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In this work, highly ionically conductive Li 6.5 La 3 Zr 1.5 Ta 0.1 Nb 0.4 O 12 (LLZTNO) fillers were blended into a poly(ethylene oxide) matrix to construct a composite solid-state electrolyte (CSSE). The LLZTNO fillers decrease the crystallinity of the PEO and provide extra lithium-ion transport pathways. Additionally, TFSI− anions are immobilized by the LLZTNO ceramic powders, and more Li ions are mobilized, increasing the overall ionic conductivity in CSSE. The PEO-LiTFSI-10% LLZTNO (i.e., PL-LLZTNO10%) CSSE shows the highest conductivity of 0.36 × 10−4 S cm−1 with a broad voltage window of 4.7 V at room temperature. The Li/ PL-LLZTNO10% CSSE/Li symmetric cells could operate for approximately 1200 h at 63 mV polarization voltage. The assembled solid-state LFP/PL-LLZTNO10%/Li cells deliver a 158.3 mAh g−1 capacity at 0.2C and 60 °C. After 300 cycles, its specific discharge capacity remains at 143.5 mAh g−1 with a Coulombic efficiency of 99.54%. [Display omitted] • PEO-LLZTNO composite electrolyte shows a broad electrochemical window of 4.7 V; • The assembled symmetric cell operates for 1200 h without short circuit; • The critical current density of PEO-LLZTNO electrolyte could reach 0.3 mA/cm2 with a capacity of 0.3 mAh/cm2; • The assembled composite solid-state lithium metal cells deliver 143.5 mAh g−1 with 99.54% Coulombic efficiency after 300 cycles. [ABSTRACT FROM AUTHOR]