Mg-rich disordered rocksalt oxide cathodes for Mg-ion batteries

Abstract

The discovery of new transition metal (TM) oxide cathodes which can act as intercalation hosts for Mg²⁺ ions is critical for the development of high energy density Mg-ion batteries. In Li-ion batteries, disordered rocksalts with sufficiently high Li⁺ charge carrier ion concentration, i.e. Li:TM >1.1, can support fast Li⁺ diffusion and therefore deliver high capacities (~300 mAh g^-1) and rate performance. Here, we investigate a range of simple Mg-rich disordered rocksalt cathodes, Mg₂TMO₃ (TM = Mn, Ni, Co), which possess similar charge carrier ion concentrations and similar ratios between ion size and interstitial tetrahedron height to the Li-rich disordered rocksalts. We show that, even with high carbon loadings, elevated cycling temperatures and reduced particle and crystallite size, it is not possible to extract Mg²⁺, indicating Mg²⁺ transport through Mg-rich cathodes is far less facile than Li⁺ in the Li-rich counterparts. Despite this, we demonstrate that Li-rich disordered rocksalts, such as Li_2-xMnO₂F (x = 1), can act as intercalation hosts for Mg²⁺, opening possible routes to activating these Mg-rich systems.