In this disclosure, we report development of porous fluoropolymers/silica composite separators for applications in redox flow batteries. These separators, conprised of a fibril matrix of fluorinated polyolefins and silica particles enmeshed therein, were prepared by shear-blending the silica particles and the aqueous dispersion of fluoropolymers, followed by calendering the resulting admixture into flat sheets. Flow batteries using these separators demonstrated high energy density and high efficiencies. These separators combine the advantages of exceptional chemical stability and significantly low cost of fluorinated polyolefins. Therefore, these separators have shown great potential to reduce the capital cost of redox flow batteries and promote their commercialization.