Two-dimensional alkaline-earth metal monohalides in unusually low oxidation states with high performance for ion batteries and electrochemical water splitting

Abstract

Exploring the low oxidation states of alkaline earth metal elements with natural abundance can be useful for renewable energy applications and is highly desirable. Although alkaline earth metal elements in the +1 oxidation state have recently been observed in organometallic compounds, the +1 oxidation state in crystal structures is extremely rare. Here, we conduct a comprehensive structure search to find stable two-dimensional (2D) metal monohalide MX crystalline materials composed of alkaline earth metals in the +1 oxidation state (M = Be, Mg, Ca, Sr, Ba) and halogens X (X = F, Cl, Br, I) with the aid of first-principles swarm structure search calculations. A subgroup of these 2D MX monolayers exhibits rich topological properties, such as being topological crystalline insulators and high-symmetry-point semimetals. These MX monolayers with inherent metallicity are also promising candidates as anode materials for ion batteries and catalysts for electrochemical water splitting. Various potential synthetic pathways for MX monolayers are proposed using top-down and bottom-up growth approaches, suggesting the feasibility of their experimental realization.