Page 354 - Handbook of Battery Materials

324 11 Separators

high separation thickness, microporous as well as sintered PVC separators also
find use.

11.2.3.3 Separators for Valve Regulated Lead–Acid (VRLA) Batteries

11.2.3.3.1 Batteries with Absorptive Glass Mat VRLA batteries are frequently
also somewhat misleadingly called sealed or recombinant batteries. Their operating
principle is – as mentioned already – based on oxygen, which is generated during
charging at the positive electrode and is able to reach the negative electrode
internally and be reduced there again. The negative electrode thus becomes
partially discharged, so that it does not enter the overcharge phase, that is, it does
not lead to hydrogen evolution. No water consumption occurs; viewed externally,
the total charging current is transformed into heat. For a more detailed description
of the system, the literature [7, 23–27, 87–97] should be consulted.
What requirements are placed by this construction on the separator? First,
the free mobility of the electrolyte has to be hampered in order to maintain
tiny open channels for oxygen transfer from the positive to the negative elec-
trode. One solution to this problem is the use of highly porous microfiber glass
mats as separators. This glass mat has to fill the space between the electrodes
completely and absorb a maximum amount of electrolyte. These requirements
imply extremely high porosity (>90%), large internal surface area, and good wet-
tability to assure a high absorption for the electrolyte. Starting from a fibrous
structure, a large internal surface means a fiber diameter as small as possible:
2
−1
glass fibers of 0.5 µm reach around 3 m g ,whereas 10 µm fibers have only
2
some 0.15 m g −1 of surface. The good wettability of glass fibers suffers if binder
is used. Of course, the separator has to have long-term resistance against var-
ious kinds of chemical and electrochemical attack inside a lead–acid battery,
and its susceptibility increases with the internal surface! It must not generate
substances that increase the gassing rate, corrosion, or self-discharge. Finally, it
has to be mechanically robust enough to be handled during the battery produc-
tion process. Sharp corners or edges should not be able to penetrate it. This
last demand competes, of course, with the desire for the least possible binder
content.
These generally defined requirements are met quite comprehensively by mi-
crofiber glass fleeces. These are blends of C-glass fibers of various diameters,
which are processed in the usual way on a Foudrinier paper machine into a
voluminous glass mat. The blending ratio gains special importance since cost
aspects have to be balanced against technical properties. The expensive microfibers
below 1 µmindiameter (∼20–30% share) give a large internal surface and the
desired pore size distribution, but do not contribute substantially to the mechanical
properties. Fibers of significantly larger diameter increase the tensile strength
and thus the processability, but tend to break more easily when the glass mat is
under compression, as it needs to be to maintain at all times sufficient contact