In recent years, automobiles using secondary batteries as energy sources have been put into practical use, for the reduction of carbon-dioxide emissions, concern over the depletion of fossil fuels such as gasoline. The secondary batteries are required to be high in output power, high in energy density, reduced in size and weight, low in price, or the like, and the improvement in safety and durability is also indispensable for the secondary batteries.
Lithium ion secondary batteries are known as high energy-density secondary batteries for automobiles. The high energy-density lithium ion secondary battery typically has an electrode assembly obtained by rolling up a positive electrode and a negative electrode stacked with a separator interposed therebetween, impregnated with an organic electrolyte solution, and enclosed in a battery can.
Because of the use of an organic electrolyte solution in the lithium ion secondary battery, in the case of reaching an overcharge condition, there is a possibility of not only increasing the battery voltage, but also increasing the pressure gas inside the battery, and further increasing the temperature in the battery, thereby leading to situations such as leakage of the electrolyte solution and rupture of the can. The secondary batteries for automobiles are expected to be subjected to the frequent repetition of charge and discharge, and measures against overcharge are thus taken to ensure safety.
As a conventional measure against overcharge, it is known that when the voltage between terminals of a zener diode exceeds a predetermined range, an opening formed in an exterior member is released by a heat generation unit to discharge gas generated by overcharge from the inside of the exterior member.
In addition, as other measure against overcharge, it is also known that safety is ensured by providing a temperature fuse inside a battery, and providing an overcharge preventer for interrupting a charging current by melting down the temperature fuse when the temperature of an electrode assembly is increased.