Sn@C Composite with an Ultra-high Initial Coulombic Efficiency via Interfacial Engineering for Sodium-Ion Battery Anodes

Tin (Sn)-based anodes are attractive candidates for sodium-ion batteries due to their high theoretical capacity and low operating potential. However, their practical viability is severely constrained by drastic volume changes, unstable solid-electrolyte interphase (SEI) formation, and intrinsically low initial Coulombic efficiency (ICE). Herein, an interfacial regulation strategy to enhance the ICE and cycling stability of Sn-based anodes is proposed. That is, we construct Sn-MOF-derived Sn@C composites with submicron Sn particles uniformly confined in a conductive carbon matrix via hydrothermal synthesis and subsequent pyrolysis. This architecture yields an exceptional ICE of 96.4% (725/752 mA h g-1 at 0.1 A g-1), far surpassing that of commercial Sn powder (77.9%). Mechanistic investigations reveal a reversible stepwise alloying pathway from Sn to Na9Sn4 and subsequently to Na15Sn4, while the carbon matrix promotes a thin, uniform NaF-rich SEI that curbs electrolyte decomposition and Na+ loss. In situ EIS-DRT analysis further corroborates the progressive reduction of interfacial resistances during sodiation-desodiation. As a result, the anode sustains 378 mA h g-1 after 500 cycles at 1 A g-1. A Sn@C-900∥Na3V2(PO4)3 full cell can deliver an energy density of 188 W h kg-1 and retain 107 mA h g-1 over 400 cycles, underscoring the practical feasibility of this high-ICE Sn@C anode.