Preparation method of polyanionic sodium ferric phosphate positive electrode material for negative-electrode-free sodium ion battery

Abstract

The invention discloses a preparation method of a polyanionic sodium ferric phosphate positive electrode material for a negative-electrode-free sodium ion battery. Comprising the following steps: S1, preparation of precursor slurry: weighing an iron source, a quick-release sodium source, a slow-release sodium source, a phosphorus source and a carbon source according to a stoichiometric ratio, mixing in a solvent, and controlling the pH value of the solution to form the uniform precursor slurry; s2, bimetal doping introduction: adding a magnesium source and a titanium source, controlling the total doping amount of Mg and Ti to be the proportion of Fe sites, and uniformly mixing to obtain doped precursor slurry; s3, presintering: presintering after the slurry is dried to form a stable precursor structure; and S4, high-temperature sintering: carrying out high-temperature sintering in a protective atmosphere to obtain the iron-based phosphate positive electrode material adaptive to the negative-electrode-free sodium ion battery. The preparation method of the polyanionic sodium ferric phosphate positive electrode material for the negative-electrode-free sodium ion battery has the characteristics of good structural stability, high pure phase performance and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing polyanionic sodium iron phosphate cathode material for negative electrode-free sodium-ion batteries. Background Technology

[0002] With the increasing volatility of lithium resource prices and the growing uneven distribution of reserves, sodium-ion batteries, due to their abundant sodium resources, low cost, and environmental friendliness, are gradually becoming an important supplementary technology to lithium-ion batteries. Currently, the cathode materials in sodium-ion battery systems mainly include layered oxides, Prussian blue compounds, and polyanionic compounds. Among these, polyanionic cathode materials are considered an important development direction for scenarios with high safety requirements due to their stable three-dimensional framework structure, high operating voltage, and good thermal stability.

[0003] Among polyanionic cathode materials, Na4Fe3(PO4)2(P2O7) (NFPP) has attracted widespread attention in recent years due to its advantages such as low cost, good safety, and high theoretical capacity. NFPP is composed of PO4³⁻ and P2O7. 4 Together, they form the framework structure. Fe²⁺ / Fe³⁺ is the main redox pair, and its sodium insertion / extraction potential is between 3.2 and 3.4 V (vs Na / Na⁺). It has shown good cycle stability in conventional sodium-ion batteries.

[0004] However, existing research and industrial applications of NFPP are almost entirely based on traditional sodium-ion battery systems with hard carbon or other sodium-intercalated materials on the negative electrode side. In this system, the negative electrode typically has a certain sodium reserve and buffering capacity, and some irreversible sodium loss during the first charge-discharge process of the positive electrode can be compensated to some extent through negative electrode redundancy or the electrolyte system. Therefore, current NFPP technology focuses more on rate performance, cycle life, and high-temperature stability, while paying insufficient attention to issues such as initial efficiency, sodium loss mechanisms, and voltage plateau smoothness.

[0005] In recent years, to further improve the mass energy density and volumetric energy density of sodium-ion batteries, academia and industry have begun to explore anode-free sodium-ion battery systems. This system initially does not contain any negative electrode active material, retaining only a metal current collector. During the first charge, sodium ions released from the positive electrode deposit metallic sodium on the surface of the negative electrode. This technical approach can significantly reduce the amount of negative electrode material used and the complexity of the process, and theoretically has a higher upper limit for energy density.

[0006] However, the anode-less sodium-ion battery system places unprecedentedly stringent requirements on the cathode material. Since the source of sodium ions in the battery system is entirely dependent on the cathode, any irreversible sodium loss directly translates into battery capacity loss, and the initial coulombic efficiency of the cathode material almost entirely determines the battery's usable lifespan. Simultaneously, the non-uniformity of sodium desorption at the cathode leads to fluctuations in the sodium metal deposition rate on the anode side, easily inducing dendrite growth and the formation of "dead sodium," severely impacting battery safety and cycle stability.

[0007] When existing NFPP materials are applied to systems without anodes, the following problems are commonly encountered:

[0008] If the sodium source dissolution and reaction rate are mismatched during the preparation process, free sodium species such as Na2CO3 and NaOH are easily formed on the material surface or at the grain boundaries, and are irreversibly consumed in the first reaction.

[0009] For example, the different sodium insertion / extraction barriers in the Fe-OP framework are quite different, which leads to the sodium removal process proceeding in multiple stages, resulting in sawtooth or slope-like voltage curves.

[0010] Conventional carbon coating often uses carbon sources such as glucose and citric acid, which easily form porous carbon layers with high specific surface area. Although this is beneficial to rate performance, it will significantly aggravate side reactions in systems without a negative electrode and consume valuable sodium reserves. Summary of the Invention

[0011] The purpose of this invention is to provide a method for preparing polyanionic sodium iron phosphate cathode material for a cathodeless sodium-ion battery, which has the characteristics of good structural stability, high purity, and excellent electrochemical performance.

[0012] This invention can be achieved through the following technical solutions:

[0013] This invention relates to a method for preparing polyanionic sodium iron phosphate cathode material for negative electrode-free sodium-ion batteries, comprising the following steps:

[0014] S1. Preparation of precursor slurry: Weigh iron source, fast-release sodium source, slow-release sodium source, phosphorus source and carbon source according to stoichiometric ratio, mix them in solvent, and control the pH of the solution to form a homogeneous precursor slurry.

[0015] S2. Introduction of bimetallic doping: Add magnesium source and titanium source to the precursor slurry, control the total doping amount of Mg and Ti to the ratio of Fe sites, mix evenly, and obtain the doped precursor slurry.

[0016] S3. Pre-sintering: The doped precursor slurry is dried and then pre-sintered to form a stable precursor structure.

[0017] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain iron-based phosphate cathode material suitable for sodium-ion batteries without negative electrodes.

[0018] Furthermore, in step S1, the pH range of the precursor slurry is controlled to be 6.0-8.0, preferably 6.5-7.5. If pH < 6, Fe³⁺ dissolves too quickly and the skeleton is unstable. When pH > 8, Na⁺ is enriched in the form of NaOH. The neutral range can achieve simultaneous release of sodium and nucleation of Fe-OP, inhibiting the generation of free sodium.

[0019] Further, in step S2, the total doping amount of Mg and Ti is 0.3-2.5% of the Fe sites, preferably 0.5-1.5%, wherein the molar ratio of Mg:Ti is (3:1)-(1:3), preferably 1:1-2:1. Mg²⁺ plays a role in stabilizing the Fe-O octahedron and inhibiting structural collapse, while Ti... 4 ⁺ can flatten the Na site insertion / extraction energy barrier and smooth the voltage plateau. However, excessive doping can introduce too many Na vacancies, leading to a decrease in first-efficiency performance.

[0020] Further, in step S3, the pre-sintering conditions are: pre-sintering temperature 280-450℃, preferably 320-380℃; heating rate 0.5-3.0℃ / min; holding time 2-8h. Pre-sintering can fix the Fe-OP-Na primary framework, suppress Na volatilization during the high-temperature stage, control the degree of PO4 to P2O7 condensation, and provide a smooth surface for the dense carbon layer.

[0021] Further, in step S4, the high-temperature sintering conditions are as follows: high-temperature sintering temperature 600-720℃, preferably 650-700℃; heating rate 1-3℃ / min; holding time 6-14h. The protective atmosphere is nitrogen and / or argon. Below 600℃, crystallization is incomplete; above 720℃, Na easily volatilizes, and P2O7 is excessively formed. A slower heating rate can prevent phase separation, forming a single-phase NFPP.

[0022] Further, in step S1, the fast-release sodium source is one or more of NaH2PO4·2H2O, NaH2PO4, Na2HPO4, and CH3COONa; the slow-release sodium source is one or more of Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7. By introducing fast-release and slow-release sodium sources to synergistically supply sodium, sodium ions are embedded into the framework during the precursor nucleation stage, and the sodium inventory is continuously replenished during the crystallization stage. This significantly suppresses free sodium species and interfacial side reactions, achieving a balance between high reversible capacity and high first-cycle coulombic efficiency.

[0023] Furthermore, in this invention, the fast-release sodium source and the slow-release sodium source need to be used synergistically in a certain molar ratio. This ratio is related to the precursor nucleation behavior, the sodium ion participation in the phase formation pathway, and the reversible sodium inventory of the final cathode material. Specifically, the fast-release sodium source provides 30–70% of the total sodium molar amount. The fast-release sodium source dissolves rapidly in solution or slurry systems and has a strong Na⁺ release capacity, which can quickly increase the effective Na⁺ concentration in the system during the initial mixing stage, allowing sodium ions to participate in framework construction during the polyhedral nucleation stage and promoting the synchronous formation of Na-OP bonds. If the proportion of the fast-release sodium source is too low, the system exhibits a "sodium-poor environment" in the early nucleation stage, with sodium ions participating in the reaction more late, easily leading to Na enrichment on the grain surface or grain boundaries, which is not conducive to the formation of structurally embedded sodium. Slow-release sodium sources (such as NaH₂PO₄, Na₂HPO₄, Na₄P₂O₇, etc.) have the characteristics of limited dissolution and reaction rates and a mild release process. Their sodium release behavior is often synchronized with the condensation and rearrangement process of the phosphorus-oxygen skeleton. By introducing a certain proportion of slow-release sodium sources, sodium ions can be continuously and slowly replenished in subsequent reaction stages, avoiding the formation of transiently high Na⁺ concentrations in some areas due to excessive fast-release sodium sources, thereby effectively inhibiting the formation of free sodium or sodium-enriched impurities.

[0024] Further, in step S1, the molar ratio of Fe to P in the iron source and phosphorus source is 2.90:4.00-3.10:4.00, preferably 2.95:4.00-3.05:4.00. If the Fe ratio is too low, it easily leads to the formation of free phosphorus-rich phases such as Na3PO4 and Na4P2O7; if the Fe ratio is too high, it causes impurity phases such as Fe2O3 and Fe3O4. Any impurity phase in the anode-free system will cause irreversible sodium loss. This range balances phase purity and the integrity of the Fe-OP framework, maximizing initial efficiency.

[0025] Further, in step S1, the molar ratio of Na to P in the sodium and phosphorus sources is 0.98:1.00-1.05:1.00, preferably 1.00:1.00-1.03:1.00. When Na < 1.00, Na sites are vacant, resulting in irreversible structural changes. When Na > 1.05, free Na⁺ readily forms Na₂CO₃ and NaOH. This range ensures that sodium enters the framework intact and effectively, without free sodium.

[0026] Further, in step S1, the carbon source is one or more of phenolic resin, epoxy resin, polydopamine, asphalt precursor, petroleum coke sol, polystyrene, and polyvinyl alcohol. The amount of carbon source added is 0.5-3.0 wt%, preferably 0.8-2.0 wt%. When the amount of carbon source added is less than 0.5%, the carbon layer is discontinuous; when the amount of carbon source added is greater than 3.0%, the specific surface area increases, and the side reactions are enhanced. A suitable carbon layer can encapsulate the main body. The amount of carbon source added is calculated as a percentage of mass based on the theoretical mass of the metal phosphate main body in the final positive electrode active material. That is, it is calculated based on a theoretical mass of 100 wt% of the carbon-free positive electrode main body material (i.e., iron-based phosphate active material).

[0027] Furthermore, the iron source is one or more of FeC2O4·2H2O, FePO4·xH2O, Fe(NO3)3·9H2O, Fe(CH3COO)2, FeSO4·7H2O, Fe2(SO4)3, FeOOH, and Fe2O3.

[0028] Furthermore, the phosphorus source is one or more of the following: Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7.

[0029] Furthermore, the magnesium source is one or more of the following: Mg(CH3COO)2·4H2O, Mg(NO3)2·6H2O, MgSO4·7H2O, MgCl2·6H2O, Mg(OH)2, and MgO.

[0030] Furthermore, the titanium source is one or more of TiO2, Ti(SO4)2, TiCl4, tetrabutyl titanate, and isopropyl titanate.

[0031] Furthermore, in step S3, the drying method is one or more of the following: spray drying, freeze drying, vacuum drying, forced-air oven drying, belt drying, and rotary evaporation drying.

[0032] This invention effectively solves key technical problems of existing iron-based polyanion cathode materials, such as uneven sodium distribution, numerous structural defects, and poor compatibility with sodium-ion batteries, by introducing a synergistic sodium supply mechanism of fast-release and slow-release sodium sources during the preparation of iron-based phosphate cathode materials, and by combining magnesium and titanium bimetallic doping to regulate the crystal structure and electronic environment. The specific beneficial effects are as follows:

[0033] First, a synergistic sodium supply system is constructed to significantly improve the integrity and structural stability of sodium sites in the material. This invention introduces both a fast-release sodium source and a slow-release sodium source, allowing sodium to play a differentiated role at different stages of material formation. The fast-release sodium source rapidly releases Na⁺ during precursor slurry preparation and low-temperature reaction stages, effectively participating in the initial construction of the iron-phosphorus framework and preventing the formation of sodium-depleted phases or intermediate impurity phases due to local sodium deficiency. The slow-release sodium source gradually decomposes during high-temperature sintering, continuously replenishing sodium and compensating for sodium volatilization and migration losses. This dual-stage sodium supply mechanism enables uniform distribution of sodium in time and space, significantly reducing the probability of Na site vacancies in the crystal lattice and improving the integrity and thermal stability of the iron-based polyanionic crystal structure.

[0034] Secondly, it effectively suppresses the formation of impurity phases, improving phase formation completeness and batch consistency. Because the sodium source release rate is highly matched with the solid-phase reaction kinetics, this invention can significantly improve the stoichiometric balance during the reaction process, reducing the risk of impurity phase formation such as FePO4 and Na3PO4 caused by insufficient or excessive sodium supply fluctuations. Through the synergistic effect of the two sodium sources, the iron, sodium, and phosphorus elements in the reaction system achieve a stable ratio at the microscale, which is beneficial to the uniform nucleation and growth of the target iron-based phosphate phase, thereby improving the phase formation completeness and batch stability of the material and enhancing its process controllability in large-scale production.

[0035] Third, this invention improves the initial sodium inventory utilization efficiency in anode-free sodium-ion battery systems, thereby enhancing first-cycle performance. Addressing the high requirements of anode-free sodium-ion batteries for the "internal sodium supply capacity" of the cathode material, the cathode material prepared in this invention optimizes the sodium source supply path and sodium site occupancy rate, enabling more stable and controllable release of sodium ions during the first charge-discharge cycle. This effectively improves the first-cycle reversible capacity and coulombic efficiency, reduces capacity loss and sodium deposition risk due to insufficient sodium, and significantly enhances the material's adaptability and application value in anode-free systems.

[0036] Fourth, by combining Mg and Ti bimetallic doping, the crystal structure and electrochemical kinetic performance are synergistically optimized. This invention, based on the synergistic sodium supply from two sodium sources, introduces magnesium and titanium bimetals for synergistic doping of iron sites. Mg²⁺ and Ti 4 The introduction of ⁺ can maintain charge balance while modulating the local electronic structure and lattice stress distribution of Fe-OP bonds, further suppressing lattice distortion and defect aggregation. This structural modulation, combined with the uniform distribution of sodium, creates a synergistic effect, which helps to reduce the Na⁺ diffusion barrier, smooth the sodium desodium insertion / extraction potential plateau, and thus improve the rate performance and cycle stability of the material.

[0037] Fifth, it broadens the process window and enhances the engineering versatility and scale-up adaptability of the preparation process. Compared with traditional preparation methods that rely on a single sodium source, this invention effectively reduces the sensitivity of the process to precise sodium ratios and sintering conditions by using a combination of fast-release and slow-release sodium sources, making the system more tolerant to temperature fluctuations, atmosphere changes, and local component deviations. It requires no complex equipment or special reaction conditions, making it suitable for direct scale-up application on existing cathode material production lines, and has good industrialization feasibility.

[0038] 6. Provides general technical ideas for the design of polyanion cathode materials.

[0039] The "dual sodium source synergistic sodium supply" concept proposed in this invention is not only applicable to the iron-based phosphate system described in this patent, but can also be extended to the preparation of other sodium-ion battery polyanion cathode materials, such as phosphate, pyrophosphate, and their composite structures. This strategy provides a universally applicable engineering solution to common problems such as high-temperature sodium volatilization, sodium site defects, and insufficient sodium in anode-less systems, possessing significant methodological value and broad application prospects. Attached Figure Description

[0040] Figure 1 SEM image of application example 1. Detailed Implementation

[0041] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0042] This invention relates to a method for preparing polyanionic sodium iron phosphate cathode material for negative electrode-free sodium-ion batteries, comprising the following steps:

[0043] S1. Preparation of precursor slurry: Weigh iron source, fast-release sodium source, slow-release sodium source, phosphorus source and carbon source according to stoichiometric ratio, mix them in solvent, and control the pH of the solution to form a homogeneous precursor slurry.

[0044] S2. Introduction of bimetallic doping: Add magnesium source and titanium source to the precursor slurry, control the total doping amount of Mg and Ti to the ratio of Fe sites, mix evenly, and obtain the doped precursor slurry.

[0045] S3. Pre-sintering: The doped precursor slurry is dried and then pre-sintered to form a stable precursor structure.

[0046] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain iron-based phosphate cathode material suitable for sodium-ion batteries without negative electrodes.

[0047] Furthermore, in step S1, the pH range of the precursor slurry is controlled to be 6.0-8.0, preferably 6.5-7.5.

[0048] Furthermore, in step S2, the total doping amount of Mg and Ti is 0.3-2.5% of the Fe sites, preferably 0.5-1.5%.

[0049] Further, in step S3, the pre-sintering conditions are: pre-sintering temperature 280-450℃, preferably 320-380℃; heating rate 0.5-3.0℃ / min; holding time 2-8h.

[0050] Further, in step S4, the conditions for high-temperature sintering are: high-temperature sintering temperature 600-720℃, preferably 650-700℃; heating rate 1-3℃ / min; holding time 6-14h. The protective atmosphere is nitrogen and / or argon.

[0051] Further, in step S1, the fast-release sodium source is one or more of NaH2PO4·2H2O, NaH2PO4, Na2HPO4, and CH3COONa; the slow-release sodium source is one or more of Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7, and the amount of sodium provided by the fast-release sodium source accounts for 30–70% of the total amount of sodium molar.

[0052] Further, in step S1, the molar ratio of Fe to P in the iron source and phosphorus source is 2.90:4.00-3.10:4.00, preferably 2.95:4.00-3.05:4.00.

[0053] Further, in step S1, the molar ratio of Na to P in the sodium source and phosphorus source is 0.98:1.00-1.05:1.00, preferably 1.00:1.00-1.03:1.00.

[0054] Further, in step S1, the carbon source is one or more of phenolic resin, epoxy resin, polydopamine, asphalt precursor, petroleum coke sol, polystyrene, and polyvinyl alcohol, and the amount of carbon source added is 0.5-3.0 wt%, preferably 0.8-2.0 wt%.

[0055] Furthermore, the iron source is one or more of FeC2O4·2H2O, FePO4·xH2O, Fe(NO3)3·9H2O, Fe(CH3COO)2, FeSO4·7H2O, Fe2(SO4)3, FeOOH, and Fe2O3.

[0056] Furthermore, the phosphorus source is one or more of the following: Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7.

[0057] Furthermore, the magnesium source is one or more of the following: Mg(CH3COO)2·4H2O, Mg(NO3)2·6H2O, MgSO4·7H2O, MgCl2·6H2O, Mg(OH)2, and MgO.

[0058] Furthermore, the titanium source is one or more of TiO2, Ti(SO4)2, TiCl4, tetrabutyl titanate, and isopropyl titanate.

[0059] Furthermore, in step S3, the drying method is one or more of the following: spray drying, freeze drying, vacuum drying, forced-air oven drying, belt drying, and rotary evaporation drying.

[0060] Example 1

[0061] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery. Its preparation method includes the following steps:

[0062] S1. Preparation of precursor slurry: Iron source, fast-release sodium source, slow-release sodium source, phosphorus source, and carbon source were weighed according to stoichiometric ratio and mixed in a solvent. The pH of the solution was controlled at 8.0 to form a homogeneous precursor slurry. Specifically, the molar ratio of Fe to P in the iron and phosphorus sources was 3.00:4.00; the molar ratio of Na to P in the sodium and phosphorus sources was 0.98:1.00; the amount of carbon source added was 3.0 wt%, and the amount of sodium provided by the fast-release sodium source accounted for 70% of the total sodium molar amount.

[0063] S2. Introduction of Bimetallic Doping: Magnesium and titanium sources are added to the precursor slurry, and the total doping amount of Mg and Ti is controlled to be the ratio of Fe sites. The mixture is then homogeneous to obtain the doped precursor slurry. Specifically, the total doping amount of Mg and Ti is 2.5% of the Fe sites.

[0064] S3. Pre-sintering: The doped precursor slurry is spray-dried and then pre-sintered to form a stable precursor structure. Specifically, the pre-sintering conditions are: pre-sintering temperature 450℃; heating rate 2℃ / min; holding time 2h.

[0065] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain an iron-based phosphate cathode material suitable for sodium-ion batteries without a negative electrode. Specifically, the high-temperature sintering conditions are: high-temperature sintering temperature 720℃; heating rate 2℃ / min; holding time 6h. The protective atmosphere is nitrogen.

[0066] In this embodiment, the fast-release sodium source is NaH2PO4·2H2O or CH3COONa; the slow-release sodium source is Na4P2O7, Na3PO4, Na2CO3, NaHCO3, or NH4H2PO4; the carbon source is phenolic resin or epoxy resin; the iron source is FeC2O4·2H2O or FePO4·xH2O; the phosphorus source is Na4P2O7, Na3PO4, or Na2CO3; the magnesium source is Mg(CH3COO)2·4H2O, MgCl2·6H2O, Mg(OH)2, or MgO; and the titanium source is TiO2, Ti(SO4)2, or isopropyl titanate.

[0067] Example 2

[0068] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery. Its preparation method includes the following steps:

[0069] S1. Preparation of precursor slurry: Iron source, fast-release sodium source, slow-release sodium source, phosphorus source, and carbon source are weighed according to stoichiometric ratio and mixed in a solvent. The pH of the solution is controlled at 7.0 to form a homogeneous precursor slurry. Specifically, the molar ratio of Fe to P in the iron and phosphorus sources is 2.90:4.00; the molar ratio of Na to P in the sodium and phosphorus sources is 1.05:1.00; the amount of carbon source added is 2.0 wt% and 0.8-2.0 wt%, with the fast-release sodium source providing 50% of the total sodium molar amount.

[0070] S2. Introduction of Bimetallic Doping: Magnesium and titanium sources are added to the precursor slurry, and the total doping amount of Mg and Ti is controlled to be the ratio of Fe sites. The mixture is then homogeneous to obtain the doped precursor slurry. Specifically, the total doping amount of Mg and Ti is 1.5% of the Fe sites.

[0071] S3. Pre-sintering: The doped precursor slurry is vacuum dried and then pre-sintered to form a stable precursor structure. Specifically, the pre-sintering conditions are: pre-sintering temperature 350℃; heating rate 0.5℃ / min; holding time 8h.

[0072] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain an iron-based phosphate cathode material suitable for sodium-ion batteries without a negative electrode. Specifically, the high-temperature sintering conditions are: high-temperature sintering temperature 680℃; heating rate 1℃ / min; holding time 14h. The protective atmosphere is argon.

[0073] In this embodiment, the fast-release sodium source is Na2HPO4 or CH3COONa; the slow-release sodium source is NaH2PO4, Na2HPO4, or Na4P2O7; the carbon source is phenolic polystyrene or polyvinyl alcohol; the iron source is Fe(CH3COO)2, FeSO4·7H2O, Fe2(SO4)3, FeOOH, or Fe2O3; the phosphorus source is NaH2PO4, Na2HPO4, or Na4P2O7; the magnesium source is MgSO4·7H2O, MgCl2·6H2O, Mg(OH)2, or MgO; and the titanium source is TiCl4, tetrabutyl titanate, or isopropyl titanate.

[0074] Example 3

[0075] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery. Its preparation method includes the following steps:

[0076] S1. Preparation of precursor slurry: Iron source, fast-release sodium source, slow-release sodium source, phosphorus source, and carbon source were weighed according to stoichiometric ratio and mixed in a solvent. The pH of the solution was controlled at 6.0 to form a homogeneous precursor slurry. Specifically, the molar ratio of Fe to P in the iron and phosphorus sources was 3.10:4.00:4.00; the molar ratio of Na to P in the sodium and phosphorus sources was 0.98:1.01:1.00, 1.00; the amount of carbon source added was 0.5 wt%, and the amount of sodium provided by the fast-release sodium source accounted for 30% of the total sodium molar amount.

[0077] S2. Introduction of Bimetallic Doping: Magnesium and titanium sources are added to the precursor slurry, and the total doping amount of Mg and Ti is controlled to be the ratio of Fe sites. The mixture is then homogeneous to obtain the doped precursor slurry. Specifically, the total doping amount of Mg and Ti is 0.3% of the Fe sites.

[0078] S3. Pre-sintering: The doped precursor slurry is spray-dried, freeze-dried, vacuum-dried, oven-dried, belt-dried, and rotary-evaporated before pre-sintering to form a stable precursor structure. Specifically, the pre-sintering conditions are: pre-sintering temperature 280℃; heating rate 3.0℃ / min; holding time 5h.

[0079] S4. High-Temperature Sintering: High-temperature sintering is performed under a protective atmosphere to obtain an iron-based phosphate cathode material suitable for sodium-ion batteries without a negative electrode. Specifically, the high-temperature sintering conditions are: sintering temperature 600℃; heating rate 3℃ / min; holding time 10h. The protective atmosphere is nitrogen and argon.

[0080] In this embodiment, the fast-release sodium source is NaH2PO4·2H2O, NaH2PO4, Na2HPO4, CH3COONa; the slow-release sodium source is Na4P2O7, Na3PO4, Na2CO3, Na4P2O7; the carbon source is phenolic resin, epoxy resin, polydopamine, polyvinyl alcohol; the iron source is Fe(CH3COO)2, FeSO4·7H2O, Fe2(SO4)3, FeOOH, Fe2O3; the phosphorus source is Na4P2O7, Na3PO4, Na2CO3, Na2HPO4, Na4P2O7; the magnesium source is MgSO4·7H2O, Mg(OH)2, MgO; and the titanium source is TiCl4, tetrabutyl titanate, and isopropyl titanate.

[0081] Example 4

[0082] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery. Its preparation method includes the following steps:

[0083] S1. Preparation of precursor slurry: Iron source, fast-release sodium source, slow-release sodium source, phosphorus source, and carbon source were weighed according to stoichiometric ratio and mixed in a solvent. The pH of the solution was controlled at 6.5 to form a homogeneous precursor slurry. Specifically, the molar ratio of Fe to P in the iron and phosphorus sources was 3.05:4.00; the molar ratio of Na to P in the sodium and phosphorus sources was 1.00:1.00; the amount of carbon source added was 1.5 wt%, and the amount of sodium provided by the fast-release sodium source accounted for 40% of the total sodium molar amount.

[0084] S2. Introduction of Bimetallic Doping: Magnesium and titanium sources are added to the precursor slurry, and the total doping amount of Mg and Ti is controlled to be the ratio of Fe sites. The mixture is then homogeneous to obtain the doped precursor slurry. Specifically, the total doping amount of Mg and Ti is 0.5% of the Fe sites.

[0085] S3. Pre-sintering: The doped precursor slurry is dried in a forced-air oven and then pre-sintered to form a stable precursor structure. Specifically, the pre-sintering conditions are: pre-sintering temperature 320℃; heating rate 1 / 5℃ / min; holding time 4h.

[0086] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain an iron-based phosphate cathode material suitable for sodium-ion batteries without a negative electrode. Specifically, the high-temperature sintering conditions are: high-temperature sintering temperature 650℃; heating rate 2℃ / min; holding time 9h. The protective atmosphere is nitrogen and argon.

[0087] In this embodiment, the fast-release sodium source is NaH2PO4·2H2O or NaH2PO4; the slow-release sodium source is Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, or (NH4)2HPO4; the carbon source is phenolic resin, epoxy resin, or polydopamine; the iron source is FeC2O4·2H2O, FePO4·xH2O, or Fe(NO3)3·9H2O; the phosphorus source is Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, or (NH4)2HPO4; the magnesium source is Mg(CH3COO)2·4H2O, Mg(NO3)2·6H2O, or MgO; and the titanium source is TiO2, Ti(SO4)2, TiCl4, tetrabutyl titanate, or isopropyl titanate.

[0088] Example 5

[0089] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery. Its preparation method includes the following steps:

[0090] S1. Preparation of precursor slurry: Iron source, fast-release sodium source, slow-release sodium source, phosphorus source, and carbon source were weighed according to stoichiometric ratio and mixed in a solvent. The pH of the solution was controlled at 7.5 to form a homogeneous precursor slurry. Specifically, the molar ratio of Fe to P in the iron and phosphorus sources was 2.95:4.00; the molar ratio of Na to P in the sodium and phosphorus sources was 1.03:1.00; the amount of carbon source added was 0.8 wt%, and the amount of sodium provided by the fast-release sodium source accounted for 60% of the total sodium molar amount.

[0091] S2. Introduction of Bimetallic Doping: Magnesium and titanium sources are added to the precursor slurry, and the total doping amount of Mg and Ti is controlled to be the ratio of Fe sites. The mixture is then homogeneous to obtain the doped precursor slurry. Specifically, the total doping amount of Mg and Ti is 1.5% of the Fe sites.

[0092] S3. Pre-sintering: The doped precursor slurry is rotary evaporated and dried before pre-sintering to form a stable precursor structure. Specifically, the pre-sintering conditions are: pre-sintering temperature 380℃; heating rate 0.5-3.0℃ / min; holding time 2-8h.

[0093] S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain an iron-based phosphate cathode material suitable for sodium-ion batteries without a negative electrode. Specifically, the high-temperature sintering conditions are: high-temperature sintering temperature 700℃; heating rate 1℃ / min; holding time 12h. The protective atmosphere is nitrogen and argon.

[0094] In this embodiment, the fast-release sodium source is NaH2PO4·2H2O or NaH2PO4; the slow-release sodium source is Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, or (NH4)2HPO4; the carbon source is phenolic resin, epoxy resin, polydopamine, polystyrene, or polyvinyl alcohol; the iron source is FeC2O4·2H2O or FePO4·xH2O; the phosphorus source is Na4P2O7, Na3PO4, Na2CO3, NaHCO3, or NH4H2PO4; the magnesium source is Mg(CH3COO)2·4H2O or Mg(NO3)2·6H2O; and the titanium source is TiO2, Ti(SO4)2, TiCl4, tetrabutyl titanate, or isopropyl titanate.

[0095] Application Example 1 (Dual Sodium Source + Mg / Ti Co-doping + Neutral pH, Baseline Example)

[0096] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery, and its preparation method includes the following steps:

[0097] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio was controlled at 1.02:1.00. Deionized water was added as a solvent, and 1.2wt% phenolic resin was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry. The amount of sodium provided by the fast-release sodium source accounted for 50% of the total sodium molar amount.

[0098] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0099] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 350°C at 1°C / min under a nitrogen atmosphere, and held for 4 hours to complete the pre-sintering.

[0100] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain a polyanionic sodium iron phosphate cathode material suitable for sodium-ion batteries without a negative electrode.

[0101] Application Example 2 (Changing the type of slow-release sodium source)

[0102] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery, and its preparation method includes the following steps:

[0103] S1. Preparation of precursor slurry: Weigh FeC2O4·2H2O, NaH2PO4·2H2O (rapid-release sodium source), and Na4P2O. 73 Using slow-release sodium source and (NH4)2HPO4 as raw materials, the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio at 1.02:1.00. Deionized water was added as a solvent, and 1.2wt% phenolic resin was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry.

[0104] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry. The sodium molar amount provided by the fast-release sodium source accounts for 40% of the total sodium molar amount.

[0105] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 350°C at 1°C / min under a nitrogen atmosphere, and held for 4 hours to complete the pre-sintering.

[0106] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain a polyanionic sodium iron phosphate cathode material suitable for sodium-ion batteries without a negative electrode.

[0107] Application Example 3 (Changing the drying method and doping ratio, carbon source)

[0108] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery, and its preparation method includes the following steps:

[0109] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio was controlled at 1.02:1.00. Deionized water was added as a solvent, and 1.0wt% polydopamine was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry. The sodium molar amount provided by the fast-release sodium source accounted for 40% of the total sodium molar amount.

[0110] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.2% of the Fe sites, wherein the Mg:Ti molar ratio is 1:2. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0111] S3. Drying and pre-sintering: The precursor powder was obtained by freeze drying and then heated to 350°C at a rate of 1°C / min under a nitrogen atmosphere and held for 4 hours to complete the pre-sintering.

[0112] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain a polyanionic sodium iron phosphate cathode material suitable for sodium-ion batteries without a negative electrode.

[0113] Application Example 4 (Low carbon content, modified sintering temperature)

[0114] This embodiment relates to a polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery, and its preparation method includes the following steps:

[0115] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio was controlled at 1.02:1.00. Deionized water was added as a solvent, and 0.8wt% asphalt was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a homogeneous precursor slurry. The sodium molar amount provided by the fast-release sodium source accounted for 40% of the total sodium molar amount.

[0116] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0117] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 330℃ at 1℃ / min under a nitrogen atmosphere, and held for 6 hours to complete the pre-sintering.

[0118] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain a polyanionic sodium iron phosphate cathode material suitable for sodium-ion batteries without a negative electrode.

[0119] Comparative Example 1 (using only fast-release sodium source, without slow-release sodium source)

[0120] This embodiment relates to a polyanionic sodium iron phosphate cathode material for sodium-ion batteries, and its preparation method includes the following steps:

[0121] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio was controlled at 1.02:1.00. Deionized water was added as a solvent, and 1.2wt% phenolic resin was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry.

[0122] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0123] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 350°C at 1°C / min under a nitrogen atmosphere, and held for 4 hours to complete the pre-sintering.

[0124] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain a polyanionic sodium iron phosphate cathode material suitable for sodium-ion batteries without a negative electrode.

[0125] Comparative Example 2 (using only slow-release sodium source, without fast-release sodium source)

[0126] This embodiment relates to a polyanionic sodium iron phosphate cathode material for sodium-ion batteries, and its preparation method includes the following steps:

[0127] S1. Preparation of precursor slurry: FeC2O4·2H2O, Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio was controlled at 1.02:1.00. Deionized water was added as solvent, and 1.2wt% phenolic resin was added as carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry.

[0128] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0129] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 350°C at 1°C / min under a nitrogen atmosphere, and held for 4 hours to complete the pre-sintering.

[0130] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain the polyanionic sodium iron phosphate cathode material for sodium-ion batteries.

[0131] Comparative Example 3 (no Mg / Ti dual doping, only single Mg doping)

[0132] This embodiment relates to a polyanionic sodium iron phosphate cathode material for sodium-ion batteries, and its preparation method includes the following steps:

[0133] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, and the Fe:P molar ratio was controlled at 3.00:4.00 and the Na:P molar ratio at 1.02:1.00. Deionized water was added as a solvent, and 1.2wt% phenolic resin was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a uniform precursor slurry. The amount of sodium provided by the fast-release sodium source accounted for 50% of the total sodium molar amount.

[0134] S2. Introduction of monometallic doping: Add Mg(CH3COO)2·4H2O to the above slurry, with a total doping amount of 1.0% of the Fe sites. Continue stirring for 2 hours to obtain a monometallic doped precursor slurry.

[0135] S3. Pre-sintering: The precursor powder is obtained by spray drying, and then heated to 350℃ at 1℃ / min under a nitrogen atmosphere and held for 4 hours to complete the pre-sintering.

[0136] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain the polyanionic sodium iron phosphate cathode material for sodium-ion batteries.

[0137] Comparative Example 4 (Excess Carbon Source)

[0138] This embodiment relates to a polyanionic sodium iron phosphate cathode material for sodium-ion batteries, and its preparation method includes the following steps:

[0139] S1. Preparation of precursor slurry: FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 were weighed as raw materials, controlling the Fe:P molar ratio to be 3.00:4.00 and the Na:P molar ratio to be 1.02:1.00; deionized water was added as a solvent, and 4.0wt% phenolic resin was added as a carbon source. The pH of the slurry was adjusted to 7.0, and the mixture was stirred at 60℃ for 6 hours to obtain a homogeneous precursor slurry. The sodium molar amount provided by the fast-release sodium source accounted for 40% of the total sodium molar amount.

[0140] S2. Introduction of bimetallic doping: Mg(CH3COO)2·4H2O and isopropyl titanate are added to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1. Stirring is continued for 2 hours to obtain bimetallic doped precursor slurry.

[0141] S3. Pre-sintering: The precursor powder is obtained by spray drying and heated to 350°C at 1°C / min under a nitrogen atmosphere, and held for 4 hours to complete the pre-sintering.

[0142] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain the polyanionic sodium iron phosphate cathode material for sodium-ion batteries.

[0143] Comparative Example 5 (without pre-sintering)

[0144] This embodiment relates to a polyanionic sodium iron phosphate cathode material for sodium-ion batteries, and its preparation method includes the following steps.

[0145] S1. Precursor preparation: Weigh FeC2O4·2H2O, NaH2PO4·2H2O (fast-release sodium source), Na2CO3 (slow-release sodium source), and (NH4)2HPO4 as raw materials, and control the Fe:P molar ratio to be 3.00:4.00 and the Na:P molar ratio to be 1.02:1.00; add deionized water as solvent, add 1.2wt% phenolic resin as carbon source, adjust the pH of the slurry to 7.0, and stir at 60℃ for 6h to obtain a uniform precursor slurry. The sodium molar amount provided by the fast-release sodium source accounts for 40% of the total sodium molar amount.

[0146] S2. Introduction of bimetallic doping: Add Mg(CH3COO)2·4H2O and isopropyl titanate to the above slurry so that the total doping amount of Mg and Ti is 1.0% of the Fe sites, wherein the Mg:Ti molar ratio is 2:1, and continue stirring for 2 hours.

[0147] S3. Pre-sintering: The precursor powder is obtained by spray drying, without a pre-sintering process.

[0148] S4. High-temperature sintering: The temperature is increased to 680℃ at 2℃ / min in a high-purity nitrogen atmosphere, held for 10h, and then cooled to obtain the polyanionic sodium iron phosphate cathode material for sodium-ion batteries.

[0149] To effectively evaluate the technical effects of the present invention, the performance of the above embodiments was evaluated, and the evaluation results are shown in Table 1:

[0150] Table 1 Performance Test Results

[0151]

[0152] In Table 1, the charge / discharge rate is 0.2C (1C = 129 mA / g).

[0153] In Table 1, from the perspective of capacity and first-cycle efficiency, Examples 1-4 using a fast-release / slow-release composite dual sodium source all exhibited significantly better overall performance than the comparative examples. Application Example 1, while maintaining a charging capacity of approximately 116 mAh·g⁻¹, achieved a reversible discharge capacity of over 102 mAh·g⁻¹ and a first-cycle coulombic efficiency of approximately 88%, indicating that sodium inventory loss was significantly controlled during the first charge-discharge cycle. Figure 1 The SEM image of Application Example 1 shows that carbon is uniformly coated on the surface. In contrast, Comparative Example 1, which uses only a fast-release sodium source, has a higher charging capacity, but its discharge capacity and first-efficiency are significantly reduced, reflecting the large-scale consumption of free sodium species in interfacial side reactions. Comparative Example 2, which uses only a slow-release sodium source, suffers from limited reversible capacity due to insufficient sodium participation in the phase formation stage. This comparison clearly demonstrates that the dual-sodium source strategy establishes an effective synergy between "phase formation sodium supply" and "stock replenishment," which is key to achieving a balance between high capacity and high first-efficiency.

[0154] From the perspectives of structural stability and electrochemical kinetics, Mg / Ti heterovalent bimetallic doping clearly contributes to the performance improvement. The materials in the examples generally exhibit a higher and more concentrated average discharge voltage plateau (approximately 3.32-3.35 V), while Comparative Example 3, with Ti doping removed, shows a decrease in both discharge capacity and voltage stability. This indicates that Ti doping helps to modulate the electronic structure of the Fe-OP bond, suppressing polarization and stabilizing the sodium deintercalation process; while Mg doping effectively alleviates lattice strain during the multi-sodium insertion / extraction process by enhancing structural rigidity. Together, they improve the material's reversibility and voltage stability.

[0155] The materials in the application examples simultaneously achieved high compaction density (approximately 2.12-2.18 g·cm⁻³) and low specific surface area (approximately 3.9-4.6 m²·g⁻¹). In particular, Application Examples 1 and 4, while maintaining capacity and first-cycle efficiency, exhibited significantly lower specific surface areas than the comparative sample. In contrast, Comparative Example 4, with its uncontrolled carbon layer structure or improper carbon addition method, had a specific surface area exceeding 9 m²·g⁻¹. Although it showed a higher apparent charge capacity, its first-cycle efficiency and discharge capacity were significantly degraded, indicating that excessively high interfacial activity exacerbates electrolyte decomposition and sodium consumption, which is detrimental to the stable operation of the anode-free system.

[0156] The results of Comparative Example 5 show that the absence of the pre-sintering step leads to insufficient phase formation and uneven distribution of sodium and phosphorus, resulting in a systematic decline in capacity, first-time performance and compaction performance, further verifying the importance of precursor structure pre-stabilization for final performance.

[0157] In summary, the above data comparison clearly demonstrates that the performance improvement of this invention is not due to a single factor, but rather the result of the combined effects of a dual sodium source supply mechanism, bimetallic doping regulation, and synergistic optimization of the dense interface and phase formation process. This technical approach effectively solves the core contradiction in anode-free sodium-ion batteries where the cathode material is difficult to balance, providing a technical solution with clear mechanistic support and repeatability for the engineering application of NFPP cathode materials in anode-free systems.

[0158] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method for preparing polyanionic sodium iron phosphate cathode material for a cathodeless sodium-ion battery, characterized in that... Includes the following steps: S1. Preparation of precursor slurry: Weigh iron source, fast-release sodium source, slow-release sodium source, phosphorus source and carbon source according to stoichiometric ratio, mix them in solvent, and control the pH of the solution to form a homogeneous precursor slurry. S2. Introduction of bimetallic doping: Add magnesium source and titanium source to the precursor slurry, control the total doping amount of Mg and Ti to the ratio of Fe sites, mix evenly, and obtain the doped precursor slurry. S3. Pre-sintering: The doped precursor slurry is dried and then pre-sintered to form a stable precursor structure. S4. High-temperature sintering: High-temperature sintering is carried out under a protective atmosphere to obtain iron-based phosphate cathode material suitable for sodium-ion batteries without negative electrodes.

2. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S1, the pH range of the precursor slurry is controlled to be 6.0-8.

0.

3. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S2, the total doping amount of Mg and Ti is 0.3-2.5% of the Fe sites.

4. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S3, the pre-sintering conditions are: pre-sintering temperature 280-450℃, heating rate 0.5-3.0℃ / min; holding time 2-8h.

5. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S4, the conditions for high-temperature sintering are: high-temperature sintering temperature 600-720℃, heating rate 1-3℃ / min, and holding time 6-14h. The protective atmosphere is nitrogen and / or argon.

6. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S1, the fast-release sodium source is one or more of NaH2PO4·2H2O, NaH2PO4, Na2HPO4, and CH3COONa; the slow-release sodium source is one or more of Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7, and the amount of sodium provided by the fast-release sodium source accounts for 30–70% of the total amount of sodium.

7. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S1, the molar ratio of Fe to P in the iron source and phosphorus source is 2.90:4.00-3.10:4.

00.

8. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S1, the molar ratio of Na to P in the sodium source and phosphorus source is 0.98:1.00-1.05:1.

00.

9. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 1, characterized in that: In step S1, the carbon source is one or more of phenolic resin, epoxy resin, polydopamine, asphalt precursor, petroleum coke sol, polystyrene, and polyvinyl alcohol, and the amount of carbon source added is 0.5-3.0 wt%.

10. The method for preparing polyanionic sodium iron phosphate cathode material for a negative electrode-free sodium-ion battery according to claim 9, characterized in that: The iron source is one or more of the following: FeC2O4·2H2O, FePO4·xH2O, Fe(NO3)3·9H2O, Fe(CH3COO)2, FeSO4·7H2O, Fe2(SO4)3, FeOOH, and Fe2O3. The phosphorus source is one or more of the following: Na4P2O7, Na3PO4, Na2CO3, NaHCO3, NH4H2PO4, (NH4)2HPO4, H3PO4, NaH2PO4, Na2HPO4, and Na4P2O7. The magnesium source is one or more of the following: Mg(CH3COO)2·4H2O, Mg(NO3)2·6H2O, MgSO4·7H2O, MgCl2·6H2O, Mg(OH)2, and MgO. The titanium source is one or more of TiO2, Ti(SO4)2, TiCl4, tetrabutyl titanate, and isopropyl titanate.

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