Page 325 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 295

time glass separators were developed and introduced. Glass separators are very
similar to the cellulosic separators already mentioned; they do not require special
machinery for processing and offer a very low electrical resistance. They succeeded
in the USA in largely displacing sintered PVC and cellulosic separators, before they
themselves were supplanted by a completely new technology, polyethylene pocket
separation. In Europe this intermediate step – glass separators – did not occur;
the transition from PVC or cellulosic leaf separators to the polyethylene pockets
proceeded directly, albeit some 5 or 10 years later than in the USA.
The decisive advantage of the polyethylene pocket is its flexibility and sealability
as well as its very small pore diameters. In the course of their efforts at efficiency
improvement, around 1970 Delco–Remy had designed a completely novel starter
battery concept: electrodes of expanded lead strip were to replace cast grids,
promising a significant saving in weight and thus in cost. Only lead–calcium
alloys proved to be suitable for expansion, which had a tendency, however, toward
increased mud shedding during battery operation. Against this the microporous
polyethylene separator film, developed by W.R. Grace & Co. in 1966 [16], promised
to be a remedy. While the mud could be accommodated by the formation of a
three-sided sealed pocket around the electrode, at the same time the micropores
eliminated the risk of penetration through the separator. A largely automated starter
battery production was the positive result of these developments. The commercial
advantages of pocketed starter batteries as well as their technical ones, such as
freedom from maintenance, high cold crank power, and prolonged battery life,
have led to a victorious worldwide advance of this technology [3].
Alternative separator materials such as organic fibers or sintered PVC did not
succeed as pocket materials due to their excessive pore size. The pore diameter
of 10–20 µm typical for such materials is insufficient to effectively prevent the
preferential formation of shorts by mass particles at the folding edge of the pocket.
A completely separate development took place in Japan. Traditionally, very porous
positive active materials are used in that country requiring support glass mats to
avoid premature capacity loss and shorting in cycling service. Since the introduction
of the cellulosic separator a version of this kind has been used in Japan without
distance ribs, achieving the total thickness of the separator by means of a thick
glass mat instead. Certainly this is an expensive type of separator, but it can be
balanced by savings in active material. In Japan, meanwhile, the fleece of organic
fibers, which was unsuitable for pocketing, has replaced the impregnated cellulosic
sheet. It distinguishes itself by favorable electrical resistance data and good acid and
oxidative stability. The transition to microporous polyethylene pockets proceeds
more slowly than in the USA or in Europe, because it requires a simultaneous
change in formulation of the positive mass.
Starting with the development of sealed lead–acid cells by The Gates Rubber
Co. in 1972 [17], this principle of internal oxygen transfer was transferred to
starter batteries around 1980. The electrolyte is absorbed in a microfiber glass mat;
this has to leave electrolyte-free channels through which the oxygen generated
at the positive electrode can diffuse to the negative, where it is reduced. Thus
one has a theoretically maintenance-free battery without any water consumption.