Thermal and Electrochemical Stability of Tetraglyme–Magnesium Bis(trifluoro­methane­sulfonyl)amide Complex: Electric Field Effect of Divalent Cation on Solvate Stability

Phase behavior of binary mixtures
of tetraglyme (G4) and Mg­[TFSA]2 (TFSA: bis­(trifluoro­methane­sulfonyl)­amide)
was
investigated. In a 1:1 molar ratio, G4 and Mg­[TFSA]2 formed
a stable complex with a melting point of 137 °C. X-ray crystallography
of a single crystal of the complex grown from a G4-Mg­[TFSA]2 binary mixture revealed that the G4 molecule wraps around Mg2+ to form a complex [Mg­(G4)]2+ cation, and the
two [TFSA]− anions also participate in the Mg2+ coordination in the crystal. The thermal stability of [Mg­(G4)]­[TFSA]2 was examined by thermogravimetry, and it was found that the
complex is stable up to 250 °C. Above 250 °C, desolvation
of the Mg2+ ion takes place and G4 evaporates. On the other
hand, the weight loss starts at around 140 °C in solutions containing
excess G4 (n > 1 in Mg­[TFSA]2:G4 =
1:n) due to the evaporation of free (uncoordinated)
G4. The
suppression of G4 volatility in the [Mg­(G4)]­[TFSA]2 complex
is attributed to strong electrostatic and induction interactions between
divalent Mg2+ and G4. In addition, complexation of G4 with
Mg2+ is effective in enhancing the oxidative stability
of G4. Linear sweep voltammetry revealed that the oxidative decomposition
of [Mg­(G4)]­[TFSA]2 occurs at electrode potentials >5
V
vs Li/Li+, while the oxidation of uncoordinated G4 occurs
at around 4.0 V. This oxidative stability enhancement occurs because
the HOMO energy level of G4 is reduced by complexation with Mg2+, which is supported by the ab initio calculations.