A new gel polymer electrolyte (GPE) based on ethyl cellulose/poly(vinylidene fluoride) (EC/PVDF) blended polymer was prepared by immersion precipitation phase inversion. The structure of the porous membrane was controlled by the addition of EC. The performance of the membranes with different ratios of EC to PVDF were characterized by X-ray diffraction spectroscopy (XRD), scan electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), linear sweep voltammetry (LSV), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The results show that as the ratio of EC to PVDF rose to 3:7 (wt.%), the membrane presented the best thermal stability. The ionic conductivity of this GPE was 1.33 mS cm−1, which was much higher than the PVDF membrane (0.69 mS cm−1) at room temperature. The electrochemical window of this GPE was established to be as high as 5.25 V (vs. Li/Li+). The Li/LiNi0.5Mn1.5O4 coin cell with this GPE displays excellent cyclic stability and rate performance at room temperature compared to PVDF membrane. This suggests that the self-supporting EC/PVDF blended GPE has a high potential for 5 V high-voltage lithium-ion batteries (LIBs).