title: Effect of Hydrofluoroether Cosolvent Addition on Li Solvation in Acetonitrile-Based Solvate Electrolytes and Its Influence on S Reduction in a Li-S Battery.
abstract: Li-S batteries are a promising next-generation battery technology. Due to the formation of soluble polysulfides during cell operation, the electrolyte composition of the cell plays an active role in directing the formation and speciation of the soluble lithium polysulfides. Recently, new classes of electrolytes termed "solvates" that contain stoichiometric quantities of salt and solvent and form a liquid at room temperature have been explored due to their sparingly solvating properties with respect to polysulfides. The viscosity of the solvate electrolytes is understandably high limiting their viability; however, hydrofluoroether cosolvents, thought to be inert to the solvate structure itself, can be introduced to reduce viscosity and enhance diffusion. Nazar and co-workers previously reported that addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to the LiTFSI in acetonitrile solvate, (MeCN)2-LiTFSI, results in enhanced capacity retention compared to the neat solvate. Here, we evaluate the effect of TTE addition on both the electrochemical behavior of the Li-S cell and the solvation structure of the (MeCN)2-LiTFSI electrolyte. Contrary to previous suggestions, Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that TTE coordinates to Li+at the expense of MeCN coordination, thereby producing a higher content of free MeCN, a good polysulfide solvent, in the electrolyte. The electrolytes containing a higher free MeCN content facilitate faster polysulfide formation kinetics during the electrochemical reduction of S in a Li-S cell likely as a result of the solvation power of the free MeCN.
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.|Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: See
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.|Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Heng-Liang
authors.initials: HL
authors.lastname: Wu
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.|Department of Physics and Astronomy, California State University , Northridge, California 91330, United States.
authors.forename: Kah Chun
authors.initials: KC
authors.lastname: Lau
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.|Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Minjeong
authors.initials: M
authors.lastname: Shin
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Lei
authors.initials: L
authors.lastname: Cheng
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Mahalingam
authors.initials: M
authors.lastname: Balasubramanian
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Kevin G
authors.initials: KG
authors.lastname: Gallagher
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Larry A
authors.initials: LA
authors.lastname: Curtiss
authors.affiliation: Joint Center for Energy Storage Research , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.|Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Andrew A
authors.identifier: http://orcid.org/0000-0003-4400-9907
authors.initials: AA
authors.lastname: Gewirth
keywords: hydrofluoroether cosolvent; in situ Raman spectroscopy; lithium−sulfur battery; solvate electrolyte; sulfur reduction kinetics