A composite iron fluoride cathode material, a preparation method thereof, and an application thereof

Abstract

The present invention belongs to the technical field of sodium ion batteries, and particularly relates to a composite iron fluoride cathode material, a preparation method thereof and an application. In the composite iron fluoride cathode material of the present invention, fluorinated graphene with high conductivity is introduced and compounded with the FeF x material to solve the problem of poor conductivity; the fluorinated graphene can provide the maximum charge polarization and improve the electrochemical performance of the composite iron fluoride cathode material; the FeF3-FeF2 micro-nano spheres are small in size, and have a high specific surface area and pore volume, which can increase the contact area between the composite iron fluoride cathode material and the electrolyte and increase the electro-chemical activity.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion batteries, and particularly relates to a composite iron fluoride cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] In the structural composition of sodium-ion batteries, the active material materials at both poles are the source of generating electric energy and are the key to determining the battery performance. Among the composition materials of currently commercialized sodium-ion batteries, the cathode material is the short board of the two electrode materials. Improving the capacity of the cathode material is the key to improving the overall power density and energy density of the battery. The cathode material is also an important factor determining the safety performance, cycle stability, and future development direction of sodium-ion batteries. Therefore, the design and development of electrode materials with high reversible capacity is the primary task for the development of sodium-ion secondary batteries.

[0003] As a new type of reversible chemical conversion mechanism cathode material - transition metal fluoride, due to its ability to provide a relatively high output voltage, relatively high theoretical specific capacity, and specific energy density, it has received extensive attention from researchers. For example, the theoretical specific capacity of FeF x is greater than 571 mAh·g -1 , and the energy density is greater than 1519 Wh·kg -1 . Due to the strong polarity of the Fe-F bond, FeF x has the advantage of a high electrode potential, that is, it can provide a relatively high output voltage, but it also has a relatively large band gap width (5.96 eV), which in turn leads to poor conductivity. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a composite iron fluoride cathode material, a preparation method thereof, and an application thereof. The composite iron fluoride cathode material provided by the present invention has excellent electrical conductivity.

[0005] To achieve the above purpose, the present invention provides the following technical solutions:

[0006] The present invention provides a composite iron fluoride cathode material, which includes fluorinated graphene and iron fluoride porous micro-nano spheres nucleated at the defect positions of the fluorinated graphene; the chemical composition of the iron fluoride porous micro-nano spheres includes FeF2 and FeF3.

[0007] Preferably, the molar ratio of FeF3 to FeF2 is (5 - 7):(3 - 5); the mass ratio of the iron fluoride porous micro-nano spheres to the fluorinated graphene is (0.0001 - 6):100.

[0008] Preferably, the mass ratio of fluorine element to carbon element in the fluorinated graphene is (0.03 - 0.05):1.

[0009] Preferably, the specific surface area of the iron fluoride composite cathode material is 15-20 m 2 / g, and the pore volume is 0.08-0.11 cm 3 / g.

[0010] Preferably, the particle size of the iron fluoride porous micro-nano spheres is 300-600 nm.

[0011] The present invention also provides a preparation method of the iron fluoride composite cathode material described in the above technical solution, including the following steps:

[0012] Mix the dispersion of graphene oxide and hydrofluoric acid, and carry out an acidothermal reaction to obtain fluorinated graphene;

[0013] Mix the fluorinated graphene, water-soluble ferric salt, hydrofluoric acid and dispersant, and carry out a solvothermal reaction to obtain a precursor;

[0014] Calcine the precursor under a protective atmosphere to obtain the iron fluoride composite cathode material.

[0015] Preferably, the temperature of the acidothermal reaction is 120-200 °C, and the heat preservation time is 10-30 h.

[0016] Preferably, the temperature of the solvothermal reaction is 100-150 °C, and the heat preservation time is 10-15 h.

[0017] Preferably, the temperature of the calcination is 350-500 °C, and the heat preservation time is 1-3 h.

[0018] The present invention also provides the application of the iron fluoride composite cathode material described in the above technical solution or the iron fluoride composite cathode material prepared by the preparation method described in the above technical solution in a sodium ion battery.

[0019] The present invention provides an iron fluoride composite cathode material, including fluorinated graphene and iron fluoride porous micro-nano spheres nucleated at the defect positions of the fluorinated graphene; the chemical composition of the iron fluoride porous micro-nano spheres includes FeF2 and FeF3. The present invention introduces fluorinated graphene into the iron fluoride composite cathode material. Fluorinated graphene itself has high conductivity and is compounded with the FeF x material to solve the problem of its poor conductivity. At the same time, fluorinated graphene can provide the largest charge polarization, enhance the electrochemical activity and electrode stability of energy-related reactions, so that the iron fluoride composite cathode material has good electrochemical performance. At the same time, the iron fluoride porous micro-nano spheres are small in size and have a high specific surface area and pore volume, which can increase the contact area between the iron fluoride composite cathode material and the electrolyte and increase the electrochemical activity.

[0020] In addition, the present invention applies the iron fluoride composite cathode material to a sodium ion battery. The pore structure of the iron fluoride composite cathode material can provide additional positions for Na + storage, which is beneficial to increasing the specific capacity and reducing the sodium ion diffusion channels, thus facilitating the improvement of the rate performance. It has a buffering effect on the volume change during the sodiation process of the iron fluoride composite cathode material, reduces the aggregation of the sodium ion cathode material, further promotes the electrochemical reaction of the sodium ion battery, and reduces the electrochemical polarization during the battery discharge process, which can significantly improve the discharge performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 SEM image of the precursor obtained in Example 1 of the present invention;

[0022] Figure 2 SEM image of the iron fluoride composite cathode material obtained in Example 1 of the present invention;

[0023] Figure 3 XRD pattern of the fluorinated graphene and iron fluoride composite cathode material obtained in Example 1 of the present invention;

[0024] Figure 4 XRD patterns of the precursor and the iron fluoride composite cathode material obtained in Example 1 of the present invention;

[0025] Figure 5 Specific surface area and pore size test diagram of the precursor obtained in Example 1 of the present invention;

[0026] Figure 6 Specific surface area and pore size test diagram of the iron fluoride composite cathode material obtained in Example 1 of the present invention;

[0027] Figure 7 Iron element distribution diagram of the iron fluoride composite cathode material obtained in Example 1 of the present invention;

[0028] Figure 8 SEM images of the fluorinated graphene and the iron fluoride composite cathode material doped with different masses of fluorinated graphene obtained in Example 2 of the present invention;

[0029] Figure 9 XRD patterns of the iron fluoride composite cathode material at different calcination temperatures obtained in Example 3 of the present invention;

[0030] Figure 10 XRD patterns of the iron fluoride composite cathode material at different calcination times obtained in Example 4 of the present invention;

[0031] Figure 11 SEM images of the iron fluoride composite cathode material at different calcination temperatures obtained in Example 3 of the present invention and the iron fluoride composite cathode material at different calcination times obtained in Example 4 of the present invention. Detailed Embodiments

[0032] The present invention provides a composite iron fluoride cathode material, comprising fluorinated graphene and iron fluoride porous micro-nano spheres nucleated at the defect positions of the fluorinated graphene; the chemical composition of the iron fluoride porous micro-nano spheres includes FeF2 and FeF3.

[0033] The composite iron fluoride cathode material provided by the present invention includes fluorinated graphene. In the present invention, the mass ratio of fluorine element to carbon element in the fluorinated graphene is preferably (0.03 - 0.05):1, more preferably 0.04:1; the fluorinated graphene is preferably a two-dimensional layered structure.

[0034] The composite iron fluoride cathode material provided by the present invention includes iron fluoride porous micro-nano spheres nucleated at the defect positions of the fluorinated graphene. In the present invention, the iron fluoride includes FeF2 and FeF3; the molar ratio of FeF3 to FeF2 is preferably (5 - 7):(3 - 5), more preferably 6:4; the particle size of the iron fluoride porous micro-nano spheres is preferably 300 - 600 nm, more preferably 400 - 500 nm.

[0035] In the present invention, the mass ratio of the iron fluoride porous micro-nano spheres to the fluorinated graphene is preferably (0.0001 - 6):100, more preferably (1 - 5):100.

[0036] In the present invention, the specific surface area of the composite iron fluoride cathode material is preferably 15 - 20 m 2 / g, more preferably 16.5 - 18 m 2 / g, and the pore volume is preferably 0.08 - 0.11 cm 3 / g, more preferably 0.08 - 0.10 cm 3 / g.

[0037] The present invention introduces fluorinated graphene into the composite iron fluoride cathode material. Fluorinated graphene itself has high conductivity. When compounded with the FeF x material, it can provide a conductive network for the FeF x to solve the problem of its poor conductivity. At the same time, fluorinated graphene can provide the maximum charge polarization, enhance the electrochemical activity and electrode stability of energy-related reactions, so that the composite iron fluoride cathode material has good electrochemical performance. At the same time, the FeF3 - FeF2 micro-nano spheres are small in size, and have a high specific surface area and pore volume, which can increase the contact area between the composite iron fluoride cathode material and the electrolyte and increase the electrochemical activity; the pore structure provides Na +The additional storage positions are beneficial to increasing the specific capacity, reducing the sodium ion diffusion channels, and facilitating the improvement of rate performance; they have a buffering effect on the volume change during the sodiation process of the iron fluoride composite cathode material, reduce the aggregation of the sodium ion cathode material, further promote the electrochemical reaction of the sodium ion battery, reduce the electrochemical polarization during the battery discharge process, and can significantly improve the discharge performance of the battery.

[0038] The present invention also provides a preparation method of the iron fluoride composite cathode material described in the above technical solution, including the following steps:

[0039] Mix a dispersion of graphene oxide and hydrofluoric acid, and perform an acidothermal reaction to obtain fluorinated graphene;

[0040] Mix the fluorinated graphene, a water-soluble ferric salt, hydrofluoric acid, and a dispersant, and perform a solvothermal reaction to obtain a precursor;

[0041] Calcine the precursor under a protective atmosphere to obtain the iron fluoride composite cathode material.

[0042] Unless otherwise specified, the present invention has no special requirements for the sources of the raw materials used in the preparation, and commercially available products well-known to those skilled in the art can be used.

[0043] The present invention mixes a dispersion of graphene oxide and hydrofluoric acid and performs an acidothermal reaction.

[0044] In the present invention, the thickness of the graphene oxide is preferably 1 - 2 nm, more preferably 2 nm; the sheet diameter of the graphene oxide is preferably 0.2 - 10 μm, more preferably 1 - 8 μm; the solvent in the dispersion of graphene oxide is preferably deionized water; the mass concentration of graphene oxide in the dispersion of graphene oxide is preferably 2 - 10 mg / mL, more preferably 2 - 5 mg / mL; the mass concentration of hydrofluoric acid is preferably 20 - 40%, more preferably 30 - 40%; the volume ratio of the dispersion of graphene oxide to hydrofluoric acid is preferably (1 - 4):1, more preferably (2 - 3):1.

[0045] In the present invention, the temperature of the acidothermal reaction is preferably 120 - 200 °C, more preferably 150 - 180 °C, and the heat preservation time is preferably 10 - 30 h, more preferably 15 - 25 h.

[0046] The present invention has no special limitation on the mixing process of the dispersion of graphene oxide and hydrofluoric acid, and a well-known mixing process in the art can be used to mix the materials evenly.

[0047] In the present invention, the equipment used for mixing the dispersion of graphene oxide and hydrofluoric acid is preferably a polytetrafluoroethylene container.

[0048] After the acidothermal reaction, the present invention preferably filters, washes and dries the product obtained from the acidothermal reaction in sequence to obtain fluorinated graphene.

[0049] In the present invention, the filtration is preferably suction filtration; the reagent used for washing is preferably water, more preferably deionized water; the number of washing times is preferably 3 times.

[0050] The present invention can remove excessive unreacted hydrofluoric acid through washing.

[0051] In the present invention, the drying temperature is preferably 70 - 80 °C, more preferably 75 - 80 °C, the time is preferably 1 - 5 h, more preferably 2 - 3 h; the drying is preferably vacuum drying; the vacuum pressure of the vacuum drying is preferably 0 - -0.2 MPa, more preferably -0.1 MPa; the drying equipment is preferably a vacuum drying oven.

[0052] After obtaining the fluorinated graphene, the present invention mixes the fluorinated graphene, water-soluble ferric salt, hydrofluoric acid and dispersant and conducts a solvothermal reaction.

[0053] In the present invention, the dispersant is preferably tetraethylene glycol; the water-soluble ferric salt preferably includes ferric nitrate and / or ferric chloride, more preferably ferric nitrate, and most preferably ferric nitrate nonahydrate. When the water-soluble ferric salt is ferric nitrate and ferric chloride, the present invention has no special limitation on the ratio of ferric nitrate to ferric chloride, and any ratio is acceptable.

[0054] In the present invention, the mass ratio of the water-soluble ferric salt to the fluorinated graphene is preferably (10 - 150):1, more preferably (40 - 100):1; the molar ratio of the water-soluble ferric salt to the hydrofluoric acid is preferably 1:(5 - 10), more preferably 1:(6 - 10), and most preferably 1:8; the mass ratio of the hydrofluoric acid to the dispersant is preferably 1:(10 - 30), more preferably 1:(10 - 20).

[0055] In the present invention, the process of mixing the fluorinated graphene, water-soluble ferric salt, hydrofluoric acid and dispersant is preferably to first mix the fluorinated graphene and the dispersant, then add the hydrofluoric acid for the first stirring, and then add the water-soluble ferric salt for the second stirring. In the present invention, the time of the first mixing is preferably 30 - 90 min, more preferably 40 - 60 min; the rate of the first stirring is preferably 1000 - 1500 r / min, more preferably 1200 - 1400 r / min; the time of the first stirring is preferably 20 - 40 min, more preferably 30 min; the rate of the second stirring is preferably 1200 - 1800 r / min, more preferably 1300 - 1600 r / min; the time of the second stirring is preferably 30 - 90 min, more preferably 40 - 60 min.

[0056] In the present invention, the solvothermal reaction is preferably carried out in an oven. The temperature of the solvothermal reaction is preferably 100-150 °C, more preferably 110-120 °C, and the heat preservation time is preferably 10-15 h, more preferably 11-12 h.

[0057] After the solvothermal reaction, the present invention preferably washes and dries the product obtained from the solvothermal reaction in sequence to obtain a precursor.

[0058] In the present invention, the reagent used for washing is preferably anhydrous ethanol; the number of washing times is preferably 4-6 times, more preferably 5 times. The present invention eliminates the residues of water and dispersant through washing.

[0059] In the present invention, the drying temperature is preferably 60-100 °C, more preferably 70-80 °C, the time is preferably 11-13 h, more preferably 12 h; the drying is preferably vacuum drying; the vacuum pressure of the vacuum drying is preferably 0--0.2 MPa, more preferably -0.1 MPa; the drying equipment is preferably a vacuum drying oven.

[0060] In the present invention, the components of the precursor are mainly FeF3·0.33H2O and graphene fluoride.

[0061] After obtaining the precursor, the present invention calcines the precursor under a protective atmosphere to obtain a composite cathode material of iron fluoride.

[0062] Before calcination, the present invention preferably dehydrates the precursor. In the present invention, the dehydration method is preferably vacuum drying; the temperature of the vacuum drying is preferably 140-160 °C, more preferably 150 °C, the time is preferably 2-4 h, more preferably 3 h; the dehydration equipment is preferably a vacuum drying oven.

[0063] Through the dehydration treatment, the present invention can completely remove the moisture and residual organic substances adsorbed by the precursor in the air, and can prevent the precursor from being oxidized due to the evaporation of moisture during the subsequent high-temperature calcination process.

[0064] In the present invention, the calcination temperature is preferably 350-500 °C, more preferably 400-500 °C, the heat preservation time is preferably 1-3 h, more preferably 1-2 h; the heating rate to the calcination temperature is preferably 3-4 °C / min, more preferably 4 °C / min; the protective atmosphere is preferably argon; the flow rate of the argon is preferably 2-6 L / min, more preferably 4 L / min.

[0065] In the present invention, hydrofluoric acid is used to etch the two-dimensional layered structure of fluorinated graphene, destroying the carbon atoms in some of the carbon hexagons to form defect positions; nucleation and growth are carried out at the defect positions of fluorinated graphene to obtain FeF x , so that fluorinated graphene and FeF x are tightly combined to achieve the purpose of providing a conductive network for FeF x .

[0066] In the present invention, the precursor is calcined under a protective atmosphere, so that the crystal water in FeF3·0.33H2O in the precursor is removed to obtain a porous morphology, and at the same time, part of FeF3 decomposes into FeF2 to form a mixed crystal form of FeF3-FeF2, making the iron fluoride composite cathode material have a higher specific surface area and pore volume. When applied to sodium-ion batteries, it can be in full contact with the electrolyte and provide more active sites.

[0067] In the present invention, fluorinated graphene is prepared by the acidothermal method as a substrate, and FeF3·0.33H2O can be uniformly grown on fluorinated graphene by the solvothermal method. The prepared precursor (FeF3·0.33H2O / FG nanocomposite) has a small size, so that when the composite material is applied to sodium-ion batteries, it can be in full contact with the electrolyte, which can not only improve the utilization rate of the cathode composite material, but also reduce the electrochemical polarization during the discharge process of the battery, significantly improving the discharge performance of the battery; at the same time, by calcining, the crystal water in the prepared FeF3·0.33H2O / FG nanocomposite is removed, finally increasing the specific surface area and pore volume of the sodium-ion battery cathode composite material, further improving the contact area between the final sodium-ion battery cathode composite material and the electrolyte, and improving the reaction activity.

[0068] The present invention also provides the application of the iron fluoride composite cathode material described in the above technical solution or the iron fluoride composite cathode material prepared by the preparation method described in the above technical solution in sodium-ion batteries.

[0069] In the present invention, the iron fluoride composite cathode material is applied to sodium-ion batteries. The pore structure of the iron fluoride composite cathode material can provide additional positions for Na + storage, which is beneficial to increasing the specific capacity and reducing the sodium ion diffusion channel, which is beneficial to improving the rate performance; it has a buffering effect on the volume change during the sodiation process of the iron fluoride composite cathode material, and reduces the aggregation of the sodium ion cathode material, further promoting the electrochemical reaction of the sodium-ion battery, reducing the electrochemical polarization during the discharge process of the battery, and can significantly improve the discharge performance of the battery.

[0070] The present invention has no special limitation on the application method of the iron fluoride composite cathode material in sodium-ion batteries, and the well-known application methods in the art can be adopted.

[0071] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0072] Example 1

[0073] 5 mL of 40 wt.% hydrofluoric acid was added to a polytetrafluoroethylene reaction kettle containing 50 mL of 5 mg / mL graphene oxide dispersion, and then acidothermal reaction was carried out at 180 °C in an oven for 16 h. After it was cooled to room temperature, suction filtration was carried out. After washing repeatedly with deionized water 3 times, it was dried in a vacuum drying oven at -0.1 MPa and 80 °C for 2 h to obtain fluorinated graphene (FG);

[0074] 50 mg of fluorinated graphene was added to a polytetrafluoroethylene reaction kettle containing 40 mL of tetraethylene glycol, stirred at 1200 r / min at room temperature for 1 h, then 2 mL of 40 wt.% hydrofluoric acid was added dropwise, stirred at 1600 r / min at room temperature for 30 min, and then 2.02 g of Fe(NO3)3·9H2O was added. After that, it was placed in an oven and heated at 120 °C for 12 h. After it was cooled to room temperature, the supernatant was poured off, washed 5 times with absolute ethanol, transferred to a vacuum drying oven, dried at -0.1 MPa and 80 °C for 12 h to obtain a precursor (FeF3·0.33H2O / FG composite material). Then the precursor was transferred to a vacuum drying oven again and dried at 150 °C for 3 h to remove water; then in a tubular furnace, it was heated to 400 °C at a heating rate of 4 °C / min in an argon atmosphere with a gas flow rate of 4 L / min and calcined for 2 h to obtain a composite positive electrode material of iron fluoride (FeF3-FeF3 / FG nanocomposite material).

[0075] Example 2

[0076] 5 mL of 40 wt.% hydrofluoric acid was added to a polytetrafluoroethylene reaction kettle containing 50 mL of 5 mg / mL graphene oxide dispersion, and then acidothermal reaction was carried out at 180 °C in an oven for 16 h. After it was cooled to room temperature, suction filtration was carried out. After washing repeatedly with deionized water 3 times, it was dried in a vacuum drying oven at -0.1 MPa and 80 °C for 2 h to obtain fluorinated graphene (FG);

[0077] 20 mg, 50 mg, and 100 mg of graphene fluoride were separately added to three PTFE reactors containing 40 mL of tetraethylene glycol, and stirred at 1200 r / min for 1 h at room temperature. Subsequently, 2 mL of 40 wt.% hydrofluoric acid was added dropwise while stirring at 1600 r / min for 30 min at room temperature. Then, 2.02 g of Fe(NO3)3·9H2O was added to each reactor, and the reactors were placed in an oven and heated at 120 °C for 12 h. After cooling to room temperature, the supernatant was poured off, and each sample was washed 5 times with absolute ethanol, transferred to a vacuum drying oven, and dried at -0.1 MPa and 80 °C for 12 h to obtain three precursors (FeF3·0.33H2O / FG composites). Then, the precursors were transferred back to the vacuum drying oven and dried at 150 °C for 3 h to remove water. Then, in a tube furnace, the precursors were placed in the tube furnace in three batches, the temperature was set at 400 °C, and calcined at a heating rate of 4 °C / min for 2 h in an argon atmosphere with a gas flow rate of 4 L / min to obtain a composite iron fluoride cathode material (FeF3-FeF3 / FG nanocomposite).

[0078] Example 3

[0079] 5 mL of 40 wt.% hydrofluoric acid was added to a PTFE reactor containing 50 mg of 5 mg / mL graphene oxide dispersion, and then subjected to acidothermal reaction at 180 °C in an oven for 16 h. After cooling to room temperature, filtration was carried out, and the sample was repeatedly washed 3 times with deionized water and then dried in a vacuum drying oven at -0.1 MPa and 80 °C for 2 h to obtain graphene fluoride (FG).

[0080] 50 mg of graphene fluoride was added to a PTFE reactor containing 40 mL of tetraethylene glycol, stirred at 1200 r / min for 1 h at room temperature, then 2 mL of 40 wt.% hydrofluoric acid was added dropwise, stirred at 1600 r / min for 30 min at room temperature, and then 2.02 g of Fe(NO3)3·9H2O was added. The reactor was placed in an oven and heated at 120 °C for 12 h. After cooling to room temperature, the supernatant was poured off, washed 5 times with absolute ethanol, transferred to a vacuum drying oven, and dried at -0.1 MPa and 80 °C for 12 h to obtain a precursor (FeF3·0.33H2O / FG composite). Then, the precursor was transferred back to the vacuum drying oven and dried at 150 °C for 3 h to remove water. Then, in a tube furnace, the precursor was placed in the tube furnace in three batches, and the temperatures were set at 400 °C, 450 °C, and 500 °C respectively, and calcined at a heating rate of 4 °C / min for 2 h in an argon atmosphere with a gas flow rate of 4 L / min to obtain a composite iron fluoride cathode material (FeF3-FeF3 / FG nanocomposite).

[0081] Example 4

[0082] 5 mL of 40 wt.% hydrofluoric acid was added to a polytetrafluoroethylene reaction kettle containing 50 mL of 5 mg / mL graphene oxide dispersion. Then, an acidothermal reaction was carried out at 180 °C for 16 h in an oven. After it cooled down to room temperature, suction filtration was performed. After washing repeatedly with deionized water three times, it was dried in a vacuum drying oven at -0.1 MPa and 80 °C for 2 h to obtain fluorinated graphene (FG).

[0083] 50 mg of fluorinated graphene was added to a polytetrafluoroethylene reaction kettle containing 40 mL of tetraethylene glycol. It was stirred at 1200 r / min at room temperature for 1 h. Subsequently, 2 mL of 40 wt.% hydrofluoric acid was added dropwise, and it was stirred at 1600 r / min at room temperature for 30 min. Then, 2.02 g of Fe(NO3)3·9H2O was added, and it was placed in an oven and heated at 120 °C for 12 h. After it cooled down to room temperature, the upper clear liquid was poured out, and it was washed 5 times with absolute ethanol. It was transferred into a vacuum drying oven and dried at 80 °C for 12 h to obtain a precursor (FeF3·0.33H2O / FG composite material). Then, the precursor was transferred into the vacuum drying oven again and dried at -0.1 MPa and 150 °C for 3 h to remove water. Then, in a tube furnace, the precursor was put into the tube furnace in three batches. The temperature was set at 400 °C, and it was calcined at a heating rate of 4 °C / min for 1 h, 1.5 h, and 2 h respectively under an argon atmosphere with a gas flow rate of 4 L / min to obtain a composite cathode material of iron fluoride (FeF3-FeF3 / FG nanocomposite material).

[0084] Performance Test

[0085] (1) The precursor and the composite cathode material of iron fluoride obtained in Example 1 were subjected to electron microscopy scanning, and the results were as shown in Figure 1 and Figure 2 respectively.

[0086] As shown in Figure 1 and Figure 2 , FeF3·0.33H2O and FeF3-FeF2 grew in-situ on fluorinated graphene and the particles were uniform. Iron, fluorine, and carbon elements were evenly distributed in the composite material.

[0087] (2) The obtained fluorinated graphene and the composite cathode material of iron fluoride in Example 1 were tested by X-ray diffraction technology, and the results were as shown in Figure 3 respectively.

[0088] As shown in Figure 3 , hydrofluoric acid fluorinated graphene oxide through an acidothermal reaction, and fluorine was successfully grafted onto graphene oxide. The fluorine-carbon ratio was about 0.04. The presence of the C-F bond can enable graphene oxide to reduce oxygen-containing functional groups and improve thermal stability while maintaining high electrical conductivity and a large specific surface area.

[0089] (3) The precursor and the composite cathode material of iron fluoride obtained in Example 1 were tested using X-ray diffraction technology, and the results are as Figure 4 shown.

[0090] As Figure 4 shown, the combination of FeF3·0.33H2O and FeF3-FeF2 with fluorinated graphene does not affect its crystallinity, and there are no impurity peaks on the diffraction peaks of FeF3-FeF2 after high-temperature calcination. This is because the good thermal stability of fluorinated graphene can ensure that FeF3·0.33H2O is not oxidized when completely dehydrated.

[0091] (4) The specific surface area and pore size of the precursor and the composite cathode material of iron fluoride obtained in Example 1 were tested, and the results are as Figure 5 and Figure 6 shown.

[0092] As Figure 5 and Figure 6 shown, the specific surface area and pore size of the precursor (FeF3·0.33H2O / FG) are 4.99 m 2 / g and 14.77 nm respectively, while those of the composite cathode material of iron fluoride (FeF3-FeF2 / FG) are 16.70 m 2 / g and 23.05 nm. It can be seen that the specific surface area and pore size of FeF3-FeF2 / FG increase after calcination.

[0093] (5) The pore volume of the precursor (FeF3·0.33H2O / FG) and the composite cathode material of iron fluoride (FeF3-FeF2 / FG) obtained in Example 1 was tested by BET, and the results are 0.015 cm 3 / g and 0.083 cm 3 / g respectively.

[0094] (6) The iron element in the composite cathode material of iron fluoride obtained in Example 1 was analyzed, and the results are as Figure 7 shown.

[0095] As Figure 1 and Figure 7 shown, the proportion of Fe 3+ in the composite cathode material of iron fluoride obtained in Example 1 is 61%, the proportion of Fe 2+ is 39%, and the content ratio of FeF3 and FeF2 is approximately 6:4.

[0096] (7) Using the composite cathode material of iron fluoride obtained in Example 1 as the cathode material and sodium sheet as the anode material, a button cell was assembled. Under a voltage window of 1.0 - 4.0 V, at 0.1C (20 mAg -1)During the charge and discharge process, the initial discharge specific capacity reaches 248.42 mAh / g.

[0097] (8) The fluorinated graphene and iron fluoride composite cathode materials obtained in Example 2 were scanned by electron microscopy, and the results are as follows Figure 8 shown, where a and b are the electron microscopy scanning results of fluorinated graphene, and c and d are the electron microscopy scanning results of iron fluoride composite materials with 20 mg and 100 mg of fluorinated graphene added respectively.

[0098] As Figure 8 shown in a and c, the surface of fluorinated graphene is uneven and presents a complete lamellar structure, indicating that the fluorination of graphene oxide by the acid-thermal method does not damage its own structure. And this complete lamellar structure is conducive to the nucleation and growth of FeF3·0.33H2O on its surface, and can also effectively prevent FeF3·0.33H2O from growing into too large particles, thereby affecting its electrochemical performance.

[0099] As Figure 8 shown in b and d, they are the electron microscopy scanning results of FeF3-FeF3 / FG nanocomposite materials with 20 mg and 100 mg of fluorinated graphene added respectively. It can be found that adding 20 mg of fluorinated graphene in the reaction cannot achieve the tight combination of fluorinated graphene and FeF3-FeF3 micro-nano spheres. The reason is that a small amount of fluorinated graphene will agglomerate seriously during the reaction process, thereby affecting the growth process of the precursor FeF3·0.33H2O on its surface. When 100 mg of fluorinated graphene is added in the reaction, although the FeF3-FeF3 micro-nano spheres are tightly combined on the fluorinated graphene, the FeF3-FeF3 micro-nano spheres themselves will agglomerate and cannot be evenly distributed on the fluorinated graphene. Therefore, it is preferred to add 50 mg of fluorinated graphene as the growth substrate of FeF3·0.33H2O to achieve the tight combination and uniform distribution of FeF3-FeF3 micro-nano spheres and fluorinated graphene.

[0100] (9) The fluorinated iron composite cathode materials at three calcination temperatures obtained in Example 3 were tested by X-ray diffraction technology, and the results are as follows Figure 9 shown.

[0101] As Figure 9 shown, when the FeF3·0.33H2O / FG nanocomposite material is calcined at different temperatures, the contents of FeF3 and FeF2 in the fluorinated iron composite cathode material will also change. As the calcination temperature increases from 400 °C to 450 °C and then to 500 °C, FeF3 in the fluorinated iron composite cathode material gradually decomposes into FeF2, resulting in a gradual decrease in the content of FeF3 and a gradual increase in the content of FeF2 until it is completely converted into FeF2.

[0102] (10) The X-ray diffraction technique was used to test the iron fluoride composite cathode materials obtained at three calcination times in Example 4, and the results are as Figure 10 shown.

[0103] As Figure 10 shown, when the FeF3·0.33H2O / FG nanocomposite material was calcined at 400 °C for different calcination times, the contents of FeF3 and FeF2 in the iron fluoride composite cathode material would further change. As the calcination time increased from 1 h to 1.5 h and then to 2 h, the content of FeF2 in the iron fluoride composite cathode material gradually increased until the ideal ratio of FeF3 to FeF2 reached 6:4.

[0104] (11) The iron fluoride composite cathode materials obtained at three calcination temperatures in Example 3 and the iron fluoride composite cathode materials obtained at three calcination times in Example 4 were scanned by electron microscopy, and the results are respectively as Figure 11 shown, where a is calcined at 400 °C for 1 h, b is calcined at 400 °C for 1.5 h, c is calcined at 400 °C for 2 h, d is calcined at 450 °C for 2 h, and e is calcined at 500 °C for 2 h.

[0105] As Figure 11 shown, Figure 11 In a, b, and c in the figure are the electron microscopy scanning results of calcination at 400 °C for 1 h, 1.5 h, and 2 h respectively. It can be seen from the figure that the iron fluoride composite cathode material at a calcination time of 1 h is in a block shape, with a relatively rough surface, and fluorinated graphene agglomerates on its surface. Such a structure is not conducive to the full contact of FeF3-FeF3 with the electrolyte, and the agglomerated fluorinated graphene cannot well improve its conductivity. The iron fluoride composite cathode material under 1.5 h of calcination is a structure in which uneven small blocks agglomerate with each other, and the structure size is large, making it difficult for sodium ions to migrate inside the particles and unable to further improve the electrochemical performance. Therefore, it is preferred that the calcination temperature of the precursor FeF3·0.33H2O / FG is 400 °C and the calcination time is 2 h. At this time, the morphology of FeF3-FeF3 is a porous structure, with the smallest size, and the specific surface area of the sample is relatively large, which can achieve the optimal actual performance.

[0106] As Figure 11 shown, Figure 11Among them, c, d, and e are the SEM results at the calcination temperatures of 400 °C, 450 °C, and 500 °C respectively. It can be seen from the figure that the iron fluoride composite cathode material at the calcination temperature of 450 °C is in a needle-like structure with uneven distribution, while the graphene fluoride used for the composite has serious agglomeration phenomenon, which is not conducive to improving the electrochemical performance of the iron fluoride composite cathode material. The iron fluoride composite cathode material at the calcination temperature of 500 °C has been completely transformed into FeF2, and the presence of FeF2 particles cannot be observed by SEM, indicating that the FeF2 particles generated after the calcination temperature of 500 °C are too small. Therefore, the preferred calcination temperature of the precursor FeF3·0.33H2O / FG is 400 °C to achieve a relatively stable crystal phase and morphology of the iron fluoride composite cathode material and further improve its electrochemical performance.

[0107] Although the above embodiments have made a detailed description of the present invention, they are only a part of the embodiments of the present invention rather than all embodiments. People can also obtain other embodiments based on this embodiment without creative efforts, and these embodiments all belong to the protection scope of the present invention.

Claims

1. Application of a composite iron fluoride cathode material in a sodium-ion battery, characterized in that, The iron fluoride composite cathode material includes fluorinated graphene and iron fluoride porous micro-nano spheres nucleated at the defect positions of the fluorinated graphene; the chemical composition of the iron fluoride porous micro-nano spheres includes FeF2 and FeF3; The molar ratio of FeF3 to FeF2 is (5-7):(3-5); the mass ratio of the iron fluoride porous micro-nano spheres to the fluorinated graphene is (0.0001-6):100; the mass ratio of fluorine element to carbon element in the fluorinated graphene is (0.03-0.05):

1.

2. The application according to claim 1, wherein The specific surface area of the iron fluoride composite cathode material is 15 to 20 m 2 / g, and the pore volume is 0.08 to 0.11 cm 3 / g.

3. The application according to claim 1, characterized in that The particle size of the iron fluoride porous micro-nano spheres is 300-600 nm.

4. The application according to any one of claims 1 to 3, characterized in that The preparation method of the iron fluoride composite cathode material includes the following steps: Mix the dispersion of graphene oxide and hydrofluoric acid, and carry out an acidothermal reaction to obtain fluorinated graphene; Mix the fluorinated graphene, water-soluble ferric salt, hydrofluoric acid and dispersant, and carry out a solvothermal reaction to obtain a precursor; Calcine the precursor under a protective atmosphere to obtain the iron fluoride composite cathode material.

5. The application according to claim 4, wherein The temperature of the acidothermal reaction is 120-200 °C, and the heat preservation time is 10-30 h.

6. The application according to claim 4, wherein The temperature of the solvothermal reaction is 100-150 °C, and the heat preservation time is 10-15 h.

7. The application according to claim 4, characterized in that, The temperature of the calcination is 350-500 °C, and the heat preservation time is 1-3 h.

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