Page 330 - Handbook of Battery Materials

300 11 Separators

USABC has set the goal so high that lead–acid batteries have been put out of the
question for this application [29]. This led to an initiative by the lead–acid battery
industry and their suppliers to set up the Advanced Lead–Acid Battery Consortium
(ALABC) with the goal of fostering development of the lead–acid battery for use in
electric vehicles, at least for an interim period until more powerful batteries with
higher energy density will become available. Here a series of complex technical
problems have to be solved [30]. Of course, such electric vehicle batteries have to be
maintenance-free, that is, of sealed construction; the resulting use of lead–calcium
alloys and thus the premature capacity loss have already been touched on.
For the separators of such batteries, gel construction and microfiber glass
fleece separators again compete: because of the deep discharge cycles, the gel
construction with its lower tendency to acid stratification and to penetration shorts
has advantages; for the required power peaks, microfiber glass fleece construction
would be the preferred solution. The work on reduction of premature capacity
loss with lead–calcium alloys has shown that considerable pressure (e.g., 1 bar)
on the positive electrode is able to achieve a significantly better cycle life [31–36].
Pressure on the electrodes produces counter pressure on the separators, which is
not unproblematic for both separation systems. New separator developments have
been presented with the goal of their being only a little deformed even at high
pressure despite high porosity, be they of ceramics [37] or highly filled polymer [38].
Because of the power requirements the trend is clearly toward thinner electrodes
and thus thinner separators, which would render a microporous pore size structure
indispensable.

11.2.2
Separators for Starter Batteries

11.2.2.1 Polyethylene Pocket Separators

11.2.2.1.1 Production Process The term ‘polyethylene separator’ is somewhat
misleading, since this separator consists mainly of agglomerates of precipitated
silica held within a network of extremely long-chained, ultrahigh-molecular-weight
polyethylene (UHMW PE) molecules. The raw materials, precipitated silica
(SiO 2 – about 60%), UHMW PE (about 23%), a mineral process oil (about 15%) – all
percentages are relative to the final product – and some processing aids (e.g., an-
tioxidants) together with an additional considerable excess of mineral oil, are mixed
intensively and fed into an extruder. Here, by the effects of heat and mechanical
shear, a viscous melt is formed which is extruded through a slit die 1 m wide into
a sheet 1–2 mm thick, which is then formed between the two profiling rolls of
a calendar into the desired separator profile. Generally this is characterized by a
backweb of about 0.2 mm, which on one side has continuous ribs 0.6–1 mm high
in the machine direction at a distance of about 10 mm. At this point the separator
material is oil-filled and thus shiny black. In a subsequent step the mineral oil
serving as pore-former is largely extracted in a solvent bath [16]. Some producers