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In this study, robust composite solid electrolytes were developed and employed to enhance the performance of Li-metal batteries significantly. The robust composite solid electrolytes are composed of a soft polymer, poly(ethylene oxide), a Li salt, bis(trifluoromethanesulfonyl)imide (LiTFSI), and super ionic conductive ceramic fillers such as Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP), and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). The main goal of this study is to enhance the electrochemical stability and ionic conductivity. The ionic conductivities of the composite solid electrolytes were found to be 2.08 × 10⁻⁴ and 1.64 × 10⁻⁴ S cm⁻¹ with the introduction of LATP and LLZTO fillers, respectively. The results prove that the fabricated solid electrolyte was electrochemical stable at voltage exceeding 4.25 V vs. Li/Li⁺. The internal resistance of the solid electrolyte significantly reduced compared to gel electrolyte. This reduction can be attributed to the alleviation of bulk electrolyte, charge-transfer, and interfacial electrolyte/electrode impedance. When LiFePO₄ cathode sheets are coated with a composite solid electrolyte containing LATP powders, the resulting Li-metal battery displays high capacity at 5 C (with a capacity retention of 65.2% compared to the original capacity at 0.2 C) as well as superior cyclic stability and excellent Coulombic efficiency (>99.5%, 200 cycles). These results confirm that the composite solid electrolyte acts as a protective layer which has the ability to prevent the growth of Li dendrites. Consequently, the fabricated electrolyte configuration can be engineered to enable high energy/power density and electrochemical stable cyclability in Li-metal batteries.