A phase-change electrolyte separator layer can include a non-reactive scaffold that has open spaces. A lithium liquid may be used that transitions into a lithium gel, the lithium liquid can include a mixture of a polymer additive, a cross-linker additive, a lithium salt; and a solvent. The lithium liquid with the polymer additive and the cross-linker additive can be filled into the open spaces within the non-reactive scaffold. The lithium liquid can then be converted into a lithium gel within the non-reactive scaffold following an application of heat while the lithium liquid is within the open spaces within the non-reactive scaffold.