With　the　solid-state　battery　(vs.　Li)　application,　the　overlithiation　mechanism　of　the　different　cathode　materials　is　worthy　to　investigate.　In　this　study,　both　LiMn2O4　and　LiFePO4　cathode　materials　at　different　over-discharge　conditions　were　tested　using　half　cell　(vs.　Li)　and　anode-free　systems.　The　cells　were　dismantled　to　study　the　electrode　structure,　surface　morphology,　and　compositional　changes.　The　study　shows　that　LiMn2O4　and　LiFePO4　still　maintain　good　crystal　morphology　during　the　deep　over-discharge　process,　showing　better　over-discharge　resistance　capability　with　different　overlithiation　mechanisms.　As　shown　by　X-ray　diffraction　and　X-ray　photoelectron　spectroscopy　with　Ar-ion　etching,　the　new　phase,　Li2Mn2O4,　appears　starting　from　2.5　V.　Until　the　voltage　is　less　than　0.2　V,　the　framework　structures　of　LiMn2O4　are　deteriorated,　and　further　overlithiation　caused　decomposition　into　Li2MnO2　and　Li2O.　LiFePO4　essentially　maintains　its　olivine-type　structure,　but　below　0.2　V,　direct　overlithiation　causes　decomposition　into　Li2O　and　Fe　metal.　Furthermore,　overlithiated　decomposition　of　LiMn2O4　and　LiFePO4　occurs　at　very　low　voltages　approximately　0.43　and　0.56　V,　respectively.　Additionally,　the　deep　over-discharge　also　leads　to　the　decay　of　the　electrolyte　structure,　associated　with　LiF,　Li2CO3　and　LixPOyFz　by-products.　The　detailed　overlithiation　mechanism　will　provide　important　theoretical　guidance　for　practical　applications.