Unlocking the Secrets of an Ideal Solid Fast Ion Conductor

All-solid-state battery (ASSB) is a possible alternative to the conventional Li-ion battery (LIB). ASSB, consisting of solid-fast-ion-conducting electrolytes and electrodes, can surpass LIB in terms of energy density, battery safety, specific power, and fast-charging capability. A highly conductive solid electrolyte is a prerequisite part of ASSB. Despite the last couple of decades of research dedicated to finding suitable materials, only a few have high ionic conductivity near room temperature (RT). To develop fast ion-conducting materials, both synthesis of novel high-conductivity materials near RT and a fundamental understanding of their ion transport mechanisms are essential for establishing suitable guidelines. However, it is a monumental task because of several challenges, such as enormous variations of the ionic conductivity, even within the same class of materials, indicating the strong influence of structural modifications on ion transport. In this review, a few selected material classes, namely layered oxides, polyhedral connections, and cluster anion types, are presented. Emphasis is placed on the inherent challenges in comprehending the intricate nature of these materials. Leveraging theoretical frameworks and insights from materials science, strategies to address these challenges are elucidated. Furthermore, this review covers the systematic trend from the previously reported results and presents a few new insights.