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Fully solid-state lithium-ion cells have the benefits of excellent safety, remarkable electrochemical stability, and extended cycle life. They are categorized as prospective candidates for new-generation energy storage devices capable of safe and stable operation for relatively long periods. In whole solid-state Li-ion cells, polymer-based solid electrolyte membranes (SEMs) are expected to serve as both the solid electrolyte and the partition material. Remarkably high ionic conductivity and efficient ion diffusion can be achieved in SEMs by incorporating inorganic filler materials. In the present work, solid electrolyte membranes are developed by a simple solution cast method. The addition of Al2O3 nanoparticles is found to enhance the ionic conductivity of SE membranes to 1.25 × 10−4 S cm−1 for optimum concentrations of lithium salt and filler material. These SE membranes show relatively high electrochemical stability window up to 4.75 V. Transparent and freestanding SE membranes developed using PEO-PVDF-LiNO3-Al2O3 have excellent thermal stability, ideal ion transport number, and good electrochemical properties, suitable for applications in solid-state lithium-ion cells. Assembled LiFePO4-MWCNT//SEM//Li metal half-cells are found to deliver an initial discharge capacity of 128 mAh g−1 at 0.1 C and an initial Coulombic efficiency of 98%. [ABSTRACT FROM AUTHOR]