A method is provided for fabricating a thin film solid-state Li-ion battery comprising a first electrode layer, a solid electrolyte layer, and a second electrode layer. The method comprises depositing, on a substrate, an initial layer stack comprising a first layer comprising a first electrode material compound, and a second layer comprising an electrolyte material compound; and afterwards performing a lithiation step comprising incorporating Li in the first layer and in the second layer, thereby forming a stack of a first electrode layer and a solid electrolyte layer. The initial layer stack may further comprise a third layer comprising a second electrode material compound. By performing the lithiation step, Li is also incorporated in the third layer, such that a stack of a first electrode layer, a solid electrolyte layer, and a second electrode layer is formed. One or more of the first, second, or third layers may be Li-free.