Page 191 - Battery Reference Book

Cadmium-air secondary batteries la3

inc-air primary batteries Table 12.1 Performance characteristics of small (I 0-7000 mA h)
primary zinc-air batteries, 675 size button cell
Metal-air and fuel cells are not discussed in detail
in this book. A brief discussion follows in which the Standard iwrenziurn
Characteristics of zinc-air primary cells are compared grade grade
with those of other types of primary cell. Details are
also given of zinc-air button cells. Energy density
In relation to their physical size, zinc-air batter- W mg-1 300 400
ies store more energy per unit of weight (in excess Whh-’ 900 1200
of 200Wh/kg) than any other primary type. Other Voltage
superior features include the ability to supply high Nominal 1.40 1.40
1.22
1.23
currents for longer periods and the maintenance of Working
an almost constant voltage throughout the discharge Nominal rate capacity
period. Up to ten times the rated current can be sup- mA/cm-’ (initial) 16 28
35
plied for pulse loads. Batteries are supplied in standard Pulse current (mA open cell) 20
international sizes and the largest has a capacity of Life
18 A h at 2 A within a size of 33 nun diameter x 6 1 mm Capacity, mA h per cell 400 540
16
long and a weight of 85 g. Service, 625 per 16 h day 12 5
Shelf life (years)
5
One type of zinc-air primary battery is that in
which the active electrodes are inserted into the elec- Temperature
trolyte, the so-c,alled mechanically rechargeable sys- Storage (“C) -40 to 50 -40 to 50
-10 to 55
Operation (“C)
-10 to 55
tem. The zinc-air system is capable of operating down
to -18°C and g,iving 80% of the 21°C performance Capacity loss per annum
at this lower temperature. Some start-up time is nec- at 21°C (%) 3 3
essary for the battery to warm itself and to achieve
proper air convection. When this is achieved, proper
operation can be sustained. At the higher temperature, 175 Wh/kg at the C/5 rate of discharge. These
there is a dryingout problem, which can be alleviated energy densities are approximately twice those of
by designing the cells with a reservoir of water. At many of the best existing rechargeable systems.
54°C approximately 60% of 21°C capacity is obtained. However, there is the problem of internal shorting,
Of course, these figures, for operation at the temper- after several chargeldischarge cycles, which occurs as
ature extremes, depend on rate of discharge. These a result of zinc dendrite growth through the separator.
zinc-air batteries are capable of 100 cycles of dis- This problem can be overcome to a great extent
charge (replacement of anode-electrolyte composites). by proper selection of separator materials. It was
This is highly desirable because of the cost of the air also found that the air electrodes, which contain
cathodes, which (contain an expensive noble metal cata- platinum as a catalyst and have been used successfully
lyst. A projected development is a one-shot throwaway in the mechanically rechargeable zinc-air battery
version of these batteries using low-cost catalysts for (Section 12.1 and Chapter 26) and the electrically
the air e1ectrode.s such as spinels and mixed oxides, rechargeable cadmium battery, do not function in the
and activated carbons such as Darco G60. Two ver- charging as well as in the discharging mode. This is
sions of the throwaway are possible; a reserve type because platinum on the anode surface acts as a low
in which water .will be added to the cells just before hydrogen overvoltage site, thereby enhancing zinc self-
use, and a non-reserve type, which will just require the discharge and reducing the available capacity of the
removal of plastics strips that cover the air ports. zinc electrode. Thus, until an adequate air electrode
The demise of the zinc-air cell has been brought is developed, a third electrode will be required for
about by econornic rather than technical factors. The charging purposes.
picture for zinc-air button cells is quite different The theoretical reversible voltage of this cell is
and Gould have manufactured these for several years 1.83 V and the practical open circuit voltage achieved
for applications such as hearing-aids and watches. is in the range of 1.75 to 1.8OV.
Gould and Berec have been marketing them in the
UK since 1980.
Performance characteristics of small (10- 1000 mA h) 12.3 Cadmium-air secondary
primary zinc air batteries are tabulated in Table 12.1. batteries
Rechargeable cadmium-air and zinc-air batteries are
inc-aiir secondary batteries currently still only at the development stage and may
not be commercially available for several years. Cer-
Development of the rechargeable zinc-air battery tainly sealed versions of these batteries can only be
is under way. Experimental cells have given 155- considered to be a project for the future. On the other