Enhancing Magnesium-Ion Storage in a Bi–Sn Anode through Dual-Phase Engineering

Magnesium-ion batteries (MIBs) are a "beyond Li-ion" technology that are hampered by Mg metal reactivity, which motivates the development of anode materials such as tin (Sn) with high theoretical capacity (903 mAh g–1). However, pure Sn is inactive for Mg2+ storage. Herein, Mg alloying with Sn is enabled within dual-phase Bi–Sn anodes, where the optimal composition (Bi66.5Sn33.5) outperformed single-phase Bi and Sn electrodes to deliver high specific capacity (462 mAh g–1 at 100 mA g–1), good cycle life (84% after 200 cycles), and significantly improved rate capability (403 mAh g–1 at 1000 mA g–1). Density functional theory (DFT) calculations revealed that Mg alloys first with Bi and the subsequent formation of the Mg3Bi2//Sn interfaces is energetically more favorable compared to the individual Mg3Bi2 and Sn phases. Mg insertion into Sn is facilitated when Mg3Bi2 is present. Moreover, dealloying Mg from Mg3Bi2:Mg2Sn systems requires the creation of Mg vacancies and subsequent Mg diffusion. Mg vacancy creation is easier for Mg2Sn compared to Mg3Bi2, while the latter has slightly lower activated Mg-diffusion pathways. The computational findings point toward easier magnesiation/demagnesiation for BiSn alloys over pure Bi or pure Sn, corroborating the superior Mg storage performance of Bi–Sn electrodes over the corresponding single-phase electrodes.