Preparation Method and Application of a Vanadium Hydrous Oxide Cathode Material for a Magnesium Ion Battery

Abstract

The present invention discloses a preparation method and application of a vanadium hydrous oxide cathode material for a magnesium ion battery, which specifically includes the following steps: Step 1: Add vanadium pentoxide into water, and obtain solution A after vigorously stirring and mixing evenly; Step 2: Add hydrogen peroxide into solution A to obtain solution B, and continuously stir solution B until vanadium pentoxide is completely pyrolyzed; Step 3: After the vanadium pentoxide in solution B is completely pyrolyzed, add absolute ethanol and stir to mix evenly to obtain solution C; Step 4: Transfer solution C to a reaction kettle for hydrothermal reaction, and obtain a reaction product after naturally cooling to room temperature; Step 5: Filter, wash multiple times and dry the reaction product to obtain a V3O7·nH2O powder product. The present invention adopts a simple one-pot hydrothermal method, can synthesize vanadium hydrous oxide nanobelts with uniform size and morphology in one step, has excellent capacity and rate performance, the material preparation process is simple, the raw material source is rich and the price is low.

Description

Technical Field

[0001] The present invention relates to the technical field of magnesium ion batteries, and specifically relates to a preparation method and application of a hydrated vanadium oxide cathode material for magnesium ion batteries. Background Art

[0002] The combustion of chemical fuels has led to serious environmental problems, which has inspired the development of efficient and green energy utilization technologies. Among them, rechargeable lithium-ion batteries have been widely used in portable consumer electronic products. However, due to the scarcity of lithium resources, energy density, cycle life, and safety limitations, the further development of lithium batteries in the market has been hindered. Magnesium ion batteries (RMBs) are considered one of the most promising candidate energy storage systems to replace lithium in electrochemical energy storage devices due to the advantages of rich magnesium resource reserves, high safety, and high volumetric capacity. The main challenge of magnesium ion batteries comes from the slow solid-state diffusion kinetics caused by the high charge radius of the electrode anion and divalent Mg 2+ . Therefore, the development of high-performance cathode materials is the key to realizing rechargeable magnesium ion batteries.

[0003] Vanadium oxide is an important vanadium-based material, which has the advantages of low cost, easy synthesis, green pollution-free, and high energy density, and has been widely used in the energy storage field. For example, Patent CN115216649A discloses a method for preparing a zinc ion battery cathode material of vanadium dioxide using waste vanadium-titanium-based SCR catalysts. The prepared vanadium dioxide cathode material has good zinc storage performance and can output a specific capacity of 204 mAh g -1 at a current density of 0.1 A g -1 . Patent CN110078121A discloses a method for preparing a magnesium ion battery cathode material of vanadium pentoxide nanoflowers by high-temperature calcination. The initial discharge specific capacity of the electrode material prepared by this invention method is 120 mAh g -1 , and its capacity retention rate is only 70% after 60 cycles. Although the above shows the feasibility of the vanadium oxide cathode materials prepared by the prior art in energy storage, however, the vanadium oxide cathode materials still have the problems of slow solid-state diffusion kinetics and poor electrochemical cycle stability. Summary of the Invention

[0004] Aiming at the above deficiencies existing in the prior art, the purpose of the present invention is to provide a preparation method and application of a hydrated vanadium oxide cathode material for magnesium ion batteries. By designing the morphology of one-dimensional nanoribbons, the diffusion rate of magnesium ions can be accelerated, and the stress distribution of the embedded magnesium ions can be made wider by utilizing the anisotropy of axial transmission, enhancing its cycle stability, so as to solve the problems of slow solid-state diffusion kinetics and poor cycle stability of the vanadium oxide cathode materials prepared by the prior art.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A preparation method of a vanadium hydrous oxide cathode material for a magnesium ion battery, specifically comprising the following steps:

[0007] Step 1: Add vanadium pentoxide into water, and after vigorously stirring and mixing evenly, obtain solution A; the molar concentration of vanadium pentoxide in solution A is 0.05 - 0.3 mol / L;

[0008] Step 2: Add hydrogen peroxide to solution A to obtain solution B, and continuously stir solution B until the vanadium pentoxide is completely pyrolyzed; the molar concentration of hydrogen peroxide in solution B is 0.4 - 0.8 mol / L;

[0009] Step 3: After the vanadium pentoxide in solution B is completely pyrolyzed, add absolute ethanol and stir and mix evenly to obtain solution C; the molar concentration of absolute ethanol in solution C is 0.25 - 1.5 mol / L;

[0010] Step 4: Transfer solution C to a reaction kettle, keep it warm at 180°C - 220°C for 24 - 60 h, and after naturally cooling to room temperature, obtain a reaction product;

[0011] Step 5: Filter the reaction product obtained in Step 4, and after repeatedly washing it with deionized water and absolute ethanol for multiple times, perform a drying treatment to obtain a V3O7·nH2O product.

[0012] Preferably, in Step 1, the molar concentration of vanadium pentoxide in solution A is 0.1 - 0.2 mol / L.

[0013] Preferably, in the reaction system of solution B, the molar concentration of hydrogen peroxide is 0.53 - 0.67 mol / L.

[0014] Preferably, in the reaction system of solution C, the molar concentration of absolute ethanol is 0.75 - 1.25 mol / L.

[0015] Preferably, in Step 4, the reaction temperature is 200°C.

[0016] Preferably, in Step 4, the reaction time is 48 h.

[0017] Preferably, in Step 5, wash it three times with deionized water and absolute ethanol respectively; the temperature of the drying treatment is 60 - 80°C, and the drying time is 8 - 12 h.

[0018] The present invention provides an application of a vanadium hydrous oxide cathode material for a magnesium ion battery, and the cathode material prepared by the above preparation method is used for the preparation of lithium ion batteries, sodium ion batteries, calcium ion batteries, and zinc ion batteries.

[0019] Preferably, the positive electrode material is used for the preparation of an aqueous magnesium ion battery.

[0020] Preferably, the battery includes a button-type symmetric battery, a button-type asymmetric battery, a symmetric soft-pack battery, and an asymmetric soft-pack battery; wherein, the battery electrode sheets are one or both of V3O7·nH2O and / or pre-magnesiated V3O7·nH2O (Mg x V3O7·nH2O). Specifically preferably, the assembly method is as follows: V3O7·nH2O / / V3O7·nH2O, V3O7·nH2O / / Mg x V3O7·nH2O, Mg x V3O7·nH2O / / Mg x Any one of them can be used. In the electrolyte used in the battery, the solute is one or more of MgCl2, Mg(NO3)2, and Mg(CF3SO3)2. The solvent is preferably an aqueous solution of polyethylene glycol, and the volume ratio of polyethylene glycol to water is 1:1, but it is not limited thereto. The separator is preferably glass fiber, but it is not limited to glass fiber. The pre-magnesiated V3O7·nH2O (Mg x V3O7·nH2O) is obtained through the following steps: using V3O7·nH2O as the positive electrode and one or more of graphite, hard carbon, graphene, and activated carbon as the negative electrode, assembling them into a battery, then performing one charge, one discharge, and then another charge, so that magnesium ions are embedded in the hydrated vanadium oxide (V3O7·nH2O) material, thereby obtaining pre-magnesiated V3O7·nH2O (Mg x V3O7·nH2O). Among them, constant current is used for charging and discharging, and the voltage for charging and discharging is -1 to 1.2V, and the current density is 0.01 to 1.00A·g -1 . The electrolyte can be an aqueous solution of polyethylene glycol, and the volume ratio of polyethylene glycol to water in this aqueous solution of polyethylene glycol is 1:1, and the dosage of Mg(CF3SO3)2 is 2M.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. Aiming at the problems existing in the prior art, the present invention improves the preparation method of the positive electrode material of the hydrated vanadium oxide (V3O7·nH2O) magnesium ion battery, so that the V3O7 layer in the prepared V3O7·nH2O product is composed of [VO5] trigonal bipyramids and [VO6] octahedrons connected by sharing edges and corners of oxygen atoms, and the V3O7 layers are connected by hydrogen bonds, ensuring its large interlayer spacing ( ) The structural water between the electrode layers is connected to the oxygen atoms of the vanadium oxide layer through hydrogen bonds. The existence of these special hydrogen bonds can appropriately buffer the expansion and contraction caused by the insertion or extraction of metal ions into the V3O7·nH2O electrode, enhance the structural stability of the electrode material, and further improve the cycle life of the battery.

[0023] 2. The prepared magnesium ion battery cathode material of vanadium oxide hydrate (V3O7·nH2O) by the method of the present invention has typical nanofiber characteristics, can provide more redox active sites, and the V 5+ / V 4+ mixed valence state can optimize the electron conductivity, and the nanostructure can greatly shorten the diffusion path of Mg 2+ and accelerate the diffusion kinetics of Mg 2+ .

[0024] 3. Using the vanadium oxide hydrate prepared by the method of the present invention as the magnesium ion battery electrode material, and assembling a symmetric full battery with 2M Mg(CF3SO3)2 (PEG:H2O = 1:1) as the electrolyte, at 0.05A g -1 , an initial reversible specific capacity of 137mAh g -1 can be exhibited. When the current density increases to 4A g -1 , a specific capacity of 45mAh g -1 can still be output, and the capacity retention rate is 33%, showing excellent rate performance; in addition, the V3O7·nH2O electrode has an extremely long cycle life (the capacity retention rate is 60% after 11000 cycles at 4A g -1 ), the Coulomb efficiency is stable at 100%, showing excellent electrochemical performance; at the same time, it also enables the battery assembled based on the positive electrode material of the present invention to have excellent electrochemical performance.

[0025] 4. The method of the present invention uses a simple one-pot hydrothermal method, which can synthesize vanadium oxide hydrate nanobelts with uniform size and morphology in one step, has excellent capacity and rate performance, the material preparation process is simple, the raw materials are rich in source and low in price, and has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the XRD analysis diagram of the V3O7·nH2O sample prepared in Example 1.

[0027] Figure 2 is the TGA analysis diagram of the V3O7·nH2O sample prepared in Example 1.

[0028] Figure 3 a to 3c are the scanning electron microscope diagrams of V3O7·nH2O prepared in Example 1; Figure 3d is the TEM image of V3O7·nH2O prepared in Example 1; Figure 3 e is the enlarged TEM image of V3O7·nH2O prepared in Example 1; Figure 3 f is the STEM-EDS diagram of the V3O7·nH2O nanofibers prepared in Example 1.

[0029] Figure 4 is the cyclic voltammogram of the first 10 cycles of the V3O7·nH2O electrode at a scanning rate of 1 mV s -1 in a three-electrode system.

[0030] Figure 5 is the rate performance diagram of a three-electrode system with a current density of 0.05 - 10 Ag -1 -1.

[0031] Figure 6 is the galvanostatic charge-discharge curve of a two-electrode system with a current density of 0.05 - 4 A·g -1 -1.

[0032] Figure 7 is the cyclic stability test diagram of a two-electrode system with a constant current density of 4 A·g -1 -1. Detailed implementation mode

[0033] The present invention will be further described below in conjunction with the drawings and examples.

[0034] I. Examples and comparative examples

[0035] Example 1

[0036] The preparation method of V3O7·nH2O in this example includes the following steps:

[0037] (1) Add vanadium pentoxide (V2O5, 1.82 g) to 70 mL of deionized water, mix and stir vigorously until evenly mixed, and record it as solution A;

[0038] (2) Add 5 mL of 30% hydrogen peroxide solution to solution A, and continuously stir magnetically for 5 h, and record it as solution B;

[0039] (3) Add 5 mL of absolute ethanol to solution B, stir for 10 min and mix evenly, and record it as solution C;

[0040] (4) Transfer solution C to a 100 mL polytetrafluoroethylene-lined autoclave, and place it in a forced-air drying oven at 180 °C for 48 h.

[0041] (5) After the reaction is completed, cool to room temperature, wash repeatedly with deionized water and absolute ethanol three times, and dry in a vacuum drying oven at 80 °C for 12 h to obtain the V3O7·nH2O powder sample.

[0042] Example 2

[0043] The preparation method of V3O7·nH2O in this example includes the following steps:

[0044] (1) Add vanadium pentoxide (V2O5, 1.82 g) to 70 mL of deionized water, mix and stir vigorously until evenly mixed, and record it as solution A;

[0045] (2) Add 3 mL of 30% hydrogen peroxide solution to solution A, and continuously stir magnetically for 5 h, and record it as solution B;

[0046] (3) Add 1 mL of absolute ethanol to solution B, stir for 10 min to mix evenly, and record it as solution C;

[0047] (4) Transfer solution C to a 100 mL polytetrafluoroethylene-lined autoclave, and place it in a forced-air drying oven at 180 °C for heat preservation for 48 h.

[0048] (5) After the reaction is completed, cool to room temperature, wash repeatedly with deionized water and absolute ethanol three times, and dry in a vacuum drying oven at 80 °C for 12 h to obtain the V3O7·nH2O powder sample.

[0049] Example 3

[0050] The preparation method of V3O7·nH2O in this example includes the following steps:

[0051] (1) Add vanadium pentoxide (V2O5, 1.82 g) to 70 mL of deionized water, mix and stir vigorously until evenly mixed, and record it as solution A;

[0052] (2) Add 6 mL of 30% hydrogen peroxide solution to solution A, and continuously stir magnetically for 5 h, and record it as solution B;

[0053] (3) Add 2 mL of absolute ethanol to solution B, stir for 10 min to mix evenly, and record it as solution C;

[0054] (4) Transfer solution C to a 100 mL polytetrafluoroethylene-lined autoclave, and place it in a forced-air drying oven at 200 °C for heat preservation for 48 h.

[0055] (5) After the reaction is completed, cool to room temperature, wash repeatedly with deionized water and absolute

[0056] The V3O7·nH2O powder sample was obtained by washing it three times with ethanol and drying it in a vacuum drying oven at 80 °C for 12 h.

[0057] Example 4

[0058] The preparation method of V3O7·nH2O in this example includes the following steps:

[0059] (1) Vanadium pentoxide (V2O5, 0.91 g) was added to 70 mL of deionized water and stirred vigorously until evenly mixed, denoted as solution A;

[0060] (2) 3 mL of 30% hydrogen peroxide solution was added to solution A and continuously stirred magnetically for 5 h, denoted as solution B;

[0061] (3) 1 mL of absolute ethanol was added to solution B and stirred for 10 min until evenly mixed, denoted as solution C;

[0062] (4) Solution C was transferred to a 100 mL polytetrafluoroethylene-lined high-pressure reactor and placed in a forced-air drying oven at 180 °C for 48 h.

[0063] (5) After the reaction ended, it was cooled to room temperature, washed three times with deionized water and absolute ethanol, and dried in a vacuum drying oven at 80 °C for 12 h to obtain the V3O7·nH2O powder sample.

[0064] Examples 5 - 8

[0065] The preparation method of V3O7·nH2O in this example is the same as that in Example 1, except that the reaction times are 24 h, 36 h, 42 h, and 60 h, respectively.

[0066] Comparative Example 1

[0067] Comparative Example 1 is a vanadium oxide prepared by the prior art. The specific preparation method is as follows:

[0068] (1) 6 mmol of ammonium metavanadate, 15 mmol of oxalic acid, and 0.5 g of cetyltrimethylammonium bromide were dissolved in a mixed solution of 15 mL of deionized water and 45 mL of ethylene glycol. After being evenly mixed by magnetic stirring, the mixed solution was transferred to a stainless steel reactor lined with polytetrafluoroethylene;

[0069] (2) The sealed reactor was transferred to an oven and kept at 180 °C for 24 h to obtain the reaction product. The product was filtered, washed repeatedly with absolute ethanol and deionized water, and then placed in a vacuum drying oven and dried at 60 °C for 10 h to obtain the vanadium oxide precursor;

[0070] (3) Place the vanadium oxide precursor obtained in step (2) in a muffle furnace, heat it to 450 °C at a heating rate of 4 °C / min, keep it for 2 h, and then cool it to room temperature with the furnace to obtain the vanadium pentoxide cathode material.

[0071] Comparative Example 2

[0072] Comparative Example 2 is vanadium oxide prepared by the prior art. The specific preparation method is as follows:

[0073] (1) Dissolve 2 mmol of ammonium metavanadate in 70 mL of deionized water and stir at room temperature for 30 min, denoted as solution A;

[0074] (2) Add 1 mL of 12.27 mol / L hydrochloric acid to solution A and stir for 10 min to obtain an orange-yellow solution, denoted as solution B;

[0075] (3) Dissolve 3 mmol of manganese acetate in 10 mL of deionized water and sonicate until dissolved, denoted as solution C;

[0076] (4) Slowly add solution C dropwise to solution B and stir and mix at room temperature to obtain a yellow transparent liquid;

[0077] (5) Transfer the yellow transparent solution obtained in step (4) to an 80 mL reaction kettle, react at 200 °C for 72 h, remove the reaction kettle and cool it to room temperature naturally. After filtration, washing and drying steps, finally obtain manganese vanadium oxide (Mn 0.04 V2O5·1.17H2O) nanobelts.

[0078] II. Perform structural characterization on the products prepared in the examples

[0079] (1) XRD characterization

[0080] Figure 1 XRD analysis pattern of V3O7·nH2O prepared in Example 1. All diffraction peaks of the sample can be indexed to orthorhombic V3O7·nH2O (JCPDS No. 85-2401), which indicates that the prepared product has the structural framework of V3O7·nH2O. The interlayer spacing of this sample was calculated based on the (200) crystal plane to be A larger interlayer spacing is beneficial to the diffusion of Mg 2+ ions.

[0081] (2) TGA characterization

[0082] Figure 2The TGA analysis chart of V3O7·nH2O prepared in Example 1 further determines the content of structural water in the V3O7·nH2O sample. As can be seen from the chart, 11.38% of the weight in the range of 210 - 606 °C is attributed to the loss of crystal water in the sample, corresponding to 1.9 crystal waters in the V3O7·nH2O sample. Therefore, the chemical formula of the sample can be determined as V3O7·1.9H2O.

[0083] The V3O7·nH2O prepared in Examples 2 - 7 was detected by the same method as above, and the results were basically consistent with those of Example 1.

[0084] (4) SEM and TEM Characterization

[0085] Figure 3 (a - c) are the scanning electron microscope images of V3O7·nH2O prepared in Example 1. It can be seen from the images that the product prepared in the present invention has typical one - dimensional nano - characteristics. The diameter of the nanobelts is about 100 - 300 nm, and the length is dozens of micrometers. The nanostructure can expose more reactive sites and can greatly shorten the diffusion path of Mg 2+ and provide fast diffusion kinetics for the embedded Mg 2+ to improve the capacity and rate performance of the electrode material. Figure 3 (d) is the TEM image of the V3O7·nH2O sample, further proving the nanofiber structure of V3O7·nH2O with a diameter of about 180 nm, which is consistent with the SEM result; Figure 3 (e) shows obvious crystal plane stripes with a spacing of 0.845 nm, corresponding to the (200) crystal plane of the crystal. Figure 3 (f) is the STEM - EDS image of the V3O7·nH2O nanofibers, and it can be seen that the V and O elements in the sample are evenly distributed.

[0086] III. Electrochemical Performance Test of the Prepared Product

[0087] The active material V3O7·nH2O, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) synthesized in Example 1 were added to an N - methylpyrrolidone (NMP) solution in a mass ratio of 7:2:1, ground and stirred into a uniform slurry, and then evenly coated on a 1×1.5 cm rectangular carbon cloth electrode using a brush. The coating area was 1 cm 2 , and then dried overnight in a vacuum drying oven at 80 °C to obtain the positive electrode plate of the magnesium - ion battery, with an active material loading of 2 mg cm -2 .

[0088] The electrochemical performance test of the present invention will adopt a traditional three - electrode system. Using the V3O7·nH2O electrode as the positive electrode, the platinum electrode as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode, a 2M Mg(CF3SO3)2 aqueous solution is used as the electrolyte to test the electrochemical performance of the V3O7·nH2O positive electrode material. In the coin - type two - electrode system, a symmetric battery is assembled with the pre - magnesiated V3O7·nH2O (Mg x V3O7·nH2O) as the electrode sheet, a 2M Mg(CF3SO3)2 solution with polyethylene glycol (PEG):H2O = 1:1 as the solvent as the electrolyte, and a glass fiber filter paper as the separator. Among them, Mg x V3O7·nH2O is obtained by pre - magnesiating a coin - type battery assembled with V3O7·nH2O as the positive electrode, one or more of graphite, hard carbon, graphene, and activated carbon as the negative electrode, a 2M Mg(CF3SO3)2 solution as the electrolyte, and a glass fiber filter paper as the separator. The pre - magnesiation treatment is specifically as follows: perform one charge, one discharge, and then one more charge. Constant - current charge - discharge is adopted, and the charge - discharge voltage is - 1~1.2V, and the current density is 0.01~1.00A·g -1 . Further, the electrochemical performance of the device assembled with the Mg x V3O7·nH2O electrode material was tested. All three - electrode tests were carried out using an electrochemical workstation (CHI660E) for cyclic voltammetry (CV) and galvanostatic charge - discharge (GCD) tests; the two - electrode was subjected to galvanostatic charge - discharge (GCD) and long - cycle tests on a multi - channel battery tester (Neware, CT4008A).

[0089] (1) Cyclic voltammogram - three - electrode

[0090] Figure 4 shows the cyclic voltammogram (CV curve) of the V3O7·nH2O electrode in the first 10 cycles with a current density of 1mV s -1 and a voltage range of - 0.8~0.9V (vs. SCE). After the first asymmetric cycle, two pairs of obvious redox peaks are shown in the CV curve, which indicates that the insertion of Mg 2+ is carried out step by step. In the subsequent cycles, it can be seen that the insertion and extraction of Mg 2+ gradually tend to be reversible, and the CV curve maintains a similar shape and voltage position, demonstrating good electrochemical reversibility. The electrochemical reaction formula is as follows:

[0091] V3O7·nH2O + xMg 2+ + xe - = Mg x V3O7·nH2O (1)

[0092] The interlayer crystal water of V3O7·nH2O is connected by hydrogen bonds, which can effectively resist the expansion / shrinkage of the structure caused during charge and discharge, and accelerate the Mg 2+ transport kinetics formula, where x in formula (1) represents the number of moles of magnesium ions embedded in V3O7·nH2O.

[0093] (2) Constant current charge and discharge - three electrodes

[0094] Figure 5 is the rate performance diagram of the V3O7·nH2O electrode material at different current densities (0.05 - 10 Ag -1 ). It can be seen that when the current density is 0.05 Ag -1 , its initial reversible specific capacity is 225 mAh g -1 . It is worth noting that when the current density increases to 10 Ag -1 , the reversible discharge specific capacity of the V3O7·nH2O electrode is still as high as 70 mAh g -1 , which is 31% of that at 0.05 Ag -1 ; when the current density is restored to 0.05 Ag -1 , its capacity can immediately recover to 212 mAh g -1 , showing excellent rate performance. When vanadium pentoxide synthesized in Comparative Example 1 is used as the cathode material of a magnesium ion battery, at 0.05 Ag-1, its reversible discharge specific capacity is only 120 mAh g -1 ; when manganese vanadium oxide synthesized in Comparative Example 2 is used as the cathode material of a magnesium ion battery, at 0.05 Ag -1 , its reversible discharge specific capacity is 145 mAh g -1 ; when the current density increases to 4 Ag -1 , the discharge specific capacity decays to about 40 mAh g -1 . Therefore, at different current densities, the reversible discharge specific capacities of Comparative Example 1 and Comparative Example 2 are much lower than those of Example 1. The magnesium ion pre-inserted layered hydrated vanadium oxide cathode material prepared by the present invention has more excellent specific capacity and rate performance.

[0095] (3) Constant current charge and discharge curve

[0096] Figure 6 is the constant current charge and discharge curve (GCD) of the Mg x V3O7·nH2O / / Mg x V3O7·nH2O symmetric full cell in 2 M Mg(CF3SO3)2 (PEG:H2O = 1:1) electrolyte at different current densities. It can be seen that under the constant current charge and discharge conditions, the approximate linear response of the symmetric cell means that no phase change occurs, and the charge and discharge are only for Mg 2+The results of insertion and deinsertion show that its pseudocapacitive characteristics endow it with high rate performance. As Figure 6 shown, at current densities of 0.05, 0.3, and 0.8 A g -1 , the reversible capacities of V3O7·nH2O / / V3O7·nH2O are 137, 87, and 68 mAh g -1 respectively. When the current density reaches 4 A g -1 , the reversible capacity remains at 45 mAh g -1 , and the capacity retention rate is 33%, indicating good reversibility of magnesium ion insertion and extraction in this material and good structural stability of the electrode material during charge and discharge processes.

[0097] (4) Cycling stability test - two electrodes

[0098] The V3O7·nH2O / / V3O7·nH2O symmetric full cell was subjected to constant current charge-discharge cycling 11,000 times at a current density of 4 A g -1 and a voltage window of 0 - 2 V to test its stability. Figure 7 shows the specific capacity and Coulombic efficiency at different cycle numbers. After 6,500 cycles, the capacity retention rate is 83%, and after 11,000 cycles, the capacity retention rate is still 60%. The Coulombic efficiency remains stable at 100% throughout the process, indicating that the V3O7·nH2O prepared in this invention has good cycling stability as an electrode material, and the insertion and extraction of magnesium ions in the V3O7·nH2O structure are highly reversible, which benefits from the large interlayer spacing and the presence of interlayer hydrogen bonds in the V3O7·nH2O structure.

[0099] This invention adopts a simple one-pot hydrothermal method to directly synthesize a hydrated vanadium oxide nanobelt magnesium ion battery cathode material with typical nanofiber characteristics. The preparation process is simple and the raw material cost is low. The assembled symmetric button cell has excellent electrochemical magnesium storage performance, mainly including a high reversible capacity (specific capacity of 137 mAh g -1 at a current density of 0.05 A g -1 ) and rate performance (it can still output a specific capacity of 45 mAh g -1 when the current density increases to 4 A g -1 ). It has an extremely long cycle life (capacity retention rate of 60% after 11,000 cycles at 4 A g -1 ), and the Coulombic efficiency remains stable at 100% all the time. This invention can solve the problem of slow diffusion kinetics in existing magnesium storage technologies. At the same time, it also solves the technical bottleneck of poor structural stability during the charge and discharge process of magnesium ion batteries. Therefore, the hydrated vanadium oxide nanobelt is a potential magnesium ion battery cathode material with excellent electrochemical performance.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the spirit and scope of the present technical solutions shall be covered by the scope of the claims of the present invention.

Claims

1. Application of a hydrated vanadium oxide cathode material for a magnesium ion battery, characterized in that, The positive electrode material is V3O7·nH2O, and the positive electrode material is used for the preparation of an aqueous magnesium ion battery; The preparation method of the positive electrode material specifically includes the following steps: Step 1: Add vanadium pentoxide to water, and stir strongly to mix evenly to obtain solution A; the molar concentration of vanadium pentoxide in solution A is 0.05 - 0.3 mol / L; Step 2: Add hydrogen peroxide to solution A to obtain solution B, and continuously stir solution B until vanadium pentoxide is completely pyrolyzed; the molar concentration of hydrogen peroxide in solution B is 0.4 - 0.8 mol / L; Step 3: After the vanadium pentoxide in solution B is completely pyrolyzed, add absolute ethanol and stir to mix evenly to obtain solution C; the molar concentration of absolute ethanol in solution C is 0.25 - 1.5 mol / L; Step 4: Transfer solution C to a reaction kettle, keep it warm at 180°C - 220°C for 24 - 60 h, and naturally cool to room temperature to obtain a reaction product; Step 5: Filter the reaction product obtained in Step 4, wash it repeatedly with deionized water and absolute ethanol for multiple times, and then perform a drying treatment to obtain V3O7·nH2O.

2. The application according to claim 1, wherein In Step 1, the molar concentration of vanadium pentoxide in solution A is 0.1 - 0.2 mol / L.

3. The application according to claim 1, wherein In the reaction system of solution B, the molar concentration of hydrogen peroxide is 0.53 - 0.67 mol / L.

4. The application according to claim 1, characterized in that In the reaction system of solution C, the molar concentration of absolute ethanol is 0.75 - 1.25 mol / L.

5. The application according to claim 1, characterized in that, In Step 4, the reaction temperature is 200°C.

6. The application according to claim 1, wherein, In Step 4, the reaction time is 48 h.

7. The application according to claim 1, characterized in that In Step 5, wash it three times with deionized water and absolute ethanol respectively; the temperature of the drying treatment is 60 - 80°C, and the drying time is 8 - 12 h.

8. The application according to claim 1, wherein In the battery, the battery electrode sheet is one of V3O7·nH2O or pre-magnesiated V3O7·nH2O; among them, the pre-magnesiated V3O7·nH2O is obtained through the following steps: using V3O7·nH2O as the positive electrode and one or more of graphite, hard carbon, graphene, and activated carbon as the negative electrode, assembling them into a battery, then performing one charge, one discharge, and then another charge, so that magnesium ions are embedded into the hydrated vanadium oxide material, thereby obtaining pre-magnesiated V3O7·nH2O.

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