NL7907908A - CATHODE FOR AN ELECTROCHEMICAL CELL AND CELL FITTED WITH THIS CATHOD. - Google Patents

Description

- 1 -

Cathode for an electrochemical cell, as well as a cell equipped with this cathode.

The invention relates to cathodes for electrochemical cells, in particular rechargeable electrochemical cells containing monovalent silver oxide, and more particularly to cathodes with improved chargeability.

In the conventional cell containing monovalent silver oxide, the silver oxide is formed into the cathode and placed in a cell housing, and electrolyte is added. A barrier or separator is placed in the cell to separate the cathode from an anode, which is also incorporated in the cell. In the finished cell, the electrolyte is in contact with the cathode, the anode, the separator, and the housing. Upon discharge of the cell, the monovalent silver oxide in the cathode is reduced to 15 elemental silver. The reduction of the silver oxide is believed to occur in layers, starting at the interface of the cathode and the separator, and continuing away from the separator.

In order to use the silver oxide cathode in a secondary cell, the layering process must be reversed during the recharge cycle. In most cases, the layering process cannot be completely reversed, and this results in an incomplete conversion of the silver to 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-charge cycles that the cell can undergo before it is completely depleted.

Due to the inability to completely reverse the layering process, attempts to recharge the cell may result in overcharging. When transferring a cell containing silver oxide, gas such as hydrogen and oxygen can form on electrodes. These gases cause a build-up of pressure in the cell, which in turn causes the cell to break.

Another factor, which limits the use of monovalent silver oxide in secondary cells, is the inability to determine the decomposition state of the cell and thereby recharge the correct time on the cell. Determination of the decomposition state is difficult because a 5 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 its full discharge that a completely new one charging is no longer possible. The number of discharge-charge cycles is then IQ. lower than if the cell had only been partially discharged. Furthermore, with multi-cell batteries, complete discharge of one cell for the other will result in reverse cell operation with possible undesirable effects such as gas evolution and leakage.

J-5 The inability to determine the decomposition state of the silver oxide cell has limited the use of such cells to primary batteries and to states involving time-controlled discharge-charge-charge cycles. However, such time-controlled cycles can still result in cells being discharged to a point where they no longer readily recharge, since only time but not actual cell capacity is measured. For these and other reasons, hitherto monovalent silver oxide has not been widely used in secondary cells.

It is now an object of the invention to provide a cathode which significantly improves the ability of a secondary monovalent silver oxide-containing cell to undergo repeated discharge-charge cycles.

To this end, the cathode according to the invention contains monovalent silver oxide and a metal, selected from the group of silver, nickel and mixtures thereof, dispersed almost homogeneously through the cathode.

It is believed that the improved rechargeable lead of this cathode is due to the fact that upon discharge of the cell, the added metal causes the elemental silver produced in the cathode to be deposited almost uniformly by the 40 cells instead of layers being formed. At 790 7 9 08-3 recharging, the deposited elemental silver is converted almost uniformly into monovalent silver oxide. The degree of cell rechargeable is increased if the cell is charged before it is completely depleted. The less elementary silver is formed and the less the anode is discharged, the better the cell can be charged, as the more silver oxide is present in the matrix of elementary silver, the more uniformly the silver is converted to silver oxide.

IQ Silver or nickel or a mixture of silver and nickel is used in the cathode, since in the presence of each (or a mixture of both, elemental silver will form substantially uniformly throughout the cathode rather than in layers. The added silver is in the form of a finely divided powder, which preferably has a particle size of 3 to 10 microns The added nickel is preferably in the form of a powder, which contains irregularly shaped particles of small cross section and with an average particle size of about 20 2 to 4 microns and a large specific surface area.

A preferred nickel powder is carbonyl nickel, especially carbonyl nickel sold under the trademark "Type 255 Carbonyl Nickel" by The International Nickel Company, Inc., New York, N.Y. 10005. This preferred nickel 25 has a very large specific surface area and a very low apparent density.

Nickel is the preferred additive for the cathode; it is cheaper than silver and due to the large specific surface area of the preferred shape for nickel 3Q, less nickel is needed in the cathode than it would be for silver to achieve the same results. With the current price increase for silver, acceptable substitutes are increasingly important.

A further function of the silver and / or nickel in the cathode is to reduce any bivalent silver peroxide present that forms during the charge cycle. The presence of silver peroxide would be very detrimental to the functioning of the cell, since the silver peroxide produces a higher voltage on discharge than 4G the monovalent silver oxide, and this span can be powered by a 79079 08-4 device powered by harm the cell.

The presence of elemental silver and / or nickel in the cathode at the time the cell is assembled allows a fully charged cell to be charged somewhat further when it is first used, since it may be bivalent silver peroxide generated during this charge will react with the metal present and be reduced to monovalent silver oxide. The amount of overcharge that the cell can tolerate depends on the amount of metal present in the cathode, and on the amount of partial discharge built into the anode in the cell. This is an important aspect, since in many applications the cell may have been subject to a charge period before any discharge has occurred.

The cathode of the present invention can be modified to contain, in addition to its solid components, a second active material selected from mercury II-20 oxide, cadmium oxide, cadmium hydroxide, and manganese dioxide, which material generates a burst voltage upon discharge. This voltage jump indicates that it is time to recharge the cell. This allows the cell to be charged before it is completely depleted, so that it more easily accepts a new charge. The jump voltage is generated after the monovalent silver oxide has been discharged to exhaustion relative to the anode, at which time the second active material begins to discharge. A measurable voltage drop or jump can then be detected, since the discharge of the monovalent silver oxide to zinc is 1.55 volts, while that of the second active material to the zinc is 1.35 volts for manganese dioxide, 1, 3 volts for mercury II oxide, 0.9 volts for cadmium hydroxide, 35 and 0.5 volts for cadmium oxide.

The voltage jump can be seen visually, e.g. when a display indicator fades, or it can be measured electronically. An electronic circuit can be used to monitor the voltage of the battery, 40 and a low-energy device such as a light-emitting diode 790 7 9 08 - 5 - can be used by the circuit to indicate that the voltage has dropped out and it is time to reload the cell. Furthermore, the electronic circuit can automatically charge the cell after the voltage drop.

The relative amounts of monovalent silver oxide, elemental silver or nickel or a mixture thereof, and the second active material contained in the cathode determine the relative discharge periods of each. Preferably, the discharge period of the monovalent silver oxide is from about 50 to 80% of the total capacity of the cell.

A cell according to the invention, discharged to this degree, can easily be charged again. Such a discharge period requires an amount of second active material from about 60 to about 20% by weight of the cathode, the remainder of the cathode being silver oxide and the metal.

The amount of silver or nickel or a mixture thereof present in the cathode should be sufficient to allow the monovalent silver oxide to be reduced substantially uniformly through the cathode and to react with any bivalent silver peroxide formed . The amount of the silver or nickel or mixture thereof in the cell cannot be large since it reduces cell capacity. The preferred amount of silver or nickel or a mixture thereof is from about 10 to about 40 wt.

% of the cathode.

The anode of a cell containing the cathode according to the invention preferably contains amalgamated zinc and zinc oxide with a suitable gelling agent. The zinc oxide is present in the anode so that the cell can be charged immediately when installed. With the zinc oxide present in the anode, the charge can take place without overloading and producing unwanted side reactions such as gas production. The gelling agent absorbs the electrolyte and swells the anode, stabilizing the electrolyte within the cell.

The electrolyte used in the cathode according to the invention is preferably of an alkaline type.

The preferred electrolyte contains 30 to 40 wt. % potassium-4Q hydroxide or sodium hydroxide, the rest being water.

790 7 9 08 v .- ** - 6 -

Electrolite can additionally add 4 to 8 wt. % zinc oxide. This zinc oxide helps to prevent gassing during the use of the cell.

The number and efficiency of the discharge-charge cycles of a cell containing the cathode of the invention can be further improved by using an improved separator or barrier member between the cathode and the anode. This separator preferably includes at least one layer selected from regenerated cellulose (cellophane), irradiated polyethylene, porous polyvinyl chloride, and microporous polypropylene.

The preferred barrier layer member contains a double layer of barrier layer material such as a 0.03 mm layer of cellophane and a 0.03 mm layer of irradiated polyethylene. The most preferred baffle member consists of two layers of 0.03 mm cellophane and two layers of 0.03 mm irradiated polyethylene.

The invention will now be further elucidated with reference to the drawing. In the drawing: Fig. 1 shows a cross section of a cell according to a preferred embodiment of the invention; T'r ': .....

Fig. 2 is a graph plotting the average discharge voltage as a function of time for a 10 cell group, Fig. 3 is a graph comparing the average capacity of a 20 cell group with two cell rows at each cycle of a series of discharge-charge cycles, and FIG. 4 is a schematic representation of the use in a possible application of a cell manufactured according to the invention.

In Figure 1, a single "button" cell is shown.

The cathode holder 12 can be of any suitable material that does not react with the cathode 14, e.g. nickel plated steel. Cathode 14 contains monovalent silver oxide, mercury II-35 oxide, and elemental silver or nickel or a mixture thereof, dispersed almost uniformly through cathode 14. The mercury II oxide can be replaced with other materials, such as manganese dioxide, cadium oxide and cadmium hydroxide, which have a discharge voltage different from that of the silver oxide. The cathode 14 is separated from a 790 7 9 08 - 7 anode 16 by a barrier layer or separator 18 and an absorption layer 19.

The baffle member 18 preferably contains two or more layers of baffle 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 form the separator. This separator resists bridging or dendrites between the cathode 10 and anode during charging. Such material bridges short-circuit the cells in which they form. The baffle member of the cell of the invention thus helps to prevent short circuits during charging.

The absorption layer 19 is shown located between the anode 16 and the separator 18. This absorption layer 19 keeps the electrolyte within the cell. The absorption layer 19 consists of a mat of non-woven cotton felt.

Anode 16 contains a mixture of zinc, zinc oxide 20 and mercury. The anode holder 20 may consist of any suitable conductive material, the preferred material being steel with nickel outer plating and copper inner plating. It goes without saying that there should be no reaction with the anode 16. The anode holder 25 is held in place and electrically separated from the cathode holder 12 by a ferrule 22. The ferrule 22 prevents any material from escaping enter or enter the cell. The electrolyte can be any suitable alkaline electrolyte of known type.

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

In Figure 2, line A graphically depicts the average discharge voltage V as a function of time T (hours) for a 10 cell group. Each cell contains a cathode made according to the invention, which contains monovalent silver oxide, mercury II oxide and elemental silver, dispersed substantially uniformly through the cathode, an amalgamated zinc and zinc oxide anode, and an aqueous potassium hydroxide electrolyte .

40 Initially, the 10 cells discharge at 1.5 volts, the formation of monovalent silver oxide. After an average time of about 102 hours, the voltage drops from 1.5 volts to 1.3 volts, indicating depletion of the silver oxide component of the cathode and initiation of the discharge of the mercury II oxide. This voltage drop 5 is the signal that it is time to recharge the cell. Of the actual 10 cells in the group, the earliest mercury II oxide discharge was after 100 hours and the latest after 104 hours.

The voltage drop between 120 hours and 130 hours is characteristic of the discharge curve of a cathode containing mercury II oxide. This voltage drop occurs because as the cell discharges, the surface area of the anode decreases and the amount of available electrolyte decreases. These two factors cause the internal resistance of the cell to increase. This increase in internal resistance is evidenced by a voltage drop in the voltage of the cell. The mercury II oxide was exhausted in all 10 cells between 150 hours and 165 hours. The average total capacity of each of the 10 cells was approximately 160 hours as was determined from the sharp debris and stress shown in Figure 2. After 165 hours, none of the cells were out of action.

Curve B shows the discharge curve of a typical cell, which has a capacity of 160 hours, and which consists of a conventional silver oxide cathode and a zinc anode.

This cell begins to discharge at 1.5 volts and continues to discharge at this voltage until substantially all of the silver oxide in the cathode is exhausted. Typically, the silver oxide cathode cell discharges at a constant voltage to nearly complete exhaustion, at which point the 3Q voltage drops rapidly. The constancy of the voltage makes it difficult to determine the state of discharge of the cell. This problem has been solved by the addition of a second active material such as mercury II oxide to produce a voltage jump at some point in the discharge of the cell, as shown in curve A.

Fig. 3 shows the ability of a cell containing the cathode of the invention to accept a charge after repeated discharge-charge cycles. The amount of charge C (mA-hour) that the cell can accept, decreased by the number of cycles N, and reaches a fixed level 790 7 9 08 Λ ~ * - 9 - of about 55 mA-hour after about 15 cycles . This fixed level has been shown to persist for at least 50 cycles.

Fig. 3 shows that a cell discharged to the point where the silver oxide is depleted but the mercury II oxide 5 and then recharged can provide much more mA-hours of useful consumption than a cell , which is simply discharged to full exhaustion.

Fig. 4 is a schematic block diagram of the use in a possible use of a cell made according to the invention with a cathode containing mercury II oxide as well as silver oxide. The cell 2 supplies power to an electronic device 1, eg a watch or calculator, and in conjunction with this 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 depletion of the silver oxide and the subsequent discharge of the mercury II oxide, the cell monitoring circuit 3 detects this change and generates a signal which makes a light-emitting diode 4 operate . The voltage generated by the mercury II oxide within the cathode supplies enough voltage to power the electronic circuit and the light-emitting diode. The signal generator 3 may, of course, be designed to generate other signals as desired, to inform the user that the voltage has dropped and it is time to restart the cell. load.

The following examples of preferred embodiments of the invention have been given to illustrate the construction and the efficiency of the invention. In these examples, as well as throughout the description and claims, all percentages are weight percentages unless otherwise indicated.

EXAMPLE I

A cell was constructed using the following materials and procedure. The cathode contains 49% monovalent silver oxide, 3% manganese dioxide, 30% mercury II oxide and 18% silver, dispersed almost homogeneously throughout the cathode. The anode contains 15% zinc oxide, 4% an amalgam of 68% zinc and 13% mercury, and 4% of a gelling agent. Four layers of a retaining layer material, consisting of 790 7 9 08 - 10 - v * * · consisting of two layers each. of cellophane and irradiated polyethylene, were used to form a barrier between the anode and the cathode. A layer of non-woven cotton felt was used as an absorbent. The cathode, the anode, the baffle member, the absorbent, and an electrolyte, which consisted of 35% potassium hydroxide, 6% zinc oxide, and 59% water, were placed in a cell housing with the electrolyte in contact with the anode, the cathode , the baffle organ, the absorbent, and the housing. The same lid IQ was then crimped into place with a ferrule.

The discharge curve A in Fig. 2 is the average of 10 cells constructed in this way.

EXAMPLE II

A cell was constructed using the following materials and procedure. The cathode contained 52% monovalent silver oxide, 3% manganese dioxide, 30% mercury II oxide, and 15% nickel dispersed almost homogeneously throughout the cathode. The anode contained 15% zinc oxide, an amalgam of 68% zinc and 13% mercury, and 4% of a gelling agent. Four layers of a barrier layer material, consisting of two layers each of cellophane and irradiated polyethylene, were used to form a barrier between the anode and the cathode. A layer of non-woven cotton felt was used as an absorbent. The cathode, the anode, the barrier, the absorbent, and the electrolyte, consisting of 35% potassium hydroxide, 6% zinc oxide, and 59% water, were placed in a cell housing, the electrolyte being in contact with the anode, the cathode, the baffle organ, the absorbent, and the housing. Then the same lid was crimped into place with a ferrule. This cell had an impurity curve similar to that of curve A in Figure 2 on discharge.

- conclusions - 790 7 9 08

Claims (10)

An electrochemical cell cathode and containing monovalent silver oxide, characterized in that a metal consisting of nickel, silver, or a mixture of nickel and silver is dispersed almost homogeneously through the cathode. Cathode according to claim 1, characterized in that the metal is nickel. Cathode according to claim 2, characterized in that the nickel is a carbonyl nickel. Cathode according to claim 1, 2 or 3, characterized in that the cathode contains the metal in an amount of from about 10 to about 40 wt. % of the cathode. Cathode according to claim 1, 2, 3 or 4, characterized in that "a second active material, selected from mercury II oxide, cadmium oxide, cadmium hydroxide and manganese dioxide, is added to the cathode. Cathode according to claim 5, characterized in that the cathode contains the second active material in an amount of about 20 to 60 wt. % of the cathode. 7. Electrochemical cell, characterized by a cathode according to any one of the preceding claims, an anode, preferably consisting of zinc, a separator between the cathode and the anode, and an electrolyte arranged inside the cell in contact with the cathode, the anode , and the separator. Cell according to claim 7, characterized in that the anode contains zinc, mercury and zinc oxide. 790 7 9 08 -12 “τν Cell according to claim 7 or 8, i e t h. It is characterized in that the separator has at least one layer selected from regenerated cellulose, irradiated polyethylene, porous polyvinyl chloride, and microporous polypropylene. Cell according to claim 7 or 8, characterized in that the separator has at least one layer of regenerated cellulose and at least one layer of irradiated polyethylene. 790 7 9 08