CN112909232A

CN112909232A - Sodium fluoride impregnated and coated vanadium-doped porous structure ferric sodium pyrophosphate cathode material and preparation method thereof

Info

Publication number: CN112909232A
Application number: CN202110083514.8A
Authority: CN
Inventors: 张大伟; 宋�莹
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2021-01-21
Filing date: 2021-01-21
Publication date: 2021-06-04

Abstract

The invention discloses a sodium fluoride impregnated and coated vanadium-doped porous structure sodium iron pyrophosphate cathode material and a preparation method thereof. The cathode material is impregnated and coated with fluorine on the surface of vanadium-doped sodium iron pyrophosphate in Sodium chloride, the general structural formula is NaF@Na ₂ Fe _1- _1.5x V _x P ₂ O ₇ . In the invention, the Na ₂ FeP ₂ O ₇ material is coated with an appropriate amount of sodium fluoride and doped with an appropriate amount of vanadium, so that the triclinic structure of sodium iron pyrophosphate can maintain a high crystal structure during the process of sodium ion intercalation and deintercalation It has excellent cycle performance and good Coulomb efficiency, effectively avoiding the sharp decrease in cycle caused by the collapse of the structure after sodium ion de-intercalation, which not only improves the stability of the material, but also improves the electrical conductivity to a certain extent. It is beneficial to improve the electrochemical performance of the material.

Description

Sodium fluoride impregnated and coated vanadium-doped porous structure ferric sodium pyrophosphate cathode material and preparation method thereof

Technical Field

The invention belongs to the technical field of lithium ion batteries and electrochemistry, and particularly relates to a sodium ferric pyrophosphate positive electrode material with a vanadium-doped porous structure and a sodium fluoride dipping coating method.

Background

Lithium ion batteries are a new chemical power source, and are considered as the most attractive candidate materials for electric vehicles (ev) and Hybrid Electric Vehicles (HEVs) due to their characteristics of high energy, high power density, long cycle life, small self-discharge, high cost performance, and the like. However, sodium ion batteries have become one of promising materials because of the low content of lithium in the earth's crust and the inability to store energy on a large scale.

Although sodium ion batteries have advantages and opportunities compared to lithium ion batteries, they are also very different: the radius of the sodium ions is larger than that of the lithium ions, so that the sodium ions are more difficult to be embedded, and the structure is more easily collapsed during de-embedding; and the energy density of the sodium ion battery is lower and the rate performance is poorer. Therefore, the exploration and development of electrode materials is crucial for future applications of sodium ion batteries.

The material of the iron-based system is a sodium ion battery anode material system with high commercial value due to the characteristics of easily available raw materials and wide sources. The ferric sodium pyrophosphate material has simple preparation method and good cycle performance, but the specific capacity is lower than that of other sodium ion anode materials. At present, the coating and doping of the material are considered to be effective means for improving the electrochemical performance of the positive electrode material of the sodium-ion battery.

Therefore, the method for coating and doping the ferric sodium pyrophosphate material has important significance.

Disclosure of Invention

Aiming at the defects of the ferric sodium pyrophosphate positive electrode material, the invention provides a sodium fluoride impregnated and coated vanadium-doped porous ferric sodium pyrophosphate positive electrode material and a preparation method thereof, aiming at improving the cycle performance and the capacity retention rate of the material in a sodium ion battery.

In order to realize the purpose of the invention, the following technical scheme is adopted:

the invention firstly discloses a sodium fluoride impregnated and coated vanadium doped porous structure ferric sodium pyrophosphate anode material which is characterized in that: the positive electrode material is prepared by coating sodium fluoride on the surface of ferric sodium pyrophosphate doped with vanadium in situ, and the structural general formula of the positive electrode material is as follows: NaF @ Na₂Fe_1-1.5xV_xP₂O₇。

Further, the positive electrode material exhibits a bulky porous structure.

Further, in the structural general formula, x is more than 0 and less than or equal to 0.2.

The invention also discloses a preparation method of the sodium fluoride impregnated and coated vanadium doped porous structure ferric sodium pyrophosphate anode material, which is characterized by comprising the following steps of:

step 1, weighing a first sodium source, an iron source, a phosphorus source and a vanadium source according to a molar ratio in a structural general formula; mixing the raw materials, adding acetone, ball-milling, and drying to obtain a precursor;

step 2, putting the precursor into a tube furnace, heating to 550-650 ℃ under an inert atmosphere, preserving heat for 12 hours, then cooling along with the furnace, and grinding to obtain Na₂Fe_1-1.5xV_xP₂O₇A material;

step 3, adding the Na₂Fe_1-1.5xV_xP₂O₇Adding the material and a second sodium source into deionized water, ultrasonically stirring uniformly, and then dropwise adding a fluorine source water solution through a peristaltic pump to obtain a suspension; continuously stirring the suspension for 2h, centrifuging, vacuum drying overnight, and grinding to obtain sodium fluoride-impregnated vanadium-doped porous structure ferric sodium pyrophosphate cathode material NaF @ Na₂Fe_1-1.5xV_xP₂O₇。

Further, the mass of the sodium fluoride accounts for 1-10% of the total mass of the positive electrode material.

Further: the first sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, sodium oxalate and sodium hydroxide; the iron source is at least one of ferrous oxalate, ferrous acetate and ferric nitrate; the phosphorus source is at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; the vanadium source is ammonium metavanadate; the second sodium source is at least one of sodium carbonate and sodium chloride; the fluorine source is at least one of ammonium fluoride and potassium fluoride.

Further, the temperature rise rate of the temperature rise is 2-5 ℃/min.

Further, the inert gas is argon or nitrogen.

Compared with the prior art, the invention has the beneficial effects that:

1. the invention passes through Na₂FeP₂O₇The material is subjected to appropriate sodium fluoride impregnation and coating and appropriate vanadium doping, so that the crystal structure of ferric sodium pyrophosphate with a triclinic structure can keep high stability in the process of sodium ion intercalation and deintercalation, the ferric sodium pyrophosphate has excellent cycle performance and good coulombic efficiency, the cycle sharp reduction caused by structure collapse after sodium ion deintercalation is effectively avoided, the material stability is improved, the conductivity can be improved to a certain extent, and the electrochemical performance of the material is further improved.

2. The preparation method is simple and easy to realize.

Drawings

FIG. 1 shows Na obtained at a sintering temperature of 600 ℃ in example 1 of the present invention₂FeP₂O₇SEM image of material.

FIG. 2 shows Na obtained at different sintering temperatures in example 1 of the present invention₂FeP₂O₇First charge and discharge curve diagram of the material.

FIG. 3 shows Na obtained at different sintering temperatures in example 1 of the present invention₂FeP₂O₇Cycle performance profile of the material.

FIG. 4 shows Na with different vanadium doping amounts in example 2 of the present invention₂Fe_1-1.5xV_xP₂O₇First charge and discharge curve diagram of the material.

FIG. 5 shows Na with different vanadium doping amounts in example 2 of the present invention₂Fe_1-1.5xV_xP₂O₇Cycle performance profile of the material.

FIG. 6 shows NaF @ Na with different sodium fluoride coating amounts in example 3 of the present invention₂Fe_0.775V_0.15P₂O₇First charge and discharge curve diagram of the material.

FIG. 7 shows NaF @ Na with different sodium fluoride coating amounts in example 3 of the present invention₂Fe_0.775V_0.15P₂O₇Cycle performance profile of the material.

FIG. 8 shows 5% NaF @ Na obtained in example 3 of the present invention₂Fe_0.775V_0.15P₂O₇SEM image of material.

Detailed Description

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below. The following disclosure is merely exemplary and illustrative of the inventive concept, and those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Example 1 preparation of Na₂FeP₂O₇Material

Firstly, 0.9085g (5mmol) of ferrous oxalate dihydrate and 1.2121g (10mmol) of anhydrous sodium dihydrogen phosphate are put into a ball milling tank of poly (tetrachloroethylene), acetone is added, ball milling is carried out for 6h, then drying is carried out for 2h in a vacuum drying oven at 50 ℃, and then grinding is carried out for 1h, thus obtaining precursor powder.

Heating the precursor powder to 560 ℃ at a heating rate of 2 ℃/min in a tube furnace filled with argon, preserving heat for 12h, then cooling along with the furnace, grinding the obtained product to obtain Na₂FeP₂O₇A material.

Adjusting the sintering temperature in the above preparation method to 600 deg.C and 640 deg.C respectively, and preparing Na by the same method₂FeP₂O₇A material. FIG. 1 shows Na obtained at 600 ℃ condition₂FeP₂O₇SEM image of the material, it can be seen from the figure that the ferric sodium pyrophosphate cathode material shows a large agglomeration of particles.

The samples obtained in this example were assembled into a CR2032 type cell, and electrochemical performance tests were performed: the positive electrode material of the prepared sodium-ion battery is taken as an active substance, SP is taken as a conductive agent, PVDF is taken as a binder, and N-methyl-2-pyrrolidone (NMP) is taken as a dispersing agent, and the components are as follows: SP: PVDF 7: 2: 1, and coating the mixture on an aluminum foil to prepare a positive plate. Then, a metal sodium sheet is used as a negative electrode, a polypropylene microporous membrane is used as a diaphragm, and 1mol/L NaPF is used₆For the electrolyte, a CR2032 type cell was produced in a glove box filled with argon gas.

And (3) performing a 0.1c constant-current charge and discharge test on each sample at normal temperature, wherein the charge and discharge cut-off voltage is 2.0V-4.0V. As shown in fig. 2 and 3, the test shows that: sintered Na at 560 ℃₂FeP₂O₇The initial discharge specific capacity of the material is 67.58mAh/g, the discharge capacity after 100 cycles is 65.98mAh/g, and the capacity retention rate is 97.63%; sintered Na at 600 deg.C₂FeP₂O₇The initial discharge specific capacity of the material is 69.71mAh/g, the discharge capacity after 100 cycles is 67.24mAh/g, and the capacity retention rate is 96.46%; na sintered at 640 deg.C₂FeP₂O₇The first discharge specific capacity of the material is 63.83mAh/g, and the discharge capacity after 100 cycles is 66.87 mAh/g. It can be seen that Na is produced at a sintering temperature of 600 deg.C₂FeP₂O₇The material has the best charge and discharge performance and cycle performance.

Example 2 preparation of Na₂Fe_1-1.5xV_xP₂O₇Material

Firstly, 0.7722g (4.25mmol) of ferrous oxalate dihydrate, 1.2121g (10mmol) of anhydrous sodium dihydrogen phosphate and 0.0591g (0.5mmol) of ammonium metavanadate are put into a ball milling tank of poly-tetrachloroethylene, acetone is added, ball milling is carried out for 6h, then drying is carried out for 2h at 50 ℃ in a vacuum drying oven, and then grinding is carried out for 1h, thus obtaining precursor powder.

Heating the precursor powder to 600 ℃ at the heating rate of 2 ℃/min in a tube furnace filled with argon, preserving heat for 12 hours, then cooling along with the furnace, grinding the obtained product to obtain Na₂Fe_0.85V_0.1P₂O₇A material.

The amount of raw materials used in the above preparation method was adjusted to 0.7041g (3.875mmol) of ferrous oxalate dihydrate, 1.2121g (10mmol) of anhydrous sodium dihydrogen phosphate, and 0.0886g (0.75mmol) of ammonium metavanadate, and the vanadium-doped sodium iron pyrophosphate material was prepared in the same manner as described above, and the obtained sample was designated as Na₂Fe_0.775V_0.15P₂O₇。

The raw material dosage in the preparation method is adjusted as follows: 0.6359g (3.5mmol) of bisA vanadium-doped ferric sodium pyrophosphate material was prepared in the same manner as above using 1.2121g (10mmol) of iron oxalate hydrate, 1.2121g (10mmol) of anhydrous sodium dihydrogen phosphate, 0.1182g (1mmol) of ammonium metavanadate, and the obtained sample was designated as Na₂Fe_0.7V_0.2P₂O₇。

The samples obtained in this example were assembled into a CR2032 type cell in the same manner as in example 1, and electrochemical performance was measured.

And (3) performing a 0.1c constant-current charge and discharge test on each sample at normal temperature, wherein the charge and discharge cut-off voltage is 2.0V-4.0V. As shown in fig. 4 and 5, after testing: na (Na)₂Fe_0.85V_0.1P₂O₇The initial discharge specific capacity of the material is 84.29mAh/g, the discharge capacity after 100 cycles is 83.76mAh/g, and the capacity retention rate is 99.37%; na (Na)₂Fe_0.775V_0.15P₂O₇The initial discharge specific capacity of the material is 89.15mAh/g, the discharge capacity after 100 cycles is 75.82mAh/g, and the capacity retention rate is 85.04%; na (Na)₂Fe_0.7V_0.2P₂O₇The initial discharge specific capacity of the material is 81.54mAh/g, the discharge capacity after 100 cycles is 79.57mAh/g, and the capacity retention rate is 97.58%. It can be seen that the first discharge capacity and capacity retention rate can be effectively improved by proper vanadium doping.

Example 3 preparation of NaF @ Na₂Fe_0.775V_0.15P₂O₇

Firstly, 0.7041g (3.875mmol) of ferrous oxalate dihydrate, 1.2121g (10mmol) of anhydrous sodium dihydrogen phosphate and 0.0886g (0.75mmol) of ammonium metavanadate are put into a ball milling tank of poly (tetrachloroethylene), acetone is added, ball milling is carried out for 6h, then drying is carried out for 2h at 50 ℃ in a vacuum drying oven, and then grinding is carried out for 1h, thus obtaining precursor powder.

Heating the precursor powder to 600 ℃ at the heating rate of 2 ℃/min in a tube furnace filled with argon, preserving heat for 12 hours, then cooling along with the furnace, grinding the obtained product to obtain Na₂Fe_0.775V_0.15P₂O₇A material.

Mixing Na₂Fe_0.775V_0.15P₂O₇MaterialAnd 0.0294g (0.49mmol) of sodium chloride are added into deionized water, the mixture is stirred evenly by ultrasonic wave, and then 49mL of 0.01mol/L potassium fluoride water solution is added dropwise through a peristaltic pump to obtain suspension; and continuously stirring the suspension for 2h, centrifuging, drying in vacuum overnight, and grinding to obtain the sodium fluoride-impregnated vanadium-doped porous structure ferric sodium pyrophosphate cathode material, which is recorded as 1% NaF @ Na₂Fe_0.775V_0.15P₂O₇。

Sodium fluoride-impregnated vanadium-doped ferric sodium pyrophosphate material was prepared in the same manner as above by adjusting sodium chloride and potassium fluoride in the above preparation methods to 0.1470g (2.45mmol) and 0.05mol/L, 0.2940g (4.9mmol) and 0.1mol/L, respectively, and the obtained sample was recorded as 5% NaF @ Na₂Fe_0.775V_0.15P₂O₇And 10% NaF @ Na₂Fe_0.775V_0.15P₂O₇。

And (3) carrying out 0.1c constant-current charge and discharge test on the sample at normal temperature, wherein the charge and discharge cut-off voltage is 2.0V-4.0V. As shown in fig. 6 and 7, after testing: 1% NaF @ Na₂Fe_0.85V_0.1P₂O₇The material has optimal performance, the first discharge specific capacity is 87.36mAh/g, the discharge capacity after 100 cycles is 85.81mAh/g, and the capacity retention rate is 98.23%; 5% NaF @ Na₂Fe_0.775V_0.15(P₂O₇)_0.8F_0.8The first discharge specific capacity is 90.31mAh/g, the discharge capacity after 100 cycles is 73.10mAh/g, and the capacity retention rate is 80.94%; 10% NaF @ Na₂Fe_0.775V_0.15(P₂O₇)_0.7F_1.2The initial discharge specific capacity of the material is 79.35mAh/g, the discharge capacity after 100 cycles is 71.45mAh/g, and the capacity retention rate is 90.04%. It can be seen that the sodium fluoride coating is beneficial to improving the capacity and the cycling stability.

FIG. 8 shows the 1% NaF @ Na obtained in this example₂Fe_0.775V_0.15P₂O₇The SEM image of the material shows that compared with the SEM image shown in FIG. 1, the sample has a loose and porous structure, and the ion transport channel can be enlarged.

The present invention is not limited to the above exemplary embodiments, and any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. a kind of sodium fluoride impregnation coating vanadium doped porous structure sodium iron pyrophosphate positive electrode material, it is characterized in that: described positive electrode material is that the surface of the sodium iron pyrophosphate doped with vanadium in situ is coated with fluoride Sodium, the general structural formula of the cathode material is: NaF@Na ₂ Fe _1-1.5x V _x P ₂ O ₇ .

2 . The sodium fluoride-impregnated and coated vanadium-doped porous structure sodium iron pyrophosphate positive electrode material according to claim 1 , wherein the positive electrode material exhibits a fluffy porous structure. 3 .

3 . The sodium fluoride-impregnated and coated vanadium-doped porous structure sodium iron pyrophosphate positive electrode material according to claim 1 , wherein in the general structural formula, 0<x≦0.2. 4 .

4. a preparation method of the sodium fluoride impregnation coating vanadium-doped porous structure sodium iron pyrophosphate positive electrode material according to any one of claims 1～3, is characterized in that, comprises the following steps:

Step 1. Weigh the first sodium source, iron source, phosphorus source and vanadium source according to the molar ratio in the general structural formula; after mixing each raw material, add acetone and ball-mill, then dry to obtain a precursor;

Step 2. Put the precursor into a tube furnace, raise the temperature to 550-650° C. in an inert atmosphere, keep the temperature for 12 hours, then cool with the furnace, and grind to obtain Na ₂ Fe _1-1.5x V _x P ₂ O ₇ Material;

Step 3. Add the Na ₂ Fe _1-1.5x V _x P ₂ O ₇ material and the second sodium source into deionized water, stir ultrasonically evenly, and then add the fluorine source aqueous solution dropwise through a peristaltic pump to obtain a suspension After the suspension was continuously stirred for 2h, centrifuged, vacuum-dried overnight, and ground to obtain a sodium fluoride-impregnated-coated vanadium-doped porous structure sodium iron pyrophosphate positive electrode material NaF@Na ₂ Fe _1-1.5× V _× P ₂ O ₇ .

5 . The preparation method according to claim 4 , wherein the mass of the sodium fluoride accounts for 1-10% of the total mass of the positive electrode material. 6 .

6. preparation method according to claim 4, is characterized in that: described first sodium source is at least one in sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, sodium oxalate and sodium hydroxide; Described iron source It is at least one of ferrous oxalate, ferrous acetate and ferric nitrate; the phosphorus source is at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; the vanadium source is a partial Ammonium vanadate.

7. preparation method according to claim 4 is characterized in that: described second sodium source is at least one in sodium carbonate and sodium chloride, and described fluorine source is at least one in ammonium fluoride and potassium fluoride A sort of.

8 . The preparation method according to claim 4 , wherein in step 2, the temperature rising rate of the temperature rising is 2-5° C./min. 9 .

9. The preparation method according to claim 4, wherein in step 2, the inert gas is argon or nitrogen.