Mechanistic insights into the delayed stabilization of the SEI Layer on silicon anodes by Fluoroethylene carbonate

초록

Fluoroethylene carbonate (FEC)'s inability to prevent severe capacity fade during the initial cycles in silicon anodes raises questions about its effectiveness as an electrolyte additive. This study explores the role of FEC in modifying electrolyte decomposition pathways and guiding the evolution of the solid electrolyte interphase (SEI) layer-a process that requires time to fully manifest. In the early cycles, electrolyte decomposition proceeds similarly in w/o FEC and w/ FEC cells, producing unstable byproducts such as lithium ethylene dicarbonate (LEDC) that dissolve into the electrolyte and destabilize the SEI layer. As cycling progresses, FEC begins to interact with reactive intermediates, gradually redirecting undesired reactions toward the formation of stable compounds such as poly(fluoroethylene carbonate) (poly(FEC)). This delayed transition mitigates SEI layer instability, seals cracks, and minimizes further decomposition, ultimately establishing a durable SEI layer. These findings highlight that the stabilizing effects of FEC are not immediate, but emerge progressively with continued cycling-underscoring the importance of accommodating its early-cycle behavior in battery design. Optimizing cycling protocols and electrolyte formulations based on these insights can enhance the long-term performance and stability of silicon-anode-based LIBs.

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