Comparing Computational Predictions and Experimental Results for Aluminum Triflate in Tetrahydrofuran

Abstract

Exploring reliable electrolytes for aluminum ion batteries requires an in-depth understanding of the behavior of aluminum ions in ethereal-organic solvents. Electrolytes comprised of aluminum trifluoromethanesulfonate (Al-triflate) in tetrahydrofuran (THF) were investigated computationally and experimentally. Optimized geometries, redox potentials, and vibrational frequencies of species likely to be present in the electrolyte were calculated by density functional theory and then measured spectroscopically and electrochemically. Aluminum appears to be electrochemically active in THF with a reduction onset near 0 V versus Al/Al 3+ . Spectroscopic measurements reveal explicit evidence for the presence of two concentrationdependent ionic environments for the triflate anions, namely, outer-shell ligands and Al-bound triflates. Additionally, ionic conductivities of ∼2.5 mS/cm were measured for these electrolytes ∼0.8M.

Aluminum Electrodeposition from Chloride-Rich and Chloride-Free Organic Electrolytes

“…Moreover, comparing the Al(OTF) 3 /THF spectra to that of pure solvent (THF), we find that the peak at 1068 cm −1 becomes broader as the concentration increases, suggesting that this band is not only associated with THF but also Al − CIP and/or Al-Agg. 63,74 The peak emerging at 883 cm −1 , as supported by our DFT calculations, can be assigned to ν(Al − O) coupled with τCH 2 of Al − THF bond. (See table S1.…”

“…Density functional theory (DFT) calculations coupled with Fourier transform infrared spectroscopy (FTIR) suggested that Al exists in these solutions as fully solvated Al-complexes, in addition to the presence of innersphere and outer-sphere trifluoromethansulfonate anions (OTF − ). 63 Further investigation at higher concentrations (>0.1M), presented herein, reveals that these solutions are dominated by [Al(THF) 2 (OTF) 2 ] + . The structure evolution of these species is depicted in (Figure 1a-c).…”

“…[69][70][71][72][73] In accordance with these reports, we revealed in our previous investigation of the Al(OTF) 3 /THF electrolytes that the peak at 1028 cm −1 is associated with symmetric stretch (ν s SO 3 ) of free OTF − and that the 1200-1400 cm −1 region of the spectra displays bands assigned to the antisymmetric stretch of the OTF − anions (ν as SO 3 ) for two types of contact ion pairs, outer sphere and inner sphere OTF − , referred to here as SSIP (for Solvent-Separated Ion Pair) and CIP 1 (for Contact Ion Pair), respectively. 63 Re-examining this region, we find that the broad peak centered at 1210 cm −1 is likely a combination of two spectral components, a peak at 1220 cm −1 , SSIP, assigned to ν as SO 3 , another, CIP 1 , at 1210 cm −1 assigned to νC − S coupled with ν s SO3 of OTF − (Table S1). Upon access of an additional OTF − to the inner solvation sphere of the Al-cation, a new peak appears both computationally and experimentally at ~1350 cm −1 .…”

“…56 58 and aluminum hexafluorophosphate (Al(PF 6 ) 3 ) in DMSO, 59 as well as explore the commercially available aluminum trifluoromethanesulfonate (Al(OTF) 3 ) in propylene carbonate and THF mixture, 12 2-methoxy ethyl ether (diglyme), 60,61 N-methyl acetamide, 62 and THF. 63 It is worth noting that employing Al(OTF) 3 as a potential Al-salt for rechargeable Al batteries has not been restricted to organic solvents, with its application in aqueous systems 64,65 and ionic liquids 66 having already been demonstrated. Despite these endeavors, the role free chlorides (Cl − ) play in Al-ion organic electrolyte chemistry, and a clear demonstration of Al electrodeposition using Al(OTF) 3 , has not yet been established.…”

“…Despite these endeavors, the role free chlorides (Cl − ) play in Al-ion organic electrolyte chemistry, and a clear demonstration of Al electrodeposition using Al(OTF) 3 , has not yet been established. In our previous report, 63 we explored the ionic speciation and the electrochemical activity of Al-complexes in Al(OTF) 3 /THF computationally and experimentally. Density functional theory (DFT) calculations coupled with Fourier transform infrared spectroscopy (FTIR) suggested that Al exists in these solutions as fully solvated Al-complexes, in addition to the presence of innersphere and outer-sphere trifluoromethansulfonate anions (OTF − ).…”