Lithium-rich　layered　oxides　(LLOs)　are　promising　candidate　cathode　materials　for　safe　and　inexpensive　high-energy-density　Li-ion　batteries.　However,　oxygen　dimers　are　formed　from　the　cathode　material　through　oxygen　redox　activity,　which　can　result　in　morphological　changes　and　structural　transitions　that　cause　performance　deterioration　and　safety　concerns.　Herein,　a　flake-like　LLO　is　prepared　and　aberration-corrected　scanning　transmission　electron　microscopy　(STEM),　in　situ　high-temperature　X-ray　diffraction　(HT-XRD),　and　soft　X-ray　absorption　spectrum　(sXAS)　are　used　to　explore　its　crystal　facet　degradation　behavior　in　terms　of　both　thermal　and　electrochemical　processes.　Void-induced　degradation　behavior　of　LLO　in　different　facet　reveals　significant　anisotropy　at　high　voltage.　Particle　degradation　originates　from　side　facets,　such　as　the　(010)　facet,　while　the　close　(003)　facet　is　stable.　These　results　are　further　understood　through　ab　initio　molecular　dynamics　calculations,　which　show　that　oxygen　atoms　are　lost　from　the　{010}　facets.　Therefore,　the　facet　degradation　process　is　that　oxygen　molecular　formed　in　the　interlayer　and　accumulated　in　the　ab　plane　during　heating,　which　result　in　crevice-voids　in　the　ab　plane　facets.　The　study　reveals　important　aspects　of　the　mechanism　responsible　for　oxygen　-anionic　activity-based　degradation　of　LLO　cathode　materials　used　in　lithium-ion　batteries.　In　particular,　this　study　provides　insight　that　enables　precise　and　efficient　measures　to　be　taken　to　improve　the　thermal　and　electrochemical　stability　of　an　LLO.