A perspective on the role of anions in highly concentrated aqueous electrolytes

Highly concentrated aqueous electrolytes enable a wider electrochemical stability window and, thus, higher energy batteries compared to conventional dilute aqueous solutions. Multiple properties of the electrolyte, e.g., ionic interactions, solvation structure, ion transport, tendency to hydrolyze, and capability to form a solid electrolyte interphase, distinctly change when the salt concentration is increased and highly depend on the salt anion. This work aims at reviewing, discussing and rationalizing the role of the salt anion in these physical and chemical properties in order to provide perspective guidelines for future developments.