Abstract:
The disclosed electrode terminal, which is used with batteries of which a positive output terminal and a negative output terminal are formed from metals dissimilar to each other, has excellent mechanical strength and can suppress electrical resistance while preventing galvanic corrosion. The electrode terminal ( 1 ) for power output is used with batteries ( 2 ) of which a pair of output terminals are each formed from a different metal, and has: a solid cylindrical shaft ( 10 ) that attaches to one output terminal and that is formed from the same metal as the one output terminal; and an outer cylinder ( 11 ) that connects with the cylindrical shaft ( 10 ) and that is formed from the same metal as the other output terminal. The shaft ( 10 ) and the outer cylinder ( 11 ) are unified by means of metallic bonding.

Description:
TECHNICAL FIELD 
     The present invention relates to an electrode terminal preferably used for a battery having a plus output terminal and a minus output terminal formed from metals dissimilar to each other, and a method for producing the electrode terminal. 
     BACKGROUND ART 
     Multiple battery cells configured as a battery pack by serially connecting a positive electrode of one of the battery cells and a negative electrode of another battery by a busbar is known as a battery installed on an electric vehicle and a hybrid vehicle (refer to a Patent Document 1, for example). Such a battery pack is characterized by a high output and a high energy density, and lithium ion batteries are used as the battery cells in most cases. The lithium ion battery has a plus output terminal formed by aluminum (Al) as a raw material, and a minus output terminal formed by copper (Cu) as a raw material. 
     There is a busbar (component for distribution of electric energy) as a component for connecting the terminals of the battery cells with each other. There is a manufacture of laser-welding members constructing busbars to each other as a method for producing the busbar as disclosed in “Technical Problem” of the Patent Document 2. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2002-373638 
     Patent Document 2: Japanese Patent Application Laid-Open No. 2003-163039 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described before, if battery cells are serially connected to each other, the plus output terminal (aluminum) and the minus output terminal (copper) are connected with each other by the busbar. As a result, whether the busbar is formed from aluminum or copper, the busbar and one of the terminals always constitute a connection of dissimilar metals. 
     It is well known that if dissimilar metals are connected with each other, water in the air generally causes galvanic corrosion (electrochemical corrosion). Thus, as the galvanic corrosion progresses, the electrical conduction between the busbar and the terminal is disconnected, or the busbar itself or the terminal itself is damaged, and such a serious problem that an electric vehicle cannot be started is finally reached. 
     Though a manufacture of a busbar by joining an aluminum piece and a cupper piece to each other by laser welding, for example, is proposed as described in the Patent Document 2 as a countermeasure for this problem, eutectic is generated by the two types of metal on a laser-welded portion on a busbar made according to this method on a trial basis, this causes defects such as an excessive electric resistance, or an extreme decrease in mechanical strengths (particularly brittleness and tensile strength), and the busbar was not practically used. 
     In other words, not only improvement of the busbar, but also improvement and development of other members such as an electrode terminal provided for a battery cell is indispensable in order to fundamentally solve the problem. 
     The present invention is devised in view of the above-mentioned problem, and has an object of providing an electrode terminal which is used for a battery having a plus output terminal and a minus output terminal formed from metals dissimilar to each other, can prevent galvanic corrosion, restrains an electric resistance, is excellent in mechanical strength, and has high performance and high reliability, and a method of producing the electrode terminal. 
     Solution to Problem 
     In order to attain the object, the electrode terminal according to the present invention is an electrode terminal for electric power output used for a battery having a pair of output terminals formed from metals dissimilar to each other, including a first connection portion connected to one output terminal, and formed from the same metal as of the one output terminal, and a second connection portion connected to the first connection portion, and formed from the same metal as of the other output terminal, where the first connection portion and the second connection portion are unified by means of metallic bonding. 
     Preferably, the first connection portion is a shaft in a solid cylindrical shape, and the second connection portion is an outer cylinder in a cylindrical shape fit over the shaft. 
     The “metallic bonding” refers to a state in which a bonding interface at which dissimilar metals to be bonded are in close contact with each other at a metal structure level, and the electric conductivity and the mechanical bonding strength are consequently increased to “values suitable for practical use as an electrode terminal”. 
     More preferably, a male thread portion is formed on an outer peripheral surface of the outer cylinder. 
     The outer cylinder may be formed so as to extend exceeding the length of the shaft in a direction opposite to a protruding side of the shaft. 
     If the electrode terminal is used for a plus output terminal of a lithium ion battery, the shaft is formed from aluminum or aluminum alloy, and the outer cylinder is formed from copper or copper alloy. 
     If the electrode terminal is used for a minus output terminal of a lithium ion battery, the shaft is formed from copper or copper alloy, and the outer cylinder is formed from aluminum or aluminum alloy. 
     On the other hand, when the above-described electrode terminal is produced, it is essential to employ a production method of providing source materials facing each other in a state in which a metal source material forming the outer cylinder is wound so as to surround a metal source material forming the shaft, and applying an extrusion process or a drawing process to the source materials facing each other by means of a die in a hydrostatic pressure environment at a high pressure. 
     The employment of this production method enables to unify the metal material forming the shaft and the metal material forming the outer cylinder by means of the metallic bonding, thereby producing an electrode terminal without presenting galvanic corrosion and the like. 
     Advantageous Effects of Invention 
     The use of this electrode terminal causes a plus output terminal and a minus output terminal of a battery to be the same metal when viewed from outside, a connection using a wire and a busbar made from the same metal as of the terminal can restrain the galvanic corrosion on terminal joint portions, and an increase in electric resistance caused thereby, resulting in an increase in reliability as a battery pack. In addition, in a preferred embodiment, the shaft and the outer cylinder of the electrode terminal are unified by means of the metallic bonding, and galvanic corrosion and an increase in electric resistance caused thereby in a bonded portion do not occur. 
     According to the present invention, the high-performance and high-reliability electrode terminal which is preferred for a battery having a plus output terminal and a minus output terminal formed by dissimilar metals, which can restrain the electric resistance while preventing the galvanic corrosion, and is excellent in mechanical strength can be realized. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A perspective view of a used form of electrode terminals according to a first embodiment. 
         FIG. 2A  A plan view of the electrode terminal according to the first embodiment. 
         FIG. 2B  A front view of the electrode terminal according to the first embodiment. 
         FIG. 3  A connection state of the electrode terminal according to the first embodiment and a busbar. 
         FIG. 4  A perspective view describing a process of producing an electrode terminal according to the present invention. 
         FIG. 5A  A plan view of the electrode terminal according to a second embodiment. 
         FIG. 5B  A front view of the electrode terminal according to the second embodiment. 
         FIG. 6  A connection state of the electrode terminal according to the second embodiment and a busbar. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A description will now be given of embodiments of the present invention with reference to drawings. 
     First Embodiment 
       FIG. 1  to  FIG. 3  show a first embodiment of an electrode terminal  1  according to the present invention. As shown in  FIG. 1 , the electrode terminal  1  can be used as a minus output terminal (minus output side) of each of battery cells  2 , for example, in a battery pack  4  and the like constructed by serially connecting the multiple battery cells  2  by busbars  3 . 
     As shown in  FIG. 3 , the battery cell  2  is a lithium ion battery, and the minus output terminal is formed from copper or copper alloy. This is because a negative-electrode-side carrier  7  (base body for fixing electrons and ions) connected to the minus output terminal inside the battery is formed from copper or copper alloy. In relation to a positive-electrode-side carrier, the plus output terminal (plus output side) is formed from aluminum or aluminum alloy. 
     As shown in  FIG. 2A  and  FIG. 2B , the electrode terminal  1  according to the present invention adopted as the minus output terminal is formed into inner/outer double shafts by a shaft  10  (first connection portion) and an outer cylinder  11  (second connection portion) fitting over and covering the shaft  10 . 
     A lower-end side of the shaft  10  protrudes from the outer cylinder  11  in the axial direction. An upper-end side of the shaft  10  and an upper end portion of the outer cylinder  11  are aligned to the same level in height. The shaft portion  10  is a round shaft, and the outer cylinder  11  is formed into a cylindrical shape. In other words, a shape on a cross section orthogonal to the axial direction of the shaft  10  and the outer cylinder  11  presents a double circle, and the thickness of the outer cylinder  11  surrounding the shaft  10  is approximately constant. 
     A base portion  12  is formed in a lower-end side of the outer cylinder  11 , and the shaft  10  protrudes downward so as to pass through the base portion  12 . Moreover, a male thread portion  13  is formed on an outer peripheral surface of the outer cylinder  11  except for the base portion  12 . 
     The base portion  12  serves to maintain a constant length of the male thread portion  13  protruding from the battery cell  2  when the electrode terminal  1  is attached to the battery cell  2 , or serves as a spacer for holding a busbar  3  lifted above the battery cell  2  when the busbar  3  is connected to the male thread portion  13 . The base portion  12  is not always necessarily provided integrally with the outer cylinder  11 , and may be provided as a separate member. 
     According to the first embodiment, the maximum diameter of the electrode terminal  1  (corresponding to an outer diameter of the base portion  12 ) is 5-25 mm, and the maximum length (corresponding to an overall length of the shaft  10 ) is 10-100 mm. The nominal outer diameter of the male thread portion  13  provided on the outer cylinder  11  is 4-12 mm. 
     The shaft  10  and the outer cylinder  11  are formed from metal having forming materials different from each other. The shaft  10  is formed by the same metal as of the negative-electrode-side carrier  7  of the battery cell  2 , namely copper or copper alloy. Moreover, the outer cylinder  11  is formed by the same metal as of a positive-electrode-side carrier and the plus output terminal of the battery cell  2 , namely aluminum or aluminum alloy as a source material. 
     A bonding interface in which the metal (Cu) of the shaft  10  and the metal (Al) of the outer cylinder  11  are brought in close contact with each other at a metal structure level by imparting deformation at an extreme high pressure (approximately 1000 MPa, for example) is formed in a gap between an outer peripheral surface of the shaft  10  and an inner peripheral surface of the outer cylinder  11 , and, as a result, the gap is brought into a state in which the electric conductivity and the mechanical bonding strength are increased to “values proper for practical use as an electrode terminal”. 
     When such an electrode terminal  1  is attached to the battery cell  2 , a portion of the shaft  10  protruding from the outer cylinder  11  is used as an internal connection portion  15 . In other words, the internal connection portion  15  is electrically connected to the negative-electrode-side carrier  7  of the battery cell  2 . Moreover, the portion of the outer cylinder  11  on which the male thread portion  13  is provided is used as an external connection portion  16 . In other words, one end portion of the busbar  3  made of aluminum, which is the same metal as the outer cylinder  11 , is connected to the external connection portion  16 . 
     Specifically, connection holes  20  are provided on both end portions of the busbar  3  as shown in  FIG. 1  and  FIG. 3 , the connection hole  20  is inserted over the external connection portion  16  (the male thread portion  13  of the outer cylinder  11 ) of the electrode terminal  1 , and an aluminum nut  21  made of the same metal as of the outer cylinder  11  is threadedly engaged with the male thread portion  13  which is passing through the connection hole  20 . 
     On this occasion, the external connection portion  16 , the busbar  3 , and the nut  21  constitute a connection of the same metal, which does not cause galvanic corrosion. In addition, though the dissimilar metals are present between the internal connection portion  15  and the external connection portion  16  (between the shaft  10  and the outer cylinder  11 ), they are metallically bonded, do not cause galvanic corrosion, and are kept in a state in which the electric resistance is restrained. 
     On the other hand, the plus output terminal preferably employs an electrode terminal in which all forming materials are formed from aluminum or aluminum alloy. The shape thereof is approximately the same as the electrode terminal  1 , and includes a base portion  23  and a male thread portion  24 . Therefore, the connection hole  20  on the other side of the busbar  3  is inserted over the male thread portion  24  of the electrode terminal on the plus side, and the nut  21  is threadedly engaged with the male thread portion  24 , which is passing through the connection hole  20 . It should be understood that connection portions between the plus output terminal and the busbar  3  constitutes a connection of the same metal, which does not cause galvanic corrosion. 
     As a result, in the battery pack  4  constructed by serially connecting the multiple battery cells  2  via the busbars  3 , galvanic corrosion is not generated in any of the connection portions, and a highly efficient electric conductivity is maintained. Moreover, the electrode terminal  1  is excellent in mechanical strength, and the electrode terminal  1  is not bent or broken in an ordinary state of use. 
     It should be noted that the busbar  3  is formed from aluminum or aluminum alloy according to the first embodiment, is light in weight, and can restrain the weight of the battery pack  4  to a small value. As a result, the battery pack  4  is advantageous in weight reduction of an electric vehicle carrying the battery pack  4  as a battery. 
     An extrusion process is carried out under a hydrostatic pressure at an extremely high pressure in order to produce the electrode terminal  1  constituted in this way as shown in  FIG. 4 . An extrusion device  30  used for this process includes a die  31  having a single opening corresponding to the maximum diameter of the electrode terminal  1  (corresponding to the outer diameter of the base portion  12 ) to be obtained, and extrusion molding can be carried out in an isostatic environment at an extremely high pressure (approximately −1000 MPa). 
     As a production sequence of the electrode terminal  1 , a source material for positive electrode  11 A (metal source material) made of the same metal as of the plus output terminal of the battery cell  2  and a source material for negative electrode  10 A (metal source material) made of the same metal as of the minus output terminal of the battery cell  2  are first prepared. In other words, the source material for positive electrode  11 A is aluminum or aluminum alloy, and the source material for negative electrode  10 A is copper or copper alloy. Then, a billet (source material facing each other) in a round shaft shape structured so that the source material for positive electrode  11 A surround the source material for negative electrode  10 A in a shaft shape is formed. 
     For example, the source material for negative electrode  10 A is formed as a round shaft member, the source material for positive electrode  11 A is formed as a hollow pipe member, and the source material for positive electrode  11 A is externally fit and inserted over the source material for negative electrode  10 A, thereby forming the billet. Alternatively, the source material for negative electrode  10 A is formed as a round shaft member, the source material for positive electrode  11 A is formed as a belt-shape member, and the source material for positive electrode  11 A is wound over the source material for negative electrode  10 A, thereby forming the billet. 
     Then, the billet is loaded in the extrusion device  30 , and the extrusion device  30  is actuated in the isostatic environment at an extremely high pressure (−1000 MPa). As described before, the billet has such a structure that the source material for positive electrode  11 A surrounds the source material for negative electrode  10 A, and the source material for positive electrode  11 A and the source material for the negative electrode  10 A are extruded in parallel. 
     As shown in  FIG. 4 , an opening area of the die  31  of the extrusion device  30  is smaller than a cross sectional area of the billet, the billet is compressed over the whole circumference, and is plastically deformed by causing the billet to pass through the die  31 . Mating surfaces of both the source materials  10 A and  11 A come out of the die  31 , and then form “the interface (metallically bonded portion) between the outer peripheral surface of the shaft  10  and the inner peripheral surface of the outer cylinder  11 ”. 
     This extrusion processing forms a formed body  1 A in an inner/outer double-shaft configuration in which the source material for positive electrode  11 A and the source material for negative electrode  10 A are integrally bonded by means of the metallic bonding. 
     The formed body  1 A acquired in this way is cut in the extruded direction at a predetermined interval. According to the first embodiment, the die  31  of the extrusion device  30  is formed into an opening shape corresponding to a cross sectional shape of the electrode terminal  1 , and the cut interval of the formed body  1 A is set to match the length dimension of the electrode terminal  1 . 
     After the cut, a turning process and a male thread cutting process are applied to the source material for positive electrode  11 A, thereby forming the male thread portion  13 , forming the base portion  12 , and forming the protruded portion of the shaft  10 , resulting in completion of the electrode terminal  1 . A surface grinding and a surface treatment may be applied according to necessity. 
     Second Embodiment 
       FIG. 5A ,  FIG. 5B , and  FIG. 6  show a second embodiment of the electrode terminal  1  according to the present invention. 
     The electrode terminal  1  according to the second embodiment is also adopted as the minus output terminal of the battery cell  2 . 
     As shown in  FIG. 5A  and  FIG. 5B , the outer cylinder  11  of the electrode terminal  1  is formed to extend upward exceeding the length of the shaft  10 . In other words, the shaft  10  is not present inside the extended portion of the outer cylinder  11 , and is thus hollow. On the other hand, the shaft  10  of the electrode terminal  1  is formed to extend downward exceeding the length of the outer cylinder  11 . In addition, the base portion  12  and the male thread portion  13  are not provided on the outer cylinder  11 , and the outer cylinder  11  is thus formed into a straight cylindrical shape. 
     It should be noted that the point that the shaft  10  is the same metal (copper or copper alloy) as of the negative-electrode-side carrier  7  of the battery cell  2 , and the outer cylinder  11  is the same metal (aluminum or aluminum alloy) as of the positive-electrode-side carrier and the plus output terminal of the battery cell  2  is the same as that of the first embodiment. Moreover, the point that the outer peripheral surface of the shaft  10  and the inner peripheral surface of the outer cylinder  11  are metallically bonded by the die processing in the isostatic environment at an extremely high pressure is the same as that of the first embodiment. 
     On the electrode terminal  1  according to the second embodiment, the portion of the shaft  10  protruding from the outer cylinder  11  is attached to the battery cell  2  as the internal connection portion  15 , and then, the hollow portion of the outer cylinder  11  is used as the external connection portion  16 . In other words, one end portion of the busbar  3  is connected to the external connection portion  16  by means of welding. 
     Specifically, as shown in  FIG. 6 , the connection hole  20  of the busbar  3  is inserted over the external connection portion  16  (corresponding to the hollow portion) of the electrode terminal  1 , and a periphery of the external connection portion  16  passing out the connection hole  20  may be welded by welding. Both the busbar  3  and the external connection portion  16  of the welded portion are formed from aluminum or aluminum alloy, are the same metal, do not generate eutectic, and the electric resistance therebetween is not excessive. 
     As shown in  FIG. 4 , in order to produce the electrode terminal  1  according to the second embodiment, as in the first embodiment, the extrusion device  30  is actuated in the isostatic environment at an extremely high pressure, thereby forming the formed body  1 A, and the outer cylinder  11  is then hollowed by boring (the shaft  10  is removed to a predetermined depth). 
     According to the second embodiment, other configuration, actions and effects, and the production method are the same as those of the first embodiment, and therefore are not detailed. 
     By the way, it should be understood that the disclosed embodiments are examples in terms of all the points, and are not limitative. The scope of the present invention is not represented by the above description but by CLAIMS, and it is intended that connotation equivalent to the scope of CLAIMS, and all changes within the scope of CLAIMS are included. 
     For example, according to the first and second embodiments, though the electrode terminal  1  used as the minus output terminal is exemplified, the electrode terminal may be adopted to the plus output terminal. In this case, preferably, the shaft  10  is formed from the same metal (aluminum or the aluminum alloy) as of the positive-electrode-side carrier of the battery cell  2 , and the outer cylinder  11  is formed from the same metal (copper or copper alloy) as of the negative-electrode-side carrier  7  of the battery cell  2 . The busbar  3  is formed from copper or copper alloy. 
     Moreover, the busbar  1  according to the present invention is highly preferred for the connection of the lithium ion batteries to be installed on an automobile, and application to connection of the lithium ion batteries used for other applications poses no problem. 
     The present application is described in detail referring to the specific embodiments, and it is apparent to a person skilled in the art that various changes and modification can be made without departing from the spirit and scope of the present invention. 
     The present application is based on Japanese Patent Application No. 2010-075916 filed on May 29, 2010, and the contents thereof are incorporated herein by reference. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Electrode terminal 
               1 A: Formed body 
               2 : Battery cell 
               3 : Busbar 
               4 : Battery pack 
               7 : Negative-electrode-side carrier 
               10 : Shaft 
               10 A: Source material for negative electrode 
               11 : Outer cylinder 
               11 A: Source material for positive electrode 
               12 : Base portion 
               13 : Male thread portion 
               15 : Internal connection portion 
               16 : External connection portion 
               20 : Connection hole 
               21 : Nut 
               23 : Base portion 
               24 : Male thread portion 
               30 : Extrusion device 
               31 : Die