A high-pressure solid iron phosphate sodium pyrophosphate cathode material, its preparation method and application

Abstract

This invention provides a high-pressure compacted sodium iron pyrophosphate cathode material, its preparation method, and its application. The preparation method of the high-pressure compacted sodium iron pyrophosphate cathode material includes: weighing sodium, iron, and phosphorus sources according to the stoichiometric ratio of sodium iron pyrophosphate, adding carbon source and solvent, and mixing and grinding to obtain a precursor slurry; spray drying the precursor slurry to obtain precursor powder; compacting and granulating the precursor powder under a pressure of 10 MPa to 80 MPa to obtain denser precursor particles; and finally sintering at 500℃ to 650℃ for 6 to 15 hours under an inert atmosphere. This invention effectively eliminates internal cavities and interparticle gaps in the precursor through compaction and granulation, and controls the pressure within the range of 10 MPa to 80 MPa. The subsequent sintering process improves the density of the cathode material while controlling the impurity phase content to below 1 wt% and maintaining good electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a high-pressure solid phosphate iron sodium pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) possesses a three-dimensional framework structure, small volume change, long cycle life, and low raw material cost, making it one of the mainstream directions in the field of cathode materials for sodium-ion batteries. Currently, the preparation of NFPP often employs a process route combining spray drying and high-temperature sintering. However, the spray drying process easily forms spherical precursors with internal cavities, and coupled with the large number of gaps between the precursor particles, this results in a high internal porosity of the sintered material. This leads to a generally low compaction density of NFPP, significantly lower than that of lithium iron phosphate (which is typically 2.45 g / cm³). 3 ~2.60g / cm 3 This limits its volumetric energy density advantage.

[0003] The reason for the low compaction density of sodium iron pyrophosphate (NFPP) due to the spray drying process is that its sintering temperature is relatively low, making it difficult to eliminate the internal cavities and interparticle gaps in the precursor formed by spray drying. Increasing the pressure or temperature can easily introduce impurity phases. Therefore, it is necessary to improve the compaction density of NFPP while avoiding excessive impurity phases and maintaining good electrochemical performance. Summary of the Invention

[0004] This invention provides a high-compact sodium iron pyrophosphate cathode material, its preparation method, and its application, in order to solve the problem of increased impurity phases and decreased electrochemical performance when increasing the compaction density of NFPP.

[0005] In a first aspect, the present invention provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, comprising the following steps: According to the stoichiometric ratio of sodium iron pyrophosphate, sodium source, iron source and phosphorus source are weighed, and carbon source and solvent are added and mixed and ground to obtain precursor slurry; The precursor slurry was spray-dried to obtain precursor powder; The precursor powder is compressed and granulated under a pressure of 10MPa to 80MPa to obtain dense precursor particles; as an example, the pressure can be 10MPa, 15MPa, 20MPa, 25MPa, 30MPa, 40MPa, 50MPa, 80MPa, or within any range of the above values. The densified precursor particles are sintered at 500℃~650℃ for 6h~15h under an inert atmosphere to obtain a high-pressure solid sodium iron pyrophosphate cathode material. As an example, the sintering temperature can be 500℃, 520℃, 550℃, 580℃, 600℃, 620℃, 650℃, or any range thereof; the sintering time can be 6h, 8h, 10h, 12h, 15h, or any range thereof.

[0006] In one optional embodiment, the mixing and grinding is performed by ball milling, and optionally, the ball milling time is 2h to 6h; as an example, the ball milling time can be 2h, 3h, 4h, 5h, 6h, or within any range of the above values.

[0007] In one optional embodiment, the heating rate of the first sintering process is 2℃ / min to 10℃ / min; as an example, the heating rate can be 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, or within any of the above values.

[0008] In one optional embodiment, the pressure of the tableting granulation densification treatment is 15MPa~50MPa, more preferably 20MPa~40MPa.

[0009] In one optional embodiment, prior to the primary sintering process, a pre-sintering treatment of the densified precursor particles at 300°C to 450°C for 2 to 5 hours is included. As an example, the pre-sintering temperature can be 300°C, 320°C, 350°C, 380°C, 400°C, 420°C, 450°C, or any range thereof; the pre-sintering time can be 2 hours, 2.5 hours, 3 hours, 4 hours, 5 hours, or any range thereof.

[0010] In one optional embodiment, the heating rate of the pre-sintering treatment is 1~5℃ / min; as an example, the heating rate of the pre-sintering treatment can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or within any range of the above values. And / or, the atmosphere for the pre-sintering treatment is an inert atmosphere.

[0011] In one optional embodiment, the inlet air temperature of the spray drying process is 200°C to 280°C; as an example, the inlet air temperature of the spray drying process can be 200°C, 220°C, 230°C, 250°C, 260°C, 280°C, or within any range of the above values. And / or, the outlet air temperature of the spray drying process is 80℃~120℃; as an example, the outlet air temperature of the spray drying process can be 80℃, 90℃, 95℃, 100℃, 110℃, 120℃, or within any range of the above values.

[0012] In one optional embodiment, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium acetate, and sodium oxalate; And / or, the iron source includes at least one of ferric phosphate, ferrous oxalate, ferric oxide, and metallic iron powder; And / or, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, and sodium pyrophosphate; And / or, the carbon source includes at least one of glucose, sucrose, citric acid, starch, and polyvinyl alcohol; And / or, the solvent includes at least one of deionized water, ethanol, and methanol; And / or, the amount of carbon source used is 5% to 20% of the molar amount of iron in the iron source; as an example, the amount of carbon source used can be 5%, 8%, 10%, 12%, 15%, 20% of the molar amount of iron in the iron source, or within any range of the above values. And / or, the amount of solvent used is 0.5 mL to 2 mL per gram of solid raw material; as an example, the amount of solvent used can be 0.5 mL, 0.8 mL, 1.0 mL, 1.2 mL, 1.5 mL, 2.0 mL per gram of solid raw material, or within any range of the above values.

[0013] In one optional embodiment, a dopant is also added during the mixing and grinding step; And / or, after the primary sintering process, further crushing and grading, and secondary sintering processes are performed.

[0014] In one optional embodiment, the pulverization and classification are carried out using an air jet mill, and the particle size D50 of the pulverized product is 3μm~8μm, and D90≤15μm.

[0015] In one optional embodiment, the dopant includes at least one of a titanium source, a vanadium source, a manganese source, an aluminum source, and a magnesium source; And / or, the amount of dopant added is 0.5% to 5% of the molar amount of iron source; as an example, the amount of dopant added can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% of the molar amount of iron source, or within any range of the above values; And / or, the temperature of the secondary sintering treatment is 500℃~600℃; as an example, the temperature of the secondary sintering treatment can be 500℃, 520℃, 540℃, 550℃, 560℃, 580℃, 600℃, or within any range of the above values. And / or, the time for the secondary sintering treatment is 2h to 8h; as an example, the time for the secondary sintering treatment can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, or within any range of the above values; And / or, the heating rate of the secondary sintering process is 1~5℃ / min; as an example, the heating rate of the secondary sintering process can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or within any range of the above values; And / or, the atmosphere for the secondary sintering treatment is an inert atmosphere.

[0016] In one optional embodiment, the titanium source is at least one of titanium dioxide, tetrabutyl titanate, and isopropyl titanate. And / or, the vanadium source includes at least one of vanadium pentoxide, ammonium metavanadate, and vanadium oxysulfate; And / or, the manganese source includes at least one of manganese dioxide, manganese tetroxide, manganese carbonate, and manganese acetate; And / or, the aluminum source includes at least one of aluminum oxide, aluminum hydroxide, and aluminum nitrate; And / or, the magnesium source includes at least one of magnesium oxide, magnesium hydroxide, magnesium carbonate, and magnesium nitrate.

[0017] In this invention, the inert atmosphere is nitrogen, argon, or a mixture of both.

[0018] In one alternative embodiment, the tableting and granulation densification process is performed using a double-roller tablet press or a tablet forming machine.

[0019] In one optional embodiment, the compaction density of the high-pressure compacted sodium iron pyrophosphate cathode material is ≥2.35 g / cm³. 3 .

[0020] Secondly, the present invention also provides a high-pressure solid phosphoric acid pyrophosphate sodium iron phosphate cathode material, which is prepared by the above-mentioned preparation method of high-pressure solid phosphoric acid pyrophosphate sodium iron phosphate cathode material.

[0021] Thirdly, the present invention also provides a positive electrode sheet, comprising a positive current collector and a positive electrode material layer coated on the positive current collector, wherein the positive electrode material layer comprises the high-pressure solid phosphate sodium iron pyrophosphate positive electrode material described in the second aspect.

[0022] Fourthly, the present invention also provides a sodium-ion battery comprising the positive electrode sheet described in the third aspect.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The method for preparing high-density sodium iron pyrophosphate cathode material provided by this invention adds a pelletizing step before sintering. Mechanical pressure is used to forcibly compact the cavities inside the precursor powder and the gaps between particles, while controlling the pressure within the range of 10MPa to 80MPa. Combined with a first sintering treatment, this ensures densification while keeping the impurity phase content at a low level (not exceeding 1wt%). Thus, a balance between high density and low impurity phase content is achieved within the narrow sintering window of NFPP. This invention is the first to apply pelletizing and densification treatment to NFPP preparation, allowing the precursor particles to achieve a high packing density before entering the sintering furnace, laying a solid foundation for the subsequent first sintering. This invention defines the pressure range as 10 MPa to 80 MPa. Within this range, the tap density of the precursor particles can be increased by more than 10% without compromising the chemical homogeneity of the particles and subsequent sintering activity. If the pressure is below 10 MPa, the densification effect is insufficient; if it is above 80 MPa, the particles are over-compressed, hindering gas escape and element diffusion during the primary sintering process, significantly increasing the impurity content, and decreasing electrochemical performance. Using the method of this invention, only a pressing step is needed after spray drying and before primary sintering. The operation is simple and low-cost, and the resulting NFPP cathode material can achieve a tap density of 2.35 g / cm³. 3 ~2.55g / cm 3 The initial discharge specific capacity is not less than 115mAh / g, and the retention rate is not less than 95% after 200 cycles at 1C.

[0024] 2. This invention adds a pre-sintering step before sintering, or a secondary sintering step after sintering. Pre-sintering allows for the initial carbonization of the carbon source, forming a uniform carbon coating layer on the surface of the precursor particles, while also allowing the components to react pre-react, reducing the risk of volume shrinkage and cracking during sintering. Secondary sintering, after sintering and pulverization, further promotes grain densification and surface carbon layer homogenization. Pre-sintering, performed after tableting and granulation but before high-temperature sintering, with a temperature controlled at 300-450℃ and a time of 2-5 hours, can improve the compaction density to a certain extent. Secondary sintering, performed after primary sintering and pulverization, with a temperature of 500-600℃ and a time of 2-8 hours, helps improve the material's cycle stability.

[0025] 3. Based on tableting and granulation, this invention also adds dopants for doping, which can further improve the specific capacity and cycle stability of the material while ensuring high compaction density. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 This is a scanning electron microscope (SEM) image of the precursor powder obtained after spray drying in Example 1 of the present invention. Figure 2 This is a scanning electron microscope image of the precursor particles obtained after tableting, granulation, and densification treatment in Example 1 of the present invention. Figure 3 This is a bar chart comparing the compaction density of the materials obtained after sintering treatment in Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0028] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0029] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0030] Example 1 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. Figure 1This is a scanning electron microscope image of the precursor powder obtained after spray drying in this embodiment, showing obvious cavities inside the powder particles; (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25MPa using a double roller tablet press to obtain densified precursor particles. Figure 2 This is a scanning electron microscope image of the precursor particles obtained after tableting, granulation, and densification treatment in this embodiment, showing that the particles are more dense. (4) First sintering treatment: The densified precursor particles are heated to 580°C for 10 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0031] Example 2 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 15MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 580°C for 10 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0032] Example 3 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 40MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 580°C for 10 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0033] Example 4 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25MPa using a double roller tablet press to obtain densified precursor particles. (4) Pre-sintering treatment: The densified precursor particles are pre-sintered at 400℃ for 3h under nitrogen atmosphere at a heating rate of 3℃ / min, and then cooled naturally. (5) First sintering treatment: The pre-sintered particles are heated to 580℃ for 10h under nitrogen atmosphere at a heating rate of 5℃ / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0034] Example 5 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 580℃ for 10h under nitrogen atmosphere at a heating rate of 5℃ / min to obtain the sintered product. (5) Crushing and classifying: The sintered product is crushed and classified by air jet mill, and the particle size D50 is controlled to be 3~8μm and D90≤15μm; (6) Secondary sintering treatment: The crushed and graded products are heated to 550℃ for 4 hours under nitrogen atmosphere at a heating rate of 3℃ / min. After natural cooling, they are lightly crushed again to obtain high-pressure solid phosphate iron sodium pyrophosphate cathode material.

[0035] Example 6 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose, titanium dioxide and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. Among them, the amount of titanium added in titanium dioxide was 2.5% of the molar amount of iron in the iron source, the amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 580°C for 10 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0036] Example 7 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 2 hours to obtain the precursor slurry. The amount of carbon source was 5% of the molar amount of iron in the iron source, and the amount of solvent was 0.5 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 200°C and an outlet air temperature of 80°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 10MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 500℃ for 15h under argon atmosphere at a heating rate of 2℃ / min to obtain high pressure solid phosphate sodium iron pyrophosphate cathode material.

[0037] Example 8 This embodiment provides a method for preparing a high-pressure solid iron phosphate sodium pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium hydroxide, ferrous oxalate and sodium phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and sucrose and ethanol were added. The mixture was ball-milled for 6 hours to obtain the precursor slurry. The amount of carbon source was 20% of the molar amount of iron in the iron source, and the amount of solvent was 2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 280°C and an outlet air temperature of 120°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is subjected to compressing and granulation densification treatment under a pressure of 80MPa using a tableting machine to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 650°C for 6 hours under nitrogen atmosphere at a heating rate of 10°C / min to obtain high-pressure solid phosphate sodium iron pyrophosphate cathode material.

[0038] Comparative Example 1 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) First sintering treatment: The precursor powder was heated to 580℃ for 10h under nitrogen atmosphere at a heating rate of 5℃ / min to obtain sodium iron pyrophosphate cathode material.

[0039] Comparative Example 2 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material. (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 100MPa using a double roller tablet press to obtain densified precursor particles. (4) First sintering treatment: The densified precursor particles are heated to 580°C for 10 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain sodium iron pyrophosphate cathode material.

[0040] Comparative Example 3 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material.

[0041] (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder.

[0042] (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25 MPa using a double roller tablet press to obtain densified precursor particles.

[0043] (4) First sintering treatment: The densified precursor particles are heated to 700℃ for 10h under nitrogen atmosphere at a heating rate of 5℃ / min to obtain sodium iron pyrophosphate cathode material.

[0044] Comparative Example 4 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material.

[0045] (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder.

[0046] (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25 MPa using a double roller tablet press to obtain densified precursor particles.

[0047] (4) First sintering treatment: The densified precursor particles are heated to 580°C for 2 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain sodium iron pyrophosphate cathode material.

[0048] Comparative Example 5 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material.

[0049] (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder.

[0050] (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25 MPa using a double roller tablet press to obtain densified precursor particles.

[0051] (4) First sintering treatment: The densified precursor particles are heated to 580°C for 20 hours under nitrogen atmosphere at a heating rate of 5°C / min to obtain sodium iron pyrophosphate cathode material.

[0052] Comparative Example 6 This comparative example provides a method for preparing sodium iron pyrophosphate cathode material, the specific steps of which are as follows: (1) Preparation of precursor slurry: Sodium carbonate, iron phosphate and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of sodium iron pyrophosphate, and glucose and deionized water were added. The mixture was ball-milled for 3 hours to obtain the precursor slurry. The amount of carbon source was 10% of the molar amount of iron in the iron source, and the amount of solvent was 1.2 mL per gram of solid raw material.

[0053] (2) Spray drying treatment: The precursor slurry is spray dried at an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder.

[0054] (3) Compressing and granulation densification treatment: The precursor powder is compressed and granulated under a pressure of 25 MPa using a double roller tablet press to obtain densified precursor particles.

[0055] (4) First sintering treatment: The densified precursor particles are heated to 580°C in air at a heating rate of 5°C / min for 10 hours to obtain sodium iron pyrophosphate cathode material.

[0056] Test case The cathode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 6 were subjected to performance tests. The specific test methods are as follows: (1) Tap density: The tap density meter is used to test the powder. After the powder is put into the measuring cylinder, it is tapped 200 times and the ratio of powder weight to bulk volume is calculated.

[0057] (2) Compacted density: The compacted density was tested using a powder compaction density meter under a pressure of 20 MPa. Figure 3 This is a bar chart comparing the compaction density of the cathode materials obtained after sintering treatment in Example 1 and Comparative Example 1 of the present invention.

[0058] (3) Phase analysis: X-ray diffraction (XRD) was used for analysis.

[0059] (4) Electrochemical performance test: A coin cell was assembled using the prepared positive electrode material as the positive electrode and sodium metal as the negative electrode. The electrolyte was a mixed solution of 1M NaPF6 dissolved in EC / DEC (volume ratio 1:1). At 25℃, the battery was charged at a constant current and constant voltage of 0.1C to 4.3V, with a cutoff current of 1 / 20C. Then, it was discharged at a constant current of 0.1C to 2.5V, and the initial discharge specific capacity was recorded. Charge-discharge cycle tests were performed at a 1C rate, and the capacity retention rate after the 200th cycle was recorded.

[0060] The test results are shown in Table 1 below.

[0061] Table 1. Performance test results of NFPP cathode materials prepared in the examples and comparative examples.

[0062] Based on the above test results, it can be seen that Examples 1 to 8 of the present invention effectively improved the compaction density of the NFPP cathode material (all reaching 2.35 g / cm³). 3The above results represent an improvement of 10% to 19% compared to Comparative Example 1. The specific capacity at 0.1C is no less than 118 mAh / g, the retention rate after 200 cycles at 1C is no less than 95.5%, and the impurity content is controlled below 0.8 wt%. Specifically, Example 4 shows further improved compaction density and specific capacity after pre-sintering; Example 5 shows improved cycle stability after secondary sintering; and Example 6 shows improved specific capacity and cycle stability after titanium doping.

[0063] In contrast, Comparative Examples 1 through 6 failed to effectively increase compaction density while maintaining good electrochemical performance. Comparative Example 1, without tableting or granulation, had a compaction density of only 2.12 g / cm³. 3 Comparative Example 2: The pressure was too high (100 MPa), and the impurity phase content increased to 4.5 wt%, resulting in a significant decrease in specific capacity and cycle retention rate. Comparative Example 3: The temperature was too high (700℃), and the impurity phase content reached 6.2 wt%, severely deteriorating the electrochemical performance. Comparative Example 4: The first sintering time was insufficient (2 h), resulting in low compaction density and specific capacity, with an impurity phase content of 5.8 wt%. Comparative Example 5: The first sintering time was too long (20 h), and although the compaction density was acceptable, the cycle retention rate decreased, with an impurity phase content of 1.2 wt%. Comparative Example 6: Sintering under an air atmosphere resulted in the appearance of Fe2O3 impurity phase, severely deteriorating the electrochemical performance.

[0064] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a high-pressure solid iron sodium pyrophosphate cathode material, characterized in that, Includes the following steps: According to the stoichiometric ratio of sodium iron pyrophosphate, sodium source, iron source and phosphorus source are weighed, and carbon source and solvent are added and mixed and ground to obtain precursor slurry; The precursor slurry was spray-dried to obtain precursor powder; The precursor powder was compressed and granulated under a pressure of 10MPa~80MPa to obtain dense precursor particles. The densified precursor particles were sintered at 500℃~650℃ for 6h~15h in an inert atmosphere to obtain high-pressure solid phosphate sodium iron pyrophosphate cathode material.

2. The preparation method according to claim 1, characterized in that, The pressure for the tablet compression granulation densification process is 15 MPa to 50 MPa.

3. The preparation method according to claim 1 or 2, characterized in that, Before the first sintering process, the process further includes a pre-sintering treatment of the densified precursor particles at 300°C to 450°C for 2 to 5 hours.

4. The preparation method according to claim 1 or 2, characterized in that, The inlet air temperature for the spray drying process is 200℃~280℃; And / or, the outlet air temperature of the spray drying process is 80℃~120℃.

5. The preparation method according to claim 1 or 2, characterized in that, The sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium acetate, and sodium oxalate; And / or, the iron source includes at least one of ferric phosphate, ferrous oxalate, ferric oxide, and metallic iron powder; And / or, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, and sodium pyrophosphate; And / or, the carbon source includes at least one of glucose, sucrose, citric acid, starch, and polyvinyl alcohol; And / or, the solvent includes at least one of deionized water, ethanol, and methanol; And / or, the amount of carbon source used is 5% to 20% of the molar amount of iron in the iron source; And / or, the amount of solvent used is 0.5 mL to 2 mL per gram of solid raw material.

6. The preparation method according to claim 1 or 2, characterized in that, In the mixing and grinding step, a dopant is also added; And / or, after the primary sintering process, further crushing and grading, and secondary sintering processes are performed.

7. The preparation method according to claim 6, characterized in that, The dopant includes at least one of titanium source, vanadium source, manganese source, aluminum source, and magnesium source; And / or, the amount of the dopant added is 0.5% to 5% of the molar amount of the iron source; And / or, the temperature of the secondary sintering treatment is 500℃~600℃; And / or, the duration of the secondary sintering treatment is 2h to 8h.

8. A high-pressure solid iron sodium pyrophosphate cathode material, characterized in that, It is prepared by the method for preparing high-pressure solid phosphate sodium iron pyrophosphate cathode material according to any one of claims 1 to 7.

9. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive electrode material layer coated on the positive current collector, wherein the positive electrode material layer comprises the high-pressure solid phosphate sodium iron pyrophosphate positive electrode material as described in claim 8.

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

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