Rational Method for Improving the Performance of Lithium-Sulfur Batteries: Coating the Separator with Lithium Fluoride

Sulfur, as a cathode material in lithium−sulfur batteries, has a very high theoretical specific capacity of 1675 mAh g−1, but there is still a large challenge, because of polysulfides’ (PSs) that cause a severe shuttle effect. To suppress this effect, a simple way of modifying separator by introducing lithium fluoride (LiF) as a coating layer was developed. Owing to the interaction between LiF and dimethoxymethane (DME, an electrolyte solvent), a dense and viscous sol layer was formed that suppresses PSs passing from the cathode to the anode. The presence of this layer was confirmed by using Fourier transform infrared spectroscopy, thermogravimetric/differential thermogravimetric, and scanning electron microscopy. The linear sweep voltammetry test had shown that the LiF-coated separator had a wide electrochemical window above 5 V vs. Li/Li+, and the cell assembled with the LiF-coated separator exhibited an excellent cycling performance with a capacity retention rate of 69.3%. Even without LiNO3 as an electrolyte additive, a high coulombic efficiency of 93% after 200 cycles at 0.2 C was achieved.