Asynchronous Crystal Cell Expansion during Lithiation of K(+)-Stabilized α-MnO2.

α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 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 α-MnO2'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(+) occupancy. The sequential Li(+) 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/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.