Model Based Evaluation of Graphene Oxide (GO)/Sulfur Composite Cathodes for High Performance Li-S Batteries
Authors: Dive Aniruddha , Song Min-Kyu , Banerjee Soumik 
Published: February 2020

Lithium-ion (Li-ion) batteries have dominated the consumer electronics market for nearly two decades as an efficient energy storage device. However, for applications such as electric vehicles, conventional lithium ion batteries have not been able to cope with the taxing energy and power densities requirements. Lithium–sulfur (Li-S) battery, with much higher theoretical specific energy (~2600Wh/kg) than those (500-600Wh/kg) of Li-ion batteries, provides a promising alternative to meet these demands. However, low electrical conductivity of pure sulfur cathode and loss of active material due to dissolution of intermediate polysulfides from the cathode during battery operation limit the performance of Li-S batteries. Development of novel cathodes with enhanced electrical conductivity and capability to reduce the dissolution of intermediate polysulfides has been the major area of research in recent times. Graphene and graphene-based materials, which has superior electrical conductivity and mechanical flexibility, are considered as potential candidate for cathodes and cathode supports in Li-S batteries. In particular, graphene oxide (GO)–sulfur composite materials provide a more economical synthesis route compared to graphene. The functional groups (mostly epoxy and hydroxyl) present in GO could potentially act like anchors to sulfur particles and help reduce dissolution of polysulfides, leading to significantly improved performance of Li-S batteries. However, leveraging the advantages of both electrical conductivity and reducing polysulfide shuttle requires tuning the types and concentration of functional groups present in GO.
In an effort to determine the favorable type and optimum concentration of functional groups in GO structure, we performed molecular dynamics (MD) simulations of a wide range of GO structures, with purely epoxy and hydroxyl functional groups, in contact with S8
2- polysulfides solvated in a standard electrolyte solvent DME – DOL in 1:1 v/v ratio. Only physical interactions between the polysulfides and the functional groups in GO were considered. Optimized potential for liquid simulations (OPLS) parameters were utilized to model GO and polysulfides along with solvent. Partial charge on carbon atoms in GO with epoxy and hydroxyl groups were evaluated from the optimized structures obtained using Density Functional Theory (DFT). Bond, angle and dihedral parameters along with partial charges on sulfur particles in polysulfide ion S8
2- were evaluated from the optimized structure using DFT. The calculated density of equilibrated solvent was within 5% of the experimental value. The time averaged distribution of polysulfides, normal to the surface of graphene oxide, was evaluated and showed that polysulfides tend to be closer to GO substrate with hydroxyl functional group as compared to ones with epoxy functional groups. The radial distribution (RDF) plots between H atom of hydroxyl functional group and S atoms of polysulfide ion suggested that a strong Coulombic interaction is responsible for physisorption of polysulfide onto the GO substrate. The diffusion co-efficients of polysulfides were also calculated and were observed to be significantly lower for GO as compared to pure graphene thus indicating a relatively stable deposition. For GO with epoxy functional groups, as the concentration of functional groups was reduced, the physisorption of polysulfides was observed to increase. The polysulfides tended to stabilize at a greater distance from the GO surface with epoxy functional groups. The study also analyzed the role of solvent molecules (especially DOL) in stabilizing polysulfides away from the GO substrate with epoxy functional groups. Overall, the results obtained in the present study show that GO with hydroxyl functional groups have a greater tendency to adsorb polysulfides onto the surface as compared to GO with epoxy groups. These results can potentially pave the way for design of molecularly-tailored cathode supports to mitigate polysulfide shuttle and therefore improve performance of Li-S batteries.