Securing cation vacancies to enable reversible Mg insertion/extraction in rocksalt oxides

Abstract

Oxide cathode materials have promising applications in rechargeable magnesium batteries (RMBs) due to their high redox potential, which allows the exploitation of the low potential of Mg metal anodes and the eventual realization of RMBs with high energy densities. However, the capacity and cyclability of oxide cathodes, such as spinel oxides, are frequently limited by the irreversible formation of rocksalt oxides during discharging because the rocksalt structure with densely packed cations hinders subsequent facile Mg extraction by charging, eventually leading to electrode deterioration. It is, therefore, of great importance to reveal a mechanism that realizes reversible Mg extraction/intercalation in rocksalt oxides to enhance the conventional oxide cathodes and exploit the rocksalt oxides as novel cathode materials for RMBs. Herein, we show an activation mechanism of rocksalt oxides as a cathode material for RMBs by exemplifying Li-extracted defect disordered rocksalt oxide obtained from Mg_0.35Li_0.3Cr_0.1Mn_0.05Fe_0.05Zn_0.05Mo_0.1O, which is capable of reversible Mg insertion/extraction. Extracting the Li cations in the first charging leads to a substantial amount of cation vacancies in the rocksalt structure, facilitating Mg diffusion in subsequent cycles. This vacancy is secured even after fully discharging the material by inserting Mg into the structure because of the lowest possible valence states of the constituent transition metal cations, contributing to the reversible charging/discharging of this material.