Effect of high concentration of polysulfides on Li stripping and deposition

Abstract

The direct reaction of polysulfide species with the Li metal anode causes several issues in the performance of lithium-sulfur batteries, from poor capacity utilization and fast capacity fade to poor Coulombic efficiency. Nevertheless, several reports indicate formation of favorable lithium SEI when allowing for its contact with polysulfides. With cycling tests, impedance spectroscopy measurements and morphology investigation, we have confirmed the beneficial effect of polysulfides on lithium metal stripping and deposition even without LiNO3, which is usually added. Most importantly, highly concentrated catholytes, which are closer to the saturated solutions encountered in lean electrolyte-to-sulfur ratios, were tested. In the 1 M Li2S8 catholyte solution, its effect produced Li electrodes without dendritic growth, but rather a wrinkled surface. To circumvent the issue of viscosity in such solutions, pre-treatment in polysulfide solutions was also tested. The performance of such electrodes showed an increase in cycle life and lower overpotentials, although dendritic growth could not be fully avoided. In order to better understand the effect of pre-treatment on transport/reaction mechanism in Li electrode, a detailed analysis of selected impedance spectra is carried out using physics-based transmission line modelling (TLM).