Intercalation is the reversible insertion of a guest species into a solid host structure such that the host maintains its basic structural features. The host structure is therefore described as a "breathing matrix", and it can be an insulator, semiconductor, or a metal. Transition metal oxides (TMO) are particularly interesting because they have been extensively studied as cathode candidates for high-energy-density secondary batteries since the 1970s. Lithium has been the dominant intercalation ion, and the efforts led to the now wide-spread use of the Li-ion technology as rechargeable batteries. TMOs are typically divided into layered and nonlayered materials. In the latter, there exists usually a size limitation for the intercalating atom. Because the magnesium ion has a comparable size to Li + , Mg 2+ intercalating into TMOs has been considered since the early 1990s 8 as a promising alternative to the Li-ion technology in order to increase energy density, address safety/toxicity concerns, and reduce costs. Hence, magnesium arguably represents the most studied multivalent ion from a guest-species perspective.