High-performance solid-state lithium-metal batteries (SSLMBs) require solid electrolytes displaying outstanding electrochemical stability, excellent ionic conductivity, and high Li+ ion transference number. On top of these, it should also be compatible with the electrodes applied and functionable under room temperature. To achieve these, a solution-casting technique is proposed herein to prepare a flexible composite polymer electrolyte (CPE), which is equipped with a high ionic conductivity and Li+ ion transference number, concurrently applicable in the construction of high-voltage solid-state Li batteries. The proposed CPE, which is made up of poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP)/polypropylene carbonate (PPC) blend with an Al-doped Li7La3Zr2O12 (Al-LLZO) filler, was sandwiched between PVDF-HFP/PPC-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) skin layers with SN plasticizer added. This formulation of PVDF-HFP/PPC/Al-LLZO/LiTFSI/SN was abbreviated as sandwich-PPA in our study. Such configuration permits notable resistance reduction at the electrode-electrolyte interface while suppressing Li dendrite growth throughout the robust charging-discharging process. This can be attributed to the excellent performance of the sandwich composite electrolyte membrane, which promises high ionic conductivity (ca. 4.04 × 10-4 S cm-1) and a high Li+ ion transference number (ca. 0.583) at room temperature. A CR2032 coin cell, which is assembled with Al2O3-C@NCA/Sandwich-PPA/Li, delivered a high specific capacity (186.20 mAh g-1 at 0.1C at room temperature), along with its excellent rate performance and cycle stability (discharge capacity of 126.23 mAh g-1; capacity retention of 80.03% after 100 cycles at a rate of 0.5C at room temperature). This verified the potential of our novelty-formulated solid-state electrolyte to secure excellent performance of SSLMBs.