SAFETY PLATE OF SECONDARY BATTERY

Abstract: A safety plate of a secondary battery is disclosed, which can prevent the explosion of the battery such as a lithium ion secondary battery which is apt to be exploded due to an excessive inner pressure. In the secondary battery, in which a separator is disposed between a cathode and an anode, a jelly role wound with the separator, the cathode an the anode is received in a battery case connected to the anode with an electrolyte, and an opening of the battery case is sealed by a top cap assembly connected to the cathode, a safety plate is formed of a curved groove on the battery case as a safety element for preventing the explosion of the battery due to the increase of internal pressure, wherein the break occurs almost simultaneously in all parts of the curved groove, reducing the danger of explosion of the secondary battery, the safety of the secondary battery is improved by reducing the break pressure deviation according to the size of the battery case and the position of the groove, and a user is prevented from being injured even after the break of the groove since the groove is formed in a curved shape.

Description: SAFETY PLATE OF SECONDARY BATTERY BACKGROUND OF THE INVENTION CROSS REFERENCE TO RELATED APPLICATION This application is based on application No. 10-2000-57507 and No. 10-2001 -5484 filed in the Korean Industrial Property Office on September 29, 2000 and February 5, 2001 respectively, the content of which is incorporated hereinto by reference (a) Field of the Invention The present invention relates to a secondary battery, and more particularly, to a safety plate of the secondary battery, which may prevent the explosion of the battery due to an internal pressure of lithium ion secondary battery. (b) Description of the Related Art With the recent, widespread trend in the variety of portable electronic equipment such as high efficient notebook computers and wireless telephones, the demands on the secondary battery, which is able to be re-charged and has a large energy density, have been increased to be utilized as a power source of the equipment. Among such a secondary battery, a lithium ion secondary battery includes a carbonaceous anode, a cathode of lithium metal oxide, a separator of polyolefin, and an electrolyte, of which charge and discharge are carried out by the electromotive force generated when lithium ions move between the cathode and the anode. However, the lithium ion secondary battery is high in its operation electric potential, so that a high energy may flow instantly and a cathode material is largely increased in chemical activation by overcharge or short circuit, thereby the cathode material reacts with the electrolyte rapidly, generating a lot of gas. As a result, pressure or temperature in the lithium ion secondary battery increases rapidly, bringing about an explosion of the battery, so that the environmental equipment may be damaged or users may be injured. Therefore, various safety elements have been developed for preventing the explosion of the lithium ion secondary battery. For example, U. S. PAT. NO. 5,738,952 discloses a structure that a safety plate is connected to a cathode lead tap by welding to be turned over at a predetermined threshold pressure and the welded portion of the safety plate is detached to interrupt a current, thereby preventing the increase of the pressure and the temperature, wherein • if the pressure exceeds the predetermined pressure, a wall of the battery case or a path through which the current flows is interrupted to prevent the increase of the pressure. However, it still has disadvantages in that it is difficult to manufacture and assemble a safety plate responsive to pressure, even though the flow of a current may be prevented by interrupting the current flow from outside the battery in case of any abnormal reaction of the battery, since such an additional safety plate is mounted outside the battery case. Japanese Patent Laying-open Publication Nos. 02-284350 and 09-320549 disclose a safety element for a secondary battery that is formed of a groove in the shape of straight line or "X" formed of such straight lines on the surface of a battery case. Such a rectilinear groove has, however, different distances to each point of the groove from the center part of a side surface which has a maximum area to be swollen the most at a predetermined threshold pressure at which the whole battery case is swollen to break, and, accordingly, pressures applied to each part of the groove become different, so that all parts of the groove are not broken simultaneously but the weakest part is broken first. Therefore, it is impossible to form the groove to be broken simultaneously in all parts but forming the whole groove of the battery case with different thickness, since it is most preferable that the whole groove becomes broken simultaneously. Further, it is another problem that it is difficult to design the safety element since a pressure deviation is different according to the position of the groove and the size of the battery case. In case of the groove in the shape of "X", it has disadvantages that it is difficult to engrave the groove on the battery case precisely and a new design is required in order to break the groove at a predetermined pressure considering the size of the battery case. Further, the rectilinear groove and the X-shaped groove may hurt users due to the sharp broken parts of the groove, even though it is possible to prevent the explosion of the battery after being broken. SUMMARY OF THE INVENTION Accordingly, the present invention is directed to a safety plate of a secondary battery that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. An object of the present invention is to provide a safety plate of a secondary battery, in which a groove is applied with a uniform pressure when the internal pressure of the secondary battery is raised to allow the groove to be broken simultaneously. Another object of the present invention is to provide a safety plate of a secondary battery, in which a pressure deviation for breaking a groove is uniform according to the size of the battery case and the position of the groove, for promoting the safety of the secondary battery. A further object of the present invention is to provide a safety plate of a secondary battery, which may prevent users from being injured due to broken parts of a groove after the groove is broken. To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in a secondary battery, in which a separator is disposed between a cathode and an anode, a jelly role wound with the separator, the cathode and the anode is received in a battery case connected to the anode with an electrolyte, and an opening of the battery case is sealed by a top cap assembly connected to the cathode, a safety plate for the secondary battery is characterized in that a groove is formed on the battery case as a safety element for preventing explosion of the battery due to the increase of internal pressure, wherein the groove is formed as a curve. The curved groove is preferably formed outside the battery case, which is formed prismatic in the shape of a hexahedron and the curved groove is formed on the side surface having a maximum area. The curved groove is more preferably formed on a diagonal line of the side surface having the maximum area of the prismatic battery case with a curvature of 3mm to 50mm. Also, it is preferable to form a rectilinear groove extended from both or either ends of the curved groove. A thickness of the groove part on the battery case is to be 10% to 30% with respect to the whole thickness of the battery case. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: Fig. 1 is a perspective view of a partially taken and expanded secondary battery formed with a safety plate according to a preferred embodiment of the present invention; Fig. 2 is a front view showing a side surface of a maximum area of the battery case formed with the safety plate according to the present invention; Fig. 3 is a front view showing a position of a safety plate according to another preferred embodiment of the present invention; Fig. 4 is a front view showing an estimated stress distribution with closed curve lines in case that internal pressure is generated in a secondary battery formed with a safety plate; Fig. 5 to Fig. 7 are front views respectively showing a secondary battery formed with a safety plate according to other preferred embodiments of the present invention; Fig. 8 is a view for comparing secondary batteries respectively formed with a safety plate according to the embodiments of the present invention; Fig. 9 is a view showing a state that the safety plate is broken due to forcible overcharge of the secondary battery formed with the safety plate according to the present invention; Fig. 10 is a graph showing a change of temperature in the secondary battery when the safety plate is broken due to the overcharge; Fig. 11 is a graph showing a change of voltage in the secondary battery when the safety plate is broken due to the overcharge; and Fig. 12 is a view showing a state that the safety plate is broken by increasing temperature of a hot box in which the secondary battery formed with the safety plate according to the present invention is received. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Fig. 1 is a perspective view showing a partially taken and expanded secondary battery formed with a safety plate according to the present invention. Among various secondary batteries, the present invention will be described with reference to a lithium ion prismatic secondary battery (hereinafter, referred to as "prismatic secondary battery"). A prismatic secondary battery includes a jelly role 8 wound with a cathode 2, a separator 4, and an anode 6 together and received in a battery case 10 which is connected to the anode. The prismatic secondary battery is mounted with a top cap assembly 12 which is connected to the cathode 2 on the battery case 10, wherein an electrolyte solution is injected into the battery case 10 via an injection hole 12a formed in the top cap assembly 12 and the injection hole 12a is sealed. The top cap assembly 12 includes a top cap 14 to be welded on the battery case 10, a lower plate 16 disposed in the center of a lower part of the top cap 14, and a cathode terminal 22 which is riveted between the top cap 14 and the lower plate 16 via upper and lower gaskets 18, 20 together with the gaskets. The lower plate 16 is connected to the anode 6 and a tap 24 of the jelly roll 8. The jelly roll 8 is disposed with an upper insulation plate 26 and a lower insulation plate 28 at an upper end part and a lower end part respectively for insulation. The prismatic secondary battery as above is formed with a safety plate 30 on a battery case as a safety element against explosion. The safety plate 30 is, as shown in Fig. 2 and Fig. 3, designed to be a curved groove formed on the battery case 10 to be torn out when internal pressure of the battery is excessively raised. The curved groove is preferably formed on a side surface having a maximum area part outside the battery case 10, wherein the side surface of the maximum area part of the battery case is the weakest part to be swollen first when the inner pressure of the secondary battery is excessively raised. The curved groove is preferably formed outside the battery case 10 to advantageously emit the internal pressure of the battery. The curved groove which is engraved on the battery case 10 according to the present invention is positioned in the center of a curvature on a diagonal line of the side surface having the maximum area part or an imaginary line extended from the diagonal line, so as to be broken at a threshold pressure more precisely than a prior rectilinear or X-shaped groove. Particularly, the curved groove is formed near each corner rather than a center part of the maximum area part and preferably has a curvature of about 3mm to 50mm. The curved groove is formed considering the concentration of stress applied to the side surface of the maximum area part. That is, it will be apparent to those skilled in the art that the center part of the maximum area part is swollen the most and the four edged corner parts are swollen the least, if the side surface of the maximum area part is swollen due to the increase of the internal pressure of the prismatic secondary battery. If the connecting points where stress is applied equally on the entire side surface of the maximum area part, the distribution of the stress points is exhibited in closed oval curve lines toward the center part of the side surface of the maximum area part, and exhibited in close rectangular lines toward the outer corner parts of side surface, as shown in Fig. 4. The stress changes rapidly between the center part and the corner parts of the maximum area part, and the curved groove of the present invention is formed in the weakest parts. Therefore, the concentration of the stress is generated on all parts of the curved groove with little deviation, so that the groove is broken simultaneously in almost all parts when the safety plate 30 is broken, thereby minimizing the pressure deviation. By the same reason, a rectilinear groove 32 as shown in Fig. 5 may be extended from both or either ends of the curved groove. The rectilinear groove 32 is preferably 5mm or less, wherein the pressure deviation may be increased if the rectilinear groove 32 is longer than 5mm. Fig. 6 and Fig. 7 respectively show a preferred embodiment of the present invention, in which the safety plate 30 is formed in the maximum area part of the battery case 10 in different shapes. The battery case 10 of a general lithium ion prismatic secondary battery is formed of any one among stainless, nickel-plated steel, aluminium alloy and plastics. The battery case 10 has a thickness designed according to the size of the battery and the material of the battery case, wherein it is preferable to form the battery case with a thickness in the range of 0.3mm to 0.7mm. The curved groove formed on the battery case 10 has a thickness smaller than that of the battery case 10, wherein the battery case in the curved groove part becomes thicker as the threshold pressure is designed higher and thinner as the threshold pressure is designed lower. In particular, the thickness of the battery case in the groove part is preferably 10% to 30% of the thickness of the battery case 10. The present invention will now be described in more detail with reference to the preferred embodiments, which do not limit the present invention thereto. [Embodiment] A jelly roll 8 in which a cathode 2 of lithium cobalt oxide is separated from an anode 6 of carbon activated material by a separator 4 of olefin is received in the battery case 10 as shown in Fig. 1. The battery case 10 is formed prismatic with a thickness of 0.45mm, a length of 63mm, and a height of 48mm. The curved groove formed on the battery case 10 has a width of 0.7mm, a length of 13mm, a thickness of 0.1mm and a curvature radius of 40mm. Under the circumstances that the jelly roll 8 is received in the battery case 10, the tap 24 attached to the cathode of the jelly roll is welded to a positive electrode terminal 22 of the top cap assembly 1 . After welding the top cap 14 by laser in close contact with an opening of the battery case 10, an electrolyte solution containing ethylene carbonate (EC), diethylene carbonate (DEC), and lithium salt(LiPF 6 ) is injected via the electrolyte solution injection hole 12a of the top cap 14 and the electrolyte solution injection hole 12a is sealed, thereby a lithium ion prismatic secondary battery is manufactured. [Comparison Example] As shown in Fig. 8, safety plates of various shapes were formed on a side surface of a maximum area part and a break pressure of the secondary battery was measured while gradually increasing the pressure to break an initial safety plate. In Fig. 8, all numerals were represented by millimeter. The grooves of the safety plates were formed by electro-discharge machining, wherein the battery cases formed with the grooves were formed of aluminium with a thickness of 0.1mm, and the grooves have a thickness of 0.4mm. In type 1 , two kinds of A are formed with a length of 2mm and 5mm, respectively. In type 3, three kinds of B are formed with a length of 3mm, 5mm, and 7mm, respectively. The result of the brake test is represented in table 1 , wherein X represents that no break was generated. [Table 1 ] As represented in Table 1 , the curved grooves are formed at the weakest portion of the maximum area part, so that the safety plates were always broken first and a pressure deviation in case of such a break is 1 kgf/cm 2 or less. That is, the lithium ion prismatic secondary batteries were broken stably without explosion by using the safety plates formed of the curved grooves. [Experimental Example 1] In order to test whether the break of the safety plate is generated stably when an abnormal internal pressure is generated in the lithium ion prismatic secondary battery, an overcharge test, a hot box test and a drop test were carried out. The drop test of UL1642 standard was carried out for checking the stability of the lithium ion prismatic secondary battery by dropping the battery case at a height of 1.9m three times to observe whether the curved groove is broken. As a result of the test, the curved groove was not broken. The overcharge test of 3C is carried out by using charge/discharge testing machines to observe a pressure at which the safety plate is broken. As a result of the overcharge test, the safety plate still operated as shown in Fig. 9. A change of internal temperature and a change of voltage of the lithium ion prismatic secondary battery, which was subject to the overcharge, are respectively represented in graphs of Fig. 10 and Fig. 11. The hot box test is carried out for observing whether the safety plate is broken while raising the temperature of the prismatic secondary battery. As a result of the hot box test, the safety plate was broken when the temperature of the inside of the hot box was raised up to about 150 ° C , as shown in Fig. 12. [Experimental Example 2] In experimental example 2, it was tested whether the safety plate designed according to the present invention in view of a size of the prismatic secondary battery is stably broken and the safety plate is broken to pressure deviation. In table 2 below, A type is a lithium ion prismatic secondary battery having a thickness of 6.3mm, a width of 3.4cm, and a height of 5.0cm, and B type is a battery having a thickness of 6.3mm, a width of 3.0cm and a height of 6.7cm. [Table 2] As shown in table 2, the break pressure is in the range of 1.1 kgf/cm 2 with reference to the average break pressure. That is, the break pressure deviation was not large so that a more stable break may be estimated, improving the stability of the prismatic secondary battery. As described hereinabove, a safety plate of a secondary battery according to the present invention exhibits improved effect in view of prior art. In other words, pressure is uniformly applied to the curved groove when the internal pressure of the secondary battery is raised, so that the break occurs almost simultaneously in all parts of the curved groove, reducing the danger of explosion of the secondary battery. Further, the safety of the secondary battery is improved by reducing the break pressure deviation considering the size of the battery case and the position of the groove. Furthermore, a user is prevented from being injured even after the break of the groove, since the groove is formed in a curved shape. While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention that come within the scope of the appended claims and their equivalents.

Claims: WHAT IS CLAIMED IS: 1. A safety plate of a secondary battery, in which a separator is disposed between a cathode and an anode, a jelly role wound with the separator, the cathode and the anode is received in a battery case connected to the anode with an electrolyte, and an opening of the battery case is sealed by a top cap assembly connected to the cathode, is characterized in that a groove is formed on the battery case as a safety element for preventing an explosion of the battery due to the increase of internal pressure, wherein the groove is formed curved. 2. A safety plate of a secondary battery as claimed in claim 1 , wherein the curved groove is formed outside the battery case. 3. A safety plate of a secondary battery as claimed in claim 2, wherein the curved groove is formed on a side surface having a maximum area in a hexagonal battery case. 4. A safety plate of a secondary battery as claimed in claim 3, wherein the curved groove is formed on a diagonal line of the side surface having the maximum area. 5. A safety plate of a secondary battery as claimed in claim 3, wherein a curvature of the curved groove is to be 3mm to 50mm. 6. A safety plate of a secondary battery as claimed in claim 4, wherein the curved groove is formed with a rectilinear groove extended from both ends of the curved groove. 7. A safety plate of a secondary battery as claimed in claim 4, wherein the curved groove is formed with a rectilinear groove extended from either ends of the curved groove. 8. A safety plate of a secondary battery as in claim 5, wherein the curved groove is formed with a rectilinear groove extended from both ends of the curved groove. 9. A safety plate of a secondary battery as in claim 5, wherein the curved groove is formed with a rectilinear groove extended from either ends of the curved groove. 10. A safety plate of a secondary battery as claimed in claim 6, wherein the thickness of the groove part on the battery case is to be 10% to 30% with respect to the whole thickness of the battery case. 11. A safety plate of a secondary battery as claimed in claim 7, wherein the thickness of the groove part on the battery case is to be 10% to 30% with respect to the whole thickness of the battery case. 12. A safety plate of a secondary battery as claimed in claim 8, wherein the thickness of the groove part on the battery case is to be 10% to 30% with respect to the whole thickness of the battery case. 13. A safety plate of a secondary battery as claimed in claim 9, wherein the thickness of the groove part on the battery case is to be 10% to 30% with respect to the whole thickness of the battery case.

Inventors: BYUN HO-KYUNG, KIM JEE-HO, KIM SOO-RYOUNG, KIM YONG-RAE, YOO KWANG-HO, BYUN, HO-KYUNG, KIM, JEE-HO, KIM, SOO-RYOUNG, KIM, YONG-RAE, YOO, KWANG-HO
Applicants: LG CHEMICAL LTD, LG CHEMICAL CO., LTD.
Application year: 2001
Main classification: H01M2
IPC codes: H01M2, H01M10