Abstract:
This review article focuses on the methods to solve the critical issue of reduction in NASICON-type
solid electrolytes such as Li1+xAlxTi2 x(PO4)3 and Li1+xAlxGe2 x(PO4)3 by Li metal. The formation of a
reduced phase on the surface of solid electrolytes degrades ionic conductivity. Numerous research
efforts were devoted to resolve this issue by depositing various types of artificial protecting layers on the
materials’ surface. Inorganics such as lithium salts, oxides, and composites, solid polymer electrolytes, as
well as hybrid composite membranes have been applied to protect the material from the side reaction
by lithium metal. Besides prevention, these layers improved the interface between solid electrolyte and
electrode. Particularly, a coating with a polymer layer prevented the lithium dendrite growth by evenly
distributing lithium. The assessments of these layers and their effect on the performance of solid
electrolytes are discussed to establish a pathway for further improvement of solid-state electrolytes and
encourage their implementation in all-solid-state Li metal batteries. Through this critical evaluation, the
present review provides a summary of the most recent works in this area, allowing more efficient and target-oriented research on improving solid-state ion conductors and their interfaces.