Superhalogen-based lithium superionic conductors

Abstract

The development of cheap, durable, and safe inorganic solid electrolytes with superionic conductivity is the key for the next generation of all-solid metal-ion batteries. The recent discovery of antiperovskites with composition Li₃OA (A = halogen) shows promise in this regard. Here, we demonstrate the potential of a new class of antiperovskites where halogens are replaced by BH₄ superhalogens. In addition to maintaining the high ionic conductivity of Li₃OA, Li₃O(BH₄) is lightweight, mechanically flexible, thermodynamically more stable, and electronically more insulating than Li₃OA. By mixing BH₄ with Cl to make Li₃O(BH₄)_0.5Cl_0.5, we further show that the conductivity will be doubled. The Li⁺-ion conductivity of the new materials is of the order of 10⁻⁴ to over 10⁻³ S cm⁻¹ at room temperature and will be well above 10⁻² S cm⁻¹ at higher temperatures below the melting point. The conduction mechanism of the material is revealed by identifying the relationship between the orientational symmetry of the BH₄⁻ rotors and the potential surface felt by the lithium ions.