A method for preparing a high-stability all-vanadium liquid flow battery electrolyte

Abstract

The present invention belongs to the technical field of all-vanadium liquid flow battery electrolyte, and discloses a method for preparing a high-stability all-vanadium liquid flow battery electrolyte, wherein ammonium metavanadate is fully mixed with an alkaline solution, and electrolyzed as a positive electrode, the obtained positive electrode product is filtered, the wet precipitate obtained after filtration is cleaned and dried to obtain a dry precipitate, the dry precipitate, ammonium metavanadate, 2-imidazole ketone, and an acid solution are fully mixed to form an acid mixed solution; a reducing agent is added, reacted, and an oxidant is added to react to obtain a negative electrode mixed solution; the negative electrode mixed solution is electrolyzed as a negative electrode electrolyte in an electrolysis process, and the negative electrode product after electrolysis is filtered, and the obtained filtrate is detected and adjusted to obtain a high-stability all-vanadium liquid flow battery electrolyte. The electrolyte prepared by the present invention using cheap ammonium metavanadate as a raw material has good stability.

Description

Technical Field

[0001] The invention relates to the technical field of all-vanadium liquid flow battery electrolyte, and in particular to a method for preparing a high-stability all-vanadium liquid flow battery electrolyte. Background Art

[0002] As one of the most important components of all-vanadium liquid flow batteries, the performance and cost of the electrolyte can significantly affect the further development and promotion of all-vanadium liquid flow batteries.

[0003] At present, the research on all-vanadium liquid flow battery electrolyte mainly focuses on high-temperature stability, conductivity, material interaction between electrolyte and membrane and electrode, and cost. Specifically, (1) high temperature stability: high-valent vanadium ions are prone to deprotonation at high temperatures, resulting in the precipitation of vanadium ions, which in turn causes the capacity of the all-vanadium flow battery to decay rapidly. The cooling device currently used not only increases the energy consumption of the all-vanadium flow battery, but also reduces the efficiency at lower temperatures. Therefore, it is necessary to improve the high temperature stability of the electrolyte so that the all-vanadium flow battery can operate stably at higher temperatures. (2) conductivity: the conductivity of the electrolyte can affect the internal resistance of the battery, affect the energy loss of the battery, and then affect the voltage efficiency and energy efficiency of the battery. (3) material interaction between the electrolyte and the membrane and electrodes: during the operation of the all-vanadium flow battery, the active material enters the electrode surface from the electrolyte to undergo oxidation or reduction reactions, and protons are transmitted between the positive and negative electrodes through the membrane and participate in the chemical reaction. However, the diffusion of vanadium ions across the membrane will also cause the battery capacity to decay. (4) cost: the electrolyte accounts for a major part of the total cost of the all-vanadium flow battery, and the cost of the electrolyte will greatly affect the total cost of the battery.

[0004] Compared with the commonly used high-purity vanadium pentoxide, ammonium metavanadate has a significant price advantage. However, in the process of preparing electrolyte with ammonium metavanadate, high-temperature decomposition and extraction are often used to remove ammonium ions. These processes are not only cumbersome, but also require high temperature or organic solvents, which will increase the preparation cost of the electrolyte and cause environmental pollution. The high-temperature stability, conductivity, material interaction between the electrolyte and the membrane and electrode, and cost of the prepared electrolyte have not been significantly improved. At the same time, the residual reducing agent used in the process will cause the battery performance to decline. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a high-stability all-vanadium liquid flow battery electrolyte to solve the problems of poor stability, high cost, residual organic impurities and complex process of the all-vanadium liquid flow battery electrolyte.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing a high-stability all-vanadium liquid flow battery electrolyte comprises the following steps:

[0008] Step S1: fully mixing ammonium metavanadate and an alkaline solution to form a positive electrode mixed solution;

[0009] Step S2: electrolyzing the cathode mixed solution, filtering the obtained cathode product, washing and drying the wet precipitate obtained after filtering to obtain a dry precipitate, and fully mixing the dry precipitate, ammonium metavanadate, 2-imidazolidinone, and the acid solution to form an acid mixed solution;

[0010] Step S3: adding a reducing agent to the acid mixture to react and obtain a reducing solution;

[0011] Step S4: adding an oxidant to the reducing solution to react and obtain a negative electrode mixed solution;

[0012] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, filtering the negative electrode product after electrolysis, and obtaining the filtrate as the electrolyte stock solution;

[0013] Step S6: testing and adjusting the electrolyte stock solution to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0014] Preferably, in step S1, the mass ratio of ammonium metavanadate to the alkaline solution is 1:(5-50); the alkaline solution is one of potassium hydroxide solution and sodium hydroxide solution, and the concentration is 0.1-1 mol / L.

[0015] Preferably, the electrolysis conditions in step S2 are: electrolysis voltage is 1-3 V, current density is 10-200 mA / cm2, and electrolysis power is Ah, where m is the mass of ammonium metavanadate, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate.

[0016] Preferably, the method of washing and drying in step S2 is: washing the wet precipitate with deionized water, and then drying it at a temperature of 50-90° C. for 2-5 hours.

[0017] Preferably, in step S2, the mass ratio of the dry precipitate, ammonium metavanadate, 2-imidazolidinone, and the acid solution is 1: (0.5-1.3): (0.005-0.05): (5-25); the acid solution is one of a sulfuric acid solution and a hydrochloric acid solution, and the concentration is 2-6 mol / L.

[0018] Preferably, the molecular formula of the reducing agent in step S3 is C x H y O z (CHO) n , the mass added is:

[0019] ;

[0020] Where: x=1~6, y=2~12, z=0~6, n=1~3, It is the sum of the masses of ammonium metavanadate in step S1 and step S2.

[0021] Preferably, the reducing agent in step S3 is an aldehyde reducing agent, which is selected from one of benzaldehyde, o-phthalaldehyde, and trialdehyde phloroglucinol.

[0022] Preferably, the oxidant in step S4 is a 3-20% hydrogen peroxide solution, and the mass ratio of the oxidant to the reducing agent in step S3 is (0.05-0.1):1.

[0023] Preferably, the electrolysis conditions in step S5 are: electrolysis voltage 1-3 V, current density 10-200 mA / cm 2 The electrolysis charge is Ah, is the sum of the masses of ammonium metavanadate in step S1 and step S2, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate, is the mass of the reducing agent in step S3.

[0024] Preferably, the detection and adjustment method in step S6 is: taking the electrolyte stock solution, detecting the vanadium ion concentration and valence by potentiometric titration, then adjusting the vanadium ion concentration to the required concentration with ammonium metavanadate and the acid solution in step S2, and finally balancing the electrolyte valence to 3.5 with a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration.

[0025] Beneficial effects of the present invention:

[0026] 1. The present invention uses ammonium metavanadate as a raw material, and achieves the purpose of decomposing and controlling ammonium ions by electrocatalytic oxidation of ammonium metavanadate, and further regulates the amount of ammonium metavanadate added for the second time, so that the electrolyte contains an appropriate amount of ammonium ions, thereby improving the conductivity and high-temperature stability of the electrolyte, reducing the transmembrane shuttling and high-temperature stability of vanadium ions, and improving the coulombic efficiency, voltage efficiency, energy efficiency and capacity retention rate of the battery;

[0027] 2. The present invention can effectively reduce the influence of reducing agent impurities on the prepared electrolyte by secondary oxidation of the electrolyte, thereby improving the stability and capacity retention rate of the electrolyte;

[0028] 3. The present invention can improve the material interaction between the electrolyte, the membrane and the electrode through the synergistic effect of 2-imidazolidinone and ammonium ions, effectively reduce concentration polarization and transmembrane diffusion of vanadium ions, and thus improve the coulombic efficiency, energy efficiency and capacity retention rate of the battery;

[0029] 4. The present invention uses cheap ammonium metavanadate as raw material to electro-oxidize ammonium metavanadate. The integration of electroreduction, chemical reduction of pentavalent vanadium compounds, and chemical oxidation of reducing agent impurities can reduce raw material costs, operating costs, simplify the process, and improve plant utilization and production efficiency, which has commercial value for large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of a method for preparing a high-stability all-vanadium liquid flow battery electrolyte according to the present invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below through the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the scope of the present invention. In addition, in the following description, the description of known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0032] like Figure 1 As shown, a method for preparing a high-stability all-vanadium liquid flow battery electrolyte specifically comprises the following steps:

[0033] Step S1: fully mixing ammonium metavanadate and alkaline solution to form a positive electrode mixed solution; the mass ratio of ammonium metavanadate to alkaline solution is 1:(5-50); the alkaline solution is one of potassium hydroxide solution and sodium hydroxide solution, and the concentration is 0.1-1 mol / L;

[0034] Step S2: electrolyzing the positive electrode mixture, filtering the obtained positive electrode product, washing and drying the wet precipitate obtained after filtration to obtain a dry precipitate, and fully mixing the dry precipitate, ammonium metavanadate, 2-imidazolidinone, and acid solution to form an acid mixture; the electrolysis conditions are: electrolysis voltage of 1-3V, current density of 10-200mA / cm 2 The electrolysis charge is Ah, where m is the mass of ammonium metavanadate, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate; the method of washing and drying is: washing the wet precipitate with deionized water, and then drying it at a temperature of 50-90°C for 2-5h; the mass ratio of dry precipitate, ammonium metavanadate, 2-imidazolidinone, and acid solution is 1: (0.5-1.3): (0.005-0.05): (5-25); the acid solution is one of sulfuric acid solution and hydrochloric acid solution, and the concentration is 2-6mol / L;

[0035] Step S3: adding a reducing agent to the acid mixture to react and obtain a reducing solution; the molecular formula of the reducing agent is C x Hy O z (CHO) n , the mass added is:

[0036] ;

[0037] Where: x=1~6, y=2~12, z=0~6, n=1~3, is the sum of the masses of ammonium metavanadate in step S1 and step S2; the reducing agent is an aldehyde reducing agent, selected from one of benzaldehyde, o-phthalaldehyde, and trialdehyde phloroglucinol;

[0038] Step S4: adding an oxidant to the reducing solution to react and obtain a negative electrode mixed solution; the oxidant is a 3-20% hydrogen peroxide solution, and the mass ratio of the oxidant to the reducing agent in step S3 is (0.05-0.1):1;

[0039] Step S5: The negative electrode mixed solution is used as the negative electrode electrolyte in the electrolysis process in step S2. The negative electrode product after electrolysis is filtered, and the obtained filtrate is the electrolyte stock solution; the electrolysis conditions are: electrolysis voltage 1~3V, current density 10~200mA / cm 2 The electrolysis charge is Ah, is the sum of the masses of ammonium metavanadate in step S1 and step S2, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate, is the mass of the reducing agent in step S3;

[0040] Step S6: Testing and adjusting the electrolyte stock solution to obtain a high-stability all-vanadium liquid flow battery electrolyte; the testing and adjustment method is: taking the electrolyte stock solution, using potentiometric titration to detect the vanadium ion concentration and valence, then using ammonium metavanadate and the acid solution in step S2 to adjust the vanadium ion concentration to the required concentration, and finally using a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration to balance the electrolyte valence to 3.5.

[0041] Embodiment 1:

[0042] In this embodiment, the electrolyte is prepared by the following steps:

[0043] Step S1: 100 g of ammonium metavanadate with a purity of 99.5% and 500 g of a sodium hydroxide solution with a concentration of 1 mol / L are fully mixed to form a positive electrode mixed solution;

[0044] Step S2: Use a 3-cell electrolysis stack, set the voltage to 1V, and the current density to 100mA / cm 2, the positive electrode mixture was electrolyzed with an electrolysis capacity of 22.79 Ah, the obtained positive electrode product was filtered, the wet precipitate obtained after filtration was washed with deionized water, and then dried at a temperature of 50 ° C for 5 h to obtain a dry precipitate, 77.35 g of the dry precipitate, 100.56 g of ammonium metavanadate, 0.62 g of 2-imidazolidinone, and 1160 g of a sulfuric acid solution with a concentration of 6 mol / L were fully mixed to form an acid mixture;

[0045] Step S3: adding 4.53 g of benzaldehyde to the acid mixture to react and obtain a reducing solution;

[0046] Step S4: adding 0.23 g of a 20% hydrogen peroxide solution into the reducing solution to react and obtain a negative electrode mixed solution;

[0047] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, and filtering the negative electrode product after the electrolysis quantity reaches 10.97 Ah, and the obtained filtrate is the electrolyte stock solution;

[0048] Step S6: Take the electrolyte stock solution and use potentiometric titration to detect that the vanadium ion concentration is 1.4 mol / L and the valence is 3.45. The required electrolyte concentration is 1.5 mol / L. Add 13.64 g of ammonium metavanadate. After the reaction, the vanadium ion concentration is tested to be 1.5 mol / L and the valence is 3.49, which meets the required concentration. Finally, add 7.7 mL of 1.5 mol / L pentavalent vanadium electrolyte to adjust the electrolyte valence to 3.5 to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0049] Embodiment 2:

[0050] In this embodiment, the electrolyte is prepared by the following steps:

[0051] Step S1: 100 g of ammonium metavanadate with a purity of 99.5% and 1000 g of a potassium hydroxide solution with a concentration of 0.5 mol / L are fully mixed to form a positive electrode mixed solution;

[0052] Step S2: Use a 3-cell electrolytic stack, set the voltage to 3V, and the current density to 10mA / cm 2 , the positive electrode mixture was electrolyzed with an electrolysis capacity of 22.79 Ah, the obtained positive electrode product was filtered, the wet precipitate obtained after filtration was washed with deionized water, and then dried at a temperature of 60 ° C for 2 h to obtain a dry precipitate, 77.35 g of the dry precipitate, 77.35 g of ammonium metavanadate, 0.77 g of 2-imidazolidinone, and 774 g of a 6 mol / L hydrochloric acid solution were fully mixed to form an acid mixture;

[0053] Step S3: adding 5 g of benzaldehyde to the acid mixture to react and obtain a reducing solution;

[0054] Step S4: adding 0.50 g of 8% hydrogen peroxide solution into the reducing solution to react and obtain a negative electrode mixed solution;

[0055] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, and after the electrolysis quantity reaches 7.03 Ah, filtering the negative electrode product, and the obtained filtrate is the electrolyte stock solution;

[0056] Step S6: Take the electrolyte stock solution and use potentiometric titration to detect that the vanadium ion concentration is 1.96 mol / L and the valence is 3.50. The required electrolyte concentration is 1.9 mol / L. Add 24 mL of 6 mol / L hydrochloric acid solution and stir evenly. The vanadium ion concentration is tested to be 1.89 mol / L to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0057] Embodiment 3:

[0058] In this embodiment, the electrolyte is prepared by the following steps:

[0059] Step S1: 100 g of ammonium metavanadate with a purity of 99.5% and 5000 g of a potassium hydroxide solution with a concentration of 0.1 mol / L are fully mixed to form a positive electrode mixed solution;

[0060] Step S2: Use a 3-cell electrolytic stack, set the voltage to 2V, and the current density to 200mA / cm 2 , the positive electrode mixture was electrolyzed with an electrolysis capacity of 22.79 Ah, the obtained positive electrode product was filtered, the wet precipitate obtained after filtration was washed with deionized water, and then dried at a temperature of 80 ° C for 3 h to obtain a dry precipitate, 77.35 g of the dry precipitate, 61.88 g of ammonium metavanadate, 0.54 g of 2-imidazolidinone, and 1934 g of a sulfuric acid solution with a concentration of 3 mol / L were fully mixed to form an acid mixture;

[0061] Step S3: adding 4.11 g of benzaldehyde to the acid mixture to react and obtain a reducing solution;

[0062] Step S4: adding 0.41 g of a 12% hydrogen peroxide solution into the reducing solution to react and obtain a negative electrode mixed solution;

[0063] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, and after the electrolysis quantity reaches 7.64 Ah, filtering the negative electrode product, and the obtained filtrate is the electrolyte stock solution;

[0064] Step S6: Take the electrolyte stock solution, use potentiometric titration to detect the vanadium ion concentration and valence, then use ammonium metavanadate and the acid solution in step S2 to adjust the vanadium ion concentration to the required concentration, and finally use a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration to balance the electrolyte valence to 3.5, to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0065] Embodiment 4:

[0066] In this embodiment, the electrolyte is prepared by the following steps:

[0067] Step S1: 100 g of ammonium metavanadate with a purity of 99.5% and 3000 g of a sodium hydroxide solution with a concentration of 0.3 mol / L are fully mixed to form a positive electrode mixed solution;

[0068] Step S2: Use a 3-cell electrolytic stack, set the voltage to 1.5V, and the current density to 50mA / cm 2 , the positive electrode mixture was electrolyzed with an electrolysis capacity of 22.79 Ah, the obtained positive electrode product was filtered, the wet precipitate obtained after filtration was washed with deionized water, and then dried at a temperature of 90 ° C for 4 hours to obtain a dry precipitate, 77.35 g of the dry precipitate, 38.68 g of ammonium metavanadate, 0.39 g of 2-imidazolidinone, and 387 g of a hydrochloric acid solution with a concentration of 5 mol / L were fully mixed to form an acid mixture;

[0069] Step S3: adding 3.77 g of o-phthalaldehyde to the acid mixture to react and obtain a reducing solution;

[0070] Step S4: adding 0.26 g of a 15% hydrogen peroxide solution into the reducing solution to react and obtain a negative electrode mixed solution;

[0071] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, and after the electrolysis quantity reaches 7.49 Ah, filtering the negative electrode product, and the obtained filtrate is the electrolyte stock solution;

[0072] Step S6: Take the electrolyte stock solution, use potentiometric titration to detect the vanadium ion concentration and valence, then use ammonium metavanadate and the acid solution in step S2 to adjust the vanadium ion concentration to the required concentration, and finally use a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration to balance the electrolyte valence to 3.5, to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0073] Embodiment 5:

[0074] In this embodiment, the electrolyte is prepared by the following steps:

[0075] Step S1: 100 g of ammonium metavanadate with a purity of 99.5% and 2000 g of a sodium hydroxide solution with a concentration of 0.7 mol / L are fully mixed to form a positive electrode mixed solution;

[0076] Step S2: Use a 3-cell electrolysis stack, set the voltage to 2V, and the current density to 150mA / cm 2 , the positive electrode mixture was electrolyzed with an electrolysis capacity of 22.79 Ah, the obtained positive electrode product was filtered, the wet precipitate obtained after filtration was washed with deionized water, and then dried at a temperature of 70 ° C for 3 h to obtain a dry precipitate, 77.35 g of the dry precipitate, 38.68 g of ammonium metavanadate, 0.39 g of 2-imidazolidinone, and 774 g of a 2 mol / L hydrochloric acid solution were fully mixed to form an acid mixture;

[0077] Step S3: adding 7.54 g of trialdehyde phloroglucinol to the acid mixture to react and obtain a reducing solution;

[0078] Step S4: adding 0.60 g of a 3% hydrogen peroxide solution into the reducing solution to react and obtain a negative electrode mixed solution;

[0079] Step S5: using the negative electrode mixed solution as the negative electrode electrolyte in the electrolysis process in step S2, and after the electrolysis quantity reaches 6.41 Ah, filtering the negative electrode product, and the obtained filtrate is the electrolyte stock solution;

[0080] Step S6: Take the electrolyte stock solution, use potentiometric titration to detect the vanadium ion concentration and valence, then use ammonium metavanadate and the acid solution in step S2 to adjust the vanadium ion concentration to the required concentration, and finally use a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration to balance the electrolyte valence to 3.5, to obtain a high-stability all-vanadium liquid flow battery electrolyte.

[0081] Comparative Example 1:

[0082] The method is the same as that of Comparative Example 1, except that the ammonium metavanadate is replaced by high-purity vanadium pentoxide.

[0083] Comparative Example 2:

[0084] The method is the same as that of Comparative Example 1, except that step S4 is not included.

[0085] Comparative Example 3:

[0086] The method is the same as that of Comparative Example 1, except that 2-imidazolidinone is not added in step S3.

[0087] The electrolytes obtained in Examples 1-5 and Comparative Examples 1-3 were subjected to charge and discharge cycle tests under the same working conditions, and the coulombic efficiency, energy efficiency, voltage efficiency and capacity retention rate after 50 cycles of the battery were recorded. The test results are shown in Table 1:

[0088] Table 1 Summary of battery test results

[0089]

[0090] It can be seen from the above table that compared with Comparative Example 1, Examples 1-5 have higher coulombic efficiency, energy efficiency, voltage efficiency and capacity retention rate after 50 cycles, mainly because high-purity vanadium pentoxide is used to replace ammonium metavanadate in the preparation process of the electrolyte in Comparative Example 1, while the electrolytes of Examples 1-5 use ammonium metavanadate as a raw material during the preparation process, and ammonium metavanadate is electrocatalytically oxidized to achieve the purpose of decomposing and controlling ammonium ions, and further regulate the amount of ammonium metavanadate added for the second time, so that the electrolyte contains an appropriate amount of ammonium ions, improves the conductivity and high-temperature stability of the electrolyte, reduces the transmembrane shuttling and high-temperature stability of vanadium ions, and improves the coulombic efficiency, voltage efficiency, energy efficiency and capacity retention rate of the battery; Compared with Comparative Example 2, Examples 1-5 have higher coulombic efficiency, energy efficiency, voltage efficiency and capacity retention rate after 50 cycles; Compared with Comparative Example 2, Examples 1-5 have higher coulombic efficiency, energy efficiency, voltage efficiency and capacity retention rate after 50 cycles. The capacity retention rate after the cycle is higher, mainly because the comparative example 2 does not perform the secondary oxidation process of step S4, so there will be some residual reducing agent and intermediate organic products, which will affect the capacity retention rate of the battery, and the present invention can effectively reduce the influence of reducing agent impurities on the prepared electrolyte by secondary oxidation of the electrolyte, thereby improving the stability of the electrolyte and the capacity retention rate; compared with comparative example 3, embodiments 1-5 have higher coulombic efficiency, energy efficiency and capacity retention rate after 50 cycles, mainly because the present invention can improve the material interaction between the electrolyte and the membrane and the electrode through the synergistic effect of 2-imidazolidinone and ammonium ions, effectively reduce concentration polarization and trans-membrane diffusion of vanadium ions, thereby improving the coulombic efficiency, energy efficiency and capacity retention rate of the battery.

[0091] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a high-stability all-vanadium redox flow battery electrolyte, characterized in that: The specific steps include: Step S1: fully mixing ammonium metavanadate and an alkaline solution to form a positive electrode mixed solution; Step S2: electrolyzing the positive electrode mixture, filtering the obtained positive electrode product, washing and drying the wet precipitate obtained after filtration to obtain a dry precipitate, and fully mixing the dry precipitate, ammonium metavanadate, 2-imidazolidinone, and acid solution to form an acid mixture; the electrolysis conditions are: electrolysis voltage of 1-3V, current density of 10-200mA / cm 2 , the electrolysis charge is Ah, where m is the mass of ammonium metavanadate, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate; Step S3: adding a reducing agent to the acid mixture to react and obtain a reducing solution; the molecular formula of the reducing agent is C x H y O z (CHO) n , the mass added is: ； Where: x=1~6, y=2~12, z=0~6, n=1~3, is the sum of the masses of ammonium metavanadate in step S1 and step S2; the reducing agent is an aldehyde reducing agent, selected from one of benzaldehyde, o-phthalaldehyde, and trialdehyde phloroglucinol; Step S4: adding an oxidant to the reducing solution to react and obtain a negative electrode mixed solution; Step S5: The negative electrode mixed solution is used as the negative electrode electrolyte in the electrolysis process in step S2. The negative electrode product after electrolysis is filtered, and the obtained filtrate is the electrolyte stock solution; the electrolysis voltage is 1~3V, and the current density is 10~200mA / cm 2 The electrolysis charge is Ah, is the sum of the masses of ammonium metavanadate in step S1 and step S2, N is the number of cells in the electrolytic stack, is the mass fraction of ammonium metavanadate, is the mass of the reducing agent in step S3; Step S6: testing and adjusting the electrolyte stock solution to obtain a high-stability all-vanadium liquid flow battery electrolyte.

2. The method for preparing a high-stability all-vanadium redox flow battery electrolyte according to claim 1, characterized in that: In step S1, the mass ratio of ammonium metavanadate to the alkaline solution is 1:(5-50); the alkaline solution is one of potassium hydroxide solution and sodium hydroxide solution, and the concentration is 0.1-1 mol / L.

3. The method for preparing a high-stability all-vanadium redox flow battery electrolyte according to claim 1, characterized in that: The method of washing and drying in step S2 is: washing the wet precipitate with deionized water, and then drying it at a temperature of 50-90° C. for 2-5 hours.

4. The method for preparing a high-stability all-vanadium redox flow battery electrolyte according to claim 1, characterized in that: In step S2, the mass ratio of the dry precipitate, ammonium metavanadate, 2-imidazolidinone, and the acid solution is 1: (0.5-1.3): (0.005-0.05): (5-25); the acid solution is one of a sulfuric acid solution and a hydrochloric acid solution, and the concentration is 2-6 mol / L.

5. The method for preparing a high-stability all-vanadium redox flow battery electrolyte according to claim 1, characterized in that: The oxidant in step S4 is a 3-20% hydrogen peroxide solution, and the mass ratio of the oxidant to the reducing agent in step S3 is (0.05-0.1):

1.

6. The method for preparing a high-stability all-vanadium redox flow battery electrolyte according to claim 1, characterized in that: The detection and adjustment method in step S6 is: taking the electrolyte stock solution, using potentiometric titration to detect the vanadium ion concentration and valence, then using ammonium metavanadate and the acid solution in step S2 to adjust the vanadium ion concentration to the required concentration, and finally using a 5-valent vanadium electrolyte and a 2-valent vanadium electrolyte with the same concentration as the required concentration to balance the electrolyte valence to 3.5.

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