Ideal two-dimensional solid electrolytes for fast ion transport: metal trihalides MX3 with intrinsic atomic pores

Abstract

Exploring ultrathin two-dimensional (2D) solid electrolytes with fast ion transport is highly desirable in nanoelectronics, ionic devices and various energy storage systems, following the rapid scaling of devices to the nanometer scale. Herein, two-dimensional (2D) metal trihalides MX₃ (ScCl₃, ScBr₃, AsI₃, ScI₃, YBr₃, SbI₃, YI₃ and BiI₃) with intrinsic atomic pore structures have been examined and found to be promising as realistic 2D solid electrolytes. Through examining the binding interactions and the diffusion barriers of monolayer MX₃–ion (Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺) systems by utilizing first principles calculations, it is found that MX₃–ion complexes are energetically favorable and the energy barriers of some MX₃–ion systems are comparable to or even smaller than those of the conventional solid electrolyte systems. More significantly, the short diffusion time of Na⁺ and K⁺ ions in some monolayers MX₃ at the nanosecond (ns) or even at the sub-ns scale indicates fast ion transport. In terms of practical applications, ultrafast Li⁺ travelling in the timescale of sub-ns to ns and Na⁺ in several tens ns in few-layer MX₃ is achieved. In addition, the insulating nature of wide band gaps for MX₃ is maintained during the ion transport, which is essential for solid electrolytes. These theoretical results provide fundamental guidance that MX₃ materials with natural atomic pores are realistic candidates for 2D solid electrolytes with broad applications in ionic devices and energy storage devices.