  The practical application of Na-based solid-state electrolytes (SSEs) is
limited by their low level of conduction. To evaluate the impact of tetrahedral
anion groups on carrier migration, we designed a set of anti-perovskite SSEs
theoretically based on the previously reported Na4OBr2, including Na4O(BH4)2,
Na4O(BF4)2, and Na4O(AlH4)2. It is essential to note that the excessive radius
of anionic groups inevitably leads to lattice distortion, resulting in
asymmetric migration paths and a limited improvement in carrier migration rate.
Na4O(AlH4)2 provides a clear example of where Na migrates in two distinct
environments. In addition, due to different spatial charge distributions, the
interaction strength between anionic groups and Na is different. Strong
interactions can cause carriers to appear on a swing, leading to a decrease in
conductivity. The low conductivity of Na4O(BF4)2 is a typical example. This
study demonstrates that Na4O(BH4)2 exhibits remarkable mechanical and dynamic
stability and shows ionic conductivity, two orders of magnitude higher than
that of Na4OBr2. This is attributed to the expansion of the carrier migration
channels by the anion groups, the moderate interaction between carriers and
anionic groups, and the paddle-wheel effect generated by the anion groups,
indicating that the paddle-wheel effect is still effective in low-dimensional
anti-perovskite structures, in which atoms are arranged asymmetrically.
