Publication date: 15th December 2025
Silicon (Si)-based negative electrodes have attracted significant attention in recent years as promising next-generation negative electrode materials for lithium-ion batteries, thanks to their high theoretical capacity. [1] However, the charge-discharge reaction of Si-based negative electrodes induces large volume expansion, which places mechanical stress on the SEI, leading to its repeated fracture. Because the SEI is primarily composed of electrolyte decomposition products, the repeated breakdown and reforming of the SEI layer leads to poor cycling stability.The use of electrolyte additives is one of the most effective strategies to enhance battery performance by stabilizing the SEI on Si-based negative electrodes. Fluoroethylene carbonate (FEC) is well known as an effective additive for the SEI stabilization. [2] However, the details of the SEI formation mechanism in the presence of FEC remain unclear.
Here, we elucidate the SEI formation process on Si-based negative electrode in FEC-containing electrolytes by directly probing interfacial reactions in real time using in situ attenuated total reflectance infrared (ATR-IR) spectroscopy. This approach enables molecular-level identification of reaction intermediates and products during SEI formation. Our results reveal a detailed decomposition pathway of FEC and demonstrate that its solvation structure critically determines its reaction destiny. In situ ATR-IR measurements show that free FEC decomposes via vinylene carbonate (VC) as an intermediate to form ROCOOLi, while an alternative FEC decomposition pathway yields Li₂CO₃. In contrast, Li⁺-coordinated FEC undergoes a distinct decomposition pathway, leading to the formation of polyether species. These findings establish that the coordination environment of FEC markedly alters its interfacial decomposition mechanism, leading to distinct SEI compositions. Our work highlights the pivotal role of solvent/additive coordination structure in governing interfacial reaction pathways and underscores the importance of solvation in rational SEI engineering.
This work was supported by the Japan Science and Technology Agency (JST) under the Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE) program( grant no. JPMJAP2422) and the New Energy and Industrial Technology Development Organization (NEDO) under the Intensive Support Program for Young Promising Researchers (grant no. JPMJAP20004).
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