Lithiation Mechanism of Tunnel-Structured MnO2 Electrode Investigated by In Situ Transmission Electron Microscopy.

Manganese oxide (α-MnO2 ) has been considered a promising energy material, including as a lithium-based battery electrode candidate, due to its environmental friendliness. Thanks to its unique 1D [2 × 2] tunnel structure, α-MnO2 can be applied to a cathode by insertion reaction and to an anode by conversion reaction in corresponding voltage ranges, in a lithium-based battery. Numerous reports have attributed its remarkable performance to its unique tunnel structure; however, the precise electrochemical reaction mechanism remains unknown. In this study, finding of the lithiation mechanism of α-MnO2 nanowire by in situ transmission electron microscopy (TEM) is reported. By elaborately modifying the existing in situ TEM experimental technique, rapid lithium-ion diffusion through the tunnels is verified. Furthermore, by tracing the full lithiation procedure, the evolution of the MnO intermediate phase and the development of the MnO and Li2 O phases with preferred orientations is demonstrated, which explains how the conversion reaction occurs in α-MnO2 material. This study provides a comprehensive understanding of the electrochemical lithiation process and mechanism of α-MnO2 material, in addition to the introduction of an improved in situ TEM biasing technique.