Composite positive electrode material with core-shell structure, preparation method of composite positive electrode material, sodium ion battery and positive electrode of sodium ion battery

Abstract

The invention discloses a core-shell structure composite positive electrode material and a preparation method thereof, a sodium ion battery and a positive electrode of the sodium ion battery, the composite positive electrode material comprises a layered oxide inner core and a sodium ferromanganese phosphate nanocrystalline coating layer, the layered oxide inner core is layered oxide secondary particles, the ferromanganese sodium phosphate nanocrystal coating layer and the layered oxide core are combined through an interface chemical bond, the thickness of the ferromanganese sodium phosphate nanocrystal coating layer is 300-500 nm, and ferromanganese sodium phosphate nanocrystals forming the ferromanganese sodium phosphate nanocrystal coating layer are magnesium and vanadium co-doped ferromanganese sodium phosphate. A specific core-shell structure of layered oxide and sodium ferromanganese phosphate is constructed, and sodium ferromanganese phosphate is directionally doped, so that the problems of poor interfacial compatibility, weak synergistic effect and the like of a traditional composite material are effectively improved, and deep integration and performance synergistic enhancement of the two materials on the microstructure level are realized; therefore, the composite positive electrode material has the advantages of high energy density, long cycle life and high safety performance.

Description

Technical Field

[0001] This invention relates to the field of sodium battery technology, and more specifically, to a core-shell structured composite cathode material and its preparation method, and a sodium-ion battery and its cathode. Background Technology

[0002] Sodium-ion batteries, as an emerging energy storage technology, have received widespread attention in recent years due to their abundant resources and low cost. The cathode material, as a core component of sodium-ion batteries, directly affects the battery's energy density, cycle life, and safety performance. Currently, the most researched cathode materials mainly include layered oxides (Na₂O₃). x The two main categories are TMO2 and polyanionic compounds.

[0003] Layered oxide cathode materials possess high theoretical specific capacity and favorable sodium-ion diffusion kinetics, showing promising application prospects in sodium-ion batteries. These materials form a layered structure through the alternating arrangement of transition metal layers and sodium ion layers, providing a two-dimensional diffusion channel for sodium ion insertion / extraction. However, this material is prone to irreversible phase transitions during charge / discharge, leading to structural collapse; simultaneously, it is sensitive to moisture and carbon dioxide in the air, easily resulting in surface side reactions; furthermore, it is susceptible to thermal decomposition at high temperatures, posing safety hazards.

[0004] Sodium ferromanganese phosphate (NaFeMnPO4, NFPP), as a representative of polyanionic compounds, possesses a stable three-dimensional framework structure. Its open Na+... + The transport channels and strong PO covalent bonds give it excellent structural stability and thermal safety, while Mn 3+ / Mn 2+ with Fe 3+ / Fe 2+ Redox couples provide a higher operating voltage. However, due to [PO4] 3- The insulating properties of polyanionic cells result in low intrinsic electronic conductivity of the material, which limits its capacity utilization and rate performance.

[0005] In existing technologies, researchers have attempted to combine the advantages of layered oxides with sodium manganese iron phosphate through simple physical mixing. However, due to significant differences between the two types of materials in terms of crystal structure, electronic conduction mechanisms, and interfacial chemistry, this simple mechanical mixing cannot achieve an effective synergistic effect. During charging and discharging, the mixed materials exhibit problems such as intensified interfacial side reactions and ion / electron transport mismatch, resulting in limited performance improvements.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a core-shell structured composite cathode material and its preparation method, as well as a sodium-ion battery and its cathode, to improve the above-mentioned technical problems.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a core-shell structured composite cathode material, comprising a layered oxide core and a sodium manganese iron phosphate nanocrystal coating layer covering the surface of the layered oxide core. The layered oxide core is a secondary particle of layered oxide, and the sodium manganese iron phosphate nanocrystal coating layer is bonded to the layered oxide core through interfacial chemical bonds. The thickness of the sodium manganese iron phosphate nanocrystal coating layer is 300 nm to 500 nm, and the sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are magnesium and vanadium co-doped sodium manganese iron phosphate.

[0009] In an optional embodiment, the mass ratio of the sodium manganese iron phosphate nanocrystal coating layer to the layered oxide core is (10~25):(75~90). Preferably, the mass of the sodium manganese iron phosphate nanocrystal coating layer accounts for 10%~20% of the total mass of the core-shell structure composite cathode material.

[0010] In an optional embodiment, the layered oxide core is O3-type or P2-type Na. x Ni y Fe z Mn w O2, where x≈1, y+z+w≈1, and the D50 of the layered oxide core is 5μm~15μm, preferably 9μm~11μm.

[0011] In an optional embodiment, the magnesium doping amount in the sodium manganese iron phosphate nanocrystals is 2 at and the vanadium doping amount is 1 at.

[0012] In an optional embodiment, the interfacial chemical bond is a PO-TM bond, where TM is a transition metal in the layered oxide.

[0013] In an optional embodiment, the sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are carbon-coated nanocrystals, and the primary grain size of the sodium manganese iron phosphate nanocrystals is 20nm~100nm.

[0014] Secondly, the present invention provides a method for preparing a core-shell structured composite cathode material as described in any of the foregoing embodiments, comprising: spray drying and granulating a mixed slurry of the layered oxide core and the sodium manganese iron phosphate precursor solution, followed by heat treatment and crystallization.

[0015] In an optional embodiment, the mixed slurry is obtained by adding the layered oxide core to a sodium manganese iron phosphate precursor solution and stirring continuously at 40°C to 60°C for 2 to 4 hours.

[0016] In an optional embodiment, the preparation steps of the sodium manganese iron phosphate precursor solution include: using sodium salt, ferrous salt, manganese salt and phosphate as raw materials, the preparation steps of the sodium manganese iron phosphate precursor solution include: using sodium salt, ferrous salt, manganese salt, magnesium salt, vanadium salt and phosphate as raw materials, dissolving them in deionized water according to the stoichiometric ratio corresponding to the chemical structural formula of sodium manganese iron phosphate, and adding citric acid with a total metal ion molar amount of 1.0 to 1.5 times, mixing to form a clear sodium manganese iron phosphate precursor solution.

[0017] In an optional implementation, during spray drying, the air inlet temperature is controlled to be 180°C to 220°C; In an optional embodiment, the heat treatment crystallization includes: first, pre-oxidation at 330°C to 370°C under static air atmosphere by heating at 2°C / min to 3°C / min; then, switching to a mixed atmosphere of hydrogen and inert gas and crystallizing at 4°C / min to 6°C / min to 580°C to 620°C; finally, switching to an inert atmosphere and annealing at 730°C to 770°C by heating at 14°C / min to 16°C / min. Preferably, the pre-oxidation time is 1.5h to 2.5h; the crystallization time is 8h to 12h; and the annealing time is 10min to 20min.

[0018] Thirdly, the present invention provides a sodium-ion battery cathode, the active component of which includes the core-shell structured composite cathode material as described in any of the foregoing embodiments.

[0019] Fourthly, the present invention provides a sodium-ion battery comprising a sodium-ion battery positive electrode as described in the foregoing embodiments.

[0020] This invention offers the following advantages: by constructing a specific core-shell structure of layered oxide and sodium manganese iron phosphate, and by directionally doping the sodium manganese iron phosphate, it effectively improves key issues such as poor interfacial compatibility and weak synergistic effects in traditional composite materials. This innovative design achieves deep integration and synergistic performance enhancement of the two types of materials at the microstructural level, thereby enabling the composite cathode material to possess high energy density, long cycle life, and high safety performance. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 Here is a SEM image of the composite cathode material of Example 1 of the present invention; Figure 2 This is a comparison chart of air stability between embodiments and comparative examples of the present invention; Figure 3 This is a comparison chart of the long-cycle performance of Embodiment 1 and Comparative Examples 1 and 2 of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0024] Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible subranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0025] In the description of this invention, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships may exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0026] The following provides a detailed description of a core-shell composite cathode material, its preparation method, and a sodium-ion battery and its cathode provided by the present invention.

[0027] Some embodiments of the present invention provide a core-shell structured composite cathode material, comprising a layered oxide core and a sodium manganese iron phosphate nanocrystal coating layer covering the surface of the layered oxide core. The layered oxide core consists of secondary particles of layered oxide, and the sodium manganese iron phosphate nanocrystal coating layer is bonded to the layered oxide core through interfacial chemical bonds. The thickness of the sodium manganese iron phosphate nanocrystal coating layer is 300 nm to 500 nm, and the sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are magnesium and vanadium co-doped sodium manganese iron phosphate.

[0028] It should be noted that the interfacial chemical bond is a PO-TM bond, where TM is a transition metal in the layered oxide. Firstly, by replacing physical contact with chemical bonding, the poor compatibility between the two materials is resolved, and the interfacial electronic conduction path is continuous, reducing charge transfer impedance. The interfacial phase provides low-barrier Na... + The improved diffusion coefficient of the transport channels significantly enhances rate performance. Secondly, the high capacity of layered oxides complements the high voltage and long cycling characteristics of NMFPs. Interfacial bonding ensures efficient electron / ion transfer between the two phases, preventing insufficient capacity utilization and improving electrode energy density. Oxygen vacancies and PO defects form continuous ion channels during bonding, further accelerating Na+ transport. + Transport. Furthermore, TM-OP bonds form "atomic anchors" with the interfacial mixed phase, confining the layered oxides in Na… +Interlayer relative displacement during insertion / extraction suppresses irreversible phase transitions such as P2-O2 / O3-P3; at the same time, it alleviates lattice distortion in NMFP caused by Fe / Mn valence state fluctuations.

[0029] The sodium manganese iron phosphate nanocrystal coating, bonded by interfacial chemical bonds, forms a continuous and dense crystal structure, isolating the active material from electrolyte corrosion and reducing Mn content. 3+ / Fe 3+ Dissolution; strong bonding prevents physical separation between particles during cycling, significantly improving cycle life.

[0030] Furthermore, Mg / V co-doped sodium manganese iron phosphate (MgFePO4) achieves dual optimization of interface and structure, synergistically improving the cycle stability and rate performance of the cathode material. It also enables the co-doped MgFePO4 nanocrystal coating to form an effective thermal barrier, thereby enhancing thermal stability. To ensure optimal co-doping effect, the magnesium doping amount in the MgFePO4 nanocrystals is 1.5~2.5 at%, for example 2 at%, and the vanadium doping amount is 0.9~1.1 at%, for example 1 at%.

[0031] In some embodiments, the mass ratio of the sodium manganese iron phosphate nanocrystalline coating layer to the layered oxide core is (10~25):(75~90), such as 10:90, 15:85, 20:80, 25:75, etc. Specifically, the mass of the sodium manganese iron phosphate nanocrystalline coating layer accounts for 10%~20% of the total mass of the core-shell composite cathode material. This suitable mass ratio allows the sodium manganese iron phosphate nanocrystalline coating layer to form a dense and continuous coating layer, effectively blocking electrolyte penetration, stabilizing the SEI film, and better synergizing performance with the layered oxide.

[0032] In some embodiments, the layered oxide core is O3-type or P2-type Na. x Ni y Fe z Mn w O2, where x≈1, y+z+w≈1. Preferably, the layered oxide core is O3-type Na. x Ni y Fe z Mn w O2, exemplarily, the layered oxide core is O3-NaNi 0.33 Fe 0.33 Mn 0.33 O2 secondary spherical particles.

[0033] To further ensure the performance of the cathode material, it is necessary to strictly control its particle size distribution. In some embodiments, the D50 of the layered oxide core is 5μm~15μm, preferably 9μm~11μm.

[0034] In some embodiments, the sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are carbon-coated nanocrystals, and the primary grain size of the sodium manganese iron phosphate nanocrystals is 20nm~100nm, preferably 40nm~60nm.

[0035] The small size effect of nanocrystals mitigates charge-discharge volume distortion, while the carbon layer inhibits nanoparticle aggregation. Interfacial chemical bonding forms "atomic anchors," synergistically preventing interlayer slip in layered oxides and Jahn-Teller distortion in NMFPs. This ensures long-term maintenance of particle integrity and lattice structure during cycling, significantly reducing the capacity decay rate. + The insertion / extraction distance is reduced from the micrometer level to the nanometer level. The carbon layer provides a 3D conductive network, and the interfacial bonding eliminates the electron / ion transport barriers between the two phases, enabling rapid and stable charge transfer at high rates. Furthermore, the carbon layer and the interfacial bonding layer provide dual isolation from the electrolyte, reducing Mn content. 3+ Dissolution, oxygen evolution, and disordered growth of SEI / CEI significantly improve coulombic efficiency and thermal stability, while reducing the risk of gas generation within the battery, ensuring the safety and reliability of the entire battery during long-term cycling.

[0036] Furthermore, some embodiments of the present invention also provide a method for preparing the core-shell structured composite cathode material in the foregoing embodiments, which includes: spray drying and granulating a mixed slurry of a layered oxide core and a sodium manganese iron phosphate precursor solution, followed by heat treatment and crystallization.

[0037] Specifically, in some embodiments, the preparation method of this core-shell structured composite cathode material includes the following steps: S1, Preparation of precursor solution Weigh out the sodium salt, ferrous salt, manganese salt, magnesium salt, vanadium salt, and phosphate according to the stoichiometric ratio corresponding to the chemical structural formula of sodium manganese phosphate. For example, if the chemical structural formula of sodium manganese phosphate is NaFeO... 49 Mn0. 49 Mg0. 02 V0. 01 For PO4, weigh the raw material salt according to the molar ratio of Na:(Fe+Mn+Mg):P=1.05:1:1 (where Mg / (Fe+Mn)=2%, V / P=1%), dissolve the weighed raw material salt in deionized water, add citric acid with a total molar amount of metal ions of 1.0~1.5 times, stir until clear, and obtain sodium manganese iron phosphate precursor solution.

[0038] S2, Liquid phase impregnation The layered oxide core material is added to the sodium manganese iron phosphate precursor solution and stirred continuously at 40℃~60℃ for 2~4 hours. The stirring speed can be selected as 200~400 rpm, so that the precursor can be fully wetted and adsorbed onto the surface and pores of the core particles.

[0039] S3, Spray drying The slurry is fed into a spray drying tower, with the inlet temperature controlled at 180℃~220℃, and the outlet temperature stabilized at 85℃~95℃ by adjusting the speed of the peristaltic pump. By controlling the above process parameters of spray drying, microspherical precursor powder with good morphology and uniform particle size can be formed.

[0040] S4, heat treatment crystallization Specifically, in some embodiments, a three-stage dynamic atmosphere sintering is adopted: first, pre-oxidation is performed in a static air atmosphere by heating to 330°C~370°C at 2°C / min~3°C / min; then, the temperature is switched to a mixed atmosphere of hydrogen and inert gas and heated to 580°C~620°C at 4°C / min~6°C / min for crystallization; finally, the temperature is switched to an inert atmosphere and annealed at 730°C~770°C at 14°C / min~16°C / min.

[0041] In some embodiments, the pre-oxidation time is 1.5h to 2.5h; the crystallization time is 8h to 12h; and the annealing time is 10min to 20min.

[0042] By employing a pre-oxidation process with a low heating rate, Fe²⁺ in the precursor can be reduced. + / Mn² + Ions towards the target valence state (Fe³) + / Mn³ + The process achieves uniform transformation of Fe2O3 and Mn3O4. By controlling the heating rate, this process effectively avoids the formation of impurity phases such as Fe2O3 and Mn3O4 due to excessive local oxidation. Simultaneously, the slow heating process helps suppress particle agglomeration, resulting in a precursor with a loose, porous structure, which provides a favorable transport channel for ion diffusion in the subsequent crystallization stage. Furthermore, the active oxygen groups generated during oxidation can induce preliminary cross-linking of the precursor particles, thereby significantly improving the uniformity of the subsequent crystallization process.

[0043] The crystallization process is based on a directional nucleation growth mechanism. Increasing the heating rate can effectively lower the activation energy barrier for crystal nucleation, thereby promoting the synchronous formation of high-density crystal nuclei and avoiding the problem of selective crystal growth caused by slow heating. Within the optimized temperature range of 580-620℃, PO4 3- Tetrahedrons and transition metal ions can achieve ordered coordination and arrangement, thereby forming a well-crystallized NMFP main phase. By adopting a gradient heating strategy, the homogeneous precursor structure formed in the pre-oxidation stage can be inherited, significantly reducing lattice defects caused by sudden temperature changes, and finally obtaining nanocrystals with uniform particle size distribution (e.g., 10~50 nm).

[0044] The annealing stage enables the repair of crystal defects and densification of the material structure. Specifically, rapid heating at high temperatures effectively eliminates various lattice defects generated during crystallization, alleviates stress concentration at grain boundaries, and thus improves the integrity of the crystal structure. Simultaneously, the rapid heating process helps shorten the material's residence time in the high-temperature region, inhibiting abnormal grain growth and the precipitation of impurity phases, which is beneficial for obtaining a uniform and stable microstructure. Furthermore, under high-temperature conditions, the graphitization degree of the carbon coating is optimized, significantly improving not only the electronic conductivity of the electrode material but also enhancing the interfacial bonding strength between particles, providing a structural basis and favorable conditions for subsequent stable interfacial chemical bonding with layered oxide cathode materials.

[0045] Furthermore, some embodiments of the present invention also provide a sodium-ion battery cathode, the active component of which includes the core-shell structured composite cathode material as described in the foregoing embodiments.

[0046] Furthermore, some embodiments of the present invention also provide a sodium-ion battery, which includes the above-described sodium-ion battery positive electrode.

[0047] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0048] Example 1 This embodiment provides a core-shell structured composite cathode material, the core of which is: O3-NaNi 0.33 Fe 0.33 Mn 0.33 O2 secondary spherical particles, with an outer shell of magnesium (Mg) and vanadium (V) co-doped sodium manganese iron phosphate (NaFeO). 49 Mn0. 49 Mg0. 02 V0. 01 PO4). The coating layer is 300nm~400nm thick and is composed of continuous and dense nanocrystals of about 50nm.

[0049] The preparation method of this core-shell structured composite cathode material is as follows: (1) Weigh out sodium acetate, ferrous sulfate, manganese sulfate, ammonium dihydrogen phosphate, magnesium sulfate, and ammonium metavanadate, and dissolve them in deionized water at a molar ratio of Na:(Fe+Mn+Mg):P=1.05:1:1 (where Mg / (Fe+Mn)=2%, V / P=1%). Add citric acid in an amount equal to 1.3 times the total molar amount of the metals, and stir in a water bath at 60°C until completely clear and transparent.

[0050] (2) Take 100g of layered oxygen core material and slowly add it to the above precursor solution. At a constant temperature of 50℃, continuously stir mechanically at a speed of 300rpm for 3 hours.

[0051] (3) The slurry obtained in step (2) is fed into the spray drying tower, the inlet temperature is controlled at 200℃, the peristaltic pump speed is adjusted to make the outlet temperature stable at 90℃±5℃, and the obtained microspherical precursor powder is collected.

[0052] (4) The precursor powder was placed in a high-temperature tube furnace and subjected to staged heat treatment according to the following procedure: Stage I (pre-oxidation): Under static air atmosphere, the temperature was increased to 350°C at a rate of 3°C / min, and held at the target temperature for 2 hours. Stage II (crystallization): The reaction atmosphere was switched to 5% H2 / Ar mixed gas, and the temperature was increased to 600°C at a rate of 5°C / min, and held at this temperature for 8 hours. Stage III (annealing): The temperature was rapidly increased to 750°C at a rate of 15°C / min by quickly switching to high-purity argon (Ar) atmosphere, and held at this temperature for 15 minutes. After the heat treatment was completed, the system automatically stopped heating, and the sample was cooled to room temperature in the furnace under the protection of high-purity Ar atmosphere. The obtained product was sieved through a 400-mesh standard sieve to obtain the target composite cathode material.

[0053] Example 2 This embodiment provides a core-shell structured composite cathode material, the material design and preparation process of which are as follows: (1) Material design: Core: Same as in Example 1, O3-NaNi0 with D50=10.2μm. 33 Fe0. 33 Mn0. 33 O2. Shell: Low-doped sodium manganese iron phosphate, target composition NaFeO. 495 Mn0. 495 Mg0. 01 V0. 005 PO4, which is 1 at% Mg doping and 0.5 at% V doping.

[0054] Mass ratio: The mass ratio of NFPP shell to layer oxygen core is 10:90.

[0055] (2) Preparation process: Except for adjusting the weighing of Mg and V raw materials in the precursor and the ratio of precursor solution to core during impregnation (to achieve a shell content of 10%), the other steps (liquid phase impregnation, spray drying, and three-stage dynamic atmosphere sintering) are exactly the same as in Example 1.

[0056] Example 3 This embodiment provides a core-shell composite cathode material, the material design and preparation process of which are as follows: (1) Material design: Core: Same as in Example 1. Shell: Highly doped sodium manganese iron phosphate, with a target composition of NaFeO. 47 Mn0. 47 Mg0. 06 V0.02 PO4, i.e., 3 at% Mg doping and 2 at% V doping. Mass ratio: The mass ratio of NFPP shell to oxygen core is 25:75.

[0057] (2) Preparation process: Except for adjusting the weighing of Mg and V raw materials in the precursor and the ratio of precursor solution to core during impregnation (to achieve a shell content of 25%), the rest of the core process is exactly the same as in Example 1.

[0058] Comparative Example 1 The only difference between this comparative example and Example 1 is that Mg and V are not added to dope the shell with sodium manganese iron phosphate; otherwise, they are the same as in Example 1.

[0059] Comparative Example 2 This comparative example is pure O3-NaNi. 0.33 Fe 0.33 Mn 0.33 O2 secondary spherical particles.

[0060] Comparative Example 3 O3-NaNi in Example 1 0.33 Fe 0.33 Mn 0.33 O2 secondary spherical particles and commercial NFPP nanoparticles (purchased from Huayou Cobalt, product model: NP1 general-purpose NFPP material, D50: 1.93μm, typical compaction density: 2.1 g / cm³, discharge specific capacity: approximately 108 mAh / g) were simply physically mixed at a mass ratio of 85:15.

[0061] Comparative Example 4 The only difference between this comparative example and Example 1 is that in step (4), only heat treatment in stages II and III is used, and pre-oxidation in stage I is not used.

[0062] Comparative Example 5 (1) Raw materials and design Core: Same as in Example 1, using O3-NaNi0 with D50=10.2μm. 33 Fe0. 33 Mn0. 33 O2 secondary spherical particles.

[0063] Outer shell objective: To prepare core-shell materials with sodium iron phosphate pyrophosphate (NaFeP2O7 / NaFePO4 composite phase, abbreviated as NFPFPP) as the shell layer.

[0064] (2) Specific operating steps Step 1: Prepare NFPFPP precursor sol Weigh the raw materials according to the stoichiometric ratio: Iron source: ferric nitrate nonahydrate (Fe(NO3)3·9H2O) or ferrous sulfate (FeSO4·7H2O).

[0065] Phosphorus and sodium sources: A mixed solution of sodium pyrophosphate (Na4P2O7) and sodium dihydrogen phosphate (NaH2PO4) to provide complex phosphate. The molar ratio can be referenced as (Fe):(total P)≈1:(1~1.1).

[0066] Carbon source: Citric acid (1.0 times the molar amount of metal ions), mainly used as a complexing agent.

[0067] The above raw materials were dissolved in deionized water and stirred at 60°C to form a uniform and clear sol.

[0068] Step 2: Sol-gel coating 100g of layered oxygen core particles were slowly added to the above sol. The mixture was continuously stirred at 70°C–80°C while water was slowly evaporated, causing the sol to gradually gel, ultimately yielding a complex encapsulated in a wet gel. The wet gel was dried at 120°C for 12 hours to obtain a dry gel precursor powder.

[0069] Step 3: Conventional sintering and crystallization Place the dry gel precursor powder in a muffle furnace or tube furnace.

[0070] Atmosphere: Sintering is carried out in static air or an inert atmosphere (such as nitrogen).

[0071] Sintering process: A single-stage heating method was adopted. The temperature was directly increased to 500°C~550°C (the lower crystallization temperature commonly found in pyrophosphates) at a rate of 2~5°C / min, and held at this temperature for 5~10 hours. After sintering, the material was cooled to room temperature in the furnace and sieved to obtain the material of Comparative Example 5.

[0072] Comparative Example 6 The only difference between this comparative example and Example 1 is that only Mg is used to dope the shell layer with sodium manganese iron phosphate; otherwise, it is the same as Example 1.

[0073] Comparative Example 7 The only difference between this comparative example and Example 1 is that only V is used to dope the shell layer with sodium manganese iron phosphate; otherwise, it is the same as Example 1.

[0074] It should be noted that all samples of the core material used in the above embodiments and comparative examples used the same batch of O3-NaNi. 0.33 Fe 0.33 Mn 0.33The O2 secondary spherical particles, with a particle size distribution determined by a laser particle size analyzer, are: D10=6.5μm, D50=10.2μm, and D90=14.8μm. This narrow and uniform particle size distribution is a prerequisite for ensuring consistent coating effects in subsequent processes.

[0075] The cathode materials of the examples and comparative examples were subjected to performance tests, and the specific test methods are as follows: Electrochemical testing: A CR2032 button half-cell was assembled with metallic sodium as the counter electrode and 1M NaClO4, EC / PC (1:1 vol%), and 5% FEC as the electrolyte. Testing was conducted on a LAND testing system within a voltage range of 2.0V to 4.3V.

[0076] Structural characterization: SEM (Hitachi SU-8600) was used. The SEM image of Example 1 is shown below. Figure 1 As shown.

[0077] Thermal stability: After charging the battery to 4.3V, the positive electrode was removed, cleaned and dried, and then tested using DSC (NetzschSTA 449 F3) at a heating rate of 10℃ / min.

[0078] Air stability: The powder sample was placed in a constant temperature and humidity chamber (25℃, 60% RH) and its capacity retention was tested periodically.

[0079] Electrochemical impedance spectroscopy (EIS): Measured on an electrochemical workstation (Bio-Logic VMP-3), with a frequency range of 100kHz to 10MHz and an amplitude of 10mV.

[0080] The material properties and structural characterization data are shown in Table 1; the electrochemical performance data are shown in Table 2; the rate performance and kinetic data are shown in Table 3; the safety and environmental stability data are shown in Table 4; and the air stability comparison is shown in Table 5. Figure 2 As shown in the figure, the long-cycle performance comparison between Example 1 and Comparative Examples 1-2 is as follows. Figure 3 As shown.

[0081] Table 1

[0082] Table 2

[0083] Table 3

[0084] Table 4

[0085] Analysis of the test results in Tables 1-4 above leads to the following conclusions: (1) Energy density: Example 1, due to the use of NFPP material and its high voltage platform, achieved an average discharge voltage of 3.46V, which is significantly higher than that of Comparative Example 4 (NFPFPP coated structure, with an average discharge voltage of 3.35V). This result fully demonstrates that the selection of cathode material system in the embodiments of the present invention has outstanding technical advantages, and the technical effect is not something that those skilled in the art could obviously expect based on the prior art.

[0086] (2) Cyclic performance and rate characteristics: Under 500-cycle testing and 5C high-rate charge-discharge conditions, Example 1 significantly outperformed Comparative Examples 5 and 6, which were doped with a single element, in terms of capacity retention and power output. The above-mentioned superior performance strongly confirms that there is a significant synergistic enhancement effect between Mg and V co-doping, and such synergistic effect cannot be reasonably predicted by conventional technical means.

[0087] (3) Safety: Example 1 showed excellent thermal stability, with its exothermic peak starting temperature reaching 278°C, which was significantly higher than that of the comparative samples. This indicates that the "doped dense nanocrystalline shell" structure effectively constructed a stable thermal barrier, significantly delaying the structural disintegration and side reactions of the material under heated conditions, thereby greatly improving the overall thermal safety performance of the battery.

[0088] (4) Interface control: The interface impedance obtained in Example 1 is only 28 Ω, which is much lower than that in Comparative Example 2 (simple physical mixing, interface impedance of 105 Ω) and Comparative Example 3 (two-stage sintering process, interface impedance of 85 Ω). This data clearly shows that the three-stage dynamic atmosphere sintering process adopted in this invention plays a decisive role in constructing a low-impedance, chemically bonded interface structure, and is the key to achieving high-performance solid-solid interface integration.

[0089] (5) Regarding structural integrity: The shell obtained in Comparative Example 3 exhibits a porous and unevenly distributed microstructure, resulting in significantly inferior cycling stability (capacity retention of 80.1%) and air stability (capacity decay of 10.8% after 7 days of exposure to air) compared to Example 1 (corresponding indicators of 94.2% and 2.1%, respectively). This comparative result clearly reveals the indispensability of the pre-oxidation step in promoting the formation of a dense and continuous coating layer. The absence of this step will directly impair the overall structural integrity and environmental tolerance of the material.

[0090] In summary, the embodiments of this invention have achieved industry-leading levels in the following five core performance dimensions: high specific capacity, long cycle life (94% capacity retention after 500 cycles), excellent rate performance (88% capacity retention at 5C), top-level thermal safety (exothermic onset temperature above 275°C), and outstanding air stability (capacity decay of only about 3% after storage). It is worth noting that any simplification or omission of key technical features of this invention (e.g., omitting the Mg / V co-doping design, changing the sintering process such as omitting pre-oxidation or adopting non-dynamic atmosphere control) will lead to a significant decrease in the key properties of the material, fully demonstrating the high synergy and indivisibility among the various technical means in the embodiments of this invention.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A core-shell structured composite cathode material, characterized in that, It includes a layered oxide core and a sodium manganese iron phosphate nanocrystal coating layer covering the surface of the layered oxide core. The layered oxide core is a secondary particle of layered oxide. The sodium manganese iron phosphate nanocrystal coating layer is bonded to the layered oxide core through interfacial chemical bonds. The thickness of the sodium manganese iron phosphate nanocrystal coating layer is 300nm~500nm. The sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are magnesium and vanadium co-doped sodium manganese iron phosphate.

2. The core-shell composite cathode material according to claim 1, characterized in that, The mass ratio of the sodium manganese iron phosphate nanocrystalline coating layer to the layered oxide core is (10~25):(75~90). Preferably, the mass of the sodium manganese iron phosphate nanocrystalline coating layer accounts for 10%~20% of the total mass of the core-shell structure composite cathode material.

3. The core-shell structured composite cathode material according to claim 1, characterized in that, The layered oxide core is O3-type or P2-type Na. x Ni y Fe z Mn w O2, where x≈1, y+z+w≈1; And / or, the D50 of the layered oxide core is 5μm~15μm, preferably 9μm~11μm; And / or, the magnesium doping amount in the sodium manganese iron phosphate nanocrystals is 1.5~2.5 at%, and the vanadium doping amount is 0.9~1.1 at%; And / or, the interfacial chemical bond is a PO-TM bond, where TM is a transition metal in the layered oxide.

4. The core-shell composite cathode material according to claim 1, characterized in that, The sodium manganese iron phosphate nanocrystals constituting the sodium manganese iron phosphate nanocrystal coating layer are carbon-coated nanocrystals, and the primary grain size of the sodium manganese iron phosphate nanocrystals is 20nm~100nm.

5. A method for preparing a core-shell structured composite cathode material as described in any one of claims 1 to 4, characterized in that, It includes: The mixture of the layered oxide core and the sodium manganese iron phosphate precursor solution was spray-dried and granulated, and then subjected to heat treatment for crystallization.

6. The method for preparing the core-shell structured composite cathode material according to claim 5, characterized in that, The mixed slurry is obtained by adding the layered oxide core to a sodium manganese iron phosphate precursor solution and stirring continuously at 40℃~60℃ for 2~4 hours.

7. The method for preparing the core-shell structured composite cathode material according to claim 5, characterized in that, The preparation steps of the sodium manganese iron phosphate precursor solution include: using sodium salt, ferrous salt, manganese salt, magnesium salt, vanadium salt and phosphate as raw materials, dissolving them in deionized water according to the stoichiometric ratio corresponding to the chemical structural formula of sodium manganese iron phosphate, and adding citric acid with a total metal ion molar amount of 1.0 to 1.5 times, mixing to form a clear sodium manganese iron phosphate precursor solution.

8. The method for preparing the core-shell structured composite cathode material according to any one of claims 5 to 7, characterized in that, During spray drying, the air inlet temperature should be controlled at 180℃~220℃; And / or, the heat treatment crystallization includes: first, pre-oxidation at 330°C to 370°C under static air atmosphere by heating at 2°C / min to 3°C / min; then, switching to a mixed atmosphere of hydrogen and inert gas and crystallizing at 4°C / min to 6°C / min to 580°C to 620°C; finally, switching to an inert atmosphere and annealing at 730°C to 770°C by heating at 14°C / min to 16°C / min; preferably, the pre-oxidation time is 1.5h to 2.5h; the crystallization time is 8h to 12h; and the annealing time is 10min to 20min.

9. A sodium-ion battery positive electrode, characterized in that, Its active ingredients include the core-shell structured composite cathode material as described in any one of claims 1 to 4.

10. A sodium-ion battery, characterized in that, It includes the sodium-ion battery positive electrode as described in claim 9.

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