Molecular insight into LiTFSI and Li2S6 transport through hydrophobic nanochannels

Abstract

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-sulphurbatteries. Herein, Carbon Nanotubes (CNTs) with diameters calibrated to the minimal cross-section of solubleLiPS were used as models of hydrophobic nanochannels. Reservoirs containing different concentrations oflithium salts were separated by CNTs, then concentration gradient and electric-field driven Molecular Dynamicswere performed. The various virtual experiments show that neither PS nor Li+ permeate through CNTs withdiameters of 1.5 and 2.0 nm and, as corroborated by free energy calculations, the ions prefer to enter into thenanotubes forming ion pairs, possibly larger ion aggregates, when the required energy is supplied. By increasingthe diameter to 4 nm, the electrolyte permeation changes dramatically from nearly frozen to bulk-like en massetransport, and a high polysulfides rejection was found. The PS density map inside this nanotube suggests apolysulfide adsorption on the CNT inner wall, which would minimize the shuttle effect while maintaining bulklikeelectrolyte transport. This result derives from a trade-off between the diameter of the nanochannel and thewall hydrophobicity.