posted on 2021-07-13, 18:04authored byGabriela Horwitz, Eneli Härk, Paula Y. Steinberg, Leide P. Cavalcanti, Sebastian Risse, Horacio R. Corti
The
increasing interest in developing safe and sustainable energy
storage systems has led to the rapid rise in attention to superconcentrated
electrolytes, commonly called water-in-salt (WiS). Several works indicate
that the transport properties of these liquid electrolytes are related
to the presence of nanodomains, but a detailed characterization of
such structure is missing. Here, the structural nano-heterogeneity
of lithium WiS electrolytes, comprising lithium trifluoromethanesulfonate
(LiTf) and bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions
as a function of concentration and temperature, was assessed by resorting
to the analysis of small-angle neutron scattering (SANS) patterns.
Variations with the concentration of a correlation peak, rather temperature-independent,
in a Q range around 3.5–5 nm–1 indicate that these electrolytes are composed of nanometric water-rich
channels percolating a 3D dispersing anion-rich network, with differences
between Tf and TFSI anions related to their distinct volumes and interactions.
Furthermore,
a common trend was found for both systems’ morphology above
a salt volume fraction of ∼0.5. These results imply that the
determining factor in the formation of the nanostructure is the salt
volume fraction (related to the anion size), rather than its molality.
These findings may represent a paradigm shift for designing WiS electrolytes.