NOVELTY - A lithium ion secondary battery (1) includes a flat-plate electrode assembly (2) having a positive electrode (3), a separator (5), and a negative electrode (4); and a non-aqueous electrolyte. Each electrode includes an active material layer and a current collector (10) including rectangular current collector body on surface of which the active material layer is formed, and a heat radiating portion (10b, 12b) that extends from at least a portion or all of two adjacent sides of the current collector body toward outside of the electrode assembly.    USE - As lithium ion secondary battery useful as a power source for portable small electronic devices, such as mobile phones, personal digital assistants (PDAs), notebook personal computers, camcorders, portable game devices, personal computers, mobile phones, portable devices, portable game consoles, and camcorders. And also useful as a secondary battery for supporting an electric motor in hybrid electric vehicles and fuel cell vehicles; a power source for electrically-powered tools, vacuum cleaners, robots; and a power supply for plug-in hybrid electric vehicles (HEVs).    ADVANTAGE - The heat caused inside the lithium ion secondary battery can be diffused efficiently to the outside, and safety of the lithium ion secondary battery can be further increased, without complicating the battery structure and decreasing mechanical strength of the battery. By forming the heat radiating portion as a single unit without dividing it into several portions, the heat radiation area of the heat radiating portion can be increased, and heat radiation efficiency can be further improved. Additionally, because the mechanical strength of the heat radiating portion and the lithium ion secondary battery is improved and the overall weight balance of the lithium ion secondary battery can be equalized, long time durability of the lithium ion secondary battery can be improved. The lead portion provides electric power to the outside of the lithium ion secondary battery, and also functions to release heat as an extended portion of the heat radiating portion, and therefore the heat radiation effect is further increased. Particularly, because the other end of the lead portion is drawn out of the lithium ion secondary battery, heat radiation by the lead portion is carried out outside the lithium ion secondary battery, increasing the heat radiation effect. The heat radiating portion is provided so that when the electrode assembly is placed on the horizontal plane with the positive electrode at its bottom, a plane of projection of the electrode assembly as viewed from vertically does not overlap with a plane of projection of the heat radiating portion of the positive electrode. In this way, possibility of decline in heat radiation efficiency is minimized.    DETAILED DESCRIPTION - A lithium ion secondary battery (1) includes a flat-plate electrode assembly (2) in which a positive electrode (3), a separator (5), and a negative electrode (4) are stacked in the thickness direction of the flat-plate electrode assembly; and a non-aqueous electrolyte. Each positive electrode and the negative electrode include an active material layer capable of absorbing and desorbing lithium and a current collector (10). The current collector includes rectangular current collector body on the surface of which the active material layer is formed; and a heat radiating portion (10b, 12b) that extends from at least a portion or all of two adjacent sides of the current collector body toward the outside of the flat-plate electrode assembly. The current collector body is present in the flat-plate electrode assembly. The heat radiating portion is formed as a single unit. The ratio of area of the heat radiating portion relative to area of the current collector body is 0.03-1.5. The ratio of length of a boundary between the current collector body and the heat radiating portion relative to peripheral length of the current collector body is greater than or equal to 0.25. A plane of projection of the heat radiating portion of positive electrode current collector does not overlap with a plane of projection of the heat radiating portion of the negative electrode current collector. The negative electrode active material layer includes an alloy-based negative electrode active material capable of absorbing and desorbing lithium.    DESCRIPTION OF DRAWING(S) - The figure shows a vertical cross sectional view of the lithium ion secondary battery.    Lithium ion secondary battery (1)    Flat-plate electrode assembly (2)    Positive electrode (3)    Negative electrode (4)    Separator (5)    Current collector (10)    Heat radiating portion (10b, 12b)