MICROSTRUCTURAL FEATURES OF ALPHA-MNO2 ELECTRODES FOR LITHIUM BATTERIES

X-ray diffraction and transmission electron microscopy, including latt ice imaging and convergent-beam electron diffraction, were used to stu dy the microstructural features of alpha-MnO2 products that had been s ynthesized from different precursors. The alpha-MnO2 products, which w ere formed by acid digestion of Mn2O3 and Li2MnO3 oxide powders, conta ined water molecules in the (2 x 2) channels of their hollandite-type structures. The alpha-MnO2 crystals had a distinct rod-shaped morpholo gy with the [001] crystallographic orientation parallel to the rod axi s. Transmission electron microscopy data showed that the nature of the precursor directly affects the grain morphology of the alpha-MnO2 pro duct. The grain morphology of alpha-MnO2 electrodes plays an important role in the electrochemical behavior of Li/alpha-MnO2 rechargeable ce lls: 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 p roducts (LixMnO2: 0.4 < x < 0.5) confirmed that the alpha-MnO2 structu re was unstable to chemical lithiation; the capacity loss that was obs erved during the initial electrochemical cycling of Li/alpha-MnO2 cell s is attributed to this structure instability.