Pyrophosphate ferric sodium phosphate as well as preparation method and application thereof

Abstract

The invention provides ferric sodium pyrophosphate as well as a preparation method and application thereof, and belongs to the technical field of sodium ion batteries. The preparation method of the ferric sodium pyrophosphate comprises the following steps: mixing an iron source, a phosphorus source, a sodium source, a carbon source, an additive and water, and grinding to obtain slurry; the iron source is iron phosphate and iron oxide red; the phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and disodium dihydrogen pyrophosphate; and drying the slurry, and carrying out heat treatment to obtain the ferric sodium pyrophosphate. The result of the embodiment shows that the tap density of the ferric sodium pyrophosphate prepared by the method is up to 1.513 g / cm < 3 >, and the compaction density is up to 2.19 g / cm < 3 >; according to the sodium ion battery prepared from the pyrophosphoric acid ferric phosphate sodium as the positive electrode material, the charging specific capacity of the first circle can reach 117.6 mAh / g and the discharging specific capacity can reach 109.4 mAh / g under the 0.1 C multiplying power.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and in particular to sodium iron pyrophosphate phosphate and a preparation method and application thereof. Background Art

[0002] In recent years, with the rapid development of industries such as electric vehicles and energy storage systems, the demand for high-performance and low-cost batteries has been increasing day by day. Sodium-ion batteries are considered to be one of the ideal alternatives to lithium-ion batteries due to their advantages such as rich resources, low cost, and high safety, and have great market potential. Polyanion compounds have the advantages of stable structure, high cycle life, and high thermal stability, and are often used as the cathode materials for sodium-ion batteries. Iron-based polyanion compounds have a wide range of raw material sources and low costs. Their three-dimensional crystal structure can provide fast and stable three-dimensional channels for sodium ions, providing high safety performance, excellent cycle performance, and high capacity. In particular, sodium iron pyrophosphate phosphate has a theoretical specific capacity of up to 129 mAh / g.

[0003] However, in the actual synthesis of sodium iron pyrophosphate phosphate, ferrous oxalate is commonly used as the iron source, ammonium dihydrogen phosphate and sodium dihydrogen phosphate are used as the phosphorus source, and sodium carbonate is used as the sodium source; these raw materials will generate water vapor or carbon dioxide during the heating process, and the generated gases will produce a large number of pores in the compound, reducing the tap density and the compaction density, and thus reducing the performance of the sodium-ion battery. For example, in the prior art, a preparation method of composite sodium iron pyrophosphate phosphate is provided. Using ammonium dihydrogen phosphate as the phosphorus source, the battery composed of the prepared compound as the cathode material has a charge-discharge capacity of only 99.4 mAh / g at a rate of 0.1C. Summary of the Invention

[0004] The purpose of the present invention is to provide sodium iron pyrophosphate phosphate and a preparation method and application thereof. The preparation method provided by the present invention can effectively improve the tap density and the compaction density of sodium iron pyrophosphate phosphate, and thus improve the battery capacity of the sodium-ion battery.

[0005] To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions: The present invention provides a preparation method of sodium iron pyrophosphate phosphate, comprising: Mixing an iron source, a phosphorus source, a sodium source, a carbon source, an additive, and water, and then grinding to obtain a slurry; the iron source is iron phosphate and iron oxide red; the phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid, and sodium dihydrogen pyrophosphate; Drying the slurry and then performing heat treatment to obtain sodium iron pyrophosphate phosphate.

[0006] Preferably, the sodium source is one or more of sodium carbonate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium citrate, and sodium hydroxide.

[0007] Preferably, the carbon source is one or more of citric acid, sucrose, glucose, starch, carbon black, and graphitized carbon.

[0008] Preferably, the additive is one or more of polyethylene glycol, polyvinylpyrrolidone, and starch polysaccharides.

[0009] Preferably, the molar ratio of iron element, phosphorus element, and sodium element in the slurry is (2 - 4):(3 - 4):(3 - 4).

[0010] Preferably, the molar amount of the iron source, the mass of the carbon source, and the mass of the additive are in the ratio of 0.3 mol:(3 - 10) g:(0.3 - 0.6) g.

[0011] Preferably, the solid content of the slurry is 30 - 50 wt%; The D 50 of the solid particles in the slurry is 0.1 - 0.5 μm.

[0012] Preferably, the heat treatment is a programmed temperature rise; The heating rate in the first stage is 1 - 10 °C / min, the holding temperature in the first stage is 300 - 400 °C, and the holding time in the first stage is 2 - 14 h; The heating rate in the second stage is 1 - 10 °C / min, the holding temperature in the second stage is 500 - 700 °C, and the holding time in the second stage is 2 - 14 h.

[0013] The present invention also provides a sodium iron pyrophosphate phosphate, which is prepared by the preparation method described in the above technical solution.

[0014] The present invention also provides an application of sodium iron pyrophosphate phosphate in a cathode material for a sodium ion battery, and the sodium iron pyrophosphate phosphate is the sodium iron pyrophosphate phosphate described in the above technical solution.

[0015] The present invention provides a method for preparing sodium iron pyrophosphate phosphate, comprising: mixing an iron source, a phosphorus source, a sodium source, a carbon source, an additive and water, and then grinding to obtain a slurry; the iron source is iron phosphate and iron oxide red; the phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and sodium dihydrogen pyrophosphate; drying the slurry and then performing heat treatment to obtain sodium iron pyrophosphate phosphate. In the present invention, iron oxide red is used to replace part of iron phosphate as the iron source of the cathode material. By utilizing the high density of iron oxide red itself, the tap density and the compression density of sodium iron pyrophosphate phosphate can be improved to a certain extent; meanwhile, the stability performance of the system can be effectively improved, so that sodium iron pyrophosphate phosphate can maintain a tight connection during the processes of tapping and compressing, effectively improving the tap density and the compression density of sodium iron pyrophosphate phosphate; at the same time, the types of phosphorus sources are limited, effectively avoiding the generation of gas products by conventional phosphorus sources during the heating process of preparing sodium iron pyrophosphate phosphate, reducing the pores inside sodium iron pyrophosphate phosphate, and further improving the tap density and the compression density, thereby effectively improving the capacity of the battery composed of sodium iron pyrophosphate phosphate. The results of the examples show that the tap density of the sodium iron pyrophosphate phosphate prepared by the present invention is as high as 1.513 g / cm 3 , and the compression density is as high as 2.19 g / cm 3 ; the first-cycle charge specific capacity of the sodium-ion battery composed of sodium iron pyrophosphate phosphate as the cathode material can reach 117.6 mAh / g at a rate of 0.1C, and the discharge specific capacity can reach 109.4 mAh / g. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is the XRD pattern of sodium iron pyrophosphate phosphate prepared in Example 1, Example 6 and Example 7; Figure 2 is the first charge-discharge curve of the sodium-ion battery composed of sodium iron pyrophosphate phosphate prepared in Example 1, Example 6, Example 7 and Comparative Example 1 as the cathode material at a rate of 0.1C; Figure 3 is the discharge specific capacity curve of the sodium-ion battery composed of sodium iron pyrophosphate phosphate prepared in Example 6 at different rates from 0.1C to 5C and the cycle performance curve at 1C; wherein, the left curve is the discharge specific capacity curve at different rates, and the right curve is the cycle performance curve at a rate of 1C. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The present invention provides a method for preparing sodium iron pyrophosphate phosphate, comprising: Mixing an iron source, a phosphorus source, a sodium source, a carbon source, an additive and water, and then grinding to obtain a slurry; the iron source is iron phosphate and iron oxide red; the phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and sodium dihydrogen pyrophosphate; Drying the slurry and then performing heat treatment to obtain sodium iron pyrophosphate phosphate.

[0018] In the present invention, an iron source, a phosphorus source, a sodium source, a carbon source, an additive and water are mixed and then ground to obtain a slurry.

[0019] In the present invention, the iron source is iron phosphate and iron oxide red. As an embodiment of the present invention, the molar ratio of the iron phosphate to the iron oxide red can be (2.0~2.8):(0.1~0.5), and can also be 2.0:0.5, 2.2:0.4, 2.4:0.3, 2.6:0.2, 2.8:0.1. In the present invention, by using iron oxide red to replace part of the iron phosphate, the electrostatic repulsion between sodium iron pyrophosphate can be effectively reduced, its binding force can be effectively improved, and the tapped density and the compacted density of sodium iron pyrophosphate can be further improved.

[0020] In the present invention, the phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and sodium dihydrogen pyrophosphate. In the embodiments of the present invention, the phosphorus source is disodium hydrogen phosphate or sodium pyrophosphate. By limiting the types of the phosphorus source in the present invention, the use of conventional phosphorus sources is avoided, the generation of by-product gases caused by heating during the preparation process is reduced, the pores inside sodium iron pyrophosphate are reduced, and the tapped density and the compacted density of sodium iron pyrophosphate are further improved.

[0021] As an embodiment of the present invention, the sodium source can be one or more of sodium carbonate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium citrate and sodium hydroxide. In the present invention, by limiting the types of the sodium source, a sodium source with sufficient reaction activity is provided for the reaction, which is beneficial to the formation of sodium iron pyrophosphate.

[0022] As an embodiment of the present invention, the carbon source can be one or more of citric acid, sucrose, glucose, starch, carbon black, graphitized carbon. In the present invention, by limiting the types of the carbon source, the growth direction of sodium iron pyrophosphate crystals can be controlled, so that sodium iron pyrophosphate can be regularly arranged during the tapping and compaction processes, and the tapped density and the compacted density are further improved; at the same time, a conductive network can be formed in sodium iron pyrophosphate, the electronic conductivity of sodium iron pyrophosphate is improved, and the capacity of the battery composed of sodium iron pyrophosphate as the cathode material is further improved.

[0023] As an embodiment of the present invention, the additive can be one or more of polyethylene glycol, polyvinylpyrrolidone and starch polysaccharides. In the present invention, by limiting the types of the additive, the components in the system can be effectively dispersed, so that sodium iron pyrophosphate grows uniformly while avoiding its agglomeration, and the compacted density and the tapped density of sodium iron pyrophosphate are further improved.

[0024] As an embodiment of the present invention, the molar ratio of iron element, phosphorus element and sodium element in the slurry can be (2-4):(3-4):(3-4), or (2.5-3.5):(3.2-4):(3.2-4), or (2.8-3.2):(3.5-4):(3.5-4), or (2.8-3):(3.8-4):(3.8-4). By limiting the molar ratio of iron element, phosphorus element and sodium element in the slurry within the above range, sodium iron pyrophosphate can be obtained and the performance of sodium iron pyrophosphate can be improved.

[0025] As an embodiment of the present invention, the molar amount of iron element in the iron source, the mass of the carbon source and the mass of the additive can be 0.3 mol:(3-10) g:(0.3-0.6) g, or 0.3 mol:(6-9.5) g:(0.4-0.5) g, or 0.3 mol:(8-9.5) g:(0.4-0.45) g, or 0.3 mol:(9-9.5) g:(0.41-0.45) g. By limiting the dosage ratio of the reaction raw materials, the full contact reaction between the raw materials is realized, and the performance of sodium iron pyrophosphate is further improved.

[0026] As an embodiment of the present invention, the grinding method can be ball milling and / or sand milling; the rotation speed of the ball milling can be 200-400 rpm, or 250-380 rpm, or 300-350 rpm; the time of the ball milling can be 6-12 h, or 7-10 h, or 8-9 h; the ball-to-material ratio of the ball milling can be (6-10):1, or (7-9):1, or (8-8.5):1; the rotation speed of the sand milling can be 1000-3000 rpm, or 1500-2800 rpm, or 2000-2500 rpm; the time of the sand milling can be 1-3 h, or 1.5-2.5 h, or 2-2.4 h. In the present invention, by limiting the type and process parameters of grinding, the process effect of grinding is ensured, the uniform contact of raw materials is realized, and it is beneficial to the reaction between raw materials.

[0027] As an embodiment of the present invention, the solid content of the slurry can be 30-50 wt%, or 35-45 wt%, or 30-32 wt%; the D 50 of the solid particles in the slurry can be 0.1-0.5 μm, or 0.2-0.45 μm, or 0.3-0.4 μm. In the present invention, by limiting the composition of the slurry and the particle size of the solid particles in the slurry, the reaction between raw materials is further ensured.

[0028] After obtaining the slurry, the present invention dries the slurry and then performs heat treatment to obtain sodium iron pyrophosphate phosphate.

[0029] As an embodiment of the present invention, the drying method can be blast drying, spray drying or freeze drying; the temperature of the blast drying can be 70-120°C, or 80-110°C, or 90-100°C; the time of the blast drying can be 8-24h, or 10-20h, or 12-16h; the inlet air temperature of the spray drying can be 100-250°C, or 120-240°C, or 150-200°C; the outlet air temperature of the spray drying can be 60-120°C, or 70-100°C, or 80-90°C; the feeding rate of the spray drying can be 0.5-40 mL / min, or 5-30 mL / min, or 10-20 mL / min; the temperature of the freeze drying can be -60 to -20°C, or -50 to -25°C, or -40 to -30°C; the pressure of the freeze drying can be 1-5 Pa, or 2-4.5 Pa, or 3-4 Pa; the time of the freeze drying can be 12-24h, or 15-20h, or 16-18h. In the present invention, by defining the drying method and process parameters, the moisture in the reaction system is removed, laying a foundation for subsequent reactions.

[0030] As an embodiment of the present invention, after drying, it can be successively crushed and sieved, and then the obtained precursor is subjected to heat treatment.

[0031] The present invention has no special limitation on the operations of crushing and sieving, and the operations of crushing and sieving commonly used by those skilled in the art can be adopted.

[0032] As an embodiment of the present invention, the particle size of the precursor can be 5-50 μm, or 10-40 μm, or 20-30 μm. In the present invention, by defining the process parameters of the crushing process, the contact between the reaction raw materials is further realized, which is beneficial to the full reaction between the raw materials.

[0033] As an embodiment of the present invention, the heat treatment can be a programmed temperature rise; the heating rate in the first stage of the programmed temperature rise can be 1-10 °C / min, or 2-8 °C / min, or 5-6 °C / min; the holding temperature in the first stage can be 300-400 °C, or 320-380 °C, or 350-360 °C; the holding time in the first stage can be 2-14 h, or 4-10 h, or 6-8 h; the heating rate in the second stage of the programmed temperature rise can be 1-10 °C / min, or 2-8 °C / min, or 5-6 °C / min; the holding temperature in the second stage can be 500-700 °C, or 550-680 °C, or 600-650 °C; the holding time in the second stage can be 2-14 h, or 4-10 h, or 6-8 h. In the present invention, by limiting the process parameters of the heat treatment, sufficient reaction between the reaction raw materials is achieved, and the product of sodium iron pyrophosphate phosphate is obtained.

[0034] As an embodiment of the present invention, the heat treatment can be carried out under a protective atmosphere; the protective atmosphere can be one of nitrogen, argon, helium, nitrogen-hydrogen mixture, and argon-hydrogen mixture. In the present invention, by limiting the conditions of the heat treatment process, the reaction raw materials are prevented from being oxidized by air to generate by-products, ensuring the smooth progress of the reaction, and further improving the tap density and bulk density of sodium iron pyrophosphate phosphate.

[0035] The preparation method provided by the present invention uses iron oxide red to replace part of the iron source of the cathode material, which can effectively reduce the electrostatic repulsion between sodium iron pyrophosphate phosphates, effectively improve the binding force and stability between sodium iron pyrophosphate phosphates, and at the same time reduce the amount of water vapor generated during the polycondensation reaction of phosphate groups; and by limiting the type of phosphorus source, the gas generation amount during the raw material reaction is further reduced, thereby effectively improving the tap density and bulk density of sodium iron pyrophosphate phosphate.

[0036] The present invention also provides sodium iron pyrophosphate phosphate prepared by the preparation method described in the above technical solution.

[0037] The sodium iron pyrophosphate phosphate provided by the present invention has a high tap density and bulk density, thereby improving the battery capacity of the sodium-ion battery; the tap density of the sodium iron pyrophosphate phosphate is as high as 1.513 g / cm 3 , and the bulk density is as high as 2.19 g / cm 3 .

[0038] The present invention also provides an application of sodium iron pyrophosphate phosphate in the cathode material of a sodium-ion battery, and the sodium iron pyrophosphate phosphate is the sodium iron pyrophosphate phosphate described in the above technical solution.

[0039] The specific manner of the application of the sodium iron pyrophosphate phosphate of the present invention is not particularly limited, and a scheme well-known to those skilled in the art can be adopted.

[0040] The sodium-ion battery prepared from the sodium iron pyrophosphate phosphate provided by the present invention can reach a first-cycle charge specific capacity of 117.6 mAh / g and a discharge specific capacity of 109.4 mAh / g at a rate of 0.1C.

[0041] To further illustrate the present invention, the sodium iron pyrophosphate phosphate provided by the present invention, its preparation method and application will be described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0042] Example 1 A preparation method of sodium iron pyrophosphate phosphate is as follows: Mix 37.75 g of ferric phosphate dihydrate, 8.07 g of iron oxide red, 28.40 g of disodium hydrogen phosphate, 9.28 g of glucose, 0.42 g of polyvinylpyrrolidone and 120 mL of water, and then carry out ball milling at 300 rpm (ball-to-material ratio is 8:1) for 10 h to obtain a slurry with a solid content of 41 wt% and the D of solid particles 50 is 0.24 μm; wherein, the molar ratio of ferric phosphate dihydrate to iron oxide red is 2.0:0.5; the molar ratio of iron, phosphorus and sodium elements in the slurry is 3:4:4, and the molar ratio of iron element in the iron source composed of ferric phosphate dihydrate and iron oxide red to the mass of glucose and the mass of polyvinylpyrrolidone is 0.3 mol:9.28 g:0.42 g; Dry the slurry in a blast dryer at 80 °C for 12 h, then crush it with a high-speed crusher and sieve it to obtain a precursor with a particle size of 300 mesh; under a nitrogen atmosphere, first perform the first-stage heat treatment of the precursor at a heating rate of 3 °C / min and 350 °C for 4 h, and then perform the second-stage heat treatment of the precursor at 3 °C / min and 500 °C for 10 h to obtain sodium iron pyrophosphate phosphate.

[0043] Use an X-ray diffractometer to detect the sodium iron pyrophosphate phosphate prepared in Example 1, and the obtained results are as Figure 1 shown. It can be seen from Figure 1 that there are obvious diffraction peaks and no obvious impurity phases, indicating that the sodium iron pyrophosphate phosphate in Example 1 has high purity and no impurities.

[0044] Example 2 The difference between Example 2 and Example 1 is only that the reaction raw materials are 41.52 g of ferric phosphate dihydrate, 6.45 g of iron oxide red, 25.56 g of disodium hydrogen phosphate (phosphorus source), 2.12 g of sodium carbonate, 9.28 g of glucose, 0.43 g of polyvinylpyrrolidone and 120 mL of water, and a slurry with a solid content of 41.5 wt% and the D of solid particles50 The slurry has a particle size of 0.24 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2.2:0.4; the molar ratio of iron element, phosphorus element and sodium element in the slurry is 3:4:4, and the molar amount of iron element in the iron source composed of ferrous phosphate dihydrate and iron oxide red is to the mass of glucose and the mass of polyvinylpyrrolidone in a ratio of 0.3 mol:9.28 g:0.43 g; the rest is the same as in Example 1.

[0045] Example 3 The difference between Example 3 and Example 1 is only that the reaction raw materials are 45.30 g of ferrous phosphate dihydrate, 4.84 g of iron oxide red, 22.72 g of disodium hydrogen phosphate (phosphorus source), 4.25 g of sodium carbonate, 9.28 g of glucose, 0.43 g of polyvinylpyrrolidone and 120 mL of water, obtaining a slurry with a solid content of 42 wt% and the D of solid particles 50 The slurry has a particle size of 0.24 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2.4:0.3; the molar ratio of iron element, phosphorus element and sodium element in the slurry is 3:4:4, and the molar amount of iron element in the iron source composed of ferrous phosphate dihydrate and iron oxide red is to the mass of glucose and the mass of polyvinylpyrrolidone in a ratio of 0.3 mol:9.28 g:0.43 g; the rest is the same as in Example 1.

[0046] Example 4 The difference between Example 4 and Example 1 is only that the reaction raw materials are 49.07 g of ferrous phosphate dihydrate, 3.23 g of iron oxide red, 19.88 g of disodium hydrogen phosphate (phosphorus source), 6.37 g of sodium carbonate, 9.28 g of glucose, 0.44 g of polyvinylpyrrolidone and 120 mL of water, obtaining a slurry with a solid content of 42.4 wt% and the D of solid particles 50 The slurry has a particle size of 0.25 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2.6:0.2; the molar ratio of iron element, phosphorus element and sodium element in the slurry is 3:4:4, and the molar amount of iron element in the iron source composed of ferrous phosphate dihydrate and iron oxide red is to the mass of glucose and the mass of polyvinylpyrrolidone in a ratio of 0.3 mol:9.28 g:0.44 g, and the rest is the same as in Example 1.

[0047] Example 5 The difference between Example 5 and Example 1 is only that the reaction raw materials are 52.85 g of ferrous phosphate dihydrate, 1.61 g of iron oxide red, 17.04 g of disodium hydrogen phosphate (phosphorus source), 8.49 g of sodium carbonate, 9.28 g of glucose, 0.45 g of polyvinylpyrrolidone and 120 mL of water, obtaining a slurry with a solid content of 42.8 wt% and the D of solid particles 50The slurry has a particle size of 0.26 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2.8:0.1; the molar ratio of iron, phosphorus, and sodium in the slurry is 3:4:4, and the molar amount of iron in the iron source composed of ferrous phosphate dihydrate and iron oxide red, the mass of glucose, and the mass of polyvinylpyrrolidone are in the ratio of 0.3 mol:9.28 g:0.45 g. The rest is the same as in Example 1.

[0048] Example 6 The difference between Example 6 and Example 1 is only that the reaction raw materials are 37.75 g of ferrous phosphate dihydrate, 8.07 g of iron oxide red, 27.55 g of sodium pyrophosphate, 9.28 g of glucose, 0.41 g of polyvinylpyrrolidone, and 120 mL of water, obtaining a slurry with a solid content of 40.9 wt% and the D of solid particles 50 The slurry has a particle size of 0.24 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2:0.5; the molar ratio of iron, phosphorus, and sodium in the slurry is 3:4:4, and the molar amount of iron in the iron source composed of ferrous phosphate dihydrate and iron oxide red, the mass of glucose, and the mass of polyvinylpyrrolidone are in the ratio of 0.3 mol:9.28 g:0.41 g. The rest is the same as in Example 1.

[0049] Using an X-ray diffractometer to detect the sodium iron pyrophosphate prepared in Example 6, the results are as Figure 1 shown. It can be seen from Figure 1 that there are obvious diffraction peaks and no obvious impurity phases, indicating that the sodium iron pyrophosphate in Example 6 has a high purity and no impurities.

[0050] Example 7 The difference between Example 7 and Example 1 is only that the reaction raw materials are 37.75 g of ferrous phosphate dihydrate, 8.07 g of iron oxide red, 27.55 g of sodium pyrophosphate, 3.67 g of flaky graphite, 0.39 g of polyvinylpyrrolidone, and 120 mL of water, obtaining a slurry with a solid content of 39.2 wt% and the D of solid particles 50 The slurry has a particle size of 0.24 μm; the molar ratio of ferrous phosphate dihydrate to iron oxide red is 2:0.5; the molar ratio of iron, phosphorus, and sodium in the slurry is 3:4:4, and the molar amount of iron in the iron source composed of ferrous phosphate dihydrate and iron oxide red, the mass of flaky graphite, and the mass of polyvinylpyrrolidone are in the ratio of 0.3 mol:3.67 g:0.39 g. The rest is the same as in Example 1.

[0051] Using an X-ray diffractometer to detect the sodium iron pyrophosphate prepared in Example 7, the results are as Figure 1 shown. It can be seen from Figure 1 that there are obvious diffraction peaks and no obvious impurity phases, indicating that the sodium iron pyrophosphate in Example 7 has a high purity and no impurities.

[0052] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is only that the reaction raw materials are 56.62 g of iron phosphate dihydrate, 14.20 g of disodium hydrogen phosphate (phosphorus source), 10.62 g of sodium carbonate, 9.28 g of glucose, 0.45 g of polyvinylpyrrolidone, and 120 mL of water, and a slurry with a solid content of 43% and a D of solid particles 50 is 0.23 μm; the molar ratio of iron element, phosphorus element and sodium element in the slurry is 3:4:4, and the molar ratio of iron element in the iron source composed of iron phosphate dihydrate to the mass of glucose and the mass of polyvinylpyrrolidone is 0.3 mol:9.28 g:0.45 g, and the rest is the same as Example 1.

[0053] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is only that the reaction raw materials are 54.51 g of ferrous oxalate dihydrate, 48.48 g of disodium hydrogen phosphate, 3.16 g of glucose, 0.32 g of polyvinylpyrrolidone, and 120 mL of absolute ethanol, and a slurry with a solid content of 46% and a D 50 is 0.23 μm; the molar ratio of iron element, phosphorus element and sodium element in the slurry is 3:4:4, and the molar ratio of iron element in the iron source composed of ferrous oxalate dihydrate to the mass of glucose and the mass of polyvinylpyrrolidone is 0.3 mol:3.16 g:0.32 g, and the rest is the same as Example 1.

[0054] Test Example 1 Respectively put 10 g of sodium iron pyrophosphate prepared in Examples 1-7 and Comparative Examples 1-2 into a 25 cm 3 cylindrical measuring cylinder, place it on a vibration device and vibrate it 3000 times at a frequency of 250 times / minute, measure the final sample volume, calculate the tapped density of the material, and the results are shown in Table 1; Respectively put 1 g of sodium iron pyrophosphate prepared in Examples 1-7 and Comparative Examples 1-2 into a cylindrical tool with a diameter of 13 mm, apply a pressure of 3 T, measure the thickness of the final sample, calculate the compacted density of the material, and the results are shown in Table 1; After mixing the sodium iron pyrophosphate prepared in Examples 1-7 and Comparative Examples 1-2 with acetylene black and polyvinylidene fluoride in a mass ratio of 94:3:3 to obtain a slurry, use a 100 μm four-sided coater to evenly coat the slurry on the aluminum foil, and then place it in a blast drying oven at 100 °C for 8 h to obtain an electrode film; use a punching machine to punch the electrode film into an electrode disc with a diameter of 14 mm; place the cut electrode disc in a vacuum drying oven at 100 °C for 4 h and then transfer it to a glove box; use Na as the counter electrode, NaPF 6Using the electrolyte and Whatman GF / D glass fiber separator, a sodium-ion battery was assembled in a glove box; The electrochemical performance of the above sodium-ion battery in the first cycle was detected at a rate of 0.1C, and the results are shown in Table 1; The first charge-discharge specific capacity curves of the sodium-ion batteries composed of sodium iron pyrophosphate in Example 1, Example 6, Example 7 and Comparative Example 1 were detected at a rate of 0.1C as Figure 2 shown; It can be seen from Figure 2 that replacing part of the iron phosphate with iron oxide red as the iron source can improve the electrochemical performance (capacity) of the sodium-ion battery composed of sodium iron pyrophosphate.

[0055] The discharge specific capacity curves of the sodium-ion battery composed of sodium iron pyrophosphate in Example 6 at different rates from 0.1C to 5C and the cycling performance at 1C were detected as Figure 3 shown, where the left curve is the discharge specific capacity curve at different rates, and the right curve is the cycling performance curve at 1C. It can be seen from Figure 3 that the sodium-ion battery composed of sodium iron pyrophosphate provided in Example 6 has a discharge specific capacity of 106.5 mAh / g at a rate of 0.1C, and the discharge specific capacity still reaches 81.6 mAh / g at 5C. It can still maintain a high capacitance after cycling 50 times at 1C, and has excellent cycling performance.

[0056] Table 1 Performance of sodium iron pyrophosphate in Examples 1-7 and Comparative Examples 1-2

[0057] It can be seen from Table 1 that replacing part of the conventional iron source iron phosphate with iron oxide red can effectively improve the tap density and bulk density of sodium iron pyrophosphate; avoiding the use of ferrous oxalate that will generate gas can further improve the tap density and bulk density of sodium iron pyrophosphate; and can effectively improve the electrochemical performance of the battery composed of sodium iron pyrophosphate as the positive electrode material.

[0058] In summary, the sodium iron pyrophosphate prepared by the present invention has a high bulk density and tap density, thereby improving the battery capacity of the sodium-ion battery.

[0059] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing sodium ferric pyrophosphate, characterized in that: include: An iron source, a phosphorus source, a sodium source, a carbon source, an additive and water are mixed and ground to obtain a slurry; the iron source is iron phosphate and red iron oxide; The phosphorus source is one or more of disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and disodium dihydrogen pyrophosphate; The slurry is dried and then heat-treated to obtain sodium ferric pyrophosphate.

2. The preparation method according to claim 1, characterized in that: The sodium source is one or more of sodium carbonate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium citrate and sodium hydroxide.

3. The preparation method according to claim 1, characterized in that: The carbon source is one or more of citric acid, sucrose, glucose, starch, carbon black and graphitized carbon.

4. The preparation method according to claim 1, characterized in that: The additive is one or more of polyethylene glycol, polyvinyl pyrrolidone and starch polysaccharide.

5. The preparation method according to any one of claims 1 to 4, characterized in that The molar ratio of iron, phosphorus and sodium in the slurry is (2-4):(3-4):(3-4).

6. The preparation method according to any one of claims 1 to 4, characterized in that The ratio of the amount of the iron element in the iron source to the mass of the carbon source and the mass of the additive is 0.3 mol: (3-10) g: (0.3-0.6) g.

7. The preparation method according to claim 1, characterized in that: The solid content of the slurry is 30-50wt%; The D of the solid particles in the slurry 50 0.1~0.5μm.

8. The preparation method according to claim 1, characterized in that: The heat treatment is programmed temperature increase; The heating rate of the first stage is 1~10℃ / min, the insulation temperature of the first stage is 300~400℃, and the insulation time of the first stage is 2~14h; The heating rate of the second stage is 1~10℃ / min, the insulation temperature of the second stage is 500~700℃, and the insulation time of the second stage is 2~14h.

9. A sodium iron pyrophosphate, characterized in that The sodium ferric pyrophosphate is prepared by the preparation method according to any one of claims 1 to 8.

10. Application of sodium iron pyrophosphate in positive electrode materials for sodium ion batteries, characterized in that: The sodium ferric pyrophosphate is the sodium ferric pyrophosphate according to claim 9.

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