Interplay between solid-electrolyte interphase and (in)active LixSi in silicon anode

Solid-electrolyte interphase (SEI) is regarded as the most important but the least understood part of lithium (Li)-ion batteries. A comprehensive understanding of the nature of the SEI and especially its interplay with active materials during cycling is crucial since it governs the charge transfer and Li+ transport. Herein, the dynamic interplay between SEI and silicon (Si) anode during cycling is revealed quantitatively and qualitatively by titration gas chromatography (TGC), cryogenic transmission electron microscopy (cryo-TEM), and other techniques to probe charge transfer, nanostructure, and equilibrium. The results show that it is difficult to construct an equilibrium interplay between the SEI and LixSi due to the intrinsic instability of some SEI components (e.g., Li2O and carbonates) and the pulverization of Si anode, resulting in the continuous formation of the SEI and inactive LixSi. The addition of fluoroethylene carbonate helps construct such equilibrium interplay through formation of a LiF-rich SEI, thus improving cyclability.