Electrode/Electrolyte Interface in Sulfolane-Based Electrolytes for Li Ion Batteries: A Molecular Dynamics Simulation Study

The double layer composition and structure of the mixed-solvent electrolyte tetramethylene sulfone/dimethyl carbonate (TMS/DMC) doped with LiPF6 near the graphite surface have been investigated using molecular dynamics simulations as a function of applied potential between the electrodes ranging from 0 to 6 V. Three solvent compositions, with TMS/DMC ratios of 1:2, 1:1, and 2:1 doped with LiPF6 salt, were investigated. At uncharged electrodes, electrolyte composition at the interfaces was found to be similar to that of bulk electrolyte for TMS/DMC ratios of 1:1 and 1:2 systems but deviated from the bulk for a TMS/DMC ratio of 2:1. At negative electrodes the polar solvent TMS preferentially adsorbs at the electrode surface displacing the almost nonpolar DMC solvent. The preferential partitioning of TMS relative to DMC to the negative electrode surface is consistent with the stronger binding of the former with Li+ that partitions to the anode surface as potential becomes more negative as well as with the ability of relatively polar TMS to better respond to the electrostatic potential near a charged surface. At the positive electrode, TMS/DMC ratios were found to be similar to bulk compositions that is different to the behavior observed in ethylene carbonate (EC)/DMC/LiPF6 electrolyte where preferential partitioning of a more polar EC molecule was observed on both electrodes. Our results also show that, in TMS/DMC/LiPF6 electrolyte, DMC is located approximately 0.8 angstrom further way from the positive electrode than in EC/DMC/LiPF6 indicating that it might be more difficult to oxidize DMC in the TMS-based electrolytes that is consistent with experimentally reported increased oxidative stability of the latter. Finally, changes of the Li+ solvation shell and double layer capacitance were analyzed as a function of electrode potential.