SECONDARY BATTERIES – ZINC SYSTEMS | Zinc–Silver

Abstract

Although the silver–zinc (Ag–Zn) system was known at least since the days of the Italian physicist Alessandro Volta (1745–1827), it was not until the late 1940s that the efforts of the French Professor Henri André transformed it from a scientific curiosity into a commercial rechargeable battery. Note that the conventional way of designating a battery system is by stating the negative electrode first, followed by the positive. Thus among the alkaline batteries, we have ‘cadmium–nickel-oxide’, ‘iron–nickel-oxide’, ‘zinc–silver-oxide’, etc., but they are more commonly referred to as nickel–cadmium, nickel–iron, silver–zinc, respectively. We have used this popular designation throughout the text of this article. The system’s electrochemistry is relatively simple: during charge the silver is oxidized first to silver monoxide (monovalent silver oxide, Ag 2 O) and then to silver peroxide (divalent silver oxide, AgO) and the zinc oxide is reduced to elemental zinc. The reverse reactions take place during discharge. The two steps of the oxidation/reduction of the silver introduce some complications to the electrochemistry and to the performance of the Ag–Zn cells and batteries, which are discussed in detail in the second section of this article. The advantages of Ag–Zn are high specific power (W kg −1 ) and power density (W L −1 ) (unsurpassed by any other battery system), high specific energy (W h kg −1 ) and energy density (W h L −1 ) (second only to Li-ion), practically no size limitations, a charge efficiency of >99%, greater safety than most other systems, and the capability of serving as either a primary (one shot) or a rechargeable power source. The disadvantages are high cost, a relatively slow recharge rate, and limited cycle life (up to 100 cycles) and wet life (up to 2 years). Ways to overcome the last two limitations, which are attributable mostly to the negative electrodes and the separators, include the addition of bismuth oxide to the zinc electrode, the manufacture of zinc powder electrodes, and the use of polyolefin separators. Since their introduction to the market, Ag–Zn cells and batteries have been used in a variety of applications, for example, submarines, torpedoes and targets, deep submergence vehicles, missiles, and space missions (experiments, astronauts’ suits and extravehicular activities, launch and reentry vehicles, lunar rover). Commercial applications include video cameras, high-intensity flashlights, and, potentially, cell phones, notebook computers, and other consumer electronics.