Activation mechanism of conventional electrolytes with amine solvents: Species evolution and hydride-containing interphase formation

Rechargeable magnesium (Mg)-metal batteries have brought great expect to overcome the safety and energy density concerns of typical lithium-ion batteries. However, interfacial passivation of the Mg-metal anode impairs the reversible Mg plating/stripping chemistries, resulting in low Coulombic efficiency and large overpotential. In this work, a facile isobutylamine (IBA)-assisted activation strategy has been proposed and the fundamental mechanism has been unveiled in a specific way of evolving active species and forming MgH₂-based solid-electrolyte interphase. After introducing IBA into a typical electrolyte of magnesium bis(trifluoromethanesulfonyl) imide (Mg(TFSI)₂) in diglyme (G2) solvents, electrolyte species of [Mg²⁺(IBA)₅]²⁺ and protonated amine-based cations of [(IBA)H]⁺ have been detected by nuclear magnetic resonance and mass spectra. This not only indicates direct solvation of IBA toward Mg²⁺ but also suggests its ionization, which is central to mitigating the decomposition of G2 and TFSI anions by forming neutrally charged [(IBAH⁺)(TFSI^–)]⁰ and other complex ions. A series of experiments, including cryogenic-electron microscopy, D₂O titration-mass spectra, and time of flight secondary ion mass spectrometry results, reveal a thin, non-passivated, and MgH₂-containing interphase on the Mg-metal anode. Besides, uniform and dendrite-free Mg electrodeposits have been revealed in composite electrolytes. Benefiting from the activation effects of IBA, the composite electrolyte displays superior electrochemical performance (overpotential is approximately 0.16 V versus 2.00 V for conventional electrolyte; Coulombic efficiency is above 90% versus <10% for conventional electrolyte). This work offers a fresh direction to advanced electrolyte design for next-generation rechargeable batteries.