Direct Probing of Lattice‐Strain‐Induced Oxygen Release in LiCoO<sub>2</sub> and Li<sub>2</sub>MnO<sub>3</sub> without Electrochemical Cycling
Dongho Kim, Jaejin Hwang, Pilgyu Byeon, Wonsik Kim, Dong Gyu Kang, Hyung Bin Bae, Sang‐Gil Lee, Seung Min Han, Jaekwang Lee, Sung‐Yoon Chung
Abstract
Abstract Since the recognition of a significant oxygen‐redox contribution to enhancing the capacity of Li transition‐metal oxide cathodes, the oxygen release and subsequent structural variations together with capacity fading are critical issues to achieve better electrochemical performance. As most previous reports dealt with the structural degradation of cathodes after electrochemical cycling, it is fairly difficult to clarify how substantial the effect of lattice strain on the oxygen release will be while exclusively ruling out any electrochemical influences. By utilizing nanoindentation and mechanical surface polishing of single‐crystal LiCoO 2 and Li 2 MnO 3 , the local variations of both the atomic structure and oxygen content are scrutinized. Atomic‐column‐resolved imaging reveals that local LiM (M = Co and Mn) disordering and further amorphization are induced by mechanical strain. Moreover, substantial oxygen deficiency in the regions with these structural changes is directly identified by spectroscopic analyses. Ab initio density functional theory calculations also demonstrate energetically favorable formation of oxygen vacancies under shear strain. Providing direct evidence of oxygen release as a consequence of lattice strain, the findings in this work suggest that efficient strain relaxation will be of great significance for longevity of the anion framework in layered oxide cathodes.