Examining Electrolyte Compatibility on Polymorphic MnO 2 Cathodes for Room-Temperature Rechargeable Magnesium Batteries.

Rechargeable magnesium batteries are promising candidates for post-lithium-ion batteries, owing to the large source abundance and high theoretical energy density. However, there remain few reports on constructing practical cells with oxide cathodes and Mg anodes at room temperature. In this work, we compare the reaction behavior of various MnO 2 polymorph cathodes in two representative electrolytes: Mg[TFSA] 2 /G3 and Mg[Al(hfip) 4 ] 2 /G3. In Mg[TFSA] 2 /G3, discharge capacities of the MnO 2 cathodes are well consistent with the changes in Mg composition, where nanorod-like α-MnO 2 and λ-MnO 2 show the capacities of about 100 mA h g -1 at room temperature. However, this electrolyte has the disadvantage that the Mg anodes are easily passivated. In contrast, Mg[Al(hfip) 4 ] 2 /G3 allows highly reversible deposition/dissolution of Mg anodes, whereas the discharge process of the MnO 2 cathodes involves a large part of side reactions, in which the MnO 2 active material takes part in some reductive reaction together with electrolyte species instead of the expected Mg 2+ intercalation. Such an unstable electrode/electrolyte interface would lead to continuous degradation on/near the cathode surface. Thus, the interfacial stability between the oxide cathodes and the electrolytes must be improved for practical applications.