Abstract:
A storage cell comprises at least two electrodes with different polarities disposed face-to-face on respective opposite sides of a separator and a metal strip fixed along an upper edge of a first of the electrodes and extending at least as far as one end of the upper edge. The part of a second of the electrodes facing the strip in line with the aforementioned one end is partly cut away by means of a cut-out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a storage cell comprising at least one electrode one edge of which is reinforced by an attached strip. 
     2. Description of the Prior Art 
     A storage cell comprises an electrode assembly comprising at least one positive electrode and at least one negative electrode disposed face-to-face on respective opposite sides of an electrically insulative separator. The separator is generally a polymer material felt or membrane impregnated with electrolyte. 
     When the storage cell is being assembled, the electrode of each polarity must be electrically connected to the respective output terminal. The connection is usually made by a thin metal band which may be cut to a required contour and is fixed to the internal part of the terminal and to the conductive support of the electrode. 
     To make the fixing of the band sufficiently reliable, the edge of the electrode is reinforced, at least in the fixing area, but more reliably so throughout its length. The band can be welded to the reinforcement or integral with it, as described in French patent 2 705 834. 
     The member reinforcing the electrode is advantageously a thin metal tape applied to one or both faces and parallel to the edge of the electrode. It is welded in place. 
     At the end of the tape there is a short distance between the edge of the tape and the last weld. This enables one of the straight corners at the end of the tape to bend as a result of mechanical impact during handling, for example by rubbing on the working surface, when the electrodes are being stacked up or the electrode assembly is being assembled. The separator is mechanically weak and easily torn by the resulting sharp corner, and so a short circuit is inevitable. The object of the present invention is to propose a storage cell in which the risk of a short circuit occurring under the conditions just described is minimized. 
     SUMMARY OF THE INVENTION 
     The present invention consists in a storage cell comprising at least two electrodes with different polarities disposed face-to-face on respective opposite sides of a separator and a metal strip fixed along an upper edge of a first of the electrodes and extending at least as far as one end of the upper edge, wherein the part of a second of the electrodes facing the strip in line with the aforementioned one end is partly cut away by means of a cut-out. 
     The part of the tape of the second electrode that is cut away is the one whose corner faces the corner of the first electrode which is the origin of the short circuits. For reasons of reliability, the cut-out encroaches partly on the part of the electrode coated with active material. 
     In a preferred embodiment of the invention, the cut-out has first and second sides merging with each other via a rounded portion and merging via respective rounded portions with an upper edge of the strip and a lateral edge of the second electrode. 
     The first end of the cut-out is a straight line segment which intersects a lower edge of the strip at an angle greater than 90°. This avoids the creation of an acute angle at the intersection which is more vulnerable to mechanical impact. 
     The second side of the cut-out intersects the lateral edge of the second electrode at an angle greater than 90°, for the same reason as previously. 
     The cut-out at one end of the strip advantageously has different dimensions to the cut-out at its other end. This slight asymmetry allows for the direction of movement of the tool for cutting the heads of the plates in the metal strip. This facilitates cutting and eliminates the wastage of material along the length of the strip of electrodes. 
     A single strip 0.2 mm thick, for example, can be applied to one face of the electrode. Two strips, each 0.1 mm thick, for example, can be placed on respective opposite sides of the electrode, and can consist of the same strip bent in two and straddling the edge of the electrode. 
     The strip is preferably spot welded or knurled wheel welded or ultrasound welded to the first electrode. The weld assures reliable electrical continuity between the current collector of the electrode and the strip. 
     The electrodes can be of the sintered or non-sintered type. A non-sintered electrode comprises a conductive support and a paste containing the electrochemically active material and a binder. 
     The electrode support can be a two-dimensional support such as expanded metal, a solid or perforated strip, a grid or a woven fabric, or a three-dimensional support such as a foam or a felt. 
     The present invention applies to storage cells with an alkaline electrolyte, such as nickel cells, or an organic electrolyte, such as lithium cells. The electrodes can be plane or spiral. 
    
    
     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 partial view of a positive electrode of a storage cell in accordance with the present invention. 
     FIG. 2 is a partial view of a first embodiment of a negative electrode of a storage cell in accordance with the present invention. 
     FIG. 3, which is analogous to FIG. 2, is a partial view of a second embodiment of a negative electrode in accordance with the present invention. 
     FIG. 4 shows an electrode assembly comprising a positive electrode and a negative electrode between which is a separator (not visible in the figure). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be 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 and in no way constitute any limitation on the scope of the invention. In particular, the various embodiments described and shown and the various numerical applications specified are given purely by way of non-limiting example. 
     The storage cell in accordance with the invention comprises an electrode assembly made up of at least one positive electrode and at least one negative electrode with a separator between them. The storage cell described by way of example is a nickel-cadmium storage cell with an alkaline electrolyte and plane electrodes. 
     The positive electrode  1  is in the form of a plane and substantially rectangular plate whose dimensions are approximately 143 mm high by 98 mm wide by 0.9 mm thick. The electrode  1  comprises a nickel foam support containing a paste containing the electrochemically active material, which in this example is a nickel hydroxide. 
     A strip  2  approximately 5 mm wide along the top edge of the electrode  1  is not coated with active material. As shown in FIG. 1, a wide nickel-plated steel tape  3  is applied to cover the strip  2  and to overlap slightly the part  4  coated with active material. The tape  3  is held in place along the edge of the electrode  1  by regularly spaced spot welds  5 . 
     The tape  3  is then cut to form a plate head  6  for electrically connecting the electrode  1  to the output terminal of the cell. Pressing or rolling after cutting reduces the overthickness created by the tape  3  in the overlap area. The connection between the plate head  6  and the terminal is made by means of a bolt passed through a perforation  7  in the plate head  6 , for example, or by welding, in particular by resistance, ultrasound or laser welding. 
     The end of the tape  3  coincides with the lateral end of the electrode  1 . After cutting, the upper part of the tape  3  is rounded with a radius R 1 =4 mm at the end  8  nearer the plate head  6  and a radius R 2 =2 mm at the opposite end  9 . The corner  10  between the lower edge  11  of the tape  3  and its end  8  covers the active material. During handling of the electrode (stacking, rubbing, etc) the corner  10  can be raised because of mechanical stresses. There is a risk of it piercing the separator and forming a short circuit to the contiguous negative electrode. In a symmetrical fashion, the corner  12  at the opposite end  9  can also be raised. 
     In a first variant of the invention, the negative electrode  21  is in the form of a plane and substantially rectangular plate whose width is in the same order of magnitude as that of the positive electrode  1 . In this example the electrode comprises a support in the form of a nickel-plated perforated strip and is coated with a paste containing cadmium hydroxide. 
     As shown in FIG. 2, in a similar manner to the positive electrode  1 , a metal tape  22  is fixed along the upper edge of the negative electrode  21  where there is no active material. To prevent the corner  10  of the tape  3  of the positive electrode  1  coming into contact with the facing part of the tape  22 , a cut-out  23  is formed in the part of the tape  22  at the same end as the edge of the negative electrode  21 . 
     The side  24  of the cut-out  23  merges with the upper edge  25  of the tape  22  through a rounded portion of radius R 3 =3 mm. The side  24  is a straight line segment approximately 2 mm long. At its intersection  26  with the lower part  27  of the tape  22 , the segment  24  is at an angle α of 100° to the part  27 . The bottom of the cut-out  23  is substantially semicircular with a radius R 4 =2 mm. 
     The other side  28  of the cut-out  23  merges with the lateral edge  29  of the electrode  21  through a rounded portion of radius R 5 =4 mm. 
     At the other end of the tape  22  is a cut-out  23 ′ similar to the cut-out  23  except that the side  28 ′ merges with the lateral edge  29 ′ of the electrode  21  through a rounded portion of radius R 6 =2 mm. This asymmetry allows for the direction of movement relative to the electrode  21  of the tool for cutting the plate head  30 . 
     In a second embodiment of the invention, the negative electrode  31  is in the form of a plane and substantially rectangular plate whose length is in the same order of magnitude as that of the positive electrode  1 . In this example the electrode comprises a support in the form of a nickel-plated perforated strip coated with a paste containing a hydridable metal alloy. 
     As shown in FIG. 3, in a similar manner to the positive electrode  1 , a metal tape  32  is fixed along the upper edge of the negative electrode  21  where there is no active material. To prevent the corner  10  of the positive electrode  1  from coming into contact with the facing part of the tape  32 , a cut-out  33  is made in the part of the tape  32  at the end of the edge of the negative electrode  31 . A plate head  40  is provided for electrically connecting the electrode  31  to an output terminal. 
     The side  34  of the cut-out  33  merges with the upper edge  35  of the electrode  31  through a rounded portion of radius R 7 =3 mm. The side  34  is a straight line segment approximately 2 mm long. At its intersection  36  with the lower part  37  of the tape  32  the side  34  is at an angle α′ of 100°. The bottom of the cut-out  33  is substantially semicircular with a radius R 8 =2 mm. 
     The other side  38  of the cut-out  33  merges with the lateral edge  39  of the electrode  31  through a rounded portion of radius R 9 =10 mm. 
     At the other end of the tape  32  is a cut-out  33 ′ similar to the cut-out  33  except that the side  38 ′ merges with the lateral edge  39 ′ of the electrode  31  through a rounded portion of radius R 10 =8 mm. 
     This embodiment has the advantage that the negative electrode  31  is better protected against impacts during handling. 
     In FIG. 4, the positive electrode  1  and the negative electrode  31  are disposed face-to-face so that the plate heads  6  and  40  of opposite polarity are offset relative to each other. The corner  10  of the tape  3  of the positive electrode  1  is opposite the cut-out  33  of the negative electrode  31 . If clumsy handling causes the corner  10  to become raised, there can be no short circuit, despite tearing of the separator, because the cut-out  33  prevents the corner  10  from coming into contact with the negative electrode  31  in accordance with the invention. 
     Type A cells comprising at least one prior art positive and negative electrode pair  1 , type B cells comprising at least one positive electrode  1 /negative electrode  21  pair in accordance with the invention and type C cells comprising at least one positive electrode  1 /negative electrode  31  pair in accordance with the invention were assembled. 
     The number of short circuits that occurred during electrical conditioning of the type A, B and C cells was counted. The results are set out in the table below. The short circuit rate is defined by the number of short circuits observed per 1000 000 cells. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Cell type 
                 Short circuit rate 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 50 
               
               
                   
                 B 
                 4 
               
               
                   
                 C 
                 0