CA2281371A1

CA2281371A1 - Rechargeable nickel-zinc cell

Info

Publication number: CA2281371A1
Application number: CA002281371A
Authority: CA
Inventors: Waltraud Taucher-Mautner; Karl Kordesch; Wayne Hartford
Original assignee: Energy Ventures Inc
Current assignee: Energy Ventures Inc
Priority date: 1999-09-03
Filing date: 1999-09-03
Publication date: 2001-03-03
Also published as: JP2003526877A; KR20020053807A; EP1218957A1; CN1372703A; AU6974700A; WO2001018897A1

Abstract

This 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 which is repeatedly filled with a nickel hydroxide rich paste made of a PVA-slurry. Then it is suitably compressed to 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, like Carbopol. In rechargeable nickel-zinc cells the anode capacity is chosen as a multiple of the cathode capacity.
The separator is preferrably of the cellulosic type. A brass nail located in the center of the cell builds the negative terminal.
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. Recombination catalysts for hydrogen gas which may evolve as corrosion product from mercury-free zinc anodes are applied. The electrolyte is 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), hermetically sealed, and thereby used in all consumer electronic devices.

Description

RECHARGEABLE NICKEL-ZINC CELL
BACKGROUND OF THE INVENTION
Alkaline nickel-zinc cells have been built as plate cells for many years, but have not achieved commercial importance to date, mainly due to the still limited life of the zinc electrode caused by electrode shape change, dendrite growth and zinc corrosion.
Electrolytes with low alkalinity containing KF and KZC03 and Ca(OH)~ additions to the anode where successfully applied to suppress solubility of zinc species and electrode shape change of flat cells. In plate type sealed nickel-zinc cells dendrite formation is mostly eliminated, since any dendrite produced is quickly oxidized by the present oxygen.
General characteristics regarding the nickel-zinc system have been summarized in detail in the contribution of M.Klein and F.McLarnon:"Nickel-Zinc Batteries", in D.
Linden, ed., Handbook of Batteries, Chapter 29, McGraw-Hill, Inc., NY, 1995. The history and the development of Ni-Zn cells is reviewed by J. Jindra, J. Power Sources, 66, 15 (1997).
The overall cell reaction of the nickel-zinc system can be written in a simplified form as follows:

2 Ni00H + Zn + 2 H20 ~ 2 Ni(OH)2 + Zn(OH)2.
In addition to this main current-generating process several parasitic reactions may occur.
At the end of charge (70-80 % state of charge) and during overcharging a cell, which is necessary for a better charge acceptance of the nickel electrode, oxygen evolution takes place. In the case of good access to the negative electrode oxygen can be directly recombined at the zinc electrode or an auxiliary electrode can be incorporated to enhance recombination. After repeated cycling also hydrogen evolution can take place at the zinc electrode. To minimize the hydrogen amount a sufficient Zn0 excess has to be provided. In general a Zn : Ni ratio between between 2 and 3 should be established. Furthermore to avoid zinc corrosion in alkaline medium corrosion inhibitors (In, Pb, Hg, organic compounds) have to be added.
In nickel-zinc cells different types of nickel electrodes are used: sintered, nonsintered and lightweight substrates. 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. Nonsintered 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 which could be hermetically sealed and showing an acceptable cycle life at deep discharge conditions.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cut through a cylindrical AA-size Ni-Zn cell made according to this invention.
FIG. 2 shows a nickel electrode with two layers from the top and a three-dimensional view.
FIG. 3 shows multiple (three) sleeves of a nickel electrode from the top and a three-dimensional view.
FIG. 4 shows the discharge capacity of cell A75, A77 and A79 with one nickel layer as a function of cycles.
FIG. 5 shows the discharge capacity of cell A71 and A86 with two nickel layers as a function of cycles.
FIG. 6 shows the discharge capacity of cell A81 and A87 with one nickel layer charged with constant current as a function of cycles.
FIG. 7 shows the discharge capacity of cell A121 containing 2 % and cell A79 with 8.6 % Ni powder / T-210 as a function of cycles.
FIG. 8 shows the discharge capacity of cell A128 containing 2 % and cell A131 with 0 % Co extra-fine powder as a function of cycles.
FIG. 9 shows the material utilization of cell C23 with one nickel layer and cell C21 with two nickel layers as a function of cycles. The highest discharge capacity value of cell C23 (6.cycle) is taken as 100 %.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 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.
FIG. 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 FIG. 3 differs from that of FIG. 2 in that, the nickel electrode is prepared by three or multiple sleeves of a nickel foam filled with nickel hydroxide mixture.

EXAMPLES OF THE INVENTION
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 FIG. 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-fme 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. Three different nickel foam types were used to prepare a nickel electrode as described above: Retec 80 PPI, 1.6 mm thick, Retec 110 PPI, 1.6 mm thick (both from RPM Ventures, ELTEC Systems Corp., Ohio) and Inco foam, 2.7 mm thick (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 gelous 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 LiOHxHzO
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 FIG. 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. FIG. 4 shows the discharge capacity of each cycle of cylindrical AA-size nickel zinc cells A75, A77 and A79 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 behaviour 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. Cell A77 contains less nickel hydroxide and therefore delivers lower capacity.
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. Two different foam types according with example 1 were taken to prepare the nickel electrode. Inco foam, 2.7 mm thick, could not be used in a douple layered arrangement because of its high thickness resulting in a deficiency of positive zinc electrode. In FIG. 5 the discharge capacity of each cycle of cylindrical AA-size nickel zinc cells A71 and A86 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 was constructed as described in Example 1. Two different foam types were used to build a cylindrical AA-size nickel zinc cell: Retec 110 PPI, 1.6 mm thick and Inco foam, 2.7 mm thick. FIG. 6 shows the dependence of discharge capacity of each cycle of cell A81 and A87 on cycle life. Both cells were charged with constant current of 66 mA (for 320 min) respectively of 81 mA (for 425 min). The cells were tested up to 60 cycles and the results are comparable to FIG. 4 with a rather flat profile of discharge curve. Cell A81 contains 42 %
less nickel hydroxide than cell A87 and delivers therefore only 300 mAh.
Example 4 A cell was assembled as described in Example 1 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 that case it was not necessary to add water to this light suspension that easy penetrates into the Inco foam, 2.2 mm thick. In FIG. 7 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 bigger amount of pasted cathode mass as indicated in the following table:
Cell No. Nickel Foam T a Nickel Cathode A79 Inco / 2.7 mm 2.88 A121 Inco / 2.2 mm 4.44 Example 5 Two cells were built as decribed in Example 1 with 2% and 0 % of cobalt 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 foam material. FIG. 8 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:
Cell No. Nickel Foam T a Nickel Cathode A128 Inco / 2.2 mm 3.44 A 131 Inco / 2.2 mm 3.51 Example 6 Cylindrical C-size nickel zinc cells were assembled as described in Example 1 with Inco foam, 2.2 mm thick. In cell C23 with one nickel layer the size of nickel foam was 38 mm x 70 mm and for cell C21 containing two nickel layers a 38 mm x 130 mm foam was used. A nickel foil current collector (70 and 130 mm respectively x 4 mm, 0.125 mm thick) was spotwelded at the longitudinal direction of both nickel foams.
Cell cycling was earned out as described in Example 1 with the exception that charging time was 760 minutes. In FIG. 9 the material utilization of each cycle of cylindrical C-size nickel zinc cell C23 and C21 can be seen. The highest discharge capacity value of cell C23 (6.cycle) is taken arbitrary as 100 %. The discharge capacity of cell C23 with one nickel layer is approximately 20 % higher compared to cell C21 with two nickel layers although both cells are pasted with the same amount of nickel hydroxide. From this experiment it is obvious that cells with thinner nickel electrodes (one layer) deliver much better utilization of active mass than cells with thicker electrodes (two layers).

Claims

What is claimed is:

1. A rechargeable galvanic element with a nickel oxide cathode and a zinc anode containing a gelled alkaline electrolyte and having a microporous separator, thereby characterized, that the cathode consists of a nickel foam structure which is filled with a nickel hydroxide-rich paste.

2. Said filled foam structure is compressed to a sheet of defined thickness, rolled up into one or more layers and inserted into a Ni-plated steel can.

3. Said filled foam can be compressed into a multiple of sleeves (e.g. 3 or 4) which are then inserted into the can from the top, also forming a cylindrical cathode.

4. Said anode consists of zinc powder, zinc oxide and a gelling agent, like Carbopol, and the anode capacity is a multiple, e.g. double or triple, of the cathode capacity.

5. The separator of the cell of claim 1 is preferably of the cellulosic type.

6. The collector (negative terminal) is a brass nail in the center of said cell.

7. The cathode of the cell of the invention contains a percentage of cobalt powder.

8. The cathode contains a silver catalyst to recombine the corrosion hydrogen and the concentration of the Ag-catalyst is preferred to be between 0.1 and 0.3 %
/wt.
of the nickel hydroxide and is incorporated into the Ni-foam cathode of claim 1 as colloidal deposit by a spray-coating process.

9. The electrolyte is a solution of potassium hydroxide with lithium hydroxide as additive. The KOH concentration is in the range of 6 to 9 molar and the LiOH is dissolved in the range of 1 % to the saturation point.

10. The charging of the nickel-zinc cell made according to claim 1 and the following claims is done by a voltage limited charging circuit, constant current charging, or an electronically controlled overflow circuit bypassing excess current above 1.95 V.