Interfacial engineering of solid electrolytes

The roles of interfaces in either blocking or enhancing ionic conduction in various types of solid electrolytes, including lithium, sodium, oxygen and other types of ion conductors as well as proton conductors, are critically reviewed. Two important fundamental interfacial phenomena, namely the formation of space charges and two-dimensional interfacial phases (complexions), can markedly alter ion transport along or across various types of interfaces, including grain and phase boundaries as well as free surfaces. Since the experiments and models of space charges have been well documented in literature, a new focus of this short review and viewpoint article is to propose and discuss an emerging opportunity of utilizing the formation and transition of interfacial phases to either alleviate the blocking effects or further enhance ionic conduction in solid electrolytes.