Identification of the Solid Electrolyte Interface on Si/C Composite Anode with FEC as the additive.

Silicon-based anodes have potential to be used in the next-generation lithium ion batteries owing to its higher lithium storage capacity. However, the large volume change during charge/discharge process and the repeated formation of a new solid electrolyte interface (SEI) on the re-exposed Si surface should be overcome in order to achieve a better electrochemical performance. Fluoroethylene carbonate (FEC) has been widely used as electrolyte additive for Si-based anodes but the intrinsical mechanism in performance improvement is not clear yet. Here, we combined solid state NMR, XPS and X-PEEM to characterize the composition, structure and inhomogeneity of the SEI on Si/C composite anodes with or without FEC additive. Similar species are observed with two electrolytes but a denser SEI formed with FEC which could prevent the small molecules (i.e. LiPF6, P-O and Li-O species) penetrating to the surface of Si/C anode. The hydrolysis of LiPF6 leading to LixPOyFz and further to Li3PO4 could also be partially suppressed by the denser SEI formed with FEC. In addition, a large amount of LiF could protect the cracking and pulverization of Si particles. This study demonstrates a deeper understanding of the SEI formed with FEC which could be a guide for optimizing the Si-based anodes for lithium ion batteries.