Magnesium bis(trifluoromethanesulfonyl)amide complexes with triglyme and asymmetric homologues: phase behavior, coordination structures and melting point reduction

Abstract

The phase behavior of binary mixtures of triglyme (G3) and Mg[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated, towards the development of a Mg-based room-temperature solvate ionic liquid (SIL) electrolyte. In a 1 : 1 molar ratio, G3 and Mg[TFSA] form a thermally stable complex (decomposition temperature, T: 240 °C) with a melting point (T) of 70 °C, which is considerably lower than that of the analogous tetraglyme (G4) system (137 °C). X-ray crystallography of a single crystal of [Mg(G3)][TFSA] revealed that a single Mg cation is coordinated by a single, distorted, tetradentate G3 molecule from one side, and two monodentate [TFSA] anions, with transoid conformation, from the reverse side to form an ion pair. Raman spectra of [Mg(G3)][TFSA] in the molten state revealed the presence of different coordination structures, as the liquid exhibits changes in the vibrational modes corresponding to G3 and the [TFSA] anion compared to those observed for the solid. Investigation of the ion pair stabilization energies by DFT calculations suggests that higher stability cation complexes and ion pairs co-exist in the molten state than those observed in the crystalline state. These results imply that the coordination structures of the ion pairs play a key role in providing SILs with low T. To decrease the T further, several asymmetric homologues of G3, which have higher conformational flexibility than G3, were investigated. Notably, a 1 : 1 mixture of Mg[TFSA] with G3Bu (where one of the terminal methyl groups of G3 is substituted for a butyl group) formed a thermally stable complex (T: 251 °C) without any distinct T and showed reasonable ionic conductivity at room-temperature, indicating partial dissociation of ions. In this electrolyte, which showed high oxidative stability, quasi-reversible Mg deposition/dissolution was achieved, indicating that Mg-based room-temperature SILs can be utilized as a new class of Mg electrolyte.

Solvate Ionic Liquids for Li, Na, K, and Mg Batteries

“…Maxima of melting point ( T m ) are typically observed at a 1 : 1 molar ratio, implying that equimolar complexes are formed (Figure ). For some limited cases, T m maxima are observed only or additionally at a 1 : 2 molar ratio, as shown in Figures and ,. This observation supports the formation of 1 : 2 complexes.…”

“…Phase behavior and complexation : As well as the Li‐based systems, certain binary mixtures of Na, K, and Mg salts and glymes (G n , CH 3 O(CH 2 CH 2 O) n CH 3 ) also form solvates at stoichiometric ratios, 1 : 1 and/or 1 : 2 for salt:glyme, irrespective of the choice of salts and glyme length ,,,,. The phase diagrams of these mixtures indeed clearly illustrate features characteristic of complex formation.…”

“…The same stepwise behavior of TG curves was observed for other salt‐glyme combinations which form stoichiometric complexes. In particular, complexation of glymes with Mg[TFSA] 2 leads to a remarkable improvement in thermal stability, indicating exceptionally strong interaction between the Mg 2+ and glymes (Figure ) . It should be noted that the thermal stability of 1 : 2 complexes is insufficient compared to the corresponding equimolar complexes ,.…”

“…In particular, complexation of glymes with Mg[TFSA] 2 leads to a remarkable improvement in thermal stability, indicating exceptionally strong interaction between the Mg 2+ and glymes (Figure ) . It should be noted that the thermal stability of 1 : 2 complexes is insufficient compared to the corresponding equimolar complexes ,. For example, [Na(G3) 2 ][FSA] (FSA: N(SO 2 F) 2 − ) and [Al(G3) 2 ][Al(TfO) 4 (OH) 2 ]⋅G3 (TfO: CF 3 SO 3 − ) easily release the coordinated glyme molecules in the molten state ,.…”

“…In addition to Li‐based SILs, non‐Li‐based SILs also show sufficient high thermal stabilities, negligible volatility, and non‐flammability with acceptable ionic conductivity ,. The electrochemical stabilities are also effectively enhanced upon complexation of glymes with metal ions.…”