Divalent silver oxide

The zinc/divalent silver oxide cell has a two-step electrochemical reaction ... 

Although the divalent silver oxide has a higher theoretieal capacity (432 mAh/g hy weight or 3200 Ah/L by volume) than the monovalent silver oxide, the use of the divalent form in button batteries has been limited and no longer marketed commercially. - This is due primarily to its instability in alkaline solutions and the fact that it exhibits a two-step discharge (Fig. 12.2). 

Divalent silver oxide is unstable in alkaline solutions, decomposing to monovalent silver... 

This instability can be improved by the addition of lead or cadmium compounds " or by the addition of gold to the divalent silver oxide. ... 

The zinc/divalent silver oxide battery exhibits a two-step discharge curve. The first occurs at 1.8 V, corresponding to the reduction of AgO to Ag20 ... 

FIGURE 12.3 Double-treatment method for divalent silver oxide. 

Double-Treatment Method for Divalent Silver Oxide—Effect of Thickness... 

A second approach to eliminate the two-step discharge was through the use of silver plumbate as a cathode additive material. Silver plumbate cathode material was prepared by reacting an excess of divalent silver oxide powder with lead sulfide (PbS) in a hot alkaline solution. The product of the reaction is a mixture of remaining divalent silver oxide (AgO), monovalent silver oxide (Ag20), and silver plumbate (Ag5Pb20g). The sulfur is oxidized to the sulfate and is washed from the reaction product ... 

The product of the reaction of bismuth sulfide with divalent silver oxide is believed to be silver bismuthate (AgBiOj) ... 

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, maintaining 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 AgjO and oxygen. Manganese dioxide is readily oxidized by AgO to alkali-soluble man-ganate compounds. 

The OCV of the zinc/divalent silver oxide battery will vary from 1.58 to 1.86 V depending on the ratio of Ag to Ag20 to AgO in the cathode. The OCV will decrease with greater Ag20 to AgO ratios and with the presence of silver metal in the cathode. With divalent silver oxide batteries, the depth of discharge does have an effect on the OCV a partially used battery will have more Ag20 and silver metal than a new one and will thus have a lower OCV. 

E. A. Megahed and D. C. Davig, Long life Divalent Silver Oxide-Zinc Primary Cells for Electronic Applications, Power Sources, Vol. 8, Academic, London, 1981. 

Performance data for monovalent silver oxide (Ag20)—zinc primary watch cells and divalent silver oxide (Ago)—zinc primary watch cells are given in Table 5.2... 

The are two types of silver oxide cell one has a cathode of monovalent silver oxide (Ag20) and the other type uses divalent silver oxide (AgO). The latter type has a higher theoretical potential (1.8V) and, because there is an additional chemical reduction from AgO to AgjO, it has a higher capacity (the theoretical energy density is 424Wh/kg). 

Further details on the construction of the latest designs of monovalent silver oxide-zinc (Ag20) and divalent silver oxide (Ago)-zinc button cells are given below. 

Again, in the button cell designs the solid cathodes are anode limited with 5—10% more cathode capacity than anode capacity. The anode is amalgamated powdered or gelled zinc and contains a gelling agent and is housed in a triclad metal top (nickel—stainless steel-copper) with the copper in direct contact with zinc. The anode is separated from the divalent silver oxide cathode by a separator or absorbent material (cotton-like material) and a semi-permeable membrane barrier. The design of the absorbent separator system depends on the chemistry of the cathode and intended use. 

A recent compilation of suppliers of primary monovalent silver oxide-zinc and divalant silver oxide-zinc cells for watch and calculator applications is given below. 

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