NOVELTY - A nonaqueous electrolyte secondary battery (10) consists of a positive electrode (11) containing an active material which occludes/releases lithium ion, a negative electrode (12) containing an active material which occludes/releases lithium ion, a separator (13), and a nonaqueous electrolyte. The positive-electrode active material contains a lithium-cobalt complex oxide containing aluminum and/or magnesium. The negative-electrode active material contains silicon oxide. The electrolyte contains e.g. ethylene carbonate and/or fluoroethylene carbonate.    USE - Nonaqueous electrolyte secondary battery e.g. lithium ion battery. Uses include but are not limited to mobile telephones, smart phones, portable computers, personal digital assistants, portable music players, electric vehicles, hybrid electric vehicles, photovoltaic power generation, and wind power generation.    ADVANTAGE - The nonaqueous electrolyte secondary battery has reduced swelling and gas generation at charge final voltage of positive electrode, and long cycle life at high temperature at a high voltage of 4.4-4.6 V.    DETAILED DESCRIPTION - A nonaqueous electrolyte secondary battery consists of a positive electrode containing an active material which occludes/releases lithium ion, a negative electrode containing an active material which occludes/releases lithium ion, a separator, and a nonaqueous electrolyte. The positive-electrode active material contains a lithium-cobalt complex oxide containing aluminum and/or magnesium. The negative-electrode active material contains silicon oxide of formula: SiOx, where x is 0.5-1.6. The electrolyte contains ethylene carbonate, lactone, and/or fluoroethylene carbonate as nonaqueous solvent.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the square nonaqueous electrolyte secondary battery.    Nonaqueous electrolyte secondary battery (10)    Positive electrode (11)    Negative electrode (12)    Separator (13)    Wound electrode (14)