Iron site doped modified sodium ferric sulfate positive electrode material and preparation method and application thereof

Abstract

The application relates to the technical field of positive electrode materials, in particular to a kind of iron site doped modified sodium ferrite sulfate positive electrode material and its preparation method and application.Preparation method: step S1: according to Na4Fe 3‑ y M1 a M2 b M3 c M4 d M5 e The stoichiometric ratio of (SO4)5 is weighed; graphene is weighed, all raw materials are mixed and dry ball milling treatment is carried out to obtain uniformly mixed powder; the obtained mixed powder is calcined under inert atmosphere; M1, M2, M3, M4 and M5 are Mg, Co, Ni, Mn and Zn or Mg, Co, Ni, Mn and Cu or Mg, Co, Ni, Mn and Ca.The application realizes the synchronous improvement of rate performance and cycle stability by stabilizing the lattice through Mg, improving electronic conductivity through Co / Ni, widening the ion channel through Mn, producing a synergistic effect by matching Zn, Cu or Ca and combining the conductive network of graphene.

Description

Technical Field

[0001] This invention relates to the field of cathode material technology, and in particular to an iron-doped modified sodium ferric sulfate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries have broad application prospects in large-scale energy storage due to their abundant resources, low cost, and good safety. The cathode material is crucial in determining battery performance. Among various cathode materials, polyanionic sodium iron sulfate stands out due to its stable structure and high operating voltage (approximately 3.8 V vs. Na). + Sodium ferric sulfate, with the chemical formula Na4Fe3(SO4)5, possesses advantages such as good thermal stability and low raw material cost, attracting considerable attention. Among them, alluaudite-type sodium ferric sulfate has a three-dimensional open framework structure, a high theoretical specific capacity (approximately 120 mAh / g), and depends solely on Fe²⁺. + / Fe³ + Redox pairs are considered to be highly promising cathode materials for sodium-ion batteries.

[0003] However, Na4Fe3(SO4)5 materials suffer from low electronic conductivity, small sodium ion diffusion coefficient, and poor air stability, which severely limit their rate performance and cycle life. To overcome these shortcomings, researchers have explored modification strategies such as elemental doping, carbon coating, and morphology control. Among these, doping with heterovalent or equivalent metal ions at iron sites is an effective means of controlling electronic structure, broadening sodium ion migration channels, and improving structural stability. Previous literature has reported single Mg²⁺... + Doping, Co² + / Ni² + The performance of sodium ferric sulfate is improved by dual doping or a small amount of three-element doping. Simultaneously, carbon coating (such as graphene or carbon nanotubes) can further enhance the surface electronic conductivity of the material. However, when only one or two types of dopants are used, the modification effect is limited, making it difficult to simultaneously optimize electronic conductivity, ion diffusion, and structural stability. Moreover, as the number of dopants increases, the risks of lattice distortion and impurity phase formation due to differences in ionic radius, valence state, and electronegativity rise sharply; therefore, "more dopants dopants do not necessarily mean better performance." Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings of the prior art by proposing an iron-doped modified sodium ferric sulfate cathode material, its preparation method, and its applications.

[0005] The first objective of this invention is to provide a method for preparing an iron-doped modified sodium ferric sulfate cathode material, comprising the following steps: Step S1: According to the general chemical formula Na4Fe 3-y M1a M2 b M3 c M4 d M5 e The stoichiometric ratio of sodium source, iron source, sulfate source and five doping element sources of (SO4)5 is weighed out. The sodium source, iron source, sulfate source and doping element sources are all sulfates. Graphene is weighed out separately. All raw materials are mixed and subjected to dry ball milling to obtain a uniform mixed powder. The obtained mixed powder was subjected to a two-stage calcination treatment under an inert atmosphere: first, it was pre-calcined at 200-300℃ for 2-4 hours, and then calcined at 350-450℃ for 6-15 hours to obtain the iron-doped modified sodium iron sulfate cathode material. Where y = 0.3, and a = b = c = d = e = 0.06; The combination of M1, M2, M3, M4, and M5 is one of the following combinations: Mg, Co, Ni, Mn, Zn, Mg, Co, Ni, Mn, Cu, Mg, Co, Ni, Mn, Ca; The amount of graphene added is 6-10 wt. of the mass of the generated iron-doped modified sodium ferric sulfate cathode material.

[0006] Furthermore, the amount of graphene added is 8 wt.% of the mass of the generated iron-doped modified sodium ferric sulfate cathode material.

[0007] Furthermore, no solvents or dispersants are added during the dry ball milling process.

[0008] Furthermore, the iron source is ferrous sulfate.

[0009] Furthermore, the inert atmosphere is nitrogen, argon, or helium, and the gas flow rate is 0.1 ~ 1 L / min.

[0010] Furthermore, in the two-stage calcination, the heating rate is 2 ~ 10℃ / min.

[0011] Furthermore, in the dry ball milling process, the ball-to-material ratio is (10~30):1, the ball milling speed is 300~600 rpm, and the milling time is 6~24 hours. The grinding jar and grinding balls can be made of zirconium oxide or agate.

[0012] A second objective of this invention is to provide an iron-doped modified sodium ferric sulfate cathode material prepared by the method described above.

[0013] A third objective of the present invention is to provide a positive electrode sheet comprising a positive current collector and a coating layer, wherein the coating layer comprises the aforementioned positive electrode material, conductive agent, and binder.

[0014] A fourth objective of the present invention is to provide a sodium-ion battery comprising the above-described positive electrode, separator, electrolyte, and negative electrode.

[0015] The synergistic mechanism of this invention: Magnesium ions (Mg²⁺) + It has a small ionic radius and low electronegativity, which can stabilize the Alluaudite framework structure, inhibit the migration and dissolution of iron ions during charging and discharging, reduce the surface activity of the material, and improve air stability. Cobalt ions (Co²) + ) and nickel ions (Ni²) + By involving d-orbital electrons, the hybridization of the third-layer d orbital in iron atoms with the second-layer p orbital in oxygen atoms is optimized, thereby improving the intrinsic electronic conductivity of the material and thus improving rate performance. Manganese ions (Mn²) + It can effectively reduce the diffusion barrier of sodium ions, broaden the migration channels of sodium ions, and significantly improve ionic conductivity. Copper ions (Cu²⁺) + ) has stable d¹ 0 Electronic configuration further stabilizes the crystal structure without destroying lattice symmetry and helps to improve electrode compaction density; Zinc ions (Zn) 2+ It suppresses the Jiang Taylor effect, stabilizes the structure, and improves conductivity, but excessive amounts will reduce capacity; Calcium ions (Ca 2+ It plays a supporting role, widens sodium ion channels, and inhibits oxygen loss under high pressure; Graphene is used to construct a three-dimensional conductive network, which significantly reduces electrode polarization and improves the ability to charge and discharge high currents.

[0016] The four core elements Mg, Co, Ni, and Mn, combined with Zn, Cu, or Ca, can produce a synergistic effect. All five ions are indispensable, and any substitution or absence of any one element will lead to a significant decrease in performance.

[0017] This invention systematically screened 462 pentagonal combinations of the above 11 elements and found that only three combinations, Mg-Co-Ni-Mn-Zn, Mg-Co-Ni-Mn-Cu, and Mg-Co-Ni-Mn-Ca, can produce significant synergistic effects. That is, only a very few specific combinations can synergistically improve electronic conductivity, ion diffusion coefficient and structural stability, and further improve conductivity by combining graphene carbon coating, thereby obtaining excellent electrochemical performance; among them, the Mg-Co-Ni-Mn-Cu combination has the best performance.

[0018] This invention achieves simultaneous improvements in rate performance and cycle stability by using Mg to stabilize the crystal lattice, Co / Ni to enhance electronic conductivity, Mn to broaden ion channels, and Zn, Cu, or Ca to optimize the structure, combined with a graphene conductive network. Compared to existing technologies, this invention overcomes the limitations of blindly using multiple dopants, providing a clear and efficient combination, and possesses outstanding substantive features and significant progress.

[0019] This invention, for the first time, uses Na4Fe3(SO4)5 as the target material and systematically screens out only three specific pentagonal combinations that can produce synergistic effects from 11 common doping elements (462 pentagonal combinations). This breaks the technical prejudice in the field that "any combination of multi-element doping is effective" or "the more doping elements, the better the performance," and points out a clear direction for the efficient doping modification of sodium iron sulfate cathode materials.

[0020] The specific pentagonal combination provided by this invention achieves multi-dimensional synergistic enhancement in electronic conductivity, ion diffusion, structural stability, and interface stability. Simultaneously, the carbon coating of graphene further constructs a three-dimensional conductive network, and the high conductivity of graphene can further enhance the electron transport capability of the electrode, significantly improving high-current charge-discharge performance. This, combined with multi-element ion doping, forms a dual modification effect of "bulk doping + surface coating."

[0021] The preparation method of this invention uses all-sulfate raw materials and a dry ball milling process, without introducing any solvents or dispersants. The process is simple, environmentally friendly, and low-cost, making it suitable for large-scale industrial production. Simultaneously, the two-stage low-temperature calcination effectively avoids Fe²⁺. + Oxidation ensures the electrochemical activity of the material. Attached Figure Description

[0022] Figure 1 shows the X-ray diffraction (XRD) patterns of the cathode materials prepared in the embodiments and comparative examples of the present invention; Figure 2 is a scanning electron microscope (SEM) image of the cathode material prepared in Example 1 of the present invention; Figure 3 shows a comparison of the discharge specific capacity of cathode materials prepared with different pentagonal combinations (Examples 1-3 and Comparative Examples 5-12) at 0.5C-10C. Figure 4 This is a comparison chart of short cycles at 5C for Examples 1, 3, and 2. Figure 5 This is a long-cycle diagram of Example 1 under an ultra-high current density of 100C; Figure 6 This is a graph showing the rate performance of Example 1 at -25 degrees Celsius; Figure 7 This is a graph showing the rate performance of Example 1 at a high temperature of 50 degrees Celsius; Figure 8 This is a graph showing the cycling performance of Example 1 at a high temperature of 50 degrees Celsius and a current density of 50°C. Figure 9 This is a comparison diagram of the in-situ impedance of Example 1 and Comparative Example 1. Detailed Implementation

[0023] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings to further illustrate the technical solutions of the present invention. However, the present invention is not limited to these embodiments.

[0024] Both the examples and comparative examples were prepared according to the following preparation method: According to the general chemical formula Na₄Fe 3-y M1 a M2 b M3 c M4 d M5 e Based on the stoichiometric ratio of (SO4)5 (y=0.3, and a=b=c=d=e=0.06), weigh out sodium sulfate (Na2SO4), ferrous sulfate (FeSO4·7H2O), and the sulfates corresponding to the five doping elements (such as magnesium sulfate MgSO4, cobalt sulfate CoSO4, nickel sulfate NiSO4, manganese sulfate MnSO4, copper sulfate CuSO4, etc.). Based on the final total mass of the cathode material, additionally weigh out 8 wt.% of graphene.

[0025] All the above raw materials (including graphene) were mixed and placed in a zirconia ball mill jar. Zirconia grinding balls were added at a ball-to-material ratio of 20:1. The mixture was then dry-milled at 500 rpm for 12 hours in a planetary ball mill to obtain a uniformly mixed powder. No solvents or dispersants were added during the ball milling process.

[0026] The mixed powder was placed in a tube furnace and calcined in two stages under a nitrogen atmosphere (gas flow rate 0.5 L / min): first, the temperature was increased to 280℃ at 5℃ / min and pre-calcined for 3 hours; then, the temperature was increased to 400℃ at 5℃ / min and calcined for 10 hours; after natural cooling, the target product was obtained, with graphene uniformly coated on its surface.

[0027] Example 1 Doping element combination: Mg, Co, Ni, Mn, Cu.

[0028] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Mn0. 06 Cu0. 06 (SO4)5+ 8 wt.% Graphene.

[0029] Example 2 Doping element combination: Mg, Co, Ni, Mn, Zn.

[0030] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Mn0. 06 Zn0. 06 (SO4)5+ 8 wt.% Graphene.

[0031] Example 3 Doping element combination: Mg, Co, Ni, Mn, Ca.

[0032] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Mn0. 06 Ca0. 06 (SO4)5+ 8 wt.% Graphene.

[0033] Comparative Example 1 (Undoped) According to the stoichiometric ratio of the chemical formula Na4Fe3(SO4)5, only sodium sulfate and ferrous sulfate are weighed, and 8 wt.% of graphene is added. The remaining steps are the same as the basic preparation method to prepare undoped sodium ferric sulfate cathode material containing graphene.

[0034] Comparative Example 2 (Single Element Doping) According to the stoichiometric ratio of the chemical formula Na4Fe2.7Mg0.3(SO4)5, sodium sulfate, ferrous sulfate and magnesium sulfate were weighed, and 8 wt.% graphene was added. The remaining steps were the same as the basic preparation method to prepare magnesium-doped sodium ferric sulfate cathode material.

[0035] Comparative Example 3 (Dual-element doping) According to the chemical formula Na₄Fe₂.₇Co₀. 15 Ni0. 15 By weighing sodium sulfate, ferrous sulfate, cobalt sulfate, and nickel sulfate according to the stoichiometric ratio of (SO4)5, and adding 8 wt.% graphene, the remaining steps are the same as the basic preparation method to prepare cobalt-nickel dual-doped sodium ferric sulfate cathode material.

[0036] Comparative Example 4 (Three-element doping) According to the stoichiometric ratio of the chemical formula Na4Fe2.7Mg0.1Co0.1Ni0.1(SO4)5, sodium sulfate, ferrous sulfate, magnesium sulfate, cobalt sulfate and nickel sulfate were weighed, and 8 wt.% graphene was added. The remaining steps were the same as the basic preparation method to prepare ternary doped sodium ferric sulfate cathode material.

[0037] Comparative Example 5 (pentavalent combination, excluding Mg) Doping element combination: Co, Ni, Mn, Zn, Cu.

[0038] Chemical formula of the product: Na₄Fe₂.₇Co₀. 06 Ni0. 06 Mn0. 06 Zn0. 06 Cu0. 06 (SO4)5+ 8 wt.% Graphene.

[0039] Comparative Example 6 (pentavalent combination, excluding Co) Doping element combination: Mg, Ni, Mn, Zn, Ca.

[0040] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Ni0. 06 Mn0. 06 Zn0. 06 Ca0. 06 (SO4)5+ 8 wt.% Graphene.

[0041] Comparative Example 7 (Pentavalent combination, excluding Ni) Doping element combination: Mg, Co, Mn, Zn, Sr.

[0042] Chemical formula of the product: Na₄Fe₂.₇Mg₀.06 Co0. 06 Mn0. 06 Zn0. 06 Sr0. 06 (SO4)5+ 8 wt.% Graphene.

[0043] Comparative Example 8 (pentavalent combination, containing Li and K, but excluding Mn) Doping element combination: Mg, Co, Ni, Li, K.

[0044] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Li0. 06 K0. 06 (SO4)5+ 8 wt.% Graphene.

[0045] Comparative Example 9 (pentavalent combination, containing Cr and Sr) Doping element combination: Mg, Co, Ni, Cr, Sr.

[0046] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Cr0. 06 Sr0. 06 (SO4)5+ 8 wt.% Graphene.

[0047] Comparative Example 10 (a pentagonal combination containing Cu and Ca, but without Mn) Doping element combination: Mg, Co, Ni, Cu, Ca.

[0048] Chemical formula of the product: Na₄Fe₂.₇Mg₀. 06 Co0. 06 Ni0. 06 Cu0. 06 Ca0. 06 (SO4)5+ 8 wt.% Graphene.

[0049] Comparative Example 11 (Five-element combination, y=0.1) Doping element combination: Mg, Co, Ni, Mn, Cu.

[0050] Product chemical formula: Na₄Fe 2.9 Mg 0.02 Co 0.02 Ni 0.02 Mn 0.02 Cu 0.02(SO4)5+ 8 wt.% Graphene.

[0051] Comparative Example 12 (Five-element combination, y=0.5) Doping element combination: Mg, Co, Ni, Mn, Cu.

[0052] Product chemical formula: Na₄Fe 2.5 Mg 0.1 Co 0.1 Ni 0.1 Mn 0.01 Cu 0.1 (SO4)5+ 8 wt.% Graphene.

[0053] Figure 1 shows the X-ray diffraction (XRD) patterns of the cathode materials prepared in the embodiments and comparative examples of the present invention. As can be seen from the figure, the XRD patterns of Examples 1, 2, 3, Comparative Examples 3, 4, and 5 are basically consistent with those of the unmodified Comparative Example 1, indicating that different doping methods did not destroy the original crystal structure of the material.

[0054] Figure 2 is a scanning electron microscope (SEM) image of the cathode material prepared in Example 1 of the present invention, showing uniform graphene coating.

[0055] Electrochemical performance testing The positive electrode materials prepared in Examples 1-3 and Comparative Examples 1-10 were mixed with conductive carbon black and polyvinylidene fluoride binder at a mass ratio of 7:2:1 (Note: the materials themselves already contain graphene; the additional conductive carbon black is a conventional component in electrode preparation). N-methylpyrrolidone was added and ground to form a slurry, which was then coated onto aluminum foil. After vacuum drying and punching, the positive electrode sheet was obtained. Using a sodium metal sheet as the counter electrode, glass fiber as the separator, and a 1.0 mol / L NaPF6 solution of ethylene carbonate / dimethyl carbonate (volume ratio 1:1) as the electrolyte, CR2032 coin cells were assembled in an argon glove box. Constant current charge-discharge tests were performed at room temperature using a Blue Battery testing system, with a voltage range of 2 ~ 4.5 V (vs. Na). + / Na). The test results are shown in Table 1 and Figure 3 As shown.

[0056] Table 1 Comparison of electrochemical performance of different samples

[0057] Results Analysis As shown in Table 1: 1. Undoped Na4Fe3(SO4)5 (Comparative Example 1) has a specific capacity of 43.58 mAh / g at 0.5 C and 26.45 mAh / g at 5 C.

[0058] 2. Single-element, dual-element, or tri-element doping (Comparative Examples 2-4) showed improvements compared to undoped, but were still significantly lower than in Examples 1-3. For example, the optimal tri-element doping (Comparative Example 4) had a specific capacity of 95.44 mAh / g at 0.5 C and 88 mAh / g at 5 C.

[0059] 3. Non-preferred pentagonal combinations (Comparative Examples 5-10) exhibited significant performance differences. Combinations lacking Mg (Comparative Example 5), Co (Comparative Example 6), or Ni (Comparative Example 7), or those incorporating Li / K (Comparative Example 8) or Cr / Sr (Comparative Example 9), showed performance even lower than some ternary doped samples, indicating that inappropriate pentagonal combinations can have negative effects. Even with Mg, Co, and Ni present, performance decreased significantly when Mn was replaced with elements other than Cu and Ca (e.g., Sr replaced Mn in Comparative Example 7, and no Mn in Comparative Example 10). For example, Comparative Example 10 (Mg, Co, Ni, Cu, Ca), although containing four core elements, lacked Mn, resulting in a 5C specific capacity of only 75.24 mAh / g, significantly lower than the 91.93 mAh / g of Example 1.

[0060] 4. Examples 1-3 all contain four core elements: Mg, Co, Ni, and Mn, and are respectively paired with Zn, Cu, and Ca. Their electrochemical performance is significantly better than all comparative examples. Among them, Example 1 (Mg-Co-Ni-Mn-Cu) has the highest performance in all three indicators, with a specific capacity of 102.75 mAh / g at 0.5 C and 91.38 mAh / g at 5 C. This indicates that Mg, Co, Ni, and Mn are four essential elements for synergistic enhancement, while Cu is the optimal pairing element. This is significantly superior to undoped Na4Fe3(SO4)5 materials, single-element doped materials, and non-preferred pentagonal combinations.

[0061] Figure 3 shows a comparison of the discharge specific capacity of cathode materials prepared with different pentagonal combinations (Examples 1-3 and Comparative Examples 5-10) at 0.5C-100C. Figure 3It can be seen that Example 1 exhibits the best rate performance, maintaining a specific capacity of nearly 90 mAh / g at 10C. Examples 2 and 3 show almost identical rate performance and discharge specific capacity. Comparative Example 1 exhibits the worst rate performance and discharge specific capacity, with a discharge specific capacity of only 40 mAh / g at 0.5C. Furthermore, Example 1 demonstrates excellent rate performance, maintaining a discharge specific capacity of 60 mAh / g even at an ultra-high rate of 100C.

[0062] Figure 4 This is a comparison graph of short cycles at 5C for Examples 1, 3, and Comparative Example 2. From... Figure 4 It can be seen that in Example 1, the capacity hardly decreased after nearly 200 cycles, while in Comparative Example 2, the capacity fluctuated after 100 cycles, indicating that the material structure was unstable.

[0063] Figure 5 This is a long-cycle diagram of Example 1 under an ultra-high current density of 100C. From Figure 5 It can be seen that Example 1 retains 82% of its capacity after 20,000 cycles at an ultra-high current density of 100C, indicating that Example 1 with the optimal element combination has excellent structural stability and cycling performance.

[0064] Figure 6 This is a rate performance graph for Example 1 at -25 degrees Celsius. From... Figure 6 It can be seen that at -25 degrees Celsius, the 0.1C discharge specific capacity is 90 mAh / g, which is nearly 90% of the discharge specific capacity at room temperature (25℃).

[0065] Figure 7 This is a rate performance graph for Example 1 at a high temperature of 50 degrees Celsius. From... Figure 7 As can be seen, at a high temperature of 50 degrees Celsius, Example 1 still has a discharge specific capacity of nearly 75 mAh / g even at an ultra-high rate of 50°C.

[0066] Figure 8 This is a graph showing the cycling performance of Example 1 at a high temperature of 50 degrees Celsius and a current density of 50°C. From... Figure 8 It can be seen that the capacity retention rate is still 81.1% after 1200 cycles at 50°C.

[0067] Figure 9 This is a comparison diagram of the in-situ impedance of Example 1 and Comparative Example 1. From... Figure 9 It can be seen that Example 1 has a smaller impedance.

[0068] Through systematic screening of 462 pentagonal combinations, this invention discovered that only combinations containing four core elements—Mg, Co, Ni, and Mn—and paired with Zn, Cu, or Ca can produce significant synergistic effects. Among these, the Mg-Co-Ni-Mn-Cu combination exhibits the best performance and unexpected technical benefits. The preparation process of dry ball milling with 8 wt.% graphene carbon coating using all-sulfate alloys is simple, environmentally friendly, and easy to scale up. The resulting material demonstrates excellent rate performance and cycle stability in sodium-ion batteries.

[0069] For any points not covered above, existing technologies shall apply.

[0070] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an iron-doped modified sodium ferric sulfate cathode material, characterized in that, Includes the following steps: Step S1: According to the general chemical formula Na4Fe 3-y M1 a M2 b M3 c M4 d M5 e The stoichiometric ratio of sodium source, iron source, sulfate source and five doping element sources of (SO4)5 is weighed out. The sodium source, iron source, sulfate source and doping element sources are all sulfates. Graphene is weighed out separately. All raw materials are mixed and subjected to dry ball milling to obtain a uniform mixed powder. The obtained mixed powder was subjected to a two-stage calcination treatment under an inert atmosphere: first, it was pre-calcined at 200-300℃ for 2-4 hours, and then calcined at 350-450℃ for 6-15 hours to obtain the iron-doped modified sodium iron sulfate cathode material. Where y = 0.3, and a = b = c = d = e = 0.06; The combination of M1, M2, M3, M4, and M5 is one of the following combinations: Mg, Co, Ni, Mn, Zn, Mg, Co, Ni, Mn, Cu, Mg, Co, Ni, Mn, Ca; The amount of graphene added is 6-10 wt. of the mass of the generated iron-doped modified sodium ferric sulfate cathode material.

2. The preparation method according to claim 1, characterized in that, The amount of graphene added is 8 wt.% of the mass of the generated iron-doped modified sodium ferric sulfate cathode material.

3. The preparation method according to claim 1, characterized in that, No solvents or dispersants are added during the dry ball milling process.

4. The preparation method according to claim 1, characterized in that, In the dry ball milling process, the ball-to-material ratio is (10~30):1, the ball milling speed is 300~600 rpm, and the ball milling time is 6~24 hours.

5. The preparation method according to claim 1, characterized in that, The iron source is ferrous sulfate.

6. The preparation method according to claim 1, characterized in that, The inert atmosphere is nitrogen, argon, or helium, and the gas flow rate is 0.1 ~ 1 L / min.

7. The preparation method according to claim 1, characterized in that, In the two-stage calcination, the heating rate is 2 ~ 10℃ / min.

8. An iron-doped modified sodium ferric sulfate cathode material prepared by the preparation method according to any one of claims 1-7.

9. A positive electrode sheet, characterized in that, It includes a positive current collector and a coating layer, said coating layer comprising the positive electrode material, conductive agent and binder as described in claim 8.

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

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