A composite electrolyte including a lithium salt; a solid electrolyte wherein the solid electrolyte is a sulfide solid electrolyte, an oxide solid electrolyte, or a combination thereof; and an ionic liquid, wherein a mixture of the ionic liquid and the lithium salt has a dielectric constant of from about 4 to about 12, and an amount of halogen ions eluted from the composite electrolyte after immersion of the solid electrolyte in the ionic liquid for 24 hours is less than about 25 parts per million by weight, based on the total weight of the composite electrolyte, as measured by ion chromatography.