Synthesis of Mn spinels from different polymorphs of MnO2

Four polymorphs of MnO2, i.e., alpha, beta, epsilon and ramsdellite, have been lithiated with Lil solutions to give LixMnO2 (0.4 <x < 0.7), Heating of these phases always results in spinel formation. beta- and epsilon-MnO2 are more easily converted into spinels, whereas the larger tunnels of alpha- and ramsdellite-MnO2 allow limitation of the LI-Mn repulsion causing the structural change. The formation of such by-products as Mn2O3 and Li2Mn3 has been demonstrated by X-ray and thermogravimetric and differential thermal analyses. Solid-state reactions of these polymorphs (with LiOH) also give rise to Mn2O3-contaminated spinels, in spite of the stoichiometric ratios. The electrochemical characteristics, as determined mainly by potential step voltammetry in the 4 V range, are similar to those of conventionally prepared spinels but the presence of by-products limits the available capacity.