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-resolution convergent-beam electron diffraction (CBED) and lattice imaging. In this paper, CBED results are used to clarify the ambiguity in the x-ray diffraction (XRD) patterns of these materials. The CBED results of KEMD used as the positive electrode material confirms that the major 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 on the low-angle sides of the XRD peaks. Both XRD and CBED analyses indicate 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 discharge phase is formed by a topotactic phase transformation during discharge and has a specific orientation relationship with the expanded pyrolusite-type phase. The symmetry and lattice parameters obtained from the single-crystal diffraction patterns of the second discharge phase are consistent with an intermediate structure in the mechanism proposed by David ct al.(1) for the formation of the spinel structure upon chemical lithiation of beta-MnO2.