Improvement of Lithium–Sulfur Battery Performance by Porous Carbon Selection and LiFSI/DME Electrolyte Optimization

High-concentration lithium bis(fluorosulfonyl)imide/1,2-dimethoxyethane (LiFSI/DME) electrolytes are promising candidates for highly reversible lithium–metal anodes. However, the performance of lithium–sulfur (Li–S) batteries with a high concentration of LiFSI/DME declines because LiFSI reacts irreversibly with lithium polysulfide, which is formed during the charge–discharge process of Li–S batteries. Hence, to apply high-concentration LiFSI/DME to Li–S batteries, we investigated carbon with an appropriate pore size for use in a sulfur composite cathode and optimized the composition of high-concentration LiFSI/DME. The results showed that the combination of carbon with mesopores of 2–3 nm diameter and 3 M LiFSI in DME/1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE) (1:1 by vol.) provided a high-rate capability (943 mA h g–1 at a rate of 2 C). Moreover, the ratio of the 50th discharge capacity to the 2nd discharge capacity (capacity retention) improved from 50.0 to 61.6% with HFE dilution of high-concentration LiFSI/DME. The improved performance was achieved by suppressing the dissolution of lithium polysulfide, decreasing the viscosity of the electrolyte, and forming a thin solid electrolyte interface on the lithium–metal anode due to HFE dilution.