A magnesium-fluorine double-substituted modified sodium iron sulfate sodium-ion battery cathode material and a preparation method thereof

Abstract

The application discloses a magnesium-fluorine double-site substitution modified sodium iron sulfate sodium-ion battery positive electrode material and a preparation method thereof. 4‑x Mg x S(O 4‑y F y )7, wherein 0.02<=x<=0.10, 0.04<=y<=0.20, and each element in the system satisfies charge balance. The application uses MgF2 as a single doping source, realizes Mg 2+ occupies Fe sites, and F ‑ substitutes O sites, and realizes one-step double-site co-doping modification of anions and cations. A uniform doped precursor is prepared by using anhydrous ethanol system wet ball milling, then the precursor is compounded with Ketjen black, a three-dimensional continuous conductive network is constructed through dry ball milling, and a segmented weak reduction atmosphere low-temperature calcination process is used. The process can effectively inhibit Fe 2+ oxidation and ensure preparation of a pure-phase material. The obtained positive electrode material is stable in structure, excellent in rate performance and long in cycle life, the preparation process is simple and low in cost, and the positive electrode material has a good application prospect in the field of sodium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery electrode materials, specifically relating to a method that uses a single MgF2 as a doping source to simultaneously achieve Mg 2+ Substitution of Fe sites, F – This study relates to a dual-site co-doping modification of sodium iron sulfate cathode material with cation and anion sites replacing the O site, and also involves low-temperature preparation processes and applications in energy storage sodium-ion batteries. Background Technology

[0002] With the rapid development of large-scale energy storage and low-speed power battery industries, sodium-ion batteries, with their advantages of abundant sodium resources, low cost, high safety, and compatibility with existing lithium battery equipment, have become a core research direction for new energy storage systems. Polyanionic sodium iron sulfate possesses advantages such as high operating voltage, good structural stability, and environmental friendliness, making it a promising cathode material for sodium-ion batteries. However, sodium iron sulfate materials suffer from low intrinsic electronic conductivity and Na+... + Poor diffusion kinetics, Fe during charging and discharging 2+ / Fe 3+ Repeated switching leads to the accumulation of lattice distortion, resulting in crystal structure collapse after long cycles and easy side reactions between the material surface and the electrolyte, severely limiting its practical application. Existing modification methods mostly employ single cation doping, composite anion doping, or simple carbon coating, making it difficult to simultaneously achieve the modification goals of strengthening the internal lattice framework, improving the sodium ion diffusion rate, and passivating and protecting the material surface. Moreover, existing doping processes often involve adding multiple dopant salts separately, resulting in poor doping uniformity, complex process flow, and easy introduction of impurity phases. There is currently no publicly available literature or patent document on the simultaneous realization of sodium ferric sulfate cathode materials with dual-site lattice substitution of anions and cations using only MgF2 as a single doping source, along with a corresponding preparation process.

[0003] Due to Mg 2+ The ionic radius is approximately 0.72 Å, similar to Fe. 2+ It is close to 0.78 Å; F - The ionic radius is approximately 1.33 Å, similar to O. 2- Since their Å values ​​are similar to 1.40 Å, both can achieve stable substitution by low-strain embedding into the sodium ferric sulfate lattice. This invention employs magnesium-fluorine dual-site synergistic substitution, segmented weak reduction low-temperature calcination, in-situ fluorination interface layer construction, and carbon network coating modification techniques to effectively address the defects of existing materials and significantly improve crystal structure stability and overall electrochemical performance. Summary of the Invention

[0004] This invention aims to provide a single-doped source anion and cation co-doped modified sodium iron sulfate cathode material and its preparation method. The material has a stable crystal structure, excellent rate discharge performance, and high capacity retention over long cycles. The entire preparation process is simple to operate, with uniform doping, high product purity, and good process repeatability, making it suitable for large-scale industrial mass production.

[0005] To achieve this objective, the present invention adopts the following technical solution: A magnesium-fluorine dual-substitution modified sodium iron sulfate cathode material, wherein the chemical formula of the cathode material is Na6Fe 4-x Mg x S(O 4-y F y )7, where 0.02 ≤ x ≤ 0.10, 0.04 ≤ y ≤ 0.20; the elements in the chemical formula satisfy charge balance. This material uses MgF2 as a single dopant source, and simultaneously completes Mg 2+ Substitutional lattice Fe sites, F Replace the sulfate O site. In the embodiments of the present invention, preferably 0.02 ≤ x ≤ 0.08, 0.04 ≤ y ≤ 0.16, and most preferably x=0.05, y=0.1.

[0006] The preparation method of the magnesium-fluorine dual-substituted modified sodium ferric sulfate cathode material specifically includes the following steps: (1) Weigh sodium source, iron source, antioxidant, dispersant and MgF2 according to the stoichiometric ratio, add them to anhydrous ethanol and wet ball mill to form a uniform suspension dispersion, and vacuum low temperature drying to remove organic solvent to obtain co-doped precursor powder; (2) The precursor powder and carbon source are ball-milled and coated under an inert gas atmosphere to obtain a uniformly coated composite powder material; (3) The composite powder is placed in a low-hydrogen-content weak reducing atmosphere for low-temperature pre-calcination to remove crystal water and volatile components; (4) The pre-calcined product is placed in a weak reducing atmosphere for sintering. After the heat preservation is completed, it is cooled to room temperature in the furnace. After grinding and sieving, magnesium-fluorine double-substituted sodium iron sulfate cathode material is obtained.

[0007] Furthermore, in step (1), the sodium source is one or more of anhydrous sodium sulfate, sodium sulfate heptahydrate, sodium sulfate decahydrate, and sodium bisulfate.

[0008] Furthermore, in step (1), the iron source is one or more of anhydrous ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate tetrahydrate, and ferrous sulfate heptahydrate.

[0009] Furthermore, in step (1), the antioxidant is one or more of ascorbic acid, citric acid, oxalic acid, tartaric acid, and polyvinylpyrrolidone (PVP); it can inhibit Fe during wet ball milling. 2+ Oxidation; antioxidants and Fe 2+ Add Fe at a molar ratio 2+ Antioxidant : Antioxidant = 10 : (1~2).

[0010] Further, in step (1), the dispersant is one or more of polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and hexadecyltrimethylammonium bromide (CTAB) to achieve uniform dispersion of metal salt and MgF2 nanoparticles without agglomeration; the amount of dispersant added is 0.5~3.0 wt% of the total mass of the raw materials.

[0011] Further, in step (1), the wet ball milling is set with a milling speed of 300~600 rpm and a time of 4~12 h; the drying temperature is 60~80 ℃, and low-temperature drying avoids oxidation of ferrous salts.

[0012] Further, in step (2), the carbon source is one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, acetylene black, Ketjen black and Super P; the mass of the added carbon source is 1.0~5.0 wt% of the mass of the sodium ferric sulfate precursor.

[0013] Furthermore, in step (2), the inert atmosphere is one or more of argon or nitrogen; the ball milling speed is 300~600 rpm, and the ball milling time is 2~6 h; dry ball milling enables carbon material to be tightly coated on the surface of the positive electrode material, thus building a continuous conductive network.

[0014] Furthermore, in step (3), the weak reducing atmosphere is 3 vol%~8 vol% H2, with the remainder being Ar; an H2 content below 3% cannot effectively suppress Fe. 2+ Oxidation at concentrations above 8% may lead to sulfate reduction and the formation of impurity phases such as FeS, therefore this range is preferred.

[0015] Furthermore, in step (3), the heating rate of low-temperature pre-firing is 2~5 ℃ / min, the pre-firing temperature is 150~250 ℃, and the holding time is 1~4 h; low-temperature graded pre-firing can gradually remove impurities, relieve internal stress, and protect the integrity of the crystal structure.

[0016] Furthermore, in step (4), the sintering temperature is controlled in the low temperature range of 350~400 ℃, the holding time is 6~12h, and the heating rate is 2~5 ℃ / min. Low temperature sintering can significantly reduce energy consumption and effectively suppress the thermal decomposition reaction of sulfate at high temperature.

[0017] Compared with existing technologies, the magnesium-fluorine dual-substitution modified iron sulfate sodium sodium-ion battery cathode material of the present invention has the following advantages: 1. Using MgF2 as a single co-doping source, Mg is simultaneously realized. 2+ Substitutional lattice Fe sites, F – The substitution of O sites does not result in the formation of any impurity phases, and the crystal structure remains intact. XRD patterns confirmed by this invention show that the diffraction peaks of the doped material shift regularly to higher angles, with no free MgF2 impurity peaks, indicating that Mg... 2+ F – Successful entry into the crystal lattice achieves dual-site substitution doping. Existing modification methods require the separate addition of two types of dopants, magnesium salt and fluorine salt, in a stepwise doping process, which generally suffers from poor doping uniformity and easy introduction of impurity phases. This invention uses only MgF2 to achieve simultaneous lattice substitution of anions and cations in one step, making the process more streamlined and significantly improving the uniformity of elemental doping. Simultaneously, Mg... 2+ With F – Synchronous embedding into the lattice forms a charge-matching synergistic modification system, which can simultaneously enhance the rigidity of the crystal framework and the stability of the surface interface. Dual modification gains of the lattice body and the material interface can be achieved by doping with only a single component.

[0018] 2. Under the synergistic modification of magnesium and fluorine, Mg 2+ By enhancing the rigidity of the skeleton, Fe can be effectively suppressed. 3+ Migration and lattice distortion, while F - This strengthens surface chemical bonds and mitigates electrolyte side reactions.

[0019] 3. Low-temperature, segmented calcination in a weakly reducing atmosphere effectively prevents Fe... 2+ Oxidation and sulfate decomposition ensure uniform doping and crystal purity.

[0020] 4. During the sintering process, fluorine (F) element is enriched on the material surface to form a fluorinated inert interface layer; after the positive electrode material is synthesized, conductive carbon is introduced through dry ball milling to build a three-dimensional conductive network in the interparticle gaps, thereby simultaneously improving interface stability and electronic conductivity.

[0021] 5. The cathode material obtained by this invention has a stable structure, excellent rate performance, and long cycle life. Moreover, the preparation process is simple and low-cost, making it suitable for large-scale production and showing good application prospects in the field of sodium-ion batteries. Attached Figure Description

[0022] Figure 1 The images show the XRD patterns of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1.

[0023] Figure 2 This is a SEM image of the sodium-ion battery cathode material prepared in Example 2.

[0024] Figure 3 The image shows a SEM image of the sodium-ion battery cathode material prepared in Comparative Example 1.

[0025] Figure 4 The charge-discharge curves of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1 at a rate of 0.1 C are shown.

[0026] Figure 5 The graph shows a comparison of the cycling performance of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1 at a rate of 1.0 C. Detailed Implementation

[0027] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. To further understand this invention, the following description, in conjunction with the specification and specific preferred embodiments, will further illustrate the invention. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0028] Example 1 Anhydrous sodium sulfate, ferrous sulfate heptahydrate, and MgF₂ were weighed and placed in a ball mill jar according to the molar ratio of Na : Fe : Mg : F = 6 : 3.98 : 0.02 : 0.04. Ascorbic acid was added in an amount of Fe. 2+ Ascorbic acid was added in a ratio of 10:2, followed by PEG at 1 wt% of the total raw material mass. An appropriate amount of anhydrous ethanol was added for wet ball milling at 400 rpm for 4 hours. After vacuum drying at 80 ℃ for 12 hours, a sodium ferric sulfate precursor was obtained. This precursor was then added to an argon-protected ball mill jar, followed by 5 wt% Ketjen black (by weight of the precursor). Argon gas was introduced, and the ball milling was performed at 400 rpm for 2 hours. A certain amount of material was spread evenly in a crucible and heated to 200 ℃ in a tube furnace under an argon / hydrogen mixed atmosphere (Ar / H2 95:5, v / v) at a heating rate of 2 ℃ / min. The temperature was held for 2 hours, then further heated to 375 ℃ at a heating rate of 3 ℃ / min and held for 8 hours. After natural cooling, the material was pulverized and sieved to obtain a magnesium-fluorine dual-substituted modified sodium ferric sulfate cathode material.

[0029] Battery assembly and electrochemical performance testing The sodium-ion battery cathode material prepared according to this invention was weighed and mixed according to the ratio of active material: acetylene black: PVDF = 7:2:1. N-methylpyrrolidone solution (NMP) was added for homogenization, and the slurry was then uniformly coated onto an aluminum foil current collector. The mixture was vacuum dried at 100 °C for 4 h, and then rolled and cut into 12 mm sheets to obtain the desired sodium-ion battery cathode sheet. Using a metallic sodium sheet as the counter electrode, a 1 mol / L NaClO4 solution of ethylene carbonate (EC) / propylene carbonate (PC) (volume ratio 1:1) was used as the electrolyte, with 5% fluoroethylene carbonate (FEC) added. A glass fiber separator was used as the separator, and CR2032 coin cells were assembled in an argon glove box. The electrochemical performance of the coin cells was tested using a Blue Battery testing system, with a test voltage range of 2.0-4.3 V and a test temperature of room temperature.

[0030] Example 2 Except for the anhydrous sodium sulfate, ferrous sulfate heptahydrate, and MgF2, which were weighed according to the molar ratio of Na:Fe:Mg:F = 6:3.95:0.05:0.1, the rest were exactly the same as in Example 1.

[0031] Example 3 Except for the anhydrous sodium sulfate, ferrous sulfate heptahydrate, and MgF2, which were weighed according to the molar ratio of Na:Fe:Mg:F = 6:3.92:0.08:0.16, the rest were exactly the same as in Example 1.

[0032] Example 4 Except for the anhydrous sodium sulfate, ferrous sulfate heptahydrate, and MgF2, which were weighed according to the molar ratio of Na:Fe:Mg:F = 6:3.9:0.1:0.2, the rest were exactly the same as in Example 1.

[0033] Comparative Example 1 Anhydrous sodium sulfate and ferrous sulfate heptahydrate were weighed and placed in a ball mill jar at a molar ratio of Na:Fe = 6:4. Ascorbic acid was added in an amount of Fe. 2+Ascorbic acid was added in a ratio of 10:2, followed by PEG at 1 wt% of the total raw material mass. An appropriate amount of anhydrous ethanol was added for wet ball milling at 400 rpm for 4 hours. After vacuum drying at 80 °C for 12 hours, sodium ferric sulfate precursor was obtained. The precursor was then added to an argon-protected ball mill jar, followed by 5 wt% Ketjen black. Argon gas was introduced, and the ball milling was performed at 400 rpm for 2 hours. A certain amount of material was spread evenly in a crucible and heated to 200 °C in a tube furnace under an argon / hydrogen mixed atmosphere (Ar / H2, 95:5, v / v) at a heating rate of 2 °C / min. The temperature was held for 2 hours, then continued at a heating rate of 3 °C / min to 375 °C and held for 8 hours. After natural cooling, the material was pulverized and sieved to obtain undoped sodium ferric sulfate cathode material.

[0034] Comparative Example 2 Except for the absence of PEG, it is exactly the same as Example 2.

[0035] Comparative Example 3 Except for replacing the argon-hydrogen mixture with a pure argon atmosphere, it is exactly the same as in Example 2.

[0036] Comparative Example 4 Except for the anhydrous sodium sulfate, ferrous sulfate heptahydrate, and MgSO4, which were weighed according to the molar ratio of Na:Fe:Mg = 6:3.95:0.05, the rest were exactly the same as in Example 2.

[0037] Comparative Example 5 Except for the anhydrous sodium sulfate, ferrous sulfate heptahydrate, and NaF, which were weighed according to the molar ratio of Na:Fe:F = 6:4:0.1, everything else was exactly the same as in Example 2.

[0038] Figure 1 The XRD patterns of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1 show that the diffraction peaks of the doped sample shift to the right and there are no impurity peaks, proving that Mg... 2+ With F - Successfully incorporated into the crystal lattice without disrupting the crystal structure of sodium ferric sulfate.

[0039] Figure 2 This is a SEM image of the sodium-ion battery cathode material prepared in Example 2. Figure 3 The image shows a SEM image of the sodium-ion battery cathode material prepared in Comparative Example 1. The comparison shows that the loose particle spacing in Example 2 facilitates electrolyte wetting and provides ample channels for sodium ion transport; in Comparative Example 1, the particles are aggregated with fewer internal voids, making it difficult for the electrolyte to fully wet the material, resulting in insufficient sodium ion transport channels and greater resistance to ion conduction.

[0040] Figure 4 The figures show the charge-discharge curves of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1 at a rate of 0.1 C. It can be seen that the magnesium-fluorine co-doped sample has higher discharge capacity, lower voltage polarization, and better sodium-ion insertion / extraction kinetics. Figure 5 The graph shows a comparison of the cycling performance of the sodium-ion battery cathode materials prepared in Example 2 and Comparative Example 1 at a rate of 1.0 C. It can be seen that the initial capacity and capacity retention of Example 2 are both higher than those of the pure phase, indicating that dual modification of the lattice and interface can effectively suppress capacity decay during cycling.

[0041] The experimental results of the above embodiments and comparative examples are recorded in Table 1 below: Table 1

[0042] Table 1 shows that there is an optimal range for magnesium-fluorine dual-site doping, with the preferred stoichiometry for F being 0.04–0.16 and for Mg 0.02–0.08. Example 2 (x = 0.05, y = 0.10) exhibits the best overall electrochemical performance, achieving a reversible discharge specific capacity of 99.21 mAh / g at 0.1 C and a capacity retention of 94.87% after 200 cycles at 1 C. Lower doping levels result in weaker modification, while excessive doping (Example 4) induces lattice distortion, blocking sodium ion transport channels and causing capacity decay. Comparative Example 1 (undoped) has a capacity of 89.64 mAh / g, lower than Example 2, demonstrating that optimal doping can increase the specific capacity by nearly 10 mAh / g. Comparative Example 2, without PEG dispersant, has a capacity of only 81.57 mAh / g, proving that the dispersant can inhibit powder agglomeration and ensure uniform dispersion of the dopant element in the precursor. Comparative Example 3, sintered under a pure argon atmosphere, produced a sample with lower discharge capacity and cycle stability than Example 2, demonstrating that a weakly reducing mixed atmosphere containing hydrogen can effectively protect Fe. 2+ It is not oxidized, which significantly improves the electrochemical performance of the material. As shown in Comparative Examples 4-5, single-doped Mg can only strengthen the crystal lattice, and single-doped F can only improve the interface. The individual modification gains of the two are limited. Co-doping of Mg and F forms a synergistic effect, which simultaneously solves the three major problems of crystal lattice distortion, interfacial side reactions and poor conductivity, thus verifying the unique advantages of the double-site substitution of this invention.

[0043] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific technical solutions or characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A magnesium-fluorine dual-substitution modified iron-sodium-sodium sulfate cathode material for sodium-ion batteries, characterized in that, The chemical formula of the positive electrode material is Na6Fe. 4-x Mg x S(O 4-y F y )7, where 0.02 ≤ x ≤ 0.10, 0.04 ≤ y ≤ 0.20; the elements in the chemical formula satisfy charge balance; the material uses a single MgF2 as the anion and cation co-doping source to simultaneously achieve Mg 2+ Occupying Fe sites in the crystal lattice, F – Replace the sulfate O site.

2. The method for preparing a magnesium-fluorine dual-substituted modified iron-sodium-sodium ion battery cathode material according to claim 1, characterized in that, Specifically, the following steps are included: (1) Weigh sodium source, iron source, antioxidant, dispersant and MgF2 according to the stoichiometric ratio, add them to anhydrous ethanol and wet ball mill to form a uniform suspension dispersion, and vacuum low temperature drying to obtain uniform co-doped precursor powder. (2) The precursor powder and carbon source are ball-milled and coated under an inert gas atmosphere to obtain a uniformly coated composite powder material; (3) The composite powder is placed in a weak reducing atmosphere for low-temperature pre-calcination to remove water of crystallization and volatile components; (4) The pre-calcined product is placed in a weak reducing atmosphere for sintering. After the heat preservation is completed, it is cooled to room temperature in the furnace. After grinding and sieving, magnesium-fluorine double-substituted sodium iron sulfate cathode material is obtained.

3. The preparation method according to claim 2, characterized in that, In step (1), the sodium source is one or more of anhydrous sodium sulfate, sodium sulfate heptahydrate, sodium sulfate decahydrate, and sodium bisulfate; the iron source is one or more of anhydrous ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate tetrahydrate, and ferrous sulfate heptahydrate.

4. The preparation method according to claim 2, characterized in that, In step (1), the antioxidant is one or more of ascorbic acid, citric acid, oxalic acid, tartaric acid, and polyvinylpyrrolidone; the antioxidant and Fe 2+ Add Fe at a molar ratio 2+ Antioxidant : Antioxidant = 10 : (1~2).

5. The preparation method according to claim 2, characterized in that, In step (1), the dispersant is one or more of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, and hexadecyltrimethylammonium bromide; the amount of dispersant added is 0.5 to 3.0 wt% of the total mass of the raw materials.

6. The preparation method according to claim 2, characterized in that, In step (1), the wet ball milling is set with a milling speed of 300~600 rpm and a time of 4~12 h; vacuum drying is performed at a temperature of 60~80 ℃.

7. The preparation method according to claim 2, characterized in that, In step (2), the carbon source is one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, acetylene black, Ketjen black and Super P; the mass of the added carbon source is 1.0~5.0 wt% of the mass of the sodium ferric sulfate precursor.

8. The preparation method according to claim 2, characterized in that, In step (2), the inert atmosphere is one or more of argon or nitrogen; the ball milling speed is 300~600 rpm, and the ball milling time is 2~6 h.

9. The preparation method according to claim 2, characterized in that, In step (3), the weak reducing atmosphere is 3 vol%~8 vol% H2, and the rest is Ar; the heating rate is 2~5 ℃ / min, the pre-fire temperature is 150~250 ℃, and the holding time is 1~4 h.

10. The preparation method according to claim 2, characterized in that, In step (4), the sintering temperature is 350~400℃, the holding time is 6~12 h, and the heating rate is 2~5℃ / min.

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