A doped sodium ferric sulfate positive electrode material, a preparation method and application thereof

Abstract

The application relates to a doped sodium sulfate positive electrode material and a preparation method and application thereof, and belongs to the technical field of positive electrode materials for sodium ion batteries. 2‑3x M x Fe2(SO4)3, M is a trivalent lanthanide element ion, and 0.005<=x<=0.05.The material adopts a trivalent lanthanide element ion doping strategy, and lattice structure is stabilized in advance to effectively inhibit irreversible structural rearrangement in the first charge-discharge process, and the cycle performance and rate performance of the material at normal temperature are simultaneously improved.

Description

Technical Field

[0001] This invention belongs to the technical field of cathode materials for sodium-ion batteries, and particularly relates to a doped sodium iron sulfate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium ferric sulfate, as a positive electrode material for polyanionic sodium-ion batteries, has shown broad application prospects in the field of sodium-ion batteries due to its outstanding advantages of abundant raw material resources, low preparation cost and excellent environmental compatibility. It has become a research hotspot in the field of energy storage materials and has attracted widespread attention.

[0003] However, the key technical bottleneck faced by this material in practical applications lies in the significant irreversible structural changes that occur during its initial charge-discharge phase. This core issue severely restricts its large-scale practical application in sodium-ion batteries. Specifically, during the initial charge, sodium ions in the sodium iron sulfate lattice gradually release from their specific Na1, Na2, and Na3 sites in a specific order. Simultaneously, Fe ions originally located at the Fe1 site... 2+ Ions migrate and occupy the Na1 sites left behind after sodium ions are released. This ion migration directly leads to an irreversible rearrangement of the sodium ferric sulfate crystal structure. During this process, the cell volume of the material shrinks by approximately 2.6%, and Fe... 2+ Ions will undergo simultaneous oxidation to produce Fe. 3+ Although sodium ferric sulfate exhibits some reversible reaction characteristics during subsequent cycles after the first charge and discharge, the irreversible structural rearrangement caused by the first charge and discharge has resulted in some sodium sites being occupied by migrating Fe ions. This not only leads to a 10%-20% initial capacity loss in the material, but also significantly increases the diffusion resistance of sodium ions in the crystal lattice, which in turn easily causes microcracks to form in the material during cycling, ultimately leading to continuous capacity decay. This problem is particularly prominent in deep charge and discharge applications.

[0004] Currently, existing technologies for improving the performance of sodium ferric sulfate materials mainly focus on optimizing synthesis process parameters or surface modification of the materials. However, these methods have failed to fundamentally solve the problem of irreversible structural rearrangement during the first charge and discharge process, and cannot achieve a substantial improvement in the material's cycle stability.

[0005] Therefore, developing a sodium ferric sulfate cathode material that can effectively suppress irreversible structural rearrangement during the first charge and discharge process and significantly improve cycle stability remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a doped sodium iron sulfate cathode material, its preparation method, and its application. This material employs a trivalent lanthanide ion doping strategy, which pre-stabilizes the crystal structure to effectively suppress irreversible structural rearrangement during the first charge and discharge process, thereby simultaneously improving the material's cycle performance and rate performance at room temperature.

[0007] The first objective of this invention is to provide a doped sodium ferric sulfate cathode material, wherein the chemical formula of the doped sodium ferric sulfate cathode material is Na. 2-3x M x Fe2(SO4)3, where M is a trivalent lanthanide ion, 0.005≤x≤0.05.

[0008] In one embodiment of the present invention, the trivalent lanthanide ion is selected from Pr 3+ 、Nd 3+ Pm 3+ Ho 3+ and Dy 3+ One or more of these; these trivalent lanthanide ions (such as Pr 3+ 、Nd 3+ Pm 3+ Ho 3+ Dy 3+ The ionic radii of Na (101.3 pm, 99.5 pm, 97.5 pm, 89.4 pm, and 90.8 pm, respectively) are compared with those of Na + The ionic radii (approximately 95 pm) differ by no more than 7%, exhibiting good matching, thus ensuring that the lattice distortion of the material after doping is minimized; moreover, after doping with these trivalent lanthanide ions, they preferentially occupy the Na1 sites in the material. By occupying this potential migration site, the migration behavior of iron ions to the Na1 site during the first charge can be effectively suppressed, thereby reducing the irreversible capacity loss of the material and alleviating structural stress.

[0009] In one embodiment of the present invention, the trivalent lanthanide ion is selected from Pr 3+ and / or Nd 3+ .

[0010] The second objective of this invention is to provide a method for preparing the doped sodium iron sulfate cathode material, wherein Na source, M source, Fe source and S source are mixed uniformly under a protective atmosphere, and the doped sodium iron sulfate cathode material is obtained by pretreatment and sintering.

[0011] In one embodiment of the present invention, the Na source is selected from one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium nitrate, and sodium sulfate; And / or, the M source is selected from one or more of trivalent lanthanide oxides, trivalent lanthanide carbonates, trivalent lanthanide nitrates, trivalent lanthanide sulfates, and trivalent lanthanide acetates; And / or, the Fe source is selected from one or more of ferric sulfate, ammonium ferric sulfate, ferric chloride, and ferric nitrate; And / or, the S source is selected from one or more of ammonium sulfate, sodium sulfate, ferric sulfate, and sodium pyrosulfate.

[0012] In one embodiment of the present invention, the pretreatment temperature is 300℃-400℃ (e.g., 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, etc.), and the time is 2h-4h (e.g., 2h, 2.5h, 3h, 3.5h, 4h, etc.); to remove volatile impurities.

[0013] In one embodiment of the present invention, the sintering temperature is 500℃-700℃ (500h, 510h, 520h, 530h, 540h, 550h, 560h, 570h, 580h, 590h, 600h, 610h, 620h, 630h, 640h, 650h, 660h, 670h, 680h, 690h, 700h, etc.), and the time is 6h-12h (for example, it can be 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, etc.).

[0014] In one embodiment of the present invention, the protective atmosphere is selected from a nitrogen atmosphere and / or an argon atmosphere; to avoid Fe 2+ Oxidation.

[0015] The third objective of this invention is to provide a sodium-ion battery cathode, prepared from a doped sodium ferric sulfate cathode material or a doped sodium ferric sulfate cathode material prepared by the method described above.

[0016] The fourth objective of this invention is to provide a sodium-ion battery prepared from the aforementioned sodium-ion battery positive electrode.

[0017] The technical solution of the present invention has the following advantages compared with the prior art: (1) The doped sodium ferric sulfate cathode material of the present invention introduces Na+ into the sodium ferric sulfate crystal structure. + With constant valence trivalent lanthanide ions of similar radius, these doped trivalent lanthanide ions preferentially occupy Na1 sites, making the crystal structure approach the stable state after the first charge rearrangement during the synthesis stage, thereby effectively suppressing iron ion migration and irreversible structural rearrangement.

[0018] (2) The doped sodium ferric sulfate cathode material of the present invention introduces trivalent lanthanide ions, which preferentially occupy the high migration barrier Na1 octahedral sites, thereby forming a strong M 3+ -O 2- Bonding to construct local energy traps to improve Fe 2+ The activation energy required for migration effectively suppresses irreversible structural rearrangement during the first charging process, significantly reducing initial capacity loss; simultaneously, the introduction of 1 mol M... 3+ It can replace 3 mol of Na + And form [FeO6] 10- -[MO3] 3- The synergistic coordination structure increases the material's conductivity by 10² through df orbital hybridization, significantly enhancing its electronic conduction performance. This doping strategy also reduces the generation of microcracks during material cycling, increasing the capacity retention rate after 100 cycles at room temperature to over 95%, greatly improving cycle stability. Furthermore, by reducing sodium ion diffusion resistance, it further improves the material's rate performance to support high-rate charge and discharge. Attached Figure Description

[0019] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is a graph showing the first and second charge-discharge curves of a sodium-ion battery made from sodium iron sulfate cathode material according to Example 1 of the present invention. Figure 2 The first and second charge-discharge curves of the sodium-ion battery made of sodium iron sulfate cathode material according to Comparative Example 1 of this invention are shown. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0021] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0022] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified. Example 1

[0024] The doped sodium ferric sulfate cathode material and its preparation method in this embodiment specifically include the following steps: According to the target product chemical formula Na 1.97 Nd 0.01 FeSO4·7H2O, Na2SO4, and Nd2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material. Example 2

[0025] The doped sodium ferric sulfate cathode material and its preparation method in this embodiment specifically include the following steps: According to the target product chemical formula Na 1.94 Nd 0.02 FeSO4·7H2O, Na2SO4, and Nd2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material. Example 3

[0026] The doped sodium ferric sulfate cathode material and its preparation method in this embodiment specifically include the following steps: According to the target product chemical formula Na 1.85 Nd 0.05 FeSO4·7H2O, Na2SO4, and Nd2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material. Example 4

[0027] The doped sodium ferric sulfate cathode material and its preparation method in this embodiment specifically include the following steps: According to the target product chemical formula Na 1.97 Pr 0.01FeSO4·7H2O, Na2SO4, and Pr2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Pr-doped sodium iron sulfate cathode material. Comparative Example 1

[0028] Basically the same as Example 1, except that no elemental doping is performed, and the specific steps include: FeSO4·7H2O and Na2SO4 raw materials were accurately weighed according to the stoichiometric ratio of the target product chemical formula Na2Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain sodium iron sulfate cathode material. Comparative Example 2

[0029] The basic principle is the same as in Example 1, except that the doping element is in excess, specifically including the following steps: According to the target product chemical formula Na 1.7 Nd 0.1 FeSO4·7H2O, Na2SO4, and Nd2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material. Comparative Example 3

[0030] Basically the same as Example 1, except that the doping element is replaced with La, specifically including the following steps: According to the target product chemical formula Na 1.97 La 0.01 FeSO4·7H2O, Na2SO4, and La2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material. Comparative Example 4

[0031] The process is basically the same as in Example 1, except that the dopant element is replaced with Eu. Specifically, the steps include: According to the target product chemical formula Na 1.97 Eu 0.01FeSO4·7H2O, Na2SO4, and Eu2O3 raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material.

[0032] Comparative Example 5 Basically the same as Example 1, except that the doping element is replaced with Ca, specifically including the following steps: According to the target product chemical formula Na 1.98 Ca 0.01 FeSO4·7H2O, Na2SO4 and CaO raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material.

[0033] Comparative Example 6 Basically the same as Example 1, except that the doping element is replaced with Sr, specifically including the following steps: According to the target product chemical formula Na 1.98 Sr 0.01 FeSO4·7H2O, Na2SO4, and SrO raw materials were accurately weighed according to the stoichiometric ratio of Fe2(SO4)3. After mixing the raw materials in an argon glove box, the mixture was ball-milled for 4 hours. Then, it was pre-calcined at 350℃ for 3 hours and sintered at 600℃ for 10 hours. After cooling, it was ground and passed through a 400-mesh sieve to obtain Nd-doped sodium iron sulfate cathode material.

[0034] Test Example 1

[0035] Battery fabrication and performance testing were conducted using sodium iron sulfate cathode materials from Examples 1-4 and Comparative Examples 1-6. Battery assembly: Positive electrode sheet: The sodium ferric sulfate positive electrode material used in Examples 1-4 and Comparative Examples 1-6 were used as the positive electrode active material, acetylene black as the conductive agent, polyvinylidene fluoride as the binder, and 6μm aluminum foil as the positive electrode current collector. The positive electrode active material, conductive agent, and binder were mixed at a mass ratio of 8:1:1, and N-methylpyrrolidone was added and stirred to form a uniform and stable positive electrode slurry. The positive electrode slurry was then uniformly coated onto the surface of the positive electrode current collector using a 200μm doctor blade coating. After drying and cold pressing, the positive electrode sheet was obtained. The mass loading of the sodium ferric sulfate positive electrode material was approximately 2.5 mg / cm³. 2 .

[0036] Counter electrode: Sodium metal sheet.

[0037] Separating membrane: Polyethylene film, 9μm thick.

[0038] Electrolyte: Sodium hexafluorophosphate is dissolved in polycarbonate to prepare an electrolyte with a concentration of 1 mol / L.

[0039] Sodium-ion battery assembly: In an argon-filled glove box, arrange the positive electrode, separator, counter electrode, and separator in sequence to assemble a CR2032 coin cell sodium-ion battery.

[0040] Performance testing: The assembled sodium-ion battery was subjected to specific capacity and cycle stability tests. (1) Specific capacity test: Within the voltage range of 2.0V-4.5V, charge and discharge tests were conducted at a rate of 0.1C. The charge and discharge voltage and capacity data of the first and second cycles were recorded to obtain the discharge capacity Q. D And the mass M of the active substance, then using the formula C=Q D / M is used to calculate the specific capacity of the active material; (2) Cyclic stability test: First, capacity calibration was performed at a 0.1C rate and the initial discharge capacity C0 was recorded. Then, charge and discharge cycles were performed at a 1C rate within the voltage range of 2.0V-4.5V (charging cut-off voltage is 4.5V, and discharge cut-off voltage is 2.0V). After every 100 cycles, a 0.1C capacity calibration was performed. When 10 0.1C capacity calibrations were completed (corresponding to 1000 charge and discharge cycles), the discharge capacity C at this time was recorded. 10 By calculating the capacity retention rate R=(C 10 / C0)×100%; Table 1 and Figures 1-2 The following are the final measured performance parameters: Table 1

[0041] Table 1 and Figures 1-2 It can be seen that the sodium ferric sulfate cathode materials of Examples 1-4 are significantly superior to those of Comparative Examples 1-6 in terms of initial discharge specific capacity, capacity reversibility (Q2 / Q1), and long-term cycle stability. This is because the examples used sodium ferric sulfate cathode materials with Na... + Trivalent lanthanide ions (Pr) with good ionic radius matching 3+ 、Nd 3+ When low-concentration doping (0.005≤x≤0.05) is performed, the trivalent lanthanide ions preferentially occupy the Na1 sites and form a stable coordination structure, which effectively suppresses the irreversible structural rearrangement during the first charge and discharge process and reduces the diffusion resistance and polarization of sodium ions. In contrast, the control group, whether doped or undoped, or with mismatched doping elements, or with excessive doping concentration, cannot achieve effective lattice structure stability, resulting in performance degradation.

[0042] Comparing Example 1 and Comparative Example 1 (undoped), it can be seen that: the initial discharge specific capacity of Example 1 (98.8 mAh / g) is higher than that of Comparative Example 1 (91.5 mAh / g), the Q2 / Q1 ratio (99.49%) is much higher than that of Comparative Example 1 (97.70%), and the capacity retention rate after 1000 cycles (92.3%) is significantly better than that of Comparative Example 1 (70.4%); moreover, the first discharge stage of Example 1 drops from 4.5V to 2.0V, and the initial stage shows a voltage plateau of about 0.8V (corresponding to Na). + (escapes from lattice sites), and then enters the slope region (corresponding to Fe). 2+ Oxidized to Fe 3+ The discharge capacity of Comparative Example 1 reached 98.8 mAh / g when the discharge ended at 2.0V. The voltage plateau disappeared in the second discharge cycle, and the entire curve showed a uniform slope (indicating structural stability), with the discharge capacity slightly decreasing to 97.9 mAh / g, and the charging cutoff voltage returning to 4.5V. In contrast, Comparative Example 1 showed no significant voltage plateau in the first discharge cycle, with the slope starting at approximately 4.5V and ending at 2.0V, resulting in a discharge capacity of 91.5 mAh / g. The charging cycle exhibited significant polarization, requiring a higher voltage to approach full charge, with only 89.4 mAh / g recovered (97.70% for Q2). The discharge capacity further decreased to 83.2 mAh / g in the second cycle, and the slope increased (corresponding to increased sodium ion diffusion resistance). This is because Comparative Example 1 did not undergo elemental doping, and Fe... 2+ The easy migration to Na1 sites leads to irreversible lattice rearrangement, causing a 10%-20% loss of initial capacity, and microcracks are easily generated during cycling, increasing the resistance to sodium ion diffusion; while in Example 1, Nd 3+ Occupying the Na1 site inhibits Fe 2+ Migration and structural rearrangement stabilize the lattice, thereby improving capacity reversibility and cycle stability.

[0043] Comparative Example 1 and Comparative Example 2 (Nd 3+ (Excessive doping) It can be seen that the initial discharge specific capacity and the second-cycle discharge specific capacity of Example 1 are much higher than those of Comparative Example 2, but the Q2 / Q1 ratio and the capacity retention rate after 1000 cycles are similar in both. This is because the Nd in Comparative Example 2... 3+ Excessive doping leads to a significant reduction in sodium ion content (Na). + The content (2-3×0.1=1.7) reduces the number of sodium ions that can be intercalated or deintercalated, directly causing a significant decrease in specific capacity; however, excessive Nd... 3+ It can still effectively occupy the Na1 site to inhibit structural rearrangement, thus maintaining good cycle stability.

[0044] Comparative Example 1 and Comparative Example 3 (La) 3+(Doping) It can be seen that the initial discharge specific capacity, Q2 / Q1, and capacity retention after 1000 cycles of Example 1 are all superior to those of Comparative Example 3. This is because La 3+ The ionic radius is 103.2 pm, similar to Na. + The ionic radii (approximately 95 pm) differ by more than 8.6%, resulting in poor matching. Doping leads to increased lattice distortion, although it can partially suppress Fe... 2+ It migrates, but cannot form a stable [FeO6] structure. 10- -[MO3] 3- The cooperative coordination structure offers limited improvement in electron conduction performance and sodium ion diffusion efficiency, resulting in lower capacity and cycling stability compared to Example 1.

[0045] Comparative Example 1 and Comparative Example 4 (Eu 3+ (Doping) It can be seen that the performance indicators of Example 1 are all superior to those of Comparative Example 4. This is because Eu 3+ The ionic radius is 94.7 pm, although it is similar to Na. + The radii are similar, but Eu 3+ The df orbital hybridization capability is weak, making it unable to effectively construct strong M orbitals. 3+ -O 2- Bond local energy trap, Fe 2+ The activation energy for migration is not sufficiently increased, and there is still a certain degree of irreversible structural rearrangement, resulting in better initial capacity loss and cycle decay than in Example 1.

[0046] Comparative Example 1 and Comparative Example 5 (Ca 2+ (Doping) It can be seen that: the initial discharge specific capacity and cycle stability of Example 1 are significantly better than those of Comparative Example 5; the charge-discharge curve of Comparative Example 5 shows severe polarization. This is because Ca 2+ The electron cloud distribution (8-electron shell) of O is fundamentally different from that of trivalent lanthanide ions (unfilled 4f orbitals), leading to its difference from O 2- The bonding strength is reduced, and the lattice structure cannot be stabilized. Therefore, the performance is far inferior to the effect of trivalent lanthanide ion doping in Example 1.

[0047] Comparative Example 1 and Comparative Example 6 (Sr 2+ (Doping) It can be seen that the performance of Example 1 is comprehensively superior to that of Comparative Example 6. This is because Sr 2+ The ionic radius is 118 pm, similar to Na. + A radius difference exceeding 24% leads to severe lattice distortion after doping, obstructing sodium ion insertion / extraction channels, and also affecting divalent Sr. 2+ Unable to form a cooperative coordination structure with [FeO6] octahedrons, thus unable to suppress Fe 2+ Migration and structural rearrangement result in poor capacity and cyclic stability.

[0048] In summary, doping with trivalent lanthanide ions can effectively improve the cycle stability of sodium iron sulfate. The key lies in the rational selection of dopant ions and the precise optimization of site occupancy. This technology successfully solves the key problem of irreversible rearrangement of materials by controlling the crystal structure of sodium iron sulfate through doping with trivalent lanthanide ions. It also has the advantages of simple and mature preparation process, low cost, and easy large-scale production, providing high-performance cathode materials for sodium-ion batteries and showing good application prospects.

[0049] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A doped sodium ferric sulfate cathode material, characterized in that, The chemical formula of the doped sodium ferric sulfate cathode material is Na. 2-3x M x Fe2(SO4)3, where M is a trivalent lanthanide ion, 0.005 ≤ x ≤ 0.05; the trivalent lanthanide ion is selected from Pr 3+ 、Nd 3+ Pm 3+ Ho 3+ and Dy 3+ One or more of them.

2. The doped sodium ferric sulfate cathode material according to claim 1, characterized in that, The trivalent lanthanide ions are selected from Pr 3+ and / or Nd 3+ .

3. The method for preparing the doped sodium ferric sulfate cathode material according to any one of claims 1-2, characterized in that, Under a protective atmosphere, Na source, M source, Fe source and S source are mixed uniformly, and the mixture is pretreated and sintered to obtain the doped sodium iron sulfate cathode material.

4. The method for preparing the doped sodium ferric sulfate cathode material according to claim 3, characterized in that, The Na source is selected from one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium nitrate, and sodium sulfate; And / or, the M source is selected from one or more of trivalent lanthanide oxides, trivalent lanthanide carbonates, trivalent lanthanide nitrates, trivalent lanthanide sulfates, and trivalent lanthanide acetates; And / or, the Fe source is selected from one or more of ferric sulfate, ammonium ferric sulfate, ferric chloride, and ferric nitrate; And / or, the S source is selected from one or more of ammonium sulfate, sodium sulfate, ferric sulfate, and sodium pyrosulfate.

5. The method for preparing the doped sodium ferric sulfate cathode material according to claim 3, characterized in that, The pretreatment temperature is 300℃-400℃, and the time is 2h-4h.

6. The method for preparing the doped sodium ferric sulfate cathode material according to claim 3, characterized in that, The sintering temperature is 500℃-700℃, and the time is 6h-12h.

7. The method for preparing the doped sodium ferric sulfate cathode material according to claim 3, characterized in that, The protective atmosphere is selected from nitrogen atmosphere and / or argon atmosphere.

8. A sodium-ion battery positive electrode, characterized in that, The doped sodium ferric sulfate cathode material is prepared from any one of the doped sodium ferric sulfate cathode materials according to any one of claims 1-2 or by any one of claims 3-7.

9. A sodium-ion battery, characterized in that, It is prepared from the sodium-ion battery cathode as described in claim 8.

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