Asynchronous Crystal Cell Expansion during Lithiation of K super(+)-Stabilized alpha -MnO sub(2)

alpha -MnO sub(2) is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li super(+). The overall electrochemical performance of alpha -MnO sub(2) is determined by the tunneled structure stability during its interaction with Li super(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K super(+)-stabilized alpha -MnO sub(2) is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 1 and 2 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the 100> projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of alpha -MnO sub(2)'s tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 2 tunnels are the preferred sites for Li super(+) occupancy. The sequential Li super(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/ alpha -MnO sub(2) batteries as well as the origin for low practical capacity and fast capacity fading of alpha -MnO sub(2) as an intercalated electrode. Keywords: in-situ TEM; Li-ion batteries; MnO sub(2); nanowires; tunneled structure