Microstructural features of α-MnO2 electrodes for lithium batteries

X-ray diffraction and transmission electron microscopy, including lattice imaging and convergent-beam electron diffraction, were used to study the microstructural features of alpha-MnO2 products that had been synthesized from different precursors. The alpha-MnO2 products, which were formed by acid digestion of Mn2O3 and Li2MnO3 oxide powders, contained water molecules in the (2 x 2) channels of their hollandite-type structures. The alpha-MnO2 crystals had a distinct rod-shaped morphology with the [001] crystallographic orientation parallel to the rod axis. Transmission electron microscopy data showed that the nature of the precursor directly affects the grain morphology of the alpha-MnO2 product. The grain morphology of alpha-MnO2 electrodes plays an important role in the electrochemical behavior of Li/alpha-MnO2 rechargeable cells: crystals with a small aspect ratio have a large electrochemically active surface [i.e., a large exposed (2 x 2) tunnel cross-sectional area] per unit volume available for lithium insertion. Lattice imaging and electron diffraction studies of chemically lithiated alpha-MnO2 products (LixMnO2: 0.4 < x < 0.5) confirmed that the alpha-MnO2 structure was unstable to chemical lithiation; the capacity loss that was observed during the initial electrochemical cycling of Li/alpha-MnO2 cells is attributed to this structure instability.