A perspective of ZnCl2 electrolytes: The physical and electrochemical properties

Molten ZnCl2 hydrates are ionic liquids at room temperature, which exhibit intriguing physical and electrochemical properties. Continuous efforts have been devoted over several decades to understanding the properties of the molten ZnCl2 hydrates that have been dubbed as water-in-salt electrolytes recently. The physical properties of molten ZnCl2 hydrates can be described from the perspectives of ions in their speciation and water molecules regarding their chemical environments. Recently, attention has been given to molten ZnCl2 hydrates as electrolytes for Zn metal batteries. It was revealed that the physical properties of such electrolytes have rich implications in their electrochemical properties. Therefore, it demands a holistic understanding of the physical and electrochemical properties of molten ZnCl2 hydrates to design functional electrolytes to serve high-performing Zn metal batteries. This perspective attempts to review the works that described the properties of concentrated ZnCl2 as an ionic liquid and as an emerging electrolyte. The author also provides a perspective to highlight the needs of future research to circumvent the limits of this electrolyte.