In the current study, a novel needleless electro-spinneret based on the Von Koch curve of Fractal Theory is developed to prepare electrospun membrane. A sandwich-like composite separator is elaborately designed with two layers of Si-O2 nanoparticles doped polyimide membranes being the sheath layers and a single layer of ethylcellulose modified PE membrane being core layer towards its application for separators in high-performance lithium-ion batteries. This tri-layered composite separator combines both thermal shutdown function at low temperature and thermal runaway function at high temperature, where ethylcellulose is used to enhance the interfacial affinity and strength between commercial polyethylene separator and the electrospun polyimide membrane. The two sheath layers of electrospun membrane prepared by SiO2 nanoparticles doped polyimide impart the separator with high thermal stability and high-temperature runaway function at, and the single core layer of ethylcellulose modified polyethylene membrane provides high mechanical property and low-temperature shutdown function. The resultant tri-layered composite membrane is of high potential as lithium-ion battery separators for electrically powered vehicles.