Page 319 - Handbook of Battery Materials

11.1 General Principles 289

Figure 11.4 Microporous polyethylene
separator (SEM).

1 mm

11.1.2.3 Electrical Resistance
The electrical resistance exerted by a separator on the ionic current is defined as
the total resistance of the separator filled with electrolyte minus the resistance
of a layer of electrolyte of equal thickness, but without the separator. The
separator resistance has to be considered as an increment over the electrolyte
resistance.

R(separator) = R(electrolyte + separator) − R(electro1yte) (11.2)

1 l
R = × (11.3)
σ q
where l is the length of the ion path and q the area of the ionic flow; σ is the specific
electrolytic conductivity, the reciprocal of the specific resistance ρ of the electrolyte,
and is a temperature-dependent material constant.
The tortuosity factor T of a separator can be expressed by

l s
T = (11.4)
d
with l s , the ion path through the separator and d, the thickness of the separating
layer.
The porosity of the separator is defined thus:

void separator volume
P =
geometric separator volume
q s l s
= (11.5)
qd
with q s as the ‘open’ area of the separator.
A transformation results in
P
q s = q ·
T