Zinc-silver oxide system

Separators must be resistant to the high pH and, since silver oxide is slightly soluble in strong bases, must prevent migration of silver ions to the anode. Such separators are further considered in Chapter 6, where secondary batteries based on the zinc-silver oxide system are described. 

The primary objective of miniature battery design is to maximize the energy density in a small container. A compromise must be reached, however, since volumetric energy density decreases as cell volume decreases and the dead volume due to containers, seals, etc. becomes increasingly significant. A plot of energy density as a function of total volume is given in Fig. 3.28 for the zinc-mercuric oxide and zinc-silver oxide systems. 

The vast majority of recent efforts in secondary silver cells has been concentrated in the areas of negative electrode and separator improvements for the zinc/silver oxide system because of its proportionally larger market volume. The major developments are summarized in Table 33.9. Some of the most recent work has been quite successful, including ... 

Battery systems of complex design and structure using—at least for one electrode—expensive materials are (for economic reasons) mainly conceived as storage batteries. Primary (and "reserve") versions of the zinc/silver oxide battery [(-) Zn/KOH/AgO (+)] — as a first example—are only used in particular cases where the question of cost is not crucial, e.g., for marine [26-28] and space applications [29]. 

Zinc-mercuric oxide, cadmium-mercuric oxide, zinc-silver oxide and related systems... 

The main features of zinc-silver oxide cells are similar to those of the zinc-mercuric oxide system, except for a higher OCV and significantly increased cost. The overall cell reaction is... 

Zinc-silver oxide reserve batteries have application as power sources for various systems in manned and unmanned space vehicles. 

The separator assembly is the most critical component of zinc-silver oxide secondary cells. In addition to its normal function of preventing contact and short circuit between electrodes of different polarity, a separator in this system must also ... 

It must be emphasized that the most appropriate charging regime is very dependent on the cell system under consideration. Some are tolerant to a considerable amount of overcharging (e.g. nickel-cadmium batteries), while for others, such as zinc-silver oxide and most lithium secondary cells, overcharging can result in permanent damage to the cell. Sealed battery systems require special care float charging should not be used and trickle charge rates should be strictly limited to the manufacturer s recommended values, since otherwise excessive cell temperatures or thermal runaway can result. 

The main features of the zinc silver oxide cell are similar to those of the zinc mercury(II) oxide system. The principal difference apart from cost is the higher open circnit voltage the emf calculated from the standard free energies of formation of ZnO and Ag20 is 1.593 V, in close agreement with the open circuit voltage of commercial cells of 1.60 V. 

The more familiar types of primary alkaline systems are the zinc/manganese dioxide, zinc/ mercuric oxide, and zinc/silver oxide batteries. These, typically, use potassium or sodium hydroxides, in concentrations from 25 to 40% hy weight, as the electrolyte, which functions primarily as an ionic conductor and is not consumed in the discharge process. In simple form, the overall discharge reaction for these metal oxide cells can be stated as... 

The performance advantages of several types of lithium batteries compared with conventional primary and secondary batteries, are shown in Secs. 6.4 and 7.3. The advantage of the lithium cell is shown graphically in Figs. 7.2 to 7.9, which compare the performance of the various primary cells. Only the zinc/air, zinc/mercuric oxide, and zinc/silver oxide cells, which are noted for their high energy density, approach the capability of the lithium systems at 20°C. The zinc/air cell, however, is very sensitive to atmospheric conditions the others do not compare as favorably on a specific energy basis nor at lower temperatures. 

The zinc/silver oxide electrochemical system was the metallic couple with which Volta demonstrated the possibility of using dissimilar metals in a pile-type multicell construction to obtain a substantial electric voltage. The system existed somewhat as a laboratory device until Professor Andr6 designed a practical secondary cell early in World War II. 

The electrochemical reactions associated with the discharge of a zinc/silver oxide battery as a primary system are generally considered to proceed as follows. The cathode or positive electrode is silver oxide and may be either Ag20 (monovalent), AgO (divalent), or a mixture of the two. The anode or negative electrode is metallic zinc, and the electrolyte is an aqueous solution of potassium hydroxide. The chemical reactions and the associated voltages at standard conditions are... 

The performance of zinc/silver oxide batteries in Amperes per unit weight and volume versus service time is given in Fig. 18.12. It can be noted, again, that this battery system is particularly sensitive to temperatures below 0°C. These data are applicable, within reasonable accuracy, for both high- and low-rate designs. 

The cost of high-performance primary zinc/silver oxide batteries is dependent on the specifications to which they are built and the quantity involved. Manual-type batteries may cost anywhere from 5 to 15 per Watthour remote-activated types will cost about 15 to 20 per Watthour. When the price of silver is high, material cost becomes one of the chief disadvantages of these batteries. There are many applications, however, in which no other technology can meet the high energy density of the zinc/silver oxide primary system. 

At one time, the zinc/potassium hydroxide/silver oxide system was also employed in spin-dependent reserve batteries. More frequently, this reserve system has been used in nonspin applications, such as missiles, where the electrolyte is driven into place by a gas generator or other activation method (Chap. 18). This system is again finding favor in some applications where the potential hazards of lithium-based systems can create safety problems. The chemistry of the zinc/silver oxide couple can be represented by either of two reactions, depending on the oxidation state of the silver oxide ... 

Operating Temperature Limits. Like most other batteries, the performance of liquid-electrolyte reserve batteries is affected by temperature. Military applications frequently demand battery operations at all temperatures between -40 and 60°C, with storage limits of -55 to 70°C. These requirements are routinely met by the lead/fluoboric acid/lead dioxide systems and, with some difficulty at the low-temperature end, by the lithium/thionyl chloride and zinc/potassium hydroxide/silver oxide systems. Provision is occasionally made to warm the electrolyte prior to the activation of the two latter systems. 

Nickel-iron (conven- tional) Nickel-zinc Zinc/silver oxide (silver-zinc) Cadmium / silver oxide (silver-cadmium) Nickel- hydrogen Nickel- metal hydride Rechargeable primary types, Zn/Mn02 lithium ion systems ... 

Most of the conventional rechargeable battery systems have a flat discharge profile, except for the silver oxide systems, which show the double plateau due to the two-stage discharge of the silver oxide electrode, and the rechargeable zinc/manganese dioxide battery. 

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