Page 360 - Handbook of Battery Materials

330 11 Separators

penetration even at narrow spacing. For medium electrical performance – that is,
electrode spacings of about 1 mm – ribbed or corrugated sintered PVC separators
are used. They largely correspond to the product used in lead–acid batteries and
have been described in that context in detail (cf. Section 11.2.2.2). This separator is
good value, but it is rather brittle and thus difficult to handle, and it has relatively
large pores (−15 µm).
For higher current loads, especially for sinter electrodes, smaller separator pores
are desired; such materials are mostly sensitive, frequently requiring multiple
layers performing different duties. Both electrodes are wrapped in a relatively
open fleece or woven fabric of polyamide (‘nylon’) or, if higher temperatures
apply, of polypropylene fibers, which provide sufficient electrolyte at the electrode
surface to keep the electrical resistance low. Between these, an ion-semipermeable
membrane, typically regenerated cellulose (‘cellophane’) [112], serves as a gas
barrier to prevent the generated oxygen from reaching the negative electrode. In
wet a condition, where it swells and achieves the desired pore sizes and properties,
cellophane is mechanically very sensitive; the aforementioned nylon fleeces offer
the required support from both sides.
Better mechanical stability can be expected from irradiated polyethylene or
microporous polypropylene (‘Celgard’) membranes, but these account for increased
electrical resistance values.
One version of the microporous, filled polyethylene separator (‘PowerSep’)
[113], which is so successful in the lead–acid battery, is also being tested in
nickel–cadmium batteries. This separator is manufactured largely in the same way
and also has similar properties to those described in Section 11.2.2.1. Of course,
silicacannot be used as afiller,but has to be replaced byanalkali-resistant substance,
for example, titanium dioxide. The resulting separator membrane excels, with very
small pore sizes and low electrical resistance as well as outstanding mechanical
properties. A comprehensive presentation of the different separation materials
follows in Section 11.3.5.
The microporous or semipermeable separators serve, as explained, to avoid
oxygen transfer and thus increased self-discharge. In special cases of severe cycling
service without extended standing periods, this oxygen transfer is actually desired,
in order to suppress – by means other than constructional techniques – hydrogen
generation and consequently water consumption. Batteries for electric vehicles
are such a case, in which freedom from maintenance is the primary goal. As
separators, several layers of macroporous fleeces of either polyamide, polyethylene,
or polypropylene fibers and blends thereof, as well as spun fleece (melt-blown) of
polypropylene, are used. This construction (‘partial recombination’) is already a
transition stage to sealed batteries.

11.3.3.1.2 Sealed Construction The working principle of sealed nickel–
cadmium batteries is based on internal oxygen consumption. The negative
electrodes have a larger capacity than the positive ones; therefore, during
the charging step the latter reach their fully charged status earlier and start to
evolve oxygen, which migrates through voids in the electrolyte to the negative