Provided is a secondary battery, specifically, a secondary battery having excellent stability and improved output characteristic and low temperature characteristic by including a cathode active material in which at least one of metals forming the cathode active material has a concentration gradient in at least a portion of a region between a central portion and a surface portion; and a conductive material mixture in which carbon nanotube is mixed with carbon black at an appropriate ratio, the carbon black being a spherical nanoparticle.