In this study, we report a newly designed crosslinked PEO-in-concentrated ionic liquid (CIL) SPEs for use in high energy solid-state LMBs. The proper combination of using CIL and UV-crosslinking shows synergistic advantages in reducing PEO crystallinity, increasing ionic conductivity, and enhancing the electrochemical stability of the designed SPEs. These innovations yield crosslinked SPE10:1:1-CL with high ambient temperature ionic conductivity of 4 x10 -4 S cm -1 and a wide electrochemical stability window reaching up to 4.9 V vs. Li + /Li. Raman analysis confirms that these promising properties are ascribed to enhanced Li + solvation and increased Li + -EO coordination after crosslinking, which significantly enhances SPE oxidation stability. The crosslinked PCIL-SPEs demonstrated stable Li symmetric plating/stripping cycling and remarkable electrochemical performance (almost full capacity, excellent coulombic efficiency, and stable cycling even at high current regimes) with both high loading LFP and, particularly, high energy LMO cathodes, highlighting their strong potential for practical applications in high energy density solidstate LMBs. This study not only advances the development of PEO-based SPEs but also underscores their potential to impact the broader field of sustainable and high-performance energy storage solutions.