The Use of Reductive Agents for Developing Capacity Balanced Aqueous Sodium‐Ion Batteries

Abstract Aqueous sodium‐ion batteries (Na‐ion batteries) represent an attractive low cost stationary energy storage technology. However, limited electrochemical stability of water and issues related to complex aqueous chemistry, component corrosion, and materials dissolution limit their lifetime. Oxygen reduction reaction is considered to be the main parasitic process in aqueous Na‐ion batteries leading to self‐discharge, materials degradation and Na‐ion inventory loss. Herein, we present a comprehensive study on novel electrolyte design which allows to mitigate the main parasitic reactions and prepare capacity balanced (N/P=1) aqueous battery cells. For this purpose, aqueous symmetric Na 2 VTi(PO 4 ) 3 |Na 2 VTi(PO 4 ) 3 system is used as a model to demonstrate that the introduction of a small concentration of strongly reducing agent such as hydrazine into purely aqueous or hybrid (water and dimethyl sulphoxide) electrolyte could chemically reduce the dissolved oxygen and significantly improve the capacity retention of N/P=1 cells during cycling. The low concentrations and self‐consuming nature of hydrazine in a closed cell do not pose any health or chemical risks and presents a viable strategy for practical design. We believe, these results to be applicable not only to aqueous Na‐ion batteries but also other aqueous or hybrid battery chemistries where such parasitic processes play a significant role.