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 (Bi_66.5Sn_33.5) 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.