Early Terminating Solid Electrolyte Interphase Formation via Nucleophilic Fluorination to Achieve High Initial Coulombic Efficiency.

Abstract

The initial Coulombic efficiency (ICE) of lithium-ion batteries, quantifying the irreversible Li⁺ loss during the first cycle, is critical for determining practical energy density. Many electrode materials exhibit substandard ICEs (<90%) due to excessive formation of solid electrolyte interphase (SEI). Traditional strategies modifying SEI formation mainly focus on the generating process but often consume extra Li⁺ and yield limited improvements. Here, a strategy is introduced that targets the terminating process of SEI formation, usually impeded by interfacial parasitic reactions, to achieve ICEs exceeding 90%. Using TiO₂ as a model electrode, it is demonstrated that equivalent chemical fluorination suppresses the parasitic reaction between phosphorus pentafluoride (PF₅) and surface hydroxyl groups (─OH), early terminating SEI formation. Interfacial analysis and theoretical simulations reveal that this approach reduces organic SEI formation while preserving the beneficial LiF-rich inner SEI layer. As a result, the fluorinated TiO₂ anode exhibits an ICE of 92.1%, significantly higher than the 74.1% of pristine TiO₂, without compromising other electrochemical performance metrics. Pouch cell tests confirm the practical applicability of the method. This work provides deep insights into mechanisms of terminating SEI formation and opens a new pathway for optimizing the battery performances through inherent SEI manipulation.