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Liquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion

TitleLiquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion
Publication TypeArticolo su Rivista peer-reviewed
Year of Publication2021
AuthorsTriolo, A., Di Lisio V., F. Celso Lo, Appetecchi Giovanni Battista, Fazio B., Chater P., Martinelli A., Sciubba F., and Russina O.
JournalJournal of Physical Chemistry B
KeywordsAnion coordination, Aqueous electrolyte, Bis(trifluoromethane sulfonyl)imide, Charge storage devices, Concentration (process), Electrochemical window, Electrolytes, Hydration, Hydrophobic nature, Liquid structures, Lithium, Lithium transference numbers, Morphology, Negative ions, Salt electrolytes, Salt systems, Virtual storage

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kgwater), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure-dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apolar nature of the anions, exploring the properties of the aqueous electrolytes of lithium salts with a strongly asymmetric anion, namely, (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl) imide. Using a synergy of experimental and computational tools, we detect a remarkable level of structural heterogeneity at a mesoscopic level between anion-rich and water-rich domains. Such a ubiquitous sponge-like, bicontinuous morphology develops across the whole concentration range, evolving from large fluorinated globules at high dilution to a percolating fluorous matrix intercalated by water nanowires at super-concentrated regimes. Even at extremely concentrated conditions, a large population of fully hydrated lithium ions, with no anion coordination, is detected. One can then derive that the concomitant coexistence of (i) a mesoscopically segregated structure and (ii) fully hydrated lithium clusters disentangled from anion coordination enables the peculiar lithium diffusion features that characterize water-in-salt systems. © 2021 American Chemical Society.


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Citation KeyTriolo2021