High-stability vanadium electrolyte and all-vanadium redox flow battery

Abstract

This invention relates to the field of electrochemical energy storage technology, and discloses a highly stable vanadium electrolyte and an all-vanadium redox flow battery, comprising vanadium active ions, an acidic medium, a main chelating agent, an auxiliary chelating agent, a viscosity modifier, and deionized water; wherein the main chelating agent comprises diethylenetriaminepentaacetic acid or its salts; and the auxiliary chelating agent is ammonium dihydrogen phosphate. This invention utilizes the synergistic effect of the DTPA main chelating agent and the ammonium dihydrogen phosphate auxiliary chelating agent to form a DTPA-vanadium-phosphate multi-component mixed complex, enhancing the inhibition of pentavalent vanadium ion hydrolysis. Simultaneously, the H2PO4 provided by the ammonium dihydrogen phosphate... ‑ / HPO4 2‑ The buffer pair can regulate in situ the pH of the electrode in response to local pH fluctuations, suppressing V-induced alkalization. 5+ Hydrolysis releases phosphate ions, which can alter the solvation structure of the solution, disrupt the formation of vanadium salt crystal nuclei at low temperatures, and effectively inhibit low-temperature crystallization.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to a highly stable vanadium electrolyte and an all-vanadium redox flow battery, and more particularly to a highly stable vanadium electrolyte containing DTPA chelating agent and its battery. Background Technology

[0002] Vanadium redox flow batteries (VRFBs) have become a research hotspot in the field of large-scale electrochemical energy storage due to their advantages such as independent power and capacity design, long cycle life, high safety, and environmental friendliness. They are widely used in scenarios such as renewable energy grid connection, grid peak shaving, and industrial backup power. As the core component of VRFBs, the electrolyte's stability, conductivity, and electrochemical activity directly affect the battery's energy efficiency, cycle life, and operating costs.

[0003] Currently, the electrolytes for vanadium redox flow batteries widely used in industry typically use sulfuric acid as the supporting electrolyte. Vanadium ions exist in the positive and negative electrode electrolytes in the form of V(IV) / V(V) redox couples and V(II) / V(III) redox couples. To suppress vanadium ion hydrolysis and precipitation and improve electrolyte stability, existing technologies (CN121215821A) often introduce chelating agents, such as citric acid, EDTA, oxalic acid, and phosphates, to stabilize vanadium ions through coordination. Although traditional chelating agents, represented by EDTA, have a certain complexing ability, complexing with vanadium ions leads to excessively high electrolyte viscosity, reduces ion mass transfer performance, increases battery charge transfer impedance, and decreases energy efficiency.

[0004] Existing technology (CN114865066A) discloses an electrolyte for iron-chromium flow batteries containing a complexing agent. It simultaneously adds DTPA and bromide to the positive and negative electrode electrolytes. The pentadentate coordination of DTPA and the monodentate coordination of bromide together form a six-coordinate structure to address the problems of chromium ion deactivation and cross-contamination. However, this technical solution mainly targets the iron-chromium system. The second component, bromide, only serves to improve the coordination number of chromium ions, and its function is relatively singular. Furthermore, it does not address the issues of inhibiting hydrolysis and controlling crystallization of high-concentration vanadium ions over a wide temperature range in vanadium redox flow batteries.

[0005] Therefore, there is an urgent need for a chelation system that can simultaneously suppress high-temperature hydrolysis and low-temperature crystallization of high-concentration vanadium ions and has multiple synergistic effects, in order to meet the requirements of long life and high-efficiency operation of vanadium redox flow batteries under complex operating conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a highly stable vanadium electrolyte and an all-vanadium redox flow battery to solve the problems of existing vanadium electrolytes, such as easy hydrolysis and precipitation of pentavalent vanadium, low-temperature crystallization, poor cycle stability, and insufficient synergy of chelate complex systems under high concentration and wide temperature range conditions.

[0007] Based on the above concept, the technical solution adopted by this invention is as follows: According to a first aspect of the present invention, a highly stable vanadium electrolyte is provided, comprising: Vanadium active ions; Acidic medium; Main chelating agent; Auxiliary chelating agents; Viscosity modifier; And deionized water; The main chelating agent includes diethylenetriaminepentaacetic acid or its salts; The auxiliary chelating agent is ammonium dihydrogen phosphate.

[0008] In some embodiments, the vanadium active ion includes V 2+ V 3+ V 4+ and V 5+ At least two of them; The total concentration of vanadium ions is 2.5~3.0 mol / L.

[0009] Optionally, the acidic medium includes any one of sulfuric acid, hydrochloric acid, and nitric acid.

[0010] Furthermore, the concentration of the acidic medium is 1.0~4.0 mol / L.

[0011] Optionally, the diethylenetriaminepentaacetic acid salt of the main chelating agent includes any one or more of diethylenetriaminepentaacetic acid pentasodium, diethylenetriaminepentaacetic acid pentapotassium, and diethylenetriaminepentaacetic acid pentamagnesium.

[0012] Furthermore, the concentration of the main chelating agent is 0.003~0.03 mol / L.

[0013] In some embodiments, the concentration of the auxiliary chelating agent is 0.02~0.08 mol / L.

[0014] Optionally, the viscosity modifier includes any one or more of propylene glycol, glycerol, ethylene glycol, and polyethylene glycol.

[0015] Furthermore, the viscosity modifier accounts for 0.01~0.05 wt% of the total electrolyte.

[0016] According to a second aspect of the present invention, an all-vanadium redox flow battery is provided, comprising: Positive electrode; Negative electrode; Positive electrolyte; Negative electrode electrolyte; And ion exchange membranes; The positive electrode electrolyte and the negative electrode electrolyte are either of the vanadium electrolytes described above; the positive electrode electrolyte includes a V(IV) / V(V) redox couple; and the negative electrode electrolyte includes a V(II) / V(III) redox couple.

[0017] The beneficial effects of this invention are as follows: This invention utilizes the synergistic effect of DTPA as the primary chelating agent and ammonium dihydrogen phosphate as the auxiliary chelating agent. DTPA, as a multidentate ligand, forms a stable five-membered ring chelate with vanadium ions, providing the basic framework for the complex structure. The phosphate ions dissociated from ammonium dihydrogen phosphate participate in coordination as auxiliary ligands, forming a multi-component mixed complex of DTPA-vanadium-phosphate, enhancing the inhibition of pentavalent vanadium ion hydrolysis. Simultaneously, the H2PO4 provided by ammonium dihydrogen phosphate... - / HPO4 2- The buffer pair can regulate in situ the pH of the electrode in response to local pH fluctuations, suppressing V-induced alkalization. 5+ Hydrolysis releases phosphate ions, which can alter the solvation structure of the solution, disrupting the formation of vanadium salt crystal nuclei at low temperatures and effectively inhibiting low-temperature crystallization. The electrolyte of this invention showed no V₂O₅ precipitation after standing at 60°C for 30 days, with vanadium ion retention rates exceeding 99%. Furthermore, no vanadium salt crystallization was observed after standing at -5°C for 7 days, achieving long-term stable operation of the electrolyte within a wide temperature range from -5°C to 60°C. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the charging process of the battery of the present invention; Figure 2 This is a schematic diagram of the battery discharge according to the present invention. Detailed Implementation

[0019] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention as detailed in the appended claims.

[0020] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, the technical or scientific terms used in this application should be understood in their ordinary sense by one of ordinary skill in the art to which this invention pertains. The words “a” or “one” and similar terms used in this application specification and claims do not indicate a limitation of quantity, but rather indicate the presence of at least one. “A plurality” means two or more. The words “comprising” or “including” and similar terms mean that the element or object preceding “comprising” or “including” covers the element or object listed following “comprising” or “including” and its equivalents, and does not exclude other elements or objects. The words “connected” or “linked” and similar terms are not limited to physical or mechanical connections and can include electrical connections, whether direct or indirect. The words “above” and / or “below” and similar terms are for ease of description only and are not limited to a location or spatial orientation. The singular forms “a,” “the,” and “the” used in this application specification and appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0021] The technical concept of this invention includes: To address the technical bottlenecks of electrolytes for vanadium redox flow batteries under high concentration and wide temperature range conditions, existing technologies mainly face the following problems: First, although traditional chelating agents, represented by EDTA, have a certain complexing ability, complexing with vanadium ions leads to excessively high electrolyte viscosity, reducing ion mass transfer performance, increasing battery charge transfer impedance, and decreasing energy efficiency; Second, high-concentration vanadium electrolytes (≥2.5...) In vanadium redox flow batteries, pentavalent vanadium ions readily hydrolyze to form V₂O₅ precipitate, especially at temperatures above 50°C, where the hydrolysis is more pronounced. Existing chelating agents are unable to effectively suppress this process. Third, existing chelating systems are prone to vanadium salt crystallization at low temperatures (e.g., -5°C), failing to meet the requirements for wide-temperature operation. Fourth, although some studies have attempted to use multi-toothed chelating agents such as DTPA in combination with the second component, these are mainly designed for iron-chromium systems. The role of the second component bromide is only to complete the coordination number of chromium ions to solve the chromium ion deactivation problem. There is no specific solution for the unique hydrolysis behavior and crystallization mechanism of vanadium ions in all-vanadium redox flow batteries, and multiple synergistic effects are lacking.

[0022] This application addresses the aforementioned problems by proposing a synergistic stabilizing system constructed from DTPA as the primary chelating agent and ammonium dihydrogen phosphate as the auxiliary chelating agent. DTPA provides the basic complex structure through multidentate coordination, while ammonium dihydrogen phosphate plays a triple synergistic role: its dissociated phosphate group acts as an auxiliary ligand to participate in coordination, enhancing the stability of the complex; its H2PO4...- / HPO4 2- The buffer can suppress hydrolysis caused by local pH increase at the electrode in situ; at the same time, the introduction of phosphate ions changes the solvation structure of the solution, disrupting the formation of crystal nuclei at low temperatures. On this basis, a viscosity modifier is further introduced to improve the mass transfer performance of the electrolyte, enabling the system to simultaneously meet multiple requirements such as high-temperature resistance to hydrolysis, low-temperature resistance to crystallization, and low viscosity with high mass transfer.

[0023] Therefore, this invention employs a compound system of DTPA and ammonium dihydrogen phosphate, combined with viscosity adjustment methods, to provide a highly stable vanadium electrolyte that can operate stably for a long time in a wide temperature range of -5 to 60°C, while possessing excellent mass transfer performance and cycle life, in order to meet the application requirements of vanadium redox flow batteries under complex operating conditions.

[0024] This application provides a highly stable vanadium electrolyte. Vanadium active ions; Acidic medium; Main chelating agent; Auxiliary chelating agents; Viscosity modifier; And deionized water; The main chelating agent includes diethylenetriaminepentaacetic acid or its salts; The auxiliary chelating agent is ammonium dihydrogen phosphate.

[0025] The benefits of insertion: This invention utilizes the synergistic effect of DTPA as the primary chelating agent and ammonium dihydrogen phosphate as the auxiliary chelating agent. DTPA, as a multidentate ligand, forms a stable five-membered ring chelate with vanadium ions, providing the basic framework for the complex structure. The phosphate ions dissociated from ammonium dihydrogen phosphate participate in coordination as auxiliary ligands, forming a multi-component mixed complex of DTPA-vanadium-phosphate, enhancing the inhibition of pentavalent vanadium ion hydrolysis. Simultaneously, the H2PO4 provided by ammonium dihydrogen phosphate... - / HPO4 2- The buffer pair can regulate in situ the pH of the electrode in response to local pH fluctuations, suppressing V-induced alkalization. 5+ Hydrolysis releases phosphate ions, which can alter the solvation structure of the solution, disrupting the formation of vanadium salt crystal nuclei at low temperatures and effectively inhibiting low-temperature crystallization. The electrolyte of this invention showed no V₂O₅ precipitation after standing at 60°C for 30 days, with vanadium ion retention rates exceeding 99%. Furthermore, no vanadium salt crystallization was observed after standing at -5°C for 7 days, achieving long-term stable operation of the electrolyte within a wide temperature range from -5°C to 60°C.

[0026] The vanadium redox flow battery described in this application includes Positive electrode; Negative electrode; Positive electrolyte; Negative electrode electrolyte; And ion exchange membranes; The positive and negative electrolytes are the vanadium electrolytes described above; the positive electrolyte includes a V(IV) / V(V) redox couple; and the negative electrolyte includes a V(II) / V(III) redox couple.

[0027] The following describes in detail a highly stable vanadium electrolyte and a full vanadium redox flow battery provided in this application, with reference to embodiments and comparative examples.

[0028] Example 1 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Subsequently, DTPA and ammonium dihydrogen phosphate were added sequentially, with DTPA reaching a final concentration of 0.003 mol / L and ammonium dihydrogen phosphate reaching a final concentration of 0.02 mol / L. The mixture was stirred continuously for 60 min until the solution became clear and free of suspended particles. Finally, polyethylene glycol was added at a mass fraction of 0.02 wt%, and the mixture was stirred for another 15 min to obtain the basic vanadium electrolyte.

[0029] The basic electrolyte was injected into the positive and negative electrode storage tanks of the vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized into a V(IV) / V(V) redox couple, and vanadium ions in the negative electrode electrolyte were reduced into a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes respectively.

[0030] Example 2 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.5 mol / L, which was then placed in a 30°C constant temperature water bath. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.8 mol / L. Subsequently, DTPA and ammonium dihydrogen phosphate were added sequentially, with DTPA reaching a final concentration of 0.015 mol / L and ammonium dihydrogen phosphate reaching a final concentration of 0.05 mol / L. The mixture was stirred continuously for 70 min until the solution became clear and free of suspended particles. Finally, polyethylene glycol was added at a mass fraction of 0.04 wt%, and the mixture was stirred for another 18 min to obtain the basic vanadium electrolyte.

[0031] The basic electrolyte was injected into the positive and negative electrode storage tanks of the vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized into a V(IV) / V(V) redox couple, and vanadium ions in the negative electrode electrolyte were reduced into a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes respectively.

[0032] Example 3 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 2.0 mol / L, which was then placed in a constant temperature water bath at 40°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 3.0 mol / L. Subsequently, DTPA and ammonium dihydrogen phosphate were added sequentially, with DTPA reaching a final concentration of 0.03 mol / L and ammonium dihydrogen phosphate reaching a final concentration of 0.08 mol / L. The mixture was stirred continuously for 90 min until the solution became clear and free of suspended particles. Finally, polyethylene glycol was added at a mass fraction of 0.05 wt%, and the mixture was stirred for another 20 min to obtain the basic vanadium electrolyte.

[0033] The basic vanadium electrolyte was injected into the positive and negative electrode storage tanks of the vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized into V(IV) / V(V) redox couples, and vanadium ions in the negative electrode electrolyte were reduced into V(II) / V(III) redox couples, thus obtaining the positive and negative electrode electrolytes respectively.

[0034] Comparative Example 1 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Stirring was continued for 60 min to obtain a basic vanadium electrolyte. This basic electrolyte was injected into the positive and negative electrode reservoirs of a vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized to a V(IV) / V(V) redox couple, while vanadium ions in the negative electrode electrolyte were reduced to a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes, respectively.

[0035] Comparative Example 2 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. DTPA was then added to bring the final DTPA concentration to 0.015 mol / L, and the solution was stirred continuously for 60 min until it became clear and free of suspended particles, yielding the basic vanadium electrolyte. This basic electrolyte was injected into the positive and negative electrode reservoirs of a vanadium redox flow battery, respectively. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized to a V(IV) / V(V) redox couple, while vanadium ions in the negative electrode electrolyte were reduced to a V(II) / V(III) redox couple, resulting in the positive and negative electrode electrolytes, respectively.

[0036] Comparative Example 3 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Subsequently, ammonium dihydrogen phosphate was added to bring the final concentration of ammonium dihydrogen phosphate to 0.05 mol / L, and the solution was stirred continuously for 60 min until it became clear and free of suspended particles, thus obtaining the basic vanadium electrolyte. This basic electrolyte was injected into the positive and negative electrode storage tanks of a vanadium redox flow battery, respectively. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized to a V(IV) / V(V) redox couple, while vanadium ions in the negative electrode electrolyte were reduced to a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes, respectively.

[0037] Comparative Example 4 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Subsequently, EDTA and ammonium dihydrogen phosphate were added sequentially to bring the final concentrations of EDTA to 0.015 mol / L and ammonium dihydrogen phosphate to 0.05 mol / L. The mixture was stirred continuously for 60 min until the solution was clear and free of suspended particles, yielding the basic vanadium electrolyte. This basic electrolyte was injected into the positive and negative electrode storage tanks of a vanadium redox flow battery, respectively. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized to a V(IV) / V(V) redox couple, while vanadium ions in the negative electrode electrolyte were reduced to a V(II) / V(III) redox couple, resulting in the positive and negative electrode electrolytes, respectively.

[0038] Comparative Example 5 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Subsequently, DTPA and ammonium dihydrogen phosphate were added sequentially to achieve final concentrations of 0.1 mol / L for both DTPA and ammonium dihydrogen phosphate, and the mixture was stirred continuously for 60 min until the solution became clear and free of suspended particles. Finally, polyethylene glycol was added at a mass fraction of 0.02 wt%, and the mixture was stirred for another 15 min to obtain the basic vanadium electrolyte. The basic electrolyte was injected into the positive and negative electrode storage tanks of the vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized into a V(IV) / V(V) redox couple, and vanadium ions in the negative electrode electrolyte were reduced into a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes respectively.

[0039] Comparative Example 6 Sulfuric acid was diluted with deionized water to prepare an acidic base solution with a sulfuric acid concentration of 1.2 mol / L, which was then placed in a constant temperature water bath at 25°C. Vanadium oxysulfate was slowly added to the acidic base solution and stirred until completely dissolved, resulting in a total vanadium ion concentration of 2.5 mol / L. Subsequently, DTPA and ammonium dihydrogen phosphate were added sequentially to achieve a final DTPA concentration of 0.015 mol / L and a final ammonium dihydrogen phosphate concentration of 0.001 mol / L, respectively. The mixture was stirred continuously for 60 min until the solution became clear and free of suspended particles. Finally, polyethylene glycol was added at a mass fraction of 0.02 wt%, and the mixture was stirred for another 15 min to obtain the basic vanadium electrolyte. The basic electrolyte was injected into the positive and negative electrode storage tanks of the vanadium redox flow battery. During the first charge of the battery, vanadium ions in the positive electrode electrolyte were oxidized into a V(IV) / V(V) redox couple, and vanadium ions in the negative electrode electrolyte were reduced into a V(II) / V(III) redox couple, thus obtaining the positive and negative electrode electrolytes respectively.

[0040] The basic vanadium electrolytes prepared in each embodiment and comparative example were injected into the positive electrode storage tank and negative electrode storage tank of the vanadium redox flow battery, respectively. After the first charge activation, the positive electrode electrolyte (V(IV) / V(V) pair) and the negative electrode electrolyte (V(II) / V(III) pair) were obtained.

[0041] High-temperature stability test: Take the positive electrode electrolyte and place it in a sealed glass bottle. Let it stand in a 60℃ constant temperature drying oven for 30 days. Observe whether V2O5 precipitation occurs and record the vanadium ion retention rate (the change in vanadium ion concentration before and after standing is measured by inductively coupled plasma atomic emission spectrometry).

[0042] Low temperature stability test: Take the positive electrode electrolyte and place it in a sealed glass bottle. Let it stand in a -5℃ low temperature test chamber for 7 days and observe whether vanadium salt crystallization occurs.

[0043] Viscosity test: The dynamic viscosity of the electrolyte was measured using a rotational viscometer at 25°C.

[0044] Battery performance testing: The activated batteries were subjected to constant current charge-discharge cycle testing at a specified current density (charging cutoff voltage 1.65V, discharging cutoff voltage 0.8V), and the capacity retention rate (based on the first discharge capacity) and energy efficiency were recorded after 1000 cycles.

[0045] Table 1. Experimental data results for the examples and comparative examples.

[0046] In this table, "-" indicates that the battery test could not be completed due to rapid precipitation; "*" indicates that the cycle test was only completed 500 times, and the table shows the data for 500 cycles.

[0047] The highly stable vanadium electrolytes provided in Examples 1 to 3 of this invention exhibit excellent performance in terms of high-temperature stability, low-temperature anti-crystallization ability, viscosity control, and battery cycle performance. No V₂O₅ precipitation occurred in the electrolytes of the three examples after standing at 60°C for 30 days, and the vanadium ion retention rate remained above 99%. In contrast, precipitation or crystallization occurred to varying degrees in all comparative examples. This indicates that the DTPA-ammonium dihydrogen phosphate synergistic system constructed in this invention can effectively inhibit the hydrolysis reaction of high-concentration vanadium ions under high-temperature conditions. The fundamental reason is that DTPA, as a multidentate ligand, forms a stable five-membered ring chelate with vanadium ions, providing a basic framework for the complex structure. The phosphate ions dissociated from ammonium dihydrogen phosphate can act as auxiliary ligands to enter the coordination layer of vanadium ions, forming a multi-component mixed complex of DTPA-vanadium-phosphate. This mixed ligand complex structure is thermodynamically more stable than a single DTPA complex, thus significantly enhancing its resistance to hydrolysis.

[0048] In terms of low-temperature stability, no vanadium salt crystallization was observed in Examples 1 to 3 after standing at -5°C for 7 days, while crystallization occurred in Comparative Examples 2 and 3 after 3 and 5 days, respectively, and slight crystallization also occurred in Comparative Example 6. This indicates that ammonium dihydrogen phosphate plays a crucial role in suppressing low-temperature crystallization. This is because the phosphate ions released from ammonium dihydrogen phosphate have a high charge density, enabling them to participate in the hydrogen bond network of water molecules and alter the ionic solvation structure. Simultaneously, the phosphate ions form a soluble complex with vanadium ions, reducing the driving force for vanadium sulfate crystal precipitation, thereby effectively suppressing low-temperature crystallization.

[0049] Comparing Examples 1-3 with Comparative Examples 2 and 3, it can be observed that when only DTPA is added and ammonium dihydrogen phosphate is lacking, precipitation occurs in the electrolyte after 15 days at high temperature and crystallization occurs after 3 days at low temperature. When only ammonium dihydrogen phosphate is added and DTPA is lacking, precipitation occurs in the electrolyte after 10 days at high temperature and crystallization occurs after 5 days at low temperature. This indicates that DTPA and ammonium dihydrogen phosphate must be used synergistically to simultaneously solve the problems of high-temperature hydrolysis and low-temperature crystallization. Their functions are complementary and neither can be omitted. DTPA provides fundamental strong complexing ability, while ammonium dihydrogen phosphate compensates for DTPA's shortcomings in wide temperature range adaptability through auxiliary coordination, pH buffering, and solvation structure regulation.

[0050] Comparative Example 4, which used EDTA instead of DTPA as the main chelating agent, showed significantly inferior high-temperature stability, low-temperature anti-crystallization ability, and battery cycle performance compared to the Example. This is because although EDTA is a multidentate ligand, its molecular structure and coordination configuration differ from DTPA, resulting in poor synergistic compatibility with ammonium dihydrogen phosphate and an inability to form a stable mixed ligand complex. Furthermore, the high viscosity of the electrolyte after EDTA complexes with vanadium ions hinders ion mass transfer, leading to a decline in battery performance.

[0051] From the perspective of concentration effects, Comparative Example 5, which increased the DTPA concentration to 0.1 mol / L, showed acceptable stability, but the excessively high viscosity led to a decrease in battery energy efficiency. This is because while excessive DTPA can effectively complex vanadium ions, too many free DTPA molecules increase the internal friction of the solution system and may form an over-complexed structure, affecting the redox reaction kinetics of vanadium ions on the electrode surface. Comparative Example 6, which reduced the ammonium dihydrogen phosphate concentration to 0.001 mol / L, showed trace precipitation in the electrolyte at high temperatures and slight crystallization at low temperatures, and its cycle performance was inferior to the example. This is because when the concentration of ammonium dihydrogen phosphate is too low, its auxiliary coordination, pH buffering, and solvation regulation effects are insufficient to fully exert their potential, failing to form an effective synergistic effect with DTPA.

[0052] Regarding viscosity control, Examples 1 to 3 added appropriate amounts of polyethylene glycol, resulting in low viscosities ranging from 9.2 to 11.8 mPa·s. In contrast, Comparative Examples 2, 3, and 4, without added polyethylene glycol, exhibited viscosities as high as 15.6, 12.3, and 18.5 mPa·s, respectively. This indicates that polyethylene glycol, as a viscosity modifier, can effectively reduce the viscosity of the electrolyte after DTPA complexation, thus improving ion mass transfer performance. Although Comparative Example 5 added polyethylene glycol, the viscosity still reached 16.2 mPa·s due to the excessively high DTPA concentration, demonstrating that the effect of viscosity modifiers has certain limitations, and the viscosity increase caused by excessive chelating agent cannot be completely offset.

[0053] From the perspective of battery cycle performance, after 1000 cycles, Examples 1 to 3 all achieved a capacity retention rate of over 93% and an energy efficiency of over 82%, while the capacity retention rates of Comparative Examples 2 and 3 dropped to 88.2% and 85.7% respectively after 500 cycles, and the capacity retention rate of Comparative Example 4 was only 82.1% after 1000 cycles. This indicates that the complexation stability of the electrolyte of the present invention can effectively reduce the dissolution and loss of vanadium ions during long-term charge and discharge processes, thereby significantly extending the battery cycle life.

[0054] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. The invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0055] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A highly stable vanadium electrolyte, characterized in that, include Vanadium active ions; Acidic medium; Main chelating agent; Auxiliary chelating agents; Viscosity modifier; And deionized water; The main chelating agent includes diethylenetriaminepentaacetic acid or its salts; The auxiliary chelating agent is ammonium dihydrogen phosphate.

2. The highly stable vanadium electrolyte according to claim 1, characterized in that, The vanadium active ions include V 2+ V 3+ V 4+ and V 5+ At least two of them; The total concentration of vanadium ions is 2.5~3.0 mol / L.

3. The highly stable vanadium electrolyte according to claim 1, characterized in that, The acidic medium includes any one of sulfuric acid, hydrochloric acid, and nitric acid; The concentration of the acidic medium is 1.0~4.0 mol / L.

4. The highly stable vanadium electrolyte according to claim 1, characterized in that, The diethylenetriaminepentaacetic acid salts of the main chelating agent include any one or more of diethylenetriaminepentaacetic acid pentasodium, diethylenetriaminepentaacetic acid pentapotassium, and diethylenetriaminepentaacetic acid pentamagnesium.

5. The highly stable vanadium electrolyte according to claim 1, characterized in that, The concentration of the main chelating agent is 0.003~0.03 mol / L.

6. The highly stable vanadium electrolyte according to claim 1, characterized in that, The concentration of the auxiliary chelating agent is 0.02~0.08 mol / L.

7. The highly stable vanadium electrolyte according to claim 1, characterized in that, The viscosity modifier includes any one or more of propylene glycol, glycerol, ethylene glycol and polyethylene glycol, and the mass fraction of the viscosity modifier accounts for 0.01~0.05 wt% of the total electrolyte.

8. A vanadium redox flow battery, characterized in that, include Positive electrode; Negative electrode; Positive electrolyte; Negative electrode electrolyte; And ion exchange membranes; The positive and negative electrolytes are vanadium electrolytes as described in any one of claims 1-7; the positive electrolyte includes a V(IV) / V(V) redox couple; and the negative electrolyte includes a V(II) / V(III) redox couple.

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