GB2033139A

GB2033139A - Bilevel rechargeable cell

Info

Publication number: GB2033139A
Application number: GB7935998A
Authority: GB
Original assignee: PR Mallory and Co Inc
Current assignee: Duracell Inc USA
Priority date: 1978-10-30
Filing date: 1979-10-17
Publication date: 1980-05-14
Also published as: IL58433A0; JPS5560275A; NL7907908A; CA1149868A; GB2033139B; SE7908806L; AU522825B2; FR2440622A1; DK456279A; BE879687A; DE2942921A1; IT7926891A0; AU5186879A; IT1124816B; IL58433A

Abstract

A rechargeable cell has an anode, a cathode, barrier means between the cathode and anode, and an electrolyte in contact with the elements of the cell, the cathode being formed from a mixture of monovalent silver oxide and finely divided silver, nickel, or a mixture of silver and nickel. The cathode may also contain a second active material, e.g. HgO, to provide a discharge voltage which drops in a step before the cell is fully discharged, to indicate that recharging is needed. The added silver and/or nickel enhances rechargeability.

Description

SPECIFICATION Bilevel rechargeable cell This invention relates to cathodes for rechargeable electrochemical cells containing monovalent silver oxide and more particularly to cathodes having improved rechargeability.

In the usual cell containing monovalent silver oxide, the silver oxide is formed into a cathode and placed in a cell casing, and an electrolyte is added. A barrier or separator is positioned within the cell to separate the cathode from an anode which is also incorporated into the cell. In the completed cell the electrolyte is in contact with the cathode, anode, separator and casing. On discharge of the cell, the monovalent silver oxide in the cathode is reduced to elemental silver. The reduction of the silver oxide is believed to occur in a generally layered fashion, beginning at the interface of the cathode and separator and proceeding away from the separator.

In order to use the silver oxide cathode in a secondary cell, the layering process must be reversed during the recharging cycle. In most cases the layering process cannot be fully reversed, and this results in incomplete conversion of the silver into monovalent silver oxide. The partial reversibility of the layering process limits the capacity of the cell to accept a charge and limits the number of discharge-recharge cycles that the cell will undergo before complete exhaustion.

As a result of the inability of the layering process to be completely reversed, attempts to recharge the cell can result in an overcharge. During overcharge of a cell containing silver, oxide, gas such as hydrogen and oxygen form at the electrodes. These gases cause a pressure build-up in the cell which in turn causes the cell to fail.

Another factor limiting the use of monovalent silver oxide in secondary cells has been an inability to determine the state of discharge of the cell and thus the proper time to recharge the cell. The determination of the state of discharge is difficult because a cell containing monovalent silver oxide produces a flat voltage curve on discharge, and there is no apparent voltage drop until the cell is so close to complete discharge that it can not be fully recharged. The number of discharge-recharge cycles is then lower than if the cell were only part discharged. Further, in multicelled batteries, complete discharge of any one cell before the other will result in cell reversal, with possible undesirable effects such as gassing and leakage.

The inability to determine the state of discharge of the silver oxide cell has limited the use of such cells to primary batteries and to situations involving timed discharge-recharge-recharge cycles. Such timed cycles can still result in cells being discharged to the point where they cannot readily accept a recharge, since only time, not true cell capacity, is measured. For this and other reasons, monavalent silver oxide has not heretofore been widely used in secondary cells.

An object of the present invention is to provide a cathode that substantially improves the ability of a secondary cell containing monovalent silver oxide to undergo repeated discharge-recharge cycles.

The cathode of the present invention comprises monovalent silver oxide and a metal selected from the group consisting of silver, nickel and mixtures thereof, substantially homogeneously dispersed throughout the cathode.

It is believed that the improved rechargeability of this cathode is due to the fact that on discharge of the cell the added metal causes the elemental silver produced in the cathode to deposit substantially uniformly throughout the cathode instead of forming layers. On recharging, the deposited elemental silver is substantially uniformly converted into monovalent silver oxide. The degree of rechargeability of the cell is enhanced if the cell is recharged before total exhaustion of the cell. The less elemental silver formed and the more undischarged the anode, the better the cell can be recharged, since the more silver oxide present within the matrix of elemental silver, the more uniformly the silver is converted into silver oxide.

Silver or nickel, or a mixture of silver and nickel, is used in the cathode since, in the presence of either (or a mixture of both) elemental silver will form substantially uniformly throughout the cathode instead of in layers. The added silver is in the form of a finely divided powder preferably having a particle size of from about 3 to 10 microns (3 to 10 x 10-6 meters). The added nickel is preferably in the form of a powder comprising irregularly shaped particles of small cross section, having an average particle size of from about 2 to 4 microns (2 to 4 x 10-6 meters) and a large specific surface area. A preferred nickel powder is carbonyl nickel in particular that sold under the trademark of "Type 255 Carbonyl Nickel" by The International Nickel Company, Inc., New York, N.Y. 10005.The preferred nickel has a large specific surface area and a low apparent density.

Nickel is the preferred additive to the cathode; it is cheaper than silver and because of the large specific surface area of the preferred form of nickel, less nickle is needed in the cathode to achieve the same results as with silver. With the recent increases in the price of silver, acceptable substitutes are taking on greater importance.

Afurtherfunction of the silver and/or nickel in the cathode is to reduce any divalent silver peroxide formed during the recharging cycle. The presence of silver peroxide could be detrimental to the functioning of the cell, since the silver peroxide produces a higher voltage on discharge than the monovalent silver oxide and this overvoltage may damage a device powered by the cell.

The presence of the elemental silver and/or nickel in the cathode at the time the cell is assembled permits a fully charged cell to be further charged to some degree when first put into service, since any divalent silver peroxide produced during this charge will react with the metal present and be reduced to monvalent silver oxide. The degree of overcharge that the cell can tolerate is dependent on the amount of metal present in the cathode and on the degree of partial discharge built into the anode in the cell. This is an important feature, since in many applications the cell can be subject to a period of charge before any discharge has occurred.

The cathode of-the present invention can be modified so that it contains among its constituents a second active material selected from mercuric oxide, cadmium oxide, cadmium hydroxide and manganese dioxide, which material generates a step voltage on discharge. The voltage step indicates that it is time to recharge the cell. This permits the cell to be recharged before it is completely exhausted, so that it can more readily accept a recharge. The step voltage is produced after the monovalent silver oxide has been discharged to exhaustion against the anode, at which time the second active material begins to discharge.A measurable voltage drop or step can then be detected since the discharge of monovalent silver oxide against zinc is 1.55 volts, while that of the second active material against zinc is 1.35 volts for manganese dioxide, 1.3 volts for mercuric oxide, 0.9 volts for cadmium hydroxide, and 0.5 volts for cadmium oxide.

The voltage step can be observed visually, as when a display dims, or it can be measured electronically. An electronic circuit can be used to monitor the voltage of the battery, and a low-drain device, such as a light emitting diode, can be utilized by the circuit to indicate that the voltage has dropped and it is time to recharge the cell. Further, the electronic circuit can automatically recharge the cell after the voltage step.

The relative amounts of monovalent silver oxide, elemental silver or nickel or mixture thereof, and second active material present in the cathode determines the relative periods of discharge of each.

Preferably the period of discharge of the monovalent silver oxide will be from about fifty to about eighty percent of the total capacity of the cell. A cell of the present invention discharged to this degree can still be readily recharged. Such a discharge period requires an amount of second active material of from about sixty to about twenty percent by weight of the cathode, with the remainder of the cathode being silver oxide and the metal.

The amount of silver or nickel or mixture thereof present in the cathode must be sufficient to permit the monovalent silver oxide to be reduced substantially uniformly throughout the cathode, and to react with any divalent silver peroxide formed;-The amount of silver or nickel or mixture thereof in the cell cannot be large since it reduces the capacity of the cell. The preferred amount of silver or nickel or mixture thereof is from about ten to about forty percent by weight of the cathode The anode of a cell containing the cathode of the present invention preferably comprises amalgamated zinc, and zinc oxide with an appropriate gelling agent. The zinc oxide is present in the anode so that the cell can be immediately charged when it is installed.With the zinc oxide present in the anode, the charging will proceed without overcharging and the production of undesirable side reactions such as the production of gas. The gelling agent absorbs the electrolyte and causes the anode to swell, this stabilizes the electrolyte within the cell.

The electrolyte used with the cathode of the present invention is preferably of the alkaline type.

The preferred electrolyte comprises thirty to forty percent by weight of potassium hydroxide or sodium hydroxide with the remainder water. It may additionally contain four to eight percent by weight zinc oxide. The zinc oxide helps to prevent gassing during the use of the cell.

The number and efficiency of the discharge recharge cycles of a cell containing the cathode of the present invention can be further improved through the use of an improved separator or barrier means between the cathode and anode. The separ ator preferably includes at least one layer selected from regenerated cellulose ("Cellophane") irradiated polyethylene, porous polyvinyl chloride, and microporous polypropylene.

The preferred barrier means comprises a double layer of barrier material such as a 0.03 millimeter layer of "Cellophane" and a 0.03 millimeter layer of irradiated polyethylene. The most preferred barrier means comprises two layers of 0.03 millimeter "Cellophane" (registered Trade Mark) and two layers of 0.03 millimeter irradiated polyethylene.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a sectional view of a cell of the preferred embodiment of the invention, Figure 2 graphically illustrates the average dis charge voltage against time for a ten-cell group, Figure 3 is a graph comparing the average capacity of a twenty-cell group, in two cell arrays, at each cycle of a series of discharge-recharge cycles, and Figure 4 is a diagrammatic illustration of the use, in one possible application, of a cell made in accordance with the invention.

Figure 1 shows a single "button" cell. The cathode container 12 can be any suitable material that does not react with the cathode 14, such as nickel plated steei. The cathode 14 comprises monovalent silver oxide, mercuric oxide, and elemental silver or nickel or a mixture thereof, substantially uniformly dispersed throughout the cathode 14. The mercuric oxide can be replaced by other materials e.g.

manganese dioxide, cadmium oxide and cadmium hydroxide, having a discharge voltage different from that of the silver oxide. The cathode 14 is separated from an anode 16 by a barrier means or separator 18 and an absorbent layer 19.

Barrier means 18 preferably comprises two or more sheets of barrier material constructed of one or more layers of regenerated cellulose ("Cellophane") and one or more layers of irradiated polyethylene. In a preferred embodiment, two layers of each material are used to from the separator. This separator resists the formation of bridges or dendrites between the cathode and anode during recharge. Such bridges of material short circuit the cells in which they form.

The-barrier means of the present cell thus helps to prevent shortxcircuiting during recharge.

Absorbent layer 19 is shown in place between the anode 16 and the separator 18. The absorbent layer 19 holds theelectrolytewithin the cell. The absorbent layer 19 is a mat of non-woven cotton felt.

The anode 16 comprises a mixture of zinc, zinc oxide and mercury. The anode container 20 can be of any suitable conductive material, a preferred material being steel with an outer plating of nickel and an inner plating of copper. There must, of course, be no reaction with the anode 16. The anode container 20 is held in place and electrically separated from cathode container 12 by a grommet 22. Grommet 22 prevents any material from escaping from or entering the cell. The electrolyte can be any suitable alkaline electrolyte of the type well known in the art.

Preferred electrolytes are potassium hydroxide solutions which may contain adjuvants such as zinc oxide.

In Figure 2, line A graphically illustrates the average discharge voltage V versus tim T (hours) for a a ten-cell group. Each cell contains a cathode made in accordance with the present invention, comprising monovalent silver oxide, mercuric oxide and elemental silver substantially uniformly dispersed through the cathode, an amalgamated zinc and zinc oxide anode, and an aqueous potassium hydroxide electrolyte.

Initally, the ten cells discharged at 1.5 volts, the discharge voltage of monovalent silver oxide. At an average of about 102 hours, the voltage dropped from 1.5 volts to 1.3 volts, signaling the exhausting of the silver oxide component of the cathode and the beginning of the discharge of the mercuric oxide.

The drop in voltage was a signal that it was time to recharge the cell. Of the actual ten cells in the group, the earliest discharge of the mercuric oxide was at 100 hours and at least at 104 hours.

The voltage drop between 120 hours and 130 hours is typical of the discharge curve of a cathode containing mercuric oxide. This voltage drop occurs because as the cell discharges, the surface area of the anode decreases and the mount of available electrolyte decreases. These two factors cause the internal resistance of the cell to increase. The increased internal resistance becomes apparent as a drop in the voltage of the cell. Between 150 hours and 165 hours the mercuric oxide in all ten cells was exhausted. The average total capacity of each of the ten cells was about 160 hours as evidenced by the sharp drop in voltage shown in Figure 2. After 165 hours all the cells had ceased to operate.

Curve B shows the discharge curve of a typical cell having a capacity of 160 hours, containing a conventional silver oxide cathode and a zinc anode. The cell begins to discharge at 1.5 volts and continues to discharge at this voltage until almost all the silver oxide in the cathode is exhausted. Characteristically the cell containing the silver oxide cathode discharges at a constant voltage until almost complete exhaustion, at which point the voltage drops rapidly.

The constancy of the voltage makes it difficult to determine the state of discharge of the cell. The problem is solved by the addition of a second active material, such as mercuric oxide, to produce a voltage step at a certain point in the discharge of the cell, as shown in curve A.

Figure 3 shows the ability of a cell containing the cathode of the present invention to accept a charge after repeated discharge-recharge cycles. The amount of charge C(mA-hours) that the cell will accept decreases with the number of cycles N and reaches a plateau of about 55 milliampere hours after about 15 cycles. This plateau has been found to continue for at least 50 cycles. Figure 3 indicates that a cell, discharged to the point where the silver oxide, but not the mercuric oxide, is exhausted, and then recharged, will deliver many more milliampere hours of total service then a cell simply discharged to total exhaustion.

Figure 4 is a diagrammatic illustration of the use, in one possible application, of a cell made in accordance with the invention, with a cathode containing mercuric oxide as well as silver oxide.

The cell 2 powers an electronic device 1 such as a watch or calculator, and concurrently an electronic circuit 3 of known construction monitors the voltage output of the cell. When the voltage output of the cell drops, due to the exhaustion of the silver oxide and the subsequent discharging of the mercuric oxide, the cell monitoring circuit 3 detects this change and generates a signal that causes a light emitting diode 4to operate. The voltage produced by the mercuric oxide within the cathode supplies sufficient voltage to powerthe electronic device and the light emitting diode. The signal generator 3 can, of course, be designed to generate other signals as desired to inform the user that the voltage has dropped and that it is time to recharge the cell.

The following examples of the preferred embodiment of the present invention are given to illustrate the construction and efficacy of the present invention. In the examples as well as throughout the specification and claims all percentages are by weight unless otherwise indicated.

Example 1 A cell was constructed using the following materials and procedures. The cathode contained 49 percent monovalentsilver oxide, 3 percent manganese dioxide, 30 percent mercuric oxide and 18 percent silver substantially homogeneously dispersed throughout the cathode. The anode contained 15 percent zinc oxide, an amalgam of 68 percent zinc and 13 percent mercury, and 4 percent of a gelling agent. Four layers of a barrier material, comprising two layers each of "Cellophane" and irradiated polyethylene, were used to construct a barrier between the anode and cathode. A layer of non-woven cotton felt was used as an absorbent.

The cathode, anode, barrier means, absorbent and an electrolyte comprising 35 percent potassium hydroxide, 6 percent zinc oxide, and 59 percent water were placed in a cell casing, with the electrolyte in contact with the anode, cathode, barrier means, absorbent and casing. The cell top was then crimped into place with a grommet.

Discharge curve A, in Figure 2, is the average of ten cells constructed in this manner.

Example II A cell was constructed using the following materials and procedures. The cathode contained 52 percent monovalent silver oxide, 3 percent manganese dioxide, 30 percent mercuric oxide and 15 percent nickel substantially homogeneously dispersed throughut the cathode. The anode contained 15 percent zinc oxide, an amalgam of 68 percent zinc and 13 percent mercury, and 4 percent of a gelling agent. Four layers of a barrier material, comprising two layers each of "Cellophane" and irradiated polyethylene, were used to construct a barrier between the anode and cathode. A layer of nonwoven cotton felt was used as an absorbent. The cathode, anode, barrier means, absorbent and an electrolyte of 35 percent potassium hydroxide, 6 percent zinc oxide and 59 percent water were placed in a cell casing the electrolyte being in contact with the anode, cathode, barrier means, absorbent and casing. The cell top was then crimped into place with a grommet. This cell on discharge, had a discharge curve similar to that of curve A in Figure 2.

Claims

1. A cathode for an electrochemical cell, comprising monovalent silver oxide and a metal comprising nickel, silver, or a mixture of nickel and silver, the said metal being substantially homogeneously dispersed throughout said cathode.

2. A cathode as claimed in claim 1 in which the said metal is nickel.

3. Acathode as claimed in claim 2 in which the nickel is a carbonyl nickel.

4. The cathode of claim 1, 2 or 3 wherein said cathode contains said metal in an amount of from about ten percent to about forty percent by weight of said cathode.

5. A cathode as claimed in claim 1,2,3 or 4 further comprising a second active material selected from mercuric oxide, cadmium oxide, cadmium hydroxide and manganese dioxide.

6. The cathode of claim 5 wherein said cathode contains said second active material in an amount of from about twenty percent to about sixty percent by weight of the cathode.

7. A cell comprising the cathode of any preceding claim, an anode, a separator between said cathode and said anode, and an electrolyte positioned within said cell in contact with said cathode, anode and separator.

8. The cell of claim 7 wherein said anode comprises zinc.

9. The cell of claim 8 wherein said anode further comprises mercury and zinc oxide.

10. The cell of claim 7, 8 or 9 wherein said separator includes at least one layer selected from regenerated cellulose, irradiated polyethylene, porous polyvinyl chloride, and microporous polypropylene.

11. The cell of claim 7, 8 or 9 wherein said separator comprises at least one layer of regenerated cellulose and at least one layer of irradiated polyethylene.