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O.E. Geiculescu et al. / Journal of Fluorine Chemistry 125 (2004) 1179–1185
insight. The primary difference among the various salts lies
in the A values, which are highest for LiTFSI but are
substantially diminished for all of the dilithium salts. The
diminished A values suggest that the major cause of the
diminished conductivity is that there are fewer mobile
charge carriers present in the dilithium salt SPEs, which
in turn is probably a consequence of the fact that the dianions
have low mobility and can partially trap the cations. A
secondary effect is that in all of the dilithium salt SPEs
the B term and the apparent activation energy Dm from the
VTF fits is diminished relative to that for LiTFSI. The latter
effect is particularly pronounced for the more dilute SPEs.
We note also that for each salt concentration the apparent
activation energy values Dm are quite close to each other for
all the dilithium salt-based SPEs.
more like two separate, non-interacting anions linked
together. Alternatively, the smaller overall size of the anion
in salt 2 may mean that the diminished electron-withdrawing
effects of the perfluoroethylene linker are partially offset by
the fact that the smaller anion should experience less viscous
drag and thus contribute more to the ionic conductivity.
4. Conclusions
Synthetic strategies are presented for making a series of
sulfonyl-imide-based dianionic lithium salts starting from
a,o-diiodoperfluoroalkylenes and triflic acid. Solid polymer
electrolytes prepared in polyethylene oxide hosts using the
resulting salts in two different salt concentrations (10:1 and
30:1) were prepared and characterized. Conductivities of
SPEs made using the monomeric salt LiTFSI were always
higher than for comparable SPEs prepared using the new
dianionic salts, which probably reflects a diminished con-
tribution of the anions in the new salts to the overall ionic
conductivity. In all cases, the highest salt content SPEs
studied (EO:Li ¼ 10:1) exhibited the highest conductivities.
A trend in the dianionic-salt-based SPEs of increasing ionic
conductivity with increasing length of the perfluoroalkylene
connector between the two anionic units in the salt is thought
to be the result of two opposing trends, one reflecting an
increase in anion size with an increased content of fluorine
which diminishes anion transport and conductivity and
another reflecting an increase in anion basicity with
increased fluorination which results in diminished ion pair-
ing and an enhancement in the number of charge carriers,
thereby increasing conductivity.
A consideration of the data in Figs. 1 and 2 and Tables 1b
and 2b reveals some unexpected trends in the relationship
between ionic conductivity and anion structure. Consider the
curves in Fig. 1 for the dilute SPEs in the region between 60
and 120 8C, where the SPEs are melted and in a fully
amorphous state. The conductivity of the SPE made from
salt 2 in which two imide anions are linked by a perfluor-
obutylene chain is diminished relative to that of an SPE
made using LiTFSI by almost an order of magnitude.
However, the conductivity of SPEs made using salts 3
and 4, in which the perfluorobutylene chain in salt 2 is
replaced by longer perfluorohexylene or perfluorooctylene
chains (respectively), increases as the linker chain length
increases. A similar effect was noted in our earlier publica-
tion, albeit for salts 2 and 3 only [15]. As was also noted in
the earlier work, the trend of increasing conductivity with
increasing anion size is counterintuitive in the sense that one
might expect that increasing anion size should result in
greater frictional drag on the anions, suppressing the anion
contribution to transport and thereby diminishing conduc-
tivity. Inspection of the VTF fitting parameters in Tables 1b
and 2b lends some insight; the A value for salt 3 is 4–5 times
larger than that for salt 2, and that for salt 4 is 10 times larger
than that for salt 2. These observations suggest that in both
cases the increased conductivity in the salts with a higher
fluorine content is a consequence of an increase in the
number of charge carriers present in the SPEs, which in
turn may reflect a diminished tendency toward ion-pairing
for anions with the greater electron-withdrawing character
of the more highly perfluorinated groups. Greater electron
withdrawing character should result in more effective delo-
calization of the negative charges, thereby minimizing anion
basicity and reducing coulomb trapping of cations.
Acknowledgements
The authors are grateful for financial support of this
research by the DoD (grant no. DAAG 55-98-1-Q004)
and DOE (BATT program, grant no. DE-AC03-76SF00098).
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