A lithium battery is provided which can promptly cut off current when it is being overheated by a short or the like, thereby preventing an explosion thereof. A shape-memory alloy spring made of a nickel-titanium alloy is included between the cover and the cap, either for making contact with both the cover and the cap, thereby electrically connecting the cover with the cap, or for separating from the cap in the case of an overheating of the battery, thereby electrically disconnecting the cover from the cap. A semi-spherical protrusion is formed at the center of the spring so as to make contact with the bottom of the cap, so that if the battery is overheated, the protrusion becomes flat, and is separated from the cap, thereby electrically disconnecting the cap from the cover. The semi-spherical protrusion of the shape-memory alloy spring repeatedly contracts and expands so as to prevent the overheating of the battery.