Assessment of thermal stability of two N-ethoxyethyl-N-methylpiperidinium borate ionic liquids by non-Arrhenian incremental kinetic method

Abstract

Thermal stability of two borate-based ionic liquids (ILs) recently proposed as effective additive for high voltage batteries, N-ethoxyethyl-N-methylpiperidinium bis(oxalato)borate (BOB) and N-ethoxyethyl-N-methylpiperidinium difluoro(oxalato)borate (DFOB), was investigated. Thermogravimetry–differential thermal analysis measurements performed under nitrogen at 1, 2, 4, and 7 °C min−1 show gradual mass loss above 220 °C for both compounds. The TG curves were processed by isoconversional kinetic analysis under the assumption of non-Arrhenian behavior. The outcome of the analysis suggests that even the initial decomposition stages occur by a complex mechanism, probably via competing evaporation–decomposition paths with relative contributions varying with the heating rate. The service lifetimes estimated by the kinetic analysis result in a stability of DFOB up to 10-times higher than that of BOB in the 25–125 °C range. Further experiments were carried out by the Knudsen effusion mass spectrometry (KEMS) in order to get information on the composition of the vapor phase produced upon heating under effusion conditions. The mass spectra of the vapor phase showed a rapid onset of decomposition at 110 °C and 130 °C for BOB and DFOB, respectively, with a large release of the ion species with m/q = 98. This decomposition path dominates the simple evaporation to neutral ion pairs, which was nonetheless observed at m/q = 172 (integer cation). All the results seem to point toward a higher stability of DFOB compared to BOB, most probably due to the charge-stabilizing inductive effect of fluorine atoms.