A sodium vanadium phosphate composite positive electrode material, a preparation method and application thereof

Abstract

The application relates to the technical field of sodium ion batteries, in particular to a sodium vanadium phosphate composite positive electrode material and a preparation method and application thereof, which comprises the following steps: S1, ammonium metavanadate, sodium carbonate, ammonium dihydrogen phosphate, chromium nitrate nine hydrate, manganese acetate four hydrate, sodium fluoride and citric acid are added into deionized water I and fully stirred and mixed, then a carbon nanotube aqueous solution with a mass fraction of 5% is added to continue stirring to obtain a mixed solution, the mixed solution is stirred and evaporated until a dry gel is obtained; S2, the dry gel is placed into an air drying oven for drying to obtain fluffy dry powder; S3, the obtained fluffy dry powder is ground and sieved to obtain powder with uniform particle size, the powder with uniform particle size is first subjected to pre-burning treatment, and then is subjected to heat treatment to obtain a target product; the problems of poor self-conductivity of the existing sodium vanadium phosphate material, low rate performance and short cycle life caused by structure instability in the sodium ion deintercalation process are solved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a sodium vanadium phosphate composite positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] As a kind of efficient energy storage equipment, lithium ion batteries have achieved great success in commercialization and are widely used in many fields such as portable electronic devices and electric vehicles. However, due to the limited reserves of lithium resources and the pressure of rising costs, it is urgent to explore various potential alternatives for lithium ion batteries. Sodium ion batteries, due to their abundant reserves and low cost, are expected to become the main battery system for grid energy storage. However, they still have some shortcomings such as short service life and low energy density, which seriously hinder the commercialization of sodium ion batteries.

[0003] Sodium vanadium phosphate, namely Na3V2(PO4)3, as a typical NASICON-type positive electrode material, has a stable three-dimensional open framework structure and has significant advantages in capacity, working voltage, structural stability and thermal safety, and is one of the most promising sodium ion battery positive electrode materials at present. However, the existing sodium vanadium phosphate material has problems such as poor self-conductivity and structural instability during sodium ion intercalation and deintercalation, which limits its application in the field of sodium ion batteries.

[0004] Therefore, it is urgent to modify the electrical conductivity and structural stability of sodium vanadium phosphate material to meet the demand of high rate and long life sodium ion battery positive electrode material. SUMMARY

[0005] The application provides a sodium vanadium phosphate composite positive electrode material and a preparation method and application thereof to solve the problems of poor self-conductivity of the existing sodium vanadium phosphate material, low rate performance and short cycle life caused by structural instability during sodium ion deintercalation.

[0006] The application is implemented by adopting the following technical scheme:

[0007] A preparation method of a sodium vanadium phosphate composite positive electrode material, comprising the following steps:

[0008] S1: ammonium metavanadate, sodium carbonate, ammonium dihydrogen phosphate, chromium nitrate nonahydrate, manganese acetate tetrahydrate, sodium fluoride and citric acid are added to deionized water I and fully stirred and mixed, then a carbon nanotube aqueous solution with a mass fraction of 5% is added and continuously stirred to obtain a mixed solution, and the mixed solution is stirred and evaporated until a dry gel is obtained;

[0009] S2: the dry gel is placed in a blast drying oven for drying to obtain fluffy dry powder;

[0010] S3: grinding and sieving the obtained fluffy dry powder to obtain a powder with uniform particle size, pre-burning the powder with uniform particle size, and then heat-treating to obtain the target product.

[0011] Further, in step S1, the molar ratio of the ammonium metavanadate, sodium carbonate, ammonium dihydrogen phosphate, chromium nitrate nine hydrate, manganese acetate four hydrate, sodium fluoride and citric acid is (2-2x):1.5:(3-x):x:x:3x:3, wherein x is in the range of 0

[0012] Further, in step S1, the temperature of the deionized water I is 60-90℃, and the volume of the deionized water I is 60-100 mL.

[0013] Further, in step S1, the preparation method of the carbon nanotube aqueous solution comprises: adding carbon nanotubes with a purity of >95% into deionized water II, ultrasonic treatment for 10-30 min, standing for 5-20 min to form a suspension, and obtaining the carbon nanotube aqueous solution.

[0014] Further, in step S2, the drying temperature of the air drying oven is 90-120℃, and the drying time is 6-10 h.

[0015] Further, in step S3, the mesh size of the sieve is 200 mesh.

[0016] Further, in step S3, the pre-burning treatment is to heat the powder with uniform particle size to 320-400℃ at a heating rate of 3-6℃ / min under argon protection for 2-5 h.

[0017] Further, in step S3, the heat treatment is to heat the powder with uniform particle size to 700-850℃ at a heating rate of 3-6℃ / min under argon protection for 4-8 h.

[0018] A sodium vanadium phosphate composite positive electrode material is prepared by the preparation method.

[0019] The application of the sodium vanadium phosphate composite positive electrode material.

[0020] The present application adopts a double carbon coating strategy of coating a carbon layer on the surface of sodium vanadium phosphate and compounding carbon nanotubes to construct a multiple conductive network, for improving the problem of insufficient conductivity of sodium vanadium phosphate itself, at the same time, through ternary double-site co-doping of vanadium site manganese and chromium double metal doping and phosphoric acid site fluorine doping, the crystal structure of sodium vanadium phosphate is optimized, the sodium ion diffusion channel is expanded, and the ion migration dynamics of sodium vanadium phosphate is effectively improved; and under the synergistic modification of double carbon coating and manganese-chromium-fluorine ternary co-doping, sodium vanadium phosphate shows higher structural stability, electronic conductivity and ionic conductivity, excellent rate performance and cycle stability, and has obvious advantages in improving the electrochemical performance of sodium vanadium phosphate; in addition, the preparation method of the sodium vanadium phosphate composite positive electrode material in the present application has the advantages of simple operation process, low production cost and can realize mass production. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a preparation flowchart of the sodium vanadium phosphate composite positive electrode material in the present application.

[0022] Figure 2 is an X-ray diffraction pattern of the sodium vanadium phosphate composite positive electrode material obtained in Example 1 of the present application.

[0023] Figure 3 is a scanning electron microscope image of the sodium vanadium phosphate composite positive electrode material obtained in Example 1 of the present application.

[0024] Figure 4 is a cyclic voltammogram of the sodium vanadium phosphate composite positive electrode material obtained in Example 1 of the present application.

[0025] Figure 5 is an X-ray photoelectron spectrum of the sodium vanadium phosphate composite positive electrode material obtained in Example 2 of the present application.

[0026] Figure 6 is a cycle performance graph of the sodium vanadium phosphate composite positive electrode material obtained in Example 3 of the present application at a 2C rate. DETAILED DESCRIPTION

[0027] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the present application will be further described in detail below in combination with the drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0028] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, however, the present application can also be implemented in other different ways from the description, therefore, the protection scope of the present application is not limited by the specific embodiments disclosed below. Example 1

[0029] The embodiment prepares a sodium vanadium phosphate composite positive electrode material, as shown in the accompanying Figure 1 The preparation method of the sodium vanadium phosphate composite positive electrode material comprises the following steps:

[0030] S1: Ammonium metavanadate (NH4VO3), sodium carbonate (Na2CO3), ammonium dihydrogen phosphate (NH4H2PO4), chromium nitrate nonahydrate (Cr(NO3)3·9H2O), manganese acetate tetrahydrate ((CH3COO)2Mn·4H2O), sodium fluoride (NaF), and citric acid (C6H8O7) are added to 60 mL of deionized water I at 60°C in a molar ratio of 1.86:1.5:2.93:0.07:0.07:0.21:3, and stirred thoroughly, then a carbon nanotube aqueous solution with a mass fraction of 5% is added for continuous stirring to obtain a mixed solution, and the mixed solution is stirred and evaporated until a dry gel is obtained.

[0031] The preparation method of the carbon nanotube aqueous solution comprises: adding carbon nanotubes with a purity of >95% to deionized water II, ultrasonic treatment for 10 min, standing for 5 min to form a suspension, and obtaining a carbon nanotube aqueous solution.

[0032] S2: The dry gel is placed in a forced air drying oven with a drying temperature of 120°C for drying for 6 h to obtain fluffy dry powder.

[0033] S3: The obtained fluffy dry powder is ground and sieved with a 200-mesh sieve to obtain powder with uniform particle size, and the powder with uniform particle size is first pre-fired at an argon gas protection and a temperature rising speed of 5°C / min to 350°C for 3 h, and then heat treated at an argon gas protection and a temperature rising speed of 5°C / min to 750°C for 6 h to obtain the target product.

[0034] The X-ray diffraction pattern of the sodium vanadium phosphate composite positive electrode material prepared in the embodiment is shown in the accompanying Figure 2 As can be seen from the figure, the sodium vanadium phosphate in the sodium vanadium phosphate composite positive electrode material has good crystallinity, and the purity of the components is very high, and no other impurity components are detected.

[0035] The scanning electron microscope image of the sodium vanadium phosphate composite positive electrode material prepared in the embodiment is shown in the accompanying Figure 3 As can be seen from the figure, the particle size of the sodium vanadium phosphate in the sodium vanadium phosphate composite positive electrode material is uniform, the porosity is rich, and the carbon nanotubes are uniformly dispersed in the particles.

[0036] The cyclic voltammetry curve of the sodium vanadium phosphate composite positive electrode material prepared in the embodiment is shown in the accompanying Figure 4 As can be seen from the figure, the polarization of the sodium vanadium phosphate composite positive electrode material in the charging and discharging process is very small, indicating that the electrochemical reversibility is good.

[0037] The sodium vanadium phosphate composite positive electrode material prepared in this embodiment is applied to the positive electrode of a sodium ion battery, and the electrochemical performance is tested by a CR2032 type button half battery. The specific steps are as follows:

[0038] First, electrode preparation. The sodium vanadium phosphate composite positive electrode material, polyvinylidene fluoride (PVDF) binder and acetylene black conductive agent are mixed in a ball mill tank in a mass ratio of 8:1:1 using N-methyl pyrrolidone (NMP) as the solvent, then uniformly coated on an aluminum foil using a coater, and then punched and accurately weighed to obtain a positive electrode sheet.

[0039] Then, battery assembly. The positive electrode sheet is transferred into an argon-protected glove box, a metal sodium sheet is used as the counter electrode and reference electrode, the electrolyte is 1.0 mol / L sodium perchlorate (NaClO4) dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC volume ratio = 1:1), and 5% fluorinated ethylene carbonate (FEC) is added, a glass fiber membrane is used as the separator, and the battery is tested after standing for 24 h to allow the electrolyte to fully soak.

[0040] Finally, performance test. The first cycle charge-discharge test is carried out at room temperature at a rate of 0.5C, and the results show that the initial specific discharge capacity is 110.37 mAh / g. After 200 cycles at a rate of 2C, the results show that the capacity retention rate is 97.48% relative to the 0.5C capacity. The results show that the rate capacity retention rate is 83.19% relative to the 0.5C capacity. Example 2

[0041] In this embodiment, a sodium vanadium phosphate composite positive electrode material is prepared. As shown in the accompanying drawings, the preparation method of the sodium vanadium phosphate composite positive electrode material comprises the following steps: Figure 1

[0042] S1: Ammonium metavanadate (NH4VO3), sodium carbonate (Na2CO3), ammonium dihydrogen phosphate (NH4H2PO4), chromium nitrate nonahydrate (Cr(NO3)3·9H2O), manganese acetate tetrahydrate ((CH3COO)2Mn·4H2O), sodium fluoride (NaF) and citric acid (C6H8O7) are added to 80 mL of deionized water I at 90°C in a molar ratio of 1.9:1.5:2.95:0.05:0.05:0.15:3, and then a carbon nanotube aqueous solution with a mass fraction of 5% is added for further stirring to obtain a mixed solution. The mixed solution is stirred and evaporated until a dry gel is obtained.

[0043] ​The method for preparing the carbon nanotube aqueous solution includes: adding carbon nanotubes with a purity >95% to deionized water II, ultrasonically treating for 20 min, and letting stand for 20 min to form a suspension, thereby obtaining the carbon nanotube aqueous solution.

[0044] S2: Place the dry gel in a forced-air drying oven at a drying temperature of 100℃ and dry for 8 hours to obtain a fluffy dry powder.

[0045] S3: Grind the obtained fluffy dry powder and sieve it through a 200-mesh sieve to obtain powder with uniform particle size. Pre-calcine the powder with uniform particle size at 340℃ for 3.5h under argon protection at a heating rate of 4℃ / min, and then heat-treat it at 720℃ for 7h under argon protection at a heating rate of 4℃ / min to obtain the target product.

[0046] Appendix Figure 5 The X-ray photoelectron spectrum of the sodium vanadium phosphate composite cathode material prepared in this embodiment shows that Na, V, P, C, O, Mn, Cr, and F are present in the sodium vanadium phosphate composite cathode material, which is consistent with the design composition of this invention, proving that Mn, Cr, and F have been successfully doped into sodium vanadium phosphate.

[0047] The sodium vanadium phosphate composite cathode material prepared in this embodiment was applied to the cathode of a sodium-ion battery, and its electrochemical performance was tested using a CR2032 button half-cell. The specific steps are as follows:

[0048] The first step is electrode preparation. Using NMP as a solvent, the sodium vanadium phosphate composite cathode material, polyvinylidene fluoride (PVDF) binder, and acetylene black conductive agent are thoroughly mixed in a ball mill jar at a mass ratio of 8:1:1. The mixture is then uniformly coated onto aluminum foil using a coating tool. After punching and precise weighing, the cathode sheet is obtained.

[0049] Then, the battery is assembled. The positive electrode is transferred into an argon-protected glove box. The counter electrode, which also serves as the reference electrode, is made of sodium metal. The electrolyte is 1.0 mol / L sodium perchlorate (NaClO4) dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC volume ratio = 1:1), with 5% by mass of fluoroethylene carbonate (FEC) added. The separator is made of glass fiber membrane. After assembly, the battery is allowed to stand for 24 hours to allow the electrolyte to fully impregnate it before battery testing.

[0050] Finally, performance tests were conducted. At room temperature, the first charge-discharge cycle was performed at a rate of 0.5C. The results showed that the initial discharge specific capacity was 98.13 mAh / g. After 200 cycles at a rate of 2C, the capacity retention rate was 95.95% relative to the 0.5C capacity. At a rate of 40C, the rate capacity retention rate was 79.77% relative to the 0.5C capacity. Example 3

[0051] This embodiment prepared a sodium vanadium phosphate composite cathode material, as shown in the attached figure. Figure 1 As shown, the preparation method of this sodium vanadium phosphate composite cathode material includes the following steps:

[0052] S1: Ammonium metavanadate (NH4VO3), sodium carbonate (Na2CO3), ammonium dihydrogen phosphate (NH4H2PO4), chromium nitrate nonahydrate (Cr(NO3)3·9H2O), manganese acetate tetrahydrate ((CH3COO)2Mn·4H2O), sodium fluoride (NaF), and citric acid (C6H8O7) were added to 100 mL of deionized water I at 60 °C in a molar ratio of 1.8:1.5:2.9:0.1:0.1:0.3:3 and stirred thoroughly. Then, 5% (w / w) of carbon nanotube aqueous solution was added and stirring was continued to obtain a mixed solution. The mixed solution was stirred and evaporated until a dry gel was obtained.

[0053] The method for preparing the carbon nanotube aqueous solution includes: adding carbon nanotubes with a purity >95% to deionized water II, ultrasonically treating for 30 min, and letting stand for 10 min to form a suspension, thereby obtaining the carbon nanotube aqueous solution.

[0054] S2: Place the dry gel in a forced-air drying oven at a drying temperature of 90℃ and dry for 10 hours to obtain a fluffy dry powder.

[0055] S3: Grind the obtained fluffy dry powder and sieve it through a 200-mesh sieve to obtain powder with uniform particle size. Pre-calcine the powder with uniform particle size at 330℃ for 4 hours under argon protection at a heating rate of 3℃ / min, and then heat treat it at 700℃ for 8 hours under argon protection at a heating rate of 3℃ / min to obtain the target product.

[0056] Appendix Figure 6 The graph shows the cycling performance of the sodium vanadium phosphate composite cathode material prepared in this embodiment at a 2C rate. As can be seen from the graph, the sodium vanadium phosphate composite cathode material has excellent cycling stability. After 200 cycles at a 2C rate, the capacity retention rate is 97.48%.

[0057] The sodium vanadium phosphate composite positive electrode material prepared in this embodiment is applied to the positive electrode of a sodium ion battery, and the electrochemical performance is tested by a CR2032 type button half battery. The specific steps are as follows:

[0058] First, the electrode preparation. The sodium vanadium phosphate composite positive electrode material, polyvinylidene fluoride (PVDF) binder and acetylene black conductive agent are mixed in a ball mill tank at a mass ratio of 8:1:1 using N-methyl pyrrolidone (NMP) as the solvent. Then, the mixture is uniformly coated on an aluminum foil using a coater, and the positive electrode sheet is obtained by punching and accurately weighing the coated aluminum foil.

[0059] Then, the battery assembly. The positive electrode sheet is transferred into an argon-protected glove box, and a metal sodium sheet is used as the counter electrode and reference electrode. The electrolyte is 1.0 mol / L sodium perchlorate (NaClO4) dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC volume ratio = 1:1), with 5% fluorinated ethylene carbonate (FEC) added. A glass fiber membrane is used as the separator. After assembly, the battery is allowed to stand for 24 hours to allow the electrolyte to fully soak, and then the battery test is performed.

[0060] Finally, the performance test. The first cycle charge-discharge test is performed at room temperature at a rate of 0.5C. The results show that the initial discharge specific capacity is 100.28 mAh / g. After 200 cycles at a rate of 2C, the capacity retention rate is 94.75% relative to the 0.5C capacity. At a rate of 40C, the rate capacity retention rate is 74.64% relative to the 0.5C capacity. Example 4

[0061] In this embodiment, a sodium vanadium phosphate composite positive electrode material is prepared. The difference between the preparation method of the sodium vanadium phosphate composite positive electrode material and that of Example 1 is that in step S3, the obtained fluffy dry powder is ground and sieved through a 200 mesh sieve to obtain a powder with uniform particle size. The powder with uniform particle size is first pre-fired at 320℃ under argon protection at a heating rate of 6℃ / min for 2h, and then heat treated at 850℃ under argon protection at a heating rate of 6℃ / min for 4h to obtain the target product. The remaining preparation steps are the same as those of Example 1. Example 5

[0062] The embodiment prepares a sodium vanadium phosphate composite positive electrode material, and the preparation method of the sodium vanadium phosphate composite positive electrode material is different from that of the embodiment 1, and the difference is that in step S3, the obtained fluffy dry powder is ground and sieved with a 200 mesh sieve to obtain a powder with uniform particle size, and the powder with uniform particle size is first pre-fired at 400 DEG C at an argon protection rate of 6 DEG C / min for 5h, and then heat treated at 750 DEG C at an argon protection rate of 5 DEG C / min for 6h to obtain the target product. The remaining preparation steps are the same as those of the embodiment 1.

[0063] In summary, the present application aims at the problem of poor sodium storage performance caused by poor conductivity of the existing single sodium vanadium phosphate material, and is modified by vanadium site manganese and chromium two-element doping, phosphoric acid site fluorine doping, and introducing double carbon sources for carbon coating. In the selection of raw materials, nine water chromium nitrate, four water manganese acetate and sodium fluoride are selected as Cr, Mn and F sources, and citric acid is used as part of the carbon source to participate in the modification process. At the same time, citric acid has the characteristics of easy sublimation at medium and high temperatures, especially in the pre-firing process, it escapes from the composite material system and forms more pores in the material, so that the sodium vanadium phosphate particles are more easily infiltrated with electrolyte, thereby enhancing the rate performance of the material and prolonging its cycle life. In addition, the present application further improves the structural stability and rate performance of the material by doping an appropriate amount of carbon nanotubes in the sodium vanadium phosphate. In the present application, the carbon nanotubes are first ultrasonically dispersed in an aqueous solution, and then added to the sodium vanadium phosphate for modification, thereby ensuring that the carbon nanotubes in the final target product have excellent dispersion uniformity and modification effect.

[0064] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can also have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a sodium vanadium phosphate composite cathode material, characterized in that: It comprises the following steps: S1: ammonium metavanadate, sodium carbonate, ammonium dihydrogen phosphate, chromium nitrate nonahydrate, manganese acetate tetrahydrate, sodium fluoride and citric acid are added into deionized water I and mixed by stirring, the molar ratio of ammonium metavanadate, sodium carbonate, ammonium dihydrogen phosphate, chromium nitrate nonahydrate, manganese acetate tetrahydrate, sodium fluoride and citric acid is (2-2x):1.5:(3-x):x:x:3x:3, wherein x is in the range of 0 S2: the dry gel is placed in a blast drying oven for drying to obtain fluffy dry powder; S3: the obtained fluffy dry powder is ground and sieved to obtain powder with uniform particle size, the powder with uniform particle size is first subjected to pre-burning treatment and then heat treatment to obtain the target product. 2.The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S1: the temperature of the deionized water I is 60-90℃, and the volume of the deionized water I is 60-100 mL. 3.The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S1: the preparation method of the carbon nanotube aqueous solution comprises: adding carbon nanotubes with a purity of >95% into deionized water II, ultrasonic treatment for 10-30 min, standing for 5-20 min to form a suspension to obtain the carbon nanotube aqueous solution. 4.The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S2: the drying temperature of the blast drying oven is 90-120℃, and the drying time is 6-10 h.

5. The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S3: the mesh size of the sieve is 200 mesh. 6.The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S3: the pre-burning treatment is to heat the powder with uniform particle size to 320-400℃ at a heating rate of 3-6℃ / min under argon protection for 2-5 h. 7.The method for preparing a sodium vanadium phosphate composite cathode material according to claim 1, characterized in that: In step S3: the heat treatment is to heat the powder with uniform particle size to 700-850℃ at a heating rate of 3-6℃ / min under argon protection for 4-8 h.

8. A sodium vanadium phosphate composite cathode material, characterized in that, It is prepared by the preparation method of any one of claims 1-7.

9. The use of the sodium vanadium phosphate composite cathode material according to claim 8, characterized in that: The material is used as a positive electrode material of a sodium ion battery.

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