Page 355 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 325

with the electrodes as they contract and expand during charging and discharging
respectively.
The conventional requirements of a separator are met fully by microfiber glass
mats: the extreme porosity guarantees – in spite of the free volume for oxygen
transfer of about 10 vol% – that acid displacement and also electrical resistance
remain very low, even though they are significantly higher (about double [23])
than is indicated by values measured on fully soaked samples. The chemical and
oxidative stability is very good. The dimensional stability of absorptive microfiber
glass fleeces is a critical parameter. On one hand, during the production of these
fleeces the thickness (i.e., weight per unit area and fiber distribution/cloudiness)
has to be maintained within narrow limits in order to assure a uniform distribution
of electrolyte and subsequently of the depth of discharge at the assembly pressure.
On the other hand, the resilience arising from the assembly pressure must not be
noticeably reduced by fiber fracture or drying-out.
The small pore size and the uniform distribution result in capillary forces
which should allow wicking heights and thus battery heights of up to 30 cm.
Due to the cavities required for gas transfer and under the effect of gravity, the
electrolyte forms a filling profile, that is, fewer cavities remain at the bottom than
at the top. Therefore with absorptive glass mats a rather flat battery construction
is preferred. Another reason for this is acid stratification: since the electrolyte
is still liquid, and acid of higher density formed, for example, during charging
will diffuse downwards – even at a delayed pace – this may detrimentally affect
especially any deep-cycling service. Furthermore, due to the severe acid limitation
of such cells during deep discharge, lead sulfate will dissolve increasingly, and
during recharge – and thus at higher acid density – it is again precipitated and
can lead after reduction to microshorts. This effect is partially counteracted by
the addition of sodium sulfate to the electrolyte. Nevertheless, sealed batteries
with microfiber glass fleece separation are therefore predominantly used in service
that rarely involves deep-discharge cycles. A special development, the addition
of a low percentage of organic fibers to microfiber glass fleeces [98], allegedly
simplifies the acid filling; excess acid is removed simply by dumping. Due to their
hydrophobicity, the organic fibers facilitate the oxygen transfer, and they should
suffice to weld such fleeces into pockets. Reports of practical experience have not
yet been published.
Developments to produce such absorptive mats totally from organic fibers even go
one step further. Only recently success came in achieving a suitable fiber diameter
and permanent hydrophilization [99]. Such materials are not yet commercially
available, however, and field experience has not been reported as yet.
Table 11.13 compares the specification data of microfiber glass fleeces from
various manufacturers.

11.2.3.3.2 Batteries with Gelled Electrolyte An advanced solution to the problem
of decreasing the free mobility of the electrolyte in sealed batteries is its gel
formation. By adding some 5–8 wt% of pyrogenic silica to the electrolyte, a gel
2
−1
structure is formed due to the immense surface area (∼200–300 m g ) of such