Recent progress in ‘water-in-salt’ and ‘water-in-salt’-hybrid-electrolyte-based high voltage rechargeable batteries

Abstract

Meeting the increasing global energy demand has pushed the energy storage research towards developing energy dense and safer electrochemical energy storage devices following sustainable and economic approach. In this regard, past few years have witnessed a rising interest in aqueous electrolyte-based energy storage systems. However, the narrow voltage window of the aqueous electrolyte (1.23 V) limits the choice of electrodes as well as the energy density of traditional aqueous batteries. To overcome this challenge, there is a quest to improve the energy density of aqueous electrolytes by extending the electrochemical stability window. In this regard, a major breakthrough was recorded by switching from conventional ‘salt-in-water’ to ‘water-in-salt’ electrolyte. In “water-in-salt” (WiS) electrolytes, dissolved salts far outnumber water molecules (salt/solvent ratio > 1) by volume or weight. Unlike the conventional aqueous electrolyte (∼1.0 m), in the ‘water-in-salt’ (WiS) system, the interionic interaction becomes dominant over the solvent–ion interaction and the solvation sheath of the metal-ion charge carrier is dominated by contact ion pairs. Such systems with a suitable anion in the contact ion pair could form stable solid electrolyte interfaces at the anode and the reduced water activity expands the electrochemical stability window (ESW) to allow the coupling of a high voltage cathode with a low voltage anode to uplift the energy density. In addition, WiS as the electrolyte offers high safety, good ionic conductivity, high chemical compatibility, and thermal stability, which are crucial to compete with non-aqueous based electrolytes. In recent years, continuous research and development have found an application of this concept in ‘water-in-bisalt’, water-in-ionomer, hybrid aqueous/non-aqueous, and hydrate-melt electrolytes, pushing ESWs up to 4.1 V. In this review, we consolidate recent developments in aforementioned aqueous electrolytes with expanded ESWs for rechargeable batteries (metal-ion, metal–sulfur, and metal–air). The formulation of electrolytes, their physicochemical properties, and electrochemical performance in the different chemistries of batteries has been discussed. This review also addresses scientific and technical issues along with insights to boost the electrochemical performance of future aqueous electrolytes.