A structural charge synergistically regulated sodium iron fluorophosphate positive electrode material and a preparation method thereof

Abstract

The application relates to a structure charge synergistically regulated sodium iron fluorophosphate positive electrode material and a preparation method thereof, and relates to a sodium ion battery positive electrode material and a preparation method. 1‑x‑ y V x Ti y PO4F@C, wherein 0.01 <= x <= 0.05, 0.01 <= y <= 0.05, and x+y < 0.1; C represents a carbon coating layer. The preparation method is as follows: after wet grinding, drying and sintering of raw materials for preparing sodium iron fluorophosphate, the raw materials are ball milled with a carbon source, and then sintered to obtain the sodium iron fluorophosphate. 0.95 V 0.03 Ti 0.02 The specific discharge capacity of the PO4F@C at 0.2C is 116.9 mAh / g, and the capacity retention rate is 94.0% after 100 cycles. The material can be used in the field of sodium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and more specifically to cathode materials for sodium-ion batteries and their preparation methods. Background Technology

[0002] With the rapid development of large-scale energy storage and low-speed electric vehicle markets, sodium-ion batteries, which are abundant in resources and inexpensive, have attracted widespread attention. Among many cathode materials, sodium iron fluorophosphate is considered one of the most promising material systems due to its stable three-dimensional framework structure, moderate operating voltage, and high theoretical specific capacity.

[0003] However, the low intrinsic electronic conductivity and ion diffusion coefficient of sodium fluorophosphate severely limit its rate performance and cycling performance. To address this issue, existing technologies typically employ carbon coating or single-element doping strategies, but these struggle to simultaneously improve structural stability and initial conductivity during long-cycle periods. Furthermore, conventional multi-element doping often focuses only on the superposition of element types, lacking in-depth design and verification of inter-element synergies and optimal atomic ratios. It also demands higher homogeneity in raw material synthesis processes, resulting in relatively high costs and failing to meet the demands for high-performance cathode materials in practical applications. Therefore, developing a novel modification scheme that synergistically improves the electronic conductivity and structural stability of sodium fluorophosphate, while also allowing for easy control of raw materials and processes and reducing preparation costs, has become a pressing technical challenge in this field. Summary of the Invention

[0004] This invention aims to address the technical problems of existing modification methods for sodium iron fluorophosphate materials, which struggle to simultaneously improve structural stability and initial conductivity during long-term cycling, and which involve high requirements for synthesis processes and high costs. Instead, it provides a sodium iron fluorophosphate cathode material with synergistically regulated structure and charge, and its preparation method. The primary objective of this invention is to prepare a sodium iron fluorophosphate cathode material with synergistically regulated structure and charge through V-Ti synergistic doping in a specific ratio, significantly enhancing its intrinsic electronic conductivity while simultaneously improving the stability of its crystal structure during long-term cycling. Another objective of this invention is to provide a simple and easily scalable method for preparing the cathode material.

[0005] The structure-charge synergistic regulation of the sodium fluorophosphate cathode material of the present invention has the general chemical formula: Na₂Fe 1-x- y V x Ti y PO4F@C, where 0.01 ≤ x ≤ 0.05, 0.01 ≤ y ≤ 0.05, and x+y < 0.1; C represents the carbon coating layer.

[0006] In a preferred embodiment of the present invention, in the general chemical formula, x = 0.03, y = 0.02, that is, the chemical formula of the positive electrode material is Na₂Fe. 0.95 V 0.03 Ti 0.02 PO4F@C. This specific ratio was found to produce the optimal synergistic effect.

[0007] The above-mentioned method for preparing the structure-charge synergistically regulated sodium fluorophosphate cathode material is carried out according to the following steps:

[0008] 1. Weigh out sodium source, iron source, vanadium source, titanium source, phosphorus source and fluorine source according to the stoichiometric ratio in the general chemical formula of sodium iron fluorophosphate cathode material, and add them to the ball mill jar in sequence. Then add an appropriate amount of anhydrous ethanol for wet milling to obtain a uniform slurry. Then dry the slurry to obtain precursor A.

[0009] 2. Place precursor A into a tube furnace, heat it to 300~400℃ under inert gas protection and hold for 4~6 hours for sintering, cool it to room temperature and take it out to obtain preliminary calcined powder;

[0010] 3. Add the pre-calcined powder back into the ball mill jar, and add a carbon source of 15% to 25% of the mass of the pre-calcined powder, and perform a second ball milling to obtain precursor B;

[0011] Fourth, the precursor B is placed in a tube furnace and heated to 450℃~600℃ under inert gas protection for 4~8 hours for re-sintering. After cooling to room temperature, the sodium iron fluorophosphate cathode material with synergistic regulation of structure and charge is obtained.

[0012] Furthermore, the sodium source mentioned in step one is sodium acetate or sodium carbonate.

[0013] Furthermore, the iron source mentioned in step one is ferrous oxalate or ferrous citrate.

[0014] Furthermore, the phosphorus source mentioned in step one is ammonium dihydrogen phosphate or diammonium hydrogen phosphate.

[0015] Furthermore, the fluorine source mentioned in step one is sodium fluoride or ammonium fluoride.

[0016] Furthermore, the vanadium source mentioned in step one is at least one of ammonium metavanadate, vanadium pentoxide, and vanadium trioxide.

[0017] Furthermore, the titanium source mentioned in step one is at least one of titanium dioxide, tetrabutyl titanate, and titanium tetrachloride.

[0018] Furthermore, the wet milling described in step one is performed by ball milling for 8 to 10 hours at a ball-to-material mass ratio of 1:(15~20) and a rotation speed of 500~800 rpm.

[0019] Furthermore, the drying process described in step one involves placing the item in a forced-air drying oven at a temperature of 65-70°C for 8-10 hours.

[0020] Furthermore, the inert gas mentioned in step two is argon or a mixture of hydrogen and argon.

[0021] Furthermore, the carbon source mentioned in step three is at least one of glucose, carbon nanotubes, or polyvinylpyrrolidone, and the carbon source forms a carbon coating layer in situ during the sintering process.

[0022] Furthermore, the inert gas mentioned in step four is argon or a mixture of hydrogen and argon.

[0023] This invention employs a V / Ti co-doping strategy to achieve vanadium (V) co-doping. 3+ The introduction of sodium ferric fluorophosphate as a "conductivity enhancer" narrows the material band gap by forming impurity energy levels, thereby improving intrinsic electronic conductivity; while titanium (Ti 4+ The introduction of ) mainly serves as a "structural stabilizer," which stabilizes Ti 4+ The ions act as a buffer, cushioning the volume changes during the insertion / extraction of sodium ions. The effect is not simply additive, but rather a synergistic one, where "1+1>2". This results in the doped material exhibiting excellent high-rate performance and ultra-long cycle life. Among these, Na₂Fe... 0.95 V 0.03 Ti 0.02 PO4F@C has a discharge specific capacity of 116.9 mAh / g at a 0.2 C rate current and a capacity retention of 94.0% after 100 cycles.

[0024] This invention utilizes a solid-state method for preparation, which is simple in process, easy to control in conditions, and suitable for large-scale production. Attached Figure Description

[0025] Figure 1 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 XRD patterns of PO4F@C and Na2FePO4F@C prepared in Comparative Example 1.

[0026] Figure 2 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 Charge-discharge curves of PO4F@C.

[0027] Figure 3 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 Rate performance curve of PO4F@C.

[0028] Figure 4 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 Cyclic performance curves of PO4F@C. Detailed Implementation

[0029] The following examples will demonstrate the beneficial effects of the present invention.

[0030] Example 1: The sodium fluorophosphate cathode material Na2Fe with synergistic structural charge regulation in this example 0.95 V 0.03 Ti 0.02 The preparation method of PO4F@C is carried out according to the following steps:

[0031] I. According to the sodium fluorophosphate cathode material Na2Fe 0.95 V 0.03 Ti 0.02 To prepare PO4F, stoichiometrically weigh 0.8202 g sodium acetate, 1.7090 g ferrous oxalate, 0.0225 g vanadium trioxide, 0.0160 g titanium dioxide, 1.1503 g ammonium dihydrogen phosphate, and 0.4199 g sodium fluoride and add them sequentially to a ball mill jar. Then, add 40 mL of anhydrous ethanol and ball mill for 9 hours at a ball-to-material mass ratio of 1:15 and a rotation speed of 500 rpm to obtain a uniform slurry. The slurry is then dried in a forced-air drying oven at 65°C for 8 hours to obtain precursor A.

[0032] 2. Precursor A is placed in a tube furnace and sintered at 350°C for 5 hours under argon protection at a rate of 5°C / min. After cooling to room temperature, it is removed to obtain preliminary calcined powder.

[0033] 3. Add the pre-calcined powder back into the ball mill jar, and add polyvinylpyrrolidone (PVP) at 25% of the mass of the pre-calcined powder as a carbon source. Ball mill for 2 hours at a ball-to-powder mass ratio of 1:15 and a rotation speed of 500 rpm to obtain precursor B.

[0034] IV. Precursor B was placed in a tube furnace and heated to 550℃ at a rate of 5℃ / min under an argon atmosphere, held for 6 hours for re-sintering, and then cooled to room temperature to obtain the sodium iron fluorophosphate cathode material Na2Fe with synergistic structural and charge regulation. 0.95 V 0.03 Ti 0.02 PO4F@C.

[0035] Comparative Example 1: Undoped Na2FePO4F@C cathode material was prepared in this comparative example. The specific steps are as follows:

[0036] 1. Weigh 0.8202g sodium acetate, 1.7989g ferrous oxalate, 1.1503g ammonium dihydrogen phosphate, and 0.4199g sodium fluoride according to the stoichiometric ratio of the molecular formula Na2FePO4F and add them to a ball mill jar. Then add 40 mL of anhydrous ethanol and ball mill for 9 hours at a ball-to-material mass ratio of 1:15 and a rotation speed of 500 rpm to obtain a uniform slurry. Then dry the slurry in a forced-air drying oven at 65℃ for 8 hours to obtain precursor A.

[0037] 2. Precursor A is placed in a tube furnace and calcined at 350°C for 5 hours at a rate of 5°C / min under an argon atmosphere. The furnace is then cooled to room temperature to obtain preliminary calcined powder.

[0038] 3. Add the pre-calcined powder to a ball mill jar, and then add polyvinylpyrrolidone (25% of the mass of the pre-calcined powder) as a carbon source. Ball mill at 500 rpm for 2 hours to obtain precursor B.

[0039] Fourth, the precursor B is placed in a tube furnace and calcined at 550°C for 6 hours under an argon atmosphere. The furnace is then cooled to room temperature to obtain a black powdery Na2FePO4F@C cathode material.

[0040] Comparative Example 2: This comparative example is the preparation of vanadium-doped Na2Fe. 0.97 V 0.03 The specific steps for using PO4F@C cathode material are as follows:

[0041] I. According to Na2Fe 0.97 V 0.03To prepare PO4F, 0.8202 g of sodium acetate, 1.7449 g of ferrous oxalate, 0.0225 g of vanadium trioxide, 1.1503 g of ammonium dihydrogen phosphate, and 0.4199 g of sodium fluoride were weighed out according to their stoichiometric ratio and added sequentially to a ball mill jar. Then, 40 mL of anhydrous ethanol was added. The mixture was ball-milled for 9 hours at a ball-to-material mass ratio of 1:15 and a rotation speed of 500 rpm to obtain a homogeneous slurry. The slurry was then dried in a forced-air drying oven at 65°C for 8 hours to obtain precursor A.

[0042] 2. Precursor A is placed in a tube furnace and sintered at 350°C for 5 hours under argon protection at a rate of 5°C / min. After cooling to room temperature, it is removed to obtain preliminary calcined powder.

[0043] 3. Add the pre-calcined powder back into the ball mill jar, and add polyvinylpyrrolidone (PVP) at 25% of the mass of the pre-calcined powder as a carbon source. Ball mill for 2 hours at a ball-to-powder mass ratio of 1:15 and a rotation speed of 500 rpm to obtain precursor B.

[0044] IV. Precursor B was placed in a tube furnace and heated to 550°C at a rate of 5°C / min under an argon atmosphere, held for 6 hours for re-sintering, and then cooled to room temperature to obtain black powdered Na2Fe. 0.97 V 0.03 PO4F@C cathode material.

[0045] Comparative Example 3: Preparation of Titanium-Doped Na2Fe 0.98 Ti 0.02 The specific method for using PO4F@C cathode material is as follows:

[0046] I. According to Na2Fe 0.98 Ti 0.02 To prepare PO4F, 0.8202 g of sodium acetate, 1.762 g of ferrous oxalate, 0.0160 g of titanium dioxide, 1.1503 g of ammonium dihydrogen phosphate, and 0.4199 g of sodium fluoride were weighed out according to their stoichiometric ratio and added sequentially to a ball mill jar. Then, 40 mL of anhydrous ethanol was added, and the mixture was ball-milled for 9 hours at a ball-to-material mass ratio of 1:15 and a rotation speed of 500 rpm to obtain a uniform slurry. The slurry was then dried in a forced-air drying oven at 65°C for 8 hours to obtain precursor A.

[0047] 2. Precursor A is placed in a tube furnace and sintered at 350°C for 5 hours under argon protection at a rate of 5°C / min. After cooling to room temperature, it is removed to obtain preliminary calcined powder.

[0048] 3. Add the pre-calcined powder back into the ball mill jar, and add polyvinylpyrrolidone (PVP) at 25% of the mass of the pre-calcined powder as a carbon source. Ball mill for 2 hours at a ball-to-powder mass ratio of 1:15 and a rotation speed of 500 rpm to obtain precursor B.

[0049] IV. Precursor B is placed in a tube furnace and heated to 550°C at a rate of 5°C / min under an argon atmosphere, held for 6 hours for re-sintering, and then cooled to room temperature to obtain black powdered Na2Fe. 0.98 Ti 0.02 PO4F@C cathode material.

[0050] Figure 1 Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 XRD patterns of Na₂FePO₄F@C prepared in Comparative Example 1 and PO₄F@C. Figure 1 It can be seen that the XRD patterns of Example 1 and Comparative Example 1 match the characteristic peaks of the standard card well, and no impurity phase peaks were observed, confirming that the synthesized materials are all pure phases. Comparing the patterns of Example 1 and Comparative Example 1, the positions and relative intensities of their diffraction peaks are basically consistent, indicating that appropriate doping did not cause significant distortion of the bulk structure of the material.

[0051] The sodium iron fluorophosphate cathode material with synergistic structure-charge regulation obtained in Example 1 and the fluorophosphate cathode material prepared in Comparative Example 1 were used as electrode active materials for electrochemical testing. The battery assembly steps were as follows: Electrode active materials, conductive agent Super P, and vinylidene fluoride (PVDF) were weighed at a mass ratio of 7:2:1, mixed in a mortar, and transferred to a weighing bottle. Then, an appropriate amount of N-methylpyridinyl ketone (NMP) was added to the weighing bottle. After the solid-liquid mixture in the bottle became viscous, a magnetic stirrer was added, and the mixture was magnetically stirred at room temperature for 4 hours to obtain a viscous but somewhat fluid electrode paste. The electrode paste was then scraped onto the electrode sheet using a spatula, resulting in an active material loading of 2-3 mg / cm³. -2 The assembly of CR2025 coin cells was carried out entirely in a glove box protected by high-purity argon, with water and oxygen content controlled below 0.1 ppm. The assembly sequence was: positive electrode shell → positive electrode → separator → electrolyte → negative electrode → negative electrode shell. The positive electrode was a circular electrode sheet, the negative electrode was a sodium metal sheet, the separator was a glass fiber separator, and the electrolyte was a 1:1 volume ratio mixture of ethylene carbonate (EC) and propylene carbonate (PC) in 1.0 mol / L NaClO4 solution. After sealing the coin cells with a sealing machine, they were left to stand for 2 hours before testing. Constant current charge-discharge tests were performed on a Newway battery tester at 25°C, with a voltage range of 2–4 V. Figure 2The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 Charge-discharge curves of PO4F@C. Figure 2 The charge-discharge curves of the sodium iron fluorophosphate cathode materials in Example 1 and Comparative Example 1 show that at a current rate of 0.2 C, the discharge specific capacity of Example 1 reaches 116.9 mAh / g, which is 5.6% higher than that of Comparative Example 1 (110.7 mAh / g). The Na2Fe in Example 1... 0.95 V 0.03 Ti 0.02 PO4F@C compared to Na2Fe monovanadium doped in Comparative Example 2 0.97 V 0.03 PO4F@C cathode material and Comparative Example 3: Na2Fe doped with titanium 0.98 Ti 0.02 The discharge specific capacity of the PO4F@C cathode material was increased by 5.0% and 9.8%, respectively.

[0052] Figure 3 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 The rate performance curves of PO4F@C are shown in Table 1.

[0053] Table 1 Rate performance and cycle performance of sodium-ion batteries assembled with sodium iron fluorophosphate cathode

[0054]

[0055] from Figure 3 It can be seen that, at a current density of 1C, the Na2Fe in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C exhibited a specific capacity of 109.9 mAh / g, while the specific capacity of Na2FePO4F@C in Comparative Example 1 was 96.4 mAh / g. At a current density of 10 C, the Na2Fe from Example 1... 0.95 V 0.03 Ti0.02 PO4F@C maintains a specific capacity of 73.1 mAh / g, while Na2FePO4F@C in Comparative Example 1 only has 62.9 mAh / g, and Na2Fe in Comparative Example 2, which is doped with vanadium, also has a specific capacity of 62.9 mAh / g. 0.97 V 0.03 The PO4F@C cathode material has a capacity of 68.1 mAh / g, compared to the titanium-doped Na2Fe in Comparative Example 3. 0.98 Ti 0.02 The PO4F@C cathode material has a capacity of 63.0 mAh / g. Example 1: Na2Fe 0.95 V 0.03 Ti 0.02 The high-rate electrochemical activity of PO4F@C stems from the dual role of V and Ti in charge regulation: stabilizing the crystal structure and ensuring high-speed ion / electron transport.

[0056] Figure 4 The Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 PO4F@C, Na2FePO4F@C prepared in Comparative Example 1, and Na2Fe prepared in Comparative Example 2 0.97 V 0.03 PO4F@C and Na2Fe 0.98 Ti 0.02 Cyclic performance curves after 100 cycles of PO4F@C. From Figure 4 It can be seen that the Na2Fe prepared in Example 1 0.95 V 0.03 Ti 0.02 The capacity retention rate of PO4F@C after 100 cycles at a current density of 1C is 94.0%, while that of Na2FePO4F@C in Comparative Example 1 is 80.7% after 100 cycles at a current density of 1C. The comparison shows that by using V-Ti for synergistic regulation of structural charge, the intrinsic electronic conductivity and crystal structure stability of sodium iron fluorophosphate cathode material are simultaneously improved, which strongly supports the technical advantages of the invention.

[0057] The above embodiments demonstrate that this invention successfully constructs a vanadium-titanium co-doped sodium iron fluorophosphate cathode material, and for the first time proposes a synergistic regulation mechanism for the structure and charge of this material system. Through the synergistic effect of vanadium and titanium, not only is structural distortion during cycling suppressed, but also the ion / electron transport channels are optimized through charge redistribution, effectively solving the bottleneck problems of poor conductivity and rate performance of sodium iron fluorophosphate. The material preparation method of this invention is simple, cost-controllable, and environmentally friendly, and the resulting product exhibits excellent electrochemical performance.

Claims

1. A sodium fluorophosphate cathode material with synergistic structural charge regulation, characterized in that, The chemical formula of this material is: Na₂Fe 1-x-y V x Ti y PO4F@C, where 0.01 ≤ x ≤ 0.05, 0.01 ≤ y ≤ 0.05, and x+y < 0.1; C represents the carbon coating layer.

2. The sodium fluorophosphate cathode material with synergistic structural charge regulation according to claim 1, characterized in that, Na2Fe 1-x-y V x Ti y In Na2Fe 1-x-y V x Ti y PO4F@C, x = 0.03 and y = 0.

02.

3. A method for preparing the sodium fluorophosphate cathode material with synergistic structural charge regulation as described in claim 1, characterized in that, This method is performed in the following steps:

1. Weigh out sodium source, iron source, vanadium source, titanium source, phosphorus source and fluorine source according to the stoichiometric ratio in the general chemical formula of sodium iron fluorophosphate cathode material, and add them to the ball mill jar in sequence. Then add an appropriate amount of anhydrous ethanol for wet milling to obtain a uniform slurry. Then dry the slurry to obtain precursor A.

2. Place precursor A into a tube furnace, heat it to 300~400℃ under inert gas protection and hold for 4~6 hours for sintering, cool it to room temperature and take it out to obtain preliminary calcined powder; 3. Add the pre-calcined powder back into the ball mill jar, and add a carbon source of 15% to 25% of the mass of the pre-calcined powder, and perform a second ball milling to obtain precursor B; Fourth, the precursor B is placed in a tube furnace and heated to 450℃~600℃ under inert gas protection for 4~8 hours for re-sintering. After cooling to room temperature, the sodium iron fluorophosphate cathode material with synergistic regulation of structure and charge is obtained.

4. The method for preparing a sodium fluorophosphate cathode material with synergistic structural charge regulation according to claim 3, characterized in that, The sodium source mentioned in step one is sodium acetate or sodium carbonate; the iron source is ferrous oxalate or ferrous citrate; the phosphorus source is ammonium dihydrogen phosphate or diammonium hydrogen phosphate; and the fluorine source is sodium fluoride or ammonium fluoride.

5. The method for preparing a sodium fluorophosphate cathode material with synergistic structural charge regulation according to claim 3 or 4, characterized in that, The vanadium source mentioned in step one is at least one of ammonium metavanadate, vanadium pentoxide, and vanadium trioxide.

6. The method for preparing a sodium fluorophosphate cathode material with synergistic structural charge regulation according to claim 3 or 4, characterized in that, The titanium source mentioned in step one is at least one of titanium dioxide, tetrabutyl titanate, and titanium tetrachloride.

7. The method for preparing a sodium iron fluorophosphate cathode material with synergistic structural charge regulation according to claim 3 or 4, characterized in that, The wet milling described in step one is performed by ball milling for 8 to 10 hours at a ball-to-material mass ratio of 1:(15~20) and a rotation speed of 500~800 rpm.

8. The method for preparing a sodium iron fluorophosphate cathode material with synergistic structural charge regulation according to claim 3 or 4, characterized in that, The carbon source mentioned in step three is at least one of glucose, carbon nanotubes, or polyvinylpyrrolidone.

9. A method for preparing a sodium fluorophosphate cathode material with synergistic structural charge regulation according to claim 3 or 4, characterized in that, The drying process described in step one involves placing the item in a forced-air drying oven at a temperature of 65-70℃ for 8-10 hours.

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