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

V.D.8 Sulfones with Additives as Electrolytes (ASU)
Angell – ASU
of features of its behavior, including its unusually high
efficiency in usage of the solute ions for charge transport,
will be documented below.
This report is divided into sections on synthetic
targets, electrochemical characterization of synthesis
products and mixed solvent systems, testing of anode half
cells (lithium in graphite intercalation), testing of cathode
half cells (stability to LiNi 0.5 Mn 1.5 O 4 ), and finally the
recently successful realization of a nanoscopic tetrahedral
mesh supporting medium for liquid electrolytes.
Approach
All technical problems are approached with the same
strategies. A sequence of rational steps and best ideas are
taken until an objective has been reached, and the sample
has been made, purified, and tested, or alternatively is
proven impossible, or too difficult, to make by available
methods.
Characterizations of successful preparations are
carried out with standard methods.
Results
1. Electrolyte Development.
(a) Fluorinated sulfone syntheses. In the first
quarter, a second fluorinated sulfone,
CF 3 CH 2 OCH 2 SO 2 CH 3 , designated FEOMMS, was
developed which is a 1,1,1 perfluoromethylated version of
one of the ethersulfones described in papers preceding this
grant (and forming part of the original proposal). Similar
to the 3F-sulfolane synthesized in the previous year, it has
a pleasingly low melting point, but its conductivity, shown
in Figure V - 167, is a factor of two lower than that of
sulfolane and little better than that of 3F-sulfolane. Its
purification was therefore not pursued, and its
electrochemical window was not evaluated.
Contrary to prior indications, no advantage in terms
of solution conductivity has been found for the fluorinated
solvents over hydrogenated equivalents, unless molecular
size is simultaneously reduced. Small molecule fluorinated
sulfones were either purchased or synthesized. Perfluoro
methyl methyl sulfone was found to have such acid
protons that bubbles of hydrogen were released in the
presence of lithium metal. 1,1,1trifluoroethylmethyl
sulfone had been prepared in an earlier study and was
known to be an excellent solvent with high conductivity,
but also with methyl hydrogens too active to be acceptable.
Finally, fluoromethyl sulfone FMS, available
commercially, was found to be very fluid, as expected
from its boiling point, and also found to be very stable
electrochemically, hence would seem interesting.
However, it is such a poor solvent that only LiTFSI,
not LiPF 6 . can be dissolved, and the LiTFSI solution
proves to be a poor conductor despite the high fluidity.
Clearly, due to low polarity, ion dissociation is a major
problem with this solvent (see also Figure V - 169, lowest
curve). However FMS has potential as a fluidity-enhancing
cosolvent, and has been studied in this role in successful
experiments reported below.
(b) Mixed solvent evaluations. For comparison,
sulfolane-DMC solutions were studied over a wide range
of compositions to find the range over which the
electrochemical stabilization of co-solvents reported
previously could be exploited, in high voltage cell
applications. In a revealing study, conductivities and
viscosities were both determined at each composition, and
a Walden plot, not used previously in non-aqueous
solution analysis, was constructed.
Figure V - 167: Arrhenius plot comparison of conductivities of 1M LiPF6 in
1:1 EC-DMC, commercial sulfolane solvents with those in the newly
synthesized solvents and their mixtures in DMC.
Figure V - 168 shows the variation of conductivity with
DMC content in wt% while the Figure V - 169 shows the
corresponding viscosity data. The densities were measured
in order that the equivalent conductivity needed for the
Walden plot could be derived. The equivalent conductivity
monitors the ionic mobilities (sum of all) and, according to
Walden, is controlled by the liquid fluidity. Thus, to first
approximation, the failure of experimental points to lie on
the ideal unit slope line (calibrated with dilute aqueous
KCl), gives information on the failure to dissociate into
ions. It is notable that in pure sulfolane as solvent, the
Walden rule is obeyed, but then there is a systematic falloff
as the low dielectric constant component is added. It is
notable that the standard electrolyte, LiPF 6 in EC:DMC 1:1
marked as red solid circle, is conductimetrically inefficient
with respect to utilizing the intrinsic mobility of its ions to
the extent of almost an order of magnitude.
Energy Storage R &D 622 FY 2011 Annual Progress Report