Poly(vinylidene ﬂuoride-co-hexaﬂuoro-propylene) doped with imidazole-modified silica nanoparticles (Z-SiO2) is coated in a polyethylene substrate to form a functional composite polymer electrolyte (PVDF-HFP-(Z-SiO2)/PE-based CPEs) and used for high voltage LiNi0.5Co0.2Mn0.3O2 cathode lithium-ion batteries (LIBs). The imidazole-based modified SiO2 nanoparticles are first prepared via a distillation precipitation polymerization. The composite separators with 30 wt% Z-SiO2 nanoparticles prepared via a dip-coating process exhibits a porous and uniformly dispersed morphology and enhanced performance, including excellent electrolyte uptake (310%), high ionic conductivity of 1.03 mS cm−1, and oxidative decomposition voltage up to 4.75 V. More importantly, a stable cathode electrolyte interphase (CEI) layer can be formed, endowing the Li/PVDF-HFP-(Z-SiO2)/PE-based CPEs/LiNi0.5Co0.2Mn0.3O2 (NCM523) cells superior cycling stability and rate capability (169 mAh g−1, 81.9%) under when the charge cut-off voltage increased to 4.5 V, which is higher than that assembled with PE separator (160 mAh g−1, 40.8%). These results demonstrate that the Z-SiO2 nanoparticles not only act as the fillers of the CPEs but also are water/acid scavengers in favor of the formation of a stable CEI film, which promotes the cycling performance and rate capability of high voltage NCM523 cathode and reveals promising prospect for practical applications in LIBs at high voltage operation.