The ion transportation mechanism in a high-concentration solution remains unclear due to the complexity of the strong ion–ion/ion–solvent interaction, resulting in the invalidation of most ionic conducting theories based on diluted solutions. Here, a superconcentrated electrolyte (water-in-salt) is investigated by multiple experimental techniques, including advanced tools (NMR, synchrotron X-ray diffraction, and spallation neutron scattering), combined with molecular dynamics (MD) simulation to draw out its unique microstructure and uncover its intrinsic relationship with the ionic transportation. Based on the results, we firstly proposed the ionic transport model for the water-in-salt electrolyte, where the solid-like nano-anion clusters construct a superfluid framework and the lithium ion is able to move freely like in an ionic atmosphere. Our model gives a unified explanation to the unique phenomena previously discovered in water-in-salt electrolytes, including the decoupling of conductivity–viscosity and the nanophase separation between the anion and water. Our findings on the microstructure of the super-high-concentrated electrolyte and the involved unique Li-conducting mechanism can fill in the gap between a solid-state conductor and a dilute liquid electrolytic solution.

Reduced graphene oxide (rGO) has numerous potential applications, such as molecular sensor, gas separation membrane, etc. The performance of these devices is often subject to the environment humidity due to the interaction of rGO with water. However, the atomically detailed information on the dynamical and thermodynamic properties of water on the surface of graphene-based materials as well as its underlying molecular mechanism is largely unknown. By performing neutron scattering on hydrated rGO powders, composed by well separated monolayer and few-layer rGO sheets, we found three components of surface water. One remains liquid at −80 °C, while the other two freeze into ice in a stepwise manner above −40 °C. Although slightly slower than the other two, the nonfreezing water diffuses an order of magnitude faster on rGO than those confined in the hydrophilic bulk phase, such as compact powder or membrane. Complementary molecular dynamics simulation revealed that the heterogeneity of surface water arises from the gradual attenuation of the electrostatic interaction between water and oxide groups on rGO within a few hydration layers. These findings are fundamental for understanding of interfacial hydration and ice formation in many materials, and valuable for various applications using graphene-based materials.

The vibrational properties of crystalline bulk materials are well described by Debye theory, which successfully predicts the quadratic ω2 low-frequency scaling of the vibrational density of states. However, the analogous framework for nanoconfined materials with fewer degrees of freedom has been far less well explored. Using inelastic neutron scattering, we characterize the vibrational density of states of amorphous ice confined inside graphene oxide membranes and we observe a crossover from the Debye ω2 scaling to an anomalous ω3 behaviour upon reducing the confinement size L. Additionally, using molecular dynamics simulations, we confirm the experimental findings and prove that such a scaling appears in both crystalline and amorphous solids under slab-confinement. We theoretically demonstrate that this low-frequency ω3 law results from the geometric constraints on the momentum phase space induced by confinement along one spatial direction. Finally, we predict that the Debye scaling reappears at a characteristic frequency ω× = vL/2π, with v the speed of sound of the material, and we confirm this quantitative estimate with simulations.

Water-in-salt electrolytes (WiSEs) have attracted extensive attention as promising alternatives to organic electrolytes. The limited electrochemical stability windows (ESWs) of aqueous electrolytes are significantly widened by WiSEs. However, the actual...