We demonstrate a new silica (SiO2) nanoparticle/polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP)-coated polyethylene terephthalate (PET) nonwoven composite separator for a lithium-ion battery as a promising alternative to a commercialized polyethylene (PE) separator. A distinctive feature of the nonwoven composite separator is its porous structure, i.e. well-connected interstitial voids formed between close-packed SiO2 nanoparticles interconnected by PVdF-HFP binders. More notably, this unusual porous structure of the nonwoven composite separator can be tuned by controlling the SiO2 particle size (herein, 40 nm and 530 nm SiO2 powders are exploited). Such morphological variation of the nonwoven composite separator has a crucial influence on separator properties such as porosity, electrolyte wettability, and ionic conductivity. The PET nonwoven is employed as a physical support to prevent thermal shrinkage of the nonwoven composite separator. Based on this understanding of the separator characteristics, the effects of the SiO2 particle size-dependent, tunable porous structure of nonwoven composite separators on the electrochemical performance of cells are investigated. In comparison to a nonwoven composite separator containing 530 nm SiO2 particles as well as the conventional PE separator, the nonwoven composite separator incorporating 40 nm SiO2 particles provides superior cell performance owing to facile ion transport and retarded growth of cell impedance during cycling.