Page 581 - Handbook of Battery Materials

17.3 Intrinsic Properties 555

Ion-pair association constants K A determined with the set of conductivity
equations (Equations 17.9–17.17) agree with those obtained from (Equations
17.20 and 17.21) [197]. Salomon and Uchiyama have shown that it is also possible
to extend directly the FuossHsia equation to include triple-ion formation [201].

17.3.3.2 Unilateral Triple Ion Formation
In contrast to bilateral triple-ion formation, unilateral triple ion formation may
also occur in solvents of high dielectric permittivity, when ion-pair association
is increased by nonCoulombic specific ion–ion interactions in solvents of low
− 0
−
+
basicity such as PC or AN. Exclusive formation of anionic triple ions, [A C A ] ,
S
−
is observed in these solvents when large organic molecular anions A S interact
+
+
with small cations such as Li or H . For example, in contrast to lithium acetate in
dimethyl sulfoxide (DMSO) [194], where ion association is moderate, ion association
as well as unilateral triple-ion formation is observed in the solvent PC [202] due to
the much lower basicity of this solvent (see Table 17.2).
According to Wooster [203, 204] conductivity measurements can be evaluated at
the level of limiting laws with the conductivity equation
(T) 2
c (2 0 − )
2 2 2 0 0
0
g(c) = /K A + (T) 1 − c (17.22)

1 − K D K A 0
0
Despite the results from various experiments such as transference number mea-
surements, polarographic studies, spectroscopic measurements, and dielectric
relaxation studies in addition to conductivity measurements, unilateral triple ions
remain a matter of debate. For experimental examples and other hypotheses for
the interpretation of conductance minima the reader is referred to Ref. [15] and the
literature cited there.
The investigation of ion-aggregate formation based on conductivity studies can
be extended up to quadruple-ion formation [205–208], which is thought to be the
reason for the conductivity maximum and the subsequent decrease of conductivity.
However, in our opinion the interdependence of up to four aggregation constants
in addition to uncertainties of the determination of the limiting conductivity
and activity coefficients makes their determination with recent equations [207]
increasingly unsure.
The following table shows a selection of ion-pair association constants, triple-ion
formation constants and limiting conductivities for various electrolytes which have
been studied in connection with the optimization of battery electrolytes.
Table 17.7 shows
• low limiting conductivities and low association constants of electrolytes based on
solvents of high permittivity and high viscosity, for example, PC,
• high limiting conductivities and high association constants for low permittivity
solvents and low viscosities,
• rather small association constants and moderate limiting conductivities for
electrolytes based on a mixture of both solvent types,