Zinc-silver oxide cell

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... 

Fig. 6.22 Discharge (a) and charge (b) characteristics of a typical 100 Ah zinc-silver oxide cell...

Fig. 6.23 Battery for Ranger lunar photography spacecraft, comprising 14 sealed 45 Ah zinc-silver oxide cells, which impacted on the moon in 1965. (By permission of Jet Propulsion Laboratory,)...

The overall cell reaction is exactly similar to the zinc-silver oxide cell, with the final discharge products being silver and Fe(OH>2. The emf of the cell is only 1.34 V, but the cell has a superior cycle life and better reliability. Batteries up to 10 kWh have been built by Westinghouse for telecommunications and submersible vehicles. 

The zinc-silver oxide cell, widely used in miniature apphcations and larger ones in mihtary apphcations, is closely... 

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. 

Unlike some other cathode materials, such as manganese dioxide, which are quite insoluble, silver oxide has a fair degree of solubility in alkaline electrolyte. If the soluble silver species were allowed to be transported to the zinc anode it would react directly with the zinc, and as a result the cell would self-discharge. In order to prevent this from happening, zinc—silver oxide cells use special separator materials such as cellophane [9005-81-6], that are designed to inhibit migration of soluble silver to the anode. 

Fig. 16. Comparison of battery efficiency for miniature zinc—silver oxide cells containing KOH or NaOH electrolyte (21).

The zinc/silver oxide cell consists of three active components a powdered zinc metal anode, a cathode of compressed silver oxide, and an aqueous electrolyte solution of potassium or sodium hydroxide with dissolved zincates. The active components are contained in an anode top, cathode can, separated hy a harrier and sealed with a gasket. 

The electrolytes used for zinc/silver oxide cells are based upon 20 to 45% aqueous solutions of potassium hydroxide (KOH) or sodium hydroxide (NaOH). Zinc oxide (ZnO) is dissolved in the electrolyte as the zincate to help control zinc gassing. The zinc oxide concentration varies from a few percent to a saturated solution. 

Because of the slight solubility of silver oxides in alkaline electrolyte, little work was done with zinc/silver oxide cells until 1941 when Andre suggested the use of a cellophane barrier. Cellophane prevents migrating silver ions from reaching the anode - " by reducing them to insoluble silver metal. The cellophane is oxidized and destroyed in the process, making it less effective for long-life cells. 

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. 

FIGURE 18.1 Typical construction of primary reserve zinc/silver oxide cell. Courtesy of Eagle-PicherIndustries.)... 

The components of a reserve zinc/silver oxide cell consist of the positive plates, the negative plates, and the separators. The components are assemhled such that each negative plate is protected from direct contact with the adjacent positive plate by a separator. The cell components are assembled and packaged in a container the plates can be prepared in either a dry and charged condition or dry and uncharged condition. 

Electrolyte. The electrolyte used for reserve zinc/silver oxide cells is an aqueous solution of potassium hydroxide. High and medium discharge rate cells use a 31% by weight electrolyte solution because this composition has the lowest freezing point and is close to the minimum resistance which occurs at 28 wt. %. Low-rate cells may use a 40-45% solution since lower rates of hydrolysis of cellulosic separators occur with the higher KOH concentrations. 

TABLE 33.7 Nominal Characteristics of Typical Vented Zinc/Silver Oxide Cells... 

A. Charkey, Long Life Zinc-Silver Oxide Cells, Proc. 26th Ann. Power Sources Symp., 1976, pp. 87-89. 

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