MXene-induced sodium vanadyl fluorophosphate composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of batteries, and discloses an MXene-induced sodium vanadyl fluorophosphate composite material as well as a preparation method and application thereof, and the preparation method comprises the following steps: (1) carrying out etching treatment on Ti3AlC2 in a hydrofluoric acid solution, and centrifuging to obtain a bottom precipitate; (2) adding tetramethylammonium hydroxide into the centrifugal precipitate, stirring for 3-10 days for intercalation, centrifuging, and taking supernatant liquid to obtain single-layer MXene dispersion liquid; and (3) adding a vanadium source, a phosphorus source, a sodium source, a fluorine source and the single-layer MXene dispersion liquid into a reaction kettle, and carrying out hydrothermal treatment to obtain the nanosheet-shaped NVOPF-coated MXene composite material. The thickness of the nano-sheet-shaped NVOPF-coated MXene composite material prepared by the invention is 5-10 nm, and the nano-sheet-shaped NVOPF-coated MXene composite material can be used as a sodium ion battery positive electrode material with long service life, high capacity and high power.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a MXene-induced sodium vanadium fluorophosphate composite material, a preparation method and an application thereof. Background Art

[0002] Compared with lithium-ion batteries, sodium-ion batteries have the advantages of abundant sodium resources, low price, and environmental friendliness, providing a new option for electrochemical energy storage, especially large-scale energy storage. However, the radius of sodium ions is larger than that of lithium ions, resulting in a large diffusion barrier for sodium ions during charging and discharging, making reversible insertion and extraction more difficult, and the energy density is still inferior to that of lithium-ion batteries, so the development of new electrode materials is crucial.

[0003] Among the currently known sodium storage electrode materials, polyanion materials have attracted much attention due to their stable structure, high safety, and open sodium ion transmission channels, showing broad development prospects. Sodium vanadyl fluorophosphate Na3(VO)2(PO4)2F (NVOPF) is one of the representatives of polyanion positive electrode materials, with a high theoretical specific capacity (~130 mAh g ‒1 ), high operating voltage (~3.8 V vs. Na + / Na) and a volume change of less than 2%, it is expected to achieve high energy density and high power density as the positive electrode of sodium-ion batteries. However, since its transition metal vanadium is separated by polyanions, there is a lack of "V‒O‒V" electron delocalization, which hinders the transfer of electrons, resulting in low intrinsic electronic conductivity of the material, poor rate performance, and relatively low power density. These factors limit the potential of this material in practical applications.

[0004] In recent years, focusing on the intrinsic conductivity of sodium vanadium oxyfluorophosphate materials, researchers have optimized them by reducing particle size, compounding with carbon-containing matrices such as graphene or carbon nanotubes, and intracrystalline doping. As a new type of two-dimensional material with excellent conductivity, MXene is mainly used in cathode materials as a conductive matrix and composited with the main material, such as patent CN 116207239 A and patent CN 114373917 A. However, the applicant found that in the preparation method provided in the prior art, MXene is only introduced as a conductive substrate. Although this has a certain improvement effect on the charge transport capacity of the material, it will occupy the mass of the active material and cannot fully exert the lamellar morphology advantages of MXene. The performance improvement effect of the composite material is limited.

[0005] The present invention introduces a monolayer MXene solution during the hydrothermal process, and uses the high conductivity of the two-dimensional monolayer MXene as a growth initiator to in-situ induce and regulate the growth of the NVOPF positive electrode material. After MXene composite induction, the NVOPF evolves from the initial nanoplate to a nanosheet, and the best NVOPF ultra-thin nanosheet with a thickness of only 8.5 nm can be obtained, which greatly shortens the migration path of sodium ions, effectively improves the electrochemical performance of the NVOPF material, and helps the development of high-rate sodium ion battery positive electrode materials. Summary of the invention

[0006] In order to overcome the problem of poor intrinsic conductivity of the existing sodium ion battery positive electrode material NVOPF, the present invention provides a method for preparing a MXene-induced nano-flake sodium vanadium oxyfluorophosphate composite material. By adjusting the amount of MXene added, MXene and NVOPF can be in situ composited, and the morphology of NVOPF is greatly improved. The formed composite material is applied to sodium ion batteries to obtain excellent electrochemical properties.

[0007] The present invention provides a method for preparing a MXene-induced nano-flaky sodium vanadium fluorophosphate composite material, comprising the following preparation steps: (1) Add Ti3AlC2 into a hydrofluoric acid solution for etching, and centrifuge to obtain a bottom precipitate; (2) Dispersing the precipitate described in step (1) into a tetramethylammonium hydroxide aqueous solution for intercalation, stirring for 3 to 10 days, and centrifuging to obtain intercalated Ti3C2T x ; (3) The intercalated Ti3C2T x Ultrasound, followed by centrifugation, the resulting supernatant is the monolayer MXene (Ti3C2T x ) dispersion; (4) Add a vanadium source to anhydrous ethanol, then add a phosphorus source, and stir to obtain solution A; add polyethylene glycol 4000 to the MXene solution obtained in the above step (3), stir until it is evenly dispersed, then add a sodium source, and stir to obtain solution B; then mix solution A and solution B, stir to form a homogeneous solution, then add a fluorine source, and continue stirring to obtain solution C; finally, subject solution C to a hydrothermal reaction, centrifuge and wash the sample, and vacuum dry to obtain a MXene-induced nano-sheet-like sodium vanadium oxyfluorophosphate composite material, denoted as NVOPF@MXene.

[0008] In the step (1), the etching temperature of MXene is 20 to 50 °C, preferably 25 °C, and the etching time is 18 to 36 h, preferably 24 h.

[0009] The mass concentration of the tetramethylammonium hydroxide is 10-30%, preferably 25%.

[0010] The centrifugal speed in step (3) is 1000 to 3000 rpm, preferably 2500 rpm, and the time is 10 to 60 min, preferably 30 min.

[0011] The vanadium source is at least one of vanadium acetylacetonate, vanadyl sulfate, ammonium metavanadate, vanadium pentoxide, and vanadium trichloride, preferably vanadium acetylacetonate; the phosphorus source is at least one of phosphoric acid, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium phosphate and ammonium phosphate, preferably phosphoric acid; the sodium source is at least one of sodium oleate, sodium fluoride, sodium hydroxide, sodium chloride, sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, and sodium phosphate, preferably sodium oleate; the fluorine source is at least one of potassium fluoride, sodium fluoride, ammonium fluoride, and magnesium fluoride, preferably ammonium fluoride.

[0012] The concentration of MXene in step (4) is 1 to 5 mg / mL, preferably 1 mg / mL.

[0013] In some preferred cases, the amount of MXene added in step (4) is 3% to 7% of the sodium vanadyl fluorophosphate.

[0014] The hydrothermal reaction temperature is 120-200°C, preferably 180°C, and the reaction time is 12-36 hours, preferably 24 hours.

[0015] In an embodiment of the present invention, the drying is performed by vacuum drying at 60-70°C.

[0016] The molar ratio of the sodium source, the vanadium source, the phosphorus source and the fluorine source required for the hydrothermal synthesis of sodium vanadyl fluorophosphate of the present invention is 3:2:2:1.

[0017] The present invention also provides a MXene-induced sodium vanadium oxyfluorophosphate composite material, which is prepared by the preparation method. The composite material is in the form of nanosheets with a thickness of less than 10 nm.

[0018] Another technical concept of the present invention is to use the MXene-induced sodium vanadium oxyfluorophosphate composite material (NVOPF@MXene) obtained by the above-mentioned preparation technology as a positive electrode active material for sodium ion batteries.

[0019] Based on this, the present invention also provides a sodium ion battery positive electrode material, wherein the positive electrode material is the MXene-induced sodium vanadium fluorophosphate composite material.

[0020] The present invention also provides a battery, comprising the sodium ion battery positive electrode material.

[0021] The NVOPF@MXene was basically characterized and its electrochemical performance was tested.

[0022] The beneficial effects of the present invention are: (1) The present invention provides a new idea for optimizing sodium vanadium fluorophosphate positive electrode materials. By adding a small amount of MXene, its conductivity and induction effect can be fully utilized to induce the evolution of nanoplate-like NVOPF into nanosheets, and the thickness is reduced from the initial 55.8nm to less than 10nm. In some embodiments, it can reach 8.5nm, which greatly shortens the migration path of sodium ions and significantly improves the electrochemical performance.

[0023] (2) The MXene-induced nano-sheet sodium vanadium fluorophosphate composite material prepared by the present invention has excellent electrochemical performance. Its initial capacity can reach 126.28 mAh g at 0.2 C. ‒1 At a high current density of 20 C, its capacity can still reach 96.8 mAh g ‒1 After being tested at different current densities of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, and 20 C, and then returning to 0.2 C, its capacity remained at 126.24 mAh g ‒1 , almost no capacity loss, and has excellent electrochemical sodium storage performance.

[0024] (3) The method proposed in the present invention is universal and scalable, and can be extended to other sodium-ion battery positive electrode materials, providing a new design idea for optimizing sodium-ion battery positive electrode materials with the same poor conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a scanning electron microscope image when the MXene addition amount in Example 1 of the present invention is 3% of NVOPF.

[0026] Figure 2 It is the XRD diagram of NVOPF when the MXene addition amount is 5% of NVOPF in Example 2 of the present invention and the NVOPF without adding MXene in the comparative example.

[0027] Figure 3 This is a scanning electron microscope image when the MXene addition amount in Example 2 of the present invention is 5% of NVOPF.

[0028] Figure 4 This is the rate performance diagram when the MXene addition amount in Example 2 of the present invention is 5% of NVOPF and the comparative example and Example 3.

[0029] Figure 5 This is the impedance diagram of Example 2 of the present invention when the MXene addition amount is 5% of NVOPF and the comparative example.

[0030] Figure 6 This is a scanning electron microscope image when the MXene addition amount in Example 3 of the present invention is 7% of NVOPF.

[0031] Figure 7 It is a scanning electron microscope image of NVOPF prepared without adding MXene in the comparative example of the present invention. DETAILED DESCRIPTION

[0032] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0033] Example 1 (the amount of monolayer MXene solution added was regulated to be 12 mL and controlled to be 3% of NVOPF) A method for preparing a MXene-induced nano-flaky sodium vanadium fluorophosphate composite material comprises the following steps: (1) First, 2 g of Ti3AlC2 powder was slowly added to 30 mL of HF solution several times, stirred for 24 h, and then centrifuged. The precipitate was washed with deionized water several times until the pH of the supernatant reached 6-7. The precipitate after centrifugation and washing was then dispersed in 20 mL of tetramethylammonium hydroxide aqueous solution and stirred at 1000-1600 rpm for 3-10 days. The obtained black suspension was centrifuged at 10000 rpm for 5 min to obtain intercalated Ti3C2T x Finally, the obtained black clay-like precipitate was ultrasonicated for 2 h and centrifuged at 2500 rpm for 30 min. The supernatant obtained was the monolayer MXene solution with a concentration of 1 mg / mL.

[0034] (2) Slowly add 2 mmol of vanadium acetylacetonate to 40 mL of anhydrous ethanol and stir for 1 h, then slowly add 140 µL of phosphoric acid and stir for 30 min to obtain a brown solution A; slowly add 1 mmol of PEG4000 to a mixed solution of 12 mL of the monolayer MXene prepared in step (1) and 18 mL of deionized water and stir for 1 h until it is completely dissolved, then slowly add 3 mmol of sodium oleate and stir for 30 min to obtain a black clear solution B; slowly drop solution B into solution A and stir for 3 h to obtain a grass green solution C; slowly add 1 mL of ammonium fluoride solution (concentration of 0.03704 g / mL) and 9 mL of deionized water to solution C and stir for 2 h to obtain a light yellow transparent solution D; pour solution D into a 100 mL reactor, hydroheat at 180 °C for 24 h, take out and cool to room temperature, centrifuge and wash the sample and vacuum dry.

[0035] The scanning electron microscopy results of the MXene-induced nanosheet sodium vanadyl fluorophosphate prepared in this example are shown in Figure 1 As shown, due to the insufficient amount of MXene added, the induction effect is incomplete, and the thickness of the obtained nanoplate is 31.8 nm, which is not significantly reduced compared with the thickness of the nanoplate in the comparative example.

[0036] Example 2 (the amount of monolayer MXene solution added was adjusted to 20 mL and controlled to be 5% of NVOPF) The amount of MXene added in step (2) of Example 1 was changed to 20 mL, and the amount of deionized water was changed to 10 mL. The other conditions were the same as those in the above Example 1.

[0037] Figure 2 The XRD diagram of this example is Figure 2 It can be observed that the phase peak position of sodium vanadium fluorophosphate after MXene composite has not shifted, indicating that the addition of MXene does not affect the phase purity of NVOPF. Figure 3 The scanning electron microscope image shows that the thickness of the sodium vanadium fluorophosphate nanoplate after MXene composite is significantly reduced to 8.5 nm, and the morphology evolves into a nano-thin sheet, which fully proves that MXene plays its inductive role. After inducing NVOPF to grow along the (200) crystal plane, the migration path of sodium ions is greatly shortened. Using metallic sodium as the negative electrode, a sodium ion half-cell was assembled and the electrochemical performance was tested. Figure 4 As shown in Figure 2, the initial capacity can reach 126.28 mAh g at 0.2 C. ‒1 At a high current density of 20 C, its capacity is still 96.8 mAh g ‒1 After being tested at different current densities of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, and 20 C, and then returning to 0.2 C, its capacity remained at 126.24 mAh g ‒1 , almost no capacity loss, showing excellent rate performance. The sample in this embodiment was subjected to electrochemical impedance test, such as Figure 5 As shown, compared with the comparative example, the impedance of this example is significantly reduced, indicating that the composite material fully utilizes the advantages of MXene conductivity, enhances the overall charge transport capacity of the material, and significantly improves the electrochemical sodium storage performance.

[0038] Example 3 (the amount of monolayer MXene solution added was adjusted to 28 mL and controlled to be 7% of NVOPF) The amount of MXene added in step (2) of Example 1 was changed to 28 mL, and the amount of deionized water was changed to 2 mL. Other conditions were the same as those in the above Example 1.

[0039] like Figure 6 SEM images and Figure 4 As shown in the electrochemical performance diagram, the MXene-induced nano-sheet sodium vanadium fluorophosphate prepared in this example has basically the same morphology and thickness as the composite material prepared in Example 2. The difference is that due to the excessive amount of MXene added in this example, it occupies the mass of the active material, resulting in a decrease in its electrochemical performance. This fully proves that there is an optimal value for the amount of MXene added to simultaneously exert its inductive effect and conductivity.

[0040] Comparative Example 4 (Synthesis of pure phase NVOPF material without adding MXene) In step (2) of Example 1, no MXene solution was added, and PEG4000 was directly dispersed in 30 mL of deionized water. Other conditions were the same as those in the above step of Example 1.

[0041] The SEM image of pure NVOPF without adding MXene is shown in Figure 2. Figure 7 As shown in Figure 2, the thickness of the prepared nanoplate-like NVOPF is 55.8 nm, which is much larger than that of the example with MXene added. Figure 5 It can be seen that the comparative sample exhibits a greater charge transfer resistance, so it exhibits Figure 4 Worst rate performance shown.

[0042] In summary, the present invention makes full use of the high conductivity and induction effect of MXene, and provides a new technology for optimizing sodium vanadium fluorophosphate. After MXene composite induction, the morphology of sodium vanadium fluorophosphate changes from nanoplate to corn flake, while showing excellent electrochemical performance. This technology provides a new idea for improving the intrinsic conductivity of sodium vanadium fluorophosphate cathode materials, and effectively helps the development of high-rate sodium ion battery cathode materials.

[0043] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations of the present invention. The embodiments and features in the embodiments of the present application can be arbitrarily combined with each other without conflict. The protection scope of the present invention shall be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for preparing a MXene-induced sodium vanadium fluorophosphate composite material, characterized in that: The method comprises the following preparation steps: (1) Add Ti3AlC2 into a hydrofluoric acid solution for etching, and centrifuge to obtain a bottom precipitate; (2) Dispersing the precipitate described in step (1) into a tetramethylammonium hydroxide aqueous solution for intercalation, stirring for 3 to 10 days, and centrifuging to obtain intercalated Ti3C2T x ; (3) The intercalated Ti3C2T x Ultrasonication followed by centrifugation, the resulting supernatant is the monolayer MXene dispersion; (4) adding a vanadium source to anhydrous ethanol, then adding a phosphorus source, and stirring to obtain a solution A; Add polyethylene glycol to the MXene solution obtained in step (3) above, stir until uniformly dispersed, then add a sodium source, and stir to obtain solution B; Then, solution A and solution B are mixed and stirred to form a homogeneous solution, and a fluorine source is added and stirred continuously to obtain solution C. Finally, solution C is subjected to a hydrothermal reaction, centrifuged, washed, and vacuum dried to obtain a MXene-induced sodium vanadium fluorophosphate composite material, recorded as NVOPF@MXene.

2. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: The etching temperature in step (1) is 20 to 50°C and the etching time is 18 to 36 hours.

3. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: The mass concentration of the tetramethylammonium hydroxide aqueous solution in step (2) is 10 to 30%; the stirring speed is controlled at 1000 to 1600 rpm.

4. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: The centrifugal speed in step (3) is 1000 to 3000 rpm, and the time is 10 to 60 min.

5. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: The vanadium source in step (4) is at least one of vanadium acetylacetonate, vanadium sulfate, ammonium metavanadate, vanadium pentoxide, and vanadium trichloride; the phosphorus source is at least one of phosphoric acid, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium phosphate, and ammonium phosphate; the sodium source is at least one of sodium oleate, sodium fluoride, sodium hydroxide, sodium chloride, sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, and sodium phosphate; and the fluorine source is at least one of potassium fluoride, sodium fluoride, ammonium fluoride, and magnesium fluoride.

6. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: In the solution B described in step (4), the added concentration of MXene is 1 to 5 mg / mL.

7. The method for preparing the MXene-induced sodium vanadium fluorophosphate composite material according to claim 1, characterized in that: The hydrothermal reaction temperature in step (4) is 120 to 200 °C and the reaction time is 12 to 36 h.

8. A MXene-induced sodium vanadium fluorophosphate composite material, characterized in that: The composite material is prepared by the preparation method according to any one of claims 1 to 7, and is in the form of nanosheets with a thickness of less than 10 nm.

9. A sodium ion battery positive electrode material, characterized in that: The positive electrode material is the MXene-induced sodium vanadium fluorophosphate composite material as described in claim 8.

10. A battery, characterized in that: Including the sodium ion battery positive electrode material as described in claim 8.

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