Composite solid-state electrolytes (CSEs) that integrate the merits of different components are considered to be promising candidates for next-generation high-energy density lithium metal batteries. Herein, we have successfully designed a flexible CSE membrane consisting of the ceramic conducting Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) filler, polyethylene oxide (PEO) matrix, and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid. In particular, the addition of ionic liquid (BMP-TFSI) can not only decrease the interface impedance between the polymer and LATP ceramic fillers and improve the ionic conductivity, but also prevent the adverse reaction between Ti⁴⁺ in LATP and Li metal and further enhance the interface stability. Benefitting from the synergistic effect of organic–inorganic hybrids, the obtained composite electrolyte membrane achieves an excellent ionic conductivity of 2.42 × 10⁻⁴ S cm⁻¹ at 30 °C and a wide electrochemical stability window of 5 V (vs. Li⁺/Li). Moreover, the CSE membrane exhibits outstanding Li dendrite suppression capability, which is proved by galvanostatic Li plating/stripping tests for 1000 h. Assembled with this solid electrolyte membrane and a commercial LiFePO₄ cathode, all-solid-state lithium metal batteries demonstrate superior rate capability and outstanding cycling stability at both 30 and 45 °C. These results demonstrate that such a flexible composite electrolyte is a promising alternative electrolyte for practical high-energy density all-solid-state batteries.