A battery includes a substrate; a composite cathode disposed on the substrate; a solid-state electrolyte disposed on the composite cathode; and a lithium anode disposed on the solid-state electrolyte, such that the composite cathode comprises a gel polymer electrolyte layer and a porous cathode active material layer. A method of forming a cathode for a solid-state battery includes mixing an active cathode material, at least one of a conductive carbon component and an electronic conductive component, and a polymer binder to form a slurry; immersing the slurry in an alcohol reagent to form a porous disc structure by phase conversion; and immersing the porous disc structure in a liquid electrolyte to form the cathode.