Abstract text (incl. figure legends and references)

The demand of high-energy lithium-ion batteries boosts the interest in increasing Ni content in cathodes and/or raising charge cutoff voltage. The Co-free LiNiO2 (LNO) has the advantage of lower production cost, high discharge capacity and high energy density compared to traditional LiCoO2, whilst the underlying mechanisms of their capacity loss and degradation when highly charged are not fully understood. The issues along with high charged states include surface O2 evolution and structural degradation. Finding pathways to unlock extra capacity is contingent upon an exact understanding of their structural behaviours. Several decades of effort worldwide have been devoted to developing characterization techniques across multiscale to study the mechanisms involved LNO charge storage but the knowledge still lacks regarding the nanoscale evolution. Here, we use the atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) combined with electron energy loss spectroscopy (EELS) to directly examine intrigue structural and chemical change mechanisms during the LNO electrochemical process. We have directly visualised the surface densification, void formation at the bulk, stacking fault evolution of LNO over charging to high cut-off voltage. We also provide detailed EELS spectroscopy analysis of Ni oxidation changes at different charged states, comparing the trends at different distances to the surface. This offers atomic-level insights of structural behaviours and degradation mechanisms of highly charge LNO in lithium-ion batteries.