Page 316 - Handbook of Battery Materials

286 11 Separators

in a highly aggressive medium. Under these conditions no substance harmful to
the electrochemical reactions may be generated.
The unhindered ionic charge transfer requires many open pores of the smallest
possible diameter to prevent electronic bridging by deposition of metallic particles
floating in the electrolyte. Thus the large number of microscopic pores form
immense internal surfaces, which inevitably are increasingly subject to chemical
attack.
Not only the electrolyte, but also the electrodes directly or indirectly exert a
chemical attack, either by an oxidation or reduction potential of the electrode
material itself or by the generation of soluble oxidizing or reducing substances.
The requirements for the separator properties are generally lower in primary
cells, that is, in nonrechargeable systems. This results from the lack of problematic
phenomena accompanying any charging of a battery, such as recrystallization of
active materials or the generation of oxidizing species during overcharge. Within
the framework of this chapter, therefore, separators mainly for secondary cells will
be described.
In the older battery literature the term ‘separator’ is frequently used very
loosely, to include all nonmetallic solid components between the electrodes,
such as supporting structures for active materials (tubes, gauntlets, glass mats),
spacers, and separators in a narrow sense. In this section, only the last of
these, the indispensable separating components in secondary cells, will be termed
‘separators,’ distinguished from the others by their microscopically small pores,
that is, with a mean diameter significantly below 0.1 mm.

11.1.2
Characterizing Properties
Some terms and properties common to all separators are defined and discussed
below.

11.1.2.1 Backweb, Ribs, and Overall Thickness
Separator backweb refers to the porous separating membrane. It is of uniform
thickness and has a macroscopically uniform pore distribution. Only in this way
can an overall uniform current density be ensured during the operation of the
storage battery, achieving a uniform charging and discharging of the electrodes
and thus a maximum utilization of the electrode materials.
The lead–acid battery has a peculiarity: the electrolyte sulfuric acid not only
serves as ion conductor (as charge-transport medium), but it actively participates
in the electrochemical reaction:
Pb + PbO 2 + 2H 2 SO 4 ↔ 2PbSO 4 + 2H 2 O (11.1)

During charging at the positive electrode one additional water molecule is con-
sumed per electron converted, which is regenerated during discharging.
In practice the desired electrolyte distribution is achieved by distance-maintaining
ribs on the porous backweb; this in addition has the advantage of maintaining