A Molecular Dynamics Study of Graphene Oxide /Sulfur Composite Cathodes for Lithium-Sulfur Batteries

Abstract

Lithium – sulfur (Li-S) battery, with highest predicted theoretical capacity of ~1675 mAh/g, is a promising technology meeting the demands of next-generation electric vehicles. Abundance of sulfur and environmental friendliness are major factors that make Li-S batteries interesting and important. However, Li-S battery is plagued by its own limitations, which include low electrical conductivity of pure sulfur cathode and loss of active material due to dissolution of intermediate polysulfides from the cathode during reduction reactions. These factors limit the performance of Li-S batteries. Developing superior cathode structures to mitigate these problems has been the major focus of research in recent times. Carbon-based materials such as fibers, nanospheres and nanotubes embedded with sulfur are being explored as cathodes for Li-S batteries. In particular, graphene, which has superior electrical conductivity and mechanical flexibility, has been studied as potential candidate for cathodes and cathode supports in Li-S batteries. However, economical synthesis of pure graphene is a major hurdle for their use in commercially-viable Li-S batteries. Graphene oxide – sulfur composite based structures are viable alternatives that could provide required electrical conductivity, based on the concentration of functional groups (epoxy and hydroxyl). Additionally, graphene oxides are believed to act like anchors to sulfur/polysulfides particles and help reduce their dissolution, leading to significantly improved performance of Li-S batteries. However, leveraging the advantages of enhanced electrical conductivity and reduced polysulfide shuttle requires tuning of the functional groups present in the graphene oxide. In an effort to determine the most favorable graphene oxide structure, we performed molecular dynamics (MD) simulations to calculate the mobility of polysulfide species in the vicinity of different graphene oxide structures with varying concentration of functional groups. Diffusion coefficients of polysulfides were calculated along the surface of these graphene oxide sheets using MD simulations. Initially graphene oxide sheets with pure epoxy and hydroxyl functional groups were simulated. Partial charge on carbon atoms in graphene oxide with epoxy and hydroxyl groups were evaluated from the optimized structures obtained using Density Functional Theory (DFT) calculations. Bond, angle and dihedral parameters along with partial charges on sulfur particles in polysulfides S8 2- were evaluated from the optimized structure using DFT. A standard electrolyte DME – DOL in 1:1 v/v ratio, modeled using OPLS force filed parameters, was used in all the MD simulations. The density of equilibrated solvent was within 5% of the experimental value. A system comprising graphene oxide solvated with electrolyte containing polysulfides was simulated at 300K. Overall charge neutrality of the system was maintained by incorporating counter ions, while using specific strategy to screen out long range electrostatic interactions between the polysulfides and counter ions. The surface diffusion coefficients of polysulfide was evaluated in the vicinity of graphene oxide and compared to that near pure graphene. Our simulations evaluated the effectiveness of various groups in reducing dissolution of polysulfides. The concentration of polysulfides in the vicinity of the graphene/graphene oxide structures tend to increase from graphene to graphene oxide with pure epoxy groups and maximizes for graphene oxide with pure hydroxyl functional groups. The relatively greater binding of polysulfides to graphene oxide structures leads to reduced surface diffusion coefficients for polysulfides on graphene oxide compared to that near graphene interfaces.