Precursor for polyanionic sodium-ion battery positive electrode material and preparation method therefor

Abstract

Disclosed in the present invention are a precursor for a polyanionic sodium-ion battery positive electrode material and a preparation method therefor. The chemical general formula of the precursor is NaxMyHzOa(POb)c·mH2O, wherein M is a transition metal element Fe and / or Mn. The relationship between x, y, z, a, b, and c is x+3y+z-2a+5c-2b×c=0, and m is greater than or equal to 0. The precursor for the polyanionic sodium-ion battery positive electrode material and the preparation method therefor of the present invention have the characteristics of high phase purity, high compaction density, excellent electrochemical performance, low costs, and wide system applicability.

Description

Precursor for polyanionic sodium-ion battery cathode material and its preparation method Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a precursor for a polyanion-type sodium-ion battery cathode material and its preparation method. Background Technology

[0002] Polyanionic sodium-ion battery cathode materials have gained market favor due to their high structural stability, thermodynamic stability, and excellent cycle stability and rate performance. Currently, the most researched polyanionic material systems include sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium phosphate, and sodium vanadium fluorophosphate. These materials consist of a three-dimensional framework structure formed by the interlacing of alkali metal tetrahedrons, transition metal tetrahedrons / octahedrons, and anionic tetrahedrons / trihedrons in a point / surface / line manner.

[0003] Currently, commercially available materials such as sodium iron phosphate pyrophosphate are synthesized using solid-state methods. These methods utilize highly crystalline anhydrous FePO4 and FeC2O4 as transition metal precursors, combined with water-soluble sodium, phosphorus, and carbon sources. The process involves grinding, mixing, spray drying, and sintering. However, highly crystalline FePO4 and FeC2O4 are difficult to completely dissolve during grinding, often remaining as solid particles ranging from tens to hundreds of nanometers in the solution. These particles contain tens of thousands of atoms, making it difficult to homogenize the sodium, transition metals, and phosphorus in the slurry. This can easily lead to phase separation during sintering, affecting the material's electrochemical performance. Furthermore, grinding highly crystalline FePO4 and FeC2O4 is time-consuming, requires high equipment parameters, consumes a lot of energy, and results in significant process costs. Therefore, in order to obtain polyanionic materials with high phase purity and low manufacturing costs, it is necessary to develop suitable precursors. These precursors must contain the elements required for the prepared materials and exist in an amorphous bonded state. Summary of the Invention

[0004] The purpose of this invention is to provide a precursor for a polyanionic sodium-ion battery cathode material and its preparation method, which has the characteristics of high phase purity, high compaction density, excellent electrochemical performance, low cost and wide applicability.

[0005] This invention can be achieved through the following technical solutions:

[0006] This invention discloses a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b )c ·mH₂O;

[0007] Where M is a transition metal element Fe and / or Mn; the relationship among x, y, z, a, and b is 1 < x ≤ 3, 2 ≤ y ≤ 3, 3 ≤ b ≤ 4, 0 < z ≤ 0.5, 0 < a ≤ 0.5, c = (-x - 3y - z + 2a) / (5 - 2b), and the value range of m is m ≥ 0.

[0008] Na x M y H z O a (PO b ) c In the Na x M y (PO b ) c ·mH₂O precursor, Na z O a is adsorbed on the surface of Na x M y (PO b ) c to maintain the conservation of positive and negative charges of elements during the precipitation process, and its content accounts for a relatively low proportion.

[0009] Another aspect of the present invention is to protect a method for preparing a precursor for a cathode material of a polyanionic sodium ion battery, including the following steps:

[0010] S1. Preparation of the precursor solution: Mix a transition metal source with an acid solution, adjust the pH of the solution to promote the dissolution of the transition metal, and generate a transparent solution;

[0011] S2. Preparation of the pre-oxidized precursor solution: Add an oxidant to the above solution to oxidize the divalent metal ions in the solution to trivalent, and generate a pre-oxidized precursor solution;

[0012] S3. Preparation of the Na x M y H z O a (PO b ) c ·mH₂O precipitate: Add a phosphorus source and a sodium source to the above pre-oxidized precursor solution, and at the same time use an alkaline compound to adjust the pH of the solution, so that the transition metal ions in the solution form a Na x M y H z O a (PO b ) c ·mH₂O precipitate;

[0013] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c ·mH2O precursor powder.

[0014] In this invention, the precursor has the general chemical formula Na. x M y H z O a (PO b ) c The precursor, ·mH₂O, is formed by the reaction of a transition metal source with acid to create a water-soluble substance. After oxidation, it bonds with phosphorus-containing compounds such as phosphate and pyrophosphate under high pH conditions, precipitating out and undergoing solid-liquid separation and washing to obtain the final product. This precursor is a long-range disordered amorphous structure composed of transition metal iron or manganese ions, phosphate, pyrophosphate, hydrogen ions, hydroxide ions, and lattice water. Its primary particle size is in the nanometer range, and the internal ionic bonding is uniform, making it easily dispersed by grinding. When mixed with other components, the dispersion is high. The polyanionic material prepared after sintering exhibits high crystallinity, high phase purity, and excellent electrochemical performance.

[0015] Furthermore, in step S1, the pH range is ≤4. When the pH is ≥4, the transition metal source cannot completely dissolve and is easily retained in the precursor precipitate, affecting the mixing uniformity between ions and hindering the preparation of high-purity polyanionic materials. In step S3, the pH range is ≥3.0. The purpose is to allow the transition metal ions in the solution to undergo a co-precipitation reaction with phosphate, pyrophosphate, hydrogen ions, hydroxide ions, etc., to form Na+. x M y H z O a (PO b ) c The precipitate precipitates in the form of mH2O. This precipitate has a disordered, non-static structure, which is beneficial for the uniform distribution of elements in the subsequent material preparation process. When the pH is below 3.0, the precipitate cannot precipitate completely, which affects the precursor yield.

[0016] Furthermore, in step S2, the molar amount of oxidant added is 1.0 to 2.0 times the molar amount of transition metal in the precursor solution. When the amount added is less than 1.0 times, the divalent transition metal in the solution is insufficient to completely oxidize the precipitate, affecting the element ratio in the precipitate. When the amount added is more than 2.0 times, the excessive oxidant will increase the input cost.

[0017] Furthermore, in step S2, the oxidant is one or more of hydrogen peroxide, peracetic acid, ammonium persulfate, sodium hypochlorite, sodium percarbonate, sodium perborate, sodium permanganate, and chromic acid; the purpose is to oxidize the divalent transition metal ions in the solution, so that they can subsequently undergo a co-precipitation reaction with other anions in the solution to precipitate out.

[0018] Further, in step S3, the phosphorus source is one or more of the following: phosphate compound, pyrophosphate compound, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, polyphosphoric acid, and sodium polyphosphate; the phosphate compound is one or more of the following: phosphoric acid, sodium phosphate and its acidic derivatives, and ammonium phosphate and its acidic derivatives; the pyrophosphate compound is one or more of the following: pyrophosphate, sodium pyrophosphate and its acidic derivatives, and ammonium pyrophosphate and its acidic derivatives.

[0019] Further, in step S3, the sodium source is an inorganic sodium-containing compound and / or an organic sodium-containing compound. The inorganic sodium-containing compound is one or more of sodium sulfate, sodium nitrate, and sodium chloride; the organic sodium-containing compound is one or more of sodium formate, sodium acetate, sodium citrate, sodium ascorbate, sodium carboxymethyl cellulose, sodium ethoxide, and sodium phenolate.

[0020] Further, in step S3, the alkaline compound is a nitrogen-containing compound and / or a sodium-containing alkaline compound. The nitrogen-containing compound is one or more of ammonia, aniline, and amine; the sodium-containing alkaline compound is one or more of sodium oxide, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, and sodium acetate.

[0021] Furthermore, in step S4, the solid-liquid separation method is one or more of centrifugation, filtration, pressure filtration, and vacuum filtration, and Na is separated by centrifugal force or gravity compression. x M y H z O a (PO b ) c • A combination of one or more methods for the rapid removal of free water from the surface by mH2O.

[0022] Further, in step S2, the transition metal source is one or more of the following: a non-water-soluble manganese source, a water-soluble manganese source, a non-water-soluble iron source, and a water-soluble iron source; the non-water-soluble manganese source is one or more of the following: elemental manganese, manganese oxalate, manganese hydroxide, manganese anhydride, manganese anhydride, permanganate anhydride, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetroxide; the water-soluble manganese source is one or more of the following: manganese sulfate, manganese nitrate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate; the non-water-soluble iron source is one or more of the following: elemental iron, ferric oxalate, ferric oxide, ferrous oxide, ferric tetroxide, ferrous hydroxide, and ferric hydroxide; and the water-soluble iron source is one or more of the following: ferric sulfate, ferric chloride, ferric acetate, ferric citrate, ferric nitrate, ferric gluconate, and ferrous ammonium sulfate.

[0023] Further, the acid solution is an inorganic acid and / or an organic acid; the acid solution is one or more of the following: nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, permanganic acid, hydrobromic acid, hydroiodic acid, sulfurous acid, phosphoric acid, oxalic acid, hydrofluoric acid, chromic acid, carbonic acid, silicic acid, nitrous acid, hydrosulfuric acid, hypochlorous acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, and succinic acid.

[0024] This invention provides a precursor for a polyanionic sodium-ion battery cathode material and its preparation method, which has the following beneficial effects:

[0025] First, the phase purity is high, Na x M y H z O a (PO b ) c The transition metals, phosphate, and pyrophosphate in the mH2O precursor are relatively uniformly mixed. When using it as a precursor to prepare polyanionic materials, only the required sodium and carbon sources need to be added, as both are water-soluble raw materials that can be dissolved by Na+. x M y H z O a (PO b ) c • The pores in the mH2O particles penetrate into the interior of the material, achieving uniform mixing between elements and thus preparing materials with high homogeneity and high phase purity;

[0026] Second, high compaction density, Na x M y H z O a (PO b ) cThe elements in the mH2O precursor are bonded to each other. When polyanionic materials are prepared using this as raw material, the elements bond to each other and grow. There is no need for long-distance diffusion between ions. Crystal nuclei can be formed in situ and epitaxial growth can be carried out. The melt forms dense single crystal particles, and there are fewer pores between crystals. The density is high and the compaction density is greatly improved.

[0027] Third, it has excellent electrochemical performance due to Na x M y H z O a (PO b ) c The high uniformity of element mixing in the mH2O precursor results in high crystallinity and phase purity of the prepared material. The elemental sites and sodium ion diffusion channels in the crystal exhibit a regular and orderly arrangement, thus giving it excellent rate performance and high capacity utilization.

[0028] Fourth, low cost, Na x M y H z O a (PO b ) c The preparation of ·mH2O can be achieved using inexpensive transition metal sources. It can be mixed with other ions through acid dissolution, and then precipitated by subsequent oxidation to obtain a high-value precursor for polyanionic materials. In addition, the amorphous nature and elemental uniformity of this precursor mean that it can achieve uniform mixing between ions without the need for special grinding equipment or long-term grinding during the preparation of polyanionic materials. This will significantly reduce grinding energy consumption and save manufacturing costs.

[0029] Fifth, the system has wide applicability, with a variety of polyanionic materials, including iron pyrophosphate / sodium manganese, iron pyrophosphate / sodium manganese, iron phosphate / sodium manganese, and non-stoichiometric sodium iron / manganese phosphorus systems. Depending on the type and content of different material elements, we can adjust the solution pH, the amount and type of raw materials added, to prepare different series of materials using Na. x M y H z O a (PO b ) c • mH2O precursor, thus matching the application of various high-performance polyanionic materials. Attached Figure Description

[0030] Figure 1 shows the application example 1. Na 1.40 Fe 2.91 H 0.13 O 0.13 (PO 7.5 Preparation of 4·2H2O precursor and Na4Fe 2.91SEM of (PO4)2P2O7 material;

[0031] Figure 2 shows the preparation of Na4Fe from the anhydrous FePO4 precursor in Comparative Example 1. 2.91 (PO4)2P2O7 material SEM. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0033] This invention discloses a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b ) c ·mH2O;

[0034] Where M is a transition metal element Fe and / or Mn; the relationship between x, y, z, a, b, and c is x + 3y + z - 2a + 5c - 2b × c = 0, and the range of m is m ≥ 0.

[0035] Another aspect of the present invention relates to a method for preparing a precursor for a polyanionic sodium-ion battery cathode material, comprising the following steps:

[0036] S1. Preparation of precursor solution: The transition metal source is mixed with acid solution, the pH of the solution is adjusted to promote the dissolution of the transition metal and generate a transparent solution.

[0037] S2. Preparation of pre-oxidation precursor solution: Add an oxidant to the above solution to oxidize the divalent metal ions in the solution to trivalent metal ions, thereby generating a pre-oxidation precursor solution;

[0038] S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted using an alkaline compound, so that the transition metal ions in the solution formed Na+ in the form of hydrated phosphides. x M y H z O a (PO b ) c ·mH2O precipitation;

[0039] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c ·mH2O precursor powder.

[0040] Furthermore, in step S1, the pH range is ≤4; in step S3, the pH range is ≥3.0.

[0041] Furthermore, in step S2, the molar amount of oxidant added is 1.0 to 2.0 times the molar amount of transition metal in the precursor solution.

[0042] Further, in step S2, the oxidant is one or more of hydrogen peroxide, peracetic acid, ammonium persulfate, sodium hypochlorite, sodium percarbonate, sodium perborate, sodium permanganate, and chromic acid.

[0043] Further, in step S3, the phosphorus source is one or more of the following: phosphate compound, pyrophosphate compound, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, polyphosphoric acid, and sodium polyphosphate; the phosphate compound is one or more of the following: phosphoric acid, sodium phosphate and its acidic derivatives, and ammonium phosphate and its acidic derivatives; the pyrophosphate compound is one or more of the following: pyrophosphate, sodium pyrophosphate and its acidic derivatives, and ammonium pyrophosphate and its acidic derivatives.

[0044] Further, in step S3, the sodium source is an inorganic sodium-containing compound and / or an organic sodium-containing compound. The inorganic sodium-containing compound is one or more of sodium sulfate, sodium nitrate, and sodium chloride; the organic sodium-containing compound is one or more of sodium formate, sodium acetate, sodium citrate, sodium ascorbate, sodium carboxymethyl cellulose, sodium ethoxide, and sodium phenolate.

[0045] Further, in step S3, the alkaline compound is a nitrogen-containing compound and / or a sodium-containing alkaline compound. The nitrogen-containing compound is one or more of ammonia, aniline, and amine; the sodium-containing alkaline compound is one or more of sodium oxide, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, and sodium acetate.

[0046] Furthermore, in step S4, the solid-liquid separation method is one or more of centrifugation, filtration, pressure filtration, and vacuum filtration, and Na is separated by centrifugal force or gravity compression. x M y H z O a (PO b ) c• A combination of one or more methods for the rapid removal of free water from the surface by mH2O.

[0047] Further, in step S2, the transition metal source is one or more of the following: a non-water-soluble manganese source, a water-soluble manganese source, a non-water-soluble iron source, and a water-soluble iron source; the non-water-soluble manganese source is one or more of the following: elemental manganese, manganese oxalate, manganese hydroxide, manganese anhydride, manganese anhydride, permanganate anhydride, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetroxide; the water-soluble manganese source is one or more of the following: manganese sulfate, manganese nitrate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate; the non-water-soluble iron source is one or more of the following: elemental iron, ferric oxalate, ferric oxide, ferrous oxide, ferric tetroxide, ferrous hydroxide, and ferric hydroxide; and the water-soluble iron source is one or more of the following: ferric sulfate, ferric chloride, ferric acetate, ferric citrate, ferric nitrate, ferric gluconate, and ferrous ammonium sulfate.

[0048] Further, the acid solution is an inorganic acid and / or an organic acid; the acid solution is one or more of the following: nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, permanganic acid, hydrobromic acid, hydroiodic acid, sulfurous acid, phosphoric acid, oxalic acid, hydrofluoric acid, chromic acid, carbonic acid, silicic acid, nitrous acid, hydrosulfuric acid, hypochlorous acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, and succinic acid.

[0049] Example 1

[0050] This embodiment relates to a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b ) c ·mH2O; where M is the transition metal element Fe; the relationship between x, y, z, a, b, and c is x+3y+z-2a+5c-2b×c=0, and the range of m is m≥0.

[0051] The precursor was prepared by the following method:

[0052] S1. Preparation of precursor solution: Mix the transition metal source with acid solution, adjust the pH of the solution to ≤4, promote the dissolution of transition metal, and generate a transparent solution;

[0053] S2. Preparation of the pre-oxidized precursor solution: An oxidant is added to the above solution to oxidize the divalent metal ions in the solution to trivalent ions, generating a pre-oxidized precursor solution. Specifically, the molar amount of the oxidant added is 2.0 times the molar amount of the transition metal in the precursor solution; the oxidant is hydrogen peroxide or peracetic acid. Specifically, the transition metal source is a non-water-soluble iron source or a water-soluble iron source; the non-water-soluble iron source is elemental iron, ferric oxalate, ferric gluconate, or ferrous ammonium sulfate. The acid solution is nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, permanganic acid, hydrobromic acid, hydroiodic acid, sulfurous acid, or succinic acid.

[0054] S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted to ≥3.0 using an alkaline compound, causing the transition metal ions in the solution to form Na+ phosphate ions in the form of hydrated phosphate compounds. x M y H z O a (PO b ) c •mH2O precipitate. Specifically, the phosphorus source is phosphate compounds and pyrophosphate compounds. Phosphate compounds include phosphoric acid, sodium phosphate, and their acidic derivatives; pyrophosphate compounds include pyrophosphate, sodium pyrophosphate, and their acidic derivatives. The sodium source is inorganic sodium-containing compounds, including sodium sulfate, sodium nitrate, and sodium chloride. The basic compounds are nitrogen-containing compounds and sodium-containing basic compounds. Nitrogen-containing compounds include ammonia and aniline; sodium-containing basic compounds include sodium oxide, sodium citrate, sodium formate, and sodium acetate.

[0055] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c •mH2O precursor powder. Specifically, the solid-liquid separation method is centrifugation and filtration.

[0056] Example 2

[0057] This embodiment relates to a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b ) c·mH2O; where M is a transition metal element Mn; the relationship between x, y, z, a, b, and c is x+3y+z-2a+5c-2b×c=0, and the range of m is m≥0.

[0058] The precursor was prepared by the following method:

[0059] S1. Preparation of precursor solution: Mix the transition metal source with acid solution, adjust the pH of the solution to ≤4, promote the dissolution of transition metal, and generate a transparent solution;

[0060] S2. Preparation of the pre-oxidized precursor solution: An oxidant is added to the above solution to oxidize the divalent metal ions in the solution to trivalent ions, generating a pre-oxidized precursor solution. Specifically, the molar amount of the added oxidant is 1.5 times the molar amount of the transition metal in the precursor solution; the oxidant is hydrogen peroxide, peracetic acid, sodium perborate, sodium permanganate, or chromic acid. Specifically, the transition metal source is a non-water-soluble manganese source or a water-soluble manganese source; the non-water-soluble manganese source is elemental manganese, manganese oxalate, or manganese hydroxide; the water-soluble manganese source is hypochlorous acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, or succinic acid.

[0061] S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted to ≥3.0 using an alkaline compound, causing the transition metal ions in the solution to form Na+ phosphate ions in the form of hydrated phosphate compounds. x M y H z O a (PO b ) c •mH2O precipitates. Specifically, the phosphorus source is metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, polyphosphoric acid, and sodium polyphosphate. The sodium source is an organic sodium-containing compound, including sodium formate, sodium acetate, sodium citrate, and sodium ascorbate. The basic compounds are nitrogen-containing compounds and sodium-containing basic compounds; the nitrogen-containing compounds are ammonia and amines; the sodium-containing basic compounds are sodium oxide, sodium citrate, and sodium carbonate.

[0062] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c•mH2O precursor powder. Specifically, the solid-liquid separation method is pressure filtration, and Na is achieved by centrifugal force or gravity extrusion. x M y H z O a (PO b ) c •mH2O rapidly removes free water from the surface.

[0063] Example 3

[0064] This embodiment relates to a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b ) c ·mH2O; where M is the transition metal element Fe and Mn; the relationship between x, y, z, a, b, and c is x+3y+z-2a+5c-2b×c=0, and the range of m is m≥0.

[0065] The precursor was prepared by the following method:

[0066] S1. Preparation of precursor solution: Mix the transition metal source with acid solution, adjust the pH of the solution to ≤4, promote the dissolution of transition metal, and generate a transparent solution;

[0067] S2. Preparation of the pre-oxidized precursor solution: An oxidant is added to the above solution to oxidize the divalent metal ions in the solution to trivalent ions, generating a pre-oxidized precursor solution. Specifically, the molar amount of the oxidant added is 1.0 times the molar amount of the transition metal in the precursor solution; the oxidant is hydrogen peroxide, peracetic acid, sodium permanganate, or chromic acid. Specifically, the transition metal source is a water-soluble manganese source or a water-soluble iron source; the water-soluble manganese source is manganese sulfate or manganese nitrate; the water-soluble iron source is ferric sulfate, ferric chloride, ferric acetate, ferric citrate, or ferric nitrate. The acid solution is formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, or succinic acid.

[0068] S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted to ≥3.0 using an alkaline compound, causing the transition metal ions in the solution to form Na+ phosphate ions in the form of hydrated phosphate compounds. x M y H z O a (POb ) c •mH2O precipitates. Specifically, the phosphorus source is phosphate compounds, polyphosphoric acid, and sodium polyphosphate, with phosphoric acid being the phosphate compound. The sodium source is inorganic and organic sodium compounds; inorganic sodium compounds include sodium sulfate, sodium nitrate, and sodium chloride; organic sodium compounds include sodium formate, sodium acetate, sodium citrate, and sodium ascorbate. The basic compounds are nitrogen-containing compounds, including ammonia, aniline, and amines.

[0069] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c •mH2O precursor powder. Specifically, the solid-liquid separation method is vacuum filtration, with Na being separated by centrifugal force or gravity extrusion. x M y H z O a (PO b ) c •mH2O rapidly removes free water from the surface.

[0070] Example 4

[0071] This embodiment relates to a precursor for a polyanionic sodium-ion battery cathode material, the precursor having the general chemical formula Na. x M y H z O a (PO b ) c ·mH2O; where M is the transition metal element Fe and Mn; the relationship between x, y, z, a, b, and c is x+3y+z-2a+5c-2b×c=0, and the range of m is m≥0.

[0072] The precursor was prepared by the following method:

[0073] S1. Preparation of precursor solution: Mix the transition metal source with acid solution, adjust the pH of the solution to ≤4, promote the dissolution of transition metal, and generate a transparent solution;

[0074] S2. Preparation of the pre-oxidized precursor solution: An oxidant is added to the above solution to oxidize the divalent metal ions in the solution to trivalent ions, generating a pre-oxidized precursor solution. Specifically, the molar amount of the oxidant added is 1.3 times the molar amount of the transition metal in the precursor solution; the oxidant is sodium hypochlorite, sodium percarbonate, sodium perborate, sodium permanganate, or chromic acid. Specifically, the transition metal source is a water-insoluble manganese source and a water-soluble iron source; the water-insoluble manganese source is elemental manganese, permanganic anhydride, manganese monoxide, manganese dioxide, manganese trioxide, or manganese tetroxide; the water-soluble iron source is ferric sulfate, ferric chloride, or ferric acetate. The acid solution is hydrosulfuric acid, hypochlorous acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, or succinic acid.

[0075] S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted to ≥3.0 using an alkaline compound, causing the transition metal ions in the solution to form Na+ phosphate ions in the form of hydrated phosphate compounds. x M y H z O a (PO b ) c •mH2O precipitation. Specifically, the phosphorus source is pyrophosphate compounds and polyphosphoric acid; the pyrophosphate compounds are pyrophosphate, sodium pyrophosphate and its acidic derivatives, and ammonium pyrophosphate and its acidic derivatives. The sodium source is inorganic sodium-containing compounds and organic sodium-containing compounds; the inorganic sodium-containing compounds are sodium sulfate, sodium nitrate, and sodium chloride; the organic sodium-containing compounds are sodium formate, sodium acetate, sodium citrate, sodium ascorbate, sodium carboxymethyl cellulose, sodium ethoxide, and sodium phenolate. The basic compounds are nitrogen-containing compounds and sodium-containing basic compounds; the nitrogen-containing compounds are ammonia, aniline, and amines; the sodium-containing basic compounds are sodium oxide, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, and sodium acetate.

[0076] S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c •mH2O precursor powder. Specifically, the solid-liquid separation method is centrifugation and pressure filtration, using centrifugal force or gravity extrusion to achieve Na… x M y H z O a (PO b) c • A combination of one or more methods for the rapid removal of free water from the surface by mH2O.

[0077] Application Example 1

[0078] Na 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 Preparation of 4·2H2O precursor and Na4Fe 2.91 Synthesis and Electrochemical Properties of (PO4)2P2O7 Materials

[0079] Step 1: Mix high-purity atomized iron powder and sulfuric acid with water at a molar ratio of 2.91:1.2. Adjust the pH of the solution to 2.0 with sulfuric acid to promote complete dissolution of the atomized iron powder and form a transparent light green solution.

[0080] Step 2: Add hydrogen peroxide to the above solution to oxidize the ferrous iron in the solution to ferric iron, generating a pre-oxidized precursor solution;

[0081] Step 3: Add phosphoric acid and pyrophosphate (molar ratio of phosphoric acid to iron is 2:2.91; molar ratio of pyrophosphate to iron is 1:2.91) to the above precursor solution, mix well, and then add sodium formate (molar ratio of sodium acetate to iron is 1.40:2.91). Subsequently, slowly add sodium hydroxide to adjust the pH of the solution to 6.0, so that the transition metal ions and phosphorus-containing functional groups in the solution can form Na+. 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 4·mH2O precipitate;

[0082] Step 4: After centrifugation and washing, the above precipitate is obtained as a polyanionic cathode material using Na. 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 )4·2H2O precursor powder.

[0083] Will Na 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75The precursor Na₄·2H₂O, sodium acetate, and sucrose (added at a molar ratio of 1:2.6, 0.2 times the molar amount of sodium acetate) were mixed with water and ground for approximately 0.1 hours. Compared to the preparation using commercially available anhydrous FePO₄ precursor in Comparative Example 1, this method significantly reduced grinding energy consumption and process costs. When the solid particle size Dmax in the slurry was ≤50 nm, the slurry was spray-dried at an inlet air temperature of 300°C and an outlet air temperature of 90°C to remove moisture and obtain a dry precursor powder. Finally, the precursor powder was calcined at 650°C for 12 hours in a nitrogen atmosphere, followed by natural cooling to obtain Na₄Fe₂O₃. 2.91 (PO4)2P2O7 material.

[0084] Figure 1 shows the use of Na 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 Na4Fe2+ prepared from 4·2H2O precursor 2.91 SEM images of the (PO4)2P2O7 material show a regular elliptical morphology, with small primary particles and dense packing with almost no obvious pores. This indicates that during sintering, the amorphous precursor melted and crystallized, resulting in larger grains with intercalation and a denser structure. Furthermore, Table 1 shows that the porosity and specific surface area of ​​this material are 1.2% and 3.2 μm, respectively. 2 The density of this material is lower than that of the material prepared from the anhydrous FePO4 precursor in Comparative Example 1, indicating that the amorphous precursor has a higher degree of melting during sintering, resulting in more compacted particle growth and a significant decrease in porosity and specific surface area. Therefore, this material exhibits a higher compaction density of 2.27 g / cm³. 3 This is better than Comparative Example 1.

[0085] Na4Fe 2.91 (PO4)2P2O7 material, SurP, and PVDF5130 were mixed with NMP in a mass ratio of 9.5:0.2:0.3. The mixture was homogenized using a high-speed homogenizer to form a uniform black slurry with high fluidity. The black slurry was then coated onto aluminum foil using a 150µm four-sided coating tool. The membrane was dried in a vacuum drying oven at 100°C for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a die-cutting machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 button cell was assembled in a glove box.

[0086] Table 1 shows the electrochemical performance test results for Na4Fe. 2.91The (PO4)2P2O7 electrode exhibits a discharge specific capacity of 125.6 mAh / g at a rate of 0.1C (1C = 129 mAh / g), with a capacity utilization rate close to its theoretical specific capacity. This is higher than the discharge specific capacity of 89.3 mAh / g in Comparative Example 1, indicating that the material has good crystallinity, high phase purity, and a low content of defects formed in the structure during sintering. This is consistent with the Na... 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 The uniformity of ion distribution in the 4·2H₂O precursor is related to the precipitation process of this precursor. During precipitation, ions interlock with each other through weak interactions, forming a uniform precipitate. This uniformity is maintained during the subsequent preparation of polyanionic materials, which is beneficial for local nucleation and melting processes during sintering. Furthermore, Table 1 shows that the capacity retention of this electrode at 10C is as high as 99.1% compared to 0.1C, which is superior to the 85.2% in Comparative Example 1. This is due, on the one hand, to the high phase purity of the material, with low content of vacancies and anti-dislocations in the structure, resulting in continuous ion diffusion channels, low diffusion barriers, and excellent ion diffusion kinetics. On the other hand, the material has low porosity, high melting degree, tight interparticle connections, and high electronic conductivity. The combined effect of these two factors gives the material its excellent rate performance. Ultimately, after 1000 cycles at a 1C rate, the electrode maintained a capacity retention of 99.4%, with almost no capacity decay. This is related to the material's small specific surface area; a smaller specific surface area results in fewer side reactions between the material interface and the electrolyte at high voltages, less dissolution of elements at the material interface, and higher structural stability. On the other hand, it is also related to the material's high phase purity. Higher phase purity means fewer impurity grain boundaries, more uniform stress dispersion due to volume expansion, and effective maintenance of crystal integrity during cycling. In summary, using Na... 1.40 Fe 2.91 H 0.13 O 0.13 (PO 3.75 Na4Fe2+ prepared from 4·2H2O precursor 2.91 (PO4)2P2O7 materials possess superior electrochemical performance due to their high phase purity.

[0087] Application Example 2

[0088] Na 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 Preparation of 4·2H2O precursor and Na4Fe 1.79 Mn 1.12Synthesis and Electrochemical Properties of (PO4)2P2O7 Materials

[0089] Step 1: Mix Fe2O3, Mn2O3, sulfuric acid and water in a ratio of 0.895:0.56:1.2. Adjust the pH of the solution to 1.0 with sulfuric acid to promote the reaction of Fe2O3 and Mn2O3 and generate a transparent brownish-green solution.

[0090] Step 2: Add hydrogen peroxide to the above brownish-green solution to oxidize the ferrous iron in the solution to ferric iron, generating a pre-oxidized precursor solution;

[0091] Step 3: Add sodium dihydrogen phosphate and sodium hydrogen pyrophosphate (molar ratio of sodium dihydrogen phosphate to Fe2O3 is 2:0.895, molar ratio of sodium hydrogen pyrophosphate to Fe2O3 is 1:0.895) to the above precursor solution, mix well, then add sodium acetate (molar ratio of sodium acetate to Fe2O3 is 2.43:0.895), and finally slowly add sodium carbonate to adjust the pH of the solution to 6.5, so that the transition metal ions in the solution can be converted into Na+. 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 4·mH2O precipitate;

[0092] Step 4: After centrifugation and washing, the above precipitate is obtained as a polyanionic cathode material using Na. 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 )4·2H2O precursor powder.

[0093] Will Na 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 The precursor 4·2H₂O, sodium acetate, and citric acid (added in an amount 0.3 times the molar amount of sodium acetate) were mixed and ground with water at a molar ratio of 1:2.37. The grinding time was approximately 0.25 hours, which is significantly shorter than that of Comparative Example 2, resulting in lower process costs. This is related to the fact that Na… 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75The Na₄·2H₂O precursor itself is non-static, and the bonding energy between ions is not high, so the particles can be dispersed by simple grinding. When the solid particle size Dmax in the slurry is ≤30nm, the slurry is spray-dried with an inlet air temperature of 290°C and an outlet air temperature of 100°C to remove moisture, obtaining a dry precursor powder. Finally, the precursor powder is calcined at 670°C for 8 hours in a nitrogen atmosphere, and after natural cooling, Na₄Fe₂O₃ is obtained. 1.79 Mn 1.12 (PO4)2P2O7 material.

[0094] The results in Table 1 show that Na4Fe 1.79 Mn 1.12 The porosity and specific surface area of ​​the (PO4)2P2O7 material are 1.5% and 2.7 m², respectively. 2 / g, which is much lower than the result of Comparative Example 2, is related to the low porosity of this material and the greater degree of melting between particles during sintering, indicating that amorphous Na 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 The 4·2H₂O precursor, due to its weak crystallinity, tends to spontaneously melt and densify during the subsequent high-temperature sintering process in material preparation. This has a significant regulatory effect on reducing the porosity and specific surface area of ​​the finished material, ultimately resulting in a material exhibiting a high compaction density (2.23 g / cm³). 3 ).

[0095] Na4Fe 1.79 Mn 1.12 (PO4)2P2O7 material, SurP, and PVDF5130 were mixed with NMP in a mass ratio of 9.5:0.2:0.3. The mixture was homogenized using a high-speed homogenizer to form a uniform black slurry with high fluidity. The black slurry was then coated onto aluminum foil using a 150µm four-sided coating tool. The membrane was dried in a vacuum drying oven at 100°C for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a die-cutting machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 button cell was assembled in a glove box.

[0096] Table 1 shows the electrochemical performance test results, indicating that the electrode exhibits a discharge capacity of 121.4 mAh / g at a rate of 0.1C (1C = 129 mAh / g), which is superior to the results of Comparative Example 2. This is related to the high phase purity and crystallinity of the material. Higher phase purity means fewer defects in the crystal structure that hinder ion and electron transport and diffusion, which is more conducive to the high capacity of the material. Furthermore, Table 1 shows that the electrode retains 98.4% of its capacity at 10C, significantly higher than the 79.2% in Comparative Example 2. This indicates a lower sodium ion diffusion barrier and faster ion conduction rate in the material structure. Simultaneously, the low porosity and tight particle contact further enhance the electron transport rate, resulting in excellent rate performance. Ultimately, after 1000 cycles at a 1C rate, the electrode maintained a capacity retention of 98.7%, demonstrating excellent cycle stability. This indicates that higher phase purity and better crystal integrity in the material result in more uniform forces generated by volume expansion during sodium insertion / extraction processes, reducing the probability of structural pulverization and improving cycle stability. Simultaneously, the material's relatively small specific surface area also reduces its catalytic effect on the electrolyte under high voltage, thereby minimizing side reactions at the material interface and further enhancing cycle stability. In conclusion, using Na... 2.43 Fe 1.79 Mn 1.12 H 0.04 O 0.04 (PO 3.75 Na4Fe2+ prepared from 4·2H2O precursor 1.79 Mn 1.12 (PO4)2P2O7 materials possess superior electrochemical performance due to their high phase purity.

[0097] Comparative Example 1: Preparation of Na4Fe from anhydrous FePO4 precursor 2.91 (PO4)2P2O7 materials and their electrochemical properties

[0098] Anhydrous FePO4, phosphoric acid, and sodium acetate were mixed with water in a molar ratio of 2.91:1.0:4.0 and added with citric acid (0.2 times the molar amount of sodium acetate). The mixture was ground for more than 10 hours, resulting in high energy consumption and low efficiency. This is related to the high crystallinity of commercially available anhydrous FePO4, which makes it difficult to reduce the particle size during grinding. When the particle size Dmax of the solid particles in the slurry was ≤50 nm, the slurry was spray-dried at an inlet air temperature of 300°C and an outlet air temperature of 90°C to remove moisture, yielding a dried precursor powder. Finally, the precursor powder was calcined at 650°C for 12 hours in a nitrogen atmosphere, followed by natural cooling to obtain Na4Fe. 2.91 (PO4)2P2O7 material.

[0099] Figure 2 shows Na4Fe2.91 SEM images of the (PO4)2P2O7 material revealed elliptical, near-spherical particles, consisting of primary particles ranging from 50 to 200 nm in size. These particles were interlocked, leaving numerous pores. Table 1 shows that the material exhibits a high porosity of 19.4% and a specific surface area of ​​14.6 μm. 2 / g, significantly higher than in Application Example 1. This is related to the inherent characteristics of the commercially available anhydrous FePO4 precursor, which is mostly a high-temperature sintered phase at around 700°C, with high crystallinity and high particle hardness, even in Na4Fe 2.91 During the grinding process of the (PO4)2P2O7 precursor, the particle size of the anhydrous FePO4 precursor decreased significantly, but this grinding process did not significantly improve its particle hardness. In the subsequent sintering process, the highly crystalline nano-FePO4 precursor was difficult to melt together to form dense particles, leading to a significant increase in porosity and specific surface area, and a decrease in compaction density (1.56 g / cm³). 3 ).

[0100] Na4Fe 2.91 (PO4)2P2O7 material, SurP, and PVDF5130 were mixed with NMP in a mass ratio of 9.5:0.2:0.3. The mixture was homogenized using a high-speed homogenizer to form a uniform black slurry with high fluidity. The black slurry was then coated onto aluminum foil using a 150µm four-sided coating tool. The membrane was dried in a vacuum drying oven at 100°C for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a die-cutting machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 button cell was assembled in a glove box.

[0101] The results in Table 1 show that the material's discharge capacity at a rate of 0.1C (1C = 129 mAh / g) is only 89.3 mAh / g, lower than that of Application Example 1. This capacity reduction is closely related to the phase purity of the material, which is determined by the ion uniformity during the precursor preparation process. This indicates that the material prepared using anhydrous FePO4 as a precursor may contain inactive sodium iron phosphate or low-capacity sodium pyrophosphate impurities due to the inability to effectively and uniformly mix elements such as sodium, iron, and phosphorus. This reduces the effective active components per unit mass, leading to a decrease in the material's capacity. Furthermore, Table 1 shows that the electrode's capacity retention at 10C is only 85.2% compared to 0.1C, worse than Application Example 1. This is partly due to the numerous pores in the material hindering electronic conductivity, and partly due to the presence of impurities generating a large number of grain boundaries. These grain boundaries impede the rapid conduction of ions between particles, ultimately leading to a decrease in the material's capacity at high rates. Ultimately, after 1000 cycles at 1C, the electrode exhibited a capacity retention of only 92.6%, showing significant degradation. This is attributed to the material's high specific surface area. A higher specific surface area leads to more electrolyte decomposition sites at higher voltages, and the resulting byproducts negatively impact the stability of the material's interface structure. Furthermore, the anisotropic expansion of the impurity phases during sodium insertion / extraction during the process easily generates microcracks between the impurity phase and the main phase, leading to continuous electrolyte decomposition and affecting the material's structural stability. In summary, the Na4Fe2O3 electrode prepared using anhydrous FePO4 precursor... 2.91 The unevenness of the material reaction caused by the low phase purity of (PO4)2P2O7 material will accelerate the deterioration of the material's electrochemical performance.

[0102] Comparative Example 2: Preparation of Na4Fe from anhydrous FePO4 and Mn2O3 precursors 1.79 Mn 1.12 (PO4)2P2O7 materials and their electrochemical properties

[0103] Anhydrous FePO4, Mn2O3, and sodium dihydrogen phosphate were mixed with citric acid (added in an amount 0.3 times the molar amount of sodium dihydrogen phosphate) and water in a molar ratio of 1.79:0.56:4.0 and ground for more than 15 hours. This grinding time is significantly longer than in Application Example 2. The main reason is that anhydrous FePO4 and Mn2O3 have higher crystallinity and harder crystals, making grinding more difficult, time-consuming, inefficient, and costly. When the particle size Dmax of the solid particles in the slurry is ≤30nm, the slurry is spray-dried at an inlet air temperature of 290°C and an outlet air temperature of 100°C to remove moisture, yielding a dried precursor powder. Finally, the precursor powder is calcined at 670°C for 8 hours in a nitrogen atmosphere, followed by natural cooling to obtain Na4Fe. 1.79 Mn1.12 (PO4)2P2O7 material.

[0104] The physicochemical test data in Table 1 show that the porosity and specific surface area of ​​this material are 16.9% and 13.8 μm, respectively. 2 / g, far exceeding that of Application Example 2, is related to the anhydrous FePO4 and Mn2O3 precursors. This type of precursor has high crystallinity, large grain size, and high crystal integrity. It is not prone to melting shrinkage after sintering, and the porosity content is high after the particles are packed together, resulting in a large specific surface area and a lower compaction density (1.68 g / cm³). 3 ).

[0105] Na4Fe 1.79 Mn 1.12 (PO4)2P2O7 material, SurP, and PVDF5130 were mixed with NMP in a mass ratio of 9.5:0.2:0.3. The mixture was homogenized using a high-speed homogenizer to form a uniform black slurry with high fluidity. The black slurry was then coated onto aluminum foil using a 150µm four-sided coating tool. The membrane was dried in a vacuum drying oven at 100°C for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a die-cutting machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 button cell was assembled in a glove box.

[0106] The results in Table 1 show that the discharge specific capacity of this material at a rate of 0.1C (1C=129mAh / g) is only 75.3mAh / g, which is lower than that of the material in Application Example 2. The capacity performance is related to the phase purity of the material, indicating that the material may contain inactive sodium iron phosphate, sodium manganese phosphate, or low-capacity sodium iron pyrophosphate, sodium manganese pyrophosphate, etc., which reduces the active components per unit mass and leads to a decrease in the specific capacity. This is mainly due to the uneven mixing of elements in the material prepared using anhydrous FePO4 and Mn2O3 as precursors. Table 1 also shows that the capacity retention rate of this electrode at a rate of 10C is only 79.2% compared to 0.1C. The lower rate performance is related to the large number of grain boundaries between the aforementioned impurities and the main phase, which hinder the sodium ion transport process and lead to a decrease in the rate performance of the material. At the same time, the large number of pores in the particles also hinders electron diffusion, thus affecting the rate performance of the material. Ultimately, after 1000 cycles at 1C, the electrode exhibited a capacity retention of only 90.2%, indicating severe capacity decay and indirectly reflecting the instability of the material structure. This is due, firstly, to the different volume expansion rates of the main phase and impurity phases, leading to cracks at the crystal interface, and the continuous decomposition of the electrolyte further damaging the material's interface structure. Secondly, the material's high specific surface area catalyzes its reaction with the electrolyte under high voltage, causing problems such as interface dissolution and structural rock formation, thus affecting the material's cycling stability. In summary, the Na4Fe prepared using anhydrous FePO4 and Mn2O3 precursors... 1.79 Mn 1.12 The unevenness of the material reaction caused by the low phase purity of (PO4)2P2O7 material will accelerate the deterioration of the material's electrochemical performance.

[0107] Table 1 Performance Test Results

[0108]

[0109] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A precursor for a polyanionic sodium-ion battery cathode material, characterized in that: The precursor has the general chemical formula Na. x M y H z O a (PO b ) c ·mH2O; Where M is a transition metal element Fe and / or Mn; the relationship between x, y, z, a, b, and c is x + 3y + z - 2a + 5c - 2b × c = 0, and the range of m is m ≥ 0.

2. A method for preparing a precursor for a polyanionic sodium-ion battery cathode material, characterized in that... Includes the following steps: S1. Preparation of precursor solution: The transition metal source is mixed with acid solution, the pH of the solution is adjusted to promote the dissolution of the transition metal and generate a transparent solution. S2. Preparation of pre-oxidation precursor solution: Add an oxidant to the above solution to oxidize the divalent metal ions in the solution to trivalent metal ions, thereby generating a pre-oxidation precursor solution; S3, Na x M y H z O a (PO b ) c Preparation of mH2O precipitate: Phosphorus and sodium sources were added to the above pre-oxidation precursor solution, and the pH of the solution was adjusted using an alkaline compound, so that the transition metal ions in the solution formed Na+ in the form of hydrated phosphides. x M y H z O a (PO b ) c ·mH2O precipitation; S4. Purification treatment: After separating the above precipitate from its solid-liquid state and washing it, the polyanion-type sodium-ion battery cathode material is obtained using Na... x M y H z O a (PO b ) c ·mH2O precursor powder.

3. The method for preparing the precursor for the polyanionic sodium-ion battery cathode material according to claim 2, characterized in that: In step S1, the pH range is ≤4; in step S3, the pH range is ≥3.

0.

4. The method for preparing the precursor for the polyanionic sodium-ion battery cathode material according to claim 2, characterized in that: In step S2, the molar amount of oxidant added is 1.0-2.0 times the molar amount of transition metal in the precursor solution.

5. The method for preparing the precursor for the polyanionic sodium-ion battery cathode material according to claim 2, characterized in that: In step S2, the oxidant is one or more of the following: hydrogen peroxide, peracetic acid, ammonium persulfate, sodium hypochlorite, sodium percarbonate, sodium perborate, sodium permanganate, and chromic acid.

6. The method for preparing the precursor for the positive electrode material of a polyanionic sodium-ion battery according to claim 2, characterized in that: In step S3, the phosphorus source is one or more of the following: phosphate compound, pyrophosphate compound, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, polyphosphoric acid, and sodium polyphosphate; the phosphate compound is one or more of the following: phosphoric acid, sodium phosphate and its acidic derivatives, and ammonium phosphate and its acidic derivatives; the pyrophosphate compound is one or more of the following: pyrophosphoric acid, sodium pyrophosphate and its acidic derivatives, and ammonium pyrophosphate and its acidic derivatives.

7. The method for preparing the precursor for the positive electrode material of a polyanionic sodium-ion battery according to claim 2, characterized in that: In step S3, the sodium source is an inorganic sodium-containing compound and / or an organic sodium-containing compound. The inorganic sodium-containing compound is one or more of sodium sulfate, sodium nitrate, and sodium chloride. The organic sodium-containing compound is one or more of sodium formate, sodium acetate, sodium citrate, sodium ascorbate, sodium carboxymethyl cellulose, sodium ethoxide, and sodium phenolate.

8. The method for preparing the precursor for the positive electrode material of a polyanionic sodium-ion battery according to claim 2, characterized in that: In step S3, the alkaline compound is a nitrogen-containing compound and / or a sodium-containing alkaline compound. The nitrogen-containing compound is one or more of ammonia, aniline, and amine. The sodium-containing alkaline compound is one or more of sodium oxide, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, and sodium acetate.

9. The precursor for the positive electrode material of a polyanionic sodium-ion battery according to claim 2, characterized in that: In step S4, the solid-liquid separation method is one or more of centrifugation, filtration, pressure filtration, and vacuum filtration, and Na is separated by centrifugal force or gravity compression. x M y H z O a (PO b ) c • A combination of one or more methods for the rapid removal of free water from the surface by mH2O.

10. The method for preparing the precursor for the polyanionic sodium-ion battery cathode material according to claim 2, characterized in that: In step S1, The transition metal source is one or more of the following: a non-water-soluble manganese source, a water-soluble manganese source, a non-water-soluble iron source, and a water-soluble iron source; The non-water-soluble manganese source is one or more of elemental manganese, manganese oxalate, manganese hydroxide, manganese anhydride, manganese anhydride, permanganate anhydride, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetroxide; the water-soluble manganese source is one or more of manganese sulfate, manganese nitrate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate. The non-water-soluble iron source is one or more of elemental iron, ferric oxalate, ferric oxide, ferrous oxide, iron(II,III) oxide, ferrous hydroxide, and ferric hydroxide; the water-soluble iron source is one or more of ferric sulfate, ferric chloride, ferric acetate, ferric citrate, ferric nitrate, ferric gluconate, and ferrous ammonium sulfate. The acid solution is an inorganic acid and / or an organic acid; the acid solution is one or more of the following: nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, permanganic acid, hydrobromic acid, hydroiodic acid, sulfurous acid, phosphoric acid, oxalic acid, hydrofluoric acid, chromic acid, carbonic acid, silicic acid, nitrous acid, hydrosulfuric acid, hypochlorous acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, malic acid, benzoic acid, salicylic acid, and succinic acid.

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