Mechanism of enhanced ionic conductivity by rotational nitrite group in antiperovskite Na3ONO2†
Sodium-rich antiperovskite (NaRAP) ionic conductors have been considered promising inorganic electrolytes for all-solid-state sodium batteries due to their low fabrication cost, high sodium ionic conductivity and good structural tolerance. The high structural flexibility of NaRAPs allows variety of chemical substitutions to improve conductivity; in particular, the substitution of the anion cluster has effectively proved promoting Na+ migration, both experimentally and theoretically. Herein, we report an unexpected boost in Na+ ionic conductivity that climbs to 0.37 mS cm−1 at 485 K by introducing NO2− groups in the antiperovskite Na3ONO2 system. Its mechanism was fully investigated by neutron powder diffraction and DFT calculations, proving that the NO2− group on the lattice center, can utilize its terminal O2− anion to facilitate the Na+ migration via the Na–O interaction, when thermally excited at 485 K. The detailed migration path is also discussed by both maximum entropy method (MEM) and DFT calculations.