A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material and its application in high-temperature sodium-ion batteries.

Abstract

This invention discloses a method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material and its application in high-temperature sodium-ion batteries, belonging to the technical field of sodium-ion battery cathode materials. The general formula of the sodium ferrous sulfate / carbon composite cathode material is Na. 2.4‑ 2a Fe 1.8‑x+a M 2+ y N 3+ z (SO4)3 / C, where 2x = 2y + 3z, 0 ≤ a ≤ 0.2, 0 < x ≤ 0.2, 0.01 ≤ y ≤ 0.1, 0.01 ≤ z ≤ 0.1; M is selected from at least four of Mg, Ca, Mn, Co, Ni, Cu, Zn, and Ba, and N is selected from Al or Cr. This invention utilizes a high configurational entropy to suppress the dissolution of transition metals, structural collapse, and abnormal grain growth under high-temperature conditions, enabling the material to maintain structural integrity and performance stability during high-temperature cycling, thus possessing broad application prospects.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, and specifically relates to a sodium ferrous sulfate cathode material that can be charged and discharged with sodium ions and its preparation method. Background Technology

[0002] With the development of new energy power generation and electric transportation, the demand for electrochemical energy storage in scenarios such as grid peak shaving and backup power continues to grow. While lithium-ion batteries are technologically mature, they are constrained by lithium resources and cost, making sodium-ion batteries, which are abundant in resources and have lower costs, an important candidate for large-scale energy storage; however, Na… + The large radius and limited intercalation / deintercalation kinetics make the development of high-energy-density and high-rate cathode materials still crucial. Currently, sodium-ion battery cathode systems include layered oxides, polyanionic compounds, Prussian blue, and organic materials. Among these, polyanionic iron-based sulfate materials offer advantages such as good structural and thermal stability, high operating voltage, and low cost. However, they generally suffer from low electronic / ionic conductivity, leading to insufficient rate performance, increased polarization, and rapid capacity decay. Especially under high-temperature or thermal management boundary conditions, problems such as transition metal dissolution, intensified electrolyte side reactions, and particle / grain coarsening become more prominent, potentially inducing a continuous increase in interfacial impedance and irreversible evolution or even collapse of the crystal structure. This accelerates material structural degradation and electrochemical performance decline, limiting their reliable application in high-temperature or wide-temperature-range energy storage.

[0003] Sodium ferrous sulfate material (general formula Na) 2+2x Fe 2-x (SO4)3 has a theoretical specific capacity of approximately 120 mAh / g under ideal stoichiometry (x=0, i.e., Na2Fe2(SO4)3). However, its practical application is limited by the difficulty in reversibly inserting and deinserting some sodium sites in the crystal, resulting in a significant reduction in actual capacity. To improve sodium ion activity, existing studies often employ a "sodium-rich, low-iron" composition design (e.g., x>0.2), but this sacrifices the theoretical capacity limit of the material. To improve the conductivity and ion diffusion kinetics of cathode materials, existing technologies often use carbon coating / carbon composites to construct conductive pathways or perform single-element doping to adjust the crystal structure and electronic structure, but limitations remain: the coating layer is prone to discontinuity, thickening, or local detachment under cycling and high-temperature conditions due to volume strain and interfacial side reactions, leading to a decline in conductivity improvement and potentially introducing additional interfacial impedance; the controllability of single-atom doping is limited, often making it difficult to simultaneously achieve phase stability and Na + Diffusion and electron transport are insufficient to suppress failure mechanisms such as phase transition, lattice strain accumulation, and transition metal migration / dissolution under high-temperature conditions.

[0004] High-entropy materials have attracted attention in the energy storage field due to their unique "cocktail effect," structural stability brought by high configurational entropy, and performance tunability. For example, invention patent CN118888712A discloses a high-entropy sodium ferrous sulfate cathode material. By introducing at least four different valence states of doping elements (including divalent, trivalent, tetravalent, and pentavalent metal ions) into sodium ferrous sulfate and coordinating with carbon coating, the aim is to improve the low-temperature performance and cycle stability of the material using the high-entropy effect. This technical solution reveals the potential of multi-element high-entropy doping in improving the overall performance of materials. However, its basis is still a sodium-rich, low-iron composition (Na... 2 .5 Fe 1 .75-x A y B z C m D n (SO4)3), limited by its low theoretical capacity, faces a bottleneck in overall performance improvement; this patent sets the doping amount to 0 < x ≤ 0.5, and a higher doping ratio has obvious drawbacks. Its doping elements include Al. 3+ Zr 4+ These are ions with no redox activity. These ions do not participate in electron transfer during battery charging and discharging, but they occupy active sites in the sodium ferrous sulfate lattice, replacing some of the electrochemically active Fe. 2+ As the doping concentration increases, the proportion of active sites occupied rises, leading to a decrease in the effective components of the material participating in redox reactions and a subsequent drop in theoretical capacity. Dopant ions of different valence states (+2, +3, +4, +5) form cation and anion vacancies. Among them, high-valence (+4, +5) cation doping expands the lattice spacing and enriches sodium ion transport channels, reducing the ion migration barrier and thus effectively improving the problem of slow sodium ion diffusion at low temperatures, significantly enhancing the low-temperature performance of the battery. However, high-valence (+4, +5) cation doping can cause lattice disorder and high distortion, which can damage the thermodynamic stability of the lattice. Under high-temperature conditions, lattice defects are prone to exacerbating the structural stress during ion insertion and extraction, leading to lattice reconstruction and loss of active sites. This high degree of distortion may ultimately accelerate the degradation of the battery structure, making it difficult to adapt to high-temperature application scenarios.

[0005] The invention patent with publication number CN116995227A discloses a composite sodium ferrous sulfate cathode material, whose general formula is Na. x Fe y (A α B βCarbon-based dispersion of (SO4)3 is carried out using conventional stirring methods with low speed (200-500 rpm) and long duration (1-3 hours). The stirring energy input is low and the shear force is weak. After spray drying, the particle size of the material powder is 5-10 μm, which requires further pulverization to 1-5 μm. The preparation process is cumbersome and the obtained material does not have a specific loose porous spherical structure. The dispersion efficiency of carbon materials and the control effect on the microstructure of materials are limited. Divalent metal cations and tetravalent and above high-valence metal cations are selected for doping. The doping system is only optimized for discharge specific capacity, rate performance and conventional cycle performance at room temperature, without considering the structural stability of materials under high temperature conditions. This results in technical defects such as insufficient high-temperature cycle performance.

[0006] Therefore, there is an urgent need to provide a high-temperature stable cathode material and its preparation technology suitable for sodium-ion batteries. This material should be able to maintain the advantages of high operating voltage and low cost, while improving the phase stability and cycle reliability of the material under high temperature environment through more effective structural stabilization strategies and microstructure control methods. It should also take into account the ion / electron transport capability, thereby meeting the comprehensive requirements of energy density, rate performance and lifespan for high-temperature or wide-temperature-range energy storage applications. Summary of the Invention

[0007] This invention addresses the limitations of existing methods for modifying sodium ferrous sulfate cathode materials and their high-temperature long-term stability by proposing a method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material and its application in high-temperature sodium-ion batteries. This invention introduces a high-entropy control strategy during the construction of the cathode active phase to achieve uniform solid solution of multiple components at lattice sites and improve the high-temperature structural stability of the material. Specifically, at least five metal cations are introduced in the precursor solution stage, forming a stable multi-metal chelate structure through a complexing agent. This ensures uniform dispersion of different metal ions at the molecular scale, while the introduction of multiple transition metal ions reduces the band gap and improves the intrinsic conductivity of the material. Subsequently, spray drying is used to rapidly immobilize the multi-metal chelate clusters inside the particles, forming a high-entropy precursor with uniform composition. After subsequent high-temperature calcination, multiple components achieve solid solution at the target lattice sites, forming a high-entropy solid solution / quasi-solid solution structure. The high configurational entropy and lattice distortion synergistically produce a diffusion retardation effect, effectively suppressing abnormal grain growth and phase separation during high-temperature calcination. It also reduces lattice strain accumulation and phase transition tendency during electrochemical sodium intercalation / deintercalation, thereby improving the structural stability and cycle performance of the cathode material under high-temperature conditions.

[0008] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0009] In a first aspect, the present invention provides a high-entropy sodium ferrous sulfate / carbon composite cathode material, wherein the molecular formula of the high-entropy sodium ferrous sulfate / carbon composite cathode material is Na. 2.4-2a Fe 1.8-x+a M2+ y N 3+ z (SO4)3 / C, where 2x = 2y + 3z, 0 ≤ a ≤ 0.2, 0 < x ≤ 0.2, 0.01 ≤ y ≤ 0.1, 0.01 ≤ z ≤ 0.1;

[0010] M is selected from at least four of Mg, Ca, Mn, Co, Ni, Cu, Zn, and Ba, and N is selected from Al or Cr;

[0011] The carbon mass fraction is 1-10% based on the total mass of the composite cathode material.

[0012] This invention utilizes a series of low-sodium-content cathode materials (0≤a≤0.2) to significantly improve the intercalation / deintercalation capability of active sodium sites while maintaining the stability of the crystal framework. By controlling the doping amount to 0<x≤0.2, this invention can improve the conductivity and cycle stability of the material through high-entropy doping, while minimizing the occupation of active sites by inactive elements, avoiding excessive capacity decay, and achieving a balance between the doping modification effect and capacity retention.

[0013] High-entropy doping introduces various divalent / trivalent metal ions (Mg, Ca, Mn, Co, Ni, Cu, Zn, Ba, Al, Cr, etc.) that are similar to Fe in ionic radius, coordination environment, and chemical bond characteristics. Therefore, these ions tend to substitute for Fe sites, forming multi-component solid solutions. The entropy stabilization effect brought about by multi-component site occupancy can reduce the driving force of phase separation / phase transformation during high-temperature and cycling processes, resulting in smoother changes in unit cell parameters and more reversible phase transformations. Simultaneously, M... 2+ / N 3+ Heterovalent co-doping triggers moderate charge compensation (such as Fe valence state distribution reformation, defect concentration and local strain redistribution), thereby playing a role in "defect pinning" and "stress buffering," inhibiting cation migration, local distortion accumulation and irreversible framework collapse at high temperatures. Overall, the doped framework is better able to withstand high-temperature thermal stress and volume changes caused by repeated insertion and extraction, thus slowing down structural degradation from the source.

[0014] High-entropy doping significantly alters the local electronic structure around Fe sites: when transition metals such as Mn, Ni, Cu, Zn, and Cr enter the crystal lattice, the hybridization of their 3d orbitals with Fe-O bonds is enhanced, typically introducing more electronic states capable of conducting electricity at the band edges, resulting in a richer density of states and a narrower band gap. Simultaneously, charge compensation caused by heterovalent doping often promotes Fe... 2+ / Fe 3+The coexistence of (and the variable valence states of doped metals) provides small polaron transition / electron hopping paths for electrons between adjacent metal sites, reducing the electron migration barrier and increasing the bulk electronic conductivity, thereby alleviating the problem of insufficient electronic conductivity that is common in sulfate cathodes.

[0015] This invention provides a method for preparing the aforementioned high-entropy sodium ferrous sulfate / carbon composite cathode material, comprising the following steps:

[0016] (1) Disperse the conductive carbon material in deionized water, stir and add anhydrous sodium sulfate, iron source, doped metal source and organic carbon source to carry out solution reaction, and the resulting suspension is dried to obtain the precursor;

[0017] (2) The precursor obtained in step (1) is ground evenly and then calcined to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0018] The conductive carbon material mentioned in step (1) is selected from at least one of graphene oxide, Ketjen black, Super P Li and CNTs. Based on 1g of conductive carbon material, each 1g of conductive carbon material is added to 150-200mL of deionized water. The stirring is carried out by using a digital display high-speed disperser with strong mechanical stirring at a speed of 1000-1500rpm for 0.5-1h.

[0019] This invention innovatively employs a short-time, high-intensity mechanical stirring carbon-based dispersion process, eliminating the need for subsequent pulverization steps. This process directly produces loose, porous spherical cathode materials with a particle size of 2.58±1.07μm. It not only significantly shortens the mixing and dispersion process time and improves the preparation efficiency, but also enables precise control of the material's microstructure and particle size. The loose, porous spherical structure increases the contact area with the electrolyte, optimizing sodium ion diffusion and electrode reaction kinetics.

[0020] The organic carbon source is selected from at least one of ascorbic acid, citric acid monohydrate, trisodium citrate dihydrate, pyrrole, and hydroquinone; the iron source is selected from at least one of FeSO4·7H2O, FeSO4·H2O, and anhydrous FeSO4; and the doped metal source is an anhydrous sulfate of the corresponding doped metal (MgSO4, CaSO4, MnSO4, CoSO4, NiSO4, CuSO4, ZnSO4, BaSO4, Al2(SO4)3, Cr2(SO4)3).

[0021] The molar ratio of the anhydrous sodium sulfate, iron source, and doped metal source is 1-1.2:1.6-2:0.045-0.2; the mass ratio of the conductive carbon material to the organic carbon source is 1:0.5-1.

[0022] The reaction time of the solution is 0.5-1 h; the drying is spray drying at a temperature of 180-220℃ and a peristaltic pump speed of 0.6-1 L / h.

[0023] The inert atmosphere for calcination in step (2) is selected from any one of nitrogen, argon and argon-hydrogen mixture, the calcination temperature is 300-400℃, the heating rate is 1-3℃ / min, and the holding time is 8-24 h.

[0024] This invention provides the application of the aforementioned high-entropy sodium ferrous sulfate / carbon composite cathode material in sodium-ion batteries.

[0025] Secondly, the present invention provides a sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, a separator, and a battery casing. The positive electrode comprises a positive current collector and an active material layer coated on the positive current collector. The active material layer comprises the aforementioned high-entropy sodium ferrous sulfate / carbon composite positive electrode material, and the positive current collector is aluminum foil. The negative electrode comprises a negative current collector and an active material layer coated on the negative current collector. The active material layer comprises metallic sodium or an active material capable of intercalating / deintercalating sodium ions, and the negative current collector is aluminum foil or copper foil. The separator is selected from at least one of modified cellulose acetate separator, polyethylene separator, polypropylene microporous membrane, glass fiber separator, and composite separator thereof. The electrolyte is an electrolyte obtained by dissolving a soluble sodium salt in an organic solvent.

[0026] The sodium-intercalating / deintercalating active material is selected from at least one of carbon materials, metal oxides, metal sulfides, and alloy materials; the conductive agent is a highly conductive carbon material; the binder is a polymer binder; the soluble sodium salt is selected from at least one of sodium hexafluorophosphate, sodium perchlorate, and sodium trifluoromethanesulfonate; and the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, diethylene glycol dimethyl ether, 1,3-cyclopentanediol, ethylene glycol dimethyl ether, and triethylene glycol dimethyl ether.

[0027] The beneficial effects of this invention are:

[0028] 1. This patent, based on high-entropy doping to regulate crystal structure, innovatively reverse-engineers and systematically synthesizes a series of low-valence (+2, +3) high-entropy doped sodium ferrous sulfate cathode materials with low sodium content. This strategy maintains the stability of the crystal framework at high temperatures, significantly improving the intercalation / deintercalation capability of active sodium sites. The synergistic doping of multiple metal ions, through multiple effects of "structural stability - electron transport - interface regulation," significantly improves the high-temperature cycling stability of the sodium ferrous sulfate cathode material. Divalent metal ions (Mg... 2+ Ca 2+ Lattice pinning of (etc.) and trivalent metal ions (Al) 3+ Cr3+ The synergistic effect of bonding reinforcement of sodium ferrous sulfate and iron sulfate at high temperatures suppresses severe lattice distortion, structural collapse and iron dissolution, reduces the rate of interfacial impedance growth, and provides structural protection for cycle stability. During high-temperature cycling, the above synergistic effect continues to play a role, enabling the material to not only maintain a high initial discharge capacity, but also effectively suppress capacity decay. After multiple cycles, the capacity retention rate is significantly improved compared with pure phase sodium ferrous sulfate, and it can still maintain stable electrochemical performance under high rate conditions. This completely solves the core pain point of short cycle life of sodium ferrous sulfate at high temperatures and broadens its application prospects in high-temperature energy storage scenarios.

[0029] 2. Experiments show that the material obtained in this invention exhibits stable cycling under high-temperature conditions, significantly outperforming comparative studies under the same testing conditions. This demonstrates that precise high-entropy doping enables efficient sodium ion insertion / extraction and excellent structural stability at a composition close to the theoretically optimal stoichiometric ratio, solving the core problem of short cycle life of sodium ferrous sulfate at high temperatures and broadening its application prospects in high-temperature energy storage scenarios. The high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in this invention has advantages such as high operating voltage, high discharge specific capacity, low electrochemical polarization, and high energy density. A sodium battery prepared using this material as the cathode and metallic sodium as the anode achieves a cycle life of up to 980 cycles at an ambient temperature of 60°C and a current density of 5C (1C = 100 mA g⁻¹), with a capacity retention of 85.55%. The material without high-entropy modification retains only 23.19% of its capacity after 400 cycles. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 Scanning electron microscope (SEM) image and particle size distribution map of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1.

[0032] Figure 2 The image shows the X-ray diffraction (XRD) pattern of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1.

[0033] Figure 3 Scanning electron microscope (SEM) images of the cross-sections of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 after 100 cycles at 0.5C under high temperature conditions.

[0034] Figure 4 The images show cryo-electron microscopy (Cryo-TEM) images of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 after cycling at 0.5C for 5 weeks under high temperature conditions.

[0035] Figure 5 The graph shows a comparison of the rate performance of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 under high-temperature conditions.

[0036] Figure 6 The graph shows a comparison of the cycling performance of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 under high-temperature conditions. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0040] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 17.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous nickel sulfate (NiSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0041] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0042] The high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in this embodiment was characterized by scanning electron microscopy (SEM) and particle size distribution. The results are as follows: Figure 1 As shown, the particle size of the prepared high-entropy sodium ferrous sulfate carbon composite material is 2.58±1.07μm, which meets the core performance requirements of high-performance materials.

[0043] X-ray diffraction (XRD) was performed on the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in this embodiment. The results are as follows: Figure 2 As shown, the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in this embodiment shows good correlation with the standard card PDF (21-1360) peaks of Na2Fe(SO4)2, indicating that the prepared sample material has good phase purity.

[0044] Example 2

[0045] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0046] (1) Disperse 0.3 g of graphene oxide powder, 0.15 g of Ketjen Black and 0.15 g of Super P Li in 120 mL of deionized water, and stir vigorously at 1000 rpm for 60 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 17.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous nickel sulfate (NiSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (220 °C, 1 L / h) to obtain the precursor.

[0047] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 400°C at a heating rate of 2°C / min. Hold it at the temperature for 8 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0048] Example 3

[0049] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0050] (1) Disperse 0.3 g of graphene oxide powder, 0.15 g of Ketjen black and 0.15 g of carbon nanotubes in 90 mL of deionized water, and stir vigorously at 1500 rpm for 30 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 17.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous nickel sulfate (NiSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (180 °C, 0.6 L / h) to obtain the precursor.

[0051] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 1°C / min, and then heat it to 300°C at a heating rate of 3°C / min. Hold it at the temperature for 24 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0052] Example 4

[0053] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0054] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7·The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 10 mmol anhydrous sodium sulfate (Na2SO4), 19.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous nickel sulfate (NiSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0055] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 24 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0056] Example 5

[0057] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0058] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 16.9 mmol anhydrous ferrous sulfate (FeSO4), 0.2 mmol anhydrous calcium sulfate (CaSO4), 0.2 mmol anhydrous magnesium sulfate (MgSO4), 0.2 mmol anhydrous manganese sulfate (MnSO4), 0.2 mmol anhydrous nickel sulfate (NiSO4), and 0.1 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0059] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0060] Example 6

[0061] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0062] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0063] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The solution reaction was carried out by magnetic stirring at 500 rpm for 30 min. The resulting suspension was spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0064] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0065] Example 7

[0066] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0067] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7·The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 17.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous copper sulfate (CuSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0068] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0069] Example 8

[0070] A method for preparing a high-entropy sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0071] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The following solutions were prepared: 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), 17.45 mmol anhydrous ferrous sulfate (FeSO4), 0.1 mmol anhydrous calcium sulfate (CaSO4), 0.1 mmol anhydrous magnesium sulfate (MgSO4), 0.1 mmol anhydrous manganese sulfate (MnSO4), 0.1 mmol anhydrous zinc sulfate (ZnSO4), and 0.05 mmol anhydrous chromium sulfate (Cr2(SO4)3). The solutions were magnetically stirred at 500 rpm for 30 min to carry out the solution reaction. The resulting suspension was then spray-dried (200 °C, 0.8 L / h) to obtain the precursor.

[0072] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

[0073] Comparative Example 1

[0074] A method for preparing a sodium ferrous sulfate / carbon composite cathode material, the specific steps of which are as follows:

[0075] (1) Disperse 0.3 g of graphene oxide powder and 0.3 g of Ketjen black powder in 100 mL of deionized water, and stir vigorously at 1200 rpm for 45 min to form a dispersion. Add 0.3 g of citric acid monohydrate (C6H8O) 7· The solution reaction was carried out by magnetic stirring at 500 rpm for 30 min, with H2O, 0.12 g ascorbic acid (C6H8O6), 12 mmol anhydrous sodium sulfate (Na2SO4), and 18 mmol anhydrous ferrous sulfate (FeSO4). The resulting suspension was then spray-dried (200℃, 0.8 L / h) to obtain the precursor.

[0076] (2) After grinding the precursor obtained in step (1) evenly, transfer it to a magnetic boat and place it in an argon atmosphere furnace. Heat it to 200°C at a heating rate of 2°C / min, and then heat it to 350°C at a heating rate of 1°C / min. Hold it at the temperature for 12 h to obtain sodium ferrous sulfate / carbon composite cathode material.

[0077] Example of implementation effect 1

[0078] Sodium-ion batteries were fabricated using the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1. Battery performance was then tested, as detailed below:

[0079] The high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 were used as cathode active materials. 0.16 g of cathode material was mixed with 0.02 g of Super P Li and 0.02 g of polyvinylidene fluoride, and added to 2 mL of 1-methyl-2-pyrrolidone. The mixture was mixed evenly to form a slurry, which was then coated onto aluminum foil. After vacuum drying at 120 °C, the foil was cut to obtain a cathode sheet with a diameter of 13 mm. A sodium metal sheet was used as the anode (with a diameter of 16 mm). A glass fiber membrane (Whatman GF / D) was used as the separator. 1 M NaClO4 dissolved in EC:PC (volume ratio of 1:1) (5 wt.% FEC additive) was used as the electrolyte. A stainless steel shell was used as the outer shell, and the cells were assembled into a CR2025 button cell.

[0080] Figure 3 The scanning electron microscope (SEM) images of the cross-sections of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 after cycling at 0.5C for 100 cycles at high temperature are shown. It can be seen that the material prepared in Example 1 has a complete internal structure, while the material prepared in Comparative Example 1 has a large degree of particle breakage. Figure 4 The images are cryo-electron microscopy (Cryo-TEM) images of the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared in Example 1 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1 after cycling at 0.5C for 5 weeks under high temperature conditions. It can be seen that the material in Example 1 formed a thinner and more uniform CEI than the material in Comparative Example 1, which effectively suppressed the side reactions at the interface.

[0081] Charge-discharge tests were conducted at a high temperature (60℃) within a potential range of 2.0-4.5 V, and the results are as follows: Figure 5 and 6 As shown, the sodium-ion battery prepared using the materials prepared in Example 1 exhibits a reversible specific capacity exceeding 100 mAh g⁻¹ at a rate of 0.2C (1C = 100 mA g⁻¹). -1 Furthermore, under high load 5C current density, the cycle life can reach up to 980 cycles with a capacity retention rate of 85.55%. In contrast, the sodium-ion battery prepared using the material prepared in Comparative Example 1 has almost zero capacity decay after 400 cycles, with a capacity retention rate of only 23.19%.

[0082] Example of implementation effect 2

[0083] Sodium-ion batteries were fabricated using the high-entropy sodium ferrous sulfate / carbon composite cathode materials prepared in Examples 1-8 and the sodium ferrous sulfate / carbon composite cathode material prepared in Comparative Example 1. Battery performance was then tested, and the results are shown in Table 1.

[0084] Table 1. Performance of sodium-ion batteries prepared using the cathode materials of Examples 1-8 and Comparative Example 1

[0085]

[0086] As shown in Table 1, the high-entropy sodium ferrous sulfate / carbon composite cathode material prepared by adjusting the high-entropy doping ratio, the type of conductive carbon, and the raw material ratio has a significantly improved high-temperature cycle life compared with the control example. When the doping ratio x=0.055 (where 2x=2y+3z, y=0.04, z=0.01), the cycle life can reach up to 980 cycles, and the capacity retention rate is 85.55%.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-entropy sodium ferrous sulfate / carbon composite cathode material, characterized in that: The molecular formula of the high-entropy sodium ferrous sulfate / carbon composite cathode material is Na. 2.4-2a Fe 1.8-x+a M 2+ y N 3+ z (SO4)3 / C, where 2x = 2y + 3z, 0 ≤ a ≤ 0.2, 0 < x ≤ 0.2, 0.01 ≤ y ≤ 0.1, 0.01 ≤ z ≤ 0.1; M is selected from at least four of Mg, Ca, Mn, Co, Ni, Cu, Zn, and Ba, and N is selected from Al or Cr.

2. The high-entropy sodium ferrous sulfate / carbon composite cathode material according to claim 1, characterized in that: The carbon mass fraction is 1-10% based on the total mass of the composite cathode material.

3. The preparation method of the high-entropy sodium ferrous sulfate / carbon composite cathode material according to claim 1, characterized in that, Includes the following steps: (1) Disperse the conductive carbon material in deionized water, stir and add anhydrous sodium sulfate, iron source, doped metal source and organic carbon source to carry out solution reaction, and the resulting suspension is dried to obtain the precursor; (2) The precursor obtained in step (1) is ground evenly and then calcined to obtain high-entropy sodium ferrous sulfate / carbon composite cathode material.

4. The preparation method according to claim 2, characterized in that: The conductive carbon material mentioned in step (1) is selected from at least one of graphene oxide, Ketjen black, Super P Li and CNTs. Based on 1g of conductive carbon material, each gram of conductive carbon material is added to 150-200mL of deionized water. The stirring is performed using a digital display high-speed disperser at a speed of 1000-1500 rpm for 0.5-1 hours with strong mechanical stirring.

5. The preparation method according to claim 4, characterized in that: The organic antioxidant mentioned in step (1) is selected from at least one of ascorbic acid, citric acid monohydrate, trisodium citrate dihydrate, pyrrole and hydroquinone; the iron source is selected from at least one of FeSO4·7H2O, FeSO4·H2O and anhydrous FeSO4; the doped metal source is an anhydrous sulfate of the corresponding doped metal; the molar ratio of anhydrous sodium sulfate, iron source and doped metal source is 1-1.2:1.6-2:0.045-0.2; the mass ratio of conductive carbon material to organic carbon source is 1:0.5-1.

6. The preparation method according to claim 5, characterized in that: The reaction time of the solution in step (1) is 0.5-1h; the drying is spray drying at a temperature of 180-220℃ and a peristaltic pump speed of 0.6-1L / h.

7. The preparation method according to claim 6, characterized in that: The inert atmosphere for calcination in step (2) is selected from any one of nitrogen, argon and argon-hydrogen mixture, the calcination temperature is 300-400℃, the heating rate is 1-3℃ / min, and the holding time is 8-24 h.

8. The application of the high-entropy sodium ferrous sulfate / carbon composite cathode material according to claim 1 in sodium-ion batteries.

9. A sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, a separator, and a battery casing, characterized in that: The positive electrode comprises a positive current collector and an active material layer coated on the positive current collector, wherein the active material layer comprises the high-entropy sodium ferrous sulfate / carbon composite positive electrode material as described in claim 1, and the positive current collector is aluminum foil; the negative electrode comprises a negative current collector and an active material layer coated on the negative current collector, wherein the active material layer comprises metallic sodium or an active material capable of intercalating / deintercalating sodium ions, and the negative current collector is aluminum foil or copper foil; the separator is selected from at least one of modified cellulose acetate separator, polyethylene separator, polypropylene microporous membrane, glass fiber separator, and composite separator thereof; the electrolyte is an electrolyte obtained by dissolving a soluble sodium salt in an organic solvent.

10. The sodium-ion battery according to claim 9, characterized in that: The sodium-intercalating / deintercalating active material is selected from at least one of carbon materials, metal oxides, metal sulfides, and alloy materials; the conductive agent is a highly conductive carbon material; the binder is a polymer binder; the soluble sodium salt is one or more of sodium hexafluorophosphate, sodium perchlorate, and sodium trifluoromethanesulfonate; and the organic solvent is one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, diethylene glycol dimethyl ether, 1,3-cyclopentanediol, ethylene glycol dimethyl ether, and triethylene glycol dimethyl ether.

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