Determination of Mg²⁺ Speciation in a TFSI^–-Based Ionic Liquid With and Without Chelating Ethers Using Raman Spectroscopy

Abstract

Raman spectroscopy was employed to assess the complex environment of magnesium salts in the n-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrTFSI) room-temperature ionic liquid (RTIL). At room temperature, Mg(TFSI)2 was miscible with BMPyrTFSI and formulated by [Mg(TFSI)2](x)[BMPyrTFSI](1-x) (x ≤ 0.55). Results suggest that at low concentrations of Mg(TFSI)2, anionic complexes in which Mg(2+) is surrounded by at least four TFSI(-) were formed. Above x = 0.2 an average of three TFSI(-) surround each Mg(2+). Below x = 0.12, there is a greater number of monodentate interactions between TFSI(-) oxygens and Mg(2+) cations, whereas above x = 0.12 bidentate ligands dominate. The fraction of TFSI(-) existing in the cis conformation increased with increasing Mg(2+) concentration. Mg(ClO4)2 was also studied as a Mg(2+) source. At equivalent mole fractions to those of the Mg(TFSI)2 salt, Mg(2+) from Mg(ClO4)2 was surrounded by only two TFSI(-) anions as ClO4(-) appeared to compete with TFSI(-) for coordination with Mg(2+). Similar behavior was also observed for the less soluble halide salts MgX2 (X = Cl, Br, I). Additions of chelating ligands were shown to effectively reduce the average number of TFSI(-) around Mg(2+) in a manner consistent with maintaining a sixfold oxygen coordination number around Mg(2+). Furthermore, an alternative class of ionic liquids, known as "solvate" ionic liquids, were produced. In this case glymes (Gm, m + 1 ether oxygens) were mixed with Mg(TFSI)2 so that glymes chelated Mg(2+), creating Mg(Gm)(y)(2+) complexes. The general formula was given by Mg(Gm)(y)(TFSI)2. These solvate ILs melt between 40 and 80 °C. Raman spectra clearly showed the glyme chelating ability and stronger coordination with Mg(2+) with respect to TFSI(-). Finally, linear sweep voltammograms showed the anodic stability of the glymes to improve due to coordination with Mg(2+).

Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes—A Review

“…Such increase in bidentate coordination, which increases the oxygen atom coordination with Mg 2+ , and CIP formation at higher concentration also negatively affects the ionic conductivity and transference number. Raman spectroscopy studies of the solvation environment of Mg(TFSI) 2 -ionic liquid also suggest the formation of both cis-and trans-conformer of TFSI − in the solution, with predominantly monodentate interactions between TFSI − and Mg 2+ at low concentrations and bidentate interactions at high concentrations [46]. Hence, it likely that at low concentrations TFSI − anions which exist as CIPs are in the minimum energy transoid (CF 3 groups on the opposite side of the S-N-S plane) state, while at higher concentrations, a slightly higher local minima cisoid (CF 3 groups on the same side of the S-N-S plane) with ~ 3.5 kJ/ mol energy barrier compared to trans-conformer exists [28,46].…”

On the Anion Exchange of PX3 (X = Cl, Br, I) in Ionic Liquids comprising Halide Anions

“…In all applications mentioned above, fundamental knowledge of the underlying mechanisms and behavior of the involved species are required to obtain reproducible results, high yields and defined morphologies. With focus on the actual molecular species in ILs, several spectroscopic in situ techniques have been employed, such as nuclear magnetic resonance (NMR),, Raman,, X‐ray photoelectron spectroscopy (XPS), or extended X‐ray absorption fine structure (EXAFS) , . Often combinations of complementary techniques are required to identify the active species.…”

A Zwitterionic Liquid Electrolyte for Magnesium Batteries

“…A better electrolyte system should be sustainable, non‐volatile, and non‐hazardous, possess low‐flammability, and show excellent electrochemical performance. Ionic liquids (ILs) may comply with all of those prerequisites, which is why some designer ionic liquids are receiving increasing attention in magnesium battery research ,,,,. In contrast, simple imidazolium based ILs (ImILs), which are accessible in a fast and efficient way from renewable resources,,,, have been abandoned by the scientific community due to instability issues of the imidazolium ring ,.…”

“…Ionic liquids (ILs) may comply with all of those prerequisites, which is why some designer ionic liquids are receiving increasing attention in magnesium battery research. [19,20,21,22,23] In contrast, simple imidazolium based ILs (ImILs), which are accessible in a fast and efficient way from renewable resources, [24,25,26,27] have been abandoned by the scientific community due to instability issues of the imidazolium ring. [22,28] Furthermore, some common 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 anions are not compatible with magnesium electrochemistry.…”