Is Mg ₃ AsN antiperovskite a promising Mg-ion conductor?

Among solid-state electrolytes (SEs), antiperovskites (APs) with an X₃AB structure stand out as SEs for monovalent-ion batteries due to their inverted perovskite framework, which supports cation-rich compositions with high ionic conductivity. For rechargeable Mg batteries (RMBs), Mg₃AsN was theoretically predicted as a potential Mg-ion conductor. Motivated by conflicting theoretical predictions regarding its electronic properties, which highlight the need for experimental validation, in the present work, we performed the first experimental investigation of the ionic and electronic properties of Mg₃AsN. Mg₃AsN was synthesized by high-energy ball milling and characterized by different structural and electrochemical techniques. Pristine Mg₃AsN exhibited mixed ionic and electronic conductivities of 5.5 × 10^-4 S cm^-1 and 4.89 × 10^-8 S cm^-1, respectively, at 100 °C. After hot pressing, the electronic conductivity was found to be 1.5 × 10^-6 S cm^-1. Heat treatment at 600 °C for 12 hours improved the total ion transport number from 0.07 to 0.615, while maintaining the electronic conductivity at 5 × 10^-8 S cm^-1 at 100 °C. To further suppress the electronic conductivity of Mg₃AsN, two approaches were performed: (i) adding electron-blocking buffering layers of metal-organic frameworks between AP and Mg electrodes and (ii) dispersing the AP powder into a polymeric matrix to block electron flow while preserving ion diffusion. Initial results from both strategies were promising and showed enhanced viability of Mg₃AsN as an SE, offering tunable solutions for RMB development and to implement mixed conductors with high ionic conductivity in solid-state batteries (SSBs). A suppressed electronic conductivity, as well as a room temperature ionic conductivity of 0.134 mS cm^-1, was achieved, affording a reversible Mg²⁺ deposition/stripping process.