A rare earth metal-doped sodium ferric sulfate positive electrode material, a preparation method thereof, and a battery

Abstract

This invention relates to a rare earth metal-doped sodium iron sulfate cathode material, its preparation method, and a battery thereof. The cathode material is expressed as Na. 2‑2x Fe 2‑2x R 2x (SO4)3, where 0.02 ≤ x ≤ 0.1, and R is selected from rare earth metal elements. The preparation uses iron, rare earth metal, and sodium sources as raw materials, and ascorbic acid as a reducing agent. The product is obtained through dissolution and drying. This invention utilizes rare earth ions to substitute Fe. 2+ The invention establishes a stable crystal structure, significantly suppresses Fe ion dissolution at high temperatures, and mitigates structural distortion during charge and discharge. The resulting material exhibits significantly improved capacity retention after 300 cycles at 45°C and 1C rate, demonstrating excellent high-temperature cycling stability. This invention utilizes a mild process, readily available raw materials, and high reproducibility, solving the problems of poor high-temperature cycling and rapid capacity decay in traditional sodium iron sulfate cathodes. It is suitable for high-performance sodium-ion batteries and possesses promising industrialization prospects.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a rare earth metal-doped sodium iron sulfate cathode material, its preparation method, and a battery. Background Technology

[0002] Sodium ferric sulfate (Na₂Fe₂(SO₄)₃) has attracted much attention due to its low material cost and high voltage plateau. However, in practical applications, the charge-discharge process of Fe... 2+ Oxidized to Fe 3+ The increased positive charge density intensifies the Coulomb repulsion between adjacent Fe ions in the crystal structure. Simultaneously, the intrinsic defect of excessively short Fe-Fe atomic spacing further amplifies this repulsion, leading to irreversible distortion of the crystal structure. This structural distortion not only disrupts the sodium ion transport channels but also causes the structure of the active material to collapse, ultimately severely deteriorating the material's high-temperature cycling performance and becoming a core bottleneck limiting its large-scale application.

[0003] To address the Coulomb repulsion problem caused by the short Fe-Fe spacing in Na2Fe2(SO4)3, CN119176583A discloses a method for preparing sodium iron sulfate composite cathode material for sodium-ion batteries. This method achieves high crystallinity and high phase purity of the sodium iron sulfate material through template reduction and solid-liquid dual-carbon source coating, resulting in significant advantages in room-temperature electrochemical performance. However, no modification is made to the Fe sites, leading to intrinsic structural instability. During long-term charge-discharge, the number of redox couples may still decrease due to Fe ion migration, potentially causing capacity decay. Furthermore, while carbon layer coating alleviates material dissolution and interfacial side reactions, the carbon layer itself may crack or detach during long-term cycling, especially under high-rate or temperature fluctuation conditions, where the protective effect of the carbon layer may fail, exacerbating side reactions between the material and the electrolyte.

[0004] Therefore, how to overcome the limitations of existing technologies, precisely modify the Fe sites from the inside of the crystal structure, widen the Fe-Fe spacing and reduce the Coulomb repulsion by controlling the lattice environment, and at the same time construct a surface protection system with both high stability and high conductivity, so as to achieve a dual improvement in the intrinsic structural stability and interface stability of Na2Fe2(SO4)3 material, and thus solve the key technical problems of its poor high-temperature cycling performance and rapid capacity decay, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The present invention aims to provide a rare earth metal-doped sodium iron sulfate cathode material, its preparation method, and a battery. By introducing rare earth metal elements containing 4f orbitals into the Fe sites of sodium iron sulfate crystals for lattice doping modification, the large ionic radius characteristics of rare earth metal ions are utilized to widen the Fe-Fe lattice spacing, thereby reducing the Coulomb repulsion between Fe ions during charging and discharging at the intrinsic structural level. This effectively suppresses the migration and dissolution of Fe ions under high-temperature conditions, as well as irreversible distortion of the crystal structure.

[0006] The solution of the present invention to the above-mentioned technical problems is as follows: a rare earth metal-doped sodium ferric sulfate cathode material, wherein the rare earth metal ions are doped into the Fe of the sodium ferric sulfate crystal. 2+ site; The expression for the cathode material is Na. 2-2x Fe 2-2x R 2x (SO4)3, where 0.02≤x≤0.1, and R is selected from one or any combination of rare earth metal elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0007] Limited rare earth ion doping in Fe 2+ The sites between, utilizing the rare earth metal ion radius greater than Fe 2+ The properties of this material increase the Fe-Fe lattice spacing. This fundamentally alleviates the problem of Fe during charging and discharging. 2+ / Fe 3+ The Coulomb repulsion induced by the phase transition solves the problem of easy distortion of the traditional sodium ferric sulfate lattice.

[0008] The method for preparing rare earth metal-doped sodium ferric sulfate cathode material as described above includes the following steps: 1) Take iron source, sodium source and rare earth metal source, add them to ethylene glycol and stir, and add ascorbic acid to prepare a mixed suspension; 2) The mixed suspension is heated for a solvothermal reaction for 4-12 hours. After the reaction is completed, it is cooled, washed, dried and ground to obtain the rare earth metal-doped sodium iron sulfate cathode material.

[0009] Ethylene glycol, acting as a solvent, and ascorbic acid, acting as a reducing agent, form a unique mixed suspension system. The high boiling point of ethylene glycol ensures the complete occurrence of the solvothermal reaction, while ascorbic acid effectively prevents Fe... 2+ Oxidation ensures the purity and electrochemical activity of the material.

[0010] Preferably, in step 1), the iron source is one or any combination of ferrous oxalate, ferrous chloride, and ferrous sulfate heptahydrate.

[0011] Among them, ferrous sulfate heptahydrate is the preferred raw material due to its good water solubility and low cost; ferrous oxalate and other alternatives can be used in specific scenarios where impurity control is sensitive. Preferably, in step 1), the sodium source is one or a mixture of two of anhydrous sodium sulfate and sodium fluoride.

[0012] Anhydrous sodium sulfate is the preferred choice, providing a standard Na source; the introduction of sodium fluoride can adjust lattice parameters or ionic conductivity.

[0013] Preferably, in step 1), the rare earth metal source is one or a mixture of any of the following rare earth metal elements: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, in their elemental form, oxide, sulfide, phosphide, nitride, or halide.

[0014] Among them, halides and nitrates have good solubility and are easy to disperse in ethylene glycol, ensuring that rare earth ions can uniformly replace Fe sites and achieve ideal doping effect.

[0015] Preferably, in step 1), the molar amount of sodium in the sodium source is used as the basis, and the molar amount of rare earth elements in the rare earth metal source is 2~10%; the molar amount of iron in the iron source is equal to the molar amount of sodium in the sodium source.

[0016] Preferably, in step 1), the concentration of ascorbic acid in the mixed suspension is 2 × 10⁻⁶. -5 ~3×10 -5 mol / L.

[0017] At this concentration, ascorbic acid can effectively inhibit Fe 2+ Oxidation ensures the initial capacity and cycle performance of the material, while preventing impurities from remaining due to excessively high concentrations.

[0018] Preferably, the solvothermal reaction temperature in step 2) is 200~430 ℃.

[0019] Within this range, sufficient precursor crystallization and uniform rare earth ion doping can be ensured, avoiding the formation of amorphous impurity phases. Based on the thermal stability of ethylene glycol solvent and the safety tolerance limit of the autoclave, solvent thermal decomposition, material thermal degradation, and safety risks are prevented. Highly crystalline and structurally complete rare earth-doped sodium iron sulfate cathode materials can be prepared.

[0020] The present invention also provides a sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is made of rare earth metal-doped sodium iron sulfate positive electrode material as described above.

[0021] Preferably, the raw materials of the positive electrode sheet include, by mass, 80-99 parts of rare earth metal-doped sodium iron sulfate positive electrode material, 1-10 parts of positive electrode conductive agent, and 1-5 parts of positive electrode binder; The positive electrode conductive agent is one or any combination of Super P, Ketchen Black, acetylene black, carbon black, carbon nanotubes, graphene, conductive graphite, carbon fiber, conductive carbon nanotubes, and mesoporous carbon. The positive electrode binder is one or any combination of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, styrene-butadiene rubber, sodium carboxymethyl cellulose, polypropylene, polyethylene, polyacrylic acid, carboxymethyl cellulose, sodium alginate, and gelatin.

[0022] The beneficial effects of this invention are as follows: This invention introduces rare-earth metal ions containing 4f orbitals into the Fe sites of sodium ferric sulfate crystals. Utilizing the larger ionic radius of rare-earth ions, the Fe-Fe lattice spacing is expanded, fundamentally alleviating the Coulomb repulsion between adjacent Fe ions caused by redox reactions during charging and discharging, thus preventing irreversible structural distortion. Simultaneously, the lattice doping effect of rare-earth metals stabilizes the electron cloud distribution and crystal structure of the Fe sites, effectively preventing the migration and dissolution of Fe ions under high-temperature conditions. The rare-earth doping amount is controlled within the range of 2-10% to avoid the formation of impurity phases. Furthermore, the antioxidant and hydrolysis-inhibiting effects of ascorbic acid, combined with a mild preparation process, ensure high crystallinity and the absence of significant defects in the material, maintaining the three-dimensional stable structure of sodium ferric sulfate.

[0023] The process mainly adopts the solvothermal method, which does not require a harsh environment, has simple steps, low operation difficulty, and is easy to scale up industrially.

[0024] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below. Specific embodiments of the present invention are given in detail in the following examples. Attached Figure Description

[0025] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 Na prepared in Example 1 1.94 Fe 1.94 Nd 0.06 XRD comparison of (SO4)3 and unmodified Na2Fe2(SO4)3. Detailed Implementation

[0026] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0027] Example 1 This embodiment provides a rare earth metal-doped sodium ferric sulfate cathode material (denoted as Na). 1.94 Fe 1.94 Nd 0.06 (SO4)3), its preparation steps are as follows: Step 1: Weigh 2.6967 g of ferrous sulfate heptahydrate (FeSO4·7H2O), 0.1081 g of neodymium sulfate octahydrate (Nd2(SO4)3·8H2O), and 0.6889 g of anhydrous sodium sulfate (Na2SO4) and dissolve them in 40 mL of ethylene glycol. Simultaneously add 0.1 g of ascorbic acid aqueous solution (0.01 mol / L). Stir at room temperature for at least 2 hours to form a uniform light purple suspension. The Nd2(SO4)3 in the neodymium sulfate octahydrate... 3+ The molar amount is 0.0003 mol, and the Fe in ferrous sulfate heptahydrate is... 2+ The molar amount is 0.0097 mol, and the Na in anhydrous sodium sulfate is... + The molar mass is 0.01 mol, Fe 2+ 、Nd 3+ Na + The molar ratio between them is 1.94:0.06:1.94.

[0028] Step 2: Transfer the entire suspension to a polytetrafluoroethylene (PTFE) liner, cover with the liner, and place in an oven. Increase the temperature to 400 °C at a rate of 2 °C / min, and maintain this temperature for 12 hours. After the reaction is complete, allow the reactor to cool naturally to room temperature in the oven.

[0029] Step 3: Pour the supernatant and precipitate from the reactor into a centrifuge tube and wash 2-3 times with anhydrous ethanol. Transfer the washed precipitate to a petri dish and place it in a vacuum drying oven at 80°C for 12 hours. Gently grind the dried powder in a glove box using an agate mortar and pestle to obtain the final product Na. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder.

[0030] Application Example 1 This application example provides a sodium-ion full battery, the preparation steps of which are as follows: Na prepared according to Example 1 1.94 Fe 1.94 Nd 0.06(SO4)3 powder: Super P: carbon nanotubes: polyvinylidene fluoride = 95%: 1.5%: 1.0%: 2.5% by mass, are mixed evenly and coated onto the current collector. After drying and cutting to the appropriate size, the positive electrode sheet is obtained.

[0031] Weigh the materials according to the mass ratio of hard carbon: Super P: sodium carboxymethyl cellulose: styrene-butadiene rubber = 94.6%: 0.8%: 1.6%: 3.0%.

[0032] First, add sodium carboxymethyl cellulose to an appropriate amount of deionized water and stir until completely dissolved to form a uniform colloid. Then, add hard carbon and Super P in sequence and stir at high speed to disperse evenly. Finally, add SBR and continue stirring until a stable negative electrode slurry is formed, mixed evenly, and coated on the current collector. After drying and cutting to a suitable size, the negative electrode sheet is obtained.

[0033] The positive electrode, negative electrode, and 12µm base film are stacked and assembled, with the separator in the middle. After the positive and negative electrodes are welded together with tabs and sealed, and baked to meet the moisture requirements, the electrolyte (a commercially available electrolyte, which can be 1 mol / L NaClO4 dissolved in a solvent with a volume ratio of 1:1 ethylene carbonate / diethyl carbonate) is injected to obtain a sodium-ion full battery.

[0034] Example 2 This embodiment provides a rare earth metal-doped sodium ferric sulfate cathode material (denoted as Na). 1.9 Fe 1.9 Nd 0.1 (SO4)3), and its preparation steps are basically the same as those in Example 1. The difference lies in step 1), adjusting the amounts of ferrous sulfate heptahydrate and neodymium sulfate octahydrate to make Fe 2+ 、Nd 3+ Na + The molar ratio between them is 1.9:0.1:1.9.

[0035] Application Example 2 This application example provides a sodium-ion full cell, the preparation steps of which are basically the same as those in Application Example 1, the difference being that the Na+ used in the positive electrode is different. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder was replaced with Na prepared in Example 2. 1.9 Fe 1.9 Nd 0.1 (SO4)3 powder.

[0036] Example 3 This embodiment provides a rare earth metal-doped sodium ferric sulfate cathode material (denoted as Na). 1.84 Fe 1.84 Nd 0.16 (SO4)3), and its preparation steps are basically the same as those in Example 1. The difference lies in step 1), adjusting the amounts of ferrous sulfate heptahydrate and neodymium sulfate octahydrate to make Fe 2+ 、Nd 3+ Na + The molar ratio between them is 1.84:0.16:1.84.

[0037] Application Example 3 This application example provides a sodium-ion full cell, the preparation steps of which are basically the same as those in Application Example 1, the difference being that the Na+ used in the positive electrode is different. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder was replaced with Na prepared in Example 3. 1.84 Fe 1.84 Nd 0.16 (SO4)3 powder.

[0038] Example 4 This embodiment provides a rare earth metal-doped sodium ferric sulfate cathode material (denoted as Na). 1.94 Fe 1.94 Ce 0.06 (SO4)3), and its preparation steps are basically the same as those in Example 1. The difference lies in step 1), where neodymium sulfate octahydrate is replaced with cerium sulfate octahydrate, and the amounts of cerium sulfate octahydrate and ferrous sulfate heptahydrate are adjusted to make Fe... 2+ Ce 3+ Na + The molar ratio between them is 1.94:0.06:1.94.

[0039] Application Example 4 This application example provides a sodium-ion full cell, the preparation steps of which are basically the same as those in Application Example 1, the difference being that the Na+ used in the positive electrode is different. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder was replaced with Na prepared in Example 4. 1.94 Fe 1.94 Ce 0.06 (SO4)3 powder.

[0040] Example 5 This embodiment provides a rare earth metal-doped sodium ferric sulfate cathode material (denoted as Na). 1.94 Fe 1.94 Y 0.06(SO4)3), and its preparation steps are basically the same as those in Example 1. The difference lies in step 1), where neodymium sulfate octahydrate is replaced with yttrium sulfate octahydrate, and the amounts of yttrium sulfate octahydrate and ferrous sulfate heptahydrate are adjusted to make Fe... 2+ Y 3+ Na + The molar ratio between them is 1.94:0.06:1.94.

[0041] Application Example 5 This application example provides a sodium-ion full cell, the preparation steps of which are basically the same as those in Application Example 1, the difference being that the Na+ used in the positive electrode is different. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder was replaced with Na prepared in Example 5. 1.94 Fe 1.94 Y 0.06 (SO4)3 powder.

[0042] Application Example 6 This application example provides a sodium-ion full cell, the preparation steps of which are basically the same as those in Application Example 1, the difference being that the Na+ used in the positive electrode is different. 1.94 Fe 1.94 Nd 0.06 (SO4)3 powder was replaced with unmodified Na2Fe2(SO4)3.

[0043] like Figure 1 As shown, the Na prepared in Example 1 1.94 Fe 1.94 Nd 0.06 Compared to Na2Fe2(SO4)3, all characteristic peaks of (SO4)3 show a slight shift towards the lower 2θ direction. The decrease in 2θ corresponds to an increase in interplanar spacing d, directly proving that the Fe sites introduce Nd containing 4f orbitals. 3+ The increased cell parameters and volume expansion of the cell result in a wider Fe-Fe lattice spacing, fundamentally mitigating the Coulomb repulsion between adjacent Fe ions caused by redox reactions during charging and discharging, thus preventing irreversible structural distortion. The diffraction peak intensities and full width at half maximum (FWHM) of the two curves are essentially equivalent, indicating that the doped material maintains high crystallinity and its crystal integrity is not significantly reduced due to ion substitution, providing a structural basis for its excellent electrochemical performance.

[0044] The sodium-ion full cell assembled in the above application example was placed in the Land CT2001A test system. Its discharge capacity was tested at 0.1C, and its high-temperature cycling performance at 1C / 1C was tested in the voltage range of 2.5-4.2V. The results are shown in Table 1 based on the Fe dissolution content of the negative electrode after the test cycle.

[0045] Table 1 Electrochemical performance of sodium-ion full batteries prepared for each application example A comparison of application examples 1, 4, and 5 with application example 6 shows that using rare earth metal Nd... 3+ Ce 3+ Y 3+ This significantly improves the high-temperature cycling stability of sodium ferric sulfate cathode materials and effectively inhibits the dissolution of iron under high-temperature conditions, providing direct data support for material structure stabilization. (3 mol% Nd doping) 3+ Ce 3+ Y 3+ Application Examples 1, 4, and 5 showed significantly improved capacity retention after 300 cycles at 45°C and 1C / 1C conditions compared to Application Example 6. Simultaneously, the amount of Fe dissolved from the electrode material decreased significantly after cycling, indicating that rare earth doping can fundamentally stabilize the crystal structure of sodium ferric sulfate and reduce the leaching caused by Fe during charge and discharge. 2+ / Fe 3+ The lattice stress caused by valence state changes reduces iron ion migration and dissolution, thereby alleviating capacity decay.

[0046] As the rare earth doping amount increases, the cycle retention rate of Application Examples 2 and 3 shows a gradual downward trend, and the Fe dissolution amount rebounds. This indicates that appropriate doping can stabilize the crystal lattice and alleviate structural distortion, while excessive doping can easily destroy the crystal integrity, introduce impurities or defects, and reduce the stability of the material.

[0047] Overall, this invention effectively solves the technical problems of poor high-temperature cycling performance, rapid capacity decay, and severe Fe dissolution in traditional sodium iron sulfate cathode materials through rare earth metal doping modification, providing reliable data support and technical basis for the preparation of high-stability, long-life sodium-ion battery cathode materials.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention according to the description and above. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, based on the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A rare earth metal-doped sodium ferric sulfate cathode material, characterized in that, The rare earth metal ions doped into sodium ferric sulfate crystals are Fe 2+ site; The expression for the cathode material is Na. 2-2x Fe 2-2x R 2x (SO4)3, where 0.02≤x≤0.1, and R is selected from one or any combination of rare earth metal elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

2. A method for preparing the rare earth metal-doped sodium ferric sulfate cathode material according to claim 1, characterized in that, Includes the following steps: 1) Take iron source, sodium source and rare earth metal source, add them to ethylene glycol and stir, and add ascorbic acid to prepare a mixed suspension; 2) The mixed suspension is heated for a solvothermal reaction for 4-12 hours. After the reaction is completed, it is cooled, washed, dried and ground to obtain the rare earth metal-doped sodium iron sulfate cathode material.

3. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, In step 1), the iron source is one or any combination of ferrous oxalate, ferrous chloride, and ferrous sulfate heptahydrate.

4. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, In step 1), the sodium source is one or a mixture of two of anhydrous sodium sulfate and sodium fluoride.

5. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, Step 1) The rare earth metal source is one or any mixture of rare earth metal elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, including their elemental form, oxide, sulfide, phosphide, nitride, and halide.

6. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, In step 1), the molar amount of sodium in the sodium source is used as the basis, and the molar amount of rare earth elements in the rare earth metal source is 2~10%; the molar amount of iron in the iron source is equal to the molar amount of sodium in the sodium source.

7. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, In step 1), the concentration of ascorbic acid in the mixed suspension is 2 × 10⁻⁶. -5 ~3×10 -5 mol / L.

8. The method for preparing a rare earth metal-doped sodium ferric sulfate cathode material according to claim 2, characterized in that, In step 2), the solvothermal reaction temperature is 200~430 ℃.

9. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode sheet is made from the rare earth metal-doped sodium iron sulfate positive electrode material as described in claim 1.

10. A sodium-ion battery according to claim 9, characterized in that, The raw materials of the positive electrode sheet include, by mass, 80-99 parts of rare earth metal-doped sodium iron sulfate positive electrode material, 1-10 parts of positive electrode conductive agent, and 1-5 parts of positive electrode binder; The positive electrode conductive agent is one or any combination of Super P, Ketchen Black, acetylene black, carbon black, carbon nanotubes, graphene, conductive graphite, carbon fiber, conductive carbon nanotubes, and mesoporous carbon. The positive electrode binder is one or any combination of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, styrene-butadiene rubber, sodium carboxymethyl cellulose, polypropylene, polyethylene, polyacrylic acid, carboxymethyl cellulose, sodium alginate, and gelatin.

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