Reaction Pathway and Performance Optimization of Tin Anodes for High‐Energy Sodium‐Ion Batteries

ABSTRACT Sn is a promising anode for sodium‐ion batteries (SIBs) owing to its high theoretical capacity, low operating potential, and high electronic conductivity. However, unclear reaction mechanisms and poor reversibility caused by severe volume changes hinder its practical application. Here, we systematically clarify the pronounced rate‐dependent electrochemical behavior of Sn and the debated Na–Sn reaction pathway through ex situ structural analyses combining X‐ray diffraction and synchrotron‐based Sn K‐edge X‐ray absorption fine structure spectroscopy. Based on these findings, a scalable Sn nanocomposite (Sn/Al 2 O 3 /C) is synthesized via one‐pot mechanochemical reduction of SnO by Al in an amorphous carbon matrix, along with a presodiated counterpart (p‐Sn/Al 2 O 3 /C) to compensate for initial Na loss. The resulting architecture features ultrafine Sn nanocrystallites uniformly dispersed within a rigid, Na‐inactive Al 2 O 3 matrix and interconnected by conductive, elastically buffering amorphous carbon networks. This design suppresses structural degradation, alleviates kinetic polarization, and enables rapid charge transport, delivering an initial Coulombic efficiency of ∼100% and stable ultrafast cycling over 2000 cycles at 20 C. A full cell paired with a Na(Ni 1/3 Fe 1/3 Mn 1/3 )O 2 cathode achieves an energy density of 263.3 Wh kg −1 with stable high‐rate operation. Overall, this study provides a reaction pathway–guided design strategy for practical, high‐performance Sn anodes in SIBs.