Abstract
We report an approach to control the reversible electrochemical activity (i.e., extraction/insertion) of Mg2+ in a cathode host through the use of phase-pure epitaxially stabilized thin film structures. The epitaxially stabilized MgMn2O4. (MMO) thin films in the distinct tetragonal and cubic phases are shown to exhibit dramatically different properties (in a nonaqueous electrolyte, Mg(TFSI)(2) in propylene carbonate): tetragonal MMO shows negligible activity while the cubic MMO (normally found as polymorph at high temperature or high pressure) exhibits reversible Mg2+ activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control multivalent cation mobility in next generation battery materials.