On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials

To initiate wider discussion about promising research directions, this paper highlights a number of challenges in the development of rechargeable Mg batteries, especially those related to the slow solid-state Mg diffusion in common hosts. With a focus on the intercalation mechanism, we compare for the first time different strategies proposed in the literature for developing Mg battery cathodes, like the use of (i) nanoscale cathode materials; (ii) hybrid intercalation compounds containing bound water or other additional anion groups that can presumably screen the charge of the inserted cations, (iii) cluster-containing compounds with efficient attainment of local electroneutrality. This comparative analysis shows that cathodes whose function is based on a combination of the two first strategies, e.g., V2O5 gels and their hybrids, can exhibit relatively high voltage and capacity upon Mg insertion, but their kinetics is insufficiently fast. A proper intercalation mechanism for such materials is still unknown, but their relatively slow cation transport seems to be intrinsic: The paradox is that the high capacity testifies Mg insertion into crystal sites with incomplete charge screening. In contrast, the high rate capability and exclusively stable cycling of cathodes based on Chevrel phases (Mo6-cluster- containing compounds) are appropriate for Mg battery design, but they offer low energy density because of the low voltage. On the basis of the knowledge of the intercalation mechanism in these phases, we believe that the future search for cathode materials in rechargeable Mg batteries should be focused on new cluster-containing intercalation compounds with higher capacity and working potential.