Ni-rich　cathode　materials　have　emerged　as　a　class　of　low-cost　and　high-energy-density　Li　ions　batteries　cathodes.　However,　the　bulk　and　surface　degradation　of　Ni-rich　cathode　remains　controversial.　By　transmission　electron　microscopy　(TEM),　the　bulk　and　surface　degradation　of　nickel-rich　cathode　are　distinguished　by　the　causes　and　development　time.　Structure　deterioration　with　rock-salt　phase　is　observed　in　the　bulk　and　surface　region.　The　triggered　phase　transition　by　defect　chain　reaction　(DCR)　mechanism　is　unveiled　by　the　first-principle-calculation　simulations.　The　critical　factor　in　DCR　mechanism　is　the　migration　barrier　of　Ni.　The　decrease　in　migration　barrier　of　Ni　due　to　the　oxygen　vacancy　and　the　lowering　of　the　system　energy　by　the　Li　ion　compensation　causes　extra　cation　mixing　and　lattice　distortion.　Hence,　Ni　ion　migration　introduces　strain　perpendicular　to　the　Ni　layers　with　oxygen　defects　and　Li　vacancies,　which　leads　to　dislocations　and　defects　with　lattice　distortion.　Based　on　the　model,　a　manganese　oxide　coating　method　was　proposed　to　adjust　the　valence　state　of　nickel　and　avoid　the　oxygen　defects.　The　treated　cathode　presented　an　excellent　long-term　cycling　performance,　about　80.6%　of　the　initial　capacity　after　200　cycles.　©　2020