Regulating solid electrolyte interphase film on fluorine‐doped hard carbon anode for sodium‐ion battery

For the performance optimization strategies of hard carbon, heteroatom doping is an effective way to enhance the intrinsic transfer properties of sodium ions and electrons for accelerating the reaction kinetics. However, the previous work focuses mainly on the intrinsic physicochemical property changes of the material, but little attention has been paid to the resulting interfacial regulation of the electrode surface, namely the formation of solid electrolyte interphase (SEI) film. In this work, element F, which has the highest electronegativity, was chosen as the doping source to, more effectively, tune the electronic structure of the hard carbon. The effect of F‐doping on the physicochemical properties of hard carbon was not only systematically analyzed but also investigated with spectroscopy, optics, and in situ characterization techniques to further verify that appropriate F‐doping plays a positive role in constructing a homogenous and inorganic‐rich SEI film. The experimentally demonstrated link between the electronic structure of the electrode and the SEI film properties can reframe the doping optimization strategy as well as provide a new idea for the design of electrode materials with low reduction kinetics to the electrolyte. As a result, the optimized sample with the appropriate F‐doping content exhibits the best electrochemical performance with high capacity (434.53 mA h g−1 at 20 mA g−1) and excellent rate capability (141 mA h g−1 at 400 mA g−1). Fluorine is introduced to realize dual‐function doping for hard carbon (HC) anodes. It modifies the physicochemical properties of HC, optimizing the sodium storage structure. The alteration of the electronic structure of HC by fluorine induces a homogenous and inorganic‐rich solid electrolyte interphase film, contributing to interfacial reaction kinetics and stable insertion/extraction of sodium ions.