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 reversiblelithium-metal anodes. However, the performance of lithium-sulfur(Li-S) batteries with a high concentration of LiFSI/DME declinesbecause LiFSI reacts irreversibly with lithium polysulfide, whichis formed during the charge-discharge process of Li-Sbatteries. Hence, to apply high-concentration LiFSI/DME to Li-Sbatteries, we investigated carbon with an appropriate pore size foruse in a sulfur composite cathode and optimized the composition ofhigh-concentration LiFSI/DME. The results showed that the combinationof carbon with mesopores of 2-3 nm diameter and 3 M LiFSI inDME/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 capacityto the 2nd discharge capacity (capacity retention) improved from 50.0to 61.6% with HFE dilution of high-concentration LiFSI/DME. The improvedperformance was achieved by suppressing the dissolution of lithiumpolysulfide, decreasing the viscosity of the electrolyte, and forminga thin solid electrolyte interface on the lithium-metal anodedue to HFE dilution.