Film-Forming Properties of Propylene Carbonate in the Presence of a Quaternary Ammonium Ionic Liquid on Natural Graphite Anode

Propylene carbonate (PC) in electrolytes is generally believed to lead to graphite exfoliation due to intensive cointercalation and ceaseless decomposition reactions. This paper reports the formation of an effective solid electrolyte interface (SEI) film on a natural graphite surface through decomposition of PC in the presence of a quaternary ammonium ionic liquid. With an ionic liquid content of at least 60%, the PC molecules are reduced in the first charge, effectively passivating the graphite anode and allowing the formation of LiC6 with high capacity and good reversibility. In the electrolyte containing 20 vol % PC, the natural graphite anode attains a discharge capacity of 322.8 mAh/g and 76.5% coulombic efficiency on the first cycle. A stable discharge capacity of around 330 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. This result compares favorably with using 20 vol % ethylene carbonate (EC) as the film-forming additive in the electrolyte. SEM and FTIR studies demonstrate the formation of a thin, homogeneous SEI layer on the graphite-electrode surface through the reduction of PC molecules. Raman spectroscopy studies show a significant decrease of the interaction between the Li ion and PC molecules in the presence of the quaternary ammonium ionic liquid. The competition for Li ions between PC and the ionic liquid reduces the solvation of PC molecules for Li ions, which in turn limits the intercalation of PC into the graphene layers during Li intercalation, an event that typically leads to exfoliation. Instead, the loosely bound PC is reduced at the graphite/electrolyte interface, forming a stable SEI.