Electrolytes in electrochemistry and energy storage

Abstract Since Arrhenius formalized the theory of electrolytic dissociation, the electrolyte has generally been treated as a settled concept: a salt dissociating into freely moving ions within a solvent [Arrhenius S. 1887 Z. Phys. Chem. 1U, 631–648. (doi:10.1515/zpch-1887-0164)]. Over the past two decades, this picture has evolved by the emergence of water-in-salt electrolytes, room-temperature ionic liquids, porous liquids and solid-state or composite electrolytes, each of which departs from the classical definition in a different way and each of which exhibits solvation structures and transport behaviour that cannot be predicted from dilute-solution theory. This theme issue brings together seven contributions spanning aqueous, non-aqueous, ionic-liquid, polymer–ceramic and solid-state electrolyte systems, applied across lithium, sodium, zinc, calcium and magnesium electrochemistries. Read together, these contributions indicate that microscopic solvation structure, rather than bulk descriptors such as conductivity in isolation, is now the primary lens through which electrolyte performance and interfacial stability are understood. We use this introduction to set out the historical background, summarize the scope of each contribution and identify the cross-cutting themes and open questions that we believe should guide the next phase of electrolyte research. This article is part of the theme issue ‘Electrolytes within the domain of electrochemistry and electrochemical energy storage’.