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Liquid structure of a water-in-salt electrolyte with a remarkably asymmetric anion

DOI: 10.1021/acs.jpcb.1c06759 DOI Help

Authors: Alessandro Triolo (Consiglio Nazionale delle Ricerche (ISM-CNR)) , Valerio Di Lisio (University of Rome Sapienza) , Fabrizio Lo Celso (Consiglio Nazionale delle Ricerche (ISM-CNR); UniversitĂ  di Palermo) , Giovanni B. Appetecchi (ENEA) , Barbara Fazio (Consiglio Nazionale delle Ricerche (IPCF-CNR)) , Philip A. Chater (Diamond Light Source) , Andrea Martinelli (University of Rome) , Fabio Sciubba (University of Rome Sapienza) , Olga Russina (Consiglio Nazionale delle Ricerche (ISM-CNR); University of Rome Sapienza)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: The Journal Of Physical Chemistry B , VOL 22

State: Published (Approved)
Published: November 2021
Diamond Proposal Number(s): 27222

Abstract: 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.

Journal Keywords: Salts; Anions; Electrolytes; Lithium; Molecules

Subject Areas: Chemistry, Energy

Instruments: I15-1-X-ray Pair Distribution Function (XPDF)

Added On: 08/11/2021 11:24

Discipline Tags:

Physical Chemistry Energy Energy Storage Chemistry

Technical Tags:

Scattering Wide Angle X-ray Scattering (WAXS)