In Situ Formation of a Bi/Mg‐Based Hybrid Interphase for Highly Reversible Magnesium Metal Anodes

Abstract Magnesium (Mg) is a promising anode material for magnesium metal batteries (MMBs) owing to its high specific capacity, excellent safety profile, and abundant availability. However, pristine Mg anodes suffer from uneven plating/stripping and surface passivation/corrosion, limiting the safety and cycling stability of MMBs. This study introduces a Bi/Mg‐based hybrid interphase protective layer on Mg foil (denoted Bi‐Mg@Mg) through an in situ quasi‐solid–solid redox reaction by immersing the foil in a bismuth oxybromide suspension. The resulting interphase layer consists of magnesiophilic components (Bi metal and Bi 2 Mg 3 alloy) and magnesiophobic species (MgO, MgBr 2 , and BiBr 3 ). These components synergistically enhance the desolvation, nucleation, and deposition kinetics, mitigate side reactions, and promote uniform electric field and ion flux distributions. As a result, the Bi‐Mg@Mg electrodes exhibit superior Mg plating/stripping reversibility, maintaining stable performance for over 4100 h in the all‐phenyl complex electrolyte and 2900 h in the Mg(TFSI) 2 electrolyte, significantly outperforming pristine Mg electrodes. Furthermore, full cells paired with Mo 6 S 8 and S cathodes demonstrate excellent capacities, rate capabilities, and long lifespans, highlighting the exceptional electrochemical performance of the Bi‐Mg@Mg anode. This study offers a promising strategy for developing highly reversible Mg anodes, paving the way for practical long‐cycle MMBs.