generated during high-temperature calcination of the precursor and air into a 4-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:7, and react at 500°C to convert SO2 and oxygen in the air into SO3.

[0065] (3) The gas passes through a heat exchanger into a sulfuric acid collector at 120°C, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector is 1:4, and the reaction directly generates sulfuric acid.

[0066] (4) Iron filings were added to a sulfuric acid solution (molar ratio of sulfuric acid to iron filings was 1:2), heated to 65°C for 8h, and then the solution was recovered by centrifugation. Hydrogen sulfide solution was added to the recovered solution cooled to room temperature (molar ratio of ferrous sulfate to hydrogen sulfide in the hydrogen sulfide solution was 1:0.5), and the solid product impurities after the redox reaction were removed by centrifugation to obtain a ferrous sulfate solution. The ferrous sulfate solution was concentrated to 0.6 of the total volume under a nitrogen atmosphere, and cooled to 25°C to precipitate FeSO4·7H2O crystals.

[0067] (5) The FeSO4·7H2O crystals obtained in step (4) were repeated in step (1) to synthesize NFPP, and assembled into CR2032 type button cells for testing.

[0068] Example 4

[0069] This example relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur recycling, which specifically comprises the following steps:

[0070] (1) Anhydrous ferrous sulfate and sodium dihydrogen phosphate dihydrate were mixed with citric acid (added amount was 2% of the total solid content), polyethylene glycol (added amount was 2% of the total solid content), and pure water in a molar ratio of 3:4:2:2 by magnetic stirring. The above solution was spray dried with an inlet temperature of 240°C and an outlet temperature of 100°C to achieve solid-liquid separation and obtain dry precursor powder. Then, the NFPP was obtained by calcining at 500°C for 10h under a nitrogen atmosphere and naturally cooling. The NFPP, acetylene black, and 4% PVDF were mixed uniformly in a mass ratio of 8:1:1. Then, the black slurry was coated on an aluminum foil using a 120um four-side coater, and dried in a 100°C oven for 2h. The electrode film was punched into a 6mm radius disc using a sheet punching machine, and assembled into a CR2032 type button cell in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0071] (2) The sulfur-containing tail gas generated during the high-temperature calcination of the precursor and air were introduced into a 5-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:5, and reacted at 600°C to convert SO2 and oxygen in the air into SO3.

[0072] (3) The mixed gas was passed through a heat exchanger to 130°C and entered a sulfuric acid collector, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector was 1:2, and sulfuric acid was directly generated by reaction.

[0073] (4) Iron powder was added into sulfuric acid solution (molar ratio of sulfuric acid to iron powder was 1:1), heated to 70 °C for 12 h, and then the solution was recovered by filtration. Hydrogen sulfide gas was introduced into the recovered solution cooled to room temperature (molar ratio of ferrous sulfate to hydrogen sulfide was 1:0.01), and the solid product impurities after redox reaction were removed by filtration to obtain a ferrous sulfate solution. The ferrous sulfate solution was concentrated to 0.5 of the total volume under argon atmosphere, and cooled to 30 °C to precipitate FeSO4·7H2O crystals.

[0074] (5) The FeSO4·7H2O crystals obtained in step (4) were repeated in step (1) to synthesize NFPP, and assembled into CR2032 type button cells for testing.

[0075] Example 5

[0076] This example relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur cycle, which specifically comprises the following steps:

[0077] (1) FeSO4·7H2O, sodium acetate, and diammonium hydrogen phosphate were uniformly mixed by ultrasonic at a molar ratio of 2.91:4:4, citric acid (added amount was 10% of the total solid content), polyvinylpyrrolidone (added amount was 10% of the total solid content), and pure water. The above solution was spray dried, with an inlet air temperature of 230 °C and an outlet air temperature of 95 °C to achieve solid-liquid separation and obtain dry precursor powder. Then, the NFPP was calcined at 550 °C for 15 h under argon atmosphere, and naturally cooled to obtain NFPP. The NFPP, acetylene black, and 4% PVDF were uniformly mixed at a mass ratio of 8:1:1. Then, the black slurry was coated on an aluminum foil using a 120 um four-side coater, and dried in a 100 °C oven for 2 h. The electrode film was punched into a 6 mm radius disc using a sheet punching machine, and assembled into a CR2032 type button cell in a glove box using metallic sodium as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0078] (2) The sulfur-containing tail gas generated during the high-temperature calcination of the precursor and air were introduced into a 3-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:8, and reacted at 450 °C to convert SO2 and oxygen in the air into SO3.

[0079] (3) The gas passed through a heat exchanger to enter a sulfuric acid collector at 100 °C, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector was 1:5, and the reaction directly generated sulfuric acid.

[0080] (4) Add ferrous oxide to sulfuric acid solution (molar ratio of sulfuric acid to ferrous oxide is 1:3), heat to 65°C for 8h, then recover the solution by pressure filtration. Add iron powder (molar ratio of ferrous sulfate to iron powder is 1:0.5) to the recovered solution cooled to room temperature, remove the solid product impurities after redox reaction by pressure filtration to obtain a ferrous sulfate solution. Evaporate the ferrous sulfate solution under a helium atmosphere to a total volume of 0.8, cool to 20°C, and precipitate FeS04-7H20 crystals.

[0081] (5) Repeat step (1) to synthesize NFPP from the FeS04-7H20 crystals obtained in step (4) and assemble into CR2032 type button cells for testing.

[0082] Example 6

[0083] This example relates to a method for preparing composite sodium iron phosphate pyrophosphate by sulfur cycle, specifically comprising the following steps:

[0084] (1) Mix FeS04-5H20, sodium nitrate, ammonium dihydrogen phosphate, glucose (added amount is 5% of total solid content), L-ascorbic acid (vitamin C, added amount is 3% of total solid content), polyethylene glycol (added amount is 8% of total solid content), and pure water by ultrasonic mixing at a molar ratio of 2.97:4:4. Spray dry the above solution at an inlet temperature of 250°C and an outlet temperature of 105°C to achieve solid-liquid separation and obtain dry precursor powder. Then calcine the NFPP at 600°C for 10h under a helium atmosphere, and obtain NFPP after natural cooling. Mix the NFPP, acetylene black, and 4% PVDF at a mass ratio of 8:1:1. Then use a 120um four-side coater to coat the black slurry on an aluminum foil, and dry in a 100°C oven for 2h. Use a sheet punching machine to punch the electrode film into a 6mm radius circle, use metallic sodium as the counter electrode, 1mol / L NaCl04EC+DEC (1:1vol%)+5%FEC as the electrolyte, and PP / PE / PP three-layer separator, and assemble into CR2032 type button cells for testing in a glove box.

[0085] (2) Pass the sulfur-containing tail gas generated during the high-temperature calcination of the precursor and air into a 4-layer plate-shaped vanadium pentoxide at a flow rate ratio of 1:7, and react at 550°C to convert SO2 and oxygen in the air into SO3.

[0086] (3) The gas passes through a heat exchanger into a sulfuric acid collector at 130°C, and the molar ratio of sulfur trioxide to water in the sulfuric acid collector is 1:4, and the reaction directly generates sulfuric acid.

[0087] (4) Iron filings were added to a sulfuric acid solution (molar ratio of sulfuric acid to iron filings was 1 : 1.5), heated to 60°C for 8h, and then the solution was recovered by centrifugation. Hydrogen sulfide solution was added to the recovered solution cooled to room temperature (molar ratio of ferrous sulfate to hydrogen sulfide in the hydrogen sulfide solution was 1 : 0.1), and the solid product impurities after the redox reaction were removed by centrifugation to obtain a ferrous sulfate solution. The ferrous sulfate solution was concentrated to 0.6 of the total volume under a nitrogen atmosphere, and cooled to 10°C to precipitate FeS04-7H20 crystals.

[0088] (5) The FeS04-7H20 crystals obtained in step (4) were repeated in step (1) to synthesize NFPP, and assembled into CR2032 type button cells for testing.

[0089] Comparative Example 1

[0090] This example relates to composite pyrophosphate sodium iron phosphate, and the preparation method thereof includes the following steps:

[0091] Fe(N03)3-9H20 was used as an iron source and mixed with sodium dihydrogen phosphate dihydrate in a molar ratio of 3:4, citric acid (added amount was 2% of the total solid content), polyethylene glycol (added amount was 2% of the total solid content), and pure water by magnetic stirring. The above solution was spray dried with an inlet temperature of 230°C and an outlet temperature of 95°C to achieve solid-liquid separation and obtain dry precursor powder. Then, NFPP was obtained by calcining at 500°C for 15h under an argon atmosphere and naturally cooling. The NFPP, acetylene black, and 4% PVDF were mixed uniformly in a mass ratio of 8:1:1. Then, the black slurry was coated on an aluminum foil using a 120um four-side coater, and dried in a 100°C oven for 2h. The electrode film was punched into a circular sheet with a radius of 6mm using a sheet puncher, and assembled into a CR2032 type button cell in a glove box using metallic sodium as the counter electrode, 1 mol / L NaC104EC+DEC (1:1 vol%) + 5% FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0092] Comparative Example 2

[0093] This example relates to composite pyrophosphate sodium iron phosphate, and the preparation method thereof includes the following steps:

[0094] Fe(NO3)3·9H2O, sodium acetate, ammonium dihydrogen phosphate were mixed with glucose (10% of the total solid content), polyvinyl alcohol (10% of the total solid content), and pure water in a molar ratio of 2.91:4:4 by ultrasonic mixing. The above solution was spray dried with an inlet temperature of 240°C and an outlet temperature of 100°C to achieve solid-liquid separation, obtaining dry precursor powder. Then, the NFPP was obtained by calcining at 600°C for 5h under nitrogen atmosphere, and then naturally cooling. The NFPP, acetylene black, and 4% PVDF were mixed uniformly in a mass ratio of 8:1:1. Then, the black slurry was coated on an aluminum foil using a 120um four-side coater, and then dried in a 100°C oven for 2h. The electrode film was punched into a 6mm radius disc using a punching machine, and then assembled into a CR2032 type button cell in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%)+5%FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0095] The constant current charge-discharge data of Na4Fe3(PO4)2P2O7(NFPP) prepared in Examples 1-6 and Comparative Examples 1, 2 are as follows:

[0096] Table 1 Performance test results

[0097]

[0098] As can be seen from Table 1, the constant current charge-discharge specific capacity of the NFPP synthesized by ferrous sulfate in Example 1-6 Step 1 is better than that of the NFPP synthesized by Fe(NO3)3·9H2O in Comparative Examples 1, 2, which shows that it is completely feasible to use low-cost and safer ferrous sulfate to synthesize Na4Fe3(PO4)2P2O7(NFPP), and the synthesized NFPP has more excellent charge-discharge performance. At the same time, the constant current charge-discharge specific capacity of the NFPP synthesized by sulfur recycling in Example 1, 2, 3, 4, 5, 6 Step 5 is also better than that of the NFPP prepared by Fe(NO3)3·9H2O in Comparative Examples 1, 2, which shows that the method for preparing composite sodium iron pyrophosphate phosphate by sulfur recycling of the present application is completely feasible.

[0099] The above examples are only specific embodiments of the present application, which are described in more detail and in detail, but should not be construed as limiting the scope of the present patent. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, and these obvious alternative forms all belong to the protection scope of the present application.

Claims

1. A process for the preparation of a complexed sodium ferric pyrophosphate by sulfur cycle characterized in that The method comprises the following steps: S1, preparation of composite pyrophosphate sodium iron phosphate: the ferrous sulfate, sodium source, phosphorus source and dispersant are mixed wetly, then the mixture is spray dried to obtain a precursor, and the precursor is calcined at high temperature under a protective atmosphere to obtain the composite pyrophosphate sodium iron phosphate; S2, tail gas recovery and conversion: the sulfur-containing tail gas generated in the high-temperature calcination process of the precursor and air are simultaneously passed through a plurality of layers of vanadium pentoxide catalyst to realize the conversion of SO2 to SO3 and obtain SO3 tail gas; S3, SO3 tail gas collection and conversion: the SO3 tail gas is transferred into a sulfuric acid collector through a heat exchanger to realize the cooling conversion of SO3 and H2O into a sulfuric acid solution; S4, preparation of ferrous sulfate: the iron source is added into the sulfuric acid solution, heated and reacted, then solid-liquid separation is performed, and then a reducing agent is added after cooling to obtain the ferrous sulfate solution used in step S1.

2. The process for the preparation of composite sodium iron pyrophosphate according to claim 1, characterized in that: In step S1, the molar ratio of sodium in the sodium source, iron in the ferrous sulfate and phosphorus in the phosphorus source is 4: (2.91-3):

4.

3. The method of claim 1, wherein the sodium iron pyrophosphate composite is prepared by sulfur recycling. In step S2, the flow rate ratio of the sulfur-containing tail gas to air is 1:5-10; the vanadium pentoxide catalyst is in a plate structure, the number of layers of the plate structure is 3-5 layers; and the temperature of the temperature control reaction is 400-600℃.

4. The method of claim 1, wherein the sodium iron pyrophosphate composite is prepared by sulfur recycling. In step S3, the temperature range of the SO3 tail gas passing through the heat exchanger for temperature control is 100-150℃; and the molar ratio of SO3 to water in the sulfuric acid collector is 1:2-5.

5. The process for the preparation of sodium iron pyrophosphate composite according to claim 1, characterized in that: In step S4, the molar ratio of sulfuric acid to iron in the iron source is 1:1-3, the reaction time is 4-12h, and the heating temperature is 60-70℃.

6. The method of claim 1, wherein the sodium iron pyrophosphate composite is prepared by sulfur recycling. In step S4, the molar ratio of ferrous sulfate to the added reducing agent is 1:0.01-1; the reducing agent is one or two or more of iron wire, iron powder, iron block, hydrogen sulfide gas and hydrogen sulfide solution.

7. The process for the preparation of sodium iron pyrophosphate composite as claimed in claim 1 wherein: In step S4, the ferrous sulfate solution is evaporated and concentrated under a protective atmosphere, and the ferrous sulfate crystals are precipitated by cooling; the ferrous sulfate crystals are directly added to step S1 for the preparation of the composite pyrophosphate sodium iron phosphate; the protective atmosphere is one or two or more of nitrogen, argon and helium; the solution is evaporated and concentrated to 0.5-0.8 of the total volume; and the cooling temperature is 10-30℃.

8. The process for the preparation of sodium iron pyrophosphate composite according to claim 7, characterized in that: In step S1, the high-temperature calcination temperature is 500-600℃, and the holding time is 5-15h; and the protective atmosphere is one or two or more of nitrogen, argon and helium.

9. The process for the preparation of composite sodium iron pyrophosphate according to claim 1, characterized in that: In step S1, a carbon source is also added, and the addition amount of the carbon source is 2%-10% of the total solid content; the carbon source is one or two or more of citric acid, starch, maltose, sucrose, glucose, L-ascorbic acid, carbon nanotubes, graphene and graphite.

10. The process for the preparation of sodium iron pyrophosphate composite according to claim 1, wherein: In step S1, the ferrous sulfate is added in the form of one or two or more of ferrous sulfate heptahydrate, ferrous sulfate pentahydrate, ferrous sulfate monohydrate and anhydrous ferrous sulfate; the sodium source is one or two or more of sodium carbonate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium sulfate, sodium oxalate, sodium citrate, sodium pyrophosphate and disodium pyrophosphate; the phosphorus source is one or two or more of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate and disodium pyrophosphate; The dispersant is one or two or more of a polyethylene glycol series, a polyvinyl alcohol series, a polyvinylpyrrolidone series, a polyacrylic acid series, and a carboxymethyl cellulose series.

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