Interplay Between Solid-Electrolyte Interphase and (In)Active Li <sub>x</sub>Si in Silicon Anode

Abstract

Solid electrolyte interphase (SEI) is regarded as the most important but the least understood part in rechargeable lithium (Li) batteries. It was formed inevitably by reacting and decomposing electrolytes on the discharged/charged anode. The interplay between the SEI and anode materials governs the charge transfer and Li+ transport, thus determining the reaction kinetic and electrochemical performance of the batteries. Having a comprehensive understanding of the SEI nature and especially its interplay with the active materials during cycling is crucial for both fundamental and applied research of rechargeable batteries. Herein, the dynamic interplay between SEI and Si anode during cycling and long-term cycles was revealed quantitively and qualitatively by titration gas chromatography (TGC), cryogenic transmission electron microscopy (cryo-TEM), and other advanced techniques in terms of charge transfer, nanostructure, and equilibrium. The results show that the SEI electrochemically forms before and through Li-Si alloy reaction, and decomposes during delithiation processes. It consumes more than 10% charges and triggers to form the inactive LixSi by isolating it from the electrical network. It is hard to construct an equilibrium interplay between the SEI and LixSi alloy 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 and thus capacity drop. Therefore, constructing and maintaining an equilibrium interplay between SEI and LixSi is essential to achieve high-performance and high-energy batteries via interfacial engineering, for example, LiF-rich interphase.