Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, medium- or large-sized devices, such as vehicles, use a medium- or large-sized battery pack having a plurality of battery cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices.
Preferably, the medium- or large-sized battery pack is manufactured with small size and small weight if possible. For this reason, a rectangular battery or a pouch-shaped battery, which has a small weight to capacity ratio, is usually used as a battery cell of the medium- or large-sized battery pack. Especially, much interest is currently generated in the pouch-shaped battery, which uses a laminate sheet comprising a metal layer and a resin layer as a sheathing member.
FIG. 1 is a perspective view typically illustrating a conventional representative pouch-shaped battery 10. The pouch-shaped battery shown in FIG. 1 is constructed in a structure in which two electrode leads 11 and 12 protrude from the upper end and the lower end of a battery cell 13, respectively, while the electrode leads 11 and 12 are opposite to each other. A sheathing member 14 comprises upper and lower sheathing parts. An electrode assembly (not shown) is received in a receiving part 15, which is defined between the upper and lower sheathing parts of the sheathing member 14. Opposite sides 14b and upper and lower ends 14a and 14c, which are contact regions of the upper and lower sheathing parts of the sheathing member 14, are bonded to each other, whereby the pouch-shaped battery 10 is manufactured. The sheathing member 14 is constructed in a laminate structure of a resin layer/a metal film layer/a resin layer. Consequently, it is possible to bond the opposite sides 14b and upper and lower ends 14a and 14c of the upper and lower sheathing parts of the sheathing member 14, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 14b and upper and lower ends 14a and 14c of the upper and lower sheathing parts of the sheathing member 14 so as to weld the resin layers to each other. According to circumstances, the opposite sides 14b and upper and lower ends 14a and 14c of the upper and lower sheathing parts of the sheathing member 14 may be bonded to each other using a bonding agent. For the opposite sides 14b of the sheathing member 14, the same resin layers of the upper and lower sheathing parts of the sheathing member 14 are in direct contact with each other, whereby uniform sealing at the opposite sides 14b of the sheathing member 14 is accomplished by welding. For the upper and lower ends 14a and 14b of the sheathing member 14, on the other hand, the electrode leads 11 and 12 protrude from the upper and lower ends 14a and 14b of the sheathing member 14. For this reason, the upper and lower ends 14a and 14b of the upper and lower sheathing parts of the sheathing member 14 are thermally welded to each other, while a film-shaped sealing member 16 is interposed between the electrode leads 11 and 12, in consideration of the thickness of the electrode leads 11 and 12 and the difference in material between the electrode leads 11 and 12 and the sheathing member 14, so as to increase sealability.
In the pouch-shaped battery 10, however, the battery cell 13 repeatedly expands and contracts during the charge and the discharge of the pouch-shaped battery 10. As a result, the thermally welded regions of the upper and lower ends 14 and 14c, especially the opposite sides 14b, of the upper and lower sheathing parts of the sheathing member 14, are easily separated from each other. Furthermore, the mechanical strength of the sheathing member 14 is low. In order to solve this problem, there have been proposed a method of applying an epoxy resin or a silicon resin to the outside sealing regions of the sheathing member 14, or attaching a sealing film to the outside sealing regions of the sheathing member 14, so as to improve the sealability, and a method of mounting batteries in additional members having sufficient mechanical strength, such as cartridges, respectively, and stacking the additional members one on another.
However, the method of using the additional sealing member is very complicated, and a battery manufactured by this method has a structure unsuitable to construct a medium- or large-sized battery pack. When the cartridges are used, on the other hand, the total weight and the total size of the battery pack are increased, and the assembly process is very complicated.
In a conventional medium- or large-sized battery module constructed by stacking a plurality of such pouch-shaped batteries one on another, the electrode leads of the unit cells are bent for electrical connection, and the bent electrode leads are connected to each other by welding, soldering, or mechanical coupling. However, internal resistance is increased due to the bending of the electrode leads. According to circumstances, portions of the bent regions may be broken, and the broken portions of the bent regions serve as heating spots, which may cause fire or explosion.