Molecular Structure and Ion Transport Near Electrode-Electrolyte Interfaces in Lithium-Ion Batteries

The performance of lithium-ion secondary batteries (LIBs) is strongly tied to ionic transport through electrode-electrolyte interfaces. Solvation structure strongly affects ion mobility, and differences in ion solvation near the interface compared to the bulk may affect conductivity. Furthermore, the formation and evolution of solid-electrolyte interphase (SEI) layers, which impede transport between electrode and electrolyte while passivating the interface, are related to molecular interactions at the interface involving solvent molecules, ions, and the electrode surface. In addition, the chemistry of the electrode surface, in particular the edge termination of graphite electrodes, can impact Li ion transport into the electrode and formation of SEI. Understanding these effects is critical to optimizing battery performance. Here we present molecular dynamics (MD) simulations of typical organic liquid LIB electrolytes in contact with graphite electrodes to understand the relationship between ion solvation structure and transport near the interface. Results for different graphite terminations are presented and the effect of an electric field is explored. Different prototypical organic liquids, including ethylene carbonate and ethyl methyl carbonate, are compared. We observe that anion mobility in an electric field is much greater than that for Li ions, a phenomenon due to the very different solvation structures. In addition, we observe a barrier to Li insertion into graphite when its surface is hydroxyl terminated. Furthermore, we compare the results of ab initio MD with the empirical reactive forcefield ReaxFF and the non-reactive, non-polarizable COMPASS forcefield and note differences in the predictive power of each technique. Prepared by LLNL under Contract DE-AC52-07NA27344.