Page 296 - Lindens Handbook of Batteries

13.2 PrImArY BATTErIES

taBle 13.1 major Advantages and Disadvantages of Zinc/Silver Oxide Primary Batteries
Advantages Disadvantages
High energy density Use limited to button and miniature cells because
Good voltage regulation, high rate capability of high cost
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

13.2 BATTERY CHEMISTRY AND COMPONENTS

The zinc/silver oxide cell consists of three main electrochemical components: fine powdered zinc
metal as the anode, an aqueous alkaline electrolyte with dissolved zincates, and a compressed silver
oxide cathode pellet. The active components are contained in an anode top, cathode can, separated
by an ionic conductive barrier membrane and sealed with a nylon gasket.
The overall electrochemical reaction of the zinc/monovalent silver oxide cell is

+
+
Zn Ag O → 2 2Ag ZnO (1.59 V)
The zinc/divalent silver oxide cell has a two-step electrochemical reaction
+
+
Step 1: Zn AgO → AgO ZnO (1.86 V)
2
+
Step 2: Zn Ag O → → + 2AgZnO (1.59V)
2
13.2.1 Zinc anode
Zinc is used for the negative electrode in aqueous alkaline batteries because of its high half-cell
2
potential, low polarization, and high limiting current density (up to 40 mA/cm in a cast electrode).
Its equivalent weight is relatively low, thus resulting in a high theoretical capacity of 820 mAh/g.
The low polarization of zinc allows for a high discharge efficiency of 85 to 95% (the ratio of useful
capacity to theoretical capacity).
The zinc is prepared as a powder by air or gas atomization of the molten zinc. As in other alkaline
zinc anode cells, care is taken during all processes to avoid contamination of the zinc with other
metals, especially iron, as the purity of the zinc is critical to the performance and leakage resistance
of the finished cells.
Zinc metal is thermodynamically unstable in aqueous alkali. Pure zinc will very slowly reduce
water to hydrogen gas and zinc oxide
+
+
Zn HO → ZnOH
2 2
Commercial zinc often contains trace heavy metal impurities that act as catalytic sites that rapidly
increase the rate of hydrogen generation. The generation of hydrogen within a tightly sealed battery
may lead to cell distortion, leakage, or, if the pressure is sufficient, rupture. Zinc alloys containing
copper, iron, antimony, arsenic, or tin are known to increase the zinc corrosion rate, while zinc
alloyed with mercury, cadmium, aluminum, bismuth, or lead will reduce the corrosion rate. In
1,2
commercial applications, anode gassing is brought to tolerable limits by the selection of zinc alloys,
by the use of organic gassing inhibitors, and by the addition of mercury to the high surface area
zinc powder. While the amount of mercury per cell is very small (typically about 3% of the anode