Asymmetric Composition of Ionic Aggregates and the Origin of High Correlated Transference Number in Water-in-Salt Electrolytes

Time-averaged appearance frequency map of number of ionic aggregates in (a) 15 m and (b) 20 m LiTFSI aqueous electrolytes. (c) Calculate uncorrelated (blue) and correlated (green) transference number. (d) a representative snapshot of the system

Scientific Achievement

MD simulations revealed the heterogeneous structure of ion and water domains/aggregates and fairly high correlated transference number in the water-in-salt electrolytes.

Significance and Impact

Li ion in the water-in-salt electrolyte can diffuse through water domains, along with the anions within the aggregates, and exchange with Li ion from water domain. The high correlated transference number originates from a weak negative correlation between the motion of cations and anions.

Research Details

  • MD simulation and SAXS technique were used to elucidate the nanostructure of water-in-salt electrolyte, which comprises ion and water domains.
  • Ionic regions are composed of more TFSI ions than Li ions based on the single-linkage algorithm.
  • Time correlation function proves the existence of two diffusion mechanism of Li ions in the ionic regions.
  • The uncorrelated transference number is consistent with the prior NMR experiment and the high correlated transference number attributes to the unique nanostructure.

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DOI:  10.1021/acs.jpclett.9b03495

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