High‐Rate Magnesium‐Ion Batteries Enabled by Boron‐Doped Bismuth Anodes

Large volume expansion upon (de)alloying faced by bismuth anodes always leads to severe capacity loss and poor durability for magnesium‐ion batteries. This failure mode becomes particularly pronounced under high current‐density impact, where the sluggish electron transport and ion diffusion across solid–solid and solid–liquid interfaces create a significant kinetic barrier. Therefore, we propose an interface engineering scheme driven by bulk doping. It involves incorporating boron (with matched electronegativity) into bismuth, aimed at enhancing Mg 2+ diffusion kinetics and solid‐electrolyte interphase (SEI) stability. Starting from the basic Bi‐Mg redox chemistry, the dual enhancement mechanism was unraveled. Density functional theory calculations and kinetic analyses revealed the origin of the high‐rate performance in Bi‐B electrodes. Furthermore, ex situ X‐ray photoelectron spectroscopy and time‐of‐flight secondary ion mass spectrometry analyses identified a SEI composed of organic species and inorganic components (MgF 2 /AlF 3 ), which supports the observed high‐rate durability at the material level. Finally, the Bi‐B anode exhibits remarkable rate capability, maintaining 167 mA h g −1 after 1000 cycles at high current density of 2 A g −1 , which represents one of the state‐of‐the‐art among all reported bismuth‐based anodes. Superior high‐rate performance is also achieved in Bi‐B||CuS full cell, highlighting its practical applicability. This study offers valuable insights for the design of high‐rate anodes, paving the way for the development of advanced alloy anodes in magnesium‐ion batteries.