Design Principles for Sodium Superionic Conductors

Motivated by the high-performance solid-state lithium batteries empowered by lithium superionic conductors as solid electrolytes, sodium superionic conductor materials share great promises for enabling novel sodium batteries with high energy, low cost, and sustainability. Designing sodium superionic conductors with high ionic conductivities is a great challenge, hindered by the lack of appropriate design principles. Here, by studying the structures and diffusion mechanisms of Li-ion versus Na-ion in solids, we reveal that fast Na⁺ Na-ion conductors exhibit the unique structural features of face-sharing high-coordination sites. By employing the new design principle based on this feature, we discover over a dozen of new families of fast Na-ion conductors in oxides, sulfides, and halides. Remarkably, a new chloride family of Na-ion conductors Na_xM_yCl₆ (M = Lanthanide) with UCl₃-type structure is discovered and experimentally confirmed with high Na ionic conductivities > 1 mS/cm at room temperature. Our results not only guide the future development of fast Na-ion conductors for new sodium batteries, but also consolidate and generalize different design principles for fast ion conductors, which can be further extendable and applicable to develop other types of ion-conducting materials for many different energy applications.