Unraveling the Fast Ionic Conduction in NASICON‐Type Materials

Abstract

AbstractNa Superionic Conductor (NASICON) materials stand out as an important class of ionic conductors, offering high ionic conductivity and notable electrochemical stability for applications such as solid electrolytes. While advances through compositional engineering have been made to enhance the ionic conductivity of NASICONs, the fundamental mechanisms underlying their superionic conducting behavior remain unresolved. In this study, the interplay between local and global factors—specifically, bottleneck volume and Na site ordering— that influence ionic conduction within the NASICON framework is elucidated. Analysis of reported ionic conductivity data indicates that optimal bottleneck volumes exceed 3.7 Å3, adjustable through the choice of metal cations and polyanions. Additionally, the findings imply that weaker Na site ordering, which enhances ionic conduction, can be induced by increasing the Na content, selecting metal cations with a d0 electron configuration, or mixing elements at the cation or polyanion sites. Moreover, it is proposed that the predominant diffusion mechanism can shift between occupancy‐conserved hopping (OCH), occurring under conditions of strong Na site ordering, and single‐ion hopping (SIH), which becomes possible when site ordering is minimal. These insights offer strategic guidelines for designing NASICONs with superionic conductivity, promoting the development of solid electrolytes for safer and more energy‐dense all‐solid‐state batteries.