Investigations of oxygen redox reactions in dialkylacetamide electrolytes for lithium air batteries

Abstract

As the world is witnessing a notable climate change mainly due to carbon dioxide emissions from fossil fuel consumption, the effort to utilize alternative forms of energy is steadily increasing. Renewable energy sources, such as wind, solar, hydropower, biomass and geothermal, are increasingly being used to generate energy to replace fossil fuels in recent years. Howerver, they have not been able to replace fossil fuels in a significant way in part because of discontinuities in the generation of energy from these alternative sources. Energy from wind or the sun is not always available when the demand arises. To solve this problem, development of efficient storage systems such as rechargeable batteries are essential to store the energy generated at peak hours. Rechargeable lithium batteries are expected to play a significant role in this respect. The first chapter summarizes the fundamentals of batteries and gives a general overview of different types of lithium batteries. The main focus in the introduction is on the basic chemistry of non-aqueous lithium air (Li-O2) batteries and the use of Hard Soft Acid-Base (HSAB) theory to explain oxygen reduction and evolution reactions (ORR/OER) in non-aqueous electrolytes. The effect of solvents' Donor Number on the ORR catalysis is explained as well. In chapter 2 we studied the chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in two high Donor Number organic solvents namely, N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc). In lithium salt (LiX) solutions (where X= PF6−, CF3SO3−, ClO4− and NO3−), solvation occurs when the Li+ bonds with the solvent's carbonyl group forming Li+[O=C(CH3)N(CH3)2]nX− ion pairs. Infrared and 13C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF6−, ClO4− or NO3−, and a contact ion pair in the case of CF3SO3−. Chemical interactions between TBA+ and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBAX and LiX containing electrolytes were measured and correlated with their viscosities. In chapter 3 the microelectrode technique was used to measure the O2 solubility and diffusion coefficient in 0.1M TBAPF6/DMAc (3.09×10−6 mol cm−3 and 5.09×10−5 cm2 s−1, respectively) as well as 0.1M TBACF3SO3, TBACLO4 and TBANO3 solutions. The values were found to be typical of these properties measured in several TBA+ solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory. Metal macrocycles are among the most important catalytic systems in electrocatalysis and biocatalysis owing to their rich redox chemistry. Precise understanding of the redox behavior of metal macrocycles under operando conditions is essential for fundamental studies and practical applications of this catalytic system. In chapter 4 we present electrochemical data for the representative iron phthalocyanine (FePc) in both aqueous and non-aqueous media coupled with in situ Raman and X-ray absorption analyses to challenge the traditional notion of the redox transition of FePc at the low potential end in aqueous media by showing that it arises from the redox transition of the ring. Our data unequivocally demonstrate that the electron is shuttled to the Pc ring via the Fe(II)/Fe(I) redox center. The Fe(II)/Fe(I) redox transition of FePc in aqueous media is indiscernible by normal spectroscopic methods owing to the lack of a suitable axial ligand to stabilize the Fe(I) state. Chapter 5 is dedicated to the study of the application of DMAc-based electrolytes and FePc-based catalysts in a Li-O2 battery cell. The analytical techniques SEM, NMR and FTIR were used to better understand the role of LiNO3 in stabilizing the lithium metal anode surface and protecting the metal from being consumed in the decomposition of DMAc. A three-electrode cell was used to study the ORR in DMAc solutions in the presence of FePc-based catalyst and little catalytic effect was observed. This behavior is typical of high Donor Number solvents (such as DMAc) which drives the ORR to proceed via an outer Helmholtz plane (OHP) reaction pathway. Li-O2 cells were constructed and tested with DMAc-based electrolytes. Both the catalyzed and uncatalyzed cell did not exhibit the ability to support long cycle life but the catalyzed cell showed slightly better performance than the uncatalyzed cells. Chapter 6 summarizes the conclusions of the research conducted in this investigation and offers suggestions for future work.