Page 302 - Lindens Handbook of Batteries

13.8 PrImArY BATTErIES

from the reaction of divalent silver oxide with 8% lead sulfide has a coulometric capacity of 345 to
360 mAh/g.
The silver plumbate process has the disadvantage that the button cells do contain a small amount
of lead, 1 to 4% of the cell weight. An alternate approach was developed to use bismuth sulfide
18
in place of the lead sulfide in the material preparation reaction. The reaction product retains the
advantages of the silver plumbate material but without the toxicity of lead. Bismuth is not consid-
ered toxic and is used in medical and cosmetic applications, both externally and internally within
19
the body. The product of the reaction of bismuth sulfide with divalent silver oxide is believed to
be silver bismuthate (AgBiO )
3
+
+
Bi S + 23 28 AgO 6 naOH → 2 AgBiO + 3 13 AgO 3 na SO + 2 4 3 HO
2
2
Like the silver plumbate compound, the silver bismuth compound is conductive and cathode
active. The monovalent silver oxide produced by the reaction coats the divalent silver oxide particles,
while the conductive silver bismuthate reduces the cell impedance, allowing for a high cell CCV.
The silver bismuthate will discharge against zinc in alkaline solutions at about 1.5 V. Therefore, in
anode-limited button cells only the monovalent silver oxide voltage is observed.
Unlike monovalent silver oxide systems, additives such as graphite or manganese dioxide
cannot be added to the divalent silver oxide. Graphite enhances the decomposition of AgO to
Ag O and oxygen. manganese dioxide is readily oxidized by AgO to alkali-soluble manganate
2
compounds.
Although the divalent silver oxide has a higher theoretical capacity (432 mAh/g by weight or
3200 Ah/L by volume) than the monovalent silver oxide, the use of the divalent form in button bat-
2,8
teries was limited and is no longer marketed commercially. This is due primarily to the difficulty in
eliminating the two-step discharge and declining prices as the zinc/silver oxide button cells became
a commodity.
13.2.3 alkaline electrolyte

The electrolytes used for zinc/silver oxide cells are based upon 20 to 45% aqueous solutions of
potassium hydroxide (KOH) or sodium hydroxide (naOH). Zinc oxide (ZnO) is dissolved in the
electrolyte as the zincate salt to help control zinc gassing. The zinc oxide concentration varies from
a few percent to a saturated solution.
The preferred electrolyte for button cells is potassium hydroxide (KOH). Its higher electrical
conductivity 20,21 allows cells to discharge over a wider range of current demands (Fig.13.7). Sodium
hydroxide (naOH) is used mainly for long life cells not requiring a high-rate discharge (Fig 13.8).
The sodium hydroxide exhibits less creep, and such cells are less apt to leak than the potassium
hydroxide cells. Leakage is evidenced as frosting or salting around the seal. However, leakage issues
with potassium hydroxide cells have been resolved by most manufacturers through improvements
in seal technology.
Electrolyte gelling agents such as polyacrylic acid, potassium or sodium polyacrylate, sodium
carboxymethyl cellulose, or various gums are generally blended into the zinc powder to improve
electrolyte accessibility during discharge.

13.2.4 Barriers and separators
A physical barrier is required to keep the zinc anode and silver cathode apart in the tight volume
constraints of a button cell. Failure of the barrier will result in internal cell shorting and cell failure.
A silver oxide cell requires a barrier with the following properties:
1. Permeable to water and hydroxyl ions
2. Stable in strong alkaline solutions