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Interfacial Speciation Determines Interfacial Chemistry: X‐ray‐Induced Lithium Fluoride Formation from Water‐in‐salt Electrolytes on Solid Surfaces
Author(s) -
Steinrück HansGeorg,
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.
Publication year - 2020
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.202007745
Subject(s) - electrolyte , lithium (medication) , electrochemistry , chemistry , salt (chemistry) , aqueous solution , lithium fluoride , inorganic chemistry , electrolysis of water , x ray photoelectron spectroscopy , electrolysis , fluoride , hydroxide , chemical engineering , electrode , engineering , endocrinology , medicine
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/H 2 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.