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 (∼2.0 V for conventional electrolytes) and greatly improved Coulombic efficiency of near 90% (almost irreversible for conventional electrolytes).