A method for manufacturing a solid electrolyte includes dissolving first polymers and second polymers in a cosolvent including a first cosolvent and a second cosolvent to provide a preparation solution; adding a lithium solution to the preparation solution to provide a mixture solution; removing the second cosolvent from the mixture solution to prepare an electrolyte paste that exhibits thixotropy; and coating the electrolyte paste onto a substrate to form an electrolyte film, wherein the electrolyte paste in a gel state is characterized by the first polymers being aligned in parallel to one another in one direction; the second polymers randomly surrounding the first polymers and having an average molecular weight that is greater than that of the first polymers; and a lithium solution provided between the first polymers and the second polymers.