Sodium ferric phosphate pyrophosphate material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of batteries, in particular to a ferric sodium pyrophosphate material as well as a preparation method and application thereof. The preparation method of the ferric sodium pyrophosphate material comprises the following steps that a mixture of a sodium source, an iron source, a phosphorus source, a carbon source, a functional additive and a solvent is subjected to sanding treatment, the sodium source comprises sodium carbonate, the iron source comprises ferrous oxalate and iron phosphate, the functional additive comprises oxalic acid, and a first system is obtained; carrying out spray drying and sintering treatment on the first system; the molar ratio n of the functional additive to the sodium carbonate is 1 < = n < = 1.1; the molar percentage of ferrous oxalate in the iron source is 10%-40%; the pH value of the first system is 4.5 to 5.5. According to the method, the pH of the sanding slurry can be adjusted, the grinding efficiency can be improved, the reaction activity during sintering can be improved, the impurity phase content can be reduced, the morphology can be optimized, the processing performance and the compaction density of the rear-end slurry can be improved, and the electrochemical performance of the material can be optimized through proper raw materials and proportions.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a sodium iron pyrophosphate material, its preparation method, and its application. Background Technology

[0002] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) is a novel cathode material for sodium-ion batteries, and its abundant raw materials, stable structure, and environmental friendliness make it a promising candidate for low-cost energy storage. To prepare high-purity NFPP cathode materials, high-purity iron-phosphate is typically used via the iron-method process, but the difficulty in grinding during this process significantly reduces production efficiency. Meanwhile, sodium carbonate is commonly used as the sodium source, but its excessively high decomposition temperature results in a high impurity content and uneven morphology in the finished product, negatively impacting downstream processing performance.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] One objective of this invention is to provide a method for preparing sodium iron pyrophosphate material. By using appropriate raw materials and proportions, the pH of the grinding slurry can be adjusted to improve grinding efficiency, increase reactivity during sintering, reduce impurity phase content, optimize morphology, improve the processing performance and compaction density of the downstream slurry, and optimize the electrochemical performance of the material.

[0005] Another object of the present invention is to provide a sodium iron pyrophosphate material.

[0006] Another object of the present invention is to provide a positive electrode.

[0007] Another object of the present invention is to provide a battery.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A method for preparing sodium iron pyrophosphate material includes the following steps: A mixture of sodium source, iron source, phosphorus source, carbon source, functional additive, and solvent is subjected to sand milling. The sodium source includes sodium carbonate, the iron source includes ferrous oxalate and ferric phosphate, and the functional additive includes oxalic acid to obtain a first system. The first system is then subjected to spray drying and sintering. The molar ratio n of the functional additive to the sodium carbonate satisfies 1 ≤ n ≤ 1.1. The molar percentage of ferrous oxalate in the iron source is 10% to 40%. The pH of the first system is 4.5 to 5.5.

[0009] In some embodiments, the oxalic acid includes at least one of anhydrous oxalic acid and oxalic acid dihydrate.

[0010] In some embodiments, the ferrous oxalate includes at least one of ferrous oxalate dihydrate and ferrous oxalate anhydrous.

[0011] In some embodiments, the carbon source includes at least one of anhydrous glucose, glucose monohydrate, and sucrose.

[0012] In some embodiments, the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, and sodium pyrophosphate.

[0013] In some embodiments, the molar ratio of elements Na, Fe, and P in the mixture is 4:(2.7~3):4.

[0014] In some embodiments, the mass percentage of the carbon source to the total mass of the sodium source, the iron source, the phosphorus source, and the functional additive is 4% to 12%.

[0015] In some embodiments, the solvent includes water.

[0016] In some embodiments, the solid content of the mixture is 10% to 40%.

[0017] In some embodiments, the preparation method of the mixture specifically includes: mixing a sodium source, an iron source, a phosphorus source and a functional additive, and then adding the carbon source.

[0018] In some embodiments, the sanding process takes 1.5 to 3 hours.

[0019] In some embodiments, the particle size of the first system is 0.2~0.4µm.

[0020] In some embodiments, the inlet air temperature of the spray dryer is 160~260°C, and the outlet air temperature is 90~120°C.

[0021] In some embodiments, the sintering temperature is 500~600℃ and the sintering time is 6~20h.

[0022] In some embodiments, the heating rate of the sintering process is 2~5℃ / min.

[0023] In some embodiments, the atmosphere for the sintering process is nitrogen, argon, or an argon-hydrogen mixture.

[0024] In some embodiments, the sintering process is followed by a cooling process and a crushing process.

[0025] A sodium iron pyrophosphate material is prepared by the method described above.

[0026] In some embodiments, the general chemical formula of the sodium iron pyrophosphate material is Na₄Fe₂O₃. x (PO4)2P2O7, 2.7≤x≤3.

[0027] In some embodiments, the D50 particle size of the sodium iron pyrophosphate material is 3~10µm, and the specific surface area is ≤9m². 2 / g, compacted density ≥2.0g / cm³ 3 .

[0028] In some embodiments, the carbon content in the sodium iron pyrophosphate material is 1% to 2.5%.

[0029] In some embodiments, the battery prepared from the sodium iron pyrophosphate material has a capacity >110 mAh / g at 0.1C.

[0030] In some embodiments, the battery prepared from the sodium iron pyrophosphate material has an initial efficiency of >90.5%.

[0031] In some embodiments, the battery prepared from the sodium iron pyrophosphate material retains >98% of its capacity after 1000 cycles at 1C.

[0032] A positive electrode comprises sodium iron pyrophosphate material prepared by the method described above, or the sodium iron pyrophosphate material described above.

[0033] A battery comprising the aforementioned positive electrode.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The preparation method of sodium iron pyrophosphate material of the present invention uses ferrous oxalate and iron phosphate in an appropriate ratio to form more uniform and lower hardness precursor particles, thereby improving grinding efficiency, optimizing morphology, and improving compaction. Ferrous oxalate has high activity, and a small amount can improve phase purity. During sintering, ferrous oxalate decomposes to produce CO2 / CO gas, which further reduces and improves phase purity and inhibits particle agglomeration during sintering. By introducing oxalic acid and sodium carbonate in combination, oxalic acid and sodium carbonate combine to form sodium oxalate, which adjusts the overall pH of the slurry to a suitable level. At the same time, it partially corrodes with iron phosphate, improving grinding efficiency and making the reaction activity higher during sintering, optimizing morphology, making the morphology more compact, reducing impurity content, and improving the processing performance and compaction density of the slurry at the end. Through the coordinated operation of each step, sodium iron pyrophosphate material with suitable particle size distribution and dense morphology can be obtained. During the homogenization process of the battery cell slurry, the gelation phenomenon is significantly reduced, and the compaction of the electrode sheet is significantly improved.

[0035] (2) The sodium iron pyrophosphate material obtained by the method of the present invention has suitable particle size, compaction density, specific surface area, suitable carbon content, and excellent electrochemical performance.

[0036] (3) The battery of the present invention has high capacity, high initial efficiency and good cycle stability. Attached Figure Description

[0037] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0038] Figure 1 This is a scanning electron microscope image of the sodium iron pyrophosphate material in Example 3 of the present invention; Figure 2 The X-ray diffraction pattern of the sodium iron pyrophosphate material in Example 4 of this invention is shown below. Figure 3 This is a scanning electron microscope image of the sodium iron pyrophosphate material in Comparative Example 1. Figure 4 The image shows the X-ray diffraction pattern of sodium iron pyrophosphate material in Comparative Example 4. Detailed Implementation

[0039] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0040] According to one aspect of the present invention, the present invention relates to a method for preparing a sodium iron pyrophosphate material, comprising the following steps: A mixture of sodium source, iron source, phosphorus source, carbon source, functional additive, and solvent is subjected to sand milling. The sodium source includes sodium carbonate, the iron source includes ferrous oxalate and ferric phosphate, and the functional additive includes oxalic acid to obtain a first system. The first system is then subjected to spray drying and sintering. The molar ratio n of the functional additive to the sodium carbonate satisfies 1 ≤ n ≤ 1.1 (e.g., 1, 1.05, 1.06, 1.08, 1.09, 1.1, etc.). The molar percentage of ferrous oxalate in the iron source is 10% to 40% (e.g., 10%, 15%, 20%, 25%, 30%, 35%, 40%, etc.). The pH of the first system is 4.5 to 5.5 (e.g., 4.5, 4.8, 5, 5.2, 5.5, etc.).

[0041] The preparation method of sodium iron pyrophosphate material of the present invention utilizes a suitable ratio of ferrous oxalate and ferric phosphate to form more uniform, low-hardness precursor particles, improving grinding efficiency, optimizing morphology, and enhancing compaction. Ferrous oxalate has high reactivity, and even a small amount can improve phase purity. During sintering, ferrous oxalate decomposes to generate CO2 / CO gas, further reducing and improving phase purity, and inhibiting particle agglomeration during sintering. By introducing oxalic acid and sodium carbonate, oxalic acid combines with sodium carbonate to form sodium oxalate, adjusting the overall pH of the slurry to a suitable level. Simultaneously, it partially corrodes with ferric phosphate, improving grinding efficiency and resulting in higher reactivity during sintering, optimizing morphology, making it denser, and reducing impurity phase content, thereby improving the processing performance and compaction density of the downstream slurry. Through the coordinated operation of each step, sodium iron pyrophosphate material with suitable particle size distribution and dense morphology can be obtained, significantly reducing gelation during the homogenization process of the battery cell slurry and significantly improving electrode compaction.

[0042] The functions of oxalic acid in this invention include: acting as a pH adjuster (reacting with sodium carbonate to produce sodium oxalate, precisely controlling the pH of the slurry); an etchant (selectively corroding the dense layer on the surface of ferric phosphate in a low pH environment, reducing particle hardness); and an auxiliary sodium / carbon source (sodium oxalate decomposes to replenish Na). + (Residual carbon improves conductivity).

[0043] The molar ratio n of the functional additive to sodium carbonate in this invention satisfies the above-mentioned range, which can control the pH of the slurry to be stable at 4.5~5.5. This ensures that the iron phosphate is moderately corroded to improve grinding efficiency, while avoiding hydrolysis / sodium source loss, thus providing a stable slurry system for spray drying. When n<1: insufficient oxalic acid and excessive sodium carbonate lead to a slurry pH>6.0, insufficient corrosion of iron phosphate particles, decreased grinding efficiency (grinding particle size is difficult to reach 0.2~0.4μm), and the slurry is prone to agglomeration and clumping during spray drying, resulting in uneven precursor morphology. When n>1.1: excessive oxalic acid leads to a slurry pH<4.5, which will cause iron phosphate to hydrolyze and generate Fe(OH)3 impurity phase. After sintering, the impurity phase content increases, and oxalic acid is easily volatilized, leading to sodium source loss. The stoichiometric ratio of the product deviates from Na4Fe. x (PO4)2P2O7 (x<2.7).

[0044] In some embodiments, the oxalic acid includes at least one of anhydrous oxalic acid and oxalic acid dihydrate.

[0045] In some embodiments, the ferrous oxalate includes at least one of ferrous oxalate dihydrate and ferrous oxalate anhydrous.

[0046] In some embodiments, the carbon source includes at least one of anhydrous glucose, glucose monohydrate, and sucrose, such as a combination of glucose monohydrate and sucrose, or a combination of sucrose and anhydrous glucose.

[0047] In some embodiments, the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, and sodium pyrophosphate, such as a combination of trisodium phosphate and sodium pyrophosphate.

[0048] In some embodiments, the molar ratio of elements Na, Fe and P in the mixture is 4:(2.7~3):4, for example 4:2.7:4, 4:2.8:4, 4:2.9:4, 4:3:4, etc.

[0049] In some embodiments, the mass percentage of the carbon source to the total mass of the sodium source, the iron source, the phosphorus source and the functional additive is 4% to 12%, for example 4%, 5%, 6%, 7%, 8%, 10%, 12%, etc.

[0050] In some embodiments, the solid content of the mixture is 10% to 40%, for example 10%, 15%, 20%, 25%, 30%, 40%, etc.

[0051] In some embodiments, the solvent includes water.

[0052] In some embodiments, the preparation method of the mixture specifically includes: mixing a sodium source, an iron source, a phosphorus source, and a functional additive, and then adding the carbon source. The appropriate feeding steps employed in this invention are more conducive to ensuring the mixing effect of the materials.

[0053] In some embodiments, the sand milling process takes 1.5 to 3 hours, for example, 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc. By employing a suitable sand milling time and coordinating it with other steps, the invention ensures that the material in the first system has a suitable particle size.

[0054] In some embodiments, the particle size of the first system is 0.2~0.4µm, for example 0.2µm, 0.3µm, 0.4µm, etc. A suitable particle size is beneficial to the effectiveness of subsequent spray drying and sintering treatments.

[0055] In some embodiments, the inlet air temperature for spray drying is 160~260℃, for example, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, etc. The outlet air temperature is 90~120℃, for example, 90℃, 100℃, 110℃, 120℃, etc. Spray drying requires the slurry to have good fluidity and atomization stability, and the suitable slurry pH of the present invention can stabilize the slurry viscosity at 100~200 mPa. With suitable spray drying conditions, uniform spherical droplets can be formed. After drying, the precursor is a spherical particle, which is beneficial to the dense morphology of the particles after subsequent sintering.

[0056] In some embodiments, the sintering temperature is 500~600℃, for example 500℃, 510℃, 520℃, 550℃, 570℃, 600℃, etc.; the sintering time is 6~20h, for example 6h, 7h, 8h, 10h, 11h, 12h, 15h, 20h, etc. In some embodiments, the heating rate of the sintering process is 2~5℃ / min, for example 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, etc. The atmosphere for the sintering process is nitrogen, argon, or an argon-hydrogen mixture. The present invention uses suitable sintering conditions to ensure the performance of the final material. If the temperature is too low, the reaction will be incomplete; if the temperature is too high, excessive particle growth will affect its electrochemical performance.

[0057] In some embodiments, the sintering process is followed by a cooling process to room temperature, and then a crushing process to a suitable particle size.

[0058] In a preferred embodiment, the preparation method of the sodium iron pyrophosphate material of the present invention includes the following steps: (a) A sodium source, an iron source, a phosphorus source, and a functional additive are mixed to obtain a mixture with a solid content of 10% to 40%, and then a carbon source is added; the sodium source includes sodium carbonate, the iron source includes ferrous oxalate and ferric phosphate, and the functional additive includes oxalic acid; the carbon source includes at least one of anhydrous glucose, glucose monohydrate, and sucrose; the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, and sodium pyrophosphate. The molar ratio n of the functional additive to the sodium carbonate satisfies 1 ≤ n ≤ 1.1; the molar percentage of ferrous oxalate in the iron source is 10% to 40%.

[0059] (b) The above mixture is subjected to sand milling for 1.5 to 3 hours to obtain a first system with a particle size of 0.2 to 0.4 µm and a pH of 4.5 to 5.5.

[0060] (c) The first system is spray-dried with an inlet air temperature of 160~260℃ and an outlet air temperature of 90~120℃.

[0061] (d) Then perform sintering treatment. The sintering temperature is 500~600℃ and the sintering time is 6~20h. The heating rate of the sintering treatment is 2~5℃ / min. The atmosphere of the sintering treatment is nitrogen, argon or argon-hydrogen mixture.

[0062] (e) The sintering process is followed by a cooling process and a crushing process.

[0063] According to another aspect of the present invention, the present invention also relates to a sodium iron pyrophosphate material, which is prepared by the method for preparing the sodium iron pyrophosphate material.

[0064] The sodium iron pyrophosphate material of the present invention has suitable particle size, compaction density, specific surface area, and excellent electrochemical performance.

[0065] In some embodiments, the general chemical formula of the sodium iron pyrophosphate material is Na₄Fe₂O₃. x (PO4)2P2O7, 2.7≤x≤3, for example 2.7, 2.8, 2.9, 3, etc.

[0066] In some embodiments, the D50 particle size of the sodium iron pyrophosphate material is 3~10µm, for example 3µm, 4µm, 5µm, 6µm, 7µm, 10µm, etc., and the specific surface area is ≤9m². 2 / g, for example 4 m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, etc.; compacted density ≥2.0g / cm³ 3For example, 2.05 g / cm³ 3 2.1g / cm 3 2.15g / cm 3 2.18 g / cm 3 2.2g / cm 3 2.3 g / cm 3 The sodium iron pyrophosphate material of the present invention has suitable D50 particle size, specific surface area and compaction density, which is beneficial to improving the electrochemical performance of the battery.

[0067] In some embodiments, the carbon content in the sodium iron pyrophosphate material is 1% to 2.5%, for example, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.5%, etc. The sodium iron pyrophosphate material of the present invention has a suitable carbon content, which is beneficial to improving the electrochemical performance of the sodium iron pyrophosphate material.

[0068] In some embodiments, the battery prepared from the sodium iron pyrophosphate material has a capacity >110 mAh / g at 0.1C, such as 110.2 mAh / g, 110.5 mAh / g, 111 mAh / g, 115 mAh / g, 120 mAh / g, etc. In some embodiments, the battery prepared from the sodium iron pyrophosphate material has an initial efficiency >90.5%, such as 90.6%, 90.8%, 91%, 92%, 93%, etc. In some embodiments, the battery prepared from the sodium iron pyrophosphate material has a capacity retention rate >98% after 1000 cycles at 1C, such as 98.1%, 98.5%, 99%, 99.1%, 99.5%, etc. The battery prepared from the sodium iron pyrophosphate material of the present invention has high capacity, high initial efficiency, and excellent cycle stability.

[0069] According to another aspect of the present invention, the present invention also relates to a positive electrode comprising a sodium pyrophosphate material prepared by the aforementioned method for preparing sodium pyrophosphate material.

[0070] The positive electrode sheet of the present invention includes a positive current collector and a positive electrode layer disposed on its surface, the positive electrode layer comprising the above-mentioned sodium iron pyrophosphate material.

[0071] According to another aspect of the invention, the invention also relates to a battery comprising the aforementioned positive electrode.

[0072] The battery of the present invention has high capacity, high first-efficiency (first coulombic efficiency) and excellent cycle stability.

[0073] The following explanation, combined with specific embodiments and comparative examples, further illustrates the point.

[0074] Example 1 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) After mixing phosphorus source, iron source, sodium source and anhydrous oxalic acid, add it to deionized water and stir. The phosphorus source is sodium dihydrogen phosphate, the iron source is iron phosphate and anhydrous ferrous oxalate, and the sodium source is sodium carbonate. A slurry with a solid content of 30% is obtained. The molar ratio of each element in the slurry is Na:Fe:P=4:2.7:4, and the molar ratio of ferrous oxalate to the total iron source is 10%. Then glucose is added as a carbon source. The mass of the carbon source is controlled to be 10% of the total mass of sodium source, iron source, phosphorus source and anhydrous oxalic acid. The molar ratio of oxalic acid to sodium carbonate is 1.0.

[0075] (2) The slurry prepared in step (1) is sand-milled for 3 hours. After the sand-milling particle size is reduced to 0.3µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 220℃, the outlet temperature is set to 110℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0076] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 550°C at 2°C / min and held for 14 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0077] (4) The sample from step (3) is crushed to obtain the final product Na4Fe. 2.7 (PO4)2P2O7.

[0078] Example 2 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) After mixing phosphorus source, iron source, sodium source and anhydrous oxalic acid, add it to deionized water and stir. The phosphorus source is sodium dihydrogen phosphate, the iron source is iron phosphate and anhydrous ferrous oxalate, and the sodium source is sodium carbonate. A slurry with a solid content of 35% is obtained. The molar ratio of each element in the slurry is Na:Fe:P=4:3:4. Ferrous oxalate accounts for 30% of the total iron source. Then glucose is added as a carbon source. The mass of the carbon source is controlled to be 10% of the total mass of sodium source, iron source, phosphorus source and anhydrous oxalic acid. The molar ratio of oxalic acid to sodium carbonate is 1.1.

[0079] (2) The slurry prepared in step (1) is sand-milled for 2.5 hours. After the sand-milling particle size is reduced to 0.35µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 200℃, the outlet temperature is set to 100℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0080] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 540°C at 2°C / min and held for 14 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0081] (4) The sample in step (3) is crushed to obtain the final product Na4Fe3(PO4)2P2O7.

[0082] Example 3 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) After mixing phosphorus source, iron source, sodium source and anhydrous oxalic acid, add it to deionized water and stir. The phosphorus source is sodium dihydrogen phosphate, the iron source is iron phosphate and anhydrous ferrous oxalate, and the sodium source is sodium carbonate. A slurry with a solid content of 40% is obtained. The molar ratio of each element in the slurry is Na:Fe:P=4:2.85:4, of which ferrous oxalate accounts for 40% of the total iron source. Then glucose is added as a carbon source. The mass of the carbon source is controlled to be 10% of the total mass of sodium source, iron source, phosphorus source and anhydrous oxalic acid. The molar ratio of oxalic acid to sodium carbonate is 1.05.

[0083] (2) The slurry prepared in step (1) is sand-milled for 3 hours. After the sand-milling particle size is reduced to 0.25µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 220℃, the outlet temperature is set to 90℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0084] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 520°C at 2°C / min and held for 20 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0085] (4) The sample from step (3) is crushed to obtain the final product Na4Fe. 2.85 (PO4)2P2O7.

[0086] Example 4 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) After mixing phosphorus source, iron source, sodium source and anhydrous oxalic acid, add it to deionized water and stir. The phosphorus source is sodium dihydrogen phosphate, the iron source is iron phosphate and anhydrous ferrous oxalate, and the sodium source is sodium carbonate. A slurry with a solid content of 20% is obtained. The molar ratio of each element in the slurry is Na:Fe:P=4:2.91:4, of which ferrous oxalate accounts for 20% of the total iron source. Then glucose is added as a carbon source. The mass of the carbon source is controlled to be 8% of the total mass of sodium source, iron source, phosphorus source and anhydrous oxalic acid. The molar ratio of oxalic acid to sodium carbonate is 1.03.

[0087] (2) The slurry prepared in step (1) is sand-milled for 2 hours. After the sand-milling particle size is reduced to 0.35 µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 200℃, the outlet temperature is set to 120℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0088] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 560°C at 2°C / min and held for 10 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0089] (4) The sample from step (3) is crushed to obtain the final product Na4Fe. 2.91 (PO4)2P2O7.

[0090] Example 5 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) After mixing phosphorus source, iron source, sodium source and anhydrous oxalic acid, add it to deionized water and stir. The phosphorus source is sodium dihydrogen phosphate, the iron source is iron phosphate and anhydrous ferrous oxalate, and the sodium source is sodium carbonate. A slurry with a solid content of 20% is obtained. The molar ratio of each element in the slurry is Na:Fe:P=4:2.85:4. Ferrous oxalate accounts for 20% of the total iron source. Then glucose is added as a carbon source. The mass of the carbon source is controlled to be 4% of the total mass of sodium source, iron source, phosphorus source and anhydrous oxalic acid. The molar ratio of oxalic acid to sodium carbonate is 1.

[0091] (2) The slurry prepared in step (1) is sand-milled for 1.5 hours. After the sand-milling particle size is reduced to 0.4µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 210℃, the outlet temperature is set to 105℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0092] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 600°C at 2°C / min and held for 4 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0093] (4) The sample from step (3) is crushed to obtain the final product Na4Fe. 2.85 (PO4)2P2O7.

[0094] Comparative Example 1 A method for preparing sodium iron pyrophosphate material includes the following steps: (1) Sodium dihydrogen phosphate, ferric phosphate, ferrous oxalate and sodium oxalate were mixed and added to deionized water and stirred to obtain a slurry with a solid content of 30%. The molar ratio of each element in the slurry was Na:Fe:P=4:2.91:4, of which ferrous oxalate accounted for 20% of the total iron source. Then glucose was added as a carbon source and the mass of the carbon source was controlled to be 10% of the total mass of sodium dihydrogen phosphate, ferric phosphate, ferrous oxalate and sodium oxalate.

[0095] (2) The slurry prepared in step (1) is sand-milled for 4 hours. After the sand-milling particle size is reduced to 0.35µm, it is transferred to the spray drying device. The spray drying inlet temperature is set to 200℃, the outlet temperature is set to 120℃, and the drying atmosphere is air. After the spray drying is completed, the precursor material is collected.

[0096] (3) The precursor material in step (2) is subjected to high-temperature heat treatment in a sintering furnace. The temperature is increased to 560°C at 2°C / min and held for 10 hours. After cooling to room temperature, the sample is taken out to obtain sodium iron pyrophosphate cathode material.

[0097] (4) The sample from step (3) is crushed to obtain the final product Na4Fe. 2.91 (PO4)2P2O7.

[0098] Comparative Example 2 A method for preparing sodium iron pyrophosphate material differs from Example 3 in that: No oxalic acid added.

[0099] The grinding time was 4 hours; the grinding was carried out until the particle size was 0.35µm.

[0100] Comparative Example 3 A method for preparing sodium iron pyrophosphate material differs from Example 3 in that: The molar ratio of acid to sodium carbonate is 1.2.

[0101] The grinding time was 2 hours; the grinding was carried out until the particle size was 0.35µm.

[0102] Comparative Example 4 A method for preparing sodium iron pyrophosphate material differs from that in Example 4 in that: No ferrous oxalate added.

[0103] The grinding time is 4 hours.

[0104] Experimental Example I. Performance Testing of Sodium Iron Pyrophosphate Material The slurries prepared by the methods of preparing sodium iron pyrophosphate materials in each embodiment and comparative example were subjected to pH testing, and the D50 particle size, specific surface area and compaction density of the sodium iron pyrophosphate materials were tested. The test results are shown in Table 1.

[0105] The scanning electron microscope (SEM) image of the sodium iron pyrophosphate material in Example 3 of this invention is as follows: Figure 1 As shown, the sodium iron pyrophosphate material of the present invention has a suitable particle size, regular and uniform morphology, smooth surface, and high density. The scanning electron microscope image of the sodium iron pyrophosphate material in Comparative Example 1 is shown below. Figure 3As shown, the particles are relatively large, have rough surfaces, and exhibit particle agglomeration. The X-ray diffraction (XRD) pattern of the sodium iron pyrophosphate material in Example 4 of this invention is shown below. Figure 2 As shown, the method of the present invention yielded a pure-phase sodium iron pyrophosphate material. The X-ray diffraction pattern of the sodium iron pyrophosphate material in Comparative Example 4 is shown below. Figure 4 As shown, it contains impurity phases.

[0106] Table 1. pH of slurry and D50 particle size, specific surface area and compaction density of sodium iron pyrophosphate material

[0107] As shown in Table 1, the preparation method of sodium iron pyrophosphate material of the present invention can stabilize the pH of the slurry at 4.5~5.5, and the obtained sodium iron pyrophosphate material has suitable D50 particle size, suitable specific surface area, and suitable compaction density. The D50 particle size is 3~10µm, and the specific surface area is ≤8.5m². 2 / g; compacted density ≥2.0g / cm³ 3 .

[0108] The slurries obtained by the methods in Comparative Examples 1 to 4 had excessively high pH, ​​excessively high specific surface area, and excessively low compaction density.

[0109] II. Battery Performance Testing The batteries prepared from sodium iron pyrophosphate material in the examples and comparative examples were subjected to performance tests, and the test results are shown in Table 2.

[0110] Table 2 Battery performance test results

[0111] As shown in Table 2, the sodium iron pyrophosphate material obtained by the methods of the various embodiments of the present invention has high capacity, high initial efficiency and good cycle performance. The battery prepared by the sodium iron pyrophosphate material has a capacity of >110mAh / g at 0.1C, an initial efficiency of >90.5%, and a capacity retention rate of >98% after 1000 cycles at 1C.

[0112] In Comparative Example 1, without the addition of oxalic acid, the pH of the slurry was slightly higher, resulting in batteries made from sodium iron pyrophosphate material with low capacity, poor initial efficiency, and poor cycle performance.

[0113] Compared with Example 3, the method of Comparative Example 2 did not add oxalic acid, and the sand milling time was 4 hours. When the sand milling reached a particle size of 0.35µm, the pH of the slurry was too high, the iron phosphate particles were not fully corroded, the grinding efficiency decreased, and the slurry was prone to agglomeration and clumping during spray drying, resulting in uneven precursor morphology. The battery prepared from the obtained sodium iron pyrophosphate material had reduced capacity, poor first-time efficiency, and poor cycle performance.

[0114] Compared with Example 3, the molar ratio of acid to sodium carbonate in Comparative Example 3 was 1.20, and the milling time was 2 hours. The slurry was milled to a particle size of 0.35µm. If the pH of the slurry was too low, it would cause the hydrolysis of iron phosphate to generate Fe(OH)3 impurity phase. After sintering, the impurity phase content increased, and the capacity of the battery prepared by the obtained sodium iron pyrophosphate material decreased, the first-time efficiency deteriorated, and the cycle performance deteriorated.

[0115] Compared to Example 4, Comparative Example 4 did not add ferrous oxalate, and the milling time was 4 hours. If the pH of the slurry was too high, the iron phosphate particles would not corrode sufficiently, resulting in decreased milling efficiency. Furthermore, the slurry was prone to agglomeration and clumping during spray drying, leading to uneven precursor morphology. The resulting sodium iron pyrophosphate material resulted in batteries with reduced capacity, poorer initial efficiency, and worse cycle performance.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a sodium iron pyrophosphate material, characterized in that, Includes the following steps: A mixture of sodium source, iron source, phosphorus source, carbon source, functional additive and solvent is subjected to sand milling. The sodium source includes sodium carbonate, the iron source includes ferrous oxalate and ferric phosphate, and the functional additive includes oxalic acid to obtain a first system. The first system is then subjected to spray drying and sintering. The molar ratio n of the functional additive to the sodium carbonate satisfies 1 ≤ n ≤ 1.1; The ferrous oxalate accounts for 10% to 40% of the molar percentage of the iron source; The pH of the first system is 4.5~5.

5.

2. The method for preparing sodium iron pyrophosphate material according to claim 1, characterized in that, Includes at least one of the following conditions: (1) The oxalic acid includes at least one of anhydrous oxalic acid and dihydrate oxalic acid; (2) The ferrous oxalate includes at least one of ferrous oxalate dihydrate and ferrous oxalate anhydrous; (3) The carbon source includes at least one of anhydrous glucose, glucose monohydrate and sucrose; (4) The phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, and sodium pyrophosphate; (5) The molar ratio of elements Na, Fe and P in the mixture is 4:(2.7~3):4; (6) The mass percentage of the carbon source to the total mass of the sodium source, the iron source, the phosphorus source and the functional additive is 4% to 12%; (7) The solvent includes water.

3. The method for preparing sodium iron pyrophosphate material according to claim 1, characterized in that, Includes at least one of the following conditions: (1) The solid content of the mixture is 10%~40%; (2) The preparation method of the mixture specifically includes: mixing sodium source, iron source, phosphorus source and functional additive, and then adding the carbon source; (3) The grinding time is 1.5~3 hours; (4) The particle size of the first system is 0.2~0.4µm.

4. The method for preparing sodium iron pyrophosphate material according to claim 1, characterized in that, The inlet air temperature of the spray dryer is 160~260℃, and the outlet air temperature is 90~120℃.

5. The method for preparing sodium iron pyrophosphate material according to claim 1, characterized in that, Includes at least one of the following conditions: (1) The sintering temperature is 500~600℃ and the sintering time is 6~20h; (2) The heating rate of the sintering treatment is 2~5℃ / min; (3) The atmosphere of the sintering process is nitrogen, argon or argon-hydrogen mixture.

6. The method for preparing sodium iron pyrophosphate material according to claim 1, characterized in that, The sintering process is followed by cooling and crushing processes.

7. A sodium iron pyrophosphate material, characterized in that, It is prepared by the method for preparing sodium iron pyrophosphate material according to any one of claims 1 to 6.

8. The sodium iron pyrophosphate material according to claim 7, characterized in that, Includes at least one of the following conditions: (1) The general chemical formula of the sodium iron pyrophosphate material is Na4Fe x (PO4)2P2O7, 2.7≤x≤3; (2) The D50 particle size of the sodium iron pyrophosphate material is 3~10µm; (3) The specific surface area of ​​the sodium iron pyrophosphate material is ≤9m². 2 / g; (4) The compacted density of the sodium iron pyrophosphate material is ≥2.0 g / cm³. 3 ; (5) The carbon content in the sodium iron pyrophosphate material is 1%~2.5%; (6) The battery prepared from the sodium iron pyrophosphate material has a capacity of >110mAh / g at 0.1C; (7) The battery prepared from the sodium iron pyrophosphate material has an initial efficiency >90.5%; (8) The battery prepared by the sodium iron pyrophosphate material has a capacity retention rate of >98% after 1000 cycles at 1C.

9. A positive electrode plate, characterized in that, The sodium ferric phosphate material prepared by the method of any one of claims 1 to 6, or the sodium ferric phosphate material of any one of claims 7 to 8.

10. A battery, characterized in that, It includes the positive electrode sheet as described in claim 9.

Images