A solid electrolyte including a composition represented by Formula 1 below is provided: aLi.sub.2O-bB.sub.2O.sub.3-cM-dX (1) wherein M is at least one selected from the group consisting of TiO.sub.2, V.sub.2O.sub.5, WO.sub.3, and Ta.sub.2O.sub.5; X is at least one selected from LiCl and Li.sub.2SO.sub.4; 0.4