Composite sodium ferric phosphate, precursor thereof, preparation method and battery

Abstract

The invention belongs to the technical field of battery materials, and discloses a composite sodium ferric phosphate material and a precursor and a preparation method thereof. The chemical expression of the composite sodium ferric phosphate material is Na4Fex( PO4) 2 (P2O7) / C, wherein x is more than or equal to 2.92 and less than or equal to 3; the crystallinity C0 of the composite sodium ferric phosphate material is greater than or equal to 3.895. A specific iron source (ammonium ferrous phosphate, ferric oxalate and ferric citrate) is adopted to prepare a precursor, a carbon source is not added in the process of preparing the precursor, the carbon source and a sodium source are added in the process of preparing the composite sodium ferric phosphate material from the precursor, sintering is performed, and the composite sodium ferric phosphate material with high compaction density is obtained. Meanwhile, the composite sodium ferric phosphate material has good electrochemical performance. The invention further discloses a battery comprising the composite sodium ferric phosphate material.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to composite sodium iron phosphate and its precursor. Background Technology

[0002] Sodium-ion batteries are rocking-chair batteries, a type of secondary battery that relies on the back-and-forth insertion and extraction of ions between the positive and negative electrodes. Research on sodium-ion batteries began in 1982 and has made significant progress in recent years, making them ready for commercial application. Currently, the main cathode materials for sodium-ion batteries include oxides, Prussian blue materials, and polyanionic materials.

[0003] Composite sodium iron phosphate (Na4Fe3(PO4)2(P2O7)) is a polyanionic cathode material for sodium-ion batteries. This material possesses structurally stable sodium-ion diffusion channels and good cycle performance, with moderate operating voltage and specific capacity, making it a cost-effective and promising cathode material for sodium-ion batteries. Currently, the main preparation process for this type of polyanionic sodium-ion battery cathode material employs solid-state ball milling. This involves mixing and ball milling water-insoluble metal salts, sodium salts, phosphorus sources, and carbon sources (glucose, citric acid, sucrose, etc.), followed by spray drying to obtain precursor powder, which is then calcined to obtain the final product. This method is a conventional solid-state one-time sintering process. The composite sodium iron phosphate material prepared by this method has a small primary particle size and low compaction density. However, existing conventional processes that increase particle size to improve compaction density often lead to a decrease in electrochemical performance, failing to balance electrochemical performance and compaction density. Summary of the Invention

[0004] To address the problems existing in the prior art, the first objective of this invention is to provide a highly crystalline composite sodium iron phosphate material and its preparation method. The second objective of this invention is to provide a precursor for the highly crystalline composite sodium iron phosphate material and its preparation method. These are intended to solve the above problems and obtain a high-reversible specific capacity, excellent cycling performance, and high-compactness composite sodium iron phosphate material.

[0005] To achieve the above objectives, the present invention provides the following specific technical solutions.

[0006] First, this invention provides a composite sodium iron phosphate material, the chemical formula of which is Na₄Fe₂O₃. x (PO4)2(P2O7) / C, where 2.92≤x≤3; the crystallinity of the composite sodium iron phosphate material is C0≥3.895; in the XRD diffraction pattern, Intensity is the diffraction peak intensity of the main phase composite sodium iron phosphate, that is, the diffraction peak intensity at 2θ=33.5±0.1°, FWHM is the full width at half maximum of the main phase, and C0=lg(Intensity / FWHM).

[0007] In a further preferred embodiment, the compacted density of the composite sodium iron phosphate material is 2.08~2.15 g / cm³. 3 .

[0008] In a further preferred embodiment, the primary particle size of the composite sodium iron phosphate material is 300~500 nm.

[0009] Secondly, this invention provides a method for preparing a precursor of a composite sodium iron phosphate material, comprising the following steps: (1) After mixing the iron source, phosphorus source and sodium source according to the stoichiometric ratio, the mixture is milled to obtain slurry I; the iron source is one or more of ferrous ammonium phosphate, ferrous oxalate and ferric citrate; (2) After drying, slurry I is sintered in an oxygen-containing atmosphere to obtain the precursor.

[0010] In a further preferred embodiment, the phosphorus source in step (1) is one or more of sodium pyrophosphate, phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0011] In a further preferred embodiment, the sodium source in step (1) is one or more of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, and sodium acetate.

[0012] In a further preferred embodiment, in step (1), the molar ratio of iron, phosphorus and sodium in slurry I is 1:x:y, where 1.334≤x≤1.375 and 0.667≤y≤0.687.

[0013] In a further preferred embodiment, in step (2), the sintering temperature is 200~300℃; the sintering time is 4~8h; and the oxygen-containing atmosphere is an air atmosphere.

[0014] Based on the same inventive concept, this invention seeks protection for the precursor of the composite sodium iron phosphate material prepared by the above preparation method.

[0015] Further preferably, the precursor has the general chemical formula Na. m FeP n O t Wherein, 0.646≤m≤0.694, 1.33≤n≤1.37, m+3+5n=2t; the precursor is an amorphous phase material with characteristic peaks at 2θ=28.8°~30.2°.

[0016] This invention also provides a method for preparing a composite sodium iron phosphate material, comprising the following steps: Step S1: Mix the aforementioned precursor, sodium source, and carbon source and then mill them to obtain slurry II; In step S2, after the slurry II is dried, it is sintered under an inert atmosphere to obtain a composite sodium iron phosphate material.

[0017] In a further preferred embodiment, in step S1, the sodium source is phosphorus-free. The sodium source is further preferably one or more of sodium carbonate, sodium bicarbonate, sodium acetate, and sodium citrate.

[0018] In a further preferred embodiment, in step S1, the carbon source is one or more of glucose, sucrose, starch, and polyethylene glycol.

[0019] In a further preferred embodiment, in step S1, the molar ratio of iron, phosphorus, and sodium in the slurry II is 1:m:n, where 1.334≤m≤1.375 and 1.334≤n≤1.375.

[0020] In a further preferred embodiment, in step S1, the amount of carbon source added is 8% to 15% of the total mass of the iron source, phosphorus source, sodium source added in step (1) to prepare the precursor and the sodium source added in step S1.

[0021] In a further preferred embodiment, in step S2, the sintering temperature is 500~550℃; and the sintering time is 8~12h.

[0022] In a further preferred embodiment, the inert atmosphere is one of nitrogen, argon, or a mixture of hydrogen and argon.

[0023] In a further preferred embodiment, the sand milling can be replaced by vibratory milling, rolling milling, or ball milling.

[0024] Based on the same inventive concept, the present invention provides the application of the above-mentioned composite sodium iron phosphate material in batteries.

[0025] Compared with the prior art, one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: (1) The present invention provides and can prepare a composite sodium iron phosphate material with crystallinity C0≥3.895.

[0026] (2) Compared with the conventional solid-state one-time sintering process, the compaction density of the composite sodium iron phosphate material prepared by this invention is significantly improved. With conventional solid-state one-time sintering, the compaction density of the composite sodium iron phosphate material is mostly between 1.85 and 1.9 g / cm³, depending on the raw materials and sintering temperature. 3 The compacted density of the composite sodium iron phosphate material prepared by this invention can reach 2.08~2.15 g / cm³. 3 .

[0027] (3) The composite sodium iron phosphate material prepared in this invention is assembled with a sodium metal sheet to form a test battery. When charged and discharged at a rate of 0.1C, it has high reversible specific capacity and excellent cycle performance.

[0028] (4) This invention clarifies the structural characteristics of the composite sodium iron phosphate precursor by selecting a specific iron source, providing a theoretical basis and practical experience for improving the performance of composite sodium iron phosphate materials.

[0029] (5) The preparation method of the present invention is simple and easy to operate, uses simple raw materials, and is easy to carry out on a large scale for industrial production. Attached Figure Description Figure 1 The figures show the XRD patterns of precursors prepared in some of the embodiments and comparative examples. In the figures, (a), (b), and (c) are the XRD patterns of precursors prepared in Example 1, Comparative Example 4, and Comparative Example 5, respectively.

[0030] Figure 2 XRD pattern of the precursor prepared in Example 4.

[0031] Figure 3 is the XRD pattern of the precursor prepared in Example 5.

[0032] Figure 4 shows SEM images of the precursors prepared in some of the embodiments and comparative examples. In the figure, (a), (b), (c), (d), and (e) are SEM images of the precursors in Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5, respectively.

[0033] Figure 5 The images show SEM images of the precursors prepared in some of the embodiments. In the images, (a), (b), (c), and (d) are SEM images of the precursors in Embodiments 2, 3, 4, and 5, respectively.

[0034] Figure 6 The images show the XRD patterns of the composite sodium iron phosphate materials obtained in Examples 1-5.

[0035] Figure 7 The XRD patterns are of the composite sodium iron phosphate materials obtained in Comparative Examples 1-5.

[0036] Figure 8 The images show SEM images of the composite sodium iron phosphate materials prepared in some of the examples and comparative examples. In the images, (a), (b), (c), (d), (e), and (f) are the SEM images of the composite sodium iron phosphate materials prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5, respectively.

[0037] Figure 9SEM images of the composite sodium iron phosphate materials prepared in some of the embodiments are shown in Figures (a), (b), (c), and (d), which are the SEM images of the composite sodium iron phosphate materials prepared in Examples 2, 3, 4, and 5, respectively.

[0038] Figures 10-19 The following are the electrical performance curves of batteries assembled from the composite sodium iron phosphate materials prepared in Examples 1, 2, 3, 4, 5, 1, 2, 3, 4, and 5, respectively. In this example, (a) represents the charge-discharge curve and (b) represents the cycle curve. Detailed Implementation

[0039] In a first aspect, some embodiments of the present invention provide a composite sodium iron phosphate material, the chemical formula of which is Na₄Fe₂O₃. x (PO4)2(P2O7) / C, where 2.92≤x≤3; the crystallinity of the composite sodium iron phosphate material is C0≥3.895; in the XRD diffraction pattern, Intensity is the diffraction peak intensity of the main phase composite sodium iron phosphate, that is, the diffraction peak intensity at 2θ=33.5±0.1°, FWHM is the full width at half maximum of the main phase, and C0=lg(Intensity / FWHM).

[0040] In some preferred embodiments, the compacted density of the composite sodium iron phosphate material is 2.08~2.15 g / cm³. 3 .

[0041] In some preferred embodiments, the primary particle size of the composite sodium iron phosphate material is 300~500nm.

[0042] Higher particle crystallinity indicates a more compact and complete crystal structure, with a higher degree of regularity in the arrangement of internal particles, fewer defects inside the crystal, lower resistance to ion migration within the material during electrochemical reactions, superior electrochemical performance, and increased compaction density.

[0043] Highly crystallizable crystals exhibit strong and sharp diffraction peaks when characterized by X-ray diffraction, while poorly crystallizable particles show broad and diffuse diffraction peaks. Therefore, the larger the ratio of the intensity of the main phase diffraction peak to the full width at half maximum (FWHM) of the main phase, the better the crystallinity of the crystal. Crystallinity is defined as C0 = log(Intensity / FWHM). Through extensive experiments, this application has found that when C0 ≥ 3.895, the prepared material exhibits good crystallinity.

[0044] Secondly, to obtain a highly crystalline composite sodium iron phosphate material, some embodiments first provide a method for preparing a precursor material for the highly crystalline composite sodium iron phosphate material, including: (1) After mixing the iron source, phosphorus source and sodium source according to the stoichiometric ratio, the mixture is milled to obtain slurry I; the iron source is one or more of ferrous ammonium phosphate, ferrous oxalate and ferric citrate; (2) After drying, slurry I is sintered in an oxygen-containing atmosphere to obtain the precursor.

[0045] In the preparation of the precursor, this invention selects specific iron sources—ferrous ammonium phosphate, ferrous oxalate, and ferric citrate. During the sintering process, these iron sources lose ammonium or organic anions, and the corresponding vacancies are filled by the exchange of substances sintered together. Therefore, it has a greater sintering driving force, can complete sintering at a lower temperature, and the exchange of substances is more complete. The prepared precursor, as detected by XRD, is an amorphous phase material with characteristic peaks in the range of 2θ = 28.8°~30.2°.

[0046] In some specific embodiments, elements other than iron, phosphorus, and sodium in the iron source, phosphorus source, and sodium source can be removed during the sintering process.

[0047] In some preferred embodiments, the phosphorus source in step (1) is one or more of sodium pyrophosphate, phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0048] In some preferred embodiments, the sodium source in step (1) is one or more of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, and sodium acetate.

[0049] In some preferred embodiments, in step (1), the molar ratio of iron, phosphorus, and sodium in slurry I is 1:x:y, where 1.334≤x≤1.375 and 0.667≤y≤0.687. Since the iron-phosphorus ratio has a significant impact on the material's performance, completing the iron-phosphorus ratio in the first step of the precursor preparation helps reduce the potential unevenness caused by introducing phosphorus during the subsequent sintering of the composite sodium iron phosphate material. However, it should be understood that the stoichiometric ratio of iron, phosphorus, and sodium sources according to chemical proportions is a standard practice for those skilled in the art in preparing the composite sodium iron phosphate material.

[0050] In some preferred embodiments, in step (2), the sintering temperature is 200~300℃; the sintering time is 4~8h; and the oxygen-containing atmosphere is an air atmosphere. The pre-sintering temperature is within this range to save energy while removing substances such as ammonia and carbon dioxide from the precursor, thereby reducing the negative impact on compaction density caused by gas volatilization during the subsequent sintering process of preparing the composite sodium iron phosphate material from the precursor, which results in more internal pores in the product.

[0051] Thirdly, some preferred embodiments of the present invention provide a precursor for the composite sodium iron phosphate material prepared by the above preparation method.

[0052] Further preferably, the precursor has the general chemical formula Na. m FeP n O t Wherein, 0.646≤m≤0.694, 1.33≤n≤1.37, m+3+5n=2t; the precursor is an amorphous phase material with characteristic peaks at 2θ=28.8°~30.2°.

[0053] Fourthly, some embodiments of the present invention provide a method for preparing a composite sodium iron phosphate material, comprising: Step S1: Mix the aforementioned precursor, sodium source, and carbon source and then mill them to obtain slurry II; In step S2, after the slurry II is dried, it is sintered under an inert atmosphere to obtain a composite sodium iron phosphate material.

[0054] Based on the precursor preparation method, it can be seen that the present invention generally adopts a two-stage sintering process to prepare composite sodium iron phosphate materials. In the initial precursor preparation, no carbon source is added, resulting in a more compact and larger-particle-sized specific precursor. In the subsequent sintering, a cathode material with even larger particles and higher compaction density is formed. The absence of a carbon source during the sintering treatment (pre-sintering) of the precursor preparation allows for particle growth without being restricted by the carbon layer, resulting in larger precursor "seeds." Simultaneously, the inventors discovered that the choice of iron source has a decisive influence on the properties of the precursor. Choosing ferrous ammonium phosphate, ferrous oxalate, or ferric citrate, the ammonium or organic anions in these iron sources are converted into gases and removed during the sintering process, providing a driving force for the insertion of other ions, thereby obtaining precursor seeds with uniform elemental distribution. Furthermore, after the precursor "seed" is broken down and thoroughly mixed with the second sodium and carbon sources, the regrowth of the crystal based on the precursor seed during sintering compensates for the original grain defects. This ultimately results in a more compact and stable crystal structure with fewer defects, higher crystallinity, and a larger primary particle size (300-500 nm), leading to a higher compaction density. The higher crystallinity contributes to the material's higher compaction density and better electrochemical performance.

[0055] In some preferred embodiments, the sodium source in step S1 is phosphorus-free. More preferably, it is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, and sodium citrate. If the phosphorus ratio in the precursor already meets the iron-phosphorus ratio of the final product, the sodium source in step S1 is phosphorus-free. If the iron-phosphorus ratio in the precursor prepared using other raw materials and proportions differs from that in the final product, a phosphorus-containing sodium source can be used in this step to adjust the final iron-phosphorus-sodium elemental ratio.

[0056] In some preferred embodiments, in step S1, the carbon source is one or more of glucose, sucrose, starch, and polyethylene glycol. Those skilled in the art should understand that other conventional carbon sources in the art are also within the selection range.

[0057] In a preferred embodiment, in step S1, the molar ratio of iron, phosphorus, and sodium in slurry II is 1:m:n, where 1.334 ≤ m ≤ 1.375 and 1.334 ≤ n ≤ 1.375. The molar ratio of iron, phosphorus, and sodium in slurry II is the ratio of iron, phosphorus, and sodium in the final product.

[0058] In some preferred embodiments, in step S1, the amount of carbon source added is 8% to 15% of the total mass of the iron source, phosphorus source, sodium source added during the preparation of the precursor, and the sodium source added in step S1. The amount of carbon source added ensures that there is a sufficient carbon layer coating on the particles to improve conductivity and guarantee the electrochemical performance of the material, while avoiding the negative effect on compaction density due to excessive carbon content causing an overly thick coating on the particle surface.

[0059] In some preferred embodiments, in step S2, the sintering temperature is 500~550℃; the sintering time is 8~12h. Temperatures below 500℃ will not form the target product, while temperatures too high will cause the target product to decompose and generate the electrochemically inert phase NaFePO4, which is detrimental to the final product performance.

[0060] In some preferred embodiments, the inert atmosphere is one of nitrogen, argon, or a mixture of hydrogen and argon.

[0061] In some preferred embodiments, the sand milling is replaced by a vibratory mill, roller mill, or ball mill, as long as it can ensure that the raw materials are fully and uniformly mixed.

[0062] Fifthly, in some embodiments of the present invention, the aforementioned composite sodium iron phosphate material is assembled into a battery, and the electrical performance of the battery is tested.

[0063] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0064] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0065] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0066] Example 1 (1) Weigh NH4FePO4, Na4P2O7, and Na2CO3 according to the stoichiometric ratio of 3:0.5:1; and weigh glucose at 10% of the total mass of NH4FePO4, Na4P2O7, and Na2CO3.

[0067] (2) Place the NH4FePO4 and Na4P2O7 weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 4 h; (3) Dry the slurry after sand milling in step (2), and then sinter it in air at 200°C for 4 hours to obtain the precursor Na. 0.66 FeP 1.33 O 5.16 ; (4) Add the Na2CO3 and glucose weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) Dry the slurry after sand milling, and then sinter it at 500°C for 10 h in a nitrogen atmosphere to obtain the composite sodium iron phosphate material Na4Fe3(PO4)2(P2O7) / C.

[0068] Comparative Example 1 (1) Weigh out NH4FePO4, Na2CO3, Na4P2O7 and glucose at a stoichiometric ratio of 3:1:0.5.

[0069] (2) Place the above-mentioned material in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0070] (3) After drying the slurry after sand milling, sinter it at 500°C for 10 h in an inert gas atmosphere to obtain the final material Na4Fe3(PO4)2(P2O7) / C.

[0071] Comparative Example 2 (1) Weigh NH4FePO4, Na4P2O7 and Na2CO3 according to the stoichiometric ratio of 3:0.5:1; and weigh glucose at 10% of the total mass of NH4FePO4 and Na4P2O7 and glucose at 10% of the mass of Na2CO3.

[0072] (2) Place the NH4FePO4, Na4P2O7 and glucose material weighed in step (1) at 10% of the total mass of NH4FePO4 and Na4P2O7 into a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h. (3) Dry the slurry after sand milling in step (2), and then sinter it in air at 200°C for 4 hours to obtain the precursor Na.0.66 FeP 1.33 O 5.16 / C; (4) Add the Na2CO3 weighed in step (1) and 10% of the mass of the weighed Na2CO3 glucose to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h. (5) Dry the slurry after sand milling, and then sinter it at 500°C for 10 h in a nitrogen atmosphere to obtain the composite sodium iron phosphate material Na4Fe3(PO4)2(P2O7) / C.

[0073] Comparative Example 3 (1) Weigh NH4FePO4, Na4P2O7, and Na2CO3 according to the stoichiometric ratio of 3:0.5:1; and weigh glucose at 10% of the total mass of NH4FePO4, Na4P2O7, and Na2CO3. (2) Place the NH4FePO4, Na4P2O7 and glucose weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 4 h; (3) Dry the slurry after sand milling in step (2), and then sinter it in air at 200°C for 4 hours to obtain the precursor Na. 0.66 FeP 1.33 O 5.16 / C; (4) Place the precursor obtained in step (3) and the Na2CO3 weighed in step (1) into a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) Dry the slurry after sand milling, and then sinter it at 500°C for 10 h in a nitrogen atmosphere to obtain the composite sodium iron phosphate material Na4Fe3(PO4)2(P2O7) / C.

[0074] Comparative Example 4 (1) Weigh FePO4, Na4P2O7, and Na2CO3 according to the stoichiometric ratio of 3:0.5:1; and weigh glucose at 10% of the total mass of FePO4, Na4P2O7, and Na2CO3. (2) Place the FePO4 and Na4P2O7 weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 4 h; (3) Dry the slurry after sand milling in step (2), and then sinter it in air at 200°C for 4 hours to obtain the precursor Na. 0.66 FeP 1.33 O 5.16 ; (4) Add the Na2CO3 and glucose weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) Dry the slurry after sand milling, and then sinter it at 500°C for 10 h in a nitrogen atmosphere to obtain the composite sodium iron phosphate material Na4Fe3(PO4)2(P2O7) / C.

[0075] Comparative Example 5 (1) Weigh Fe2O3, Na4P2O7, Na2CO3, and NH4H2PO4 according to the stoichiometric ratio of 1.5:0.5:1:3; and weigh glucose at 10% of the total mass of Fe2O3, Na4P2O7, Na2CO3, and NH4H2PO4. (2) Place the Fe2O3, Na4P2O7 and NH4H2PO4 weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 4 h; (3) Dry the slurry after sand milling in step (2), and then sinter it in air at 200°C for 4 hours to obtain the precursor Na. 0.66 FeP 1.33 O 5.16 ; (4) Add the Na2CO3 and glucose weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) Dry the slurry after sand milling, and then sinter it at 500°C for 10 h in a nitrogen atmosphere to obtain the composite sodium iron phosphate material Na4Fe3(PO4)2(P2O7) / C.

[0076] Example 2 (1) Weigh FeC2O4, Na4P2O7, NH4H2PO4, Na2CO3, and glucose, which together constitute 10% of the total mass of FeC2O4, Na4P2O7, NH4H2PO4, and Na2CO3, according to a stoichiometric ratio of 3:0.5:3:1.

[0077] (2) Place the FeC2O4, Na4P2O7 and NH4H2PO4 weighed in step (1) into a sand mill, add sand milling media (low boiling point solvents such as ethanol and acetone), and wet sand mill for 4 h.

[0078] (3) After drying the slurry after sand milling, sinter it in air at a heating rate of 5℃ / min to 300℃ for 4h to obtain the precursor Na. 0.66 FeP 1.33 O 5.16 .

[0079] (4) Place the precursor obtained in step (3) and the Na2CO3 and glucose weighed in step (1) into a sand mill, add sand milling media (low boiling point solvents such as ethanol and acetone), and wet sand mill for 4 h.

[0080] (5) After drying the slurry after sand milling, sinter it at 550°C for 10 h in an inert gas atmosphere to obtain Na4Fe3(PO4)2(P2O7) / C.

[0081] Example 3 (1) Weigh FeC6H5O7, H3PO4, Na2CO3, and CH3COONa according to the stoichiometric ratio of 1:1.37:0.347:0.676; and weigh polyethylene glycol at 15% of the total mass of the above substances. (2) Place the FeC6H5O7, H3PO4 and Na2CO3 weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 5 h; (3) Dry the slurry after sand milling in step (2), and then place it in an air atmosphere and heat it to 300°C at a heating rate of 5°C / min for 4 hours to obtain the precursor Na. 0.694 FeP 1.37 O 5.27 ; (4) Add CH3COONa and polyethylene glycol weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) The slurry after sand milling is dried, and then sintered at 520℃ for 8 hours in an argon atmosphere to obtain the composite sodium iron phosphate material Na4Fe. 2.92 (PO4)2(P2O7) / C.

[0082] Example 4 (1) Weigh NH4FePO4, NaH2PO4, CH3COONa, and C6H5Na3O7 according to the stoichiometric ratio of 1.0:0.36:0.32:0.227 respectively; and weigh starch at 10% of the total mass of the above substances. (2) Place the NH4FePO4, NaH2PO4 and CH3COONa weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 5 h; (3) Dry the slurry after sand milling in step (2), and then place it in an air atmosphere and heat it to 280°C at a heating rate of 5°C / min for 8 hours to obtain the precursor Na. 0.68 FeP 1.36 O 5.24 ; (4) Add the C6H5Na3O7 and starch weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) The slurry after sand milling is dried, and then sintered at 500℃ for 12h in an argon atmosphere to obtain the composite sodium iron phosphate material Na4Fe. 2.94 (PO4)2(P2O7) / C.

[0083] Example 5 (1) Weigh out NH4FePO4, Na2HPO4, NaH2PO4, and Na2CO3 in stoichiometric ratios of 1.0:0.3:0.046:0.35 respectively; and weigh out glucose at 10% of the total mass of the above substances. (2) Place the NH4FePO4, Na2HPO4 and NaH2PO4 weighed in step (1) into a sand mill, add sand milling media (ethanol and acetone), and wet sand mill for 5 h; (3) Dry the slurry after sand milling in step (2), and then place it in an air atmosphere and heat it to 280°C at a heating rate of 5°C / min for 8 hours to obtain the precursor Na. 0.646 FeP 1.346 O 5.19 ; (4) Add the Na2CO3 and glucose weighed in step (1) to the precursor obtained in step (3), place it in a sand mill, add sand milling media (ethanol, acetone), and wet sand mill for 4 h; (5) The slurry after sand milling is dried, and then sintered at 500℃ for 12h in an argon atmosphere to obtain the composite sodium iron phosphate material Na4Fe. 2.97 (PO4)2(P2O7) / C.

[0084] The precursors in Examples 1-5 and Comparative Examples 1-5 were characterized and analyzed.

[0085] Figure 1 Figures (a), (b), and (c) are the XRD patterns of the precursors prepared in Example 1, Comparative Example 4, and Comparative Example 5, respectively. As can be seen from the figures, the precursor in Example 1 is an amorphous phase material with a characteristic peak at 2θ = 29.8°; the precursor in Comparative Example 4 is a material containing certain crystals within an amorphous state; and the precursor in Comparative Example 5 is a special material resembling a solid solution, containing a crystalline phase within an amorphous phase.

[0086] Elemental analysis was performed on the precursors of Example 1, Comparative Example 4, and Comparative Example 5, and the results are shown in Table 1.

[0087] Table 1 Figure 2 The XRD pattern of the precursor in Example 4 shows that the precursor is an amorphous phase material with a characteristic peak at 2θ=30.2°.

[0088] Figure 3 The XRD pattern of the precursor in Example 5 shows that the precursor is an amorphous phase material with a characteristic peak at 2θ = 28.8°.

[0089] Figure 4 SEM images of the precursors of the composite sodium iron phosphate materials obtained in Example 1 and Comparative Examples 2-5.

[0090] Figure 5 The images show SEM images of the precursor materials for the composite sodium iron phosphate materials obtained in Examples 2-5.

[0091] X-ray diffraction analysis was performed on the composite sodium iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-5 to determine the intensity and half-width of the main phase diffraction peaks. The crystallinity C0 was calculated according to C0=log(Intensity / FWHM). Figure 6 The images show the XRD patterns of the composite sodium iron phosphate materials obtained in each embodiment. Figure 7 The XRD patterns of the composite sodium iron phosphate materials obtained in each comparative example are shown.

[0092] The compaction density of the composite sodium iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-5 was tested using a multi-channel powder resistance compaction density tester.

[0093] The results of crystallinity calculation and compaction density test are shown in Table 2.

[0094] Table 2 Figure 8 The images show SEM images of the composite sodium iron phosphate materials obtained in Example 1 and Comparative Examples 1-5. Figure 9 The images show SEM images of the composite sodium iron phosphate materials obtained in Examples 2-5.

[0095] The size of the primary particles of the composite sodium iron phosphate material was determined based on SEM images, and the results are shown in Table 3.

[0096] Table 3 Compared to Example 1, Comparative Example 1 involved solid-state single-stage sintering, while Comparative Examples 2 and 3 added a carbon source during the first ball milling. The addition of a carbon source during the first sintering or the initial sintering process may have restricted the growth of the precursor particles by coating them on the particle surface. Furthermore, the carbon layer encapsulated and agglomerated the particles, reducing the effectiveness of the second ball milling. This resulted in insufficient mass exchange during the second sintering process, limiting crystal regrowth and leading to a decrease in crystallinity. Comparative Example 4 used iron phosphate. Iron phosphate has a relatively stable crystal structure, but the uneven particle size distribution during sand milling resulted in a large range in the primary particle size distribution of the final product. Additionally, the iron phosphate crystal structure is close to that of the impure phase sodium iron phosphate, which may induce the formation of impure phases during sintering, thus reducing electrochemical performance. Comparative Example 5 used iron oxide as the iron source. Iron oxide itself has a loose and porous structure, resulting in smaller primary particle sizes after sintering. Moreover, its loose structure, retained in the precursor, negatively impacts the crystallinity of the final product.

[0097] The composite sodium iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-5 were assembled into batteries in the following manner: Preparation of positive electrode sheet: The composite sodium iron phosphate material prepared in each example and comparative example is used as the positive electrode material, the conductive agent is conductive carbon black and the binder is polyvinylidene fluoride. The positive electrode material, conductive agent and binder are mixed in a mass ratio of 90:5:5. After mixing, the mixture is dissolved in N-methylpyrrolidone solvent, ground into a slurry and coated onto aluminum foil. It is then dried at 90°C for 10 hours to obtain the positive electrode sheet.

[0098] Fabrication of coin cells: Sodium metal sheet is used as the negative electrode, glass fiber is used as the separator, the solute of the electrolyte is 0.1 mol / L sodium perchlorate, and the solvent is ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1. The positive electrode, separator, negative electrode, gasket, and spring are installed in the CR2023 coin cell in the following order: positive electrode, separator, negative electrode, gasket, and spring. 100 μL of electrolyte is injected and the cells are then encapsulated.

[0099] The charge-discharge curves and cycle curves of the battery were tested under conditions of 1.7V-3.7V, and the results are as follows: Figures 10-19 As shown in Table 4.

[0100] Table 4 A comparative analysis of the 0.1C discharge capacity and capacity retention of batteries using the composite sodium iron phosphate material of Example 1 and Comparative Examples 1-5 as the cathode material revealed the following: While conventional solid-state single-stage sintering cathode materials resulted in higher 0.1C discharge capacity, they also exhibited lower capacity retention. Adding carbon during precursor preparation further reduced the 0.1C discharge capacity of the resulting cathode material. Using other iron sources resulted in lower 0.1C discharge capacity and capacity retention. In summary, the composite sodium iron phosphate material prepared using the specific iron source and two-stage sintering process of this invention resulted in batteries with higher 0.1C discharge capacity and capacity retention.

[0101] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A composite sodium iron phosphate material, characterized in that, The chemical formula of the composite sodium iron phosphate material is Na4Fe x (PO4)2(P2O7) / C, wherein 2.92≤x≤3; the crystallinity Co of the composite sodium iron phosphate material is ≥3.895; in the XRD diffraction pattern, Intensity is the diffraction peak intensity of the main phase composite sodium iron phosphate, that is, the diffraction peak intensity at 2θ=33.5±0.1°, FWHM is the half peak width of the main phase, and Co=lg(Intensity / FWHM).

2. The composite iron sodium phosphate material of claim 1, wherein, The compacted density of the composite sodium iron phosphate material is 2.08-2.15 g / cm 3 .

3. The composite sodium iron phosphate material of claim 1 or 2, wherein, The primary particle size of the composite sodium iron phosphate material is 300-500 nm.

4. A method for preparing a precursor of a composite sodium iron phosphate material, characterized in that, The method comprises the following steps: (1) mixing an iron source, a phosphorus source and a sodium source according to a stoichiometric ratio, and then sand grinding to obtain slurry I; the iron source is one or more than two of ferrous ammonium phosphate, iron oxalate and iron citrate; (2) drying the slurry I, and then sintering in an oxygen-containing atmosphere to obtain a precursor.

5. The production method according to claim 4, wherein In step (1), the phosphorus source is one or more than two of sodium pyrophosphate, phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate; and the sodium source is one or more than two of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate and sodium acetate.

6. The production method according to claim 4 or 5, characterized by, In step (1), the molar ratio of iron, phosphorus and sodium in the slurry I is 1:x:y, wherein 1.334≤x≤1.375 and 0.667≤y≤0.

687.

7. The production method according to claim 4, wherein In step (2), the sintering temperature is 200-300°C; the sintering time is 4-8 hours; and the oxygen-containing atmosphere is an air atmosphere.

8. A precursor of a composite sodium iron phosphate material, characterized in that, The composite sodium iron phosphate material is prepared by the preparation method of any one of claims 4-7.

9. A precursor of a composite sodium iron phosphate material, characterized in that, The chemical general formula of the precursor is Na m FeP n O t Wherein, 0.646≤m≤0.694, 1.33≤n≤1.37, m+3+5n=2t; the precursor is an amorphous substance with a characteristic peak at 2θ=28.8°~30.2°.

10. A method of preparing a composite sodium iron phosphate material, characterized by, The method comprises the following steps: Step S1, mixing the precursor of claim 8 or 9, a sodium source and a carbon source, and then sand grinding to obtain slurry II; Step S2, drying the slurry II, and then sintering in an inert atmosphere to obtain a composite sodium iron phosphate material.

11. The production method according to claim 10, wherein In step S1, the sodium source is one or more than two of sodium carbonate, sodium bicarbonate, sodium acetate and sodium citrate; and in step S1, the carbon source is one or more than two of glucose, sucrose, starch and polyethylene glycol.

12. The production method according to claim 10 or 11, characterized by, In step S1, the molar ratio of iron, sodium and phosphorus in the slurry II is 1:m:n, wherein 1.334≤m≤1.375 and 1.334≤n≤1.

375.

13. The production method according to claim 10, wherein In step S1, the addition amount of the carbon source is 8%-15% of the total mass of the iron source, the phosphorus source and the sodium source added in step (1) for preparing the precursor and the sodium source added in step S1.

14. The production method according to claim 10, wherein In step (2), the sintering temperature is 500-550°C; and the sintering time is 8-12 hours.

15. The production method according to claim 10 or 14, wherein The inert atmosphere is one of nitrogen, argon and hydrogen-argon mixed gas.

16. The production method according to claim 10, wherein The sand grinding can be replaced by vibration grinding, rolling grinding or ball grinding.

17. A battery, characterized by The composite sodium iron phosphate material is prepared by the method of any one of claims 1-3 or the method of any one of claims 10-16.

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