Page 102 - Battery Reference Book

Secondary batteries 2/19

2.2.4 Silver-cadmium batteries not be commercially available for several years. Cer-
tainly, sealed versions of these batteries can only be
The silver-cadmium battery combines the high energy considered to be a prospect for the future. The cad-
and excellent space and weight characteristics of the mium-air system has a theoretical energy density of
silver-zinc ba.tte1-y with the long-life, low-rate char- 445 W hkg. The cadmium anode used is the type that
acteristics and some resistance to overcharge of the has demonstrated good stability and low seif-discharge
nickel-cadmium battery. The battery also provides in other alkaline systems. The air cathode is similar to
high efficiency on extended shelf life in charged or that used in the mechanically rechargeable zinc-air
uncharged conditions, level voltage and mechanical battery. The battery consists of two air cathodes in
ruggedness. Watt hour capacity per unit of weight parallel, positioned on the two sides of a plastics
and volume are two to three times greater than those frame, and one anode. The bifunctional air electrode
of a comparable nickel-cadmium battery and it has
superior charge retention. The silver-cadmium battery can be used for both discharge and charge. Over 300
promises great weight and space savings and superior cycles have been obtained with the best combination
life characteristics to those of the nickel-cadmium bat- of materials.
tery currently used as storage batteries in most satellite There are several problems associated with the
programmes. operation of the cadmium-air battery, such as the
Today a silver-zinc system offers the greatest avail- loss of cadmium on cycling, cadmium penetration and
able energy dlensity in terms of watt hours per kilo- poisoning of the air electrode by a soluble cadmium
gram. There are newer so-called high energy density species migrating to the air electrode. Water must
couples which have been under development for many be added to this cell periodically because water loss
years; the effective energy density of many of these occurs by transpiration of water vapour through the
systems tends to decline as they are developed close air electrode - a characteristic of batteries using an
to the point of practical utilization. In addition, chronic air electrode. Operating cell voltage varies between
safety problems have already caused serious difficul- 0.70 and 0.85V at the C/2 to C/lQ rate (that is, the
ties with the lithium systems and are potentially dan- discharge current is numerically equal to one-half to
gerous in others, most of which are high-temperature one-tenth of capacity, C). At these rates practical bat-
systems based on volatile materials. The use of silver teries produce 80-90 W hkg and 14-24 W h/drn3.
as a couple obviously increases initial costs (although
silver costs are recoverable) when compared to other
existing systems such as lead-acid, nickel-cadmium,
etc. When space and weight are limiting factors, the 2.2.7 Zinc-air batteries
silver-zinc system is a very attractive proposition. Development of the zinc-air electrically rechargeable
Other metal couples that are considered at present
to be of great potential are the nickel-hydrogen and battery is under way. Experimental cells have given
155-175 Whkg at the C/5 rate of discharge. These
nickel-zinc systems. These may be batteries of the
future in applications such as utilities load levelling energy densities are approximately twice those of the
and electric vehicles; the latter type is, in fact, now in best existing rechargeable systems. However, there is
commercial production. the problem of internal shorting after several cycles
as a result of zinc dendrite growth. This problem can
be overcome to a great extent by proper selection
2.2.5 Nickel-zinc batteries of separator materials. It was also found that the
air electrodes, which contain platinum as the catalyst
With the development of new separators and improved and have been used successfully in the mechanically
zinc electrodes, the nickel-zinc battery has now
become competitive with the more familiar battery rechargeable zinc-air battery and the cadmium-air
systems. It has a good cycle life and has load-voltage electrically rechargeable battery, do not function in
characteristics higher than those of the silver-zinc the charging as well as the discharging mode. This
system. The energy per unit of weight and volume is because platinum on the anode surface acts as a low
are slightly lower than those of the silver-cadmium hydrogen overvoltage site, thereby enhancing zinc self-
system. Good capacity retention (up to 6 months) discharge and reducing the available capacity of the
has made thle nickel-zinc battery a more direct zinc electrode. Thus, until an adequate air electrode
competitor of he silver-zinc and silver-cadmium is developed, a third electrode will be required for
systems. Nickel-zinc batteries are not yet available charging purposes.
in a sealed form. Table 2.11 compares some basic parameters of the
five main types of rechargeable battery system. Some
2.2.6 Cadrn.ium-air batteries of these data must, of course, be interpreted with cau-
tion - particularly energy density data. Discharge char-
Rechargeable cadmium-air and zinc-air batteries are acteristics for these five types of battery are compared
currently still only at the development stage and may in Figure 2.10.