Energy zinc-mercuric oxide

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 most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Volumetric energy density is, at times, a more useful parameter than gravimetric specific energy, particularly for button and small batteries, where the weight is insignificant. The denser batteries, such as the zinc/mercuric oxide battery, improve their relative position when compared on a volumetric basis, as shown in Table 7.4 and Fig. 7.9. The chapters on the individual battery systems include a family of curves giving the hours of service each battery system will deliver at various discharge rates and temperatures. 

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 button cells that provide the energy for watches, electronic calculators, hearing aids, and pacemakers are commonly alkaline systems of the silver oxide-zinc or mercuric oxide-zinc variety. These alkaline systems provide a vei y high energy density, approximately four times greater than that of the alkaline zinc-manganese dioxide battery. 

The only commercially important primary metalair batteries today are zinc-air batteries. They dominate the hearing aid battery market, having largely replaced silver oxide (Zn/Ag20) and mercuric oxide (Zn/HgO) batteries. They provide nearly twice the energy of silver oxide batteries... 

Miniature applications have become more important in recent years with the general aeeeptance of the behind-the-ear hearing-aid and the advent of the electronic watch. High energy density per unit volume is the prime requirement for a battery in these products. The mercuric oxide-zinc, silver oxide-zinc, zinc-air and lithium-based systems appear to be likely contenders for this market. Although the last two types of battery have been produced in sizes suitable for miniature applications, they are not widely available in this format. These systems will therefore be discussed later in their usual cylindrical form, and the conclusions drawn then may explain the difficulties that have prevented their wide acceptance. 

---