Fluorobenzene diluted low-density electrolyte for high-energy density and high-performance lithium-sulfur batteries

FB-diluted electrolyte shows low density and high ionic conductivity. The resulting cells shows a high capacity of 9.48 mAh cm −2 and excellent capacity retention under practical conditions, realizing high energy density of Li-S batteries. The mass fraction of electrolytes is the crucial factor affecting the energy density of lithium-sulfur (Li-S) batteries. Due to the high porosity within the C/S cathode, high concentration of polysulfides, and side reaction in lithiun metal anode under lean electrolyte, it is extremely challenging to improve performance while reducing the electrolyte volume. Here, we report a novel electrolyte with relatively low density (1.16 g cm −3 ), low viscosity (1.84 mPa s), and high ionic conductivity, which significantly promotes energy density and cyclability of Li-S batteries under practical conditions. Moreover, such electrolyte enables a hybrid cathode electrolyte interphase (CEI) and solid electrolyte interface (SEI) layer with plentiful LiF, which leads to fast kinetics of ions transport and stable cyclability even under low temperatures. Compared to Li-S batteries in electrolyte employing 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) diluent, the ultra-thick cathode (20 mg cm −2 ) shows a high capacity of 9.48 mAh cm −2 and excellent capacity retention of 80.3% over 191 cycles at a low electrolyte-to-sulfur ratio ( E / S = 2) and negative-to-positive capacity ratio ( N / P = 2.5), realizing a 19.2% improvement in energy density in coin cells (from 370 to 441 Wh kg −1 ) and a high energy density up to 467 Wh kg −1 in pouch cells. This study not only provides guidance for the electrolyte design but also paves the way for the development of high performance Li-S batteries under practical conditions.