New Insights into Electrochemical Lithiation/Delithiation Mechanism of α‑MoO<sub>3</sub> Nanobelt by in Situ Transmission Electron Microscopy

The α-MoO<sub>3</sub> nanobelt has great potential for application as anode of lithium ion batteries (LIBs) because of its high capacity and unique one-dimensional layer structure. However, its fundmental electrochemical failure mechanism during first lithiation/delithiation process is still unclear. Here, we constructed an electrochemical setup within α-MoO<sub>3</sub> nanobelt anode inside a transmission electron microscope to observe in situ the mircostructure evolution during cycles. Upon first lithiation, the α-MoO<sub>3</sub> nanobelt converted into numerous Mo nanograins within the Li<sub>2</sub>O matrix, with an obvious size expansion. Interestingly, α-MoO<sub>3</sub> nanobelt was found to undergo a two-stage delithiation process. Mo nanograins were first transformed into crystalline Li<sub>1.66</sub>Mo<sub>0.66</sub>O<sub>2</sub> along with the disappearance of Li<sub>2</sub>O and size shrink, followed by the conversion to amorphous Li<sub>2</sub>MoO<sub>3</sub>. This irreversible phase conversion should be responsible for the large capacity loss in first cycle. In addition, a fully reversile phase conversion between crystalline Mo and amorphous Li<sub>2</sub>MoO<sub>3</sub> was revealed accompanying the formation and disapperance of the Li<sub>2</sub>O layer during the subsequent cycles. Our experiments provide direct evidence to deeply understand the distinctive electrochemical lithiation/delithiation behaviors of α-MoO<sub>3</sub> nanobelt, shedding light onto the development of α-MoO<sub>3</sub> anode for LIBs.