Page 362 - Handbook of Battery Materials

332 11 Separators

The reason for this limited cycle life is the high solubility of the zinc electrode
in alkaline electrolyte; the zincate ions formed are deposited again during the
subsequent charging in the form of dendrites, that is, of fernlike crystals. They
grow in the direction of the counterelectrode and ultimately cause shorts.
A remedy could be achieved by a decrease in the zinc solubility in the elec-
trolyte or by suppression of dendrite formation; oxides of cadmium, lead, or
bismuth, as well as calcium hydroxide or aluminum hydroxide, have been added
to the zinc electrode or the electrolyte for this purpose, but not with long-lasting
effectiveness.
Thus in this system, in addition to the usual requirements, the separator has
the task of delaying penetration for as long as possible. A membrane would be
regarded as perfect which lets hydroxyl ions pass, but not the larger zincate ions.
This requirement is best met by regenerated cellulose (‘cellophane’) [10, 11], which
in swollen condition shows such ion-selective properties but at the same time
is also chemically very sensitive and allows only a limited number of cycles; the
protective effects of additional fleeces of polyamide or polypropylene have already
been taken into account.
Chemically more stable systems with microporous properties, such as stretched
polypropylene films (‘Celgard’), irradiated, coated polyethylene, or filled polyethy-
lene separators (‘PowerSep’) offer a compromise: smaller pore diameters have been
shown to increase the number of cycles to penetration. However, a different failure
mode occurs at an earlier stage, namely ‘shape change’ of the negative electrode
[116]. If the off-diffusion of zincate ions into the bulk electrolyte is obstructed,
for example, by small separator pores, concentration gradients on the electrode
surface cause a shifting of the zinc deposit from the edges toward the center of
the electrode [117]. In summary it may be noted that these opposing effects have
prevented a breakthrough of the nickel–zinc system, as yet.

11.3.4.2 Zinc–Manganese Dioxide Secondary Cells
The system known as primary alkaline manganese cells has been further developed
since 1975 into secondary cells [118, 119]. The above-mentioned problems of the
zinc electrode apply here as well, although safety is assured for these sealed cells
by constructional measures. Depending on the depth of discharge, between 20
and 200 cycles can be attained, which may be sufficient for many applications, for
example, as low-cost rechargeable power sources for children’s toys. The described
combination of a few layers of fleece of polyamide or polypropylene fibers with an
ion-selective film of regenerated cellulose (‘cellophane’) is being used as a means of
separation to prevent shorting by dendrites. A further development of the separator
has been achieved by impregnation of a polyamide fleece with regenerated cellulose
in order to obtain a single, stable, ion-semipermeable separator layer.

11.3.4.3 Zinc–Air Batteries
A completely different way has been taken to render zinc–air elements of very high
energy density rechargeable for the use in electric vehicles [120]. In the vehicle they
are used exclusively as primary cells to be ‘mechanically’ recharged at a central