Layered oxide positive electrode material of sodium ion battery and preparation method of layered oxide positive electrode material

Abstract

The invention discloses a layered oxide positive electrode material of a sodium-ion battery and a preparation method of the layered oxide positive electrode material, and belongs to the technical field of batteries. The method comprises the following steps: preparing a NaNi < 0.33 > Mn < 0.33 > Fe < 0.33 > O2 precursor, mixing the precursor with a Na2S and P2S5 mixture under argon, carrying out heat treatment at 500 DEG C, and sintering with sodium hydroxide in an oxygen atmosphere at 800 DEG C to obtain the material. The core of the material is that a continuous super-ion conductor phase is formed on a grain boundary, a three-dimensional ion transmission network and a mechanical strengthening framework are constructed, the ion diffusion efficiency and the structural stability are remarkably improved, high-rate and long-cycle performance breakthrough is achieved, the process is controllable, and the industrialization potential is large.

Description

Technical Field

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

[0002] Sodium-ion batteries, with their significant advantages such as abundant and widely distributed sodium resources and low cost, have shown great application potential in large-scale energy storage, low-speed electric vehicles, and other fields, becoming one of the current research hotspots in the field of battery technology. As the core component of sodium-ion batteries, the performance of the cathode material directly determines key indicators such as energy density, rate performance, and cycle life. Therefore, developing high-performance sodium-ion battery cathode materials has significant practical importance and industrial value.

[0003] O3-type layered oxides (such as NaNi) 0.33 Mn 0.33 Fe 0.33 O2 (sodium-ion) has become a promising cathode material system for sodium-ion batteries due to its high theoretical specific capacity, suitable operating voltage, and simple preparation process. However, this type of material faces two major bottlenecks in practical applications, which severely restrict its commercialization process: First, ion diffusion kinetics are sluggish. Sodium ions have a larger radius than lithium ions, resulting in greater migration resistance within the lattice and grain boundaries of O3-type layered oxides, leading to a slow ion diffusion rate. This problem is particularly pronounced in high-rate charge-discharge scenarios, where a large number of sodium ions cannot be inserted into or extracted from the electrode material lattice in time, causing rapid capacity decay and making it difficult to meet the demands of high-power applications. Existing improvement strategies, such as bulk element doping and particle nanostructuring, can reduce ion transport resistance within the lattice to some extent, but they cannot fundamentally solve the problem of ion transport blockage at grain boundaries. Grain boundaries, as the interface regions between grains in polycrystalline materials, have disordered atomic arrangement and high defect density, acting as "thrombi" for sodium ion transport and significantly limiting the overall ion diffusion efficiency of the material.

[0004] Secondly, the grain boundary structure has poor stability. During battery charge-discharge cycles, the lattice of O3-type layered oxides undergoes periodic volume expansion and contraction due to the insertion / extraction of sodium ions. This volume effect generates continuous mechanical stress at the grain boundaries within the secondary particles, making them prone to microcracks. These microcracks not only disrupt the structural integrity of the material but also expose more electrode material surfaces, becoming active sites for side reactions between the electrolyte and the electrode. This leads to problems such as the dissolution of transition metal ions and electrolyte decomposition, resulting in a surge in battery impedance and deterioration of cycle stability. Traditional surface modification techniques (such as carbon coating and oxide coating) can only protect the outer surface of the secondary particles and cannot reach the grain boundary regions inside the particles, making it difficult to suppress the initiation and propagation of microcracks within the grain boundaries.

[0005] Currently, related research and patented technologies mostly focus on adjusting the crystal structure of materials through composition optimization (such as doping with heterogeneous elements) or improving the interfacial compatibility between materials and electrolytes through surface coating. However, none of them address the design of the internal grain boundaries of polycrystalline materials themselves. Therefore, developing a novel technical solution that can penetrate deep into the particle interior and modify the grain boundaries to improve conductivity has become the core breakthrough for solving the above-mentioned key problems and promoting the practical application of O3-type layered oxide cathode materials. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a layered oxide cathode material for sodium-ion batteries and its preparation method. The material forms a continuous superionic conductor phase at the grain boundaries, constructs a three-dimensional ion transport network and a mechanically reinforced framework, significantly improves ion diffusion efficiency and structural stability, achieves breakthroughs in high-rate and long-cycle performance, has controllable process, and has great industrialization potential.

[0007] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a layered oxide cathode material for sodium-ion batteries, comprising the following steps: S1 Preparation of NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor; S2 mixes the precursor with a mixture of Na2S and P2S5 under argon protection; S3 The mixture obtained in step S2 is heat-treated at 500-550℃ for 2-2.5h in an argon atmosphere; This 500-550℃ heat treatment aims to react Na2S and P2S5 to generate Na3PS4, and take advantage of its low viscosity at this temperature to melt and penetrate into the gaps (grain boundaries) of the oxide grains that have not yet been densely sintered. At this time, the main material has not yet crystallized and maintains a porous structure. In an oxygen atmosphere, the heat-treated mixture is sintered with sodium hydroxide at 780-800℃ for 2-2.5h to completely crystallize the layered oxide, thus obtaining the layered oxide cathode material for sodium-ion batteries.

[0008] Preferably, step S1 specifically includes the following steps: S11 Preparation of metal salt solution: According to NaNi 0.33 Mn 0.33 Fe 0.33 The stoichiometric ratio of O2 precursors is as follows: nickel sulfate, manganese sulfate, and ferrous sulfate are weighed and dissolved in water to obtain a metal salt solution. Preparation of S12 precipitant solution: Dissolve sodium carbonate in water and add ammonia to reduce the NH4+ concentration in the solution. +The concentration was maintained at 0.5 mol / L to obtain the precipitant solution; Under the protective atmosphere of continuous argon gas, S13, the metal salt solution and the precipitant solution are pumped into the reactor at the same flow rate and in parallel to carry out the reaction, and the reaction temperature is controlled at 50-60℃ and the pH is 10-11. After the S14 reaction is completed, continue stirring and aging. Then filter the precipitate and wash it several times with deionized water and ethanol until the filtrate is neutral. S15 vacuum-dry the filter cake to obtain NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor.

[0009] Preferably, in step S2, the molar ratio of Na2S to P2S5 is (3-3.2):1, and the mixture accounts for 4-6% of the precursor mass.

[0010] Preferably, in step S3, the temperature is increased to 500-550°C at a rate of 4-6°C / min.

[0011] Preferably, in step S4, the heat-treated mixture and sodium hydroxide are added at a ratio of 1.02:1 between the total moles of Ni, Mn, and Fe and the moles of Na.

[0012] Preferably, in step S4, the temperature is increased to 780-800°C at a rate of 4-6°C / min.

[0013] On the other hand, the present invention provides a layered oxide cathode material for sodium-ion batteries, which is prepared by the above-described method for preparing layered oxide cathode materials for sodium-ion batteries.

[0014] This invention utilizes an innovative "fusion infiltration welding" grain boundary engineering technique to reconstruct and optimize the internal structure of O3-type layered oxide cathode materials for sodium-ion batteries. Compared with existing technologies, it achieves the following significant advantages: 1. This invention constructs a continuous, dense, low-melting-point sodium superionic conductor phase (such as Na3PS4) between primary grains, which has a sodium ion conductivity as high as 2.5 × 10⁻⁶. -4 The S / cm ratio replaces the traditional disordered, high-resistivity grain boundary medium. By forming a three-dimensional sodium ion ultrafast transport network that runs through the entire secondary particle, sodium ions do not need to struggle through the internal lattice of the grain; they can migrate rapidly through the grain boundary "ion highway," increasing the material's apparent ion diffusion coefficient by 1-2 orders of magnitude, reaching 10. -11 cm 2 / s. This breakthrough completely solves the "thrombosis" problem of grain boundary ion transport blockage in traditional materials, enabling the material to retain more than 85% of its 0.1C capacity at an ultra-high rate of 50C, which is far superior to traditional materials and greatly expands the application potential of sodium-ion batteries in high-power scenarios.

[0015] 2. In this invention, the superionic conductor phase forms a strong chemical bond with the main grains through a "welding" effect, rather than simple physical adhesion, significantly strengthening the grain boundaries, a mechanically weak point. This conductor phase possesses both toughness and elasticity, effectively buffering the stress between adjacent grains and inhibiting the initiation and propagation of microcracks during the lattice expansion / contraction process of charge-discharge cycles, as if constructing a "mechanically reinforced skeleton" within the secondary particles. This structural design fundamentally solves the problems of easy pulverization, transition metal dissolution, and impedance surge during the cycling process of traditional materials. After 2000 cycles at 1C, the capacity retention rate remains as high as 96%, far exceeding that of traditional polycrystalline materials, achieving ultra-long-term cycling stability.

[0016] 3. The "ion highway" and "mechanically reinforced framework" of this invention form a highly synergistic effect: the stable mechanical framework ensures that the ion transport network remains continuous and intact during long-term cycling, avoiding transport channel failure due to grain boundary cracking; while efficient ion transport reduces local concentration polarization and lowers the stress caused by uneven lattice expansion, which in turn further protects structural stability. This synergistic effect enables the material to maintain a high specific capacity (up to 145 mAh / g at 0.1C) while also achieving ultra-high rate performance and ultra-long cycle life, breaking through the bottleneck of traditional modification schemes where "capacity-rate-cycle" is difficult to balance, and its overall performance is superior to existing technologies.

[0017] 4. Existing technologies mostly focus on material doping or surface modification, failing to address the fundamental modification of grain boundaries within the particles. This invention, for the first time, achieves a dual design of ion conduction and mechanical strengthening of grain boundaries within polycrystalline materials, fundamentally solving the core bottleneck of O3-type layered oxides. Its modification approach and technical effects differ from traditional physical mixing and surface coating methods, providing a completely new path for performance breakthroughs in sodium-ion battery cathode materials and offering important reference for grain boundary engineering modification of other polycrystalline electrode materials. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1 The method for preparing the layered oxide cathode material for sodium-ion batteries in this embodiment includes the following steps: S1 Preparation of NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor S11 Preparation of metal salt solution: According to NaNi 0.33 Mn 0.33 Fe 0.33 The stoichiometric ratio of O2 precursors is as follows: nickel sulfate, manganese sulfate and ferrous sulfate are weighed and dissolved in deionized water to obtain a 2 mol / L metal salt solution. S2 Preparation of precipitant solution: Dissolve sodium carbonate in deionized water and add ammonia to reduce the NH4+ concentration in the solution. + The concentration was maintained at 0.5 mol / L, resulting in a 2.5 mol / L precipitant solution; Under the protective atmosphere of continuous argon gas, S3 pumps the metal salt solution and the precipitant solution into the reactor at the same flow rate in parallel to carry out the reaction. The reaction temperature is controlled at 55℃, the stirring speed is 600rpm, and the pH is maintained at 10.5. After the S4 reaction is completed, continue stirring and aging for 12 hours. Then, filter the precipitate and wash it several times with deionized water and ethanol until the filtrate is neutral. S5 dried the filter cake in a vacuum oven at 100℃ for 24 hours to obtain NaNi. 0.33 Mn 0.33 Fe 0.33 O2 precursor.

[0020] S2: In an argon-filled glove box (H2O, O2 < 0.1ppm), the precursor obtained in S1 was mixed with Na2S and P2S5 "solder" powders in a molar ratio of 3:1, with the total mass of Na2S and P2S5 powders being 5% of the precursor mass. The resulting mixture was placed in a ball milling jar containing zirconia grinding beads, sealed, and removed from the glove box. The mixture was then ball-milled at 400 rpm for 4 hours in a high-energy planetary ball mill to ensure the "solder" was uniformly adhered to the precursor surface and between particles.

[0021] S3: The ball-milled composite powder is placed in an alumina boat and then placed in a tube furnace. Under a continuously flowing argon atmosphere (flow rate 50 sccm), it is heated to 500°C at a heating rate of 5°C / min and held at this temperature for 2 hours. During this process, Na3PS4 is generated and melts and penetrates into the grain boundaries.

[0022] S4: The intermediate product from step S3 is mixed with sodium hydroxide powder at a Na to Ni+Mn+Fe molar ratio of 1.02:1 (to provide excess sodium to compensate for high-temperature volatilization). The mixture is placed in a muffle furnace and heated to 800°C at a rate of 5°C / min under a flowing oxygen atmosphere, and held at that temperature for 2 hours to form layered oxides NaNi. 0.33 Mn 0.33 Fe 0.33 O2 crystallized completely. After cooling in the furnace, the sintered product was ground and passed through a 400-mesh sieve to obtain the cathode material of Example 1.

[0023] Example 2 The method for preparing the layered oxide cathode material for sodium-ion batteries in this embodiment includes the following steps: S1 Preparation of NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor S11 Preparation of metal salt solution: According to NaNi 0.33 Mn 0.33 Fe 0.33 The stoichiometric ratio of O2 precursors is as follows: nickel sulfate, manganese sulfate and ferrous sulfate are weighed and dissolved in deionized water to obtain a 2 mol / L metal salt solution. Preparation of S12 precipitant solution: Dissolve sodium carbonate in deionized water and add ammonia to reduce the NH4+ concentration in the solution. + The concentration was maintained at 0.5 mol / L, resulting in a 2.5 mol / L precipitant solution; Under the protective atmosphere of continuous argon gas, S13, the metal salt solution and the precipitant solution are pumped into the reactor at the same flow rate in parallel to carry out the reaction. The reaction temperature is controlled at 50°C, the stirring speed is 600 rpm, and the pH is maintained at 10.5. After the S14 reaction is completed, continue stirring and aging for 12 hours. Then, filter the precipitate and wash it several times with deionized water and ethanol until the filtrate is neutral. S15 dried the filter cake in a vacuum oven at 100℃ for 24 hours to obtain NaNi. 0.33 Mn 0.33 Fe 0.33 O2 precursor.

[0024] S2: In an argon-filled glove box (H2O, O2 < 0.1ppm), the precursor obtained in S1 was mixed with Na2S and P2S5 "solder" powders in a molar ratio of 3:1, with the total mass of Na2S and P2S5 powders being 4% of the precursor mass. The resulting mixture was placed in a ball milling jar containing zirconia grinding beads, sealed, and removed from the glove box. The mixture was then ball-milled at 400 rpm for 4 hours in a high-energy planetary ball mill to ensure the "solder" was uniformly adhered to the precursor surface and between particles.

[0025] S3: The ball-milled composite powder was placed in an alumina boat and then placed in a tube furnace. Under a continuously flowing argon atmosphere (flow rate 50 sccm), it was heated to 520°C at a heating rate of 4°C / min and held at this temperature for 2 hours. During this process, Na3PS4 was generated and melted and penetrated into the grain boundaries.

[0026] S4: The intermediate product from step S3 is mixed with sodium hydroxide powder at a Na to Ni+Mn+Fe molar ratio of 1.02:1 (to provide excess sodium to compensate for high-temperature volatilization). The mixture is placed in a muffle furnace and heated to 780°C at a rate of 4°C / min under a flowing oxygen atmosphere, and held at that temperature for 2 hours to form layered oxides NaNi. 0.33 Mn 0.33 Fe 0.33 O2 crystallized completely. After cooling in the furnace, the sintered product was ground and passed through a 400-mesh sieve to obtain the cathode material of Example 2.

[0027] Example 3 The method for preparing the layered oxide cathode material for sodium-ion batteries in this embodiment includes the following steps: S1 Preparation of NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor S11 Preparation of metal salt solution: According to NaNi 0.33 Mn 0.33 Fe 0.33 The stoichiometric ratio of O2 precursors is as follows: nickel sulfate, manganese sulfate and ferrous sulfate are weighed and dissolved in deionized water to obtain a 2 mol / L metal salt solution. Preparation of S12 precipitant solution: Dissolve sodium carbonate in deionized water and add ammonia to reduce the NH4+ concentration in the solution. + The concentration was maintained at 0.5 mol / L, resulting in a 2.5 mol / L precipitant solution; Under the protective atmosphere of continuous argon gas, S13, the metal salt solution and the precipitant solution are pumped into the reactor at the same flow rate in parallel to carry out the reaction. The reaction temperature is controlled at 60℃, the stirring speed is 600rpm, and the pH is maintained at 10.5. After the S14 reaction is completed, continue stirring and aging for 12 hours. Then, filter the precipitate and wash it several times with deionized water and ethanol until the filtrate is neutral. S15 dried the filter cake in a vacuum oven at 100℃ for 24 hours to obtain NaNi. 0.33 Mn 0.33 Fe 0.33 O2 precursor.

[0028] S2: In an argon-filled glove box (H2O, O2 < 0.1ppm), the precursor obtained in S1 was mixed with Na2S and P2S5 "solder" powders at a molar ratio of 3.2:1, with the total mass of Na2S and P2S5 powders being 6% of the precursor mass. The resulting mixture was placed in a ball milling jar containing zirconia grinding beads, sealed, and removed from the glove box. The mixture was then ball-milled at 400 rpm for 4 hours in a high-energy planetary ball mill to ensure the "solder" was uniformly adhered to the precursor surface and between particles.

[0029] S3: The ball-milled composite powder was placed in an alumina boat and then placed in a tube furnace. Under a continuously flowing argon atmosphere (flow rate 50 sccm), it was heated to 550°C at a heating rate of 6°C / min and held at this temperature for 2.5 h. During this process, Na3PS4 was generated and melted and penetrated into the grain boundaries.

[0030] S4: The intermediate product from step S3 is mixed with sodium hydroxide powder at a Na to Ni+Mn+Fe molar ratio of 1.02:1 (to provide excess sodium to compensate for high-temperature volatilization). The mixture is placed in a muffle furnace and heated to 800°C at a rate of 6°C / min under a flowing oxygen atmosphere, and held at that temperature for 2.5 hours to form layered oxides NaNi. 0.33 Mn 0.33 Fe 0.33 O2 was fully crystallized. After cooling in the furnace, the sintered product was ground and passed through a 400-mesh sieve to obtain the cathode material of Example 3.

[0031] Comparative Example 1: Traditional polycrystalline NaNi 0.33 Mn 0.33 Fe 0.33 Preparation of O2 materials The difference from Example 1 is that steps S2 and S3 are not performed, and the precursor of step S1 is used instead of the intermediate product after step S3 in step S4.

[0032] Comparative Example 2: Preparation of Na3PS4 Physically Modified Materials Includes the following steps: S1 prepared conventional polycrystalline NaNi according to the method of Comparative Example 1. 0.33 Mn 0.33 Fe 0.33 O2 materials; S2 synthesized Na3PS4 powder separately using a high-temperature solid-state method: Na2S and P2S5 were mixed in a molar ratio of 3:1, sintered at 550℃ for 10h in argon, and then ground to obtain Na3PS4 powder. S3. The material from step S1 and the Na3PS4 powder from step S2 are ball-milled at a mass ratio of 95:5 at 400 rpm for 2 hours. S4 The powder mixed in step S3 is heat-treated at 300℃ (this temperature is below the significant flow point of Na3PS4, which is intended to remove surface adsorbates and maintain a physically mixed state to avoid melt penetration) for 2 hours under argon protection to obtain the cathode material of Comparative Example 2.

[0033] Comparative Example 3: Surface coated with Na3Zr2Si2PO 12 Preparation of (NZSP) materials Includes the following steps: S1 prepared conventional polycrystalline NaNi according to the method of Comparative Example 1. 0.33 Mn 0.33 Fe 0.33 O2 materials; S2 was surface coated using the sol-gel method. S21 coating solution preparation: Tetraethyl orthosilicate (TEOS), zirconium nitrate, ammonium dihydrogen phosphate and sodium nitrate are used as raw materials and dissolved in an ethanol-water mixed solvent at a molar ratio of Na:Zr:Si:P=3.2:1.9:1.2:0.8 and stirred to form a transparent sol; S22 will use the NaNi from step S1 0.33 Mn 0.33 Fe 0.33 O2 material is added to the above sol, stirred and ultrasonicated to disperse it evenly, and then stirred continuously at 80°C to allow the solvent to slowly evaporate and form a gel. S23 dried the gel-like product at 120℃ for 12 hours, then placed it in a muffle furnace and heated to 750℃ at a rate of 5℃ / min under air atmosphere, and held at that temperature for 4 hours to crystallize the surface gel layer into Na3Zr2Si2PO4. 12 (NZSP) coating; After cooling and grinding, S3 yielded the cathode material of Comparative Example 3. This cathode material had an NZSP surface coating layer, but the internal grain boundaries were not modified in any way.

[0034] The positive electrode materials of Examples 1-3 and Comparative Examples 1-3 were assembled into coin cells: (1) Electrode preparation: Weigh the positive electrode material, conductive carbon black and polyvinylidene fluoride (PVDF) of Examples 1-3 and Comparative Examples 1-3 respectively and mix them in a mass ratio of 8:1:1. Add N-methylpyrrolidone (NMP) and stir until a uniform slurry is formed. Coat the slurry on aluminum foil, dry it under vacuum at 80°C for 12 hours, and then cut it into circular electrodes with a diameter of 12 mm.

[0035] (2) Battery assembly: In a glove box filled with argon (H2O, O2<0.1ppm), a sodium metal sheet is used as the counter electrode, glass fiber is used as the separator, and 1mol / L NaPF6 ethylene carbonate / dimethyl carbonate (EC / DMC, volume ratio 1:1) is used as the electrolyte. The batteries are stacked in the order of “positive electrode shell → positive electrode sheet → separator → electrolyte → sodium sheet → gasket → spring sheet → negative electrode shell” and then sealed with a button battery sealing machine to obtain a button battery.

[0036] The battery performance was tested, and the results are shown in Table 1. Table 1 Performance test results of the batteries assembled in Examples 1-3 and Comparative Examples 1-3 Comparative Example 1: NaNi was prepared directly using the traditional solid-state sintering process. 0.33 Mn 0.33 Fe 0.33 The poor performance of O2 materials, without any grain boundary modification treatment, stems from the dual defects of "ion transport blockage" and "weak structural stability" at the grain boundaries. (1) Extremely poor ion diffusion kinetics: The disordered atomic arrangement and dense defects at the grain boundaries of traditional polycrystalline materials form a "natural barrier" for sodium ion transport, with a grain boundary ion conductivity of only 10. -8 The S / cm is four orders of magnitude lower than that of the example. Sodium ions have to struggle to cross the crystal lattice and high-resistivity grain boundaries inside the particles, resulting in a sharp drop in transport efficiency at high rates. The 50C capacity retention is only 15%, far lower than the more than 85% of the example.

[0037] (2) Collapse of cyclic structure stability: Grain boundaries are weak points in mechanical strength. During the lattice expansion / contraction process of charge-discharge cycles, they lack an effective stress buffer mechanism and are prone to microcracks that continue to propagate. This leads to secondary particle pulverization, dissolution of transition metals, and a surge in impedance. The capacity retention rate after 2000 cycles at 1C is only 70%, which is significantly lower than that of the example.

[0038] Comparative Example 2 uses a physical ball milling method to modify Na3PS4 powder with traditional polycrystalline materials. Although it introduces Na3PS4 with high ionic conductivity, it does not form an effective grain boundary phase, resulting in limited performance improvement that is far inferior to the example. (1) Discontinuous ion transport network: Physical mixing only allows Na3PS4 particles to adhere to the surface of secondary particles or between particles, and cannot penetrate into the grain boundary region of the internal primary grains. Therefore, the interior of the material is still a high-resistivity traditional grain boundary, with low-resistivity channels only existing on the surface and in local areas. The grain boundary ion conductivity (10) -6 The sodium ion transport capacity (S / cm) is two orders of magnitude lower than in the previous example. Sodium ion transport remains limited by internal grain boundary blockage, with only 35% of the 50C capacity retention.

[0039] (2) Ineffective grain boundary mechanical strengthening: The physically mixed Na3PS4 only has physical contact with the main grains and no chemical bonding effect, so it cannot play a "welding" strengthening effect on the internal grain boundaries. During the cycle, the initiation and propagation of grain boundary microcracks were not suppressed, and the problems of transition metal dissolution and structural pulverization still exist. The capacity retention rate after 2000 cycles at 1C is only 78%.

[0040] Comparative Example 2 failed to overcome the limitations of "surface modification" and failed to reach the core bottleneck region of grain boundaries inside polycrystalline materials, thus the high ionic conductivity and mechanical strengthening potential of Na3PS4 could not be fully utilized.

[0041] Comparative Example 3 uses the sol-gel method to coat the surface of a traditional polycrystalline material with NZSP superionic conductors. This method only achieves surface protection and cannot improve the internal grain boundary properties, resulting in limited performance improvement. (1) The bottleneck of internal ion transport has not been overcome: The NZSP coating layer exists only on the outer surface of secondary particles, which can reduce the side reactions between the material and the electrolyte, but cannot change the high resistance characteristics of the traditional grain boundaries inside. Grain boundary ion conductivity (10 -7 The S / cm ratio is still much lower than in the examples. The transport of sodium ions inside the particles is still limited by grain boundary blockage. The 50C capacity retention rate is only 50%, which is better than comparative examples 1-2, but still less than 60% of the examples.

[0042] (2) The internal grain boundary structure is not strengthened: the surface coating layer cannot buffer the grain boundary stress inside the secondary particles. During the cycling process, internal grain boundary microcracks will still be generated and propagate, resulting in the destruction of structural integrity. At the same time, the surface coating layer may fall off due to the pulverization of internal particles, further aggravating the performance degradation. The capacity retention rate is only 85% after 2000 cycles at 1C.

[0043] (3) Additional defects: The surface coating may hinder the transport of sodium ions from the electrolyte to the interior of the particles to a certain extent, resulting in the 0.1C specific capacity (142mAh / g) of Comparative Example 3 being slightly lower than that of the Example, forming a contradiction between "surface protection and internal transport", and failing to achieve comprehensive performance optimization.

[0044] In summary, the embodiments of this invention, through a "fusion infiltration welding" process, achieve continuous infiltration and chemical bonding of the superionic conductor phase within the internal grain boundaries, constructing a "three-dimensional ion transport network" and a "mechanically reinforced framework," fundamentally solving the grain boundary bottleneck of traditional materials. In contrast, Comparative Examples 1-3 did not address the functional reconstruction of the internal grain boundaries: Comparative Example 1 retained the dual defects of traditional grain boundaries, while Comparative Examples 2-3 only achieved surface or external modification, failing to form a continuous and effective internal grain boundary phase. This resulted in all three examples significantly lagging behind the embodiments in ionic conductivity, high-rate performance, and cycle stability, highlighting the innovation and superiority of the grain boundary engineering design of this invention.

Claims

1. A method for preparing a layered oxide cathode material for sodium-ion batteries, characterized in that, Includes the following steps: S1 Preparation of NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor; S2 mixes the precursor with a mixture of Na2S and P2S5 under argon protection; S3. The mixture obtained in step S2 is heat-treated in an argon atmosphere at 500-550℃ for 2-2.5h. S4 is prepared by sintering the heat-treated mixture with sodium hydroxide at 780-800℃ for 2-2.5h in an oxygen atmosphere to obtain a layered oxide cathode material for sodium-ion batteries.

2. The method for preparing the layered oxide cathode material for sodium-ion batteries as described in claim 1, characterized in that, Step S1 specifically includes the following steps: S11 Preparation of metal salt solution: According to NaNi 0.33 Mn 0.33 Fe 0.33 The stoichiometric ratio of O2 precursors is as follows: nickel sulfate, manganese sulfate, and ferrous sulfate are weighed and dissolved in water to obtain a metal salt solution. Preparation of S12 precipitant solution: Dissolve sodium carbonate in water and add ammonia to reduce the NH4+ concentration in the solution. + The concentration was maintained at 0.5 mol / L to obtain the precipitant solution; Under the protective atmosphere of continuous argon gas, S13, the metal salt solution and the precipitant solution are pumped into the reactor at the same flow rate in parallel to carry out the reaction, and the reaction temperature is controlled at 50-60℃ and the pH is 10-11. After the S14 reaction is completed, continue stirring and aging. Then filter the precipitate and wash it several times with deionized water and ethanol until the filtrate is neutral. S15 vacuum-dry the filter cake to obtain NaNi 0.33 Mn 0.33 Fe 0.33 O2 precursor.

3. The method for preparing the layered oxide cathode material for sodium-ion batteries as described in claim 1, characterized in that, In step S2, the molar ratio of Na2S to P2S5 is (3-3.2):1, and the mixture accounts for 4-6% of the precursor mass.

4. The method for preparing the layered oxide cathode material for sodium-ion batteries as described in claim 1, characterized in that, In step S3, the temperature is increased to 500-550℃ at a rate of 4-6℃ / min.

5. The method for preparing the layered oxide cathode material for sodium-ion batteries as described in claim 1, characterized in that, In step S4, the heat-treated mixture and sodium hydroxide are added at a ratio of 1.02:1 between the total moles of Ni, Mn, and Fe and the moles of Na.

6. The method for preparing the layered oxide cathode material for sodium-ion batteries as described in claim 1, characterized in that, In step S4, the temperature is increased to 780-800℃ at a rate of 4-6℃ / min.

7. A layered oxide cathode material for sodium-ion batteries, characterized in that, It is prepared by the method for preparing sodium-ion battery layered oxide cathode material as described in any one of claims 1-7.

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