Solid-State Electrolytes for Sodium Metal Batteries: Recent Status and Future Opportunities

Solid-state sodium metal batteries (SSMBs) possess superior safety and high energy density over liquid counterparts, and therefore hold great promise for large-scale energy storage application. In this review, the recent development of the advanced sodium-based solid-state electrolytes (SSEs) for high-performance SSMBs is systematically summarized. First, the fundamental mechanisms and key parameters of SSEs, including ionic conductivity, electrochemical stability and Na+ transference number, are introduced in details, and then various inorganic (Na-beta-alumina, Na superionic conductors, and sulfides), polymer SSEs (solid polymer SSEs and gel polymer SSEs), and inorganic-polymer hybrid SSEs are elaborately discussed, emphasizing the unique roles of structure optimization and interface engineering of SSEs for boosting electrochemical performance in SSMBs. Finally, the key challenges and promising prospects of advanced SSEs in terms of interface optimization of inorganic SSEs, molecular design and screening of polymer SSEs, objective evaluation, and multiscale characterizations of SSEs are briefly proposed.