Abstract:
A sealed battery according to the present invention includes a sealing body  18 A having a terminal cap  19′  made of an iron-based material and a safety valve  25  made of an aluminum-based material. In the sealing body  18 A, at least one of the flange of the terminal cap  19′  and the flange of the safety valve  25  has a portion defining a space  30 . The flanges of the terminal cap and of the safety valve are welded at a position corresponding to the space with high-energy rays. It is thus possible to provide the sealed battery including the sealing body having a current interrupt function in which resistance between the terminal cap and the safety valve is kept low, thereby being suitable for large-current applications.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a sealed battery. More particularly, the present invention relates to a sealed battery including a sealing body having a current interrupt function in which a negative electrode or positive electrode terminal cap and a safety valve are directly welded with a laser beam or other high-energy rays for increasing the welding strength between the two without increasing the electrical resistance of the battery in use. 
       BACKGROUND OF THE INVENTION 
       [0002]    The ongoing surge in the development of lightweight and space economical electronic devices has raised the need for lighter and smaller batteries as their power supplies. To meet this requirement, sealed nonaqueous electrolyte secondary batteries represented by lithium-ion secondary batteries, which are compact, lightweight, high capacity, chargeable and dischargeable, have been in practical use for compact camcorders, mobile phones, notebook computers, and other mobile electronics and communications equipment, for example. 
         [0003]    Referring to  FIGS. 4 and 5 , a typical structure of such sealed nonaqueous electrolyte secondary batteries will be described.  FIG. 4  is a perspective view showing a longitudinal section of a cylindrical nonaqueous electrolyte secondary battery disclosed in JP-2001-15155-A.  FIG. 5  is an enlarged partial cutaway view of the sealing body shown in  FIG. 4 . This nonaqueous electrolyte secondary battery  10  is manufactured by the following process: rolling a positive electrode plate  11  and a negative electrode plate  12  with a separator  13  therebetween to provide a spiral electrode unit  14 , placing insulators  15  and  16  on the top and bottom of the electrode unit  14  and thereafter laying the electrode unit in a steel cylindrical outer can  17  also serving as a negative electrode terminal, welding a current collecting tab  12   a  of the negative electrode plate  12  to the inside bottom of the outer can  17  and welding a current collecting tab  11   a  of the positive electrode plate  11  to a bottom plate of a sealing body  18 D with a built-in safety device, injecting a predetermined nonaqueous electrolyte from an opening of the outer can  17 , and tightly sealing the outer can  17  with the sealing body  18 D. 
         [0004]    Referring to  FIG. 5 , the sealing body  18 D includes an inverted-dish-shaped terminal cap  19  and a dish-shaped bottom plate  20  both of which are made of stainless steel. The terminal cap  19  has a convex portion  21  protruding outwardly from the battery, and a flat flange  22  serving as the base of the convex portion  21 . At the corner edge of the convex portion  21 , a plurality of gas vent holes  21   a  are formed. The bottom plate  20  has a concave portion  23  protruding inward of the battery, and a flat flange  24  serving as the base of the concave portion  23 . At the corner edge of the concave portion  23 , a gas vent hole  23   a  is formed. 
         [0005]    Accommodated inside the terminal cap  19  and bottom plate  20  is an aluminum safety valve  25  whose shape changes when the battery&#39;s internal gas pressure increases to reach a predetermined level. The safety valve  25  has a concave portion  25   a  and a flange  25   b , and made of aluminum foil that is 0.2 mm thick with a 0.005-mm concavo-convex surface, for example. The bottommost part of the concave portion  25   a  is placed so as to be in contact with the upper surface of the concave portion  23  of the bottom plate  20 . The flange  25   b  is sandwiched between the flange  22  of the terminal cap  19  and the flange  24  of the bottom plate  20 . On a part upon the flange  25   b  of the safety valve  25 , a positive temperature coefficient (PTC) thermistor element  26  is provided. The terminal cap  19  and bottom plate  20  are liquid-tightly fixed to each other. Specifically, the flange  24  of the bottom plate  20  is fixed to the terminal cap  19  side with a polypropylene (PP) insulating gasket  27  for a sealing body therebetween, for example. 
         [0006]    In the nonaqueous electrolyte secondary battery  10 , when an excess current flows in the battery to an extent that causes abnormal heat, the resistance of the PTC thermistor element  26  in the sealing body  18 D having a current interrupt function increases, thereby suppressing the excess current. Furthermore, the shape of the concave portion  25   a  of the safety valve  25  changes when the battery&#39;s internal gas pressure increases to reach a predetermined level in order to interrupt contact between the safety valve  25  and the concave portion  23  of the bottom plate  20 , thereby interrupting an excess or short-circuit current. It is therefore possible to provide a sealed nonaqueous electrolyte secondary battery that is highly safe. 
         [0007]    JP-2000-90892-A discloses a secondary battery for large current discharge including a sealing body whose electrical resistance is kept low. Referring to  FIG. 6 , which is an enlarged sectional view of this sealing body  40 , the body includes from the inside of the battery a dish-shaped bottom plate  41  made of aluminum, a thin-plate safety valve  42  made of aluminum, and a positive electrode terminal cap  43  made of nickel-plated iron that are placed on top of each other. Provided between the bottom plate  41  and safety valve  42  is a ring-like valve retainer  44  made of butyl rubber for tight sealing. The terminal cap  43  and bottom plate  41  are fixed to each other on the periphery, and moreover, the bottom plate, safety valve, and terminal cap are spot-welded at four points  47  with a diameter of 3 mm on a flange  46  on the inside of a fixing member  45  provided to the periphery, thereby completing the united sealing body  40 . 
         [0008]    Against the background of intensifying calls for environmental protection, regulations on emissions of carbon dioxide and similar gases have been tightened. In the automobile world, development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) is being vigorously pursued in addition to vehicles using gasoline, diesel oil, natural gas and other fossil fuels. Furthermore, the soaring rise in the price of fossil fuels over recent years has given a boost to the development of EVs and HEVs. In addition, sealed batteries represented by lithium-ion secondary batteries have been developed for use in machine tools. 
         [0009]    Batteries for EVs, HEVs, and machine tools are required not only to be environmentally friendly, but also to have high-level basic performance as automobiles or tools, that is, large power supply capacity. However, the PTC thermistor element serving as a safety device in nonaqueous electrolyte secondary batteries as disclosed in JP-2001-15155-A restricts the amount of current to be supplied. Therefore, secondary batteries without requiring such a thermistor element have been developed to achieve large power supply. 
         [0010]      FIG. 7  is a sectional view illustrating a sealing body  18 E having a current interrupt function without requiring a PTC thermistor element. Since this sealing body  18 E has the same structure as that of the sealing body  18 D shown in  FIG. 5  except for the presence of a PTC thermistor element  26 , the like numerals indicate like elements in  FIGS. 5 and 7  and the description thereof will be omitted here. 
         [0011]    If the sealing body  18 E having a current interrupt function without requiring a PTC thermistor element is used in the batteries for EVs, HEVs, and machine tools, the batteries can supply large power thanks to their low internal resistance. However, the temperature of the batteries may reach 80 degrees Celsius or more, whereby repeated use may cause heat effects on the PP insulating gasket  27  provided between the terminal cap  19  and bottom plate  20 . If the elasticity of the gasket  27  decreases, the contact pressure between the terminal cap  19  and safety valve  25  decreases, thereby increasing or fluctuating the batteries&#39; internal resistance. 
         [0012]    As regards the secondary battery for large current discharge disclosed in JP-2000-90892-A, since the positive electrode terminal cap, safety valve, and bottom plate are spot-welded, electrical resistance between the terminal cap and bottom plate is kept low and relatively constant without increasing the area of the battery&#39;s output terminal. Accordingly, the battery has an advantage in that its output current can be increased without increasing its weight. However, the sealing body used here has a problem in that a current keeps flowing even if something abnormal happens in the battery, for example, the shape of the safety valve changes or the battery opens with an increased internal gas pressure, since the positive electrode terminal cap, safety valve, and bottom plate are spot-welded and there is no current interrupt means provided between the terminal cap and bottom plate. 
         [0013]    Typically, while the positive electrode terminal cap is made of a hard iron-based material, such as nickel-plated iron or stainless steel, the safety valve is made of a thin aluminum-based material with a need for flexibility. The bottom plate is made of aluminum to avoid dissimilar metal contact, since a positive electrode current collecting body in a lithium-ion secondary battery is typically made of aluminum. 
         [0014]    To enhance manufacturing efficiency, spot-welding with high-energy rays such as laser or electron beams are widely used these days. Since aluminum has extremely high heat conductivity compared with iron-based materials such as nickel-plated iron and stainless steel, welding with high-energy rays from the terminal cap side, for example, can melt the surface of the terminal cap desirably, while failing to melt the contact portions of the terminal cap and safety valve with an insufficient temperature increase in these portions. Accordingly, it is difficult to strongly weld the terminal cap and safety valve. 
       SUMMARY 
       [0015]    In consideration with the above-mentioned issues in the related art, the present invention provides a sealed battery including a sealing body having a current interrupt function in which a negative electrode or positive electrode terminal cap and safety valve are strongly welding while the resistance of the battery in use is kept low. 
         [0016]    The present invention provides the following features. According to a first aspect of the invention, a sealed battery includes a sealing part having an inverted-dish-shaped terminal cap made of an iron-based material and provided to at least one of a negative electrode and a positive electrode of the battery, and a dish-shaped safety valve made of an aluminum-based material for interrupting electrical connection between inside and outside of the battery. At least one of a flange face of the terminal cap and a flange face of the safety valve facing each other has a portion defining a space. The flange of the terminal cap and the flange of the safety valve are welded at a position corresponding to the space with a high-energy ray. 
         [0017]    It is preferable that the space be an annular groove. 
         [0018]    It is preferable that the space is at least 0.05 mm deep and at most half as deep as any side having the space. 
         [0019]    It is preferable that the high-energy ray be a laser beam or an electron beam. 
         [0020]    According to a second aspect of the invention, a sealed battery includes a sealing part having an inverted-dish-shaped terminal cap made of an iron-based material and provided to at least one of a negative electrode and a positive electrode of the battery, a dish-shaped safety valve made of an aluminum-based material for interrupting electrical connection between inside and outside of the battery, and a ring-like interposed material interposed between the terminal cap and the safety valve, made of a material identical to a material for the safety valve, and being as thick as or thinner than the safety valve. The interposed material and the safety valve are welded with a high-energy ray. At least one of a flange face of the terminal cap and a face of the interposed material facing each other has a portion defining a space. The flange of the terminal cap and the interposed material are welded at a position corresponding to the space with a high-energy ray. 
         [0021]    It is preferable that the interposed material be at most 70% as thick as the terminal cap. 
         [0022]    It is preferable that the space be an annular groove. 
         [0023]    It is preferable that the space is at least 0.05 mm deep and at most half as deep as any side having the space. 
         [0024]    It is preferable that the high-energy ray be a laser beam or an electron beam. 
         [0025]    The above-described features according to the invention provide a sealed battery with the following advantages. According to the first aspect of the invention, since at least one of the flange faces of the terminal cap and of the safety valve facing each other has a space, the portion having this space has low heat conductivity. Since the diffusion rate of heat generated by the high-energy ray applied to a position corresponding to the space in the flange of the terminal cap toward the safety valve side decreases, the flange of the safety valve melts only after the flange of the terminal cap irradiated with the high-energy ray melts sufficiently. Accordingly, welding strength between the flanges of the terminal cap and of the safety valve is significantly high, and electrical resistance between the terminal cap and safety valve in use is kept low. It is therefore possible to provide a sealed battery whose in-use resistance is kept low. In addition, the sealed battery is highly safe, provided with a current interrupt function for interrupting electrical connection between inside and outside of the battery. For example, a current interrupt mechanism for interrupting electrical connection by changing the shape of the safety valve with an increase in the battery&#39;s internal pressure can be used here. 
         [0026]    In this aspect, the iron-based material may be nickel-plated iron or stainless steel, while the aluminum metal may be flexible aluminum or aluminum alloy. The number of welding points (e.g. four) can be appropriately set in consideration of required welding strength and resultant electrical resistance, and the flange is not necessarily required to be thoroughly welded. Furthermore, the space can be provided to either the terminal cap or the safety valve, and can be provided to both. 
         [0027]    Preferably in this aspect, since the space is annular, it can be easily processed and allow discretionary welding positioning around the terminal cap, thereby increasing design flexibility. 
         [0028]    Preferably in this aspect, if the space is at least 0.05 mm deep, the welding strength between the flanges of the terminal cap and of the safety valve is significantly high because of this space. The space that is at most half as deep as any side having space provides desirable welding strength, since too deep a space will decrease the strength of the flange of the terminal cap or the safety valve having the space. On the contrary, if the space is shallower than 0.05 mm, there is no decrease in heat conductivity between the flanges of the terminal cap and of the safety valve because of this space, thereby providing no increase in welding strength. 
         [0029]    Preferably in this aspect, since both laser and electron beams are widely used as welding high-energy rays, it is possible to provide a sealed battery with a reliable and high quality welded portion. 
         [0030]    According to the second aspect of the invention, since the interposed material and the safety valve are made of the same material and the interposed material is thinner than the safety valve, the two are strongly welded with a high-energy ray without perforation in the safety valve. Furthermore, the space is provided between the terminal cap and interposed material in the same manner as in the first aspect. Accordingly, the terminal cap and safety valve are strongly welded with a high-energy ray. Accordingly, since electrical resistance between the terminal cap and the safety valve in use is kept low, it is possible to provide a sealed battery whose electrical resistance in use is kept low. In addition, the sealed battery is highly safe, provided with a current interrupt function for interrupting electrical connection between inside and outside of the battery. For example, a current interrupt mechanism for interrupting electrical connection by changing the shape of the safety valve with an increase in the battery&#39;s internal pressure can be used as in the first aspect. It is noted that making the interposed material thicker than the safety valve, which is likely to cause perforation in the safety valve while welding with the high-energy ray, is undesirable. 
         [0031]    Preferably in this aspect, the thickness of the interposed material exceeding 70% of that of the terminal cap makes it difficult for the interposed material to melt due to its good heat conductivity, thereby resulting in a decrease in welding strength between the terminal cap and the interposed material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is an enlarged sectional view of a sealing body used in a first embodiment according to the present invention. 
           [0033]      FIG. 2  is an enlarged sectional view of a sealing body used in a second embodiment according to the invention. 
           [0034]      FIG. 3  is an enlarged sectional view of a sealing body used in a comparative example. 
           [0035]      FIG. 4  is a perspective view showing a longitudinal section of a related-art cylindrical nonaqueous electrolyte secondary battery. 
           [0036]      FIG. 5  is an enlarged partial cutaway view of a sealing body shown in  FIG. 4 . 
           [0037]      FIG. 6  is an enlarged sectional view of a sealing body used in a related-art secondary battery for large current discharge. 
           [0038]      FIG. 7  is an enlarged sectional view of a sealing body having a current interrupt function without requiring a related-art PTC thermistor element. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Preferred embodiments according to the invention will be described with reference to  FIGS. 1 to 3 . It should be noted that the description of the embodiments below is given to illustrate a sealing body having a current interrupt function that can be used in the related-art lithium-ion nonaqueous electrolyte secondary battery shown in  FIG. 4  as an example of a sealed battery that embodies the concept of the invention and is not intended to limit the invention to lithium-ion nonaqueous electrolyte secondary batteries including sealing bodies with this function. The invention is also applicable to various types of sealed batteries without departing from the spirit and scope of the claims appended hereto.  FIG. 1  is an enlarged sectional view of a sealing body used in a first embodiment according to the invention.  FIG. 2  is an enlarged sectional view of a sealing body used in a second embodiment according to the invention.  FIG. 3  is an enlarged sectional view of a sealing body used in a comparative example. The like numerals indicate like elements in these drawings and in  FIGS. 5 and 7  showing the related-art sealing bodies with a current interrupt function. 
         [0040]    A manufacturing process of a sealing body  18 A according to the first embodiment will now be described. The dish-shaped bottom plate  20  made of aluminum and having the same structure as that used in the above-described related-art sealing bodies is prepared. The bottom plate  20  has the concave portion  23  protruding inwardly of the battery, and the flat flange  24  serving as the base of the concave portion  23 . At the corner edge of the concave portion  23 , the gas vent hole  23   a  is formed. On the flange  24  of the bottom plate  20 , the annular insulating gasket  27  made of PP is mounted. Also as in the above-described related art, the safety valve  25  made of aluminum foil that is 0.2 mm thick, for example, and including the concave portion  25   a  and the flange  25   b  is placed so that the flange  25   b  is laid upon the insulating gasket  27  and the concave portion  25   a  of the safety valve  25  is laid upon the concave portion  23  of the bottom plate  20 . The bottommost part of the concave portion  25   a  of the safety valve  25  and the concave portion  23  of the bottom plate  20  are ultrasonic-welded. 
         [0041]    The terminal cap  19 ′ having the same structure (inverted dish) as in the above-described related-art sealing bodies is prepared. Here, the terminal cap is made of nickel-plated iron with a diameter of 23.0 mm and a thickness of 0.3 mm. The terminal cap  19 ′ has the convex portion  21  protruding outwardly of the battery, and the flat flange  22  serving as the base of the convex portion  21 . At the corner edge of the convex portion  21 , the plurality of gas vent holes  21   a  are formed. In the first embodiment, the face of the flange  22  of the terminal cap  19 ′ facing the flange  25   b  of the safety valve  25  is cut to have an annular groove  30  with a width of 1.5 mm and a depth of 0.1 mm as shown in  FIG. 1 . Accordingly, this terminal cap  19 ′ used in the first embodiment differs from the related-art terminal cap  19  in than it has the annular groove  30 . 
         [0042]    The terminal cap  19 ′ with the groove  30  is placed on the safety valve  25 . Subsequently, four points on the width centerline of the groove  30  are laser-welded at regular intervals from the terminal cap  19 ′ side for fixing the flange  24  of the bottom plate  20 . The sealing body  18 A having a current interrupt function according to the first embodiment is thus completed. 
         [0043]    A sealing body  18 B according to a second embodiment of the invention is manufactured through a process similar to that of the first embodiment, except that an interposed material  32  is added between the safety valve  25  and terminal cap  19 ′. Here, the interposed material  32  is annular, half as thick as the safety valve  25 , and made of the same aluminum-based material as the safety valve  25 . The material is placed on the flange  25   b  of the safety valve  25 . Subsequently, four points are laser-welded at regular intervals from the surface of the interposed material  32  for uniting the interposed material  32  and safety valve  25 . With the terminal cap  19 ′ having the annular groove  30  as in the first embodiment placed on the surface of the interposed material  32 , four points corresponding to the groove  30  are laser-welded at regular intervals from the terminal cap  19 ′ side as in the first embodiment for fixing the flange  24  of the bottom plate  20 . The sealing body  18 B having a current interrupt function according to the second embodiment is thus completed. 
       COMPARATIVE EXAMPLE 
       [0044]    A sealing body  18 C having a current interrupt function according to a comparative example is manufactured through a process similar to that of the first embodiment, except that the terminal cap  19  having no groove  30  is used instead of the terminal cap  19 ′ having the annular groove  30 . Referring to  FIG. 3 , the sealing body  18 C in the comparative example has a shallow recess in a welded portion  31  as it has no groove  30 . 
         [0045]    Vibration, post-drop resistance increase, and drop strength tests were conducted as described in detail below. Each test used thirty pieces of the sealing bodies  18 A to  18 C each having a current interrupt function according to the first and second embodiments and comparative example. Table 1 shows measurement results. 
       Vibration Test 
       [0046]    In the vibration test, the pieces were vibrated at an acceleration of 2.5 G, amplitude of 1.5 mm, and frequency of 30 Hz for three hours with a marketed vibration tester. Internal resistance was measured before and after the vibration. For each of the sealing bodies  18 A to  18 C, the average of resistance increases was calculated if the welded portion remained engaged in all of the thirty pieces after the vibration. If the welded portion was disengaged in any of the pieces, the number of such pieces was counted. 
       Post-Drop Resistance Increase Test and Drop Strength Test 
       [0047]    The post-drop resistance increase test used thirty pieces of the sealing bodies  18 A to  18 C each to be dropped freely from a height of 1.9 meters onto a concrete floor. Internal resistance was measured before and after the drop. The average of resistance increases was calculated if the welded portion remained engaged in all of the thirty pieces after up to thirty times of the drop. If the welded portion was disengaged in any of the pieces, the number of such pieces was counted. Furthermore, the number of times for which the welded portion was disengaged in the process of this test was counted and averaged in the drop strength test. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 After vibration test* 
                 After drop test** 
                   
               
             
          
           
               
                   
                 Re- 
                 # of welded 
                 Re- 
                 # of welded 
                 Drop 
               
               
                   
                 sistance 
                 portions 
                 sistance 
                 portions 
                 strength*** 
               
               
                   
                 increase 
                 disengaged 
                 increase 
                 disengaged 
                 (times) 
               
               
                   
                   
               
             
          
           
               
                 First 
                 0.1 mΩ 
                 0/30 
                 — 
                 30/30 
                 15 
               
               
                 em- 
               
               
                 bodiment 
               
               
                 Second 
                 0.1 mΩ 
                 0/30 
                 0.1 mΩ 
                  0/30 
                 &gt;30 
               
               
                 em- 
               
               
                 bodiment 
               
               
                 Com- 
                 — 
                 10/30  
                 — 
                 30/30 
                 2 
               
               
                 parative 
               
               
                 example 
               
               
                   
               
               
                 *At 30 Hz, for three hours 
               
               
                 **From 1.9 m, up to 30 times 
               
               
                 ***Average times before welded portions were disengaged in the drop test 
               
             
          
         
       
     
         [0048]    Table 1 reveals the following facts. Regarding the sealing body  18 C of the comparative example having no space, the welded portion was disengaged in ten out of thirty pieces in the vibration test, and in all of the thirty pieces in the post-drop resistance increase test. The number of drop times for which the welded portion of the sealing body  18 C was disengaged was two on average. 
         [0049]    As regards the sealing body  18 A according to the first embodiment including a terminal cap having a space (annular groove), no welded portion was disengaged in the vibration test. The internal resistance increase after the vibration test was 0.1 mΩ on average. While the welded portion of this sealing body  18 A was disengaged in all of the thirty pieces in the post-drop resistance increase test, the number of drop times for which the welded portion was disengaged was 15 on average, which means the welded portion is stronger than its counterpart in the sealing body  18 C of the comparative example. Since the only difference in the sealing bodies  18 A and  18 C was the presence of an annular groove in their terminal caps, the higher strength of the sealing body  18 A is clearly attributed to this groove. 
         [0050]    As for the sealing body  18 B according to the second embodiment including a terminal cap having a space (annular groove) and also having an interposed material between its safety valve and terminal cap, no welded portion was disengaged in the vibration test. The internal resistance increase after the vibration test was 0.1 mΩ on average. Furthermore, no welded portion of the sealing body  18 B was disengaged through thirty drops in the post-drop resistance increase test. Here, the internal resistance increase was 0.1 mΩ on average. Accordingly, the sealing body  18 B of the second embodiment provided the best results. 
         [0051]    The sealing body  18 B of the second embodiment, however, requires one more welding compared with the sealing body  18 A of the first embodiment. Therefore, the user may select either the sealing body  18 A or  18 B in consideration with necessary strength. 
         [0052]    While laser welding is employed in the first and second embodiments, electron beam welding can also achieve the same advantages as described above. Also, while the space provided to the terminal cap is an annular groove in the first and second embodiments, it may be a circular or square recess. In this case, since the recess is not readily identifiable from the backside to which the terminal cap is welded, it is necessary to provide a certain means to show the position of the recess from the backside. Furthermore, while the groove was provided to the terminal cap in the first and second embodiments, the groove may be provided to the safety valve or both the terminal cap and safety valve, as a space between the terminal cap and safety valve can provide the same advantages as described above. 
         [0053]    In addition, while a sealed battery including a sealing body whose terminal cap and safety valve are fixed to a bottom plate is described in the above-described embodiments, other types of sealed batteries, for example, in which an outer can containing an electrode unit has a constricted part near its opening, and a sealing gasket, a safety valve, and a terminal cap are mounted and fixed with the opening edge of the outer can may provide the same advantages as described above.