This critical review presents the state of the art research progress, proposes strategies to improve the conductivity of solid electrolytes, discusses the chemical and electrochemical stabilities, and uncovers future perspectives for solid state batteries.

Elucidation of the structure of a new sodium superionic conductor, Na11Sn2PS12via single crystal XRD and AIMD simulations reveal isotropic 3D Na+-ion conduction pathways.

Na super ion conductor (NaSICON), Na 1+n Zr 2 Si n P 3-n O 12 is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na 3 Zr 2 Si 2 PO 12 is unveiled. It is revealed that Na + -ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single-ion migration barrier. Furthermore, computational work uncovers the origin of the improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na + -ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., σ bulk = 4.0 mS cm −1 , σ total = 2.4 mS cm −1 at 25 °C) are achieved by increasing the Na content from 3.0 to 3.30-3.55 mol formula unit −1 . These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na + -ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.