Halide-Cu⁺-Mediated Dynamic Interfacial Reconstruction in Magnesium-Aluminum Chloride Complex Electrolyte Triggers...

The development of rechargeable magnesium batteries (RMBs) is largely constrained by severe interfacial passivation of Mg anodes and sluggish Mg²⁺ 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 Mg²⁺ 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.