NOVELTY - A lithium ion secondary battery consists of positive electrode having positive mix layer containing positive electrode active material provided on surface(s) of collector, negative electrode having negative mix layer containing carbon material as negative electrode active material provided on surface(s) of collector, separator and nonaqueous electrolyte. The positive electrode contains lithium-nickel complex oxide (I) as positive electrode active material. The ratio of swollen microparticles in the total volume of the structural component is 6-95 vol.%.    USE - Lithium ion secondary battery used as power supply. Uses included but are not limited to mobile telephone, notebook personal computer, electric vehicle and hybrid vehicle.    ADVANTAGE - The lithium ion secondary battery has high output characteristics and excellent reliability.    DETAILED DESCRIPTION - A lithium ion secondary battery consists of positive electrode having positive mix layer containing positive electrode active material provided on surface(s) of collector, negative electrode having negative mix layer containing carbon material as negative electrode active material provided on surface(s) of collector, separator and nonaqueous electrolyte. The positive electrode contains lithium-nickel complex oxide of formula: Li1-aNixMnyMzO2 (I) as positive electrode active material. The ratio of swollen microparticles in the total volume of the structural component is 6-95 vol.%, when the electric potential of the negative electrode is 0.05 V. The structural component contains swollen microparticles and heat-resistant microparticles absorbed in the separator and swollen by heating. A porous layer (A) containing heat-resistant microparticles and having thickness of 3 mu m or more, and heat-resistant porous layer (B) with thickness of 3 mu m or more and resin film (C) made of polyolefin are contained in the separator. The layer (B) is formed on the surface of layer (C). The ratio of heat-resistant microparticles in the layer (A) is less than 6 vol.% with respect to the total volume of the structural component excluding the swollen microparticles. The battery has power density of 1000 W/kg or more.    M=titanium, chromium, iron, cobalt, copper, zinc, aluminum, germanium, tin, magnesium, or zirconium;    a=0.4-0.7;and    x+y+z=1.    x,y,z satisfy the relations: x greater than or equal to y greater than 0 and x greater than or equal to z greater than 0.