Composite sodium iron phosphate positive electrode material, precursor and preparation method thereof, and sodium ion battery

Abstract

The present invention provides a composite sodium iron phosphate positive electrode material, a precursor and a preparation method thereof, and a sodium ion battery, belonging to the technical field of inorganic material preparation. The present invention innovatively adopts a wet chemical method to successfully prepare amorphous ferric hydrogen phosphate, and mixes the amorphous ferric hydrogen phosphate precursor with a sodium salt, a phosphorus source and a carbon source, and obtains a composite sodium iron phosphate positive electrode material through a solid-phase sintering process. The composite sodium iron phosphate positive electrode material prepared by the present invention has high purity, high compaction density, good structural stability, and excellent discharge specific capacity and rate performance. The use of this positive electrode material can effectively improve the reversible specific capacity and service life of sodium ion batteries, providing a high-quality sodium ion battery supply for the energy storage field.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic material preparation, and in particular relates to a composite sodium iron phosphate positive electrode material, a precursor and a preparation method thereof, and a sodium ion battery. Background Art

[0002] In the global energy transformation and electrification process, lithium-ion batteries dominate the fields of portable electronic devices and electric vehicles due to their high energy density and excellent cycle stability; however, the uneven geographical distribution of lithium resources and high mining costs have seriously restricted the promotion and application of lithium-ion batteries in large-scale energy storage systems; in comparison, sodium-ion batteries have broad application prospects in the field of large-scale energy storage due to their sodium resource abundance of up to 2.74%, significant cost advantages, and electrochemical mechanisms and device structure characteristics similar to lithium-ion batteries.

[0003] As a key component of sodium-ion batteries, cathode materials directly determine the overall performance and manufacturing cost of the battery system. Among the mainstream sodium-ion battery cathode material systems, polyanionic compounds are particularly suitable for energy storage scenarios with stringent requirements on safety and durability due to their unique three-dimensional skeleton structure, which gives them high thermal stability, high operating voltage and long cycle life. Among them, composite sodium iron phosphate (NFPP) with both phosphate and pyrophosphate groups has a high structural stability (Na + Volume change during intercalation and deintercalation is less than 4%), and a moderate average operating voltage (3.1 V vs. Na + / Na) and high theoretical specific capacity (129mAh / g) have become research hotspots.

[0004] Currently, composite sodium iron phosphate cathode materials are mostly synthesized using solid-phase sintering technology. This involves refining the sodium source, iron source, supplementary phosphorus source, and carbon source into particles and mixing them evenly through physical means such as ball milling and sand milling. The mixed raw materials are then solid-phase sintered in a single step to form a composite sodium iron phosphate cathode material. In existing technologies, iron phosphate is the most commonly used iron and phosphorus source for synthesizing NFPP. However, its stable crystalline structure and extremely low water solubility make it difficult to fully dissolve / dissociate during ball milling or sand milling, resulting in it ultimately existing in the slurry in the form of submicron-sized crystallites. This incompletely dissociated initial state can cause multiple problems during the subsequent sintering process:

[0005] 1) Difficulty in ion mass transfer: The large particle size and compact internal structure of iron phosphate seriously hinder the ion diffusion during the sintering process. 3+ →Fe 2+ ), the extension of the mass transfer path and the increase of diffusion resistance inside the microcrystal lead to the Fe 3+ Cannot be completely reduced, and the unreduced Fe 3+ With reduced Fe2+ Due to the phosphate (PO4 3- ) have different binding forces and the diffusion rates are significantly different. 2+ Migrate outward faster and Fe 3+ It is retained inside the microcrystals, which eventually leads to local iron enrichment and destroys the uniformity of the intrinsic structure of the material.

[0006] 2) Very easy to induce phase separation: During the sintering process, phosphate (PO4 3- ) need to be passed with H + Combined with high temperature dehydration condensation to convert into pyrophosphate (P2O7 4- ), however, the larger molecular weight PO4 3- Trapped in the crystallites, it is difficult to migrate and + Combined with the uneven temperature field of large particles, the conversion energy barrier is further raised, resulting in PO4 3- Insufficient conversion. Under the thermodynamic drive of the iron-rich region, the unconverted PO4 3- Will diffuse Na + Combined with the preferential generation of the electrochemically inert impurity phase - sodium iron phosphate, it seriously damages the reversible capacity and cycle life of the battery;

[0007] 3) Difficulty in controlling phase purity: During the sintering process, ion mass transfer barriers and phase separation problems form a vicious cycle, leading to the uncontrollable formation of electrochemically inert sodium iron phosphate impurities. This ultimately significantly reduces the NFPP phase purity and severely restricts its electrochemical performance.

[0008] 4) Product particle morphology and compaction density defects: The iron phosphate raw material is micron-sized secondary particles formed by the agglomeration of nano-primary particles. The structural characteristics of this multi-level agglomerate are directly inherited to the final product NFPP during the solid-phase sintering process, making the final material present an irregular porous agglomerate morphology and a low compaction density, which significantly restricts the volume energy density of the material.

[0009] The above-mentioned process bottlenecks and material performance defects caused by the characteristics of iron phosphate raw materials have seriously restricted the performance stability, large-scale production and application promotion of composite sodium iron phosphate materials. Summary of the Invention

[0010] The object of the present invention is to provide a method for preparing a composite sodium iron phosphate positive electrode material to solve the problems raised in the above background technology.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is:

[0012] A first aspect of the present invention provides a precursor of a composite sodium iron phosphate positive electrode material, wherein the precursor is amorphous ferric hydrogen phosphate, and its chemical formula is FeHPO4.

[0013] The second aspect of the present invention provides a method for preparing a precursor of the above-mentioned composite sodium iron phosphate positive electrode material, comprising the following steps: placing a divalent iron source and a phosphorus source with a molar ratio of 1:1-1.2, and a grinding aid in a mechanical stirred mill, and conducting a sufficient grinding reaction under an inert atmosphere; after washing and drying the reaction product, low-temperature calcination is carried out in an inert atmosphere to obtain an amorphous FeHPO4 precursor.

[0014] Preferably, the ferrous iron source is one or more of ferrous sulfate, ferrous acetate, ferrous citrate, and ferrous sulfate hydrate, the phosphorus source is one or more of ammonium dihydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate, and the grinding aid is one or more of ethylene glycol, polyethylene glycol, acetone, glycerol, diformate, or sulfolane; and the solid-to-liquid ratio of the total mass of the ferrous iron source and the phosphorus source to the grinding aid is 1:1-4.

[0015] Preferably, the inert atmosphere is argon or nitrogen; the stirring speed of the mechanical stirring mill is 300-600 rpm, and the grinding time is 1-10 h; the calcination temperature is 200-400° C., and the time is 3-8 h.

[0016] A third aspect of the present invention provides a method for preparing a composite sodium iron phosphate positive electrode material, comprising the following steps: dispersing a sodium salt, a phosphate salt, a carbon source, and the above-mentioned FeHPO4 in a solvent, sand milling, spray drying, and then calcining at a temperature of 500-600°C for 6-16 h under an inert atmosphere to obtain a composite sodium iron phosphate positive electrode material.

[0017] Preferably, the sodium salt is one or more of sodium carbonate, sodium bicarbonate, sodium oxalate, sodium dihydrogen phosphate, and sodium phosphate; the phosphate salt is one or more of ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; and the carbon source is one or more of glucose, sucrose, and starch.

[0018] Preferably, the molar ratio of Na:P:Fe provided by the sodium salt, FeHPO4 and phosphorus source is (4-4.05):(3.95-4.05):(2.9-3), and the mass of the carbon source is 1%-20% of the mass of FeHPO4.

[0019] Preferably, the solvent is at least one of water and ethanol, and the total solid mass of the FeHPO4 powder, phosphorus source, sodium salt and carbon source accounts for 20%-40% of the total solid mass of the solvent.

[0020] The third aspect of the present invention provides a composite sodium iron phosphate positive electrode material prepared by the above-mentioned preparation method. The chemical formula of the composite sodium iron phosphate positive electrode material is Na4Fe3(PO4)2P2O7@C, wherein C is the outer carbon coating layer; the material has high purity, spherical secondary particles, high compaction density, and excellent discharge specific capacity and rate performance.

[0021] A fourth aspect of the present invention provides a sodium ion battery comprising the composite sodium iron phosphate positive electrode material as described above.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present invention innovatively uses a wet chemical method to successfully prepare amorphous ferric hydrogen phosphate. The ferric hydrogen phosphate is monodisperse submicron particles with smooth primary particle surfaces and no hard agglomerates, regular and rounded morphology, and high particle density. The composite sodium iron phosphate positive electrode material prepared using this amorphous ferric hydrogen phosphate as a precursor has high purity, spherical secondary particles, high compaction density, and excellent discharge specific capacity and rate performance.

[0024] 2. The present invention directly uses amorphous FeHPO4 to provide Fe 2+ It replaces the traditional trivalent iron source and avoids the carbon thermal reduction step. 2+ (Fe2+) and PO4 3- The binding energy is significantly lower than that of Fe 3+ (trivalent iron), which increases the migration rate of iron ions. At the same time, the divalent iron source eliminates the problem of different diffusion rates of iron elements with different valences caused by incomplete reduction of trivalent iron, thereby avoiding local enrichment of iron elements and achieving atomic-level uniform distribution of iron elements in the NFPP lattice, significantly improving the uniformity of the material's intrinsic structure;

[0025] The present invention utilizes the phase transformation of amorphous FeHPO4 at around 500℃: FeHPO4→Fe2P2O7+H2O to directly generate pyrophosphate (P2O7 4- ), this process circumvents PO4 3- The kinetic and thermodynamic barriers of the high-temperature condensation step effectively suppress the formation of the electrochemically inert phase NaFePO4 from a thermodynamic perspective by precisely providing the pyrophosphate required by the composite sodium iron phosphate cathode material, thus avoiding phase separation and the formation of the inert phase.

[0026] 3. In the process of synthesizing the composite sodium iron phosphate positive electrode material using FeHPO4 as the iron source and phosphorus source in the present invention, the amorphous non-crystalline structure of FeHPO4 gives it ultra-high chemical reaction activity during the solid-phase sintering process. The amorphous FeHPO4 has no lattice periodic constraints, and the random atomic arrangement reduces the ion migration energy barrier, significantly accelerating the ion diffusion during the reaction process. The ultra-high specific surface area of ​​FeHPO4 and the large number of unsaturated coordination sites provide an atomic-level contact interface for the sodium salt / phosphorus salt, effectively improving the reaction kinetics; in addition, the loose pores formed in the FeHPO4 by high-temperature dehydration condensation provide efficient ion diffusion channels for the inward diffusion of sodium ions and the outward diffusion of iron ions, greatly promoting the ion mass transfer process;

[0027] 4. In the process of producing the composite sodium iron phosphate cathode material of the present invention, the amorphous FeHPO4 precursor achieves a significant improvement in the phase purity of the NFPP material through four-dimensional synergy: high activity breaks down the reaction kinetics and thermodynamic barriers, in-situ pyrophosphate cuts off the impurity phase thermodynamic path, dehydration and pore formation break down the mass transfer barrier, and divalent iron circumvents the reduction and diffusion problems of trivalent iron with divalent iron. This provides key technical support for the large-scale application of composite sodium iron phosphate cathode materials.

[0028] 5. The composite sodium iron phosphate positive electrode material obtained by the preparation method of the present invention has the advantages of high electronic conductivity and ion diffusion rate, good structural stability, good rate and cycle performance, etc. It can be used as a positive electrode material for nano-ion batteries with large capacity and high power, and the battery has good specific capacity and long service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the XRD pattern of the ferric hydrogen phosphate precursor prepared in Example 1-5 and Comparative Example 1-2;

[0030] Figure 2 is the XRD pattern of the composite sodium iron phosphate positive electrode material samples prepared in Examples 6-8 and Comparative Examples 3-5;

[0031] Figure 3 1-4 and Comparative Examples 1-2 are SEM images of the ferric hydrogen phosphate precursor FHP prepared in Example 1-4 and Comparative Examples 1-2;

[0032] Figure 4 is an SEM image of the iron phosphate precursor used in Comparative Example 3;

[0033] Figure 5 is a SEM image of the composite sodium iron phosphate positive electrode material sample NFPP-A1 prepared in Example 6;

[0034] Figure 6 This is an SEM image of the composite sodium iron phosphate positive electrode material sample NFPP-B1 prepared in Comparative Example 1;

[0035] Figure 7 This is the first cycle charge and discharge curve of test battery 1 and test battery 2 at 0.2C;

[0036] Figure 8 is the rate performance graph of test cell 1 and test cell 2;

[0037] Figure 9 Figure 2 is a cycle performance diagram of test battery 1 and test battery 2 at 2C. DETAILED DESCRIPTION

[0038] The present invention is described in detail below through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-mentioned content of the present invention. In the following examples, reagents and instruments not otherwise specified are commercially available, and experimental operations not otherwise specified are performed according to the manufacturer's instructions or conventional techniques in the art. Unless otherwise defined, all professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to those described herein can be applied to the present invention; the endpoints and any values ​​of the ranges disclosed herein are not limited to the exact ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein; the professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0039] A first aspect of the present invention provides a precursor of a composite sodium iron phosphate positive electrode material, wherein the precursor is amorphous ferric hydrogen phosphate, and its chemical formula is FeHPO4.

[0040] The second aspect of the present invention provides a method for preparing a precursor of the composite sodium iron phosphate positive electrode material, comprising the following steps:

[0041] A divalent iron source and a phosphorus source with a molar ratio of 1:1-1.2, as well as a grinding aid, are placed in a mechanical stirring mill and subjected to a thorough grinding reaction under an inert atmosphere; the reaction product is washed and dried, and then calcined at a low temperature in an inert atmosphere to obtain an amorphous FeHPO4 precursor.

[0042] In some preferred embodiments of the present invention, the product is dried at 50-60° C. to remove residual deionized water from washing;

[0043] In some preferred embodiments of the present invention, the inert atmosphere is argon or nitrogen. The role of the inert atmosphere is to avoid Fe 2+ oxidation.

[0044] In some preferred embodiments of the present invention, the mass ratio of the sum of the divalent iron source and the phosphorus source to the grinding aid is 1:1-4, and the mass ratio of the sum of the iron source and the phosphorus source to the grinding aid can also be 1:2 or 1:3.

[0045] In some preferred embodiments of the present invention, the stirring speed of the mechanical stirring mill is 300-600 rpm, and the grinding reaction time is 1-10 h; the stirring rate can also be selected as 350 rpm, 400 rpm, 450 rpm, 500 rpm, 550 rpm; the grinding reaction time can also be selected as 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h.

[0046] In some preferred embodiments of the present invention, the divalent iron source is one or more of ferrous sulfate, ferrous acetate, ferrous citrate, and ferrous sulfate hydrate; the phosphorus source is one or more of ammonium dihydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate; the grinding aid is one or more of ethylene glycol, polyethylene glycol, acetone, glycerol, diformate, or sulfolane; the function of the grinding aid is to reduce the surface energy of the particles, prevent agglomeration, and make the raw materials evenly dispersed.

[0047] In some preferred embodiments of the present invention, the calcination temperature is 200-400°C, and the time is 3-8 hours; the calcination temperature can also be selected as 250°C, 300°C, or 350°C; the calcination time can also be selected as 4 hours, 5 hours, 6 hours, or 7 hours.

[0048] The third aspect of the present invention provides a method for preparing a composite sodium iron phosphate positive electrode material using the above-mentioned FeHPO4 precursor.

[0049] Amorphous FeHPO4 powder, sodium salt, phosphate salt and carbon source are dispersed in a solvent, sand-milled and spray-dried, and then calcined at 500-600°C for 6-16 hours under an inert atmosphere to obtain a composite sodium iron phosphate positive electrode material.

[0050] In some preferred embodiments of the present invention, the sodium salt is one or more of sodium carbonate, sodium bicarbonate, sodium oxalate, sodium dihydrogen phosphate, and sodium phosphate; the phosphate salt is one or more of ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; the carbon source is one or more of glucose, sucrose, and starch;

[0051] In some preferred embodiments of the present invention, the molar ratio of Na:P:Fe provided by the sodium salt, FeHPO4 and phosphorus source is (4-4.05):(3.95-4.05):(2.9-3), and the mass of the carbon source is 1%-20% of the mass of FeHPO4; the molar ratio of Na:P:Fe provided by the sodium salt, FeHPO4 and phosphorus source can also be (4.04 / 4.08):(3.95 / 4 / 4.05):(2.9 / 2.95); the mass of the carbon source can also be selected as 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, or 18% of the mass of the FeHPO4 powder.

[0052] In some preferred embodiments of the present invention, the solvent is at least one of water and ethanol, and the total solid mass of the FeHPO4 powder, phosphorus source, sodium salt and carbon source accounts for 20%-40% of the total mass of the solid and the solvent; the total solid mass can also account for 25%, 30% or 35% of the total mass of the solid and the solvent.

[0053] When FeHPO4 is used as raw material for sintering to prepare composite sodium iron phosphate, taking disodium hydrogen phosphate and sodium carbonate as raw materials as an example, the reactions occurring in the process are as follows:

[0054] After sand milling and spray drying, soluble sodium salts and phosphorus sources are wrapped around the FeHPO4 particles.

[0055] At around 500℃, FeHPO4 undergoes a phase transition and pyrophosphate is generated in situ:

[0056] FeHPO4→Fe2P2O7+H2O (1)

[0057] During the sintering process, Na + Diffusion into the FeHPO4 and Fe2P2O7 particles, Fe 2+ Diffusion from the inside of FeHPO4 and Fe2P2O7 particles, PO4 3- and P2O7 4- Because the groups are large, diffusion can be ignored; the overall reaction is shown in (2):

[0058] 3FeHPO4+Na2HPO4+Na2CO3→Na4Fe3(PO4)2P2O7+CO2↑+H2O↑ (2)

[0059] The third aspect of the present invention provides a composite sodium iron phosphate positive electrode material prepared by the above-mentioned preparation method. The chemical formula of the composite sodium iron phosphate positive electrode material is Na4Fe3(PO4)2P2O7@C, wherein C is the outer carbon coating layer; the material has high purity, spherical secondary particles, high compaction density, and excellent discharge specific capacity and rate performance.

[0060] A fourth aspect of the present invention provides a sodium ion battery comprising the composite sodium iron phosphate positive electrode material as described above.

[0061] The preparation method and application of the composite sodium iron phosphate are further described below in conjunction with specific examples and comparative examples.

[0062] Example 1

[0063] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 121.35 g of ethylene glycol were weighed and placed in a mechanical stirred mill. The mixture was ground at 500 rpm for 4 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and dried, and calcined at 300°C for 6 h under a nitrogen atmosphere to obtain an amorphous ferric hydrogen phosphate precursor, which was designated as FHP-A1.

[0064] Example 2

[0065] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 121.35 g of ethylene glycol were weighed and placed in a mechanical stirred mill. The mixture was ground at 500 rpm for 4 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and dried, and calcined at 200°C for 8 h in a nitrogen atmosphere to obtain an amorphous ferric hydrogen phosphate precursor, which was designated as FHP-A2.

[0066] Example 3

[0067] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 121.35 g of ethylene glycol were weighed and placed in a mechanical stirred mill. The mixture was ground at 500 rpm for 4 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and dried, and calcined at 400°C for 3 h in a nitrogen atmosphere to obtain an amorphous ferric hydrogen phosphate precursor, which was designated as FHP-A3.

[0068] Example 4

[0069] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 80.9 g of acetone were weighed and placed in a mechanical stirred mill. The mixture was ground at 600 rpm for 1 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and dried, and calcined at 300°C for 6 h in a nitrogen atmosphere to obtain an amorphous ferric hydrogen phosphate precursor, which was designated as FHP-A4.

[0070] Example 5

[0071] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 161.8 g of propylene glycol were weighed and placed in a mechanical stirred mill. The mixture was ground at 300 rpm for 10 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and dried, and calcined at 300°C for 6 h under a nitrogen atmosphere to obtain an amorphous ferric hydrogen phosphate precursor, which was designated as FHP-A5.

[0072] Example 6

[0073] This example uses the method of Example 1 to prepare ferric hydrogen phosphate powder;

[0074] According to the molar ratio of Na:Fe:P=4.04:2.9:4, the mass of glucose is 10% of the mass of the ferric phosphate powder, and the ferric phosphate powder prepared in step 1), disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of the ferric phosphate powder, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the sample is calcined at 580°C for 10 hours under an argon atmosphere to obtain a composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-A1.

[0075] Example 7

[0076] This example uses the method of Example 1 to prepare ferric hydrogen phosphate powder;

[0077] According to the molar ratio of Na:Fe:P=4.04:2.9:4, the mass of glucose is 10% of the mass of the ferric phosphate powder, and the ferric phosphate powder prepared in step 1), disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of the ferric phosphate powder, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the sample is calcined at 500°C for 16 hours under an argon atmosphere to obtain a composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-A2.

[0078] Example 8

[0079] This example uses the method of Example 1 to prepare ferric hydrogen phosphate powder;

[0080] According to the molar ratio of Na:Fe:P=4.04:2.9:4, the mass of glucose is 10% of the mass of the ferric phosphate powder, and the ferric phosphate powder prepared in step 1), disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of the ferric phosphate powder, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the sample is calcined at 600°C for 6h under an argon atmosphere to obtain a composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-A3.

[0081] Comparative Example 1

[0082] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 121.35 g of deionized water were weighed and placed in a mechanical stirred mill. The mixture was ground at 500 rpm for 4 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and filtered, and calcined at 300 °C for 6 h in a nitrogen atmosphere to obtain a ferrous hydrogen phosphate precursor, which was designated as FHP-B2.

[0083] Comparative Example 2

[0084] 27.80 g of ferrous sulfate heptahydrate, 12.65 g of ammonium dihydrogen phosphate, and 121.35 g of ethylene glycol were weighed and placed in a mechanical stirred mill. The mixture was ground at 500 rpm for 4 h under a nitrogen atmosphere for sufficient reaction. The ground product was then washed and filtered, and calcined at 450°C for 6 h under a nitrogen atmosphere to obtain a precursor, which was designated as FHP-B3.

[0085] Comparative Example 3

[0086] In this comparative example, ferric phosphate was used as an iron source, and the NFPP@C material was prepared by a method of sand milling-spray drying-solid phase sintering, including the following steps:

[0087] According to the molar ratio of Na:Fe:P=4.04:2.9:4, the mass of glucose is 10% of the mass of iron phosphate, and iron phosphate, disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of iron phosphate, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of solids and solvent. After sand grinding and spray drying, the sample is calcined at 580°C for 10 hours under argon atmosphere to obtain the composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-B1.

[0088] Comparative Example 4

[0089] This comparative example adopts the method of comparative example 1 to prepare amorphous ferric hydrogen phosphate powder;

[0090] According to the molar ratio of Na:Fe:P=4.04:2.90:4, the mass of glucose is 10% of the mass of the ferric phosphate powder. The ferric phosphate powder prepared in step 1), disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of the ferric phosphate powder, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the sample is calcined at 580°C for 10 hours under an argon atmosphere to obtain a composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-B2.

[0091] Comparative Example 5

[0092] This comparative example adopts the method of Example 1 to prepare ferric hydrogen phosphate powder;

[0093] According to the molar ratio of Na:Fe:P = 4.04:2.9:4, the mass of glucose is 10% of the mass of the ferric phosphate powder. The ferric phosphate powder FHP-A1 prepared in step 1) of Example 1, disodium hydrogen phosphate, sodium carbonate, and glucose are dispersed in water so that the total solid mass of the ferric phosphate powder, disodium hydrogen phosphate, sodium carbonate, and glucose accounts for 30% of the total mass of the solids and solvent. After sand milling and spray drying, the sample is calcined at 650°C for 10 hours under an argon atmosphere to obtain a composite sodium iron phosphate positive electrode material NFPP@C sample, which is recorded as NFPP-B3.

[0094] Experimental Example 1

[0095] The precursors FHP-A1, FHP-A2, FHP-A3, FHP-A4, and FHP-A5 prepared in Examples 1-5 and the precursors FHP-B2 and FHP-B3 prepared in Comparative Examples 1-2 were subjected to X-ray diffraction (XRD) characterization analysis. The results are as follows: Figure 1 As shown;

[0096] from Figure 1 It can be seen that FHP-A1, FHP-A2, FHP-A3, FHP-A4, FHP-A5 prepared in Examples 1-5 and FHP-B2 prepared in Comparative Example 1 have no sharp diffraction peaks and have a broad peak centered at 15-35° 2θ, indicating that Examples 1-5 and Comparative Example 1 have successfully prepared amorphous ferric hydrogen phosphate precursors; FHP-B3 prepared in Comparative Example 2 has a distinct peak shape, which shows that a crystalline product is generated in Comparative Example 2, which is Fe2P2O7;

[0097] from Figure 1It can also be seen that with the increase of calcination temperature, the relative diffraction intensity of FHP-A2, FHP-A1, and FHP-A3 increases slightly gradually; from the XRD patterns of FHP-A1, FHP-A4, and FHP-A5, it can be seen that different grinding aids have little effect on the XRD peak shape;

[0098] Experimental Example 2

[0099] The composite sodium iron phosphate positive electrode material samples NFPP-A1, NFPP-A2, NFPP-A3 prepared in Examples 6-8 and the composite sodium iron phosphate positive electrode materials NFPP-B1, NFPP-B2, NFPP-B3 prepared in Comparative Examples 3-5 were subjected to XRD characterization analysis. The results are as follows: Figure 2 As shown;

[0100] from Figure 2 It can be seen that the characteristic diffraction peaks of NFPP-A1, NFPP-A2, NFPP-A3 and NFPP-B2 prepared in Examples 6-8 and Comparative Example 4 correspond to the standard Pn21a space group orthorhombic crystal structure of sodium iron phosphate pyrophosphate and the peak shape is symmetrical and sharp, indicating that Examples 6-8 and Comparative Example 4 have successfully prepared composite sodium iron phosphate positive electrode materials and the product phase purity is high; the diffraction peak corresponding to NFPP-B1 prepared in Comparative Example 3 has a relatively low relative intensity and the 32-33°2θ ​​diffraction peak corresponds to the NaFePO4 standard card, indicating that there are more NaFePO4 impurity phases in the product, further proving that the composite sodium iron phosphate positive electrode material is prepared using amorphous FeHPO4 as the iron source, and the product phase (NFPP) has higher purity; the NFPP-B3 prepared in Comparative Example 5 has a diffraction peak at 32-33°2θ ​​corresponding to the NaFePO4 standard card, which also indicates that there are more NaFePO4 impurity phases in the product of this comparative example.

[0101] from Figure 2 It can also be seen that with the decrease of sintering temperature, the relative diffraction intensities of NFPP-A3, NFPP-A1, and NFPP-A2 decrease in turn.

[0102] The XRD refinement of the composite sodium iron phosphate positive electrode material samples NFPP-A1, NFPP-A2, NFPP-A3 prepared in Examples 6-8 and the composite sodium iron phosphate positive electrode materials NFPP-B1, NFPP-B2, NFPP-B3 prepared in Comparative Examples 3-5 was performed to obtain the purity of the target phase (NFPP). The results are shown in Table 1 below. wp Less than 10%, indicating that the refinement results are reliable.

[0103] Table 1 Purity of NFPP in each sample

[0104] sample purity(%) Rwp (%) Example 6 96.2 2.68 Example 7 97.1 2.59 Example 8 91.6 2.96 Comparative Example 3 82.5 6.91 Comparative Example 4 93.8 3.62 Comparative Example 5 72.6 7.58

[0105] As can be seen from Table 1, the purity of the target phase (NFPP) in the composite sodium iron phosphate positive electrode materials prepared in Examples 6-8 and Comparative Example 4 is very high (>90%), and the purity of the target phase (NFPP) in the composite sodium iron phosphate positive electrode materials prepared in Comparative Examples 3 and 5 is greatly reduced; this shows that at a suitable sintering temperature, using amorphous FeHPO4 as a precursor can effectively inhibit the formation of the electrochemically inert phase NaFePO4 and improve the purity of the target phase (NFPP) in the product, while using iron phosphate as a precursor to prepare the composite sodium iron phosphate positive electrode material will generate an electrochemically inert impurity phase - sodium iron phosphate, which will greatly reduce the purity of the target phase (NFPP) in the product. The lower purity of Comparative Example 5 is because the calcination temperature is too high, and the NFPP in the product decomposes to generate NaFePO4.

[0106] Experimental Example 3

[0107] The FHP-A1, FHP-A2, FHP-A3, and FHP-A4 prepared in Examples 1-4 and the FHP-B2 and FHP-B3 prepared in Comparative Examples 1-2 were observed using a scanning electron microscope (SEM). Figure 3 As shown, Figure 3 a is the SEM image of FHP-A1, b is the SEM image of FHP-A2, c is the SEM image of FHP-A3, d is the SEM image of FHP-A4, e is the SEM image of FHP-B2, and f is the SEM image of FHP-B3;

[0108] The iron phosphate precursor was observed by scanning electron microscopy (SEM), and the results were as follows Figure 4 As shown;

[0109] from Figure 3 It can be seen that the FHP-A1, FHP-A2, FHP-A3, and FHP-A4 prepared in Examples 1-4 are all monodisperse submicron particles with a size of 100-500 nm. The primary particle surface is smooth and free of hard agglomerates, the morphology is regular and rounded, and the particle density is high. Due to the lower calcination temperature, the FHP-A2 particles are slightly smaller than the FHP-A1, and due to the higher sintering temperature, the FHP-A3 particles are slightly larger than the FHP-A1. The FHP-B2 particles prepared in Comparative Example 1 are submicron particles, the primary particles are irregular in shape and agglomerated severely, and the particle density is low, indicating that deionized water as a grinding aid has a significant effect on the morphology of ferric hydrogen phosphate. The FHP-B3 particles prepared in Comparative Example 2 are larger in size, at the micron level, because the calcination temperature is too high, causing the amorphous FeHPO4 to be converted into Fe2P2O7, and having a higher degree of crystallinity.

[0110] from Figure 4It can be seen that the iron phosphate precursor is an irregular particle assembled from flake structures and solid spheres, with a particle size ranging from several microns to more than ten microns, an uneven surface and agglomeration, and a low particle density.

[0111] Experimental Example 4

[0112] The composite sodium iron phosphate positive electrode material NFPP-A1 prepared in Example 1 and the composite sodium iron phosphate positive electrode material NFPP-B1 prepared in Comparative Example 3 were observed by scanning electron microscopy (SEM). Figure 5 and Figure 6 As shown;

[0113] from Figure 5 It can be seen that the NFPP-A1 material prepared in Example 1 has spherical secondary particles with good sphericity and high particle density, which is due to the regular and rounded morphology, high particle density and small particle size of the ferric hydrogen phosphate precursor;

[0114] from Figure 6 It can be seen that the NFPP-B1 material prepared in Comparative Example 3 has some particles with poor sphericity and some broken particles, which further indicates that the compacted density of the material is relatively low. This is because when iron phosphate is used as a precursor, its morphology is irregular and the density is low. The larger particles are difficult to fully dissolve / dissociate during sand milling and can only exist in the raw material in the form of nanocrystals of 200-400 nm.

[0115] Experimental Example 5

[0116] The composite sodium iron phosphate positive electrode material NFPP-A prepared in Examples 6-8 and the composite sodium iron phosphate positive electrode material NFPP-B prepared in Comparative Examples 3-5 were used as the positive electrode of sodium ion batteries to assemble CR2032 button-type half-cells, with the metal sodium sheet as the negative electrode. The batteries were then subjected to charge and discharge tests using a Xinwei system.

[0117] Preparation of button cells: The composite sodium iron phosphate positive electrode material, Super P and polyvinylidene fluoride (PVDF) were dispersed in N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a uniform slurry. The slurry was then coated on a carbon-containing aluminum foil current collector and dried in a vacuum oven at 80°C overnight. Circular electrodes with a diameter of 12 mm were cut out using a sheet puncher. Finally, CR2032 button cells were assembled in an argon-filled glove box using GF / D glass fiber filter paper as the separator, a metal sodium sheet as the negative electrode, and 1M NaPF6 / propylene carbonate (PC) / ethylene carbonate (EC) (volume ratio 1:1, containing 5% FEC) as the electrolyte.

[0118] 1. Test the battery at a constant current charge and discharge rate of 0.2C (1C = 129 mAh / g) and a voltage range of 1.5-3.8 V (vs. Na + / Na), the discharge specific capacity of the composite sodium iron phosphate cathode material was tested at 0.2C, and the results are shown in Table 2;

[0119] 2. The composite sodium iron phosphate positive electrode material NFPP-A prepared in Examples 6-8 and the composite sodium iron phosphate positive electrode material NFPP-B prepared in Comparative Examples 3-5 were tested for compaction density using a compaction tester. The results are shown in Table 2.

[0120] Table 2 Discharge specific capacity and compaction density at 0.2C obtained in the examples and comparative examples

[0121] sample 0.2C discharge specific capacity (mAh / g) Compacted density (g / cm3) Example 6 109.24 2.32 Example 7 102.21 2.03 Example 8 106.8 2.38 Comparative Example 3 91.69 1.90 Comparative Example 4 107.40 1.96 Comparative Example 5 96.40 2.26

[0122] As can be seen from Table 2, the composite sodium iron phosphate positive electrode materials prepared in Examples 6-8 all exhibited high discharge specific capacity and compaction density, the composite sodium iron phosphate positive electrode material prepared in Comparative Example 3 had significantly reduced performance, the composite sodium iron phosphate positive electrode material prepared in Comparative Example 4 had a high discharge specific capacity but a low compaction density, and the composite sodium iron phosphate positive electrode material prepared in Comparative Example 5 had a low discharge specific capacity but a high compaction density;

[0123] The above results show that at a suitable sintering temperature, the use of ferric hydrogen phosphate as a precursor to prepare a composite sodium iron phosphate positive electrode material can avoid the formation of impurity phases in the product, improve the purity of the target phase (NFPP), and thus obtain a positive electrode material with a higher discharge specific capacity; similarly, it further proves the decisive role of the precursor morphology on the compaction density. The ferric hydrogen phosphate precursor particles used in Examples 6-8 and Comparative Example 5 are submicron-sized and highly dense, so the prepared positive electrode material has a high compaction density; the ferric hydrogen phosphate precursor particles used in Comparative Example 4 are irregular in shape, severely agglomerated, and have low particle density, so the prepared positive electrode material has a low compaction density; the iron phosphate precursor used in Comparative Example 3 has an irregular morphology and low density, so the prepared positive electrode material has a low compaction density.

[0124] Experimental Example 6

[0125] Based on the above embodiments, a battery using the composite sodium iron phosphate positive electrode material NFPP-A1 prepared in Example 1 as the battery positive electrode is selected as test battery 1, and a battery using the composite sodium iron phosphate positive electrode material NFPP-B1 prepared in Comparative Example 1 as the battery positive electrode is selected as test battery 2 for electrochemical performance testing.

[0126] Performance testing and results

[0127] Test cells 1 and 2 were tested for constant current charge and discharge at a rate of 0.2C (1C = 129 mAh / g) and a voltage range of 1.5-3.8 V (vs. Na + / Na), the results are as follows Figure 6 As shown;

[0128] like Figure 7 As shown, the first discharge specific capacities of test cell 1 and test cell 2 at 0.2C are 109.24 mAh / g and 91.69 mAh / g, respectively. In addition, the first coulombic efficiency of test cell 1 (97%) is much greater than that of test cell 2 (91%) (first coulombic efficiency = first discharge specific capacity / first charge specific capacity). This is because the NFPP-A1 prepared using FeHPO4 precursor as the iron source in Example 1 has higher phase purity and better crystallinity than the NFPP-B1 prepared using iron phosphate as the iron source.

[0129] 2. Test battery 1 and test battery 2 were charged and discharged at rates of 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C (1C = 129mAh / g). The results are as follows: Figure 7 shown.

[0130] Depend on Figure 8 It can be seen that the discharge specific capacity of test battery 1 at rates of 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C are 109.24 mAh / g, 103.99 mAh / g, 101.53 mAh / g, 98.49 mAh / g, 93.85 mAh / g, 90.48 mAh / g, and 86.15 mAh / g, respectively; the discharge specific capacity of test battery 2 at rates of 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C are 92.98 mAh / g, 89.46 mAh / g, 86.98 mAh / g, 84.64 mAh / g, 82.27 mAh / g, 79.91 mAh / g, and 75.98 mAh / g, respectively.

[0131] It can be seen from the above test results that there is a certain gap between the discharge specific capacity of test battery 1 and test battery 2 at low rate and high rate, and the discharge specific capacity of test battery 1 is significantly better than that of test battery 2; this shows that the composite sodium iron phosphate positive electrode material prepared by the present invention has more excellent discharge specific capacity and rate performance, which is attributed to the fact that the composite sodium iron phosphate positive electrode material prepared by the present invention has higher purity and compaction density. The high purity ensures the maximum active material content and the minimum impurity hindrance, and the high compaction density effectively increases the active material load per unit volume and constructs an efficient, low-resistance electron / ion conduction network. The synergistic effect of the two ensures that the material can achieve more sufficient electrochemical reactions, faster charge transfer rates and smaller polarization losses during the charging and discharging process, thereby significantly improving its specific capacity and rate performance.

[0132] (3) Test battery 1 and test battery 2 were tested for cycle performance at a rate of 2C (1C = 129 mAh / g). The results are as follows: Figure 8 shown.

[0133] Depend on Figure 9 It can be seen that the test battery 1 still has a capacity retention rate of 99.1% after 300 cycles at a 2C rate, while the capacity retention rate of the test battery 2 is 96.5% after 300 cycles at a 2C rate; this is because the electrochemically inert phase of NaFePO4 in the NFPP-B1 synthesized in comparative example 1 hinders the transmission of ions and electrons, and the polarization accumulation during the cycle is serious, which accelerates the capacity decay; at the same time, the mismatch between the expansion / contraction behavior of NFPP and NaFePO4 phases will produce significant mechanical stress at the phase interface, which will lead to the breakage of particles and accelerate the capacity decay; this further illustrates that the composite sodium iron phosphate positive electrode material prepared in this application has significant cycle stability, providing a key material basis for long-life sodium ion batteries.

[0134] In summary, it should be noted that the above is only a preferred embodiment of the present invention and does not overly limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still make some simple deductions, replacements, or equivalent replacements of some of the technical features of the technical solutions described in the aforementioned embodiments without departing from the concept of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a composite sodium iron phosphate positive electrode material, characterized in that: The steps include: A divalent iron source and a phosphorus source in a molar ratio of 1:1-1.2, as well as a grinding aid, are placed in a mechanical stirring mill and subjected to a thorough grinding reaction under an inert atmosphere; the reaction product is washed and dried, and then calcined at a temperature of 200-400° C. for 3-8 hours in an inert atmosphere to prepare an amorphous ferric hydrogen phosphate precursor; wherein the grinding aid is one or more of ethylene glycol, polyethylene glycol, acetone, glycerol, diformate, or sulfolane; and the chemical formula of the amorphous ferric hydrogen phosphate precursor is FeHPO4; Sodium salt, phosphate salt, carbon source and amorphous ferric hydrogen phosphate precursor are dispersed in a solvent, sand-milled and spray-dried, and then calcined at 500-600 °C for 6-16 h under an inert atmosphere to obtain a composite sodium iron phosphate positive electrode material.

2. The method for preparing the composite sodium iron phosphate positive electrode material according to claim 1, wherein: The divalent iron source is one or more of ferrous sulfate, ferrous acetate, ferrous citrate, and ferrous sulfate hydrate; the phosphorus source is one or more of ammonium dihydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate; and the total mass ratio of the divalent iron source and the phosphorus source to the mass ratio of the grinding aid is 1:1-4.

3. The method for preparing the composite sodium iron phosphate positive electrode material according to claim 1, wherein: The inert atmosphere is argon or nitrogen; the stirring speed of the mechanical stirring mill is 300-600 rpm, and the grinding reaction time is 1-10 h.

4. The method for preparing the composite sodium iron phosphate positive electrode material according to claim 1, wherein: The sodium salt is one or more of sodium carbonate, sodium bicarbonate, sodium oxalate, sodium dihydrogen phosphate, and sodium phosphate; the phosphate salt is one or more of ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; and the carbon source is one or more of glucose, sucrose, and starch.

5. The method for preparing the composite sodium iron phosphate positive electrode material according to claim 1, wherein: The molar ratio of Na:P:Fe provided by the sodium salt, the amorphous ferric hydrogen phosphate precursor and the phosphate salt is (4-4.05):(3.95-4.05):(2.9-3), and the mass of the carbon source is 1%-20% of the mass of the amorphous ferric hydrogen phosphate precursor.

6. The method for preparing the composite sodium iron phosphate positive electrode material according to claim 1, wherein: The solvent is at least one of water and ethanol, and the total mass of the amorphous ferric hydrogen phosphate precursor, phosphate salt, sodium salt and carbon source solid accounts for 20%-40% of the total mass of the solid and the solvent.

7. A composite sodium iron phosphate positive electrode material, characterized in that: Prepared by the method according to any one of claims 1-6, the chemical formula of the composite sodium iron phosphate positive electrode material is Na4Fe3(PO4)2P2O7@C, wherein C is the outer carbon coating layer.

8. A sodium ion battery, characterized in that: The sodium ion battery comprises the composite sodium iron phosphate positive electrode material according to claim 7.

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