Iron-based polyphosphate-type sodium-ion battery positive electrode material, preparation method therefor and use thereof

Abstract

The present disclosure relates to the technical field of sodium-ion batteries, and in particular to an iron-based polyphosphate-type sodium-ion battery positive electrode material, a preparation method therefor and a use thereof. An organic ferrous source, a sodium source, a phosphorus source and a dopant are mixed and then ground until D50 is less than 180 nm, so that the reactivity of the material is improved, a solid-phase reaction is facilitated, and doped high-valence metal elements such as vanadium, niobium, titanium, zirconium and tin can be better doped into lattices of the iron-based polyphosphate-type sodium-ion battery positive electrode material (NFPP), so as to replace the iron site to form lattice defects such as vacancies, thereby improving the ionic conductivity of the material, inhibiting generation of NaFePO4 impurity phase, and improving the capacity and cycling stability of the material.

Description

Iron-based polyphosphate sodium ion battery positive electrode material and preparation method and application thereof Technical Field

[0001] The present disclosure belongs to the technical field of sodium ion batteries, and in particular, relates to an iron-based polyphosphate sodium ion battery positive electrode material, a preparation method thereof, and applications thereof. Background Art

[0002] In recent years, with the increasing depletion of fossil energy and the growing severity of environmental issues such as global warming, green secondary batteries have gradually come into the spotlight. Lithium-ion batteries, due to their superior performance, have been widely commercialized in electric vehicles, portable electronic devices, and energy storage. However, the scarcity and uneven distribution of lithium resources have severely limited the further development of lithium-ion batteries. Sodium-ion batteries, which share similar charging and discharging principles with lithium-ion batteries and have abundant sodium reserves worldwide, are an important development direction for green energy storage.

[0003] The cathode materials for sodium ion batteries mainly include layered transition metal oxides, Prussian blue compounds, and polyanion compounds. Among them, layered transition metal oxides have low reversible capacity and contain precious metals such as cobalt, nickel, and copper, which are expensive. Prussian blue compounds have lattice defects and coordinated water, resulting in poor thermal stability. In addition, the material contains toxic CN. - Polyanionic compounds have the advantages of high working voltage, stable structure, small volume change during the cycle, etc., and therefore have attracted widespread attention.

[0004] Iron-based mixed phosphate polyanion compound Na4Fe3(PO4)2P2O7 (NFPP for short) combines the advantages of all iron-based phosphates, and has a high theoretical specific capacity (129mAh / g), a high average working voltage (3.1VS.Na + / Na), and boasts excellent cycling stability, low cost, environmental friendliness, and abundant reserves, making it considered the most promising sodium-ion battery cathode material for large-scale energy storage. However, conventional synthesis methods often result in the formation of an electrochemically inactive NaFePO4 impurity phase, and also suffer from inherently low electronic and ionic conductivity, which severely impacts the material's capacity, rate capability, and cycling performance.

[0005] Therefore, there is an urgent need to develop a method that can significantly improve the capacity, rate performance and cycle performance of NFPP, while also having the advantages of low cost and industrial preparation.

[0006] Summary of the Invention

[0007] The present invention aims to provide an iron-based polyphosphate sodium ion battery cathode material, a preparation method thereof, and an application thereof, in order to significantly improve the electrochemical performance of the sodium ion battery cathode material.

[0008] In order to achieve the above-mentioned purpose of the present disclosure, the following technical solutions can be adopted:

[0009] The present disclosure provides a method for preparing an iron-based polyphosphate sodium-ion battery cathode material, comprising: mixing an organic ferrous source, a sodium source, a phosphorus source, and a dopant, followed by grinding and drying to obtain a mixed powder; sintering the mixed powder in stages; wherein, during the grinding process, the D50 of the ground particles is controlled to be less than 180 nm;

[0010] The doping element in the dopant is selected from at least one of vanadium, niobium, titanium, zirconium and tin;

[0011] By adjusting the amount of dopant added, the mass ratio of the amount of doped metal element to the theoretical amount of positive electrode material generated is (0.05-0.40):100.

[0012] In some embodiments of the present disclosure, the grinding is performed by wet grinding, and the D50 of the particles in the slurry after grinding is controlled to be less than 150 nm.

[0013] In some embodiments of the present disclosure, the wet grinding process includes: mixing the raw materials with a dispersant to obtain a grinding slurry, and wet grinding the grinding slurry; wherein the dispersant is selected from at least one of water and ethanol.

[0014] In some embodiments of the present disclosure, the solid content of the polishing slurry is 20%-40% by mass.

[0015] In some embodiments of the present disclosure, spray drying is used for drying, and the air inlet temperature is controlled to be 220°C-250°C, and the air outlet temperature is controlled to be 105°C-120°C.

[0016] In some embodiments of the present disclosure, by adjusting the amount of dopant added, the mass ratio of the amount of doping metal element to the theoretical amount of positive electrode material generated is (0.05-0.40):100.

[0017] In some embodiments of the present disclosure, the dopant is selected from at least one of vanadyl oxalate, vanadium pentoxide, ammonium metavanadate, niobium pentoxide, titanium dioxide, zirconium dioxide, and tin dioxide.

[0018] In some embodiments of the present disclosure, a carbon source is further added to the mixed feedstock before grinding;

[0019] In terms of mass fraction, the mass ratio of the added amount of carbon source to the theoretical generated amount of positive electrode material is (8-14):100.

[0020] In some embodiments of the present disclosure, the carbon source is selected from at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, activated carbon, carbon nanotubes, and graphene.

[0021] In some embodiments of the present disclosure, the iron source is an organic ferrous source, and the organic ferrous source is selected from at least one of ferrous oxalate and ferrous acetate.

[0022] In some embodiments of the present disclosure, the sodium source is selected from at least one of sodium pyrophosphate, sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium hydroxide, sodium phosphate, sodium monohydrogen phosphate, sodium oxalate, and sodium acetate.

[0023] In some embodiments of the present disclosure, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0024] In some embodiments of the present disclosure, the molar ratio of sodium, iron and phosphorus in the raw material is 4:3:4 by adjusting the added amounts of the iron source, sodium source and phosphorus source.

[0025] In some embodiments of the present disclosure, the sintering process includes low-temperature pre-sintering and high-temperature sintering performed sequentially, the sintering temperature of the low-temperature pre-sintering is 360°C-420°C, and the sintering temperature of the high-temperature sintering is 480°C-550°C.

[0026] In some embodiments of the present disclosure, the sintering temperature of the low-temperature pre-firing is 400° C.-420° C., and the sintering time is 2 h-6 h.

[0027] In some embodiments of the present disclosure, the sintering temperature of the high-temperature sintering is 500° C.-550° C., and the sintering time is 8 h-14 h.

[0028] In a second aspect, the present disclosure further provides an iron-based polyphosphate sodium ion battery positive electrode material, which is prepared by the preparation method in any of the above embodiments.

[0029] In a third aspect, the present disclosure further provides a positive electrode plate comprising the iron-based polyphosphate sodium ion battery positive electrode material in the above embodiment.

[0030] In a fourth aspect, the present disclosure further provides a sodium ion battery comprising the positive electrode sheet in the above embodiment.

[0031] In a fifth aspect, the present disclosure further provides an electrical device comprising the sodium ion battery in the above embodiment.

[0032] After mixing the iron source, sodium source, phosphorus source and dopant, they are first ground to a D50 of less than 180nm to improve the reaction activity of the material, which is conducive to the solid-phase reaction. The doped high-valent metal elements such as vanadium, niobium, titanium, zirconium and tin can be better doped into the lattice of the iron-based polyphosphate sodium ion battery positive electrode material (NFPP), replacing the iron site to form lattice defects such as holes, thereby improving the ionic conductivity of the material and inhibiting the formation of NaFePO4 impurity phase, which is beneficial to improving the capacity and cycle stability of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0034] FIG1 is a SEM image of the NFPP / C positive electrode material prepared in Example 1 of the present invention.

[0035] FIG2 is an XRD pattern of the NFPP / C positive electrode material prepared in Example 1 of the present invention.

[0036] FIG3 is an XRD pattern of the NFPP / C positive electrode material prepared in Comparative Example 7 of the present invention.

[0037] FIG4 is a charge and discharge curve diagram of the NFPP / C positive electrode material prepared in Example 1 of the present invention.

[0038] FIG5 is a charge and discharge curve diagram of the NFPP / C positive electrode material prepared in Comparative Example 7 of the present invention.

[0039] FIG6 shows the capacity retention rate of the NFPP / C positive electrode material prepared in Example 1 of the present invention after 200 cycles of 1C. DETAILED DESCRIPTION

[0040] The embodiments of the present disclosure will be described in detail below with reference to the examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. Where specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0041] The endpoints of the ranges and any values ​​disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0042] The present disclosure provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0043] S1. Grinding and drying

[0044] The iron source, sodium source, phosphorus source and dopant are mixed, ground and dried in sequence to obtain a mixed powder for later use.

[0045] In some embodiments of the present disclosure, the iron source can be an organic ferrous source, selected from at least one of ferrous oxalate and ferrous acetate, and can be any one or both of the above. Using the organic ferrous source as the raw material and the organic ferrous source as the growth template during the synthesis process, NFPP spherical powder can be synthesized in situ, and the particle size of the product can be controlled. During subsequent low-temperature pre-calcination, the decomposition of the ferrous oxalate can form a uniform carbon coating in situ on the surface of the NFPP grains, inhibiting grain growth and shortening the diffusion path of sodium ions, thereby improving the material's rate performance.

[0046] In some embodiments of the present disclosure, the sodium source is selected from at least one of sodium pyrophosphate, sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium hydroxide, sodium phosphate, sodium monohydrogen phosphate, sodium oxalate, and sodium acetate. The above sodium sources are all suitable for the preparation methods provided in the embodiments of the present disclosure, and the sodium source can be any one or more of the above. The phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. The above phosphorus sources are all suitable for the preparation methods provided in the embodiments of the present disclosure, and the phosphorus source can be any one or more of the above.

[0047] In some embodiments of the present disclosure, by adjusting the amount of iron source, sodium source and phosphorus source added, the molar ratio of sodium, iron and phosphorus elements in the raw material is 4:3:4, satisfying the element ratio of Na4Fe3(PO4)2P2O7.

[0048] In some embodiments of the present disclosure, a carbon source is also added to the mixed raw materials before grinding. The introduced carbon source decomposes at high temperature to produce a large number of micropores, thereby forming a porous carbon matrix. The NFPP particles coated with a uniform carbon layer formed in situ are embedded in the porous carbon matrix, thereby forming a continuous conductive network. The composite carbon coating process significantly improves the electronic conductivity of the material, which can solve the problem of poor electronic conductivity of the NFPP material.

[0049] In some embodiments of the present disclosure, the carbon source is selected from at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, activated carbon, carbon nanotubes, and graphene, and may be any one or more of the above. In terms of mass fraction, the mass ratio of the added amount of the carbon source to the theoretical amount of the positive electrode material produced is (8-14):100, such as 8:100, 9:100, 10:100, 11:100, 12:100, 13:100, 14:100, etc.

[0050] It should be noted that the theoretical production amount of the positive electrode material is obtained by conventional conversion based on the amount of each raw material used. For example, the theoretical production mass is calculated based on the molar amount of Na4Fe3(PO4)2P2O7, and then the corresponding amount of doping elements and carbon content are added based on the theoretical production mass of the positive electrode material.

[0051] In some embodiments of the present disclosure, the doping element in the dopant is selected from at least one of vanadium, niobium, titanium, zirconium, and tin. By doping with a high-valent metal element and controlling the grinding particle size (D50 less than 180nm), it can be successfully doped into the lattice of NFPP at a relatively low sintering temperature during a solid-phase reaction, replacing the iron site to form lattice defects such as holes, thereby improving the ionic conductivity of the material. At the same time, the Na:Fe ratio in the material is increased, which can inhibit the formation of the NaFePO4 (Na:Fe=1:1) impurity phase. This can solve the problem of the inactive impurity NaFePO4 often produced during the high-temperature solid-phase synthesis of Na4Fe3(PO4)2P2O7 (Na:Fe=1:0.75), significantly improving the capacity of the NFPP material.

[0052] Furthermore, by adjusting the amount of dopant added, the mass ratio of the amount of the doping metal element to the theoretical amount of the positive electrode material produced is (0.05-0.40):100, such as 0.05:100, 0.10:100, 0.15:100, 0.20:100, 0.25:100, 0.30:100, 0.35:100, 0.40:100, etc. The dopant is selected from at least one of vanadyl oxalate, vanadium pentoxide, ammonium metavanadate, niobium pentoxide, titanium dioxide, zirconium dioxide, and tin dioxide, and can be any one or more of the above.

[0053] To improve the reactivity of the solid-phase reaction, the particle size after grinding should not be too large, so that the D50 of the particles is less than 180nm. In some disclosed embodiments, grinding is performed by wet grinding, and the D50 of the particles in the slurry after grinding is controlled to be less than 150nm to further improve the reactivity of the material and facilitate the solid-phase reaction. In principle, the smaller the grinding particle size, the better, but if the grinding particle size is too small, the grinding efficiency drops sharply, which will significantly increase the process cost.

[0054] Furthermore, the wet grinding process includes: mixing the raw materials with a dispersant to obtain a grinding slurry, and transferring the grinding slurry to a nano sand mill for wet grinding. The dispersant is selected from at least one of water and ethanol, and may be any one or both of these. The solid content of the grinding slurry is controlled by adjusting the amount of dispersant. The solid content of the grinding slurry is preferably 20% to 40% by mass, to ensure optimal grinding and achieve the desired particle size.

[0055] Specifically, the solid content of the polishing slurry may be 20%, 25%, 30%, 35%, 40%, etc.

[0056] In some embodiments of the present disclosure, spray drying is performed, with the inlet air temperature controlled between 220°C and 250°C, and the outlet air temperature between 105°C and 120°C. By controlling the inlet and outlet air temperatures, a uniform spherical dry powder is obtained. Specifically, the inlet air temperature can be 220°C, 230°C, 240°C, 250°C, etc., and the outlet air temperature can be 105°C, 110°C, 115°C, 120°C, etc.

[0057] S2. Sintering

[0058] The mixed powder is sintered by a staged sintering process, but is not limited thereto.

[0059] In some embodiments of the present disclosure, the sintering process includes sequential low-temperature pre-sintering and high-temperature sintering, with the low-temperature pre-sintering temperature being 360°C-420°C and the high-temperature sintering temperature being 480°C-550°C. During the low-temperature pre-sintering, the organic ferrous source decomposes, forming a uniform carbon coating in situ on the surface of the NFPP grains. This inhibits grain growth and shortens the diffusion path of sodium ions, thereby improving the material's rate performance. NFPP is sensitive to sintering temperature, and the sintering temperature during high-temperature sintering should not be too high, as this can negatively impact the product's electrochemical performance.

[0060] Specifically, the sintering temperature for low-temperature pre-sintering can be 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, etc.; the sintering temperature for high-temperature sintering can be 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, etc. Both low-temperature pre-sintering and high-temperature sintering can be carried out in an inert atmosphere in a tubular furnace, and the inert atmosphere can be nitrogen, argon, etc.

[0061] In some embodiments of the present disclosure, the sintering temperature for low-temperature pre-sintering is 400°C-420°C, and the sintering time is 2 hours-6 hours. The sintering temperature for high-temperature sintering is 500°C-550°C, and the sintering time is 8 hours-14 hours. By controlling the temperature and time of low-temperature and high-temperature sintering to further promote the reaction, the electrochemical performance of the material can be further improved.

[0062] Specifically, the sintering time for low-temperature pre-sintering can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc., and the sintering time for high-temperature sintering can be 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, etc. The heating rate for low-temperature pre-sintering and high-temperature sintering is not limited and can be 1 to 5°C / min.

[0063] The embodiments of the present disclosure also provide an iron-based polyphosphate sodium ion battery positive electrode material, which is prepared by the above-mentioned preparation method. The prepared NFPP / C material is basically free of impurities and has excellent electrochemical performance, specifically manifested in having high charge and discharge capacity, rate performance and cycle performance.

[0064] In some embodiments of the present disclosure, the positive electrode material includes Na4Fe3(PO4)2P2O7 and a composite carbon coating layer on its surface, the inner layer of the carbon coating layer is produced by the decomposition of an organic ferrous source, and the outer layer of the carbon coating layer is produced by the decomposition of an introduced carbon source.

[0065] The embodiments of the present disclosure also provide a positive electrode plate, comprising the above-mentioned iron-based polyphosphate type sodium ion battery positive electrode material, and may also include a positive electrode current collector, wherein the positive electrode active coating on the positive electrode current collector contains the above-mentioned iron-based polyphosphate type sodium ion battery positive electrode material, and the positive electrode plate can be prepared by conventional coating and drying processes.

[0066] In some embodiments, the positive electrode material, binder, conductive agent, dispersant, etc. can be mixed and homogenized to obtain a positive electrode active slurry, which is then coated on a positive electrode current collector and dried to obtain a positive electrode sheet.

[0067] The present disclosure also provides a sodium-ion battery comprising the aforementioned positive electrode sheet, and further comprising a negative electrode sheet, an electrolyte, a separator, and other structures to form a complete sodium-ion battery. Due to improvements in the positive electrode material, the electrochemical performance of the sodium-ion battery, such as capacity and cycle performance, has been improved to a certain extent.

[0068] It should be noted that the NFPP / C cathode material powder prepared using the method provided in the examples of this disclosure, when assembled into a button-type half-cell, exhibited a 0.1C discharge capacity of 116.5 mAh / g, a 1C discharge capacity of 103.8 mAh / g, and a capacity retention rate of 98.8% after 200 cycles at 1C. The resulting NFPP / C cathode material exhibits excellent electrochemical performance and can provide support for the industrialized preparation of cathode materials for polyanionic sodium-ion batteries.

[0069] The embodiments of the present disclosure further provide an electrical device, including the sodium ion battery described above, and may further include an electrical appliance, wherein the sodium ion battery is used to power the electrical appliance, and the specific type of the electrical appliance is not limited.

[0070] The features and performance of the present disclosure are further described in detail below with reference to the embodiments.

[0071] Example 1

[0072] This embodiment provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0073] (1) Ferrous oxalate is used as an iron source, sodium pyrophosphate is used as both a sodium source and a phosphorus source, and phosphoric acid is used as a supplementary phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P = 4:3:4, and 8% of glucose, 0.5% of carbon nanotube slurry (calculated by the amount of carbon nanotubes, the same below) and 0.25% of vanadyl oxalate (calculated by the mass of the doped metal element, the same below) are added to the mixture.

[0074] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 25%, and the mixed slurry was obtained after stirring and dispersing for 40 minutes. The mixed slurry was ground with a sand mill to a D50 of 100 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 110°C).

[0075] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 400°C at a heating rate of 2°C / min and kept at this temperature for 4 h to completely decompose the ferrous oxalate in the raw material. The powder was then heated to 500°C at a heating rate of 3°C / min and kept at this temperature for 10 h. The powder was then naturally cooled to obtain the NFPP / C positive electrode material.

[0076] Example 2

[0077] This embodiment provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0078] (1) Ferrous oxalate is used as an iron source, sodium dihydrogen phosphate is used as both a sodium source and a phosphorus source, and the ingredients are prepared according to the element molar ratio of Na / Fe / P = 4:3:4. Glucose (8% of the theoretical mass of NFPP), 0.5% of carbon nanotube slurry, and 0.20% of vanadyl oxalate are added.

[0079] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 30%, and the mixed slurry was obtained after stirring and dispersing for 45 minutes. The mixed slurry was ground with a sand mill to a D50 of 100 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 105°C).

[0080] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 400°C at a heating rate of 2°C / min and kept at this temperature for 3 h to completely decompose the ferrous oxalate in the raw material. The temperature was then increased to 520°C at a heating rate of 4°C / min and kept at this temperature for 10 h. After natural cooling, the NFPP / C positive electrode material was obtained.

[0081] Example 3

[0082] This embodiment provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0083] (1) Ferrous oxalate is used as the iron source, sodium carbonate is used as the sodium source, and ammonium dihydrogen phosphate is used as the phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P=4:3:4, and 10% of glucose, 0.5% of carbon nanotube slurry, and 0.30% of niobium pentoxide are added to the mixture to form NFPP.

[0084] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to 40%, and the mixed slurry was obtained after stirring and dispersing for 45 minutes. The mixed slurry was ground with a sand mill to a D50 of 100 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 105°C).

[0085] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace and heated from room temperature to 420°C at a heating rate of 3°C / min and kept warm for 3 h to completely decompose the ferrous oxalate in the raw material. The powder was then heated to 550°C at a heating rate of 4°C / min and kept warm for 12 h. The powder was then naturally cooled to obtain the NFPP / C positive electrode material.

[0086] Example 4

[0087] This embodiment provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0088] (1) Ferrous oxalate is used as an iron source, sodium carbonate is used as a sodium source, and phosphoric acid is used as a phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P=4:3:4, and 12% of glucose, 0.5% of polyethylene glycol, and 0.40% of vanadyl oxalate are added to theoretically generate NFPP by mass.

[0089] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 35%, and the mixed slurry was obtained after stirring and dispersing for 45 minutes. The mixed slurry was ground with a sand mill to a D50 of 150 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 240°C, outlet air temperature 110°C).

[0090] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 400°C at a heating rate of 2°C / min and kept at this temperature for 5 h to completely decompose the ferrous oxalate in the raw material. The temperature was then increased to 550°C at a heating rate of 5°C / min and kept at this temperature for 12 h. The NFPP / C positive electrode material was obtained after natural cooling.

[0091] Example 5

[0092] This embodiment provides a method for preparing an iron-based polyphosphate sodium ion battery cathode material, comprising the following steps:

[0093] (1) Ferrous acetate is used as the iron source, sodium pyrophosphate is used as both the sodium source and the phosphorus source, and phosphoric acid is used as the supplementary phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P = 4:3:4, and 8% of glucose, 0.5% of carbon nanotube slurry and 0.25% of vanadyl oxalate are added to the mixture to generate NFPP by theoretical weight.

[0094] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 25%, and the mixed slurry was obtained after stirring and dispersing for 40 minutes. The mixed slurry was ground with a sand mill to a D50 of 100 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 110°C).

[0095] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 400°C at a heating rate of 2°C / min and kept at this temperature for 4 h to completely decompose the ferrous oxalate in the raw material. The powder was then heated to 500°C at a heating rate of 3°C / min and kept at this temperature for 10 h. The powder was then naturally cooled to obtain the NFPP / C positive electrode material.

[0096] Comparative Example 1

[0097] This comparative example provides a method for preparing an iron-based polyphosphate sodium ion battery positive electrode material, comprising the following steps:

[0098] (1) Ferrous oxalate is used as the iron source, sodium pyrophosphate is used as both the sodium source and the phosphorus source, and phosphoric acid is used as the supplementary phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P = 4:3:4, and 8% of glucose, 0.5% of carbon nanotube slurry and 0.25% of vanadyl oxalate are added to the mixture to generate NFPP by mass.

[0099] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 25%, and the mixed slurry was obtained after stirring and dispersing for 40 minutes. The mixed slurry was ground with a sand mill to a D50 of 200 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 110°C).

[0100] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 400°C at a heating rate of 2°C / min and kept at this temperature for 4 h to completely decompose the ferrous oxalate in the raw material. The powder was then heated to 500°C at a heating rate of 3°C / min and kept at this temperature for 10 h. The powder was then naturally cooled to obtain the NFPP / C positive electrode material.

[0101] It should be noted that the only difference between Comparative Example 1 and Example 1 is that the wet grinding was performed until D50 was 200 nm.

[0102] Comparative Example 2

[0103] The only difference from Example 1 is that the particle size D50 of the sand-milled slurry after wet grinding in step (2) is 300 nm.

[0104] Comparative Example 3

[0105] The only difference from Example 1 is that the particle size D50 of the sand-milled slurry after wet grinding in step (2) is 300 nm and is not doped with vanadium pentoxide.

[0106] Comparative Example 4

[0107] The only difference from Example 1 is that the amount of vanadium pentoxide added is 0.50%.

[0108] Comparative Example 5

[0109] The only difference from Example 1 is that the amount of glucose added is 18%.

[0110] Comparative Example 6

[0111] The only difference from Example 1 is that the iron source used is ferric phosphate, and the ingredients are still prepared according to the element molar ratio of Na / Fe / P=4:3:4.

[0112] Comparative Example 7

[0113] This comparative example provides a method for preparing an iron-based polyphosphate sodium ion battery positive electrode material, comprising the following steps:

[0114] (1) Ferric phosphate is used as the iron and phosphorus sources, sodium carbonate is used as the sodium source, and phosphoric acid is used as the supplementary phosphorus source. The ingredients are prepared according to the element molar ratio of Na / Fe / P = 4:3:4, and 8% of glucose and 0.5% of carbon nanotube slurry are added to theoretically generate NFPP by mass.

[0115] (2) The mixed raw materials were mixed with pure water, the solid content was controlled to be 25%, and the mixed slurry was obtained after stirring and dispersing for 40 minutes. The mixed slurry was ground with a sand mill to a D50 of 300 nm, and spherical dry powder was obtained by centrifugal spray drying equipment (inlet air temperature 230°C, outlet air temperature 105°C).

[0116] (3) The spherical dry powder was placed in a nitrogen tubular atmosphere furnace, and the temperature was increased from room temperature to 500°C at a heating rate of 2°C / min and kept at this temperature for 10 h. After natural cooling, the NFPP / C positive electrode material was obtained.

[0117] Test Example 1

[0118] The SEM and XRD patterns of the NFPP / C cathode material prepared in Test Example 1 are shown in Figures 1 and 2. The XRD pattern of the NFPP / C cathode material prepared in Test Comparative Example 7 is shown in Figure 3.

[0119] The results show that the NFPP grains are essentially spherical, with a uniform distribution of large and small particles, indicating a good carbon coating effect. Figure 2 shows the XRD pattern of the NFPP / C cathode material prepared in Example 1. It can be seen that the NFPP / C material has high crystallinity, that vanadium doping does not disrupt the NFPP crystal structure, and that there are no impurity peaks due to NaFePO4 and Na2FeP2O7.

[0120] FIG3 is an XRD pattern of the NFPP / C positive electrode material prepared in Comparative Example 7. It can be seen from the figure that the characteristic diffraction peaks of NaFePO 4 exist at diffraction angles of 32.92° and 33.18°.

[0121] Test Example 2

[0122] The electrochemical properties of the positive electrode materials prepared in the examples and comparative examples were tested, and the results are shown in Table 1.

[0123] Testing method: The prepared NFPP / C cathode material, acetylene black, and polyvinylidene fluoride (PVDF) were dissolved in NMP at a ratio of 8:1:1. The mixture was stirred into a slurry and coated onto aluminum foil. After drying and lamination, the cathode material pole piece was formed. Sodium metal was used as the counter electrode, and the electrolyte consisted of 1 mol / L NaClO₄ / (EC+DMC+EMC) containing 5% fluoroethylene carbonate (FEC). The separator was Cellgard 2035. Coin cells (model CR2016) were assembled in a glove box and electrochemically tested using a Land battery tester.

[0124] Table 1 Performance test results of positive electrode materials prepared in Examples and Comparative Examples

[0125] From the comparison between Example 1 and Comparative Examples 1-7, it can be seen that the particle size of the ground particles should not be too large, and the amount of metal elements and carbon doped should not be too large. In addition, the staged sintering and the addition of organic iron source also have a certain impact on the product performance.

[0126] The charge and discharge curves of Example 1 and Comparative Example 7 are shown in Figures 4 and 5 , and the capacity retention test results of the NFPP / C positive electrode material prepared in Example 1 after 200 cycles of 1C are shown in Figure 6 .

[0127] Figure 4 shows the charge-discharge curves of the NFPP / C positive electrode material prepared in Example 1. The 0.1C discharge capacity is 116.5 mAh / g, and the 1C discharge capacity is 103.8 mAh / g. Figure 5 shows the charge-discharge curves of the NFPP / C positive electrode material prepared in Comparative Example 7. The 0.1C discharge capacity is 90.8 mAh / g, and the 1C discharge capacity is 84.3 mAh / g. Figure 6 shows the capacity retention rate of the NFPP / C positive electrode material prepared in Example 1 after 200 cycles at 1C. The discharge capacity after 200 cycles at 1C is 102.5 mAh / g, and the capacity retention rate is as high as 98.8%. It can be seen that compared with Comparative Example 7, the discharge capacity of Example 1 is significantly improved, and the capacity retention rate of the Example is maintained at a very high level. Industrial Applicability

[0128] The present invention discloses a method for preparing a ferrous metal sulfide by mixing an iron source, a sodium source, a phosphorus source, and a dopant, grinding the mixture to a D50 value of less than 180 nm, and then performing staged sintering. The synthesis method is simple, and the raw materials used, including an organic ferrous source, a sodium source, a phosphorus source, and a carbon source, are inexpensive, readily available, and widely distributed. The entire preparation process is environmentally friendly, simple, and amenable to large-scale industrial production, thus possessing excellent industrial applicability.

Claims

1. A preparation method of an iron-based polyphosphate-type sodium ion battery cathode material, characterized in that, comprising: mixing an organic ferrous source, a sodium source, a phosphorus source and a dopant, and then successively grinding and drying to obtain a mixed powder; subjecting the mixed powder to segmented sintering; wherein, during the grinding process, the D50 of the particles after grinding is controlled to be less than 180 nm; the doping element in the dopant is selected from at least one of vanadium, niobium, titanium, zirconium and tin; by adjusting the addition amount of the dopant, the mass ratio of the amount of the doped metal element to the theoretical production amount of the cathode material is (0.05 - 0.40):

100.

2. The preparation method according to claim 1, characterized in that, the grinding is carried out by means of wet grinding, and the D50 of the particles in the slurry after grinding is controlled to be less than 150 nm.

3. The preparation method according to claim 2, characterized in that, the process of the wet grinding includes: mixing each raw material with a dispersant to obtain a grinding slurry, and carrying out wet grinding on the grinding slurry; wherein, the dispersant is selected from at least one of water and ethanol.

4. The preparation method according to claim 3, characterized in that, by mass fraction, the solid content of the grinding slurry is 20% - 40%.

5. The preparation method according to any one of claims 1 - 4, characterized in that, spray drying is adopted for drying, and the inlet air temperature is controlled to be 220°C - 250°C, and the outlet air temperature is 105°C - 120°C.

6. The preparation method according to any one of claims 1 - 5, characterized in that, the dopant is selected from at least one of vanadyl oxalate, vanadium pentoxide, ammonium metavanadate, niobium pentoxide, titanium dioxide, zirconium dioxide and tin dioxide.

7. The preparation method according to any one of claims 1 - 6, characterized in that, a carbon source is further added to the mixed raw materials before grinding; by mass fraction, the mass ratio of the addition amount of the carbon source to the theoretical production amount of the cathode material is (8 - 14):

100.

8. The preparation method according to claim 7, characterized in that, the carbon source is selected from at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, activated carbon, carbon nanotubes and graphene.

9. The preparation method according to any one of claims 1 - 8, characterized in that, the iron source is an organic ferrous source, and the organic ferrous source is selected from at least one of ferrous oxalate and ferrous acetate.

10. The preparation method according to any one of claims 1 - 9, characterized in that, the sodium source is selected from at least one of sodium pyrophosphate, sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium hydroxide, sodium phosphate, disodium hydrogen phosphate, sodium oxalate and sodium acetate.

11. The preparation method according to any one of claims 1 - 10, characterized in that, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate and sodium phosphate.

12. The preparation method according to any one of claims 1 - 11, characterized in that, By adjusting the addition amounts of the iron source, the sodium source, and the phosphorus source, the molar ratio of sodium, iron, and phosphorus elements in the raw materials is 4:3:

4.

13. The preparation method according to any one of claims 1-12, wherein, the sintering process includes low-temperature pre-sintering and high-temperature sintering carried out in sequence. The sintering temperature of the low-temperature pre-sintering is 360°C - 420°C, and the sintering temperature of the high-temperature sintering is 480°C - 550°C.

14. The preparation method according to claim 13, wherein, the sintering temperature of the low-temperature pre-sintering is 400°C - 420°C, and the sintering time is 2h - 6h.

15. The preparation method according to claim 13 or 14, wherein, the sintering temperature of the high-temperature sintering is 500°C - 550°C, and the sintering time is 8h - 14h.

16. An iron-based polyphosphate-type sodium-ion battery cathode material, wherein, it is prepared by the preparation method according to any one of claims 1-15.

17. A positive electrode sheet, wherein, it includes the iron-based polyphosphate-type sodium-ion battery cathode material according to claim 16.

18. A sodium-ion battery, wherein, it includes the positive electrode sheet according to claim 17.

19. An electrical device, wherein, it includes the sodium-ion battery according to claim 18.

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