Structure, Principle, and Development Status of Vanadium Batteries

Vanadium batteries were first proposed by Skyllas KazacosM in Australia in 1985. Sumitomo Corporation of Japan built a 450kW vanadium battery stack in 1996. As of the end of 2022, the cumulative installed capacity of global flow batteries (mainly vanadium flow batteries) is 274.2 MW, accounting for 0.6% of the world's new energy storage. Among them, the cumulative installed capacity of flow batteries in China is 157.2 MW, accounting for 1.2% of the country's new energy storage; Industrialization is still in its early stages. Due to the vast application potential of vanadium batteries in the field of long-term energy storage, governments around the world have introduced policies to actively support them. China has actively introduced detailed rules from various aspects such as overall planning, implementation details, and safety regulations to accelerate the industrialization process of vanadium batteries. It is expected that by 2025, the installed capacity of vanadium battery energy storage in China will reach 4GW. Vanadium flow batteries mainly include stacks (as power units), electrolytes (as energy units), electrolyte delivery systems, battery management systems, and energy storage inverters. The working principle of vanadium flow battery is to use an external pump to inject electrolytic water into the fuel cell. Under the action of mechanical power, the electrolyte circulates in a closed loop of different storage tanks and half cells, flows over the electrode surface and undergoes electrochemical reactions. Then, the dual electrode plate collects and conducts current, thereby converting the chemical energy stored in the solution into electrical energy. The stack and electrolyte of a vanadium flow battery are independent of each other, and the number and size of the stack affect the power of the vanadium battery. The volume and concentration of the electrolyte determine the energy storage capacity of the vanadium battery. The vanadium electrolyte has the highest cost proportion in the vanadium battery energy storage system. As the charging and discharging time increases, the electrolyte cost proportion of the battery also increases. In a 4-hour energy storage system, the electrolyte cost proportion reaches 50%. VRFB is generated by the change of valence states of all vanadium ions, resulting in current flow without the problem of cross contamination. As of now, the number of charge and discharge cycles of VRFB is over 200000. It has the advantages of high safety, good reliability, large energy storage capacity, long service life, recyclable electrolyte, and environmental friendliness, and has become one of the preferred technologies in the field of power grid peak shaving and backup power supply. However, VRFB has problems such as low vanadium concentration, low energy density, and narrow applicable temperature range. To promote the development of VRFB, address the shortcomings of vanadium batteries, and improve the performance of electrolytes is an important technological approach. By selecting materials with better performance, the preparation cost of the electrolyte has been reduced, the preparation process has been optimized, losses have been reduced, and the stability and electrochemical performance of the electrolyte have been improved. According to reports, the use of mixed acid electrolytes can effectively improve the drawbacks of vanadium batteries. Research status of VRFB supported electrolyte

Vanadium battery electrolyte is a key material for vanadium batteries. The electrolyte directly serves as the positive and negative electrodes of the battery system, consisting of vanadium salts in different valence states and supporting electrolytes. The electrolytes of the positive and negative electrodes of vanadium batteries exist independently in external storage tanks. The positive electrode electrolyte is an acidic solution containing V+O2 (V valent vanadium) and VO22+(IV valent vanadium), while the negative electrode electrolyte is an acidic solution containing V2+and V3+. The electrolyte of the all vanadium liquid battery is a high concentration vanadium containing solution obtained by mixing an acidic system with vanadium pentoxide. The main sources of V2O5 are vanadium titanium magnetite, vanadium extraction from coal, and waste catalyst raw materials. Currently, the mainstream vanadium extraction method in China is to extract V2O5 from vanadium slag generated during the steelmaking process of vanadium titanium magnetite (accounting for about 87% in 2020). The battery capacity and energy density of VRFB mainly depend on the concentration and volume of vanadium ions in the electrolyte, with higher vanadium ion concentration resulting in higher energy capacity. However, the increase in vanadium concentration is limited by the solubility of vanadium salts. For example, a vanadium ion concentration exceeding 2M in sulfuric acid solution below 40 ℃ can lead to the formation of V2O5 precipitates in the V+O2 electrolyte; At a low temperature of 5 ℃, V2+and V3+tend to precipitate, greatly reducing the capacity of the battery. Sediments are affected by conditions such as temperature, sulfuric acid solution, vanadium ion concentration, and electrolyte charge. Therefore, suitable conditions are crucial for the stability of the positive and negative electrode electrolytes. Many researchers mainly improve the stability of batteries from two aspects: electrolyte composition and additives. Adding some organic or inorganic chemical stabilizers to the electrolyte can improve the stability of the positive and negative electrode electrolytes to a certain extent. However, even with additives in the temperature range of 10-40 ℃, the vanadium concentration of most VRFB systems still needs to be below 2M. Moreover, excessive concentration of sulfuric acid can also affect the concentration of vanadium ions.