Vanadium redox flow battery (VRFB) is a type of liquid redox rechargeable battery with vanadium ions as the active material. The system mainly consists of an electrochemical cell stack, electrolyte, and other components. The electrolyte is a key material for vanadium batteries. Unlike lithium-ion batteries, the electrolyte in vanadium batteries directly serves as the positive and negative electrodes of the battery system.

The positive electrode electrolyte of the vanadium battery is a sulfuric acid solution containing tetravalent and pentavalent vanadium ions, while the negative electrode electrolyte is a sulfuric acid solution containing divalent and trivalent vanadium ions. During the charging and discharging of the battery, the positive and negative electrode electrolytes undergo redox reactions on the two sides of the ion exchange membrane. As the energy storage medium, the volume and concentration of the electrolyte determine the maximum energy that the vanadium redox flow battery energy storage system can store. Moreover, factors such as the purity of the electrolyte (which generally needs to be above 99.9%), stability, and applicable temperature range will also have a significant impact on the operating efficiency and lifespan of the vanadium redox flow battery. Therefore, the development and preparation capabilities of the electrolyte are the key breakthrough points for the performance improvement of vanadium redox flow batteries. The preparation process of the electrolyte has high requirements for the control of impurities and valence states. At present, the main preparation methods of vanadium electrolyte include physical method, chemical reduction method, and electrolysis method. The physical dissolution method involves directly dissolving high-purity VOSO4 solid in sulfuric acid to obtain the electrolyte. The chemical reduction method uses reducing agents (such as hydrogen, sulfur, alcohols, etc.) to reduce high-valence vanadium oxides or vanadates to prepare the electrolyte. The electrolysis method can be further divided into direct electrolysis and indirect electrolysis. Direct electrolysis involves dissolving V2O5 in sulfuric acid and feeding it into the negative electrode of an electrolytic cell. After electrification, a reduction reaction occurs at the negative electrode, yielding equimolar amounts of trivalent and tetravalent vanadium ions. The reaction principle is as follows:

Indirect electrolysis, on the other hand, involves using a reducing substance (such as oxalic acid, sulfur dioxide, etc.) to react with V2O5 dissolved in sulfuric acid through a redox reaction to obtain a VO2+ solution, which is then fed into the negative electrode for further electrolysis to produce equimolar amounts of trivalent and tetravalent vanadium ions. Both methods have the characteristics of high energy consumption and slow kinetic processes. Therefore, researchers are constantly optimizing the preparation process and technology of vanadium electrolyte.

At present, there have been reports of optimized electrolyte preparation methods, including directly electrolyzing high-purity VOCl3 as the vanadium precursor and catalytically reducing VOSO4 as the precursor to obtain equimolar amounts of trivalent and tetravalent vanadium ions. Each method has its own characteristics and advantages. Electrolysis can continuously produce a large amount of high-concentration vanadium electrolyte, is simple to operate, and is easy to industrialize. However, it also has disadvantages such as slow rate, high equipment requirements, high energy consumption, and high cost. The article "Technical Analysis of Electrolysis for Vanadium Electrolyte Preparation—Taking Patents on Electrolysis for Vanadium Electrolyte Preparation as an Example" briefly outlines the patent situations of relevant companies using electrolysis to prepare vanadium electrolyte for all-vanadium redox flow batteries, helping readers gain a deeper understanding of the technology of electrolysis for vanadium electrolyte preparation. In general, both chemical reduction and electrolysis have their pros and cons, and the choice should be made based on specific circumstances.

Electrolyte is the largest cost source of vanadium batteries, accounting for more than 50%. Reducing the cost of vanadium electrolyte can effectively reduce the cost of vanadium battery energy storage systems. The utilization rate of vanadium electrolyte still needs to be further improved. The volume and concentration of the electrolyte determine the maximum energy that the all-vanadium redox flow battery energy storage system can store. V2O5 accounts for more than 80% of the cost of the electrolyte. Theoretically, 5.6 kg of V2O5 is needed to store 1 kWh of electrical energy. However, the actual utilization rate of the electrolyte is currently only about 70%, and the amount of V2O5 used is about 8 kg. Improving the utilization rate of the electrolyte will reduce the amount of V2O5 used and, in turn, reduce investment costs. After the vanadium battery reaches the end of its life, the vanadium electrolyte can be recycled and reused. Leasing electrolyte can effectively reduce the cost of the electrolyte. If a closed-loop industry of manufacturing, using, and recycling can be established, the residual value of the electrolyte can be further increased, and the cost of the vanadium battery over its entire life cycle can be significantly reduced.

Since the preparation process requires consideration of factors such as electrolyte proportion, temperature, and additives to achieve high-efficiency production of high-performance vanadium electrolyte, the industry has high barriers to entry. The vanadium electrolyte production equipment independently developed by ZH Energy is deeply integrated with the company's leading stack and electrolyte control technologies, which greatly enhances the operational stability of the stack, effectively ensures the service life of the stack, and reduces the costs of equipment maintenance and replacement. It has successfully delivered a production line project for 60,000 cubic meters of all-vanadium redox flow battery electrolyte per year to customers, providing a highly competitive technical solution for the vanadium electrolyte preparation industry.

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