Preparation method of polyanionic sodium-ion battery cathode material

Abstract

The application discloses a preparation method of a polyanionic sodium ion battery positive electrode material, and comprises the following steps: S1, preparation of a precursor solution: a transition metal M source, an acid solution and a complexing agent are mixed and dissolved to form a precursor solution; S2, preparation of a pre-oxidized precursor solution: an oxidizing agent is added to the solution to generate a pre-oxidized precursor solution; S3, preparation of M(OH)x precipitation: a pH value of the pre-oxidized precursor solution is adjusted so that transition metal ions in the solution are precipitated in the form of M(OH)x; S4, preparation of a precursor powder: the M(OH)x precipitation, a sodium source, a phosphorus source and a carbon source are mixed wetly to obtain a precursor slurry; S5, drying of the precursor powder: the precursor slurry is dried and separated to obtain a dried precursor powder; and S6, high-temperature sintering: the precursor powder is sintered at a high temperature to obtain a polyanionic material. The polyanionic sodium ion battery positive electrode material has the characteristics of excellent electrochemical performance, high compaction density and low cost.

Description

Preparation method of polyanion sodium-ion battery cathode material Technical Field

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

[0002] Polyanionic sodium-ion battery cathode materials have gained market favor due to their high structural stability, thermodynamic stability, excellent cycle stability, and rate performance. Currently, the most researched polyanionic material systems include sodium iron phosphate pyrophosphate, sodium vanadium phosphate, and sodium vanadium fluorophosphate. These materials consist of a three-dimensional framework structure formed by the interlacing of alkali metal tetrahedra, transition metal tetrahedra / octahedrons, and anionic tetrahedra / trihedra in a point / surface / line manner. This type of polyanionic material is mostly prepared using solid-state methods, resulting in materials with low compaction density, high porosity, low phase purity, and poor electrochemical performance.

[0003] Taking commercially available sodium iron phosphate pyrophosphate as an example, its synthesis process is similar to that of lithium iron phosphate. It typically uses highly crystalline FePO4 as the iron source, along with sodium, phosphorus, and carbon sources, through grinding, spray drying, and sintering. However, highly crystalline FePO4 solids are mostly inactive materials sintered at 600-700℃, with low interfacial energy, making them difficult to dissolve during grinding. They exist as nanoparticles of 200-500 nm in the slurry, preventing effective mixing of sodium ions, transition metal ions, and anions. This leads to phase separation during sintering, negatively impacting the material's electrochemical performance.

[0004] In addition, after the slurry is dried, the spherical particles formed by the stacking of FePO4 nanoparticles contain a large number of pores. These pores are difficult to eliminate due to crystal growth during the sintering process, which is related to the low interfacial energy of FePO4. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a polyanionic sodium-ion battery cathode material, which has the characteristics of excellent electrochemical performance, high compaction density and low cost.

[0006] This invention can be achieved through the following technical solutions:

[0007] This invention discloses a method for preparing a polyanionic sodium-ion battery cathode material, comprising the following steps:

[0008] S1. Preparation of precursor solution: The transition metal M source, acid solution and complexing agent are mixed and dissolved to form a precursor solution;

[0009] S2. Preparation of pre-oxidized precursor solution: Add an oxidant to the above precursor solution to oxidize the easily oxidized divalent ions in the solution to trivalent ions, thereby generating a pre-oxidized precursor solution.

[0010] The preparation of S3 and M(OH)x precipitates involves adjusting the pH of the pre-oxidized precursor solution to precipitate the transition metal ions in the solution as M(OH)x, where x is greater than 0 and depends on the valence state of the transition metal M, with common valence states being +2 or +3.

[0011] S4. Preparation of precursor powder: M(OH)x precipitate, sodium source, phosphorus source and carbon source are wet-mixed and ground to form a uniform precursor slurry;

[0012] S5. Drying of precursor powder: The above precursor slurry is dried and the solid and liquid are separated to obtain dried precursor powder.

[0013] S6. High-temperature sintering: The above precursor powder is sintered at high temperature in a protective atmosphere, and then cooled naturally to obtain the polyanionic material.

[0014] In this invention, to obtain a high-purity, high-compact, and low-cost polyanionic sodium-ion battery material, a suitable precursor is developed. This precursor needs to possess high surface energy, small particle size, and exist in a long-range disordered bonded state. The method for preparing the polyanionic sodium-ion battery cathode material of this invention involves dissolving an inexpensive transition metal element or its oxide in an acid by heating, followed by oxidation with an oxidant. After adjusting the pH with alkali, a high-surface-energy, long-range disordered amorphous bonded state M(OH)x precipitate is generated. This precipitate is used as the transition metal M source, and is ground and mixed uniformly with sodium, phosphorus, and carbon sources. After drying and sintering, the final product material is obtained. This method uses low-cost raw materials and has low manufacturing costs. Simple grinding and mixing are sufficient to achieve uniform diffusion between elements during sintering. Furthermore, because the M(OH)x precipitate particles are fine and have high surface energy, the arrangement between particles is denser after drying, resulting in higher ion migration kinetic energy during sintering, larger crystal growth, denser material particles, and superior compaction density and electrochemical performance.

[0015] Furthermore, between steps S4 and S5, the following step is also included: S41, M(OH)x precipitation purification: The above precipitate is filtered and washed to obtain high-purity M(OH)x precipitate. After purification in this step, the standard for completion of filtration and washing is that the purity of the M(OH)x precipitate is ≥95%. If the purity is lower than this value, the presence of impurity elements will cause impurity phases to be generated during the subsequent synthesis of polyanionic materials, affecting their electrochemical performance.

[0016] Furthermore, in step S3, the pH adjustment range is 2.5-7.0. Within this range, transition metal ions in the solution can slowly combine with hydroxide ions to form M(OH)x precipitates. When the pH is below 2.5, the solution is strongly acidic, and transition metals cannot precipitate out in the form of precipitates. When the pH is above 7.0, the reaction rate of transition metal ions with hydroxide ions is too fast, and large M(OH)x aggregates are easily formed, which is not conducive to the uniform mixing of elements in the subsequent material synthesis.

[0017] Furthermore, in step S6, the conditions for high-temperature sintering are: sintering temperature of 450-700℃ and holding time of 0.1-20H. When the sintering temperature is below 450℃, M(OH)x is not easy to melt and decompose, which can easily lead to uneven local ion distribution and affect the purity of the material phase. When the temperature is above 700℃, the polyanionic material is easy to decompose into a mixture of multiple phases, which causes it to lose its electrochemical activity.

[0018] Furthermore, in step S4, the mixing and grinding method is one or more of the following: sand milling, ball milling, homogeneous dispersion, high-energy grinding, and mechanical pulverization, to achieve uniform dispersion of materials in the form of shear force or mechanical force.

[0019] Furthermore, in step S5, the slurry drying method is one or more of spray drying, high-temperature evaporation, flash drying, and high-temperature pyrolysis; the purpose is to use high temperature to achieve rapid separation between solid and liquid in the slurry, and to prevent ion segregation caused by slow evaporation, which would affect the ion uniformity in the precursor.

[0020] Further, in step S6, the protective atmosphere is one or more of nitrogen, argon, carbon monoxide, argon / hydrogen mixed gas, and nitrogen / hydrogen mixed gas.

[0021] Further, in step S1, the acid solution is an inorganic acid and / or an organic acid, wherein the inorganic acid is one or more of sulfuric acid, hydrochloric acid, and nitric acid, and the organic acid is one or more of citric acid, ascorbic acid, malic acid, formic acid, acetic acid, benzoic acid, and salicylic acid.

[0022] Furthermore, the transition metal M source is one or more of manganese, cobalt, and iron sources. The manganese source is one or more of elemental manganese, manganese dioxide, manganese trioxide, manganese tetroxide, manganese oxalate, manganese carbonate, and basic manganese carbonate. The cobalt source is one or more of elemental cobalt, cobalt carbonate, cobalt oxalate, cobalt oxide, and cobalt tetroxide. The iron source is one or more of elemental iron, iron oxide, iron carbonate, iron tetroxide, and ferrous oxalate.

[0023] Furthermore, the complexing agent is one or more of the following: triacetic acid, ethylenediaminetetraacetic acid and its sodium salt, cyclohexanetetraacetic acid, and ethylene glycol diethyl ether diaminetetraacetic acid.

[0024] Further, in step S2, the oxidant is one or more of the following: hydrogen peroxide, sodium peroxide, potassium peroxide, magnesium peroxide, sodium perchlorate, sodium hypochlorite, peracetic acid, sodium dichromate, potassium dichromate, ammonium persulfate, and sodium perborate.

[0025] Furthermore, in step S3, a weak alkaline substance is used to adjust the pH. This weak alkaline substance is one or more of ammonia, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, and sodium sulfite.

[0026] Further, in step S4, the sodium source is one or more of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium nitrate, sodium phosphate, and sodium monohydrogen / dihydrogen phosphate.

[0027] Furthermore, the phosphorus source is one or more of the following: phosphoric acid, sodium monohydrogen phosphate / sodium dihydrogen phosphate, sodium phosphate, pyrophosphate, sodium pyrophosphate, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, ammonium monohydrogen phosphate / ammonium dihydrogen phosphate, and ammonium phosphate.

[0028] Furthermore, in step S4, the carbon source is one or more of the following: carbon black, graphite, graphene, carbon nanotubes, carbon fibers, etc.; sucrose, citric acid, glucose, starch, cellulose, maltose, lactose, etc.

[0029] This invention discloses a method for preparing a polyanionic sodium-ion battery cathode material, which has the following beneficial effects:

[0030] First, excellent electrochemical performance: The disordered structure of M(OH)x precipitate makes it easier to mix uniformly with sodium, phosphorus, and carbon sources during grinding. During sintering, the elements melt and mix in a localized environment, further increasing the uniformity of ion mixing and facilitating the formation of a structure with lower defects, higher crystallinity, and higher phase purity. In this pure phase structure, the sodium ion migration path is smoother, with lower resistance from vacancy defects, antisite defects, and grain boundaries. The sodium ion migration rate and insertion / extraction quantity will be significantly improved, resulting in excellent rate performance and high capacity characteristics in the material.

[0031] Secondly, high compaction density: In this invention, a complexing agent is introduced into the M(OH)x preparation process. This complexing agent can bond with dissolved transition metal ions to form a stable coordination structure. When the pH of the solution is adjusted with a weakly alkaline substance, the bonding energy between transition metal ions and hydroxide ions gradually increases. Accompanied by the slow release of transition metal ions by the complexing agent, M(OH)x nanoprecipitates are gradually formed. The smaller M(OH)x nanoparticles have a larger surface energy. After being mixed evenly with sodium, phosphorus, and carbon sources, the resulting particle packings after drying are more dense and have lower porosity. During sintering, the larger surface energy of the M(OH)x nanoparticles is more conducive to the mutual melting of ions and epitaxial growth, forming a more dense material with even lower porosity, and its compaction density is correspondingly increased.

[0032] Third, low cost: The M(OH)x precursor is a weakly crystalline substance with short-range order and long-range disorder, formed by the connection of transition metal ions and hydroxide ions through van der Waals forces and weak interionic interactions. This substance is produced by reacting inexpensive transition metals with inorganic / organic acids and then alkalizing them. The raw materials are all bulk chemicals, resulting in large production volumes and low costs. In addition, the M(OH)x precursor has low crystal integrity and weak crystallization. It is easier to grind and disperse evenly during the grinding process with sodium, phosphorus, and carbon sources, resulting in high efficiency and low energy consumption. Attached Figure Description

[0033] Figure 1 shows the Na4Fe prepared from the nano-disordered Fe(OH)3 precursor in Example 1. 2.91 SEM of (PO4)2P2O7 material;

[0034] Figure 2 shows Na₄Fe₂ from Comparative Example 1. 2.91 (PO4)2P2O7 material SEM. Detailed Implementation

[0035] 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.

[0036] This invention discloses a method for preparing a polyanionic sodium-ion battery cathode material, comprising the following steps:

[0037] S1. Preparation of precursor solution: The transition metal M source, acid solution and complexing agent are mixed and dissolved to form a precursor solution;

[0038] S2. Preparation of pre-oxidized precursor solution: Add an oxidant to the above precursor solution to oxidize the easily oxidized divalent ions in the solution to trivalent ions, thereby generating a pre-oxidized precursor solution.

[0039] The preparation of S3 and M(OH)x precipitates involves adding pH-adjusted solutions to the pre-oxidized precursor solution to precipitate transition metal ions in the solution as M(OH)x.

[0040] S4. Preparation of precursor powder: M(OH)x precipitate, sodium source, phosphorus source and carbon source are wet-mixed and ground to form a uniform precursor slurry;

[0041] S5. Drying of precursor powder: The above precursor slurry is dried and the solid and liquid are separated to obtain dried precursor powder.

[0042] S6. High-temperature sintering: The above precursor powder is sintered at high temperature in a protective atmosphere, and then cooled naturally to obtain the polyanionic material.

[0043] Furthermore, between steps S4 and S5, the following step is also included: S41, M(OH)x precipitation purification: The above precipitate is filtered and washed to obtain high-purity M(OH)x precipitate. After purification in this step, the standard for completion of filtration and washing is that the purity of the M(OH)x precipitate is ≥95%. If the purity is lower than this value, the presence of impurity elements will cause impurity phases to be generated during the subsequent synthesis of polyanionic materials, affecting their electrochemical performance.

[0044] Furthermore, in step S3, the pH adjustment range is 2.5-7.0. Within this range, transition metal ions in the solution can slowly combine with hydroxide ions to form M(OH)x precipitates. When the pH is below 2.5, the solution is strongly acidic, and transition metals cannot precipitate out in the form of precipitates. When the pH is above 7.0, the reaction rate of transition metal ions with hydroxide ions is too fast, and large M(OH)x aggregates are easily formed, which is not conducive to the uniform mixing of elements in the subsequent material synthesis.

[0045] Furthermore, in step S6, the conditions for high-temperature sintering are: sintering temperature of 450-700℃ and holding time of 0.1-20H. When the sintering temperature is below 450℃, M(OH)x is not easy to melt and decompose, which can easily lead to uneven local ion distribution and affect the purity of the material phase. When the temperature is above 700℃, the polyanionic material is easy to decompose into a mixture of multiple phases, which causes it to lose its electrochemical activity.

[0046] Furthermore, in step S4, the mixing and grinding method is one or more of the following: sand milling, ball milling, homogeneous dispersion, high-energy grinding, and mechanical pulverization, to achieve uniform dispersion of materials in the form of shear force or mechanical force.

[0047] Furthermore, in step S5, the slurry drying method is one or more of spray drying, high-temperature evaporation, flash drying, and high-temperature pyrolysis; the purpose is to use high temperature to achieve rapid separation between solid and liquid in the slurry, and to prevent ion segregation caused by slow evaporation, which would affect the ion uniformity in the precursor.

[0048] Further, in step S6, the protective atmosphere is one or more of nitrogen, argon, carbon monoxide, argon / hydrogen mixed gas, and nitrogen / hydrogen mixed gas.

[0049] Further, in step S1, the acid solution is an inorganic acid and / or an organic acid, wherein the inorganic acid is one or more of sulfuric acid, hydrochloric acid, and nitric acid, and the organic acid is one or more of citric acid, ascorbic acid, malic acid, formic acid, acetic acid, benzoic acid, and salicylic acid.

[0050] Furthermore, the transition metal M source is one or more of manganese, cobalt, and iron sources. The manganese source is one or more of elemental manganese, manganese dioxide, manganese trioxide, manganese tetroxide, manganese oxalate, manganese carbonate, and basic manganese carbonate. The cobalt source is one or more of elemental cobalt, cobalt carbonate, cobalt oxalate, cobalt oxide, and cobalt tetroxide. The iron source is one or more of elemental iron, iron oxide, iron carbonate, iron tetroxide, and ferrous oxalate.

[0051] Furthermore, the complexing agent is one or more of the following: triacetic acid, ethylenediaminetetraacetic acid and its sodium salt, cyclohexanetetraacetic acid, and ethylene glycol diethyl ether diaminetetraacetic acid.

[0052] Further, in step S2, the oxidant is one or more of the following: hydrogen peroxide, sodium peroxide, potassium peroxide, magnesium peroxide, sodium perchlorate, sodium hypochlorite, peracetic acid, sodium dichromate, potassium dichromate, ammonium persulfate, and sodium perborate.

[0053] Furthermore, in step S3, a weak alkaline substance is used to adjust the pH. This weak alkaline substance is one or more of ammonia, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, and sodium sulfite.

[0054] Further, in step S4, the sodium source is one or more of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium nitrate, sodium phosphate, and sodium monohydrogen / dihydrogen phosphate.

[0055] Furthermore, the phosphorus source is one or more of the following: phosphoric acid, sodium monohydrogen phosphate / sodium dihydrogen phosphate, sodium phosphate, pyrophosphate, sodium pyrophosphate, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, ammonium monohydrogen phosphate / ammonium dihydrogen phosphate, and ammonium phosphate.

[0056] Furthermore, in step S4, the carbon source is one or more of the following: carbon black, graphite, graphene, carbon nanotubes, carbon fibers, etc.; sucrose, citric acid, glucose, starch, cellulose, maltose, lactose, etc.

[0057] Application Example 1: Preparation of Nanodisordered Fe(OH)3 Precursor and Na4Fe 2.91Synthesis and Electrochemical Properties of (PO4)2P2O7 Materials

[0058] In this embodiment, nano-disordered Fe(OH)3 precursor and Na4Fe are involved. 2.91 The preparation steps of (PO4)2P2O7 material include:

[0059] Step 1: Dissolve iron oxide red and sulfuric acid in water at a molar ratio of 1:3. After the reaction is complete, the solution will be a transparent light green. Add 0.25 times the molar amount of iron oxide red disodium ethylenediaminetetraacetate as a complexing agent to complex the ferrous ions in the solution and prevent the subsequent reaction from being too fast.

[0060] Step 2: Slowly add hydrogen peroxide to the above light green solution to convert the ferrous ions in the solution into ferric ions, generating an oxidized brownish-red solution.

[0061] Step 3: Slowly add sodium carbonate solid to the above brownish-red solution to neutralize the excess hydrogen ions in the solution and adjust the pH of the solution to 4.0. At this time, red nano Fe(OH)3 precipitate will slowly form in the solution. Due to its large surface energy, the precipitate will be accompanied by the aggregation between particles, which will appear as flocculent precipitate.

[0062] Step 4: Use a 400-mesh filter to filter the above precipitate and wash it repeatedly with deionized water to obtain Fe(OH)3 precipitate with a purity greater than 95%.

[0063] Step 5: Mix Fe(OH)3 precipitate, sodium acetate, and phosphoric acid with water in a molar ratio of 2.91:4:4. Add glucose as a carbon source, with the amount added being 5% of the weight of Fe(OH)3. Grind the mixture using a sand mill until the particle size of the solid particles in the slurry is less than 50nm, forming a uniform dark red precursor slurry.

[0064] Step 6: Spray dry the above-mentioned dark red precursor slurry to remove the moisture in the slurry and obtain dry red precursor powder.

[0065] Step 7: In a nitrogen atmosphere, the above-mentioned red precursor powder is kept at 500°C for 5 hours, and then naturally cooled to obtain Na4Fe. 2.91 (PO4)2P2O7 material.

[0066] Figure 1 shows Na4Fe 2.91SEM images of the (PO4)2P2O7 material show irregularly elliptical particles with a solid structure and no obvious pores. Primary particles are almost entirely absent, indicating that during sintering, all elements undergo melting and crystallization. This is related to Fe(OH)3 as a precursor, which is a nano-disordered state with high surface energy and a metastable structure. During heating, the hydroxide ions in this precursor readily decompose, and the remaining transition metal ions melt with elements such as sodium and phosphorus, crystallizing and growing to form dense, solid particles. Table 1 shows that, with the same carbon content (compared to Comparative Example 1), the specific surface area of ​​this material is only 5.67 m². 2 / g, far lower than the 12.13m in Comparative Example 1. 2 The density of the material is 2.32 g / cm³, indicating low porosity and high densification, consistent with the results shown in the SEM images. Furthermore, Table 1 shows that the compacted density of this material is as high as 2.32 g / cm³. 3 It is much higher than the 1.83 g / cm³ in Comparative Example 1. 3 This indirectly indicates that the material has high density during growth. At the same time, because its surface melt growth is relatively smooth, when subjected to external pressure, the material interfaces slide against each other, making it easier to interlock and increase density, thus reflecting the increase in compaction density.

[0067] Will Na4Fe 2.91 (PO4)2P2O7 material SurP and PVDF5130 are added in a mass ratio of 9.5:0.2:0.3. NMP is mixed, and the above materials are mixed evenly using a high-speed homogenizer to form a black slurry with uniform color and high fluidity. Of The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0068] Table 1 shows the Na4Fe 2.91 The electrochemical performance of the (PO4)2P2O7 electrode was examined. At a rate of 0.1C (1C = 129 mAh / g), the electrode exhibited a discharge capacity of 124.6 mAh / g, significantly higher than the 96.7 mAh / g in Comparative Example 1. This demonstrates the effectiveness of the Na4Fe2O3 electrode prepared using Fe(OH)3 as a precursor. 2.91The (PO4)2P2O7 material exhibits higher phase purity due to its more uniform ion mixing, localized ion migration and crystal growth during sintering, resulting in fewer defects, a higher number of redox transfer charges, and thus improved capacity performance. Furthermore, Table 1 shows that this electrode maintains a capacity retention of 98.2% at 10C compared to 0.1C, significantly higher than the 78.3% in Comparative Example 1. This is likely due to faster ion migration kinetics within the material's structure. Numerous studies have demonstrated that pure-phase structures offer more unobstructed ion diffusion channels, lower migration barriers, faster migration rates, and superior rate performance. Ultimately, this electrode maintained a capacity retention of 99.5% at 1C after 1000 cycles, with virtually no capacity decay. This may be attributed to the material's smaller specific surface area, reduced side reactions with the electrolyte at high potentials, and preserved interfacial structural stability, thereby enhancing its interfacial cycling stability. Furthermore, the cycling stability of this electrode is also related to its crystallinity. Higher crystallinity helps reduce the isotropy caused by crystal volume expansion during sodium insertion / extraction, thereby reducing stress concentration and release phenomena caused by volume expansion, and thus improving its cycling stability. In summary, Na4Fe prepared using Fe(OH)3 precursor... 2.91 (PO4)2P2O7 materials possess superior electrochemical performance due to their high phase purity and small specific surface area.

[0069] Application Example 2: Preparation of Nanodisordered Mn(OH)2 Precursor and Synthesis and Electrochemical Properties of Na2MnP2O7 Material

[0070] In this embodiment, the preparation steps involve a nano-disordered Mn(OH)2 precursor and Na2MnP2O7 material, which include:

[0071] Step 1: Dissolve elemental manganese and sulfuric acid in water at a molar ratio of 1:1. After the reaction is complete, the solution will be a transparent light red color. Add cyclohexanetetraacetic acid (0.3 times the molar amount of elemental manganese) as a complexing agent to complex the manganese ions in the solution and prevent the subsequent reaction from being too fast.

[0072] Step 2: Slowly add hydrogen peroxide to the above light red solution to cause the easily oxidized ions in the solution to undergo an oxidation reaction, generating a pre-oxidized solution;

[0073] Step 3: Slowly add ammonia water to the above pre-oxidized solution to neutralize the excess hydrogen ions in the solution and adjust the pH of the solution to 3.5. At this time, brown nano-Mn(OH)2 precipitate will slowly form in the solution. Because the surface energy of this nano-precipitate is large, the precipitate often presents as flocculent aggregates.

[0074] Step 4: Use a 350-mesh filter to filter the above precipitate and wash it repeatedly with deionized water to obtain Mn(OH)2 precipitate with a purity greater than 95%.

[0075] Step 5: Mix Mn(OH)2 precipitate and sodium dihydrogen phosphate with water at a molar ratio of 1:2, add sucrose as a carbon source, and add sucrose at 6.5% of the weight of Mn(OH)2. Grind the mixture with a sand mill until the particle size of the solid particles in the slurry is less than 70nm to form a uniform dark brown precursor slurry.

[0076] Step 6: Spray dry the above dark brown precursor slurry to remove the moisture from the slurry and obtain dry brown precursor powder.

[0077] Step 7: In a nitrogen atmosphere, the above brown precursor powder is kept at 550°C for 7 hours, and then naturally cooled to obtain Na2MnP2O7 material.

[0078] The results in Table 1 show that, with the same carbon content (compared to Comparative Example 2), the specific surface area of ​​this material is only 4.38 m². 2 / g, far lower than the 13.89m in Comparative Example 2. 2 The / g indicates that during the sintering process, the nano-disordered Mn(OH)2 has a large surface energy and its structure is in an amorphous metastable state, making it prone to melting at high temperatures. It then melts and crystallizes with sodium and phosphorus sources, forming a relatively dense material with low porosity. Furthermore, Table 1 shows that this material has a higher compaction density (2.35 g / cm³). 3 This indirectly indicates the compactness of the material's growth. Furthermore, due to the melting phenomenon during the high-temperature sintering process, the surface of the material is smoother. When subjected to external pressure, the material interfaces slide against each other, making it easier for them to interlock and increase the compaction density.

[0079] Will Na2MnP2O7 material SurP and PVDF5130 were mixed with NMP at a mass ratio of 9.5:0.2:0.3. The above materials were mixed evenly using a high-speed homogenizer to form a black slurry with a uniform color and high fluidity. The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0080] Table 1 shows the electrochemical performance test results. At a rate of 0.1C (1C = 98 mAh / g), the electrode exhibits a discharge capacity of 92.3 mAh / g, significantly higher than the 64.8 mAh / g discharge capacity of Comparative Example 2. This is related to the high phase purity of the material. Higher phase purity, fewer grain boundaries, vacancies, defects, and heterojunctions in the crystal, higher crystal integrity and crystallinity, and more redox transfer charges all contribute to better capacity performance. Furthermore, Table 1 shows that at a rate of 10C, the electrode retains a capacity of 92.5% compared to 0.1C, far exceeding the 64.3% in Comparative Example 2. This indicates that higher phase purity, fewer crystal defects, more unobstructed ion migration channels, lower transition barriers, faster migration rates, and superior rate performance. Finally, at a rate of 1C, after 1000 cycles, the electrode retains a capacity of 99.3%, exhibiting low capacity decay. The high capacity retention is related to the material's low specific surface area, which to some extent reduces the side reactions between the material interface and the electrolyte, reduces the dissolution of manganese ions at the interface, and ensures the stability of the material structure. At the same time, the material's high phase purity also reduces the crystal cracks caused by volume expansion during the sodium insertion / extraction process, effectively reducing the decomposition of the electrolyte at this point and improving the material's cycle stability.

[0081] Comparative Example 1 Na4Fe 2.91 Synthesis and Electrochemical Properties of (PO4)2P2O7 Materials

[0082] In this embodiment, Na4Fe is involved. 2.91 The preparation steps of (PO4)2P2O7 material include:

[0083] Step 1: Mix commercial FePO4, sodium acetate, and phosphoric acid with water in a molar ratio of 2.91:4:1.09. Add glucose as a carbon source, with the amount added being 4.5% of the weight of FePO3. Grind the mixture using a sand mill until the particle size of the solid particles in the slurry is less than 50nm, forming a uniform dark green precursor slurry.

[0084] Step 2: Spray dry the above-mentioned dark green precursor slurry to remove moisture and obtain a dry gray precursor powder; Step 3: In a nitrogen atmosphere, heat the above-mentioned gray precursor powder at 500°C for 5 hours, and then allow it to cool naturally to obtain Na4Fe. 2.91 (PO4)2P2O7 material.

[0085] Figure 2 shows Na4Fe 2.91SEM images of (PO4)2P2O7 material show that it consists of a single-particle stack of approximately 50-100 nm particles with numerous micropores between them. These micropores are difficult to fill due to the rigid connections between the particles and are not easily filled by external forces. The formation of this micropore structure is related to a commercially available highly crystalline FePO4 precursor. This precursor, after high-temperature sintering, forms a highly crystalline state with low surface energy and high hardness. The particles formed after spray drying tend to accumulate, leaving numerous pores. Furthermore, due to its low surface energy, it cannot effectively melt and grow during sintering, resulting in a large number of micropores remaining in the final product. Table 1 shows that, with the same carbon content (compared to Application Example 1), the specific surface area of ​​this material is as high as 12.13 m². 2 / g, with a compacted density of only 1.83g / cm³. 3 The results were far lower than those in Application Example 1, indicating that the porous material has a high porosity and low compaction, which leads to a significant increase in the exposed interface. At the same time, the roughness of the interface makes it difficult for the materials to slide against each other under external pressure, and a large number of pores are more likely to remain between the particles, resulting in a decrease in compaction density.

[0086] Will Na4Fe 2.91 (PO4)2P2O7 material SurP and PVDF5130 are added in a mass ratio of 9.5:0.2:0.3. NMP is mixed, and the above materials are mixed evenly using a high-speed homogenizer to form a black slurry with uniform color and high fluidity. Of The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0087] Table 1 shows the Na4Fe 2.91 The electrochemical performance of the (PO4)2P2O7 electrode was examined. At a rate of 0.1C (1C = 129 mAh / g), its discharge capacity was only 96.7 mAh / g, significantly lower than the 124.6 mAh / g in Comparative Example 1. This reduction in capacity is correlated with the phase purity of the material. When the material contains a large amount of non-electrochemically active phase, its capacity will decrease substantially. This indicates that the Na4Fe2O3 electrode prepared using commercially available highly crystalline FePO4 as a precursor... 2.91The (PO4)2P2O7 material suffers from phase separation during grinding because FePO4 is difficult to mix uniformly with sodium and phosphorus sources. This leads to uneven ion concentrations and phase separation during sintering, ultimately resulting in a reduced specific capacity. Furthermore, Table 1 shows that the capacity retention of this electrode at 10C is only 78.3% compared to 0.1C, significantly lower than the 98.2% in Application Example 1. This lower rate performance is related to the presence of impurity phases (sodium iron phosphate or sodium iron pyrophosphate) in the material. The presence of numerous grain boundaries between these impurity phases and the main phase results in a lower sodium ion transport rate at these grain boundaries, exhibiting a certain degree of hysteresis and contributing to the decreased rate performance. Ultimately, after 1000 cycles at a 1C rate, the electrode's capacity retention was only 92.4%, a significant decrease compared to Application Example 1. This is related to the material's large specific surface area (or porosity), which to some extent increases side reactions between the electrolyte and the interface, leading to continuous electrolyte decomposition and material interface pulverization under high voltage, thus affecting the material's cycle stability. Furthermore, the presence of impurity phases often affects the material's cycle stability. Due to the different electrochemical activities of different phases within the material, their volume expansion rates during charge and discharge due to sodium insertion / extraction are inconsistent, causing cracking and pulverization at grain boundaries between different phases, accelerating side reactions between the electrolyte and the material, and further affecting the material's cycle stability.

[0088] Comparative Example 2: Synthesis and Electrochemical Performance of Na2MnP2O7 Material

[0089] In this embodiment, Na2MnP2O7 material is involved, and its preparation steps include:

[0090] Step 1: Mix commercially available Mn3(PO4)2, sodium acetate, and phosphoric acid with water in a molar ratio of 0.11:2.0:1.78. Add sucrose as a carbon source at a weight of 4.2% of Mn3(PO4)2. Grind the mixture using a sand mill until the particle size of the solid particles in the slurry is less than 70 nm to form a uniform dark red precursor slurry.

[0091] Step 2: Spray dry the above-mentioned dark red precursor slurry to remove the moisture in the slurry and obtain dry brown precursor powder; Step 3: In a nitrogen atmosphere, keep the above-mentioned brown precursor powder at 550°C for 7 hours, and after natural cooling, obtain Na2MnP2O7 material.

[0092] The results in Table 1 show that, with the same carbon content (compared to Application Example 2), the specific surface area of ​​this material is as high as 13.89 m². 2The / g indicates that the material has a relatively large porous structure, which is related to the use of commercially available Mn3(PO4)2 as a precursor. This precursor is a high-temperature sintering phase with low surface energy and high crystallinity. After spraying, the particles formed have a low packing density and large gaps. During the sintering process, due to the low surface energy, they cannot melt and agglomerate together, resulting in a large amount of residual porosity, which in turn increases the specific surface area of ​​the material. In addition, the results in Table 1 show that the compaction density of this material is only 1.91 g / cm³. 3 This indirectly proves that the material is relatively smooth and has a low degree of deformation under external pressure. The rigid structure of the material's pores is strong and cannot be filled by external pressure, resulting in a low compaction density.

[0093] Will Na2MnP2O7 material SurP and PVDF5130 were mixed with NMP at a mass ratio of 9.5:0.2:0.3. The above materials were mixed evenly using a high-speed homogenizer to form a black slurry with a uniform color and high fluidity. The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0094] Table 1 shows the electrochemical performance test results. At a rate of 0.1C (1C = 98 mAh / g), the electrode's discharge capacity is only 64.8 mAh / g, far lower than the 92.3 mAh / g in Application Example 2. This indicates that the material's phase purity is low. This is related to the high crystallinity and low surface energy of Mn3(PO4)2. This characteristic makes it difficult for Mn3(PO4)2 crystals to melt with sodium and phosphorus sources during high-temperature sintering, hindering the uniform mixing of local ionic states. Consequently, impurity phases are generated locally, affecting the material's capacity. Furthermore, Table 1 shows that at a rate of 10C, the electrode's capacity retention is only 64.3% compared to 0.1C. This lower rate performance is related to the low phase purity. Numerous grain boundaries often exist between the impurity phase and the main phase. These grain boundaries hinder the migration of sodium ions, easily leading to impaired local ion diffusion, reduced migration kinetics, and poorer rate performance. Ultimately, after 1000 cycles at a 1C rate, the electrode exhibited a capacity retention of only 94.2%, a significantly higher capacity decay than in Application Example 2. This issue is related to the material's large specific surface area, which to some extent increases side reactions between the material interface and the electrolyte. Partial dissolution of manganese ions at the interface leads to interfacial structural damage, ultimately affecting the material's cycling stability. Furthermore, the material's low phase purity results in inconsistent volume deformation during sodium insertion / extraction processes in different phases. Grain boundaries are prone to cracking and pulverization due to varying phase forces, exacerbating the reaction with the electrolyte and ultimately worsening the material's cycling stability.

[0095] Table 1 Performance Test Results

[0096]

[0097] 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 method for preparing a polyanionic sodium-ion battery cathode material, characterized in that... Includes the following steps: S1. Preparation of precursor solution: The transition metal M source, acid solution, and complexing agent are mixed and dissolved to form a precursor solution; the complexing agent is one or more of the following: nitrotriacetic acid, ethylenediaminetetraacetic acid and its sodium salt, cyclohexanetetraacetic acid, and ethylene glycol diethyl ether diaminetetraacetic acid; the transition metal M source is one or more of the following: manganese source, cobalt source, and iron source; S2. Preparation of pre-oxidized precursor solution: An oxidizing agent is added to the above precursor solution to oxidize the easily oxidized ions in the solution, thereby generating a pre-oxidized precursor solution; S3. Preparation of M(OH)x precipitate: Adjust the pH of the pre-oxidized precursor solution to 2.5-7.0 to precipitate the transition metal ions in the solution as M(OH)x; S4. Preparation of precursor powder: Wetly mix and grind the M(OH)x precipitate, sodium source, phosphorus source, and carbon source to form a uniform precursor slurry; S5. Drying of precursor powder: Dry the precursor slurry, separate the solid and liquid, and obtain dried precursor powder; The slurry drying method is one or more of spray drying, high-temperature evaporation, flash drying, and high-temperature pyrolysis; S6. High-temperature sintering: Sinter the precursor powder at high temperature in a protective atmosphere, and allow it to cool naturally to obtain the polyanionic material; The high-temperature sintering conditions are: sintering temperature 450-700℃, holding time 0.1-20h.

2. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: Between steps S3 and S4, the following steps are also included: S41, M(OH)x precipitation purification: The above precipitate is filtered and washed to obtain high-purity M(OH)x precipitate.

3. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: The grinding method is one or more of sand milling, ball milling, and homogeneous dispersion; in step S6, the protective atmosphere is one or more of nitrogen, argon, carbon monoxide, argon / hydrogen mixture, and nitrogen / hydrogen mixture.

4. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: In step S1, the acid solution is an inorganic acid and / or an organic acid. The inorganic acid is one or more of sulfuric acid, hydrochloric acid, and nitric acid. The organic acid is one or more of citric acid, ascorbic acid, malic acid, formic acid, acetic acid, benzoic acid, and salicylic acid. The manganese source is one or more of elemental manganese, manganese dioxide, manganese trioxide, manganese tetroxide, manganese oxalate, manganese carbonate, and basic manganese carbonate. The cobalt source is one or more of elemental cobalt, cobalt carbonate, cobalt oxalate, and cobalt oxide. The iron source is one or more of elemental iron, iron oxide, iron carbonate, and ferrous oxalate.

5. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: In step S2, the oxidant is one or more of the following: hydrogen peroxide, sodium peroxide, potassium peroxide, magnesium peroxide, sodium perchlorate, sodium hypochlorite, peracetic acid, sodium dichromate, potassium dichromate, ammonium persulfate, and sodium perborate.

6. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: In step S3, a weak alkaline substance is used to adjust the pH. The weak alkaline substance is one or more of ammonia, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, and sodium sulfite.

7. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: In step S4, the sodium source is one or more of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium nitrate, sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate; the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, pyrophosphate, sodium pyrophosphate, metaphosphoric acid, sodium metaphosphate, phosphorus pentoxide, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate.

8. The method for preparing the polyanionic sodium-ion battery cathode material according to claim 1, characterized in that: In step S4, the carbon source is one or more of the following: carbon black, graphite, graphene, carbon nanotubes, carbon fibers, sucrose, citric acid, glucose, starch, cellulose, maltose, and lactose.

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