Disclosed is a secondary battery including a cathode, an anode, a membrane and an electrolyte, wherein the cathode contains a mixture of a first cathode material defined herein and a second cathode material selected from the group consisting of a second-(a) cathode material defined herein and a second-(b) cathode material defined herein, and a combination thereof, wherein a mix ratio of the two cathode materials (first cathode material: second cathode material) is 50:50 to 90:10, and the membrane is an organic/inorganic composite porous membrane including (a) a polyolefin-based membrane substrate and (b) an active layer in which one or more areas selected from the group consisting of the surface of the substrate and a portion of pores of the substrate are coated with a mixture of inorganic particles and a binder polymer, wherein the active layer has a structure in which the inorganic particles are interconnected and fixed through a binder polymer and porous structures are formed by the interstitial volume between the inorganic particles.