The origin of anode-electrolyte interfacial passivation in rechargeable Mg-metal batteries

Understanding the electrolyte-metal anode interface passivation mechanism is crucial for the buildup of sustainable and low cost alkali (earth) metal batteries. Trace H2O-assisted Mg2+-anion ion pair decomposition on a model Mg metal electrode is studied here using a nuclear magnetic resonance and cryogenic electron microscopy technique, accompanied by molecular dynamic simulation and density functional theory calculations. The electrolyte chemical species transitions, from [Mg2+(diglyme)(2)](2+) and [Mg2+(diglyme)(2)(TFSI)(-)](+) to [Mg2+(diglyme)(TFSI-)(2)(H2O)](0), [Mg2+(H2O)(n)(TFSI-)](+) (n = 1, 4, 6), and [Mg2+(H2O)(6)](2+), have been unraveled upon introducing trace H2O impurities into the conventional electrolyte. These H2O competitively solvating complexes not only induce the preferential decomposition of anions, but also reduce the cation transference number. The electrodeposits with a primary fractal nano-seaweed morphology and a secondary dendrite-in-ball microstructure were seriously passivated by MgO and Mg(OH)(2) nanocrystals derived from the parasitic reactions of anions and H2O molecules. The reversibility of Mg stripping/plating processes were thus impaired along with the reproducibility of electrochemical experiments. By introducing isobutylamine and trace di-N-butylmagnesium, the ternary electrolytes displayed extra-low overpotential of lower than 0.15 V (similar to 2.0 V for conventional electrolytes) and greatly improved Coulombic efficiency of near 90% (almost irreversible for conventional electrolytes).