Fast Water-Assisted Lithium Ion Conduction in Restacked Lithium Tin Sulfide Nanosheets

While two-dimensional (2D) materials may preserve some intrinsic properties of the corresponding layered bulk material, new characteristics arise from their pronounced anisotropy or confinement effects. Recently, exceptionally high ionic conductivities were discovered in 2D materials such as graphene oxide and vermiculite. Here, we report on the water-assisted fast conduction of lithium ions in restacked lithium tin sulfide nanosheets. Li0.8Sn0.8S2 exfoliates spontaneously in water and can be restacked into homogeneous films in which the lithium content is decreased, and a partial substitution of sulfur with hydroxyl groups takes place. Using a recursive supercell refinement approach in reciprocal space along with real-space pair distribution function analysis, we describe restacked lithium tin sulfide as a partially turbostratically disordered material composed of lithium-containing and lithium-depleted layers. In humid air, the material takes up multiple layers of water that coordinate lithium ions in the space between the layers, increasing the stacking distance and screening the interaction between lithium ions and the anionic layers. This results in a 1000-fold increase in ionic conductivity up to 47 mS cm–1 at high humidities. Orientation-dependent impedance spectroscopy suggests a facile in-plane conduction and a hindered out-of-plane conduction. Pulsed field gradient nuclear magnetic resonance spectroscopy reveals a fast, simultaneous diffusion of a majority and a minority species for both 7Li and 1H, suggesting water-assisted lithium diffusion to be at play. This study enlarges the family of nanosheet-based ionic conductors and helps to rationalize the transport mechanism of lithium ions enabled by hydration in a nanoconfined 2D space