Overlithiation　of　lithium　ion　batteries　often　causes　a　structural　transformation　of　the　electrode　and　capacity　degradation　and　may　even　lead　to　severe　safety　problems.　In　this　study,　the　electrode　structure,　surface　morphology　and　compositions　at　the　different　overlithiation　depths　of　LiCoO2　cathode　material　were　investigated　in　detailed　by　examining　the　LCO/Li　cells　and　anode-free　cells,　combined　with　post-mortem　characterizations.　When　LiCoO2　is　found　in　a　slight　overlithiation　state,　the　cycle　capacity　fades　slightly　and　minority　of　LiCoO2　reduces　to　CoO　and　Li2O.　After　LiCoO2　undergoes　a　deep　overlithiation　reaction,　a　long　flat　voltage　plateau　appears　at　1.2　V　and　the　cycle　performance　is　greatly　deteriorated.　SEM　and　EIS　tests　show　that　the　single-crystal　particles　are　fractured　into　small　particles　with　thick-layer　formation　at　the　surface　and　the　shape　of　the　EIS　curve　changes　strongly　with　increasing　overlithiation　depth.　Additionally,　as　shown　by　XPS　Ar-ion　etching　and　TEM　characterization,　the　LiCoO2　surface　is　converted　into　greater　amounts　of　Li2O,　CoO　and　Co　metal　associated　with　the　severe　electrolyte　decomposition,　and　LiF,　Li2CO3　and　LixPOyFz　are　the　main　electrolyte　decomposition　products.　The　detailed　investigation　of　the　overlithiation　mechanism　of　LiCoO2　provides　comprehensive　and　thorough　guidance　for　understanding　the　overlithiation　mechanism　of　cathode　materials　as　well　as　important　reference　information　for　practical　all-solid-state　battery　applications.　(C)　2021　The　Electrochemical　Society　(＂ECS＂).　Published　on　behalf　of　ECS　by　IOP　Publishing　Limited.