Narrowly distributed spherical polyanionic sodium cathode material and preparation method thereof

Abstract

The present application relates to a kind of narrow distribution spherical polyanionic sodium electric positive electrode material and its preparation method, its method includes: S1: cationic surfactant, organic ligand, metal salt are mixed to carry out supramolecular encapsulation, obtain pre-encapsulation solution;S2: pre-encapsulation solution, water, sodium source, cobalt source, phosphorus source and carbon source are mixed and ground, obtain nano slurry, then spray drying, obtain precursor particle;S3: precursor particle is pyrolyzed at 250 DEG C-450 DEG C, then it is densified at 450 DEG C-850 DEG C, obtain positive electrode material;Wherein, cationic surfactant is alkyl quaternary ammonium salt of carbon atom number 12-18;Organic ligand is polybasic organic carboxylic acid;Metal salt is at least one kind of water-soluble salt of magnesium, aluminum, zirconium, titanium in metal.The present application can solve the technical problems that existing sodium cobalt phosphate pyrophosphate positive electrode material preparation is difficult, morphology is poor and is prone to instability in prior art.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a narrowly distributed spherical polyanion sodium-ion cathode material and its preparation method. Background Technology

[0002] With the increasing global demand for renewable energy storage, sodium-ion batteries are considered one of the most promising large-scale energy storage technologies in the post-lithium era due to their abundant sodium resources, low cost, and similar working principle to lithium-ion batteries. The cathode material, as a key component of sodium-ion batteries, directly determines the battery's energy density, cycle life, and safety performance.

[0003] Among numerous sodium-ion battery cathode material systems, polyanionic materials have attracted considerable attention due to their stable three-dimensional framework structure, good thermal stability, and excellent long-cycle performance. Among these, sodium cobalt pyrophosphate (Na4Co3(PO4)2P2O7), as a highly active polyanionic cathode material, exhibits a high V0 of up to 4.5V (vs. Na0). + The material has a high theoretical energy density and the introduction of cobalt effectively improves its redox potential, making it a promising candidate for high-voltage sodium-ion batteries.

[0004] However, existing sodium cobalt pyrophosphate materials still have some limitations in practical applications: Firstly, the preparation process is challenging, resulting in irregular particle morphology. Existing sodium cobalt pyrophosphate materials are generally prepared via high-temperature solid-state sintering or sol-gel methods. However, as a highly active polyanionic cathode material, the synthesis of sodium cobalt pyrophosphate involves complex multiphase reactions, easily leading to the formation of various impurity phases. While the conventional high-temperature solid-state method is relatively simple and suitable for large-scale production, it struggles to effectively suppress impurity phases. Furthermore, the sintered particles often exhibit irregular morphology, with a wide particle size distribution and a tendency to agglomerate, resulting in low tap density and poor electrode processing performance. This significantly impacts the electrode's processing performance and volumetric energy density, hindering the fabrication of high-energy-density electrode sheets. While the sol-gel method can improve particle uniformity to some extent, it is complex and costly, typically involving the use of large amounts of organic solvents and prolonged liquid-phase heat treatment and control. Moreover, the products obtained through this method are often micron-sized aggregates composed of submicron-sized primary particles, requiring subsequent secondary crushing, carbon coating, and sintering to obtain usable cathode materials—a cumbersome and inefficient process.

[0005] Secondly, insufficient interfacial stability under high voltage leads to a decline in cycle life. Sodium cobalt pyrophosphate has high reactivity but poor thermal stability, and its electrode / electrolyte interface is prone to severe side reactions during high-voltage charge and discharge. These side reactions can cause electrolyte decomposition, forming an unstable solid electrolyte interfacial film, or repeated intercalation / deintercalation (sodium deintercalation) of cobalt ions on its surface, resulting in severe lattice distortion. These interfacial side reactions lead to interfacial instability and a continuous increase in interfacial impedance. Furthermore, after interfacial instability, the deterioration of the surface structure rapidly spreads into the interior of the sodium cobalt pyrophosphate particles, ultimately leading to a rapid degradation of the overall material structure. This manifests as a gradual decline in the discharge voltage plateau and a rapid decrease in reversible capacity, severely limiting the long cycle life of sodium cobalt pyrophosphate.

[0006] Therefore, existing technologies require a method suitable for large-scale production that can stably prepare sodium cobalt pyrophosphate cathode materials with regular morphology, narrow particle size distribution, and long cycle life. Summary of the Invention

[0007] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a narrowly distributed spherical polyanionic sodium cathode material and its preparation method, which solves the technical problems of the existing technology, such as the difficulty in preparing sodium cobalt pyrophosphate cathode material, poor morphology and easy instability.

[0008] (II) Technical Solution To achieve the above objectives, the main technical solutions adopted by the present invention include: In a first aspect, the present invention provides a method for preparing a narrowly distributed spherical polyanionic sodium cathode material, comprising the following steps: S1: Cationic surfactant, organic ligand, metal salt and water are mixed and supramolecularly co-assembled to encapsulate the metal ions in the metal salt between the cationic surfactant and the organic ligand, forming a supramolecular encapsulation structure to obtain a pre-encapsulated solution. S2: The pre-encapsulation solution, water, sodium source, cobalt source, phosphorus source and carbon source are mixed and ground to obtain nano slurry, which is then spray-dried to obtain precursor particles. The supramolecular encapsulation structure is enriched on the surface of the precursor particles. S3: The precursor particles are placed in a protective atmosphere and subjected to pyrolysis at 250℃-450℃, followed by densification at 450℃-850℃ to obtain the cathode material. Among them, the cationic surfactant includes at least one alkyl quaternary ammonium salt with 12-18 carbon atoms; the organic ligand includes at least one polycarboxylic acid; and the metal salt is a water-soluble salt of at least one metal selected from magnesium, aluminum, zirconium, and titanium.

[0009] According to a preferred embodiment of the present invention, S1 includes: first mixing and stirring a cationic surfactant with an aqueous solution containing a metal salt to form a pre-coordinated solution; then adding an organic ligand to the pre-coordinated solution and stirring at 25°C-80°C for 0.5-4 hours to complete supramolecular co-assembly and obtain a pre-encapsulated solution.

[0010] According to a preferred embodiment of the present invention, in S1, the molar ratio of the cationic surfactant, the organic ligand and the metal ions in the metal salt is 1-4:0.5-2:1; the viscosity of the pre-encapsulated solution at 25°C is 10-30 mPa·s; and the concentration of the metal ions in the pre-encapsulated solution is 0.01-0.5 mol / L.

[0011] According to a preferred embodiment of the present invention, in S1, the metal salt is at least one selected from magnesium nitrate, magnesium chloride, aluminum nitrate, zirconium chloride, and titanium oxysulfate; the cationic surfactant is at least one selected from hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and dodecyltrimethylammonium bromide; and the organic ligand is at least one selected from citric acid, oxalic acid, tartaric acid, malic acid, and gallic acid.

[0012] According to a preferred embodiment of the present invention, S2 includes: mixing water, sodium source, cobalt source, phosphorus source and carbon source and dispersing them by sand milling, then adding a pre-encapsulation solution and continuing sand milling to obtain a nano-slurry; the particle size of the solid particles in the nano-slurry is 50-200 nm; the dynamic viscosity of the nano-slurry at 25°C is 100-300 mPa·s, and the solid content is 30%-50%; the molar ratio of cobalt element in the cobalt source to metal ions in the pre-encapsulation solution is 1:0.01-0.1.

[0013] According to a preferred embodiment of the present invention, in S2, the molar ratio of sodium, cobalt, and phosphorus in the nano-slurry is controlled to be 4:3:4; the amount of carbon source added is 5%-20% of the total weight of sodium, cobalt, and phosphorus sources. The cobalt source is at least one of cobalt carbonate, cobalt hydroxide, cobalt nitrate, and cobalt acetate; the phosphorus source is at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and sodium pyrophosphate; the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium nitrate, disodium hydrogen phosphate, and sodium pyrophosphate; the carbon source is an organic carbon source and / or an inorganic carbon source; the organic carbon source is at least one of sucrose, starch, cyclodextrin, glucose, maltose, lactose, polyvinyl alcohol, and polyacryl alcohol; and the inorganic carbon source is at least one of graphene, graphite, hard carbon, carbon nanotubes, and carbon black.

[0014] According to a preferred embodiment of the present invention, in S2, the carbon source contains an organic carboxylic acid; the organic carboxylic acid is a polycarboxylic acid that is the same as or different from the organic ligand; The amount of organic carboxylic acid added accounts for 50wt%-100wt% of the total weight of the carbon source; the organic carboxylic acid is selected from at least one of citric acid, oxalic acid, tartaric acid, malic acid and gallic acid.

[0015] According to a preferred embodiment of the present invention, in S3, the heating rate of the pyrolysis treatment is 1-5℃ / min, the holding temperature is 250-350℃, and the holding time is 2-8 hours; the heating rate of the densification treatment is 4-10℃ / min, the holding temperature is 600-750℃, and the holding time is 3-12 hours; the protective atmosphere is a weakly reducing atmosphere or an inert atmosphere.

[0016] In a second aspect, the present invention also provides a spherical polyanionic sodium cathode material prepared by the preparation method described in any one of the first aspects, which is a core-shell structured composite particle comprising, from the inside out, a sodium cobalt pyrophosphate core and a composite functional layer; the composite functional layer comprises a carbon matrix and metal oxide nanodots dispersed and embedded within the carbon matrix, the metal oxide nanodots being physically separated by the carbon matrix; the metal oxide nanodots are at least one selected from magnesium oxide, aluminum oxide, zirconium oxide and titanium oxide; the particle size of the metal oxide nanodots does not exceed 20 nm; the metal oxide nanodots are enriched in the subsurface of the composite functional layer to form a subsurface.

[0017] According to a preferred embodiment of the present invention, the chemical formula of sodium cobalt pyrophosphate is Na4Co3(PO4)2P2O7; the particle size of the composite particles is 1-10 micrometers, and the sphericity is ≥0.9; the total thickness of the composite functional layer is 30-120 nm; the particle size of the metal oxide nanodots does not exceed 10 nm; and the metal oxide nanodots account for 0.1wt%-2wt% of the total mass of the cathode material.

[0018] (III) Beneficial Effects The beneficial effects of this invention are as follows: This invention provides a narrow-distribution spherical polyanionic sodium electrode material and its preparation method. Due to the use of supramolecular co-assembly technology, cationic surfactants, organic ligands, and metal salts are pre-encapsulated in an aqueous phase through non-covalent bonding to form a supramolecular encapsulation structure. This encapsulation structure is then co-ground with sodium, cobalt, phosphorus, and carbon sources. Compared to existing technologies, this method, on the one hand, utilizes the dispersing effect of the surfactants in the supramolecular encapsulation structure to promote uniform dispersion of the raw material components, improve grinding efficiency, and reduce the risk of impurity phase formation. On the other hand, because the metal ions are firmly encapsulated in the supramolecular encapsulation structure, it also prevents inert metal ions from doping into the crystal lattice of sodium cobalt pyrophosphate during mixing or sintering, ensuring the integrity of the sodium cobalt pyrophosphate main material structure and its purity after sintering. This allows the invention to use a method of spray drying and sintering with other raw materials to obtain regular secondary particles with an outer carbon layer in a single step, eliminating the need for secondary processing. The processing method is simple and highly efficient.

[0019] Meanwhile, since this invention uses a spray drying process to instantly dry and shape nano-slurry containing supramolecular encapsulation structures, compared with the prior art, the droplets sprayed during spray drying will spontaneously form precursor particles with high sphericity and uniform particle size distribution during the evaporation process under the action of surfactants in the supramolecular encapsulation structure. This results in the final cathode material having a narrow distribution and high sphericity morphology, which can significantly improve the tap density of the cathode material and the slurry dispersion during the electrode processing, thereby improving the coating uniformity and volumetric energy density of the electrode.

[0020] During spray drying, the supramolecular encapsulation structure migrates and accumulates on the droplet surface under the guidance of the surfactant, forming a gradient distribution where metal ion precursors are enriched on the surface and relatively sparse inside. Due to the spatial confinement characteristics of the encapsulation structure, the metal ions are concentrated and anchored within a certain depth below the surface. Subsequently, after segmented sintering, metal oxide nanodots are enriched and distributed below the surface of the composite functional layer, forming a subsurface. Compared with existing technologies, the metal oxide nanodots in the subsurface formed by this invention are closer together and more densely packed, enabling a stronger shielding and stabilizing effect on the electrode / electrolyte interface with a smaller dosage, effectively preventing the spread of side reactions to the interior under high voltage. Meanwhile, the concentrated distribution of metal oxide nanodots also guides the surrounding organic matter to form a more compact and ordered microstructure during pyrolysis. Combined with the pinning and supporting effect of the metal oxide nanodots themselves, the overall strength of the carbon layer in the subsurface region can be greatly improved. This allows the material to better resist stress concentration caused by volume changes during cyclic charging and discharging, suppress the generation and propagation of microcracks, and further extend the cycle life of the cathode material of this invention. In addition, since the oxide nanodots of this invention are confined to a specific depth in the subsurface rather than penetrating the entire carbon layer, the outer surface of the carbon layer still maintains a complete conductive network, resulting in a more uniform current distribution. This also avoids hot spot effects caused by local conductivity differences or excessive local current.

[0021] This invention, through the synergistic effect of the above methods, ultimately produces a core-shell structured cathode material composed of a sodium cobalt pyrophosphate core and a composite functional layer. The composite functional layer comprises a carbon matrix and inert metal oxide nanoparticles (magnesium oxide, aluminum oxide, zirconium oxide, and titanium oxide) uniformly dispersed and embedded within the carbon matrix. The nanoparticles are physically separated by the carbon matrix and enriched in the subsurface region. This structure, while maintaining the intrinsic high-voltage plateau of sodium cobalt pyrophosphate, effectively suppresses side reactions at the electrode / electrolyte interface under high voltage and their propagation into the bulk phase, significantly improving the cycle stability of the cathode material. Simultaneously, the narrow-distribution, high-sphericity particle morphology improves the electrode processing performance, simultaneously solving the problems of irregular morphology, high preparation difficulty, and poor cycle stability under high voltage in existing technologies. Attached Figure Description

[0022] Figure 1 This is a scanning electron microscope (SEM) image of the positive electrode material of Embodiment 1 of the present invention; Figure 2 This is a scanning electron microscope (SEM) image of the cathode material of Comparative Example 2 of the present invention. Detailed Implementation

[0023] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] This invention provides a method for preparing a narrowly distributed spherical polyanionic sodium cathode material, comprising the following steps: S1: A cationic surfactant, an organic ligand, a metal salt, and water are mixed. The organic ligand coordinates with the metal ions to form a negatively charged complex anion. This complex anion then electrostatically binds to the positively charged cationic surfactant. At the same time, the long alkyl chains of the cationic surfactant further aggregate through hydrophobic interactions to form a supramolecular encapsulation structure that encapsulates the metal ions between the cationic surfactant and the organic ligand, thus obtaining a pre-encapsulated solution.

[0025] S2: The pre-encapsulated solution obtained in S1 is mixed and ground with water, sodium source, cobalt source, phosphorus source, and carbon source. The cationic surfactant in the pre-encapsulated solution promotes the uniform dispersion of each raw material component, resulting in a nano-slurry. Subsequently, spray drying is performed. During droplet evaporation, the cationic surfactant in the supramolecular encapsulation structure migrates to the droplet surface due to its amphiphilicity, simultaneously carrying the bound metal ions to the surface. After rapid drying and solidification, a metal ion precursor is formed and enriched on the particle surface, creating a gradient distribution structure with surface enrichment and internal sparseness. Simultaneously, the directional migration of the surfactant homogenizes the droplet surface tension, guiding the droplets to maintain a regular spherical shape during contraction, ultimately yielding precursor particles with high sphericity and narrow particle size distribution.

[0026] S3: The precursor particles obtained in S2 are placed in a protective atmosphere and first subjected to pyrolysis at 250℃-450℃ to partially decompose and carbonize the organic components in the supramolecular encapsulation structure to form a porous carbon framework (amorphous carbon). At the same time, the encapsulated metal salt is initially pyrolyzed to form metal oxide nanodots and / or their crystal nuclei. Then, densification treatment is carried out at 450℃-850℃ to fully crystallize the sodium cobalt pyrophosphate main phase and further graphitize the carbon layer. Meanwhile, the metal salt is further fully decomposed in a confined environment to form a large number of metal oxide nanodots, and finally, the cathode material is obtained.

[0027] In S1, the cationic surfactant is selected from at least one alkyl quaternary ammonium salt with 12-18 carbon atoms to ensure that its hydrophobic chain length is moderate, which can provide sufficient hydrophobic interaction to drive assembly and maintain good water solubility; the organic ligand is at least one polycarboxylic acid containing multiple carboxyl groups, which can form a stable chelate structure with metal ions, and can also maintain negative charge after binding with metal ions, and can generate strong electrostatic interaction with the cationic surfactant; the metal salt is a water-soluble salt of at least one metal selected from magnesium, aluminum, zirconium and titanium. The oxides of these metals have excellent high-temperature stability and chemical inertness, which can ensure that they can maintain a good interface even at high potentials.

[0028] Furthermore, it should be noted that cationic surfactants, when present alone, tend to undergo complete thermal decomposition and vaporization under an inert atmosphere. In this invention, since the cationic surfactant has already formed a robust supramolecular encapsulation structure with the metal ions in step S1, and the metal ions can also act as an in-situ catalyst for dehydrogenation, coupled with temperature control during the pyrolysis process in S3, the cationic surfactant will carbonize rather than completely vaporize. Simultaneously, the carbon skeleton formed by the pyrolysis of polycarboxylic acids will further combine with the carbonaceous fragments generated by the surfactant decomposition to jointly constitute a continuous carbon matrix, ensuring the continuity and density of the composite functional layer, regulating particle morphology, and preventing metal oxide nanodots from being directly exposed on the surface of the cathode material.

[0029] Preferably, S1 includes: first mixing and stirring a cationic surfactant with an aqueous solution containing a metal salt to form a pre-coordinated solution, then adding an organic ligand to the pre-coordinated solution, stirring at 25℃-80℃ for 0.5-4 hours to complete supramolecular co-assembly and obtain a pre-encapsulated solution.

[0030] In this process, the cationic surfactant and metal salt are first mixed, allowing the cationic head group of the surfactant to pre-adsorb onto the metal ions via electrostatic interactions, forming a preliminary ion-pair complex. This ensures that the subsequently added organic ligand will first coordinate with the metal ions, rather than directly with the cationic surfactant. The complex anion (chelate) formed by the organic ligand and the metal ions further generates stronger electrostatic (coordination) binding with the hydrophilic head group of the surfactant. The metal ions are encapsulated within the polar region jointly constructed by the hydrophilic head group of the cationic surfactant and the organic ligand, and further aggregate into micelles under the action of their hydrophobic chains. Ultimately, this results in a more robust and uniformly distributed supramolecular encapsulation structure, avoiding encapsulation failures that may occur with direct mixing.

[0031] The specific reaction temperature and time can be determined based on the type of materials selected. It is sufficient to ensure that the content of free metal ions in the final pre-encapsulated solution is less than 0.00001 mol / L, so as to avoid introducing impurities and prevent free metal ions from being doped into sodium cobalt pyrophosphate, which would affect its purity.

[0032] In addition, methods such as centrifugal drying can be used to obtain a further concentrated supramolecular encapsulated structure, but it is necessary to avoid drying and removing it, which could lead to structural collapse and other problems.

[0033] Preferably, the stirring speed when forming the pre-coordinated solution is 500-1500 rpm, and ultrasonic dispersion treatment for 10-30 minutes can be used as an adjunct. Appropriate stirring helps the cationic surfactant and metal ions to come into full contact, and ultrasonic assistance can further promote uniform mixing at the molecular level, ensuring the quality of subsequent supramolecular assembly.

[0034] Preferably, in S1, the molar ratio of the cationic surfactant, the organic ligand, and the metal ions in the metal salt is 1-4:1-1.5:1, ensuring that the cationic surfactant and the organic ligand can fully bind the metal ions and form a complete supramolecular encapsulation structure. If the proportion of the cationic surfactant or the organic ligand is too low, it may be difficult to effectively "encapsulate" the metal ions. If the proportion is too high, it may form too many empty micelles, affecting the uniform distribution of the metal oxide nanodots and resulting in a large amount of waste.

[0035] The concentration, or content, of metal ions in the pre-encapsulation solution is 0.01-0.5 mol / L to ensure efficient encapsulation and avoid leaving a large number of free metal ions after encapsulation. The metal ion concentration refers to the total concentration of metal ions in the solution, including the portion already encapsulated in the supramolecular structure and a small amount that may be free. Through the aforementioned molar ratio and viscosity control, it can be ensured that the vast majority of metal ions are effectively encapsulated.

[0036] Preferably, in S1, the viscosity of the pre-encapsulated solution at 25°C is 10-30 mPa·s, ensuring that a supramolecular encapsulation structure has been formed in the pre-encapsulation solution and that its aggregates are of moderate size and uniformly distributed, enabling sufficient dispersion during subsequent use. Too low a viscosity indicates insufficient assembly, leading to a large number of non-concentrated ion dopants, resulting in problems such as decreased stability and conductivity of the final interface. Too high a viscosity may cause excessive aggregation, leading to agglomeration of metal oxides during sintering, preventing the formation of nanodots and potentially creating weak points in the composite functional layer.

[0037] Preferably, in S1, the metal salt is at least one selected from magnesium nitrate, magnesium chloride, aluminum nitrate, zirconium chloride, and titanium oxysulfate. The cationic surfactant is at least one selected from hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and dodecyltrimethylammonium bromide.

[0038] Preferably, in S1, the organic ligand is selected from at least one of citric acid, oxalic acid, tartaric acid, and malic acid. All of the above-mentioned organic carboxylic acid molecules are polycarboxylic acids containing multiple carboxyl functional groups, and they can provide abundant coordination sites to form stable multidentate chelate structures with metal ions. Furthermore, they can electrostatically bind with cationic surfactants to achieve encapsulation. In addition, these organic carboxylic acids can also serve as part of the carbon source during subsequent pyrolysis, fully carbonizing to form a continuous carbon layer. Moreover, the above-mentioned organic carboxylic acids are all biomass-derived, non-toxic, environmentally friendly, and inexpensive, suitable for large-scale procurement and use. Of course, the materials listed above are preferred choices. In actual operation, if sufficient encapsulation and sintering effects can be ensured, other types of cationic surfactants, polycarboxylic acids, and water-soluble metal salts can also be used.

[0039] Preferably, S2 includes: mixing water, sodium source, cobalt source, phosphorus source, and carbon source, dispersing them by sand milling, and then adding a pre-encapsulation solution for further milling (sand milling) to obtain a nano-slurry. The sand milling of the main raw materials (sodium source, cobalt source, phosphorus source, and carbon source) requires high energy input to break them down to the nanoscale; the supramolecular structure in the pre-encapsulation solution may be partially damaged if subjected to high-intensity shear throughout the process. Therefore, by first completing the sand milling of the main raw materials, and then adding the pre-encapsulation solution for a short-term sand milling, the raw materials can be pulverized and the components can be uniformly mixed, while maximizing the protection of the integrity of the encapsulation structure.

[0040] Preferably, in S2, a multi-stage grinding method can be used for grinding.

[0041] Preferably, in S2, the particle size of the solid particles in the nano-slurry is 50-200 nm to ensure that the subsequent sintering reaction is sufficient and that a pure phase product can be obtained. If the particle size is too coarse, the solid phase reaction will be incomplete, and impurities will easily remain. It will also make it difficult to obtain a good morphology in the subsequent spray drying. If the particle size is too fine, it may increase the surface energy, leading to abnormal growth or distortion during sintering.

[0042] The nano-slurry has a dynamic viscosity of 100-300 mPa·s at 25℃ and a solid content of 30%-50%, ensuring good flowability and further ensuring atomization effect. This avoids problems such as low spray drying efficiency, difficulty in atomization, and nozzle clogging, and ensures that the sprayed particles have a good basic morphology.

[0043] The molar ratio of cobalt in the cobalt source to metal ions in the pre-encapsulated solution is 1:0.01-0.1, more preferably 0.01-0.05. In this invention, a relatively small amount of metal ions is sufficient to form a subsurface with densely distributed metal oxide nanodots, achieving excellent interface stability. It is necessary to accurately control the addition ratio of metal ions to avoid a poor stability effect due to an excessively low proportion of metal oxide nanodots in the final composite functional layer, and to avoid an excessively high proportion that leads to excessive aggregation of nanodots, affecting the continuity of the carbon layer or causing interfacial side reactions and decreased conductivity.

[0044] Preferably, in S2, the molar ratio of sodium, cobalt, and phosphorus in the nano-slurry is controlled at 4:3:4 to ensure that pure-phase sodium cobalt pyrophosphate is obtained after sintering, avoiding the formation of impurity phases due to ratio deviations. The amount of carbon source added is 5%-20% of the total weight of sodium, cobalt, and phosphorus sources, which is sufficient to form a continuous conductive carbon layer while avoiding excessive carbon content that would reduce the material's volumetric energy density.

[0045] Preferably, in S2, the cobalt source is at least one selected from cobalt carbonate, cobalt hydroxide, cobalt nitrate, and cobalt acetate. The phosphorus source is at least one selected from phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and sodium pyrophosphate. The sodium source is at least one selected from sodium carbonate, sodium bicarbonate, sodium nitrate, disodium hydrogen phosphate, and sodium pyrophosphate. The carbon source is an organic carbon source and / or an inorganic carbon source. The organic carbon source is at least one selected from sucrose, starch, cyclodextrin, glucose, maltose, lactose, polyvinyl alcohol, and polyacryl alcohol, and the inorganic carbon source is at least one selected from graphene, graphite, hard carbon, carbon nanotubes, and carbon black.

[0046] More preferably, in S2, both organic and inorganic carbon sources are used. The organic carbon source forms amorphous carbon during pyrolysis, exhibiting good coating properties and interfacial compatibility with nanodots. This allows for the formation of a basic carbon layer structure, preventing the agglomeration of metal oxides formed during pyrolysis, and subsequent densification to form a graphitized carbon layer. The inorganic carbon source provides a highly dense graphitized region with a shorter sintering time, improving the overall strength and conductivity of the carbon layer and accelerating the densification rate of the surrounding amorphous carbon. The synergistic effect of both sources allows for the acquisition of a composite carbon layer with good conductivity and a high degree of graphitization within a shorter sintering time, avoiding problems such as damage to the sodium cobalt pyrophosphate structure due to excessively long sintering times.

[0047] Preferably, in step S2, the carbon source contains an organic carboxylic acid. The amount of organic carboxylic acid added accounts for 50wt%-100wt% of the total weight of the carbon source. The organic carboxylic acid is selected from at least one of citric acid, oxalic acid, tartaric acid, malic acid, and gallic acid. By adding an organic carboxylic acid to the carbon source, the overall concentration of organic carboxylic acid in the system can be increased, further enhancing the encapsulation effect on metal ions and preventing the dissociation of the supramolecular encapsulation structure. The gas generated by the pyrolysis of the organic carboxylic acid as a carbon source can also form uniform micropores in the carbon layer, which is beneficial for electrolyte wetting and ion transport.

[0048] Preferably, in S2, the organic carboxylic acid can be the same as or different from the organic ligand in S1, as long as the above-mentioned function can be achieved. More preferably, the second organic carboxylic acid is the same as the organic ligand in S1, ensuring that the excess organic carboxylic acid in the carbon source and the organic carboxylic acid in the supramolecular encapsulation structure can form a continuous cross-linked network during pyrolysis, and together with the surfactant, construct a denser carbon precursor framework.

[0049] Preferably, in step S2, the pre-encapsulated solution is first mixed and stirred with the second organic carboxylic acid in the carbon source and water for 0.5-2 hours, and then sodium source, cobalt source, phosphorus source and other carbon sources are added and milled to ensure that the newly added organic carboxylic acid can fully interact with the supramolecular structure in the pre-encapsulated solution, enhance the "encapsulation" effect on metal ions, and avoid the escape of metal ions due to competition when other raw materials are added later.

[0050] Preferably, in step S2, the inlet temperature of the spray dryer is 180℃-250℃, and the outlet temperature is 80℃-130℃. If the inlet temperature is too low, insufficient drying and particle adhesion may occur; if it is too high, premature decomposition of the surfactant may occur, affecting morphology control. The outlet temperature needs to be controlled within a suitable range to ensure that the particles do not absorb moisture or stick together. The specific feed rate during spray drying needs to be matched with the atomizer to ensure uniform droplet size. In this invention, preferably, the feed rate is 10-100 mL / min.

[0051] Preferably, in S2, the precursor particles obtained by spray drying have a particle size D50 of 1-10 μm and a width-to-length ratio sphericity (SWL) of not less than 0.85, preferably not less than 0.9, exhibiting high sphericity. It should be noted that, since the raw material components in the nano-slurry of this invention have been milled to the nanoscale and uniformly dispersed under the action of surfactants in the supramolecular encapsulation structure (and possibly some surfactants that have not formed a supramolecular encapsulation structure), and these surfactants can rapidly and uniformly migrate to the droplet surface during droplet evaporation, resulting in highly consistent evaporation and shrinkage behavior of individual droplets, this significantly reduces the particle size difference between different particles within the same batch. In S2, the particle size distribution span (D90-D10) / D50 of the precursor particles is stably controlled below 1.0, preferably 1.0-0.2, more preferably within the range of 0.6-0.2, and can extend to the final sintered cathode material particles, achieving a narrow distribution.

[0052] Preferably, in step S3, the heating rate of the pyrolysis treatment is 1-5℃ / min, more preferably 1-3℃. The holding temperature is 250-350℃, and the holding time is 2-8 hours, more preferably 2-4 hours. Excessive heating or holding temperature may cause the surfactant in the supramolecular encapsulation structure to vaporize, causing metal ions to lose their spatial confinement. This increases the risk of metal oxide nanodot aggregation and direct exposure to the surface of the composite functional layer, disrupting the continuity of the carbon layer and affecting its conductivity. Conversely, excessively low holding temperature or short holding time may lead to incomplete decomposition of metal salts or polycarboxylic acids, potentially generating large amounts of gas during densification and affecting the densification effect. Excessively long holding time may cause excessive aggregation or growth of nanodots within the loose carbon layer.

[0053] The densification treatment involves a heating rate of 4-10℃ / min, a holding temperature of 600-750℃, and a holding time of 3-12 hours, preferably 4-6 hours. If the temperature is below 600℃ or the holding time is too short, the main phase of sodium cobalt pyrophosphate will not crystallize completely. If the temperature is above 750℃ or the holding time is too long, it may lead to excessive growth or aggregation of metal oxide nanodots, which may not even exceed 50-100 nm, resulting in severe damage to the carbon layer structure and high internal stress. A heating rate that is too low will also cause excessive growth of nanodots and deterioration of the carbon layer structure. A heating rate that is too fast will cause a mismatch between the main phase crystallization and the graphitization process of the carbon layer, generating internal stress and potentially leading to cracks in the composite functional layer.

[0054] The protective atmosphere is either inert or weakly reducing, preferably inert, as sintering is easier and more widely applicable with an inert atmosphere. If low-valence or multi-valence metal oxide nanodots are required, a weakly reducing atmosphere can be chosen to avoid excessive oxidation. The inert atmosphere is nitrogen or argon, while the weakly reducing atmosphere is a mixture of 2%-5% hydrogen and nitrogen or argon by volume.

[0055] Preferably, in S3, the total thickness of the composite functional layer formed after sintering is 30-120 nm. If the thickness is too thin, the interface protection performance will be insufficient, and the side reaction will still easily erode the surface of sodium cobalt pyrophosphate under high voltage. If the thickness is too thick, the overall energy density of the material will be excessively diluted. Moreover, an excessively thick functional layer is prone to internal stress during subsequent electrode pressing or further secondary particle preparation, which will lead to interlayer delamination, affect the contact effect between the particles and the conductive agent and binder, and is not conducive to the preparation of high-load electrodes or further compaction molding.

[0056] Preferably, the particle size of the metal oxide nanodots is no more than 20 nm, more preferably no more than 10 nm, and even more preferably no more than 5 nm. It should be noted that nanodots are three-dimensional nanoparticle structures. This invention chooses to embed nanodots with a particle size of no more than 20 nm into the composite functional layer. This allows the nanodot particles to utilize their sufficiently high specific surface area and surface activity at this nanoscale, forming a densely distributed "shielding network" (i.e., a subsurface) in the carbon layer with extremely low addition amounts. This ensures that direct contact between the electrolyte and the surface of sodium cobalt pyrophosphate is effectively blocked without affecting the conductivity of the composite functional layer, thus stabilizing the current. Simultaneously, the nanodot size is much smaller than the carbon layer thickness (30-120 nm), allowing them to be completely embedded within the carbon matrix without compromising the continuity and mechanical strength of the carbon layer. When the particle size exceeds 20 nm, the specific surface area decreases, the interfacial shielding effect weakens significantly, and larger particles may become stress concentration points in the carbon layer, inducing microcracks during cyclic charging and discharging. Controlling the particle size to below 10 nm ensures that the nanodots are uniformly and densely distributed in the subsurface region, further reducing their interference with the integrity of the carbon layer structure. Further controlling the particle size to below 5 nm enhances the surface activity of the nanodots, achieving superior interface stability. The nanodots produced by the confined sintering method of this invention generally have a particle size not exceeding 10 nm.

[0057] This invention also provides a narrowly distributed spherical polyanionic sodium electrode material with a core-shell structure, comprising a sodium cobalt pyrophosphate core and a composite functional layer from the inside out. The composite functional layer includes a carbon matrix and metal oxide nanoparticles dispersed and embedded within the carbon matrix, with the metal oxide nanoparticles physically separated by the carbon matrix. The chemical formula of sodium cobalt pyrophosphate is Na4Co3(PO4)2P2O7. The metal oxide nanoparticles are selected from at least one of magnesium oxide, aluminum oxide, zirconium oxide, and titanium oxide. The particle size of the metal oxide nanoparticles does not exceed 20 nm. The metal oxide nanoparticles are enriched at a certain depth below the surface of the composite functional layer (the specific depth range is affected by the amount of cationic surfactant used, the carbon chain length, and the collapse of the carbon chain during calcination, but is generally enriched in the range of 1-20 nm below the surface), forming a subsurface rich in metal oxide nanoparticles.

[0058] The subsurface enrichment structure concentrates metal oxide nanodots in the core region of the electrode / electrolyte interface, forming a "shielding layer." This effectively prevents direct contact between the electrolyte and the sodium cobalt pyrophosphate surface with relatively low addition amounts, suppressing side reactions under high voltage. Simultaneously, because the metal oxide nanodots are non-conductive, the dense metal oxides force current to be shunt within the carbon layer between the nanodots as it passes through the subsurface, resulting in a more uniform current distribution. This avoids issues such as localized high voltage or hot spots, improving the electrode's rate performance and further suppressing electrolyte decomposition caused by excessively high localized overpotentials, thus enhancing interfacial stability. Furthermore, because the nanodots are confined to the subsurface rather than dispersed throughout the entire carbon layer, the outer surface of the carbon layer maintains a complete conductive network, ensuring unaffected electron transport.

[0059] Furthermore, since the metal oxide nanodots of this invention originate from the confined pyrolysis of supramolecular encapsulation structures, and are isolated from each other by the carbon matrix, preventing direct contact, this avoids problems such as nanodot aggregation forming conductive pathways and the introduction of additional electron channels into the carbon layer, thus preventing uneven current distribution and the generation of local hot spots. Simultaneously, the metal oxide nanodots are tightly encapsulated by the carbon matrix, resulting in strong interfacial bonding and preventing them from easily detaching during cyclic charging and discharging, thus maintaining interfacial stability for a relatively long time.

[0060] Preferably, the metal oxide nanodots account for 0.1wt%-2wt% of the total mass of the cathode material, more preferably 0.1wt%-0.8wt%, which ensures the interfacial stability of the composite functional layer while avoiding the side effects that may be caused by excessive addition of metal oxides.

[0061] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0062] Example 1 This embodiment provides a narrowly distributed spherical polyanionic sodium cathode material and its preparation method, the preparation method including: S1: Preparation of pre-packaged solution Hexadecyltrimethylammonium bromide was dissolved in water, and after stirring until dissolved, magnesium nitrate was added. Stirring continued for 30 minutes to form a pre-coordinated solution. Citric acid was then added to this solution, and the mixture was stirred at 50°C for 2 hours to obtain a pre-encapsulated solution. The molar ratio of hexadecyltrimethylammonium bromide, citric acid, and magnesium ions was 2:1:1. The magnesium ion concentration in the pre-encapsulated solution was 0.1 mol / L, and the viscosity of the resulting solution at 25°C was 18 mPa·s.

[0063] S2: Preparation of nano-slurry and spray molding Sodium pyrophosphate (sodium source), cobalt carbonate (cobalt source), and ammonium dihydrogen phosphate (phosphorus source) were weighed at a sodium:cobalt:phosphorus molar ratio of 4:3:4. These were then mixed with glucose and sucrose (carbon source) and deionized water, and dispersed by sand milling at 2500 rpm. First, 0.6-0.8 mm zirconium beads were used for milling for 30 minutes, then 0.1 mm zirconium beads were used to continue milling until the solid particle size D50 in the slurry was <100 nm. Subsequently, the pre-encapsulated solution obtained in S1 was added to the slurry, and milling continued for 30 minutes until homogeneous, yielding a nano-slurry. The total amount of glucose and sucrose added was 12% of the total weight of the sodium, cobalt, and phosphorus sources, and the molar ratio of cobalt to magnesium ions in the pre-encapsulated solution was 1:0.02. The resulting nano-slurry had a solid content of 40% and a dynamic viscosity of 180 mPa·s at 25°C.

[0064] The nano-slurry was spray-dried at an inlet temperature of 200℃, an outlet temperature of 90℃, and a feed rate of 50mL / min to obtain spherical precursor particles with a particle size D50 of 3.6μm and a width-to-length ratio sphericity (SWL) of 0.9.

[0065] S3: Segmented sintering The precursor particles were placed in a tube furnace and sintered in stages under a nitrogen atmosphere: first, the temperature was increased to 300℃ at a heating rate of 2℃ / min and held for 3 hours for pyrolysis treatment; then, the temperature was increased to 650℃ at a heating rate of 5℃ / min and held for 6 hours for densification treatment, finally obtaining the following... Figure 1 The cathode material shown.

[0066] like Figure 1 As shown, the cathode material obtained in this embodiment has a spherical core-shell structure, with the core being sodium cobalt pyrophosphate and the shell being a composite functional layer. Its particles exhibit a good spherical morphology. The cathode material particles have a D50 of 2.5 μm and a (D90-D10) / D50 ratio of 0.8, exhibiting a narrow particle distribution range. The total thickness of the composite functional layer is 60 nm (average), embedding magnesium oxide nanodots with a particle size not exceeding 8 nm (approximate value), and these magnesium oxide nanodots are enriched in the subsurface region of the composite functional layer.

[0067] Example 2 This embodiment provides a narrowly distributed spherical polyanionic sodium electrode material and its preparation method. The difference from Embodiment 1 is that the metal salt in S1 is replaced with aluminum nitrate instead of magnesium nitrate.

[0068] The cathode material obtained in this embodiment has a spherical core-shell structure, with the core being sodium cobalt pyrophosphate and the shell being a composite functional layer. The cathode material particles have a D50 of 2.5 μm and a (D90-D10) / D50 ratio of 0.7. The total thickness of the composite functional layer is 55 nm, which contains embedded alumina nanoparticles with a particle size not exceeding 7 nm, and the alumina nanoparticles are enriched in the subsurface region of the composite functional layer.

[0069] Example 3 This embodiment provides a narrowly distributed spherical polyanionic sodium cathode material and its preparation method. The difference from Embodiment 1 is that in S2, the carbon source includes citric acid. In S2, the pre-encapsulated solution obtained in S1 is first mixed with a second portion of citric acid (accounting for 60% of the total carbon source mass) and deionized water, and stirred for 1 hour to obtain a pre-encapsulated solution containing excess citric acid. Sodium pyrophosphate, cobalt carbonate (cobalt source), and ammonium dihydrogen phosphate (phosphorus source) are weighed according to a sodium:cobalt:phosphorus molar ratio of 4:3:4, and then mixed with glucose, sucrose (both accounting for 40% of the total carbon source mass), and deionized water. The mixture is then dispersed by sand milling at 2500 rpm. First, 0.6-0.8 mm zirconium beads are used for milling for 30 minutes, then 0.1 mm zirconium beads are used to continue milling until the particle size D50 of the solid particles in the slurry is <100 nm. Subsequently, the pre-encapsulated solution containing excess citric acid is added to the slurry, and milling continues for 30 minutes until homogeneous, obtaining a nano-slurry.

[0070] The cathode material obtained in this embodiment has a spherical core-shell structure, with the core being sodium cobalt pyrophosphate and the shell being a composite functional layer. The cathode material particles have a D50 of 2.4 μm and a (D90-D10) / D50 ratio of 0.4. The total thickness of the composite functional layer is 55 nm, which contains embedded magnesium oxide nanodots with a particle size not exceeding 6 nm (approximate value), and the magnesium oxide nanodots are enriched in the subsurface region of the composite functional layer.

[0071] Example 4 This embodiment provides a narrowly distributed spherical polyanionic sodium electrode material and its preparation method. The difference from Example 1 is that, in S1: hexadecyltrimethylammonium bromide is mixed and stirred with magnesium nitrate solution for 30 minutes, then citric acid is added, and the mixture is stirred at 60°C for 3 hours to perform supramolecular co-assembly. The molar ratio of hexadecyltrimethylammonium bromide, citric acid, and magnesium ions is 2.5:1.5:1, the magnesium ion concentration is 0.15 mol / L, and the viscosity of the pre-encapsulated solution at 25°C is 22 mPa·s.

[0072] S2: The pre-encapsulated solution obtained in S1 is mixed with the second portion of citric acid (80% of the total carbon source mass) and deionized water, and stirred for 1.5 hours. Then, sodium pyrophosphate, cobalt carbonate, ammonium dihydrogen phosphate, glucose, and sucrose (the remaining carbon source) are added, and the mixture is dispersed by sand milling until the particle size D50 of the solid particles in the slurry is <80 nm. The total amount of carbon source is 15% of the total weight of sodium, cobalt, and phosphorus sources, and the molar ratio of cobalt to magnesium ions is 1:0.03. The solid content of the slurry is 45%, and the dynamic viscosity at 25℃ is 220 mPa·s. The spray drying inlet temperature is 210℃, the outlet temperature is 95℃, and the feed rate is 40 mL / min.

[0073] S3: Place the precursor particles in an argon atmosphere, first heat to 280℃ at 1.5℃ / min and hold for 4 hours; then heat to 680℃ at 4℃ / min and hold for 5 hours.

[0074] The cathode material obtained in this embodiment has a spherical core-shell structure, with the core being sodium cobalt pyrophosphate and the shell being a composite functional layer. The cathode material particles have a D50 of 2.4 μm and a (D90-D10) / D50 of 0.4. The composite functional layer has a thickness of 50 nm and is embedded with magnesium oxide nanodots with a particle size not exceeding 5 nm. These nanodots are more uniformly distributed and have a more concentrated subsurface enrichment.

[0075] Comparative Example 1 This comparative example provides a narrow-distribution spherical polyanionic sodium electrode material and its preparation method. The difference from Example 1 is that in S1, water, magnesium nitrate, and citric acid are mixed and stirred at 50°C for 2 hours. The molar ratio of citric acid to magnesium ions is 1:1, and the magnesium ion concentration in the pre-encapsulated solution is 0.1 mol / L. In S2, sodium pyrophosphate, cobalt carbonate, ammonium dihydrogen phosphate, glucose, and sucrose are weighed in the same proportions as in Example 1, and mixed with hexadecyltrimethylammonium bromide (its addition amount is in a molar ratio of 2:1 to magnesium ions in S1) and water. The mixture is then dispersed by sand milling at 2500 rpm. First, 0.6-0.8 mm zirconium beads are used for milling for 30 minutes, then 0.1 mm zirconium beads are used to continue milling until the particle size D50 of the solid particles in the slurry is <100 nm. Then, the pre-encapsulated solution is added, and milling continues for another 30 minutes until homogeneous.

[0076] The cathode material obtained in this comparative example has an irregularly shaped core-shell structure, with a core of sodium cobalt pyrophosphate and a shell of magnesium oxide-doped carbon layer. The cathode material particles have a D50 of 2.8 μm and a (D90-D10) / D50 ratio of 1.2. The total thickness of the composite functional layer varies considerably, ranging from 40 to 100 nm. It contains embedded magnesium oxide nanodots with a particle size of 10-60 nm. The magnesium oxide nanodots are unevenly distributed and some exhibit severe agglomeration, even penetrating the composite functional layer.

[0077] Comparative Example 2 This comparative example provides a narrow-distribution spherical polyanionic sodium electrode material and its preparation method. The difference from Comparative Example 1 is that in step S2, sodium pyrophosphate, cobalt carbonate, ammonium dihydrogen phosphate, glucose, and sucrose are weighed in the same proportions as in Example 1, mixed with deionized water, and then dispersed by sand milling at 2500 rpm. First, 0.6-0.8 mm zirconium beads are used for milling for 30 minutes, then 0.1 mm zirconium beads are used to continue milling until the particle size D50 of the solid particles in the slurry is <100 nm. Then, a pre-encapsulation solution is added, and milling continues for another 30 minutes until homogeneous.

[0078] like Figure 2 As shown, the cathode material obtained in this comparative example has an irregular core-shell structure. The core is sodium cobalt pyrophosphate, and the shell is a carbon layer with dispersed magnesium oxide. Its particle structure is broken and loose. The particle size of the cathode material is 9.5 μm, which is coarse. The ratio of (D90-D10) / D50 is 1.6. The magnesium oxide particles have a particle size of 20-150 nm, which is severely agglomerated and extremely unevenly distributed.

[0079] Comparative Example 3 This comparative example provides a narrowly distributed spherical polyanionic sodium cathode material and its preparation method. The difference from Example 1 is that in S2, 0.1 mm zirconium beads are not used for sand milling, and the D50 particle size of the sand-milled particles is 750 nm.

[0080] The cathode material obtained in this comparative example has an irregularly shaped core-shell structure, with a core of sodium cobalt pyrophosphate and a shell of magnesium oxide-doped carbon layers. The cathode material particles have a particle size D50 of 3.8 μm, a (D90-D10) / D50 ratio of 1.3, and a composite functional layer thickness of 80-160 nm, which contains embedded magnesium oxide nanodots with a particle size of 15-50 nm, but the distribution is poor.

[0081] Comparative Example 4 This comparative example provides a narrowly distributed spherical polyanionic sodium cathode material and its preparation method. The difference from Example 1 is that in S3, the temperature is directly raised to 650°C at a heating rate of 5°C / min under a nitrogen atmosphere and held for 9 hours. The cathode material obtained in this comparative example has a spherical core-shell structure, with a core of sodium cobalt pyrophosphate and a shell of magnesium oxide-doped carbon layers. The cathode material particles have a particle size D50 of 2.7 μm, a (D90-D10) / D50 ratio of 1.1, and a total thickness of approximately 65 nm in the composite functional layer. It contains embedded magnesium oxide nanodots with a particle size of 10-30 nm, and some areas show obvious agglomeration, resulting in a loose composite functional layer structure.

[0082] The positive electrode active material, SP, and PVDF obtained in the above examples and comparative examples were mixed in a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil. The film was then dried in a vacuum drying oven at 110°C for 4 hours. The electrode film was punched into a disc with a radius of 0.6 cm using a punching machine. Sodium metal was used as the counter electrode, and 1 mol / L NaClO4 + EC + PC (1:1, vol%) + 5% FEC was used as the electrolyte, where EC is ethylene carbonate, PC is propylene carbonate, and FEC is a fluoroethylene carbonate additive. The separator was glass fiber. The CR2032 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C = 129 mAh / g). The test results are shown in Table 1 below. Table 1: Experimental test results of each embodiment and comparative example

[0083] Based on the above results, it can be found that Example 1, by employing supramolecular co-assembly to pre-encapsulate magnesium ions in an encapsulation structure constructed from hexadecyltrimethylammonium bromide and citric acid, dispersed them into a nanoscale slurry through sand milling, and spray-dried to obtain precursor particles with high sphericity and narrow particle size distribution, followed by segmented sintering to generate magnesium oxide nanodots in situ and enrich them on the subsurface of the composite functional layer, yields cathode material particles with uniform particle size distribution, controllable nanodot size, and enrichment in the subsurface region, exhibiting good discharge specific capacity, initial coulombic efficiency, and cycle stability.

[0084] Compared to Example 1, Example 2 replaced the metal salt with aluminum nitrate instead of magnesium nitrate. The resulting alumina nanodots could also achieve uniform distribution and subsurface enrichment. The electrochemical properties of alumina were similar to those of magnesium oxide, with slight changes in various indicators.

[0085] In Example 3, a second portion of citric acid was added to S2 and preferentially mixed with the pre-encapsulated solution, which further enhanced the encapsulation effect and anchoring effect on metal ions, making the magnesium oxide nanodots more uniformly distributed and smaller in size. The particle size distribution was further narrowed, and the discharge specific capacity and cycle stability were improved compared with Example 1.

[0086] Example 4 further optimizes the raw material ratio, sand milling particle size, spray drying parameters and segmented sintering temperature based on Example 3, so that the magnesium oxide nanoparticles are smaller and more uniformly distributed, the composite functional layer structure is more compact, the subsurface enrichment characteristics are more significant, and it has better comprehensive performance.

[0087] Compared to Example 1, Comparative Example 1 did not add a cationic surfactant in S1, and only mixed magnesium ions with citric acid, thus failing to form a supramolecular encapsulation structure. Although the surfactant added in S2 could play a certain role in dispersion and morphology guidance during sand milling and spray drying, controlling the particle size to 2.8 μm, it could not effectively control the dispersion of magnesium ions under complex chemical environments. After sintering, the magnesium oxide nanoparticles were unevenly distributed, large in size, and partially agglomerated, even penetrating the carbon layer, resulting in a weakened interfacial stabilization effect. Therefore, the capacity and cycle performance were significantly reduced compared to Example 1.

[0088] Comparative Example 2, due to the lack of a pre-encapsulation structure in S1 of Comparative Example 1, further lacks a cationic surfactant in S2, thus losing its dispersion and morphology guiding effects. The slurry stability after sand milling is poor, and the surface tension of the droplets is uneven during spray drying, resulting in severely uncontrolled particle morphology, large particle size (9.5 μm), and extremely wide distribution. At the same time, magnesium ions can freely agglomerate, and after sintering, magnesium oxide agglomerates severely and is extremely unevenly distributed, failing to play an effective interfacial stabilizing role. All electrochemical performances are poor.

[0089] In Comparative Example 3, the sand milling dispersion did not reach the nanoscale, resulting in coarse raw material particles that led to insufficient solid-phase reaction, insufficient crystallinity of sodium cobalt pyrophosphate, poor distribution of nanoparticles, and irregular particle morphology. Consequently, the discharge specific capacity and cycle stability were significantly lower than those in Example 1.

[0090] Comparative Example 4 changed the segmented sintering to a single-stage sintering process. The pyrolysis and densification processes were mixed, the organic components decomposed violently and destroyed the particle structure, the nanodots agglomerated significantly and the carbon layer was loose, the interface stability decreased, and all electrochemical performances were inferior to those of Example 1.

[0091] In summary, the present invention can solve the technical problems of existing technologies, such as the difficulty in preparing sodium cobalt pyrophosphate cathode materials, poor morphology, and easy instability.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a narrowly distributed spherical polyanionic sodium cathode material, characterized in that, Includes the following steps: S1: A cationic surfactant, an organic ligand, a metal salt, and water are mixed and supramolecularly co-assembled to encapsulate the metal ions in the metal salt between the cationic surfactant and the organic ligand, forming a supramolecular encapsulation structure to obtain a pre-encapsulated solution. S2: The pre-encapsulated solution, water, sodium source, cobalt source, phosphorus source and carbon source are mixed and ground to obtain a nano slurry, which is then spray-dried to obtain precursor particles. The supramolecular encapsulation structure is enriched on the surface of the precursor particles. S3: The precursor particles are placed in a protective atmosphere and subjected to pyrolysis at 250℃-450℃, and then subjected to densification at 450℃-850℃ to obtain the cathode material. The cationic surfactant comprises at least one alkyl quaternary ammonium salt having 12-18 carbon atoms; the organic ligand comprises at least one polycarboxylic acid; and the metal salt is a water-soluble salt of at least one metal selected from magnesium, aluminum, zirconium, and titanium.

2. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, S1 includes: first, mixing and stirring the cationic surfactant with an aqueous solution containing the metal salt to form a pre-coordinated solution; then, adding the organic ligand to the pre-coordinated solution and stirring at 25℃-80℃ for 0.5-4 hours to complete supramolecular co-assembly and obtain a pre-encapsulated solution.

3. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, In S1, the molar ratio of the cationic surfactant, the organic ligand, and the metal ions in the metal salt is 1-4:0.5-2:1; the viscosity of the pre-encapsulated solution at 25°C is 10-30 mPa·s; and the concentration of the metal ions in the pre-encapsulated solution is 0.01-0.5 mol / L.

4. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, In S1, the metal salt is at least one of magnesium nitrate, magnesium chloride, aluminum nitrate, zirconium chloride, and titanium oxysulfate; the cationic surfactant is at least one of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and dodecyltrimethylammonium bromide; and the organic ligand is at least one selected from citric acid, oxalic acid, tartaric acid, malic acid, and gallic acid.

5. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, S2 include: Water, sodium source, cobalt source, phosphorus source and carbon source are mixed and dispersed by sand milling, and then the pre-encapsulated solution is added and sand milling is continued to obtain nano slurry; the particle size of solid particles in the nano slurry is 50-200 nm; the dynamic viscosity of the nano slurry at 25℃ is 100-300 mPa·s, and the solid content is 30%-50%; the molar ratio of cobalt element in cobalt source to metal ions in pre-encapsulated solution is 1:0.01-0.

1.

6. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1 or 5, characterized in that, In S2, the molar ratio of sodium, cobalt, and phosphorus in the nano-slurry is controlled at 4:3:4; the amount of carbon source added is 5%-20% of the total weight of sodium, cobalt, and phosphorus sources. The cobalt source is at least one of cobalt carbonate, cobalt hydroxide, cobalt nitrate, and cobalt acetate; the phosphorus source is at least one of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and sodium pyrophosphate; the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium nitrate, disodium hydrogen phosphate, and sodium pyrophosphate; the carbon source is an organic carbon source and / or an inorganic carbon source; the organic carbon source is at least one of sucrose, starch, cyclodextrin, glucose, maltose, lactose, polyvinyl alcohol, and polyacryl alcohol; and the inorganic carbon source is at least one of graphene, graphite, hard carbon, carbon nanotubes, and carbon black.

7. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, In S2, the carbon source contains an organic carboxylic acid; the organic carboxylic acid is a polycarboxylic acid that is the same as or different from the organic ligand; The amount of the organic carboxylic acid added accounts for 50wt%-100wt% of the total weight of the carbon source; the organic carboxylic acid is at least one selected from citric acid, oxalic acid, tartaric acid, malic acid and gallic acid.

8. The method for preparing the spherical polyanionic sodium cathode material as described in claim 1, characterized in that, In S3, the heating rate of the pyrolysis treatment is 1-5℃ / min, the holding temperature is 250-350℃, and the holding time is 2-8 hours; the heating rate of the densification treatment is 4-10℃ / min, the holding temperature is 600-750℃, and the holding time is 3-12 hours; the protective atmosphere is a weakly reducing atmosphere or an inert atmosphere.

9. A spherical polyanionic sodium cathode material prepared by the preparation method according to any one of claims 1-8, characterized in that, It is a core-shell structured composite particle, comprising a sodium cobalt pyrophosphate core and a composite functional layer from the inside out; the composite functional layer includes a carbon matrix and metal oxide nanoparticles dispersed and embedded within the carbon matrix, the metal oxide nanoparticles being physically separated by the carbon matrix; the metal oxide nanoparticles are at least one selected from magnesium oxide, aluminum oxide, zirconium oxide and titanium oxide; the particle size of the metal oxide nanoparticles does not exceed 20 nm; the metal oxide nanoparticles are enriched below the surface of the composite functional layer to form a subsurface.

10. The spherical polyanionic sodium cathode material as described in claim 9, characterized in that, The chemical formula of the sodium cobalt pyrophosphate is Na4Co3(PO4)2P2O7; the particle size of the composite particles is 1-10 micrometers, and the sphericity is ≥0.9; the total thickness of the composite functional layer is 30-120 nm; the particle size of the metal oxide nanodots does not exceed 10 nm; the metal oxide nanodots account for 0.1wt%-2wt% of the total mass of the cathode material.

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