Unlocking the secrets of ideal fast ion conductors for all-solid-state batteries

All-solid-state batteries (ASSBs) are promising alternatives to conventional lithium-ion batteries. ASSBs consist of solid-fast-ion-conducting electrolytes and electrodes that offer improved energy density, battery safety, specific power, and fast-charging capability. Despite decades of intensive research, only a few have high ionic conductivity at ambient temperature. Developing fast ion-conducting materials requires both synthesis of high-conducting materials and a fundamental understanding of ion transport mechanisms. However, this is challenging due to wide variations of the ionic conductivity, even within the same class of materials, indicating the strong influence of structural modifications on ion transport. This Review discusses three selected material classes, namely layered oxides, polyhedral connections, and cluster anion types, as promising fast ion conductors. Emphasis is placed on the inherent challenges and the role of the framework structure on mobile ion conduction. We elucidate strategies to address these challenges by leveraging theoretical frameworks and insights from materials science. Designing fast ionic conductors for all-solid-state batteries is challenging due to the large variations of ionic conductivity even within the same material class. Here, the challenges and trends in layered oxide, polyhedral connection, and cluster anion type fast ion conductors are Reviewed.