Enhancing carrier transfer properties of Na-rich anti-perovskites, NaOM with tetrahedral anion groups: an evaluation through first-principles computational analysis

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 Na 4 OBr 2 , including Na 4 O(BH 4 ) 2 , Na 4 O(BF 4 ) 2 , and Na 4 O(AlH 4 ) 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. Na 4 O(AlH 4 ) 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 Na 4 O(BF 4 ) 2 is a typical example. This study demonstrates that Na 4 O(BH 4 ) 2 exhibits remarkable mechanical and dynamic stability and shows ionic conductivity of 1.09 × 10 −4 S cm −1 , two orders of magnitude higher than that of Na 4 OBr 2 . 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. Sodium ions migrate through migration gates (MGs) in Na 4 O(BH 4 ) 2 and cause rotation of the [BH 4 ] − clusters that make up MGs.