Sodium iron high-entropy sulfate positive electrode material, preparation method thereof and sodium ion battery

Abstract

The present invention provides a high-entropy sodium iron sulfate positive electrode material, a preparation method thereof, and a sodium-ion battery. The high-entropy sodium iron sulfate positive electrode material includes a doped sodium iron sulfate material and a carbon coating layer. The molecular formula of the doped sodium iron sulfate material is Na 2.5 Fe 1.75‑ x A y B z C m D n (SO4)3, where 0 < x ≤ 0.5, 0 ≤ y ≤ 0.06, 0 ≤ z ≤ 0.06, 0 ≤ m ≤ 0.06, 0 ≤ n ≤ 0.06, 2x = 2y + 3z + 4m + 5n, and at least two of y, z, m, and n are not 0; A is selected from at least one of Mn 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Cu 2+ , Sn 2+ , Ca 2+ , Co 2+ ; B is selected from Al 3+ and / or Cr 3+ ; C is selected from Zr 4+ and / or Ti 4+ ; D is selected from Nb 5+ and / or Ta 5+ , and at least contains four doping elements, and the contained doping elements satisfy equimolar or near-equimolar doping. The present invention conducts high-entropy doping on sodium iron sulfate to improve the low-temperature performance and cycling performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy, and particularly relates to a high-entropy sodium iron sulfate cathode material, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] Sodium-ion batteries have the advantages of low cost, high safety, high power, and excellent low-temperature performance, and are an important supplement to lithium-ion batteries, which have received extensive attention at the present stage. So far, great progress has been made in representative hard carbon anode materials, while the energy density and cycle life of cathode materials are still one of the bottlenecks restricting the practical application of sodium-ion batteries. Among the developed sodium-ion battery cathode materials, polyanion compounds have good ion diffusion channels and high safety, and their unique structural framework endows them with excellent structural stability. Therefore, sodium-ion battery cathode materials based on polyanion compounds have great commercial prospects.

[0003] Polyanion compounds have a three-dimensional framework structure composed of strong covalent bonds. On the one hand, the strong covalent bonds endow the material with outstanding structural stability, ensuring the rapid diffusion of sodium ions; at the same time, the three-dimensional framework structure endows the material with abundant lattice vacancies, effectively suppressing the large volume change and complex phase change during the ion insertion / extraction process; on the other hand, the strong inductive effect of polyanion groups weakens the transition metal (M)-O bond, increasing the redox potential of transition metal ions, and thus enabling the material to have a high working potential. However, polyanion-type cathode materials also face challenges such as low electronic conductivity and poor charge-discharge performance at high rates. In the actual design and preparation process, carbon coating, particle nanosizing, ion doping, etc. are usually used for modification to improve their electronic conductivity.

[0004] Sodium iron sulfate has a relatively high redox potential, and has the advantages of easy synthesis, high safety, and environmental friendliness. However, sodium-ion batteries based on sodium iron sulfate cathode materials still have problems such as low conductivity, poor low-temperature performance, and cycle performance.

[0005] Due to their diverse components and simple crystal structures, high-entropy materials have been proven to have good application prospects in the field of energy storage. They offer great compositional possibilities for achieving target functions and have a positive impact on improving the cycling stability, low-temperature performance, and Coulomb efficiency of electrode materials. Chinese Patent CN117638027A discloses a high-entropy iron-based polyanion compound cathode material, its preparation method, and application. By using the sol-gel method to replace the iron element in the structure, high-entropy doping without destroying the original structure is achieved, which can improve the electrochemical performance of the battery to a certain extent. However, its preparation process is complex, which is not conducive to large-scale production, and the ratio of doping elements with similar properties is not controlled, easily forming interfering impurity phases, which is not conducive to the synthesis of high-entropy materials. Therefore, how to use the high-entropy strategy to construct cathode materials with excellent performance remains to be further developed and practiced by researchers. Summary of the Invention

[0006] To solve all or part of the above technical problems, the present invention provides the following technical solutions:

[0007] One of the purposes of the present invention is to provide a high-entropy sodium iron sulfate cathode material, which includes a doped sodium iron sulfate material and a carbon coating layer coated on the doped sodium iron sulfate material; the molecular formula of the doped sodium iron sulfate material is Na 2.5 Fe 1.75-x A y B z C m D n (SO4)3, where 0 < x ≤ 0.5, 0 ≤ y ≤ 0.06, 0 ≤ z ≤ 0.06, 0 ≤ m ≤ 0.06, 0 ≤ n ≤ 0.06, 2x = 2y + 3z + 4m + 5n, and at least two of y, z, m, and n are not 0 (that is, the doped sodium iron sulfate material includes at least two types of doping elements among A, B, C, and D);

[0008] Among them, A is selected from one or a combination of more of Mn 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Cu 2+ , Sn 2+ , Ca 2+ , Co 2+ , B is selected from Al 3+ and / or Cr 3+ , C is selected from Zr 4+ and / or Ti 4+ , D is selected from Nb 5+ and / or Ta 5+and at least four doping elements are contained in the doped sodium iron sulfate material, and the contained doping elements satisfy equi-proportion or near-equi-proportion doping.

[0009] "High entropy" as described in the present invention refers to a material containing five or more chemically independent elements in a single-phase lattice of the material, or a compound with configurational entropy (ΔS conf ) ≥ 1.5R.

[0010] Moreover, in the high-entropy doping scheme of the present invention, at least four of the above-mentioned metal elements are doped, and the doped metal elements belong to at least two of A, B, C, and D.

[0011] The near-equi-proportion doping as described in the present invention means that the absolute value of the difference in the content ratio of any two doped metal elements is less than or equal to 0.01.

[0012] In the present invention, by doping a variety of metal elements with different valence states in the sodium iron sulfate structure, the "cocktail effect" of the high-entropy material is generated, the degree of disorder of the atomic arrangement in the structure is increased, the functions of different elements are coordinated, so that the electrochemical performance of the sodium iron sulfate-based cathode material is further improved, especially excellent low-temperature performance and cycle stability are shown, and at the same time, the skeleton of the sulfate material is effectively stabilized, and the diffusion of sodium ions is enhanced; in the present invention, a carbon-based material is used to modify the sodium iron sulfate-based cathode material, and the sodium iron sulfate particles are tightly attached to the carbon material, a high-speed electron conductive network is constructed between different particles, effectively improving the electron transfer efficiency of the material. The carbon material and the high-entropy doping strategy synergistically improve the electron conductivity on the surface and inside of the material, so that the electrochemical performance of the sodium iron sulfate cathode material is better exerted.

[0013] In some embodiments, n in the molecular formula of the doped sodium iron sulfate material is not 0, that is, the doping elements include class D, and at least one of A, B, and C is also included. The present invention finds that in the high-entropy doping strategy of the sodium iron sulfate-based cathode material, selecting Nb 5+ and / or Ta 5+ as one of the doping elements can further improve the electrochemical performance of the cathode material; specifically, Nb 5+ and / or Ta 5+ have higher valence states, which can help the doped metal generate mixed valence, reduce the band gap of electron transition in the structure, thereby improving the intrinsic electron conductivity of the material; moreover, Nb 5+ and / or Ta 5+ have larger ionic radii and inertness, enabling them to act as "pillars" in the lattice, and further stabilizing the crystal structure of the material during the Na + insertion and extraction process, and improving the cycle stability of the cathode material.

[0014] In some embodiments, the content of the carbon coating layer is 0.01-10 wt% of the doped sodium iron sulfate material.

[0015] In some preferred embodiments, the content of the carbon coating layer is 0.5-7 wt% of the doped sodium iron sulfate material.

[0016] In some embodiments, the high-entropy sodium iron sulfate cathode material is spherical particles with a median particle size of 6-15 μm.

[0017] In some embodiments, the thickness of the carbon coating layer is 3-8 nm.

[0018] The second object of the present invention is to provide a method for preparing a high-entropy sodium iron sulfate cathode material, comprising:

[0019] Providing a precursor mixture containing a carbon-based material, a dispersant, a sodium source, an iron source, and a doping source;

[0020] Using an inert atmosphere to protect a closed-loop aerosol spray drying process for the precursor mixture to obtain a precursor powder;

[0021] Under an inert atmosphere condition, subjecting the precursor powder to solid-phase sintering to obtain a high-entropy sodium iron sulfate cathode material;

[0022] Wherein, the doping source is used to provide at least two types of doping elements among A, B, C, and D; A is selected from one or a combination of more than one of Mn 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Cu 2+ , Sn 2+ , Ca 2+ , Co 2+ , B is selected from one or a combination of more than one of Al 3+ , Cr 3+ , C is selected from Zr 4+ and / or Ti 4+ , D is selected from Nb 5+ and / or Ta 5+ , and there are at least four types of doping elements, and the doping elements contained satisfy equimolar or near-equimolar doping.

[0023] The present invention uses an aerosol spray drying composite carbon-based material, a doping source and sodium ferrous sulfate. Aerosol spray drying can greatly avoid the sedimentation of insoluble substances and the agglomeration of powder materials through instant drying, and the obtained precursor powder has good fluidity and compositional uniformity. In addition, micro-nano level precursor powder can be obtained by the aerosol spray drying method. The micron-level spherical morphology is beneficial to improving the packing density of the material and the volume specific energy of the battery, and maintaining the cycle stability of the material. The nano-level particles can effectively shorten the diffusion path of Na + and improve the solid-phase diffusion coefficient of Na + . The obtained micro-nano level particles can seek a balance between improving the packing density of the material and shortening the sodium ion diffusion distance to obtain better electrochemical performance. Further, an inert protective atmosphere is filled during the spray drying process, which can prevent the oxidation of the material without using an antioxidant, avoid the decrease in electrochemical performance that may be caused by using an antioxidant, and the problem that the chemical or physical properties of the material may change due to improper selection of the antioxidant. And it can effectively avoid the risk of explosion and fire of organic solvents, and can provide a dust-free and sterile operation to improve the product purity. The inert atmosphere is, for example, nitrogen, but is not limited thereto. A dispersant is added during the preparation process of the present invention, which can inhibit the sedimentation of the slurry during the aerosol spray drying process, and can form an adsorption layer on the surface of solid particles, increasing the charge on the surface of the solid particles to form a double-layer structure, so as to increase the degree of wetting of the solid particles by water, and help the solid particles to disperse due to electrostatic repulsion to obtain a more uniform precursor powder.

[0024] In some embodiments, the precursor mixture further includes an organic acid. Compared with inorganic acids, organic acids can not only play a role in adjusting the pH value, but also in-situ form a coating layer on the powder surface during the subsequent high-temperature sintering process, which is beneficial to improving the surface / interface electron conductivity and ion conductivity of the cathode material, inhibiting the side reaction between the sodium ferrous sulfate cathode material and the electrolyte, and cooperating with the Nb 5+ and / or Ta 5+ doping strategy can synergistically improve the cycle stability; and organic acids are not easy to introduce impurities, have mild acidity and are easy to adjust.

[0025] In some embodiments, the organic acid includes one or more combinations of formic acid, acetic acid, oxalic acid, pyruvic acid, and lactic acid.

[0026] In some embodiments, the content of the organic acid is 0.02 - 6 wt% of the total mass of the sodium source, iron source and doping source; and / or, the addition amount of the organic acid adjusts the pH of the precursor mixture to 5 - 7.

[0027] In some embodiments, the preparation method specifically includes:

[0028] Provide suspension A containing carbon-based materials and a dispersant;

[0029] Provide dispersion B containing a sodium source, an iron source, and a doping source;

[0030] Uniformly mix the suspension A with the dispersion B, and add an organic acid to obtain the precursor mixture.

[0031] In some embodiments, the doping element includes type D and also includes at least one of A, B, and C.

[0032] In some embodiments, the carbon-based material includes an organic carbon source and / or an inorganic carbon source. The organic carbon source includes, but is not limited to, one or a combination of more of citric acid, sucrose, glucose, and maltose. The inorganic carbon source includes, but is not limited to, one or a combination of more of carbon nanotubes, conductive carbon black, Ketjen black, activated carbon, and graphene oxide.

[0033] In some embodiments, the content of the carbon-based material is 0.5 to 12 wt% of the total mass of the sodium source, the iron source, and the doping source.

[0034] In some preferred embodiments, the carbon-based material includes an organic carbon source and an inorganic carbon source. Compared with the scheme of using only an organic carbon source or an inorganic carbon source alone, using both an organic carbon source and an inorganic carbon source can enhance the surface electron conductivity and reduce battery polarization. On the one hand, the organic carbon source can protect the electrode material from the influence of volume expansion and contraction due to its flexible structure. On the other hand, it can assist in acting as a reducing agent to avoid the formation of Fe 3+ and improve the product purity, thereby improving the electrochemical performance of the material.

[0035] In some embodiments, the mass ratio of the inorganic carbon source to the organic carbon source is 1:10 to 10:1.

[0036] In some embodiments, the dispersant includes one or a combination of more of polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyvinyl butyral resin (PVB), polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and sodium carboxymethyl cellulose (CMC), but is not limited thereto. The dispersant can not only promote the dispersion of the carbon-based material but also inhibit the sedimentation of the slurry during the aerosol spray drying process.

[0037] In some preferred embodiments, the dispersant includes one or a combination of polyethylene glycol, polyvinyl alcohol, polyvinyl butyral resin, polyvinylpyrrolidone, sodium carboxymethyl cellulose, etc. These polymer dispersants can form an adsorption layer on the surface of solid particles during the spray drying process, increasing the charge on the surface of the solid particles and forming a bilayer structure, thereby increasing the degree of wetting of the solid particles by water, helping the solid particles to disperse due to electrostatic repulsion, and obtaining a more uniform precursor powder.

[0038] In some embodiments, the content of the dispersant is 1-10 wt% of the total mass of the sodium source, iron source, and doping source. If the content of the dispersant is too low, the solid particles cannot be effectively dispersed, and obvious sedimentation will still occur in the slurry; if the dosage of the dispersant is too high, the viscosity of the slurry will be too high, and it is difficult to obtain solid particles with good fluidity during spray drying. At the same time, due to the increase of inactive components, the electrochemical capacity is reduced.

[0039] In some embodiments, the sodium source includes sodium sulfate.

[0040] In some embodiments, the iron source includes ferrous sulfate, such as ferrous sulfate heptahydrate.

[0041] In some embodiments, the source of the doping element includes one or a combination of sulfates and oxides of the doping element, but is not limited thereto.

[0042] In some embodiments, the solvents of the suspension A and the dispersion liquid B include one or a combination of water, ethanol, and ethylene glycol.

[0043] In some embodiments, the inlet temperature of the aerosol spray drying is 130-230 °C, the outlet temperature is 80-160 °C, the feeding rate is 600-1800 mL / h, and the aerosol pressure is 0.1-0.3 MPa. By regulating the process parameters of the aerosol spray drying, the present invention can obtain a micro-nano scale cathode material with a reasonable morphology and structure design, thereby improving the discharge specific capacity and cycle performance of the sodium iron sulfate material.

[0044] In some preferred embodiments, the inlet temperature of the aerosol spray drying is 150-200 °C, the outlet temperature is 80-135 °C, the feeding rate is 800-1400 mL / h, and the aerosol pressure is 0.15-0.25 MPa.

[0045] In some embodiments, the calcination temperature of the solid-phase sintering is 320-420 °C, and the holding time is 6-18 h.

[0046] In some embodiments, the heating rate of the solid-phase sintering is 1-3 °C / min.

[0047] In some embodiments, the inert atmosphere includes, but is not limited to, one or more of nitrogen, argon, etc.

[0048] In some typical embodiments, the preparation method specifically includes:

[0049] Disperse the dispersant in deionized water and use heating magnetic stirring to make it uniformly dispersed, then add the carbon-based material and disperse it by ultrasonic wave to obtain suspension A;

[0050] Uniformly disperse the sodium source, ferrous source, and doping source in deionized water to obtain dispersion B;

[0051] Mix suspension A and dispersion B, and add an organic acid to adjust the pH to 5 - 7, and stir at room temperature to obtain the precursor mixture.

[0052] In some embodiments, the temperature of the heating magnetic stirring is 40 - 80 °C, and the stirring time is 0.1 - 2 h.

[0053] In some embodiments, the time of ultrasonic dispersion is 1 - 5 h.

[0054] In some embodiments, the stirring time at room temperature is 0.2 - 5 h.

[0055] The third object of the present invention is to provide a high-entropy sodium ferrous sulfate cathode material obtained by using the preparation method in any one of the technical solutions.

[0056] The fourth object of the present invention is to provide the application of the high-entropy sodium ferrous sulfate cathode material described in any one of the technical solutions in the preparation of a cathode for a sodium-ion battery or a sodium-ion battery.

[0057] The fifth object of the present invention is to provide a sodium-ion battery, including a cathode, an anode, an electrolyte, and a separator. The cathode includes a current collector and an active material layer formed on the current collector, and the active material layer includes the high-entropy sodium ferrous sulfate cathode material described in any one of the technical solutions.

[0058] In some typical embodiments, the preparation method of the cathode includes:

[0059] Weigh the high-entropy sodium ferrous sulfate cathode material, carbon black (SuperP), and polyvinylidene fluoride (PVDF) according to a mass ratio of 80:10:10 - 90:5:5, disperse the three materials in N-methylpyrrolidone (NMP), and disperse and stir at high speed for 3 - 6 h to make a conductive slurry; then mix the conductive slurry evenly and coat it on an aluminum foil, and after vacuum drying, roll and punch it to obtain a cathode electrode sheet.

[0060] In some embodiments, the areal density of the cathode electrode sheet is 2 - 4 mg / cm2 。

[0061] In some embodiments, the material of the negative electrode is metallic sodium.

[0062] In some embodiments, the electrolyte includes sodium hexafluorophosphate (NaPF6), a solvent, and a functional additive. The functional additive includes 2-5 wt% of fluoroethylene carbonate (FEC) and 4-10 wt% of ethylene sulfite (ES). The functional additive can weaken the interaction between Na+ and the solvent, endowing the electrolyte with high ionic conductivity and interfacial charge transfer kinetics, and enabling the sodium-ion battery to obtain better low-temperature performance without sacrificing room-temperature electrochemical performance.

[0063] In some embodiments, the concentration of sodium hexafluorophosphate in the electrolyte is 0.8-1.5 mol / L.

[0064] In some embodiments, the solvent of the electrolyte includes ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 to 1:1:4.

[0065] In some embodiments, the separator includes GF / A, GF / D, or GF / F glass fiber filter paper membrane.

[0066] Compared with the prior art, the present invention has at least the following beneficial effects:

[0067] (1) By doping with multiple metal elements of different valence states, the present invention realizes the regulation of the structure of the sodium iron sulfate material. The doped ions of different valence states form cation and anion vacancies in the bulk phase, which can improve the defective activity of sodium iron sulfate and effectively enhance its conductivity. At the same time, the different ionic radii of the doped elements can cause lattice distortion, resulting in the effects of expanding the lattice spacing and increasing the types of active ions, thereby suppressing the short-range order in the structure to enhance the diffusion of Na + and finally enabling the sodium-ion battery to obtain good low-temperature performance and cycling performance;

[0068] (2) Further, the present invention discovers that selecting Nb 5+ and / or Ta 5+ as the doping element can further improve the electrochemical performance of the positive electrode material; Nb 5+ and / or Ta 5+ have higher valence states, which can help the doped metal generate mixed valence, reduce the band gap of electron transition in the structure, and thus improve the intrinsic electronic conductivity of the material. Moreover, Nb 5+ and / or Ta 5+ have relatively large ionic radii and inertness, enabling them to act as "pillars" in the lattice, thereby stabilizing Na +During the delithiation process, the crystal structure of the material is adjusted to improve the cycle stability of the cathode material;

[0069] (3) The present invention adopts an inert gas protected closed-loop aerosol spray drying method. During the spray drying process, an inert protective gas is filled, which can prevent the oxidation of the material without using an antioxidant, avoiding the decrease in electrochemical performance that may be caused by the use of an antioxidant, and the problem that the chemical or physical properties of the material may change due to improper selection of the antioxidant;

[0070] (4) Further, the organic acid added during the preparation process of the present invention can not only adjust the pH value, but also in-situ form a coating layer on the surface of the powder during the subsequent high-temperature sintering process, which is beneficial to improving the surface / interface electron conductivity and ionic conductivity of the cathode material, inhibiting the side reaction between the sodium ferrous sulfate cathode material and the electrolyte. The present invention unexpectedly finds that its combination with Nb 5+ and / or Ta 5+ doping strategy can synergistically improve the cycle stability;

[0071] (5) The preparation method provided by the present invention is simple and has good repeatability, and can realize large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0073] Figure 1 is a process flow diagram for preparing a high-entropy sodium ferrous sulfate cathode material in an embodiment of the present invention;

[0074] Figure 2 is an X-ray diffraction pattern of the high-entropy sodium ferrous sulfate cathode material in Example 1 of the present invention;

[0075] Figure 3 is a scanning electron microscope image of the high-entropy sodium ferrous sulfate cathode material in Example 1 of the present invention;

[0076] Figure 4 is the first two charge-discharge curves of the sodium-ion battery in Example 1 of the present invention at room temperature and a current density of 0.2C;

[0077] Figure 5 is the cycle performance graph of the sodium-ion battery in Example 1 of the present invention at room temperature and a current density of 10C;

[0078] Figure 6It is the particle size distribution diagram of the high-entropy sodium iron sulfate cathode material in Embodiment 2 of the present invention;

[0079] Figure 7 It is the C-V curve of the sodium-ion battery in Embodiment 3 of the present invention at room temperature;

[0080] Figure 8 It is the rate performance diagram of the sodium-ion battery in Embodiment 3 of the present invention at room temperature;

[0081] Figure 9 It is the comparison diagram of the cycling performance of the sodium-ion battery in Embodiment 4 of the present invention at room temperature and low temperature (-15 °C) under a current density of 1C. Detailed Description of the Invention

[0082] The technical solution of the present invention will be described in detail below in conjunction with specific embodiments, so that those skilled in the art can better understand and implement the technical solution of the present invention. The specific functional details disclosed herein should not be construed as limiting, but only as the basis of the claims and as a representative basis for teaching those skilled in the art to adopt the present invention in different ways in any appropriate detailed embodiments.

[0083] Unless otherwise specified, the raw materials and reagents used in the embodiments and comparative examples of the present invention are obtained commercially.

[0084] Embodiment 1

[0085] This embodiment provides a high-entropy sodium iron sulfate cathode material and a preparation method thereof. Figure 1 It is the process schematic diagram for preparing the high-entropy sodium iron sulfate cathode material in this embodiment, specifically as follows:

[0086] (1) 11.4 g of polyethylene glycol was dispersed in 140 mL of ethylene glycol. After magnetic stirring at 40 °C for 0.3 h, 2 g of carbon nanotubes and 2.45 g of citric acid were added, and after ultrasonic dispersion for 2 h, a uniformly dispersed suspension A was obtained;

[0087] (2) 0.775 mol of ferrous sulfate heptahydrate, 0.563 mol of anhydrous sodium sulfate, 0.00225 mol of stannous sulfate, 0.00225 mol of nickel sulfate hexahydrate, 0.00113 mol of aluminum sulfate, and 0.0009 mol of tantalum pentoxide were added to 810 mL of deionized water to obtain a dispersion B;

[0088] (3) The dispersion B was added to the suspension A, and 4.12 g of oxalic acid was added. After magnetic stirring at room temperature for 0.8 h, a precursor suspension was obtained;

[0089] (4) The precursor liquid obtained in step (3) is subjected to aerosol spray drying by a spray dryer, and nitrogen is introduced for closed-loop protection during the spray drying process. The aerosol pressure is set to 0.2 MPa, the inlet temperature is 150 °C, the outlet temperature is 100 °C, and the feeding rate is 1150 mL / h to obtain the precursor powder;

[0090] (5) The precursor powder obtained in step (4) is placed in a porcelain boat and heated to 400 °C at a heating rate of 1.5 °C / min in an argon atmosphere and calcined for 10 h to obtain the high-entropy sodium iron sulfate cathode material.

[0091] Figure 2 is the XRD pattern of the cathode material prepared in this example, Figure 3 is the SEM pattern of the cathode material prepared in this example.

[0092] A sodium-ion battery is prepared using the above high-entropy sodium iron sulfate cathode material, and the preparation method is as follows:

[0093] Weigh the above high-entropy sodium iron sulfate cathode material, carbon black (Super P), and polyvinylidene fluoride (PVDF) according to a mass ratio of 80:10:10. Disperse these three materials in N-methylpyrrolidone (NMP) and stir well for 6 h to form a homogeneous slurry; then coat the slurry onto an aluminum foil and obtain a positive electrode plate after vacuum drying. The areal density of the obtained electrode plate is 2.4 mg / cm2;

[0094] The negative electrode material is metallic sodium;

[0095] The composition of the electrolyte used: 1 mol / L sodium hexafluorophosphate (NaPF6) and a solvent. The solvent is ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) with a volume ratio of 1:1:4, and 5% fluoroethylene carbonate (FEC) and 6% ethylene sulfite (ES) are added thereto;

[0096] The separator is a GF / F glass fiber filter paper membrane;

[0097] Assemble the above positive electrode, negative electrode, separator, and electrolyte into a sodium-ion battery.

[0098] Figure 4 are the first two charge-discharge curves of the sodium-ion battery in this example at room temperature and a current density of 0.2C, Figure 5 is the cycling performance graph of the sodium-ion battery in this example at room temperature and a current density of 10C.

[0099] Example 2

[0100] This example provides a high-entropy sodium iron sulfate cathode material and a preparation method thereof, specifically as follows:

[0101] (1) 6.12 g of polyvinyl alcohol was dispersed in 120 mL of ethylene glycol. After magnetic stirring at 70 °C for 1 h, 2.4 g of carbon nanotubes and 1 g of glucose were added, and after ultrasonic dispersion for 3 h, a uniformly dispersed suspension A was obtained;

[0102] (2) 0.77 mol of ferrous sulfate heptahydrate, 0.563 mol of anhydrous sodium sulfate, 0.0045 mol of stannous sulfate, 0.00227 mol of nickel sulfate hexahydrate, 0.0045 mol of titanium sulfate, and 0.0005 mol of tantalum pentoxide were added to 800 mL of deionized water to obtain a dispersion B;

[0103] (3) Dispersion B was added to suspension A, 1.7 g of oxalic acid was added, and magnetic stirring was carried out at room temperature for 1.2 h to obtain a precursor suspension;

[0104] (4) The precursor liquid obtained in step (3) was subjected to aerosol spray drying by a spray dryer, and nitrogen was introduced for closed-loop protection during the spray drying process. The aerosol pressure was set to 0.25 MPa, the inlet temperature was 170 °C, the outlet temperature was 125 °C, and the feeding rate was 1000 mL / h to obtain a precursor powder;

[0105] (5) The precursor powder obtained in step (4) was placed in a porcelain boat, and in an argon atmosphere, it was heated to 380 °C at a heating rate of 2 °C / min and calcined for 15 h to obtain a high-entropy sodium ferrous sulfate cathode material.

[0106] Figure 6 is the particle size distribution diagram of the cathode material prepared in this example.

[0107] A sodium-ion battery was prepared according to the same method as in Example 1.

[0108] Example 3

[0109] This example provides a high-entropy sodium ferrous sulfate cathode material and its preparation method, specifically as follows:

[0110] (1) 3.2 g of cetyltrimethylammonium bromide was dispersed in 200 mL of deionized water. After magnetic stirring at 40 °C for 0.5 h, 4.5 g of activated carbon and 5 g of glucose were added, and after ultrasonic dispersion for 2 h, a uniformly dispersed suspension A was obtained;

[0111] (2) 0.771 mol of ferrous sulfate heptahydrate, 0.563 mol of anhydrous sodium sulfate, 0.0025 mol of copper sulfate pentahydrate, 0.0036 mol of nickel sulfate hexahydrate, 0.001 mol of chromium sulfate hexahydrate, and 0.0018 mol of niobium oxide were added to 740 mL of deionized water to obtain a dispersion B;

[0112] (3) Add dispersion B to suspension A, add 1.88 g of oxalic acid, and magnetically stir for 3 h at room temperature to obtain a precursor suspension;

[0113] (4) Spray the precursor liquid obtained in step (3) through a spray dryer for aerosol spray drying, and introduce nitrogen for closed-loop protection during the spray drying process. Set the aerosol pressure to 0.2 MPa, the inlet temperature to 160 °C, the outlet temperature to 100 °C, and the feeding rate to 800 mL / h to obtain precursor powder;

[0114] (5) Place the precursor powder obtained in step (4) in a porcelain boat, heat it to 410 °C at a heating rate of 1 °C / min in an argon atmosphere, and calcine for 6 h to obtain a high-entropy sodium ferrous sulfate cathode material.

[0115] Prepare a sodium-ion battery according to the same method as in Example 1. Figure 7 This is the C-V curve of the sodium-ion battery in this example at room temperature, Figure 8 This is the rate performance diagram of the sodium-ion battery in this example at room temperature.

[0116] Example 4

[0117] This example provides a high-entropy sodium ferrous sulfate cathode material and its preparation method, which are as follows:

[0118] (1) Disperse 10 g of polyvinyl alcohol in 150 mL of ethanol, magnetically stir for 0.1 h at 40 °C, then add 3.5 g of carbon nanotubes and 15 g of sucrose, and ultrasonically disperse for 1.5 h to obtain a uniformly dispersed suspension A;

[0119] (2) Add 0.7747 mol of ferrous sulfate heptahydrate, 0.5625 mol of anhydrous sodium sulfate, 0.0025 mol of copper sulfate pentahydrate, 0.001125 mol of aluminum sulfate, 0.0036 mol of titanium sulfate, and 0.00045 mol of niobium oxide to 800 mL of deionized water to obtain dispersion B;

[0120] (3) Add dispersion B to suspension A, add 5 g of pyruvic acid, and magnetically stir for 1.5 h at room temperature to obtain a precursor suspension;

[0121] (4) Spray the precursor liquid obtained in step (3) through a spray dryer for aerosol spray drying, and introduce nitrogen for closed-loop protection during the spray drying process. Set the aerosol pressure to 0.25 MPa, the inlet temperature to 200 °C, the outlet temperature to 135 °C, and the feeding rate to 1200 mL / h to obtain precursor powder;

[0122] (5) Place the precursor powder obtained in step (4) in a porcelain boat, and under an argon atmosphere, heat it to 400 °C at a heating rate of 2 °C / min and calcine for 8 h to obtain the high-entropy sodium iron sulfate cathode material.

[0123] Prepare a sodium-ion battery according to the same method as in Example 1.

[0124] Example 5

[0125] This example provides a high-entropy sodium iron sulfate cathode material and a preparation method thereof, specifically as follows:

[0126] (1) Disperse 14.3 g of polyvinylpyrrolidone in 220 mL of deionized water, magnetically stir at 60 °C for 1.3 h, then add 10 g of conductive carbon black and 2.2 g of citric acid, and ultrasonically disperse for 4 h to obtain a uniformly dispersed suspension A;

[0127] (2) Add 0.7722 mol of ferrous sulfate heptahydrate, 0.5625 mol of anhydrous sodium sulfate, 0.0045 mol of stannous sulfate, 0.0018 mol of titanium sulfate, 0.00225 mol of zinc sulfate monohydrate, and 0.0008 mol of niobium oxide to 700 mL of deionized water to obtain a dispersion B;

[0128] (3) Add dispersion B to suspension A, add 4 g of formic acid, and magnetically stir at room temperature for 2.5 h to obtain a precursor suspension;

[0129] (4) Spray the precursor liquid obtained in step (3) through a spray dryer for aerosol spray drying, and introduce nitrogen closed-loop protection during the spray drying process. Set the aerosol pressure to 0.18 MPa, the inlet temperature to 175 °C, the outlet temperature to 120 °C, and the feeding rate to 900 mL / h to obtain a precursor powder;

[0130] (5) Place the precursor powder obtained in step (4) in a porcelain boat, and under a nitrogen atmosphere, heat it to 360 °C at a heating rate of 2 °C / min and calcine for 17 h to obtain the high-entropy sodium iron sulfate cathode material.

[0131] Prepare a sodium-ion battery according to the same method as in Example 1.

[0132] Example 6

[0133] This example provides a high-entropy sodium iron sulfate cathode material and a preparation method thereof, specifically as follows:

[0134] (1) Disperse 3.73 g of sodium dodecyl sulfate in 110 mL of deionized water, magnetically stir at 50 °C for 1.5 h, then add 6.5 g of Ketjen black and 5 g of citric acid, and ultrasonically disperse for 5 h to obtain a uniformly dispersed suspension A;

[0135] (2) 0.7736 mol of ferrous sulfate heptahydrate, 0.5625 mol of anhydrous sodium sulfate, 0.00315 mol of calcium sulfate, 0.00068 mol of aluminum sulfate, 0.0027 mol of titanium sulfate, and 0.00034 mol of tantalum pentoxide were added to 760 mL of deionized water to obtain dispersion B;

[0136] (3) Dispersion B was added to suspension A, 8 g of lactic acid was added, and magnetic stirring was carried out at room temperature for 2.5 h to obtain a precursor suspension;

[0137] (4) The precursor liquid obtained in step (3) was subjected to aerosol spray drying by a spray dryer, and nitrogen was introduced for closed-loop protection during the spray drying process. The aerosol pressure was set to 0.2 MPa, the inlet temperature was 150 °C, the outlet temperature was 100 °C, and the feeding rate was 1000 mL / h to obtain a precursor powder;

[0138] (5) The precursor powder obtained in step (4) was placed in a porcelain boat, and in a nitrogen atmosphere, it was heated to 370 °C at a heating rate of 1 °C / min and calcined for 16 h to obtain a high-entropy polyanionic cathode material.

[0139] A sodium-ion battery was prepared in the same manner as in Example 1.

[0140] Example 7

[0141] This example provides a high-entropy sodium iron sulfate cathode material and its preparation method, which are as follows:

[0142] (1) 6 g of polyvinyl butyral resin was dispersed in 250 mL of ethylene glycol. After magnetic stirring at 80 °C for 1.2 h, 1.5 g of graphene oxide and 10.5 g of maltose were added, and after ultrasonic dispersion for 1 h, a uniformly dispersed suspension A was obtained;

[0143] (2) 0.773 mol of ferrous sulfate heptahydrate, 0.563 mol of anhydrous sodium sulfate, 0.0018 mol of magnesium sulfate, 0.0036 mol of copper sulfate pentahydrate, 0.00225 mol of zirconium sulfate tetrahydrate, and 0.0009 mol of tantalum pentoxide were added to 750 mL of deionized water to obtain dispersion B;

[0144] (3) Dispersion B was added to suspension A, 6.6 g of oxalic acid was added, and magnetic stirring was carried out at room temperature for 2 h to obtain a precursor suspension;

[0145] (4) The precursor liquid obtained in step (3) is subjected to aerosol spray drying by a spray dryer, and nitrogen is introduced for closed-loop protection during the spray drying process. Set the aerosol pressure to 0.22 MPa, the inlet temperature to 170 °C, the outlet temperature to 110 °C, and the feeding rate to 1050 mL / h to obtain the precursor powder;

[0146] (5) Place the precursor powder obtained in step (4) in a porcelain boat, and in an argon atmosphere, heat it to 400 °C at a heating rate of 3 °C / min and calcine for 9 h to obtain the high-entropy sodium iron sulfate cathode material.

[0147] Prepare the sodium-ion battery according to the same method as in Example 1.

[0148] Example 8

[0149] This example provides a high-entropy sodium iron sulfate cathode material and its preparation method, which are as follows:

[0150] (1) Disperse 8 g of polyethylene glycol in 100 mL of deionized water, magnetically stir for 1 h at 40 °C, then add 1 g of carbon nanotubes and 7 g of citric acid, and ultrasonically disperse for 0.8 h to obtain a uniformly dispersed suspension A;

[0151] (2) Add 0.7745 mol of ferrous sulfate heptahydrate, 0.5625 mol of anhydrous sodium sulfate, 0.0027 mol of stannous sulfate, 0.00225 mol of manganese sulfate monohydrate, 0.0009 mol of chromium sulfate hexahydrate, and 0.00108 mol of niobium oxide to 840 mL of deionized water to obtain a dispersion B;

[0152] (3) Add the dispersion B to the suspension A, add 7.44 g of formic acid, and magnetically stir at room temperature for 2 h to obtain a precursor suspension;

[0153] (4) The precursor liquid obtained in step (3) is subjected to aerosol spray drying by a spray dryer, and nitrogen is introduced for closed-loop protection during the spray drying process. Set the aerosol pressure to 0.25 MPa, the inlet temperature to 180 °C, the outlet temperature to 125 °C, and the feeding rate to 1300 mL / h to obtain the precursor powder;

[0154] (5) Place the precursor powder obtained in step (4) in a porcelain boat, and in an argon atmosphere, heat it to 380 °C at a heating rate of 2 °C / min and calcine for 14 h to obtain the high-entropy sodium iron sulfate cathode material.

[0155] Prepare the sodium-ion battery according to the same method as in Example 1. Figure 9 It is a comparison diagram of the cycling performance of the sodium-ion battery in this example at room temperature and low temperature (-15 °C) under a 1C current density condition.

[0156] Example 9

[0157] The difference between Example 9 and Example 1 is only that cobalt sulfate heptahydrate is used to replace nickel sulfate hexahydrate, and the rest is the same as in Example 1.

[0158] Example 10

[0159] The difference between Example 10 and Example 2 is only that the composition of the electrolyte used is: 1 mol / L sodium hexafluorophosphate (NaPF6) and a solvent, and the solvent is ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) with a volume ratio of 1:1:4, and 2% fluoroethylene carbonate (FEC) and 4% vinylene sulfite (ES) are added thereto.

[0160] Example 11

[0161] This example provides a high-entropy sodium iron sulfate cathode material and a preparation method thereof, which are specifically as follows:

[0162] (1) 5.82 g of sodium dodecyl sulfate is dispersed in 240 mL of ethanol. After magnetic stirring at 55 °C for 1 h, 1.9 g of Ketjenblack and 8.5 g of sucrose are added, and after ultrasonic dispersion for 4 h, a uniformly dispersed suspension A is obtained;

[0163] (2) 0.7677 mol of ferrous sulfate heptahydrate, 0.5625 mol of anhydrous sodium sulfate, 0.0036 mol of magnesium sulfate, 0.0036 mol of stannous sulfate, 0.0036 mol of nickel sulfate hexahydrate, and 0.0018 mol of niobium oxide are added to 720 mL of deionized water to obtain a dispersion B;

[0164] (3) The dispersion B is added to the suspension A, 5 g of oxalic acid is added, and magnetic stirring is carried out at room temperature for 2 h to obtain a precursor suspension;

[0165] (4) The precursor liquid obtained in step (3) is subjected to aerosol spray drying by a spray dryer, and nitrogen is introduced for closed-loop protection during the spray drying process. The aerosol pressure is set to 0.15 MPa, the inlet temperature is 150 °C, the outlet temperature is 80 °C, and the feeding rate is 800 mL / h to obtain a precursor powder;

[0166] (5) The precursor powder obtained in step (4) is placed in a porcelain boat, and in an argon atmosphere, it is heated to 420 °C at a heating rate of 3 °C / min and calcined for 6 h to obtain a high-entropy sodium iron sulfate cathode material.

[0167] The difference from the method for preparing a sodium-ion battery in Example 1 is only that 3% fluoroethylene carbonate (FEC) and 8% vinylene sulfite (ES) are added to the electrolyte used, and the rest is the same as in Example 1.

[0168] Example 12

[0169] The difference between Example 12 and Example 1 is only that in step (1), only 4.45 g of carbon nanotubes are added as the carbon source.

[0170] Example 13

[0171] The difference between Example 13 and Example 1 is only that in the electrolyte, the functional additives of vinyl fluorocarbonate and vinylene sulfite are not added.

[0172] Comparative Example 1

[0173] The difference between Comparative Example 1 and Example 1 is only that a sodium iron sulfate cathode material without doping elements is used, that is, 0.00225 mol of stannous sulfate, 0.00225 mol of nickel sulfate hexahydrate, 0.00113 mol of aluminum sulfate, and 0.0009 mol of tantalum pentoxide are not added during preparation, and the rest is the same as in Example 1.

[0174] Comparative Example 2

[0175] The difference between Comparative Example 2 and Example 1 is only that a sodium iron sulfate cathode material is prepared by doping only 0.0009 mol of tantalum pentoxide, and the rest is the same as in Example 1.

[0176] Comparative Example 3

[0177] The difference between Comparative Example 3 and Example 1 is only that carbon nanotubes and citric acid are not used during preparation, that is, the prepared sodium iron sulfate cathode material does not contain a carbon coating layer.

[0178] Comparative Example 4

[0179] The difference from Example 1 is only that oxalic acid in step (3) is replaced by hydrochloric acid.

[0180] Table 1 Related performance of sodium-ion batteries in the examples and comparative examples of the present invention

[0181]

[0182]

[0183] In summary, in the present invention, by selecting appropriate metal ions to replace the electrochemically active iron sites in sodium iron sulfate, high-entropy doping is formed to adjust the electronic structures of the sodium iron sulfate material and specific atoms; by introducing various defects and stabilizing the structure of the host material, the material exhibits a more flexible bulk structure, increasing the degree of disorder in the atomic arrangement in the material structure and improving the electrochemical performance and cycle stability. In the present invention, by modifying the cathode material with a carbon material, the inherent electron insulation of the material can be effectively improved, and a cathode material with good electrochemical performance can be obtained. The present invention uses an aerosol spray drying method to prepare a high-entropy polyanionic cathode material, and a uniform three-dimensional spherical morphology precursor powder can be obtained. Moreover, this preparation method is simple and is conducive to realizing the large-scale production of this cathode material. In the present invention, by adding a functional additive to the electrolyte, this additive can participate in the solvation structure, participate in cation coordination, and weaken the interaction between Na + and solvent molecules, so that the electrolyte has high ionic conductivity and interfacial charge transfer kinetics, and more excellent low-temperature performance can be obtained without sacrificing the room-temperature electrochemical performance.

[0184] All aspects, embodiments, features and examples of the present invention should be considered illustrative in all respects and are not intended to limit the present invention. The scope of the present invention is only defined by the claims. Without departing from the spirit and scope of the claimed invention, those skilled in the art will appreciate other embodiments, modifications and uses.

[0185] In addition, the inventors of this case also referred to the foregoing embodiments and conducted experiments with other raw materials, process operations and process conditions described in this specification, and all obtained relatively ideal results.

[0186] Although the present invention has been described with reference to illustrative embodiments, those skilled in the art will understand that various other changes, omissions and / or additions can be made without departing from the spirit and scope of the present invention and elements of the embodiments can be replaced with substantial equivalents. Additionally, many modifications can be made to adapt a particular situation or material to the teachings of the present invention without departing from the scope of the present invention. Therefore, the present invention is not intended to be limited to the specific embodiments disclosed for carrying out the present invention, but is intended to cover all embodiments falling within the scope of the appended claims. Moreover, unless specifically stated, any use of the terms first, second, etc. does not denote any order or importance, but the terms first, second, etc. are used to distinguish one element from another.

Claims

1. A high-entropy sodium iron sulfate cathode material, characterized in that, Comprising: A doped sodium iron sulfate material and a carbon coating layer coated on the doped sodium iron sulfate material; The chemical formula of the doped sodium ferrous sulfate material is Na 2.5 Fe 1.75-x A y B z C m D n (SO4)3, 0 < x ≤0.5, 0 ≤ y ≤0.06, 0 ≤ z ≤0.06, 0 ≤ m ≤0.06, 0 < n ≤0.06, 2 x = 2 y + 3 z + 4 m + 5 n , and y , z , m at least one of them is not 0; A is selected from at least one of Mn 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Cu 2+ , Sn 2+ , Ca 2+ , Co 2+ ; B is selected from Al 3+ and / or Cr 3+ , C is selected from Zr 4 + and / or Ti 4+ , D is selected from Nb 5+ and / or Ta 5+ , and the doped sodium ferrous sulfate material contains four doping elements in equal proportion or nearly equal proportion, and the nearly equal proportion means that the absolute value of the difference between the content ratios of any two doping elements is less than or equal to 0.01; The high-entropy sodium iron sulfate cathode material is spherical particles with a median particle size of 6-15 μm, wherein the doped sodium iron sulfate material is tightly attached to the carbon material in the carbon coating layer, and the thickness of the carbon coating layer is 3-8 nm; The preparation method of the high-entropy sodium iron sulfate cathode material includes: Providing a suspension A containing a carbon-based material and a dispersant, and a dispersion B containing a sodium source, an iron source, and a doping source. The suspension A and the dispersion B are uniformly mixed and an organic acid is added to obtain a precursor mixture; the organic acid is selected from at least one of formic acid, acetic acid, oxalic acid, pyruvic acid, and lactic acid, and its content is 0.02-6 wt% of the total mass of the sodium source, the iron source, and the doping source; the doping source provides doping element D and also provides at least one of A, B, and C; the sodium source includes sodium sulfate, the iron source includes ferrous sulfate, the doping source includes sulfates and / or oxides of doping elements, and the dosages of the sodium source, the iron source, and the doping source satisfy the chemical formula; Using an inert atmosphere to protect a closed-cycle aerosol spray drying process for the precursor mixture, the inlet temperature of the spray drying is 130-230 °C, the outlet temperature is 80-160 °C, the feeding speed is 600-1800 mL / h, and the aerosol pressure is 0.1-0.3 MPa to obtain a precursor powder; Under an inert atmosphere condition, the precursor powder is solid-phase sintered at 320-420 °C and kept warm for 6-18 h to obtain a high-entropy sodium iron sulfate cathode material.

2. The high-entropy sodium iron sulfate cathode material according to claim 1, wherein: The carbon-based material includes an organic carbon source and / or an inorganic carbon source. The organic carbon source includes one or a combination of citric acid, sucrose, glucose, and maltose, and the inorganic carbon source includes one or a combination of carbon nanotubes, conductive carbon black, activated carbon, and graphene oxide.

3. The high-entropy sodium iron sulfate cathode material according to claim 1, characterized in that: The dispersant includes one or a combination of polyethylene glycol, polyvinyl alcohol, polyvinyl butyral resin, polyvinylpyrrolidone, sodium dodecyl sulfate, cetyltrimethylammonium bromide, and sodium carboxymethylcellulose.

4. The high-entropy sodium iron sulfate cathode material according to claim 1, characterized in that: The solvents of the suspension A and the dispersion B include one or a combination of water, ethanol, and ethylene glycol.

5. The high-entropy sodium iron sulfate cathode material according to claim 1, wherein: The content of the carbon-based material is 0.5-12 wt% of the total mass of the sodium source, the iron source, and the doping source.

6. The high-entropy sodium iron sulfate cathode material according to claim 1, characterized in that: The content of the dispersant is 1-10 wt% of the total mass of the sodium source, the iron source, and the doping source.

7. The high-entropy sodium iron sulfate cathode material according to claim 1, characterized in that: The organic acid adjusts the pH of the precursor mixture to 5-7.

8. The high-entropy sodium iron sulfate cathode material according to claim 1, characterized in that: The heating rate of the solid-phase sintering is 1-3 °C / min.

9. Application of the high-entropy sodium iron sulfate cathode material according to any one of claims 1-8 in preparing a cathode for a sodium-ion battery or a sodium-ion battery.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The cathode includes a current collector and an active material layer formed on the current collector, and the active material layer includes the high-entropy sodium iron sulfate cathode material according to any one of claims 1-8.

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