Halide‐Cu ⁺ ‐Mediated Dynamic Interfacial Reconstruction in Magnesium‐Aluminum Chloride Complex Electrolyte Triggers Divergent Mg Deposition Behaviors on Active and Inert Electrodes

The development of rechargeable magnesium batteries (RMBs) is largely constrained by severe interfacial passivation of Mg anodes and sluggish Mg 2+ transport kinetics. Here, slightly soluble Cu(I)‐halide (CuX, X = Cl, Br, I) reservoirs were introduced into the classic magnesium‐aluminum chloride complex (MACC) electrolytes, where the continuous Cu + release via dissolution equilibrium drives spontaneous Mg/Cu displacement alloying throughout cycling. The resulting dynamically reconstructed interface ensures the persistent exposure of magnesiophilic sites, enabling ultralow Mg deposition/stripping overpotentials (<0.15 V) at 0.2 mA cm − 2 (1 mA h cm − 2 ) over 800 h in Mg||Mg symmetric cells. Electric field simulations further reveal that the in situ formed alloy interphase homogenizes the local electric‐field distribution and regulates Mg 2+ flux, thereby facilitating rapid and uniform Mg electrodeposition. In contrast, on inert electrodes that cannot undergo displacement alloying, Cu + is preferentially reduced to metallic Cu, which further promotes Mg deposition. Consequently, Mg||Cu, Mg||SS, and Mg||Mo asymmetric cells using CuX‐modified MACC exhibit greatly improved cycling stability compared with cells employing unmodified MACC. These findings establish a new paradigm for understanding and engineering multi‐metallic interfacial chemistries to precisely regulate deposition behavior in multivalent metal batteries.