Tuning the Polysulfides Chemistry in Li-S Batteries Via Electrolyte Salt Selection

Abstract

Fast capacity fading due to polysulfide dissolution is a significant challenge for Li-S batteries. Porous carbon is widely studied for physically trapping polysulfide species. Here, we report a different chemical approach through suppressing polysulide dissolution using lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI) as a supporting salt in electrolytes. The electrolyte to sulfur ratio(E/S) is carefully controlled and the results show that under the optimized E/S ratio the cycling stability of the lithium sulfur cell is improved in the LiTDI supported electrolyte. The Combined experimental and simulation results reveal the dissolution mechanism of lithium polysulfides and suggest that the dominating disproportionation product of Li2S8 is Li2S4 in the LiTDI electrolyte due to a coordination between Li+ and TDI anion, rather than separated Li2S3 and Li2S5 as in the LiTFSI electrolyte. The Li2S4 would continuously form a Li4S8 dimer and precipitate out. Detailed DFT calculation and Li NMR results suggest lithium ion cation has a higher solvation level in the TDI based electrolyte. Furthermore, the Li-S cells with LiTDI based electrolyte with optimized sulfur to electrolyte ratio was firstly applied into a high loading cathode and achieve a stable cycling. The use of the electrolyte with the LiTDI salt (with polysulfide and LiNO3 additives) enables a cell with a high sulfur loading (3 mg-S cm-2) to deliver a 1.67 mAh cm-2 areal capacity after 300 stable cycles at a high current (2.4 mA cm-2) density. The fading of Li metal anode is also suppressed. Figure 1 Capacity retention of cells with (a) 1M LiTFSI and (b) 1M LiTDI in DOL/DME electrolytes (no additives) with different sulfur to electrolyte (S/E) ratios at a C/5 rate. 1C = 1675 mA g-1 of S and the cathode was prepared with 64 wt% S (i.e., S8), 16 wt% MWCNTs, 10 wt% Super P carbon additive and 10 wt% PVDF binder. Figure 1