RECHARGEABLE NICKEL-ZINC CELLS

Abstract: A rechargeable electrochemical cell having a nickel based cathode and a zinc based anode. The cathode comprises a porous nickel material such as nickel foam coated with a nickel hydroxide paste. The anode comprises a gelled zinc and zinc hydroxide mixture. The cell further includes an electrolyte comprising KOH and LiOH.

Description: RECHARGEABLE NICKEL-ZINC CELLS BACKGROUND OF THE INVENTION The present invention relates to rechargeable nickel-zinc alkaline cells. Alkaline nickel-zinc cells in the form of plate cells are commonly known but have not achieved commercial importance to date, mainly due to the limited life of the zinc electrode. Deterioration of the zinc electrode is caused by a change in the shape of the electrode, the growth of zinc dendrites, and corrosion of the electrode. In order to reduce the solubility of zinc and, thereby reduce any shape change of the electrode, cells have been formed using electrolytes with low alkalinity and containing KF and K CO 3 and Ca (OH) 2 as anode additives. In plate type sealed nickel-zinc cells dendrite formation is mostly eliminated since any dendrite produced is quickly oxidized by the oxygen present in the system. The general characteristics of nickel-zinc cell systems have been summarized by M. Klein and F. McLarnon ("Nickel-Zinc Batteries", D. Linden (ed.), Handbook of Batteries, Chapter 29, McGraw-Hill, Inc., NY, 1995) and the history and development of Ni-Zn cells is reviewed by J. Jindra (J. Power Sources, 66, 15 (1997)). The contents of these publications are incorporated herein by reference. The Ni-Zn cell system follows the following reaction: 2 NiOOH + Zn + 2 H 2 O <=> 2 Ni(OH) 2 + Zn(OH) 2 In addition to this main current-generating process, several parasitic reactions may occur. Oxygen evolution has been found to occur at the end of a charge cycle (i.e. at a charge state of approximately 70-80%) and during overcharging of a cell (which is necessary for a better charge acceptance of the nickel electrode). In cases where the negative electrode can be easily accessed, oxygen can be directly recombined at the zinc electrode or an auxiliary electrode can be incorporated to enhance recombination. After repeated cycling, hydrogen evolution can also occur at the zinc electrode. To minimize the amount of hydrogen produced, a sufficient excess of ZnO has to be provided. In general a Zn:Ni ratio between about 2 and 3 should be established. Furthermore, to avoid zinc corrosion in the alkaline medium, corrosion inhibitors such as In, Pb, Hg, or organic compounds, should be added. In nickel-zinc cells different types of nickel electrodes are used: sintered, nonsintered and lightweight substrates. A description of such electrodes is provided in "Handbook of Batteries" (David Lindon (ed.), pg. 29.3), the contents of which are incorporated herein by reference. Sintered nickel electrodes are prepared by sintering carbonyl nickel powder into a porous plaque containing a nickel screen and is then filled with active nickel hydroxide. Typically sintered nickel electrodes have a ratio of inactive to active nickel between 1 to 1.4 :1 providing excellent cycle life and stability, but with the disadvantage of being very heavy. Non-sintered nickel electrodes are made by kneading and calendering an electrode strip consisting of nickel hydroxide, graphite and plastic binder laminated on both sides of an appropriate current collector. Applying lightweight substrates based on a fiber structure filled with active electrode mass has the advantage of reducing electrode weight as well as material costs. Cylindrical cells with spirally rolled nickel electrode/separator/zinc electrode assemblies, quite similar to Ni-Cd cells, have been tentatively produced by some manufacturers, but they suffered from serious short circuit troubles due to zinc dendrites growing during the charge cycles across the narrow (open) spiral distances between cathodes and anodes. The objectives of this invention are mainly to produce high current, high capacity, cylindrical consumer cells that could be hermetically sealed and showing an acceptable cycle life at deep discharge conditions. SUMMARY OF THE INVENTION In a preferred embodiment, the present invention provides a rechargeable electrochemical cell comprising: a generally cylindrical container having an interior surface and an exterior surface; a generally cylindrical cathode contacting the container, the cathode being coaxial with the container; a generally cylindrical anode contained within the cathode and being coaxial therewith; a separator for physically separating the anode and the cathode; and, an electrolyte for electrically contacting the anode and the cathode; wherein the anode comprises a zinc material, the cathode comprises a nickel material. In a further embodiment, the cathode material comprises a porous nickel material coated a with nickel hydroxide paste. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a cut through a cylindrical AA-size Ni-Zn cell made according to this invention. Figure 2 shows a nickel electrode with two layers from the top and a three- dimensional view. Figure 3 shows multiple (three) sleeves of a nickel electrode from the top and a three- dimensional view. Figure 4 shows the discharge capacity of cell A75 and A79 with one nickel layer as a function of cycles. Figure 5 shows the discharge capacity of cell A71 and A86 with two-nickel layers as a function of cycles. Figure 6 shows the discharge capacity of cell A121 containing 2 % and cell A79 with 8.6 % Ni powder / T-210 as a function of cycles. Figure 7 shows the discharge capacity of cell A128 containing 2 % and cell A131 with 0 % Co extra- fine powder as a function of cycles. DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment, the invention is directed to fabrication of a rechargeable galvanic element with a positive nickel oxide electrode and a negative zinc electrode containing an alkaline electrolyte and a separator. The cathode consists of a nickel foam structure that is filled with a nickel hydroxide rich paste made of a polyvinylalcohol (PVA) slurry. The nickel hydroxide is suitably compressed or compacted into a sheet or tape of defined thickness, rolled up into one or more layers and inserted into a nickel-plated steel can. In this way the nickel electrode is shaped into a very tight cylindrical cathode. Alternatively the filled foam can be compressed into a multiple of sleeves which are inserted exactly the same, also forming a cylindrical cathode. Such nickel foam based cathodes are exhibiting an exceptionally low resistance and high efficiency leading to a sharp cut-off after the capacity is completely exhausted thereby establishing a cathode limited cell. The anode consists of zinc powder, zinc oxide and a gelling agent, such as for example, Carbopol. In rechargeable nickel-zinc cells the anode capacity is chosen as a multiple of the cathode capacity. The separator is preferably of the cellulose type. A brass nail located in the center of the cell builds the negative terminal. Other materials for the negative current collector will be apparent to persons skilled in the art. The cell is characterized by prevention of excessive swelling of the cathode due to the cylindrical design in contrast to plate cells. It is further distinguished by the use of special additives to improve recharging. In a preferred embodiment the cathode is provided with hydrogen recombination catalysts for eliminating any hydrogen gas that may evolve. Such catalysts can comprise those used in mercury- free zinc anodes. A most preferred catalyst is silver (Ag). In such embodiment, the silver catalyst may be provided in an amount of between about 0.1% to 0.3% (wt.) of the nickel hydroxide. Such Ag catalyst may be incorporated into the Ni foam in the form of a colloidal deposit by means of a spray coating process as known in the art. The electrolyte is preferably a solution of potassium hydroxide with lithium hydroxide as additive. The rechargeable nickel-zinc cells built according to the invention can be manufactured in all conventional cylindrical sizes (e.g. AAA, AA, C and D) but are not limited to these formats. Further the cells of the invention are hermetically sealed and can be used in all consumer electronic devices. In a preferred embodiment, the cathode is provided with nickel foil strips to assist the can of the cell in its capacity as a current collector. Figure 1 of the drawings shows a cut through a cylindrical AA-size Ni-Zn cell embodying the present invention. The cell comprises a Ni-plated steel can 1 housing a porous nickel oxide cathode 2, a zinc anode 3 and a separator 8 as the main components of a rechargeable galvanic element. The cathode 2 may comprise one or several layers of a porous nickel substrate filled with nickel hydroxide, additives and a binder, and is separated from anode 3, which may comprise zinc powder, zinc oxide and gelling agent, by an electrolyte permeable separator 8. The electrolyte, which may consist of aqueous potassium and lithium hydroxide, permeates the nickel cathode 2 and zinc anode 3 through separator 8. A current collector nail 7, that is connected to the negative cap 5 and embedded into the plastic top seal 4, is located in the center of the nickel-zinc cell. For safety reasons the plastic top seal 4 is provided with a safety vent break area 6. Figure 2 illustrates the embodiment of a nickel electrode made of two layers of a nickel foam, pasted with a mixture of nickel hydroxide, nickel powder, cobalt powder and a binder (PVA-solution), that is shaped into a very tight cylindrical arrangement. The embodiment of Figure 3 differs from that of Figure 2 in that, three or multiple sleeves of a nickel foam prepare the nickel electrode filled with nickel hydroxide mixture. Separators according to a preferred embodiment of the invention comprise two overlapping layers of a laminated product comprising one piece of regenerated high purity cellulose bonded to a non-woven polyamide synthetic fiber. However, other separators known in the art may also be used. As discussed below in more detail, the process of making the cells of the present invention according to a preferred embodiment comprises: 1) forming a sheet of Ni foam; 2) applying a paste of nickel powder, cobalt powder, PVA solution, and nickel hydroxide to the Ni foam; 3) forming a separator in the form of a cylindrical tube open at one end (referred to herein as "separator bag"); 4) placing the separator bag on a mandrel or other such support; 5) rolling the Ni foam sheet around the bag; 6) placing such Ni foam coated bag within a Ni plated steel can; 7) filling the interior of the bag with a zinc anode material. As described below, the cathode preferably comprises a nickel foam coated with 8.6% Ni powder, 4.3 % Co powder, 30% poluvinylalcohol (PVA) solution, and 57.1% Ni hydroxide. The anode preferably comprises 59% zinc oxide, 10% zinc powder, 0.5% Carbopol, and 30.5%o KOH. The anode is preferably in the form of a gel paste. The preferred electrolyte is KOH/LiOH solution. In a preferred embodiment, the electrolyte comprises KOH in a concentration in the range of 6 to 9 M and LiOH dissolved in about 1% to the saturation point. The charging of the nickel-zinc cell made according to the preferred embodiment is done by a voltage limited charging circuit, constant current charging, or an electronically controlled overflow circuit bypassing excess current above 1.95 V. The following examples serve to illustrate the present invention and are not intended to limit the scope thereof. Example 1 A cylindrical AA-size nickel zinc cell was fabricated which consisted of one positive nickel electrode layer and a negative zinc electrode assembled in an arrangement as shown in Figure 1. The nickel electrode was prepared by blending a mixture of 8.6 % of nickel T-210 powder (from Inco Technical Services Ltd., Missisauga, Ontario), 4.3 % of cobalt extra-fine powder (UNION MINIERE, INC. - Carolmet Cobalt Products, Laurinburg, N.C.), 30.0 % of PVA-solution (1.17 % PVA in water/ethanol) and 57.1 % of nickel hydroxide (from Inco Technical Services Ltd., Missisauga, Ontario). Some water was added to obtain a light suspension. The slurry was pasted into a nickel foam of 38 mm x 36 mm provided with a spotwelded nickel foil current collector (36 mm x 4 mm, 0.125 mm thick, 99.98 %, from Goodfellow Cambridge Ltd.,) at the longitudinal direction. The pasting procedure was carried out a few times on both sides of the nickel foam with a spatula to ensure that the slurry completely penetrates into the foam. Wet surplus material was removed from the foam surface. The nickel electrode was dried at 110°C for one hour. Two different nickel foam types were used to prepare a nickel electrode as described above: Retec 80 PPI (pores per inch), 1.6 mm thick, (foam from RPM Ventures, ELTEC Systems Corp., Ohio) and Inco foam, 2.7 mm thick of the same porosity (from Inco Technical Services Ltd., Missisauga, Ontario). The zinc electrode was prepared by mixing up 59 % of zinc oxide (from Merck), 10 % of zinc / type 004F (from Union Miniere S.A., Overpelt, Belgium), 0.50 % of Carbopol 940 (from Nacan, Toronto) and 30.5 % of 7 M KOH to a gel paste. Two overlapping layers of a laminated product comprising one piece of regenerated high purity cellulose bonded to a non- woven polyamide synthetic fiber (from Berec Components Ltd., Co. Durham) were used to construct the separator bag. The nickel electrode was rolled up around the separator bag, inserted into the nickel plated steel can, filled with 27 % KOH - 10 g/1 LiOHxH 2 O electrolyte and allowed to soak for 24 hours. The zinc anode paste was filled into the separator bag and the cylindrical AA-size nickel zinc cell was closed with the negative cap unit as shown in Figure 1. Cell cycling was carried out with constant voltage taper charging at 1.90 Volts for approximately 500 minutes followed by the discharge process at 3.9 Ohms to a cut-off voltage of 800 mV. Figure 4 shows the discharge capacity of each cycle of cylindrical AA- size nickel zinc cells A75 (Retec 80, 1.6 mm thick) and A79 (Inco, 2.7 mm thick) containing one layer of nickel electrode consisting of the above mentioned nickel foam types and a pasted zinc electrode as a function of cycle life. The results obtained show a stable discharge behavior for at least 100 cycles with a relatively flat discharge profile and a small capacity decline during cycling. The first few cycles are formation cycles that run under the cycling condition described above. Example 2 A cell was assembled as described above with the exception that the positive electrode was made of two nickel layers and the appropriate dimension of the nickel foam was 38 mm x 70 mm. In the case of cylindrical cell design the assembly is volume limited and therefore cells with two layers contain less zinc. The nickel foam types used in this example were Retec 80 PPI and Retec 110 PPI both of the same thickness of 1.6mm but of different porosity's as indicated by PPI (pores per inch). Inco foam, 2.7 mm thick, could not be used in a double layered arrangement because of its high thickness resulting in a deficiency of positive zinc electrode. In Figure 5 the discharge capacity of each cycle of cylindrical AA-size nickel zinc cells A71 (Retec 80) and A86 (Retec 110) with 2 layers of nickel electrode and a pasted zinc electrode is shown. It turned out that the cells had high values of discharge capacity (600-500 mAh) for the first twenty cycles but due to the Zn/Ni ratio of only 1.2 the discharge capacity decreased with increasing cycles. Example 3 A cell, A121, was assembled as described in Example 1 except with a thinner Inco 2.2 mm nickel foam but of the same porosity as the 2.7 mm Inco foam and with 2 % of nickel T- 210 powder and 63.7 % of nickel hydroxide. The other components of nickel hydroxide slurry were the same as in Example 1. In this case, it was not necessary to add water to this light suspension that easy penetrates into the Inco foam, 2.2 mm thick. In Figure 6, the discharge capacity of each cycle of cylindrical AA-size nickel zinc cell A121 and A79 (from Example 1) can be seen. In comparison with cell A79, that is also constructed with one nickel layer, cell A121 delivers a 150-50 mAh higher discharge capacity up to 50 cycles due to its composition with more active nickel hydroxide (63.7 % instead of 57.1 %) and to a larger amount of pasted cathode mass as indicated in the following table: Example 4 Two cells were built as described in Example 1 but with 2% (A128) and 0 % (A131) of cobalt (Co) extra fine powder and with 59.4 % and 61.4 % of nickel hydroxide. The other components of nickel hydroxide slurry were the same as in Example 1 and Inco foam, 2.2 mm thick was used as the foam material. Figure 7 shows the discharge capacity of each cycle of cylindrical AA-size nickel zinc cell A128 and A131. The discharge capacity of cell A128 with 2 % cobalt is approximately 200 mAh higher than that of cell A131 containing 0 % cobalt since the addition of cobalt increases electronic conductivity of nickel electrode mass. The following table summarizes foam type and nickel cathode mass of both cells: Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims: THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: 1. A rechargeable electrochemical cell comprising: a generally cylindrical container having an interior surface and an exterior surface; a generally cylindrical cathode contacting said container, said cathode being coaxial with said container; a generally cylindrical anode contained within said cathode and being coaxial therewith; a separator for physically separating said anode and said cathode; and, an electrolyte for electrically contacting said anode and said cathode; wherein said anode comprises a zinc material, said cathode comprises a nickel material. 2. The cell of claim 1 wherein said cathode comprises a porous nickel material. 3. The cell of claim 2 wherein said cathode comprises a nickel foam. 4. The cell of claim 3 wherein said nickel foam is coated with a paste including nickel powder and nickel hydroxide. 5. The cell of claim 4 wherein said paste further includes a cobalt component. 6. The cell of claim 5 wherein said cobalt is present in an amount of about 4.3%(wt) of said paste. 7. The cell of claim 6 wherein said cathode further includes a hydrogen recombination catalyst. 8. The cell of claim 7 wherein said hydrogen recombination catalyst comprises silver. 9. The cell of claim 8 wherein said silver catalyst is present in an amount between 0.1 % and 0.3%o (wt.) of said nickel hydroxide component. 10. The cell of claim 8 wherein said silver catalyst is applied to the nickel foam as a colloidal deposit. 11. The cell of claim 10 wherein said silver catalyst is applied by a spray coating process. 12. The cell of claim 1 wherein said container is a nickel plated steel can, wherein said nickel plating in applied on the interior surface thereof and contacting said cathode. 13. The cell of claim 1 wherein said cathode further includes nickel foil current collectors, said current collectors comprising strips of nickel foil positioned axially within said cathode. 14. The cell of claim 1 wherein said electrolyte comprises a solution of KOH and LiOH. 15. The cell of claim 1 wherein said anode comprises a mixture of zinc powder, zinc oxide powder and a gelling agent. 16. The cell of claim 15 wherein said anode is present in the form of a gel. 17. The cell of claim 16 wherein said anode material is present in an amount to provide an anode capacity that is more than twice the cathode capacity of the cathode.

Inventors: HARTFORD WAYNE, KORDESCH KARL, TAUCHER-MAUTNER WALTRAUD, HARTFORD, WAYNE, KORDESCH, KARL, TAUCHER-MAUTNER, WALTRAUD
Applicants: ENERGY VENTURES INC CANADA, HARTFORD WAYNE, KORDESCH KARL, TAUCHER MAUTNER WALTRAUD, ENERGY VENTURES INC. (CANADA), HARTFORD, WAYNE, KORDESCH, KARL, TAUCHER-MAUTNER, WALTRAUD
Application year: 2000
Main classification: H01M10
IPC codes: H01M2, H01M4, H01M10