A method for preparing an NFPP / C sodium-ion battery cathode material by using double iron sources
By preparing NFPP/C materials using a dual iron source, the Fe2+ ratio is controlled through the combination of ferrous oxalate and a secondary iron source to form a carbon coating layer. This solves the problems of low conductivity and poor cycle performance of NFPP materials, achieving high specific capacity and long cycle stability, and is suitable for the large-scale production of cathode materials for sodium-ion batteries.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- 兴荣新源(厦门)科技有限公司
- Filing Date
- 2026-05-06
- Publication Date
- 2026-06-23
AI Technical Summary
In the existing technology, sodium iron phosphate pyrophosphate (NFPP) cathode materials have low intrinsic conductivity and poor cycle performance. Furthermore, the single iron source preparation method results in an excessively high Fe3+/Fe2+ ratio, which affects the charging capacity of sodium-ion batteries and makes it difficult to balance the specific capacity, rate performance and cycle stability of the material.
A dual-iron-source preparation method was adopted, using ferrous oxalate as the main iron source and other iron compounds as secondary iron sources. Carbon-coated NFPP/C materials were synthesized by high-temperature solid-phase reduction. By controlling the Fe2+ ratio and combining the addition of carbon source and reducing agent, an amorphous carbon coating layer was formed, thereby optimizing the material structure and electrochemical performance.
It significantly improves the specific capacity and rate performance of NFPP/C materials, has good cycle stability, and is suitable for large-scale production applications. The discharge specific capacity reaches 108 mAh/g at 1C rate, and the capacity retention rate is as high as 95% or more.
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Figure CN122267178A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of sodium-ion battery cathode material preparation technology, and in particular to a method for preparing NFPP / C sodium-ion battery cathode material using dual iron sources. Background Technology
[0002] Lithium-ion batteries are widely used in portable electronic devices, electric vehicles, and other fields due to their high energy density and long cycle life. However, the uneven distribution and limited reserves of lithium resources in the Earth's crust, along with high extraction costs, limit their application in large-scale energy storage. Sodium-ion batteries, on the other hand, are considered one of the most promising alternative technologies to lithium-ion batteries due to the abundance of sodium resources, low extraction costs, and a similar electrochemical mechanism.
[0003] Sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) is a polyanionic cathode material with advantages such as high theoretical capacity, high operating voltage of 3.1V, and low volume expansion rate of <4%, making it an important research direction for cathode materials in sodium-ion batteries. However, NFPP has inherent problems such as low intrinsic conductivity and poor cycle performance. Existing technologies mainly modify it by carbon coating and introducing crystal defects, but the modification effect is limited and it is difficult to simultaneously achieve the desired specific capacity, rate performance, and cycle stability. In addition, existing technologies for preparing NFPP mostly use single iron source synthesis, which cannot balance electrochemical performance and raw material cost. For example, the mainstream iron source, ferric nitrate nonahydrate, is prone to generating nitrides during sintering, polluting the environment, and the prepared material has poor capacity; the core reason is that the Fe in the material prepared by single iron source is low. 3+ / Fe 2+ The proportion is too high, while the charging capacity of sodium-ion batteries is increased by Na... + The migration quantity is determined by the conservation of electrons gained and lost, Na + Fe in cathode material during migration 2+ An oxidation reaction occurs, Fe 2+ The content of Na directly determines + The migration number directly affects the battery charging capacity. Therefore, it is necessary to develop a method that can effectively reduce Fe in NFPP materials. 3+ / Fe 2+ A suitable modification method that combines a specific ratio with simple preparation process and low cost has become an urgent need in the field of sodium-ion battery cathode materials. Summary of the Invention
[0004] The purpose of this invention is to provide a method for preparing NFPP / C sodium-ion battery cathode materials using a dual iron source. The method involves synthesizing carbon-coated NFPP / C via a high-temperature solid-state reduction process. The resulting NFPP / C exhibits high purity, good crystallinity, and high ionic and electronic conductivity, significantly improving the specific capacity and rate performance of the cathode material, making it suitable for large-scale production applications. This addresses the problems in the prior art.
[0005] To achieve the above objectives, this invention provides a method for preparing NFPP / C sodium-ion battery cathode materials using dual iron sources, comprising the following steps: (1) Raw material mixing and drying: Sodium source compound, iron source compound, phosphorus source compound, carbon source compound and reducing agent are mixed evenly and dried to obtain mixed raw materials; The iron source compound includes a primary iron source and a secondary iron source. The primary iron source is ferrous oxalate, and the secondary iron source is one or more of the following: ferrous citrate, ferrous citrate, ferrous nitrate, ferrous nitrate, ferrous sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II,III) oxide, ferric oxide, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate, and ferrous ammonium sulfate. (2) Segmented sintering: The mixed raw materials obtained in step (1) are placed in an inert atmosphere or a reducing atmosphere, and pre-sintered at 200-250℃ for 2-4h, and then heated to 350-600℃ and held for 4-20h to obtain NFPP / C sodium ion battery cathode material.
[0006] Preferably, in step (1), the ratio of sodium source compound, iron source compound and phosphorus source compound satisfies the following: the molar ratio of sodium to iron is 4:3 and the molar ratio of sodium to phosphorus is 1:1.
[0007] Preferably, in step (1), the molar ratio of carbon source compound to iron source compound is (0.1-5):1, and the molar ratio of reducing agent to iron source compound is (0.1-5):1.
[0008] Preferably, in step (1), the sodium source compound is one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.
[0009] Preferably, in step (1), the phosphorus source compound is one or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.
[0010] Preferably, in step (1), the carbon source compound and the reducing agent are one or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid monohydrate, malic acid, oxalic acid, adipic acid, citric acid, soluble starch, ascorbic acid, sucrose, and glucose.
[0011] Preferably, in step (1), the method of uniform mixing is either ball milling or aqueous solution dissolution.
[0012] Preferably, in step (2), the inert atmosphere is nitrogen or argon; the reducing atmosphere is a nitrogen-hydrogen mixture or an argon-hydrogen mixture, wherein the volume percentage of hydrogen is 5%.
[0013] Preferably, in step (2), the heating rate is 2-4℃ / min.
[0014] Preferably, in step (2), the mass ratio of Na4Fe3(PO4)2P2O7 to C in the prepared NFPP / C sodium-ion battery cathode material is 9.5:0.5-9.9:0.1.
[0015] This invention employs a dual iron source system with ferrous oxalate as the primary iron source and a single secondary iron source. The core principle is to utilize the characteristics of the primary and secondary iron sources to achieve Fe... 2+ It retains the dual benefits of preserving and sintering structural stability. Ferrous oxalate not only directly provides Fe... 2+ Participating in NFPP lattice synthesis increases the Fe content in the material. 2+ The ratio of [specific components] increases the discharge specific capacity, rate capability, and cycle performance of Na4Fe3(PO4)2P2O7 / C. Furthermore, the reducing gases such as CO produced during sintering decomposition create a localized reducing atmosphere, inhibiting Fe [reduction]. 2+ Oxidation to Fe 3+ Transformation to achieve Fe in NFPP lattice 2+ The secondary iron source effectively prevents excessive gas production during sintering by diluting the proportion of ferrous oxalate, allowing for slow and uniform gas release and fundamentally avoiding agglomeration and bulging issues. Simultaneously, the iron from the secondary iron source can supplement NFPP synthesis, ensuring a 4:3 molar ratio of total iron to sodium source without affecting the material's crystal lattice formation. This flexible ratio of primary and secondary iron sources achieves a balance between capacity enhancement and structural stability.
[0016] Furthermore, the addition of carbon source and reducing agent synergizes with the dual iron source system. The reducing agent consumes oxidizing substances in the system, enhancing the reducing power to provide secondary protection for Fe. 2+ The amorphous carbon coating layer formed by high-temperature carbonization of carbon source not only improves the intrinsically low electronic conductivity of NFPP, but also inhibits particle agglomeration and buffers lattice volume expansion, further optimizing the material structure and electrochemical performance.
[0017] To adapt to the reaction characteristics of the dual-iron source system, this invention optimizes the process through segmented sintering, atmosphere control, and precise molar ratio regulation, ensuring the synthesis of high-performance NFPP / C. Pre-sintering at 200-250℃ completes the dehydration of raw materials and the initial decomposition of organic components; slow heating avoids lattice defects caused by direct high temperatures. High-temperature sintering at 350-600℃ achieves NFPP crystallization and stable formation of the carbon coating layer; slow heating reduces lattice cracking caused by thermal stress. An inert or reducing sintering atmosphere isolates O2 from the air, preventing Fe... 2+ Oxidation and a reducing atmosphere can form a triple reduction protection; strict control of the stoichiometric ratio of sodium to iron (4:3) and sodium to phosphorus (1:1) avoids raw material waste and impurity phase formation, ensuring the high purity and crystallinity of NFPP.
[0018] Therefore, the method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source provided by the present invention has the following beneficial effects: (1) This invention breaks with convention by employing a "dual iron source" combination, with ferrous oxalate as the primary iron source and other iron compounds as secondary iron sources. The reducing gas released by the decomposition of ferrous oxalate during sintering can inhibit Fe 2+ To Fe 3+ The transformation, and it directly provides highly active Fe. 2+ This significantly increases the Fe content in the final product. 2+ The proportion of higher Fe 2+ The initial content directly determines how much Na the material can release during charging. + This significantly improves the discharge specific capacity of the material.
[0019] (2) The introduction of a secondary iron source effectively alleviated the problems of material agglomeration and finished product bulging caused by intense gas generation during the sintering process of ferrous oxalate alone, thus improving the stability of the process and the consistency of the product. Simultaneously, by adjusting the ratio of the primary and secondary iron sources, the Fe content in the material can be controlled. 3+ / Fe 2+ The ratio is used to optimize electrochemical performance.
[0020] (3) The NFPP / C composite material prepared by this invention has high specific capacity, excellent rate performance and long cycle stability. The data from the examples show that the coin cell prepared by the method of this invention can achieve a discharge specific capacity of 108 mAh / g after 500 cycles at 1C rate, and the capacity retention rate is as high as 95% or more.
[0021] (4) The preparation method of the present invention is simple, the operation is controllable, no additional complex synthesis equipment is required, and it is easy to realize large-scale production and application.
[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description
[0023] Figure 1 The XRD pattern is shown in Example 1. Figure 2 This is the XRD pattern for Comparative Example 1; Figure 3 This is a SEM image of Comparative Example 1 at a magnification of 5 μm; Figure 4 This is the SEM image of Comparative Example 1 at a magnification of 20 μm; Figure 5 Here is a SEM image of Example 1 at a magnification of 5 μm; Figure 6 This is a SEM image of Example 1 at a magnification of 20 μm; Figure 7 XPS image of Example 1; Figure 8 The XPS graph is for Comparison 1; Figure 9 This is a comparison chart of the rate performance of Example 1 and Comparative Example 1; Detailed Implementation
[0024] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.
[0025] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.
[0026] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.
[0027] Unless otherwise specified, the reagents, instruments, and equipment used in this invention are all commonly used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.
[0028] Example 1 This embodiment provides a method for preparing NFPP / C sodium-ion battery cathode materials using dual iron sources, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate is selected as the sodium source and phosphorus source, ferrous oxalate as the main iron source, ferric oxide as the secondary iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0029] 0.4 mol of sodium dihydrogen phosphate, 0.25 mol of ferrous oxalate, 0.025 mol of ferric oxide, and 0.4 mol of citric acid monohydrate were weighed out and mixed evenly in an aqueous solution. The mixture was placed in a water bath and stirred at 80°C for 4 hours to form a complex precursor. The complex precursor was then dried in a vacuum oven at 100°C for 4 hours to obtain a dry powder. The dried powder was then placed in a polytetrafluoroethylene ball mill jar, and zirconium dioxide ball milling beads were added. The mixture was ball milled at 600 r / min for 18 hours to obtain a mixed raw material.
[0030] (2) Segmented sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is raised to 200℃ at a heating rate of 3℃ / min for pre-sintering for 2h. The temperature is then raised to 550℃ at the same heating rate and held for sintering for 10h. After natural cooling, the NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 98.3:1.7.
[0031] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0032] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 108mAh / g, and the capacity retention rate reaches 95.8%.
[0033] Example 2 This embodiment provides a method for preparing NFPP / C sodium-ion battery cathode materials using dual iron sources, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate is selected as the sodium source and phosphorus source, ferrous oxalate as the main iron source, ferric oxide as the secondary iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0034] 0.4 mol of sodium dihydrogen phosphate, 0.29 mol of ferrous oxalate, 0.005 mol of ferric oxide, and 0.4 mol of citric acid monohydrate were weighed out and mixed evenly in an aqueous solution. The mixture was placed in a water bath and stirred at 80°C for 4 hours to form a complex precursor. The complex precursor was then dried in a vacuum oven at 100°C for 4 hours to obtain a dry powder. The dried powder was then placed in a polytetrafluoroethylene ball mill jar, and zirconium dioxide ball milling beads were added. The mixture was ball milled at 600 r / min for 18 hours to obtain a mixed raw material.
[0035] (2) Segmented sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is raised to 200℃ at a heating rate of 3℃ / min for pre-sintering for 2h. The temperature is then raised to 550℃ at the same heating rate and held for sintering for 10h. After natural cooling, the NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 98:2.
[0036] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0037] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 105mAh / g, and the capacity retention rate reaches 93.5%.
[0038] Example 3 This embodiment provides a method for preparing NFPP / C sodium-ion battery cathode materials using dual iron sources, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate is selected as the sodium source and phosphorus source, ferrous oxalate as the main iron source, ferric oxide as the secondary iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0039] 0.4 mol of sodium dihydrogen phosphate, 0.2 mol of ferrous oxalate, 0.05 mol of ferric oxide, and 0.4 mol of citric acid monohydrate were weighed out and mixed evenly in an aqueous solution. The mixture was placed in a water bath and stirred at 80°C for 4 hours to form a complex precursor. The complex precursor was then dried in a vacuum oven at 100°C for 4 hours to obtain a dry powder. The dried powder was then placed in a polytetrafluoroethylene ball mill jar, and zirconium dioxide ball milling beads were added. The mixture was ball milled at 600 r / min for 18 hours to obtain a mixed raw material.
[0040] (2) Segmented sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is raised to 200℃ at a heating rate of 3℃ / min for pre-sintering for 2h. The temperature is then raised to 550℃ at the same heating rate and held for sintering for 10h. After natural cooling, the NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 97.4:2.6.
[0041] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0042] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 105mAh / g, and the capacity retention rate reaches 90.1%.
[0043] Example 4 This embodiment provides a method for preparing NFPP / C sodium-ion battery cathode materials using dual iron sources, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate is selected as the sodium source and phosphorus source, ferrous oxalate as the main iron source, ferric oxide as the secondary iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0044] 0.4 mol of sodium dihydrogen phosphate, 0.25 mol of ferrous oxalate, 0.025 mol of ferric oxide, and 0.4 mol of citric acid monohydrate were weighed out and mixed evenly in an aqueous solution. The mixture was then placed in a water bath and stirred at 80°C for 4 hours to form a complex precursor. The complex precursor was then dried in a vacuum oven at 100°C for 4 hours to obtain a dry powder. The dried powder was then placed in a polytetrafluoroethylene ball mill jar, and zirconium dioxide ball milling beads were added. The mixture was then ball milled at 500 r / min for 12 hours to obtain a mixed raw material.
[0045] (2) Segmented sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is raised to 200℃ at a heating rate of 3℃ / min for pre-sintering for 2h. The temperature is then raised to 550℃ at the same heating rate and held for sintering for 10h. After natural cooling, the NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 98.3:1.7.
[0046] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0047] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 102mAh / g, and the capacity retention rate reaches 88%.
[0048] Comparative Example 1 This comparative example provides a method for preparing NFPP / C sodium-ion battery cathode material using a single iron source, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate was selected as the sodium and phosphorus source, ferrous oxalate as the iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0049] 0.4 mol of sodium dihydrogen phosphate, 0.3 mol of ferrous oxalate, and 0.4 mol of citric acid monohydrate were weighed and mixed evenly in an aqueous solution. The mixture was then placed in a water bath and stirred at 80°C for 4 hours to form a complex precursor. The complex precursor was then dried in a vacuum oven at 100°C for 4 hours to obtain a dry powder. The dried powder was then placed in a polytetrafluoroethylene ball mill jar, and zirconium dioxide ball milling beads were added. The mixture was then ball milled at 600 r / min for 18 hours to obtain a mixed raw material.
[0050] (2) Segmented sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is raised to 200℃ at a heating rate of 3℃ / min and held for 2h. The temperature is then raised to 550℃ at the same heating rate and held for sintering for 10h. After natural cooling, the NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 98.5:1.5.
[0051] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0052] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 95mAh / g, and the capacity retention rate is 85.8%.
[0053] Comparative Example 2 This comparative example provides a method for preparing NFPP / C sodium-ion battery cathode material using a single iron source, including the following steps: (1) Raw material mixing and drying: Sodium dihydrogen phosphate is selected as the sodium source and phosphorus source, ferric oxide as the iron source, and citric acid monohydrate as the carbon source and reducing agent.
[0054] Weigh out 0.4 mol of sodium dihydrogen phosphate, 0.3 mol of ferrous oxalate, and 0.4 mol of citric acid monohydrate, mix them thoroughly, and place them directly into a polytetrafluoroethylene ball mill jar. Add zirconium dioxide ball milling beads and ball mill at 600 r / min for 18 h to obtain the mixed raw material.
[0055] (2) Sintering: The mixed raw materials are placed in an atmosphere tube furnace and a hydrogen-argon mixed reducing atmosphere (hydrogen volume ratio of 5%) is introduced. The temperature is directly raised to 550℃ at a heating rate of 3℃ / min and held for sintering for 10h. After natural cooling, NFPP / C sodium-ion battery cathode material is obtained, in which the mass ratio of Na4Fe3(PO4)2P2O7 to C is 98.5:1.5.
[0056] The NFPP / C material prepared in this embodiment was used as the positive electrode material and assembled with a sodium sheet to form a coin cell sodium-ion battery. The entire battery assembly process was completed in an argon glove box with a water and oxygen content of 0.08 ppm. First, the NFPP / C active material, conductive agent Super P, PVDF and NMP were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil and subjected to gradient vacuum drying and stamping to obtain a positive electrode sheet with a diameter of 12 mm (active material loading of 1.5 mg / cm³). 2 Then, using the CR2032 negative electrode shell as a base, the gasket, sodium sheet, and glass fiber separator Whatman GF / D wetted with electrolyte are stacked in sequence. Sufficient electrolyte (1mol / L NaPF6 in EC / DEC=3 / 7+5%FEC) is added dropwise. Then, the NFPP / C positive electrode sheet, gasket, and positive electrode shell are stacked in sequence. The battery is sealed under a pressure of 12MPa. After sealing, the battery is left to stand for 18 hours. After the electrolyte has fully wetted each component, standard electrochemical performance tests are performed.
[0057] Test results show that after 500 cycles at 1C rate, the battery's discharge specific capacity reaches 90mAh / g, and the capacity retention rate is 82.6%.
[0058] Depend on Figure 1 and Figure 2 The comparison shows that the XRD pattern of Example 1 has a higher peak intensity, indicating that it has better crystallinity.
[0059] Depend on Figure 3 and Figure 4 It can be seen that the product of Comparative Example 1 has an irregular blocky structure. Figure 5 and Figure 6 As can be seen, the product morphology of Example 1 is a regular porous spherical structure. This multi-level rough microstructure not only improves the wettability of the electrolyte but also expands the electrode-electrolyte contact interface. Furthermore, the small size of the spherical particles can effectively shorten the Na... + This improves the transport path and enhances ion diffusion dynamics.
[0060] according to Figure 7 and Figure 8 Integrating it yields the Fe obtained in Example 1. 2+ / Fe 3+ The peak area ratio is greater than that of Fe in Comparative Example 1. 2+ / Fe 3+ The peak area ratio indicates the Fe content in the material of Example 1. 2+ The content is relatively high, with a relatively high Fe content. 2+ The content of Na causes the battery to have higher Na content during charging and discharging. + There is sufficient Fe during the migration process. 2+ To provide sufficient energy for oxidation, thus enabling Na to... + Completely migrated to the negative electrode.
[0061] Depend on Figure 9 It can be seen that the capacity of Example 1 remains good even at high magnification.
[0062] The test results of the above embodiments and comparative examples show that the dual iron source preparation method of the present invention can significantly improve the electrochemical performance of NFPP / C materials and solve the problems of low capacity, structural defects and poor cycle stability in the preparation of single iron sources.
[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.
Claims
1. A method for preparing NFPP / C sodium-ion battery cathode material using dual iron sources, characterized in that, Includes the following steps: (1) Raw material mixing and drying: Sodium source compound, iron source compound, phosphorus source compound, carbon source compound and reducing agent are mixed evenly and dried to obtain mixed raw materials; The iron source compound includes a primary iron source and a secondary iron source. The primary iron source is ferrous oxalate, and the secondary iron source is one or more of the following: ferrous citrate, ferrous citrate, ferrous nitrate, ferrous nitrate, ferrous sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II,III) oxide, ferric oxide, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate, and ferrous ammonium sulfate. (2) Segmented sintering: The mixed raw materials obtained in step (1) are placed in an inert atmosphere or a reducing atmosphere, and pre-sintered at 200-250℃ for 2-4h, and then heated to 350-600℃ and held for 4-20h to obtain NFPP / C sodium ion battery cathode material.
2. The method for preparing NFPP / C sodium-ion battery cathode material using dual iron sources according to claim 1, characterized in that, In step (1), the ratio of sodium source compound, iron source compound and phosphorus source compound satisfies the following: the molar ratio of sodium to iron is 4:3 and the molar ratio of sodium to phosphorus is 1:
1.
3. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (1), the molar ratio of carbon source compound to iron source compound is (0.1-5):1, and the molar ratio of reducing agent to iron source compound is (0.1-5):
1.
4. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (1), the sodium source compound is one or more of the following: sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.
5. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (1), the phosphorus source compound is one or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.
6. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (1), the carbon source compound and the reducing agent are one or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid monohydrate, malic acid, oxalic acid, adipic acid, citric acid, soluble starch, ascorbic acid, sucrose, and glucose.
7. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (1), the method of uniform mixing is either ball milling or aqueous solution dissolution.
8. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (2), the inert atmosphere is nitrogen or argon; the reducing atmosphere is a nitrogen-hydrogen mixture or an argon-hydrogen mixture, wherein the volume percentage of hydrogen is 5%.
9. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (2), the heating rate is 2-4℃ / min.
10. The method for preparing NFPP / C sodium-ion battery cathode material using a dual iron source according to claim 1, characterized in that, In step (2), the mass ratio of Na4Fe3(PO4)2P2O7 to C in the prepared NFPP / C sodium-ion battery cathode material is 9.5:0.5-9.9:0.1.