Patent Publication Number: US-2023148394-A1

Title: Holder for an electrode of a button battery and a battery provided therewith

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. 21207278.9, filed on Nov. 9, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention is related to button-shaped batteries, in particular to a support means for the cathode of such a battery. 
     STATE OF THE ART 
     Button-shaped batteries, also referred to shortly as ‘button batteries’ or ‘button cells’ are well known. Various types of button batteries are in use, distinguished by various dimensions and by the materials used for the electrodes and the electrolyte. A commonly used type for low voltage appliances such as wristwatches or the like is often referred to as a ‘CR’ type battery, the C referring to the chemistry of the electrodes and the R to the round shape of the battery. CR batteries comprise a lithium-based anode and a cathode comprising manganese dioxide as active material. The electrolyte may be a solid or liquid organic material. Other material combinations are known under various other letter codes, such as BR, SR, etc. 
     The cathode of a CR battery is usually in the form of a tablet produced by compacting a powder mixture comprising the active material, a binder, carbon and/or graphite to reduce the internal resistance, and other additives. According to one well-known technique, the mixture is pressed into the shape of a tablet with a mesh at the bottom. The mesh is added to increase the strength of the tablet and to provide a better electrical contact between the tablet and the metal can which lies under the tablet and which forms the positive terminal of the battery. 
     During discharge, the tablet expands because of the ingress of lithium ions migrating from the anode. As the amount of lithium ions increases in the tablet, the tablet expands in all directions. Problems resulting from this expansion include: 
     Losing contact with the mesh: the mesh is pressed onto the tablet without any binding agent. As the tablet expands, the grip on the mesh by the powders becomes looser, and finally the two become separated. 
     While the tablet expands during discharge, the anode, made of metallic lithium, is consumed. In an ideal situation, the expansion rate of the cathode tablet in the height direction is greater than the rate at which the lithium anode recedes, so that a conductive path between the two electrodes is maintained. However, as there is no restriction in the direction of the tablet&#39;s expansion, the tablet not only grows in the height direction, but also in the radial direction. This causes the rate of expansion toward the lithium anode to slow down, eventually leading to a loss of a conductive path between the electrodes. 
     A known solution to these problems is to apply a metal ring-shaped holder instead of a mesh, with the tablet pressed inside the ring-shaped holder. The holder forms a wall around at least part of the tablet that directs the expansion of the tablet in the height direction only, towards the anode, thus maintaining the conductive path between the anode and the cathode. However, the ring has an open bottom and therefore only substantially supports the tablet along its circumference. For this reason, the tablet is now more likely to expand towards the positive can during discharge, in the direction away from the anode. This expansion pushes itself against the can, and lifts the holder away from the can, resulting in the loss of electrical contact between the cathode and the can. 
     Other holder designs have been proposed to alleviate these particular problems, including a holder not having an open bottom but provided with a grid on the bottom of the holder, as disclosed for example in patent publication documents CN103606684 and CN203553277. In these latter examples, the grid exhibits a staggered pattern, consisting of parallel strips which are bent repeatedly inward and outward relative to the bottom plane of the holder. The inwardly bent portions of the strips may imbed themselves into the powder during tablet pressing, increasing the strength of the tablet. The outwardly bent portions of the strips are in direct contact with the can of the battery and exhibit a degree of elasticity, which enables maintaining a good contact between the tablet and the can. Also, when a liquid electrolyte is used, the outwardly bent portions provide a space between the can and the bottom of the tablet, where electrolyte can be collected as a reserve. 
     However, when a tablet is pressed into a holder of this type during the manufacturing process, the force required to press the powders together is applied directly on the inwardly and outwardly bent portions. Unless the holder is made from metals with very high strength, these portions may be flattened, thereby reducing or eliminating the intended beneficial effect of the staggered patterns. In particular, when the outwardly bent portions are flattened during tablet pressing, said space for collecting a liquid electrolyte reserve is eliminated. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a solution to the above-described problems. This aim is achieved by an electrode holder, a holder/tablet assembly and a battery in accordance with the appended claims. 
     According to the invention, an electrically conductive holder is provided that is suitable for holding a tablet electrode of a button battery pressed into the holder. The holder comprises a bottom portion having a circular circumference. The holder further comprises at least three supports protruding outward from said bottom portion, said supports being positioned along the circumference of the holder in such a manner that the holder may be supported on a planar surface by said supports. The holder may further comprise a wall portion rising upward from the circumference of the bottom portion with the supports protruding from the bottom portion in the direction opposite the wall portion. The invention is equally related to an assembly comprising said holder and a battery electrode tablet pressed into the holder, and to a button battery comprising said assembly. The invention is further related to a method for producing a holder/tablet assembly by a compression process that uses a stamp provided with depressions configured for deforming the bottom portion of the holder during said compression process. 
     The supports ensure that a space is available between the bottom of the cathode tablet of a button battery and the bottom of the can of the battery. This space allows a degree of expansion of the cathode while also representing a reserve area for the battery&#39;s electrolyte. The space is ensured by the supports even when the holder comprises outwardly bent portions, which are flattened during the compression process. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    illustrates a ring-shaped holder for the cathode of a CR battery, as known in the prior art. 
         FIGS.  2   a  and  2   b    illustrate the holder of  FIG.  1    with a cathode tablet compressed therein. 
         FIGS.  3   a  to  3   g    illustrate the steps of the compression process for producing the assembly of a holder and cathode tablet as presently known in the art. 
         FIGS.  4   a  and  4   b    show two section views of a CR battery with the holder/tablet assembly of  FIGS.  2   a  and  2   b    mounted therein. 
         FIGS.  5   a  and  5   b    illustrate a holder/tablet assembly in accordance with an embodiment of the invention. 
         FIGS.  6   a  and  6   b    show two section views of a CR battery with the holder/tablet assembly of  FIGS.  5   a  and  5   b    mounted therein. 
         FIG.  7    shows a stamp suitable for producing a holder/tablet assembly in accordance with the invention, using the compression process illustrated in  FIGS.  3   a    to  3   g.    
         FIG.  8    shows a holder/tablet assembly according to another embodiment of the invention. 
         FIG.  9    illustrates a further alternative embodiment of a holder/tablet assembly according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG.  1    illustrates a ring-shaped holder  1  for a round cathode tablet for a CR battery as known in the art and referred to above in the introduction, i.e. a ring with an open bottom. The ring has a bottom portion  3  in the form of a flat ring-shaped strip  3  and a round wall portion  2  rising upward from the circular circumference of the strip  3 . The wall portion  2  is preferably perpendicular to the bottom portion  3 . The holder  1  is formed of an electrically conductive material, preferably a metal that is mechanically, chemically, and electrochemically stable under the environment inside the battery, for example stainless steels (SS) such as SS304, SS316, aluminium and most precious metals. 
       FIGS.  2   a  and  2   b    show the same holder  1  with a cathode tablet  4  pressed into said holder, respectively viewed from above and from below. In the latter view, it is seen that the bottom of the ring-shaped strip  3  is level with the cathode tablet  4 . This is a consequence of the process by which the tablet assembly shown in  FIGS.  2   a  and  2   b    is produced. An example of such a process is schematically illustrated in  FIGS.  3   a  to  3   g   . As seen in  FIG.  3   a   , the installation for producing the tablet assembly comprises a cavity  5  and a stamp  6  having mutually corresponding cross-sections so that the stamp  6  may be inserted into the cavity  5  with a small clearance. The cavity  5  comprises a planar bottom  5   a  and a round upstanding sidewall  5   b.  The stamp  6  is configured to be pushed by a controlled force into the cavity  5 . The cross-section of the cavity  5  corresponds to the cross-section of the ring-shaped cathode holder  1 . As seen in  FIG.  3   b   , a cathode powder mixture  7  is introduced into the cavity  5 , followed by a first compression step ( FIG.  3   c   ) for lightly pre-compacting the powder  7 . Then the stamp  6  is raised again out of the cavity  5  ( FIG.  3   d   ), and the ring-shaped holder  1  is introduced into the cavity  5 , with the bottom side up ( FIG.  3   e   ). Then the stamp  6  is again pushed down, now with a higher force ( FIG.  3   f   ), thereby fully compressing the cathode tablet  4  and pressing the cathode material into the ring-shaped holder  1 . The stamp  6  is finally raised ( FIG.  3   g   ), allowing the assembly of the cathode tablet  4  and the ring-shaped holder  1  to be taken out of the cavity  5 . 
     The stamp  6  used for producing this assembly has a flat front surface  8 , hence the level bottom surface of the assembly, as illustrated in  FIG.  2     b.    
       FIGS.  4   a  and  4   b    show a CR battery  11  equipped with the compacted cathode/holder assembly described above. The images represent the battery  11  at the start of its battery life, i.e. before any significant volume change of the electrodes has taken place. The ring-shaped holder  1  sits inside the can  12  that forms the positive terminal of the battery  11 , with the ring-shaped strip  3  contacting the bottom of the can  12 . As described above, the compacted cathode material  4  is level with the bottom surface of the ring-shaped strip  3 , and is therefore in physical contact with the bottom of the can  12 . 
     Above the tablet  4  is a separator sheet  13 , that separates the cathode tablet  4  from the battery&#39;s anode  14  located on top of the separator sheet  13 . The anode  14  is covered by a metal lid  15  that forms the negative terminal of the battery  11 . An electrically insulating gasket  16  is inserted between the outer rims of the can  12  and the lid  15 , effectively separating the positive and negative terminals and closing off the interior of the battery  11  from the external atmosphere. 
     The separator  13  may be a sheet of solid electrolyte laminated onto the tablet  4 , in which case the tablet  4  also comprises the solid electrolyte mixed together with the active material. Alternatively, the battery  11  may include a liquid electrolyte. In this case, the tablet  4  is soaked in this liquid electrolyte and the separator  13  may be a porous polymeric film that is itself electrically insulating, but that can absorb the liquid electrolyte in order to conduct ions from the anode  14  to the cathode  4 . 
     The disadvantages of the type of ring-shaped holder  1  have been stated in the introduction and may be re-summarized here. When the tablet  4  expands, it is likely to push itself away from the can  12 . Also, when a liquid electrolyte is used, no empty space is available between the cathode tablet  4  and the bottom of the can  12 , so that no electrolyte reserve can be formed in that area. 
       FIGS.  5   a  and  5   b    show a holder/tablet assembly comprising a ring-shaped holder  1 ′ and a compacted cathode tablet  4  in accordance with an embodiment of the invention. The holder  1 ′ again comprises a wall portion  2  and a bottom portion in the form of a ring-shaped strip  3 . Viewed from above ( FIG.  5   a   ), the assembly looks the same as the prior art assembly shown in  FIG.  2   a   . The bottom view in  FIG.  5   b    shows that the ring-shaped strip  3  is still level with the bottom plane of the compacted cathode  4 . However, now the holder  1 ′ is provided with a number of supports  20  which protrude outward from the ring shaped-strip  3  in the direction opposite from the wall portion  2 , so as to form support legs for the assembly when said assembly is placed on a planar surface, such as the bottom surface of the can  12  of a CR battery  11 ′ in accordance with the invention, as illustrated in  FIGS.  6   a    and  6   b.    
     In the embodiment shown, the ring-shaped holder  1 ′ comprises 4 supports  20 , placed at angular intervals of 90° along the circumference of the ring-shaped holder  1 ′. The number of supports  20  may be any realistic number of 3 or more, distributed along the circumference in such a manner that the supports realize a clearance  21  (see  FIG.  6   a   ) between the bottom of the can  12  and the cathode tablet  4 , when the holder/tablet assembly is installed in the can. According to preferred embodiments, the height of the supports  20  is essentially the same for all the supports, so that the clearance  21  is essentially constant along the surface of the cathode tablet  4 , i.e. the bottom of the tablet  4  is essentially parallel to the bottom of the can  12 . The clearance  21  allows a degree of expansion of the tablet  4  in the direction of the can  12 , without breaking the electrical contact between the holder  1 ′ and the can  12 . Also, the clearance  21  may accommodate an electrolyte reserve when a liquid electrolyte is used. 
     The shape of the supports  20  is not limited to the rounded half-circular shape illustrated in  FIG.  5   b   , but may be any other suitable shape. 
     A preferred method for realising the supports  20  is by forming these supports  20  during the tablet compression process illustrated in  FIGS.  3   a  to  3   g   , by using an adapted compression stamp. A stamp  6 ′ that is suitable for producing the tablet assembly of  FIGS.  5   a  and  5   b    is illustrated in  FIG.  7   . The stamp  6 ′ is provided with 4 depressions  22  of half-circular shape along the circumference of the stamp&#39;s front surface  8 . When the compression process shown in  FIGS.  3   a - 3   g    is applied with this stamp  6 ′, the compression force realises a plastic deformation of the ring-shaped strip  3  relative to the 4 depressions, resulting in the formation of the supports  20  integral with the bottom portion  3  of the holder  1 ′. 
     The height and the planar dimensions of the supports  20  are not limited to any particular value. As an example, for a ring-shaped holder having a diameter of about 17 mm, semi-circular supports  20  may be provided having diameters of about 0.5 mm and a height between 0.02 mm and 0.06 mm. The force required for the formation of the supports by plastic deformation depends on the hardness of the material used for the holder. A material that is commonly used for these holders is SS316, having a Vickers hardness number of  140 . Deforming such a material by indentation would require a force of about 1.8 kN. The force applied for compressing the tablet  4  in the method shown in  FIGS.  3   a  to  3   g    is upwards of 60 kN, therefore it is clear that the compression process is able to produce the supports  20 , when the stamp  6 ′ provided with depressions  22  is used. 
     The invention is applicable to any other holder type for an electrode tablet of CR batteries besides the holder type  1 ′ shown in the drawings so far. A general characteristic of a holder to which the invention is applicable is that it comprises a bottom portion  3  having a circular circumference. In the ring-shaped holder  1 ′ of the preceding figures, the bottom portion is the ring-shaped strip  3 , i.e. besides this strip the bottom of the holder  1 ′ is open. In other holder types, this is not the case, such as for example in the holder type described in previously cited patent publications CN103606684 and CN203553277, in which the bottom portion of the holder comprises a staggered pattern, consisting of parallel strips which are bent repeatedly inward and outward relative to the bottom plane of the holder, i.e. the bottom plane of the bottom portion  3 . When the compression process shown in  FIGS.  3   a  to  3   g    is applied with this holder type and with a flat stamp  6 , the outwardly bent portions of the strips are flattened between the stamp  6  and the bottom  5   a  of the cavity  5 , leaving no space for an electrolyte reserve when the holder/tablet assembly is placed in the battery can. When the stamp of  FIG.  7    is used, the outwardly bent portions are still flattened, but at the same time the supports  20  are formed on the bottom of the holder, thereby ensuring an empty space when the holder/tablet assembly is installed in the can. 
     According to an embodiment, the holder  1 ′ shown in the drawings so far could be provided with a mesh inside the inner surface of the ring shaped strip  3  for further improving the ability of holding the compressed cathode tablet. 
       FIG.  8    shows a holder/tablet assembly comprising another type of a holder  1 ′ provided with supports  20  in accordance with the present invention. The number of supports  20  is three in the embodiment shown, but could also be four or more. This holder type (without the supports) has been the subject of earlier patent application no 20214346.7, and comprises an upstanding wall  2  and a bottom portion  3  provided with apertures  25 , which are preferably equal-sized and regularly distributed on the plane of the bottom surface. For this type of holder, the supports  20  represent the same advantages as described above, i.e. allowing space for the cathode tablet  4  to expand towards the can  12  without losing the electrical connection, and providing the possibility of having an electrolyte reserve. 
       FIG.  9    shows a holder/tablet assembly according to another embodiment. The holder  1 ′ now consists only of the bottom portion  3  that has the shape of planar ring having an inner wall  30  and an outer wall  31  arranged around a central axis  29 , and wherein the cathode tablet  4  has been partially pressed into the space defined by the inner wall  30  of the bottom portion  3 . The thickness of the bottom portion  3  is preferably higher than in the previously shown embodiments, so that sufficient cathode material can be pressed inside the inner wall  30  in order for the cathode tablet  4  to be held in place. The supports  20  can be produced by the method illustrated in  FIGS.  3   a   - 3   g,  by placing a ring with sufficient thickness inside the cavity  5  on top of a preferably pre-compressed cathode powder, and compressing the powder using a stamp  6 ′ as illustrated in  FIG.  7   . 
     According to a further embodiment, the holder may comprise the ring-shaped bottom portion  3  shown in  FIG.  9   , but further comprising a mesh arranged inside the inner wall  30  of the ring. The mesh will further aid in holding the compressed tablet  4  in place. Instead of a mesh, any other suitable holding structure can be applied, such as a grid forming a staggered pattern of bent strips, as known from CN103606684 and CN203553277. Once again, the supports  20  can be produced by the method illustrated in  FIGS.  3   a   - 3   g.    
     All holder types described above can be used in the compression process shown in  FIGS.  3   a  to  3   g    combined with the stamp  6 ′ shown in  FIG.  7   , for producing a holder/tablet assembly in accordance with the invention. The invention is however not limited to the use of the compression process for producing the supports  20 . For example, strips of the holder material could be attached to the bottom surface of the holder, after the standard compression process shown in  FIG.  3   , said strips forming the supports  20 . The supports  20  could also be formed on the holder prior to the compression process, provided that the materials and dimensions of the holder are designed so that the compression process does not substantially deform the bottom portion  3  of the holder relative to the supports. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.