NOVELTY - A secondary battery comprises positive electrode, negative electrode, separator membrane and electrolyte. The positive electrode contains mixture of primary positive electrode material and secondary positive electrode material in ratio of 50:50-90:10. The separator membrane is organic/inorganic composite microporous separator membrane comprising polyolefin-based separator membrane base and active layer. The primary positive electrode material includes lithium-cobalt-metal oxide. The secondary positive electrode material contains lithium-nickel-manganese cobaltate.    USE - Secondary battery e.g. lithium secondary battery (claimed) used as power supply for wireless communication apparatus and hybrid electric vehicles.    ADVANTAGE - The lithium secondary battery is highly safe, and has high capacitance and charging/discharging characteristics.    DETAILED DESCRIPTION - A secondary battery comprises positive electrode, negative electrode, separator membrane and electrolyte. The positive electrode contains mixture of primary positive electrode material and secondary positive electrode material in ratio of 50:50-90:10. The separator membrane is organic/inorganic composite microporous separator membrane comprising polyolefin-based separator membrane base and active layer in which its surface and portion of pores are coated with mixture of inorganic particles and binder polymer. The active layer has structure in which inorganic particles are connected and fixed by the binder polymer, and in which micropores are formed by the interstitial volume between inorganic particles. The primary positive electrode material includes lithium-cobalt-metal oxide of formula: Lix(CoyAmD'z)Ot. The secondary positive electrode material contains lithium-nickel-manganese cobaltate of formula: Lix(Ni1-a-bMnaCob)yO2, where A is magnesium or calcium, D' is titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, tellurium, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, platinum, palladium, copper, gold, silver, zinc, cadmium, mercury, boron, aluminum, gallium, indium, thallium, carbon, silicon, germanium, tin, lead, nitrogen, phosphorus, arsenic, antimony or bismuth, x is 0.8 to 1.2, t is 1.8 to 4.2, y is (0.8-m-z) to (2.2-m-z), z and m are 0 to 0.3, a is 0.05 to 0.4, b is 0.1 to 0.4, x is 0.95 to 1.05, sum of x and y is 1.9 to 2.3, and difference between 1-a-b is 0.4 to 0.7. The ratio of the content of nickel with respect to content of manganese, is more than 1.1 to less than 1.5. The ratio of the content of nickel having oxidation state of 2 or more with respect to content of manganese having oxidation state of 4 or more, is more than 0.4 to less than 1.