Solid-state　electrolytes　(SSE)　exhibit　great　promise　in　enhancing　the　safety　of　Li　metal　batteries　by　replacing　flammable　liquid　electrolytes.　However,　the　practical　application　of　SSE　is　hampered　mainly　due　to　the　poor　electrode-electrolyte　interface,　low　ion　conductivity,　and　inferior　electrochemical　stability.　Herein,　superior　nonflammable　solid　polymer　electrolytes　are　elaborately　designed　by　in　situ　encapsulating　succinonitrile　(SN)-based　deep　eutectic　solvent　(DES)　into　the　ethoxylated　trimethylolpropane　triacrylate　(ETPTA)　matrix　(DES-ETPTA).　Benefiting　from　strong　polarity　and　high　anti-oxidation　capability,　as-prepared　DES-ETPTA　electrolyte　shows　high　ionic　conductivity　(9.55　x　10-4　S　cm-1　at　30　degrees　C),　high　Li+　transference　number　(0.68),　and　good　electrochemical　stability.　As　a　result,　the　assembled　LiFePO4　||　Li　full　cells　based　on　the　designed　DES-ETPTA　electrolyte　deliver　a　high　reversible　capacity　and　capacity　retention　at　-10　degrees　C　and　room　temperature.　Furthermore,　considering　the　compatibility　with　high-voltage　layered　oxide　cathode,　the　electrochemical　stability　of　the　ETPTA　is　further　improved　through　the　decoration　of　cyanoacrylate　(CA)　with　strong　electron-withdrawing　characteristic　of　C　equivalent　to　N.　Consequently,　the　constructed　4.5　V　LiCoO2　||　Li　full　cells　using　DES-ETPTA-CA　electrolyte　deliver　a　high　reversible　capacity　of　144　mAh　g-1　and　a　superior　retention　rate　of　93%　after　200　cycles　at　0.5　C.　This　work　paves　a　new　pathway　to　design　high-safety　and　high-voltage　solid　polymer　electrolytes　for　lithium　metal　batteries.　In　situ　polymerization　is　proposed　to　design　high　ionic　conductivity　and　good　interface　contact　solid　polymer　electrolytes.　The　constructed　LiFePO4　(LFP)　||　Li　cells　with　elaborately　designed　electrolyte　exhibit　100%　capacity　retention　after　100　cycles　at　-10　degrees　C.　The　electrochemical　stability　of　electrolyte　is　further　enhanced　by　cyanoacrylate　decoration.　And　the　constructed　LiCoO2　(LCO)　||　Li　cell　with　modified　electrolyte　shows　93%　of　capacity　retention　under　4.5　V　cut-off　voltage　after　200　cycles.　image