The present application claims priority to Japanese Application No. P10-303084 filed Oct. 23, 1998 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a nonaqueous-electrolyte battery incorporating an encapsulating medium constituted by a laminate film and arranged to encapsulate a unit cell.
2. Description of the Related Art
In recent years, electronic apparatuses represented by portable telephones and notebook personal computers have been formed into cordless structures and their sizes have been reduced. Therefore, a variety of portable electronic apparatuses have sequentially been researched and developed, the thicknesses, sizes and weights of which have been reduced. Since the apparatuses have been varied, the quantities of electric power which is consumed by the apparatuses have been enlarged. Therefore, there arises a need for enlarging the capacities of batteries which are energy sources for the foregoing electronic apparatuses, and more particularly enlarging the capacities of secondary batteries.
The conventional secondary batteries are represented by lead batteries and nickel-cadmium batteries. Moreover, new secondary batteries have been put into practical use, the secondary batteries including nickel-hydrogen batteries and lithium-ion batteries. The foregoing secondary batteries arranged to use liquid to serve as an electrolyte, however, suffer from a problem in that liquid leaks.
To overcome the above-mentioned problem, a polymer lithium-ion secondary battery has been developed which uses polymer gels swelled by the electrolytic solution as the electrolyte. Since the polymer lithium-ion secondary battery has been developed, the fear of leakage of the solution from the battery can be eliminated. As a result, a secondary battery exhibiting a small size, light weight and a small thickness and having a high energy density can be realized.
The structure of the polymer lithium-ion secondary battery will now be described. Active materials, made of, for example, LiCoO2 and graphite, are laminated on a positive electrode collector, while active materials made of carbon, cokes and graphite are laminated on a negative electrode constituted by a thin copper plate. Thus, electrodes are formed. Moreover, a separator is sandwiched between the electrodes, the separator being a thin film made of polypropylene, polyethylene or the like and having pores. In addition, a space between the electrode and the separator is filled with polymer gels made of polyacrylonitrile (PAN), polyethylene oxide (PEO) or polyvinylidene fluoride (PVDF). Thus, a sandwich structure is employed.
Each unit cell having the sandwich structure is packaged in an encapsulating medium serving as an encapsulating container constituted by a thin metal film, such as aluminum foil and a plastic film made of nylon, polyethylene, polypropylene or polyethylene terephthalate.
In general, a battery of the foregoing type is mounted in a small space formed in the electronic apparatus in which elements are densely integrated. Therefore, it is preferable that a sheathing material is employed which has satisfactory flexibility against a variety of stresses. The inventors of the present invention have developed a method of manufacturing a card-type battery which is capable of maintaining characteristics required for the battery and which incorporates a flexible encapsulate medium in which the unit cell is encapsulated (refer to Japanese Patent Laid-Open No. 8-83596).
A lithium secondary battery of the foregoing type has characteristics that the battery produces a large output and exhibits a high energy density. Therefore, if short circuit occurs between the positive electrode and the negative electrode owing to a problem of a charging unit or the peripheral circuit of the battery or an incorrect operation of the battery performed by a user, a large electric current flows in the battery. As a result, there is apprehension that rapid heat generation and rise in the internal pressure occur and the battery is broken. If heat is generated in the unit cell owing to the short circuit, there is apprehension that a serious accident occurs. As a safety mechanism capable of preventing the serious accident, a current limiter (for example, a PTC circuit or a fuse) is provided for the body of the battery or the peripheral circuit of the battery, the current limiter being able to limit an instantaneous output current.
The conventional safety mechanism, however, has a possibility that generated heat is accumulated in the battery even after the electric current has been limited. Therefore, development of a new safety mechanism has been required which is capable of preventing generation of heat in addition to the conventional current limiter.
To overcome the foregoing problems experienced with the conventional techniques, an object of the present invention is to quickly disperse heat which is generated when short circuit occurs between a positive electrode and a negative electrode to prevent accumulation of latent heat in the battery.
To achieve the foregoing object, according to one aspect of the present invention, there is provided a nonaqueous-electrolyte battery comprising an encapsulating medium constituted by a laminated film in which a unit cell is accommodated, wherein the laminated film of the encapsulating medium contains a metal material having a heat conductivity k at room temperatures which is 230 Wmxe2x88x921Kxe2x88x921 or higher.
Moreover, the ratio R of the volume of the metal material portion of the encapsulating medium with respect to a capacity of 1 mAh of the unit cell is 0.0002 cm3/mAhxe2x89xa6Rxe2x89xa60.05 cm3/mAh.
Since a metal material having high heat conductivity is employed to constitute the encapsulating medium, dispersion of heat from the unit cell can be accelerated.
When the volume of the metal material portion is made to be a proper value, a sufficient heat radiation effect can be obtained while a high energy density is being maintained.
Other objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.