Lithium ion batteries (LIBs) are quickly becoming the mainstream power sources for environmentally friendly vehicles such as hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and electric vehicles (EV) due to their high energy density. On the other hand, a battery system with even higher energy density is required for the long-range PHV and EV applications, and therefore, post lithium ion batteries (PLIB) such as Li-sulfur batteries and Li-air batteries have been getting more attention in recent years. Rechargeable magnesium batteries are also a candidate for the PLIB thank to the natural abundance of magnesium and the absence of dendrite formation when magnesium metal is used as the anode. In addition, the magnesium batteries are expected to have high energy density because of its divalent nature. However, there is not much progress on the development of novel cathodes since an innovation of Chevrel phase materials such as Mg_xMo₆S₈. The difficulty lies in the strong polarization character of the small and divalent Mg²⁺ and consequently the intercalation and diffusion of Mg²⁺ ions is somewhat difficult and complicated.

Because of the so-called “diagonal relationship” in the periodic table stating the chemical similarity between lithium and magnesium, significant amount of efforts have been attempted to use compounds analogous to classical Li-ion battery cathode as Mg-intercalation hosts. As olivine LiFePO₄ is one of the commercialized Li-ion battery cathode, it is of great interest to examine the performance of olivine compounds as Mg battery cathode. In this work, a comprehensive study about the performance of olivine FePO₄ as Mg battery cathode is performed. With a combined theoretical and experimental approach we show that the intercalation of magnesium does not occur in the electrochemical operations. On the contrary to the common belief that the intercalation of magnesium is critically determined by the mobility of Mg²⁺ ions in the lattice, we demonstrate that the failure of FePO₄ cathode is due to the formation of amorphous layer in the electrochemical process that passivates the active particle, similar to the phenomenon that we reported before for α-MnO₂ cathode. Our research provides new insight for the magnesiation mechanism of a classical cathode and elucidate several directions towards future rechargeable Mg battery.