Study of structure and dynamics of Protic Ionic Liquids (PILs) as electrolytes and components for polymer electrolytes

The use of energy storage devices is exponentially growing, especially for mobile devices and the transportation sector, therefore the development of optimized systems is the key to the successful and entire transition to renewable energy sources. Nowadays, lithium-ion batteries (LIBs) are the technology of choice for most electrochemical applications thanks to their high specific energy, high efficiency, and long cycle life. However, safety is still an issue, and some electrolyte components must be improved. Recently, protic ionic liquids (PILs) emerged as potential electrolyte components in LIBs. When replacing flammable and volatile organic solvents, PILs are expected to improve the safety and performance of electrochemical devices. For implementing PILs as electrolyte components, a challenging task is still to understand the key factors governing their physicochemical and transport properties. To this end, this PhD work deeply investigates the effects of the structural features and intermolecular interactions on the properties of promising PILs based on the 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBUH+) cation and the (trifluoromethanesulfonyl-nonafluorobutylsulfonyl)imide (IM14-), trifluoromethanesulfonate (TFO-) and bis(trifluoromethanesulfonyl)imide (TFSI-) anions. Here, a complete characterization of the PILs by using multinuclear NMR methods and different physicochemical analyses provided a comprehensive understanding of the features governing the properties of the selected PILs. The initial results unveiled the peculiar behavior of DBUH-IM14 in terms of macroscopic properties, which may be related to the structural characteristics of IM14- anion, such as the presence of a C4 perfluorinated chain, the asymmetric distribution of the F atoms at the side of the sulfonylimide functional group, the steric hindrance, the capability of establishing fluorinated domains in the bulk liquid, etc. Yet, to promote the application of the PILs as electrolyte components, an in-depth understanding of the role played by the ions in the bulk system became crucial to scaling their transport properties. In this regard, both neat PILs and PIL-based electrolytes (DBUH-IM14, DBUH-TFSI and DBUH-TFO doped with lithium salt containing the same anion, i.e., LiTFO, LiIM14, and LiTFSI) were investigated by conductivity, diffusion and relaxation NMR. These techniques allowed the achievement of a broad overview of the ion dynamics of these systems. Accordingly, the presence of Li+ showed a peculiar effect on the dynamics of DBUH-IM14, and from the molecular point of view, DBUH-IM14 electrolytes deserve further investigation. It is known that polymer electrolytes can further improve the safety issues of the LIBs. Besides, polymer electrolytes are shapable materials and can improve the volumetric design concerns associated with liquid electrolytes. Then, to expand the application of PILs as innovative electrolyte components, the selected PILs were confined into a polymeric matrix using poly(methyl methacrylate) (PMMA) as a polymer host. Through the thermal analysis and solid-state NMR, this work could probe the structural features and molecular level interactions between the ions and the PMMA. The combined approach showed that (i) even when housed by the polymeric matrix, the PIL retains some internal mobility, and (ii) DBUH-IM14 has a stronger interaction with the polymeric matrix than DBUH-TFSI and DBUH-TFO. The unpredictable behavior of DBUH-IM14-polymeric system prompted to further investigate the transport properties of DBUH-IM14-based polymer electrolytes. Then, the effect of PILs confinement onto their transport properties was probed by pulsed field gradient NMR and fast field cycling NMR. When blended with PMMA, a marked slowing of the overall dynamics was observed. In the case of the polymer electrolyte system, Li+ showed a minor effect on the DBUH+ dynamics, whereas rotational and translational dynamics of the IM14- anion was quite responsive to the presence of Li+. Still, a descriptive model is currently being developed to fully explain the transport properties and relaxation profiles of these systems. From all the achievements of this PhD work, I can certainly state that DBUH-IM14-based electrolytes are of interest in LIB application and the structural features and nature of the intermolecular interactions in the IM14- anion play a crucial role in their molecular and macroscopic properties.