STRUCTURAL CHARACTERIZATION OF HEAT-TREATED ELECTROLYTIC MANGANESE-DIOXIDE AND TOPOTACTIC TRANSFORMATION OF DISCHARGE PRODUCTS IN THE LI-MNO2 CELLS

The crystal structures of heat-treated electrolytic manganese dioxide (HEMD) and the discharge products are characterized by high-spatial-re solution convergent-beam electron diffraction (CBED) and lattice imagi ng. In this paper, CBED results are used to clarify the ambiguity in t he x-ray diffraction (XRD) patterns of these materials. The CBED resul ts of KEMD used as the positive electrode material confirms that the m ajor phase has the pyrolusite space group, P4(2)/mnm, but with a range of cia ratios. The experimentally observed variability of the lattice parameters from grain to grain is found to coincide with broadening o n the low-angle sides of the XRD peaks. Both XRD and CBED analyses ind icate that the unit cell volume of the pyrolusite-type phase increases during discharge, which is consistent with previous work by Ohzuku et al. CBED patterns of the discharge products suggest that a second dis charge phase is formed by a topotactic phase transformation during dis charge and has a specific orientation relationship with the expanded p yrolusite-type phase. The symmetry and lattice parameters obtained fro m the single-crystal diffraction patterns of the second discharge phas e are consistent with an intermediate structure in the mechanism propo sed by David ct al.(1) for the formation of the spinel structure upon chemical lithiation of beta-MnO2.