CN107611429B - Sodium-rich vanadium iron phosphate sodium material, preparation method thereof and application thereof in sodium-ion battery - Google Patents

Abstract

The invention discloses a sodium-rich vanadium iron phosphate sodium material, a preparation method thereof and application thereof in a sodium-ion battery, wherein Na is_{3+ x}Fe³⁺ _1‑xFe²⁺ _xV(PO₄)₃The material has a NASICON type structure, wherein x is more than or equal to 0.05 and less than or equal to 0.9; the preparation method is that cheap sodium source, iron source, phosphorus source and vanadium source are adopted to be formed by ball milling and sintering, and Na with single crystal phase and high electrochemical activity can be obtained_3+xFe³⁺ _1‑xFe²⁺ _xV(PO₄)₃A material; the preparation method is simple and easy to implement, mild in condition and high in yield, and when the prepared material is used as a positive electrode material of a sodium-ion battery, the prepared material shows higher first-turn charging capacity, high specific capacity, high working voltage, good cycling stability and excellent rate capability.

Description

Sodium-rich vanadium iron phosphate sodium material, preparation method thereof and application thereof in sodium-ion battery

Technical Field

The invention relates to a positive electrode material of a sodium-ion battery, in particular to Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃Materials and solid phase Synthesis of Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃Method of (1), and Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The application of the sodium ion positive electrode material belongs to the field of sodium ion batteries.

Background

With the rapid development of the lithium ion battery in the field of 3C products and electric vehicles, and the good development prospect, the lithium ion battery is difficult to satisfy the large-scale application in the field of large-scale energy storage due to the shortage of the abundance of the metal lithium resource in the earth crust, and the manufacturing cost of the lithium ion battery also tends to rise continuously along with the shortage of the lithium resource. Compared with lithium element, sodium element has abundant reserves in earth crust and wider sources, and sodium element and lithium are in the same main group in the periodic table of elements, so the sodium element has similar physicochemical properties with lithium. Therefore, sodium ion batteries, which are relatively inexpensive to manufacture and comparable to lithium ion batteries, are the most promising battery system for industrial mass storage. However, the ionic radius of sodium ions is larger than that of lithium ions, so that the sodium ions are more difficult to insert and extract in the electrode material than the lithium ions in dynamics, and the positive redox potential and the larger atomic mass of the sodium ions lead to the lower voltage and the lower energy density of the positive electrode material of the sodium-ion battery. Therefore, it is important to improve the voltage and energy density of the positive electrode material of the sodium ion battery.

Similar to lithium ion batteries, sodium ion battery positive electrode materials are more typically represented by P2 type and O3 type layered oxide systems, such as P2-Na_2/3[Fe_1/2Mn_1/2]O₂，O3-NaFe_0.5Co_0.5O₂However, the layered material is unstable in an organic electrolyte and is easily decomposed at a high voltage, resulting in poor cycle performance of the battery. In the polyanionic positive electrode material system, Na₃V₂(PO₄)₃The nano-composite material has an NASICON type structure and good thermal stability and electrochemical stability, and sodium ions have excellent conductivity in the NASICON crystal structure, so that the nano-composite material has outstanding high-rate charge and discharge performance. The single vanadium source is expensive, the earth crust abundance is low, and compared with the iron source, the iron source has low price and wide source. The use amount of vanadium element can be reduced by adopting the iron source, and the cost and the environmental pressure can be effectively reduced. But Na₃Fe₂V(PO₄)₃The ferric iron of (a) is not electrochemically active during the first charge cycle, which greatly limits the application of the material in full cells as full electricityWhen the cell positive electrode material is used, the sodium source is completely derived from the positive electrode material, and the quantity of sodium which is extracted from the positive electrode material is reduced, which means that the sodium which can be extracted from the cell is reduced, the cell capacity is low, and the cell positive electrode material is not beneficial to being used in a commercial full cell.

Disclosure of Invention

Aiming at Na in the prior art₃Fe₂V(PO₄)₃The invention aims to provide sodium-rich Na with stable property, single crystal phase and high electrochemical activity_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃A material.

The invention also aims to provide a method for realizing high-purity Na based on all-solid-phase synthesis under mild conditions_{3+ x}Fe³⁺ _1-xFe²⁺ _xV(PO₄)₃The preparation method of the material has the advantages of short steps, simple operation and low cost, and can realize large-scale production.

It is a third object of the present invention to provide the Na-forming compound_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is applied as the positive electrode material of the sodium-ion battery, and the sodium-ion battery has high coulombic efficiency of a first circle, high energy density, high working voltage, good circulation stability and excellent rate performance.

In order to achieve the technical purpose, the invention provides Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃A material having a NASICON-type structure; na (Na)_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃X in the material is more than or equal to 0.05 and less than or equal to 0.9.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material not only contains ferrous ions, but also is rich in sodium ions, and is relatively Na₃Fe₂V(PO₄)₃The electrochemical performance is greatly improved, and the rate performance is betterAnd the contained ferrous ions have higher electrochemical activity in the first charging process, can effectively improve the first coulomb efficiency of the material and overcome Na₃Fe₂V(PO₄)₃The first turn of coulomb has low efficiency. While Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃Has NASICON (fast ion conductor) type crystal structure and single crystal phase structure, can realize the fast transmission of sodium ions, and thus has good rate performance.

Preferred embodiment, the Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is particles with the particle size of 50-3000 nm.

Preferred embodiment, Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃Has a trigonal system of space group

The invention also provides Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The preparation method of the material comprises the steps of mixing a sodium source, a phosphorus source, a vanadium source and an iron source through ball milling, placing the mixture in a protective atmosphere or a reducing atmosphere, and heating to 500-750 ℃ for sintering to obtain the material.

According to the preferable scheme, the sodium source, the phosphorus source, the vanadium source and the iron source are metered according to the molar ratio of Na to P to V to Fe of 3.05-4.05 to 3 to 1. More preferably, the molar ratio is 3.1 to 3.8:3:1: 1. The amount of the sodium source determines the amount of the synthesized sodium-rich Na_{3+ x}Fe³⁺ _1-xFe²⁺ _xV(PO₄)₃The proportion of ferrous ions in the phase affects the coulombic efficiency of the first turn of the material. An inappropriate ratio of the sodium source may result in the synthesis of a sodium-rich phase or in the production of a large amount of a heterogeneous phase.

In a preferred embodiment, the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium sulfate, sodium bisulfate, sodium citrate, and sodium hydroxide; a more preferred sodium source is sodium carbonate.

Preferably, the phosphorus source comprises at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate and sodium dihydrogen phosphate; a more preferred source of phosphorus is diammonium phosphate.

In a preferred embodiment, the vanadium source includes at least one of vanadium pentoxide, ammonium metavanadate, vanadium, vanadyl acetylacetonate; a more preferred vanadium source is vanadium pentoxide.

In a preferred embodiment, the iron source includes at least one of ferrous chloride, ferrous sulfate, ferric nitrate, ferric chloride, ferric ammonium citrate, ferrous oxide, and ferric oxide, and a more preferred iron source is ferrous sulfate.

In a preferred embodiment, the ball milling conditions are as follows: the mass ratio of the ball materials is 30-120: 1; the rotating speed of the main machine is 300-1200 r/min, and the ball milling time is 6-24 h. The more preferable ball material mass ratio is 60-90: 1. More preferably, the rotating speed of the main machine is 500-1000 r/min, and the ball milling time is 10-14 h. The optimized ball milling condition can fully mix the solid powder, improve the reactivity of the raw materials, and facilitate the subsequent solid phase reaction so as to reduce the generation of impurity phases.

More preferably, the ball milling is carried out in an organic solvent medium. The preferable organic solvent is acetone and/or absolute ethyl alcohol, and the like, and the organic solvents have better wettability to various solid principles, improve the ball milling mixing effect, fully and uniformly mix various raw materials and reduce the generation of impure phases.

According to the preferred scheme, after the sodium source, the phosphorus source, the vanadium source and the iron source are subjected to ball milling and mixing, the mixture is dried at the temperature of 80-120 ℃, and is sieved by a 100-400-mesh sieve, and the sieved powder is taken out for sintering.

In a more preferred embodiment, the sintering process is as follows: heating to 500-750 ℃ at a heating rate of 2-10 ℃/min, and sintering for 6-48 h; the protective atmosphere is nitrogen and/or argon; the reducing atmosphere is a mixed atmosphere of hydrogen and inert gas. By controlling the sintering temperature, time and heating rate, Na with less impurity phase, complete crystallization and moderate particle size can be obtained_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is beneficial to improving the electrical property of the material. The sintering atmosphere may be appropriately adjusted depending on the selected raw material, and for example, a reducing atmosphere may be selected when a ferric iron source is selected, or a protective atmosphere or a reducing atmosphere may be selected when a ferrous iron source and a ferric iron source are mixed. The firing atmosphere is preferably a reducing atmosphere, preferably a mixed atmosphere of hydrogen and argon (hydrogen volume concentration: 5% to 10%, most preferably 10%).

The invention also provides Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The application of the material is characterized in that: the material is applied as a sodium ion positive electrode material.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is applied as a positive electrode material of a sodium ion battery, the sodium ion battery is assembled by adopting the existing method, and the performance of the sodium ion battery is tested: weighing the above Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is prepared by adding 10 wt.% of conductive carbon black as a conductive agent and 10 wt.% of PVDF as a binder, fully grinding, adding a small amount of NMP, mixing to form uniform black paste slurry, coating the slurry on an aluminum foil current collector as a test electrode, and assembling a button cell by taking a metal sodium sheet as a contrast electrode, wherein an electrolytic liquid system is 1M NaClO₄and/PC. The charge-discharge current density for testing the cycle performance is 100mAh g^-1(1C magnification).

Compared with the prior art, the technical scheme of the invention has the following beneficial effects:

na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material has a sodium fast ion conductor structure with a single crystal phase, so that Na is added_{3+ x}Fe³⁺ _1-xFe²⁺ _xV(PO₄)₃The ionic conductivity of the material is increased, and the electrochemical activity of the material is improved.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is synthesized by a solid phase method, and Na is synthesized by a ball milling mixing combination method_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material improves the contact between solid raw materials through high-energy ball milling, so that the reaction is more sufficient, the generation of impurity phases is reduced, and meanwhile, the high-energy mechanical ball milling can improve the reaction activity of the raw materials, so that the material can synthesize sodium-rich Na in the subsequent sintering process_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃A phase pure material.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material is rich in sodium ions and is used in a full battery, the deintercalated sodium element is derived from the positive electrode material, and the positive electrode material deintercalates more sodium element in the first circle, which means that the battery has higher stable charge-discharge capacity, and the significance of effectively improving the first circle coulomb efficiency of the positive electrode material on the full battery is great.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The first charging process of the material has a 2.5V platform corresponding to ferrous ions, compared with Na₃Fe³⁺ ₂V(PO₄)₃Capacity of material charged first cycle, rich in Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃The first circle of the material appears a 2.5V charging platform, and more sodium atoms can be removed, so that the sodium atoms which can be removed and inserted in the full-cell are correspondingly increased, and the capacity is increased.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material has good rate performance and excellent cycle performance, and the material has higher power density due to the two discharge platforms of 2.5V and 3.4V.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The preparation process of the material uses cheap sodium source, phosphorus source, vanadium source and iron source as raw materials and adopts a solid phaseBall milling to prepare pure phase Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The material reduces the cost, simplifies the synthesis process and is suitable for large-scale production.

Na of the invention_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃When the material is used as a positive electrode material of a sodium-ion battery, the material has high coulombic efficiency, high energy density, high working voltage, good cycle stability and excellent rate capability.

Drawings

FIG. 1 shows Na prepared in example 1_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃An X-ray diffraction pattern of the material;

FIG. 2 is Na prepared in example 1_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃Scanning electron micrographs of the material;

FIG. 3 is Na prepared in example 1_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃First turn charging profile of the material.

FIG. 4 is Na prepared in comparative example 1₃Fe³⁺ ₂V(PO₄)₃First turn charging profile of the material.

Detailed Description

The following examples are intended to illustrate the invention in further detail; and the scope of the claims of the present invention is not limited by the examples.

Example 1

The embodiment comprises the following steps:

step (1): this example is designed to yield 0.03mol of the desired product Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃Adding 1400g of zirconia ball grinding beads into 0.0525mol of sodium carbonate, 0.09mol of diammonium hydrogen phosphate, 0.015mol of vanadium pentoxide and 0.0015mol of ferrous sulfate, and adding a certain amount of acetone as a grinding medium;

step (2): ball-milling for 12h at the rotating speed of 1000r/min, drying in an oven at 80 ℃, crushing, grinding, and sieving with a 100-400 mesh sieve to obtain Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃A material precursor;

and (3): sintering the precursor obtained in the step (2) at 600 ℃ for 12h in a hydrogen-argon mixed atmosphere (containing 10% of hydrogen), wherein the heating rate is as follows: cooling at 5 deg.C/min to obtain Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃A material;

the button cell is assembled by the sodium-ion battery composite positive electrode material prepared by the embodiment and a sodium sheet, and the material characteristics and the electrochemical properties of the button cell are shown in the figure:

FIG. 1 shows the successful synthesis of Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃A material.

FIG. 2 shows the synthesis of Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃The material has uniform particle size distribution and average particle size of 400 nm.

FIG. 3 is Na_3.5Fe³⁺ _0.5Fe²⁺ _0.5V(PO₄)₃The first-loop charging specific capacity of the button cell assembled by the material and the sodium sheet is 68.3mAh g under 1C multiplying power^-1And in comparative example Na₃Fe³⁺ ₂V(PO₄)₃First cycle charge of the material is rich in Na_3.5Fe³⁺ _0.5Fe^{2 +} _0.5V(PO₄)₃The first circle of the material appears a 2.5V charging platform, and more sodium atoms can be removed, so that the sodium atoms which can be removed and inserted in the full-cell are correspondingly increased, and the capacity is increased.

Example 2

The embodiment comprises the following steps:

step (1): this example is designed to yield 0.03mol of the desired product Na_3.2Fe³⁺ _0.8Fe²⁺ _0.2V(PO₄)₃Adding 1200g of zirconia ball grinding beads into 0.048mol of sodium carbonate, 0.09mol of diammonium hydrogen phosphate, 0.015mol of vanadium pentoxide and 0.0015mol of ferrous sulfate, and adding a certain amount of acetone as a grinding medium;

step (2): ball-milling at the rotating speed of 800r/min for 12h, drying in an oven at the temperature of 80 ℃, crushing, grinding, and sieving with a 100-400-mesh sieve to obtain Na_3.2Fe³⁺ _0.8Fe²⁺ _0.2V(PO₄)₃A material precursor;

and (3): sintering the precursor obtained in the step (2) at 580 ℃ for 14h in a high-purity argon atmosphere, wherein the heating rate is as follows: cooling at 5 deg.C/min to obtain Na_3.2Fe³⁺ _0.8Fe²⁺ _0.2V(PO₄)₃A material;

the cell assembly and testing method for the material obtained in this example was the same as in example 1, Na_3.2Fe³⁺ _0.8Fe²⁺ _0.2V(PO₄)₃The average particle size of the material was 500 nm. The first charging specific capacity is 63mAh g^-1。

Example 3

Step (1): this example is designed to yield 0.03mol of the desired product Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃The material is prepared by adding 0.0555mol of sodium carbonate, 0.09mol of diammonium hydrogen phosphate, 0.03mol of ammonium metavanadate and 0.0015mol of ferrous chloride into 1000g of zirconia ball grinding beads, and adding a certain amount of acetone as a grinding medium;

step (2): ball-milling at the rotating speed of 800r/min for 14h, drying in an oven at the temperature of 80 ℃, crushing, grinding, and sieving with a 100-400-mesh sieve to obtain Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃A material precursor;

and (3): sintering the precursor obtained in the step (2) at 580 ℃ for 12h in a high-purity argon atmosphere, wherein the heating rate is as follows: cooling at 3 deg.C/min to obtain Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃A material;

the cell assembly and testing method for the material obtained in this example was the same as in example 1, Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃The average particle size of the material was 450 nm. The first charging specific capacity is 70mAh g^-1，

Comparative example 1

The content of the raw material elements is adjusted to be 3:3:1 molar ratio of Na to P to V to Fe

Step (1): this example is designed to yield 0.03mol of the desired product Na₃FeV(PO₄)₃Adding 1400g of zirconia ball grinding beads into 0.045mol of sodium carbonate, 0.09mol of ammonium dihydrogen phosphate, 0.015mol of vanadium pentoxide and 0.03mol of ferrous sulfate, and adding a certain amount of acetone as a grinding medium;

step (2): ball-milling at the rotating speed of 800r/min for 12h, drying in an oven at the temperature of 80 ℃, crushing, grinding, and sieving with a 100-400-mesh sieve to obtain Na₃FeV(PO₄)₃A material precursor;

and (3): heating the precursor obtained in the step (2) to 600 ℃ at 350 ℃ under the atmosphere of high-purity argon, and sintering for 12h, wherein the heating speed is as follows: cooling at 5 deg.C/min to obtain Na₃FeV(PO₄)₃A material.

The cell assembly and testing method for the material obtained in this example was the same as in example 1, Na₃FeV(PO₄)₃The average particle size of the material was 800 nm. The first charging specific capacity is 56mAh g^-1As shown in fig. 4.

Comparative example 2

The raw material mixing is changed into: general grinding

Step (1): in this embodiment, 0.03mol of a target product material is generated, wherein 1000g of zirconia ball grinding beads are added with 0.0555mol of sodium carbonate, 0.09mol of diammonium hydrogen phosphate, 0.03mol of ammonium metavanadate and 0.0015mol of ferrous chloride, and a certain amount of acetone is added as a grinding medium;

step (2): grinding raw materials, and sieving with a 100-400 mesh sieve to obtain Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃A material precursor;

and (3): sintering the precursor obtained in the step (2) at 580 ℃ for 12h in a high-purity argon atmosphere, wherein the heating rate is as follows: cooling at 3 deg.C/min to obtain Na_3.7Fe³⁺ _0.3Fe²⁺ _0.7V(PO₄)₃A material;

the material obtained in this comparative example contains a heterogeneous phase of unknown name, and was assembled into a half-cell, the cell assembly and testing method was the same as in example 1, and the average particle size of the material was 2000 nm. The first charging specific capacity is 30mAh g^-1。

Claims (8)

1. Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that:

has a NASICON type structure;

Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃x in the positive electrode material of the sodium-ion battery is more than or equal to 0.05 and less than or equal to 0.9; the Na is_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is particles with the particle size of 50-3000 nm;

said Na_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is prepared by the following method: and (2) mixing a sodium source, a phosphorus source, a vanadium source and an iron source by ball milling, placing in a protective atmosphere or a reducing atmosphere, and heating to 500-750 ℃ for sintering to obtain the vanadium-containing iron-based catalyst.

2. Na according to claim 1_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that: the ratio of the sodium source, the phosphorus source, the vanadium source and the iron source is measured according to the molar ratio of Na to P to V to Fe of 3.05-4.05 to 3 to 1.

3. Na according to claim 2_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that:

the sodium source comprises at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium sulfate, sodium bisulfate, sodium citrate and sodium hydroxide;

the phosphorus source comprises at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate and sodium dihydrogen phosphate;

the vanadium source comprises at least one of vanadium pentoxide, ammonium metavanadate, vanadium and vanadyl acetylacetonate;

the iron source comprises at least one of ferrous chloride, ferrous sulfate, ferric nitrate, ferric chloride, ferric ammonium citrate, ferrous oxide and ferric oxide.

4. Na according to any one of claims 1 to 3_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that: the ball milling conditions are as follows: the mass ratio of the ball materials is 30-120: 1; the rotating speed of the main machine is 300-1200 r/min, and the ball milling time is 6-24 h.

5. Na according to claim 4_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that: the ball milling is carried out in an organic solvent medium.

6. Na according to any one of claims 1 to 3 and 5_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that: and (3) performing ball milling and mixing on a sodium source, a phosphorus source, a vanadium source and an iron source, drying at the temperature of 80-120 ℃, sieving by a 100-400-mesh sieve, and taking powder below the sieve for sintering.

7. Na according to any one of claims 1 to 3 and 5_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The positive electrode material of the sodium-ion battery is characterized in that: the sintering process comprises the following steps: heating to 500-750 ℃ at a heating rate of 2-10 ℃/min, and sintering for 6-48 h; the protective atmosphere is nitrogen and/or argon; the reducing atmosphere is a mixed atmosphere of hydrogen and inert gas.

8. Na according to claim 1_3+xFe³⁺ _1-xFe²⁺ _xV(PO₄)₃The application of the positive electrode material of the sodium-ion battery is characterized in that: the material is applied as a sodium ion positive electrode material.

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