Page 665 - Handbook of Battery Materials

18.2 Solvent-Free Polymer Electrolytes 639

There is experimental evidence to suggest that anion and cation diffusion
can have different mechanisms [70]. The temperature variation of the diffusion
7
coefficients of 31 Pand Li in aPEO–LiPF 6 shows quite different trends. The 31 P
diffusion coefficients follow a VTF-type dependence at all concentrations and
are always significantly faster than those of Li -based species. Anions do not
+
form strong bonds with the polymer hosts, so their transport is likely to depend
principally on the rate of polymer rearrangements. Such a mechanism may be
described in terms of configurational entropy or free-volume theories, both of which
7
predict a VTF-like temperature dependence. Li diffusion shows a change in the
ion-diffusion mechanism from a process controlled by VTF kinetics to a thermally
activated mechanism as salt concentration is increased. Ionic conductivity for the
cation appears to be an average of two distinct processes, with an ion-hopping
mechanism predominating at high salt concentrations.

18.2.7
An Analysis of Ionic Species

For a salt MX dissolved in a polymer host solvent, the formation of neutral ion pairs,
0
[MX] , leads to a drop in the concentration of charge carriers. Larger aggregates
may also exist in some media, and although they may be charged, for example,
[M 2 X] or [MX 2 ] , their mobilities will be impaired by size in comparison with free
−
+
ions, which again adversely affects conductivity. The nature of the anionic species
is of course of paramount importance for the type of speciation. Both spectral and
molar conductivity studies reveal marked change in the type and concentration
of species involved in charge transport as salt concentration changes [71, 72].
Figure 18.5 compares these data.
At salt concentrations below those shown in Figure 18.5, molar conductivity
behavior has been identified with the formation of electrically neutral ion pairs
[8]. Between concentrations of 0.01 and ∼0.1 mol L −1 (up toanO : M ratioof ∼50
: l) the molar conductivity rises and this can be explained by the formation of
mobile charged clusters such as triple ions, a progressive dissociation of ion pairs,
∼
or a combination of both. Up to O : M = 50 : 1, however, spectral data indicate
very little change in the species concentrations, and this may instead indicate
an enhancement in ionic mobility. With a charge separation <5 ˚ A and polymer
motion restricted by ion coordination, an anion-assisted (Grotthus-like) transport
mechanism could be envisaged as Equations 18.5 and 18.4.
−
XM + X + X ←→ X + MX + X − (18.4)
−
−
−
OM + X ←→ O + MX (18.5)
18.2.8
Cation-Transport Properties
The mobility of lithium ions in cells based on cation intercalation reactions is
clearly a crucial factor in terms of fast and/or deep discharge, energy density, and