High　nickel　content　worsens　the　thermal　stability　of　layered　cathodes　for　lithium-ion　batteries,　raising　safety　concerns　for　their　applications.　Thoroughly　understanding　the　thermal　failure　process　can　offer　valuable　guidance　for　material　optimization　on　thermal　stability　and　new　opportunities　in　monitoring　battery　thermal　runaway　(TR).　Herein,　this　work　comprehensively　investigates　the　thermal　failure　process　of　a　single-crystal　nickel-rich　layered　cathode　and　finds　that　the　latent　thermal　failure　starts　at　similar　to　120　degrees　C　far　below　the　TR　temperature　(225　degrees　C).　During　this　stage　of　heat　accumulation,　sequential　structure　transition　is　revealed　by　atomic　resolution　electron　microscopy,　which　follows　the　layered　-＞　cation　mixing　layered　-＞　LiMn2O4-type　spinel　-＞　disordered　spinel　-＞　rock　salt.　This　progression　occurs　as　a　result　of　the　continuous　migration　and　densification　of　transition　metal　cations.　Phase　transition　generates　gaseous　oxygen,　initially　confined　within　the　isolated　closed　pores,　thereby　not　showing　any　thermal　failure　phenomena　at　the　macro-level.　Increasing　temperature　leads　to　pore　growth　and　coalescence,　and　eventually　to　the　formation　of　open　pores,　causing　oxygen　gas　release　and　weight　loss,　which　are　the　typical　TR　features.　We　highlight　that　latent　thermal　instability　occurs　before　the　macro-level　TR,　suggesting　that　suppressing　phase　transitions　caused　by　early　thermal　instability　is　a　crucial　direction　for　material　optimization.　Our　findings　can　also　be　used　for　early　warning　of　battery　thermal　runaway.