Abstract:
An electrode comprises a porous three-dimensional conductive support and at least one reinforcing metal band fixed along a longitudinal edge portion of the support and having a coefficient of elongation to rupture equal to at least 20% in a direction parallel to the edge portion of the support. Two longitudinal edge portions of the band are bent against the same face of the band, at least part of whose surface is pressed against the support.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrode intended to be wound into a spiral, of the paste-coated type whose three-dimensional support is a conductive metal foam, for example. It relates more particularly, although not exclusively, to alkaline electrolyte cylindrical nickel-cadmium (Ni—Cd) or nickel-hydridable metal (Ni—MH) storage cells used for portable applications. 
     It further encompasses the method of manufacturing the electrode and its use in a storage cell. 
     2. Description of the Prior Art 
     A paste-coated electrode comprises a conductive support which can be a two-dimensional or plane support, such as solid or perforated strip, expanded metal, mesh or woven fabric, or a three-dimensional support such as foam or felt. The support is coated with a layer of paste containing the electrochemically active material and usually a binder. 
     When the storage cell is assembled, the electrode must be electrically connected to the current output terminal, generally by means of a connection fixed to the inside part of the terminal and to the conductive support of the electrode. The connection is preferably welded to the support to produce a reliable electrical contact. 
     In the case of cells with a spiral-wound electrode assembly it is advantageous to be able to weld the connection directly to the edge of the electrode. When the electrode support is a three-dimensional porous support with a large volume, its edge does not have the solidity needed for a connection to be welded directly to it. A number of documents therefore propose to reinforce the edge portion of the support before fixing the connection to it. 
     The support can be reinforced by fixing a reinforcing band in the form of a metal tape along the longitudinal edge portion. 
     Japanese patent application JP-62 219 462 describes a fibrous support electrode whose edge portion is reinforced by a metal band bent into a U-shape straddling and spot-welded to the support. A connecting tongue is then fixed to the band. 
     The use of a reinforcing band with a cut-out edge portion has been suggested. The band is compressed onto the support so that the cut-outs are inserted into the pores of the support to anchor the band to the support. 
     Japanese patent application JP-62 237 665 proposes to use a metal plate bent into a U-shape with a crenellated edge. The plate is compressed onto the felt support so that the upstanding portions are anchored to the support. 
     British patent GB-2 055 899 describes a porous support electrode. The reinforcement is a conductive plate whose edge is cut to a sawtooth shape and bent at right angles to the plate. Pressure is applied to the teeth so that they penetrate into the support. A metal deposit then consolidates the assembly. 
     A reinforcing band of this kind is costly to make and difficult to fit in a reliable and reproducible manner. 
     Another solution is to use a reinforcing band consisting of a very porous material, which can be the same as the material of the support. The reinforcing band is compressed onto the support so that the two materials interpenetrate. 
     European patent EP-0 658 946 describes a plane electrode whose edge portion is reinforced by compressing a plurality of layers of foam similar to that constituting the support. 
     According to Japanese patent application JP-57 080 672 the edge portion of a felt support is bent on itself one or more times and compressed. 
     All the reinforcing methods previously described stiffen the longitudinal edge portion of the electrode, making the flexing imposed by subsequently winding the electrode into a spiral very difficult. 
     European patent application EP-0 516 535 proposes to provide on each side of the edge portion of the electrode an expanded metal or perforated strip connection with a voids ratio exceeding 40% and fixed by compression. 
     This solution is difficult to use because of the high risk of short circuits. The short circuits are caused by the free metal strands at the edge of the connection, which can perforate the separator and come into contact with the opposite polarity electrode. 
     The object of the present invention is to propose a spiral-wound electrode with a three-dimensional support having a longitudinal edge portion which is reinforced in order to receive a connection welded to its edge, which is easier to wind into a spiral and for which there is a lower risk of short circuits than for prior art electrodes. 
     SUMMARY OF THE INVENTION 
     The invention consists in an electrode comprising a porous three-dimensional conductive support and at least one reinforcing metal band fixed along a longitudinal edge portion of the support and having a coefficient of elongation to rupture equal to at least 20% in a direction parallel to the edge portion of the support, wherein two longitudinal edge portions of the band are bent against the same face of the band, at least part of whose surface is pressed against the support. 
     This prevents short circuits associated with the presence of metal strands escaping from the edge portion of the band by enclosing those strands between the band and the electrode support. 
     To preserve the longitudinal flexibility which is indispensable for winding the electrode into a spiral, the reinforcing band must itself be highly flexible, which is possible only if the band is capable of sufficient elongation without deformation. This property is assessed by the coefficient of elongation to rupture, defined as the difference between the length at rupture and the initial length divided by the initial length of the sample, which is measured in the following manner. 
     A sample of the material to be characterized is inserted between the jaws of a tensile test machine. The length L 0  between the jaws is 100 mm and the width of the sample is that of the band that will be used to reinforce the edge portion of the electrode. The strain is applied at a rate of 100 mm/min. The length at rupture Lr is determined from the response curve of the traction force as a function of elongation. 
     The coefficient of elongation to rupture y expressed as a percentage is calculated from the equation: 
     
       
         γ=100 (Lr−L 0 ) /L 0   
       
     
     The band is preferably expanded metal, mesh or perforated strip with a voids ratio of at least 40%, and preferably at least 50%. The voids ratio is defined as the ratio of the surface area of the voids to the total surface area of the band. 
     The band can be bent into a U-shape in its lengthwise direction so that it can straddle the edge portion of the electrode, its bent edge portions being trapped inside the U-shape. Two bands with both edge portions bent in this way can be provided on respective opposite sides of the support. 
     The band is preferably provided on only one side of the support, advantageously the side of the support which is on the outside during winding into a spiral. The band is then subjected to traction and stretches without deforming. If it were instead placed on the inside of the spool, it could become deformed and this deformation could cause increases in thickness compromising the spiral winding. 
     The support is usually a nickel foam. The band can be made of nickel, nickel-plated steel or stainless steel. 
     The band is advantageously welded to the support. The fixing of the band to the support is then more solid and the electrical continuity is reliable. The welding process can be a spot welding, knurled wheel welding with contiguous spots or ultrasound welding. 
     The invention also consists in a storage cell comprising a spiral-wound electrode of the above kind forming a spool and electrically connected to a terminal by a connection member welded to the edge of the spool and to the terminal. 
     The material of the connection member can be nickel, nickel-plated steel or stainless steel. The band is preferably made of the same metal as the connection member to facilitate welding. 
     In a first embodiment of the invention the electrode contains electrochemically active material in the form of nickel-based hydroxide. 
     In a second embodiment, the electrode contains a cadmium-based electrochemically active material. 
     In a third embodiment of the invention, the electrode contains electrochemically active material in the form of a metal alloy capable of absorbing hydrogen to form a hydride. 
     In a fourth embodiment of the invention the electrode contains electrochemically active material in the form of a carbon-containing material into whose structure lithium can be inserted, for example graphite, coke, carbon black and vitreous carbon. 
     In a fifth embodiment of the invention the electrode contains electrochemically active material in the form of an oxide of a transition metal, such as vanadium oxide, lithium-containing oxides of manganese, nickel and cobalt and lithium-containing oxides containing nickel, cobalt, manganese or aluminum, for example. 
     The invention will be better understood and other advantages and features of the invention will become apparent on reading the following description which is given by way of non-limiting example and with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a first embodiment of an electrode in accordance with the present invention. 
     FIG. 2 is a side view of a first embodiment of the electrode in accordance with the invention. 
     FIG. 3 is a diagrammatic view of a spool and a connection member. 
     FIG. 4 shows a second embodiment of the invention. 
     FIG. 5 shows a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be clearly understood that the drawings and the corresponding parts of the description are given entirely by way of illustration of the subject matter of the invention, of which they do not in any way constitute a limitation. 
     EXAMPLE 1 
     An electrode  1  in accordance with the invention shown in FIGS. 1 and 2 was manufactured with a current collector support in the form of a nickel foam band approximately 250 mm long and 32 mm wide. A paste containing a binder and the electrochemically active material, which was in the form of a nickel-based hydroxide, was introduced into the pores of the foam. 
     A band  2  of stainless steel mesh approximately 6 mm wide and the same length as the electrode  1  was cut so that its longitudinal axis was parallel to the diagonals of the mesh. The longitudinal edge portions  3  and  4  were bent against the same face of the mesh  2 , partially overlapping to form a narrower band whose edge portions did not have any free strands. 
     The active material was removed from one longitudinal edge portion  5  of the electrode  1  to a distance of approximately 1 mm to 2 mm by blowing with compressed air and ultrasound treatment. The mesh  2  was applied to the edge portion  5  so that the bent edge portions  3 ,  4  of the mesh  2  were placed against the electrode  1 . This trapped the free strands of the mesh against the foam. 
     The mesh  2  was fixed to the electrode  1  by electric spot welding with the spots 1 cm apart. The electrode  1  was then compressed to its final thickness. 
     The positive electrode  1  formed as above, a polyamide separator and a negative electrode with a cadmium-based electrochemically active material were then superposed to form a stack which was wound into a spiral to form a spool  30  as shown in FIG. 3, taking care to place the side of the electrode carrying the band toward the outside of the spool  30 . 
     A connection member  31  in the form of a nickel-plated steel disc incorporating openings  32  flanked by two lips which came into contact with the spool was then welded to the mesh  2  reinforcing the edge of the spiral-wound positive electrode  1 . 
     After placing it in a container, the spool was impregnated with electrolyte using in particular the central orifice  33  in the piece  31 . The alkaline electrolyte was a mixture of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium hydroxide (LiOH) in aqueous solution. 
     The connection member  31  carried a lug  34  whose opposite end was fixed to the inside part of a current output terminal after adding the electrolyte. The cell  5  was closed by crimping a lid to the container. 
     EXAMPLE 2 
     An electrode  41  in accordance with the invention shown in FIG. 4 was manufactured using a current collector support and an active layer analogous to those of example 1. 
     An expanded nickel-plated steel band  42  approximately 4 mm wide and the same length as the electrode  41  was prepared. The longitudinal edge portions  43  and  44  were bent onto the same face of the expanded metal  42 . The bent portion of each edge portion  43 ,  44  had a width at most equal to half that of the band  42  so that the edge portions  43 ,  44  did not overlap. A narrower band whose edges did not have any free strands was obtained in this way. 
     As in example 1, the active layer was removed from one longitudinal edge portion  45  of the electrode  41  and the expanded metal  42  was applied along the edge portion  45  so that the bent edge portions  43 ,  44  of the expanded metal  42  were placed against the electrode  41 . The free strands of the deployed metal were trapped against the foam in this way. 
     The expanded metal  42  was fixed to the electrode  41  by knurled wheel welding with contiguous spots. The electrode  41  was then compressed to its final thickness. 
     The positive electrode  41  formed as above, a polyolefin separator and a negative electrode whose electrochemically active material was a metal alloy capable of absorbing hydrogen were then superposed to form a stack which was then spiral-wound to form a spool. 
     A nickel-plated steel connection member analogous to that of FIG. 3 was welded to the expanded metal  42  reinforcing the edge of the spiral-wound positive electrode  41  and carried a lug whose opposite end was fixed to the inside part of a current output terminal. 
     After inserting it into a container, the spool was impregnated with an alkaline electrode analogous to that of example 1. The cell was closed by crimping a lid to the container. 
     EXAMPLE 3 
     An electrode  51  in accordance with the invention as shown in FIG. 5 was manufactured. A band  52  of a mesh analogous to that of example 1 was prepared. The longitudinal edge portions  53  and  54  were bent against the same face of the mesh  52  so that the edge portions  53 ,  54  did not touch. The band  52  was then bent in two to form a U-shape along a longitudinal axis. The bent band  52  then straddled the edge portion  55  of the electrode from which the active layer had previously been removed. The band  52  was finally welded to the edge portion  55 .