Linden D., Reddy T.B. (eds.) Handbook of batteries
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11.8 CHAPTER ELEVEN
11.4.5 Low-Current-Drain Structures
Batteries designed for operation at low current drain require modification of the structure to
prevent internal electrical discharge paths from forming from the conductive materials in
both anode and cathode. After partial discharge, metallic mercury globules are particularly
troublesome in this respect. The problem can be minimized by the use of silver powder in
the cathode.
All available passages through which material could form an electrical track need to be
blocked if long-term discharge is to be realized. A typical button battery for watch appli-
cations used multiple barrier layers and a polymer insulator washer effectively sealing off
the anode from the cathode by compressing these layers against the support ring. These
batteries are discharged at the 1 to 2-year rate.
8
11.5 PERFORMANCE CHARACTERISTICS OF ZINC /MERCURIC
OXIDE BATTERIES
11.5.1 Voltage
The open-circuit voltage of the zinc /mercuric oxide battery is 1.35 V. Its voltage stability
under open-circuit or no-load conditions is excellent, and these batteries have been widely
used for voltage reference purposes. The no-load voltage is nonlinear with respect to both
time and temperature. A voltage-time curve is shown in Fig. 11.6. The no-load voltage will
remain within 1% of its initial value for several years. A voltage-temperature curve is shown
in Fig. 11.7. Temperature stability is even better than age stability. From
⫺20 to ⫹50⬚C the
total no-load voltage range is in the region of 2.5 mV.
Batteries containing manganese dioxide as a cathode additive to the mercuric oxide do
not show the no-load voltage stability illustrated in Figs. 11.6 and 11.7.
FIGURE 11.6 No-load voltage vs. time, zinc / mercuric oxide battery, 20⬚C.
MERCURIC OXIDE BATTERIES 11.9
FIGURE 11.7 No-load voltage vs. temperature, zinc / mercuric oxide battery.
11.5.2 Discharge Performance
A flat discharge curve, characteristic of the zinc /mercuric oxide battery, is shown in Fig.
11.8 for a pressed-powder anode battery at 20
⬚C. The end-point voltage is generally consid-
ered to be 0.9 V, although at higher current drains the batteries may discharge usefully below
this voltage. At low current drains the discharge profile is very flat and the curve is almost
‘‘squared off.’’
The capacity or service of the zinc /mercuric oxide battery is about the same on either
continuous or intermittent discharge regimes over the recommended current drain range,
irrespective of the duty cycle.
Under overload conditions, however, a considerable shift in available capacity can be
realized by the use of ‘‘rest’’ periods, which may increase service life considerably.
Problems are not encountered at low rates of discharge with batteries designed for the
purpose unless a high-current-drain pulse is superimposed on a continuous low-drain base
current; special designs are necessary to cope with this situation.
FIGURE 11.8 Discharge curves, zinc/ mercuric oxide battery, 1000 mAh size, 20⬚C.
11.10 CHAPTER ELEVEN
11.5.3 Effect of Temperature
The zinc/ mercuric oxide battery is best suited for use at normal and elevated temperatures
from 15 to 45
⬚C. Discharging batteries at temperatures up to 70⬚C is also possible if the
discharge period is relatively short. The zinc/ mercuric oxide battery generally does not
perform well at low temperatures. Below 0
⬚C, discharge efficiency is poor unless the current
drain is low. Figure 11.9 shows the effect of temperature on the performance of two types
of zinc /mercuric oxide batteries at nominal discharge drains.
The wound-anode or ‘‘dispersed’’-powder anode structures are better suited to high rates
and low temperatures than the pressed-powder anode.
FIGURE 11.9 Effect of temperature on performance of zinc/ mercuric
oxide batteries.
11.5.4 Impedance
Impedance for the zinc/mercuric oxide button batteries was usually measured at a frequency
of 1 kHz because of their use in hearing-aids.
9
The impedance curve is almost a mirror image of the voltage discharge curve, rising very
steeply at the end of the useful discharge life, as illustrated in Fig. 11.10. The value obtained
is frequency-dependent to some degree, particularly above 1 MHz, and a fixed frequency
has to be specified. A frequency versus impedance curve under no-load conditions is shown
in Fig. 11.11.
FIGURE 11.10 Internal impedance, zinc / mercuric oxide bat-
tery, 350 mAh size, 20⬚C, 1 kHz, 250-⍀ load.
MERCURIC OXIDE BATTERIES 11.11
FIGURE 11.11 Variation of impedance with frequency, zinc/ mercuric oxide
battery, 210 mAh size, 20⬚C.
11.5.5 Storage
Zinc/ mercuric oxide batteries have good storage characteristics. In general they will store
for over 2 years at 20
⬚C with a capacity loss of 10 to 20% and 1 year at 45⬚C with about a
20% loss. Storage at lower temperatures, such as down to
⫺20⬚C, will, as with other battery
systems, increase storage life.
The storability will depend on the discharge load and also on the cell structure. Failure
in storage is usually due to the breakdown of cellulosic compounds within the cell which,
at first, results in a reduction of the limiting-current density at the anode. Further breakdown
produces low-drain internal electrical paths and a real loss of capacity due to self-discharge.
Eventually, complete self-discharge can occur, but at 20
⬚C and below these processes take
many years.
Long storage lives are within the capabilities of the mercuric oxide system. For example,
a wound-anode cell with a noncellulosic barrier has a capacity loss in the region of only
15% over 6 years. With cells designed for long-term storage, dissolution of mercuric oxide
from the cathode and its transfer to the anode become a significant factor in capacity loss.
11.5.6 Service Life
The performance of the zinc /mercuric oxide cell at various temperatures and loads is sum-
marized in Figs. 11.12 and 11.13 on a weight and volume basis. These data, based on the
performance of a 800 mAh battery with a dispersed anode, can be used to approximate the
performance of a zinc/mercuric oxide battery.
11.12 CHAPTER ELEVEN
FIGURE 11.12 Service life of typical zinc / mercuric oxide battery (dis-
persed anode) on a weight basis.
FIGURE 11.13 Service life of typical zinc / mercuric oxide battery (dis-
persed anode) on a volume basis.
MERCURIC OXIDE BATTERIES 11.13
11.6 PERFORMANCE CHARACTERISTICS OF CADMIUM /
MERCURIC OXIDE BATTERIES
11.6.1 Discharge
An outstanding feature of the cadmium /mercuric oxide battery is its ability to operate over
a wide temperature range. The usual operating range is from
⫺55 to ⫹80⬚C, but with the
low gassing rate and thermal stability of the cell, operating temperatures to 180
⬚C have been
achieved with special designs.
Figure 11.14 shows the discharge curves for a typical button battery at various tempera-
tures. Excellent voltage stability and flat discharge curves are characteristic of these but at
a low operating voltage (open-circuit voltage is only 0.9 V). Figure 11.15 shows the effect
of temperature on the capacity at various discharge loads. A high percentage of the 20
⬚C
capacity is available at the lower temperatures. The end-point voltage is usually taken as
0.6 V, although at higher current densities and lower temperatures more useful life can be
obtained to lower end voltages.
The performance of the cadmium /mercuric oxide battery is summarized in Figs. 11.16
and 11.17 on a weight and volume basis, respectively. The data were derived from the
performance of typical button batteries.
FIGURE 11.14 Discharge curves—cadmium / mercuric oxide button battery (500-mAh size).
FIGURE 11.15 Effect of temperature on capacity of a cadmium / mercuric
oxide button battery (3000-mAh size).
11.14 CHAPTER ELEVEN
FIGURE 11.16 Service life of typical cadmium / mercuric
oxide batteries on a weight basis.
FIGURE 11.17 Service life of typical cadmium / mercuric ox-
ide, batteries on a volume basis.
11.6.2 Storage
Storage life over the temperature range of ⫺55 to ⫹80⬚C is remarkably good, and if the
barrier-absorbent system is designed to withstand elevated-temperature storage, the major
self-discharge mechanism is by dissolution of the mercuric oxide and its transfer to the anode.
A shelf life of 10 years at ambient temperatures with less than 20% capacity loss is within
the capabilities of the system. Elevated-temperature storage is exceptionally good (approxi-
mately 15% loss per year at 80
⬚C), and since neither electrode should generate hydrogen,
the cells can be hermetically sealed with minimal risk of electrolyte leakage or cell distor-
tion.
8
MERCURIC OXIDE BATTERIES 11.15
REFERENCES
1. C. L. Clarke, U.S. Patent 298,175 (1884).
2. S. Ruben, ‘‘Balanced Alkaline Dry Cells,’’ Proc. Electrochem. Soc. Gen. Meeting, Boston, Oct. 1947.
3. B. Berguss, ‘‘Cadmium-Mercuric Oxide Alkaline Cell,’’ Proc. Electrochem. Soc. Meeting, Chicago,
Oct. 1965.
4. P. Ruetschi, ‘‘The Electrochemical Reactions in Mercuric Oxide-Zinc Cell,’’ in D. H. Collins (ed.),
Power Sources, vol. 4, Oriel Press, Newcastle-upon-Tyne, England, 1973, p. 381.
5. D. P. Gregory, P. C. Jones, and D. P. Redfearn, ‘‘The Corrosion of Zinc Anodes in Aqueous Alkaline
Electrolytes,’’ J. Electrochem. Soc. 119:1288 (1972).
6. T. P. Dirkse, ‘‘Passivation Studies on the Zinc Electrode,’’ in D. H. Collins (ed.), Power Sources, vol.
3, Oriel Press, Newcastle-upon-Tyne, England, 1971, p. 485.
7. D. Weiss and G. Pearlman, ‘‘Characteristics of Prismatic and Button Mercuric Oxide-Cadmium Cells,’’
Proc. Electrochem. Soc. Meeting, New York, Oct. 1974.
8. P. Ruetschi, ‘‘Longest Life Alkaline Primary Cells,’’ in J. Thompson (ed.), Power Sources, vol. 7,
Academic, London, 1979, p. 533.
9. S. A. G. Karunathilaka, N. A. Hampson, T. P. Haas, R. Leek, and T. J. Sinclair. ‘‘The Impedance of
the Alkaline Zinc-Mercuric Oxide Cell. I. Behaviour and Interpretation of Impedance Spectra.’’ J.
Appl. Electrochem. 11 (1981).
12.1
CHAPTER 12
SILVER OXIDE BATTERIES
Sid A. Megahed, Joseph Passaniti and John C. Springstead
12.1 GENERAL CHARACTERISTICS
The energy density of the zinc /silver oxide system (zinc /alkaline electrolyte/ silver oxide)
is among the highest of all battery systems making it ideal for use in small, thin ‘‘button’’
batteries. The monovalent silver oxide battery will discharge at a flat, constant voltage at
both high- and low-current rates. The battery has long storage life, retaining more than 95%
of its initial capacity after 1 year of room temperature. It also has good low temperature
discharge capabilities, delivering about 70% of its nominal capacity at 0
⬚C and 35% at
⫺20⬚C. These features have enabled the zinc/silver oxide battery to be an important micro-
power source for electronic devices and equipment, such as watches, calculators, hearing
aids, glucometers, cameras, and other applications that require small, thin, high-capacity,
long-service-life batteries that discharge at a constant voltage. The use of this battery system
in larger sizes is limited by the high cost of silver.
The primary zinc/ silver oxide batteries are manufactured mainly in the button cell con-
figuration in a wide range of sizes from 5 to 250 mAh. Most of these batteries are now
prepared from the monovalent silver oxide (Ag
2
O).
The divalent silver oxide (AgO) has the advantage of a higher theoretical capacity and
the capability of delivering about a 40% higher capacity in the same battery size than the
monovalent silver oxide, but it has the disadvantages of dual voltage discharge curves and
greater instability in alkaline solutions. The gain of more service hours for the same weight
of silver, a cost advantage over monovalent silver oxide, brought the divalent oxide into
limited commercial use about a decade or so ago. They were marketed as ‘‘Ditronic’’ or
‘‘Plumbate’’ batteries using heavy metals to stabilize the reactivity of the divalent oxide.
However, early in the 1990’s, these batteries were withdrawn from the market because of
the environmental restrictions that were imposed on the use of these metals. The information
on the use of the divalent silver oxide has been included in this chapter, nevertheless, for
reference and possible future applications of silver oxide batteries.
The major advantages and disadvantages of the zinc /monovalent silver oxide battery are
summarized in Table 12.1.
12.2 CHAPTER TWELVE
TABLE 12.1 Major Advantages and Disadvantages of Zinc/ Silver Oxide Primary Batteries
Advantages Disadvantages
High energy density
Good voltage regulation, high rate capability
Flat discharge curve can be used as a reference voltage
Comparatively good low-temperature performance
Leakage and salting negligible
Good shock and vibration resistance
Good shelf life
Use limited to button and miniature cells
because of high cost
12.2 BATTERY CHEMISTRY AND COMPONENTS
The zinc/silver oxide cell consists of three active components: a powdered zinc metal anode,
a cathode of compressed silver oxide, and an aqueous electrolyte solution of potassium or
sodium hydroxide with dissolved zincates. The active components are contained in an anode
top, cathode can, separated by a barrier and sealed with a gasket.
The overall electrochemical reaction of the zinc/monovalent silver oxide cell is
Zn
⫹ Ag O → 2Ag ⫹ ZnO (1.59 V)
2
The zinc /divalent silver oxide cell has a two-step electrochemical reaction:
Step 1: Zn
⫹ AgO → Ag O ⫹ ZnO (1.86 V)
2
Step 2: Zn ⫹ Ag O → 2Ag ⫹ ZnO (1.59 V)
2
12.2.1 Zinc Anode
Zinc is used for the negative electrode in aqueous alkaline batteries because of its high half-
cell potential, low polarization, and high limiting current density (up to 40 mA/cm
2
in a
cast electrode). Its equivalent weight is fairly low, thus resulting in a high theoretical capacity
of 820 mAh/g. The low polarization of zinc allows for a high discharge efficiency 85 to
95% (the ratio of useful capacity to theoretical capacity).
Zinc purity is very important. Zinc is thermodynamically unstable in aqueous alkali,
reducing water to hydrogen:
Zn
⫹ HO→ ZnO ⫹ H
22
Zinc alloys containing copper, iron, antimony, arsenic, or tin are known to increase the zinc
corrosion rate while zinc alloyed with cadmium, aluminum, or lead will reduce corrosion
rates.
1,2
If enough gas pressure is generated, the sealed button cell could leak or rupture. In
commercial use, the high surface area zinc powder is amalgamated with small amounts of
mercury (3 to 6%) to bring the corrosion rate within a tolerable limit.