CuCl as a Mg²⁺ reservoir for in situ interface layer engineering and highly stable Mg plating/stripping

Although rechargeable magnesium batteries (RMBs) represent a promising post‑lithium energy storage technology, the formation of a non-conductive Mg²⁺ passivation layer on Mg anodes remains a significant challenge. Here, we develop a novel all-inorganic electrolyte with low overpotential for Mg plating/stripping by introducing electrochemical active CuCl into a MgCl₂/THF solution (MgCl₂-CuCl/THF). During cycling, a multi-functional interface layer composed of Cu and Cu_xMg_yCl_z is formed in situ on the Mg anode surface, which not only promotes MgCl₂ salt dissociation but also offer rapid-electron transport pathways. Moreover, the Cu⁺ with reversible redox nature in Cu_xMg_yCl_z can accommodate and release Mg²⁺ ions, thereby facilitating rapid and homogenous Mg²⁺ deposition. Consequently, the optimized MgCl₂-CuCl/THF electrolyte enables Mg//Mg symmetric cells to cycle stably for over 1300 h at 0.5 mA cm^-2, with an overpotential as low as 0.15 V (vs. Mg²⁺/Mg). Furthermore, excellent Mg electro-plating/stripping properties have also been demonstrated in Mg//Mo, Mg//Cu, and Mg//SS asymmetric cells, achieving nearly 100 % Coulombic efficiency and stable cycling for over 138, 138, and 55 days, respectively. This work provides a promising strategy for the design of high-performance electrolyte systems for advanced RMBs.