Unsuccessful Coupling of Weakly Coordinating Anion-Based Mg Salts and Alkoxyalkylamine Solvents for Rechargeable Mg Battery Application: Trace Water-Driven Interfacial Failure.

Rechargeable magnesium metal batteries (RMMBs) are promising candidates for large-scale energy storage owing to the advantageous properties of magnesium metal negative electrodes. Recent advances, particularly in electrolyte chemistry, have accelerated progress toward practical RMMBs. Here, we combined two emerging concepts-weakly coordinating anion (WCA)-based Mg salts and alkoxyalkylamine solvents-in electrolyte design. Unexpectedly, the combination of a representative WCA-based Mg salt, Mg[Al(Ohfip)4]2, and a conventional alkoxyalkylamine, 2-methoxyethylamine (MOEA), resulted in inferior electrochemical performance, whereas MOEA favored strongly coordinating anion-based Mg salts such as Mg[TfO]2. Comprehensive analyses revealed distinct anion-dependent solvation structures and interfacial chemistries. In Mg[Al(Ohfip)4]2 systems, the anion and the solvent complexes are only minimally involved in initial Mg-interface formation. Yet, the presence of trace water in MOEA induces the MOEA-mediated partial hydroxylation of the anion [Al(Ohfip)4]-, which leads to the formation of unstable oxygen-rich interfaces upon cycling. On the contrary, Mg2+, [TfO]-, and MOEA form associative ion-paired complexes that jointly contribute to the formation of fluorine- and nitrogen-rich hybrid interfaces that suppress parasitic reactions and enhance performance. Despite these benefits, the associative Mg[TfO]2-MOEA electrolyte still exhibits limitations for practical RMMB application. The complex dilemma in developing rational electrolyte materials for RMMB applications will also be discussed.