ELECTRODE FOR AN ELECTROCHEMICAL CELL

Abstract: An electrochemical cell is provided comprising a first electrode (30) including a plurality of stacked disks (35), a second electrode disposed about the periphery of the disks (20), and an electrolyte. Also, an electrode is disclosed that includes a plurality of stacked disks. The electrode is preferably a negative electrode of an alkaline cell and the disks are preferably formed of zinc. By forming the negative electrode of such zinc disks, the discharge capacity of the negative electrode may be increased. Consequently, the high-rate service of an alkaline cell having the inventive negative electrode is improved.

Description: ELECTRODE FOR AN ELECTROCHEMICAL CELL This invention generally relates to electrochemical cells. More particularly, the present invention relates to electrochemical cells, especially alkaline cells, having an improved electrode structure, and to electrodes for such cells. Typical alkaline electrochemical cells include a positive electrode made of manganese dioxide (MnO 2 ), a negative electrode made of zinc, and an alkaline electrolyte made of potassium hydroxide (KOH), or the like. The positive electrode is normally formed as a hollow cylinder with its outer surface contacting the inner surface of a cell housing shaped as a can. A separator is disposed within the inside of the positive electrode to physically separate the positive electrode from the negative electrode while allowing ionic transport between the two electrodes. The negative electrode is formed by mixing the zinc active material in the form of a zinc alloy powder with the alkaline electrolyte and a gelling agent. The mix is dispensed within the hollow middle area defined by the inner surface of the separator within the positive electrode. Subsequently, a collector assembly is inserted into the open end of the cell housing with a collector nail extending down within the negative electrode/electrolyte gel. An outer cover is then placed over the collector assembly and the cell housing walls are then crimped over the outer cover to seal the cell. Japanese Kokai Patent Application No. Hei 7-254406 discloses the use of a gelled zinc negative electrode in which a gelling agent and an alkaline electrolyte are mixed, and the negative electrode active material comprises non-amalgamated zinc powder in the shape of balls and long slender elements to increase the surface area exposed to alkali electrolyte. In the manufacture and use of these known batteries or cells, the lowest zinc volume percent in the negative electrode that manufacturers utilise is about no less than 28 percent in the negative electrode gel in order to both match the positive electrode's rate of electrochemical output and provide sufficient particle-to-particle and particle-to- collector contact to maintain the negative electrode's electrical conductance. Below this amount, voltage instability occurs, as well as the resulting production of a cell structure having high sensitivity to shock and vibration which cause the zinc particles to migrate away from the current collector nail thereby decreasing cell efficiency. In order to provide the maximum electrochemical activity and a minimum of limiting polarisation, it is desirable to operate a battery at as low a current density on the zinc as possible while still producing the required amount of total current from the system. Accordingly, alkaline batteries conventionally employ electrodes made from powdered active materials to obtain the highest possible surface area per unit weight or volume, and thus minimise the current density. Conventional zinc powder is powder that has been produced by air-jet atomisation of molten zinc. It consists of irregularly shaped particles, ranging from lumpy or distorted spheroids to elongated, tuberous forms. In typical battery grade zinc powder, the full population of material consists of many individual particles of a wide range of sizes and shapes. The median value of the particle size for negative electrodes, as determined by sieving, is approximately 100 to 300 micrometers (μm). The extremes of particle sizes range from 20 to 1000 μm. While zinc powder negative electrodes are relatively efficient at low discharge rates, such electrodes are much less efficient when discharged at high rates. Given that most new battery-powdered devices have high current demands, causing the batteries to discharge at high rates, there exists a strong demand for batteries having greater high- rate performance. In WO-A-98/20569, a negative electrode is disclosed that includes zinc flakes. The zinc flakes differ from the prior zinc powder particles in that the zinc flakes have a thickness many times smaller than both their length and width, for example, 10 to 20 times smaller. The disclosed flakes have a thickness on the order of 25 μm (0.001 inch) and lengths and widths of 610 to 1020 μm (0.024 to 0.04 inch). While the use of zinc flakes improves the high-rate performance of the negative electrode of an alkaline electrochemical cell, there remains room for further improving negative electrode performance particularly at high drain rates. It has been discovered that discharge of zinc in an alkaline cell starts near the positive electrode and then proceeds away from the positive electrode. Because the reaction product (e.g., zinc oxide and zinc hydroxide) resulting from the discharge of zinc is more voluminous than the zinc itself, a reaction product skin tends to form between the positive and negative electrodes if there is not enough space to accommodate the reaction product. While such a skin still allows some electrolyte to pass through, the reacting zinc behind the skin does not receive hydroxyl ions from where they are formed in the positive electrode fast enough to offset those consumed by the reacting zinc. As a result, polarisation occurs leading to premature cell failure. In most cell designs, the current collector, which is often in the form of a nail, is located in the centre of the negative electrode. Because most of the zinc discharge occurs at the outer periphery of the negative electrode near the positive electrode interface, it is necessary to maintain a continuous path of connected zinc from the reacting site to the collector nail to facilitate electron transfer. When zinc powders or flakes are used, many particles must touch to form an electron conduction path back to the collector nail. However, because the zinc powder or flakes only constitute approximately 30 percent of the negative electrode volume, any physical shocks to the cell may cause the particles to shift and lose contact. Thus, excess zinc is often added to the negative electrode only to serve as an electron conductor. The excess zinc, however, is not discharged during the life of the cell and takes up valuable space within the cell that could otherwise be used for extra electrolyte to fuel reactions or to hold discharge reaction product while still leaving space for ion transfer. Alternatively, some of the space could be used to increase the amount of MnO 2 in the positive electrode. We have now found, surprisingly, that it is possible to solve some or all of the above problems associated with known electrochemical cells. Accordingly, in a first aspect, the present invention provides an electrochemical cell comprising a first electrode including a plurality of stacked disks, a second electrode disposed about the periphery of the disks, and an electrolyte. Preferably, the electrochemical cell comprises a negative electrode including a plurality of stacked zinc disks, a positive electrode disposed about the periphery of the zinc disks, and an alkaline electrolyte. In a second aspect, the present invention provides an electrode for an electrochemical cell, the electrode comprising a plurality of stacked disks. It is an advantage of the present invention that a zinc negative electrode for an alkaline cell having improved high-rate performance can be provided. More specifically, the present invention can provide a zinc negative electrode having improved electron conduction between the collector nail and the peripheral reaction sites in the negative electrode. An additional advantage of the present invention is that a zinc negative electrode can be provided in which the zinc is more concentrated in the periphery of the negative electrode where it is more likely to discharge. Correspondingly, it is also an advantage that the present invention can provide an alkaline electrochemical cell including the improved zinc negative electrode. According to the present invention, the first electrode, which preferably forms the negative electrode, includes a plurality of stacked disks. For an alkaline cell, the disks are preferably made of zinc, and more preferably made of a zinc alloy including one or more of the metals selected from the group consisting of bismuth, indium, calcium, and aluminium. However, while the present invention is described herein primarily by reference to a negative electrode, it will be appreciated that the electrode having the stacked disks could be the positive electrode, depending on the cell type. Additionally, although the cell according to the present invention is preferably a primary alkaline cell, the inventive electrode structure may be utilised in other primary cell chemistries, such as carbon-zinc or lithium cells, or in rechargeable cells, such as nickel cadmium, nickel metal hydride, or Li-ion cells. Thus, the disks could be made of any of the electrochemically active materials used for the positive or negative electrodes in such cells. For example, the disks could be made of a compound including at least one material selected from the group consisting of cadmium, nickel, metal hydride, lithium, cobalt, manganese dioxide, zinc, and carbon. The disks have a diameter that is approximately equal to the inner diameter of the layer of separator that lines the hollow cavity of the positive electrode. A central aperture is preferably formed in each of the disks, that has a diameter substantially equal to that of the current collector. In this manner, a plurality of such disks may be threaded onto the current collector with an interference fit so as to be in physical and electrical contact with the current collector. The disks preferably further include a plurality of apertures that may be aligned in the stacked assembly. By providing apertures, electrolyte more readily flows axially within the cell, and hence may be dispensed in the cell without creating any unwanted air pockets. Also, the apertures facilitate transfer of hydrogen gas formed on the zinc by corrosion out of the negative electrode volume upward to the void space underlying the seal where it is consumed by the MnO 2 in the positive electrode. Accordingly, by increasing the amount of hydrogen gas consumed by the positive electrode, internal pressure build-up within the cell is reduced, thereby enabling simpler seal designs to be used. The disks may for example be uniformly flat or corrugated. Preferably the central portion of the disks is generally flat, while the peripheral portion is preferably corrugated in order to increase the surface area of the negative electrode in the vicinity of the electrode interface. By maximising the surface area in the vicinity of the separator interface and by maintaining an excellent conductive path between the disks and the current collector, the discharge capacity of the negative electrode improves. The increased discharge efficiency of a zinc negative electrode of the present invention can result in an electrochemical cell having improved high rate service, which is becoming increasingly important as more and more battery-powered devices are designed that draw current at higher rates. For example, a prototype including a plurality of zinc disks was constructed and found to be capable of being subjected to a 2.5 A current load without becoming polarised. The disks are preferably between 50 and 510 μm (2 and 20 mils) thick and are more preferably between 130 and 250 μm (5 and 10 mils) thick. Preferably, the disks have a radius approximately equal to the internal radius of the separator-lined cavity defined within the positive electrode. It will be appreciated by those skilled in the art that the number, sizes, and shapes of apertures in the disks may be varied to optimise performance for different cell constructions. Also, the disks may have non-uniform thickness, such that the disks are thinner in their central regions in order to distribute more of the zinc to the periphery of the negative electrode. In addition, the depth and frequency of the corrugation may also be varied without departing from the scope of the present invention. By forming the negative electrode of a plurality of spaced-apart disks arranged perpendicular to the surface of the positive electrode inner surface, the electron conduction path between the periphery of the negative electrode and the current collector may be greatly improved. Moreover, the disks provide a conduction path that is stable and not subject to physical shock. The spacing of the disks provides chutes for the reaction product formed at the electrode interface to easily travel and expand toward the central void area provided by any apertures in the disks. Thus, the reaction product skin found in most conventional cells is much less likely to form between the electrodes and inhibit transfer of for example hydroxyl ions from the positive electrode to zinc in the negative electrode. In addition, for a zinc alkaline cell, by introducing the zinc as disks, the distribution and location of the zinc may be more closely controlled. The disks also eliminate problems associated with zinc particles migrating through the separator or otherwise becoming located on the wrong side of the separator and in contact with the positive electrode. Because the distribution of zinc may be calculated and controlled, low volume percentage zinc may be used and variability in zinc weight per cell is very low. The negative electrode preferably further includes a plurality of washers altematingly disposed between the disks. The washers are provided to prevent adjacent disks from nesting with one another. If adjacent disks are allowed to nest in a zinc- MnO 2 cell, the zinc would either not be evenly distributed throughout the negative electrode or the zinc density would become excessive relative to the density of MnO 2 in the positive electrode. Further, by maintaining void space between adjacent disks, the surface area is further increased near the electrode interface and electrolyte flows more freely across the entire surface of the disks. The washers may themselves be formed of zinc or may be formed of any inert material such as a separator material. By making the washers of a separator material, the washers can absorb electrolyte and will not hamper the transport of ions and water within the negative electrode. Preferably, the cell includes a cylindrical cell housing in which a positive electrode is located adjacent the inner sidewalls of the cell housing. The positive electrode is shaped as a hollow cylinder that may for example be impact moulded inside of the housing or inserted as a plurality of rings after moulding. However, although the present invention is shown and described herein primarily with reference to cylindrical cells, it will be appreciated by those skilled in the art that the stacked disk electrode of the present invention may be employed in other electrochemical cells, such as prismatic cells. Additionally, the stacked disk electrode of the present invention may be used in cells having essentially any cell construction. In the preferred embodiment, of a zinc alkaline cell, the positive electrode is preferably made primarily of manganese dioxide (MnO ). However, other suitable electrode materials, for alkaline or other types of cell, may be used in accordance with the present invention. The cell further includes a separator that lines the inner walls of the hollow cavity within the positive electrode. As described in further detail below, a negative electrode is deposited within the separator-lined hollow cavity of the positive electrode. An electrolyte, preferably an alkaline electrolyte such as potassium hydroxide (KOH), is also dispensed within the lined hollow cavity of the positive electrode. The cell is preferably closed and sealed by a collector assembly and an outer terminal cover. In general, the collector assembly includes an inner cover, a seal, and a current collector. As known in the art, the collector assembly and outer terminal cover are electrically coupled to the negative electrode and are insulated from the remainder of cell housing. In this manner, the outer terminal cover may serve as a negative contact terminal for the cell. A second outer terminal cover may be secured to a closed end of the electrically conductive cell housing to serve as a positive terminal for the cell. However, it will be appreciated that other suitable seal designs and collector assembly constructions that permit the provision of stacked electrode disks may be used in accordance with the present invention. The negative electrode of the present invention may be easily assembled in mass production. The disks may be readily moulded or stamped and then altematingly fed onto a long collector pin along with the washers to form a plurality of negative electrodes resembling a sausage link. The plurality of linked negative electrode assemblies may be separated by a spacer washer fed onto the collector pin at periodic intervals. The negative electrode assemblies may then be cut apart and readily inserted into the separator lined hollow cavity defined by the positive electrode. Because of the nature of the negative electrode construction, gelling of electrolyte may be delayed until air is displaced in the cell by the dispensing of the electrolyte into the cell. In this manner, the gelling agent may be dispensed into the cell separate from the electrolyte so that the electrolyte may be dispensed gel-free. Because such a gel-free electrolyte is much less viscous, air is less likely to become trapped within the negative electrode during manufacture. In a preferred embodiment, an alkaline electrochemical cell is provided comprising: a cylindrical cell housing; a positive electrode disposed within the housing and having a central cavity; a separator lining the central cavity; an electrolyte within the central cavity; a current collector extending along a longitudinal central axis of the cell housing; and a negative electrode including a plurality of stacked spaced-apart zinc disks having a radius sufficient to extend between the separator and the current collector and disposed perpendicular to the inner surface of the cavity. In this preferred embodiment, preferably the disks include a central aperture having a radius approximately equal to the radius of the current collector. The zinc disks preferably have corrugated peripheral edges. The zinc disks preferably include a plurality of apertures. The preferred thickness of the zinc disks is between about 50 and 510 μm (2 and 20 mils), more preferably between about 130 and 250 μm (5 and 10 mils). A plurality of washers is preferably interposed between the disks, and preferably the washers are made of separator material or are made of zinc. The present invention will be further understood by reference to the following drawings, in which: Figure 1 is a cut-away schematic of an electrochemical cell constructed in accordance with a first embodiment of the present invention; and Figure 2 is an exploded perspective view of a negative electrode constructed in accordance with the first embodiment of the present invention. Figure 1 shows an electrochemical cell 10 constructed in accordance with a first embodiment of the present invention. As shown, cell 10 includes a cylindrical cell housing 15 in which a positive electrode 20 is located adjacent the inner sidewalls of cell housing 15. Positive electrode 20 is shaped as a hollow cylinder that may be impact moulded inside of housing 15 or inserted as a plurality of rings after moulding. In this alkaline cell, positive electrode 20 is made primarily of MnO 2 . Cell 10 further includes a separator 25 that lines the inner walls of the hollow cavity within positive electrode 20. A negative electrode 30 is deposited within the separator-lined hollow cavity of positive electrode 20. An alkaline electrolyte, such as KOH, is also dispensed within the lined hollow cavity of positive electrode 20. The cell is closed and sealed by a collector assembly 40 and an outer terminal cover 45. Collector assembly 40 includes an inner cover 42, a seal 44, and a current collector 46. Collector assembly 40 and outer terminal cover 45 are electrically coupled to negative electrode 30 and are insulated from the remainder of cell housing 15. In this manner, outer terminal cover 45 serves as a negative contact terminal for cell 10. A second outer terminal cover 48 is secured to a closed end 49 of the electrically conductive cell housing 15 to serve as a positive terminal for cell 10. As shown in Figures 1 and 2, negative electrode 30 includes a plurality of stacked disks 35 made of zinc or zinc alloy. Disks 35 have a diameter that is approximately equal to the inner diameter of the layer of separator 25 that lines the hollow cavity of positive electrode 20. A central aperture 50 is formed in each of disks 35 that has a diameter substantially equal to that of current collector 46. In this manner, the plurality of disks 35 is threaded onto current collector 46 with an interference fit so as to be in physical and electrical contact with current collector 46. Disks 35 further include a plurality of apertures 52 that are aligned in the stacked assembly so that electrolyte more readily flows axially within the cell, and to facilitate transfer of hydrogen gas formed on the zinc by corrosion. While the central portion of disks 35 is flat, the peripheral portion is corrugated in order to increase the surface area of the negative electrode in the vicinity of the electrode interface. As best shown in Figure 2, negative electrode 30 further includes a plurality of washers 56 altematingly disposed between disks 35 to prevent adjacent disks from nesting with one another. Washers 56 are formed of zinc or formed of an inert material such as a separator material. The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention.

Claims: CLAIMS: 1. An electrochemical cell comprising a first electrode including a plurality of stacked disks, a second electrode disposed about the periphery of the disks, and an electrolyte. 2. An electrochemical cell according to claim 1 and further including a cylindrical cell housing and a current collector extending along a longitudinal central axis of the cell housing, wherein the disks have a radius sufficient to extend between the electrode and the current collector. 3. An electrochemical cell according to claim 1 or claim 2, wherein the disks include a central aperture having a radius approximately equal to the radius of the current collector. 4. An electrochemical cell according to any preceding claim, wherein the disks have a thickness between 50 and 510 μm (2 and 20 mils), preferably between 130 and 250 μm (5 and 10 mils). 5. An electrochemical cell according to any preceding claim, wherein the disks have corrugated peripheral edges. 6. An electrochemical cell according to any preceding claim, wherein the disks are made of zinc, zinc alloy or a zinc compound. 7. An electrochemical cell according to claim 6, wherein the disks are made of a zinc alloy including at least one of the metals selected from bismuth, indium, calcium, and aluminium. 8. An electrochemical cell according to any preceding claim and further including a plurality of washers interposed between the disks. 9. An electrochemical cell according to claim 8, wherein the washers are made of a separator material. 10. An electrochemical cell according to claim 8, wherein the washers are made of zinc, zinc alloy or a zinc compound. 11. An electrochemical cell according to any preceding claim, wherein the disks include a plurality of apertures. 12. An electrochemical cell according to any preceding claim, wherein the electrolyte is an alkaline electrolyte. 13. An electrochemical cell according to any preceding claim, wherein the second electrode is a positive electrode. 14. An electrochemical cell according to claim 13, wherein the positive electrode includes manganese dioxide (MnO 2 ). 15. An electrochemical cell according to any preceding claim, wherein the disks are disposed perpendicular to an inner surface of the second electrode. 16. An electrochemical cell according to claim 1, wherein the disks are made of a compound containing a material selected from cadmium, nickel metal hydride, lithium, cobalt, manganese dioxide, zinc, and carbon. 17. An alkaline electrochemical cell comprising: a cylindrical cell housing; a positive electrode disposed within the housing and having a central cavity; a separator lining the central cavity; an electrolyte within the central cavity; a current collector extending along a longitudinal central axis of the cell housing; and a negative electrode including a plurality of stacked spaced-apart zinc disks having a radius sufficient to extend between the separator and the current collector and disposed perpendicular to the inner surface of the cavity. 18. An electrode for an electrochemical cell, the electrode comprising a plurality of stacked disks as defined in any preceding claim.

Inventors: URRY LEWIS F, URRY, LEWIS, F.
Applicants: EVEREADY BATTERY INC, EVEREADY BATTERY COMPANY, INC.
Application year: 1999
Main classification: H01M4
IPC codes: H01M4, H01M6, H01M10