Ion-doped sodium ferric sulfate positive electrode material with egg yolk shell structure as well as preparation method and application thereof

Abstract

The invention relates to an egg yolk shell structure ion-doped sodium ferric sulfate positive electrode material, a preparation method and an application thereof, and the material is an egg yolk shell sphere structure carbon composite sodium ferric sulfate positive electrode material formed by doping cations A and B at Na or Na / Fe ion sites of sodium ferric sulfate and enabling anions to occupy part of S-O tetrahedral positions to change the arrangement mode of Fe-O octahedrons of the sodium ferric sulfate, the shell and the inner core of the egg yolk shell sphere are uniform in material, a gap exists between the shell and the inner core, and carbon is uniformly distributed and coated inside and outside the material. The invention also discloses a preparation method and application of the egg yolk shell structure ion-doped sodium ferric sulfate positive electrode material. The material is large in specific surface area, the reversible charge-discharge capacity of an assembled battery is high, and the cycle performance and the high-rate performance are good. The method is low in raw material cost, simple, short in production period and suitable for large-scale continuous production.

Description

Technical Field

[0001] The present invention relates to a sodium iron sulfate cathode material, a preparation method and an application thereof, and particularly relates to a yolk-shell structured ion-doped sodium iron sulfate cathode material, a preparation method and an application thereof. Background Art

[0002] In recent years, sodium-ion batteries have been rapidly developed as a new generation of energy storage batteries. Among them, polyanionic cathode materials are far superior to other materials in terms of cycle stability and are also much lower in cost, meeting the material advantages required by the future large-scale and commercial sodium-ion energy storage market. As a sodium-ion battery cathode material with multiple Na + storage sites, its chemical formula is NaA’A”M y (X a O b ) z D w , where A’ and A” are different sodium storage sites, M is one or several of Ti, V, Cr, Ni, Mn, Fe, Co, Ca, Mg, Al, etc., X is S, P, Si, B, Mo, etc., and Z is F, OH, etc. As an iron-based polyanionic material, sodium iron sulfate cathode material has attracted the attention of many researchers due to its high working voltage, stable three-dimensional structure and ultra-low raw material cost.

[0003] Na 2.6 Fe 1.7 (SO 4 ) 3 / C, as a key sodium-ion cathode material for energy storage batteries and the lead-to-sodium market, has attracted the attention of many energy storage researchers due to its low raw material cost, high working voltage and good cycle performance. Sodium-ion batteries achieve the conversion between electrical energy and chemical energy by the extraction and insertion of Na + between the cathode and anode materials. However, compared with lithium-ion batteries, since the radius of Na + (1.02 Å) is larger than that of Li + (0.76 Å), the energy required for its intrinsic ion diffusion is relatively higher, which results in a slower migration rate of Na + and poor rate performance, thus limiting its promotion and application. To meet the challenges of the fast charge rate market, the modification research on the rate performance of cathode materials generally achieves by doping substitution, controlling the micro-morphology and other modification means.

[0004] CN118851275A discloses a cation-doped sodium iron sulfate composite cathode material, its preparation method and application. A microspherical metal-ion-doped sodium iron sulfate material is prepared by spray drying. The metal ions are doped at the iron site to change the bond length of Fe-O in the metal-oxygen octahedron to form distortion, thereby changing the ion transport channels. However, this method cannot change the fact that only the c-axis serves as the main Na + transport channel in the sodium iron sulfate material. Therefore, the rate performance of the material is not significantly improved.

[0005] CN118099392A discloses an element-doped dual-carbon modified sodium iron sulfate cathode material, its preparation method and application. A sodium iron sulfate material is prepared by combining metal element doping and carbon coating. Although the metal ions are doped and anchored at the iron site to change the types of redox couples and thereby improve the discharge specific capacity of the material, there is no improvement effect on the rate performance of the material.

[0006] CN118888712A discloses a high-entropy sodium iron sulfate cathode material, its preparation method and a sodium-ion battery. A sodium iron sulfate material is prepared by co-doping with multiple metal elements. Its modification method is similar to that of CN118099392A, that is, multiple metal ions are doped at the iron site, and stronger lattice distortion is formed by utilizing the difference in the coordination bond lengths between different metals and oxygen, thereby improving the cycle performance of the material.

[0007] In summary, there is an urgent need to find a yolk-shell structured ion-doped sodium iron sulfate cathode material and its application with a large specific surface area, a high reversible charge-discharge capacity of the battery assembled with the made cathode electrode sheet, good cycle performance and high rate performance, as well as a preparation method of the yolk-shell structured ion-doped sodium iron sulfate cathode material with low raw material cost, simple synthesis method, short production cycle and suitable for large-scale continuous production. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the above-mentioned defects existing in the prior art and provide a yolk-shell structured ion-doped sodium iron sulfate cathode material and its application with a large specific surface area, a high reversible charge-discharge capacity of the battery assembled with the made cathode electrode sheet, good cycle performance and high rate performance.

[0009] The further technical problem to be solved by the present invention is to overcome the above-mentioned defects existing in the prior art and provide a preparation method of the yolk-shell structured ion-doped sodium iron sulfate cathode material with low raw material cost, simple synthesis method, short production cycle and suitable for large-scale continuous production.

[0010] The technical solution adopted by the present invention to solve its technical problems is as follows: The yolk-shell structure ion-doped sodium iron sulfate cathode material is a yolk-shell spherical structure carbon composite sodium iron sulfate cathode material formed by doping cations A and B at the Na or Na / Fe ion sites of sodium iron sulfate, and anions occupying part of the S-O tetrahedron positions to change the arrangement of part of the Fe-O octahedrons in sodium iron sulfate. Among them, the outer shell and the inner core of the yolk-shell sphere have uniform materials, and there is a gap between the two. Carbon is evenly distributed and coated inside and outside the material; its chemical formula is: Na 2.6-x-2y A x B y Fe 1.7 (SO 4 ) 3-z (C 2 O 4 ) z / C, where A includes alkali metal elements, B includes alkaline earth metals and / or transition metal elements, 0 < x ≤ 0.05 (more preferably 0.01 ≤ x ≤ 0.04), 0 < y ≤ 0.03 (more preferably 0.01 ≤ y ≤ 0.025), 0 < z ≤ 0.05 (more preferably 0.01 ≤ z ≤ 0.04), and the law of conservation of charge is satisfied.

[0011] The yolk-shell structure and the core-shell structure are two different material structures: The core-shell structure is composed of two parts, the inner core and the outer shell. The inner core is completely coated by the outer shell, and there is no gap between the core and the shell, which is a layered material of different types inside and outside that is completely enclosed; while the yolk-shell structure is based on the core-shell structure, and a gap part is introduced between the outer shell and the inner core, forming a structural variant of outer shell - gap - inner core, which further increases the specific surface area of the material. In addition, in addition to the advantages of the ordinary core-shell structure, in the process of charge and discharge, the internal gap between the core and the shell can play a buffering role in the volume shrinkage and expansion caused by the insertion and extraction of sodium ions in the material. Compared with ordinary core-shell and spherical shell materials, the stress change inside the material is greatly reduced; it alleviates the volume change of the CEI film, thereby reducing the side reaction between the electrolyte and the material. Different from the existing single metal ion doped at the iron site, the material of the present invention uses dual-cation lattice site directional doping, and the doping site is at the sodium site, rather than the iron site; the anion C 2 O 4 2- is arranged in a monomer two-dimensional manner. Its introduction causes local lattice distortion of the material, improves the migration channel of sodium ions, and promotes ion transport; at the same time, it provides additional redox behavior and improves the reversible charge / discharge capacity of the material. In summary, through the synergistic effect of macroscopic surface morphology control and microscopic element lattice site regulation, the material of the present invention reduces the sodium ion migration energy barrier and improves the migration rate of Na + inside the material and at the interface.

[0012] The selected metal element has an ionic radius larger than that of sodium ions and can reduce the band gap of the material, thereby reducing the reaction energy barrier for ion deintercalation and improving the rate performance and discharge specific capacity. Among them, monovalent metal ion A + is doped into the Na2 site to achieve the effect of broadening the sodium ion transport channel through its larger ionic radius; divalent metal B 2+ is doped into the Na1 site to inhibit the irreversible migration of Fe 3+ during the first charge and discharge process, avoid the irreversible reduction in the number of redox couples in the material, and reduce the band gap of the material.

[0013] Preferably, the average particle size of the yolk-shell structure ion-doped sodium iron sulfate cathode material is 5-35 μm (more preferably 12-32 μm). Smaller spherical particles will increase the specific surface area of the material, but the side reactions between the electrolyte and the electrode material will also increase, resulting in a significant reduction in the initial Coulomb efficiency of the material; while too large a particle size of the material will lead to an increase in voids, which is not conducive to improving the tap density and bulk density of the material.

[0014] Preferably, in the yolk-shell structure ion-doped sodium iron sulfate cathode material, the doping amounts of cations A and B are 0.01-2.00 at% (more preferably 0.8-1.8 at%) and 0.01-1.20 at% (more preferably 0.3-1.1 at%) respectively, and the doping amount of oxalate ions is 0.01-1.60 at% (more preferably 0.1-1.4 at%). Too much or too little doping amount of metal ions will lead to a reduction in the overall electrochemical performance. If the doping amount of metal ions is too much, it will occupy too many Na sites, resulting in a decrease in the content of Na + participating in ion transport during the redox process, and the electrochemical performance cannot be fully exerted; if the doping amount of metal ions is too little, it is not sufficient to fully improve the electrochemical performance of the material. If the doping amount of oxalate ions is too much, it will lead to severe lattice distortion of the material and cause structural collapse during the charge and discharge cycle; if the doping amount of oxalate ions is too little, it is difficult to play a role in the lattice distortion of the material, and it is difficult to improve the rate performance of the material. The doping amounts of cations A and B are relative to the doping amount of Na + , and the doping amount of oxalate ions is relative to the doping amount of SO 4 2- .

[0015] Preferably, in the yolk-shell structure ion-doped sodium iron sulfate cathode material, the mass fraction of uniformly distributed and coated carbon is 0.3-4.0% (more preferably 1.5-3.5%). If the content of uniformly distributed and coated carbon is too much, it will hinder the Na +Migration results in reduced electrochemical performance; if the uniformly distributed and coated carbon content is too low, it is difficult to improve the electronic conductivity of the material, leading to a reduction in the comprehensive electrochemical performance of the material.

[0016] Preferably, the initial specific surface area of the yolk-shell structured ion-doped sodium iron sulfate cathode material is 10-30 m 2 / g. The cathode material of the present invention has a structure with a larger initial specific surface area, enabling the electrolyte to fully contact the material surface and providing more active sites and Na + -insertion / extraction transport channels for electrochemical reactions, thereby promoting the rapid insertion and extraction of Na + in the structure. Through the design of broadening the multi-directional sodium ion migration channels, the present invention activates Na + in different directions, designs a material with a larger initial specific surface area, and greatly improves the rate performance and discharge specific capacity of the material.

[0017] Preferably, the alkali metal element includes K and / or Li, etc.

[0018] Preferably, the alkaline earth metal element includes Mg and / or Ca, etc.

[0019] Preferably, the transition metal element includes Zn and / or Cu, etc.

[0020] The technical solution adopted by the present invention to further solve its technical problems is as follows: A preparation method of a yolk-shell structured ion-doped sodium iron sulfate cathode material, comprising the following steps: (1) Add a sodium source, a ferrous source, a sulfuric acid source, a cation dopant, an oxalate anion dopant, and an organic acid antioxidant to water in sequence. After stirring and dissolving, add an organic carbon source template agent and an inorganic carbon source conductive slurry, stir and mix, and then perform spray drying to obtain a yolk-shell structured ion-doped sodium iron sulfate cathode material precursor; (2) Sinter the yolk-shell structured ion-doped sodium iron sulfate cathode material precursor obtained in step (1) under a protective atmosphere to obtain a yolk-shell structured ion-doped sodium iron sulfate cathode material.

[0021] The inventive concept of the method of the present invention is as follows: In step (1), first add an organic acid antioxidant to provide a suitable pH environment to ensure the stable existence of ferrous ions, and at the same time carbonize during the sintering process as a conductive coating layer; after adding the organic carbon source template agent and the inorganic conductive carbon source, stir evenly, and use spray drying to make the material precipitate homogeneously and avoid the generation of associated crystal water; in step (2), perform low-temperature sintering on the microsphere particles obtained in the previous step to remove the crystal water in the material, increase the crystallinity of the material, promote the diffusion of metal ions to form the obtained phase, and finally the template agent decomposes to promote the change of the microsphere structure to form a yolk-shell structure of shell - void - core.

[0022] Preferably, in step (1), the molar ratio of sodium element in the sodium source, ferrous element in the ferrous source, sulfate radical in the sulfuric acid source, A and B cations in the cation dopant, and oxalate radical in the oxalate anion dopant matches the molar ratio of the corresponding substances in the chemical formula. The cation is doped in trace amounts. If the doping amount is too large, the amount of Na participating in the redox reaction will decrease, and it is difficult to fully exert the electrochemical performance. If the doping amount is too small, it will be difficult to fully broaden the ion migration channels at specific sodium sites and the degree of lattice distortion is small. The reason for choosing oxalate radical as the doped anion is that, compared with pyrophosphate radical and phosphate radical, oxalate radical will form two-dimensional lattice distortion. + Preferably, in step (1), the dosage of the organic acid antioxidant is such that the mass fraction of its carbonized mass in the sodium iron sulfate doped with ions in the eggshell structure of the cathode material is 0.1 - 1.5% (more preferably 0.6 - 1.4%). The organic acid antioxidant can control the pH value of the solution environment to be maintained between 2 and 8, ensuring the stable existence of Fe during the stirring process and preventing the occurrence of oxidation or hydrolysis reactions. At the same time, during the sintering process of the material, the organic acid antioxidant carbonizes and decomposes to form a conductive carbon coating layer. If the dosage of the organic acid antioxidant is too small, it is difficult to ensure the stable existence of divalent iron ions throughout the reaction process. If the dosage of the organic carbon antioxidant is too large, the proportion of the active substance of the material will be too low, resulting in low conductivity of the material and increased electrochemical polarization during the charge and discharge process. Different organic acid antioxidants have different carbonized masses at different sintering temperatures. The carbonized mass = the mass of the organic acid antioxidant × the residual percentage after carbonization at the corresponding temperature, and the residual percentage after carbonization at the corresponding temperature is obtained by thermogravimetric analysis.

[0023] Preferably, in step (1), the dosage of water is such that the concentration of ferrous ions is 0.8 - 1.8 mol / L (more preferably 1.0 - 1.6 mol / L). During the spray drying process, generally the volume of the atomized droplets is constant. If the concentration of the prepared material is too low, the proportion of solvent water molecules in it will increase, and the hollow volume will increase greatly when forming microsphere particles, seriously affecting the cycle stability of the material during the charge and discharge process. If the concentration of the prepared material is too high, the formed microsphere particles will have a rough and thick wall layer. 2+ Preferably, in step (1), the temperature of the stirring and dissolution is 25 - 30 °C, the rotation speed is 300 - 500 rpm, and the time is 15 - 25 min.

[0024] Preferably, in step (1), the temperature of the stirring and dissolution is 25 - 30 °C, the rotation speed is 300 - 500 rpm, and the time is 15 - 25 min.

[0025] Preferably, in step (1), the temperature of the stirring and dissolution is 25 - 30 °C, the rotation speed is 300 - 500 rpm, and the time is 15 - 25 min.

[0026] Preferably, in step (1), 20-80 g (more preferably 40-60 g) of the organic carbon source template agent is added per liter of water. The organic carbon source template agent carbonizes and decomposes during the high-temperature sintering process of the material to form a hollow separation layer, and then a shell-void-core structure is formed. If the concentration of the organic carbon source template agent is too low, it is difficult to exert the template effect; if the concentration of the organic carbon source template agent is too high, the proportion of the active substance in the material will be too low, resulting in low conductivity of the material and difficult effective exertion of the electrochemical performance.

[0027] Preferably, in step (1), the solid content of the inorganic carbon source conductive paste is 3-10% (more preferably 4-8%). If the solid content is too high, the consistency and stability of the paste will be poor, and it is easy to agglomerate into a gel state, which is not conducive to long-term use, and at the same time, the raw material cost of the material increases; if the solid content is too low, the solid content of the whole system will be reduced, affecting the particle size and conductivity of the material, and at the same time reducing the electrochemical performance of the material.

[0028] Preferably, in step (1), the dosage of the inorganic carbon source conductive paste is such that the mass fraction of the inorganic carbon source in the yolk-shell structure ion-doped sodium iron sulfate cathode material is 0.1-2.2% (more preferably 1.1-2.1%). The intrinsic electronic conductivity of the sodium iron sulfate cathode material is low, and the addition of the inorganic carbon source can improve the electronic conductivity performance of the material. If the dosage of the inorganic carbon source is too high, the proportion of the active substance in the material system will be reduced, and at the same time, the production cost of the material will increase. If the dosage of the inorganic carbon source is too low, the conductivity of the material is low, and the electrochemical performance is difficult to be effectively exerted.

[0029] Preferably, in step (1), the temperature of the stirring and mixing is 25-30 °C, the rotation speed is 700-900 rpm, and the stirring time is 15-25 min.

[0030] Preferably, in step (1), while feeding the spray drying, the feeding solution is continuously stirred at 700-900 rpm.

[0031] Preferably, in step (1), the process parameters of the spray drying are: the frequency of the induced draft fan is 20-40 Hz (more preferably 30-40 Hz), the inlet air temperature is 200-300 °C (more preferably 200-245 °C), and the outlet air temperature is 90-130 °C (more preferably 90-110 °C). During the spray drying process, the atomized droplets enter a relatively high temperature environment from the room temperature environment instantaneously, causing the water solvent to evaporate instantaneously and enabling the solute to precipitate in a better homogeneous phase. The inlet air temperature is matched with the frequency of the induced draft fan to control the size of the atomized droplets; if the inlet air temperature is too high, the oxalate in the material will decompose into carbonate; the outlet air temperature ≥ the solvent evaporation temperature to reduce the generation of crystal water.

[0032] Preferably, in step (1), the sodium source includes one or more of sodium sulfate, sodium acetate, sodium formate, sodium nitrate, and their hydrates, etc. When the sodium source is sodium sulfate, it can be partially used as a sulfuric acid source.

[0033] Preferably, in step (1), the ferrous source includes one or more of ferrous sulfate and / or ferrous nitrate, and their hydrates, etc. When the ferrous source is ferrous sulfate, it can be partially used as a sulfuric acid source.

[0034] Preferably, in step (1), the sulfuric acid source includes sulfuric acid, and one or more of ammonium sulfate or sodium sulfate and their hydrates, etc.

[0035] Preferably, in step (1), the cation dopant includes one or more of sulfates, nitrates or chlorides of alkali metal elements, alkaline earth metals or transition metal elements, and their hydrates, etc. More preferably, the cation dopant includes one or more of sulfates or nitrates of K, Li, Mg, Ca, Zn or Cu, and their hydrates, etc. When the cation dopant is a sulfate, it can be partially used as a sulfuric acid source.

[0036] Preferably, in step (1), the oxalate anion dopant includes oxalic acid, and one or more of sodium oxalate, potassium oxalate or zinc oxalate and their hydrates, etc. When the oxalate anion dopant is sodium oxalate, it can be partially used as a sodium source.

[0037] Preferably, in step (1), the organic acid antioxidant includes one or more of citric acid, tannic acid or ascorbic acid, etc.

[0038] Preferably, in step (1), the organic carbon source template includes one or more of cyclodextrin, starch, polydextrose or polyvinylpyrrolidone, etc. The present invention uses an organic carbon source as a template for controlling the morphology to form a yolk-shell structure without considering the carbon residue. More preferably, the organic carbon source template is cyclodextrin and / or polyvinylpyrrolidone.

[0039] Preferably, in step (1), the inorganic carbon source includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P, acetylene black or graphene, etc.

[0040] Preferably, in step (2), before the sintering, first, at room temperature, purge with a protective atmosphere for 1.5 - 2.5 h to ensure that the sintering system is filled with the protective atmosphere and other gases are excluded.

[0041] Preferably, in step (2), the sintering means: heating from room temperature to 220 - 300 °C (more preferably 220 - 280 °C) at a rate of 1 - 5 °C / min, and sintering for 8 - 24 h (more preferably 8 - 16 h). Through one-step calcination, the dehydration reaction of crystal water in the sodium iron sulfate precursor can be realized, and at the same time, a sodium iron sulfate cathode material with a well-crystallized yolk-shell structure can be formed. If the sintering temperature is too high or the time is too long, the oxalate radical in the material will decompose into carbonate radical, the material structure will be distorted, and the specific capacity and cycle performance will decrease. If the sintering temperature is too low, the crystallinity of the material will be poor.

[0042] Preferably, in step (2), the protective atmosphere includes one or more of nitrogen, argon, and hydrogen-argon or hydrogen-nitrogen mixed gas, etc. The nitrogen, argon or the atmosphere used to prepare the mixed gas in the present invention are all high-purity atmospheres with a purity ≥ 99.999%.

[0043] The technical solution adopted by the present invention to further solve its technical problems is as follows: Application of a yolk-shell structure ion-doped sodium iron sulfate cathode material, and using the positive electrode sheet made of the yolk-shell structure ion-doped sodium iron sulfate cathode material to assemble a sodium ion battery.

[0044] The beneficial effects of the present invention are as follows: (1) The material of the present invention has a specific yolk-shell structure, which greatly increases the contact area between the electrode material and the electrolyte, and provides an initial specific surface area as high as 11.97 m 2 / g, providing more reactive sites for electrochemical reactions; the yolk-shell structure alleviates the volume expansion and contraction caused by stress changes during the charge and discharge cycles of the material, maintaining good cycle stability; through the synergistic effect of in-situ co-doping of anions and cations, the alkali metal and transition metal elements in the material lattice are directionally regulated to occupy the Na2 and Na1 sites respectively, broadening the sodium ion migration channels and inhibiting the irreversible migration of trivalent iron ions at the Fe1 site during the first charge and discharge process; by utilizing the redox behavior of additional anions, the reversible charge / discharge capacity of the material is increased, and the irreversible capacity loss is reduced; after testing, the battery assembled with the positive electrode sheet made of the obtained material has a reversible discharge specific capacity as high as 92.29 mAh / g at a 0.1C rate, an initial Coulomb efficiency as high as 88.04%, a hundred-cycle capacity retention rate as high as 94.97% at a 1C rate, a discharge specific capacity as high as 82.58 mAh / g at a 10C high rate, and a discharge specific capacity as high as 78.76 mAh / g at a 20C high rate; it shows that the present invention realizes the regulation of atomic lattice sites through the macroscopic morphology structure design and the in-situ co-doping of metal ions and anions from the microscopic perspective, comprehensively improves the cycle performance, and improves the rate performance of the material, and successfully prepares a sodium iron sulfate cathode material for high-power sodium ion batteries; (2) The raw material cost of the method of the present invention is extremely low, the synthesis method is simple, the production cycle is short, and it is suitable for large-scale continuous production. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is the SEM diagram of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention; Figure 2 It is the XRD diagram of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1; Figure 3 It is the EDS energy spectrum diagram of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention; Figure 4 It is the FTIR diagram of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1; Figure 5 It is the Raman diagram of the yolk-shell structured ion-doped sodium iron sulfate cathode material (a) in Example 1 of the present invention and Comparative Example 1 (b); Figure 6 It is the BET, isothermal adsorption and desorption curve (a) and pore size analysis diagram (b) of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1; Figure 7 It is the first charge and discharge curve diagram of the battery assembled with the positive electrode sheets made of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1, 2 in the voltage range of 2.0 - 4.5V and the current rate of 0.1 C (11 mA / g); Figure 8 It is the cycle curve diagram of the battery assembled with the positive electrode sheets made of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1 in the voltage range of 2.0 - 4.5V and the current rate of 1 C (1C = 110 mAh / g); Figure 9 It is the current rate curve diagram of the battery assembled with the positive electrode sheets made of the yolk-shell structured ion-doped sodium iron sulfate cathode material in Example 1 of the present invention and Comparative Example 1 in the voltage range of 2.0 - 4.5V; Figure 10 It is the SEM diagram of Comparative Example 1 of the present invention (wherein, the scale of (a) is 20 μm, and the scale of (b) is 10 μm). DETAILED DESCRIPTION OF THE INVENTION

[0046] The present invention will be further described below in conjunction with examples and drawings.

[0047] In the examples and comparative examples of the present invention, the solid content of the aqueous carbon nanotube conductive paste used is 5%, which is purchased from Xiamen Kaina Graphene Technology Co., Ltd.; the nitrogen used in the examples and comparative examples of the present invention is high-purity gas with a purity of ≥99.999%; in the examples and comparative examples of the present invention, through thermogravimetric analysis, it is obtained that the residual percentages after carbonization of ascorbic acid at 280 °C, 250 °C, and 220 °C are 44.57%, 64.48%, and 87.87% respectively; the raw materials or chemical reagents used in the examples and comparative examples of the present invention are obtained through conventional commercial channels without special instructions.

[0048] Example 1 of the egg yolk shell structure ion-doped sodium iron sulfate cathode material The egg yolk shell structure ion-doped sodium iron sulfate cathode material is composed of cations K + , Zn 2+ doped at the sodium ion sites of sodium iron sulfate, and the anion C 2 O 4 2- occupies part of the S-O tetrahedral positions to change the arrangement of some Fe-O octahedrons in sodium iron sulfate to form an egg yolk shell sphere structure carbon composite sodium iron sulfate cathode material. Among them, the outer shell and the inner core of the egg yolk shell sphere have uniform materials, and there are voids between the two. Carbon is evenly distributed and coated inside and outside the material; its chemical formula is: Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C; the average particle size of the egg yolk shell structure ion-doped sodium iron sulfate cathode material is 25 μm; in the egg yolk shell structure ion-doped sodium iron sulfate cathode material, the doping amounts of K + , Zn 2+ and C 2 O 4 2- are 1.59 at%, 0.996 at%, and 1.01 at% respectively, and the mass fraction of uniformly distributed and coated carbon is 3.018%; the initial specific surface area of the egg yolk shell structure ion-doped sodium iron sulfate cathode material is 11.97 m 2 / g.

[0049] As Figure 1 shown, the average particle size of the egg yolk shell structure ion-doped sodium iron sulfate cathode material in the example of the present invention is 25 μm, and its morphology has an obvious egg yolk shell sphere structure, and there are voids between the outer shell and the inner core.

[0050] As Figure 2As shown, the crystal plane diffraction peaks of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention can all correspond to the PDF#97-025-2403 standard card. Among them, at 15.52°, 22.74°, 28.71°, and 32.12°, they respectively correspond to the main crystal plane reflection peaks such as (200), (130), (-112), and (240), proving the successful synthesis of the sodium iron sulfate cathode material. At the same time, there are also partial crystal plane reflection peaks in the XRD diffraction pattern corresponding to the PDF standard cards of potassium sulfate and zinc sulfate, indicating that the material contains K and Zn doping elements.

[0051] As Figure 3 shown, in the material cross-section diagram of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention, it is found by EDS that the K and Zn elements are evenly distributed in the material, which not only proves that the K and Zn elements have been successfully doped into the material lattice, but also proves that the outer shell and inner core materials of the yolk-shell sphere are homogeneous.

[0052] As Figure 4 shown, in the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention, at a wave number of 591 cm -1 , 988 cm -1 and 1045 cm -1 , strong Fe-O vibration peaks, S-O symmetric stretching vibration peaks, and S=O asymmetric stretching vibration peaks are found, indicating the successful preparation of the sodium iron sulfate cathode material. At the same time, at a wave number of about 850 cm -1 , a weak and additional K-O vibration peak is found, proving the existence of trace K element doping.

[0053] As Figure 5 (a) shown, in the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention, at a wave number of 665 m -1 , an S=O asymmetric stretching reflection peak is found, indicating the successful preparation of the sodium iron sulfate cathode material. At the same time, at wave numbers of 628 cm -1 and 758 cm -1 , symmetric and asymmetric bending reflection peaks corresponding to the O-C-O part are found; at 960 cm -1 and 1496 cm -1 , symmetric and asymmetric stretching reflection peaks corresponding to the C-O bond are found; in summary, it indicates that the oxalate has been successfully doped into the material.

[0054] As Figure 6 (a), (b) shown, the initial specific surface area of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention is 11.97 m 2 / g, with an average pore diameter of 8.47 nm, indicating that the pores in the material mainly exist in the form of mesopores and have a larger initial specific surface area compared to the microsphere material; there is an obvious inflection point in the adsorption curve in the low-pressure range, judging that the material is of the mesoporous capillary condensation type; through the analysis of the desorption curve of the hysteresis loop, the material conforms to the H4 type, verifying that the material has no obvious saturated adsorption plateau, indicating that the pore structure is very irregular, with the occurrence of mixed adsorption of micropores and mesopores, and containing narrow slit pores, further proving the existence of the void structure in the yolk-shell structure.

[0055] Example 1 of the Preparation Method of Yolk-Shell Structure Ion-Doped Sodium Iron Sulfate Cathode Material (1) Add 6.96 g (0.049 mol) of anhydrous sodium sulfate, 18.91 g (0.068 mol) of ferrous sulfate heptahydrate, 0.14 g (0.0008 mol) of anhydrous potassium sulfate, 0.288 g (0.001 mol) of zinc sulfate heptahydrate, 0.16 g (0.0012 mol) of sodium oxalate, and 0.42 g of ascorbic acid to 50 mL of deionized water in sequence. Stir and dissolve for 20 min at 25 °C and a rotation speed of 400 rpm, then add 2.5 g of cyclodextrin and 7.3 g of aqueous carbon nanotube conductive paste. Stir and mix for 20 min at 25 °C and a rotation speed of 800 rpm, and then continuously stir the obtained feed solution at 800 rpm. Carry out spray drying at a blower frequency of 30 Hz, an inlet air temperature of 245 °C, and an outlet air temperature of 100 °C to obtain the yolk-shell structure ion-doped sodium iron sulfate cathode material precursor Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 ·xH 2 O / Vc / CNTs (x = 2, 4); (2) Place the yolk-shell structure ion-doped sodium iron sulfate cathode material precursor obtained in step (1) in a tube furnace. First, purge with a high-purity nitrogen atmosphere at room temperature for 2 h to ensure that the tube furnace is filled with a high-purity nitrogen atmosphere and other gases are removed. Then, under a high-purity nitrogen atmosphere, heat from room temperature to 280 °C at a rate of 3 °C / min and sinter for 12 h to obtain 18.30 g of the yolk-shell structure ion-doped sodium iron sulfate cathode material Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C.

[0056] Application Example 1 of the Yolk-Shell Structure Ion-Doped Sodium Iron Sulfate Cathode Material The positive electrode sheet made of the yolk-shell structure ion-doped sodium iron sulfate cathode material described in Example 1 of the present invention is used to assemble a sodium-ion battery.

[0057] Battery Assembly: Under the condition that the dew point is lower than -30 °C, 0.0800 g of the yolk-shell structure ion-doped sodium iron sulfate cathode material described in Example 1 of the present invention is weighed, 0.0100 g of conductive carbon black is added as a conductive agent and 0.0100 g of PVDF (polyvinylidene fluoride) is added as a binder. After mixing evenly, it is coated on aluminum foil to make a positive electrode sheet. In a vacuum glove box, a sodium metal sheet is used as the negative electrode, a Whatman GF / D glass fiber diaphragm is used as the battery separator, and the electrolyte is 1 mol / L NaClO 4 (EC:DMC = 1:1 (vol%) + 5% FEC), and a CR2032 coin cell is assembled.

[0058] Battery Performance Test: In the voltage range of 2 - 4.5 V, the above-assembled battery is subjected to electrochemical performance test and cycling performance test. Specifically, the initial charge-discharge specific capacity and initial Coulomb efficiency at 0.1C, the discharge specific capacity at 1C current rate, the capacity retention rate after 100 cycles, and the discharge specific capacity at different high current rates are tested.

[0059] As Figure 7 shown, for the sodium-ion battery assembled with the positive electrode sheet made of the yolk-shell structure ion-doped sodium iron sulfate cathode material described in Example 1 of the present invention, at 0.1C, the initial discharge specific capacity is 92.29 mAh / g, and the initial charge-discharge Coulomb efficiency is 88.04%. This indicates that Zn ion doping can occupy the Na1 site to prevent the irreversible migration of Fe 3+ during the initial charge-discharge process, and improve the discharge specific capacity of the material.

[0060] As Figure 8 shown, for the sodium-ion battery assembled with the positive electrode sheet made of the yolk-shell structure ion-doped sodium iron sulfate cathode material described in Example 1 of the present invention, at 1C rate (starting from the 4th cycle), the discharge specific capacity is 89.00 mAh / g. After 100 cycles, the discharge specific capacity is 84.52 mAh / g, and the capacity retention rate is 94.97%. This shows that the doping of K ions can effectively expand the ion channels and contribute to improving the cycling performance of the material.

[0061] As Figure 9As shown, the sodium-ion battery assembled with the positive electrode sheet made of the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping in Example 1 of the present invention has discharge specific capacities of 82.58 mAh / g and 78.76 mAh / g at 10C and 20C, respectively. This indicates that the doping of K ions broadens the sodium-ion transport channels, and the doping of Zn ions reduces the band gap of the material, enabling the material to have a higher Na + deintercalation rate and excellent rate performance; at the same time, it still maintains a similar discharge specific capacity under repeated rate tests, indicating that the material has excellent structural reversibility.

[0062] Example 2 of the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping The sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping is composed of cations K + , Mg 2+ doped at the sodium-ion sites of sodium iron sulfate, and anions C 2 O 4 2- occupying part of the S-O tetrahedral positions to change the arrangement of some Fe-O octahedrons in sodium iron sulfate to form a yolk-shell spherical structure carbon composite sodium iron sulfate positive electrode material. Among them, the outer shell and the inner core of the yolk-shell sphere have uniform materials, and there are voids between the two. Carbon is evenly distributed and coated inside and outside the material; its chemical formula is: Na 2.52 K 0.04 Mg 0.02 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C; the average particle size of the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping is 25 μm; in the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping, the doping amounts of K + , Zn 2+ and C 2 O 4 2- are 1.59 at%, 0.79 at%, and 1.01 at%, respectively, and the mass fraction of the uniformly distributed and coated carbon is 3.024%; the initial specific surface area of the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping is 11.90 m 2 / g.

[0063] After testing, the average particle size of the sodium iron sulfate positive electrode material with a yolk-shell structure and ion doping in the example of the present invention is 25 μm, and its morphology has an obvious yolk-shell spherical structure, and there are voids between the outer shell and the inner core.

[0064] After testing, the crystal plane diffraction peaks of the egg yolk shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention can all correspond to the PDF#97-025-2403 standard card, proving the successful synthesis of the sodium iron sulfate cathode material; at the same time, there are also partial crystal plane reflection peaks corresponding to the PDF standard cards of potassium sulfate and magnesium sulfate, indicating that the material contains K and Mg doping elements.

[0065] After testing, in the material cross-section diagram of the egg yolk shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention, it is found by EDS that the K and Mg elements are uniformly distributed in the material, which not only proves that the K and Mg elements have been successfully doped into the material lattice, but also proves that the outer shell and inner core materials of the egg yolk shell sphere are homogeneous.

[0066] After testing, for the egg yolk shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention, at the wave numbers of 630 cm -1 and 752 cm -1 , symmetric and asymmetric bending reflection peaks corresponding to the O-C-O part are found; at 965 cm -1 and 1494 cm -1 , symmetric and asymmetric stretching reflection peaks corresponding to the C-O bond are found; in summary, it indicates that the oxalate has been successfully doped into the material.

[0067] After testing, the initial specific surface area of the egg yolk shell structure ion-doped sodium iron sulfate cathode material in the embodiments of the present invention is 11.90 m 2 / g.

[0068] Example 2 of the preparation method of the egg yolk shell structure ion-doped sodium iron sulfate cathode material The difference between the embodiment of the present invention and Example 1 of the method is only that: in step (1), 6.96 g (0.049 mol) of anhydrous sodium sulfate and 0.288 g (0.001 mol) of zinc sulfate heptahydrate are respectively replaced with 6.99 g (0.0492 mol) of anhydrous sodium sulfate and 0.097 g (0.0008 mol) of anhydrous magnesium sulfate, and finally the precursor of the egg yolk shell structure ion-doped sodium iron sulfate cathode material Na 2.52 K 0.04 Mg 0.02 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 ·xH 2 O / Vc / CNTs (x = 2, 4) is obtained; in step (2), finally 18.26 g of the egg yolk shell structure ion-doped sodium iron sulfate cathode material Na 2.52 K 0.04 Mg 0.02 Fe 1.7(SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C. The same as in the method example 1.

[0069] Application Example 2 of the Yolk-Shell Structured Ion-Doped Sodium Iron Sulfate Cathode Material The positive electrode sheet made of the yolk-shell structured ion-doped sodium iron sulfate cathode material described in Example 2 of the present invention is used to assemble a sodium ion battery.

[0070] Battery assembly: The same as in Application Example 1.

[0071] Battery performance test: The same as in Application Example 1, and the test results are shown in Table 1.

[0072] Example 3 of the Yolk-Shell Structured Ion-Doped Sodium Iron Sulfate Cathode Material The yolk-shell structured ion-doped sodium iron sulfate cathode material is a yolk-shell spherical structure carbon composite sodium iron sulfate cathode material formed by doping cations Li + , Ca 2+ at the sodium ion sites of sodium iron sulfate, and anions C 2 O 4 2- occupying part of the S-O tetrahedral positions to change the arrangement of some Fe-O octahedrons in sodium iron sulfate. Among them, the outer shell and the inner core of the yolk-shell sphere have the same material, and there is a gap between the two. Carbon is evenly distributed and coated inside and outside the material; its chemical formula is: Na 2.54 Li 0.03 Ca 0.015 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C; the average particle size of the yolk-shell structured ion-doped sodium iron sulfate cathode material is 22 μm; in the yolk-shell structured ion-doped sodium iron sulfate cathode material, the doping amounts of Li + , Ca 2+ and C 2 O 4 2- are 1.18 at%, 0.59 at% and 1.01 at% respectively, and the mass fraction of the evenly distributed and coated carbon is 3.038%; the initial specific surface area of the yolk-shell structured ion-doped sodium iron sulfate cathode material is 11.86 m 2 / g.

[0073] After detection, the average particle size of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention is 22 μm, and its morphology has an obvious yolk-shell spherical structure, with a gap between the outer shell and the inner core.

[0074] After detection, the crystal plane diffraction peaks of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention can all correspond to the PDF#97-025-2403 standard card, proving the successful synthesis of the sodium iron sulfate cathode material; at the same time, there are also partial crystal plane reflection peaks corresponding to the PDF standard cards of lithium sulfate and calcium sulfate, indicating that the material contains Li and Ca doping elements.

[0075] After detection, in the material cross-section diagram of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention, it is found by EDS that the Li and Ca elements are evenly distributed in the material, which not only proves that the Li and Ca elements have been successfully doped into the material lattice, but also proves that the outer shell and the inner core of the yolk-shell sphere are made of the same material.

[0076] After detection, for the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention, at the wave numbers of 631 cm -1 and 750 cm -1 , symmetric and asymmetric bending reflection peaks corresponding to the O-C-O part are found; at 958 cm -1 and 1490 cm -1 , symmetric and asymmetric stretching reflection peaks corresponding to the C-O bond are found; in summary, it shows that the oxalate has been successfully doped into the material.

[0077] After detection, the initial specific surface area of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention is 11.86 m 2 / g.

[0078] Example 3 of the preparation method of the yolk-shell structure ion-doped sodium iron sulfate cathode material The difference between the embodiment of the present invention and Example 1 of the method is only that: in step (1), 6.96 g (0.049 mol) of anhydrous sodium sulfate, 0.14 g (0.0008 mol) of anhydrous potassium sulfate, 0.288 g (0.001 mol) of zinc sulfate heptahydrate, and 0.42 g of ascorbic acid are respectively replaced with 7.05 g (0.0496 mol) of anhydrous sodium sulfate, 0.08 g (0.0006 mol) of lithium sulfate monohydrate, 0.082 g (0.0006 mol) of anhydrous calcium sulfate, and 0.39 g of ascorbic acid, and 7.3 g of aqueous carbon nanotube conductive paste is replaced with 7.6 g of aqueous carbon nanotube conductive paste, and finally the yolk-shell structure ion-doped sodium iron sulfate cathode material precursor Na 2.54 Li 0.03 Ca 0.015 Fe 1.7 (SO4 ) 2.97 (C 2 O 4 ) 0.03 ·xH 2 O / Vc / CNTs (x = 2, 4); In step (2), finally, 18.23 g of the yolk-shell structured ion-doped sodium iron sulfate cathode material Na 2.54 Li 0.03 Ca 0.015 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C. The rest is the same as in Example 1 of the method.

[0079] Application Example 3 of the Yolk-Shell Structured Ion-Doped Sodium Iron Sulfate Cathode Material The positive electrode sheet made of the yolk-shell structured ion-doped sodium iron sulfate cathode material described in Example 3 of the present invention is used to assemble a sodium ion battery.

[0080] Battery assembly: The same as in Application Example 1.

[0081] Battery performance test: The same as in Application Example 1, and the test results are shown in Table 1.

[0082] Example 4 of the Yolk-Shell Structured Ion-Doped Sodium Iron Sulfate Cathode Material The yolk-shell structured ion-doped sodium iron sulfate cathode material is a yolk-shell sphere structured carbon composite sodium iron sulfate cathode material formed by doping cations K + , Zn 2+ into the sodium ion sites of sodium iron sulfate, and anions C 2 O 4 2- occupying part of the S-O tetrahedron positions to change the arrangement of some Fe-O octahedrons in sodium iron sulfate. Among them, the outer shell and the inner core of the yolk-shell sphere have the same material, and there is a gap between the two. Carbon is evenly distributed and coated inside and outside the material; its chemical formula is: Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.99 (C 2 O 4 ) 0.01 / C; The average particle size of the yolk-shell structured ion-doped sodium iron sulfate cathode material is 23 μm; in the yolk-shell structured ion-doped sodium iron sulfate cathode material, K + , Zn 2+ and C 2 O 42- The doping amounts are 1.59 at%, 0.996 at% and 0.33 at% respectively, and the mass fraction of uniformly distributed and coated carbon is 3.005%; the initial specific surface area of the yolk-shell structured ion-doped sodium iron sulfate cathode material is 11.92 m 2 / g.

[0083] It is detected that the average particle size of the yolk-shell structured ion-doped sodium iron sulfate cathode material in the embodiment of the present invention is 23 μm, and its morphology has an obvious yolk-shell spherical structure, and there are voids between the outer shell and the inner core.

[0084] It is detected that the crystal plane diffraction peaks of the yolk-shell structured ion-doped sodium iron sulfate cathode material in the embodiment of the present invention can all correspond to the PDF#97-025-2403 standard card, proving that the sodium iron sulfate cathode material is successfully synthesized; at the same time, there are also partial crystal plane reflection peaks corresponding to the PDF standard cards of potassium sulfate and zinc sulfate, indicating that the material contains K and Zn doping elements.

[0085] It is detected that in the material sectional view of the yolk-shell structured ion-doped sodium iron sulfate cathode material in the embodiment of the present invention, it is found by EDS that the K and Zn elements are uniformly distributed in the material, which not only proves that the K and Zn elements have been successfully doped into the material lattice, but also proves that the materials of the outer shell and the inner core of the yolk-shell sphere are homogeneous.

[0086] It is detected that for the yolk-shell structured ion-doped sodium iron sulfate cathode material in the embodiment of the present invention, at the wave numbers of 625 cm -1 and 763 cm -1 symmetric and asymmetric bending reflection peaks corresponding to the O-C-O part are found; at 962 cm -1 and 1491 cm -1 symmetric and asymmetric stretching reflection peaks corresponding to the C-O bond are found; in summary, it shows that the oxalate is successfully doped into the material.

[0087] It is detected that the initial specific surface area of the yolk-shell structured ion-doped sodium iron sulfate cathode material in the embodiment of the present invention is 11.92 m 2 / g.

[0088] Example 4 of the preparation method of the yolk-shell structured ion-doped sodium iron sulfate cathode material (1) 8.24 g (0.1004 mol) of sodium acetate, 18.91 g (0.068 mol) of ferrous sulfate heptahydrate, 0.162 g (0.0016 mol) of potassium nitrate, 0.19 g (0.001 mol) of zinc nitrate, 0.036 g (0.0004 mol) of oxalic acid, 0.0516 mol of dilute sulfuric acid solution, and 0.31 g of ascorbic acid were successively added to 50 mL of deionized water. At 25 °C and a rotation speed of 350 rpm, after stirring and dissolving for 22 min, 2.0 g of polyvinylpyrrolidone and 7.0 g of aqueous carbon nanotube conductive paste were added. At 25 °C and a rotation speed of 850 rpm, after stirring and mixing for 18 min, the obtained feed solution was continuously stirred at 850 rpm. Spray drying was carried out at a blower frequency of 30 Hz, an inlet air temperature of 225 °C, and an outlet air temperature of 100 °C to obtain a yolk-shell structured ion-doped sodium iron sulfate cathode material precursor Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.99 (C 2 O 4 ) 0.01 ·xH 2 O / Vc / CNTs (x = 2, 4); (2) The yolk-shell structured ion-doped sodium iron sulfate cathode material precursor obtained in step (1) was placed in a tubular furnace. First, at room temperature, it was purged with high-purity nitrogen gas for 2 h to ensure that the tubular furnace was filled with high-purity nitrogen gas and other gases were excluded. Then, under a high-purity nitrogen gas atmosphere, it was heated from room temperature to 250 °C at a rate of 5 °C / min and sintered for 10 h to obtain 18.30 g of yolk-shell structured ion-doped sodium iron sulfate cathode material Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.99 (C 2 O 4 ) 0.01 / C.

[0089] Application Example 4 of Yolk-Shell Structured Ion-Doped Sodium Iron Sulfate Cathode Material The positive electrode sheet made of the yolk-shell structured ion-doped sodium iron sulfate cathode material described in Example 4 of the present invention was used to assemble a sodium ion battery.

[0090] Battery Assembly: The same as Application Example 1.

[0091] Battery Performance Test: The same as Application Example 1, and the test results are shown in Table 1.

[0092] Example 5 of the Yolk-Shell Structure Ion-Doped Sodium Iron Sulfate Cathode Material The yolk-shell structure ion-doped sodium iron sulfate cathode material is composed of cations K + , Zn 2+ doped at the sodium ion sites of sodium iron sulfate, and anions C 2 O 4 2- occupying part of the S-O tetrahedral positions to change the arrangement of some Fe-O octahedrons in sodium iron sulfate, forming a yolk-shell sphere structure carbon composite sodium iron sulfate cathode material. Among them, the outer shell and the inner core of the yolk-shell sphere have uniform materials, and there are voids between the two. Carbon is uniformly distributed and coated inside and outside the material; its chemical formula is: Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C; The average particle size of the yolk-shell structure ion-doped sodium iron sulfate cathode material is 32 μm; in the yolk-shell structure ion-doped sodium iron sulfate cathode material, the doping amounts of K + , Zn 2+ and C 2 O 4 2- are 1.59 at%, 0.996 at% and 1.01 at% respectively, and the mass fraction of uniformly distributed and coated carbon is 3.004%; the initial specific surface area of the yolk-shell structure ion-doped sodium iron sulfate cathode material is 11.89 m 2 / g.

[0093] After testing, the average particle size of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention is 32 μm, and its morphology has an obvious yolk-shell sphere structure, and there are voids between the outer shell and the inner core.

[0094] After testing, the crystal plane diffraction peaks of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention can all correspond to the PDF#97-025-2403 standard card, proving the successful synthesis of the sodium iron sulfate cathode material; at the same time, there are also some crystal plane reflection peaks corresponding to the PDF standard cards of potassium sulfate and zinc sulfate, indicating that the material contains K and Zn doping elements.

[0095] After testing, it is found by EDS in the material cross-section diagram of the yolk-shell structure ion-doped sodium iron sulfate cathode material in the embodiment of the present invention that the K and Zn elements are uniformly distributed in the material, which not only proves that the K and Zn elements have been successfully doped into the material lattice, but also proves that the outer shell and the inner core of the yolk-shell sphere have uniform materials.

[0096] Upon detection, in the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping of the embodiment of the present invention, at a wave number of 645 cm -1 and 748 cm -1 , symmetric and asymmetric bending reflection peaks corresponding to the O-C-O part are found; at 965 cm -1 and 1494 cm -1 , symmetric and asymmetric stretching reflection peaks corresponding to the C-O bond are found; in summary, it shows that oxalate ions are successfully doped into the material interior.

[0097] Upon detection, the initial specific surface area of the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping of the embodiment of the present invention is 11.89 m 2 / g.

[0098] Example 5 of the preparation method of the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping The difference between the embodiment of the present invention and Example 1 of the method is only that: in step (1), 0.42 g of ascorbic acid is replaced with 0.21 g of ascorbic acid, and the process parameters of spray drying are: the frequency of the induced draft fan is 40 Hz, the inlet air temperature is 200 °C, and the outlet air temperature is 100 °C; in step (2), at a rate of 5 °C / min, it is heated from room temperature to 220 °C and sintered for 8 h, and finally 18.29 g of the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping Na 2.51 K 0.04 Zn 0.025 Fe 1.7 (SO 4 ) 2.97 (C 2 O 4 ) 0.03 / C is obtained. The rest is the same as Example 1 of the method.

[0099] Example 5 of the application of the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping The positive electrode sheet made of the sodium iron sulfate positive electrode material with yolk-shell structure and ion doping described in Example 5 of the present invention is used to assemble a sodium ion battery.

[0100] Battery assembly: The same as Example 1 of the application.

[0101] Battery performance test: The same as Example 1 of the application, and the test results are shown in Table 1.

[0102] Comparative Example 1 The difference between this comparative example and Method Example 1 is only as follows: In step (1), 7.39 g (0.052 mol) of anhydrous sodium sulfate, 18.92 g (0.068 mol) of ferrous sulfate heptahydrate, and 0.42 g of ascorbic acid were successively added to 50 mL of deionized water. After stirring and dissolving for 20 min at room temperature and a rotation speed of 400 rpm, 7.3 g of aqueous carbon nanotube conductive paste was added, followed by spray drying, and finally the inorganic carbon composite sodium iron sulfate cathode material precursor Na 2.6 Fe 1.7 (SO 4 ) 3 ·xH 2 O / Vc / CNTs (x = 2, 4) was obtained; in step (2), the microsphere-structured composite sodium iron sulfate cathode material Na 2.6 Fe 1.7 (SO 4 ) 3 / C was finally sintered. The rest was the same as Method Example 1.

[0103] As shown in Figure 10 (a) and (b), the average particle size of the microsphere-structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention is 25 μm. Although it has a similar particle size to the yolk-shell structure ion-doped sodium iron sulfate cathode material of Example 1 of the present invention, it can be seen from the figure that the material obtained in the comparative example of the present invention is a partially hollow and non-layered microsphere structure.

[0104] As shown in Figure 2 , the crystal plane diffraction peaks of the microsphere-structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention can all correspond to the PDF#97-025-2403 standard card. Among them, at 15.52°, 22.74°, 28.71° and 32.12° respectively correspond to the main crystal plane reflection peaks such as (200), (130), (-112) and (240), proving the successful synthesis of the sodium iron sulfate cathode material, and there are no other crystal plane reflection peaks in the XRD diffraction pattern.

[0105] As shown in Figure 4 , the microsphere-structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention only shows strong Fe-O vibration peaks, S-O symmetric stretching vibration peaks and S=O asymmetric stretching vibration peaks at wavenumbers of 591 cm -1 , 988 cm -1 and 1045 cm -1 , indicating the successful preparation of a pure-phase sodium iron sulfate cathode material and the absence of doping with other elements.

[0106] As shown in Figure 5 (b), the microsphere-structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention only shows at a wavenumber of 667 m -1On it, an asymmetric stretching reflection peak of S=O was found; in the wavenumber range of about 620 - 650 cm -1 , 740 - 770 cm -1 , 950 - 980 cm -1 and 1480 - 1490 cm -1 , no symmetric and asymmetric bending (stretching) reflection peaks of the O - C - O bond and C - O bond were found, indicating that no oxalate was doped into the interior of the material.

[0107] As Figure 6 (a), (b) shown, the initial specific surface area of the microsphere - structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention was only 6.46 m 2 / g, and the average pore diameter was only 7.97 nm, indicating that the material did not have a good specific surface area, and the pores mainly existed in the form of mesopores; there was an obvious inflection point in the adsorption curve in the low - pressure range, and it was judged that the material was of the central - control capillary condensation type; through the analysis of the desorption curve of the hysteresis loop, the material conforms to the H4 type, verifying that the material has no obvious saturated adsorption platform, indicating that the pore structure is very irregular, with a mixed adsorption of micropores and mesopores, and contains narrow slit pores; however, in the high - pressure range, the slope of the adsorption - desorption curve did not increase significantly and the proportion of the adsorption amount was not large, proving that there may be a small cavity part in the microsphere structure.

[0108] The positive - electrode sheet made of the microsphere - structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention was used to assemble a sodium - ion battery.

[0109] Battery assembly: The same as Application Example 1.

[0110] Battery performance test: The same as Application Example 1.

[0111] As Figure 7 shown, for the sodium - ion battery assembled with the positive - electrode sheet made of the microsphere - structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention, at 0.1C, the initial discharge specific capacity was 88.17 mAh / g, and the initial charge - discharge Coulomb efficiency was 90.77%.

[0112] As Figure 8 shown, for the sodium - ion battery assembled with the positive - electrode sheet made of the microsphere - structured composite sodium iron sulfate cathode material obtained in the comparative example of the present invention, at a 1C rate (starting from the 4th cycle), the discharge specific capacity was 85.91 mAh / g. After 100 cycles, the discharge specific capacity was only 79.5 mAh / g, and the capacity retention rate was only 92.54%.

[0113] As Figure 9As shown, for the sodium-ion battery assembled with the positive electrode sheet made of the microsphere structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention, the discharge specific capacities at 10C and 20C are only 70.3 mAh / g and 65.12 mAh / g respectively.

[0114] Comparative Example 2 The difference between this comparative example and Method Example 1 is only that: in step (1), 7.39 g (0.052 mol) of anhydrous sodium sulfate, 18.92 g (0.068 mol) of ferrous sulfate heptahydrate and 0.42 g of ascorbic acid are successively added to 50 mL of deionized water, and stirred and dissolved at room temperature and a rotation speed of 400 rpm for 20 min, then 2.5 g of cyclodextrin and 7.3 g of aqueous carbon nanotube conductive paste are added, and spray drying is carried out to finally obtain the yolk-shell structure composite sodium iron sulfate positive electrode material precursor Na 2.6 Fe 1.7 (SO 4 ) 3 ·xH 2 O / Vc / CNTs (x = 2, 4); in step (2), the yolk-shell structure composite sodium iron sulfate positive electrode material Na 2.6 Fe 1.7 (SO 4 ) 3 / C is finally sintered. The rest is the same as Method Example 1.

[0115] It is detected that the average particle size of the yolk-shell structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention is 23 μm, with an obvious yolk-shell spherical structure, and there are voids between the outer shell and the inner core.

[0116] It is detected that the crystal plane diffraction peaks of the yolk-shell structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention can all correspond to the PDF#97-025-2403 standard card, proving that the sodium iron sulfate positive electrode material is successfully synthesized, and there are no other crystal plane reflection peaks in the XRD diffraction pattern.

[0117] It is detected that in the material cross-section diagram of the yolk-shell structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention, it is found by EDS that the Na, Fe, and S elements in the material are evenly distributed, and the outer shell and the inner core materials of the yolk-shell sphere are homogeneous, and no other elements are detected.

[0118] It is detected that the initial specific surface area of the yolk-shell structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention is only 11.84 m 2 / g.

[0119] The positive electrode sheet made of the yolk-shell structure composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention is used to assemble a sodium-ion battery.

[0120] Battery assembly: same as Application Example 1.

[0121] Battery performance test: same as Application Example 1, and the test results are shown in Table 1.

[0122] Comparative Example 3 (1) 1.42 g (0.01 mol) of sodium sulfate, 4.73 g (0.017 mol) of ferrous sulfate heptahydrate, 0.402 g (0.003 mol) of sodium oxalate, and 0.42 g of ascorbic acid were successively added to 50 mL of deionized water. At room temperature and a rotation speed of 400 rpm, after stirring and dissolving for 20 min, 7.3 g of aqueous carbon nanotube conductive paste was added. At 25 °C and a rotation speed of 800 rpm, after stirring and mixing for 20 min, the obtained feed solution was continuously stirred at 800 rpm. Spray drying was carried out at a blower frequency of 30 Hz, an inlet air temperature of 180 °C, and an outlet air temperature of 80 °C to obtain the precursor of sodium iron sulfate cathode material with organic-inorganic carbon source composite, Na 2.6 Fe 1.7 (SO 4 ) 2.7 (C 2 O 4 ) 0.3 ·xH 2 O / Vc / CNTs (x = 2, 4); (2) The yolk-shell structure ion-doped sodium iron sulfate cathode material precursor obtained in step (1) was placed in a tube furnace. First, at room temperature, it was purged with high-purity nitrogen gas for 2 h to ensure that the tube furnace was filled with high-purity nitrogen gas and other gases were removed. Then, under a high-purity nitrogen gas atmosphere, it was heated from room temperature to 350 °C at a rate of 5 °C / min and sintered for 10 h to obtain the microsphere structure multi-carbon composite sodium iron sulfate cathode material Na 2.6 Fe 1.7 (SO 4 ) 2.7 (C 2 O 4 ) 0.3 / C.

[0123] It was detected that the microsphere structure multi-carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention had an average particle size of 24 μm and a microsphere structure with partial hollow and no layering.

[0124] It was detected that the crystal plane diffraction peaks of the microsphere structure multi-carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention could all correspond to the PDF#97-025-2403 standard card, proving that the sodium iron sulfate cathode material was successfully synthesized and there were no other crystal plane reflection peaks in the XRD diffraction pattern.

[0125] After detection, for the microsphere-structured multi-carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention, an S=O asymmetric stretching reflection peak was only found at a wave number of 637 m -1 ; within the wave number range of about 580 - 590 cm -1 , 760 - 770 cm -1 , 910 - 920 cm -1 and 1480 - 1490 cm -1 , symmetric and asymmetric bending (stretching) reflection peaks of the O - C - O bond and C - O bond were not found, indicating that no oxalate was doped into the interior of the material. This is because too high a sintering temperature will cause the decomposition of oxalate ions in the material into carbon dioxide and carbonate, thus seriously affecting the electrochemical performance of the material.

[0126] After detection, the initial specific surface area of the microsphere-structured multi-carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention was only 6.42 m 2 / g.

[0127] The positive electrode sheet made of the microsphere-structured multi-carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention was used to assemble a sodium-ion battery.

[0128] Battery assembly: The same as Application Example 1.

[0129] Battery performance test: The same as Application Example 1, and the test results are shown in Table 1.

[0130] Battery assembly: The same as Application Example 1.

[0131] Comparative Example 4 The difference between this comparative example and Method Example 1 is only that: in step (1), 7.39 g (0.052 mol) of anhydrous sodium sulfate, 16.68 g (0.06 mol) of ferrous sulfate heptahydrate, 1.35 g (0.008 mol) of manganese sulfate monohydrate and 0.42 g of ascorbic acid were successively added to 50 mL of deionized water, and stirred and dissolved for 20 min at room temperature and a rotation speed of 400 rpm, then 7.3 g of aqueous carbon nanotube conductive paste was added, and spray drying was carried out to finally obtain the precursor of the inorganic carbon composite sodium iron sulfate cathode material Na 2.6 Mn 0.2 Fe 1.5 (SO 4 ) 3 ·xH 2 O / Vc / CNTs (x = 2, 4); in step (2), the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material Na 2.6 Mn 0.2 Fe 1.5 (SO 4 ) 3 / C. The same as in Method Example 1.

[0132] After testing, the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of the present invention has a microsphere structure with an average particle size of 21 μm, partially hollow and without stratification.

[0133] After testing, the crystal plane diffraction peaks of the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of the present invention can all correspond to the PDF#97-025-2403 standard card, proving the successful synthesis of the sodium iron sulfate cathode material; at the same time, there are also partial crystal plane reflection peaks corresponding to the PDF standard card of manganese sulfate, indicating that the material contains Mn doping elements.

[0134] After testing, it is found by EDS in the material cross-section diagram of the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of the present invention that the elements Na, Fe, S, and Mn are evenly distributed in the material, indicating that the Mn element has been doped into the interior of the material.

[0135] After testing, the initial specific surface area of the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of the present invention is only 6.40 m 2 / g.

[0136] The positive electrode sheet made of the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of the present invention is used to assemble a sodium ion battery.

[0137] Battery assembly: The same as in Application Example 1.

[0138] Battery performance test: The same as in Application Example 1, and the test results are shown in Table 1.

[0139] Battery assembly: The same as in Application Example 1.

[0140] Comparative Example 5 The difference between this comparative example and Method Example 1 is only that in step (1), 0.33 g (0.002 mol) of sodium phosphate, 4.73 g (0.017 mol) of ferrous sulfate heptahydrate, 0.411 g (0.005 mol) of sodium acetate, 0.71 g (0.005 mol) of anhydrous sodium sulfate, and 0.42 g of ascorbic acid are successively added to 50 mL of deionized water, stirred and dissolved at room temperature and a rotation speed of 400 rpm for 20 min, and then 7.3 g of aqueous carbon nanotube conductive paste is added for spray drying, and finally the composite sodium iron sulfate cathode material precursor Na 2.6 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 0.2 ·xH 2O / Vc / CNTs (x = 2, 4); In step (2), the final sintering yields the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material Na 2.6 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 0.2 / C.

[0141] After testing, the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention has a microsphere structure with an average particle size of 22 μm and is partially hollow and without stratification.

[0142] After testing, the crystal plane diffraction peaks of the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention can all correspond to the PDF#97 - 025 - 2403 standard card, proving the successful synthesis of the sodium iron sulfate cathode material, and there are no other crystal plane reflection peaks in the XRD diffraction pattern.

[0143] After testing, the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention has a Raman reflection caused by the vibration of the PO -1 group at 984.3 cm 4 3- .

[0144] After testing, using EDS in the material cross-section diagram of the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention, it is found that the elements Na, Fe, S, and P in the material are evenly distributed, indicating that the P element has been doped into the material interior.

[0145] After testing, the initial specific surface area of the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention is only 6.45 m 2 / g.

[0146] The positive electrode sheet made of the microsphere-structured composite anion in-situ doped sodium iron sulfate cathode material obtained in the comparative example of the present invention is used to assemble a sodium-ion battery.

[0147] Battery assembly: The same as Application Example 1.

[0148] Battery performance test: The same as Application Example 1, and the test results are shown in Table 1.

[0149] Table 1 Comparison table of the electrical performance and cycling performance test results of the batteries assembled in Examples 1 - 5 and Comparative Examples 1 - 5 of the present invention

[0150] As can be seen from Table 1, the materials of Examples 1 to 5 of the present invention have a specific yolk-shell structure, which provides more reactive sites for electrochemical reactions; the yolk-shell structure alleviates the volume expansion and contraction caused by stress changes in the material during the charge and discharge cycle, and maintains good cycle stability; through the synergistic effect of in-situ co-doping of anions and cations, the alkali metal and transition metal elements in the material lattice are directionally regulated to occupy the Na2 and Na1 sites respectively, broaden the sodium ion migration channel, and inhibit the irreversible migration of trivalent iron ions at the Fe1 site during the first charge and discharge process; by utilizing the redox behavior of additional anions, the reversible charge / discharge capacity of the material is increased, and the irreversible capacity loss is reduced; after testing, the battery assembled with the positive electrode sheet made of the obtained material has a reversible discharge specific capacity of up to 92.29 at a rate of 0.1C. mAh / g, the first coulombic efficiency is as high as 88.04%, at 1C rate, the capacity retention rate of 100 cycles is as high as 94.97%, at 10C high rate, the discharge specific capacity is as high as 82.58mAh / g, at 20C high rate, the discharge specific capacity is as high as 78.76mAh / g; This shows that the present invention achieves atomic lattice site regulation through macroscopic morphology structure design and microscopic metal ion and anion co-doping, comprehensively improves the cycle performance, and improves the material rate performance, and successfully prepares high-power sodium ferric sulfate positive electrode material for sodium ion batteries.

[0151] As shown in Table 1, in Comparative Example 1, no other metal elements and oxalate doping were added, and no template agent was used. The obtained sodium iron sulfate positive electrode material had a microsphere structure. First, Zn was not introduced in Comparative Example 1. 2+ Occupies the Na1 site. During the first charge and discharge process of the material, after the sodium ions at the Na1 site are released, Fe 3+ The irreversible migration of Na1 sites leads to an irreversible decrease in the number of redox couples in the material. + The synergistic effect of doping results in the inability to widen the sodium ion migration channel in the direction of the Na2 site, and the material rate performance is further attenuated. Finally, the microsphere structure has a smaller initial specific surface area than the yolk-shell structure, and the Na + In summary, the electrochemical performance of comparative example 1 is lower than that of example 1 of the present invention in terms of rate and discharge specific capacity.

[0152] As shown in Table 1, in Comparative Example 2, an organic carbon source template was added to prepare yolk-shell structure microspheres, which increased the initial specific surface area of ​​the material, increased the contact area between the electrolyte and the electrode material, and increased the external Na + However, due to the low intrinsic ion diffusion rate inside the material, the larger initial specific surface area leads to the aggravation of the interface side reactions, resulting in a decrease in the first coulombic efficiency and cycle retention rate.

[0153] As can be seen from Table 1, in Comparative Example 3, only oxalate doping was added and the temperature conditions of the material during sintering were changed. Although the higher sintering temperature improved the crystallinity of the material, since the temperature range for the stable existence of oxalate was exceeded, the decomposition of oxalate produced carbonate, resulting in an uncontrollable change and collapse of the material structure, and ultimately leading to a significant reduction in the discharge specific capacity, cycle performance, and rate performance of the material.

[0154] As can be seen from Table 1, in Comparative Example 4, the transition metal Mn was doped into the Fe site. Due to the strong inductive effect of SO 4 2- , the redox potential of the Mn element was about 4.6V, and at the same time, the number of divalent iron redox couples decreased. Compared with the material of Example 1 of the present invention, the discharge specific capacity decreased significantly.

[0155] As can be seen from Table 1, in Comparative Example 5, trace amounts of phosphate doping were used instead of oxalate. Theoretically, PO 4 3- and SO 4 2- both exist in a three-dimensional tetrahedral structure in the sodium iron sulfate cathode material, and the P-O bond has a stronger binding energy, making the material structure more stable. However, after the oxalate enters the lattice, it exists in a two-dimensional planar arrangement, and the lattice of local regions of the material will be distorted, and the local charge density will also change. Therefore, the migration barrier of Na + is reduced, and the channels for sodium ion migration are increased, enabling the full play of the rate performance of the material. Therefore, doping with oxalate can obtain better high-rate performance than doping with phosphate.

Claims

1. A yolk-shell structure ion-doped sodium iron sulfate positive electrode material, characterized in that: It is a yolk-shell sphere structure carbon composite sodium ferric sulfate positive electrode material formed by doping cations A and B in the Na or Na / Fe ion sites of sodium ferric sulfate, and anions occupying part of the SO tetrahedron position to change the arrangement of the Fe-O octahedron of part of the sodium ferric sulfate. The outer shell and the inner core of the yolk-shell sphere are made of uniform materials, and there is a gap between the two. The carbon is evenly distributed and coated inside and outside the material. Its chemical formula is: Na 2.6-x- 2y A x B y Fe 1.7 (SO4) 3-z (C2O4) z / C, wherein A includes alkali metal elements, B includes alkaline earth metals and / or transition metal elements, 0<x≤0.05, 0<y≤0.03, 0<z≤0.05, and the law of conservation of charge is satisfied.

2. The yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to claim 1, characterized in that: The average particle size of the yolk-shell structure ion-doped sodium iron sulfate positive electrode material is 5 to 35 μm; in the yolk-shell structure ion-doped sodium iron sulfate positive electrode material, the doping amounts of cations A and B are 0.01 to 2.00 at and 0.01 to 1.20 at, respectively, and the doping amount of oxalate ions is 0.01 to 1.60 at; in the yolk-shell structure ion-doped sodium iron sulfate positive electrode material, the mass fraction of uniformly distributed and coated carbon is 0.3 to 4.0%; the initial specific surface area of ​​the yolk-shell structure ion-doped sodium iron sulfate positive electrode material is 10 to 30 m 2 / g; the alkali metal elements include K and / or Li; the alkaline earth metal elements include Mg and / or Ca; the transition metal elements include Zn and / or Cu.

3. A method for preparing the yolk-shell structure ion-doped sodium iron sulfate positive electrode material as claimed in claim 1 or 2, characterized in that: The following steps are involved: (1) A sodium source, a ferrous source, a sulfuric acid source, a cationic dopant, an oxalate anionic dopant and an organic acid antioxidant are sequentially added into water, stirred and dissolved, and then an organic carbon source template and an inorganic carbon source conductive slurry are added, stirred and mixed, and then spray-dried to obtain an egg yolk-shell structure ion-doped sodium iron sulfate positive electrode material precursor; (2) Sintering the yolk-shell structure ion-doped sodium ferric sulfate cathode material precursor obtained in step (1) under a protective atmosphere to obtain a yolk-shell structure ion-doped sodium ferric sulfate cathode material.

4. The method for preparing the yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to claim 3, characterized in that: In step (1), the molar ratio of the sodium element in the sodium source, the ferrous element in the ferrous source, the sulfate in the sulfate source, the A and B cations in the cationic dopant, and the oxalate in the oxalate anion dopant matches the molar ratio of the corresponding substances in the chemical formula; the amount of the organic acid antioxidant is such that the mass fraction of its carbonized mass in the yolk-shell structure ion-doped sodium ferric sulfate positive electrode material is 0.1 to 1.5%; the amount of water is such that the ferrous ion concentration is 0.8 to 1.8 mol / L; the temperature of the stirring and dissolving is 25 to 30° C., the rotation speed is 300 to 500 rpm, and the time is 15 to 25 min.

5. The method for preparing the yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to claim 3 or 4, characterized in that: In step (1), 20 to 80 g of the organic carbon source template is added to each liter of water; the solid content of the inorganic carbon source conductive slurry is 3 to 10%; the amount of the inorganic carbon source conductive slurry is such that the mass fraction of the inorganic carbon source in the yolk-shell structure ion-doped sodium iron sulfate positive electrode material is 0.1 to 2.2%; the stirring and mixing temperature is 25 to 30° C., the rotation speed is 700 to 900 rpm, and the stirring time is 15 to 25 min; while the spray drying feed is being fed, the feed solution is continuously stirred at 700 to 900 rpm; the process parameters of the spray drying are: the induced draft fan frequency is 20 to 40 Hz, the air inlet temperature is 200 to 300° C., and the air outlet temperature is 90 to 130° C.

6. The method for preparing the yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to any one of claims 3 to 5, characterized in that: In step (1), the sodium source includes sodium sulfate, sodium acetate, sodium formate or sodium nitrate, and one or more of their hydrates; the ferrous source includes ferrous sulfate and / or ferrous nitrate, and one or more of their hydrates; the sulfuric acid source includes sulfuric acid, and one or more of ammonium sulfate or sodium sulfate and their hydrates; the cationic dopant includes sulfates, nitrates or chlorides of alkali metal elements, alkaline earth metals or transition metal elements, and one or more of their hydrates; the oxalate anion dopant includes oxalic acid, and one or more of sodium oxalate, potassium oxalate or zinc oxalate and their hydrates; the organic acid antioxidant includes one or more of citric acid, tannic acid or ascorbic acid; the organic carbon source template includes one or more of cyclodextrin, starch, polydextrose or polyvinyl pyrrolidone; the inorganic carbon source includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P, acetylene black or graphene.

7. The method for preparing the yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to any one of claims 3 to 6, characterized in that: In step (2), before sintering, the sintered product is first ventilated with a protective atmosphere at room temperature for 1.5 to 2.5 hours to ensure that the sintering system is filled with the protective atmosphere and other gases are excluded; the sintering refers to: heating from room temperature to 220 to 300° C. at a rate of 1 to 5° C. / min, and sintering for 8 to 24 hours; the protective atmosphere includes one or more of nitrogen, argon, and a mixture of hydrogen and argon or hydrogen and nitrogen.

8. An application of the yolk-shell structure ion-doped sodium iron sulfate positive electrode material as claimed in claim 1 or 2, characterized in that: The positive electrode sheet made of the egg yolk-shell structure ion-doped sodium iron sulfate positive electrode material according to claim 1 or 2 is used to assemble a sodium ion battery.

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