Advisor(s)

Kuzhikalail M. Abraham

Contributor(s)

Sanjeev Mukerjee (1960-), Graham B. Jones, David E. (David Edward) Budil, Max Diem, Eugene S. Smotkin

Date of Award

2010

Date Accepted

4-2010

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Arts and Sciences. Department of Chemistry and Chemical Biology.

Keywords

aprotic solvents, electrochemistry, hard acid soft base, lithium-air battery, non-aqueous, oxygen reduction

Subject Categories

Lithium cells, Electrolytes - Conductivity

Disciplines

Materials Chemistry

Abstract

Unlocking the true energy capabilities of the lithium metal negative electrode in a lithium battery has until now been limited by the low capacity intercalation and conversion reactions at the positive electrodes. This is overcome by removing these electrodes and allowing lithium to react directly with oxygen in the atmosphere forming the Li-air battery. The Li/O2 battery redox couple has a theoretical specific energy of 5200Wh/Kg and represents the ultimate energy density, environmentally friendly battery.

Chapter 2 discusses the intimate role of electrolyte, in particular the role of ion conducting salts on the mechanism and kinetics of oxygen reduction in non-aqueous electrolytes designed for such applications and in determining the reversibility of the electrode reactions. Such fundamental understanding of this high energy density battery is crucial to harnessing its full energy potential. The kinetics and mechanisms of O2 reduction in solutions of hexafluorophosphate salts of the general formula X+PF6-, where, X = tetra butyl ammonium (TBA), K, Na and Li, in acetonitrile have been studied on glassy carbon electrodes using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. Our results show that cation choice strongly influences the reduction mechanism of O2. Large cations such as TBA facilitate reversible O2 reduction involving the one electron reduction product, O2- which is stabilized by the large TBA cation. In contrast small cations like Li (and other alkali metals), promote an irreversible electrochemical reaction. The initial reaction again is one-electron reduction of O2 to LiO2 or other alkali metal superoxides. The LiO2 formed initially either decomposes to Li2O2 or undergoes further reduction to Li2O2 and Li2O. Electrochemical data supports the view that alkali metal oxides formed via electrochemical and chemical reactions passivate the electrode surface inhibiting the kinetics and reversibility of the processes. The O2 reduction mechanisms in the presence of the different cations have been supplemented by kinetic parameters determined from detailed analyses of the CV and RDE data. The Lewis acid characteristics of the cation appear to be crucial in determining the reversibility of the system. The organic solvent present in the Li+-conducting electrolyte has a major role on the reversibility of each of the O2 reduction products as found from the work discussed in the next chapter.

A fundamental study of the influence of solvents on the oxygen reduction reaction (ORR) in a variety of non-aqueous electrolytes was conducted in chapter 4. In this work special attention was paid to elucidate the mechanism of the oxygen electrode processes in the rechargeable Li-air battery. Towards this end, using either tetrabutylammonium hexafluorophosphate (TBAPF6) or lithium hexafluorophosphate (LiPF6) electrolyte solutions in four different solvents, namely, dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME), possessing a range of properties, we have determined that the solvent and the supporting electrolyte cations in the solution act in concert to influence the nature of reduction products and their rechargeability. In solutions containing TBA+, O2 reduction is a highly reversible one-electron process involving the O2/O2- couple in all of the electrolytes examined with little effect on the nature of the solvent. On the other hand, in Li+-containing electrolytes relevant to the Li-air battery, O2 reduction proceeds in a stepwise fashion to form O2-, O22- and O2- as products. These reactions in presence of Li+ are irreversible or quasi-reversible electrochemical processes and the solvents have significant influence on the kinetics, and reversibility or lack thereof, of the different reduction products. The stabilization of the one-electron reduction product, superoxide (O2-) in TBA+ solutions in all of the solvents examined can be explained using Pearson’s Hard Soft Acid Base (HSAB) theory involving the formation of the TBA+---O2- complex. The HSAB theory coupled with the relative stabilities of the Li+-(solvent)n complexes existing in the different solvents also provide an explanation for the different O2 reduction products formed in Li+-conducting electrolyte solutions. Reversible reduction of O2 to longlived superoxide in a Li+-conducting electrolyte in DMSO has been shown for the first time here.

Chapter 5 is the culmination of the thesis where the practical application of the work is demonstrated.. We designed electrolytes that facilitate Li-Air rechargeability, by applying the knowledge gained from chapters 2-4. A rechargeable Li-air cell utilizing an electrolyte composed of a solution of LiPF6 in tetraethylene glycol dimethyl ether, CH3O(CH2CH2O)4CH3 was designed, built and its performance studied. It was shown that the cell yields high capacity and can be recharged in spite the absence of catalyst in the carbon cathode. From the X-ray diffraction patterns of the discharged carbon electrodes, the discharge product of the cell was identified to be Li2O2 during normal discharge to 1.5 V. Discharging the cell to 1.0 V produces Li2O as well. The application of X-ray diffraction to identify these products formed in a porous carbon electrode is shown here for the first time. The rechargeability of the cell was investigated by repeated charge/discharge cycling of the cell, and the factors limiting the cycle life of the cell were studied using AC impedance spectra of the cells as a function of cycle number.

In conclusion, the work carried out in this research has shown that the O2 electrochemistry in organic electrolytes is substantially different from that in aqueous electrolytes. Our work has uncovered the key roles the ion conducting salts and the organic solvents play in determining the nature of the reduction products and their reversibility. The results presented here for the first time provide a rational approach to the design and selection of organic electrolyte solutions for use in the rechargeable Li-air battery. Factors affecting the cycle life limitations of the Li-air cell have been identified from the cycling performance and the associated impedance changes of Li/air laboratory test cells. Our work is expected to contribute to the rapid development of the rechargeable Li-air battery.

Document Type

Dissertation

Rights Information

Copyright 2010

Rights Holder

Cormac Míchéal Ó’Laoire

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