The high gravimetric capacity of lithiated silicon has motivated research as a potential anode for lithium ion batteries; however, major challenges remain for the implementation of silicon in commercial devices. During the lithiation process, a portion of the Li ions are consumed as the electrolyte is reduced to inactive side products from parasitic reactions, forming a solid electrolyte interphase (SEI). The SEI components should be electronically insulating and passivate the active material’s surface from further solvent reduction.¹ However, due to the mechanical instability of silicon during lithiation, it prohibits the formation of a stable SEI causing capacity loss during cycling. In order to solve this issue, fluoroethylene carbonate (FEC) electrolyte additive has been widely used.² However, the connection between SEI stability and FEC needs to be further investigated on a silicon model system.

Herein, SEI formed from LiPF₆-carbonate based electrolytes, with and without FEC were investigated on 50nm amorphous silicon thin film electrodes. Figure 1 (a) demonstrates the excellent cycling performance of FEC over prolonged cycles compared to its counterpart. Anhydrous and anoxic X-ray photoelectron spectroscopy (Figure 2 (b)) and time-of-flight secondary ion mass spectrometry depth profiling techniques were used to accurately characterize the SEI structure and composition.  These results show that FEC reduction leads to fluoride ion and LiF formation, consistent with previous results.³ We believe 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 formation of a kinetically stable SEI comprised of predominately inorganic compounds. This work is funded by the Office of Vehicle Technologies, U.S. Department of Energy under the Advanced Battery Materials Research (BMR) Program.

Reference:

[1] Verma, P.; Maire, P.; Novák, P. Electrochim. Acta 55 (2010) 6332–6341.

[2] Xu, C.; Lindgren, F.; Philippe, B.; Gorgoi, M.; Björefors, F.; Edstrom, K.; Gustafsson, T. Chem. Mater. 27 (2015) 2591-2599.

[3] Leung, K.; Rempe, S. B.; Foster, M. E.; Ma, Y.; Martinez del la Hoz, J. M.; Sai, N.; Balbuena, P. B. J. Electrochem. Soc. 161 (2013) A213–A221.