Page 327 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 297

In the second half of the 1960s, at the same time but independently, three
basically different plastic separators were developed. One was the polyethylene
separator [16] already referred to in starter batteries, used only rarely in stationary
batteries, but successful in traction batteries. The others were the microporous
phenolic resin separator (DARAK) [18] and a microporous PVC separator [19],
both of which became accepted as the standard separation for stationary batteries.
They are distinguished by high porosity (about 70%) and thus very low electrical
resistance and very low acid displacement, both important criteria for stationary
batteries.
The desire for maintenance-free service, for example, in decentralized single
emergency lights for panic lighting, had led, around 1960, to the development of
small, sealed, lead–acid batteries with gelled electrolyte [20, 21]. An idea that was
already known – the gelling of electrolyte – became applicable to the sulfuric acid
electrolyte with the industrial availability of silica types of very high surface area.
2
These fumed silicas have an internal surface of, say, 200 − 300 m g −1 and convert
sulfuric acid into a thixotropic gel. By vigorous stirring, the electrolyte is liquefied
and can be filled into the battery cell, where it gels again, rendering it leakproof and
serviceable in all positions. Microporous separators, for example, of phenolic resin
or PVC like the ones referred to above are required for maintaining the spacing
and preventing shorts.
However, because of the viscous electrolyte, the charging gases can no longer
escape! The application of the principle of sealed nickel–cadmium batteries, known
since around 1933 [22], letting the oxygen generated at the positive electrode diffuse
to the negative electrode for reduction with partial discharge there, was successful.
After an initial water loss, the gel starts to dry out, thereby forming cracks and
allowing the oxygen to find a path. Unfortunately this technology is rather costly
and therefore can only be justified for special applications.
Uninterrupted power supply for computers, initially only for central units, grew
significantly with the introduction of personal computers (PCs): the batteries
became smaller and, since they had to be located in offices, had to be in a
sealed version, since no aggressive charging gases were permitted to escape. This
advanced the breakthrough of the development of so-called recombination or
valve-regulated batteries, a version with an absorptive glass mat and an internal
oxygen cycle. At about the same time British Telecom phased them out because
of the high maintenance, especially the water replenishment required for large
stationary batteries with liquid electrolyte, and also started using recombination
batteries. The liquid electrolyte is completely absorbed by a microfiber glass fleece,
although some channels have to remain free from electrolyte to permit oxygen
transfer. The electrolyte absorption occurs on the surface of the microfibers,
which – as known – increases steeply with decreasing diameter. Frequently such
separators are therefore also characterized by means of their internal surface
2
−1
area (∼1m g !). The pore distribution is anisotropic, which is desirable; in the
plane between the electrodes – because of the fiber diameters – very small pores
are formed effecting a large capillary force, while perpendicular to this plane,
due to the high porosity of >90%, pores of 10–20 µm diameter are found, which