Catecholamine Functionalized Reduced Graphene Oxide-Sulfur Composite As a Cathode Material for Lithium-Sulfur Batteries

Abstract

Among various next generation rechargeable batteries, Lithium-Sulfur batteries have emerged as a potential candidate to replace Li-ion batteries because the former has high theoretical energy density of 2500 Wh/Kg, which is almost 7 times higher than that of the currently used Li-ion batteries. To address the low conductivity of sulfur and polysulfide dissolution in the sulfur cathode during discharge cycling, we propose the use of functionalized reduced graphene oxide-sulfur composite as a better candidate compared to graphene oxide-sulfur composite (GO/S) which has relatively lower conductivity. The reduced graphene oxide structure with simultaneous introduction of catecholamine functional groups will have the advantage of higher electrical conductivity along with a better ability to adsorb polysulfides through chemical interaction. Catecholamine functionalized reduced graphene oxide (c-rGO) was prepared starting from GO as a raw material, followed by sulfur infiltration (c-rGO/S) at 155°C with an intended sulfur content of 74-75%. To emphasize the importance of introducing catecholamine functional groups in rGO/S composite for polysulfide adsorption by chemical interaction, thermally reduced graphene oxide- sulfur composite (t-rGO/s) with no functional groups and bare GO/S composite was tested too. The use of c-rGO/S composite maintains the capacity to as high as 700 mAh/g after 100 cycles at 0.5C whereas the high electrical resistivity of oxidized graphene-sulfur composite lower the capacity to as low as 300 mAh/g after 100 cycles at 0.5C. Even with the higher electrical conductivity, the t-rGO/S composite delivered a discharge capacity of only 400 mAh/g at 0.5C after 100 cycles amid the absence of any polysulfide adsorbing functional groups like the one present in c-rGO/S composite. Moreover, the evidence of effective polysulfide adsorption by chemical interaction in c-rGO/S in comparison to GO/S has be shown by in-situ UV-Vis spectroscopy results. Keywords: Catecholamine, Graphene-oxide, Polysulfide adsorption, In-situ UV-Vis spectroscopy