A plurality of unit cells is stacked on each other in a secondary battery, and each of the plurality of unit cells includes first and second collector layers, which are spaced apart from each other, and a 3-dimensional (“3D”) electrode structure provided between the first and second collector layers and having an outer side surface that is externally exposed and insulated, wherein, in the plurality of unit cells, the first collector layers are stacked to face each other and the second collector layers are stacked to face each other.