Lithium ion secondary battery

A lithium ion secondary battery comprising:

• an electrode body having a positive electrode and a negative electrode;
• and
• a nonaqueous electrolytic solution,
• the lithium ion secondary battery further comprising a pressure-type safety mechanism for changing an energization state according to a battery internal pressure,
• wherein the nonaqueous electrolytic solution includes a gas generating agent which reacts at a voltage equal to or higher than a predetermined voltage to generate a gas,
• the positive electrode has a positive electrode active material layer including a positive electrode active material,
• the positive electrode active material includes a first lithium transition metal composite oxide represented by the general formula LiNiaCobMncO2 wherein a, b and c are values satisfying the following conditions:
• 0.35≦a≦0.45, 0.15≦b≦0.25, 0.35≦c≦0.45, and a+b+c=1, and a second lithium transition metal composite oxide represented by the general formula LiNixCoyMnzO2 wherein x, y and z are values satisfying the following conditions:
• 0.35≦x≦0.45, 0.45≦y≦0.55, 0.05≦z≦0.15, and x+y+z=1, and
• a mass ratio of the first lithium transition metal composite oxide and the second lithium transition metal composite oxide is 60:40 to 85:15.