Molecular insight into LiTFSI and Li2S6 transport through hydrophobic nanochannels

Transport mechanisms of polysulfides (PS), Li+ and electrolyte through nanochannels are not yet fully understood, limiting the design of nanostructured separators aimed at blocking the PS shuttle effect in lithium-sulphur batteries. Herein, Carbon Nanotubes (CNTs) with diameters calibrated to the minimal cross-section of soluble LiPS were used as models of hydrophobic nanochannels. Reservoirs containing different concentrations of lithium salts were separated by CNTs, then concentration gradient and electric-field driven Molecular Dynamics were performed. The various virtual experiments show that neither PS nor Li+ permeate through CNTs with diameters of 1.5 and 2.0 nm and, as corroborated by free energy calculations, the ions prefer to enter into the nanotubes forming ion pairs, possibly larger ion aggregates, when the required energy is supplied. By increasing the diameter to 4 nm, the electrolyte permeation changes dramatically from nearly frozen to bulk-like en masse transport, and a high polysulfides rejection was found. The PS density map inside this nanotube suggests a polysulfide adsorption on the CNT inner wall, which would minimize the shuttle effect while maintaining bulk-like electrolyte transport. This result derives from a trade-off between the diameter of the nanochannel and the wall hydrophobicity. [Display omitted] •Molecular insights into ion transport mechanisms in CNT-based nanochannels.•Effects of nanochannel diameter on polysulfide rejection and lithium permeation.•Li2S6 rejection and Li permeation unveiled by Molecular Dynamics experiments.•En masse Li diffusion and confinement effect predicted by in-silico experiments.