WO1993012552A1

WO1993012552A1 - Alkaline zinc cell with improved current collector

Info

Publication number: WO1993012552A1
Application number: PCT/CA1992/000551
Authority: WO
Inventors: Karl Kordesch; Josef Daniel-Ivad
Original assignee: Battery Technologies Inc.
Priority date: 1991-12-19
Filing date: 1992-12-21
Publication date: 1993-06-24
Also published as: AU3154393A; HU914041D0

Abstract

An alkaline zinc cell having an improved current collector, comprising a cathode, a zinc anode, a metal current collector in contact with the anode, a separator between the anode and cathode and an alkaline electrolyte. A gold coating is provided on the current collector, which prevents corrosion of the zinc in the anode if the cell is overdischarged.

Description

ALKALINE ZINC CELL WITH IMPROVED CURRENT COLLECTOR

FIELD OF THE INVENTION:

The invention relates to an alkaline zinc cell with improved current collector, more particularly to the current collector of a low mercury or mercury free secondary cell.

BACKGROUND OF THE INVENTION:

The properties of the state of the art secondary alkaline zinc electrodes includin g those of the current collectors are well summarized in the paper of F.R. McLarnon and E.J. Cairns: "The Secondary Alkaline Zinc Electrode" published in the J. Elechtrochem. Soc. Vol 138 No. 2. February 1991 (pp. 645 to 664).

The zinc electrode current collector must exhibit a sufficiently high electronic conductivity to avoid excessive ohmic potential losses at high current densities, yet be lightweight and inexpensive. Typical current collectors are high area sheets such as expanded copper foil or perforated copper foil. Copper current collectors can be coated with a high hydrogen overpotential metal, such as lead, cadmium, indium, tin or bismuth to prevent copper dissolution during overdischarge. Organic compounds, such as polyvinyl alcohol and carboxy ethyl cellulose, have also been used as protective coatings for current collectors.

In case of rechargeable (secondary) cells with low mercury or zero mercury content, the corrosion of the zinc electrode material is the basic hazard that should be minimized. If mercury is present, amalgamation takes place at the surface of the current collector and of zinc particles which reduces corrosion.

If the current collector, which is generally brass or copper, is coated by the metals suggested in the prior art like cadmium, indium and tin, etc. the coating material dissolves in the electrolyte if the cell is overdischarged, and this leads to a partial dissolution of the copper-based substrate. The dissolved metals contaminate the zinc particles resulting in an increased gassing. The object of the invention is to provide a current collector for alkaline zinc cells, particularly for low mercury or mercury free cells, in which there is a substantially decreased corrosion when the cell is overdischarged.

It was found according to the invention that a gold coating on the current collector protects the base metal from dissolution under overdischarge conditions, and thereby protects the zinc anode from parasitic impurities from the current collector. The impurities that would otherwise come from the base metal would increase the hydrogen gas evolution from zinc, which is detrimental in low mercury or mercury free cells.

This property of the gold coating is quite unexpected, since noble metals like platinum, palladium etc. increase the rate of hydrogen evolution from zinc in alkaline solutions. Noble metals, including gold, have low hydrogen overpotential which, according to prior art principles, is unacceptable as a coating material for the current collector. Gold, however, does not accelerate gassing because it gets rapidly plated with zinc when immersed in a conventional zinc anode gel. When used in cells having zinc-limited anodes, the zinc is completely used up at complete discharge, and metal gold appears but it does not dissolve. The metallic gold thus isolates the underlying base metal from dissolution.

BRIEF DESCRIPTION OF THE DRA INGS:

The invention will now be described in connection with a preferred embodiment thereof, in which reference will be made to the accompanying drawing. In the drawing:

Figure 1 is a sectional elevational view of a cell embodying the invention; and

Figure 2 is a sketch of the test cell arrangement for studying the behavior of the current collector. DESCRIPTION OF THE PREFERRED EMBODIMENTS :

Figure 1 shows a cross sectional elevation view of an alkaline manganese dioxide zinc rechargeable cell 10. The cell comprises the following main units: a steel can 12 defining a cylindrical inner space, a manganese dioxide cathode 14 formed by a plurality of hollow cylindrical pellets 16 pressed in the can, a zinc anode 18 made of an anode gel and arranged in the hollow interior of the cathode 14, and a cylindrical separator 20 separating the anode 18 from the cathode 14. The ionic conductivity between the anode and the cathode is provided by the presence of potassium hydroxide electrolyte added into the cell in a predetermined quantity.

The can 12 is closed at the bottom and it has a central circular pip 22 serving as positive terminal. The upper end of the can 12 is hermetically sealed by a cell closure assembly which comprises a negative cap 24 formed by a thin metal sheet, a current collector nail 26 attached to the negative cap 24 and penetrating deeply into the anode gel to provide electrical contact with the anode, and a plastic top 28 electrically insulating the negative cap 24 from the can 12 and separating gas spaces formed beyond the cathode and anode structures, respectively. The preferred material for making the current collector nail 26 is brass (Cu-Zn alloy), bronze (Cu-Sn alloy) or copper, with either of these alloys plated by gold. The coating thickness has no specific significance; it is sufficient if a uniform coating is provided.

In the test arrangement shown in Figure 2 the behavior of the current collector nail 26 was studied, with the visual control under a microscope.

The test cell contains a discharged alkaline zinc anode gel (ZnO/KOH), a counter electrode as the positive terminal, and the nail of interest as the negative terminal. The circuit is completed via a power supply. The surface reactions of the nail were monitored via a microscope that was located above the nail of interest.

A current of 25 mA was driven through the test cell arrangement with the anodic reaction at the tested nail. The test was carried out with a brass nail and with a brass nail having a gold coating. Under the microscope, dissolution of the brass nail was noticed; whereas the gold plated brass nail behaved in a stable manner, and the anodic reaction was oxygen evolution without any dissolution of the metal.

The reactions taking place in the test arrangement were as follows:

Anodic Oxidation

Brass Nail Gold Plated Brass Nail

Cu + 2 0H~ > Cu(OH)₂ + 2e^" 2 OH > | 0₂ + H₂0 + 2e~

Zn + 2 OH^" > Zn (OH) ₂ + 2e^~

Cathodic Reduction

At The Counter Electrode Zn (OR) ₂ + 2e~ > Zn + 2 0H^~

To demonstrate the positive influence of the gold plated current collector nail, two groups of mercury free AA size rechargeable alkaline manganese dioxide-zinc cells were made, each cell being as shown in Figure 1. Each group comprised 20 cells. In the first group, the current collector nail was gold plated according to the present invention, while in the other (control) group the current collector nail was made of brass without plating or gold coating of any sort. All other components of the cells in the two groups were the same (i.e. standard cathode, mercury free anode, separator, and electrolyte — both as to type and amount of each).

The zinc capacity of all cells was discharged by applying a 50 mA constant current load until the voltage dropped to 0.9 V. These discharged cells were then further discharged with 25 mA constant current for 1 hour. The forced overdischarge was necessary to remove the zinc plating and to determine the effect of the collector base metal, since both the brass and the gold plated nails become zinc plated j α the alkaline anode gel. After the forced overdischarge, the cells were charged for 15 hours with a conventional taper charger using 1.72 V constant charging voltage. The charged cells were then stored at an elevated temperature of 55^* C temperature, and no deformation or venting was observed. This indicates that the pressure was within the permitted limits — i.e. no bulging occurred.

The advantages of the gold coating of the current collector are most apparent in secondary cells. However, the performance of mercury free alkaline zinc primary cells can also be improved by using the present invention. In primary cells, the corrosion causes problems in the case of intermittent discharge combined with long storage at elevated temperatures.

Claims

WHAT IS CLAIMED IS:

1. An alkaline zinc cell (10) having an improved current collector, where said cell comprises a cathode (14), a zinc anode (18), a metal current collector (26) in contact with said anode, a separator (20) between said anode and cathode, and an alkaline electrolyte, c h a r a c t e r i z e d in that there is a metallic gold coa ing on said current collector.

2. The cell as claimed in claim 1, wherein said current collector is made of a metal or alloy selected from the group consisting of brass, copper and bronze.

3. The cell as claimed in claim 2, wherein said cell is a low mercury or mercury free manganese dioxide-zinc cell.

. The cell as claimed in claim 2, wherein said cell has a cylindrical design, and said current collector is a brass nail extending axially in the middle of the cell.

5. The cell as claimed in claim 2, wherein said cell is a rechargeable cell.

6. The cell as claimed in claim 3, wherein said cell is a rechargeable cell.

7. The cell as claimed in claim 4, wherein said cell is a rechargeable cell.