The history of metal-ion batteries has been plagued by ubiquitous side reactions that occur at the electrode surface during charge cycling.  In contrast to the ion insertion mechanisms at the electrode, these redox processes degrade the liquid electrolyte and inhibit ion transport. Given its technological relevance, many studies have focused on the breakdown of carbonate solvent molecules at carbon electrode surfaces common to lithium-ion energy storage.  A peculiar observation that remains the subject of debate is the difference in behaviors of ethylene carbonate (EC) and propylene carbonate (PC) solvent molecules.  Even though they only differ by a single methyl group, their behavior during charge cycling differs dramatically:  EC forms a stable surface layer while PC renders the carbon surface inoperable.  Several experiments to probe these divergent behaviors have considered varying the compositions of the electrolyte by mixing EC and PC in different ratios.  It has also been shown that adding a high concentration of lithium salt can stabilize PC-containing electrolytes.  The analyses of these experiments have focused on the importance of bulk solvation structures, however our previous studies have shown the importance of the electrode surface in modifying the solvation structure surrounding lithium ions in solution.  Hence, we have performed molecular dynamics simulations on liquid electrolytes of similar composition to those studied in the experiments.  We considered solutions made from varying ratios of EC, PC, and lithium hexafluorophosphate (LiPF<sub>6</sub>) at an idealized graphite surface.  By including an applied bias voltage, we have shown that the electrode interface plays an important role in altering composition from bulk structure and could explain the changes in electrolyte performance.  In specific, we find that at voltages near the insertion threshold for metal ions, the surface remains EC enriched.  Hence the observed stability changes might have more to do with exclusion of PC rather than a solvation preference on behalf of the lithium ion.  Similarly, our results show that the prevalence of salt at the electrode surface switches at sufficient concentration, which connects with the seemingly universal behavior seen for electrolytes with more than 3.0M salt concentration.  Agreement with the qualitative features of these experiments indicates the relevance of interface effects as an alternative to interpretations based solely on bulk behavior.

Surface selectivity: The importance of the electrode interface in steering electrolyte stability

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