Page 297 - Lindens Handbook of Batteries

BUTTOn CELL BATTErIES: SILVEr OxIDE–ZInC AnD ZInC-AIr SYSTEmS 13.3

weight), the battery industry in general is eliminating the use of mercury in all batteries in favor of
organic gassing inhibitors and the use of lower gassing zinc alloys.
The oxidation of zinc during discharge is a complex phenomenon. The reactions can be written
simplistically as 3,4

Zn+ OH → 2 - Zn OH + ( ) E° = 2e +1 249 V
.
2
+
+
Zn 4OH → - ZnO - 2 + 2 HO+ 2e E° = +1 215 V
.
2 2
As the electrolyte becomes saturated, zinc oxide will precipitate, releasing the bound water
+
Zn OH) → 2 ZnOH O
(
2
13.2.2 silver oxide Cathode
2
Silver oxide is known to have three oxidation states: monovalent (Ag O), divalent (AgO), and tri-
2
valent (Ag O ). The trivalent silver oxide is very unstable and is not used for batteries. The divalent
2
3
form had been used in button cells, generally mixed with other metal oxides. The monovalent silver
oxide is the most stable and is the one primarily used for commercial button cell batteries.
The reaction product of the discharge of the monovalent silver oxide cathode is highly conduc-
tive silver metal.
+
+
Ag OH O 2 → + e 2 Ag 2 OH - E° =+ 0 342 V
.
2 2
Prior to discharge, however, the monovalent silver oxide is a very poor conductor of electricity.
Without any additives, a monovalent silver oxide cell would initially exhibit a very high cell imped-
ance and an unacceptably low closed-circuit voltage (CCV). To improve the initial CCV, the monova-
lent silver oxide is generally blended with 1 to 5% powdered graphite. As the cathode discharges, the
silver metal produced maintains a low internal cell resistance and a high CCV. The theoretical capac-
ity of the monovalent silver oxide is 231 mAh/g by weight or 1640 Ah/L by volume. The addition of
graphite reduces the cathode capacity due to lower packing density and lower silver oxide content.
Unlike the other silver oxides, the monovalent silver oxide is stable to decomposition in alkaline
solutions. Some decomposition to silver metal may occur due to the impurities brought into the cath-
ode material by the graphite. The decomposition rate is dependent upon the quality of the graphite
blended into the cathode, the amount of graphite, and the cell storage temperature. Greater graphite
impurities and higher cell storage temperatures result in greater silver oxide decomposition rates. 5
Due to the high cost of silver bullion, some manufacturers may reduce the amount of silver in
the cathode by the use of other cathode active additives. One common additive is manganese dioxide
(mnO ). With increasing amounts of mnO added to the cathode, the voltage curve changes from
2
2
a constant voltage throughout the discharge to a curve where the voltage gradually decreases as the
cathode is depleted (Fig. 13.1). This gradual drop in voltage has been considered an indicator of the
state of cell depletion; the decreasing voltage indicates the cell is nearing its end of useful life.
Another additive that can serve a dual function is silver nickel oxide (AgniO ). Silver nickel
2
oxide is produced by the reaction of nickel oxyhydroxide (niOOH) with monovalent silver oxide in
hot aqueous alkaline solution 6,7
+
Ag O 2 niOOH → 2 AgniO + H O
2 2 2
The dual feature of silver nickel oxide is that it is electrically conductive, like graphite, as well
as cathode active, like mnO . The coulometric capacity of silver nickel oxide (263 mAh/g) is higher
2
than Ag O and discharges at 1.5 V against zinc (Figs. 13.2 and 13.3). Silver nickel oxide can replace
2
both the graphite and part of the monovalent silver oxide, reducing the cost of the cell.