Patent Publication Number: US-2021167455-A1

Title: Battery module and method for manufacturing same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/479,409, filed Jul. 19, 2019, which is a U.S. national stage application of the PCT International Application No. PCT/JP2018/001099 filed on Jan. 17, 2018, which claims the benefit of foreign priority of Japanese patent application 2017-015070 filed on Jan. 31, 2017, respectively, the contents all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a battery module and a method for manufacturing the battery module. 
     BACKGROUND ART 
     Conventionally, a battery module configured of a plurality of battery cells electrically connected by a lead plate has been known. As a battery cell, a secondary battery such as a lithium ion battery or a nickel hydride battery is preferably used. As a lead plate, a nickel thin plate or a copper thin plate is used. Besides it, for example, an aluminum thin plate may be used in consideration of weight, heating by a current flow, cost, and the like. 
     A lead plate has a plurality of lead parts corresponding to the respective battery cells. For example, a lead part can be formed in a piece shape connected with the base plate of the lead plate in a cantilever state by means of punching applied to an aluminum thin plate in a plate shape. When the lead part formed as described above is connected to each of the terminals of the battery cells, each battery cell is electrically connected via the lead plate. 
     In the case where a terminal of a battery cell is configured of a plate material made of aluminum or aluminum alloy, metallurgical bonding can be made by applying spot welding to the lead part of the lead plate formed of an aluminum thin plate of a same type. However, in the case where a terminal of a battery cell is made of a material other than aluminum such as copper, copper alloy, or iron-based metal, since aluminum does not form an alloy layer with copper or iron, metallurgical bonding cannot be made. Further, when a lead plate is joined to a battery cell by metallurgical bonding such as spot welding, it is concerned that heat at the time of welding may be conducted to an electrode inside the battery cell via a terminal, and consequently an electrode material (such as active substance) may be degraded by the influence of the heat so that the battery performance may be lowered. 
     Recently, as a technology of joining different types of metal to each other, solid-phase bonding is widely used. For example, PTL 1 describes an assembled battery in which a metal line is connected to metal terminals of a plurality of unit cells adjacent to each other to thereby connect the unit cells in series or in parallel by the metal line. The metal line is connected by ultrasonic welding to a joining surface of a metal terminal of each unit cell. 
     CITATION LIST 
     Patent Literature 
     
         
         
           
             PTL 1: Japanese Patent No. 5078282 
           
         
       
    
     SUMMARY OF THE INVENTION 
     PTL 1 describes that a metal line formed of a metal body is layered on a metal terminal of a unit cell, and in a state where the metal line is pressed onto the metal terminal, the metal line is applied with ultrasonic vibration so as to be welded to the metal terminal by ultrasonic welding. Pressing force, vibration frequency, and power of an ultrasonic vibrator at the time of pressing and vibrating the metal line are illustrated with specific numerical values. 
     However, in the case of joining a lead plate to a terminal of a battery cell by solid-phase bonding as described above, it is preferable to improve the reliability of a battery module due to an increase in joining strength of the lead plate and to improve productivity due to reduction of the joining time, by thinking out parameters other than welding process conditions of an ultrasonic welding machine. 
     An object of the present disclosure is to improve reliability of a battery module by firmly joining a lead plate formed of an aluminum thin plate to terminals of battery cells, and to improve productivity of the battery module by reduction of joining time, through parameter adjustment other than process conditions of ultrasonic welding. 
     A battery module according to the present disclosure is a battery module including a plurality of battery cells each having a terminal, and a lead plate having a lead part to be joined to the terminal of each of the battery cells to electrically connect the battery cells to each other. The lead part includes an aluminum thin plate having aluminum purity higher than or equal to 99.0%. Surface roughness Ra of a joining surface of the lead part to the terminal is less than or equal to 10 μm. The lead part is electrically connected to the terminal by solid-phase bonding. 
     Further, a method of manufacturing a battery module, according to the present disclosure, is a method of manufacturing a battery module that includes a plurality of battery cells each having a terminal, and a lead plate having a lead part joined to the terminal of each of the plurality of battery cells to electrically connect the battery cells to each other, the lead part being connected to the terminal by solid-phase bonding through ultrasonic joining. The method includes a preparing step of preparing the plurality of the battery cells and the lead plate, the lead part of the lead plate including an aluminum thin plate having aluminum purity higher than or equal to 99.0%, a surface roughness Ra of a joining surface of the lead part to the terminal being less than or equal to 10 μm; a pressing step of pressing the joining surface of the lead part to the terminal of one of the battery cells by an ultrasonic vibrator; a vibration step of vibrating the lead part with use of the ultrasonic vibrator; and a heating step of heating at least the lead part in the lead plate. Here, the pressing step, the vibration step, and the heating step may be performed simultaneously. 
     According to a battery module and a method for manufacturing the battery module according to the present disclosure, a lead plate formed of an aluminum thin plate can be firmly joined to terminals of battery cells through solid-phase bonding even between different types of metals, reliability of the battery module is improved, and productivity of the battery module is improved by reduction of bonding time. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a battery module according to an embodiment. 
         FIG. 2A  is an enlarged plan view of a lead part of a lead plate joined to a terminal of a battery cell by solid-phase bonding, and  FIG. 2B  is a cross-sectional view taken along IIB-IIB of  FIG. 2A . 
         FIGS. 3A to 3C  are views similar to  FIG. 2A , showing a modification of a shape of a lead part. 
         FIG. 4  shows a state where a lead part of a lead plate is joined to a terminal of a battery cell by ultrasonic joining. 
         FIG. 5  is a flowchart showing steps of manufacturing a battery module by joining a lead part of a lead plate to a terminal of a battery cell through ultrasonic joining. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An exemplary embodiment of the present invention will now be described in detail with reference to the attached drawings. In the following description, specific shapes, materials, numerical values, directions, and other elements are examples shown to facilitate understanding of the present invention and may be changed as appropriate to suit uses, purposes, specifications, or other requirements. It is initially envisaged that when exemplary embodiments and modifications are included in the following description, some distinctive elements in the exemplary embodiments and the modifications are suitably combined and used. 
     Hereinafter, description will be given of a case where a battery cell is a cylindrical battery. However, the present disclosure may be applied to a battery module configured of a plurality of square-shaped battery cells connected with each other. Further, in the below description, a term “ultrasonic joining” is appropriately used instead of a typical term “ultrasonic welding”, in order to clearly distinguish it from metallurgical bonding such as spot welding. 
       FIG. 1  is an exploded perspective view of battery module  10  according to an exemplary embodiment of the present disclosure. First, with reference to  FIG. 1 , an overview of battery module  10  will be described. As shown in  FIG. 1 , battery module  10  includes a plurality of cylindrical battery cells  11  and battery holder  20  having a plurality of cylindrical containers to hold battery cells  11 . 
     Battery cell  11  includes cell case  12  made of metal, a battery element (not shown) contained in cell case  12 , positive-electrode terminal  14 , and negative-electrode terminal  15 . The battery element includes a pair of electrode assemblies, a non-aqueous electrolyte that allows transfer of electric charges, and the like. Cell case  12  is configured of cell case body  13  that is formed in a bottomed cylindrical shape to contain the battery element, and a sealing body that seals an opening of cell case body  13 . The sealing body constitutes positive-electrode terminal  14  that protrudes in a flat columnar shape, for example, at one end of battery cell  11 . A gasket (not shown) made of resin is disposed between cell case body  13  and positive-electrode terminal  14 . 
     Positive-electrode terminal  14  has a layered or stacked structure including a valve, a cap, and the like, and is electrically connected to a positive-electrode assembly of the battery element. In the present embodiment, positive-electrode terminal  14  may be formed of an aluminum plate or an aluminum alloy plate, or may be formed of a metal plate made of copper, copper alloy, iron-based metal, or the like. Cell case body  13  is electrically connected with a negative-electrode assembly of the battery element and functions as a negative electrode of battery cell  11 . In general, however, an outer peripheral side surface of cell case body  13  is covered with an insulating resin film and a circular flat bottom surface of cell case body  13  serves as negative-electrode terminal  15 . In the present embodiment, cell case body  13  constituting negative-electrode terminal  15  may be formed of an aluminum plate or an aluminum alloy plate, or may be formed of a metal plate made of copper, copper alloy, iron-based metal, or the like. 
     Battery cell  11  is contained in hole  21  of each cylindrical container of battery holder  20 , and is arranged in an array. Battery module  10  includes a pair of posts  30  attached to battery holder  20 . Posts  30  are plate-shaped members that cover both lateral faces of battery holder  20 . Each post  30  has protrusion  31  on one surface. Posts  30  are disposed so as to face each other over battery holder  20 , with protrusions  31  facing battery holder  20 . Protrusions  31  are fitted into recesses  25  of battery holder  20 . 
     Positive-electrode lead plate (lead plate)  41  is disposed above battery holder  20  via positive-electrode insulation board  42  in a state of being electrically connected to positive-electrode terminals  14  of battery cells  11 . Positive-electrode current collector plate  40  is disposed above positive-electrode lead plate  41  in a state of being electrically connected to positive-electrode lead plate  41 . Positive-electrode lead plate  41  and positive-electrode current collector plate  40  are integrated with each other by welding or the like, for example. 
     Meanwhile, negative-electrode lead plate (lead plate)  46  is disposed below battery holder  20  via negative-electrode insulation board  53  in a state of being electrically connected to negative-electrode terminals  15  of battery cells  11 . Negative-electrode current collector plate  45  is disposed below negative-electrode lead plate  46  in a state of being electrically connected to negative-electrode lead plate  46 . Negative-electrode lead plate  46  and negative-electrode current collector plate  45  are integrated with each other by welding or the like, for example. 
     Battery cells  11  are connected in parallel with each other by positive-electrode lead plate  41  and negative-electrode lead plate  46 . Positive-electrode lead plate  41  includes positive-electrode base plate (base plate)  43  and positive-electrode lead parts (lead parts)  47 . Positive-electrode lead parts  47  are formed in a number corresponding to a number of battery cells  11  included in battery module  10 . Positive-electrode base plate  43  is electrically connected to positive-electrode terminals  14  of battery cells  11  via positive-electrode lead parts  47 . 
     Negative-electrode lead plate  46  includes negative-electrode base plate (base plate)  48  and negative-electrode lead parts (lead parts)  50 . Negative-electrode lead parts  50  are formed in a number corresponding to a number of battery cells  11  included in battery module  10 . Negative-electrode base plate  48  is electrically connected to negative-electrode terminals  15  of battery cells  11  via negative-electrode lead parts  50 . 
     Positive-electrode insulation board  42  is disposed between battery holder  20  and positive lead plates  41 , positive-electrode insulation board  42  having circular holes  49  to expose terminals  14  of battery cells  11 . Negative-electrode insulation board  53  is disposed between battery holder  20  and negative-electrode lead plates  46 , negative-electrode insulation board  53  having circular holes  54  to expose terminals  15  of battery cells  11 . Each of positive-electrode insulation board  42  and negative-electrode insulation board  53  is configured of a resin board material, for example. Each of circular holes  49 ,  54  is formed to have a larger diameter than that of positive-electrode terminal  14  and have a smaller diameter than that of cylindrical cell case body  13 . 
     Positive-electrode current collector plate  40 , positive-electrode insulation board  42 , and other components are fixed to the pair of posts  30  using screws (not shown), for example. Negative-electrode current collector plate  45  and negative-electrode insulation board  53  are also fixed to the pair of posts  30  using screws (not shown), for example. Thereby, battery module  10  is integrally assembled. The battery module integrated in this way is connected in series to another battery module  10  adjacently disposed, via, for example, positive-electrode current collector plate  40  and negative-electrode current collector plate  45 . 
       FIG. 2A  is an enlarged plan view of a lead part of a positive-electrode lead plate joined to positive-electrode terminal  14  of battery cell  11  by solid-phase bonding, and  FIG. 2B  is a cross-sectional view taken along IIB-IIB of  FIG. 2A . 
     As shown in  FIGS. 2A and 2B , in positive-electrode lead plate  41 , positive-electrode lead part  47  is formed as a piece connected with base plate  43  in a cantilever shape by forming through hole  44  in an almost U shape by applying punching to an aluminum thin plate, for example. For example, positive-electrode lead part  47  includes lead tip  47   a  in an almost circular shape, and lead neck  47   b  that connects lead tip  47   a  and base plate  43 . In the present embodiment, lead neck  47   b  is bent at a base end side close to base plate  43  so as to be inclined obliquely and is bent near lead tip  47   a  such that lead tip  47   a  takes a posture along base plate  43 . 
     Maximum width W 1  of lead tip  47   a  is preferably formed to be larger than width W 2  (for example, between 0.15 mm and 2 mm, inclusive) of lead neck  47   b  in a range from 1.0 time to 10 times, approximately, and more preferably, formed to be larger in a range from 2 times to 5 times, approximately. In other words, width W 2  of lead neck  47   b  is preferably formed to have a value approximately ranging from a same value to 1/10 of maximum width W 1  of lead tip  47   a , and more preferably, formed to have a value approximately ranging from ½ to ⅕. By narrowing width W 2  of lead neck  47   b  as described above, in the case of joining positive-electrode lead part  47  to positive-electrode terminal  14  by ultrasonic joining as described above, lead tip  47   a  easily vibrates, and ultrasonic joining can be performed in a good condition in a short time. 
     Further, for example, plate thickness t of positive-electrode lead plate  41  integrally having lead tip  47   a  is preferably between 0.05 mm and 0.5 mm (inclusive), and more preferably, less than or equal to 0.3 mm. Furthermore, it is preferable that lead tip  47   a  is formed to be wider than lead neck  47   b . In particular, it is preferable that lead tip  47   a  is formed in a symmetrical shape with respect to center line C in a lateral direction of lead neck  47   b . When lead tip  47   a  is formed in a symmetrical shape as described above, lead tip  47   a  easily vibrates. Therefore, it is advantageous in performing ultrasonic joining in a good condition in a short time. 
     Note that the shape of lead tip  47   a  is not limited to the aforementioned semicircular shape. For example, lead tip  47   a  may be formed in an almost circular shape as shown in  FIG. 3A , or in an almost rectangular shape as shown in  FIG. 3B , or in an almost trapezoidal shape as shown in  FIG. 3C . 
     Moreover, lead neck  47   b  of positive-electrode lead part  47  may have width W 2  for example, and may be formed to have a narrow width that is about 1/10 of width W 1  of lead tip  47   a  to function as a fuse. In that case, when overcurrent flows due to an internal short circuit or the like of battery cell  11 , by fusing lead neck  47   b , it is possible to electrically cut off battery cell  11  so as to prevent other battery cells  11  from being affected. 
     As shown in  FIG. 2A , positive-electrode lead part  47  is joined to the surface of positive-electrode terminal  14  by ultrasonic joining at a center of lead tip  47   a . Joining portion  60  is illustrated as a circular region of a two dot chain line. Next, ultrasonic joining of positive-electrode lead part  47  to positive-electrode terminal  14  of battery cell  11  will be described with reference to  FIG. 4 . 
       FIG. 4  shows a state where positive-electrode lead part  47  of positive-electrode lead plate  41  is joined to positive-electrode terminal  14  of battery cell  11  by ultrasonic joining.  FIG. 5  is a flowchart showing steps of manufacturing battery module  10  by joining positive-electrode lead part  47  of positive-electrode lead plate  41  to positive-electrode terminal  14  of battery cell  11  through ultrasonic joining. 
     As shown in  FIG. 4 , in the present embodiment, positive-electrode lead part  47  is joined to positive-electrode terminal  14  by using ultrasonic welding machine  70  having columnar ultrasonic vibrator  72 , for example. In the present embodiment, cell case body  13  of battery cell  11 , in the case of being configured of an iron-based metal plate, is illustrated as an example. Positive-electrode terminal  14  of battery cell  11  is configured of an iron-based metal plate. Positive-electrode lead part  47  is configured of an aluminum thin plate having aluminum purity higher than or equal to 99.0%. It is preferable that positive-electrode lead part  47  is configured of an aluminum thin plate having aluminum purity higher than or equal to 99.5% (corresponding to JIS standard: A1050). The surface roughness of the joining surface of positive-electrode lead part  47  to be joined to the surface of positive-electrode terminal  14  is less than or equal to Ra10 μm, and more preferably, less than or equal to Ra1 μm. 
     The purity and surface roughness of positive-electrode lead part  47  as described above are realized by using an aluminum thin plate having such purity and surface roughness as described above, as positive-electrode lead plate  41 . By using an aluminum thin plate having high purity and less additives and impurities as described above, it is advantageous in reducing the inter-atom distance between an atom of a metallic material constituting positive-electrode terminal  14  and an aluminum atom of positive-electrode lead part  47 . Thereby, strong solid-phase bonding (eutectic state) can be realized. 
     In the case of joining positive-electrode lead plate  41  to battery cell  11  by ultrasonic joining, first, a plurality of battery cells  11  and positive-electrode lead plate  41  are prepared (step S 10  of  FIG. 5 ). In addition, positive-electrode insulation board  42  is also prepared in the present embodiment. In the present embodiment, the case where positive-electrode lead plate  41  is previously integrated with positive-electrode current collector plate  40  by welding or the like is shown as an example. However, it is also acceptable that after positive-electrode lead plate  41  is joined to battery cell  11  by ultrasonic joining, positive-electrode lead plate  41  may be welded with positive-electrode current collector plate  40  to be integrated. 
     Next, battery cell  11  is set on a support table (not shown) of ultrasonic welding machine  70 . Then, a state in which battery cell  11  is overlaid with positive-electrode insulation board  42 , positive-electrode lead plate  41 , and positive-electrode current collector plate  40  is set. At this time, positive-electrode terminal  14  of battery cell  11  is in a state of being exposed via holes  49 ,  51  of positive-electrode insulation board  42  and positive-electrode current collector plate  40 , and lead tip  47   a  of positive-electrode lead part  47  faces or is laid on the surface of positive-electrode terminal  14 . 
     In this state, by ultrasonic vibrator  72 , lead tip  47   a  is pressed against positive-electrode terminal  14  with pressing force F between 1 N and 50 N (inclusive), for example, and more preferably, between 10 N and 35 N (inclusive) (step S 12  of  FIG. 5 ). 
     Then, in the present embodiment, lead tip  47   a , pressed by ultrasonic vibrator  72 , is irradiated with laser beam LB. Thereby, lead tip  47   a  is heated locally, rather than the entire positive-electrode lead plate  41  (step S 14  of  FIG. 5 ). The laser wavelength of laser beam LB at this time is between 300 nm and 1100 nm (inclusive), for example (more preferably, between 800 nm and 1100 nm, (inclusive)), and the laser power may be between 100 W and 5000 W (inclusive), for example. With the local heating as described above, when lead tip  47   a  is joined by ultrasonic joining under a room temperature environment, temperature rise on the boundary surface between lead tip  47   a  and positive-electrode terminal  14  is accelerated. Therefore, it is advantageous in realizing a solid-phase state in a shorter time. 
     Here, it is confirmed that in the local heating by laser beam LB, in the case where a temperature rise approximately ranging from 20° C. to 50° C. is achieved, for example, joining strength of lead tip  47   a  is increased in a range from 5% to 10% approximately. With the configuration of heating locally by laser beam LB as described above, downsizing and cost reduction of manufacturing facility of battery module  10  can be realized. To heat lead tip  47   a  from the outside, it is conceivable to spray hot air by a nozzle at a spot or perform ultrasonic joining in a high-temperature tank. In that case, however, battery module constituent members (for example, battery cell  11 ) other than lead tip  47   a , not related to solid-phase bonding, may also be heated and affected by the heat (for example, degradation of electrode material or resin packing, or the like). Therefore, it is preferable to use local heating by irradiation of laser beam. 
     Next, in the pressing state and the local heating state, for example, ultrasonic vibrator  72  is vibrated with vibration frequency f ranging from 60 kHz to 100 kHz, and more preferably, from 70 kHz to 90 kHz to vibrate lead tip  47   a  (step S 16 ). For example, the power of ultrasonic welding machine  70  at that time may range from 5 W to 300 W, and more preferably, from 40 W to 80 W. 
     Thereby, the temperature of lead tip  47   a  rises by the friction heat with positive-electrode terminal  14 , and lead tip  47   a  is softened. The temperature of the joining surface of lead tip  47   a  (that is, a contact surface with positive-electrode terminal  14 ) at that time becomes a temperature (approximately between 150° C. and 300° C. (inclusive), for example) lower than a melting point (about 660° C.) of an aluminum thin plate constituting lead tip  47   a . Further, as lead tip  47   a  is in friction with the surface of positive-electrode terminal  14  in the pressed state, the aluminum oxide layer formed on the joining surface of lead tip  47   a  is broken and aluminum atoms are exposed to the joining surface. Therefore, solid-phase bonding is not hindered. 
     With the softening due to the temperature rise as described above, aluminum atoms having high purity that are exposed to the joining surface of lead tip  47   a  are enlarged. Therefore, the distance with atoms of the metallic material constituting positive-electrode terminal  14  is decreased and a eutectic state is achieved. Consequently, a solid-phase bonding state is achieved in a short time, and firm joining can be realized. In that case, the area of the joining surface between lead tip  47   a  and positive-electrode terminal  14  is between 0.1 mm 2  and 3.0 mm 2  (inclusive), for example, and preferably, larger than or equal to 0.3 mm 2 . Therefore, lead tip  47   a  and positive-electrode terminal  14  can be joined with sufficient intensity. 
     Note that in the present embodiment, as a heating means from the outside of positive-electrode lead part  47 , an example of laser beam irradiation has been described. However, the present disclosure is not limited thereto. It is possible to perform irradiation by condensing light such as LED, for example. Further, in the case where the lead tip and the terminal are made of same type of metal and solid-phase bonding is easily performed, for example, local heating by a laser beam can be omitted. 
     After positive-electrode lead part  47  of positive-electrode lead plate  41  is connected to positive-electrode terminal  14  of each battery cell  11  through ultrasonic joining, as for a negative-electrode terminal  15  of battery cell  11 , negative-electrode lead part  50  of negative-electrode lead plate  46  can be applied with solid-phase bonding by ultrasonic joining, similarly. 
     As described above, according to battery module  10  and the method for manufacturing battery module  10  according to the present embodiment, lead plates  41 ,  46  each formed of an aluminum thin plate can be firmly joined by solid-phase bonding to positive-electrode and negative-electrode terminals  14 ,  15  of battery cell  11  even between different types of metals to say nothing of same types of metals. Therefore, reliability of battery module  10  is improved, and productivity of battery module  10  is also improved by reduction of the joining time. 
     Note that the battery module and the method for manufacturing the battery module according to the present disclosure should not be limited to the exemplary embodiment and modification thereof. It is needless to say that various modifications and alterations can be made within the scope of the appended claims of the present application and their equivalents.