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

The performance of lithium-ion secondary batteries is strongly tied to ionic transport through electrode-electrolyte interfaces. Ion solvation structures near the interface as well as the chemistry of the electrode surface, in particular the edge termination of graphite electrodes, can impact Li ion transport into and out of the electrode. Understanding these effects is critical to optimizing battery performance. We have performed molecular dynamics simulations of organic liquid electrolytes in contact with graphite electrodes, with and without electric field, to understand the relationship between ion solvation structure and transport near the interface. Prototypical electrolytes consisting of ethylene carbonate (EC) or ethyl methyl carbonate (EMC) organic solvent containing dissolved LiPF₆ salt were studied, in contact with armchair-edge oriented graphite electrodes. Various surface terminations of the graphite, including H and OH, were compared. We find that Li⁺ maintains a well-structured solvation shell even when an electric field is present. The bulkier PF₆^– anion does not present a strong solvation structure and exhibits much higher mobility in an electric field. We further observe an energy barrier for Li⁺ insertion into graphite, particularly when the surface is hydroxyl terminated. Lithium extraction, however, is facile even against an applied electric field. The energetics of Li⁺ insertion and extraction were analyzed using density functional theory to sample the free energy surfaces.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Support for this work was provided through Scientific Discovery through Advanced Computing (SciDAC) program funded by U.S. Department of Energy, Office of Science, Advanced Scientific Computing Research and Basic Energy Sciences.