Interfacial Chemistry and Mechanics of Fluoroethylene Carbonate (FEC)-Derived Electrode/Electrolyte Interfaces

Fluoroethylene carbonate (FEC) has been commonly used as an electrolyte additive to improve the stability of the solid electrolyte interphase (SEI) at silicon and graphite-based anodes. Although the impact of FEC is well known, there is still lack of understanding how FEC has influence on interfacial chemistry and mechanics at the anode/electrolyte interface. In addition, relatively little attention has been paid to understanding of the effects of FEC on the cathode side. In this work, we symmetrically investigated the effects of FEC on the graphite/electrolyte and LiMn 2 O 4 /electrolyte interfaces at room and elevated temperatures and revealed associated interfacial chemistry and mechanics at the interfaces, using various electrochemical and material characterizations. We demonstrated that FEC has different effects on the interfacial behavior of the anode and the cathode. In the anode side, the SEI layer derived from FEC exhibited lower interfacial resistance and excellent thermal stability, showing excellent rate capability and improved cycle retention of cells. While the chemical and mechanical instability of the SEI layer were overcome by using FEC additive in the electrolyte, adverse effects of FEC on the cathode side were observed, especially at elevated temperatures. Poor cycling retention and a rapid increase in the interfacial resistance of the cathode were observed at elevated temperature. The poorer performance of the cathode in the FEC-containing cell at elevated temperature was attributed to the formation of a thicker surface layer and to increased Mn dissolution catalyzed by hydrofluoric acid (HF), which resulted from FEC dehydrofluorination initiated or accelerated by elevated temperature. Accordingly, it is suggested that the amount of FEC in a full cell must be optimized to minimize the adverse effects of FEC on cathode. Based on this work, the necessity of evaluating the effects of the electrolyte additive used on the cathode side should be emphasized in order to further improve the thermal stability of Li-ion batteries.