a body including positive electrodes and negative electrodes stacked in one direction with solid electrolyte layers interposed therebetween;

a first external electrode disposed on one surface of the body from which the positive electrodes extend, and connected to the positive electrodes; and

a second external electrode disposed on the other surface of the body, which opposes the one surface in another direction different from the one direction and from which the negative electrodes extend, and connected to the negative electrodes,

wherein one of the positive electrodes comprises a positive electrode active material layer and a first electrolytic mixing portion disposed at an interface of the one of the positive electrodes in contact with one of the solid electrolyte layers,

wherein one of the negative electrodes comprises a negative electrode active material layer and a second electrolytic mixing portion disposed at an interface of the one of the negative electrodes in contact with the one of the solid electrolyte layers,

wherein the first electrolytic mixing portion and the second electrolytic mixing portion are in direct contact with the one of the solid electrolyte layers and are disposed on opposing sides of the one of the solid electrolyte layers,

wherein the first electrolytic mixing portion is a mixture of a positive electrode active material and a liquid phase and/or gel phase electrolyte, and the second electrolytic mixing portion is a mixture of a negative electrode active material and a liquid phase and/or gel phase electrolyte,

wherein the solid electrolyte layers comprise oxides,

wherein a center portion of the one of the solid electrolyte layers, disposed between the first electrolytic mixing portion and the second electrolytic mixing portion in the one direction, is spaced apart from the liquid phase and/or gel phase electrolyte of the first electrolytic mixing portion and the liquid phase and/or gel phase electrolyte of the second electrolytic mixing portion,

wherein a center portion of the one of the positive electrodes in the one direction is spaced apart from any liquid phase and/or gel phase electrolyte,

wherein a center portion of the one of the negative electrodes in the one direction is spaced apart from any liquid phase and/or gel phase electrolyte,

wherein the one of the positive electrodes comprises at least one or more of a recess or a pore on a surface of the one of the positive electrodes, and

wherein the liquid phase or gel phase electrolyte is disposed in the recess and/or the pore.