Microstructural Observation of LiNi_0.8Co_0.15Al_0.05O₂after Charge and Discharge by Scanning Transmission Electron Microscopy

A lithium cell of pelletized LiNi 0.8 Co 0.15 Al 0.05 O 2 without carbon or binder was operated in a voltage window of 4.2 – 2.5 V at C/40 rate at 20°C for ten cycles, and the microstructure for the ten-cycled sample was examined by scanning transmission electron microscopy (STEM), which was compared with those for pristine or first-cycled one. Microcracks appeared after the first cycle at grain boundaries, and widened after ten cycles, which are derived from 1 – 2% change in unit cell volume during charge and discharge. Grain boundary layers extended during the first cycle are almost invariable for subsequent cycles. This suggests that the extended grain boundary layers formed during the first cycle act as protection layers to prevent structural degradation during charge and discharge, which enable the materials to exhibit topotactic reaction in terms of a bulk crystal structure.