Intercalation cathodes serve as host networks, where ions can be reversibly inserted and extracted. Typical insertion compounds used are transition metal oxides, metal chalcogenides and polyanion framework compounds, which provide stable performances and are currently used in commercial Li-ion batteries59. One of the major limitations is the number of charge-carrying cations that can be inserted into the crystal structure and thus moderate specific capacity (Fig. 6b). Among the systems being investigated are layered Li2MeO2 (where Me=Co, Ni and Mn) and Li2VOPO4 (ref. 60), where two Li ions can be accommodated. With Li2MeO2, the Li ions occupy tetrahedral sites between the MeO2 layers. On extraction of Li+, there is a two-phase reaction to the LiMeO2 phase, with the remaining Li+ staying in octahedral sites between the layers. The major drawback is that subtle shifting of the layers impacts the long-term stability of the lattice. As a potential solution, the use of Mg2+ instead of Li+ can be considered: the layer shifting may be avoided, provided Mg2+ intercalates reversibly into only the octahedral sites, as fewer sites are required to transfer the same or larger charge.