Phase transitions in a LiMn2O4 nanowire battery observed by operando electron microscopy

Fast charge-discharge process has been reported to give a high capacity loss. A nanobattery consisting of a single LiMn2O4 nanowire cathode, ionic liquid electrolyte and lithium titanium oxide anode was developed for in situ transmission electron microscopy. When it was fully charged or discharged within a range of 4 V in less than half an hour (corresponding average C rate: 2.5C), Li-rich and Li-poor phases were observed to be separated by a transition region, and coexisted during whole process. The phase transition region moved reversibly along the nanowire axis which corresponds to the [011] direction, allowing the volume fraction of both phases to change. In the electron diffraction patterns, the Li-rich phase was seen to have the (100) orientation with respect to the incident electron beam, while the Li-poor phase had the (111̅) orientation. The orientation was changed as the transition region moved. However, the nanowire did not fracture. This suggests that a LiMn2O4 nanowire has the advantage of preventing capacity fading at high charge rates.