Page 364 - Handbook of Battery Materials

334 11 Separators

Occasionally the zinc electrode is wrapped in a polypropylene fleece filled with
inorganic substances, such as potassium titanate, in order to reduce the solubility of
zinc, since the problem of dendrite growth is aggravated even by the metallization
of the cellophane separator due to the aforesaid silver reduction and its promoting
the generation of shorts.
After these comments it is understandable that this expensive and life-limited
system could succeed only in a few special applications where the high energy and
power density could not be achieved by other systems.

11.3.5
Separator Materials for Alkaline Batteries

In the product range of alkaline power sources, each manufacturer has developed
for each special application an optimum type of separator. Generally, however,
these consist of the combination of a relatively small variety of proven materials.
They are presented here jointly, even though they can hardly be compared with
each other. They may be divided into three groups, depending on their application:
macroporous wetting fleeces (Table 11.15), microporous separators (Table 11.16),
and ion-semipermeable membranes (Table 11.17).
Of all possible manufacturing processes for macroporous separators to be
employed in alkaline batteries, the wet-fleece process using paper machines is the
predominant one [125]; it permits a very uniform (‘cloud-free’) production of such
material and the use of different types of fibers as well as of short and very thin
fibers, thus achieving a uniform structure of small pores (Table 11.15).
Whereas PVA fleeces are used only in primary cells, polyamide fleeces compete
with polyolefin, preferably polypropylene fleeces. The latter are more stable at
higher temperatures and do not contribute to electrolyte carbonation, but they wet
only after a pretreatment either by fluorination [126] or by coating and crosslinking
with hydrophilic substances (e.g., polyacrylic acid [127]) on the surface of the fiber.
Only very recently, the production of melt-blown polypropylene fleeces with
considerably thinner fiber diameter became possible [99], thus making it possible
to achieve attractive properties with regard to small pore size and excellent tensile
performance for use in highly automated assembly processes, provided that a
low-cost hydrophilization is available.
Very different microporous separators for alkaline batteries are included in
Table 11.16. The very thin (∼25 µm) films of stretched polypropylene (‘Celgard’)
are generally employed in combination with fleeces, while separators of sintered
PVC or filled UHMW PE find use also in single separation of alkaline industrial
batteries. Their production process corresponds to the analogous version for
lead–acid batteries and is described in detail in Sections 11.2.2.1 and 11.2.2.2
respectively.
As ion-semipermeable membranes, which – despite good permeability to hy-
droxyl ions – hinder the transfer of zincate and silver ions, essentially only
regenerated cellulose is being used in alkaline batteries. In a complex conver-
sion process, a pulp of wood cellulose, primarily from eucalyptus trees, is dissolved