Graphene quantum dots: structural integrity and oxygen functional groups for high sulfur/sulfide utilization in lithium sulfur batteries

Abstract

Lithium–sulfur (Li–S) batteries are expected to overcome the limit of current energy storage devices by delivering high specific energy with low material cost. However, the potential of Li–S batteries has not yet been realized because of several technical barriers. Poor electrochemical performance is mainly attributed to the low electrical conductivity of the fully charged and discharged species, the irreversible loss of polysulfide anions and the decrease in the number of electrochemically active reaction sites during battery operation. Here, we report that the introduction of graphene quantum dots (GQDs) into the sulfur cathode dramatically enhanced sulfur/sulfide utilization, yielding high performance. In addition, the GQDs induced structural integrity of the sulfur–carbon electrode composite by oxygen-rich functional groups. This hierarchical architecture enabled fast charge transfer while minimizing the loss of lithium polysulfides, which is attributed to the physicochemical properties of GQDs. The mechanisms through which excellent cycling and rate performance are achieved were thoroughly studied by analyzing capacity versus voltage profiles. Furthermore, experimental observations and theoretical calculations further clarified the role played by GQDs by proving that C–S bonding occurs. Thus, the introduction of GQDs into Li–S batteries will provide an important breakthrough allowing their use as high-performance and low-cost batteries for next-generation energy storage systems. High-capacity cathodes with enhanced stability have been prepared by using graphene quantum dots (GQDs). Lithium–sulphur batteries are considered one of the most promising candidates for future battery systems. However, intermediate compounds readily dissolve into the electrolyte during battery operation, which results in a loss of active materials. A team led by Yung-Eun Sung and Byung Hee Hong of Seoul National University introduced a hierarchical structure with GQDs via wet chemical techniques. Oxygen-rich functional groups of GQDs gave rise to a tightly packed structure between carbon black and sulphur, enabling fast charge transfer. Moreover, experimental observations and theoretical analysis helped to clarify the role of GQDs. In particular, the introduction of GQDs significantly was found to improve cycling performance and rate capability through the formation of carbon–sulphur bonds. Graphene quantum dots (GQDs) decorated sulfur–carbon hierarchical structure serve as a high sulfur/sulfide utilization in Li–S battery. The oxygen rich functionalities of the GQDs induced structural integrity of the sulfur–carbon electrode composite. This hierarchical structure enables fast charge transfer, minimizes the loss of soluble polysulfides and completes reaction of sulfur/sulfide.