Multi-doped carbon-coated na3v 1.94 (TM) 0.06 Method for preparing sodium-ion battery cathode material of (po4)2f3

Abstract

Multi-element doped carbon-coated Na3V 1.94 (TM) 0.06 Preparation method of Na3V2(PO4)2F3 sodium ion battery positive electrode material, it relates to the preparation method of sodium ion battery positive electrode material, it is to solve the existing Na3V2(PO4)2F3 positive electrode material capacity is low, the problem of poor rate performance.The present application adopts multi-element or even high-entropy doping strategy, introduces multiple transition metal elements to cooperatively dope vanadium site, uses sol-gel method to prepare multi-element doped carbon-coated Na3V 1.94 (TM) 0.06 Na3V2(PO4)2F3 sodium ion battery positive electrode material, wherein TM is 2-5 kinds of Al, Fe, Mn, Co and Ni.The first week capacity of the material is 115.69-130.25mA / g, the retention rate after 1100 cycles is 77.48%-81.85%, and the capacity at 30C high rate is as high as 112.18mA / g.It can be used in the field of high-power sodium ion battery.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a sodium ion battery positive electrode material and belongs to the technical field of material synthesis. BACKGROUND

[0002] The sodium ion battery system has become an important candidate technology in the field of large-scale energy storage because of its reliance on abundant resources and low price and the advantage of inheriting mature industrial technology of lithium batteries. Among many positive electrode material systems, Na3V2(PO4)2F3 exhibits better thermal stability than layered oxides due to its high redox platform and NASICON-type rigid skeleton structure. However, the industrialization process is still subject to the following key bottlenecks: 1) limited capacity release: although the theoretical specific capacity reaches 128.3 mAh / g (corresponding to a two-electron transfer reaction), the actual discharge capacity of commercial materials is generally lower than 115 mAh / g, which is caused by the low number of electron transfer due to the localization of vanadium ion 3d orbit; 2) kinetic lag: the sodium ion diffusion coefficient in the material is only ~10-12 cm / s, which leads to a sharp drop in capacity under high-rate working conditions, and the charge transfer impedance increases exponentially with cycles; 3) poor cycle stability: the anisotropic volume change caused by the V-O octahedron distortion in the charging and discharging process causes microcracks in the particle interior, leading to structural instability, and in the long-term cycle process, the material structure is unstable, and the capacity decays seriously. Although the current modification strategies such as single-element doping or surface modification engineering can partially improve the conductivity, it is difficult to realize the synergistic optimization of electron / ion transport. 2 SUMMARY

[0003] The application aims to solve the technical problems of low capacity and poor rate performance of the existing Na3V2(PO4)2F3 positive electrode material, and provides a preparation method of a multi-element doped carbon-coated Na3V 1.94 (TM) 0.06 (PO4)2F3 sodium ion battery positive electrode material. The application proposes a multi-element synergistic doping strategy based on the high-entropy doping design concept to solve the key defects of limited electron / ion conduction and insufficient structural stability of the Na3V2(PO4)2F3 positive electrode material. By constructing a multi-element or high-entropy doped carbon-coated Na3V 1.94 ( TM) 0.06 (PO4)2F3 (TM= Al, Fe, Mn, Co, Ni) sodium positive electrode material structure, the electronic state density reconstruction and sodium ion diffusion channel are synergistically optimized.

[0004] The preparation method of the multi-element doped carbon-coated Na3V 1.94 (TM) 0.06 (PO4)2F3 sodium ion battery positive electrode material is carried out in the following steps: ​

[0005] I. According to the stoichiometric ratio of Na3V 1.94 (TM) 0.06 (PO4)2F3, wherein TM is a combination of 2-5 kinds of Al, Fe, Mn, Co and Ni, the vanadium source is a trivalent vanadium source or a pentavalent vanadium source, the sodium salt, the transition metal TM source, the phosphate and the fluoride are weighed in;

[0006] For the trivalent vanadium source, the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.73-0.83):1;

[0007] For the pentavalent vanadium source, the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.73-0.83):1; for the carbon source without reducibility, a reducing agent is weighed in, wherein the amount of the reducing agent is just enough to completely reduce the pentavalent vanadium source V 5+ to V 3+ ;

[0008] II. The carbon source or the carbon source and the reducing agent are dissolved in water, and the pH value is adjusted to 4-4.5, then the vanadium source is added to obtain a mixed solution;

[0009] III. The mixed solution is heated to a temperature of 80-90℃, stirring is started, and the sodium salt, the transition metal TM source are sequentially added to the mixed solution in the process of stirring, after the addition is completed, stirring is continued for 30-50 minutes, then the phosphate and the fluoride are sequentially added and stirring is continued until a wet gel is formed;

[0010] IV. The wet gel is vacuum dried at a temperature of 80-85℃ for 12-15h to obtain a dry gel;

[0011] V. The dry gel is ground into powder, placed in a high-temperature furnace, heated to 200-400℃ for pre-sintering for 4-5h, then heated to 500-700℃ for high-temperature sintering for 7-9h to obtain a multi-doped carbon-coated Na3V 1.94 (TM) 0.06 (PO4)2F3 sodium-ion battery positive electrode material, Na3V 1.94 (TM) 0.06 (PO4)2F3, wherein TM is a combination of 2-5 kinds of Al, Fe, Mn, Co and Ni.

[0012] Further, the sodium salt in step I is one or a combination of several of sodium metavanadate, sodium chloride, sodium fluoride, sodium hydroxide and sodium acetate.

[0013] Further, the pentavalent vanadium source in step one is one or a combination of ammonium metavanadate, sodium metavanadate, and vanadium pentoxide; the trivalent vanadium source in step one is one or a combination of vanadium acetate, vanadium fluoride, vanadium phosphate, and vanadium trioxide.

[0014] Further, the aluminum source in step one is one or a combination of aluminum nitrate and aluminum acetate.

[0015] Further, the iron source in step one is one or a combination of iron nitrate and iron acetate.

[0016] Further, the manganese source in step one is one or a combination of manganese nitrate, manganese acetate, and manganese oxalate.

[0017] Further, the cobalt source in step one is one or a combination of cobalt nitrate, cobalt acetate, and cobalt oxalate.

[0018] Further, the nickel source in step one is one or a combination of nickel nitrate, nickel acetate, and nickel oxalate.

[0019] Further, the phosphate in step one is one or a combination of ammonium dihydrogen phosphate, phosphoric acid, and sodium phosphate.

[0020] Further, the fluoride in step one is one or a combination of ammonium fluoride, sodium fluoride, and vanadium fluoride.

[0021] Further, the carbon source in step one is one or a combination of glucose, sucrose, ethylene glycol, polyethylene glycol (PEG), citric acid, and ascorbic acid.

[0022] Further, the reducing agent in step one is one or a combination of ethylene glycol, citric acid, glucose, oxalic acid, ammonium carbamate, urea, ascorbic acid, and sodium borohydride.

[0023] Further, the mass ratio of solute to water in step two is 1:(15-30). The concentration of the solution affects the time of reaction. Too high a concentration leads to a faster sol-gel process, insufficient reduction of V elements, and poor complexing effect, affecting uniformity. Too low a concentration leads to insufficient reaction and the formation of impurities.

[0024] Further, the material used to adjust the pH value in step two is one or a combination of ammonia and sodium hydroxide.

[0025] Further, the inert atmosphere in step seven is nitrogen or argon. In an inert gas atmosphere, transition metal oxidation can be avoided, and the uniformity of carbon coating can be improved.

[0026] In the field of electrochemistry, in the prior art of sodium ion battery positive electrode materials, although the Na3V2(PO4)2F3 material has a stable NASICON type crystal structure, its intrinsic electronic conductivity is low, which makes it difficult to fully exert the actual specific capacity, and there is a problem of structural degradation in the long cycle process. Element doping is one of the most simple and efficient methods to improve the intrinsic electronic conductivity of the material. The traditional single element doping modification effect is limited, and the capacity retention rate is usually less than 75% after 1000 cycles.

[0027] The multi-element high-entropy doped carbon-coated Na3V 1.94 (AlFeMnCoNi) 0.06 (PO4)2F3 sodium ion battery positive electrode material preparation method, by introducing iron, aluminum, manganese, cobalt, nickel multi-element doping system, the crystal structure can be stabilized by using the synergistic effect between the dopants to increase the configuration entropy. And high-entropy cation doping can reduce the band gap between the conduction band and the valence band of the material, adjust the charge distribution, and change the electronic state density near the Fermi level, thereby improving the intrinsic conductivity of the material.

[0028] The method designs ion radius gradient doping. The V³⁺ ion radius in the original material is 0.058 nm. The doping of different ion radius elements has a synergistic effect to form an isotropic micro-strain field in the crystal lattice, which effectively alleviates the volume change caused by sodium ion deintercalation. The doping of anions with larger atomic radii in the crystal structure can slightly cause anisotropic expansion of the crystal lattice, which can expand the unit cell parameters and reduce the migration energy barrier. Large radius ions (such as Co 2+ , 0.745Å; Ni 2+ ,0.69Å) usually have lower electronegativity, which can adjust the electronic band structure of the material. Ni 2+ doping can increase the V-O bond length, reduce the 3d-2p orbital hybridization strength, and move the conduction band bottom to the Fermi level, thereby improving the electronic mobility. When small radius ions (such as Al 3+ , 0.535Å) replace the host metal sites, a uniform distribution of compressive stress field is formed. This micro-strain can buffer the anisotropic expansion caused by sodium ion deintercalation, thereby improving the cycle stability. Therefore, the cycle stability of the material after 1100 cycles is much higher than that of the undoped sample (63.97%), which is 81.85%.

[0029] The method also designs ion valence gradient doping. The redox pairs of Ni²⁺ / Ni³⁺, Mn²⁺ / Mn³⁺ and Co²⁺ / Co³⁺ can provide an additional charge compensation path in cooperation with V³⁺ / V 4 ⁺, thereby improving the specific capacity of the material. Therefore, the material can have the ability to release more than the rated capacity, and the high conductivity Ni 2+The overall electron mobility can also be improved, effectively improving the internal conductivity of the material, significantly improving the capacity, cycle life and rate performance of the material, especially the rate performance under large rate working conditions. The first week capacity of the material prepared by the application breaks through the theoretical capacity of Na3V2(PO4)2F3 material, reaching 115.69-130.25 mA / g, and the capacity retention rate reaches 77.48%-81.85% after 1100 cycles, and the capacity under 30C large rate is as high as 112.18 mA / g.

[0030] The method of the application prepares a multi-element or even high-entropy doped carbon-coated Na3V 1.94( TM) 0.06 The Na-ion battery cathode material of Na3V2(PO4)2F3 (TM= Al, Fe, Mn, Co, Ni) is a new type of cathode material with excellent performance, simple process and controllable cost, which can meet the application requirements of current high-rate performance sodium-ion batteries. It provides an industrialized solution for building a low-cost, high-safety and long-life sodium battery energy storage system, and can be used in high-energy-density sodium-ion batteries in special equipment, low-speed vehicles, power grid energy storage and emergency backup power supply fields. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1 is compared with the material prepared in Comparative Example 1 in terms of XRD spectrum; 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 XRD spectrum of the five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1 and the material prepared in Comparative Example 1;

[0032] Figure 2 The five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1 is compared with the material prepared in Comparative Example 1 in terms of SEM graph; 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 SEM graph of the five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1 and the material prepared in Comparative Example 1;

[0033] Figure 3 The five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1 is compared with the material prepared in Comparative Example 1 in terms of XPS full spectrum; 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 XPS full spectrum of the five-element high-entropy doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material prepared in Example 1;

[0034] Figure 4 Na3V2(PO4)2F3 prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Comparison chart of the first three circle differential capacity curves of the Na-ion battery cathode material (PO4)2F3 and the materials prepared in Example 2, Comparative Examples 1-2;

[0035] Figure 5 Na3V2(PO4)2F3 prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Comparison chart of the first circle charge-discharge of the Na-ion battery cathode material (PO4)2F3 and the materials prepared in Example 2, Comparative Examples 1-2;

[0036] Figure 6 Na3V2(PO4)2F3 prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Comparison chart of the rate performance of the Na-ion battery cathode material (PO4)2F3 and the materials prepared in Example 2, Comparative Examples 1-2.

[0037] Figure 7 Na3V2(PO4)2F3 prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Performance chart of (a) half-cell and (b) full-cell charge-discharge of the Na-ion battery cathode material (PO4)2F3.

[0038] Figure 8 Na3V2(PO4)2F3 prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Comparison chart of the cycle performance of the Na-ion battery cathode material (PO4)2F3 and the materials prepared in Example 2, Comparative Examples 1-2 at 5C rate. DETAILED DESCRIPTION

[0039] The beneficial effects of the present application are verified by the following examples.

[0040] Example 1: In this example, the multi-element doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 The preparation method of the sodium-ion battery cathode material (PO4)2F3 is carried out in the following steps:

[0041] I. 0.0090 grams of aluminum nitrate nonahydrate, 0.0096 grams of iron nitrate nonahydrate, 0.0060 grams of manganese nitrate tetrahydrate, 0.0070 grams of cobalt nitrate hexahydrate, 0.0070 grams of nickel nitrate hexahydrate, 0.2550 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and 0.6147 grams of citric acid as a carbon source and reducing agent are weighed. The molar ratio of Na, V, Al, Fe, Mn, Co, Ni, P, and F is 3:1.94:0.012:0.012:0.012:0.012:0.012:2:3, and the molar ratio of citric acid to transition metal atoms is 0.8:1.

[0042] II. Dissolve the citric acid in water, add ammonia water to adjust the pH value to 4.4, and then add the vanadium pentoxide to obtain a mixed solution.

[0043] III. Increase the temperature of the mixed solution to 85°C and start stirring. During the stirring process, add the aluminum nitrate nonahydrate, iron nitrate nonahydrate, manganese nitrate tetrahydrate, cobalt nitrate hexahydrate, and nickel nitrate hexahydrate in sequence. After the addition is complete, continue stirring for 30 minutes. Then add the ammonium dihydrogen phosphate and sodium fluoride in sequence. Continue stirring the mixed solution at 85°C until it becomes a wet gel.

[0044] IV. Dry the wet gel under vacuum at a temperature of 80°C for 12 hours to obtain a dry gel.

[0045] V. Grind the dry gel into powder and place it in a high-temperature furnace. In an inert atmosphere, pre-sinter at 300°C for 4 hours, and then sinter at a high temperature of 600°C for 8 hours to obtain the multi-element doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 (PO4)2F3 sodium-ion battery cathode material.

[0046] Example 2: In this example, only two elements (Al 3+ and Fe 3+), the difference between this embodiment and embodiment 1 is that the operation of step one is as follows: I. Take 0.0225 grams of aluminum nitrate nonahydrate, 0.0242 grams of iron nitrate nonahydrate, 0.2550 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and then take 0.6147 grams of citric acid as the carbon source and reducing agent;

[0047] The other steps and parameters are the same as those in embodiment 1, and a multi-element doped carbon-coated Na3V 1.94 Al 0.03 Fe 0.03 (PO4)2F3 sodium ion battery positive electrode material is obtained.

[0048] Embodiment 3: This embodiment dopes three elements (Al 3+ , Fe 3+ , and Mn 2+ ), the difference between this embodiment and embodiment 1 is that the operation of step one is as follows:

[0049] I. Take 0.0150 grams of aluminum nitrate nonahydrate, 0.0161 grams of iron nitrate nonahydrate, and 0.0100 grams of manganese nitrate tetrahydrate, 0.2521 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and then take 0.6147 grams of citric acid as the carbon source and reducing agent;

[0050] The other steps and parameters are the same as those in embodiment 1. A multi-element doped carbon-coated Na3V 1.94 Al 0.02 Fe 0.02 Mn 0.02 (PO4)2F3 sodium ion battery positive electrode material is obtained.

[0051] Embodiment 4: This embodiment is a quaternary meso-doping, doping four elements (Al 3+ , Fe 3+ , Mn 2+ , and Co 2+ ), the difference between this embodiment and embodiment 1 is that the operation of step one is as follows:

[0052] I. Take 0.0113 grams of aluminum nitrate nonahydrate, 0.0121 grams of iron nitrate nonahydrate, 0.0075 grams of manganese nitrate tetrahydrate, and 0.0087 grams of cobalt nitrate hexahydrate, 0.2545 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and then take 0.6147 grams of citric acid as the carbon source and reducing agent;

[0053] The other steps and parameters are the same as those in embodiment 1. A multi-element doped carbon-coated Na3V 1.94 Al 0.015 Fe 0.015 Mn 0.015 Co0.015 A sodium-ion battery cathode material of Na3V

[0054] Example 5: This example is a quaternary medium-entropy doping, doping four elements (Al 3+ , Fe 3+ , Mn 2+ and Ni 2+ ), and the difference between this example and example 1 is that the operation of step one is as follows:

[0055] I. Take 0.0113 grams of aluminum nitrate nonahydrate, 0.0121 grams of iron nitrate nonahydrate, 0.0075 grams of manganese nitrate tetrahydrate, and 0.0087 grams of nickel nitrate hexahydrate, 0.2545 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and then take 0.6147 grams of citric acid as a carbon source and a reducing agent;

[0056] The other steps and parameters are the same as in example 1, and a multi-element doped carbon-coated Na3V 1.94 Al 0.015 Fe 0.015 Mn 0.015 Ni 0.015 (PO4)2F3 sodium-ion battery cathode material is obtained.

[0057] Example 6: This example is a five-element high-entropy doping, and the ratio between the five elements is Al 3+ : Fe 3+ : Mn 2+ : Co 2+ : Ni 2+ =2:2:3:2:3. The difference between this example and example 1 is that the operation of step one is as follows:

[0058] I. Take 0.0075 grams of aluminum nitrate nonahydrate, 0.0081 grams of iron nitrate nonahydrate, 0.0075 grams of manganese nitrate tetrahydrate, 0.0058 grams of cobalt nitrate hexahydrate, and 0.0087 grams of nickel nitrate hexahydrate, 0.2553 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and then take 0.6147 grams of citric acid as a carbon source and a reducing agent;

[0059] The other steps and parameters are the same as in example 1, and a multi-element doped carbon-coated Na3V 1.94 Al 0.010 Fe 0.010 Mn 0.015 Co 0.010 Ni 0.015 (PO4)2F3 sodium-ion battery cathode material is obtained.

[0060] Example 7: This example is a five-element high-entropy doping, and the ratio between the five elements is Al3+ : Fe 3+ : Mn 2+ : Co 2+ : Ni 2+ =5:7:6:5:7, the difference between this embodiment and example 1 is that step one is operated as follows:

[0061] I. Take 0.0075 grams of aluminum nitrate nonahydrate, 0.011312 grams of iron nitrate nonahydrate, 0.0060 grams of manganese nitrate tetrahydrate, 0.0058 grams of cobalt nitrate hexahydrate, and 0.0081 grams of nickel nitrate hexahydrate, 0.2550 grams of sodium fluoride, 0.3528 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and 0.6147 grams of citric acid as a carbon source and reducing agent;

[0062] The other steps and parameters are the same as those in example 1, and a multi-element doped carbon-coated Na3V2(PO4)2F3 sample is obtained. 1.94 Al 0.010 Fe 0.014 Mn 0.012 Co 0.010 Ni 0.014 (PO4)2F3 sodium ion battery positive electrode material.

[0063] Comparative Example 1: This comparative example is to prepare a sample of Na3V2(PO4)2F3 without doping, and the specific steps are as follows:

[0064] I. Take 0.2520 grams of sodium fluoride and 0.364 grams of vanadium pentoxide, 0.46 grams of ammonium dihydrogen phosphate, and 0.6147 grams of citric acid as a carbon source and reducing agent;

[0065] II. Dissolve the citric acid in water, add ammonia to adjust the pH value to 4.4, and then add the vanadium pentoxide to obtain a mixed solution; increase the temperature of the mixed solution to 85°C and start stirring until it becomes a wet gel;

[0066] III. Dry the wet gel in a vacuum at a temperature of 80°C for 12 hours to obtain a dry gel;

[0067] IV. Grind the dry gel into powder and place it in a high-temperature furnace, heat it to 300°C for pre-sintering for 4 hours, and then heat it to 600°C for high-temperature sintering for 8 hours to obtain Na3V2(PO4)2F3.

[0068] Comparative Example 2: This comparative example only dopes one element (Al 3+ ), and the difference between this comparative example and example 1 is that the operation in step one is replaced by the following operation:

[0069] I. Weigh 0.2520 g of sodium fluoride, 0.0450 g of aluminum nitrate nonahydrate, 0.3528 g of vanadium pentoxide, 0.46 g of ammonium dihydrogen phosphate, and 0.6147 g of citric acid as a carbon source and a reducing agent;

[0070] The other steps and parameters are the same as those in Example 1, and the product obtained is Na3V 1.94 Al 0.6 (PO4)2F3.

[0071] Example 1 is prepared by using a high-entropy strategy to dope five elements, Examples 2-7 are materials prepared by doping two or more elements, and Comparative Examples 1-2 are materials prepared by not doping and doping one element, respectively.

[0072] The undoped material prepared in Example 1 and Comparative Example 1 was subjected to X-ray diffraction (XRD) testing, and the obtained XRD spectrum is shown in Figure 1 From Figure 1 it can be seen that the main diffraction peak position of the sample prepared in Example 1 by five-element high-entropy doping is basically the same as that of the undoped sample, and the main diffraction peak does not split or appear a new phase peak, indicating that the doping elements are dispersed into the vanadium sites at the atomic level to form a substitutional solid solution, and the five-element high-entropy doping strategy does not significantly change the crystal structure of the material and does not cause amorphization due to lattice distortion caused by doping.

[0073] Figure 2 The XPS full spectrum of the high-entropy doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 (PO4)2F3 sodium ion battery cathode material prepared in Example 1 and the scanning electron microscope photo of the undoped material Na3V2(PO4)2F3 prepared in Comparative Example 1, wherein a is Example 1, and b is Comparative Example 1, and it can be seen from Figure 2 that doping does not change the cubic block structure of the material itself, but doping can increase the size and uniformity of the secondary particles formed by aggregation of the original primary particles, and the surface of the larger particles can load a more complete carbon coating layer and shorten the ion diffusion path between the primary particles.

[0074] Figure 3 The XPS full spectrum of the high-entropy doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 (PO4)2F3 sodium ion battery cathode material prepared in Example 1, and it can be seen from Figure 3It can be seen that the spectral peaks of the five transition metals can be measured, proving that all five metals are doped into the material.

[0075] The materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were mixed with a conductive agent (super P) and a binder (PVDF) at a mass ratio of 7:2:1, respectively. An appropriate amount of N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred to form a slurry. This slurry was then coated onto aluminum foil, with the coating thickness controlled at 200 μm. The coating was dried in a vacuum oven at 80°C for 12 hours, cut into 10 mm diameter discs, and compacted to a thickness of approximately 60 μm to form electrodes. The half-cell used a pure sodium metal sheet as the negative electrode, and the full-cell used a hard carbon material as the negative electrode, with glass fiber (GF-D) as the separator. 1 mol·L⁻¹ -1 A CR2025 coin cell was assembled using a NaClO4-based organic solution of dimethyl carbonate (DMC), ethylene carbonate (EC) (volume ratio 1:1), and 5% fluoroethylene carbonate (FEC). The cells were then assembled into a 2032 type coin cell for testing. The test conditions were as follows:

[0076] (1) First week charge and discharge test: 1C (1C=128.3mA / g), voltage range 2-4.5V;

[0077] (2) Cyclic performance test: 5C, voltage range 2.5-4.5V;

[0078] (3) Rate performance test: 1C, 5C, 10C, 15C, 20C, 30C, voltage range 2-4.5V.

[0079] Figure 4 Example 1: High-entropy carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 (PO4)2F3 sodium-ion battery cathode material, multi-component doped carbon-coated Na3V prepared in Example 2 1.94 Al 0.03 Fe 0.03 A comparison of the first three differential capacity curves of the (PO4)2F3 sodium-ion battery cathode material and the materials prepared in Comparative Examples 1-2. The figure shows that the reversibility of the first three cycles is generally similar between the doped and undoped samples. The voltages marked in the figure are the peak voltages of the third cycle. The charging curves at the top show that... Figure 4As shown in (b), (c), and (d), doping modification can significantly improve the voltage of the first charging platform, with the pentagonal high-entropy doped material of Example 1 showing the greatest improvement. For the second high-voltage charging platform, only the Al-doped material in Comparative Example 2 has a lower second charging voltage than the undoped material in Comparative Example 1. Example 1 shows the greatest voltage increase, reaching 0.0073V, thus the high-entropy doped material of Example 1 possesses a higher specific charging capacity. The discharge curves below show that the high-entropy doped material of Example 1 mainly improves the voltage of the relatively large low-voltage platform. The area of ​​the relatively large low-voltage platform (3.5179V) is also larger than that of Example 2 and Comparative Examples 1-2, thus significantly improving the discharge capacity of this reaction stage. Furthermore, it adds a 3.2472V reaction, thus comprehensively improving the material's specific discharge capacity.

[0080] Figure 5 Example 1: High-entropy carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 A comparison of the first charge-discharge curves of the (PO4)2F3 sodium-ion battery cathode material and Examples 2 and 1-2. The graph shows the first plateau (~3.4V vs. Na) and the first charge-discharge curves of Examples 2 and 1-2. + / Na) corresponds to V 3+ → V 4+ The oxidation process, the second plateau (~4.0V vs. Na) + / Na) corresponds to V 4+ → V 5+ Further oxidation, as Figure 4 Analysis shows that the high-entropy doped material of Example 1 can significantly improve the charging capacity. Both of its charging voltage plateaus are longer than those of Example 2 and Comparative Examples 1-2, indicating that high-entropy doping promotes more efficient utilization of the two redox pairs. In the discharge curve, not only are the voltage plateaus longer, but also the redox pairs Ni²⁺ / Ni³⁺, Mn²⁺ / Mn³⁺, and Co²⁺ / Co³⁺ are more efficient than V³⁺ / V³⁺. 4 The ⁺ synergistic effect provides an additional charge compensation path, with an additional supplementary plateau appearing at around 3.2V, thus enabling the release of a specific capacity of 130.25mA / g that exceeds the theoretical capacity at 1C discharge.

[0081] Figure 6 The multi-component high-entropy doped carbon-coated Na3V prepared in Example 1 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012Discharge specific capacity comparison of Example 2 and the material prepared in Comparative Example 1-2 at different rates. From the figure, it can be seen that the material of Comparative Example 2 with single Al doping has worse performance than the undoped sample in Comparative Example 1, which is due to the structural distortion caused by ion radius mismatch and the charge transport bottleneck caused by inactive doping. The 3p orbit of Al 3+ may not be able to effectively participate in electron conduction, especially the doping of Al 3+ may cause insufficient change in the oxidation state of V, which can reduce the electronic conductivity of the material. The material of Example 2 with doped Al and Fe has improved at low rates, but the performance has decreased at high rates greater than 15C, because Al 3+ with a smaller radius can cause local lattice contraction and form a compressive stress field. At low rates, slow sodium ion deintercalation allows the lattice to fully relax, and structural stability dominates performance, Fe 2+ / Fe 3+ redox pairs react synergistically with V 3+ / V 4+ , which can provide additional capacity contribution, but the lattice contraction caused by Al 3+ doping at high rates inhibits the rapid migration of sodium ions, so the capacity decreases. And the high-entropy doped material is about 10mA / g higher than the undoped sample of Comparative Example 1 at each rate, because high-entropy doping can reduce the band gap between the conduction band and the valence band, promoting electron transport and thus improving the electronic conductivity of the material. At the same time, the gradient radius design can widen the ion diffusion channel, significantly improve the diffusion efficiency of Na⁺ ions, and enhance the electrode reaction kinetics, so the material has excellent rate performance.

[0082] Figure 7 Multi-element high-entropy doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 PO4)2F3 sodium ion battery cathode material (a) half-cell charge-discharge diagram and (b) full-cell charge-discharge curve diagram. From the figure, it can be seen that in the half-cell test, the material maintains a good platform in high-rate charge-discharge, so the capacity attenuation is low, and the polarization of the material is also low, and the attenuation is less with the increase of rate. At the same time, the full-cell test also has excellent performance, and can release a specific capacity of nearly 120mA / g in 0.5C charge-discharge test.

[0083] Figure 8 Multi-element high-entropy doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn0.012 Co 0.012 Ni 0.012 A comparison of the cycling performance of the (PO4)2F3 sodium-ion battery cathode material and the materials of Examples 2 and Comparative Examples 1-2 at a 5C rate. The figure shows that the high-entropy doped material in Example 1 has a better discharge capacity and cycle retention rate than that of Examples 2 and Comparative Examples 1-2. This is because the gradient radius design can induce local lattice contraction, forming a uniformly distributed compressive stress field within the material. This micro-strain can buffer the anisotropic expansion caused by sodium ion insertion / extraction, thereby improving cycle stability.

[0084] Example 1: Preparation of multi-component high-entropy doped carbon-coated Na3V 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 Table 1 shows the electrical performance data of the (PO4)2F3 sodium-ion battery cathode material and the materials prepared in Examples 2-7 and Comparative Examples 1-2.

[0085] Table 1 Electrical properties of the materials prepared in Examples 1-7 and Comparative Examples 1-2

[0086] Sample Discharge specific capacity at 1C rate (mA / g) Discharge specific capacity at 30C rate (mA / g) Capacity retention rate after 5C cycle for 1100 times (%) Example 1 130.25 112.18 81.85 Comparative Example 1 117.10 100.90 63.97 Comparative Example 2 110.60 87.61 54.21 Example 2 119.53 92.00 77.48 Example 3 119.16 96.10 83.03 Example 4 115.69 95.69 84.36 Example 5 120.27 102.07 -- Example 6 124.51 112.36 -- Example 7 118.95 99.71 --

[0087] from Figures 4~8 As can be seen from Table 1, the Na3V prepared in Example 1 using the multi-element high-entropy carbon coating method... 1.94 Al 0.012 Fe 0.012 Mn 0.012 Co 0.012 Ni 0.012 The (PO4)2F3 sodium-ion battery cathode material exhibits higher first-cycle capacity, better cycle stability, and better rate performance compared to the materials prepared in Examples 2-7 and Comparative Examples 1-2. This is achieved through the use of Al³⁺ and Fe³⁺... 3 ⁺、Mn 2The doping of five elements of Na+, Co2+, Ni2+ not only improves the capacity of the material, but also effectively inhibits the structural degradation of the material in the cycle process. For undoped, single-element doped or multi-element doped, binary (Example 2), ternary doped (Example 7), quaternary doped (Example 5) and quinary doped (Example 1, Example 6 and Example 7), the discharge specific capacity at 1C rate is higher than that of the single-element doped material, which indicates that the band structure reconstruction caused by multi-element doping can improve the electronic state density near the Fermi level, thereby affecting the electronic conductivity of the material. It is a complex synergistic effect, so in the quaternary doping (Example 4), only the cycle life is improved, and in the large rate 30C charge-discharge test, only the quaternary entropy doping (Example 5) and the quinary high-entropy doping (Example 1 and Example 6) have an improvement effect on the undoped sample (Comparative Example 1), which may be caused by the fact that part of the doped elements cannot complete the valence state transition at high rate or the local lattice distortion caused by different ionic radii makes the sodium ion diffusion path tortuous. Basically, doping more than two elements can greatly improve the cycle performance of the material, because in high-entropy doping, the internal structure of the material can be strengthened, and the structural degradation of the material in the cycle process is slowed down, so the cycle performance is improved. In quinary high-entropy doping, various proportions can be used, not limited to equal proportions, and different proportions (Example 6 and Example 7) can also improve the material performance. In Example 1, the quinary high-entropy doped material has the best comprehensive performance, with a first-week capacity of 130.25 mA / g (1C rate), which breaks through the theoretical capacity of the material, and a capacity retention rate of 81.85% after 1100 cycles, and a capacity of 112.18 mA / g at 30C rate. The improved material has improved performance in various aspects.

[0088] The method of the present application, on the basis of the sol-gel method, breaks through the rated capacity of the traditional Na3V2(PO4)2F3 material by simply doping two to five elements, enhances the structural stability of the material while improving the ion migration ability. The capacity, cycle stability and rate performance of the material are significantly improved. The modification method is simple and easy to implement, and the cost is low, which can be applied on a large scale. The strategy of high-entropy doping provided by the present application for material preparation has the following advantages: (1) simple operation, only the doping elements are added in the original preparation process, without changing the original production line, easy to mass production; (2) the process does not produce toxic gas, environmentally friendly; (3) the improvement effect is remarkable, and multiple electrochemical performance indicators can be improved at the same time; (4) wide application range, can be popularized to other types of sodium ion battery positive electrode materials. This simple and effective modification method provides a new idea for the development of high-performance sodium vanadium fluorophosphate positive electrode material, and is expected to promote the industrial development of high-capacity polyanion type sodium ion battery.

Claims

1. A multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V2(PO4)2F3, characterized in that, The method is carried out according to the following steps: I. stoichiometric ratio of Na3V 1.94 (TM) 0.06 sodium salt, vanadium source, transition metal TM source, phosphate and fluoride salt, wherein TM is a combination of 5 kinds of Al, Fe, Mn, Co and Ni, and the vanadium source is a trivalent vanadium source or a pentavalent vanadium source; For the trivalent vanadium source, the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source (0.73-0.83):1; For the pentavalent vanadium source, the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source (0.73-0.83):1; for the carbon source without reducibility, a reducing agent is further weighed, wherein the amount of the reducing agent is just the amount of the pentavalent vanadium source V 5+ completely reduced to V 3+ . II. Dissolve the carbon source or the carbon source and the reducing agent in water, and then adjust the pH value to 4-4.5, add the vanadium source, and obtain a mixed solution; III. Heat the mixed solution to a temperature of 80-90℃, start stirring, and then add the sodium salt, the transition metal TM source, and the phosphorus salt and fluorine salt in sequence to the mixed solution during stirring. After the addition is completed, continue stirring for 30-50 minutes, and then continue stirring until a wet gel is formed; IV. Vacuum dry the wet gel at a temperature of 80-85℃ for 12-15h to obtain a dry gel; V. Grinding the xerogel into powder, pre-sintering at 200~400℃ for 4~5h in an inert atmosphere, and then high-temperature sintering at 500~700℃ for 7~9h in an inert atmosphere to obtain the multi-doped carbon-coated Na3V 1.94 (TM) 0.06 (PO4)2F3 sodium-ion battery cathode material, Na3V 1.94 (TM) 0.06 TM in Na3V (PO4)2F3 is a combination of 5 kinds of Al, Fe, Mn, Co and Ni.

2. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The sodium salt in step I is one or a combination of several of sodium metavanadate, sodium chloride, sodium fluoride, sodium hydroxide, and sodium acetate.

3. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The pentavalent vanadium source in step I is one or a combination of two of ammonium metavanadate, sodium metavanadate, and vanadium pentoxide; the trivalent vanadium source in step I is one or a combination of two of vanadous acetate, vanadous fluoride, vanadous phosphate, and vanadium trioxide.

4. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The Al source in step I is one or a combination of two of aluminum nitrate and aluminum acetate.

5. The multi-doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material of claim 1 or 2, wherein the carbon coating has a thickness of 1-100 nm. 1.94 (TM) 0.06 A preparation method of a Na3V2(PO4)2F3 sodium-ion battery cathode material, characterized in that, The Fe source in step I is one or a combination of two of iron nitrate and iron acetate.

6. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The Mn source in step I is one or a combination of two of manganese nitrate, manganese acetate, and manganese oxalate.

7. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The Co source in step I is one or a combination of two of cobalt nitrate, cobalt acetate, and cobalt oxalate.

8. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The Ni source in step I is one or a combination of two of nickel nitrate, nickel acetate, and nickel oxalate.

9. The multi-doped carbon-coated Na3V 1.94 (TM) 0.06 A preparation method of a sodium-ion battery cathode material Na3V The carbon source in step I is one or a combination of several of glucose, sucrose, ethylene glycol, polyethylene glycol, citric acid, and ascorbic acid.

10. The multi-doped carbon-coated Na3V2(PO4)2F3 sodium-ion battery cathode material of claim 1 or 2, wherein the carbon coating has a thickness of 1-50 nm. 1.94 (TM) 0.06 A method for preparing a Na3V2(PO4)2F3 sodium-ion battery cathode material, characterized in that, The mass ratio of the solute to water in step II is 1:(15-30).

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