context: The use of Mg-alloyable metallic anodes is another promising approach to enhancing the viability of MIBs. This is mainly because the Mg alloying/dealloying process occurs slightly above the Mg plating/stripping potentials, and the surface passivation problems of Mg in conventional electrolytes is expected to be alleviated to a great extent. Indeed, reversible Mg alloying/dealloying behavior on bismuth has been confirmed in conventional electrolytes by several researchers (e.g., Bi nanoparticles in Mg(N(SO2CF3)2)2/acetonitrile; Bi nanotubes in Mg(N(SO2CF3)2)2/diglyme; and Bi nanocrystals in Mg(N(SO2CF3)2)2/diglyme). Bi anodes have shown reasonable cyclability and rate performance in a nanometric form. Mg-alloyable Sn was also studied for use as MIB anodes. Sn can deliver a higher specific capacity than Bi (903 mA h g−1 for Sn + 2Mg2+ + 4e− → Mg2Sn and 385 mA h g−1 for 2Bi + 3Mg2+ + 6e− → Mg3Bi2) and can alloy/dealloy Mg at a lower potential than Bi (0.15 and 0.25 V vs. Mg/Mg2+ for Sn and Bi, respectively). Despite the theoretical prediction that nanometric Sn can act as a high-capacity MIB anode, pure Sn has shown limited electrochemical performance. For example, though pure Sn delivered a capacity of ca. 900 mA h g−1 during the 1st magnesiation (complete conversion of Sn to Mg2Sn), the capacity abruptly dropped to ca. 200–300 mA h g−1 after the 1st magnesiation, and continuously declined with repeated charge/discharge (C/D). Furthermore, pure Sn often requires the application of potentials close to, or even lower than, the thermodynamic Mg plating potential for the activation of an alloying process.
answer: Bi