Electrochemical Reactions and Failure Mechanism Study of Sodium‐Aqueous Polysulfide Conversion Reactions in Redox Flow Batteries

Low‐cost aqueous polysulfide (APS) chemistry has recently attracted intensive research as it is a promising candidate for redox flow batteries. However, the intrinsic properties, electrochemical stability, and compatibility with sodium super ionic conductor (NASICON) membrane have been rarely demonstrated to clarify the challenges on the road to practical application. Herein, a detailed study on the electrochemical reactions and failure mechanism of aqueous polysulfides is presented. Starting from investigating the intrinsic stability, the optimized concentration of NaOH is determined. Through constructing a hybrid Na‐APS cell to avoid the effect from the counterelectrode, the cycling behavior is found to be stable under the capacity cut‐off condition, while severe decay occurs after ≈80 cycles. Comprehensive analysis of the decay mechanism shows the decomposition and loss of active materials during the long‐term cycles. Meanwhile, the NASICON separator surface is found to be corrosive after electrochemical exposure with APS solution under a high‐pH environment. It is anticipated that the systematic study of the APSs’ electrochemical reactions and decay mechanism could be beneficial for their further application in redox flow batteries.