Abstract:
The invention relates to a method for connecting two energy storage assemblies ( 10 ) together, each energy storage assembly including a sealed metal housing, in which: a connector strip ( 30 ), which is sized so as to contact the end surface of each of the housings, is positioned on the end surfaces ( 24 ) of two housings arranged side by side; and the strip is friction-stir welded to each of the housings.

Description:
[0001]    The present invention concerns the technical field of electric energy storage assemblies. 
         [0002]    In the invention by “electric energy storage assembly ” is meant either a capacitor (i.e. a passive system comprising electrodes and an insulator) or a supercapacitor (i.e. a passive system comprising at least two electrodes, an electrolyte and at least one separator) or a battery (i.e. a system comprising an anode, a cathode, and an electrolyte solution between the anode and the cathode) e.g. of lithium battery type. 
         [0003]    Said energy storage assemblies are most often associated in energy storage modules allowing specifications to be met that are specific to some applications. 
       STATE OF THE ART 
       [0004]    In the state of the art an energy storage module is known comprising a plurality of energy storage assemblies, each energy storage assembly being contained in a cylindrical housing. The cylindrical housings are placed side by side in the module and are electrically connected together, in series, by connector strips. 
         [0005]    Each connector strip is placed on one of the substantially planar end walls of the cylinder and on the corresponding end wall of the neighbouring cylinder. 
         [0006]    In the state of the art the connector strip is welded onto each housing by a LASER welding process as described in document FR 2 915 626 for example. 
         [0007]    LASER welding is transparency welding which creates a highly localized melt point in the two materials to be welded. The molten materials then mix together and the assembly solidifies again almost instantly having regard to the highly localised nature of the melt. However the forming of the weld depends on the fine-tuned adjusting of a plurality of parameters in particular the focus of the laser, the power thereof, etc. 
         [0008]    Yet the connection of the housing of an energy storage assembly with the connector strip is performed after the electrolyte has been placed in the housing and the impregnation hole has been re-plugged. It is therefore most important to maintain the seal of the housing otherwise impurities may be added to the electrolyte of the storage assembly and substantially reduce its lifetime and even lead to the discarding thereof. 
         [0009]    It has been ascertained that the LASER welding process, on account of the different parameters having an influence on the melting of the zone to be welded, is scarcely repeatable and can lead to piercing of the lid during the melt process if the beam is too powerful or too focused. 
         [0010]    To obtain a reliable weld process, it is therefore necessary to provide a housing for the storage assembly that is relatively thick and/or of special design, thereby preventing any slight disturbance of the LASER parameters from deteriorating the seal of the housing, and/or the different LASER parameters used to weld the strip onto the housing of the storage assembly must be monitored using highly accurate means thus entailing a non-negligible increase in manufacturing costs. 
         [0011]    It is one objective of the present invention to propose a simple, low-cost method for manufacturing an energy storage module which nevertheless allows maintaining of the seal of each of the energy storage assemblies. 
       SUMMARY OF THE INVENTION 
       [0012]    For this purpose a method is proposed for connecting two energy storage assemblies, each energy storage assembly comprising a sealed housing in which: 
         [0013]    on the end surfaces of two housings placed side by side a connector strip is positioned sized so that it is contact with the end surface of each of the housings; 
         [0014]    the strip is friction-stir welded onto each of the housings. 
         [0015]    In the present invention by “end surface ” is meant the upper or lower housing surface intended to lie opposite the strip. For a housing having a longitudinal axis (e.g. a cylindrical housing) the end surfaces are, for example, the surfaces perpendicular to the longitudinal axis of the housing. 
         [0016]    Each storage assembly preferably comprises at least two electrodes and an electrolyte. The assemblies may be identical, either of same shape and same type (supercapacitor, battery, etc.) or of different shape and/or type. 
         [0017]    Said method is of great advantage. Friction-stir welding is a welding technique in which a tool is very rapidly driven in rotation on the parts to be welded. The tool enters into the material heating it to a paste. Heating associated with stirring allows welding of the constituent material of the parts to be welded. Welding is performed in the paste state and the tool only heats the material with which it comes into contact. 
         [0018]    This method is therefore very easy to control since there is no risk of material polluting the electrolyte subsequent to welding if the tool itself does not pass through the housing. Control over the method therefore amounts to verifying the movement of the tool which is easy to perform. It is not necessary either to increase the thickness of the lid to the extent required for LASER welding. 
         [0019]    Since welding is performed in the paste state, it will also be noted that there is no risk of hot cracking, which may also deteriorate the seal of the housing. 
         [0020]    A method according to the invention therefore allows the connecting together of energy storage assemblies in simple, low-cost manner whilst maintaining the seal of each storage assembly. 
         [0021]    The method of the invention also has numerous other advantages. 
         [0022]    In particular, it allows electric conduction to be increased between the different energy storage assemblies without requiring any major increase in cycle time. 
         [0023]    It is effectively found that with the method of the invention the weld bead corresponds to the size of the stir-welding tool, which is not limited by constraints of tool power unlike the case in the prior art for LASER welding (power largely related to the size of the beam). It is therefore possible to increase the size of the bead or weld points by increasing the size of the tool and hence the electric conduction between the strip and each housing, without the need however to perform several passes of the welding tool on the zone to be welded thereby maintaining a short manufacturing time. 
         [0024]    For the same reasons (bead width) the heat conduction from the upper part of the storage assembly towards the strip is improved. The evacuation of heat is therefore optimized by the method of the invention. 
         [0025]    In addition, the strip may be more economical being of simpler shape. It is effectively not necessary to hollow out the strip to obtain local reduction of thicknesses as carried out in the prior art to facilitate LASER welding. The use of a non-hollowed strip also allows maximization of the surface of strip in contact with the conducting mat of a module, which also ensures better evacuation of heat towards outside the module. 
         [0026]    Finally, the method of the invention is performed by heating the material of the housing to a far lesser extent than in the prior art. The risks of damaging the electrolyte inside the housing or the actual energy storage element are therefore reduced compared with LASER welding. 
         [0027]    The method of the invention may also comprise one or more of the following characteristics:
       during the weld step a device mobile in rotation can be placed in contact with the housing and/or strip, and the device is translated along a predetermined axis so that it enters into the material of the housing and/or strip. The axis generally corresponds to the normal to the end surface of the housing. If the housing is a tubular element the predetermined axis generally corresponds to the axis of symmetry of the tubular element;   each housing may comprise a tubular element and at least one lid closing the tubular element at its end. The housing may particularly comprise two lids, each positioned at one end of the tubular element. Preferably the strip is welded onto the lid. In this case, one surface of the lid forms the end surface of the assembly;   the strip and the housing can be welded over a distance of at least 1 cm in length, which allows the electric conduction of the assembly to be increased. However it is also possible to perform spot welding of the strip onto the lid;   the end surface may comprise one or more protuberances forming positioning means for the strip thereupon. The protuberance may be formed for example by a centring pin, the strip then comprising at least one mating orifice; the protuberance may also be formed by one or more pins intended to delimit the contour of the strip;   the housing and the strip can be superimposed over a contact surface normal to the predetermined axis, in particular the end surface, so that welding passes through the strip to reach the housing;   the strip and the housing may be in contact upon a contact surface comprising the predetermined axis, so that the strip and the housing are welded edge to edge. In this case, the tool may enter simultaneously into the material of the housing and of the strip. This embodiment is generally applied when the housing comprises a protuberance forming positioning means for the strip;   the strip may be made in a conductive material, copper in particular;   the housing may be made at least in part in a metal material, aluminium in particular;   the end surface of the housing can be made in a first material, the strip being made in a second material different from the first material. With the method of the invention it is effectively possible to have a strip and housing in different materials. Since welding is performed when the material is in a paste state, with the method it is possible without difficulty to weld together two different materials having different properties. In the state of the art it is difficult to weld together two different materials (using the LASER weld technique) particularly if their respective melt temperatures lie fairly distant apart. This is a problem in particular if it is desired to weld aluminium, recommended for manufacturing the housing on account of its properties of rigidity and lightweight, with copper which has excellent electrical conduction properties and able to be given advantageous use for fabricating the strip. Far apart melt temperatures may generate numerous technical difficulties notably including difference in shrinkage which does not allow a reliable, durable weld to be obtained of the two materials. By means of the method of the invention it is possible to optimize the choice of constituent materials of the different parts to be assembled (e.g. copper for the strip, aluminium for the housing) since the method does not add any constraint which would limit this choice.   Similarly, the method of the invention can be used to weld aluminium alloys (6000-series type for example) having advantageous mechanical properties but difficult to weld using the LASER weld method.       
 
         [0038]    A further subject of the invention is a module of at least two energy storage assemblies, each energy storage assembly comprising a sealed housing, the assemblies being connected in pairs by means of a connector strip, conforming to the above-described method. 
         [0039]    The weld bead of the strip on the housing is larger than  3  mm, in particular larger than 5 mm in all directions of the plane of the end surface. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0040]    Other characteristics, objectives and advantages of the present invention will become further apparent from the following description which is solely illustrative and non-limiting and is to be read in connection with the appended drawings in which: 
           [0041]      FIG. 1  is radial sectional view of an energy storage assembly of a module according to one embodiment of the invention; 
           [0042]      FIG. 2  is an overhead view of a module comprising two energy storage assemblies according to a first embodiment; 
           [0043]      FIG. 3  is a radial sectional illustration of a friction-stir device allowing implementation of the method according to one embodiment of the invention; 
           [0044]      FIGS. 4A and 4B  are detailed cross-sectional views of the interfaces of the storage assembly and of a connector strip once welded; 
           [0045]      FIGS. 5A to 5C  are perspective views of modules according to variants of embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0046]    A more detailed description will now be given of a method for connecting two assemblies, supercapacitors in particular, according to one embodiment of the invention. 
         [0047]    With reference to  FIG. 1 , an example of embodiment is illustrated of a supercapacitor for which the method of the invention is implemented. 
         [0048]    The supercapacitor  10  comprises a winding  12  positioned in a housing  14  which, in the described embodiment, is a casing  16  comprising a cylindrical side wall  16 A and a bottom  16 B, and a lid  18  capping and closing the casing  16 . The housing also contains a liquid electrolyte, not shown in the Figure for reasons of clarity. 
         [0049]    The casing  16  and the lid  18  are glued over their entire periphery to ensure the sealing of the supercapacitor. The casing  16  and the lid are generally made in aluminium. They are also electrically insulated from each other by means of the continuous bead of adhesive connecting them together to avoid short-circuiting. 
         [0050]    The winding  12  is formed of a unit comprising two foil electrodes  20 ,  21  and an insulating separator  22  that are superimposed, the separator being inserted between the two foil electrodes  20 ,  21 . 
         [0051]    Each foil electrode comprises a collector and an electrode formed of an active material notably containing activated carbon and deposited on the two opposite surfaces of the collector. The collector of each of the electrodes is respectively connected to the casing  16  and to the lid  18 , the casing and the lid therefore respectively forming the positive and negative terminals of the energy storage assembly. 
         [0052]    The lid  18  comprises an upper surface  24  and a cylindrical turned-over edge  26  following the contour of the casing  16 . The upper surface  24  of the lid and the bottom  16 B of the casing form the end surfaces of the housing. The lid also comprises a central protuberance  28  called centring pin in the remainder hereof. 
         [0053]    As indicated above, the energy storage assemblies are often assembled in series to form a module dedicated to a particular application. To assemble the different assemblies in series, it is generally necessary firstly to connect the casing  16  of one assembly to at least one adjacent assembly and secondly to connect the lid  18  to another adjacent assembly. The assemblies connected to the terminals of the module are only connected to one adjacent assembly. 
         [0054]      FIG. 2  gives an overhead view of a module containing two storage assemblies as shown in  FIG. 1 . 
         [0055]    It can be seen that the lids  18  of these two assemblies are assembled via a connector strip  30  arranged on the upper surface  24  of each of the lids. The connector strip  30  is flat and of elongate shape and in the vicinity of each of its ends in its longitudinal direction it comprises an orifice  32  of dimensions mating with those of the centring pin  28  of the strip of the lid  14 . The length of the strip is evidently greater than the diameter of a storage assembly. In particular, its length is between one and three times the diameter of the assembly. The space between the two orifices  32  is also larger than the diameter of the assembly. 
         [0056]    As can be seen in the Figure, the strip is solid but could also comprise hollows and its shape is not fixed. 
         [0057]    A description will now be given of the method for connecting a strip onto the lids. 
         [0058]    At a first step, the strip  30  is positioned on the lids in particular by positioning each of the orifices  32  on a centring pin  28  of the lids  18  of the energy storage assemblies  10 . The centring pins  28  are sized so that they are essentially of the same thickness as the strip. 
         [0059]    Once the strip is in place it is welded onto each of the lids by FSW (Friction Stir Welding). 
         [0060]    For this welding a friction-stir device is used such as described below. This step of friction-stir welding will be explained after first describing the device used. 
         [0061]    The device comprises a body  50  of cylindrical shape extending along an axis of revolution A-A′. The constituent material of the body  50  is steel for example or any type of material of greater hardness than the constituent material of the part to be welded. 
         [0062]    The device also comprises a head  51  extending as far as one of the axial ends of the body  50 . The head comprises a tip  52  of flattened cone shape and a peripheral shoulder  54  extending over a plane substantially perpendicular to the axis of revolution A-A′ of the body  50 . 
         [0063]    The body  50  and the head  51  of the device are adapted so that they can be driven in rotation about an axis of rotation corresponding to the axis of revolution A-A′ of the body  50 . During the weld operation the head of the device enters into the material, heating it to a paste. The heating associated with stirring allows welding of the constituent material of the parts to be welded, here the strip  30  and the lid  18 . After cooling, the finished weld is obtained. 
         [0064]    By means of the device of the invention, the heating of the part is reduced since welding is performed in the paste state. This reduces risks of deterioration of the storage assembly. 
         [0065]    The flattened cone shape of the head of the device allows the material to be trapped and thereby limits chip formation. This flattened cone shape also allows the heated material to be directed against the shoulder to re-plug the hole formed by the head of the device as it travels forward. 
         [0066]    The device also comprises a motor (not illustrated) to drive the body  50  and head  51  of the device in rotation. For example the motor is capable of rotating the body and head of the device at a rate of between 500 and 5000 rpm, and preferably of 1000 rpm. 
         [0067]    Therefore, during this weld step the device  50  is set in operation and it is placed on the assembly so that the axis of revolution A-A′ of the tool merges with the axis of symmetry of the assembly. First the device  50  is positioned so that its head  51  covers both the centring pin  28  of the lid and the strip  30  in the vicinity of the orifice  32 . 
         [0068]    The two parts (strip  30  and centring pin  28 ) lying flush as illustrated in  FIG. 4A , the device  50  is translated along axis A-A′ so that it enters almost simultaneously into the two parts. On account of its rotational movement, it locally turns the material to a paste and mixes the material of the strip and of the centring pin. The centring pin  28  and the strip  30  are then welded edge to edge by means of a weld bead  60 . 
         [0069]    Once the device  50  has entered into the two parts, it is directed so that it moves along the contour of the centring pin  28  after which the device is withdrawn. Once these steps have been performed the material cools and the weld is formed. The bead  60  has the width of the head of the tool i.e. a width of 3 to 5 mm and its pathway is illustrated by the dotted line in  FIG. 2 . 
         [0070]    Next, to reinforce the welding of the strip and lid, the device  50  is positioned on the periphery of the strip  30 . The head of the device is pressed down so that it passes through the strip after the material thereof has been heated, until it reaches the upper surface  24  of the lid which it also enters. The materials of the lid and strip are also mixed through the movement of the device  50  as can be seen in  FIG. 4B , and the device  50  is moved along the periphery of the strip in its part superimposed over the lid. The weld bead  62  also illustrated by a dotted line in  FIG. 2  is continuous and may be several centimetres in length. 
         [0071]    On completion of the welding of the strip  30  onto a storage assembly  10 , the operation is repeated with the other assembly. 
         [0072]    The method of the invention is most advantageous since it allows welding without overheating the material and there is therefore no risk of damaging the electrolyte located inside the housing. 
         [0073]    In addition, it allows weld beads of larger size to be obtained than with LASER welding, which is of advantage to reduce the electric resistance of the module. 
         [0074]    It is noted that the invention is not limited to the described embodiment. 
         [0075]    For example, the energy storage assembly is not necessarily such as described above. It may be parallelepiped and non-cylindrical and/or may comprise two lids positioned at each end of an open tube. The lid may also have a flat end surface and not comprise any centring pin as illustrated in the variants in  FIGS. 5A to 5C . Other centring mean may optionally replace the centring pin such as protuberances following the outer contour of the strip. 
         [0076]    The connector strip may also differ from the description given. It may not have an orifice as illustrated in the variants in  FIGS. 5A to 5C . It may also connect two casings together or a casing to a lid. 
         [0077]    Similarly, the method is not limited to the foregoing description. The strip may only comprise one of the two weld beads described above. It may also be welded to the lid at points other than those described. For example it could be welded using weld spots  70  (as illustrated in  FIG. 5A ) using a circular continuous weld bead  80  forming a circle of radius equal to about one half of the radius of the lid (as illustrated in  FIG. 5B ) or using multiple beads  80 A,  80 B,  80 C,  80 D that are V-shaped and regularly distributed over the end surface of the lid of an assembly (as illustrated in  FIG. 5C ). It is noted that in the variants illustrated in  FIGS. 5A to 5C  the weld beads pass through the strip to reach the lid of the storage assembly and are therefore similar to the bead illustrated in  FIG. 4B . 
         [0078]    It could also be envisaged to implement a method of the invention using a welding device having a different configuration to the one described. 
         [0079]    Finally, a module comprising more than two energy storage assemblies evidently lies within the definition of the invention. The assemblies may also be batteries and/or capacitors, or they may be of different types (one of the assemblies being a battery, the other being a supercapacitor for example).