Forming a lithium lanthanum zirconate thin film includes disposing zirconium oxide on a substrate to yield a zirconium oxide coating, contacting the zirconium oxide coating with a solution including a lithium salt and a lanthanum salt, heating the substrate to yield a dried salt coating on the zirconium oxide coating, melting the dried salt coating to yield a molten salt mixture, reacting the molten salt mixture with the zirconium oxide coating to yield lithium lanthanum zirconate, and cooling the lithium lanthanum zirconate to yield a lithium lanthanum zirconate coating on the substrate. In some cases, the zirconium oxide coating is contacted with an aqueous molten salt mixture including a lithium salt and a lanthanum salt, the molten salt mixture is reacted with the zirconium oxide coating to yield lithium lanthanum zirconate, and the lithium lanthanum zirconate is cooled to yield a lithium lanthanum zirconate coating on the substrate.