Enhancing\nMagnesium-Ion Storage in a Bi–Sn\nAnode through Dual-Phase Engineering

Magnesium-ion batteries\n(MIBs) are a “beyond Li-ion”\ntechnology that are hampered by Mg metal reactivity, which motivates\nthe development of anode materials such as tin (Sn) with high theoretical\ncapacity (903 mAh g<sup>–1</sup>). However, pure Sn is inactive\nfor Mg<sup>2+</sup> storage. Herein, Mg alloying with Sn is enabled\nwithin dual-phase Bi–Sn anodes, where the optimal composition\n(Bi<sub>66.5</sub>Sn<sub>33.5</sub>) outperformed single-phase Bi\nand Sn electrodes to deliver high specific capacity (462 mAh g<sup>–1</sup> at 100 mA g<sup>–1</sup>), good cycle life\n(84% after 200 cycles), and significantly improved rate capability\n(403 mAh g<sup>–1</sup> at 1000 mA g<sup>–1</sup>).\nDensity functional theory (DFT) calculations revealed that Mg alloys\nfirst with Bi and the subsequent formation of the Mg<sub>3</sub>Bi<sub>2</sub>//Sn interfaces is energetically more favorable compared to\nthe individual Mg<sub>3</sub>Bi<sub>2</sub> and Sn phases. Mg insertion\ninto Sn is facilitated when Mg<sub>3</sub>Bi<sub>2</sub> is present.\nMoreover, dealloying Mg from Mg<sub>3</sub>Bi<sub>2</sub>:Mg<sub>2</sub>Sn systems requires the creation of Mg vacancies and subsequent Mg\ndiffusion. Mg vacancy creation is easier for Mg<sub>2</sub>Sn compared\nto Mg<sub>3</sub>Bi<sub>2</sub>, while the latter has slightly lower\nactivated Mg-diffusion pathways. The computational findings point\ntoward easier magnesiation/demagnesiation for BiSn alloys over pure\nBi or pure Sn, corroborating the superior Mg storage performance of\nBi–Sn electrodes over the corresponding single-phase electrodes.