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Open Access Publications from the University of California

Interfacial Speciation Determines Interfacial Chemistry: X-ray-Induced Lithium Fluoride Formation from Water-in-salt Electrolytes on Solid Surfaces.

  • Author(s): Steinrück, Hans-Georg;
  • Cao, Chuntian;
  • Lukatskaya, Maria R;
  • Takacs, Christopher J;
  • Wan, Gang;
  • Mackanic, David G;
  • Tsao, Yuchi;
  • Zhao, Jingbo;
  • Helms, Brett A;
  • Xu, Kang;
  • Borodin, Oleg;
  • Wishart, James F;
  • Toney, Michael F
  • et al.

Super-concentrated "water-in-salt" electrolytes recently spurred resurgent interest for high energy density aqueous lithium-ion batteries. Thermodynamic stabilization at high concentrations and kinetic barriers towards interfacial water electrolysis significantly expand the electrochemical stability window, facilitating high voltage aqueous cells. Herein we investigated LiTFSI/H2 O electrolyte interfacial decomposition pathways in the "water-in-salt" and "salt-in-water" regimes using synchrotron X-rays, which produce electrons at the solid/electrolyte interface to mimic reductive environments, and simultaneously probe the structure of surface films using X-ray diffraction. We observed the surface-reduction of TFSI- at super-concentration, leading to lithium fluoride interphase formation, while precipitation of the lithium hydroxide was not observed. The mechanism behind this photoelectron-induced reduction was revealed to be concentration-dependent interfacial chemistry that only occurs among closely contact ion-pairs, which constitutes the rationale behind the "water-in-salt" concept.

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