Fluorinated Reduced Graphene Oxide as an Interlayer in Li–S Batteries

We report on the synthesis of fluorinated reduced graphene oxide (F-rGO) by a direct fluorination of reduced graphene oxide (rGO) with F2 or XeF2/BF3 in anhydrous HF. Characterization performed by high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) and NMR confirmed the formation of C–F bonds, which is also supported by the color change from graphite gray to light yellow. F-rGO has been used as an interlayer additive supported by a glass fiber separator in lithium–sulfur (Li–S) batteries in order to block the diffusion/migration of polysulfides from the porous positive electrode to the metallic lithium electrode and to prevent the redox shuttle effect. Electrochemical cycling of Li–S batteries has confirmed the beneficial role of F-rGO separators, with a more pronounced effect observed for high degrees of fluorination. X-ray photoelectron spectroscopy (XPS) studies have shown a direct effect on the amount of Li2S and polysulfides found on the lithium electrode and evidenced a better reversibility of reduction/oxidation mechanisms of sulfur at the positive electrode upon discharge/charge.