A Li/Mg Double‐Salt Strategy Based on Amine Solvent Achieves Bulk Phase‐Interface‐Electrode Multi‐Scale Optimization for Mg Metal Batteries

Abstract Achieving good compatibility between anodes and cathodes with electrolytes still faces great challenges in Mg metal batteries (MMBs). Recently developed amine‐based electrolytes have enabled reversible Mg anodes with conventional Mg salts through solvation structure regulations but still suffer from low conductivity and poor cathode compatibility. Herein, a Li/Mg double‐salt strategy is proposed to achieve the bulk phase‐interface‐electrode multi‐scale optimization for the amine‐based electrolyte, including ionic conductivity, anode stability, and cathode compatibility. Lithium triflate (LiOTf) serves as a multifunctional additive to compensate for the limitations of single Mg salt electrolyte (MgCl 2 /3‐methoxypropylamine). Li + ions accelerate the ions transport in the bulk phase and ions insertion at the cathode side, benefitting to the conductivity and the cathode compatibility. Attributed to the high reduction stability of OTf − anions, the stable Mg anode/interface is also retained. Therefore, the Mg//SS cell achieves an ultralong cycling life for 2100 cycles with the coulombic efficiency of 99.8% at 1.0 mA cm −2 and the full cell also exhibits a superior cycling performance for 1000 cycles at 5 C. Additionally, the batteries with the optimized electrolyte are verified as a proof‐of‐concept for the air‐assembled MMBs. This work emphasizes the crucial role of multifunctional additives in enhancing amine‐based electrolyte performances.