Patent Publication Number: US-10770730-B2

Title: Through-wall current collector for a pouch cell

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to battery cell that includes a stacked or rolled arrangement of electrode plates, and a current collector device disposed in the battery cell that forms an electrical connection with the electrode plates within the battery cell and permits transfer of current out of the cell via an opening in the battery cell pouch housing. 
     2. Description of the Related Art 
     Battery packs provide power for various technologies ranging from portable electronics to renewable power systems and environmentally friendly vehicles. For example, hybrid electric vehicles (HEV) use a battery pack and an electric motor in conjunction with a combustion engine to increase fuel efficiency. Battery packs are formed of a plurality of battery modules, where each battery module includes several electrochemical cells. The cells are arranged in two or three dimensional arrays and are electrically connected in series or in parallel. Likewise, the battery modules within a battery pack are electrically connected in series or in parallel. 
     Different cell types have emerged in order to deal with the space requirements of a very wide variety of installation situations, and the most common types used in automobiles are cylindrical cells, prismatic cells, and pouch cells. Regardless of cell type, each cell may include a cell housing and an electrode assembly disposed in the cell housing. The electrode assembly includes a series of stacked or rolled positive electrode plates that alternate with negative electrode plates and are separated by an intermediate separator plates. Each cell may also include a first current collector that is electrically connected to the positive electrode plates and joins the positive electrode plates to a positive cell terminal disposed outside the cell housing, and a second current collector that is electrically connected to the negative electrode plates and joins the negative electrode plates to a negative cell terminal disposed outside the cell housing. 
     In a pouch cell, the first and second current collectors typically each include a lead tab that passes out of the pouch between two stacked layers of pouch fabric and along a weld line that joins the layers of pouch fabric together and forms a sealed joint. The lead tab is used to pass current from inside the pouch cell housing to the outside where it can be electrically connected to an external structure such as a terminal. A special sealing tape is used to surround the lead tab at the sealed joint where the lead tab passes between the layers and out of the pouch. The sealing tape is relatively expensive since it is required to have very specific material properties. For example, the sealing tape is required to remain solid, tacky and pliable in all conditions except when localized heat is applied as occurs during the pouch fabric welding operation. When localized heat is applied, the sealing tape is required to melt, flow into open gaps between materials, and bond to both the pouch material and the respective lead tabs. A need exists for a relatively simple and less costly structure for passing current from inside the pouch cell housing to the outside where it can be electrically connected to an external structure such as a terminal or another cell. 
     SUMMARY 
     A pouch cell includes an electrode assembly that is sealed within a pouch-type, metal laminated film cell housing along with an electrolyte to form a power generation and storage unit. The electrode assembly may be a “stacked” electrode assembly that includes a series of stacked positive electrode plates alternating with negative electrode plates and separated by an intermediate separator plates. In addition, the pouch cell includes a current collector device that forms an electrical connection with a corresponding one of the electrode plates and allows passage of current generated in the electrode assembly to pass through the current collector and out of the cell while maintaining a hermetically sealed pouch cell housing. 
     In some aspects, an electrochemical cell includes a cell housing formed of a flexible sheet. The cell housing has a first housing portion, and a second housing portion that is joined to the first housing portion along a sealed joint to form a pouch. The cell includes an electrode assembly disposed in the cell housing. The electrode assembly includes positive electrode portions alternating with negative electrode portions. The positive electrode portions and the negative electrode portions are separated by at least one separator and stacked along a stack axis. In addition, the cell includes a current collector device that is electrically connected to one of the positive electrode portions and the negative electrode portions and exits the cell housing via an opening formed in the cell housing. The first housing portion includes a base and a sidewall that protrudes from a perimeter of the base and surrounds the base to form an open-ended container. The stack axis extends in a direction perpendicular to the base. The opening is formed in the sidewall at a location spaced apart from the sealed joint and at a location facing the stack axis. The current collector device includes a current collecting plate and a terminal plate. The current collecting plate is disposed between the sidewall and the one of the positive electrode portions and the negative electrode portions, is electrically connected to the one of the positive electrode portions and the negative electrode portions, is oriented parallel to the sidewall, and overlies the opening. The terminal plate is oriented parallel to the sidewall, overlies the opening and is disposed outside the cell housing. The terminal plate directly contacts the current collecting plate via the opening so as to form an electrical connection with the current collecting plate. 
     The electrochemical cell may include one or more of the following features: A force is applied to the terminal plate that urges the terminal plate into direct contact with the current collecting plate. The terminal plate and the current collecting plate form a weld-free electrical connection. The current collecting plate is welded to the terminal plate, and the weld is positioned in the opening. The cell includes a second current collector that is electrically connected to another one of the positive electrode portions and the negative electrode portions and exits the cell housing via a second opening formed in the cell housing. The cell housing is rectangular including a first end, an opposed second end, and four sides that extend between the first end and the second end, and the first opening is formed in one of the four sides, and the second opening is formed in another of the four sides. The first opening is formed in one of the four sides, and the second opening is formed in a side opposed to the one of the four sides. The first opening is formed in one of the four sides, and the second opening is formed in a side adjacent to the one of the four sides. One the terminal plate and the current collecting plate includes a protrusion that protrudes through the opening toward the other of the terminal plate and the current collecting plate and forms an electrical connection with the other of the terminal plate and the current collecting plate. The current collector device includes a seal that surrounds the opening, and the seal is disposed between the sidewall and one of the terminal plate and the current collecting plate. Each of the terminal plate and the current collecting plate include a first side and an opposed second side, and each of the first side and the second side is generally planar and free of surface features. The current collector device consists of the current collecting plate and the terminal plate. 
     In some aspects, an electrochemical cell includes a cell housing formed of a flexible sheet, the cell housing having a first housing portion, and a second housing portion that is joined to the first housing portion along a sealed joint to form a pouch. The cell includes an electrode assembly disposed in the cell housing. The electrode assembly includes positive electrode portions alternating with negative electrode portions, and the positive electrode portions and the negative electrode portions are separated by at least one separator and stacked along a stack axis. The cell includes a current collector device that is electrically connected to one of the positive electrode portions and the negative electrode portions and exits the cell housing via an opening formed in the cell housing. The first housing portion comprises a base and a sidewall that protrudes from a perimeter of the base and surrounds the base to form an open-ended container. The stack axis extends in a direction perpendicular to the base. The opening is formed in the sidewall at a location spaced apart from the sealed joint and at a location facing the stack axis. The current collector device consists of a current collecting plate. The current collecting plate is disposed between the sidewall and the one of the positive electrode portions and the negative electrode portions, electrically connected to the one of the positive electrode portions and the negative electrode portions, oriented parallel to the sidewall and positioned so as to overlie the opening, and includes a protrusion that extends through the opening and forms a terminal. 
     The electrochemical cell may include one or more of the following features: The protrusion is formed integrally with the plate and terminates in a planar surface. The cell includes a second current collector device that is electrically connected to another one of the positive electrode portions and the negative electrode portions and exits the cell housing via a second opening formed in the cell housing. 
     In some aspects, a battery pack includes electrochemical cells. Each electrochemical cell includes a cell housing formed of a flexible sheet. The cell housing has a first housing portion, and a second housing portion that is joined to the first housing portion along a sealed joint to form a pouch. Each cell includes an electrode assembly disposed in the cell housing. The electrode assembly includes positive electrode portions alternating with negative electrode portions. The positive electrode portions and the negative electrode portions are separated by at least one separator and stacked along a stack axis. Each cell also includes a current collector device that is electrically connected to one of the positive electrode portions and the negative electrode portions and exits the cell housing via an opening formed in the cell housing. The first housing portion comprises a base and a sidewall that protrudes from a perimeter of the base and surrounds the base to form an open-ended container. The stack axis extends in a direction perpendicular to the base. The opening is formed in the sidewall at a location spaced apart from the sealed joint and at a location facing the stack axis, and a portion of the current collector device of one of the electrochemical cells directly contacts and forms an electrical connection with a portion of a current collector device of an adjacent electrochemical cell. 
     The battery pack may include one or more of the following features: The current collector device includes a current collecting plate and a terminal plate. The current collecting plate is disposed between the sidewall and the one of the positive electrode portions and the negative electrode portions, is electrically connected to the one of the positive electrode portions and the negative electrode portions, is oriented parallel to the sidewall, and overlies the opening. The terminal plate is oriented parallel to the sidewall, overlies the opening and is disposed outside the cell housing, the terminal plate directly contacting the current collecting plate via the opening so as to form an electrical connection with the current collecting plate. One of the terminal plate and the current collecting plate includes a protrusion that protrudes through the opening toward the other of the terminal plate and the current collecting plate and forms an electrical connection with the other of the terminal plate and the current collecting plate. The current collector device includes a seal that surrounds the opening, and the seal is disposed between the sidewall and one of the terminal plate and the current collecting plate. The current collector device consists of a current collecting plate that is disposed between the sidewall and the one of the positive electrode portions and the negative electrode portions, electrically connected to the one of the positive electrode portions and the negative electrode portions, oriented parallel to the sidewall and positioned so as to overlie the opening, and includes a protrusion that extends through the opening and forms a terminal. 
     Advantageously, the current collector device is disposed at a location that is spaced apart from the sealed joint used to form the pouch cell, and particularly at a location overlying an opening in the pouch sidewall, where the opening faces (lies in a plane parallel to) the stack axis of the electrode assembly. By forming the opening at this location rather than at the sealed joint, commonly available sealing materials can be used to prevent leakage about the current collector device, whereby the use of the relatively costly sealing tape can be eliminated and the cost to manufacture the cell is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a partially exploded perspective view of a battery pack including an array of pouch cells. 
         FIG. 2  is a perspective view of a pouch cell including the current collector devices. 
         FIG. 3  is a schematic cross sectional view of the pouch cell of  FIG. 2  as seen across line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a perspective view of an electrode pair including a positive electrode plate, a negative electrode plate, and separator plates alternating with the positive and negative electrode plates. 
         FIG. 5  is a cross-sectional view of the electrode plate stack illustrating the positive folded portions of the positive electrode plates arranged in an overlapped configuration and in electrical contact with a current collecting plate, with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 6  is cross-sectional view of the current collector device as seen across line  5 - 5  of  FIG. 2 , with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 7  is cross-sectional view of an alternative embodiment current collector device, with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 8  is cross-sectional view of another alternative embodiment current collector device, with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 9  is cross-sectional view of another alternative embodiment current collector device, with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 10  is cross-sectional view of an alternative embodiment current collector device, with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 11  is a cross-sectional view of an electrical connection formed between adjacent pouch cells, each pouch cell including the current collector device of  FIG. 9 , with the negative electrode plates and separator plates omitted for clarity. 
         FIG. 12  a partially exploded perspective view of a battery pack including an array of pouch cells. 
         FIG. 13  is a partial cross sectional view of an alternative embodiment electrode stack. 
         FIG. 14  is a side view of the cell illustrating an exemplary opening in the side of the cell. 
         FIG. 15  is a side view of the cell illustrating another exemplary opening in the side of the cell. 
         FIG. 16  is a side view of the cell illustrating another exemplary opening in the side of the cell. 
         FIG. 17  is a side view of the cell illustrating another exemplary opening in the side of the cell. 
         FIG. 18  is a side view of the cell illustrating yet another exemplary opening in the side of the cell. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-3 , a battery pack  1  used to provide electrical power includes electrochemical cells  20  that are electrically interconnected and stored in an organized manner within a battery pack housing  2 . The battery pack housing  2  includes a pack container portion  3  and a detachable pack lid  4 . The cells  20  are lithium-ion pouch cells that include an electrode assembly  60  ( FIGS. 3 and 4 ) that is sealed within a cell housing  21  along with an electrolyte to form a power generation and storage unit. In some embodiments, groups of cells  20  may be bundled together to form battery modules (not shown), which in turn are stored within the battery pack housing  2 . However, in the illustrated embodiment, the cells  20  are not bundled into modules and instead are directly electrically connected to battery pack housing terminals  6 ,  7 . Within the battery pack housing  2 , the cells  20  are electrically connected in series or in parallel. 
     Each cell  20  includes a pouch-type cell housing  21 . The cell housing  21  has a rectangular shape. In the illustrated embodiment, the cell housing  21  is cube shaped, and includes six orthogonal surfaces. The surfaces include a first end  22 , a second end  23  that is opposed to the first end  22 , a first side  24 , a second side  25  adjoining the first side  24 , a third side  26  adjoining the second side  25  and being opposed to the first side  24 , and a fourth side  27  adjoining the third side  26  and the first side  24 , the fourth side  27  being opposed to the second side  25 . Each of the first side  24 , the second side  25 , the third side  26  and the fourth side  27  extend between the first end  22  and the second end  23 , and the six surfaces together define a sealed interior space occupied by the electrode assembly  60 . 
     The pouch-type cell housing  21  is an assembly of two blanks of a metal laminated polymer film sheet. Each blank is folded to form the shape of an open-ended box. The first blank corresponds to a relatively deep box that serves as a first cell housing portion or container  42  dimensioned to receive the electrode assembly  60 . The container  42  includes a base corresponding to the cell housing second end  23 , and a sidewall corresponding to the closed section defined by the cell housing sides  24 ,  25 ,  26 ,  27 . The second blank corresponds to a relative shallow box that serves as a second housing portion or cover  44  that closes the open end of the container  42 . The cover  44  corresponds to the cell housing first end  22 , and is joined to each of the sides  24 ,  25 ,  26 ,  27  for example via welding. In particular, a continuous sealed joint  40  is formed along an intersection between the sides  24 ,  25 ,  26 ,  27  and the cell housing first end  22  to form a sealed rectangular pouch having a depth that is greater than the depth of some conventional pouch-type cell housings. For example, in some embodiments, the depth is greater than 20 mm. In the illustrated embodiment, the cell housing  21  is cube shaped and has length, width and height dimensions that are 90 mm or more. 
     The cell housing  21  includes openings  28 ,  29  formed in the cell housing sides  24 ,  26  that cooperate with a current collector devices  70 ,  71  to permit transfer of current out of the cell housing  21 . The first opening  28  is formed in one of the sides, e.g. the first side  24 , and the second opening  29  is formed in the opposed side, e.g. the third side  26 . In addition, the first opening  28  and the second opening  29  are spaced apart from the sealed joint  40 . For example, in the illustrated embodiment, the first opening  24  and the second opening  26  are each generally centered on the respective side  24 ,  26  whereas the sealed joint  40  adjoins the housing first end  22 . By this arrangement, the first and second openings  28 ,  29  lie in a plane perpendicular to a plane defined by the cell housing second end  23  and thus face a stack axis  66  (discussed below) of the electrode assembly  60 . 
     Referring to  FIGS. 3 and 4 , the electrode assembly  60  is disposed in the cell housing  21  and includes a series of stacked positive electrode plates  61  alternating with negative electrode plates  62  and separated by intermediate separator plates  30 ,  32 . The separator plates  30 ,  32  are each a permeable membrane that functions to keep the positive and negative electrodes  61 ,  62  apart to prevent electrical short circuits while also allowing passage of ionic charge carriers provided in the electrolyte and that are needed to close the circuit during the passage of current within the cell  20 . The separator plates  30 ,  32  are formed of, for example, an electrically insulating material such as a tri-layer polypropylene-polyethylene-polypropylene membrane. 
     The series of stacked electrode plates  61 ,  62  and separator plates  30 ,  32  will be referred to herein as the “plate stack”  64 , and a stack axis  66  of the plate stack  64  extends through a center of the plate stack  64  in a direction parallel to the stacking direction. When the electrode assembly  60  is disposed in the cell housing  21 , the electrode plates  61 ,  62  are parallel to the cell housing first and second ends  22 ,  23  and the stack axis  66  extends in a direction perpendicular to the cell housing first and second ends  22 ,  23 . The electrode plates  60 ,  61  are very thin (e.g., having a thickness on the order of about 0.095 to 0.145 mm) compared to the overall cell thickness (e.g. having a thickness on the order of tens or hundreds of mm) and thus are illustrated schematically in  FIG. 3 . 
     During stacking, the positive electrode plates  61 , the negative electrode plates  62  and the separator plates  30 ,  32  that form the electrode assembly  60  are arranged in a layered or stacked configuration in the stacking direction. In the stacked configuration, the separator plates  30 ,  32 , are stacked along the stack axis  66  such that peripheral edges of all the separator plates  30 ,  32  of the stack  64  are aligned in a direction parallel to the direction of the stack axis  66 . 
     In addition, the positive and negative electrode plates  61 ,  62  are partially offset in a direction transverse to the stack axis (i.e., a length direction) relative to the respective separator plates  30 ,  32 . In particular, the positive electrode plates  61  are stacked along the stack axis  66  such that peripheral edges of the positive electrode plates  61  are aligned with each other in a direction parallel to the direction of the stack axis  66  but are partially offset relative to the separator plates  30 ,  32  in a first direction parallel to the length direction. The first direction is represented in  FIG. 4  by arrow  34 . Thus, one edge  61   a  of each of the positive electrode plates  61  extends beyond a corresponding edge  30   a ,  32   a  of the separator plates  30 ,  32  resulting in a positive “clear lane”  63  of exposed conductive material. 
     The negative electrode plates  62  are stacked along the stack axis  66  such that peripheral edges of the negative electrode plates  62  are aligned with each other in a direction parallel to the direction of the stack axis  66  but are partially offset relative to the separator plates  30 ,  32 , in a second direction, where the second direction is parallel to the length direction and opposed to that of the first direction. The second direction is represented in  FIG. 4  by arrow  35 . Thus, one edge  62   b  of each of the negative electrode plates  62  extends beyond a corresponding edge  30   b ,  32   b  of the separator plates  30 ,  32  resulting in a negative “clear lane”  65  of exposed conductive material. 
     Referring to  FIG. 5 , the clear lane  63  of each positive electrode plate  61  may be folded against a side of the plate stack  64 . Likewise, the clear lane  65  of each negative electrode plate  62  may be folded against the opposed side of the plate stack  64  (not shown). Due to the relative spacing of the electrode plates  61 ,  62  along the stack axis  66 , the folded clear lanes  63 ,  65  form an overlapping louvered configuration in which a portion of each clear lane  63 ,  65  is exposed and faces a side (i.e., side  24 ) of the cell housing  21 . The folded clear lanes on a given side of the plate stack  64  cooperate to form a generally planar electrically conductive surface that can be used to form an electrical connection with a current collection device  70 ,  71  as discussed further below. 
     Referring to  FIGS. 2, 3 and 6 , each cell  20  also includes a first current collector device  70  and a second current collector device  71  that form a weld-free electrical connection with the plates  61 ,  62  of the plate stack  64 . The first current collector device  70  is disposed at the first opening  28  and the second current collector device  71  is disposed at the second opening  29 . The first current collector device  70  and the second current collector device  71  are identical, and thus only the first current collector device  70  will be described in detail. Elements common to both the first and second current collector devices  70 ,  71  will be referred to with common reference numbers. 
     The first current collector device  70  is a two-piece device that includes a current collecting plate  72  and a terminal plate  82 . The current collecting plate  72  is a thin, electrically conductive plate that includes an electrode-facing surface  73 , a side-facing surface  74  that is opposed to the electrode-facing surface  73 , and a peripheral edge surface  75  that extends between the electrode-facing surface  73  and the side-facing surface  74 . In the illustrated embodiment, the peripheral edge surface  75  defines a rectangular shape to correspond to the rectangular shape of a side of the electrode stack  64 . Both of the electrode-facing surface  73  and the side-facing surface  74  are generally planar and free of surface features. In addition, the electrode-facing surface  73  and the side-facing surface  74  are dimensioned to have the same shape and nearly the same size as the facing side of the electrode plate stack  64 . The current collecting plate  72  is disposed between the housing first side  24  and the electrode assembly  60 . In particular, the electrode-facing surface  73  directly contacts and forms an electrical connection with the folded clear lanes  63  of the positive electrode plates  61 . In addition, the current collecting plate  72  is oriented so that the side-facing surface  74  is parallel to the housing first side  24  and overlies the first opening  28 . The cell housing  21  and the electrode assembly  60  are dimensioned so that the side-facing surface  74  of the collecting plate  72  abuts the inner surface of the housing first side  24  while also being in contact with the folded clear lanes  63  of the positive electrode plates  61 . 
     The terminal plate  82  is a thin, electrically conductive plate that includes a housing-facing surface  83 , an outward-facing surface  84  that is opposed to the housing-facing surface  83 , and a peripheral edge surface  85  that extends between the housing-facing surface  83  and the outward-facing surface  84 . In the illustrated embodiment, the peripheral edge surface  85  defines a generally rectangular shape, but is not limited to a generally rectangular shape. A central portion  81  of the terminal plate  82  is slightly offset relative to the periphery, and thus each of the housing-facing surface  83  and the outward-facing surface  84  are slightly non-planar. In particular, in the central region  81 , the housing-facing surface  83  is bulged while the outward-facing surface  84  is depressed. The terminal plate  82  is disposed outside the cell housing  21  and abuts the outer surface of the housing first side  24  so as to overlie the first opening  28 . The terminal plate  82  is oriented so that the housing-facing surface  83  is parallel to the housing first side  24 , and faces the current collecting plate  72  through the first opening  28 . In addition, the terminal plate  82  is positioned so that the bulge of the central portion  81  protrudes into the first opening  28  and abuts the current collecting plate  72 , whereby a direct electrical connection is formed between the current collecting plate  72  and the terminal plate  82 . 
     A bead  48  of sealing material is provided around the opening  28  to form a hermetic seal and prevent leakage of electrolyte from the cell housing  21  through the opening  28 . In the illustrated embodiment, the bead  48  is provided between the terminal plate housing-facing surface  83  and the housing first side  24 . It is understood that the bead  48  could alternatively be provided between the current collecting plate side-facing surface  74  and the housing first side  24 , or at both locations. Advantageously, since the sealing material is used at a location remote from the seal joint  40 , it requires fewer specialized material properties. For this reason, the sealing material used to provide the bead  48  may be a commonly available material. The sealing material may be, for example, a pressure sensitive adhesive or a gasket material. As a result, the sealing material may be low in cost relative to some sealing materials used in some conventional pouch cells to form a seal within the seal joint. 
     The second current collector device  71  is disposed at the second opening  29  ( FIG. 3 ). The current collecting plate  72  of the second current collector device  71  is disposed between the housing third side  26  and the electrode assembly  60 , and is oriented so that the side-facing surface  74  is parallel to the housing third side  26  and overlies the second opening  29 . In addition, electrode-facing surface  73  of the second current collector device  71  directly contacts and forms an electrical connection with the folded clear lanes  65  of the negative electrode plates  62 , while the side-facing surface  74  abuts an inner surface of the cell housing third side  26 . In addition, the terminal plate  82  of the second current collector device  71  is disposed outside the cell housing  21  and abuts an outer surface of the housing third side  26  so as to overlie the second opening  29 . The terminal plate  82  is oriented so that the housing-facing surface  83  is parallel to the housing third side  26 , and faces the current collecting plate  72  through the second opening  29 . In addition, the terminal plate  82  is positioned so that the bulge of the central portion  81  protrudes into the second opening  29  and abuts the current collecting plate  72 , whereby a direct electrical connection is formed between the current collecting plate  72  and the terminal plate  82 . 
     In both the first current collector device  70  and the second current collector device  71 , a direct electrical connection is formed between the current collecting plate  72  and the terminal plate  82  via direct contact between the bulged central portion  81  of the terminal plate  82  with the side facing surface  74  of the current collecting plate  72 . In particular, a weld-free, direct electrical connection is formed between the side-facing surface  74  of the current collecting plate  72  and the housing-facing surface of the terminal plate  82  in the vicinity of the opening  28 ,  29 . In order to ensure that an electrical connection is formed between the facing surfaces  83 ,  74  of the terminal plate  82  and the current collecting plate  72 , an external force may optionally be provided. For example, a force F (represented by an arrow in  FIG. 6 ) is applied to outward-facing surface  84  of the terminal plate  82  that urges the terminal plate  82  into direct contact with the current collecting plate  72  via the opening  28 ,  29 . The force F may be applied to the outward-facing surface  84  by, for example, a spring (not shown) or by an adjacent cell  20 , as discussed further below. 
     Referring to  FIG. 7 , an alternative embodiment current collector device  170  includes a welded connection between the current collecting plate  72  and the terminal plate  82 . For example, the current collecting plate  72  may be welded to the terminal plate  82  via resistance welding, ultrasonic welding or other methods, with the weld passing through the opening  28 ,  29  in order to ensure a reliable electrical connection between these elements. 
     Referring to  FIG. 8 , another alternative embodiment current collector device  270  is similar to the current collector device  70  described above with respect to  FIG. 6 , and common elements are referred to using common reference numbers. The current collector device  270  includes the current collecting plate  72  and a terminal plate  282  that is modified relative to the earlier disclosed terminal plate  82 . In particular, the terminal plate  282  includes a protrusion  286  that is formed on the housing-facing surface  83  and extends toward the current collecting plate  72 . The protrusion  286  is provided at a location corresponding to the opening  28 ,  29 . In addition, the protrusion  286  has a height that corresponds to the sum of the thickness of the cell housing  21  and the bead  48  of sealant material, and thus is configured to protrude through the opening  28 ,  29 , directly contact the current collecting plate  72  and form an electrical connection with the current collecting plate  72 . The protrusion  286  terminates in a flat contact surface  287  to maximize the contact area between the contact surface  287  and the current collecting plate  72 . In the illustrated embodiment, the protrusion  286  has a circular cross-sectional shape, but is not limited to having this shape. As in the earlier embodiments, the bead  48  of sealing material is provided on the housing-facing surface  83  of the terminal plate  282  around the opening  28 ,  29  to form a hermetic seal and prevent leakage of electrolyte from the cell housing  21 . In the illustrated embodiment, the electrical connection between the terminal plate  282  and the current collecting plate  72  is a weld-free, direct electrical connection. 
     Referring to  FIG. 9 , an alternative embodiment current collector device  370  includes a welded connection  346  between the current collecting plate  72  and the terminal plate  282 . For example, the current collecting plate  72  may be welded to the terminal plate  282  via resistance welding, ultrasonic welding or other methods, with the weld passing through the opening  28 ,  29  in order to ensure a reliable electrical connection between these elements. 
     Referring to  FIGS. 10 and 11 , another alternative embodiment current collector device  470  is similar to the current collector device  70  described above with respect to  FIG. 6 , and common elements are referred to using common reference numbers. The current collector device  470  is a one-piece element that includes only a current collector plate  472  that is modified relative to the earlier disclosed current collector plate  72 . The current collector plate  472  includes a protrusion  486  that is formed on the side-facing surface  74  and extends toward the side  22 ,  26  of the cell housing  21 . The protrusion  486  is provided at a location corresponding to the opening  28 ,  29 . In addition, the protrusion  486  has a height that corresponds to, or is slightly greater than, the sum of the thickness of the cell housing  21  and the bead  48  of sealant material, and thus is configured to protrude through the opening  28 ,  29 . By this configuration, the protrusion  486   a  of one cell  20   a  can form a direct electrical connection with a protrusion  486   b  of an adjacent cell  20   b . The protrusion  486  terminates in a flat contact surface  487  to maximize the contact area between the contact surface  487  and an external structure such as an adjacent protrusion  486   b . In the illustrated embodiment, the protrusion  486  has a generally rectangular cross-sectional shape, but is not limited to having this shape. As in the earlier embodiments, the bead  48  of sealing material is provided on the side-facing surface  74  of the current collector plate  472  that surrounds the opening  28 ,  29  to form a hermetic seal and prevent leakage of electrolyte from the cell housing  21 . 
     Referring now to  FIG. 12 , the force F that optionally provides the direct contact, for example between the current collecting plate  72  and the terminal plate  82  within each current collector device  70 ,  71 , or between the protrusions  486   a ,  486   b  of adjacent cells  20   a ,  20   b , may be generated externally with respect to the cell housing  20 .  FIG. 12  illustrates the battery pack  1  including an array of cells  20  arranged in rows R 1 , R 2 , R 3 , R 4  and columns C 1 , C 2 , C 3 , C 4 , C 5  within the battery pack housing  2 . The electrical connection between the current collecting plate  72  and the terminal plate  82  within each current collector device  70 ,  71 , or between the protrusions  486   a ,  486   b  of adjacent cells  20   a ,  20   b , is generated and/or assured by urging the cells  20  of a row together and the cells  20  of a column together. Specifically, a compression force along the cell rows is achieved by providing an elastic member  13  between the cells  20  of the row and the sidewall  3   a  of the container portion  3  of the battery pack housing  2 . For example, an elastic member such as the wave spring  13  can be disposed at one or both ends of each row R 1 , R 2 , R 3 , R 4  to ensure an electrical connection between the current collecting plate  72  and the terminal plate  82  within each current collector device  70 ,  71  of the row, or between the protrusions  486   a ,  486   b  of adjacent cells  20   a ,  20   b  of the row. Similarly, a wave spring  13  can be disposed at one or both ends of each column C 1 , C 2 , C 3 , C 4 , C 5  to ensure an electrical connection between the current collecting plate  72  and the terminal plate  82  within each current collector device  70 ,  71  of the column, or between the protrusions  486   a ,  486   b  of adjacent cells  20   a ,  20   b  of the column. In other embodiments, the elastic member  13  may alternatively be disposed between adjacent cells  20   a ,  20   b  along the rows and columns to ensure that an electrical is formed within each current collector device  70 ,  71 . 
     In some embodiments, the current collecting plate  72  and the terminal plate  82  of the first collector device  70  are formed of, or plated with, a first electrically conductive material that corresponds to the material used to form the positive electrode plates  61 , such as aluminum. In addition, the current collecting plate  72  and the terminal plate  82  of the second collector device  71  are formed of, or plated with, a second electrically conductive material that corresponds to the material used to form the negative electrode plates  62 , such as copper. 
     Referring to  FIG. 13 , although the current collecting plate  72  of the first current collecting device  70  and the second current collecting device  71  are described above as being pressed against a folded portion (e.g., the clear lane  63 ,  65 ) of the corresponding positive or negative electrode plate  61 ,  62  to form the weld-free electrical connection, other connection configurations may be employed to form the weld-free electrical connection. For example, the current collecting plate  72  of the first current collecting device  70  may be electrically connected via direct contact to peripheral edges  61   a  of non-folded positive electrode plates  61 . Similarly, the current collecting plate  72  of the second current collecting device  71  may be electrically connected via direct contact to the peripheral edges  62   b  of non-folded negative electrode plates  62 . Thus, the first and second current collector devices  70 ,  71  may form a direct electrical connection with edge surfaces of the electrode plates  61 ,  62  via a weld-free pressure contact. 
     Referring to  FIGS. 14-18 , the location and arrangement of the openings  28 ,  29  formed in the cell housing can affect overall cell storage capacity since current density is localized at the openings  28 ,  29 . This is because cell regions having a high current density tend to age more quickly than other regions of lower current density, thus reducing cell storage capacity. Thus, for a given side (i.e., side  24 ) of the cell housing  21 , it can be beneficial to provide the opening  28  as one or more openings arranged in such a way as to minimize current density at any one location and/or to more evenly distribute current density across a side of the cell  20 . However, the benefits of reducing current density at any one location have to be balanced with the technical challenges associated with ensuring that the cell housing  21  is sealed along a perimeter of the opening  28  to prevent leakage of electrolyte from the cell  20  and moisture entry into the cell  20 . These challenges are minimized by minimizing a total length of the peripheral edge of the opening or openings in the cell housing  21 . 
     Referring to  FIG. 14 , in one exemplary embodiment, there is a single cell opening  28 ,  29  (only opening  28  is shown) disposed on a corresponding cell side  24 ,  26 . The cell opening  28  is circular in shape. In the illustrated embodiment, the opening  28  is located closer to the housing first end  22  than the housing second end  23 , but the opening  28  is not limited to this location. Although a circular shape is illustrated, it is understood that the opening  28  could have a polygonal or an irregularly curved shape (i.e., a kidney shape). 
     Referring to  FIG. 15 , in another exemplary embodiment, there are multiple, mutually spaced cell openings  128 ,  129  (only openings  128  are shown) on each side  24 ,  26 . In the illustrated embodiment, there are two elongated openings  128  that are generally rectangular in shape. The openings  128  are arranged so that long axes of the openings  128  extend in parallel to each other and are oriented to extend between the first and second ends  22 ,  23  of the cell housing  21 . Although a generally rectangular shape is illustrated, it is understood that the openings  128  could have rounded ends or have an oval, or other, elongated shape. 
     Referring to  FIG. 16 , in another exemplary embodiment, there are multiple, mutually spaced cell openings  228 ,  229  (only openings  228  are shown) on each side  24 ,  26 . In the illustrated embodiment, there are five elongated openings  228  that are generally rectangular in shape. The openings  228  are arranged so that long axes of the openings  228  extend in parallel to each other and are oriented to extend between the second and fourth sides  25 ,  27  of the cell housing  21 . Although a generally rectangular shape is illustrated, it is understood that the openings  228  could have rounded ends or have an oval, or other, elongated shape. 
     Referring to  FIG. 17 , in another exemplary embodiment, there are multiple, mutually spaced cell openings  328 ,  329  (only openings  328  are shown) on each side  24 ,  26 . In the illustrated embodiment, there are two rows of five openings  228  that are generally square in shape. The rows are arranged in parallel and so that axes parallel to the rows are oriented to extend between the first and second ends  22 ,  23  of the cell housing  21 . Although a generally square shape is illustrated, it is understood that the openings  228  could have a circular or other polygonal shape. It is also understood that the rows could alternatively extend between the second and fourth sides  25 ,  27  of the cell housing  21  or in a diagonal orientation. 
     Referring to  FIG. 18 , in yet another exemplary embodiment, there is a single cell opening  428 ,  429  (only opening  428  is shown) disposed on a corresponding cell side  24 ,  26 , and the cell openings  428 ,  429  are irregular in shape. In the illustrated embodiment, the opening  428  includes a circular central portion  428   a  with elongated slots  428   b  extending radially outward from the central portion  428   a.    
     In the embodiment illustrated in  FIG. 6 , the central portion  81  of the terminal plate  82  is bulged and protrudes through the opening  28  to form an electrical connection with the current collecting plate  72 . It is understood that in some alternative embodiments, the central portion of the current collecting plate  72  may be bulged rather than the central portion of the terminal plate  82 . In still other alternative embodiments, both the terminal plate  82  and the current collecting plate  72  may be substantially planar (e.g., bulge-free and protrusion-free), and the force F may be applied to outward-facing surface  84  of the terminal plate  82  to urge the terminal plate  82  into direct contact with the current collecting plate  72  via the opening  28 ,  29 . 
     In the illustrated embodiments, the cell housing  21  is described as having the first opening  28  that is formed in one of the sides and the second opening  29  that is formed in a side that is opposed to the one side. However, the cell housing  21  is not limited to this configuration. For example, in some embodiments, the cell housing  21  may include the first opening  28  that is formed in one of the sides and the second opening  29  that is formed in side that is adjacent to the one side. In other embodiments, the cell housing  21  may include two first openings  28  where one first opening  28  is disposed on each of opposed sides, and two second openings  29  where one second opening is disposed on each of the sides adjacent to the opposed sides. 
     Although the electrode assembly  60  is described herein as being a “stacked” electrode assembly that includes a series of stacked plates  61 ,  62 , the electrode assembly  60  is not limited to this configuration. For example, in some embodiments, the electrode assembly  60  may include a rolled electrode assembly (e.g., a jelly roll assembly), a folded electrode assembly (i.e., a Z-fold assembly), or other electrode arrangement. 
     Although the cell  20  has a cube-shaped cell housing  21 , the cell housing  21  is not limited to a cube shape. For example, the cell housing  21  may be rectangular in shape ( FIG. 4 ). In another example, the cell housing  21  may have other polygonal shapes that permit close packing such as an eight surface structure having hexagonally arranged sides (not shown). 
     Moreover, the cells  20  are not limited to being a lithium-ion battery. For example, the cells  20  may be aluminum-ion, alkaline, nickel-cadmium, nickel metal hydride, or other type of cell. 
     Selective illustrative embodiments of the battery system including the cell are described above in some detail. It should be understood that only structures considered necessary for clarifying these devices have been described herein. Other conventional structures, and those of ancillary and auxiliary components of the battery system, are assumed to be known and understood by those skilled in the art. Moreover, while working examples of the battery system and battery cell have been described above, the battery system and/or battery cell are not limited to the working examples described above, but various design alterations may be carried out without departing from the devices as set forth in the claims.