Strong solvent coordination effect inducing gradient solid-electrolyte-interphase formation for highly efficient Mg plating/stripping

Developing simple Mg-salt electrolytes without chlorine-containing additives is vital for the technical viability of rechargeable magnesium batteries (RMBs). However, the formation of an ion-blocking passivation layer on Mg anode in these electrolytes severely hinders the reversible Mg plating/stripping. Here we successfully designed a highly efficient and chlorine-free Mg(SO3CF3)2/ether electrolyte by introducing 2-methoxyethylamine (MOEA) with a strongly nucleophilic amidogen (-NH2) donor for competing solvation coordination. The preferred decomposition of MOEA in MOEA-coordinated Mg-ion complex generates organic N-containing species on surface of Mg anode. Accompanied by the SO3CF3− decomposition, the gradient organic-inorganic solid-electrolyte-interphase (SEI) could be formed. This gradient SEI enables a fast interfacial Mg2+ migration and uniform Mg plating/stripping at 5 mA cm−2 with 5 mAh cm−2. The Mg(SO3CF3)2/ether/MOEA electrolyte exhibits high oxidation stability (3.5 V vs. Mg/Mg2+), high average Coulombic efficiency (99.3%, 1000 cycles), excellent rate performance (low overpotentials of 75, 270 and 570 mV at 0.5, 3.0 and 7.5 mA cm−2, respectively) and realizes a stable cycle life of Mg||Mo6S8 coin cell (300 cycles at 1 C), when using 50 μm Mg anode. Importantly, the relationships between solvation configuration, SEI chemistry and battery performance were successfully built to guide the design of simple Mg-salt electrolytes in RMBs.