Direct imaging of oxygen shifts associated with the oxygen redox of Li-rich layered oxides

Abstract

Li-rich metal oxides, such as Li<inf>1.2</inf>Ni<inf>0.13</inf>Mn<inf>0.54</inf>Co<inf>0.13</inf>O<inf>2</inf>, can deliver high specific capacities because of the redox of lattice O²⁻ in addition to the cations. Observing oxygen distortions is key to understand the redox process. Electron ptychography is a phase-reconstruction method in 4D scanning transmission electron microscopy, providing atomic-resolution phase images with high signal-to-noise ratio and dose efficiency. Herein, we use electron ptychography to image the oxygen shift in Li<inf>1.2</inf>Ni<inf>0.13</inf>Mn<inf>0.54</inf>Co<inf>0.13</inf>O<inf>2</inf> during the first cycle. The picometer-scale precision measurement shows distinct oxygen shifts in the bulk and surface after charging and compares with various theoretical anionic redox models. The shift after discharging is not seen to recover in the bulk accounting for voltage hysteresis; however, it recovers close to the surface, although with a phase change. We suggest that Li<inf>1.2</inf>Ni<inf>0.13</inf>Mn<inf>0.54</inf>Co<inf>0.13</inf>O<inf>2</inf> proceeds distinct oxygen redox in the bulk and surface. The altered oxygen sublattice after first cycle potentially explains the changed voltage profiles of following cycles.