A high specific capacity sodium iron vanadium phosphate positive electrode material, a preparation method thereof and a sodium ion battery

Abstract

The application discloses a high-specific-capacity sodium vanadium phosphate iron sodium positive electrode material, a preparation method thereof and a sodium ion battery. The application adopts acetylacetone iron and acetylacetone vanadyl as iron sources and vanadium sources, the preparation method is simple, the synthesized high-sodium positive electrode material is prepared by replacing vanadium with iron, the material has the advantages of high rate performance and high specific capacity, long cycle life and good cycle stability. The application has very important significance for promoting the application of the sodium ion battery in the large-scale energy storage field.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and more particularly relates to a high-specific-capacity sodium iron vanadium phosphate positive electrode material and a preparation method thereof and a sodium ion battery. BACKGROUND

[0002] Sodium ion batteries are considered to be an ideal choice for stationary energy storage. Electrode materials are key components of sodium ion batteries, which determine the overall performance energy density of sodium ion batteries. Therefore, optimizing the working potential and weight / volume capacity of electrode materials is still an important prerequisite for designing competitive sodium ion batteries. However, Na + has a larger radius Its diffusion kinetics is slow, resulting in a large volume change of the electrode material during sodiumization / desodiation. Therefore, most sodium ion battery electrode materials exhibit relatively low energy density and undesirable cycle performance. It is essential to develop electrode materials with high stability and high capacity for practical applications.

[0003] Sodium ion battery positive electrode materials are generally divided into Prussian blue analogues, layered metal oxides and polyanion compounds. Among the many choices, polyanion compounds are widely studied due to their structural stability, thermodynamic stability, adjustable working voltage, high safety and other advantages. The NASICON structure in phosphate has been widely studied, and its general formula is Na3M2(PO4)3 (M = transition metal). One of the widely studied materials, Na3V2(PO4)3, is a very promising cathode material. Chinese patent CN 110993942A discloses a high-performance sodium-deficient positive electrode material Na 3.4 □ 0.6 Fe 2+ 0.4 Fe 3+ 0.6 V 3+ (PO4)3, which has good discharge capacity and cycle performance at low rate performance, but poor high rate performance. Chinese patent CN 112429712A synthesizes phosphate Na4FeV(PO4)3 with a NASICON structure through a sol-gel process and uses it as a sodium ion battery positive electrode material. In the voltage range of 1.8-3.8V, the theoretical capacity is 110mAh g -1 . However, the reversible specific capacity of the positive electrode material is low, only 81.6mAh g -1 at a current density of 5C, and it also has the defect of poor high rate performance.

[0004] Therefore, it is necessary to develop a positive electrode material with high rate performance and high specific capacity. SUMMARY

[0005] The application aims to provide a high specific capacity sodium iron vanadate phosphate positive electrode material, a preparation method thereof and a sodium ion battery. 3+ Substituting V 3+ , the production cost of the positive electrode material is reduced, and the reversible specific capacity and high rate performance of the positive electrode material are improved.

[0006] To achieve the above-mentioned purpose, the application provides the following technical scheme.

[0007] A preparation method of a high specific capacity sodium iron vanadate phosphate positive electrode material comprises the following steps.

[0008] S1, sodium, vanadium, iron and phosphorus are weighed according to the measurement ratio of sodium:vanadium:iron:phosphorus=4:2-x:x:3, and anhydrous citric acid is weighed according to the ratio of carbon:vanadium=1.5:1, wherein x=0.2-1;

[0009] S2, sodium hydroxide, acetylacetone iron and acetylacetone vanadyl are dissolved in anhydrous ethanol to obtain solution A; phosphoric acid and citric acid are dissolved in anhydrous ethanol to obtain solution B; under stirring, solution B is added dropwise into solution A;

[0010] S3, after the dropwise addition is completed, the powder is dried and ground; in a nitrogen atmosphere, the temperature is raised to 350-400 DEG C and kept for 3-5 h, and then the temperature is raised to 500-700 DEG C and kept for 3-5 h to obtain the sodium iron vanadate phosphate positive electrode material.

[0011] Further, in step S1, x=0.4-1.

[0012] Further, in step S3, the drying temperature is 80-90 DEG C; and the drying time is 10-14 h.

[0013] Further, in step S3, the heating rate is 5-10 DEG C / min.

[0014] The positive electrode material prepared by the preparation method.

[0015] A sodium ion battery comprises the positive electrode material.

[0016] Compared with the prior art, the application has the following beneficial effects:

[0017] The positive electrode material preparation method is simple, acetylacetone iron and acetylacetone vanadyl are selected as iron source and vanadium source respectively, the organic salt is dissolved in ethanol, and the carbon-containing functional group forms a carbon coating layer after calcination, dropwise adding the solution is beneficial to forming sodium iron vanadate phosphate with better performance, the specific capacity of the positive electrode material can be improved, and the cost is reduced by substituting iron for vanadium. The specific capacity of the positive electrode material of the application is 94.9 mAh g -1The specific capacity is 79.3 mAh g at 20C current density -1 , and has good high-rate performance. In the prepared sodium iron phosphate positive electrode material of the application, sodium ions occupy more Na2 sites, thereby improving the capacity of the material. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 SEM image of the positive electrode material prepared in Example 1.

[0019] Figure 2 XRD spectrum of the positive electrode material prepared in Examples 1-3.

[0020] Figure 3 is the charge-discharge performance test diagram of the positive electrode material prepared in Example 1 of the application.

[0021] Figure 4 is the charge-discharge performance test diagram of the positive electrode material prepared in Comparative Example 1 of the application.

[0022] Figure 5 is the charge-discharge performance test diagram of the positive electrode material prepared in Comparative Example 2 of the application.

[0023] Figure 6 is the charge-discharge performance test diagram of the positive electrode material prepared in Comparative Example 3 of the application. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the application clearer, the application will be further described in detail below in combination with examples, but the implementation of the application is not limited to this. Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the application belongs. The terms used in the specification of the application herein are only for the purpose of describing the specific examples and are not intended to limit the application. It should be noted that, unless otherwise specified, the reagents and the like used in the examples are all ordinary commercially available products.

[0025] Example 1

[0026] The sol-gel method for preparing sodium iron vanadium phosphate (Na4VFe(PO4)3) positive electrode material for sodium ion battery includes the following steps:

[0027] 1) The molar ratio of iron acetylacetone (C 15 H 21 FeO6), phosphoric acid (H3PO4), sodium hydroxide (NaOH), anhydrous citric acid (C6H8O7), vanadium acetylacetone oxide (C 10 H 14 O5V) used is 1:3:4:0.5:1;

[0028] 2) Slowly add the powder obtained in step 1) except for phosphoric acid and citric acid to a 100mL beaker (containing a magnetic stir bar), and then add 50mL of anhydrous ethanol as a solvent to form solution A; add phosphoric acid and citric acid to a 100mL beaker, and then add 10mL of anhydrous ethanol as a solvent to form solution B; place the beaker containing the magnetic stir bar on a stirrer and stir for 12h, and then add solution B dropwise.

[0029] 3) Transfer the suspension obtained in step 2) to a forced-air drying oven, and then dry it at 80°C for 24 hours to obtain a completely dried solid powder. Then grind it in an agate mortar for 30 minutes.

[0030] 4) Place the powder obtained in step 3) into a tube furnace filled with nitrogen. First, ventilate at room temperature for one hour, then calcine it by raising the temperature to 400°C at a rate of 5°C / min and holding for 4 hours. Then raise the temperature to 650°C and hold for 5 hours to obtain sodium vanadium iron phosphate polyanionic cathode material.

[0031] Example 2

[0032] The difference compared to Example 1 is that the molar ratio of ferric acetylacetone and vanadium acetylacetone oxide is 0.8:1.2.

[0033] Example 3

[0034] The difference compared to Example 1 is that the molar ratio of ferric acetylacetone and vanadium acetylacetone oxide is 0.6:1.4.

[0035] Figure 1 The morphology of the cathode material synthesized in Example 1 is shown. The particle size of the prepared material is about 100 nm. Figure 2 The XRD spectra of the cathode materials prepared in Examples 1-3 show that the synthesized materials in Examples 1-3 are consistent with the standard card PDF#00-040-0019 (Na8Fe4(PO4)6), and the space group is [missing information]. A small amount of Na is present. x FePO4 is a miscellaneous substance, with a typical crystal structure of Na4V. 2-x Fe x (PO4)3, composed of MO6 (M=Fe,V) octahedrons and PO4 tetrahedra connected by angle sharing, provides two types of occupancy sites for Na+ with different oxygen environments. + The presence of more Na2 ions (Na1 and Na2) is beneficial for increasing capacity.

[0036] Comparative Example 1

[0037] The difference from Example 1 is that it is not doped with iron.

[0038] Comparative Example 2

[0039] Compared with Example 1, the iron source was changed to ferric citrate, the vanadium source was changed to ammonium metavanadate, and the solution was changed to deionized water.

[0040] Comparative Example 3

[0041] Compared with Example 1, the difference is that all raw materials are mixed together and added.

[0042] Performance test

[0043] The vanadium iron sodium phosphate positive electrode material powder prepared in Example 1 and Comparative Examples 1-3 was mixed with conductive carbon black and polyvinylidene fluoride according to a mass ratio of 8:1:1, and then ground for 30 min, and then coated on a carbon-containing aluminum foil, dried, and cut into pieces to obtain a positive electrode sheet; a metal sodium sheet was used as a metal negative electrode, a glass fiber (GF / D) was used as a separator, and a 1 mol / L sodium perchlorate salt dissolved in propylene carbonate and 5% fluoroethylene carbonate solvent was used as an organic electrolyte, and a 2032 type sodium ion button cell was assembled in an argon-filled glove box. The battery was tested by a NEWARE battery test system for constant current cycle charging and discharging test, and the battery test was carried out in a voltage range of 1.5-4.2 V.

[0044] Figure 3 The vanadium iron sodium phosphate prepared in Example 1 was subjected to rate test at different current densities of 0.1, 0.5, 1, 2, 5, 10, 15, and 20 C during the charging and discharging process, and the average discharge capacity was 147.1, 118.7, 107.7, 102.3, 94.9, 88.1, 83.4, and 79.3 mAh g -1 .

[0045] Figure 4 The vanadium sodium phosphate prepared in Comparative Example 1 was subjected to rate test at different current densities of 0.1, 0.5, 1, 2, 5, and 10 C during the charging and discharging process, and the average discharge capacity was 88.7, 84.9, 82.6, 79.8, 72.7, and 53.5 mAh g -1 . At a higher rate, the capacity almost decayed to 0, which was not as good as the performance of vanadium iron sodium phosphate.

[0046] Figure 5 The vanadium iron sodium phosphate prepared in Comparative Example 2 with different raw materials was subjected to rate test at different current densities of 0.1, 0.5, 1, 2, 5, 10, 15, and 20 C during the charging and discharging process, and the average discharge capacity was 151.54, 117.0, 106.0, 97.0, 79.7, 60.8, 46.8, and 37.7 mAh g -1At low rate, the average discharge capacity is comparable to that of the positive electrode material of sodium iron vanadium phosphate prepared in Example 1, but the discharge capacity at high rate is significantly reduced, which may be because the structure of sodium iron vanadium phosphate synthesized by iron citrate is not as stable as that of iron acetylacetone, as shown in the figure, the capacity of iron gradually decreases with the increase of rate.

[0047] Figure 6 The sodium iron vanadium phosphate prepared by directly mixing all raw materials in Comparative Example 3 has a performance of 70.8 mAh g -1 , which is not as good as the performance of 79.3 mAh g -1 . This may be because direct mixing leads to uneven coating of citric acid, and the carbon layer formed after firing is also uneven, resulting in reduced conductivity of the material and unsatisfactory capacity.

[0048] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A method for preparing a high specific capacity sodium iron vanadium phosphate cathode material, characterized in that, It comprises the following steps: S1. Sodium hydroxide, acetylacetone iron, acetylacetone vanadyl, phosphoric acid are weighed according to the ratio of sodium:vanadium:iron:phosphorus=4:2-x:x:3, and anhydrous citric acid is weighed according to the ratio of carbon:vanadium=1.5:1, wherein x=0.2-1; S2. Sodium hydroxide, acetylacetone iron, acetylacetone vanadyl are dissolved in anhydrous ethanol to obtain solution A; phosphoric acid and citric acid are dissolved in anhydrous ethanol to obtain solution B; solution B is added dropwise into solution A under stirring; S3. After the dropwise addition is completed, the obtained powder is dried and ground; the temperature is raised to 350-400℃ under nitrogen atmosphere for 3-5h, and then the temperature is raised to 500-700℃ for 3-5h to obtain the positive electrode material of sodium vanadium iron phosphate.

2. The production method according to claim 1, characterized by, In step S1, x=0.4-1.

3. The production method according to claim 1, characterized by, In step S3, the drying temperature is 80-90℃; and the drying time is 10-14h.

4. The method of claim 1, wherein, In step S3, the heating rate is 5-10℃ / min.

5. The positive electrode material prepared by the preparation method of any one of claims 1-4.

6. A sodium-ion battery, characterized in that, It comprises the positive electrode material of claim 5.

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