In this work, the scalable ceramic-polymer composite electrolytes composed of Li_6.4La₃Zr_1.4Ta_0.6O₁₂, poly (ethylene oxide), lithium bis(trifluoromethane)sulfonimide, and solid plasticizer succinonitrile are prepared in the form of flexible membranes. The ionic transport properties, electrochemical stability, and interfacial behaviors against lithium electrode of this electrolyte are systematically investigated. Among these electrolytes, the sample containing 60 wt.% Li_6.4La₃Zr_1.4Ta_0.6O₁₂ and 10 wt.% succinonitrile presents a maximum conductivity of 1.22 × 10⁻⁴ S cm^-1 at 30 °C, and exhibits a broadened electrochemical stability window of 5.5 V vs. Li/Li⁺. Moreover, the ionic transference number of this electrolyte is improved to 0.41, and the interfacial compatibility against lithium electrode is excellent under both static and dynamic conditions. Excellent cycling and rate performance of the Li/LiFePO₄ cells are resulted from the enhanced ionic transport properties and improved interfacial contact between electrolyte and electrodes. The cell run at 0.5C delivers a discharge specific capacity of 151.1 mAh·g⁻¹ after 200 cycles under 60 °C, and retains 98% of the maximum specific capacity. Notably, this cell also can be successfully charged and discharged at 45 °C and still delivers a discharge capacity of 124.1 mAh·g⁻¹ after 70 cycles at 0.5 C.