Phase-separated bimetal enhanced sodium storage: Dimer-like Sn-Bi@C heterostructures with high capacity and long cycle life

Abstract

Phase boundaries facilitate the charge transportation and alleviate the intrinsic stress upon cycles. Therefore, how to achieve regular phase boundaries is very attractive. Herein, dimer-like Sn-Bi@C nanostructures, where is a well-defined phase boundary between Sn and Bi, have been prepared by a two-step process for the first time. The phase boundary not only provides additional and fast transportation for Na+, but also mitigates the structure stress/strain upon cycling. Therefore, Sn-Bi@C exhibits a high capacity (472.1 mA h g−1 at 2 A g−1 for 200 cycles), an ultra-long cyclic life (355.6 mA h g−1 at 5 A g−1 for 4500 cycles) and an excellent rate performance (372 mA h g−1 at 10 A g−1) for sodium storage, much higher than those of Sn@C, Bi@C, and Sn@C + Bi@C. Notably, the full cells of Sn-Bi@C//Na3V2(PO4)3/rGO (Sn-Bi@C//NVP/rGO) demonstrate impressive performance (323 mA h g−1 at 2 A g−1 for 300 cycles). The underlying mechanism for such an excellent performance is elucidated by in-situ X-ray diffraction, ex-situ scanning electron microscopy /high-resolution transmission electron microscopy and atomic force microscopy, revealing the good electrode stability and improved mechanical properties of Sn-Bi@C. The synthetic method is extended to dimer-like Sn-Pb@C and Sn-Ag@C heterostructures, which also exhibit the good cycle stability for sodium storage.