V. Focused Fundamental Research - EERE - U.S. Department of ...

V.D.2 Interfacial Behavior of Electrolytes (LBNL)
Kerr – LBNL
Conductivity of the SIC-EC:EMC
7.1 kJ/mol
Conductivity (S/cm)
1E-4
1E-5
10.6 kJ/mol
17.0 kJ/mol
7.4 kJ/mol
19 kJ/mol
9.7 kJ/mol
14.8 kJ/mol
PEO-MDFB (17.0 kJ)
PEO-FMDFB (10.6 kJ)
PEO-TFSI (7.4 kJ)
PS-MDFB (14.8 kJ)
PS-FMDFB (9.7 kJ)
PS-MMDFB (19.0 kJ)
PS-TFSI (7.1 kJ)
2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
1000/T (1000/K)
Figure V - 139: Conductivities of polyelectrolyte gels as a function of temperature. Gels are prepared with EC:EMC solvent.
The activation energies for Li ion conduction are also
listed on the plot. It can be seen that switching from the
polyether backbone to the polyether-polysulfone backbone
resulted in a significant increase in conductivity without
much change in the activation energy. However, the
activation energy almost doubled on changing the solvent
to tetraethyleneglycoldimethyl (TEGDME) ether from the
carbonate solvents and the absolute value of the
conductivity decreased by an order of magnitude that
probably reflects the effect of the lower dielectric constant
of the medium on the ion-pairing of the salts. Similar
trends were also observed with the interfacial impedance
of the gels at lithium metal where the impedance with the
polyether-polysulfone is much smaller. Figure V - 140 shows
the impedance of the PS-TFSI gel at lithium metal at 80°C
which is an order of magnitude less than the polyether
material. Even at 25°C the impedance is only 200 ohm.cm 2
which indicates that these single-ion conductor materials
are now very close to being practical.
More important is the performance of these materials
when used as binders in composite electrodes. Figure V - 141
shows the impedance of half cells of LiFePO 4 one with the
single-ion conductor separator and binder and one with a
conventional binary salt electrolyte. Figure V - 142 shows the
discharge capacities of these two cells as a function of rate.
Figure V - 142 shows the much better capacity at high
rates of the single-ion conductor binder which is expected
due to the lack of concentration polarization. The stability
of the PS-TFSI SIC gel at high voltages is illustrated in
Figure V - 143, where it can be seen that the electrolyte
system is stable to at least 4.5V. This indicates the material
will be stable with the spinel –type cathodes and tests on
this stability are under way.
The interfacial impedance of these SIC materials is
still high but maybe reduced by functionalization of the
surfaces in composite electrodes as illustrated in Figure V -
136 (b). Very encouraging results have already been
obtained that will merit a patent application in due course.
The results will be reported on in future.
Energy Storage R &D 600 FY 2011 Annual Progress Report