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 F-2 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.