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

Angell – ASU
V.D.8 Sulfones with Additives as Electrolytes (ASU)
impressive in view of the immediate exfoliation that
occurs when EMS is used alone as solvent. We note that
the fluorosulfonyl group is also implicated in the efficacy
of the bis-fluorosulfonylimide anion as an SEI-forming
agent (observed in Japanese laboratories).
Figure V - 168: Conductivities of sulfolane-DMC mixtures
Figure V - 169: Walden plot for assessing ionicities.
Conversely, and keeping in mind that it enhances cell
safety, it should be recognized that although sulfolane is
more than an order of magnitude more viscous than
EC:DMC, it only gives up a factor of two or so in
conductivity. At the same time the solution gains in
electrochemical stability. The gain should be even greater
for the acyclic sulfone EMS, the “window” of which is the
widest on record. It should be noted that sulfolane as
solvent has already passed tests of stability against the
LNMO high voltage cathode in the 1,000 cycle tests
reported by Amine and colleagues, at ANL.
To consider solutions based on EMS, data are
presented in Figure V - 170. It is seen that the mixture of EMS
and FMS has a very favorable temperature dependence
such that at 70ºC it has a conductivity not only as high as
the EMS-DMC solvent, but almost as high as that of
EC:DMC in Figure V - 167. Figure V - 171 shows that the electrochemical
stability is above 5.5 volts, and Figure V - 172 shows
that a graphite anode half cell quickly establishes a stable
SEI and then intercalates lithium reversibly. This is
Figure V - 170: Conductivities with EMS and FMS co-solvents
EMS with FMS co-solvent also supports cycling at
the graphite anode, though the SEI does not form as
rapidly and efficiently as with sulfolane.
Turning to the cathode half cell, less success has been
had, but the preparation of the cathode is more complex
and there has been no access to proper cathode preparation
facilities (spin-coaters etc). The results are shown in Figure
V - 173, and it is found, in follow-up experiments, that the
electrodes prepared in the ASU lab cannot be charged
beyond 4.7 V before some irreversibility is encountered. In
view of the success reported by the Amine lab, we will not
consider the EMS-FMS solvent based electrolyte a failure
until tests on more satisfactory cathodes can be carried out.
Dr. Ueno, now back in Japan, has offered to use
sophisticated electrode fabrication tools available in his
current laboratory to prepare more reliable cathodes and
thereby to establish the true efficacy of the sulfone-based
electrolytes for high voltage cells of the LNMO type.
Figure V - 171: Oxidative stabilities of EMS and FMS
FY 2011 Annual Progress Report 623 Energy Storage R&D