Page 317 - Handbook of Battery Materials

11.1 General Principles 287

a maximum distance between the origin of oxidizing substances located at the
positive electrode and the highly porous separating membrane, sensitive due to its
large inner surface.
The total or overall thickness thus comprises the backweb thickness and the
rib height. For achieving a uniform current distribution the thickness is normally
specified very precisely and is acceptable only within rather narrow tolerances.
Besides technical difficulties in the production, this also presents a problem in
measurement: since all separator materials are more or less compressible, a
specified measuring pressure has to be used. Moreover, the measuring area is also
significant; one can easily imagine an extended area touching only the microscopic
elevations of the separator, whereas a measuring tip may very well hit ‘valleys.’

11.1.2.2 Porosity, Pore Size, and Pore Shape
Porosity of a separator is defined as the ratio of void volume to apparent geometric
volume. High porosity is desirable for unhindered ionic current flow.
The pores of the separating membrane are to be most uniformly distributed
and of minimum size to avoid deposition of metallic particles and thus electronic
bridging. One distinguishes between macroporous and microporous separators,
the latter having to show pore diameters below 1 micron (µm), that is, below
one-thousandth of a millimeter. Thus the risk of metal particle deposition and
subsequent shorting is quite low, since active materials in storage batteries usually
have particle diameters of several microns.
However, even these small pores cannot prevent the formation of so-called
‘microshorts,’ arising by metal deposition (e.g., dendrites) from the solution phase.
The pores of modern separators have a diameter of about 0.1 µm, equal to 100 nm,
while metal ions have a diameter of few angstroms, equal to 0.5–1 nm. On an
atomic scale even micropores are barn doors!
Micropores are invisible to the naked human eye; thus for outsiders it is always
surprising that separators of typically 60% porosity (i.e., 60% void volume, 40%
solid material) present the impression of a compact, hole-free, nontransparent
sheet.
In a first approximation the average size of pore diameter has no effect on
porosity, even though a superficial view leads to other conclusions. A mental
experiment may be of assistance: imagine a pore and its outside wall, decrease both
to identical scale, then the ratio of void to outside volume remains constant. Of
course the requirements as to pore sizes and their uniform distribution increase
with decreasing separator backweb thickness. The risk of defects also increases;
so-called ‘pinholes’ can originate, for example, by bubble inclusion within the
separator membrane during the production process.
Pores generally are not of a hose-like configuration of constant diameter in a
straight-line direction from one electrode to the other. In practice, a separator’s
pores are formed as void between fibers (Figure 11.1) or spherical bodies in
amorphous agglomerates (Figure 11.2), thus being very different in their form and
size. Statements of any pore diameter are always to be viewed with the above in
mind. Figures 11.1 and 11.2 represent macroporous systems, whereas Figures 11.3