With remarkable development of information technology (IT), a great variety of portable information communication devices has been popularized. As a result, in the 21st century, we are moving toward a ubiquitous society in which high-quality information service is possible regardless of time and place.
A lithium secondary battery is very important to realize such a ubiquitous society. The lithium secondary battery may be manufactured by receiving an electrode stack in a receiving part of a battery case together with an electrolyte and thermally bonding the outer edge of the receiving part.
Such an electrode stack may be classified as a stacked type electrode stack configured to have a structure in which positive electrode plates, separator plates, and negative electrode plates, each having a predetermined size, are repeatedly stacked such that the separator plates are disposed respectively between the positive electrode plates and the negative electrode plates, a wound (jelly-roll) type electrode stack configured to have a structure in which a positive electrode sheet, a separator sheet, and a negative electrode sheet, each having a predetermined size, are stacked such that the separator sheet is disposed between the positive electrode sheet and the negative electrode sheet, and then the separator sheet is wound in one direction, or a stacked and folded type electrode stack configured to have a structure in which a predetermined number of stacked type electrode stacks is arranged on a separator sheet, and then the separator sheet is wound in one direction such that the stacked type electrode stacks are stacked.
FIG. 1 is a view typically showing a state of an electrode stack, including a positive electrode 1, a separator 2, and a negative electrode 3, configured to have a structure in which the separator 2 is disposed between the positive electrode 1 and the negative electrode 3, and one end of each of the positive electrode 1 and the negative electrode 3 does not intersect the other end of each of the positive electrode 1 and the negative electrode 3, before and after the electrode stack is bent. For the convenience of description, a structure in which one positive electrode 1, one separator 2, and one negative electrode 3 are stacked is shown in FIG. 1. Alternatively, the electrode stack may include a plurality of positive electrodes, a plurality of separators, and a plurality of negative electrode. Before the electrode stack is bent, the lengths of the separator 2 and the negative electrode 3 are greater than that of the positive electrode 1 (see A and A′). After the electrode stack is bent, however, opposite ends of the positive electrode 1, which is located inside the separator 2 and the negative electrode 3, protrude further outward from corresponding ends of the separator 2 and the negative electrode 3 (see A and A′). As a result, lithium is separated from the electrode stack during charge of the electrode stack, whereby safety of a lithium secondary battery including the curved electrode stack is lowered. This is because a conventional positive electrode, separator, and negative electrode used for a non-curved electrode stack are applied to the curved electrode stack without any change.
FIG. 2 is a typical view showing a curved jelly-roll type electrode stack 20. Referring to FIG. 2, when the jelly-roll type electrode stack 20 is bent along an imaginary line X′-X″ perpendicular to a direction in which electrode terminals protrude, stress is concentrated on bent opposite ends A′ and A″ of the jelly-roll type electrode stack 20 with the result that the electrodes of the jelly-roll type electrode stack 20 may be broken.
Meanwhile, in recent years, designs of electronic devices have played a very important part in consumers' selection of products, and the size and thickness of the electronic devices have been gradually reduced in response to consumers' taste. In order to minimize unnecessary waste of spaces in the electronic devices, therefore, miniaturization and slimness of a lithium secondary battery are required, and it is necessary to modify the shape of the lithium secondary battery to reflect the shapes of such electronic devices.
In connection with this, Japanese Patent Application Publication No. 1999-307130 discloses a method of manufacturing a crooked battery by thermally pressing a positive electrode impregnated with an electrolytic solution containing a plasticizer, a negative electrode impregnated with an electrolytic solution containing a plasticizer, and a gel type electrolyte layer impregnated with an electrolytic solution containing a plasticizer using two rolls having different diameters.
FIG. 3 shows a crooked electrode stack disclosed in Japanese Patent Application Publication No. 1999-307130. Referring to FIG. 3, the electrode stack includes a crooked positive electrode 1 including a positive electrode layer 4 and a positive electrode current collector 5, a crooked negative electrode 2 including a negative electrode layer 6 and a negative electrode current collector 7, and a crooked gel type electrolyte layer 3. A positive electrode terminal 8 is connected to the positive electrode current collector 5, and a negative electrode terminal 9 is connected to the negative electrode current collector 7. The positive electrode terminal 8 and the negative electrode terminal 9 are formed at non-crooked portions of the electrode stack.
Japanese Patent Application Publication No. 1999-307130 discloses a technology for solving a conventional problem that the crooked electrode stack returns to the original shape thereof. Protection circuit modules known to date each have a planar structure with no curved surface. In consideration of the fact that no conventional curved protection circuit modules have been proposed, therefore, it is understood that Japanese Patent Application Publication No. 1999-307130 does not disclose or recognize a battery pack having a curved protection circuit module mounted thereto.
Furthermore, in a case in which the curved protection circuit module is applied, a very complicated manufacturing process is needed. For example, in a case in which components, such as an integrated circuit (IC), are welded to a crooked surface of the curved protection circuit module, the welding process is very complicated, it is difficult to exhibit desired coupling force, and it is also difficult to manufacture a jig used to mount the curved protection circuit module to a crooked battery cell. Consequently, the curved protection circuit module is limited in its applicability to a battery pack, including a plurality of battery cells, the mass production of which is required.
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