Synthesis of Sulfur-Loaded Monodisperse Carbon Nanocapsules Anchored on Graphene Nanosheets As Cathodes for High Performance Lithium–Sulfur Batteries

Abstract

Among various next-generation battery systems, rechargeable lithium–sulfur batteries have earned distinction because of their high specic energy density, environmental friendliness, and the natural abundance of sulfur. Even with such great advantages, practical applications of Li-S battery are still greatly restricted by several challenges. First of all, electronic insulating nature and the rapid capacity fading caused by the high solubility of the intermediate products in organic electrolyte solutions hinder it from direct using as an electroactive material. In addition, the large volume expansion of the sulfur particles during cycling can result in a structural collapse of the electrode. A reasonable strategy to overcome these problems is to use carbon materials as effective hosts for sulfur loading. Such carbon materials include carbon nanotubes, graphene, hollow carbon spheres, and porous carbon that can improve the electrical conductivity of the active materials and minimize the dissolution of lithium polysulfide into the electrolyte. Among these carbon materials, graphene has been intensively studied as a matrix for the electrodes of Li–S batteries due to its high specific surface area, excellent electrical conductivity, and good mechanical strength. However, the ability of graphene as a host material to effectively confine the intermediate products in the cathodes during cycling is limited due to its open structure. Thus, polysulfides can still easily diffuse out of the cathode, which leads to gradual capacity decay. As a partial solution, considerable effort has been made to incorporate other carbon materials that can confine sulfur into the graphene structure, such as carbon nanocapsules, mesoporous carbon, and conducting polymers. Specifically, the combination of carbon nanocapsules with graphene has been considered as an interesting host because this composite may offer improved electrical conductivity as well as confine the sulfur or polysulfides under the carbon shell. In this work, a facile strategy for the synthesis of monodisperse carbon nanocapsule anchored on graphene nanosheet composites as effective host materials to confine sulfur for Li–S batteries is propose. The strategy involves mixing of iron-oleate and graphene, heat treatment, and finally, acid etching of the iron oxide nanoparticles. The composites comprised highly uniform, hollow structured carbon with a diameter of about 20 nm that were densely deposited on the surface of the graphene nanosheets. After heat-treatment, elemental sulfur was impregnated into the uniform void spaces of the carbon material and partially deposited on the graphene nanosheets. The impregnation of elemental melted sulfur into the unique pore structure of carbon material imparts significantly improved the electrochemical performance to the carbon–sulfur composite compared to that of bare graphene nanosheets. We believe that these uniquely structured monodisperse carbon nanocapsules anchored on graphene nanosheets can be promising candidates for other energy storage applications.