Abstract:
Disclosed is a sealed secondary battery provided with a current cut-off device which employs an inexpensive and uncomplicated construction, without the need to mount a separate current cut-off device on the battery body. A sealed secondary battery ( 10 ) comprises a battery casing ( 11 ) and a sealing plate ( 12 ), an electrode body ( 21 ) and current collecting plates ( 22, 22 ) provided on the inside of this casing ( 11 ). A contact section ( 11   c ) formed as part of the bottom ( 11   a ) of this battery casing ( 11 ) is bowed further inwards towards the negative electrode-side current collecting plate ( 22   b ) than the peripheral section of the bottom ( 11   a ) of this battery casing ( 11 ). At this contact section ( 11   c ), the inside face of the bottom ( 11   a ) of the battery casing contacts the current collecting plate ( 22   b ) at the negative electrode end, but this electrical contact between the contact section ( 11   c ) and the current collecting plate ( 22   b ) at the negative electrode end is cut off when outward swelling of the contact section ( 11   c ) takes place, produced by a rise in internal pressure in the casing in a sealed condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application of International Application No. PCT/JP2010/054830, filed Mar. 19, 2010, and claims the priority of Japanese Application No. 2009-119249, filed May 15, 2009, the contents of both of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a sealed secondary battery. Concretely, the present invention relates to an art for constructing easily a current cut-off device formed in the sealed secondary battery so as to cut off a current. 
     BACKGROUND ART 
     Conventionally, a sealed secondary battery used widely for home electric appliances and the like. Especially, a lithium-ion secondary battery is actually used for an electric vehicle because of its high energy density. 
     When the secondary battery mentioned above is charged with electricity further than the state at which the battery is charged enough (full charge), overcharge occurs, whereby an internal short circuit may be caused between positive and negative electrodes so as to generate gas by resolution of an electrolyte. As a result, the internal pressure of the secondary battery rises, thereby causing exothermicity and failure of the secondary battery. 
     In the case in which such secondary batteries are employed as an assembled battery, by providing a control circuit in each of the batteries and watching the batteries totally, dispersion of capacities of the batteries and change of degradation speed can be tackled so as to prevent the overcharge. However, providing the control circuit in each of the batteries is difficult because it increases the cost and increases the weight and space. 
     On the other hand, as a countermeasure for the overcharge of the secondary battery, an art using a current cut-off device of so-called internal pressure reaction type is actually employed in which an electric connection section is disconnected when the internal pressure of the secondary battery becomes not less than a fixed value. Concretely, an art concerning the current cut-off device is well known in which an electrically conductive diaphragm (metal foil) is provided in a lid or a battery casing of the secondary battery, and when the internal pressure of the secondary battery reaches the reversal pressure of the diaphragm, the diaphragm is reversed oppositely to a current collecting plate, whereby the connection between the diaphragm and the current collecting plate is disconnected and the current is cut off (for example, see JP 2007-87730 A and JP 2007-227283 A). 
     PTL 1: JP 2007-87730 A 
     PTL 2: JP 2007-227283 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to the conventional arts described in the Patent Literatures, in the current cut-off device, the battery body such as the lid or the battery casing and the electrically conductive diaphragm are separated from each other, that is, the diaphragm is attached separately from the battery body. Accordingly, the size of the battery is large considering the battery capacity, and the number of the parts and the design space are increased so as to increase the cost. 
     Then, the purpose of the present invention is to provide a sealed secondary battery provided with a current cut-off device which employs an inexpensive and uncomplicated construction so as to reduce the size of the battery and suppress the number of the parts and the design space. 
     Technical Solution 
     A sealed secondary battery according to a first embodiment of the present invention includes a cylindrical housing which has electrical conductivity and each of whose ends is formed therein with a bottom, an electrode body housed inside the housing and fixed inside the housing, and current collecting plates respectively disposed in both ends of the electrode body in the lengthwise direction inside the housing. A part of at least one of the bottoms of the housing is formed as a contact section contacting the current collecting plate and the contact section is electrically connected to the current collecting plate so as to make one of the bottom be a positive electrode and to make the other bottom be a negative electrode. The contact section is expanded outwards by a rise in internal pressure of the housing which is sealed so that the electric connection between the contact section and the current collecting plate is cut off. 
     Preferably, when the contact section is expanded outwards by the rise in the internal pressure of the housing and then the internal pressure of the housing drops, the state at which the electric connection between the contact section expanded outwards and the current collecting plate is cut off is maintained. 
     Preferably, the contact section is formed to be bowed towards the current collecting plate further than the peripheral section of the bottom of the housing, and the concave amount of the contact section from the peripheral section is larger than the board thickness of the part at which the contact section is formed. 
     Preferably, the housing comprises a battery casing having electrical conductivity and formed to be a close-end cylinder in which one of the ends thereof is opened, and a sealing plate having electrical conductivity and provided to the battery casing insulatedly so as to close the opening of the battery casing, and the contact section is formed in at least one of the sealing plate and the bottom of the battery casing so that one of the sealing plate and the bottom of the battery casing is made be a positive electrode and the other thereof is made be a negative electrode. 
     Preferably, the housing comprises a battery casing formed to be a cylinder in which both of the ends thereof are opened, and two sealing plates provided to the battery casing insulatedly so as to close the openings of the battery casing, and the contact section is formed in at least one of the sealing plates so that one of the sealing plates is made be a positive electrode and the other sealing plate is made be a negative electrode. 
     Preferably, the board thickness of the part of the bottom of the housing at which the contact section is formed is not more than the board thickness of the side face of the housing. 
     Advantageous Effect of the Invention 
     According to the present invention, in addition to using the housing as the positive and negative electrodes, the bottom of the housing is used as the diaphragm of the current cut-off device, whereby the sealed secondary battery provided with the current cut-off device which employs the inexpensive and uncomplicated construction in which it is not necessary to attach separately any current cut-off device to the battery body. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a drawing partially in section of entire construction of a sealed secondary battery according to a first embodiment. 
         FIG. 2  is an end view of a bottom of a battery casing.  FIG. 2(   a ) is an end view of the bottom of the sealed secondary battery according to a first embodiment at the normal state, and  FIG. 2(   b ) is an end view of the bottom of the sealed secondary battery at the state at which internal pressure rises. 
         FIG. 3  is a sectional view showing the board thickness of the battery casing and a bowed amount of a contact section. 
         FIG. 4  is a diagram of results of an overcharge test of a current cut-off device. 
         FIG. 5  is a drawing partially in section of entire construction of a sealed secondary battery according to a second embodiment. 
         FIG. 6  is a drawing partially in section of entire construction of a sealed secondary battery according to a third embodiment. 
     
    
    
     REFERENCE SIGNS LIST 
       10 : sealed secondary battery 
       11 : battery casing 
       11   c : contact section 
       12 : sealing plate 
       21 : electrode body 
       22 : current collecting plate 
     DESCRIPTION OF EMBODIMENTS 
     Next, explanation will be given on the mode for carrying out the present invention. 
     The technical field of the present invention is not limited to below embodiments and ranges widely to the entire range of technical idea, which the present invention means, defined by the matters described in the specification and the drawings. 
     [Sealed Secondary Battery  10  (First Embodiment)] 
     Firstly, explanation will be given on a sealed secondary battery  10  according to a first embodiment of the present invention referring to  FIG. 1 . As shown in  FIG. 1 , the sealed secondary battery  10  has a battery casing  11  which is a cylindrical housing having electrical conductivity and a sealing plate  12 . 
     The battery casing  11  is formed to be a close-end cylinder and one of the ends thereof is opened. The sealing plate  12  which is discal is disposed in the battery casing  11  via an insulator  24 , whereby the sealing plate  12  closes the opening of the battery casing  11  while being insulated from the battery casing  11 . 
     Accordingly, the housing is sealed by closing the opening of the battery casing  11  by the sealing plate  12 . Namely, in this embodiment, the housing has two bottoms, i.e., a bottom  11   a  of the battery casing  11  opposite to the opening of the battery casing  11  and the sealing plate  12 . The side face of the battery casing  11  forms a side face  11   b  of the battery casing  11 . 
     Each of the battery casing  11  and the sealing plate  12  is formed by metal such as iron or aluminium and has electrical conductivity as mentioned above, but the material of each of the battery casing  11  and the sealing plate  12  is not limited. In this specification, the housing is supposed to be a circular cylinder. However, the housing may alternatively be a square cylinder, and the shale of the housing is not limited to the shape described in this specification. 
     A relief valve  12   a  is formed in the sealing plate  12  and opens when the pressure inside the sealed secondary battery  10  rises extremely, and discharges gas inside the sealed secondary battery  10  so as to secure the safety. 
     In the sealed secondary battery  10 , an electrode body  21  formed to be a cylinder slightly smaller than the housing and two current collecting plates  22  respectively disposed at the ends of the electrode body  21  in the lengthwise direction are provided inside the housing. 
     While the current collecting plates  22  are disposed at the ends of the electrode body  21  and the current collecting plates  22  are surrounded by two insulation members  25  which are circular, the electrode body  21  is inserted into the battery casing  11  before closed its opening by the sealing plate  12 . 
     Accordingly, the electrode body  21  and the current collecting plates  22  are fixed via the insulation members  25  to the battery casing  11 , and then the sealing plate  12  closes the opening of the battery casing  11 , whereby the electrode body  21  and the current collecting plates  22  are housed inside the battery casing  11 . 
     The electrode body  21  is a wound body in which a positive electrode and a negative electrode are laminated via a separator formed by a porous body and then wound a plurality of times around an axial core  21   a . Each of plied timbers respectively including a positive electrode active material and a negative electrode active material are held in corresponding one of the lamination sections of the positive electrode and the negative electrode. In other words, in the electrode body  21 , a plied timber supporting section of the positive electrode and a plied timber supporting section of the negative electrode are laminated via the separator, and the electrode body  21  is charged and discharged by a chemical reaction between the positive electrode and the negative electrode in this lamination section. 
     The positive electrode is an electronic conductive collector foil (for example, a metal foil such as aluminum) wherein a part of surface of the foil is applied thereto with a plied timber including a positive electrode active material, and the negative electrode is an electronic conductive collector foil (for example, a metal foil such as copper) wherein a part of surface of the foil is applied thereto with a plied timber including a negative electrode active material. 
     The positive electrode is projected at one of the ends of the electrode body  21  in the lengthwise direction, and the negative electrode is projected at the other end. In this embodiment, in the electrode body  21 , the positive electrode is projected at the end at the side of the sealing plate  12 , and the negative electrode is projected at the end at the side of the bottom  11   a . In the electrode body  21 , a positive electrode-side current collecting plate  22   a  is disposed at the end at the side of the sealing plate  12 , and a negative electrode-side current collecting plate  22   b  is disposed at the end at the side of the bottom  11   a.    
     In the sealed secondary battery  10  constructed as mentioned above, the sealing plate  12  and the battery casing  11  are electrically connected respectively to the positive electrode-side current collecting plate  22   a  and the negative electrode-side current collecting plate  22   b , whereby electric power generated by the electrode body  21  is taken out from the sealing plate  12  and the battery casing  11 . 
     Concretely, a reed member  26  is welded to the inside face of the sealing plate  12  (the face of the sealing plate  12  at the side of the electrode body  21 ) and the reed member  26  is connected electrically to the positive electrode-side current collecting plate  22   a , whereby the sealing plate  12  is connected electrically to the positive electrode-side current collecting plate  22   a.    
     On the other hand, a contact section  11   c  formed as a part of the bottom  11   a  is bowed curvedly (dome-like) further inwards towards the negative electrode-side current collecting plate  22   b  than the peripheral section of the bottom  11   a . At the contact section  11   c , the inside face of the bottom  11   a  contacts and is connected electrically to the negative electrode-side current collecting plate  22   b , whereby the battery casing  11  is connected electrically to the negative electrode-side current collecting plate  22   b.    
     According to the construction, the sealing plate  12  serves as the positive electrode of the sealed secondary battery  10 , and the bottom  11   a  serves as the negative electrode of the sealed secondary battery  10 . 
     In this embodiment, for reducing the electric resistance at the contact section  11   c , a plurality of projections  11   d  are formed in the contact section  11   c  and the projections  11   d  contact the negative electrode-side current collecting plate  22   b , whereby the contact section  11   c  is electrically connected to the negative electrode-side current collecting plate  22   b . Instead of forming the projections  11   d  in the contact section  11   c , it may alternatively be constructed that the contact section  11   c  is formed flat and the contact section  11   c  contacts the negative electrode-side current collecting plate  22   b.    
     In this embodiment, the contact section  11   c  is bowed curvedly further inwards than the peripheral section of the bottom  11   a . However, for example, the contact section  11   c  may alternatively become hollow cylindrically further inwards than the peripheral section of the bottom  11   a , and the shape of the contact section  11   c  is not limited. 
     [Current Cut-Off Device] 
     Next, explanation will be given on a current cut-off device of the sealed secondary battery  10  referring to  FIGS. 2 and 3 . 
     In the sealed secondary battery  10 , when the internal pressure in the housing rises, the contact section  11   c  is expanded outwards, whereby the electric connection between the contact section  11   c  and the negative electrode-side current collecting plate  22   b  is cut off. 
     At the normal use, as shown in  FIGS. 1 and 2(   a ), the contact section  11   c  contacts and is electrically joined to the negative electrode-side current collecting plate  22   b , whereby the contact section  11   c  is electrically connected to the negative electrode-side current collecting plate  22   b . However, when the sealed secondary battery  10  is overcharged and the overcharge causes an internal short circuit between the positive and negative electrodes so as to generate gas by resolution of an electrolyte, the internal pressure in the sealed secondary battery  10  becomes not less than a fixed value so that the contact section  11   c  is expanded outwards as shown in  FIG. 2(   b ), whereby the electric connection is cut off. 
     In detail, when the internal pressure in the sealed secondary battery  10  reaches a reversal pressure of the bottom  11   a  by the gas, the contact section  11   c  is reversed oppositely to the negative electrode-side current collecting plate  22   b  and the joining between the contact section  11   c  and the negative electrode-side current collecting plate  22   b  is cut off, whereby the current is cut off. In other words, in the sealed secondary battery  10 , the bottom  11   a  which is a part of the housing is constructed as a diaphragm so as to construct the current cut-off device. 
     As mentioned above, the battery casing  11  which is the main body of the battery and the bottom  11   a  which is the diaphragm integrally constitute the current cut-off device according to this embodiment. Namely, the bottom  11   a  which is the part of the main body of the battery serves as the diaphragm. 
     Accordingly, parts and a space for attaching a separate diaphragm which is necessary conventionally is not required, whereby the sealed secondary battery  10  can be downsized with an inexpensive and uncomplicated construction and the number of the parts and the design space can be prevented from being increased. 
     As shown in  FIG. 3 , preferably, a board thickness Tb of the part of the bottom  11   a  at which the contact section  11   c  is formed is not more than a board thickness T of the side face  11   b  of the housing. 
     According to the construction, the rigidity of the part at which the contact section  11   c  is formed is lower than the rigidity of the side face  11   b  of the housing. 
     Accordingly, when the internal pressure in the sealed secondary battery  10  rises, the contact section  11   c  can be reversed easily oppositely to the negative electrode-side current collecting plate  22   b . Namely, when the state of emergency occurs which is the rise of the internal pressure in the sealed secondary battery  10  caused by the overcharging, the contact section  11   c  is reversed outwards while the side face  11   b  of the housing maintains its shape so that the current is cut off immediately, whereby problems such as exothermicity and failure of action can be prevented. 
     Furthermore, when the contact section  11   c  is expanded outwards by the rise of the internal pressure in the sealed secondary battery  10 , the contact section  11   c  preferably maintains its expanded state even if the internal pressure of the housing drops subsequently so as not to contact electrically the negative electrode-side current collecting plate  22   b  again. 
     According to the construction, the operation of the current cut-off device can be made irreversible. Namely, the sealed secondary battery  10  in which the current cut-off device is operated and the current is cut off once is prevented from being reused. 
     Concretely, as shown in  FIG. 3 , a concave amount H of the contact section  11   c  in the peripheral section of the bottom  11   a  is larger than the board thickness Tb of the part of the bottom  11   a  at which the contact section  11   c  is formed. 
     According to the construction, the power required for reversing the contact section  11   c  is increased, whereby when the contact section  11   c  is expanded outwards once, the contact section  11   c  is not reversed inwards even if the internal pressure of the housing drops subsequently. 
     [Overcharge Test] 
     Next, explanation will be given on results of an overcharge test made by the inventor on the current cut-off device of the sealed secondary battery  10  according to this embodiment referring to  FIG. 4 . 
     The inventor made the overcharge test of the sealed secondary battery  10  while the board thickness T of the side face  11   b  of the battery casing  11  in the sealed secondary battery  10  was set be 1.2 mm, the board thickness Tb of the part of the bottom  11   a  at which the contact section  11   c  was set be 0.6 mm, 0.7 mm, 0.8 mm, 1.0 mm or 1.2 mm, and the concave amount H of the contact section  11   c  in the peripheral section of the bottom  11   a  was set be 0.5 mm or 1.0 mm. 
     Concretely, in the sealed secondary battery  10  under each of the conditions mentioned above, the internal pressure of the casing was raised by the overcharge test, and then it was checked whether the contact section  11   c  was expanded outwards and the electric connection between the contact section  11   c  and the negative electrode-side current collecting plate  22   b  was cut off or not. It was also checked whether the contact section  11   c  returned to the side of the negative electrode-side current collecting plate  22   b  and the electric connection therebetween was restored or not when the internal pressure of the casing drops after the rise in the internal pressure of the casing by the overcharge test. 
     The results of the overcharge test is shown in  FIG. 4 . 
     In  FIG. 4 , the case in which the current cut-off device operated normally and the current was cut off at the pressure not more than the internal pressure set by the inventor at the time at which the internal pressure of the casing rose (at the time of rise in the internal pressure) is indicated by ◯, and the case in which the current was not cut off at the internal pressure set by the inventor is indicated by X. The case in which the contact section  11   c  expanded outwards once was not reversed inwards again even if the internal pressure of the housing dropped subsequently (after the drop of the internal pressure) is indicated by ◯, and the case in which the contact section  11   c  was reversed inwards again is indicated by X. 
     As shown in  FIG. 4 , in the case in which the concave amount H of the contact section  11   c  in the peripheral section of the bottom  11   a  was set be 0.5 mm, when the board thickness Tb of the part at which the contact section  11   c  was not more than the board thickness T of the side face  11   b  of the battery casing  11 , the effect of current cut-off was obtained in all the tests. In the case in which the concave amount H of the contact section  11   c  was set be 1.0 mm, when the board thickness Tb of the part at which the contact section  11   c  was 0.6 mm, 0.7 mm or 0.8 mm and was further smaller than the board thickness T of the side face  11   b  of the battery casing  11  which is 1.2 mm, the higher effect of current cut-off was obtained. 
     Therefore, according to the overcharge test made by the inventor, it is confirmed that by making the board thickness Tb of the part of the bottom  11   a  at which the contact section  11   c  not more than the board thickness T of the side face  11   b  of the housing, the contact section  11   c  can be reversed easily oppositely to the negative electrode-side current collecting plate  22   b  when the internal pressure in the sealed secondary battery  10  rises. Namely, it is confirmed that pertinent effect of current cut-off is obtained when the state of emergency occurs which is the rise of the internal pressure in the sealed secondary battery  10  caused by the overcharging. 
     On the other hand, as shown in  FIG. 4 , in the case in which the concave amount H of the contact section  11   c  in the peripheral section of the bottom  11   a  was made not more than the board thickness Tb of the part at which the contact section  11   c , when the contact section  11   c  was expanded outwards once and subsequently the internal pressure of the housing drops, the contact section  11   c  was reversed inwards again. To the contrary, in the case in which the concave amount H was made more than the board thickness Tb, the contact section  11   c  expanded outwards once was not reversed inwards again even if the internal pressure of the housing drops subsequently. 
     Namely, according to the overcharge test made by the inventor, it is confirmed that by making the concave amount H more than the board thickness Tb, the contact section  11   c  expanded outwards once is prevented from being reversed inwards again even if the internal pressure of the housing drops subsequently. 
     [Sealed Secondary Battery  110  (Second Embodiment)] 
     Next, explanation will be given on a sealed secondary battery  110  according to a second embodiment of the present invention referring to  FIG. 5 . Explanation on the parts of the sealed secondary battery explained in the below embodiment which are in common with those in the embodiment mentioned above is omitted. 
     As shown in  FIG. 5 , the sealed secondary battery  110  has a battery casing  111  which is a cylindrical housing having electrical conductivity and a sealing plate  112 . 
     Similarly to the first embodiment, the battery casing  111  is formed to be a close-end cylinder and one of the ends thereof is opened. The sealing plate  112  which is discal is disposed in the battery casing  111  via an insulator  124 , whereby the sealing plate  112  closes the opening of the battery casing  111  while being insulated from the battery casing  111 . 
     In the sealed secondary battery  110 , an electrode body  121  formed to be a cylinder slightly smaller than the housing and two current collecting plates  122  (a positive electrode-side current collecting plate  122   a  and a negative electrode-side current collecting plate  122   b ) respectively disposed at the ends of the electrode body  121  in the lengthwise direction are provided inside the housing. Namely, the electrode body  121  and the current collecting plates  122  are fixed via two insulation members  125  to the battery casing  111 , and then the sealing plate  112  closes the opening of the battery casing  111 . 
     In the sealed secondary battery  110  constructed as mentioned above, the sealing plate  112  and the battery casing  111  are electrically connected respectively to the positive electrode-side current collecting plate  122   a  and the negative electrode-side current collecting plate  122   b , whereby electric power generated by the electrode body  121  is taken out from the sealing plate  112  and the battery casing  111 . 
     Concretely, a contact section  112   c  is formed at a part of the sealing plate  112  so as to become hollow towards the positive electrode-side current collecting plate  122   a  further than the peripheral section of the sealing plate  112 . At the contact section  112   c , the inside face of the sealing plate  112  contacts and is electrically joined to the positive electrode-side current collecting plate  122   a , whereby the sealing plate  112  is connected electrically to the positive electrode-side current collecting plate  122   a.    
     On the other hand, a part of the inside face of the bottom  111   a  (the face at the side of the electrode body  121 ) contacts directly the negative electrode-side current collecting plate  122   b , whereby the battery casing  111  is connected electrically to the negative electrode-side current collecting plate  122   b.    
     According to the construction, the sealing plate  112  serves as the positive electrode of the sealed secondary battery  110 , and the bottom  111   a  serves as the negative electrode of the sealed secondary battery  110 . 
     In the sealed secondary battery  110  according to this embodiment, when the internal pressure in the housing rises, the contact section  112   c  is expanded outwards, whereby the electric connection between the contact section  112   c  and the positive electrode-side current collecting plate  122   a  is cut off. 
     Namely, when the internal pressure in the sealed secondary battery  110  reaches a reversal pressure of the sealing plate  112 , the contact section  112   c  is reversed oppositely to the positive electrode-side current collecting plate  122   a  and the joining between the contact section  112   c  and the positive electrode-side current collecting plate  122   a  is cut off, whereby the current is cut off. In other words, in the sealed secondary battery  110  according to the present invention, the sealing plate  112  which is a part of the housing is constructed as a diaphragm so as to construct the current cut-off device. 
     As mentioned above, in the current cut-off device according to this embodiment, the sealing plate  112  which is a part of the main body of the battery serves as the diaphragm. 
     Accordingly, parts and a space for attaching a separate diaphragm which is necessary conventionally is not required, whereby the sealed secondary battery  110  can be downsized with an inexpensive and uncomplicated construction and the number of the parts and the design space can be prevented from being increased. 
     [Sealed Secondary Battery  210  (Third Embodiment)] 
     Next, explanation will be given on a sealed secondary battery  210  according to a third embodiment of the present invention referring to  FIG. 6 . As shown in  FIG. 6 , the sealed secondary battery  210  has a battery casing  211  which is a cylindrical housing having electrical conductivity, and a positive electrode-side sealing plate  212   a  and a negative electrode-side sealing plate  212   b.    
     The battery casing  211  is formed to be a cylinder and both ends thereof are opened. The positive electrode-side sealing plate  212   a  which is discal is arranged at the positive electrode side of the battery casing  211  via an insulator  224  at the insulated state, and the negative electrode-side sealing plate  212   b  is arranged at the negative electrode side of the battery casing  211  via an insulator  224  at the insulated state, whereby both of the openings of the battery casing  211  are closed by the positive electrode-side sealing plate  212   a  and the negative electrode-side sealing plate  212   b.    
     In the sealed secondary battery  210 , an electrode body  221  formed to be a cylinder slightly smaller than the housing and two current collecting plates  222  (a positive electrode-side current collecting plate  222   a  and a negative electrode-side current collecting plate  222   b ) respectively disposed at the ends of the electrode body  221  in the lengthwise direction are provided inside the housing. Namely, the electrode body  221  and the current collecting plates  222  are fixed via two insulation members  225  to the battery casing  211 , and then both of the openings of the battery casing  211  are closed by the positive electrode-side sealing plate  212   a  and the negative electrode-side sealing plate  212   b.    
     In the sealed secondary battery  210  constructed as mentioned above, the positive electrode-side sealing plate  212   a  and the negative electrode-side sealing plate  212   b  are electrically connected respectively to the positive electrode-side current collecting plate  222   a  and the negative electrode-side current collecting plate  222   b , whereby electric power generated by the electrode body  221  is taken out from the positive electrode-side sealing plate  212   a  and the negative electrode-side sealing plate  212   b.    
     Concretely, a reed member  226  is welded to the inside face of the positive electrode-side sealing plate  212   a  (the face of the positive electrode-side sealing plate  212   a  at the side of the electrode body  221 ) and the reed member  226  is connected electrically to the positive electrode-side current collecting plate  222   a , whereby the positive electrode-side sealing plate  212   a  is connected electrically to the positive electrode-side current collecting plate  222   a.    
     On the other hand, at a part of the negative electrode-side sealing plate  212   b , a contact section  212   c  is formed so as to become hollow towards the negative electrode-side current collecting plate  222   b  further than the peripheral section of the negative electrode-side sealing plate  212   b . At the contact section  212   c , the inside face of the negative electrode-side sealing plate  212   b  contacts and is connected electrically to the negative electrode-side current collecting plate  222   b , whereby the negative electrode-side sealing plate  212   b  is connected electrically to the negative electrode-side current collecting plate  222   b.    
     According to the construction, the positive electrode-side sealing plate  212   a  serves as the positive electrode of the sealed secondary battery  210 , and the negative electrode-side sealing plate  212   b  serves as the negative electrode of the sealed secondary battery  210 . 
     In the sealed secondary battery  210  according to this embodiment, when the internal pressure in the housing rises, the contact section  212   c  is expanded outwards, whereby the electric connection between the contact section  212   c  and the negative electrode-side current collecting plate  222   b  is cut off. 
     Namely, when the internal pressure in the sealed secondary battery  210  reaches a reversal pressure of the negative electrode-side sealing plate  212   b , the contact section  212   c  is reversed oppositely to the negative electrode-side current collecting plate  222   b  and the joining between the contact section  212   c  and the negative electrode-side current collecting plate  222   b  is cut off, whereby the current is cut off. In other words, in the sealed secondary battery  210 , the negative electrode-side sealing plate  212   b  which is a part of the housing is constructed as a diaphragm so as to construct the current cut-off device. 
     As mentioned above, in the current cut-off device according to this embodiment, the negative electrode-side sealing plate  212   b  which is a part of the main body of the battery serves as the diaphragm. 
     Accordingly, parts and a space for attaching a separate diaphragm which is necessary conventionally is not required, whereby the sealed secondary battery  210  can be downsized with an inexpensive and uncomplicated construction and the number of the parts and the design space can be prevented from being increased. 
     In this specification, one of the bottoms of the housing as the contact section is electrically connected to the current collecting plate, and when the internal pressure in the housing rises, the contact section is expanded outward, whereby the electric connection between the contact section and the current collecting plate is cut off. However, the contact section may alternatively be formed in each of the bottoms of the housing. Namely, each of the bottoms may alternatively be constructed as the current cut-off device. 
     Industrial Applicability 
     The present invention can be employed for a sealed secondary battery having a current cut-off device therein, and is especially suitable for an art for constructing the current cut-off device easily.