Patent Publication Number: US-10784475-B2

Title: Cap assembly for a second battery and second battery

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims priority to Chinese Patent Application No. 201710935598.7, filed on Oct. 10, 2017, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to the technical field of energy storage devices, and in particular, to a cap assembly for a secondary battery and a secondary battery. 
     BACKGROUND 
     New energy vehicles are widely promoted in the country and even in the world. However, in order to completely replace fuel vehicles, there are many areas for improvement. For example, there are a few problems such as low travelling mileages of vehicles, high cost of battery packs, reliability of battery packs and the like, which need to be further solved. 
     Currently, a power battery usually adopts a square hard shell structure. A shell of the power battery includes a case and a cap assembly. The shell of the power battery provides a closed space for accommodating an electrode assembly and electrolyte. The power of the electrode assembly is led out of the closed space through a terminal of the cap assembly. 
     In a conventional cap assembly, a manner of fixing the terminal is as follows: a cap plate is provided with a through hole; the terminal includes a base portion and an extension portion; and the base portion has a cross-sectional area larger than an area of the through hole. During assembly, the base portion is located under the cap plate (i.e. inside the case), and the extension portion is fixed by a clamp spring or a riveting member after passing through the through hole. In this way, the terminal is fixed to the cap plate. With such a fixing manner, the cap assembly may employ a large number of mechanical components, thereby increasing the cost of the secondary battery and reducing the reliability of the cap assembly. Meanwhile, since the base portion is located inside the case, space utilization inside the case may be reduced and thus energy density of the power battery may be reduced. In order to solve this problem, a terminal board may be employed to be disposed at a side of the cap plate; a fixing member and a connecting member may be provided; the fixing member may be fixed to the cap plate through the connecting member; and at least part of an outer peripheral surface of the terminal board may be surrounded by the fixing member so that the terminal board may be fixed to the fixing member. However, when the cap plate is deformed to form an arch due to a large amount of gas generated inside the case of the secondary battery (the cap plate is deformed to form an arch because two edges of the cap plate in its width direction are welded to the case, the deformations of the two edges in the width direction are small, and the deformation of a middle region of the cap plate in the width direction is large), due to a large rigidity of the fixing member, the fixing member may not form an arch along with the cap plate, so the gaps between two sides of the cap plate in the width direction and the fixing member may be increased (even the connecting member may be pulled off). Thus it may not be possible to tightly press a sealing member between the terminal board and the cap plate, thereby causing air leakage or liquid leakage of the secondary battery. 
     SUMMARY 
     According to an aspect of the present disclosure, a cap assembly for a secondary battery is provided. The cap assembly includes a cap plate, a fixing member, a connecting member and an electrode terminal, wherein the cap plate has an electrode lead-out hole; the fixing member is fixed to the cap plate through the connecting member and provided with a weakened portion that is close to a center line of the cap plate in a width direction of the cap plate; and the electrode terminal includes a terminal board, wherein the terminal board has an outer peripheral surface at least partially surrounded by the fixing member so that the electrode terminal is fixed to the fixing member, and the terminal board is provided on a side of the cap plate and covers the electrode lead-out hole. 
     According to an aspect of the present disclosure, the fixing member includes at least two weakened portions that are respectively located on two sides of the electrode lead-out hole in a length direction of the cap plate as well as on the center line of the cap plate in the width direction of the cap plate. 
     According to an aspect of the present disclosure, the weakened portion includes an opening portion that is formed on a surface of the fixing member on a side away from the cap plate. 
     According to an aspect of the present disclosure, the opening portion penetrates through the fixing member along a length direction of the cap plate; or the opening portion penetrates through the fixing member along a thickness direction of the cap plate. 
     According to an aspect of the present disclosure, one of the terminal board and the fixing member is provided with a recess, and the other one of the terminal board and the fixing member is provided with a protrusion to be matched with the recess, so that relative rotation between the electrode terminal and the fixing member is limited by engagement of the recess and the protrusion. 
     According to an aspect of the present disclosure, the recess is provided on the outer peripheral surface of the terminal board, and the protrusion to be matched with the recess is provided on an inner surface of the fixing member in contact with the terminal board. 
     According to an aspect of the present disclosure, the terminal board is provided with a notch on a surface of the terminal board on a side close to the cap plate or on a side away from the cap plate, and the notch extends along a length direction of the cap plate and is close to the center line of the cap plate in the width direction of the cap plate. 
     According to an aspect of the present disclosure, the connecting member includes an undercut and an undercut hole; the undercut hole is provided on a surface of the cap plate facing the fixing member; the undercut hole has an aperture gradually increasing along a direction from the fixing member to the cap plate; and the undercut is connected to the fixing member and has a shape adapted to a shape of the undercut hole so as to be engaged with the undercut hole. 
     According to an aspect of the present disclosure, the cap assembly further includes a sealing member that is provided between the terminal board and the cap plate and surrounds the electrode lead-out hole so as to seal the electrode lead-out hole. 
     According to an aspect of the present disclosure, the sealing member is provided with an annular groove on a surface of the sealing member on a side facing the cap plate; the cap plate is provided with an annular flange to be matched with the groove on a surface of the cap plate on a side facing the sealing member; and the flange is inserted into the groove. 
     According to another aspect of the present disclosure, a secondary battery is provided. The secondary battery includes a case having an opening; an electrode assembly accommodated in the case, including a first electrode plate, a second electrode plate and a separator disposed between the first electrode plate and the second electrode plate; and the cap assembly as described above, wherein the cap assembly covers the opening of the case so as to enclose the electrode assembly in the case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood from the following descriptions of specific embodiments of the present disclosure by taken in conjunction with the accompanying drawings, in which: 
       Other features, objects, and advantages of the present disclosure will become more apparent by reading the following detailed descriptions of non-limiting embodiments with reference to the accompanying drawings, in which the same or similar reference signs denote the same or similar features. 
         FIG. 1  is a schematic diagram of an exploded structure of a cap assembly according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic top view of a structure of the cap assembly of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a cross-sectional structure of the cap assembly of  FIG. 2  taken along a section line A-A; 
         FIG. 4  is a schematic diagram of a cross-sectional structure of a cap plate in the cap assembly of  FIGS. 1 to 3  taken along a length direction; 
         FIG. 5  is an enlarged partial view of a portion A of the cap assembly of  FIG. 3 ; 
         FIG. 6  is a schematic diagram of a three-dimensional structure of a certain state of a fixing member according to a first embodiment; 
         FIG. 7  is a schematic diagram of a three-dimensional structure of a terminal board according to a first embodiment of the present disclosure; 
         FIG. 8  is a schematic diagram of a three-dimensional structure of a terminal board according to a second embodiment of the present disclosure; 
         FIG. 9  is a schematic diagram of a three-dimensional structure of another state of the fixing member of  FIG. 6 ; 
         FIG. 10  is a schematic diagram of a three-dimensional structure of a fixing member according to a second embodiment of the present disclosure; and 
         FIG. 11  is a schematic diagram of a three-dimensional structure of a terminal board according to a third embodiment of the present disclosure. 
     
    
    
     REFERENCE LABELS IN THE FIGURES 
     
         
           100 —Cap assembly 
           10 —Cap plate 
           11 —Electrode lead-out hole 
           12 —Depressed portion 
           13 —Undercut hole 
           14 —Liquid injection hole 
           15 —Anti-explosion valve assembly 
           16 —Flange 
           20 —Terminal assembly 
           21 —Terminal board 
           211 —Depressed portion 
           212 —Recess 
           213 —Recess 
           214 —Notch 
           22 —Fixing member 
           221 —Through hole 
           222 —Accommodation space 
           223 —Protrusion 
           224 —Opening portion 
           225 —Undercut 
           226 —Opening portion 
           23 —Sealing member 
           231 —Groove 
           30 —Terminal assembly 
           32 —Fixing member 
           33 —Sealing member 
           40 —Lower insulator 
           41 —Undercut 
       
    
     DETAILED DESCRIPTION 
     The features and exemplary embodiments of the various aspects of the present disclosure will be described in detail below. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to those skilled in the art that the present disclosure may be practiced without some of these specific details. The following description of embodiments is only provided by illustrating examples for a better understanding of the present disclosure. In the drawings and the following description, at least a part of well-known structures and techniques are not shown in order to avoid unnecessarily obscuring the present disclosure. Further, for clarity, the size of a part of the structures may be exaggerated. The same reference numerals in the drawings denote the same or similar structures, and thus their detailed description will be omitted. Furthermore, the features, structures, or characteristics described below can be combined in any suitable manner in one or more embodiments. 
     The terms denoting directions that appear in the following description indicate directions shown in the drawings, and do not limit specific structures of the cap assembly and the secondary battery of the present disclosure. In the description of the present disclosure, it should also be noted that the terms “mounted”, “connected” and “connection” should be interpreted in a broad sense unless explicitly defined and limited otherwise. For example, it may indicate “fixed connection”, “disassemble connection” or “integral connection”; it may indicate a direct connection or an indirect connection. For those skilled in the art, specific meanings of the above terms in the present disclosure may be understood depending on specific situations. 
     The cap assembly of the secondary battery provided by an embodiment of the present disclosure can be connected at an opening of a case of the secondary battery to seal an electrode assembly and electrolyte into the case and enable an electrical connection between the electrode assembly and conductive parts outside the case. The cap assembly according to the embodiment of the present disclosure can reduce the occupancy of an internal space of the battery case while ensuring the sealing effect of the battery case. Therefore, energy density of the secondary battery can be increased, and reliability of the secondary battery in use can be ensured. 
     For a better understanding of the present disclosure, a cap assembly of a secondary battery and the secondary battery according to embodiments of the present disclosure will be described in detail below with reference to  FIG. 1  to  FIG. 11 . 
       FIG. 1  is a schematic diagram of an exploded structure of a cap assembly  100  according to an embodiment of the present disclosure;  FIG. 2  is a schematic top view of a structure of the cap assembly  100  of  FIG. 1 ;  FIG. 3  is a cross-sectional structural diagram of the cap assembly  100  of  FIG. 2  taken along a section line A-A. The structure diagram of the secondary battery is not shown in the figures. The secondary battery may generally include a cap assembly  100 , a case and an electrode assembly located inside the case. 
     According to an embodiment of the present disclosure, the case may be made of metal material, such as aluminum, aluminum alloy, or nickel-plated steel. The case is formed in a rectangular box shape and has an opening to communicate to its inside accommodation space. 
     The electrode assembly may be formed by stacking or winding a first electrode plate, a second electrode plate, and a separator together, where the separator is an insulator interposed between the first electrode plate and the second electrode plate. In this embodiment, as an example, the description will be provided by taking the first electrode plate as a positive electrode plate and the second electrode plate as a negative electrode plate. Similarly, in other embodiments, the first electrode plate may be a negative electrode plate, and the second electrode plate may be a positive electrode plate. In addition, active materials for the positive electrode plate may be coated on a coating area of the positive electrode plate, and active materials for the negative electrode plate may be coated on a coating area of the negative electrode plate. A portion extending from the coating area of the first electrode plate is referred to as an electrode tab, i.e., a first electrode tab; and a portion extending from the coating area of the second electrode plate is referred to as a negative electrode tab, i.e., a second electrode tab (not shown in the figures). 
     The cap assembly  100  may be used to seal the case. The electrode assembly may be sealed in the case by the cap assembly  100  that is connected at the opening of the case. According to an embodiment of the present disclosure, the cap assembly  100  may generally include a cap plate  10 , a terminal assembly  20 , a terminal assembly  30  and a lower insulator  40 . 
       FIG. 4  is a schematic diagram of a cross-sectional structure of a cap plate in the cap assembly of  FIGS. 1 to 3  taken along a length direction. As shown in  FIG. 4 , the cap plate  10  may be in the shape of a thin plate and have a size and shape matched with the opening of the case  200  so as to be capable of being connected at the opening of the case. The cap plate  10  may be made of metal material, for example, the same metal material as that of the case. In the embodiment, the cap plate  10  is provided with an electrode lead-out hole  11 , a depressed portion  12 , an undercut hole  13 , a liquid injection hole  14  and an anti-explosion valve assembly  15 . 
     The liquid injection hole  14  is formed on the cap plate  10  in a predetermined size, so that the electrolyte can be injected into the case through the liquid injection hole  14  after the cap plate  10  covers the opening of the case and is hermetically connected with the case. The anti-explosion valve assembly  15  may have a conventional structure (for example, by providing a rupture disk) and may be disposed at a substantially central position of the cap plate  10 . When the gas pressure inside the second battery becomes too large as a result of over-charging, over-discharging, or over-heating of the second battery, the rupture disk in the anti-explosion valve assembly  15  can be broken so that the gas produced inside the secondary battery can be exhausted to the outside via a through hole of the anti-explosion valve assembly  15 , thereby being able to prevent the secondary battery from exploding. 
       FIG. 5  is an enlarged partial view of a portion A of the cap assembly  100  of  FIG. 3 ;  FIG. 6  is a schematic diagram of a three-dimensional structure of a certain state of a fixing member  22  according to a first embodiment;  FIG. 7  is a schematic diagram of a three-dimensional structure of a terminal board  21  according to a first embodiment of the present disclosure. With reference to  FIG. 1  and  FIGS. 5 to 7 , according to an exemplary embodiment, the cap plate  10  may be provided with two electrode lead-out holes  11  for leading electric energy in the electrode assembly inside the case out of the cap plate  10 . In an exemplary embodiment, the terminal assembly  20  may generally include a first electrode terminal, a fixing member  22  and a sealing member  23 ; likewise, the terminal assembly  30  may generally include a second electrode terminal, a fixing member  32  and a sealing member  33 . The following description is merely given by way of example with the structure of the terminal assembly  20  and its mounting form on the cap plate  10 . Also by way of example, the following description is given by assuming the terminal assembly  20  is a positive electrode terminal assembly and the terminal assembly  30  is a negative electrode terminal assembly. 
     The first electrode terminal may include a terminal board  21 . In the embodiment, for example, the terminal board  21  has a structure of a circular (and alternatively square) sheet or plate, and an outer peripheral surface of the terminal board  21  is at least partially surrounded by the fixing member  22 . In this way, the terminal board  21  may be mounted to the fixing member  22  and thus fixed to the cap plate  10  through the fixing member  22 . In an alternative embodiment, the first electrode terminal may be a plate-like structure, and a surface of the first electrode terminal close to the cap plate  10  does not exceed a surface of the cap plate  10  close to the first electrode terminal (i.e., the first electrode terminal is composed of the terminal board  21  and does not protrude into the electrode lead-out hole  11 ). 
     The fixing member  22  may be a plastic member and integrally molded to the outer periphery of the terminal board  21 . In an exemplary embodiment, the fixing member  22  is a hard plastic member. For example, the fixing member  22  may be made of high-temperature-resistant insulating plastic material such as one or more of polyphenylene sulfide (PPS), perfluoroalkoxy resin (PFA) or polypropylene (PP) by an integral injection molding process. 
     The fixing member  22  as shown in  FIG. 6  is the fixing member  22  in the cap assembly  100  of  FIGS. 1 to 3 . In the embodiment, as an example, the fixing member  22  has a rotary structure and has an accommodation space  222  adapted to the shape of the terminal board  21 . The accommodation space  222  is an annular receiving groove formed on an inner wall surface of a through hole  221  in a circumferential direction, so that the terminal board  21  may be accommodated in the accommodation space  222  and fixed to the terminal board  21 . In this case, a part of the fixing part  22  may be clamped between the terminal board  21  and the cap plate  10 . 
     In an alternative embodiment, in order to increase the fastening force with which the terminal board  21  and the fixing member  22  are engaged with each other, one of the terminal board  21  and the fixing member  22  may be provided with a recess and the other one of the terminal board  21  and the fixing member  22  may be provided with a protrusion adapted to the recess. Thus the electrode terminal and the fixing member  22  are restrained from rotating relative to each other by the fitted engagement of the recess and the protrusion. In the embodiment, the terminal board  21  is provided with a recess  212 , and the fixing member  22  is provided with a protrusion  223  corresponding to the recess  212  of the terminal board  21 . 
     With reference to both  FIG. 6  and  FIG. 7 , exemplarily, the outer peripheral surface of the terminal board  21  is provided with a plurality of recesses  212  which are radially opened notches along the outer peripheral surface of the terminal plate  21 , and each of the recesses  212  penetrates through the terminal board  21  in the thickness direction of the terminal board  21 . The plurality of recesses  212  are spaced apart and evenly arranged in the circumferential direction of the terminal board  21 . Correspondingly, protrusions  223  matched with the recesses  212  of the terminal board  21  are provided on an inner wall surface of the fixing member  22  in contact with the terminal board  21 , that is, in the accommodation space  222 . Again, the protrusions  223  of the fixing member  22  are radially protruded from the inner wall surface of the accommodation space  222 . As such, when the terminal board  21  is placed in the accommodation space  222  of the fixing member  22 , the recesses  212  and the protrusions  223  are engaged with each other in a snap-fit manner so that the terminal board  21  can be firmly connected with the fixing member  22 . Therefore, it is possible to prevent the terminal board  21  and the fixing member  22  from rotating relative to each other to affect the structural stability of the cap assembly  100 . At the same time, by providing the recesses  212  and the protrusions  223  that are fitted with each other, the contact area between the terminal board  21  and the fixing member  22  can be increased, and thus the engaging force between the terminal board  21  and the fixing member  22  can be further increased. 
     Since the terminal board  21  and the fixing member  22  in the embodiment of the present disclosure are formed by integrally injection molding, the specific shape of the recess  212  provided on the terminal board  21  may not be limited, and the recess  212  may be a regular or irregular notch. In addition, in the embodiment of the present disclosure, the depth of the recess  212  in the radial direction of the terminal board  21  is not limited, as long as the recess  212  can be fitted with the protrusion provided on the fixing member  22  and the provision of the recess  212  may not affect the performance of basic functions of the terminal board  21 . 
     Of course, as shown in  FIG. 7 , the terminal board  21  may be provided with four recesses  212  in the circumferential direction, and two of the recesses  212  may be opposite to the other two of the recesses  212  respectively. Alternatively, only two recesses  212  opposite to each other may be provided, or more recesses  212  may be provided along the circumferential direction of the terminal board  21 . 
     In addition, in order to fit with the accommodation space  222 , an annular depressed portion  211  may be provided on the surface of the side of the terminal board  21  far away from the cap plate  10  in the circumferential direction. As such, the fixing member  22  can wrap the surface of the terminal board  21  on the side far away from the cap plate  10 , so that the fixing member  22  can be more firmly engaged with the terminal board  21 . Meanwhile, after the terminal board  21  and the fixing member  22  are engaged with each other and mounted on the cap plate  10 , the overall thickness of the cap assembly  100  will not be increased, so that the space occupied by the secondary battery in the height direction can be saved to improve the energy density of the secondary battery. After the first electrode terminal and the fixing member  22  are connected to each other, the terminal board  21  covers the electrode lead-out hole  11  and the outer peripheral surface of the terminal board  21  protrudes from the inner wall of the electrode lead-out hole  11  (i.e., as shown in  FIG. 4 , the cross-sectional size of the terminal board  21  is greater than the cross-sectional size of the electrode lead-out hole  11 ), and the surface of the terminal board  21  far away from the cap plate  10  protrudes from the surface of the fixing member  22  far away from the cap plate  10  (i.e., the top surface of the terminal board  21  is higher than the top surface of the fixing member  22 ). Also, the through hole  221  is provided on the side of the fixing member  22  close to the cap plate  10  so that the terminal board  21  can be exposed inside the case via the through hole to be electrically connected with the electrode assembly. 
     Of course, the embodiment of the present disclosure is not limited thereto. In other embodiments, a protrusion may be provided on the outer circumferential surface of the terminal board  21 , and correspondingly, a recess to be engaged with the protrusion may be provided on the inner wall surface of the fixing member  22  in contact with the terminal board  21 . Likewise, it is also possible to achieve the purpose of restricting the relative rotation between the terminal board  21  and the fixing member  22 . In addition, the dimensions of the recess and the protrusion in the thickness directions of the terminal plate  21  and the fixing member  22  are not limited in the embodiment of the present disclosure. In the above embodiment, the recesses are all provided to penetrate through the thickness of the terminal board  21 . However, in other embodiments, the recess may be provided not to penetrate through the thickness of the terminal board  21 , but may also be engaged with the corresponding protrusion of the fixing member  22  to prevent relative rotation between the terminal board  21  and the fixing member  22 . 
       FIG. 8  is a schematic diagram of a three-dimensional structure of a terminal board  21  according to a second embodiment of the present disclosure. In the embodiment, the same parts in the terminal board  21  as in the terminal board  21  of the first embodiment are denoted by the same reference numerals, and the already described parts will not be described again. As shown in  FIG. 8 , different from the terminal board  21  of the first embodiment, in the present embodiment, in order to prevent relative rotation between the terminal board  21  and the fixing member  22 , a recess  213  is provided on the terminal board  21 , and the recess  213  is a through hole penetrating through the terminal board  21  in the thickness direction thereof. Correspondingly, a post (i.e., a protrusion, not shown in the figure) to be matched with the recess  213  is protruded from the surface of the fixing member  22  in contact with the terminal board  21 . Since the peripheral edge of the terminal board  21  is embedded in the accommodation space  222  of the fixing member  22 , the fixing member  22  can be provided with the above-mentioned post on an upper surface and/or a lower surface of the accommodation space  222  to be matched with the recess  213  of the terminal board  21 . 
     Therefore, when the terminal board  21  is received in the accommodation space  222 , the post provided in the fixing member  22  may be inserted into the recess  213  of the terminal board  21 , so that the terminal board  21  can be firmly connected to the fixing member  22  by the snap fit of the recess  213  and the post. Of course, in other embodiments, the recess  213  may also be disposed to not penetrate through the terminal board  21  but only penetrate a predetermined depth in the thickness direction of the terminal board  21  to be engaged with the corresponding post provided in the fixing member  22 . Again, it is also possible to achieve the purpose of restricting the relative movement between the terminal board  21  and the fixing member  22 . 
       FIG. 9  is a schematic diagram of a three-dimensional structure of another state of the fixing member  22  of  FIG. 6 . As shown in  FIG. 1 ,  FIG. 5  and  FIG. 9 , in order to fix the fixing member  22  to the cap plate  10 , a connecting member including an undercut  225  and an undercut hole  13  may be further provided. The undercut  225  and the undercut hole  13  are correspondingly provided between the fixing member  22  and the cap plate  10  so as to be engaged with each other. 
     Particularly, at a surface of the cap plate  10  facing the terminal board  21 , a plurality of undercut holes  13  may be disposed around the electrode lead-out hole  11 . The plurality of undercut holes  13  may be arranged with regular intervals along the circumference, and an aperture of each undercut hole  13  may gradually increase along the direction from the fixing member  22  to the cap plate  10 . Of course, the increasing of the aperture as mentioned herein does not require that the aperture of the undercut hole  13  regularly increases along the direction from the fixing member  22  to the cap plate  10 , but means that the aperture of one end of the undercut hole  13  far away from the fixing member  22  is larger than that of the other end of the undercut hole  13  close to the fixing member  22 . In other words, it is only required that the aperture of the bottom of the undercut hole  13  is larger than the aperture of the top of the undercut hole  13  so as to allow for the engagement between the undercut  225  and the undercut hole  13 . Correspondingly, the undercut  225  adapted to the shape of the undercut hole  13  is provided at a surface of the fixing member  22  facing the cap plate  10 . Thus, the fixing member  22  can be firmly fixed to the cap plate  10  by the engagement between the undercut  225  provided on the fixing member  22  and the undercut hole  13  provided on the cap plate  10 . In this way, the fixing member  22  can be tightly fixed to the cap plate  10  by a simple structure to simplify the structure of the cap assembly  100 , and meanwhile, the productivity and structural stability of the cap assembly  100  can be improved. 
     As shown in  FIGS. 1 and 5 , the sealing member  23  is provided between the terminal board  21  and the cap plate  10  and surrounds the electrode lead-out hole  11 . In this embodiment, specifically, the sealing member  23  may be of an annular shape and provided with an annular groove  231  on the surface thereof facing the cap plate  10 . The cap plate  10  may be provided with an annular flange  16  surrounding the electrode lead-out hole  11  on the side of the cap plate  10  facing the sealing member  23 , so that the flange  16  of the cap plate  10  can be engaged with the groove  231 . The terminal board  21  can press the sealing member  23  tightly toward the cap plate  10  under the fastening force of the fixing member  22 , so that the sealing member  23  between the flange  16  and the terminal board  21  can be pressed tightly to make the sealing member  23  be in close contact with the terminal board  21  through the flange  16  and form a sealing line between the terminal board  21  and the cap plate  10 . Thus, it is possible to improve the sealing between the terminal board  21  and the cap plate  10  and avoid the problem of liquid leakage due to bad sealing between the terminal board  21  and the cap plate  10 , thereby further improve the reliability of use of the secondary battery. 
     In this embodiment, since the terminal board  21  is mounted on the cap plate  10  from top to bottom, the terminal board  21  may always apply a pressing force to the sealing member  23  under the effect of the fastening force of the fixing member  22  and the gravity of the terminal board  21  and the fixing member  22 . Accordingly, the sealing performance of the sealing member  23  can be further improved. Also, the provision of the sealing member  23  outside the case can reduce the probability that the sealing member  23  may contact with the electrolyte inside the case. 
     During use of the secondary battery, when the pressure inside the case increases, the cap plate  10  may arch substantially along a center line in its width direction. When the cap plate  10  is deformed, the fixing member  22  generally does not deform due to the rigidity of the fixing member  22 . In this case, it is possible to cause increased gaps between the fixing member  22  and both sides of the cap plate  10  in the width direction, so that the sealing member  23  cannot be pressed tightly between the terminal board  21  and the cap plate  10 . For example, the undercuts  225  close to both sides of the cap plate  10  in the width direction of  FIG. 5  may be disengaged from the undercut holes  13 , so that the sealing member  23  cannot be pressed tightly by the terminal board  21  and the cap plate  10 . In order to avoid an increase of gaps between the fixing member  22  and both sides of the cap plate  10  in the width direction and make the sealing member  23  be always pressed tightly between the terminal board  21  and the cap plate  10 , the fixing member  22  may be further provided with a weakened portion so as to allow the fixing member  22  to deform along with the cap plate  10 . 
     With continued reference to  FIG. 1  and  FIG. 6 , in an alternative embodiment, the weakened portion of the fixing member  22  is close to the center line of the cap plate  10  in the width direction thereof. In this embodiment, since the fixing member  22  is a rotating body and the fixing member  22  includes two weakened portions, the two weakened portions are disposed on two opposite sides of the fixing member  22  in the radial direction. That is, the two weakened portions are respectively located on two sides of the electrode lead-out hole  11  in the length direction of the cap plate  10  (i.e., two sides of the through hole  221  in the length direction of the cap plate  10 ) and close to the center line of the cap plate  10  in the width direction thereof. The weakened portion of the fixing member  22  is close to the center line of the cap plate  10  in the width direction thereof means that the position of the weakened portion of the fixing member  22  needs to be distributed along the center line of the cap plate  10  in the width direction thereof so that the fixing member  22  can arch along the center line of the cap plate  10  in the width direction thereof and deform together with the cap plate  10 . However, a certain error is allowed between the position of the weakened portion and the center line of the cap plate  10  in the width direction thereof, as long as the error does not affect the deformation of the fixing member together with the cap plate  10  so that the undercut  225  and the undercut hole  13  can keep in the status of being engaged with each other. 
     As an example, the weakened portion may be formed on the fixing member  22  by the provision of an opening portion  224 . As shown in  FIG. 6 , the opening portion  224  may be formed on the side of the fixing member  22  facing away from the cap plate  10  along the thickness direction of the fixing member  22 . The opening portion  224  is formed to have a predetermined depth in the thickness direction of the fixing member  22 . The opening portion  224  is respectively opened on two sides of the fixing member  22  in the radial direction of the fixing member  22 , so as to form notches that are provided on opposite sides of the through hole  221  of the fixing member  22  and connected to each other through the through hole  221  (i.e., the opening portion  224  penetrates through the fixing member  22  along the length direction of the cap plate  10 ). Therefore, by providing the opening portion  224  on opposite sides of the through hole  221  to form the weakened portion, the connection area of the fixing member  22  can be effectively reduced so that the fixing member  22  can be easily deformed along the weakened portion. 
     Thus, by providing the weakened portion on two sides of the through hole  221  of the fixing member  22  along the center line of the cap plate  10  in the width direction thereof, the fixing member  22  can be more easily deformed along the weakened portion. When the cap plate  10  is deformed and arched by a thrust force of the gas generated inside the case, the fixing member  22  can be arched along the weakened portion at the center line of the cap plate  10  in the width direction thereof together with the cap plate  10 , so as to ensure that the undercut  225  and the undercut hole  13  are always in the snap fit state. Therefore, it is possible to prevent the fixing member  22  from being detached from the cap plate  10 , improve the reliability of the use of the secondary battery and increase the service life of the secondary battery. At the same time, since it is not necessary to increase the strength of the cap plate  10  by increasing the thickness of the cap plate  10 , the manufacturing cost of the cap assembly  100  can also be saved. 
     Of course, there is no limitation on the number of weakened portions on the fixing member  22 . In other embodiments, more weakened portions may be provided on the fixing member  22 , and preferably arranged on two sides of the through hole  221  along the length direction of cap plate  10 . As a result, the fixing member  22  can be more easily deformed along with the cap plate  10  when the cap plate  10  is arched due to the gas inside the case. Also, the fixing member  22  can be more fitted with the arc formed by the cap plate  10  when the fixing member  22  is deformed along with the cap plate  10 , so as to further ensure the stability of the connection between the undercut  225  and the undercut hole  13 . It should be noted that it is necessary to provide the weakened portion on the fixing member  22  without affecting the structural stability of the fixing member  22  itself and ensure that the terminal plate  21  can be fixed to the cap plate  10  by the fixing member  22 . 
       FIG. 10  is a schematic diagram of a three-dimensional structure of a fixing member  22  according to a second embodiment of the present disclosure. For ease of understanding, the same structures as in the fixing member  22  of the above embodiment are denoted by the same reference numerals, and the already described structures will not be described again. In the second embodiment, a weakened portion may be formed by the provision of an opening portion  226 . Likewise, the opening portion  226  may also be provided on two sides of the through hole  221  along the length direction of the cap plate  10  and close to the center line of the cap plate  10  in the width direction thereof, and preferably be located along the center line of the cap plate  10  in the width direction thereof. However, the difference from the opening portion  224  in the above embodiment is that the opening portion  226  is a hole disposed on the surface of the fixing member  22  far away from the cap plate  10  and the opening portion  226  penetrates through the fixing member  22  in the thickness direction of the fixing member  22  (i.e., penetrates through the fixing member  22  in the thickness direction of the cap plate  10 ). 
     Thus, by providing the opening portion  226  on two sides of the through hole  221  to form the weakened portion, the connection area of the fixing member  22  can be effectively reduced so that the fixing member  22  can be easily deformed along the weakened portion. When the cap plate  10  is deformed and arched by a thrust force of the gas generated inside the case, the fixing member  22  can be arched along the weakened portion at the center line of the cap plate  10  in the width direction thereof together with the cap plate  10 , so as to ensure that the undercut  225  and the undercut hole  13  are always in the snap fit state. 
     Based on the above embodiments, a recess  212  and a protrusion  223  capable of fitting with the recess  212  are correspondingly disposed between the terminal board  21  and the fixing member  22 . In an alternative embodiment, the recess  212  provided along the outer peripheral surface of the terminal board  21  is located on the center line of the cap plate  10  in the width direction thereof. That is, the outer periphery of the terminal board  21  has at least two recesses  212  opposed to each other in the radial direction, and the two recesses  212  are located on the center line of the cap plate  10  in the width direction thereof. Therefore, by providing openings at both ends of the center line of the terminal board  21  in the width direction of the cap plate  10 , a weakened area is formed at a portion of the terminal board  21  between the openings at both ends. In this way, the terminal board  21  can be allowed to deform along the weakened area. Thus, after the terminal board  21  and the fixing member  22  are engaged with each other and mounted on the cap plate  10 , when the cap plate  10  is deformed under the thrust force of the gas inside the case, the terminal plate  21  and the fixing member  22  can be deformed along with the cap plate  10 . Therefore, it is possible to avoid that the terminal board  21  and the fixing member  22  cannot be deformed along with the deformation of the cap plate  10 , which may cause the fixing member  22  and the cap plate  10  to be disconnected from each other, eventually cause the terminal board  21  to be detached from the cap plate  10  and raise the problem of air leakage or liquid leakage. As a result, the reliability of the secondary battery can be further increased and the service life of the secondary battery can be lengthened. 
       FIG. 11  is a schematic diagram of a three-dimensional structure of a terminal board  21  according to a third embodiment of the present disclosure. For ease of understanding, the same structures as in the terminal board  21  of the first embodiment are denoted by the same reference numerals, and the already described structures will not be described again. As shown in  FIG. 11 , in other alternative embodiments, in order to enhance the deformability of the terminal board  21 , it is also possible to provide a weakened portion on the terminal board  21 . As an example, in the embodiment, the surface of the terminal board  21  on the side close to the cap plate  10  may be provided with a notch  214  along the radial direction. When the terminal board  21  is assembled to the cap plate  10 , the notch  214  extends in the length direction of the cap plate  10  and is located on the center line of the cap plate  10  in the width direction thereof. In the embodiment, the notch  214  is engaged between two recesses  212  opposite to each other, but in other embodiments, the notch  214  may be provided only on the surface of the terminal board  21  on the side close to the cap plate  10 . 
     Thus, by providing the notch  214  to form the weakened portion of the terminal board  21  along the center line of the cap plate  10  in the width direction thereof, the terminal board  21  can be allowed to have deformation along the weakened portion. After the terminal board  21  and the fixing member  22  are engaged with each other and mounted on the cap plate  10 , the terminal board  21  and the fixing member  22  can be deformed along with the cap plate  10  when the cap plate  10  is deformed by a thrust force of the gas inside the case. Therefore, it is also possible to further improve the reliability of the secondary battery and increase the service life of the secondary battery. 
     In addition, in the above embodiments, in order to maintain the insulation between the cap plate  10  and the electrode assembly inside the case as well as between the cap plate  10  and a wiring board, the lower insulator  40  may be generally made of plastic material, have a substantially plate shape, and be attached at the surface of the cap plate  10  on the side facing the interior of the case. The lower insulator  40  may include two through holes and first insulators disposed around the through holes. The two through holes are respectively opposite to the two electrode lead-out holes  11  provided on the cap plate  10 , while the first insulators are respectively disposed around the periphery of the through holes. 
     Furthermore, in an alternative embodiment, an undercut  41  may be provided on the surface of the lower insulator  40  facing the cap plate  10 . Correspondingly, an undercut hole to be matched with the undercut  41  may be provided on the surface of the cap plate  10  facing the case. With the similar way of engagement between the undercut  225  and the undercut hole  13 , the lower insulator  40  can be fixed to the cap plate  10  through the undercut  41  and the undercut hole on the cap plate  10 , which will not be described in detail here. 
     According to another embodiment of the present disclosure, there is also provided a secondary battery including a case, an electrode assembly, and a cap assembly  100  in any of the above embodiments. The case has an opening. The electrode assembly is accommodated in the case and includes a first electrode plate, a second electrode plate, and a separator disposed between the first electrode plate and the second electrode plate. The cap assembly  100  covers the opening of the case to enclose the electrode assembly in the case. Since the secondary battery has the same advantages as the cap assembly  100  in the above embodiments, it will not be described again. 
     In summary, in the secondary battery and the cap assembly of the secondary battery according to embodiments of the present disclosure, the terminal board is fixed to the fixing member and the fixing member is fixed to the cap plate through the connecting member, so that the terminal board is located on a side of the cap plate and covers the electrode lead-out hole of the cap plate. Therefore, the electrode terminal does not need to be provided with the base portion on the surface of the cap plate on the side facing the interior of the battery case. The space inside the case may not be occupied, thus the space occupancy in the case of the secondary battery can be increased and the energy density of the secondary battery can be further improved. In addition, by providing the weakened portion on the fixing member and close to the center line of the cap plate in the width direction thereof, when the cap plate is deformed to form an arch due to the gas generated inside the case of the secondary battery, the fixing member can be deformed along the weakened portion together with the cap plate. Thus it is possible to prevent an increase of gaps between both sides of the cap plate in the width direction and the fixing member and make the sealing member be always pressed tightly between the terminal board and the cap plate. Therefore, the structural reliability of the cap assembly of the secondary battery can be improved. 
     The invention may be embodied in other specific forms without departing from the spirit and essential characteristics thereof. The embodiments discussed therefore should be considered in all aspects as illustrative and not restrictive. The scope of the invention is defined by the appended claims rather than by the foregoing description, and those modifications falling within the meaning and equivalents of the claims are thus intended to be embraced by the scope of the invention. Different technical features in different embodiments may be combined to obtain beneficial effects. Other variations of the described embodiments can be understood and practiced by those skilled in the art upon studying the drawings, the specification and the claims herein.