Abstract:
The invention relates to electric cells which may comprise, for example, two trough-shaped elementary units mounted in a casing, each elementary unit comprising a relatively concavo-convex or trough-shaped positive electrode of compressed, plastified or sintered material, tightly sandwiched between two thin concavo-convex trough-shaped negative electrode parts obtained by folding a single electrode sheet around the positive electrode. Application of the invention is non-exclusively to alkaline storage cells.

Description:
RELATED APPLICATIONS 
     No related applications are co-pending. 
     BACKGROUND OF THE INVENTION 
     This invention relates to cylindrical electric cells and is particularly, but not exclusively, applicable to alkaline storage cells. 
     Such storage cells are known in which the electrodes of the thin sintered carrier type are wound in a spiral with one or two interleaved separators. This technology, while enabling the production of storage cells capable of supplying high currents is nevertheless fairly expensive and becomes relatively more expensive as the dimensions of the storage cell are decreased. 
     There have been proposals to use relatively thicker electrodes in the form of coaxial cylinders which are alternatively positive and negative, the number of coaxial layers of one polarity being then limited to one or two and each cylindrical layer being made of a plurality of cylindrical arcs, for example, two half-cylinders in order to facilitate assembly. A central current collector formed by a metal plate folded in a V, acts as a terminal of one polarity while at the same time urging the half-cylinders radially outwards against one another and against a metal casing which acts as an outer terminal of the other polarity. A disadvantage of such a cell is that the end innermost and outermost electrodes (in the center and at the periphery) have a low efficiency since they are relatively thick and operate with one face only, i.e. the respective faces adjoining the electrodes of opposite polarity. 
     An object of the present invention is to mitigate this disadvantage at reasonable cost. 
     BRIEF SUMMARY OF INVENTION 
     The present invention provides a cylindrical electric cell comprising a casing containing a plurality of parallel-connected elementary units each of which is concavo-convex or trough-shaped and extends over a portion of the arc of a cylinder, the elementary units being juxtaposed within the casing to obtain a substantially entire cylinder with appropriate connections being made between electrodes of the elementary units and the terminals of the cell, wherein each elementary unit comprises a relatively thick concavo-convex or trough-shaped, positive electrode tightly sandwiched with interleaved layers of separator material between inner and outer relatively thin, concavo-convex, trough-shaped, negative electrode parts which are electrically interconnected. 
     The thick positive electrode (having, for example, a thickness of 1 to 3 mm) is thus surrounded on both its faces, by a thin negative electrode (having a thickness of less than 1 mm) and so operates with high efficiency. 
     In a preferred embodiment, the elementary units are substantially half cylinders and, consequently, there are two of them in the casing or can. 
     The positive electrode can be of the compressed type and in that case will, to great advantage, be directly compressed into its concavo-convex shape, or of the plastified type, i.e. in which the active material, to which an electric conductor may be added in divided form, is mixed with a binding agent and pasted on a current collector. The plastified type of electrode will preferably be shaped to requisite concavo-convex form after manufacture as a plane electrode. 
     The negative electrode parts are also concavo-convex in shape and preferably of the thin sintered carrier type and are preferably connected together by a portion of their common collector free from sintered material folded along an end edge of the arcuate positive electrode, and preferably coated with an insulating material such as insulative varnish at that portion. 
     In another embodiment, the cell comprises at least two coaxial cylindrical sets of elementary concavo-convex or arcuate units having the same height but different diameters with at least one outer set surrounding one inner set. 
     In order to keep the positive and negative electrodes closely pressed against each other, the sets pressed against each other and the outer set pressed against the casing which can be made of metal and can be used as a negative current collector, the central axially extending tubular space of the cell can be occupied by an insulating rod equipped with resiliently deformable fins which exert an outwardly directed force on the sets when inserted into said space. 
    
    
     Embodiments of the present invention are described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 is a longitudinal sectional view of a cell embodying the invention in cross-section taken along a line I--I of FIG. 2; 
     FIG. 2 is a horizontal cross-section of the cell of FIG. 1 along the line II--II of FIG. 1; 
     FIG. 3a is a cross-section of a positive electrode of the plastified type before shaping; 
     FIG. 3b is a cross-section view of the positive electrode of FIG. 3a after shaping to arcuate form; 
     FIG. 4a is a cross-section view of a negative electrode after sintering and impregnation but before shaping; 
     FIG. 4b is also a cross-section view of the negative electrode of FIG. 4a after arcuate shaping of parts intended to form inner and outer negative electrode components adapted to lie respectively in the concave and surround the convex faces of the arcuate positive electrode; 
     FIG. 5 shows an intermediate stage in the assembly of the shaped electrodes of FIGS. 3b and 4b; 
     FIG. 6 is a partial cross-section view of an insulating rod shown in FIG. 1, but before assembly in the cell, and 
     FIG. 7 is a horizontal cross-section of another embodiment of a cell according to the invention. 
    
    
     DETAILED DESCRIPTION 
     The examples of the invention described are of alkaline nickel-cadmium storage cell. The invention also is applicable to other types of electrical cells. 
     In FIG. 1, the cell has a cylindrical casing 1 made of nickel-plated steel. It is closed at its upper part by a cover 2, also made of nickel-plated steel, which rests on a shoulder defined by an inwardly projecting groove 3 formed at the upper part of the casing 1. An insulative sealing ring 4 surrounds the periphery of the cover 2 and serves to insulate it electrically from the casing 1. The upper rim part of the casing 1 is turned down onto the sealing ring 4 after assembly of the cells components therein and ensures a fluid-tight seal by pressing the assembled sealing ring 4 and cover 2 against the shoulder defined by the groove 3. The cover 2 supports a valve which comprises a metal cap 5, welded to the cover 2, and containing an elastomer disk 8 plugging a hole 7 formed in the center of the cover 2. The cap 5 serves as one of the cell terminals. All these arrangements are well-known and do not form a part of the invention. 
     Referring to FIGS. 1 and 2, the casing 1 contains two elementary units electrically connected in parallel. Each of the elementary units is concavo-convex or trough-shaped, extending over an arc of substantially 180° and comprises a positive electrode 91 or 92 respectively sandwiched in the radial direction between an inner negative ultimately concavo-convex electrode part 82 or 83 and an outer ultimately concavo-convex negative electrode part 81 or 84. The corresponding inner and outer negative electrode parts, 82 and 81 or 83 and 84 are electrically interconnected by portions 85 and 86 as will be presently described. The outer convex surfaces of negative electrode parts 81 and 84 are pressed against the inner convex surface of the wall of the casing 1 by the action of radially-resilient fins 16 of insulating rod 15 which occupies the tubular axially extending space in the casing 1 defined by assembled units in said casing and which fins urge the elementary units outwardly, thus providing mechanical support for the units in the casing 1 as well as promoting the electrical connection between the casing 1 and the abutting convex surfaces of the negative electrode parts 81 and 84. The positive electrodes 91 and 92 are electrically connected to the cover 2 and hence to the positive terminal cap 5, by respective leads 10 and 11. 
     The negative electrode parts are made of pulverulent nickel layers sintered on a core of perforated metal foil and impregnated with active material. Corresponding pairs of negative electrode parts 81 and 82 or 83 and 84 share the same metal core 85 or 86 which provides the electrical connection between the parts of each pair. The base or sinter free portion of the metal core 85 or 86 which interconnects the corresponding pairs of negative electrode parts is folded around one of the straight or end edges of the positive electrode 91 or 92. These bare portions of the metal cores 85 and 86 are preferably and advantageously coated with an insulating varnish or the like to prevent short circuits to the positive electrodes. 
     The positive electrodes 91 and 92 are made of plastified material pasted on respective current collecting cores 93 and 94 which are electrically connected to the leads 10 and 11. 
     The respective positive electrode and negative electrode parts of each elementary unit are separated by separators 12 and 13 which fold round the same straight end edges of the positive electrodes 91 and 92 respectively as the edges round which the metal cores 85 and 86 of the negative electrode parts are folded. It will be observed, in FIG. 2, that even if the ends of the separators 12 and 13 surrounding the positive electrodes are long enough to fold around the opposite edges of the positive electrodes there is nevertheless a danger of short circuiting with the bare portion of the negative electrode of the other elementary unit (i.e. the positive electrode 92 may short to the negative electrode core 85 and similarly 91 to 86) hence it is preferable as has been said above, to coat these bare portions with an insulating varnish. Moreover, if this precaution is not taken, there is further risk of these bare portions becoming a seat of gas evolution which is to be avoided, especially in sealed storage cells. 
     The bottom of the casing 1 is provided with an insulating washer 14 which prevents any contact between the positive electrodes 91, 92 and the casing 1. 
     An insulating rod 15 is made of plastic material and is provided with deformable fins 16 which are compressed into the form of truncated cones whose generator lines are slightly curvilinear when the rod is installed in the tubular space of the casing 1 defined by the innermost concave surfaces of the innermost negative electrode, and which fins or disks 16 provide the above-mentioned radially outward resilience. 
     FIGS. 3a to 5 show different components of an elementary unit at various stages of assembly. 
     FIG. 3a is a cross-sectional view of the positive electrode 92 taken perpendicular to its longest side. A mixture of nickel hydroxide, graphite (or nickel powder) and a binding agent such as polytetrafluoro-ethylene is pasted on the current collecting core 94 which is, for example, made of expanded metal such as nickel-plated steel. The final thickness of the electrode is 2 to 3 millimeters. The electrode is then arcuately shaped into a concavo-convex or trough-shaped half cylinder as shown in FIG. 3b. Positive electrode 91 is similarly made and shaped. Lead wires 10 and 11 are respectively provided for cores 93 and 94 of said positive electrodes 91 and 92. 
     FIG. 4a is a cross-sectional view of the parts 83 and 84 of the negative electrode such as they are after impregnation, (and after severing from a continuous length, if they are obtained through a continuous method). As shown, the sintered parts 84 and 83 borne by the nickel-plated foil collector 86 are separated by a bare portion of the foil which has been bared as by wiping off therefrom in portion 86 after coating with nickel powder and before the sintering operation. The electrode is then impregnated by the usual methods with cadmium hydroxide. The arcuate shaping of the respective parts 84 and 83 of this unit does not cause any difficulty since the thickness of the negative electrode parts 83 and 84 is about 0.7 mm. As shown in FIG. 4b the concave surfaces or troughs of the concavo-convex half cylinders 84 and 83 face in opposite directions. Half cylinder 83 is dimensioned to fit into the concave surface of a positive electrode e.g. 92, while half cylinder 84 is dimensioned to fit around the convex surface of said positive electrode. FIG. 5 shows the assembling of the positive half-cylinder electrode 92 with the negative electrode parts to form one of the elementary units. As shown, the separator 13 is first placed in the concave trough of the negative half cylinder 84, allowing a much greater length of separator to over-hang on the side of the part 86 than on the opposite side. The positive electrode 92, whose convex surface must closely engage the concave face of the negative electrode part 84 covered by the separator 13, is then positioned thereon. The half cylinder part 83 is then turned down together with the overhanging part of the separator 13, so as to locate itself in the concave trough of the half cylinder positive electrode 92, it being necessary for the convex face of the half cylinder negative electrode part 83 closely to engage the concave face of the positive electrode 92 when lined with the separator 13. The other elementary unit consisting of positive electrode 91 surrounded by separator 12 and sandwiched between negative electrode parts 81 and 82 is similarly assembled. The assembled elementary units are then placed in the casing 1, portions of the groove 3 being of slight depth to facilitate insertion of said units into the casing. The leads 10 and 11 are welded to protruding parts of the positive current collecting cores 93 and 94 and to the cover 2 which is already fitted with the sealing ring 4. The requisite electrolyte is added to the casing and then the rod 15 is inserted in the center of the casing 1. 
     The rod 15 is shown in a cross-sectional view in FIG. 6 before insertion into the axially extending tubular space within casing 1. This rod 15 is made of insulative plastic material provided with thin circular discs or fins 16 projecting perpendicular to its axis, thus allowing it to be directly manufactured as by moulding. These discs or fins 16 having an outside diameter greater than that of the hollow axial tubular space defined by the concave sides of the half cylinder negative electrode parts 82 and 83 they deflect during insertion of rod 15 into said space and thereby exert a centripetal bearing force on the two elementary units in the casing so that the electrodes of each unit are pressed hard against each other and the outermost faces of the outermost negative electrode parts 81 and 84 are pressed hard against the inner wall of the casing 1, thus insuring good electrical contact therewith. The cover 3 is then placed in position and a crimping operation is performed on the upper edge of the cup or casing 1 to crush the sealing ring 4 against the shoulder defined by the groove, thus completing the cycle of operations of the cell. 
     If storage cells having a higher capacity and a larger diameter are required, another analogous set of elementary units, whose inside diameter is equal to the outside diameter of the first described set is disposed concentrically around the first set. This is depicted in FIG. 7 in which the same reference characters as in FIG. 2 have been given to the same parts of the innermost set of elementary units. A second set, also made of two units, concentrically surrounds the first set, the concave trough of the innermost negative half cylinders 82&#39; and 83&#39; of this outer set is in contact with the convex face of the outermost negative half cylinders 84 and 81 of the inner set, the two sets then being coaxial. The positive electrodes 91&#39; and 92&#39; are sandwiched between the negative electrode parts 82&#39; and 81&#39;, 83&#39; and 84&#39; respectively and provided with the interleaved separators 12&#39; and 13&#39;. The two sets are urged outwardly against each other and the second set is urged against the casing 1 by the pressure exerted by the fins 16 of central rod 15. 
     Of course, additional sets such as a third set or more could be added to the first two in the same way for still larger cells. However, the invention is more particularly advantageous for application to cells having a small diameter. 
     While specific embodiments of the invention have been described and shown, variations in structure within the scope of the appended claims are possible and are contemplated. There is no intention of limitation to the exact disclosure herein presented.