Provided are an energy storage device including an electrode in which lithium is introduced into a silicon layer and a method for manufacturing the energy storage device. A silicon layer is formed over a current collector, a solution including lithium is applied on the silicon layer, and heat treatment is performed thereon; thus, at least lithium can be introduced into the silicon layer. By using the solution including lithium, even when the silicon layer includes a plurality of silicon microparticles, the solution including lithium can enter a space between the microparticles and lithium can be introduced into the silicon microparticles which are in contact with the solution including lithium. Moreover, even when the silicon layer is a thin silicon film or includes a plurality of whiskers or whisker groups, the solution can be uniformly applied; accordingly, lithium can be included in silicon easily.