Development of solid polymer electrolytes (SPEs) with high thermal and mechanical stability and fast ion conductivity is the key for high-performance lithium metal batteries (LMBs) with high safety. In this work, a flexible poly-(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide) (LiTFSI) SPE assisted with a bifunctional ionic liquid (IL) of tetrabutylphosphonium 2-hydroxypyridine (TBPHP) as well as a garnet-type fast-ion conducting ceramic of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was fabricated via a solvent-free procedure and applied for the LMBs. It is found that the tetrabutylphosphonium cation (TBP+) and 2-hydroxypyridine anion (HP-) of IL effectively tuned the crystallinity of PEO and enhanced lithium ion (Li+) transference, respectively, and the TBPHP and LLZTO played a synergistic role in improving the performance of SPEs. Meanwhile, the density functional theory (DFT) study was performed to understand the interaction between TBPHP and LiTFSI. As a result, the obtained PEO8-LiTFSI-TBPHP-12.5% LLZTO composite SPE possessed a high ionic conductivity of 9.39 x 10(-4) S cm(-1) at 50 degrees C and a wide electrochemical stability window (more than 5 V) with significantly promoted uniform Li plating/stripping properties. The quasi-solid-state-LMBs assembled with LiFePO4 cathode also rendered excellent cycling stability with a high discharge capacity above 150 mA h.g(-1) even after 100 cycles at 0.2 C and 50 degrees C. Besides, the fabricated flexible pouch cell showed excellent performance. It is expected that such a bifunctional ionic liquid and conducting ceramic co-assisted polymer composite could be a promising solid electrolyte for the next-generation of safe LMBs.