Synergy between the coordination and trace ionization of co-solvents enables reversible magnesium electroplating/stripping behavior

Electrolyte chemistries, including the composition of chemical species and their transformation processes, are of utmost importance for safe and stable operation of any battery, but the corresponding understanding remains incomplete. Prevalent viewpoints on direct solvation are not satisfactory sometimes in describing the implicit relationship among solvents, active ionic species and electrode/electrolyte interphases. Here we report the synergy between the coordination and trace ionization of amine/ether co-solvents to enable a reversible magnesium (Mg)-metal anode for low cost rechargeable Mg batteries. Unique bivalent [Mg(amino)₆]²⁺ is firstly identified as the active ionic species via single-crystal X-ray diffraction, mass spectroscopy, and NMR tests. Partially protonated alkylamine/ether cations, formed due to trace solvent ionization, can chemically associate with the trifluoromethanesulfonimide (TFSI) anion to form neutrally charged ion pairs, mitigating the decomposition of the TFSI anion on the Mg-metal anode. Cryo-electron microscopy images along with the deuterium-oxide (D₂O) titration tests reveal a much thin electrolyte-anode interface and one distinctive component of MgH₂ in electrodeposits, closely related to the ionization process of co-solvents. A much better Mg plating/stripping reversibility (an overpotential of ∼0.10 V vs. ∼2.0 V for the pristine electrolyte) and prolonged cycle life (∼320 h vs. ∼38 h for the pristine electrolyte) are achieved by regulating the coordination/ionization processes of co-solvents.