A composite separator includes: a porous substrate; and a composite electrolyte on a surface of the porous substrate, the composite electrolyte including block copolymer, an ionic liquid, and a particle, wherein a size of the particle is larger than a pore size of the porous substrate, the particle includes an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, and the particle has a particle size of greater than about 1 micrometer to about 100 micrometers.