Lithiation mechanism of tunnel-structured MnO₂ electrode investigated by in situ transmission electron microscopy

Abstract

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 Li2O 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.