Sulfolane-Based Highly Concentrated Electrolytes of Lithium Bis(trifluoromethanesulfonyl)amide: Ionic Transport, Li-Ion Coordination, and Li–S Battery Performance

Following our recent
study demonstrating predominant Li-ion hopping
conduction in sulfolane (SL)-based highly concentrated electrolytes
with LiBF4, LiClO4, and lithium bis­(fluorosulfonyl)­amide,
herein a systematic study on transport properties and Li-ion coordination
of SL-based electrolytes with lithium bis­(trifluoromethanesulfonyl)­amide
was performed. In the highly concentrated region, Li ions clearly
diffuse faster than SL and TFSA anions. The two oxygen atoms of the
SL sulfonyl group tend to coordinate to two different neighboring
Li ions and TFSA anions form ionic clusters with Li ions, verifying
the previous observation of the unusual Li-ion conduction and its
relevance to the SL- and anion-bridged, chainlike Li-ion coordination
structure for the SL-based concentrated systems with other Li salts.
Moreover, addition of hydrofluoroether (HFE) to the SL-based concentrated
electrolytes greatly enhances diffusion coefficients but fragments
the chainlike Li-ion coordination to smaller clusters, leading to
a reduced contribution of Li-ion hopping to the overall Li-ion conduction.
The SL-based concentrated electrolyte and its mixtures with HFE showed
lower lithium polysulfide solubility and higher rate capability for
lithium–sulfur (Li–S) cells compared with previously
reported tetraglyme-based electrolytes. The SL-based electrolytes
were found to manifest a significant improvement in Li-ion mass transfer
as a sparingly solvating electrolyte, enabling the solid-state sulfur
redox reactions in high-performance Li–S batteries.