The Effect of Fluoroethylene Carbonate as an Additive on the Solid Electrolyte Interphase on Silicon Lithium-Ion Electrodes

Fluoroethylene carbonate (FEC) has become a standard electrolyte additive for use with silicon negative electrodes, but how FEC affects solid electrolyte interphase (SET) formation on the silicon anode's surface is still not well understood. Herein, SET formed from LiPF6-based carbonate electrolytes, with and without FEC, were investigated on 50 nm thick amorphous silicon thin film electrodes to understand the role of FEC on silicon electrode surface reactions. In contrast to previous work, anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SET films. This allowed for accurate characterization of the SEI structure and composition by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling. These results show that FEC reduction leads to fluoride ion and LiF formation, consistent with previous computational and experimental results. Surprisingly, we also find that these species decrease lithium-ion solubility and increase the reactivity of the silicon surface. We conclude that the effectiveness of FEC at improving the Coulombic efficiency and capacity retention is due to fluoride ion formation from reduction of the electrolyte, which leads to the chemical attack of any silicon-oxide surface passivation layers and the formation of a kinetically stable SET comprising predominately lithium fluoride and lithium oxide.