Page 363 - Handbook of Battery Materials

11.3 Separators for Alkaline Storage Batteries 333

depot. The zinc electrodes are removed from the discharged battery; then they are
mechanically crushed, chemically dissolved, and electrolytically deposited, again
compacted, and supplied with a separator pocket before being reinstalled in the
battery. A woven fabric of polyamide (‘nylon’) fibers serves a separator, which is
sufficient to prevent shorts during discharge.

11.3.4.4 Zinc-Bromine Batteries
Even though zinc–bromine batteries operate with a slightly acidic electrolyte
(pH 3), they are discussed here briefly, because they offer another way of escaping
the problems of zinc deposition. At this pH value both zinc corrosion as well
as the tendency toward dendrite formation are low; the latter, furthermore, is
prevented by electrolyte circulation [121]. The separator, besides meeting the usual
requirements, has to perform an additional duty: although it must permit the charge
transfer of zinc and bromide ions, it should suppress the transfer of dissolved
bromine, of polybromide ions, or of the complex phase. Due to mechanical
and chemical susceptibility, ion-selective membranes did not prove effective.
Microporous polyethylene separators are usually used; in their manufacture and
properties they are quite similar to those described in Section 11.2.3.1.
11.3.4.5 Zinc–Silver Oxide Storage Batteries
Zinc–silver oxide batteries as primary cells are known both as button cells, for
example, for hearing aids, watches, or cameras, and for military applications,
usually as reserve batteries. Since the latter after activation have only a very short
life (a few seconds to some minutes), a separation by cellulosic paper is generally
sufficient.
Rechargeable zinc–silver oxide batteries have to struggle against the same
problems as the zinc electrode, which have been described in detail for the
nickel–zinc systems. To make matters even worse, the silver oxide electrode
contributes an additional problem: silver ions – even to a small extent – dissolve,
deposit on the negative electrodes, and poison them by forming local corrosion
elements and causing self-discharge with hydrogen evolution. In order to prevent
this, several layers of semipermeable cellophane membranes are used [122], among
other methods. The beneficial effect is caused by a sacrificial action: the silver ions
migrate through the electrolyte and oxidize (i.e., they thus destroy) cellophane
film sites, simultaneously being reduced to metallic silver and thereby becoming
less harmful. The life of the cellophane is therefore limited; together with wetting
fleeces to prevent also direct contact with the silver oxide electrode, this is fully
sufficient for primary cells. For rechargeable batteries, cycle lives of 10–100 cycles
are quoted [123, 124], depending on type of separation and depth of discharge; in
special cases of very shallow discharges of only a few percent, however, 3000 cycles
and three years of life have been reported.
Advanced development of ion-selective films has been attempted by radiation
grafting of methacrylic acid onto polyethylene films, and combinations of this with
cellophane are also being tested. Polyamide fleece impregnated with regenerated
cellulose is another option for zinc-silver oxide batteries.