DE2708372C3 - Galvanic element with negative zinc electrode and positive silver oxide electrode - Google Patents

Description

The invention relates to a galvanic element with a negative zinc electrode and with an electronic one Non-conductive plastic layer from the positive conductor insulated positive electrode made of 2-valent Silver oxide (AgO), whereby the cell has the discharge voltage of the Ag2O / Zn system during operation.

The silver oxide / zinc systems, which are distinguished by their high energy density, come in the form of handy ones Button cells are increasingly being used to supply power to electronic wristwatches. It is due the more energetic AgO / Zn system takes precedence, although it has the disadvantage that the discharge depending on the current load only during the first third of the discharge time with a Voltage around 1.8 volts occurs, while the remaining discharge takes place at voltages below 1.6 volts, as it corresponds to the Ag2O / Zn system. The discharge is thus with a voltage jump of approx. mV, which would interfere with the synchronization of an electronically operated clock.

For a long time, efforts have therefore been made to keep the discharge step-free at the lower voltage level of 1.6 V. to lead, but nevertheless to fully utilize the capacity of the divalent silver oxide used.

TP Dirk se (J. Elektrochem. Soc. 109 (1962) 3, pages 173 to 177) has shown practical ways to this goal by finding that the unloaded tension of AgO tablets actually corresponds to Ag ₂ O / Ag System if they had previously been subjected to a partial electrochemical discharge.
Since then, many suggestions have emerged as to how

ίο A thin layer of Ag2O can be produced on an AgO tablet, for example by reducing it with chemical means or by pressing it on, so that the lower cell voltage of the Ag ₂ O / Zn system is set even in the unloaded state, if in addition to the Ag ₂ O metallic Ag is also present on the arrester. For the arrester, the bivalent silver oxide, the actual active electrode substance, is more or less masked, and only the layer of Ag and Ag ₂ O closest to it determines the potential.

Regardless of their specific embodiment, cells on this basis have the advantage that one can with immediately provide you with the desired operating voltage at the lower voltage level of 1.6VoIt The voltage is stabilized by the reduction at the level of the Ag2O / Zn system, which is why a direct exchange against this system is possible.

Is detrimental to the stabilized cell

on the other hand, the necessary reduction of the AgO and the deformation of the tablet, which is often unavoidable during assembly. The reduction costs the electrode a considerable part of its AgO capacity and represents an additional work step. When deformed, the AgjO layer tears, so that there is now no separation between AgO and Ag, but an electrolyte-conductive connection and a corrosion current within the silver oxide electrode can flow. This has the consequence that the metallic silver on the contact surface is _{oxidized to Ag 2} O and AgO is reduced _{to Ag 2 O.}

If the surface of the metallic silver is small compared to the surface of the AgO, the Ag will finally be completely _{oxidized to Ag 2} O, while part of the AgO surface is retained, so that at the contact now instead of the Ag ₂ O / Ag electrode an AgO / Ag ₂ O electrode is present. Accordingly, as the corrosion reaction progresses, the cell will have intermediate voltage values between 1.6VoIt and 1.85 volts and will eventually adjust to the undesirable high voltage of the AgO / Zn system.

The practical design of the cells has also taken into account the trend towards downsizing electronic wristwatches, which is only possible because the cells are also made smaller. With a button cell with the desired dimensions of z. B. 73 mm 0 χ 2.1 mm, the usable volume in relation to the inactive components such as housing, separator, etc. is already so small that the more energetic AgO / Zn system stabilized at 1.6 volts compared to the Ag ₂ O / Zn system no longer offers any significant advantage, since a considerable part of its capacity is lost because of the required reduction in AgO

According to DE-OS 25 06 399, it is possible to keep this loss of capacity within limits that the AgO positive electrode tablet in a plastic cup is fitted, which she from the metal cup,

the positive arrester, isolates and so only the open side of the tablet for a chemical reduction treatment before assembly releases the electrical contact of the positive ground with the metal cup is produced by a contact ring resting on the open edge of the tablet

The reduction of the AgO tablet after completion of the cell by:? Inen adequate cathodic current can be made.

The functional principle of the stabilized AgO / Zn cell was not changed with this construction either, since the AgO electrode is still _{coated on the contact side with Ag 2} O, of which in turn a part is reduced to Ag directly on the contact surface, so that an Ag2O / Ag electrode is produced electrochemically and the cell voltage is 1.6 V.

With the measures taken so far was so in terms of maintaining the largest possible Useful capacity has not yet achieved a decisive breakthrough in the aforementioned miniaturization of the cell.

The invention is therefore based on the object of maximizing the capacity design of the watch cell using 2-valent silver oxide as the positive electrode substance and still using the entire capacity when drawing current up to about 15 microamps and less at a voltage of 1.6 volts corresponding to the Ag ₂ O / Zn system available.

According to the invention, this object is achieved in that the positive electrode is only connected to the positive arrester via an electronically conductive contact whose contact surface with the silver oxide is so small that even with current draws of less than 15μΑ the formation of monovalent silver oxide (Ag ₂ O) and metallic silver on the contact occurs faster than the oxidation of the silver formed on the contact by the divalent silver oxide connected to it in an electrolyte-conducting manner.

This rating rule should be particularly preferred for current draws in the range from 0.2 to 15 uA in the range from 03 to 10 μΑ, apply.

It is essential for the invention that the contact area between the contact and the electrode body is so small that with currents, in particular between 03 and 10 microamps, currents of a few mA · 10 −1 / cm ^{2 result} at the contact.

Under this condition, the cathodic current density prevailing at the microcontact is high enough to relatively quickly reduce the closest AgO powder to Ag ₂ O and then to Ag. The speed of formation of the metallic silver at the contact must be greater than the speed of its oxidation by the AgO, to which the silver is in an electrolyte-conductive connection _{through the very thin, porous reduction layer of Ag 2 O}

In inverse proportion to the increase in the surface area of the silver, however, the current density must decrease, the formation of new silver must be slowed down and, on the other hand, its oxidation by the AgO must come more into play. After a certain current flow, the formation and oxidative degradation of Ag are in equilibrium with each other.The equilibrium is canceled if the cathodically formed silver is not sufficient to replace the amount oxidized at the same time due to insufficient current load and therefore it disappears only one AgO / Ag ₂ O electrode determines the potential, 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. B. obtained by the Contact ring in the cell construction according to DE-OS 25 06 399 through a 0.1 mm thick metal wire replaced, which diametrically runs through the surface of the AgO tablet

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

With a smaller cell of 7.9 mm 0 χ 2.1 mm, the voltage setting to 1.6 Volts only lasted 40 minutes.

If you initially give a current pulse of z. B. 15 mA χ 2.5 min. = 0.63 mAh, an unloaded voltage of 1.6 volts is then observed, which remains for several days constant If thereupon a load resistance of z. B. 1.5 megohms placed on the cell, which is about one One third of the consumer load corresponds to a quartz watch with LCD display, so the voltage remains until End of discharge in the range 1.5 to 1.6 volts

The inventive measure increases the useful capacity of the button cell mentioned 11.6 mm 0 χ 3.6 mm from 12OmAh to approx. 150mAh, d. H. increased by around 25%. The charge equivalent used for the current pulse falls from less than 1 mAh as a loss factor as good as negligible

With the current cell dimensions, the contact area between the AgO powder of the electrode tablet and the contact should be 1 mm ^{2 for extremely small operating currents of approx. 2 microamps and 5 mm 2} for larger currents of up to 10 microamps. This corresponds to a current density of approx. 0.2 mA / cm ² . Under this condition, the high output voltage drops from 1.85 volts within 2.5 hours to 1.6 volts, so that the accuracy of the clock is then guaranteed by the cell.

The demands placed on the clock cell are therefore met by a very special construction. The operating principle of the microcontact, which was tried to explain above by the existence of a dynamic equilibrium of two opposing processes, namely the formation of metallic silver on the contact surface and its corrosive conversion to Ag ₂ O, is surprising, especially since not only the voltage drop occurs for a short time, but also the internal resistance of the cell goes back to Vi _{0 of} the starting amount.

Certain observations speak for the correctness of the assumptions about the effect of the microcontact, the were employed on known button cells according to DE-OS 25 06 399: when discharging such a cell with Plastic-coated AgO electrode, which, however, was not reduced, with a current of 4 microamps the output voltage fell from 1.85 volts to 1.6 volts after 1.5 months. After that, about 90% of the Nominal capacity taken at this voltage level. Was the one insulating against the metal cup If the plastic cover was removed, the low SDannune did not appear for about 6 months. These

Findings make it clear that between the length of time that occurs during the passage through the voltage level elapses, and the manner of contact must have a causal relationship.

In Fig. 1, the invention is illustrated by a cross section of the cell (Fig. La) and a plan view of the plane AA (Fig. Ib).

The cell consists of the cell cup 1, the AgO mass 2, the separator 3 and the electrolyte source sheet 4, the negative mass 5, the cell cover 6 and the sealing ring 7. From Zeüenbecher 1 is the AgO electrode with its edge and the bottom through a thin-walled plastic cup 8, z. B. off Teflon, insulated. The contact between the AgO mass 2 and the cell cup 1, at the same time a positive arrester, is made by a 0.1 mm thick metal wire 9 made of fine silver or silver-plated nickel, which the Electrode surface diametrically traversed and with its ends between the edges of cell cup 1 and plastic cup 8 is fixed.

Instead of the overlapping wire, the contact could also be made, for example, with the aid of an or several wire pins are made, which protrude from the cell cup into the interior of the electrode.

Two embodiments of the microcontact that are particularly advantageous for manufacturing reasons are shown in FIGS. 2 and 3 shown. According to FIG. 2, the contact consists of a metal wire pin 10 which is attached by welding to the bottom of the cell cup and which, when the AgO mass 2 is pressed in, slightly pierces the M thin-walled plastic cup 8 and is firmly embedded in the AgO powder.

According to Fig. 3 is an annular disc 11 made of fine silver sheet, the underside of which with an insulating layer 12 is covered, sunk so deep into the AgO mass that it is on the edge of the plastic cup 8 rests. In this way, the disc 11 is with the Part of the AgO mass recessed by the ring opening is only in contact along its inner edge 13, while its outer edge 14 is frictionally and conductively connected to the metal cup 1-40.

F i ε ,. 4 shows the time course of the voltage setting on a cell according to the invention. The area! of the time axis periods 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 Ri = oo to R ₁ . = 100 ohms a current of size 12 to 14 mA impressed for 2.5 minutes.

This pulse results in an instantaneous drop in voltage from 1.85 volts to 1.2 volts, whereby it recovers to 1.4 Volts towards the end of the pulse load (t = 2.5 min). During the same period, the internal resistance of the cell goes from / ?,> 100 ohms (rest) to / ?, «10 ohms (t = 0 min) to / ?, ·« 8 ohms.

During a subsequent idle time of several days without load (Rl = °°) , a constant no-load voltage Ua of 1.6VoIt is established. The small internal resistance Ri is essentially retained.

If the cell is now discharged via the load resistance Ri. - 1 megohm, the loaded voltage Ug is always in the range 1.5 to 1.6 volts during the entire, months-counting discharge time.

In FIG. 5, the change in voltage in the case of a miniature cell according to the invention of 7.9 mm 0 by simply applying a laser resistance Rl = 0.5 megaohms corresponding to a normal consumer load is shown. After that, the required 1.6VoIt operating voltage has become constant after just 40 minutes.

The advantages of the invention over known cells are in particular that the Reduction of the AgO required for voltage-stabilized cells as a special operation is omitted and that with the discontinuation of the reduction, a larger useful volume filled by AgO is retained, the one Corresponds to an increase of more than 25% usable capacity. Due to the microcontacting, there is a current load 0.2 to 15 microamps, especially 0.3 to 10 microamps, the nominal voltage within 1.6 volts 2.5 hours. This applies to the cell with the dimensions 11.6 mm 0 χ 3.6 mm. By means of an impulse task The reduction in cell voltage from 1.85 to 1.6VoIt can be seen in a period of minutes to reduce. A cell that has not been pretreated can also be inserted into the watch, as the one with the Time setting associated power consumption equals a pulse load, so that the desired Operating voltage 1.6 full is available as soon as possible.

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

The unloaded voltage or open circuit voltage of the still unused cell is 1.8 volts to 1.85 volts This indicates that the cell still has its full capacity or that it has not yet been used has taken place

For this purpose 3 sheets of drawings

Claims (6)

Patent claims: 1. Galvanic element with negative zinc electrode and with a positive electrode made of 2-valent silver oxide (AgO) isolated from the positive conductor by an electronically non-conductive plastic layer, the cell having the discharge voltage of the Ag ₂ O / Zn system during operation, characterized that the positive electrode is only connected to the positive arrester via an electronically conductive contact, the contact area of which with the silver oxide is so small that even with a current draw of less than 15 μΑ the formation of 1-valent silver oxide (Ag ₂ O) and metallic silver on the contact occurs faster than the oxidation of the silver formed on the contact by the 2-valent silver oxide connected to it in an electrolyte-conducting manner. 2. Galvanic element according to claim 1, characterized in that the contact area between the silver oxide and the contact is less than 10 mm ² 3. Galvanic element according to claims 1 and 2, characterized in that with discharge currents between 0.2 and 15 microamps, preferably between 03 and 10 μΑ, the current density at the contact surface of the order of 10- 'milliamps / cm ² 4. Galvanic element according to claims 1 to 3, characterized in that the contact is formed by a metal wire (9) passing through the surface of the positive electrode (2) 5. Galvanic element according to claims 1 to 3, characterized in that the contact is a Metal wire pin (10) protruding into the positive electrode (2) and connected to the cell cup (1) is. 6. Galvanic element according to claims 1 to 3, characterized in that the contact of the Edge (13) of an inserted into the surface of the positive electrode (2), at least on its Underside insulated metal ring (11) is.