Synergistic γ‐In(2)Se(3)@rGO Nanocomposites with Beneficial Crystal Transformation Behavior for High‐Performance Sodium‐Ion Batteries

Crystal transformation of metal compound cathodes during charge/discharge processes in alkali metal‐ion batteries usually generates profound impact on structural stability and electrochemical performance, while the theme in anode materials, which always occurs and completes during the first redox cycle, is rarely explored probably due to the fast transformation dynamics. Herein, for the first time, a unique crystal transformation behavior with slow dynamics in anode of sodium‐ion batteries (SIBs) is reported, which further promotes electrochemical performance. Specifically, irreversible γ → β crystal transformation of In(2)Se(3) is observed, induced by the persistent size degradation of In(2)Se(3) particles during repeated sodiation/desodiation, supported by a series of ex situ characterizations, such as HRTEM, XRD, and XPS of γ‐In(2)Se(3)/reduced graphene oxide (γ‐In(2)Se(3)@rGO) nanocomposite. The hybrid electrode shows ultrahigh long‐term cycling stability (378 mA h g(−1) at 1.0 A g(−1) after 1000 cycles) and excellent rate capability (272 mA h g(−1) at 20.0 A g(−1)). Full battery with Na(3)V(2)(PO(4))(3) cathode also manifests superior performance, promising β‐In(2)Se(3) dominated electrode materials in high‐power and long‐life SIBs. The first‐principle calculations suggest the crystal transformation enhances electric conductivity of β‐In(2)Se(3) and facilitates its accessibility to sodium. In combination with the synergistic effect between rGO matrix, substantially enhanced electrochemical performance is realized.