(Invited) Conductivity and Dielectric Relaxations in New Morpholinium- and Piperidinium-Based Ionic Liquids

Abstract

Room Temperature Ionic Liquids (RTILs) are key materials for the development of new energy storage and conversion systems [1]. In the past decade several relevant properties such as their negligible vapour pressure, non-flammability, and high room-temperature conductivity were explored in order to obtain safer and better performing devices [2]. Despite of this, the complex interplay existing between the IL nanostructure and the conductivity mechanism is still under discussion within the scientific community [3]. Following this, the synthesis of new ILs and the investigation of their electrical properties are of crucial importance and could pave the way to new applications in the field of ion-conducting materials [4, 5]. This contribution summarizes the electrical study of a family of ILs based on 1-methylmorpholinium (MM+) and 1-methylpiperidinium (MP+) cations. The MM+ and MP+ cations have been synthesized comprising two different alkyl side chain substituents on the nitrogen atom, namely 1-methoxyethyl (ME) and 1-ethoxyethyl (EE) groups. A set of four cations (ME-MM+, EE-MM+, ME-MP+ and EE-MP+) neutralized by TFSI-anions is obtained, thus giving rise to four new RTILs. The structure of these ILs allows the systematic investigation of the presence of oxygen atoms on the aliphatic ring (see morpholinium vs. piperidinium cations) with respect to the different substituent side chain length (see methoxyethyl vs.ethoxyethyl groups). The effect of the oxygen atom position on the diffusion of conformational states as well as the charge transfer mechanism is particularly interesting for future Lithium battery applications in which the oxygen atoms are expected to play a crucial role on the migration of Li cations. At this regard, Broadband Electrical Spectroscopy (BES) measurements were undertaken to elucidate the electrical response of the ILs in terms of dielectric relaxations and polarization phenomena. At T<Tg (Tg=glass transition temperature) dielectric relaxations are present, associated with the rotational motions of morpholinium and piperidinium cations bearing permanent dipole moment; at T>Tg, interdomain polarization events are detected. These polarization events are associated with the presence of cation and anion nanocluster aggregates with different permittivities. The results allow us to correlate the dielectric relaxations of the different cations with the overall long-range charge migration, thus elucidating the interplay existing between conductivity and nanostructure of this new ILs.