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Abstract: Computational and experimental work directed at exploring the electrochemical properties of tetrahedrally coordinated Mn in the +5 oxidation state is presented. Specific capacities of nearly 700mAhg−1 are predicted for the redox processes of Li x MnO4 complexes based on two two-phase reactions. One is topotactic extraction of Li from Li3MnO4 to form LiMnO4 and the second is topotactic insertion of Li into Li3MnO4 to form Li5MnO4. In the experiments, it is found that the redox behavior of Li3MnO4 is complicated by disproportionation of Mn5+ in solution to form Mn4+ and Mn7+ and by other irreversible processes; although an initial capacity of about 275mAhg−1 in lithium cells was achieved. Strategies based on structural considerations to improve the electrochemical properties of MnO4 n− complexes are given. [Copyright &y& Elsevier]