Mitigating first charge overpotential of Li2S-based lithium-sulfur batteries by leveraging PVDF reaction with the LiOH/Li2O layer

Abstract

The high overpotential observed during the first charging process of Li2S-based lithium sulfur batteries (Li2S-LSBs) poses a challenge for their development, as it leads to irreversible side reactions. One possible cause for this overpotential is the high hygroscopic nature of Li2S, which results in the formation of a highly insulating LiOH/Li2O layer on the surface of Li2S particles. This study proposes a novel method to reduce the first charge overpotential of Li2S-LSBs by utilizing the reaction between the polyvinylidene fluoride (PVDF) binder and the LiOH/Li2O layer on the surface of the Li2S particles. Model reactions demonstrate that LiOH or Li2O can readily react with PVDF through dehydrofluorination at room temperature. While a battery made with a Li2S cathode slurry containing PVDF, prepared using conventional grinding for 30 min, displays high charging voltages with an overpotential spike of 3.74 V during the first charge, a battery prepared using a slurry stirred for 48 h exhibits significantly reduced first charge voltages, with an overpotential spike of only 2.75 V. The study also reveals that PVDF can undergo similar dehydrofluorination with Li2S, which leads to a decrease in discharge capacity due to the consumption of some Li2S. This study provides new insights into the origin of overpotential during the first charging process of Li2S-LSBs and suggests that stirring Li2S cathode slurry in the presence of PVDF can effectively reduce the first charge overpotential of Li2S-LSBs.