Water‐in‐Salt Electrolyte (WiSE) for Aqueous Batteries: A Long Way to Practicality

Abstract The sustainability of battery components is becoming a key parameter for storing renewable energy at large scale. Toward that goal, several strategies are currently being explored. Great hopes are placed in the use of superconcentrated aqueous electrolytes, which enlarge the electrochemical stability window well beyond 1.2 V. Although fundamentally elegant, the practicability of such an approach remains unknown. Therefore, an in‐depth analysis of the stability and cycling behavior of water‐in‐salt (WiSE) and water‐in‐bisalt (WiBS) electrolytes as a function of concentration and temperature is carried out by monitoring via combined operando gas monitoring, cyclic voltammetry, and self‐discharge experiments the solid electrolyte interphase (SEI) growth and stability. The SEI formed on the negative electrode is found to not be able to protect the battery against continuous electrolyte degradation through water reduction during both cycling and storage; this inefficiency is increased at elevated temperatures. This result contrasts with the less severe impact of water oxidation. The data are benchmarked against other commercial batteries. It is shown that WiSE‐based batteries in their current form cannot compete with lead‐acid, Ni‐Cd or nickel‐metal hydride commercial aqueous batteries in terms of price, operating temperature range, lifetime, and capacity fading upon storage. So the practical outcome of the superconcentrated aqueous electrolyte remains highly uncertain.