Preparation method of polyanionic positive material na4fe3(po4)2p2o7 and sodium ion battery

Abstract

The application discloses a preparation method of a polyanionic positive electrode material Na4Fe3(PO4)2P2O7 and a sodium ion battery, and relates to the technical field of battery positive electrode material preparation.The application provides a new solid-phase synthesis method of the NFPP material based on a "nucleation induction-carbon coating synergistic regulation" strategy, through defect engineering design in the precursor stage and synergistic coating of multiple sources of carbon, while keeping process controllability and scalability, multiple goals such as material densification, uniform particle size, impurity phase inhibition and conductivity improvement are achieved, the bottleneck of the prior art is solved, and the performance improvement and engineering development of the core material of the sodium ion battery are promoted.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material preparation technology, and in particular to a method for preparing a polyanionic cathode material Na4Fe3(PO4)2P2O7 and a sodium-ion battery. Background Technology

[0002] With the transformation of the global energy structure and the advancement of carbon neutrality goals, the development of efficient, low-cost, and sustainable energy storage systems has become a research hotspot. Sodium-ion batteries, due to their abundant sodium resources, low cost, and compatibility with existing lithium-ion battery manufacturing systems, are considered one of the next-generation energy storage battery systems suitable for large-scale application.

[0003] In sodium-ion battery systems, the cathode material, as the key carrier for sodium-ion intercalation / deintercalation reactions, directly determines the battery's energy density, operating voltage, safety, and cycle stability. Currently, the most researched cathode materials include layered oxides (such as Na+). X MO2), Prussian blue compounds (such as NaFe[Fe(CN)6]), NASICON-type materials (such as Na3V2(PO4)3), and polyanionic materials (such as Na4Fe3(PO4)2P2O7, NFPP), etc. Among them, NFPP is a polyanionic structural material, belonging to the pyrophosphate-phosphate composite system, and has the following significant advantages: (1) High theoretical specific capacity: NFPP can provide a theoretical capacity of about 129 mAh·g-1 in the fully sodium state; (2) Stable working voltage platform: Its average working voltage is about 3.1 V, which takes into account both energy density and electrolyte stability; (3) Good thermal stability and high safety: The polyanionic skeleton structure has better stability than the layered structure, and has excellent thermal stability and intrinsic safety. (4) Iron-based system is rich in resources, non-toxic and environmentally friendly: Fe is a major valence transition metal, which is low in cost and environmentally friendly, and is suitable for large-scale development.

[0004] Despite the numerous theoretical advantages of NFPP, its practical application is still limited by the following factors: (1) Poor electronic conductivity and ion diffusion performance NFPP is a typical electrical insulator, and the Fe-OPO bridging path results in extremely low electron mobility, severely limiting its rate performance and fast charging capability. Furthermore, Na... + The diffusion path is constrained by the rigid framework, which also affects its kinetic behavior. Although conductivity can be improved by carbon coating, doping modification, and other methods, existing coating techniques often suffer from uneven carbon layer distribution and poor continuity.

[0005] (2) The material has low density and compaction density. For practical battery engineering applications, in addition to specific capacity per unit mass, energy density per unit volume is equally crucial. Most NFPP materials in existing literature are loose, porous powders, and their density after compaction is difficult to reach 2 g·cm³. -3 The above factors significantly limit its application potential in power batteries and energy storage power stations. Improving the bulk density of materials and the packing efficiency between particles is one of the core goals of materials engineering.

[0006] (3) Impurity phases are easily generated, and the synthesis window is narrow. NFPP is a multi-component phosphate system, and its synthesis process is sensitive to process parameters such as the Na / Fe / P ratio, reaction atmosphere, and heating rate. Slight deviations can easily lead to the formation of impurity phases (such as NaFePO4 and Na2FeP2O7), affecting product purity and electrochemical performance.

[0007] (4) Nucleation and particle size control are difficult. In conventional solid-state reactions, the formation of NFPP is severely hampered by nucleation barriers due to the low reactivity and slow lattice diffusion rate of FePO4. This results in a wide grain size distribution, severe agglomeration, and difficulty in forming a uniform structure. Disordered nucleation can also induce problems such as incomplete carbon coating, incomplete reaction, and a large number of impurity phases.

[0008] For example, patent CN 107069012 B, entitled "Hollow Spherical Na4Fe3(PO4)2P2O7 / C Composite Cathode Material and Preparation Method Thereof," discloses a method for preparing a hollow spherical NFPP / C composite, employing a surfactant-assisted, liquid-phase mixing followed by segmented calcination process. This method focuses on morphology control (hollow spherical shape).

[0009] For example, patent CN 118352495 A, entitled "Nitrogen-Doped Carbon-Coated Composite Na4Fe3(PO4)2P2O7 Sodium-Ion Battery Cathode Material and Its Preparation Method," discloses a method for preparing a nitrogen-doped carbon-coated composite NFPP cathode material. This method involves mixing, milling, and drying raw materials such as iron phosphate, sodium dihydrogen phosphate, and sodium carbonate, followed by pre-calcination, then adding a carbon source and ball milling, and finally sintering to obtain the product. It employs carbon coating and segmented sintering.

[0010] Therefore, there is an urgent need to develop a novel solid-phase reaction pathway that simultaneously possesses nucleation regulation mechanisms, carbon coating structure optimization, impurity phase suppression capabilities, and high-density formation mechanisms, in order to achieve comprehensive optimization of the structure and performance of NFPP, thereby promoting the practical application of this material in sodium-ion batteries. Summary of the Invention

[0011] In view of the shortcomings of the prior art, the present invention provides a new solid-phase synthesis method for NFPP materials based on the strategy of "nucleation induction-carbon coating synergistic regulation". Through defect engineering design in the precursor stage and synergistic coating of multi-source carbon, the method achieves multiple objectives such as material densification, particle size uniformity, impurity phase suppression and conductivity improvement while maintaining process controllability and scalability. This solves the bottleneck of the existing technology and promotes the performance improvement and engineering development of core materials for sodium-ion batteries.

[0012] This invention includes the following steps: (1) Preparation of precursor I: FePO4 and organic carbon source are mixed in a certain mass ratio, added to deionized water and stirred to form a mixed slurry, which is then milled to form a uniform suspension, and then spray dried to obtain precursor I; (2) Preparation of precursor II: Precursor I is sintered for the first time at 500~700℃ under an inert atmosphere (such as argon or nitrogen) and held for 2~4h to obtain FePO4 powder with surface defects, i.e. precursor II; (3) Preparation of precursor III: Precursor II is mixed with sodium source, supplementary phosphorus source and supplementary carbon source, and then spray-dried after being milled evenly to obtain precursor III. The molar ratio of sodium to phosphorus in precursor III is set to 1:1, and the molar ratio of phosphorus to iron is (1.36~1.4):1. The supplementary carbon source accounts for 2%~5% of the total mass of precursor III; (4) Sintering of NFPP material: Precursor III is subjected to two-stage temperature-controlled sintering treatment under an inert or weak reducing atmosphere. First, it is kept at 280~350℃ for 2~5 hours, and then kept at 480~600℃ for 8~20 hours to obtain the final NFPP cathode material.

[0013] Preferably, in step (1), the organic carbon source may be one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol or polyacrylonitrile.

[0014] Preferably, in step (1), the organic carbon source accounts for 5% to 10% of the mass of precursor I.

[0015] Preferably, in step (2), the first sintering is carried out in a nitrogen or argon atmosphere, with a temperature range of 500~700℃ and a holding time of 2~4 hours.

[0016] Preferably, in step (2), the carbon content of the precursor II is 1% to 2%.

[0017] Preferably, in step (3), the sodium source includes one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium nitrite, sodium phosphate, sodium pyrophosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0018] Preferably, in step (3), the supplementary phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

[0019] Preferably, in step (3), the molar ratio of sodium to phosphorus in precursor III is 1:1, and the molar ratio of phosphorus to iron is (1.36~1.4):1.

[0020] Preferably, in step (3), the supplementary carbon source is an organic supplementary carbon source or an inorganic supplementary carbon source. The organic carbon source is one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol, and polyacrylonitrile. The inorganic carbon source is one or more of carbon black, carbon nanotubes, and graphite.

[0021] Preferably, in step (3), the supplementary carbon source accounts for 2% to 5% of the total mass of precursor III.

[0022] Preferably, in step (4), the second sintering includes two heat preservation processes: the pre-sintering temperature is 280~350℃ and the time is 2~5h; the subsequent heat preservation temperature is 480~600℃ and the time is 8~20h, and the atmosphere is argon or nitrogen gas.

[0023] A sodium-ion battery, comprising a sodium-ion battery cathode material prepared by the aforementioned method for preparing the polyanionic cathode material Na4Fe3(PO4)2P2O7.

[0024] The present invention provides a method for preparing Na4Fe3(PO4)2P2O7, a nucleation-induced polyanionic cathode material. Through a "nucleation-induced followed by sodium formation" strategy, FePO4 is first guided to form a defect-rich structure, lowering the nucleation energy barrier and improving sodium formation uniformity. Simultaneously, the coating effect of multi-source carbon reduces the interface energy and improves electronic conductivity, ultimately yielding a Na4Fe3(PO4)2P2O7 cathode material with uniform particle size, stable structure, and high compaction density. Compared with existing technologies, the present invention has the following advantages: (1) Control the nucleation pathway and reduce the formation of impurity phases; (2) Construct a multi-scale carbon coating network to improve electrical conductivity; (3) The balance between nanoparticle size control and compaction density is beneficial to improving the volumetric energy density of the material; (4) The process is suitable for large-scale preparation and is safe and controllable.

[0025] Therefore, the preparation method provided by this invention offers a feasible technical route for achieving high performance of NFPP, and has important practical significance and industrial application prospects for the development of cathode materials for sodium-ion batteries. Attached Figure Description

[0026] Figure 1 The image shows the XRD pattern of the FePO4 / C material rich in surface defects prepared in Example 1 of this invention.

[0027] Figure 2 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Example 1 of this invention.

[0028] Figure 3 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Example 1 of this invention is shown at 0.1C.

[0029] Figure 4 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Example 2 of this invention.

[0030] Figure 5 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Example 2 of this invention is shown at 0.1C.

[0031] Figure 6 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Example 3 of this invention.

[0032] Figure 7 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Example 3 of this invention is shown at 0.1C.

[0033] Figure 8 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 1 of this invention.

[0034] Figure 9 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 1 of this invention is shown at 0.1C.

[0035] Figure 10 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 2 of this invention.

[0036] Figure 11 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 2 of this invention is shown at 0.1C.

[0037] Figure 12 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 3 of this invention.

[0038] Figure 13 The charge-discharge curve of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 3 of this invention is shown at 0.1C. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0040] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0042] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0043] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0044] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0045] This invention provides a method for preparing the nucleation-induced polyanionic cathode material Na4Fe3(PO4)2P2O7, comprising the following steps: (1) FePO4 and organic carbon source are mixed in a certain mass ratio, added to deionized water and stirred to form a mixed slurry. After sand milling, a uniform suspension is formed, and then the precursor I is obtained by spray drying. (2) Precursor I is sintered for the first time at 500~700℃ under an inert atmosphere (such as argon or nitrogen) and held for 2~4h to obtain FePO4 powder with surface defects, i.e., precursor II; (3) Precursor II is mixed with sodium source, supplementary phosphorus source and supplementary carbon source, and then spray-dried after being milled evenly to obtain precursor III. The molar ratio of sodium to phosphorus in precursor III is set to 1:1, the molar ratio of phosphorus to iron is (1.36~1.4):1, and the supplementary carbon source accounts for 2%~5% of the total mass of precursor III; (4) Precursor III is subjected to two-stage temperature-controlled sintering treatment under an inert or weak reducing atmosphere. First, it is kept at 280~350℃ for 2~5 hours, and then kept at 480~600℃ for 8~20 hours to obtain the final NFPP cathode material.

[0046] Among them, the aforementioned steps (1) and (2) serve as the nucleation induction stage, used to prepare a highly reactive precursor, thereby creating favorable nucleation sites for the generation of NFPP.

[0047] The aforementioned steps (3) and (4) serve as the sodium-based reaction stage, used to mix the prepared active precursor with sodium and phosphorus sources, and complete the final crystal synthesis under controlled conditions.

[0048] In this invention, in step (1), the organic carbon source may be one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol or polyacrylonitrile.

[0049] In this invention, in step (1), the organic carbon source accounts for 5% to 10% of the mass of precursor I.

[0050] In step (1), spray drying instantly dries the uniform suspension into microsphere powder. During this process, the organic carbon source is uniformly coated around the FePO4 particles, forming precursor I. This structure ensures that carbon can uniformly form a coating layer on the FePO4 surface during the next heat treatment.

[0051] In this invention, in step (2), the first sintering is carried out under a nitrogen or argon atmosphere, with a temperature range of 500~700℃ and a holding time of 2~4 hours.

[0052] In this invention, in step (2), the carbon content of the precursor II is 1% to 2%.

[0053] In step (2), precursor I is sintered for the first time at 500~700℃ in an inert atmosphere (such as nitrogen) to obtain FePO4 powder rich in surface defects (precursor II).

[0054] In this step, during the heat treatment, the organic carbon source coated on the FePO4 surface undergoes carbonization and decomposition. This process disturbs the crystal structure of the FePO4 surface, introducing a large number of surface defects. These defects become "hot spots" for preferential nucleation of NFPP crystals in the subsequent sodiumization reaction, significantly reducing the nucleation energy barrier and guiding the uniform and orderly formation of NFPP crystals.

[0055] Moreover, after the organic carbon source is carbonized, a dense carbon coating layer (carbon content controlled at 1%-2%) is formed on the surface of FePO4 particles. This carbon layer not only improves conductivity, but more importantly, it reduces the surface energy of FePO4, making it more stable when mixed with sodium and phosphorus sources in the future, reducing agglomeration, and facilitating the uniform adsorption and distribution of sodium and phosphorus components on its surface.

[0056] Therefore, in step (2), the purpose of creating surface defects and forming a carbon protective layer that reduces surface energy is achieved.

[0057] In this invention, in step (3), the sodium source includes one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium nitrite, sodium phosphate, sodium pyrophosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0058] In this invention, in step (3), the supplementary phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

[0059] In this invention, in step (3), the molar ratio of sodium to phosphorus in the precursor III is 1:1, and the molar ratio of phosphorus to iron is (1.36~1.4):1.

[0060] In this invention, in step (3), the supplementary carbon source is an organic supplementary carbon source or an inorganic supplementary carbon source. The organic carbon source is one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol, and polyacrylonitrile. The inorganic carbon source is one or more of carbon black, carbon nanotubes, and graphite.

[0061] In this invention, in step (3), the supplementary carbon source accounts for 2% to 5% of the total mass of precursor III.

[0062] In step (3), it serves as a transition step from the induction to the reaction.

[0063] The purpose of repeated sand milling and spray drying is to ensure that the sodium source, phosphorus source, and supplementary carbon source are uniformly dispersed around the FePO4 particles with highly active surfaces (defects). This close and uniform contact ensures a full and thorough solid-phase reaction, avoiding the formation of impurity phases due to local compositional deviations.

[0064] The supplementary carbon source will be carbonized in the final sintering process, and together with the carbon coating layer formed in the first sintering, it will construct a complete multi-scale carbon coating network, further improving the electronic conductivity of the final product and making up for the defect of poor intrinsic conductivity of NFPP; at the same time, it will continue to suppress excessive grain growth and agglomeration during the sintering process.

[0065] In this invention, in step (4), the second sintering includes two heat preservation processes. The pre-sintering temperature is 280~350℃ and the time is 2~5h. The subsequent heat preservation temperature is 480~600℃ and the time is 8~20h. The atmosphere is argon or nitrogen gas.

[0066] In step (4), the sodium-based reaction is completed. Pre-sintering at a relatively low temperature allows for a gentle initial reaction between the sodium and phosphorus sources and the prepared defective, highly reactive FePO4 surface, promoting the preferential and abundant formation of NFPP nuclei at the defect sites of precursor II. This avoids the problem of excessively rapid, uneven, and coarse-grained reactions caused by sudden high temperatures.

[0067] Then, the mixture is held at a higher temperature for an extended period to provide sufficient energy for the full growth and crystallization of NFPP nuclei. This ultimately results in an NFPP product with good crystallinity, uniform particle size, and high purity. This allows the NFPP crystal structure to fully develop and crystallize, forming a stable polyanionic framework. The prolonged holding time ensures the completeness of the reaction. The carbon coating layer also plays a role in inhibiting excessive grain growth during this process, which is beneficial for obtaining nanoscale particles.

[0068] The technical solution of the present invention will be clearly and thoroughly described below with reference to specific embodiments. Example

[0069] Na4Fe3(PO4)2P2O7 material was prepared using FePO4, Na4P2O7, Na2CO3, and glucose as raw materials and employing a "nucleation-induced-carbon coating synergistic regulation" strategy. FePO4 served as both the iron and phosphorus source, Na4P2O7 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0070] (1) FePO4 and glucose were mixed in a mass ratio of 9:1, and added to deionized water and stirred to form a mixed slurry. The mixture was then milled to form a uniform suspension, and then spray-dried to obtain precursor I. (2) Precursor I was sintered at 500°C under a nitrogen atmosphere for the first time and held for 2 hours to obtain FePO4 powder with surface defects, namely precursor II; (3) Precursor II is mixed with Na4P2O7, Na2CO3, and glucose, and then milled evenly before being spray-dried to obtain precursor III. The molar ratio of sodium to phosphorus in precursor III is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 2% of the total mass of precursor III; (4) Precursor III was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0071] Figure 1 The image shows the XRD pattern of the FePO4 / C material rich in surface defects prepared in this embodiment. The XRD results show that the crystal structure of FePO4 was successfully changed by carbonization with an organic carbon source, indicating that a high-defect FePO4 material was successfully induced and prepared. Figure 2 The XRD pattern of the NFPP material prepared in this embodiment shows that Na4Fe3(PO4)2P2O7 material was successfully prepared, and the material has excellent crystallinity and no obvious impurity phase. Figure 3 The first charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared in this embodiment at 0.1C is shown. The charging capacity can reach as high as 121 mAh / g, and the discharge capacity can reach 104.6 mAh / g. Example

[0072] Na4Fe3(PO4)2P2O7 material was prepared using FePO4, NaH2PO4, Na2CO3, and glucose as raw materials and employing a "nucleation-induced-carbon coating synergistic regulation" strategy. FePO4 served as both the iron and phosphorus source, NaH2PO4 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0073] (1) FePO4 and glucose were mixed in a mass ratio of 9:1, and added to deionized water and stirred to form a mixed slurry. The mixture was then milled to form a uniform suspension, and then spray-dried to obtain precursor I. (2) Precursor I was sintered at 500°C under a nitrogen atmosphere for the first time and held for 2 hours to obtain FePO4 powder with surface defects, namely precursor II; (3) Precursor II is mixed with NaH2PO4, Na2CO3, and glucose, milled evenly, and then spray-dried to obtain precursor III. The molar ratio of sodium to phosphorus in precursor III is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 2% of the total mass of precursor III; (4) Precursor III was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0074] Figure 4 The image shows the XRD pattern of the NFPP material prepared in this embodiment. The pattern shows that the Na4Fe3(PO4)2P2O7 material was successfully prepared, and the material has excellent crystallinity and a small amount of Na2FeP2O7 impurity phase. Figure 5 The first charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared in this embodiment at 0.1C is shown to be as high as 113.9 mAh / g for charging and 97.4 mAh / g for discharging. Example

[0075] Na4Fe3(PO4)2P2O7 material was prepared using FePO4, Na2HPO4, Na2CO3, and glucose as raw materials and employing a "nucleation-induced-carbon coating synergistic regulation" strategy. FePO4 served as both the iron and phosphorus source, Na2HPO4 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0076] (1) FePO4 and glucose were mixed in a mass ratio of 9:1, and added to deionized water and stirred to form a mixed slurry. The mixture was then milled to form a uniform suspension, and then spray-dried to obtain precursor I. (2) Precursor I was sintered at 500°C under a nitrogen atmosphere for the first time and held for 2 hours to obtain FePO4 powder with surface defects, namely precursor II; (3) Precursor II is mixed with Na2HPO4, Na2CO3, and glucose, milled evenly, and then spray-dried to obtain precursor III. The molar ratio of sodium to phosphorus in precursor III is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 2% of the total mass of precursor III; (4) Precursor III was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0077] Figure 6 The image shows the XRD pattern of the NFPP material prepared in this embodiment. The pattern shows that Na4Fe3(PO4)2P2O7 material was successfully prepared, and the material has excellent crystallinity and trace amounts of Na2FeP2O7 impurity phase. Figure 7 The first charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared in this embodiment at 0.1C is shown. The charging capacity can reach as high as 118.8 mAh / g, and the discharge capacity can reach 107 mAh / g.

[0078] Comparative Example 1: Na4Fe3(PO4)2P2O7 material was prepared using FePO4, Na4P2O7, Na2CO3, and glucose as raw materials and a one-step sintering process with single feeding. FePO4 served as both the iron and phosphorus source, Na4P2O7 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0079] (1) FePO4, Na4P2O7, Na2CO3 and glucose are mixed in a certain proportion, added to deionized water and stirred to form a mixed slurry. After sand milling, a uniform suspension is formed. Then, the precursor is obtained by spray drying. The molar ratio of sodium to phosphorus is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 10% of the total mass of the precursor. (2) The precursor was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0080] Figure 8 The XRD pattern of the NFPP material prepared in this comparative example shows that Na4Fe3(PO4)2P2O7 material was successfully prepared. However, the NFPP material prepared by this process has poor crystallinity and contains a large amount of Na2FeP2O7 and some NaFePO4 impurity phases. Among them, NaFePO4 does not have electrochemical activity, which reduces the overall capacity of the material. Figure 9 The comparison example shows the initial charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared at 0.1C, with a charging capacity of 107.8 mAh / g and a discharge capacity of 100.1 mAh / g.

[0081] Comparative Example 2: Na4Fe3(PO4)2P2O7 material was prepared using FePO4, NaH2PO4, Na2CO3, and glucose as raw materials and a one-step sintering process with single feeding. FePO4 served as both the iron and phosphorus source, NaH2PO4 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0082] (1) FePO4, NaH2PO4, Na2CO3 and glucose are mixed in a certain proportion, added to deionized water and stirred to form a mixed slurry. After sand milling, a uniform suspension is formed. Then, the precursor is obtained by spray drying. The molar ratio of sodium to phosphorus is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 10% of the total mass of the precursor. (2) The precursor was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0083] Figure 10 The XRD pattern of the NFPP material prepared in this comparative example shows that Na4Fe3(PO4)2P2O7 material was successfully prepared. However, the NFPP material prepared by this process has poor crystallinity and contains a large amount of Na2FeP2O7 and NaFePO4 impurity phases, which seriously affects the overall capacity of the material. Figure 11 The comparison example shows the initial charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared at 0.1C, with a charging capacity of 71.3 mAh / g and a discharge capacity of 72.9 mAh / g.

[0084] Comparative Example 3: Na4Fe3(PO4)2P2O7 material was prepared using FePO4, Na2HPO4, Na2CO3, and glucose as raw materials and a one-step sintering process with single feeding. FePO4 served as both the iron and phosphorus source, Na2HPO4 as both the phosphorus and sodium source, Na2CO3 as the carbon source, and glucose as the carbon source.

[0085] (1) FePO4, Na2HPO4, Na2CO3 and glucose are mixed in a certain proportion, added to deionized water and stirred to form a mixed slurry. After sand milling, a uniform suspension is formed. Then, the precursor is obtained by spray drying. The molar ratio of sodium to phosphorus is set to 1:1, the molar ratio of phosphorus to iron is 1.38:1, and glucose accounts for 10% of the total mass of the precursor. (2) The precursor was subjected to a two-stage temperature-controlled sintering treatment under a nitrogen atmosphere. First, it was held at 300℃ for 3 hours, and then at 500℃ for 10 hours to obtain the final NFPP cathode material. The material was characterized and assembled into a sodium-ion battery for electrochemical performance testing.

[0086] Figure 12 The XRD pattern of the NFPP material prepared in this comparative example shows that Na4Fe3(PO4)2P2O7 material was successfully prepared. However, the NFPP material prepared by this process has poor crystallinity and contains a large amount of Na2FeP2O7 and NaFePO4 impurity phases, which reduces the overall capacity of the material. Figure 13 The comparison example shows the initial charge-discharge specific capacity of the Na4Fe3(PO4)2P2O7 material prepared at 0.1C, with a charging capacity of 96.9 mAh / g and a discharge capacity of 93.9 mAh / g.

[0087] Comparing the examples and comparative examples, it can be found that by regulating the reaction pathway of Na4Fe3(PO4)2P2O7, the capacity of Na4Fe3(PO4)2P2O7 can be effectively improved, while the crystallinity of Na4Fe3(PO4)2P2O7 can be significantly increased. This indicates that the "nucleation-induced-carbon coating synergistic regulation" strategy for preparing Na4Fe3(PO4)2P2O7 material is beneficial to promoting the reaction, inhibiting the formation of impurity phases, and increasing the content of the main phase. This stems from the fact that treating FePO4 with an organic carbon source is beneficial to preparing FePO4 powder rich in surface defects, which in turn is conducive to solid-phase nucleation. Ultimately, an NFPP cathode material with nanoscale particle size, high compaction density, high main phase content, and high reversible capacity is prepared.

[0088] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a polyanionic cathode material Na4Fe3(PO4)2P2O7, characterized in that, The method includes the following steps: (1) FePO4 and organic carbon source are mixed at a set mass ratio, added to deionized water and stirred to form a mixed slurry. The mixed slurry is ground by sand milling to obtain a uniformly dispersed suspension, and precursor I is obtained by spray drying. (2) Precursor I was subjected to a first sintering treatment in an inert atmosphere to obtain precursor II, which is FePO4 powder rich in surface defects; (3) The precursor II is mixed evenly with sodium source, supplemented phosphorus source and supplemented carbon source, and then sand-milled and spray-dried to obtain precursor III; (4) The precursor III is subjected to a second sintering treatment in an inert or weakly reducing atmosphere to finally obtain a high-compact, nano-sized Na4Fe3(PO4)2P2O7 cathode material.

2. The method according to claim 1, characterized in that: The organic carbon source is one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol, and polyacrylonitrile; the organic carbon source accounts for 5% to 10% of the mass of precursor I.

3. The method according to claim 1, characterized in that: The first sintering is carried out under a nitrogen or argon atmosphere at a temperature range of 500-700°C for 2-4 hours.

4. The method according to claim 1, characterized in that: The carbon content in precursor II is 1% to 2%.

5. The method according to claim 1, characterized in that: The sodium source includes one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium nitrite, sodium phosphate, sodium pyrophosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

6. The method according to claim 1, characterized in that: The supplemental phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

7. The method according to claim 1, characterized in that: The precursor III has a sodium to phosphorus molar ratio of 1:1 and a phosphorus to iron molar ratio of (1.36~1.4):

1.

8. The method according to claim 1, characterized in that: The supplementary carbon source is an organic supplementary carbon source or an inorganic supplementary carbon source. The organic carbon source is one or more of starch, glucose, sucrose, citric acid, cyclodextrin, phenolic resin, polyvinyl alcohol, and polyacrylonitrile. The inorganic carbon source is one or more of carbon black, carbon nanotubes, and graphite. The supplementary carbon source accounts for 2% to 5% of the total mass of precursor III.

9. The method according to claim 1, characterized in that: The second sintering process includes two heat preservation processes: the pre-sintering temperature is 280~350℃ and the time is 2~5h; the subsequent heat preservation temperature is 480~600℃ and the time is 8~20h, and the atmosphere is argon or nitrogen gas.

10. A sodium-ion battery, characterized in that, Sodium-ion battery cathode material prepared by the preparation method of nucleation-induced polyanionic cathode material Na4Fe3(PO4)2P2O7 as described in any one of claims 1 to 9.

Images