A method for manufacturing a gel membrane for a battery cell includes supplying a first substrate to a first roller; heating a gel membrane solution in a tank, wherein the gel membrane solution includes a polymer and a liquid electrolyte comprising one or more lithium salts; arranging a slot die within a predetermined distance of the first substrate located on the first roller; pumping the gel membrane solution into an inlet of the slot die; depositing a gel membrane layer from an outlet of the slot die onto the first substrate supported by the first roller; and cooling the gel membrane layer on the first substrate.