The Critical Role of Fluoroethylene Carbonate in the Gassing of Silicon Anodes for Lithium-Ion Batteries

The use of functionalized electrolytes is effective in mitigating the poor cycling stability of silicon (Si), which has long hindered the implementation of this promising high-capacity anode material in next-generation lithium-ion batteries. In this Letter, we present a comparative study of gaseous byproducts formed by decomposition of fluoroethylene carbonate (FEC)-containing and FEC-free electrolytes using differential electrochemical mass spectrometry and infrared spectroscopy, combined with long-term cycling data of half-cells (Si vs Li). The evolving gaseous species depend strongly on the type of electrolyte; the main products for the FEC-based electrolyte are H<sub>2</sub> and CO<sub>2</sub>, while the FEC-free electrolyte shows predominantly H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, and CO. The characteristic shape of the evolution patterns suggests different reactivities of the various Li<sub><i>x</i></sub>Si alloys, depending on the cell potential. The data acquired for long-term cycling confirm the benefit of using FEC as cosolvent in the electrolyte.