Patent Publication Number: US-2021175562-A1

Title: Cell assembly, cell sub-module, energy storage module and method for assembling the same

Description:
The present disclosure relates generally to the field of energy storage cells and energy storage modules, in particular a lithium-ion starter battery. More specifically, the present disclosure relates to lithium-ion cell assemblies that may be used in vehicular contexts, as well as other energy storage/expending applications. Furthermore, the present disclosure relates to a method for manufacturing/assembling such cell assemblies, cell sub-modules, and energy storage modules, respectively. 
     This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present disclosure, which are described and/or claimed below. The discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior-art. 
     A vehicle generally refers to any means of transportation using one or more battery system for providing a starting power and/or at least a portion of a motion power for the vehicle. The vehicle may refer to a motor-powered and/or electrically powered vehicle such as an air- or watercraft, a rail-guided vehicle, or preferably a street vehicle. The street vehicle may in particular refer to cars, trucks, buses or recreational vehicles. 
     In vehicles, different types of batteries are used, such as traction batteries (especially for electric or hybrid electric vehicles) and starter batteries. In automotive applications, a starter battery is used for providing the necessary energy/power required for starting a vehicle. In more detail, a starter battery generally refers to a battery or energy storage module, which provides at least a portion of the energy/power, preferably the total energy/power, required when starting a vehicle and/or required for providing power to vehicle-internal electrical systems (such as, e.g., lights, pumps, ignition and/or alarm systems). 
     Conventionally, 12 Volt (V) lead-acid batteries are used as starter batteries for vehicles. However, lead-acid batteries have a rather heavy weight, in particular, due to their low energy densities. Quite to the contrary, lithium-ion energy storage modules are known for their high energy densities. In addition, lithium-ion energy storage modules have, for example, a longer service life, less self-discharge, improved rapid charging capability and shorter maintenance intervals than conventional lead-acid batteries. However, the lithium-ion chemistry has different needs and requirements as the conventional lead-acid battery. 
     As battery technology evolves, there is a need to provide improved power sources, particularly energy storage modules for vehicles. For example, lithium-ion batteries or battery cells tend to be very susceptible to heating or overheating, which may negatively affect components of the energy storage module. Also, lithium-ion batteries or battery cells tend to be very sensitive with respect to overcharging and deep-discharging of the respective cells or battery. 
     Accordingly, an objective of the present application is to provide a cell assembly, a cell sub-module and an energy storage module, which overcome the disadvantages of the conventional systems, and which are at the same time easy to manufacture, economical and versatile, and which can be easily adapted and assembled, while meeting the specific demands posed by a lithium-ion battery chemistry. A further objective is to provide a method for assembling such an energy storage module, in an easy, flexible and cost efficient manner. 
     These objectives are solved by a cell assembly, a cell-sub-module, an energy storage module and a method for assembling the same according to the independent claims. Advantageous embodiments are defined in the dependent claims. 
     In more detail, the objective is solved by a cell assembly comprising a cell frame, into which bus bars and a thermal plate are integrated. The cell assembly further comprises a lithium-ion pouch cell comprising a first face and a second face opposite the first face, wherein the pouch cell further comprises positive and negative cell terminals arranged at a top end of the pouch cell, and a compression element, wherein the cell frame is configured to receive and house the pouch cell and the compression element in a space defined by the thermal plate and the cell frame. 
     The inventive proposal to form the cell assembly such that the lithium-ion pouch cell and the compression element are received in a space defined by the cell frame and the integrated thermal element achieves an exceptionally compact design of the cell assembly, which can be realized easily and with only few standard components. Furthermore, the thermal management of the cell assembly can be ensured in a reliable way by means of a (rather) large contact surface between the lithium-ion pouch cell and the thermal plate. 
     According to another aspect, the bus bars and the thermal plate can be in-molded in the cell frame, which is preferably made of a polymeric material, which increases the stability of the cell frame-bus bars-thermal plate arrangement and provides an easy and precise way for arranging the bus bars and thermal plate in the cell frame. 
     According to another aspect, a bottom portion of the thermal plate can extend through a bottom wall of the cell frame, wherein the bottom portion of the thermal plate is preferably configured to contact a thermal management feature. This ensures structural integrity of the cell frame and also enhances the thermal management of the cell assembly. 
     According to another aspect, the cell terminals of the pouch cell can be pre-bent such as to contact the bus bars. Thereby, the electrical conduct can be ensured in an easy and cost efficient manner. 
     According to another aspect, the pouch cell can be secured to the thermal plate by means of a supported or non-supported adhesive layer, which is at least partially applied on the thermal plate, preferably a glue layer. Thus, the pouch cell can securely arranged in a simple manner. 
     According to another aspect, the compression element can comprise at least one foam layer. 
     According to another aspect, the cell frame can comprise geometric features for supporting appropriate placement of the cell terminals. 
     In an embodiment, the geometric features can comprise recesses, which comprise a shape corresponding to the cell terminals of the pouch cell. Thereby, the arrangement and positioning of the pouch cell in the frame is simplified. 
     According to another aspect, the bus bars can further comprise a Cu-bus bar for coupling to a negative terminal of the pouch cell and an Al-bus bar for coupling to a positive terminal of the pouch cell. 
     According to another aspect, the cell terminals can be welded to the bus bars, preferably laser welded to the bus bars. Thereby, enhancing the electrical conductivity between the cell terminals and the bus bars by a tight contact. 
     Furthermore, the objective of the present application is solved by a cell sub-module according to claim  11 , which comprises at least two cell assemblies as described above, in particular three cell assemblies as described above, wherein the at least two cell assemblies are stacked such that the thermal plate of a first cell assembly contacts the compression element of an adjacent cell assembly and such that the cell terminals of each cell assembly are arranged on a first side of the cell sub-module. 
     According to another aspect, the cell sub-module comprises three cell assemblies. 
     Moreover, an energy storage module is provided which comprises a housing having a thermal management feature, a plurality of cell sub-modules as described above, which is arranged in the housing, wherein the housing comprises a plurality of cavities, each configured to receive a corresponding one of the plurality of cell sub-modules, the cavities being defined either by at least one wall of the housing and an internal partition of the housing or by at least two internal partitions of the housing. 
     The inventive proposal to form an energy storage module of a plurality of cell sub-modules each comprising two or more cell assemblies, achieves a highly versatile product. In more detail, the desired qualities (e.g. total voltage, total capacity, energy density etc.) of the energy storage module can be easily and cost efficiently adapted by providing a corresponding amount of cell sub-modules having a respective number of cell assemblies. 
     According to another aspect, the (aligned) bus bars of two adjacent cell sub-modules are connected to each other by means of a bimetallic plate such that the two adjacent cell sub-modules are electrically connected in series, wherein the respective cell assemblies of each cell sub-module are simultaneously electrically connected in parallel to each other. Thereby, a simple way of electrically connecting the respective cell sub-modules and cell assemblies is provided. 
     According to another aspect, the bimetallic plate comprises a Cu-portion for connecting to the Cu-bus bar of the negative terminal and Al-portion for connecting to the Al-bus bar of the positive terminal. Thus, welding of the portions of the bimetallic plate to the respective positive and negative bus bars is simplified, which leads to a better electrical connectivity and reduced manufacture costs. 
     According to another aspect, the energy storage module can further comprise a sense line for measuring the voltage of a cell assembly, and/or of one or more cell sub-module of the plurality of cell sub-modules. Thus, the operation of the cell assemblies and/or cell sub-modules can be monitored and defects or malfunctions can be detected more easily. 
     According to another aspect, the sense line further comprises at least one temperature sensor integrated into the sense line, which increases the operation security of the energy storage module as, e.g., a thermal runaway can be detected swiftly. 
     According to another aspect, the housing of the energy storage module is closable or closed by means of a cover element. 
     According to another aspect, the energy storage module is a 12 Volt lithium-ion starter battery comprising four cell sub-modules, each cell sub-module preferably comprising three cell assemblies. 
     Moreover, the objective of the present application is also solved by a method for assembling an energy storage module as described above, which comprises the steps of: providing a plurality of cell sub-modules, preferably four cell sub-modules, each comprising at least two, in particular three, cell assemblies; arranging each cell sub-module into a corresponding cavity of the battery housing; and electrically connecting the plurality of cell sub-modules in series by means of a plurality of bimetallic plates, which simultaneously electrically connects the at least two cell assemblies in parallel. Thus, the cell assemblies of a cell sub-module, and the cell sub-modules of the energy storage module can be electrically connected in a simplified manner, which reduces manufacture cost and time necessary for assembly. 
     According to another aspect, the bimetallic plates are welded, in particular laser welded or ultrasonically welded, to the plurality of bus bars integrated in the cell frame and electrically connected to the corresponding terminals of the corresponding pouch cell. 
     According to another aspect, positive and negative end connection pieces are welded, in particular laser welded or ultrasonically welded, to respective positive and negative cell terminals of the two end cell sub-modules of the energy storage module for electrically coupling to respective main terminals of the energy storage module. 
    
    
     
       These and other features, aspects and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1A  is a perspective view of a cell assembly; 
         FIG. 1B  is an exploded view of the cell assembly; 
         FIG. 2A  is a detailed view of a cell frame, bus bars and a thermal plate of the cell assembly; 
         FIG. 2B  is a cross-sectional view of the cell frame, bus bars and thermal plate as shown in  FIG. 2A ; 
         FIG. 3  is a perspective view of a cell sub-module; 
         FIG. 4A  is a perspective view of the battery housing; 
         FIG. 4B  is a perspective view of the battery housing receiving a plurality of cell sub-modules; 
         FIG. 5A  is an illustrative view of an energy storage module wherein the casing is depicted in a transparent way; and 
         FIG. 5B  is a perspective view of the energy storage module. 
     
    
    
     It should be noted that terms such as “above”, “below”, “on top of”, and “beneath” may be used to indicate relative positions for elements (e.g., stacked components of the cell sub-module  100  and energy storage module  1000  described below) and are not limiting embodiments to either of a horizontal or vertical stack orientation. Further, it should be noted that terms such as “above”, “below”, “proximate”, or “near” are intended to indicate the relative positions of two layers in the stack that may or may not be in direct contact with one another. 
     Also, terms such as “top”, “bottom”, and “side” are configured to describe relative position with respect to the cell assembly  1 , cell sub-module  100  and/or energy storage module  1000  in the mounted state (e.g. when mounted in a vehicle). 
     Additionally, the geometric references are not intended to be strictly limiting. For example, the use of the term “perpendicular” does not require an exact right angle, but defines a relationship that is substantially perpendicular, as would be understood by one of ordinary skill in the art. Similarly, for example, the term “parallel” used in reference to geometric relationships does not require a perfect mathematical relationship, but indicates that certain features are generally extending in the same directions. Additionally, the term “planar” is used to describe features that are substantially flat that does not require perfect mathematical planarity. 
     In more detail, “substantially parallel” and “substantially planar” means that an angle between ±10°, preferably ±5°, most preferably ±2° to an exact parallel or planar orientation are considered as substantially parallel or substantially planar. In the same sense, a “substantially perpendicular” or “substantially right” angle is considered as an angle of 80° to 110°, preferably 85° to 95°, most preferably 88° to 92°. 
     Lithium-ion battery systems such as used in automotive applications, may be used in conjunction with or as a replacement for lead-acid batteries traditionally used in vehicles. 
     Described herein are various embodiments and design features of lithium-ion cell assemblies  1  and cell sub-modules  100  which may be arranged in a lithium-ion energy storage module  1000  for use in an automobile or other motive environments. 
     Those cell assemblies  1 , cell sub-modules  100  and energy storage modules  1000  can also be used in various different environments, e.g. recreational purposes (e-bikes, scooters etc.) and so forth. 
     A perspective view of an embodiment of a cell assembly  1  is shown in  FIG. 1A , along with the constituent layers of the cell assembly  1  which are shown in an exploded view in  FIG. 1B . 
     Therein, the cell assembly  1  includes a cell frame  20  for housing at least a lithium-ion pouch cell  10  and a compression element  30 . 
     The cell frame  20  preferably comprises four walls defining a space for receiving the pouch cell  10  and the compression element  30 . In more detail, the cell frame  20  can comprise a top wall, a bottom wall opposite the top wall and two side walls connecting the top wall and bottom wall at respective ends. The top wall may be configured with recesses in order to receive and arrange cell terminals  12   i    12   ii  of the lithium-ion pouch cell  10 . 
     The cell frame  20  can be made of a polymeric material such as for example polyethylene, polypropylene, polyamide, polyimide, acrylnitril-butadien-styrol etc. and combinations thereof. 
     Bus bars  22   i ,  22   ii  for coupling to a respective terminal of the lithium ion pouch cell  10  and a thermal plate  24  for thermal management purposes can be in-molded into the cell frame  20 . In some embodiments, the bus bars  22   i ,  22   ii  can be received in the top wall of the cell frame  20 . Moreover, the top wall of the cell frame  20  may be provided with gripping features (e.g. a slot) in which the thermal plate  24  is arranged (e.g. the thermal plate  24  may be in-molded into the cell frame  20 ). 
     The lithium-ion pouch cell  10  can be secured to the thermal plate  24  using an adhesive  40 . The adhesive  40  can be provided in form of an adhesive layer, a supported or non-supported transfer tape layer, or by means of adhesive portions provided only at selective portions of the thermal plate  24 . 
     The thermal plate  24  can be made of a thermally conductive material, in particular, a metal like aluminum, magnesium, copper, etc. In an embodiment, the thermal plate  24  can be made of aluminum and the surface facing the pouch cell  10  can be coated with aluminum oxide, which is electrically insulative. 
     The pouch cell  10  may include an outer electrically insulating layer (e.g. a polyimide film or another suitable electrically insulating polymer). Additionally, the pouch cell  10  may also include a metallic foil layer (e.g., an aluminum foil layer, or an aluminum oxide foil layer that may provide enhanced structural integrity to be more resilient to pin holes deformities, to provide a better gas barrier layer, and so forth, compared to the use of insulating polymer films alone). Further, the pouch cell  10  can include an inner electrically insulating layer (e.g., a polyimide film or another suitable electrically insulating polymer) to electrically isolate the metallic foil layer from the internal components of the pouch cell  10 . The above-described layers can be individually applied to the pouch cell  10  or may be provided as a single film including the layers, which may be collectively referred to as pouch material film. 
     The pouch material film may be sealed (e.g., sonically welded, sealed with epoxy, or another suitable seal) around the cell terminals  12   i ,  12   ii  to isolate the internal components of the pouch cell  10 . 
     Inside the pouch cell  10 , a positive cell terminal  12   i  may be electrically coupled to one or more cathode layers while the negative cell terminal  12   ii  may be electrically coupled to one or more anode layers. In certain embodiments the coupled layers may be made from an aluminum plate that are coated with a cathode active material (e.g., including a lithium metal oxide such as lithium nickel cobalt manganese oxide (NMC) (e.g., LiNiCoMnO 2 ), lithium nickel cobalt aluminum oxide (NCA) (e.g., LiNiCoAlO 2 ), or lithium cobalt oxide (LCO) (e.g., LiCoO 2 ). In certain embodiments the anode layers may be made from copper plates that are coated with an anode active material (e.g., including graphite or graphene). It should be appreciated that these materials are merely provided as examples and that the present approach may be applicable to a number of differently lithium-ion and nickel metal hydride battery modules. 
     The at least one cathode layer and the at least one anode layer are configured to form an electrochemical stack which may be implemented as a “jelly roll”, wherein the positive cell terminal  12   i  and the at least one cathode layer may be formed from a single continuous strip of aluminum foil and the negative cell terminal  12   ii  and the at least one anode layer may be formed from a single, continuous strip of copper foil. For such an implementation, the aluminum foil strip and the copper foil strip may be stacked, along with a number of electrically insulating layers and wound to provide the electrochemical stack. In more detail, the aluminum foil strip and the copper foil strip may be stacked along with a number of electrically insulating layers and wound about a mandrel to provide the electrochemical stack. 
     Furthermore, an electrolyte (e.g., including carbonate solvents and LiPeF 6  as salt) is provided in the pouch cell  10 . However, the present invention is not limited by a solvent (aqueous) electrolyte. Rather, a non-aqueous electrolyte can be used instead. 
     The negative cell terminal  12   ii  and the positive cell terminal  12   i  are preferably arranged on the same side of the pouch cell  10 , and spaced apart from each other by a predetermined distance. In more detail, the cell terminals  12   i ,  12   ii , which can be provided in form of tabs, may be pre-bent in order to accommodate securement to the bus bars  22   i ,  22   ii  integrated into the cell frame  20 , as illustrated, e.g., in  FIG. 1B . 
     The compression element  30  is arranged at a second planar face of the pouch cell  10 , which is opposite to a first planar face of the pouch cell  10  contacting the thermal plate  24  via the adhesive  40 . The compression element  30  can be formed as a foam layer. The compression element  30  helps to accommodate differences in sizes between the pouch cells  10  and furthermore serves to provide a minimum amount of compression such that the pouch cell  10  and the thermal plate  24  contact each other firmly; thus enhancing the thermal conduct. 
     Accordingly, the compression element  30  can equalize at least to some extent cell tolerances existing when manufacturing lithium-ion pouch cells  10 . 
     The cell assembly  1  as shown in  FIGS. 1A and 1B  can be assembled or manufactured by inserting the thermal plate  24  and the bus bars  22   i ,  22   ii  into a molding tool, molding the cell frame  20  such as to integrate the bus bars  22   i ,  22   ii  and the thermal plate  24  into the cell frame  20 , and applying an adhesive  40  at least partially to the surface of the thermal plate  24  facing the space for receiving the pouch cell  10  and the compression element  30 . Then, the pouch cell  10  is inserted into the space such that the respective cell terminals  12   i ,  12   ii  are received by recesses formed in the cell frame  20 , preferably in the top wall of the cell frame  20 . 
     The cell terminals  12   i ,  12   ii  may be bent at approximately a right angle such that they can easily be connected to the respective bus bars  22   i ,  22   ii.    
     In some embodiments, the terminals can be pre-bent to an angle slightly less than 90° (e.g. 85°, 83° or 80°) such that the terminals are pressed to the bus bars  22   i ,  22   ii  by an elastic force, while the pouch cell  10  can still be easily inserted into the cell frame  20 . Thereby, the electrical conductivity between the cell terminals  12   i ,  12   ii  and the bus bars  22   i ,  22   ii  is enhanced. 
     Then, a compression element  30  is inserted into the space of the cell frame  20 . The compression element  30  can be provided as a cut sheet of a foam material. The compression element  30  can be secured in the cell frame  20  by means of an adhesive and/or by means of pressing the compression element  30  into the frame to form a press fit. Respective retaining features can therefore be provided in the cell frame  20  (e.g. in the sidewalls of the cell frame  20 ) in order to hold and retain the compression element  30  in the space. 
     Alternatively, the compression element  30  could also be formed on the pouch cell  10  by directly applying the foam layer to the pouch cell  10 . In other words, by foaming the compression element  30  on the pouch cell  10 . 
     An exemplary compression element  30  could be made of a polyurethane, polypropylene, a polyethylene, a polystyrene, a polyethylene terephthalate material. 
     Then, each cell terminal  12   i ,  12   ii  can be secured to the corresponding bus bar  22   i ,  22   ii  by means of laser-welding or ultrasonically welding. 
       FIG. 2A  shows a more detailed view of the cell frame  20  and the thermal plate  24 , which can be provided in form of a metal sheet, a metal oxide sheet or a combination thereof (e.g. a sheet made of aluminum coated with aluminum oxide). 
     As shown in  FIG. 2A , the cell frame  20  can be made of a polymeric material and may include geometrical features to support appropriate placement of the cell terminals  12   i ,  12   ii . In more detail, the top wall of the cell frame  20  can comprise two recesses configured to receive a respective cell terminal  12   i ,  12   ii.    
     As illustrated in  FIG. 2A , the bus bars  22   i ,  22   ii  and the thermal plate  24  are integrated into the cell frame  20 . In more detail, the bus bars  22   i ,  22   ii  and the thermal plate  24  can be in-molded or over-molded by the cell frame  20 . In an embodiment, the bus bars  22   i ,  22   ii  are in-molded into the top wall of the cell frame  20 . 
       FIG. 2B  shows in a cross-sectional view of the cell frame  20  with in-molded bus bars  22   i ,  22   ii  and thermal plate  24 . As shown in  FIG. 2B , the thermal plate  24  may extend through the bottom wall of the cell frame  20  and may be bent at an approximately right angle such as to form a two-dimensional bottom portion  24   ii  parallel to and substantially covering the bottom wall of the cell frame  20 . The bottom portion  24   ii  of the thermal plate  24  is configured for contacting a thermal management feature  50   i  of an energy storage module  1000 . Thereby, heat can be conducted to and from the pouch cell  10  very efficiently from or to the thermal management feature  50   i  of the energy storage module  1000 . 
     The top wall of the cell frame  20  can be over-molded on the thermal plate  24  such that a top portion  24   i  of the thermal plate  24  is received in a slot formed in the top wall of the cell frame  20 . In some embodiments, one or more apertures or undercuts may be provided in the top portion  24   i  of the thermal plate  24  such that portions of the top wall of the cell frame  20  are molded and extend through the apertures in order to provide a secure grip of the thermal plate  24  in the cell frame  20 . 
     The top wall of the cell frame  20  is configured to receive the bus bars  22   i ,  22   ii  of the cell assembly  1 , e.g. by means of in-molding or over-molding. In some embodiments, a first (positive) bus bar  22   i  for contacting the positive cell terminal  12   i  of the pouch cell  10  can be made of aluminum, and a second (negative) bus bar  22   ii  for contacting the negative cell terminal  12   ii  of the pouch cell  10  can be made of copper. 
       FIG. 3  shows a perspective view of a stack of cell assemblies  1 , which produces a cell sub-module  100 . Therein, three cell assemblies  1  are arranged in a stack such that the cell terminals  12   i ,  12   ii  of a first cell assembly  1  substantially align with the cell terminals  12   i ,  12   ii  of an adjacent cell assembly  1 . The three cell assemblies  1  are stacked such that the thermal plate  24  of a first cell assembly  1  faces and preferably contacts the compression element  30  of a second adjacent cell assembly  1 . 
     Each cell assembly  1  is preferably arranged such that all negative cell terminals  12   ii  and corresponding negative bus bars  22   ii  are aligned and all positive cell terminals  12   i  and corresponding positive bus bars  22   i  are aligned with each other. Thereby, the cell-assemblies of the cell sub-module  100  can be connected in parallel more easily. 
     Although only a cell-submodule  100  having three cell assemblies  1  is shown in the  FIG. 3 , a cell-submodule  100  may comprise any suitable number of cell assemblies  1  greater than or equal to two cell assemblies  1 , which result in a desired requirement of the cell sub-module  100  (e.g., total voltage or total capacity of the cell sub-module  100 ). 
     A plurality of such cell sub-modules  100  is configured to form an energy storage module  1000 . In more detail, at least two, e.g. four, cell sub-modules  100  are arranged into a casing of an energy storage module  1000 . 
     The casing comprises a housing  50  having internal partitions  52  in order to form respective cavities for receiving a corresponding one of the at least two cell sub-modules  100  and a cover element  80  for closing the housing  50 . 
     As shown in  FIGS. 4A and 4B , the housing  50  includes a plurality of cavities to enable placement and securement of a corresponding number of cell sub-modules  100  in the housing  50 . The cavities are formed by a plurality of internal partitions  52  in the housing  50 . The housing  50  may also include the thermal management feature  50   i , which can be provided as a heat sink or cold plate to enable passive cooling of the battery cells. Once the cell sub-modules  100  are positioned in the case, the cell sub-modules  100  may be electrically connected using bimetallic plates  60 . 
     In more detail,  FIG. 4A  shows an exemplary housing  50  of an embodiment of the present application. In this regard, the housing  50  comprises four side walls, three internal partitions  52  defining four cavities for receiving a respective cell sub-module  100 . Furthermore, the thermal management feature  50   i  is provided at the bottom of the housing  50  to close and seal the bottom of the housing  50 . The thermal management feature  50   i  may be a metallic heat sink, e.g. an aluminum heat sink, to support passive cooling of the respective cell sub-modules  100  and thereby of the respective cell assemblies  1 . 
       FIG. 4B  shows the housing  50  of  FIG. 4A , wherein four cell sub-modules  100  are placed within each corresponding cavity. Each cell sub-module  100  may be fixed by means of the internal partitions  52  and/or an epoxy layer or a thermal paste provided on the thermal management feature  50   i.    
     In  FIG. 4B , the cell sub-modules  100  are arranged such that the negative cell terminals  12   ii  and negative bus bars  22   ii  of a first cell sub-module  100  join or align with the positive cell  12   i  terminals and positive bus bars  22   i  of a second adjacent cell sub-module  100 . 
     The plurality of cell sub-modules  100  can then be electrically coupled by means of a plurality of bimetallic plates  60 , which are configured to connect two adjacent cell sub-modules  100 . In more detail, the bimetallic plate  60  comprises a first portion made of copper and a second portion made of aluminum, wherein the first portion contacts the negative cell bus bars  22   ii  of a first cell sub-module  100  and the second portion contacts the positive bus bars  22   i  of a second adjacent cell sub-module  100 . Thus, the cell sub-modules  100  are connected in series with each other, whereas the cell assemblies  1  of each cell sub-module  100  are simultaneously connected in parallel. 
     The bimetallic plates  60  may be welded to the cell bus bars  22   i ,  22   ii  by laser welding. 
     Moreover, a positive end connection piece  60   i  and a negative end connection piece  60   ii  are provided for electrically connecting the series-connected cell sub-modules  100  with respective positive and negative main terminals  82   i ,  82   ii  of the energy storage module  1000 . In this regard, the negative end connection piece  60   ii  can be connected to the negative bus bar  22   ii  of the first cell assembly  1 , and the positive end connection piece  60   i  can be connected to the positive bus bar  22   i  of the last (in the embodiment of  FIG. 4B , the fourth) cell assembly  1 . 
     The main terminals  82   i ,  82   ii  are provided in the cover element  80  for connection to electronics. 
     A sense line  70  may be connected and secured to the cell sub-modules  100 , wherein the sense line  70  includes voltage and temperature sense features. 
       FIG. 5A  is a perspective view of the energy storage module  1000  once assembled, wherein the housing  50  and the cover element  80  of the energy storage module  1000  is transparent in order to illustrate the internal components of the energy storage module  1000 . As shown, the energy storage module  1000  includes various electronics such as a control module, a relay, signal harness and so forth positioned above the cell sub-modules  100 . 
       FIG. 5B  is a perspective view of the energy storage module  1000  once assembled. Therein, the cover element  80  comprises a receiving portion for the two main terminals  82   i ,  82   ii  of the energy storage module  1000 . Furthermore, the cover element  80  can be connected and secured to the housing  50  in order to provide a sealed energy storage module  1000  casing. Therefore, the cover element  80  may be welded, in particular laser welded or ultrasonically welded to the housing  50 . 
     In a preferred embodiment, the energy storage module  1000  is a 12V lithium-ion starter battery, comprising four cell sub-modules  100  electrically connected in series and each comprising three stacked cell assemblies  1  electrically connected in parallel as described above. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. In more detail, depending upon the desired voltage and/or capacity of the energy storage module  1000 , any suitable number of cell sub-modules  100  or cell assemblies  1  can be used in order to meet the desired demands. 
     The technical effects and technical problems in the specification are exemplary and are not limiting. It should be noted that the embodiments described in the specification may have other technical effects and may have other technical problems. 
     While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g. variations and sizes, dimensions, structures, shapes, proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors orientations, etc.) without materially departing from the novel teachings and advantageous of the subject-matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. 
     REFERENCE SIGNS 
     
         
           1  cell assembly 
           10  lithium-ion pouch cell 
           12   i ,  12   ii  (cell) terminal 
           20  cell frame 
           22   i ,  22   ii  (cell) bus bar 
           24  thermal plate 
           24   i  top portion of thermal plate 
           24   ii  bottom portion of thermal plate 
           30  compression element 
           40  adhesive 
           50  housing 
           50   i  thermal management feature 
           52  internal partition of housing 
           60  bimetallic plate 
           60   i ,  60   ii  positive/negative end connection pieces (of bimetallic plate) 
           70  sense line 
           80  cover element 
           82   i ,  82   ii  main terminals 
           100  cell sub-module 
           1000  energy storage module