High-energy-density　Li-rich　layered　oxides　(LLOs)　as　promising　cathodes　for　Li-ion　batteries　suffer　from　the　dissolution　of　transition　metals　(especially　manganese)　and　severe　side　reactions　in　conventional　electrolytes,　which　greatly　deteriorate　their　electrochemical　performance.　Herein,　an　in　situ　＇anchoring　+　pouring＇　synergistic　cathode–electrolyte　interphase　(CEI)　construction　is　realized　by　using　1,3,6-hexanetricarbonitrile　(HTCN)　and　tris(trimethylsilyl)　phosphate　(TMSP)　electrolyte　additives　to　alleviate　the　challenges　of　an　LLO　(Li1.13Mn0.517Ni0.256Co0.097O2).　HTCN　with　three　nitrile　groups　can　tightly　anchor　transition　metals　by　coordinative　interaction　to　form　the　CEI　framework,　and　TMSP　will　electrochemically　decompose　to　reshape　the　CEI　layer.　The　uniform　and　robust　in　situ　constructed　CEI　layer　can　suppress　the　transition　metal　dissolution,　shield　the　cathode　against　diverse　side　reactions,　and　significantly　improve　the　overall　electrochemical　performance　of　the　cathod　with　a　discharge　voltage　decay　of　only　0.5　mV　cycle−1.　Further　investigations　based　on　a　series　of　experimental　techniques　and　theoretical　calculations　have　revealed　the　composition　of　in　situ　constructed　CEI　layers　and　their　distribution,　including　the　enhanced　HTCN　anchoring　effect　after　lattice　densification　of　LLOs.　This　study　provides　insights　into　the　in　situ　CEI　construction　for　enhancing　the　performance　of　high-energy　and　high-voltage　cathode　materials　through　effective,　convenient,　and　economical　electrolyte　approaches.　©　2020　Wiley-VCH　GmbH