Rational Defect and Fluorine Chemistry in Tin Oxide Enables Reversible Na Intercalation with a Stable NaF‐Rich SEI Formation

Significant efforts have been devoted to optimizing the morphology and synthesizing composite materials to activate SnO₂ for sodium-ion batteries. However, challenges arising from its intrinsic crystal structure remain insufficiently addressed. This study aims to introduce both oxygen vacancies and fluorine ions into the SnO₂ lattice, yielding a modified compound with a chemical composition of SnO_1.74£_0.16F_0.1. Notably, SnO_1.74£_0.16F_0.1 anode exhibits an enhanced discharge capacity and improved cycling stability. These enhancements are attributed to the accelerated Na⁺ diffusion and increased electrochemical activity, as confirmed by kinetics investigations and in situ X-ray diffraction (XRD) characterizations. Furthermore, cryogenic transmission electron microscopy (cryo-TEM) analyses reveal that the coexistence of oxygen vacancies and fluorine anions promotes the formation of a NaF-rich solid electrolyte interface (SEI) layer, showing an excellent compatibility in sodium-ion batteries, which holds prospects for the realization of the next generation of energy storage systems.