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 mA h 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 the ion size and interstitial tetrahedron height to Li-rich disordered rocksalts. However, even with high carbon loadings, elevated cycling temperatures and reduced particle and crystallite size, no significant Mg²⁺ deintercalation was observed, indicating that conventional design rules established for Li-rich DRS cannot be simply translated to Mg-rich systems. Despite the lack of activity in Mg-rich oxides, we demonstrate that Mg²⁺ intercalation into close-packed cubic disordered rocksalts, such as Li_2−xMnO₂F (x = 1), is possible, opening possible routes to activating Mg-rich systems.