In-Situ TEM Study of Intercalation- and Conversion-Driven Lithiation in Single α-MnO2 Nanowire

Nano-sized transitional metal oxides (TMOs, M=Fe, Co, Cu, Mn, etc.) have been increasingly recognized as promising candidate anode for lithium ion battery (LIB) with high specific capacity (700–1200 mAh/g). The high capacity is achieved via a so-called reversible “conversion reaction” with Li⁺, as shown in Equation (1):

                                 M_xO_y + 2yLi⁺ + 2ye^- = xM + yLi₂O                                                 (1)

where TMO, after reaction with Li⁺, is reduced to metal (M), and Li₂O also forms via combination of Li⁺ and O^2-. In spite of their high capacity, most TMOs suffer from poor rate performance, poor cycle performance and low coulombic efficiency in 1^st cycle, preventing its commercialization. The origins and mechanisms underlying these problems are still unclear.

        In this study, single crystalline α-MnO₂ nanowire is used as a TMO representative to study the electrochemical lithiation mechanism by constructing an all-solid open-cell inside Transmission Electron Microscope (TEM) with MnO₂ as the target electrode and Li metal as Li⁺ source. Morphological evolution, structural change and phase transition are simultaneously characterized inside TEM. A novel two-step expansion behavior of α-MnO₂ nanowire is first visually observed during lithiation. These two expansion stages are sharply different in terms of expansion-needed time, propagation speed of each expansion front and volume change during each expansion. High resolution TEM and Selected Area Electron Diffraction analysis confirm that the first expansion stage is lead by 2×2 tunnel-driven lithiation and formation of Li_xMnO₂ solid solution, while the second expansion stage is caused by thorough conversion of Li_xMnO₂ to Mn and Li₂O accompanied by fracture formation and electrode pulverization. These findings provide possible origins for electrochemical drawbacks of TMOs as LIB electrode and also cast light on potential electrode improvement by proper structural and compositional modification.