Improved broad temperature adaptability and energy density of vanadium redox flow battery based on sulfate-chloride mixed acid by optimizing the concentration of electrolyte

In order to improve the energy density and broad temperature adaptability of vanadium redox flow battery based on sulfate-chloride mixed acid electrolyte, the stability and charge/discharging behavior of electrolytes at a broad temperature range (-20-50 °C) are investigated systematically. The static stability tests of vanadium ions show that the 2.4 M vanadium concentration can keep stable for 10 days with the electrolyte composition of chloride ion concentration is 6.0–7.0 M and sulfate concentration is 2.0–3.0 M. However, the static stability tests of chloride ions indicate that volatilization of hydrogen chloride can be avoided when chloride ion concentration is not exceeding 6.4 M. The cell performance of vanadium redox flow battery with optimized electrolyte compositions indicates that the sulfate-chloride mixed acid electrolyte can operate at a wider temperature range and the charge cut-off voltage must be less than 1.7 V to prevent the chlorine evolution. This work not only systematically optimizes the composition of mixed acid electrolyte, but also provides theoretical direction to the engineering application of vanadium redox flow battery at extreme environment.