Dias Ferreira, Lais
(2021)
Fundamental studies of ionic liquid electrolytes aiming at application in Na metal-based batteries.
PhD thesis, University of Nottingham.
Abstract
In this thesis, the tunability of ionic liquids was explored to design a series of ILs and evaluate the impact of different IL-based electrolytes on metallic Na electrode cells. This was done by performing a comprehensive investigation involving Na+ plating/stripping and electrode surface resistance over time (resting and cycling time). In addition, a novel study of solid-electrolyte interphase (SEI) structure and composition was carried out with 3D OrbitrapTM secondary ion mass spectroscopy (3D OrbiSIMS) in depth profiling mode. The unique sensitivity and mass resolution of the 3D OrbiSIMS, and the use of an Ar cluster primary ion beam as a sputtering and analysis beam allowed us to obtain vast and detailed information relating to the nature and distribution of the SEI-products in the interphase. Consequently, we were able to accurately determine the effects of anion and cation variation on the SEI structure. This allowed us to identify key structural features to design high performing stable electrolytes for use in Na-electrode cells. It was demonstrated that the distribution of SEI-products into an organised structure is critical to achieve a stable SEI. In addition, low overpotential during Na symmetrical cycling, and stabilisation of surface resistance was achieved with the IL that demonstrated the most organised SEI structure. Furthermore, the impact of the commonly used [PF6]-based salt as an additive was also investigated. It was found that the presence of [PF6] additive improves SEI stability and cycling time of symmetrical cells. 3D OrbiSIMS studies of the SEI with the [PF6] additive revealed that the organisation of the structure of the SEI has a greater influence on the improved performance of cells than the presence of fluoride products in the SEI itself.
The designer aspect of ILs was further used to obtain electrolytes aiming for application in Na-O2 cells. The fundamentals of the oxygen reduction reaction (ORR) in the ILs were thoroughly studied, and O2 concentration and diffusion coefficients in new series of ILs were determined as well. It was found that [C1(C2OC1)Pyrr][NTf2] and [C1(C2OC1)Pyrr][C2F5COO] displayed high O2 solubility, whilst the higher O2 diffusion coefficients were found for [NC1,C1,C2,(C2OC1)][NTf2] and [NC1,C1,C2,C4][NTf2].
In the presence of Na+, both the IL used and the concentration of Na+ seemed to influence the nature of the ORR product. It was found that with low Na+ concentration O2 .-interacted with both Na+ and IL cations in [NC1,C1,C2,C4 [NTf2], [C1(C2OC1)Pyrr][C2F5COO] and [C1CdmaPyrr][NTf2]. Furthermore, multiple oxidation peaks and/or shift of the oxidation peaks over cycling were found for [NC1,C1,C2,(C2OC1)][NTf2], [C1CdmaPyrr][NTf2], [C1(C2OC1)Pyrr][NTf2] and [C1(C2OC1)Pyrr][C2F5COO]. With the aid of rotating ring and disc (RRDE) studies, these peaks were suggested to be associated with formation of NaxOy product in solution or at the electrode surface. This was found to be significantly impacted by the H-bond ability of the IL, and [C1(C2OC1)Pyrr][C2F5COO] displayed soluble products even at high Na+ concentration.
Finally, following the step-by-step approach of this thesis based on studies of IL electrolytes with metallic Na, O2 and Na+/O2, a full Na-O2 cell was assembled. Assembly of a full Na-O2 cell was challenging, and a brief overview is given considering the difficulties and solutions for the cell preparation. Preliminary charge and discharge profiles were obtained, and the post discharge carbon electrode was analysed by scanning electron microscopy. It was suggested that O2 diffusion in the IL electrolytes is one of the major challenges in applying IL-based electrolytes to full Na-O2 cells.
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