Direct Probing of Lattice-Strain-Induced Oxygen Release in LiCoO2 and Li2MnO3 without Electrochemical Cycling

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 LiCoO2 and Li2MnO3, the local variations of both the atomic structure and oxygen content are scrutinized. Atomic-column-resolved imaging reveals that local Li-M (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.

Automated summary

Since the recognition of the significant oxygen-redox contribution to Li transition-metal oxide cathodes' capacity enhancement, the issues of oxygen release, structural variations, and capacity fading have become critical for achieving better electrochemical performance. This study utilizes nanoindentation and surface polishing to investigate the local variations of atomic structure and oxygen content in LiCoO2 and Li2MnO3. The findings provide direct evidence of oxygen release due to lattice strain and suggest the importance of efficient strain relaxation for the longevity of the anion framework in layered oxide cathodes.