As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source has also sharply increased.
Depending upon kinds of external devices in which secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to one another. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the power and the capacity of one battery. On the other hand, a battery pack including a plurality of batteries needs to be used in middle or large-sized devices, such as laptop computers, portable digital versatile disc (DVD) players, small-sized personal computers (PCs), electric vehicles, and hybrid electric vehicles, because high power and large capacity are necessary for the middle or large-sized devices.
Among secondary batteries, a lithium secondary battery is widely used since the lithium secondary battery has high power and large capacity.
However, various kinds of combustible materials are contained in the lithium secondary battery. As a result, the lithium secondary batter may be heated or explode due to overcharge of the lithium secondary battery, overcurrent in the lithium secondary battery, or other external physical impact applied to the lithium secondary battery. That is, the safety of the lithium secondary battery is very low.
Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the lithium secondary battery, such as overcharge of the lithium secondary battery or overcurrent in the lithium secondary battery, are loaded on a battery cell in a state in which the safety elements are connected to the battery cell.
Generally, the PCM is electrically connected to the battery cell via conductive nickel plates by welding or soldering. That is, nickel plates are connected to electrode tabs of the PCB by welding or soldering, and the nickel plates are connected to electrode terminals of the battery cell by welding or soldering. In this way, the PCM is connected to the battery cell to manufacture a battery pack.
It is required for the safety elements, including the PCM, to be maintained in electrical connection with the electrode terminals of the battery cell and, at the same time, to be electrically isolated from other parts of the battery cell. To this end, a plurality of parts is necessary, which complicates an assembly process of the battery pack. In addition, a space necessary to receive the battery cell is reduced.
Furthermore, a battery case to receive the parts is made of conductive metal exhibiting high electric conductivity, such as metal plated with aluminum, an aluminum alloy, or nickel. As a result, a short circuit may be caused in the battery cell when the battery case contact other parts of the battery cell.
Consequently, there is a high necessity for a battery cell that is capable of securing a receiving space and insulativity of the battery cell and, at the same time, preventing the outer surface of a battery case from being corroded or worn while using a conventional battery cell and a conventional assembly method.