Composite all-vanadium redox flow battery electrolyte additive, all-vanadium redox flow battery electrolyte and preparation method

Abstract

The invention discloses a composite all-vanadium redox flow battery electrolyte additive, an all-vanadium redox flow battery electrolyte and a preparation method, and belongs to the technical field of redox flow batteries. The composite electrolyte additive for the all-vanadium redox flow battery comprises MgCl2, (NH4) 2SO4 and CH3SO3H, wherein the molar ratio of the MgCl2 to the (NH4) 2SO4 to the CH3SO3H is (0.5-1.5): (1-3): (10-20). The all-vanadium redox flow battery electrolyte comprises vanadium ions, the composite all-vanadium redox flow battery electrolyte additive and water. According to the invention, the concentration and high-temperature stability of vanadium ions in the electrolyte can be improved, the problems of low energy density and poor stability of the electrolyte are solved, and the cost is relatively low.

Description

Technical Field

[0001] The present invention relates to an additive for an electrolyte of a composite vanadium redox flow battery, an electrolyte of a vanadium redox flow battery, and a preparation method thereof, belonging to the technical field of redox flow batteries. Background Art

[0002] Renewable energy sources such as wind power, hydropower, and solar energy have developed rapidly. However, the power generation of such clean energy sources is intermittent and unstable, and there is a problem of "time + space" mismatch. Therefore, at both the grid side and the power generation side, a suitable energy storage method is required to smooth out power fluctuations and ensure the stability of the power system and the power quality. Vanadium redox flow batteries have received extensive attention due to their high safety, long service life, good scalability, independent design of power and energy, and suitability for large-scale energy storage. They are currently the most commercially developed and technically mature redox flow battery technologies. However, there are also obvious technical bottlenecks in the specific applications of vanadium redox flow batteries. Among them, the electrolyte accounts for about 45% of the total cost. Problems such as poor high-temperature stability, low electrochemical activity, and low energy density of the electrolyte seriously restrict its further development. Breaking through the existing technical bottlenecks is an inevitable choice for the further wide application of vanadium redox flow batteries.

[0003] Currently, the problems existing in the application process of the electrolyte of vanadium redox flow batteries mainly include:

[0004] (1) Cost problem: Vanadium is a rare metal and is expensive. Most of the existing methods for preparing electrolytes use VOSO4 dissolved in sulfuric acid. The price of VOSO4 is relatively high, which is also an important reason for the relatively large proportion of the electrolyte cost in the total cost of vanadium redox flow batteries.

[0005] (2) Poor high-temperature stability of the electrolyte: During the operation of the battery, pentavalent vanadium ions have poor stability at high temperatures and are extremely prone to precipitation, changing the composition and concentration of the electrolyte and thus affecting the charge-discharge performance and capacity of the battery.

[0006] (3) Low energy density of the electrolyte: The energy density of the electrolyte of vanadium redox flow batteries is closely related to the concentration of active substances in the electrolyte. The solubility of vanadium ions in the electrolyte is limited, resulting in fewer active substances that can participate in the reaction per unit volume, and thus it is difficult to significantly increase the energy density.

[0007] In view of the above problems existing in the electrolyte of all-vanadium redox flow batteries, the prior art has a solution of using mixed acids as supporting electrolytes. For example, CN118970126A discloses a method for preparing all-vanadium electrolyte with multi-mixed acids. In this method, V2O5 powder with a concentration ≥ 99.9% is dissolved in a sulfuric acid solution, fully activated for a certain period of time, cooled, oxalic acid is added, after reacting for a certain period of time and cooling, a methylsulfonic acid-hydrochloric acid mixed solution is added for volume fixing to prepare a vanadyl sulfate tetravalent stock solution. Then the vanadyl sulfate tetravalent stock solution is evenly divided and filled into the positive and negative electrode barrels, and electrolyzed with an electrolysis device to obtain a 3.5-valent all-vanadium electrolyte. Using mixed acids as the supporting electrolyte of the electrolyte can improve the stability of the electrolyte, and at the same time can solve the problems of high viscosity, low conductivity, and poor electrochemical activity existing in high-concentration electrolytes. However, the mixed acid system has strong corrosiveness, which will corrode components such as the electrodes, diaphragms, and storage containers of the battery, reduce the service life of these components, increase the maintenance cost and replacement frequency of the battery, and may also cause safety hazards such as internal leakage of the battery.

[0008] The prior art also has solutions of using a single organic additive or a single inorganic additive. Using a single additive can effectively improve a certain property of the electrolyte, such as enhancing the conductivity of the electrolyte or inhibiting the precipitation of vanadium ions, etc., but it cannot take into account multiple performance indicators. At the same time, a single additive may bring other negative effects when improving a certain property. For example, although some additives can enhance the stability of the electrolyte, they will also reduce the reaction activity of the electrode, resulting in a decrease in the charge and discharge efficiency of the battery.

[0009] The prior art also has solutions of using a single organic or inorganic composite additive. To solve the problem of single effect existing in a single additive, some researchers have proposed composite additives at present, but most of them are limited to the combination of inorganic additives with inorganic additives or the combination of organic additives with organic additives. For example, CN105762395A discloses a positive electrode electrolyte of an all-vanadium redox flow battery containing a composite additive, and a composite additive containing phosphate and tungstate is added to the positive electrode electrolyte of the all-vanadium redox flow battery. Another example is that CN118231723A discloses an electrolyte of an all-vanadium redox flow battery containing a composite additive and a preparation method thereof, and the composite additive includes a phosphorylated hydroxyquinone sulfonate complex. However, a single composite additive cannot combine the advantages of organic additives and inorganic additives.

[0010] In view of the problems of poor high-temperature stability and low energy density of the electrolyte in the current all-vanadium redox flow battery, the existing technologies basically rely on changing the supporting electrolyte or adding additives to improve the performance of the electrolyte. However, changing the supporting electrolyte will affect other components of the battery, and there are certain limitations in the research of additives. At the same time, the raw materials used in the existing commercial methods for preparing electrolytes are relatively expensive, which increases the cost of the electrolyte for all-vanadium redox flow batteries. Therefore, it is necessary to develop a new type of composite additive for all-vanadium redox flow battery electrolyte and the all-vanadium redox flow battery electrolyte. Summary of the Invention

[0011] To solve the above technical problems, the purpose of the present invention is to provide a composite additive for all-vanadium redox flow battery electrolyte, the all-vanadium redox flow battery electrolyte and a preparation method. The present invention can increase the concentration of vanadium ions in the electrolyte and its high-temperature stability, improve the problems of low energy density and poor stability of the electrolyte, and has a relatively low cost.

[0012] To achieve the above purpose, in the first aspect of the present invention, a composite additive for all-vanadium redox flow battery electrolyte is provided, which comprises MgCl2, (NH4)2SO4 and CH3SO3H with a molar ratio of (0.5 - 1.5):(1 - 3):(10 - 20).

[0013] According to the specific embodiments of the present invention, preferably, the molar ratio of MgCl2, (NH4)2SO4 and CH3SO3H is (0.5 - 1.5):2:(10 - 20). More preferably, the molar ratio of MgCl2, (NH4)2SO4 and CH3SO3H is 1:2:10.

[0014] In the second aspect of the present invention, an all-vanadium redox flow battery electrolyte is provided, which comprises vanadium ions, the above-mentioned composite additive for all-vanadium redox flow battery electrolyte and water; wherein, the total concentration of vanadium ions in the electrolyte is 1.8 - 2.2 mol / L, the concentration of MgCl2 in the electrolyte is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.1 - 0.3 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 - 2.0 mol / L.

[0015] According to the specific embodiments of the present invention, preferably, the total concentration of vanadium ions in the electrolyte is 2 mol / L.

[0016] According to the specific embodiments of the present invention, preferably, the concentration of MgCl2 in the electrolyte solution is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte solution is 0.2 mol / L, and the concentration of CH3SO3H in the electrolyte solution is 1.0 - 2.0 mol / L. More preferably, the concentration of MgCl2 in the electrolyte solution is 0.1 mol / L, the concentration of (NH4)2SO4 in the electrolyte solution is 0.2 mol / L, and the concentration of CH3SO3H in the electrolyte solution is 1.0 mol / L.

[0017] According to the specific embodiments of the present invention, preferably, the vanadium ions include tetravalent vanadium ions and / or 3.5-valent vanadium ions. Those skilled in the art can understand that the tetravalent vanadium ions are V 4+ , and the 3.5-valent vanadium ions are a mixture of V 4+ and V 3+ with a molar ratio of 1:1.

[0018] The third aspect of the present invention provides a method for preparing the above-mentioned all-vanadium redox flow battery electrolyte solution, which includes the following steps:

[0019] (1) Dissolve V2O5 in the first H2SO4 solution to obtain a mixed solution. Use the mixed solution as the cathode electrolyte of a two-chamber electrolytic cell, and use the second H2SO4 solution as the anode electrolyte of the two-chamber electrolytic cell. Perform constant current electrolysis to obtain an electrolyte mother liquor with a total vanadium ion concentration of 1.8 - 2.2 mol / L;

[0020] (2) Add MgCl2, (NH4)2SO4, and CH3SO3H to the electrolyte mother liquor obtained in step (1) so that the concentration of MgCl2 in the electrolyte solution is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte solution is 0.1 - 0.3 mol / L, and the concentration of CH3SO3H in the electrolyte solution is 1.0 - 2.0 mol / L to obtain the all-vanadium redox flow battery electrolyte solution.

[0021] According to the specific embodiments of the present invention, preferably, in step (1), dissolve V2O5 in the first H2SO4 solution at a molar ratio of V2O5:H2SO4 of 1:(2 - 6).

[0022] According to the specific embodiments of the present invention, preferably, in step (1), the concentration of the first H2SO4 solution is 2 - 6 mol / L.

[0023] According to the specific embodiments of the present invention, preferably, in step (1), the concentration of the second H2SO4 solution is 2 - 6 mol / L, and its volume is the same as the volume of the mixed solution. More preferably, the concentration of the first H2SO4 solution is the same as the concentration of the second H2SO4 solution.

[0024] According to a specific embodiment of the present invention, preferably, in step (1), the current density of the constant-current electrolysis is 0.1 to 1 A / cm 2 , and the electrolysis time is 90 to 240 min. More preferably, the current density of the constant-current electrolysis is 0.5 A / cm 2 , and the electrolysis time is 120 min to obtain a vanadium redox flow battery electrolyte with tetravalent vanadium ions; or the current density of the constant-current electrolysis is 0.5 A / cm 2 , and the electrolysis time is 180 min to obtain a vanadium redox flow battery electrolyte with 3.5-valent vanadium ions. The present invention does not impose special restrictions on the positive electrode, negative electrode, and diaphragm of the two-chamber electrolytic cell, and conventional positive and negative electrodes and diaphragms in the art can be used. For example, the positive electrode and the negative electrode can be lead plate electrodes, etc., and the diaphragm can be a Nafion diaphragm, etc.

[0025] The present invention has at least the following beneficial effects:

[0026] The present invention uses MgCl2, (NH4)2SO4 and CH3SO3H with a molar ratio of (0.5 to 1.5):(1 to 3):(10 to 20) as additives for the electrolyte of a composite vanadium redox flow battery, which can inhibit the formation of V2O5 precipitation in the electrolyte at high temperatures, improve the stability of the electrolyte, and further enhance the stability of the battery during operation. Among them, MgCl2 can prevent the nucleation of V2O5 precipitation by occupying the nucleation sites of the hydrated structure of vanadium ions in the electrolyte through chloride ions, and improve the high-temperature stability of V(V) ions; (NH4)2SO4 can complex with the hydrated structure of vanadium ions in the electrolyte through ammonium ions to block the crystal growth of V2O5 precipitation and increase the concentration of vanadium ions in the electrolyte; CH3SO3H can form complexes with the hydrated structure of vanadium ions or minute precipitates in the electrolyte through sulfonic acid groups (-SO3H), reduce the further aggregation of V2O5 precipitation, improve the stability of the electrolyte, and broaden the working temperature range of the electrolyte. Through the combination of the three additives of the present invention in a specific molar ratio and controlling the contents of the three additives within the scope of the present invention, they produce a synergistic effect, which can prevent the nucleation, growth and aggregation of precipitates in the electrolyte, thereby reducing the generation of precipitates during the operation of the battery, significantly improving the stability of the electrolyte, increasing the concentration of vanadium ions in the electrolyte, and enhancing the overall performance of the battery. Compared with the current single-component additives or single-type composite additives, the action mode of the composite additive of the present invention is more comprehensive and has broad application prospects. In addition, the present invention uses V2O5 and H2SO4 solution as raw materials for preparing the electrolyte, and prepares the electrolyte by means of constant-current electrolysis, which reduces the preparation cost of the electrolyte from the raw material aspect, and does not require heating activation, reducing energy consumption and process flow, and obtaining an electrolyte with a relatively high vanadium ion concentration. Therefore, the present invention improves the concentration and high-temperature stability of vanadium ions in the electrolyte, solves the problems of low energy density and poor stability of the electrolyte, has good electrochemical activity, and has a relatively low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Optical diagrams obtained by diluting the electrolyte mother liquor of 3.5-valent vanadium ions and the electrolyte mother liquor of 4-valent vanadium ions prepared in the examples of the present invention respectively.

[0028] Figure 2 Conductivity and viscosity of the electrolytes of Examples 1 to 9 and the control group.

[0029] Figure 3 CV curves of the electrolytes of Examples 1 to 9.

[0030] Figure 4 CV curves of the electrolytes of the control group and Examples 1 to 9 at a scanning rate of 30 mV / s.

[0031] Figure 5Impedance spectra of the electrolytes for the control group and Examples 1-9.

[0032] Figure 6 Battery efficiency diagrams of the electrolytes for the control group and Examples 1-9.

[0033] Figure 7 Battery charge-discharge efficiency and capacity diagrams of the electrolytes for the control group and Examples 1-9.

[0034] Figure 8 Efficiency, charge-discharge curves, discharge capacity, and capacity retention diagrams of the battery tests for the electrolytes of Example 10 and the control group.

[0035] Figure 9 Static stability test results of the electrolytes for the control group and Example 10.

[0036] Figure 10 Conductivity and viscosity of the electrolytes with different concentrations of ZnCl2 added in Comparative Example 1.

[0037] Figure 11 CV curves of the electrolytes with different concentrations of ZnCl2 added in Comparative Example 1 and CV curves of different electrolytes at the same scanning rate.

[0038] Figure 12 Impedance spectra of the electrolytes with different concentrations of ZnCl2 added in Comparative Example 1.

[0039] Figure 13 Conductivity and viscosity of the electrolytes with different concentrations of MgCl2 added in Comparative Example 2.

[0040] Figure 14 CV curves of the electrolytes with different concentrations of MgCl2 added in Comparative Example 2 and CV curves of different electrolytes at the same scanning rate.

[0041] Figure 15 Impedance spectra of the electrolytes with different concentrations of MgCl2 added in Comparative Example 2.

[0042] Figure 16 Conductivity and viscosity of the electrolytes with different concentrations of FeCl2 added in Comparative Example 3.

[0043] Figure 17 CV curves of the electrolytes with different concentrations of FeCl2 added in Comparative Example 3 and CV curves of different electrolytes at the same scanning rate.

[0044] Figure 18 Impedance spectra of the electrolytes with different concentrations of FeCl2 added in Comparative Example 3.

[0045] Figure 19The battery performance of the electrolytes with different chloride additives in Comparative Example 4 at a charge-discharge current density of 120 mA / cm 2 when.

[0046] Figure 20 The conductivity and viscosity of the electrolytes with different concentrations of (NH4)2SO4 added in Comparative Example 5.

[0047] Figure 21 The CV curves of the electrolytes with different concentrations of (NH4)2SO4 added in Comparative Example 5 and the CV curves of different electrolytes at the same scan rate.

[0048] Figure 22 The impedance spectra of the electrolytes with different concentrations of (NH4)2SO4 added in Comparative Example 5.

[0049] Figure 23 The conductivity and viscosity of the electrolytes with different concentrations of (NH4)2HPO4 added in Comparative Example 6.

[0050] Figure 24 The CV curves of the electrolytes with different concentrations of (NH4)2HPO4 added in Comparative Example 6 and the CV curves of different electrolytes at the same scan rate.

[0051] Figure 25 The impedance spectra of the electrolytes with different concentrations of (NH4)2HPO4 added in Comparative Example 6.

[0052] Figure 26 The battery performance of the electrolytes with different ammonium salt additives in Comparative Example 7 at a charge-discharge current density of 120 mA / cm 2 when.

[0053] Figure 27 The conductivity and viscosity of the electrolytes with different concentrations of CH3SO3H added in Comparative Example 8.

[0054] Figure 28 The CV curves of the electrolytes with different concentrations of CH3SO3H added in Comparative Example 8 and the CV curves of different electrolytes at the same scan rate.

[0055] Figure 29 The impedance spectra of the electrolytes with different concentrations of CH3SO3H added in Comparative Example 8.

[0056] Figure 30 The conductivity and viscosity of the electrolytes with different concentrations of NH2SO3H added in Comparative Example 9.

[0057] Figure 31 The CV curves of the electrolytes with different concentrations of NH2SO3H added in Comparative Example 9 and the CV curves of different electrolytes at the same scan rate.

[0058] Figure 32 The impedance spectra of electrolytes with different concentrations of NH2SO3H added in Comparative Example 9.

[0059] Figure 33 The battery performance of electrolytes with different organic sulfonic acid additives added in Comparative Example 10 at a charge-discharge current density of 120 mA / cm 2 at that time.

[0060] Figure 34 The static stability test results of electrolytes without additives, electrolytes with only 0.1 mol / L MgCl2 added, electrolytes with only 0.2 mol / L (NH4)2SO4 added, and electrolytes with only 1 mol / L CH3SO3H added in Comparative Example 11.

[0061] Figure 35 The electrochemical performance test results of electrolytes without additives, electrolytes with 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4 added, electrolytes with 0.1 mol / L MgCl2 + 1 mol / L CH2SO3H added, and electrolytes with 0.2 mol / L (NH4)2SO4 + 1 mol / L CH2SO3H added in Comparative Example 12.

[0062] Figure 36 The battery cycle performance test results of electrolytes without additives, electrolytes with 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4 added, electrolytes with 0.1 mol / L MgCl2 + 1 mol / L CH2SO3H added, and electrolytes with 0.2 mol / L (NH4)2SO4 + 1 mol / L CH2SO3H added in Comparative Example 12. Detailed implementation manners

[0063] In order to have a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will be described in detail below, but it should not be construed as a limitation on the scope of implementation of the present invention.

[0064] It should be noted that unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0065] All kinds of raw materials, reagents, instruments, and equipment used in the present invention, unless otherwise specifically stated, can be obtained through market purchases or can be prepared by existing methods.

[0066] It should be understood that the terms "comprising", "including" and / or "containing", as used herein, specify the presence of the stated features, integers, steps, components or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or combinations thereof.

[0067] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0068] Test method:

[0069] Physical property test: Conductivity and viscosity tests are carried out on the electrolyte. The conductivity is measured by a conductivity meter. The viscosity is measured by an Ubbelohde viscometer. The test uses a tetravalent vanadium ion electrolyte.

[0070] Electrochemical property test: Cyclic voltammetry test (CV) and electrochemical impedance spectroscopy test (EIS) are carried out on the electrolyte. The cyclic voltammetry test uses a CHI660E electrochemical workstation from Shanghai Chenhua. The test conditions include: room temperature, scanning range: 0 - 1.8V, scanning rates: 10, 20, 30, 40, 50 mV / s. The electrochemical impedance spectroscopy test uses a CHI660E electrochemical workstation from Shanghai Chenhua. The test conditions include: room temperature, scanning range: 0.01 - 100000 Hz. The test uses a tetravalent vanadium ion electrolyte.

[0071] Battery performance test: The electrolyte is put into a YTH-1 battery system for battery performance cycle test. The test uses a 3.5-valent vanadium ion electrolyte.

[0072] Static stability test: Stability is an important index for evaluating the performance of the electrolyte. During the operation of the battery, due to chemical reactions and polarization and other effects, the temperature of the electrolyte increases, which in turn leads to the precipitation of the precipitate in the electrolyte, blocking the flow channels and coating the carbon felt, affecting the operation of the battery. The test method includes the following steps: Take the same volume of electrolyte and put it into a glass bottle, and keep it at a constant temperature in a water bath at room temperature and 50 °C for one week respectively, and record the time when precipitation appears in different samples and the volume of the final precipitate. The test uses a pentavalent vanadium ion electrolyte. The preparation method of the pentavalent vanadium ion electrolyte is: Put the 3.5 or tetravalent electrolyte into the battery system, charge to 1.8V, and then take out the electrolyte on the positive electrode side, which is the V 5+ electrolyte.

[0073] Examples 1 - 9

[0074] Examples 1 to 9 respectively provide electrolytes for all-vanadium redox flow batteries, and the preparation method thereof includes the following steps:

[0075] (1) Dissolve V2O5 in a H2SO4 solution with a concentration of 4 mol / L at a molar ratio of V2O5:H2SO4 of 1:4 to obtain a mixed solution. Place the mixed solution at the cathode of a two-chamber electrolytic cell as the cathode electrolyte, and place a H2SO4 solution with the same volume as the mixed solution and a concentration of 4 mol / L at the anode of the two-chamber electrolytic cell as the anode electrolyte. Use lead plate electrodes for the positive and negative electrodes respectively, with an electrode area of 2×2 cm. Use a Nafion membrane as the diaphragm, and connect an external power supply to conduct constant current electrolysis with a current density of 0.5 A / cm 2 , and 75 mL of electrolyte is prepared at one time in these examples. The electrolysis time for preparing V 4+ is 120 min, and the electrolysis time for preparing V 3+ is 180 min, respectively obtaining an electrolyte mother liquor of trivalent vanadium ions with a total vanadium ion concentration of 2 mol / L and an electrolyte mother liquor of tetravalent vanadium ions with a total vanadium ion concentration of 2 mol / L; among them, the optical diagrams of the diluted electrolyte mother liquor of trivalent vanadium ions and tetravalent vanadium ions are as shown in Figure 1 shown, Figure 1 the left figure in Figure 1 is the diluted electrolyte mother liquor of trivalent vanadium ions,

[0076] (2) Add MgCl2, (NH4)2SO4 and CH3SO3H to the electrolyte mother liquor obtained in step (1) so that the concentrations of MgCl2, (NH4)2SO4, and CH3SO3H in the electrolytes of each example are as shown in Table 1, to obtain electrolytes for all-vanadium redox flow batteries.

[0077] Table 1

[0078]

[0079] For comparison, use the electrolyte mother liquor with a total vanadium ion concentration of 2 mol / L prepared in the above step (1) as the control group.

[0080] Conduct conductivity and viscosity tests on the above examples and the control group, and the test results are as shown in Figure 2 shown. Figure 2 are the test results of the electrolyte of tetravalent vanadium ions. It can be seen from Figure 2 that there are certain changes in the conductivity and viscosity between the electrolytes with composite additives prepared in Examples 1 to 9 and the electrolyte of the control group, but generally they fluctuate within a fixed range and the changes are not significant.

[0081] The above-mentioned examples and the control group were tested for their electrochemical performance, and the test results are as Figures 3 - 5 shown. Figures 3 - 5 The test results are for the electrolyte of tetravalent vanadium ions. Figure 3 Fig. Figure 3 (a)-(i) in Figure 4 are the CV curves of the electrolytes of Examples 1-9 respectively; Figure 5 Fig.

[0082] The above-mentioned examples and the control group were tested for battery performance. The active area of the battery was 3×3 cm 2 , the charge-discharge current density was 120 mA / cm 2 , and the number of cycles was 25. The test results are as Figures 6 - 7 shown. Figures 6 - 7 The test results are for the electrolyte of 3.5-valent vanadium ions. Figure 6 Fig. Figure 7 is the battery efficiency diagram of the electrolytes of the control group and Examples 1-9; Figure 7 Fig. Figure 6 in Figure 7 shows that the Coulomb efficiency of the electrolyte with the composite additive is basically improved, but the voltage efficiency decreases slightly, resulting in an improvement in the energy efficiency of Examples 1 and 6 among the various examples. From

[0083] Among them, the capacity retention rate refers to the ratio of the discharge capacity of the battery after being fully charged through multiple cycles to the discharge capacity after the first full charge, which is a key indicator for measuring the battery life and is related to the stability of the battery. Table 2 lists the additive contents of each embodiment and the capacity retention rate data of the battery after the battery test is completed.

[0084] Table 2

[0085]

[0086] As can be seen from Table 2, the capacity retention rates of all embodiments are relatively high. Among them, the capacity retention rates of Embodiments 4-6 and Embodiment 8 are higher, and the capacity retention rate of Embodiment 5 is the highest. At the same time, as described above, the energy efficiency of Embodiment 6 has been improved.

[0087] Embodiment 10

[0088] The all-vanadium redox flow battery electrolyte is prepared by the methods of Embodiments 1-9, such that the concentration of MgCl2 in the electrolyte is 0.1 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.2 mol / L, and the concentration of NH2SO3H in the electrolyte is 1.0 mol / L.

[0089] The graphs of the efficiency, charge-discharge curve, discharge capacity, and capacity retention rate of the battery tests of the electrolytes of Embodiment 10 and the control group are as Figure 8 shown, Figure 8 wherein Figure (a) is the efficiency, Figure (b) is the charge-discharge curve, Figure (c) is the discharge capacity, and Figure (d) is the capacity retention rate. Figure 8 These are the test results of the electrolyte of 3.5-valent vanadium ions. From Figure 8 Figures (a) and (b), it can be seen that the energy efficiency of Embodiment 10 is significantly higher than that of the control group. At the same time, the charge-discharge efficiency graph shows that the capacity of Embodiment 10 is larger than that of the control group. From Figure 8 Figures (c) and (d), it can be seen that by plotting the discharge capacity and capacity retention rate of each cycle after cycling, the discharge capacity of Embodiment 10 is significantly higher than that of the control group, and the attenuation rate is low, and the lifespan is longer.

[0090] The electrolytes of the control group and Embodiment 10 are respectively placed at room temperature and in a 50°C water bath for one week to test the static stability of the electrolyte. The test results are as Figure 9 shown, Figure 9 wherein Figure (a) is the room temperature test results of the control group and Embodiment 10, Figure (b) is the 50°C test results of the control group and Embodiment 10, Figure (c) is the precipitation height of the control group electrolyte after the 50°C test, and Figure (d) is the precipitation height of the Embodiment 10 electrolyte after the 50°C test. Figure 9These are the test results of the electrolyte of pentavalent vanadium ions. From the static stability test results, it can be seen that after maintaining at room temperature for one week, no precipitation occurred in both the control group and Example 10; after maintaining in a water bath at 50 °C for one week, the precipitation amount of the electrolyte in Example 10 with the addition of the composite additive was significantly less than that of the electrolyte without the addition of the composite additive. The control group and Example 10 used the same glass bottle with a bottom diameter of 3 cm. The precipitation height of the control group was 1.60 cm, and the precipitation height of Example 10 was 1.05 cm. The volumes of the precipitation were 11.31 cm 3 and 7.42 cm 3 , indicating that the addition of the composite additive in Example 10 of the present invention has an obvious effect on improving the stability of the electrolyte at high temperatures.

[0091] The precipitation volumes of the electrolytes of the above Examples 1 to 9 after maintaining in a water bath at 50 °C for one week are shown in Table 3, which shows the test results of the electrolyte of pentavalent vanadium ions.

[0092] Table 3

[0093]

[0094] From the above results, it can be seen that in Example 10, the concentration of MgCl2 in the electrolyte is 0.1 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.2 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 mol / L, achieving the best effects in terms of electrochemical performance and static stability.

[0095] Comparative Example 1

[0096] This comparative example provides a vanadium redox flow battery electrolyte containing different concentrations of ZnCl2, and its preparation method is basically the same as that of the above examples, except that: only ZnCl2 is added to the electrolyte mother liquor (tetravalent vanadium ions) obtained in step (1) to make the concentration of ZnCl2 in the electrolyte be 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, and 0.2 mol / L respectively. And, the electrolyte mother liquor (tetravalent vanadium ions) without the addition of ZnCl2 (i.e., 0 mol / L) is used as the control group.

[0097] The conductivity, viscosity, and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 10 - 12 shown. Figure 10 are the conductivity and viscosity of the electrolytes with different concentrations of ZnCl2 added; Figure 11 are the CV curves of the electrolytes with different concentrations of ZnCl2 added and the CV curves of different electrolytes at the same scanning rate, Figure 11In Figure (a), it is 0 mol / L; in Figure (b), it is 0.05 mol / L; in Figure (c), it is 0.1 mol / L; in Figure (d), it is 0.15 mol / L; in Figure (e), it is 0.2 mol / L; in Figure (f), the scanning rate is 30 mV / S. Figure 12 They are impedance spectra of electrolytes with different concentrations of ZnCl₂ added.

[0098] From Figure 10 It can be seen that with the addition of ZnCl₂, the conductivity of the electrolyte slightly increases. When the addition amount is 0.15 mol / L, the conductivity of the electrolyte reaches the highest. The viscosity fluctuates to some extent with the addition of ZnCl₂, but the overall change is not significant, and the viscosity of the electrolyte is lower when the addition amount is 0.15 mol / L. From Figure 11 It can be seen that for the CV curves of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, the oxidation-reduction peak current value of the sample with the ZnCl₂ additive is not much different from that of the sample without the additive, but the oxidation-reduction peak potential difference increases, and the reaction kinetics slightly decreases. From Figure 12 It can be seen that with the addition of ZnCl₂, the solution resistance of the sample changes little, but the charge transfer resistance shows an obvious difference. Among them, the charge transfer resistance of the electrolyte is the smallest when the addition amount is 0.15 mol / L. However, the electrochemical performance of this comparative example is lower than that of the above-mentioned examples.

[0099] Comparative Example 2

[0100] This comparative example provides a vanadium redox flow battery electrolyte containing different concentrations of MgCl₂. Its preparation method is basically the same as that of the above-mentioned examples, except that: only MgCl₂ is added to the electrolyte mother liquor (tetravalent vanadium ions) obtained in step (1), so that the concentration of MgCl₂ in the electrolyte is 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, and 0.2 mol / L respectively. And the electrolyte mother liquor (tetravalent vanadium ions) without adding MgCl₂ (i.e., 0 mol / L) is used as a control group.

[0101] The conductivity, viscosity, and electrochemical performance of the above electrolytes are tested, and the test results are as Figures 13 - 15 shown. Figure 13 They are the conductivity and viscosity of electrolytes with different concentrations of MgCl₂ added. Figure 14 They are the CV curves of electrolytes with different concentrations of MgCl₂ added and the CV curves of different electrolytes at the same scanning rate. Figure 14In figure (a), it is 0 mol / L; in figure (b), it is 0.05 mol / L; in figure (c), it is 0.1 mol / L; in figure (d), it is 0.15 mol / L; in figure (e), it is 0.2 mol / L; in figure (f), the scanning rate is 30 mV / S. Figure 15 They are impedance spectra of electrolytes with different concentrations of MgCl₂ added.

[0102] From Figure 13 it can be seen that with the addition of MgCl₂, the conductivity of the electrolyte first increases and then decreases. The highest point is when the addition amount of MgCl₂ is 0.1 mol / L. The viscosity increases to a certain extent with the addition of MgCl₂, but the viscosity of the electrolyte is relatively low when the addition amount is 0.1 mol / L. From Figure 14 it can be seen that for the CV curves of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, there is not much change in the oxidation-reduction peak current value of the sample with the additive compared to the sample without the additive, but the oxidation-reduction peak potential difference increases, and the reaction kinetics decreases slightly. From Figure 15 it can be seen that with the addition of MgCl₂, the solution resistance of the sample slightly increases, but the charge transfer resistance shows an obvious difference, presenting a trend of first decreasing and then increasing. Among them, the charge transfer resistance of the electrolyte is the smallest when the addition amount is 0.1 mol / L. However, the electrochemical performance of this comparative example is lower than that of the above examples.

[0103] Comparative Example 3

[0104] This comparative example provides a vanadium redox flow battery electrolyte containing different concentrations of FeCl₂. Its preparation method is basically the same as that of the above examples, except that: only FeCl₂ is added to the electrolyte mother liquor (4-valent vanadium ions) obtained in step (1) to make the concentration of FeCl₂ in the electrolyte 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, and 0.2 mol / L respectively. And the electrolyte mother liquor (4-valent vanadium ions) without adding FeCl₂ (i.e., 0 mol / L) is used as a control group.

[0105] The conductivity, viscosity, and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 16 - 18 shown. Figure 16 They are the conductivity and viscosity of electrolytes with different concentrations of FeCl₂ added. Figure 17 They are the CV curves of electrolytes with different concentrations of FeCl₂ added and the CV curves of different electrolytes at the same scanning rate. Figure 17In Figure (a), it is 0 mol / L; in Figure (b), it is 0.05 mol / L; in Figure (c), it is 0.1 mol / L; in Figure (d), it is 0.15 mol / L; in Figure (e), it is 0.2 mol / L; in Figure (f), the scanning rate is 30 mV / S. Figure 18 They are impedance spectra of electrolytes with different concentrations of FeCl2 added.

[0106] From Figure 16 it can be seen that with the addition of FeCl2, the conductivity of the electrolyte first increases and then decreases. The conductivity reaches the maximum value when the addition amount is 0.15 mol / L. The viscosity increases to a certain extent with the addition of FeCl2, but the viscosity of the electrolyte is relatively low when the addition amount is 0.15 mol / L. From Figure 17 it can be seen that for the CV curves of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, the oxidation-reduction peak current value of the sample with the additive added is significantly higher than that of the sample without the additive added, and the kinetics and reversibility of the reaction are improved to a certain extent. Among them, when the additive content is 0.15 mol / L, the current value is the largest. From Figure 18 it can be seen that with the addition of FeCl2, the solution resistance of the sample slightly increases, but the change is not significant. The law of the charge transfer resistance with the addition of FeCl2 is not obvious. However, the physical properties and electrochemical properties of this comparative example are lower than those of the above-mentioned examples.

[0107] Comparative Example 4

[0108] In this comparative example, electrolytes containing 0.15 mol / L ZnCl2, 0.1 mol / L MgCl2, and 0.15 mol / L FeCl2 with a valence of 3.5 were prepared. The preparation methods of these electrolytes are the same as those of the above-mentioned comparative examples. The electrolytes were filled into the battery system for battery performance testing, where the active area of the battery was 3×3 cm 2 , and the charge-discharge current density was 120 mA / cm 2 , and the number of cycling was 15 cycles. Figure 19 They are the battery performances of electrolytes with different chloride additives at a charge-discharge current density of 120 mA / cm 2 . From the perspective of battery efficiency, when the additive is MgCl2, the Coulomb efficiency, voltage efficiency, and energy efficiency of the electrolyte are all the highest, showing a significant improvement compared to the electrolyte without the additive (control group); from the charge-discharge curves, it can be seen that except for ZnCl2, the battery capacity increases after the addition of the other two additives. However, after the addition of MgCl2, the overpotential of the electrolyte decreases, and the battery performance improves.

[0109] Comparative Example 5

[0110] This comparative example provides vanadium redox flow battery electrolytes containing different concentrations of (NH4)2SO4. The preparation method is basically the same as that of the above-mentioned examples, except that: only (NH4)2SO4 is added to the electrolyte mother liquor (tetravalent vanadium ions) obtained in step (1), so that the concentration of (NH4)2SO4 in the electrolyte is 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, and 0.4 mol / L respectively. And the electrolyte mother liquor (tetravalent vanadium ions) without adding (NH4)2SO4 (i.e., 0 mol / L) is used as a control group.

[0111] The conductivity, viscosity and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 20 - 22 shown. Figure 20 is the conductivity and viscosity of the electrolytes with different concentrations of (NH4)2SO4 added; Figure 21 is the CV curve of the electrolytes with different concentrations of (NH4)2SO4 added and the CV curves of different electrolytes at the same scanning rate. Figure 21 In the figure, (a) is 0 mol / L, (b) is 0.1 mol / L, (c) is 0.2 mol / L, (d) is 0.3 mol / L, (e) is 0.4 mol / L, and (f) is the scanning rate of 20 mV / S; Figure 22 is the impedance spectrum of the electrolytes with different concentrations of (NH4)2SO4 added.

[0112] From Figure 20 it can be seen that with the addition of (NH4)2SO4, the conductivity of the electrolyte first increases and then decreases, and the conductivity reaches the maximum value when the addition amount is 0.2 mol / L. The viscosity first rises, then decreases and then rises with the addition of (NH4)2SO4, and the viscosity of the electrolyte is lower when the addition amount is 0.2 mol / L. From Figure 21 it can be seen that for the CV curve of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general law. At the same scanning rate, the oxidation-reduction peak current value of the sample with the additive added is significantly higher than that of the sample without the additive added, and the kinetics and reversibility of the reaction are improved to a certain extent. Among them, when the content of (NH4)2SO4 is 0.2 mol / L, the current value is the largest. From Figure 22 it can be seen that with the addition of (NH4)2SO4, the solution resistance of the sample changes little, and the charge transfer resistance first becomes smaller and then larger with the addition of (NH4)2SO4, and the charge transfer resistance is the smallest when the content is 0.2 mol / L. However, the electrochemical performance of this comparative example is lower than that of the above-mentioned examples.

[0113] Comparative Example 6

[0114] This comparative example provides vanadium redox flow battery electrolytes containing different concentrations of (NH4)2HPO4. The preparation method is basically the same as that of the above-mentioned examples, except that: only (NH4)2HPO4 is added to the electrolyte mother liquor (tetravalent vanadium ions) obtained in step (1), so that the concentrations of (NH4)2HPO4 in the electrolyte are 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, and 0.4 mol / L respectively. And, the electrolyte mother liquor (tetravalent vanadium ions) without adding (NH4)2SO4 (i.e., 0 mol / L) is used as the control group.

[0115] The conductivity, viscosity and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 23 - 25 shown. Figure 23 are the conductivity and viscosity of the electrolytes with different concentrations of (NH4)2HPO4 added; Figure 24 are the CV curves of the electrolytes with different concentrations of (NH4)2HPO4 added and the CV curves of different electrolytes at the same scan rate. Figure 24 In the (a) figure of , it is 0 mol / L, in the (b) figure it is 0.1 mol / L, in the (c) figure it is 0.2 mol / L, in the (d) figure it is 0.3 mol / L, in the (e) figure it is 0.4 mol / L, and in the (f) figure the scan rate is 20 mV / S; Figure 25 are the impedance spectra of the electrolytes with different concentrations of (NH4)2HPO4 added.

[0116] It can be seen from Figure 23 that with the addition of (NH4)2HPO4, the conductivity of the electrolyte first increases and then decreases, and the viscosity gradually increases with the addition of (NH4)2HPO4. It can be seen from Figure 24 that for the CV curve of the same sample, as the scan rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scan rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general law. At the same scan rate, the oxidation-reduction peak current value of the sample with the additive added is significantly higher than that of the sample without the additive added, and the kinetics and reversibility of the reaction are improved to a certain extent. When the additive content is 0.3 mol / L, the current value is the largest. It can be seen from Figure 25 that with the addition of (NH4)2HPO4, the solution resistance of the sample slightly increases, and the charge transfer resistance gradually decreases with the addition of (NH4)2HPO4. When the content is 0.3 mol / L, the charge transfer resistance is the smallest. However, the physical and electrochemical properties of this comparative example are lower than those of the above examples.

[0117] Comparative Example 7

[0118] This comparative example prepared 3.5-valent electrolytes containing 0.2 mol / L (NH4)2SO4 and 0.3 mol / L (NH4)2HPO4 respectively. The preparation methods of these electrolytes are the same as those of the above comparative example. The electrolyte was filled into the battery system for battery performance testing, where the active area of the battery was 3×3 cm 2 , and the charge-discharge current density was 120 mA / cm 2 , and the number of cycling times was 15 cycles. Figure 26 shows the battery performance of electrolytes with different ammonium salt additives at a charge-discharge current density of 120 mA / cm 2 . From the perspective of battery efficiency, when (NH4)2SO4 was added, the Coulomb efficiency, voltage efficiency, and energy efficiency of the electrolyte were all improved compared with the electrolyte without additive (control group). After adding (NH4)2HPO4, the Coulomb efficiency of the electrolyte increased, but the voltage efficiency decreased; from the charge-discharge curve, it can be seen that after adding (NH4)2SO4, the capacity of the electrolyte remained basically unchanged, but the overpotential decreased.

[0119] Comparative Example 8

[0120] This comparative example provided a vanadium redox flow battery electrolyte containing different concentrations of methanesulfonic acid (CH3SO3H). Its preparation method was basically the same as that of the above examples, except that: only CH3SO3H was added to the electrolyte mother liquor (4-valent vanadium ions) obtained in step (1) to make the concentration of CH3SO3H in the electrolyte be 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, and 2.0 mol / L respectively. And the electrolyte mother liquor (4-valent vanadium ions) without adding CH3SO3H (i.e., 0 mol / L) was used as the control group.

[0121] The conductivity, viscosity, and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 27 - 29 shown. Figure 27 shows the conductivity and viscosity of electrolytes with different concentrations of CH3SO3H; Figure 28 shows the CV curves of electrolytes with different concentrations of CH3SO3H and the CV curves of different electrolytes at the same scanning rate. Figure 28 In (a) of, it is 0 mol / L, in (b) it is 0.5 mol / L, in (c) it is 1.0 mol / L, in (d) it is 1.5 mol / L, in (e) it is 2.0 mol / L, and in (f) it is the scanning rate of 20 mV / S; Figure 29 shows the impedance spectra of electrolytes with different concentrations of CH3SO3H.

[0122] From Figure 27 it can be seen that with the addition of CH3SO3H, the conductivity of the electrolyte first increases and then decreases, and the viscosity gradually increases with the addition of CH3SO3H. FromFigure 28 It can be seen that for the CV curves of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, the oxidation-reduction peak current value of the sample with the additive changes little compared to the sample without the additive. However, after adding CH3SO3H, the oxidation-reduction peak potential difference of the electrolyte decreases, and the reaction kinetics is improved. From Figure 29 It can be seen that with the addition of CH3SO3H, the solution resistance of the sample decreases, and the solution resistance is the smallest when the content is 1 mol / L. However, the electrochemical performance of this comparative example is lower than that of the above examples.

[0123] Comparative Example 9

[0124] This comparative example provides a vanadium redox flow battery electrolyte containing different concentrations of sulfamic acid (NH2SO3H). Its preparation method is basically the same as that of the above examples, except that: only NH2SO3H is added to the electrolyte mother liquor (4-valent vanadium ions) obtained in step (1) to make the concentration of NH2SO3H in the electrolyte 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, and 0.4 mol / L respectively. And the electrolyte mother liquor (4-valent vanadium ions) without adding NH2SO3H (i.e., 0 mol / L) is used as a control group.

[0125] The conductivity, viscosity and electrochemical performance of the above electrolytes were tested, and the test results are as Figures 30 - 32 shown. Figure 30 are the conductivity and viscosity of the electrolytes with different concentrations of NH2SO3H added; Figure 31 are the CV curves of the electrolytes with different concentrations of NH2SO3H added and the CV curves of different electrolytes at the same scanning rate. Figure 31 In the (a) figure, it is 0 mol / L, in the (b) figure, it is 0.1 mol / L, in the (c) figure, it is 0.2 mol / L, in the (d) figure, it is 0.3 mol / L, in the (e) figure, it is 0.4 mol / L, and in the (f) figure, the scanning rate is 20 mV / S; Figure 32 are the impedance spectra of the electrolytes with different concentrations of NH2SO3H added.

[0126] From Figure 30 it can be seen that with the addition of NH2SO3H, the conductivity of the electrolyte first decreases and then rises slightly, and the viscosity gradually rises with the addition of NH2SO3H. From Figure 31It can be seen that for the CV curves of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase. This is because as the scanning rate increases, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, the oxidation-reduction peak current value of the sample with the additive is slightly increased compared to the sample without the additive, and the peak potential difference is slightly decreased, improving the reaction kinetics. From Figure 32 It can be seen that with the addition of NH2SO3H, the solution resistance of the samples all decreases, and the solution resistance is the smallest when the content is 0.4 mol / L. However, it was found during the experiment that adding the NH2SO3H additive resulted in poor solubility of the electrolyte and easy precipitation.

[0127] Comparative Example 10

[0128] In this comparative example, electrolytes containing 1 mol / L CH3SO3H and 0.4 mol / L NH2SO3H at +3.5 valence were prepared, and the preparation methods of these electrolytes were the same as those of the above comparative examples. The electrolytes were filled into the battery system for battery performance testing, where the active area of the battery was 3×3 cm 2 , and the charge-discharge current density was 120 mA / cm 2 , and the number of cycles was 15. Figure 33 For the electrolytes with different organic sulfonic acid additives at a charge-discharge current density of 120 mA / cm 2 . From Figure 33 It can be seen that after adding CH3SO3H and NH2SO3H respectively, both the Coulombic efficiency and energy efficiency of the battery are improved, but the electrolyte capacity decreases, and the performance of CH3SO3H is more stable than that of the NH2SO3H additive.

[0129] Comparative Example 11

[0130] In this comparative example, the static stabilities of the electrolyte without additive (control group), the electrolyte with only 0.1 mol / L MgCl2 added, the electrolyte with only 0.2 mol / L (NH4)2SO4 added, and the electrolyte with only 1 mol / L CH3SO3H added were tested. The preparation methods of these electrolytes were the same as those of the above comparative examples, and the electrolyte of +5 valence vanadium ions was used for the test. The test results are as Figure 34 shown, Figure 34In Figure (a), the electrolyte without additives is shown; in Figure (b), the electrolyte with only 0.1 mol / L MgCl₂ added is shown; in Figure (c), the electrolyte with only 0.2 mol / L (NH₄)₂SO₄ added is shown; in Figure (d), the electrolyte with only 1 mol / L CH₃SO₃H added is shown. The bottom diameter of the sample bottles is 3 cm for all. The heights of the precipitates formed in the electrolyte without additives, the electrolyte with only 0.1 mol / L MgCl₂ added, the electrolyte with only 0.2 mol / L (NH₄)₂SO₄ added, and the electrolyte with only 1 mol / L CH₃SO₃H added are 1.60 cm, 1.45 cm, 1.30 cm, and 1.70 cm respectively, and the volumes of the precipitates are 11.31 cm 3 ³ 3 ³ 3 ³ 3 ³ respectively. From the volumes of the precipitates formed, it can be seen that the addition of additives has a certain impact on the stability of the electrolyte. The addition of MgCl₂ and (NH₄)₂SO₄ reduces the amount of precipitate in the electrolyte and enhances the stability. The addition of CH₃SO₃H increases the height of the precipitate in the electrolyte, but there are obvious gaps in the middle of the precipitate, which may be due to the addition of CH₃SO₃H reducing the aggregation of the precipitate. Combining the static stability test results of the above examples, it can be seen that the high-temperature stabilities of the electrolytes with only 0.1 mol / L MgCl₂ added, only 0.2 mol / L (NH₄)₂SO₄ added, and only 1 mol / L CH₃SO₃H added are lower than those of the above examples.

[0131] Comparative Example 12

[0132] In this comparative example, electrolytes without additives (control group), electrolytes with 0.1 mol / L MgCl₂ + 0.2 mol / L (NH₄)₂SO₄ added, electrolytes with 0.1 mol / L MgCl₂ + 1 mol / L CH₂SO₃H added, and electrolytes with 0.2 mol / L (NH₄)₂SO₄ + 1 mol / L CH₂SO₃H added were prepared. The preparation methods of these electrolytes are the same as those of the above comparative examples, with the only difference being the different additives added, and electrolytes of 4-valent and 3.5-valent vanadium ions were prepared.

[0133] Electrochemical performance tests (using electrolytes of 4-valent vanadium ions) and battery performance tests (using electrolytes of 3.5-valent vanadium ions) were carried out on the above electrolytes, and the test results are as Figures 35 - 36 shown.

[0134] Figure 35Electrochemical performance test results of electrolytes without additives (control group), electrolytes with 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4, electrolytes with 0.1 mol / L MgCl2 + 1 mol / L CH2SO3H, and electrolytes with 0.2 mol / L (NH4)2SO4 + 1 mol / L CH2SO3H Figure 35 In (a) of Figure 35 is the impedance spectrum of the electrolyte. In (b)-(e) are the CV curves of the electrolyte without additives (control group), the electrolyte with 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4, the electrolyte with 0.1 mol / L MgCl2 + 1 mol / L CH2SO3H, and the electrolyte with 0.2 mol / L (NH4)2SO4 + 1 mol / L CH2SO3H at different scanning rates, respectively. (f) is the CV curve of the electrolyte at a scanning rate of 50 mV / s. From the impedance spectrum of the electrolyte, the addition of additives greatly reduces the resistance in the electrolyte and improves the electrochemical reaction activity of the electrolyte. At the same time, for the CV curve of the same sample, as the scanning rate increases, the oxidation-reduction peak current and oxidation-reduction peak potential gradually increase because, with the increase of the scanning rate, the slower kinetic process requires more response time, increasing the reaction barrier, which conforms to the general rule. At the same scanning rate, the oxidation-reduction peak potential difference of the electrolyte with additives is smaller than that of the control group electrolyte, and both the battery reaction kinetics and reaction rate are improved.

[0135] Figure 36 Battery cycle performance test results of electrolytes without additives (control group), electrolytes with 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4, electrolytes with 0.1 mol / L MgCl2 + 1 mol / L CH2SO3H, and electrolytes with 0.2 mol / L (NH4)2SO4 + 1 mol / L CH2SO3H. The active area of the battery is 3×3 cm 2 , the charge-discharge current density is 120 mA / cm 2 , and the number of cycles is 25. Figure 36Figures (a)-(c) therein are respectively the Coulomb efficiency, voltage efficiency, and energy efficiency diagrams of different electrolytes, figure (d) is the charge-discharge curve of different electrolytes, figure (e) is the discharge capacity diagram of different electrolytes during the cycle, and figure (f) is the discharge capacity retention rate of different electrolytes during the cycle. From the perspective of the battery cycle performance, the voltage efficiency of the electrolyte has been improved after the addition of the additive, but the current efficiency and energy efficiency have not been significantly improved, and the efficiency stability at different cycle numbers is poor. Considering the data of the comprehensive discharge capacity and capacity retention rate, the performance of the 0.1 mol / L MgCl2 + 0.2 mol / L (NH4)2SO4 electrolyte has been slightly improved, but its attenuation rate is faster than that of the electrolyte without the additive. Therefore, considering the overall cycle data of the battery, the effect of using the above pairwise combinations of the additive is very small, far lower than that of the above embodiments of the present invention.

[0136] As can be seen from the above examples and comparative examples, through the combination of the three additives in the embodiments of the present invention in a specific molar ratio and controlling the contents of the three additives within the scope of the present invention, they have a synergistic effect, can prevent the nucleation, growth, and aggregation of precipitates in the electrolyte, and thus reduce the generation of precipitates during the operation of the battery, significantly improve the high-temperature stability of the electrolyte, and enhance the overall performance of the battery. However, each comparative example using one additive or pairwise combined additives cannot achieve the effects of the embodiments of the present invention. Therefore, the embodiments of the present invention increase the concentration and high-temperature stability of vanadium ions in the electrolyte, improve the problems of low energy density and poor stability of the electrolyte, have good electrochemical activity, and have a low cost.

Claims

1. A composite vanadium redox flow battery electrolyte additive, comprising: MgCl2, (NH4)2SO4 and CH3SO3H with a molar ratio of (0.5 - 1.5):(1 - 3):(10 - 20).

2. The composite vanadium redox flow battery electrolyte additive according to claim 1, wherein, The molar ratio of MgCl2, (NH4)2SO4 and CH3SO3H is (0.5 - 1.5):2:(10 - 20); Preferably, the molar ratio of MgCl2, (NH4)2SO4 and CH3SO3H is 1:2:

10.

3. A vanadium redox flow battery electrolyte, comprising: Vanadium ions, the composite vanadium redox flow battery electrolyte additive described in any one of claims 1 - 2, and water; wherein, the total concentration of vanadium ions in the electrolyte is 1.8 - 2.2 mol / L, the concentration of MgCl2 in the electrolyte is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.1 - 0.3 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 - 2.0 mol / L.

4. The vanadium redox flow battery electrolyte according to claim 3, wherein, The concentration of MgCl2 in the electrolyte is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.2 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 - 2.0 mol / L; Preferably, the concentration of MgCl2 in the electrolyte is 0.1 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.2 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 mol / L.

5. The vanadium redox flow battery electrolyte according to claim 3, wherein The vanadium ions include tetravalent vanadium ions and / or 3.5 - valent vanadium ions.

6. A method for preparing the vanadium redox flow battery electrolyte described in any one of claims 3 - 5, which comprises the following steps: (1) Dissolve V2O5 in the first H2SO4 solution to obtain a mixed solution, use the mixed solution as the cathode electrolyte of a two - chamber electrolytic cell, use the second H2SO4 solution as the anode electrolyte of the two - chamber electrolytic cell, and perform constant - current electrolysis to obtain an electrolyte mother liquor with a total vanadium ion concentration of 1.8 - 2.2 mol / L; (2) Add MgCl2, (NH4)2SO4 and CH3SO3H to the electrolyte mother liquor obtained in step (1) so that the concentration of MgCl2 in the electrolyte is 0.05 - 0.15 mol / L, the concentration of (NH4)2SO4 in the electrolyte is 0.1 - 0.3 mol / L, and the concentration of CH3SO3H in the electrolyte is 1.0 - 2.0 mol / L to obtain the vanadium redox flow battery electrolyte described above.

7. The preparation method of the all-vanadium redox flow battery electrolyte according to claim 6, wherein, In step (1), dissolve V2O5 in the first H2SO4 solution with a molar ratio of V2O5:H2SO4 of 1:(2 - 6).

8. The preparation method of the all-vanadium redox flow battery electrolyte according to claim 6, wherein, In step (1), the concentration of the first H2SO4 solution is 2 - 6 mol / L.

9. The preparation method of the all-vanadium redox flow battery electrolyte according to claim 6, wherein, In step (1), the concentration of the second H2SO4 solution is 2 - 6 mol / L, and its volume is the same as the volume of the mixed solution; Preferably, the concentration of the first H2SO4 solution is the same as the concentration of the second H2SO4 solution.

10. The preparation method of the vanadium redox flow battery electrolyte according to claim 6, wherein, In step (1), the current density of the constant-current electrolysis is 0.1 - 1 A / cm 2 , and the electrolysis time is 90 - 240 min.

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