Composite polymer electrolytes (CPEs) offer a promising route toward practical lithium metal batteries. However, their performance in simple organic-inorganic composite systems remains limited. Herein, we engineer fast Li⁺ conductor Li_6.4La₃Zr_1.4Ta_0.6O₂ (LLZTO) as a solid inorganic plasticizer (SIP) through interfacial modification with acidic ionic liquid (1-carboxyethyl-3-methylimidazolium trifluoromethanesulfonate). This design enables stable electrochemical performance in CPE-based high-voltage lithium-metal batteries (HV-LMBs) across a wide temperature range. Combined experimental and theoretical investigations demonstrate that the achieved wide-temperature stability stems from markedly enhanced composite compatibility and the construction of multidimensional ion-transport pathways. The SIP sustains efficiently fast Li⁺ conduction at low temperatures and simultaneously preserves stable and unobstructed Li⁺ transport pathways at elevated temperatures. Furthermore, the SIP significantly improves the membrane fracture strength and fracture strain compared with conventional organic-inorganic composites. Thus, the resulting CPEs enable wide-temperature stability of HV-LMBs paired with Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathodes over a range from −20 °C to 80 °C. This work effectively enhances the compatibility of organic-inorganic composites in CPEs by designing an ionic liquid interfacial layer, offering a new approach for developing CPEs-based HV-LMBs with high specific energy density and a wide operating temperature range.