Superionic conduit of alkaline earth metals confined by two-dimensional boron–carbon layers

Superionic conductors feature fast super-ion diffusion in the solid-state framework, making them ideal materials for safe, high-performing electrolytes. It is, therefore, in hot pursuit of seeking solid-state electrolyte materials with high energy density and flexible operation conditions. Here, we verified a class of two-dimensional superionic conduction in A(BC)2, in which A are alkaline earth metals such as Be, Mg, and Ca, and boron–carbon (BC) form graphene-like layers. Our first-principles molecular dynamics simulation, boosted by high-accuracy machine-leaning potentials, shows that alkaline metal becomes super-ions under high-temperature conditions, moving freely between BC layers. Differences in superionic conduit lead to the diffusion in Be(BC)2 driven by the vacancy mechanism. In contrast, the diffusion in Mg(BC)2 and Ca(BC)2 is jointly driven by both the vacancy and cooperative mechanisms. We demonstrate that the superionic transition temperature is controlled by the deficiency of mobile super-ions, tuning from 1300 to 1600 K, with up to 2.5% cation defects. With superior thermal stability, these two-dimensional compounds are promising electrolyte materials with ultrahigh heat resistivity capable of operating under high-temperature environments such as deep drills and aerospace devices.