A poly(vinylidene fluoride)/ethyl cellulose and amino-functionalized nano-SiO2 (PVDF-EC-(A-SiO2)) composite coated on polyethylene (PE) is developed as the functional separator of 5 V high-voltage lithium ion batteries with enhanced performance. Compared with the PE separator, the composite membrane has a uniform interconnected structure, an increased melting temperature from 138 °C (PE) to 140.8 °C, a decreased contact angle from 49.5° to 23.2°, and higher ionic conductivity from 0.34 mS cm−1 to 0.79 mS cm−1. In particular, the electrochemical window is raised to 5.3 V (vs. Li/Li+). LiNi0.5Mn1.5O4/Li batteries with this separator display a high discharge capacity of 131.6 mAh g−1 and a superior cycling stability with a capacity retention of 95.7% after 200 cycles at 0.5 C. Those results demonstrate that the PVDF-EC-(A-SiO2)/PE composite separator exhibits a good-performance in cycling stability and thermal stability, implying promising potential application in high-voltage lithium ion batteries.