Glycol ethers, or glymes, have been
recognized as good candidates
as solvents for lithium–air batteries because they exhibit
relatively good stability in the presence of superoxide radicals.
Diglyme (bis(2-methoxy-ethyl)ether), in spite of its low donor number,
has been found to promote the solution mechanism for the formation
of Li
2
O
2
during the discharge reaction, leading
to large deposits, that is, high capacities. It has been suggested
that lithium salt association in these types of solvents could be
responsible for this behavior. Thus, the knowledge of the speciation
and transport behavior of lithium salts in these types of solvents
is relevant for the optimization of the lithium–air battery
performance. In this work, a comprehensive study of lithium trifluoromethanesulfonate
(LiTf) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in
1,2-di-methoxyethane (DME) and diglyme, over a wide range of concentrations,
have been performed. Consistent ion pairs and triplet ions formation
constants have been obtained by resorting to well-known equations
that describe the concentration dependence of the molar conductivities
in highly associated electrolytes, and we found that the system LiTf/DME
would be the best to promote bulky Li
2
O
2
deposits.
Unexpected differences are observed for the association constants
of LiTf and, to a lesser extent, for LiTFSI, in DME and diglyme, whose
dielectric constants are similar. Molecular dynamics (MD) simulations
allowed us to rationalize these differences in terms of the competing
interactions of the O-sites of the ethers and the SO
x
groups of the corresponding anions with Li
+
ion.
The limiting Li
+
diffusivity derived from the fractional
Walden rule agrees quite well with those obtained from MD simulations,
when solvent viscosity is conveniently rescaled.
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