SYNTHESIS OF MN SPINELS FROM DIFFERENT POLYMORPHS OF MNO2

Four polymorphs of MnO2, i.e., alpha, beta, epsilon and ramsdellite, h ave 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 large r tunnels of alpha- and ramsdellite-MnO2 allow limitation of the LI-Mn repulsion causing the structural change. The formation of such by-pro ducts as Mn2O3 and Li2Mn3 has been demonstrated by X-ray and thermogra vimetric and differential thermal analyses. Solid-state reactions of t hese polymorphs (with LiOH) also give rise to Mn2O3-contaminated spine ls, in spite of the stoichiometric ratios. The electrochemical charact eristics, as determined mainly by potential step voltammetry in the 4 V range, are similar to those of conventionally prepared spinels but t he presence of by-products limits the available capacity.