The Zn/silver oxide system has a high energy and a flat potential. Furthermore, it performs well at low temperatures and has a good shelf life. These characteristics make this system ideal for electronic devices requiring a small, high-capacity, long-lasting and constant-voltage cell. As a primary battery, it is mainly produced in button sizes, while its use in larger batteries is limited by the high cost of silver.
Zn/Ag2O cells were introduced in the early 1960s as power sources for electronic watches, with currents ranging from a few microamperes (LCD displays) to hundreds of microamperes (LED displays). These cells are also used in pocket calculators, hearing aids, cameras, glucometers, etc.
The anode is zinc powder, the cathode is monovalent silver oxide, Ag2O, and the electrolyte is a KOH or NaOH aqueous solution (20-45%). The Zn powder has to be highly pure, as already pointed out for alkaline cells. Indeed, impurities (such as Cu, Fe and Sn) favour Zn corrosion and formation of H2, which results in an overpressure. In commercial cells, the Zn powder is amalgamated with Hg to keep corrosion under control. A low percentage of Hg is permitted in these button cells, in view of the small amount of Zn: indeed, the maximum capacity is 165 mAh. Gelling agents, such as polyacrylic acid and the like, are added to the anode to facilitate electrolyte accessibility.
Ag2O is now preferred as a cathode in commercial cells. Unlike AgO, used until the early 1990s, it has a stable potential and does not need to be stabilized by heavy metals, as its reactivity with alkalis is low. As Ag2O is a poor semiconductor, some graphite (<5%) is added. Furthermore, the reduction of Ag2O produces Ag, which helps to decrease the cathode resistance.
The alkaline electrolyte contains some zincate to control gassing. KOH is preferred over NaOH in button cells submitted to high drains, as its conductivity is higher. Instead, KOH is preferred if the cells are to be used at low temperature: with this electrolyte, a working temperature as low as 28C can be attained.
Button cells have the cathode contained in the can and the anode attached to the cap. A barrier of cellophane or grafted plastic membrane is set on top of the cathode pellet to prevent Agþ migration to the anode (Ag2O is slightly soluble in alkalis). A separator, usually fibrous polyvinyl alcohol, is added on top of the barrier to act as a further protection. Zn/Ag2O button cells are available in a variety of sizes with capacities from 8 to 165 mAh. Large batteries (up to hundreds of Ah) can be used in space applications (especially for launch vehicles).
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1. Alkaline Batteries
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