Abstract
ABSTRACTWith an objective to overcome the cyclability problems of manganese oxides, solution-based procedures are pursued to synthesize metastable manganese oxides. Reduction of permanganate with lithium iodide in an acetonitrile medium followed by heating at 250 °C in vacuum gives an amorphous lithium sodium manganese oxyiodide that is intimately mixed with crystalline NaIO3. On the other hand, oxidation of manganese acetate with lithium or hydrogen peroxide in presence of lithium hydroxide followed by firing at T < 500 °C gives the metastable spinel oxides, Li4Mn5O12 and Li2Mn4O9-δ. The amorphous manganese oxide exhibits excellent cyclability with a capacity > 275 mAh/g at 4.3-1.5 V. The presence of NaIO3 and a unique microstructure are found to play a critical role in the electrochemical properties. Although Li4Mn5O12 could be achieved without much oxygen vacancies, Li2Mn4O9-δ has significant amount of oxygen vacancies with δ > 0.35. Both Li4Mn5O12 and Li2Mn4O9-δ exhibit capacities around 150 mAh/g with good cyclability in the 3 V region.
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