Enhancing carrier transfer properties of Na-rich anti-perovskites, Na4OM2 with tetrahedral anion groups: an evaluation through first-principles computational analysis

Abstract

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 of 1.09 × 10⁻⁴ S cm⁻¹, 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.