Chemical Hazard Assessment of Vanadium–Vanadium Flow Battery Electrolytes in Failure Mode

The growing demand for energy storage and the rising frequency of lithium ion battery failure events worldwide underscore the urgency of addressing the battery safety challenges. Ensuring the safe and reliable deployment of advanced battery technologies is paramount. Flow batteries present a promising solution for long-duration energy storage, yet their electrolytes pose potential hazards to human health and the environment. The largest scale vanadium–vanadium flow batteries have been reported in China, with a 100 MW/400 MWh system reportedly commissioned in 2022 and a 175 MW/700 MWh battery completed in December 2024. This is equivalent to 150–200 million liters of vanadium electrolyte. This study aims to assess the chemical hazards of the electrolytes in vanadium–vanadium flow battery during failure mode. There is little or no chemical hazard data for the electrolyte mixtures, and the hazard assessment was thus based on chemical reactivity and toxicity data for the individual electrolyte components. Potential failure modes are identified with overcharging (or high cell voltages) in particular presenting potential hazards due to the possible production of toxic gases. Depending on the electrolyte composition, these conditions could result in the evolution of Cl2, SO2, H2S, or PH3 gases, with immediate associated risks for human health. The two main all-vanadium flow battery chemistries use either sulfuric acid or sulfuric acid/HCl mixtures as the supporting electrolyte, with low concentrations of phosphoric acid often included in the sulfuric acid systems. The sulfuric acid–based cells generate oxygen and hydrogen at the positive and negative half-cell electrodes, respectively, during overcharge. On the other hand, the mixed H2SO4/HCl-based chemistries produce chlorine at the positive electrode during overcharge and potentially under normal charging conditions at high states-of-charge if the flow-rate is insufficient. Vanadium electrolytes containing chloride ions therefore present the most significant toxicity hazards in failure mode. The inherently safe design of battery management and control systems, along with electrolyte containment, is an essential measure to ensure safe flow battery operation. The next step involves designing controlled experiments to study overcharging effects on the electrolyte stability and degradation.