Abstract
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.
Highlights
Several technological applications require improved development and understanding of materials with fast ion transport that can be fabricated on a large scale
Li0.8Sn0.8S2 in water can be described by the formal removal of Li2S, which is realized by evaporation of H2S and the formation of LiOH in aqueous solution according to eq 1
Li0.8Sn0.8S2 + 2xH2O → Li0.8−2xSn0.8S2−x + 2xLiOH + xH2S↑ (1). This exfoliation process is distinct from the exfoliation of Liintercalated LixSnS2, featuring partly solution into exfoliated SnS2, LiOH, reduced Sn(II), in aqueous and H2.44−46 As previously shown by Kuhn et al.,[29] the in-plane structure of the exfoliated material is reminiscent of a “defective” variant of SnS2, with a lower electron density at the S positions indicating the presence of sulfur vacancies
Summary
Several technological applications require improved development and understanding of materials with fast ion transport that can be fabricated on a large scale. Vermiculite layers were assembled into a nanofluidic device showing high proton conductivity using near-neutral solutions (σ = 6 × 10−3 S cm−1) with the advantage of higher thermal stability than GO In this system, the conductance of lithium ions stemming from a LiCl solution and being transported through the vermiculite layers was demonstrated.[21] Besides, a remarkably high in-plane hydroxyl conductivity of 10−1 S cm−1 was observed in singlelayer, layered double hydroxides.[22] thin films fabricated from the exfoliated and restacked phosphatoantimonates HSbP2O823 and H3Sb3P2O14,24 which show a large swelling upon water exposure, exhibit an increase in proton conductance of several orders of magnitude upon exposure to relative humidities from 0 to 100%.
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