DE2708372A1 - Prim. cell using zinc and silver oxide electrodes - where initial voltage can be rapidly reduced to constant value for driving watches - Google Patents

Abstract

Prim cell uses a negative Zn electrode, and a positive AgO electrode sepd. from its current collector by an isolating plastic layer (a); in operation, the cell has the discharge voltage of the Ag2O/Zn system. The AgO is joined to its collector only by a contact (b) with a surface (bs) so small that even with a cell current below 15 mu A, the formation of Ag2O and Ag on bs occurs more rapidly than the oxidn. of the Ag formed on surface bs. Contact (b) is pref. metal wire passing through the AgO metal pegs projecting through layer (a) into the AgO, or a ring. Surface (bs) touching the AgO is pref. below 10 mm2; and for discharge currents of 0.2-15, esp. 0.3-10 mu A, the current density on surface (bs) is pref. ca 10-1 milliamps/cm2. Used as button cells e.g. for electronic wrist watches.

Description

Galvanic element with negative zinc electrode

and positive silver oxide electrode The invention relates to a galvanic Element with negative zinc electrode and with an electronically non-conductive one Plastic layer from the positive arrester insulated positive electrode made of 2-valent Silver oxide (AgO), the cell using the discharge voltage of the Ag20 / Zn-S # stems owns.

The silver oxide / zinc systems with their high energy density are increasingly used for energy supply in the form of the handy button cell electronic wristwatches used. The more energetic AgO / Zn system is due the Vorrarlg, although it has the disadvantage that the discharge depends on Current load only during the first third of the discharge time with a voltage around 1.8 volts, while the remainder of the Erit charge takes place at voltages below 1.6 Volt takes place, as it corresponds to the Ag2O / Zn system.

The discharge is therefore associated with a voltage jump of approx. 250 mV, which would seriously disturb the synchronization of an electronically operated clock.

For a long time, efforts have therefore been made to ensure that the discharge is step-free on the lower voltage level of 1.6 V, but the capacity of the one used To take full advantage of divalent silver oxides.

T. P. Dirkse (J. Elektrnchem. Soc. 109 (1962) 3, pages 173 to 177) by finding that the unencumbered The tension of AgO tablets actually corresponded to the Ag20 / Ag system if they were previously an electrochemical partial discharge had been subjected.

Since then, many suggestions have become known, such as on an Ag0 tablet, for example by reducing it with chemical means or by pressing on, a thin one Layer of Ag20 can be produced so that the sets a lower cell voltage of the Ag20 / Zn system if, in addition to the Ag20, am Metallic Ag is also present in the arrester. For the arrester then that is 2-valent silver oxide, the actual active electrode substance, virtually masked, and only the top layer of Ag and Ag20 that is closest to it determines the potential.

Cells based on this regardless of their specific design the advantage that you immediately get the desired operating voltage on the lower Voltage level of 1.6 volts is available.

Due to the reduction, the tension is at the level of the Ag20 / Zn system stabilized, which is why a direct exchange against this system is possible.

On the other hand, it has an adverse effect on the stabilized cell the necessary reduction of the AgO as well as that which is often unavoidable during assembly Deformation of the tablet. The reduction costs the electrode a considerable amount Part of their AgO capacity and represents an additional work step Deformation tears open the Ag20 layer so that there is now no between AgO and Ag Separation, but there is an electrolyte-conductive connection and a corrosion stream can flow within the silver oxide electrode. This has the consequence that the metallic Silver is oxidized to Ag20 on the contact surface and AgO is reduced to Ag20.

Now the surface of the metallic silver is small compared to the While on the surface of the AgO, the Ag will eventually be entirely oxidized to Ag2O part of the AgO surface is retained, so that on the contact now instead of the Ag20 / Ag electrode an AgO / Ag20 electrode is available.

Accordingly, as the corrosion reaction proceeds, the cell becomes have intermediate voltage values between 1.6 volts and 1.85 volts and are on Adjust the end to the undesirable high tension of the AgO / Zn system.

In the practical execution of the cells one also has to follow the trend a downsizing of the electronic wristwatches, which is only due to this it is possible that the cells are also made smaller. With a button cell the desired dimensions of e.g. 8. 7.9 mm ~ x 2.1 mm is the relative usable volume to the inactive components such as housing, separator, etc. already so small that the more energetic AgO / Zn system, stabilized at 1.6 volts, compared to the Ag20 / Zn system no longer offers any significant advantage because of the required reduction of the AgO a considerable part of its capacity is lost.

According to DT-OS 2 506 399 it is possible to reduce this capacity loss to keep within limits that the positive AgO electrode plate in a Kunotetoffbecher that isolates them from the metal cup, the positive arrester, and so on only the open side of the tablet for chemical reduction treatment before Assembly releases. The electrical contact of the positive ground with the metal cup is produced by a contact ring lying on top of the open edge of the tablet.

This contact ring can be used to reduce the Agû tsblette Completion of the cell made by an adequate cathodic current will.

However, on the functional principle of the stabilized AgO / Zn cell Nothing changed with this construction either, since the AgO electrode is still used is coated on the contact side with Ag20, of which in turn a part is immediately adjacent the Kontaktobarfläche is reduced to Ag, so that an Ag20 # Ag electrode and the cell voltage is 1.6 V.

With the measures taken so far, so with regard to the Maintaining the greatest possible usable capacity with the aforementioned miniaturization the cell has not yet achieved a decisive breakthrough.

The invention is therefore based on the object. the capacity design the clock cell using divalent silver oxide as positive Electrode substance to design as much as possible and still use the entire capacity for power consumption up to to about 15 microamps and less at a voltage of 1.6 volts accordingly available to the Ag20 / Zn system.

This object is achieved according to the invention in that the positive Electrode with the positive arrester only via an electronically conductive one Contact is connected, the contact area of which with the silver oxide is so small is that even with a current draw of less than 151um the formation of 1-valent Silver oxide (Ag20) and metallic silver on the contact occurs faster than that Oxidation of the silver formed on the contact by being electrolyte-conductive connected bivalent silver oxide.

This sizing rule is intended in particular for power consumption in the area from 0.2 to 15 / uA, preferably in the range from 0.3 to 10 / uA, apply.

It is essential for the invention that the contact surface between the contact and the electrode body is so small that with currents in particular between 0S3 and 10 microamps result in current densities of a few t 10 1 / cm2 at the contact.

Under this condition, the prevailing one on the micro-account is cathodic current density high enough to generate the closest Agü powder relatively quickly to reduce to Ag20 and then to Ag. The speed of formation of the metallic silver at the contact must be greater than the rate of its oxidation through the AgO, to which the silver, yes, through the very thin, porous reduction layer from Ag2O is in electrolyte-conducting connection.

Inversely proportional to the surface area of the silver must however, the current density declines and the formation of new silver slows down and, on the other hand, its oxidation by the AgO come more into play.

After a certain flow of current there is formation and oxidative degradation the Ag in equilibrium with each other. The equilibrium is broken when the cathodically formed silver is insufficient due to insufficient current load, to replace the amount oxidized in the same time and therefore disappears completely. Then again only one Ag0 / Ag20 electrode would be potentialbastimmand at the contact, and the cell would have the voltage of the AgO / Zn system.

A practical embodiment of the cell according to the invention is e.g. obtained by placing the contact ring in the cell construction according to DT-OS 2 506 399 was replaced by a 0.1 mm thick metal wire, which the surface of the AgO tablet diametrically through it.

A button cell provided with such a contact according to the invention Dimensions 11.6 mm VI x 3.6 mm showed after 2.5 h when loaded with 4 microamps the voltage 1.6 volts. Surprisingly, the internal resistance decreased during the same period from 100 ohms to about 10 ohms.

For a smaller cell of 7.9 mm VI x 2.1 mm, the voltage adjustment took time to 1.6 volts only 40 minutes.

If you initially apply a current pulse of e.g. 15 mA x 2.5 min. = 0.63 mAh, an unloaded voltage is then observed of 1.6 volts. This remains constant for several days. It then becomes a load resistance of e.g. 1.5 megsohms are applied to the cell, which is about a third of the consumer load corresponds to a quartz watch with LCD display, the voltage remains until the end of the discharge in the range 1.5 to 1.6 volts.

Due to the inventive measure, the Nutzkapszltät the mentioned Button claw 11.6 mm VI x 3.6 mm from 120 mAh to approx. 150 mAh, i.e. increased by around 25%. The charge equivalent used for the current pulse falls by less than 1 mAh as a loss sector is almost negligible.

The contact area between the AgO powder of the electrode tablet and the contact should with the current cell dimensions for extremely small Operating currents of approx. 2 microamps at 1 nm for larger currents up to 10 microamps at 5 mm2. This corresponds to a current density of approx.

0.2 mA / cm2. The high output voltage falls under this condition from 1.85 volts within 2.5 hours from 1.6 volts, eo that then the accuracy of the clock is guaranteed by the cell.

The demands placed on the clock cell are therefore through a very special construction fulfills. The working principle of the microcontact, the above about the existence of a dynamic equilibrium between two opposing forces Processes, namely the formation of metallic silver on the contact surface and attempt to explain its corrosive transformation to Ag20 is surprising, especially since not only the voltage drop occurs briefly, but also the internal one Resistance of the cell decreases to 1/10 of the initial amount.

Speak for the correctness of the assumptions about the effect of the microcontact certain observations made on known button cells according to DT-OS 2 506 399 were: when discharging such a cell with a plastic-coated AgO electrode, however. was not reduced, with a current of 4 microamps the output voltage decreased 1.85 volts after 1.5 months from 1.6 volts. After that, approx. 90% of the nominal capacity was still used taken at this voltage level. Was the one insulating against the metal cup Plastic cover, however, was removed, the low voltage was about only after 6 months. These findings make it clear that between the period of time which elapses during the passage of the voltage level, and the contacting avalanche there must be a causal relationship.

In FIG. 1, the invention is illustrated by a cross section of the cell (Fig.

1 a) and a plan view of the plane A-A (Fig. 1 b) illustrates.

The cell consists of the cell cup 1, the AgO mass 2, the separator 3, the electrolyte source sheet 4, the negative ground 5, the cell lid 6 and the Sealing ring 7. From the cell cup 1 is the AgO electrode with its edge and the Bottom by a thin-walled plastic cup 8, e.g.

made of Teflon, insulated. The contact between the AgO-Massa 2 and the Cell cup 1, which is also a positive arrester, is replaced by an approx. 0.1 mm thick Metal wire 9 made of fine silver or silver-plated nickel, which is the electrode surface diametrically traversed and with its ends between the edges of the cell cup 1 and plastic cup 8 is fixed.

Instead of the overarching wire, the contact could be, for example can also be done with the help of one or more wire pins that come from the cell cup protrude into the interior of the electrode.

Two embodiments which are particularly advantageous for manufacturing reasons of the microcontact are shown in FIGS.

According to Figure 2, the contact consists of a cell cup on the bottom by Aufachwaißan attached metal wire pin 10, which when pressing the AgO-Mssse 2 slightly pierced the thin-walled plastic cup 8 and put it in the AgO powder is firmly embedded.

According to Figure 3 is an annular disc 11 made of fine silver sheet, the underside of which is covered with an insulating layer 12, in the AgO mass as above sunk deep so that it rests on the edge of the plastic cup 8. To this Way is the disc 11 with the recessed through the Ringöff voltage part of the AgO mass only in contact along its inner edge 13, while its outer edge 14 is frictionally and conductively connected to the metal cup 1.

Figure 4 shows the time course of the voltage adjustment on a cell according to the invention again. Area I of the time axis denotes true time of minutes, area II means periods of several days and area III means Periods of months. Before commissioning, the cell is switched on at time t = 0 min by reducing the load resistance from RL = oo to RL = 100 Ohm a current i der Size 12 to 14 mA impressed for 2.5 minutes. This impulse has an instantaneous one The voltage drops from 1.85 volts to 1.2 volts as a result, being against it Recovered to 1.4 volts at the end of the pulse loading (t = 2.5 min). In the same period the internal resistance of the cell goes from Ri # 100 Ohm (rest) to Mix 10 Ohni (t = O Min) back to Ri # @ Ohm.

During a subsequent idle time of several days without load (RL P oo) a constant no-load voltage Uo of 1.6 volts is established. The small Internal resistance Ri is essentially retained.

If the cell is now discharged via the load resistance RL = 1 megohm, so the loaded voltage UB is during the entire month counting Discharge time always in the range 1.5 to 1.6 volts.

In the figure, the voltage change is in an inventive Small cell of 7.9 mm VI by simply applying a load resistor RL = 0.5 Megaohm shown according to a normal consumer load. After that it has the required operating voltage of 1.6 volts is set constant after 40 minutes.

The advantages of the invention over known cells are in particular in that the reduction required in the voltage stabilized cells of the AgO as a special operation is omitted and that with the discontinuation of the reduction a larger useful volume fulfilled by AgO is retained, which is an increase of corresponds to more than 25% usable capacity. Due to the microcontacting, one Current load 0.2 to 15 microamps, in particular from 0.3 to 10 microamps, the nominal voltage of 1.6 volts is established within 2.5 hours. This applies to the cell with the dimensions 11.6 mm VI x 3.6 mm. The lowering can be achieved by means of an impulse task reduce the cell voltage from 1.85 to 1.6 volts over a period of minutes. A cell that has not been pretreated can also be inserted into the watch as the the power consumption associated with the time setting equates to a pulse load, su that the desired operating voltage 1.6 volts will be available soon.

In the case of LCD clocks, the current load in the microampere range is sufficient the electronics from the fact that the cell assumes this voltage within a few hours.

The unsteady voltage or open circuit voltage of the cell that is not yet in use is 1.8 volts to 1.85 volts, indicating that the cell is still its has full capacity or that it has not yet been used.

- patent claims -

Claims (6)

Claims 1. Galvanic element with negative zinc electrode and with an olektroninch non-conductive plastic layer from the positive arrester insulated positive electrode made of bivalent silver oxide (AgO), the cell being im Operation has the discharge voltage of the Ag2O / Zn system, characterized that the positive electrode with the positive arrester only has an electronic conductive contact is in connection, whose contact surface with the silver oxide is so small that even with a current draw of less than 15 µA the formation of Monovalent silver oxide (Ag20) and metallic silver come into contact more quickly than the oxidation of the silver deposited on the contact by that with it Electrolyte-conductive connected 2-valent silver oxide. 2. Galvanic element according to claim 1, characterized in that the contact area between the silver oxide and the contact is less than 10 mm2 amounts to. 3. Galvanic element according to claims 1 and 2, characterized in that that with discharge currents between 0.2 and 15 microamps, preferably between 0.3 and 10 uA, the current density at the contact surface on the order of 10-4 milliamps / cm² is. 4. Galvanic element according to claims 1 to 3, characterized in that that the contact is made by a surface pulling through the positive electrode (2) Metal wire (9) is formed. 5. Galvanic element according to claims 1 to 3, characterized in that that the contact is a protruding into the positive electrode (2) with the cell cup (1) connected metal wire pin (10). 6. Galvanic element according to claims 1 to 3, characterized in that that the contact of the edge (13) of one in the surface of the positive electrode (2) used, at least on its underside insulated metal ring (11).