Synthesis of Mn spinels from different polymorphs of MnO_2

摘要

Four polymorphs of MnO_2, i.e., α, β, ε and ramsdellite, have been lithiated with LiI solutions to give Li_xMnO_2 (0.4 < x < 0.7). Heating of these phases always results in spinel formation, β- and ε-MnO_2 are more easily converted into spinels, whereas the larger tunnels of α- and ramsdelIite-MnO_2 allow limitation of the Li-Mn repulsion causing the structural change. The formation of such by-products as Mn_2O_3 and Li_2MnO_3 has been demonstrated by X-ray and thermogravimetric and differential thermal analyses. Solid-state reactions of these polymorphs (with LiOH) also give rise to Mn_2O_3-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.