Page 282 - Lindens Handbook of Batteries

12.2 PRIMARY BATTERIES

TABLE 12.1 Characteristics of the Zinc/Mercuric Oxide and Cadmium/Mercuric Oxide Batteries
Advantages Disadvantages
Zinc/mercuric oxide battery
High energy-to-volume ratio, 450 Wh/L Batteries were expensive; although widely used in
Long shelf life under adverse storage conditions miniature sizes, but only for special applications
Over a wide range of current drains, recuperative in the larger sizes
periods are not necessary to obtain a high capacity After long periods of storage, cell electrolyte tends
from the battery to seep out of seal, which is evidenced by white
High electrochemical efficiency carbonate deposit at seal insulation
High resistance to impact, acceleration, and Moderate energy-to-weight ratio
vibration Poor low-temperature performance
Very stable open-circuit voltage, 1.35 V Batteries for disposal are considered hazardous
Flat discharge curve over wide range of current drains wastes under environmental regulations

Cadmium/mercuric oxide battery
Long shelf life under adverse storage conditions Batteries are more expensive than zinc/mercuric
Flat discharge curve over wide range of current drains oxide batteries due to high cost of cadmium
Ability to operate efficiently over wide temperature System has low output voltage (open-circuit
range, even at extreme high and low temperatures voltage = 0.90 V)
Can be hermetically sealed because of inherently Moderate energy-to-volume ratio
low gas evolution level Low energy-to-weight ratio
Batteries for disposal are considered hazardous
wastes under environmental regulations

12.2 CHEmISTRy

It is generally accepted that the basic cell reaction for the zinc/mercuric oxide cell is

Zn + HgO → ZnO + Hg

For the overall reaction, ∆G = 259.7 kJ. This gives a thermodynamic value for E at 25°C of 1.35 V,
0
0
which is in good agreement with the observed values of 1.34 to 1.36 V for the open-circuit voltage
4
of commercial cells. From the basic reaction equation it can be calculated that 1 g of zinc provides
819 mAh and 1 g of mercuric oxide provides 247 mAh.
Some types of zinc/mercuric oxide cells exhibit open-circuit voltages between 1.40 and 1.55 V.
These cells contain a small percentage of manganese dioxide in the cathode and are used where
voltage stability is not of major importance for the application.
The basic cell reaction for the cadmium/mercuric oxide cell is
Cd + HgO + H O → Cd(OH) + Hg
2
2
0
0
For the overall reaction, ∆G = –174.8 kJ. This gives a thermodynamic value for E at 25°C of 0.91 V,
which is in good agreement with the observed values of 0.89 to 0.93 V. From the basic reaction it
can be calculated that 1 g of cadmium should provide 477 mAh.