Unraveling the New Role of an Ethylene Carbonate Solvation Shell in Rechargeable Metal Ion Batteries

Electrolytes play a critical role in controlling metal-ion battery performance. However, the molecular behavior of electrolyte components and their effects on electrodes are not fully understood. Herein, we present a new insight on the role of the most commonly used ethylene carbonate (EC) cosolvent both with the bulk and at the electrolyte-electrode interface. We have discovered a new phenomenon that contributes to stabilizing the electrolyte, besides the well-known roles of dissociating metal salt and forming a solid electrolyte interphase (SEI). As a paradigm, we confirm that EC can form an Li+–EC pair in a priority compared to other kinds of solvents (e.g., ethyl methyl carbonate) and then alter the Li+–solvent interactions in the electrolyte. The Li+–EC pair can dominate the desolvation structure at the electrode interface, therefore suppressing Li+–solvent decomposition due to the higher stability of Li+–EC. Our viewpoint is confirmed in different electrolytes for lithium, sodium, and potassium ion batteries, where the SEI is shown to be limited for stabilizing the electrode in the case of the less stable Li+–solvent pair. Our discovery provides a general explanation for the effect of EC and provides new guidelines for designing more reliable electrolytes for metal (ion) batteries.