Superhalogen-Based Double Antiperovskite Li₆NBr(BH₄)₂: Structural Stability and Li Ion Transport Characteristics

Antiperovskites provide an intriguing platform for designing functional materials due to their structural diversity. Especially, double antiperovskites further expand the antiperovskites family with more tunability. Going beyond atom-based double antiperovskites, superhalogens can be used to replace anions for adding more freedom of regulating properties. This study proposes superhalogen-based double antiperovskite Li6NBr(BH4)2 with a unique trigonal crystal structure and dynamic stability. Li6NBr(BH4)2 is found to exhibit a significant band gap of 4.40 eV with the ability to hinder the conduction of electrons. The nonspherical geometry of superhalogen BH4 intrinsically introduces a paddle-wheel mechanism for enhancing the transport of Li+ ions, resulting in a Li+ ion conductivity of 0.54 mS/cm and an activation energy of 0.26 eV at room temperature, which outperforms its atom-based counterpart Li6NBrBr2 with corresponding values of 0.32 eV and 0.13 mS/cm, respectively. Furthermore, Li6NBr(BH4)2 shows superior mechanical properties, including a Poisson’s ratio of 0.28 and a Pugh ratio of 1.89, indicating good ductility and flexibility, crucial for preventing cracking or delamination during battery operation.