A Cubic Mg2MnO4 Cathode for non-aqueous Magnesium Batteries

The feasibility of cubic Mg 2 MnO 4 (c-Mg 2 MnO 4 ) cathode for rechargeable magnesium batteries is scrutinized by both theoretical and experimental methods. To exploit the high valence state of Mn 4+ upon magnesium insertion in c-Mg 2 MnO 4 , we substituted Mn 3+ (0.645 Å) by the larger Mg 2+ (0.72 Å) and studied a series of Mg 1+y Mn 3+ 2-2y Mn 4+ y O 4 . The results prove that this Mg/Mn substitution results in a progressive increase of the unit cell parameter “a” due to the increasement in the average oxidation of Mn. We also distinguish the importance of having Mn(IV) in c-Mg 2 MnO 4 vs. the known Mn(III) in tetragonal MgMn 2 O 4 (t-MgMn 2 O 4 ). The geometry optimization and energy calculations on the c-Mg 2 MnO 4 system are carried out within the density functional theory (DFT) calculations indicating the Mg insertion occurs at 2.23 V with a volume expansion of ca. 8.7 %. Galvanostatic curves revealed a two-step profile with an average voltage of 2.1 V and 160 mA h g −1 (0.5 Mg 2+ ).