Positive electrode electrolyte, preparation method thereof and all-vanadium redox flow battery

Abstract

The invention provides a positive electrode electrolyte and a preparation method thereof, and an all-vanadium redox flow battery, the positive electrode electrolyte comprises: an active material, the active material comprising + 5-valent vanadium ions; the additive comprises a carbon material, at least part of the surface of the carbon material is provided with a modification group, and the modification group comprises at least one of nitro, sulfo, carbonyl, amino, carboxyl and hydroxyl. Therefore, the carbon material can inhibit agglomeration and precipitation of the + 5-valent vanadium ions, the dispersity of the + 5-valent vanadium ions in the positive electrode electrolyte is improved, and the stability of the positive electrode electrolyte is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of all-vanadium redox flow battery, in particular, the present application relates to the positive electrolyte and its preparation method and all-vanadium redox flow battery. BACKGROUND

[0002] With the increasingly serious problems of fossil energy resources depletion and environmental pollution, finding new energy and energy storage technology has become an urgent problem in the development of energy. All-vanadium redox flow battery is a new type of high-efficiency electrochemical energy storage battery, which has super-long cycle, flexible capacity design, fast response rate, high safety and other characteristics, which also makes it widely concerned in the field of new energy. Many research institutions and scholars have also carried out a lot of research work, and strive to improve and perfect the performance of all-vanadium redox flow battery. Electrolyte as the energy storage medium of all-vanadium electrolyte, it determines the capacity and energy density of all-vanadium redox flow battery, therefore, the research of electrolyte has very important significance for the development of all-vanadium redox flow battery.

[0003] In the charging and discharging process of all-vanadium redox flow battery, there will be different valence state of vanadium ion conversion in the electrolyte, in the positive electrolyte, +5 valence and +4 valence vanadium ions are active substances, +5 valence and +4 valence vanadium ions exist in the form of VO2 + ion and VO 2+ ion in acidic solution, among them, the solubility of +5 valence vanadium ion is low, and it is greatly affected by temperature, when the temperature is higher than 40℃, the positive electrolyte after charging is easy to hydrolyze and precipitate V2O5(2VO2 + +H2O=V2O5+2H + ). And +5 valence vanadium ion is the main component of all-vanadium redox flow battery positive side electrolyte, in the process of charging and discharging of the battery, the conversion between +4 valence vanadium ion and +5 valence vanadium ion occurs constantly, therefore, the instability of +5 valence vanadium ion will lead to precipitation, loss of vanadium ion, affect the capacity and cycle life of the battery. The positive electrolyte has good stability, which is the basis for the normal work of the battery and good charging and discharging performance.

[0004] Therefore, it is urgent to develop a positive electrolyte with good stability and can improve the solubility of +5 valence vanadium ion and avoid its precipitation, the development of this electrolyte is beneficial to improve the capacity and energy density of all-vanadium redox flow battery, and can solve the problem of capacity disadvantage of all-vanadium redox flow battery. SUMMARY

[0005] The present application aims to at least solve one of the technical problems in the prior art to at least some extent.

[0006] Therefore, in a first aspect of the present application, the present application provides a positive electrolyte. According to an embodiment of the present application, the positive electrolyte comprises: an active substance, the active substance comprising +5 valence vanadium ions; and an additive, the additive comprising a carbon material, at least part of a surface of the carbon material having a modification group, the modification group comprising at least one of a nitro group, a sulfonic acid group, a carbonyl group, a carboxyl group, and a hydroxyl group. Thus, the +5 valence vanadium ions can be inhibited from agglomerating and precipitating, the dispersibility of the +5 valence vanadium ions in the positive electrolyte can be improved, and the stability of the positive electrolyte can be improved.

[0007] According to an embodiment of the present application, the positive electrolyte can further comprise at least one of the following additional technical features:

[0008] According to an embodiment of the present application, the mass ratio of the carbon material to the +5 valence vanadium ions is 1:(3-80). Thus, by setting the mass ratio of the carbon material to the +5 valence vanadium ions in the above range, the probability of the +5 valence vanadium ions forming precipitates is reduced, the solubility of the +5 valence vanadium ions is improved, and the stability of the electrolyte and the energy density of the battery are improved.

[0009] According to an embodiment of the present application, the mass percentage of the carbon material is 0.1%-2% based on the total mass of the positive electrolyte. Thus, while the +5 valence vanadium ions are inhibited from agglomerating and forming precipitates, the content of the carbon material that does not undergo adsorption in the positive electrolyte is reduced, and the influence of the carbon material on the charge-discharge efficiency of the battery is reduced.

[0010] According to an embodiment of the present application, the molar concentration of the +5 valence vanadium ions in the positive electrolyte is not less than 0.9 mol / L. Thus, the concentration of the +5 valence vanadium ions in the positive electrolyte is increased, and the energy density of the battery is improved.

[0011] According to an embodiment of the present application, the molar concentration of the +5 valence vanadium ions in the positive electrolyte is 0.9 mol / L-1.5 mol / L. Thus, the concentration of the +5 valence vanadium ions in the positive electrolyte is increased, and the energy density of the battery is improved.

[0012] According to an embodiment of the present application, the carbon material comprises at least one of carbon nanotubes, superconducting carbon, graphene, acetylene black, carbon black, Ketjen black, and carbon nanofibers. Thus, the carbon material of the above types can be adsorbed on the surface of the +5 valence vanadium ions, the probability of the +5 valence vanadium ions agglomerating and precipitating is reduced, the dispersibility of the +5 valence vanadium ions and the stability of the electrolyte are improved, and the energy density and the cycle life of the battery are further improved.

[0013] According to an embodiment of the present application, the modification group comprises at least one of a carboxyl group, a hydroxyl group, a carbonyl group, and a sulfonic acid group. Thus, the modification group of the above-mentioned kind has strong electronegativity, which can reduce the probability of +5 valence vanadium ions agglomerating to form precipitates, thereby improving the stability of the electrolyte and the energy density and cycle life of the battery.

[0014] According to an embodiment of the present application, the carbon material comprises at least one of the carbon nanotube having the carboxyl group on at least part of the surface, the carbon nanotube having the hydroxyl group on at least part of the surface, the carbon nanotube having the carbonyl group on at least part of the surface, and the carbon nanotube having the sulfonic acid group on at least part of the surface. Thus, the carbon nanotube can be adsorbed on the surface of +5 valence vanadium ions, inhibit the agglomeration of +5 valence vanadium ions to form precipitates, improve the solubility of +5 valence vanadium ions, and improve the stability of the electrolyte.

[0015] According to an embodiment of the present application, the positive electrode electrolyte further comprises a sulfuric acid aqueous solution. Thus, the low pH of the electrolyte can be maintained, the hydrolysis of +5 valence vanadium ions can be inhibited, the electrical conductivity of the electrolyte can be increased, and the energy efficiency of the battery can be improved.

[0016] According to an embodiment of the present application, the sulfuric acid aqueous solution has a molar concentration of 2 mol / L to 4 mol / L. Thus, the low pH of the electrolyte can be maintained, the hydrolysis of +5 valence vanadium ions can be inhibited, the electrical conductivity of the electrolyte can be increased, and the energy efficiency of the battery can be improved.

[0017] In a second aspect of the present application, a method for preparing the positive electrode electrolyte of the first aspect is provided. According to an embodiment of the present application, the method comprises: performing a first mixing treatment on a vanadium source solution and an additive, and electrolysis, to obtain the positive electrode electrolyte, wherein the vanadium source solution comprises +4 valence vanadium ions, and the additive comprises a carbon material having at least one of a nitro group, a sulfonic acid group, a carbonyl group, an amino group, a carboxyl group, and a hydroxyl group on at least part of the surface. Thus, the positive electrode electrolyte prepared by the method can reduce the probability of +5 valence vanadium ions forming precipitates, improve the stability of the electrolyte, and improve the cycle stability of the battery.

[0018] According to an embodiment of the present application, the method for preparing the positive electrode electrolyte can further comprise at least one of the following additional technical features:

[0019] According to an embodiment of the present application, the +4 valence vanadium ions have a molar concentration of no less than 1.5 mol / L. Thus, the concentration of +5 valence vanadium ions in the positive electrode electrolyte after charging can be increased, and the energy density of the battery can be improved.

[0020] According to the embodiment of the present application, the molar concentration of the +4 valence vanadium ions is 1.5 mol / L to 2.5 mol / L. Therefore, the concentration of the +5 valence vanadium ions in the positive electrolyte after charging is increased, and the energy density of the battery is increased.

[0021] According to the embodiment of the present application, the current of the electrolysis treatment is 100 mA to 1000 mA. Therefore, the conversion rate of the +4 valence vanadium ions to the +5 valence vanadium ions can be increased.

[0022] According to the embodiment of the present application, the time of the electrolysis treatment is 1 h to 10 h. Therefore, the conversion degree of the +4 valence vanadium ions to the +5 valence vanadium ions can be ensured.

[0023] A full vanadium redox flow battery is provided in the third aspect of the present application. The full vanadium redox flow battery has all the features and advantages of the positive electrolyte described above, which will not be repeated here. In general, at least a higher energy density and cycle life are provided.

[0024] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0025] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, including the appended drawings.

[0026] Figure 1 is a schematic diagram of the state of the +5 valence vanadium ions in an aqueous solution according to the embodiment of the present application;

[0027] Figure 2 is a UV-Vis spectrum detection diagram of vanadium electrolyte with different additives added according to the embodiment of the present application;

[0028] Figure 3 is a UV-Vis spectrum detection diagram of vanadium electrolyte with different concentrations of carbon nanotubes with carboxyl groups added according to the embodiment of the present application;

[0029] Figure 4 is a UV-Vis spectrum detection diagram of the addition of carbon nanotubes with carboxyl groups to electrolyze vanadium electrolyte with different concentrations of +4 valence vanadium ions. DETAILED DESCRIPTION

[0030] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0031] For the sake of brevity, this article only discloses some specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0033] In this document, it should be understood that the terms “center,” “longitudinal,” “lateral,” “length,” “width,” “thickness,” “upper,” “lower,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” “outer,” “clockwise,” “counterclockwise,” “axial,” “radial,” and “circumferential” indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0034] In this document, the terms “comprising” or “including” are open-ended expressions, meaning that they include the contents specified in this application but do not exclude other contents.

[0035] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this application, unless otherwise stated, "multiple" means two or more.

[0036] If there is no special indication, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0037] The all-vanadium redox flow battery, also known as vanadium battery, is a kind of liquid flow battery, which is a liquid redox renewable battery taking metal vanadium ions as active substances. The all-vanadium redox flow battery takes vanadium ion solution in +4 and +5 valence states as the active substance of the positive electrode and takes vanadium ion solution in +2 and +3 valence states as the active substance of the negative electrode, which are respectively stored in the electrolyte storage tank. When the battery is charged and discharged, the positive and negative electrolytes undergo oxidation and reduction reactions on both sides of the ion exchange membrane. At the same time, through the action of the pump outside the battery, the electrolyte in the storage tank is continuously sent into the positive and negative chambers to maintain the ion concentration and realize the charging and discharging of the battery.

[0038] The vanadium battery stores electrical energy in the form of chemical energy in the sulfuric acid electrolyte of vanadium ions in different valence states, and the electrolyte is pressed into the battery stack by an external pump. Under the action of mechanical power, it circulates in the closed loop of different storage tanks and half cells. Proton exchange membrane is used as the separator of the battery pack. The electrolyte solution flows parallelly over the electrode surface and undergoes electrochemical reaction. The current is collected and conducted through the double electrode plate, so that the chemical energy stored in the solution is converted into electrical energy. This reversible reaction process enables the vanadium battery to successfully complete charging, discharging and recharging. The positive electrolyte is composed of vanadium ion solution in +4 and +5 valence states, and the negative electrolyte is composed of vanadium ion solution in +2 and +3 valence states. After the battery is charged, the positive electrode is +5 valence state vanadium ion solution, and the negative electrode is +2 valence state vanadium ion solution. After the battery is discharged, the positive and negative electrodes are +4 valence and +3 valence vanadium ion solutions respectively. H + Conductivity. Vanadium ions in +5 and +4 valence states exist in the form of VO2 + ions and VO 2+ ions in acidic solution, so the positive and negative reactions of the vanadium battery can be expressed as follows:

[0039] Positive electrode during charging: VO 2+ +H2O→VO2 + +2H + +e -

[0040] Negative electrode during charging: V 3+ +e - →V 2+

[0041] Positive electrode during discharging: VO2 + +2H + +e -→ VO 2+ + H2O

[0042] Discharge negative electrode: V 2+ → V 3+ + e -

[0043] The transition of vanadium ions of different valence states is the basis for the charge and discharge cycle of the all-vanadium redox flow battery, and the solubility of the four ions involved in the charge and discharge cycle of vanadium ions is not the same. The +5 valence state of vanadium ion is an important component of the positive electrolyte, and the mutual transformation between the +5 valence state of vanadium ion and the +4 valence state of vanadium ion is the prerequisite for the smooth progress of the charge and discharge cycle. However, the solubility of +5 valence state of vanadium ion at room temperature is not as high as that of other valence states, and the stability and solubility of +5 valence state of vanadium ion are directly related to the capacity and stability of the all-vanadium redox flow battery, so improving the stability and solubility of the +5 valence state of vanadium ion can improve the stability of the positive electrolyte, and thus obtain a more stable all-vanadium redox flow battery system. The related technology improves the electrolyte, for example, during the preparation of the positive electrolyte, acetonitrile is used as an additive, which can effectively improve the stability of the electrolyte, but acetonitrile has strong toxicity, which limits its application in vanadium batteries.

[0044] The application provides a positive electrolyte, which comprises a carbon material, at least part of the surface of the carbon material comprises at least one modification group of a nitro group, a sulfonic acid group, a carbonyl group, an amino group, a carboxyl group and a hydroxyl group, the carbon material can not only improve the dispersibility of +5 valence state of vanadium ion in the positive electrolyte, but also can prevent +5 valence state of vanadium ion from agglomerating and precipitating, increase the stability of the electrolyte, and improve the energy density and cycle stability of the battery.

[0045] Positive electrolyte

[0046] Therefore, the application provides a positive electrolyte. According to an embodiment of the application, the positive electrolyte comprises: an active substance, the active substance comprises +5 valence state of vanadium ion; and an additive, the additive comprises a carbon material, at least part of the surface of the carbon material has a modification group, and the modification group comprises at least one of a nitro group, a sulfonic acid group, a carbonyl group, a carboxyl group and a hydroxyl group.

[0047] It should be noted that the main component of the positive electrolyte of the all-vanadium redox flow battery in the application is a vanadium ion (vanadium oxygen ion)-sulfuric acid system, wherein the +5 valence state of vanadium ion exists in the form of VO2 + , and the +4 valence state of vanadium ion exists in the form of VO 2+ .

[0048] The +5 valence state of vanadium ion exists in the form of VO2 + , although VO2 + has a positive charge, but VO2 +There is no repulsion between them because VO2 + There are van der Waals forces and covalent bonds within V₂O₅ between them, and these forces are stronger than the repulsive forces between them. Therefore, VO₂ + They attract each other, leading to aggregation, which in turn causes VO2 to... + The phenomenon of hydrolysis transforming into V2O5 precipitate. In addition, VO2... + It undergoes solvation in aqueous solution, binding with water molecules, thereby increasing VO2. + Size increases (e.g.) Figure 1 As shown), the size is comparable to that of the additive, so the additive can be adsorbed onto its surface. This application utilizes the additive added to the positive electrode electrolyte to reduce VO2... + Solvation binds water molecules, increasing their radius. Additives with negatively charged surface groups can adsorb onto positively charged VO2. + The surface allows VO2 containing additives to be adsorbed. + The surfaces, being negatively charged, repel each other, thus reducing the VO2 content. + The mutual attraction between them improves the dispersibility of +5 vanadium ions in the positive electrode electrolyte and reduces VO2. + The probability of precipitation due to aggregation is reduced, thus improving the stability of the positive electrode electrolyte and enhancing the cycle capacity retention and energy density of the vanadium redox flow battery.

[0049] In some embodiments of this application, the mass ratio of the carbon material to the +5-valent vanadium ions is 1:(3-80). For example, it can be 1:3, 1:4, 1:5, 1:6, 1:8, 1:10, 1:16, 1:17, 1:27, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80, etc., or any range of the above values. By keeping the mass ratio of carbon material to +5-valent vanadium ions within the above range, the content of carbon material can be adjusted according to the content of +5-valent vanadium ions in the positive electrode electrolyte, thereby improving the adsorption effect of +5-valent vanadium ions and increasing the concentration of VO2+ in the positive electrode electrolyte. + The surface of vanadium ions in the form of +5 valence ions has a negative charge, VO2 + They repel each other, reducing VO2. + The phenomenon of hydrolysis producing precipitation increases VO2. + The solubility of the carbon material improves the stability of the positive electrode electrolyte. In some embodiments of this application, the mass ratio of the carbon material to the +5 vanadium ions is 1:(5-30).

[0050] In some embodiments of the present application, the mass percentage of the carbon material can be 0.1% to 2% based on the total mass of the positive electrolyte. For example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, or a range formed by any of the above values. By making the content of the carbon material in the above range, the carbon material can interact with VO2+ + in the positive electrolyte, and be adsorbed on the surface of VO2+ + , so that VO2+ + can be uniformly dispersed in the positive electrolyte. At the same time, the content of the additive that does not undergo adsorption in the positive electrolyte is reduced, the probability of its adhesion to the electrode pores is reduced, and the influence on the charge and discharge efficiency of the battery is reduced. In some embodiments of the present application, the mass percentage of the carbon material can be 0.3% to 1% based on the total mass of the positive electrolyte.

[0051] In some embodiments of the present application, the molar concentration of the +5 valence vanadium ion in the positive electrolyte is not less than 0.9 mol / L. For example, it can be 0.9 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, or 3 mol / L, or a range formed by any of the above values. In this way, the molar concentration of the +5 valence vanadium ion in the positive electrolyte is increased, and the energy density and cycle life of the battery are improved.

[0052] In some embodiments of the present application, the molar concentration of the +5 valence vanadium ion in the positive electrolyte is 0.9 mol / L to 1.5 mol / L. For example, it can be 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or a range formed by any of the above values. By making the concentration of the +5 valence vanadium ion in the above range, both a larger output current and normal electrochemical reaction can be ensured.

[0053] It should be noted that when the molar concentration of +5 valence vanadium ions in the positive electrolyte is 0.9-1.5 mol / L, the addition of carbon material can reduce the probability of precipitation of the electrolyte and the electrode during the charging and discharging process. This is because the +5 valence vanadium ions in this concentration range provide sufficient reactants to enable the positive and negative reactions to proceed smoothly, and the added carbon material can effectively adsorb the +5 valence vanadium ions in this concentration range, thereby improving the stability of the +5 valence vanadium ions and reducing the probability of precipitation. In order to avoid the occurrence of precipitation, the theoretical concentration of +5 valence vanadium ions is generally set to be within the range of 0.9-1.5 mol / L (calculated by a +4 valence vanadium ion conversion rate of 60%).

[0054] In some embodiments of the present application, the carbon material includes at least one of carbon nanotubes, superconducting carbon, graphene, acetylene black, carbon black, ketjen black, and carbon nanofibers. Thus, the above-mentioned types of carbon materials do not react with vanadium ions or other substances in the electrolyte, and after being modified by electronegative groups, they still have electronegativity in an aqueous solution environment and can exist stably in the aqueous solution environment, thereby being able to adsorb on the surface of +5 valence vanadium ions to inhibit their agglomeration and precipitation.

[0055] In some embodiments of the present application, the modification group includes at least one of a carboxyl group, a hydroxyl group, a carbonyl group, and a sulfonic acid group. Thus, the above-mentioned types of modification groups can make the carbon material electronegative and enable the carbon material to adsorb on the surface of +5 valence vanadium ions to increase the repulsive force between +5 valence vanadium ions, reduce their agglomeration and precipitation, and improve the stability of the electrolyte and the energy density of the battery.

[0056] In some embodiments of the present application, the carbon material includes at least one of the carbon nanotubes having at least part of the surface with the carboxyl group, the carbon nanotubes having at least part of the surface with the hydroxyl group, the carbon nanotubes having at least part of the surface with the carbonyl group, and the carbon nanotubes having at least part of the surface with the sulfonic acid group. Thus, the above-mentioned types of carbon materials can interact with the +5 valence vanadium ions with positive charges in the positive electrolyte under the action of the modification group, adsorb on the surface of the +5 valence vanadium ions, cause mutual repulsion between the +5 valence vanadium ions, prevent their agglomeration and precipitation, improve the dispersibility of the +5 valence vanadium ions in the positive electrolyte and the stability of the positive electrolyte, and further improve the electrochemical activity and cycle stability of the battery.

[0057] In some embodiments of the present application, the carbon material includes the carbon nanotube having the carboxyl group on at least part of the surface. Thus, the carbon nanotube having the carboxyl group can interact with the +5 valence vanadium ion with positive charge in the positive electrolyte under the action of the carboxyl group, be adsorbed on the surface of the +5 valence vanadium ion, cause mutual repulsion between the +5 valence vanadium ions, prevent the +5 valence vanadium ions from agglomerating to form a precipitate, improve the dispersibility of the +5 valence vanadium ions in the positive electrolyte and the stability of the positive electrolyte, and further improve the electrochemical activity and cycle stability of the battery.

[0058] In some embodiments of the present application, the positive electrolyte further includes a sulfuric acid aqueous solution. The electrolyte matrix of the all-vanadium redox flow battery is generally a sulfuric acid aqueous solution, which functions to maintain a low pH of the electrolyte, inhibit the hydrolysis of vanadium ions, and increase the conductivity of the electrolyte, reduce ohmic polarization, and improve the energy efficiency of the battery.

[0059] In some embodiments of the present application, the sulfuric acid aqueous solution can have a molar concentration of 2 mol / L to 4 mol / L. For example, the molar concentration can be 2 mol / L, 2.2 mol / L, 2.4 mol / L, 2.5 mol / L, 2.6 mol / L, 2.8 mol / L, 3 mol / L, 3.2 mol / L, 3.4 mol / L, 3.5 mol / L, 3.6 mol / L, 3.8 mol / L, 4 mol / L, or a range formed by any of the above values. Thus, the molar concentration of the sulfuric acid aqueous solution in the range can balance the relationship between the viscosity, conductivity, and electrochemical activity of the electrolyte, enable the best battery performance and cycle stability, and improve the energy efficiency of the battery.

[0060] Method for preparing positive electrolyte

[0061] The present application provides a method for preparing a positive electrolyte. According to an embodiment of the present application, the method includes: performing a first mixing treatment on a vanadium source solution and an additive, and electrolyzing to obtain the positive electrolyte, the vanadium source solution including +4 valence vanadium ions, and the additive including a carbon material having a modification group on at least part of the surface, the modification group including at least one of a nitro group, a sulfonic acid group, a carbonyl group, an amino group, a carboxyl group, and a hydroxyl group.

[0062] In the method described in the present application, the +4 valence vanadium ions in the vanadium source solution are converted into +5 valence vanadium ions after electrolysis, wherein the +5 valence vanadium ions are in the form of VO2 +The carbon material with electronegative modification group can interact with the +5 valence vanadium ions in the positive electrolyte and be adsorbed on the surface of the +5 valence vanadium ions by adding the carbon material in the electrolyte, so as to enhance the mutual repulsion force between the +5 valence vanadium ions, make the +5 valence vanadium ions uniformly dispersed in the positive electrolyte, reduce the phenomenon of aggregation and precipitation of the +5 valence vanadium ions, improve the stability of the electrolyte, and improve the cycle stability and capacity retention of the all-vanadium redox flow battery.

[0063] In some embodiments of the present application, the molar concentration of the +4 valence vanadium ions in the vanadium source solution is not less than 1.5 mol / L. For example, it can be 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, or 4 mol / L, etc., or can be a range consisting of any of the above values. In this way, sufficient +4 valence vanadium ions can provide sufficient reaction materials, so that after electrolysis, sufficient +5 valence vanadium ions can be formed, thereby improving the energy density of the battery.

[0064] In some embodiments of the present application, the molar concentration of the +4 valence vanadium ions can be 1.5 mol / L to 2.5 mol / L. For example, it can be 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2.0 mol / L, 2.1 mol / L, 2.2 mol / L, 2.3 mol / L, 2.4 mol / L, 2.5 mol / L, etc., or can be a range consisting of any of the above values. By making the +4 valence vanadium ions in this range, the +4 valence vanadium ions can provide sufficient reaction materials, so that after electrolysis, sufficient +5 valence vanadium ions can be formed, thereby improving the energy density of the battery.

[0065] In some embodiments of the present application, the electrolysis is carried out under the condition of 100 mA to 1000 mA for 1 h to 10 h. Therefore, the conversion of +4 valence vanadium ions to +5 valence vanadium ions can be promoted, and the normal occurrence of electrochemical reaction is ensured.

[0066] In some embodiments of the present application, the electrolysis can be carried out under the condition of 100 mA, 200 mA, 300 mA, 400 mA, 500 mA, 600 mA, 700 mA, 800 mA, 900 mA, 1000 mA, etc., or can be a range consisting of any of the above values. Therefore, the conversion of +4 valence vanadium ions to +5 valence vanadium ions can be promoted, and the normal occurrence of electrochemical reaction is ensured.

[0067] In some embodiments of the present application, the electrolysis time can be, for example, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, etc., or can be a range consisting of any of the above values. Thus, the degree of conversion of +4 valence vanadium ions to +5 valence vanadium ions can be ensured, and the normal occurrence of electrochemical reactions can be ensured.

[0068] In some embodiments of the present application, the vanadium source solution is obtained by subjecting a compound containing +4 valence vanadium ions to a second mixing treatment with an aqueous sulfuric acid solution, and subjecting the second mixing treatment product to a heating treatment to obtain the vanadium source solution. The +4 valence vanadium ions, as a positive electrode electrolyte active material, participate in the redox reaction of the battery, and under the action of an external power source, the conversion of +4 valence vanadium ions to +5 valence vanadium ions is realized. The aqueous sulfuric acid solution serves as an electrolyte for conducting vanadium ions.

[0069] In some embodiments of the present application, the compound containing +4 valence vanadium ions includes, but is not limited to, vanadyl sulfate (VOSO4) or vanadyl dichloride (VOCl2), etc., wherein the +4 valence vanadium ions exist in the form of VO 2+ in the positive electrode electrolyte.

[0070] In some embodiments of the present application, the molar concentration of the aqueous sulfuric acid solution can be 2 mol / L to 4 mol / L. For example, it can be 2 mol / L, 2.2 mol / L, 2.4 mol / L, 2.5 mol / L, 2.6 mol / L, 2.8 mol / L, 3 mol / L, 3.2 mol / L, 3.4 mol / L, 3.5 mol / L, 3.6 mol / L, 3.8 mol / L, 4 mol / L, etc., or can be a range consisting of any of the above values. The concentration of the aqueous sulfuric acid solution will affect the viscosity, conductivity and electrochemical activity of the electrolyte by changing the concentration of hydrogen ions in the electrolyte. Therefore, the molar concentration of the aqueous sulfuric acid solution in this range can balance the relationship between the viscosity, conductivity and electrochemical activity of the electrolyte, and the best battery performance and cycle stability can be obtained.

[0071] In order to enable the compound containing +4 valence vanadium ions to be quickly and uniformly dissolved in the aqueous sulfuric acid solution, magnetic stirring is performed at the same time as the heating treatment. The heating treatment is performed at a temperature of 40-70°C, and the time of the heating treatment is 15-30 min.

[0072] In some embodiments of the present application, the heating treatment can be, for example, performed at a temperature of 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, etc., or can be a range consisting of any of the above values.

[0073] In some embodiments of the present application, the heating treatment time can be 15 min, 16 min, 17 min, 18 min, 19 min, 20 min, 21 min, 22 min, 23 min, 24 min, 25 min, 26 min, 27 min, 28 min, 29 min, 30 min, or the like, or can be a range consisting of any of the above values.

[0074] All-vanadium redox flow battery

[0075] The present application provides an all-vanadium redox flow battery. According to embodiments of the present application, the all-vanadium redox flow battery comprises the aforementioned positive electrolyte. As described above, the better stability of the positive electrolyte of the present application can not only reduce the side reactions of the electrolyte during charging and discharging, thereby prolonging the cycle life of the battery, but also help to improve the energy conversion efficiency of the battery. In addition, the increase in the stability of +5 valence vanadium ions in the positive electrolyte enables better transmission during charging and discharging, thereby helping to improve the energy efficiency and cycle stability of the battery, thereby enhancing the performance of the battery.

[0076] In some embodiments of the present application, the all-vanadium redox flow battery further comprises a negative electrolyte, a positive electrode, a negative electrode, and a separator.

[0077] In some embodiments of the present application, the negative electrolyte comprises an active substance, a base, and an additive, wherein the active substance comprises +2 valence vanadium ions and / or +3 valence vanadium ions, the base comprises but is not limited to an aqueous sulfuric acid solution, and the additive is not limited herein as long as it can promote the occurrence of overall oxidation and reduction and promote the transmission of electric charge.

[0078] In some embodiments of the present application, the positive electrode or negative electrode material comprises a metal type, a carbon type, or a composite material type, wherein the metal type can be selected from Pb, Ti, and the like; the carbon type can be selected from graphite, carbon cloth, carbon felt, and the like; and the composite material type can be selected from conductive polymers, polymer composites, and the like.

[0079] In some embodiments of the present application, the separator can be selected from an ion exchange membrane as long as it can inhibit the cross-mixing of vanadium ions of different valence states in the positive and negative electrolytes without hindering the passage of hydrogen ions through the separator to transfer electric charge.

[0080] The scheme of the present application will be explained below in conjunction with embodiments. Those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. If the specific technology or condition is not specified in the embodiments, it is performed according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument used is not specified by the manufacturer, it is a conventional product that can be obtained by purchase.

[0081] Example 1

[0082] Electrolyte preparation: A certain amount of VOSO4 was dissolved in 3.0 mol / L aqueous sulfuric acid solution to prepare a mixed solution with a +4 valence vanadium ion concentration of 2.5 mol / L. The mixed solution was placed on a magnetic stirrer, and a stirrer was placed in it. VOSO4 was dissolved in sulfuric acid under heating. After VOSO4 was fully dissolved, 0.8wt% of carbon nanotubes with carboxyl groups were added to the VOSO4 sulfuric acid solution and uniformly distributed under the action of magnetic stirring. Then it was transferred to the anode tank of the H-type electrolytic cell, and constant current electrolysis was carried out at a current density of 500 mA for 4 h. At this time, the anode will undergo an oxidation reaction, converting +4 valence vanadium ions to +5 valence vanadium ions, with a conversion rate of 60%, to prepare a positive electrolyte. The collected electrolyte was observed for precipitation. The electrolyte was diluted and tested with a UV-visible spectrophotometer to determine the valence state and concentration of vanadium ions from the position and intensity of the UV absorption peak.

[0083] Constant current electrolysis conditions: electrode: graphite felt electrode, area 1.5 cm x 3 cm; electrolytic cell: H type; proton exchange membrane: perfluorosulfonic acid membrane; constant current electrolysis current density 500 mA; electrolyte volume: 25 mL; electrolysis time: 4 h.

[0084] Example 2

[0085] The electrolyte was prepared according to the method described in Example 1, except that 0.8wt% of carbon nanotubes with hydroxyl groups were added in Example 2.

[0086] Example 3

[0087] The electrolyte was prepared according to the method described in Example 1, except that 0.8wt% of carbon nanotubes with carbonyl groups were added in Example 3.

[0088] Example 4

[0089] The electrolyte was prepared according to the method described in Example 1, except that 0.8wt% of sulfonated carbon nanotubes were added in Example 4.

[0090] Example 5

[0091] The electrolyte was prepared according to the method described in Example 1, except that 0.1wt% of carbon nanotubes with carboxyl groups were added in Example 5.

[0092] Example 6

[0093] The electrolyte was prepared according to the method described in Example 1, except that 1wt% of carbon nanotubes with carboxyl groups were added in Example 6.

[0094] Example 7

[0095] The electrolyte was prepared according to the method described in Example 1, except that 0.5wt% of the carbon nanotubes with carboxyl groups were added in Example 7.

[0096] Example 8

[0097] The electrolyte was prepared according to the method described in Example 1, except that 2wt% of the carbon nanotubes with carboxyl groups were added in Example 8.

[0098] Example 9

[0099] The electrolyte was prepared according to the method described in Example 1, except that 0.3wt% of the carbon nanotubes with carboxyl groups were added in Example 9.

[0100] Example 10

[0101] The electrolyte was prepared according to the method described in Example 1, except that a mixed solution with a +4 valence vanadium ion concentration of about 0.9 mol / L was configured in Example 10, so that the theoretical concentration of +5 valence vanadium ions reached about 0.5 mol / L (conversion rate was calculated as 60%).

[0102] Example 11

[0103] The electrolyte was prepared according to the method described in Example 1, except that a mixed solution with a +4 valence vanadium ion concentration of about 1.5 mol / L was configured in Example 11, so that the theoretical concentration of +5 valence vanadium ions reached about 0.9 mol / L (conversion rate was calculated as 60%).

[0104] Example 12

[0105] The electrolyte was prepared according to the method described in Example 1, except that a mixed solution with a +4 valence vanadium ion concentration of about 4 mol / L was configured in Example 12, so that the theoretical concentration of +5 valence vanadium ions reached about 2.5 mol / L (conversion rate was calculated as 60%).

[0106] Comparative Example 1

[0107] The electrolyte was prepared according to the method described in Example 1, except that no additive was added in Comparative Example 1, and the specific method was as follows:

[0108] The electrolyte formula: a certain amount of VOSO4 was dissolved in a 3.0 mol / L aqueous sulfuric acid solution, and a mixed solution with a V 4+ concentration of 2.5 mol / L was configured. The mixed solution was placed on a magnetic stirrer, and a stirring rod was placed in it. VOSO4 was dissolved in sulfuric acid under heating. After VOSO4 was fully dissolved, it was transferred to the anode tank of the H-type electrolytic cell, and constant current electrolysis was carried out at a current density of 500 mA for 4 h. At this time, oxidation reaction occurred at the anode, and V4+ Convert to V 5+ A positive electrode electrolyte was prepared. The collected electrolyte was examined for precipitation. After dilution, the electrolyte was tested using a UV-Vis spectrophotometer. The position and intensity of the UV absorption peaks were used to determine the valence state and concentration of vanadium ions.

[0109] Conditions for constant current electrolysis: Electrode: graphite felt electrode, area 1.5cm×3cm; Electrolytic cell: H type; Proton exchange membrane: perfluorosulfonic acid membrane; Constant current electrolysis current density 500mA; Electrolyte volume: 25mL; Electrolysis time: 4h.

[0110] Comparative Example 2

[0111] The electrolyte was prepared according to the method described in Example 1, except that 0.8 wt% of unmodified carbon nanotubes were added to Comparative Example 2.

[0112] The types, mass fractions, and concentrations of +5 vanadium ions of additives used in Examples 1-12 and Comparative Examples 1-2 are shown in Table 1.

[0113] Table 1

[0114]

[0115]

[0116] The method for calculating the mass ratio of additives to +5 vanadium ions is as follows:

[0117] (1) Calculate the actual mass of the additive in the electrolyte of the example based on its mass fraction;

[0118] (2) Based on the +5 valent vanadium ion (in VO2) + The concentration of vanadium ions (calculated using a +4 vanadium ion conversion rate of 60%) was calculated. The concentration of vanadium ions (in VO2) was also calculated. + The mass of the calculated vanadium (+5 valent ion concentration in each embodiment is shown in Table 1).

[0119] Performance testing

[0120] Ultraviolet-Vis Spectroscopy Detection

[0121] Figure 1 It represents VO2 + The way sulfuric acid combines with water molecules.

[0122] The characteristic peak position of +5 vanadium ions is at 270-290 nm, such as Figure 2As shown in the UV-Vis spectrum, after adding the carbon nanotube additive with a modified group, a very strong absorption peak appeared in this wavelength range, which corresponds to the +5 valence vanadium ion in the electrolyte, indicating that the concentration of +5 valence vanadium ion in the solution is much higher than that of the electrolyte without adding the additive (Comparative Example 1). The carbon material additive with a modified group promotes the solubility of +5 valence vanadium ion in acid.

[0123] A series of carbon nanotube materials with different groups were added to the electrolyte, including carbon nanotubes with carboxyl groups, carbon nanotubes with hydroxyl groups, carbon nanotubes with carbonyl groups, sulfonated carbon nanotubes, and carbon nanotubes (CNT) without groups. The results are shown in Figure 2 , which shows that the group-modified CNT greatly helps to increase the solubility of pentavalent vanadium ions in sulfuric acid.

[0124] Among them, the concentration of pentavalent vanadium ions in the vanadium electrolyte with carboxyl-containing carbon nanotubes is the highest, corresponding to Figure 2 with the strongest UV absorption peak. Therefore, the subsequent study continued to explore the effect of the amount of carboxyl-containing carbon nanotubes added to the vanadium electrolyte on the concentration of vanadium ions. From Figure 3 , it can be seen that the amount of carbon material with modified carboxyl groups added to the electrolyte has a certain regularity on the concentration of +5 valence vanadium ions. With the increase of the amount of carboxyl-containing carbon nanotubes, the concentration of +5 valence vanadium ions in the electrolyte first increases and then decreases, indicating that only when the amount of carbon material with modified groups is within a certain range can it improve the concentration of +5 valence vanadium ions in the electrolyte. The concentration range is 0.3-2.0wt%, and the effect of improving the concentration of +5 valence vanadium ions is most obvious at 0.8wt%.

[0125] The effect of carboxyl-containing carbon nanotubes on the conversion of pentavalent vanadium ions in different concentrations of tetravalent vanadium ion electrolyte environment was further explored. The concentration of pentavalent vanadium ions in electrolytes with different concentrations was tested, and the results are shown in Figure 4 , which shows that the concentration of pentavalent vanadium ions in electrolytes with tetravalent vanadium ion concentrations of 2.5M and 4.0M is not much different, indicating that when the total vanadium concentration in the solution is above 2.5M, the concentration of pentavalent vanadium ions in the electrolyte is not very obvious. And when the concentration of tetravalent vanadium ions is 4.0M, the concentration of +5 valence vanadium ions after electrolysis is too high, even with the addition of carbon nanotube additives, precipitation will occur. In order to avoid the formation of precipitation and affect the cycle performance of the battery, the concentration of tetravalent vanadium ions is generally set to 1.5mol / L-2.5mol / L (the concentration of pentavalent vanadium ions is 0.9mol / L-1.5mol / L).

[0126] The experimental results show that the addition of the modified carbon nanotubes to the electrolyte can improve the concentration of the pentavalent vanadium ions in the electrolyte to different degrees. The addition of 0.8wt% of the carbon nanotubes containing carboxyl groups has the most obvious effect. Within the total vanadium concentration of 2.5M, the additive can help to improve the concentration of the pentavalent vanadium ions in the positive electrolyte to a certain degree. The development of the application is conducive to improving the energy density of the all-vanadium redox flow battery.

[0127] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

[0128] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. Furthermore, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

Claims

1. A positive electrode electrolyte, characterized in that, include: The active substance includes vanadium ions in the +5 valence state; An additive comprising a carbon material having at least a portion of its surface having modifying groups, the modifying groups including at least one of nitro, sulfonic acid, carbonyl, carboxyl, and hydroxyl groups.

2. The positive electrode electrolyte according to claim 1, characterized in that, The mass ratio of the carbon material to the +5 vanadium ions is 1:(3-80).

3. The positive electrode electrolyte according to claim 2, characterized in that, Based on the total mass of the positive electrode electrolyte, the carbon material accounts for 0.1% to 2% of the total mass.

4. The positive electrode electrolyte according to claim 2, characterized in that, The molar concentration of the +5 vanadium ions in the positive electrode electrolyte is not less than 0.9 mol / L; optionally, the molar concentration of the +5 vanadium ions in the positive electrode electrolyte is 0.9 mol / L to 1.5 mol / L.

5. The positive electrode electrolyte according to any one of claims 1-4, characterized in that, The carbon material includes at least one of carbon nanotubes, superconducting carbon, graphene, acetylene black, carbon black, Ketjen black, and carbon nanofibers. And / or, the modifying group includes at least one of carboxyl, hydroxyl, carbonyl, and sulfonic acid groups.

6. The positive electrode electrolyte according to any one of claims 1-4, characterized in that, The carbon material includes at least one of the following: carbon nanotubes having at least a portion of the carboxyl group on their surface; carbon nanotubes having at least a portion of the hydroxyl group on their surface; carbon nanotubes having at least a portion of the carbonyl group on their surface; and carbon nanotubes having at least a portion of the sulfonic acid group on their surface.

7. The positive electrode electrolyte according to any one of claims 1-4, characterized in that, It further includes an aqueous solution of sulfuric acid; optionally, the molar concentration of the aqueous solution of sulfuric acid is 2 mol / L to 4 mol / L.

8. A method for preparing the positive electrode electrolyte according to any one of claims 1-7, characterized in that, include: The vanadium source solution and the additive are first mixed and then electrolyzed to obtain the positive electrode electrolyte. The vanadium source solution includes vanadium ions in the +4 valence state, and the additive includes a carbon material. At least a portion of the surface of the carbon material has a modifying group, which includes at least one of nitro, sulfonic acid, carbonyl, amino, carboxyl, and hydroxyl groups.

9. The method according to claim 8, characterized in that, In the vanadium source solution, the molar concentration of the +4 vanadium ions is not less than 1.5 mol / L, and optionally, the molar concentration of the +4 vanadium ions is 1.5 mol / L to 2.5 mol / L.

10. The method according to claim 9, characterized in that, The electrolysis current is 100mA to 1000mA; And / or, the electrolysis time is 1h to 10h.

11. A vanadium redox flow battery, characterized in that, Includes the positive electrode electrolyte as described in any one of claims 1-7.

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