A secondary battery is a device that stores electrical energy in chemical form and converts the stored chemical energy into electrical energy to generate electricity when needed. The secondary battery is also referred to as a “rechargeable battery” because it can be recharged repeatedly. A common secondary battery includes a lead accumulator, a NiCd battery, a NiMH accumulator, a Li-ion battery, and a Li-ion polymer battery. When compared to a disposable primary battery, not only is the secondary battery more economically efficient, it is also more environmentally friendly.
Secondary batteries are currently used in applications where low power is needed, for example, devices for assisting in the starting of car engines, mobile devices, tools, and uninterrupted power supply systems. Recent developments in wireless communication technologies have led to the popularization of mobile devices and have brought about a tendency to connect many types of existing devices to wireless networks. Under such circumstances, demand for secondary batteries is growing explosively. Hybrid vehicle and electric vehicle have been put into practical use in the aspect of environmental pollution prevention. These next-generation automobiles reduce costs and weight and increase their life span by employing technologies based on secondary batteries.
Generally, most secondary batteries have a cylindrical, prismatic, or pouch shape. This is because the secondary batteries are fabricated by mounting an electrode assembly composed of a negative electrode, a positive electrode and a separator in a cylindrical or prismatic metal can or a pouch-shaped case of an aluminum laminate sheet, and by injecting an electrolyte into the electrode assembly. Thus, because a predetermined mounting space for the secondary battery is required, the cylindrical, prismatic or pouch shape of the secondary battery is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for a new type of secondary battery that is easily adaptable in shape.
To fulfill this need, a cable-type secondary battery having a very high ratio of length to cross-sectional diameter has been proposed. A packaging for protecting the cable-type secondary battery requires both flexibility and waterproof properties. When a general tube packaging of a polymer material is used, water or air may permeate through the micropores of the polymer and contaminate an electrolyte in a battery, causing battery performance degradation.
To overcome the problem, a packaging formed of a metal foil layer may be used, but due to a stiff property of the metal foil layer itself, in the event that a battery is bent, the battery is not completely bent, and folding or wrinkling occurs on the surface of the metal foil layer, the result being the metal foil layer tearing.