There has been limited study of negative electrodes for multivalent ions based on transition metal compounds. There are a number of transition metal oxides that offer a range of oxidation states (Nb2O5, TiO2 and so on) but the ability to reversibly insert/remove ions such as Mg2+ has only been reported for MnO2 (ref. 68) and V2O5 (ref. 69). Instead, it would seem that carbides and 1T-dichalcogenides16 are worth considering because of their multiple oxidation states and significant electrical conductivities.