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.

Concentration polarization in an electrolyte comprising dissociated ions and a solvent is often modeled using concentrated solution theory developed by Newman. This theory is built upon two differential equations for electrolyte concentration and solvent velocity fields. We characterize the concentration and solvent velocity fields in a polystyrene-block-polyethylene oxide (SEO) block copolymer electrolyte using operando X-ray transmission measurements and X-ray photon correlation spectroscopy, respectively. Using calculations based on the assumption that the SEO chain behaves as a single species, we show that the experimental data are consistent with a cation transference number, t+0, ≈ 0.7. Previously published electrochemical experiments using small polarizations led to the conclusion that t+0 is less than 0.3. The discrepancy indicates that the block copolymer electrolyte cannot be approximated as a three-component system (cation, anion, and a single solvent), and frictional interactions involving the glassy polystyrene cannot be lumped with those involving rubbery poly(ethylene oxide) segments.