Advisor(s)

Sanjeev Mukerjee, K.M. Abraham

Contributor(s)

David E. Budil, Geoffrey Davies, Eugene Smotkin

Date of Award

6-2012

Date Accepted

6-2012

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Science. Department of Chemistry and Chemical Biology.

Keywords

Li-air batteries, Li-ion batteries, XAS

Disciplines

Chemistry | Materials Chemistry

Abstract

Today we hold high expectations that supporting technologies will keep pace with the ever-changing lifestyles most of us lead. Batteries are perhaps the most important of these supporting technologies as they provide the energy to power the lifestyle devices. Various redox chemistries have been developed and engineered in the form of batteries over the past two centuries, paving the way for many such conveniences. Despite the success, they are still lacking the energy density required for future high energy demands such as hybrid, full electric vehicles and mobile devices with step changes in computing and transmission power. This is a critical limitation of intercalation based cathode chemistries for Li-ion batteries. Advancements in this arena as well as metal-O2 batteries are welcome as portable power sources for the next line of energy hungry applications. The scope of chapter 1 is to provide a summary of current state-of-the-art lithium-ion and lithium-O2 battery technologies.

In chapter 2, the synthesis and characterization of the Li-ion battery positive electrode materials α-LiVOPO4, β-LiVOPO4 and α-Li3V2(PO4)3 are discussed. They have been prepared as individual phases from a common precursor mixture by controlling the synthesis conditions. These phases result because of their distinct crystal symmetries and vanadium oxidation states. The synthesis involves the decomposition of a precursor mixture prepared from NH4VO3, NH4H2PO4, LiF and hexanoic acid, which when heat-treated under different conditions produced the three LiwVxOy(PO4)z products. The materials were characterized by means of elemental analysis, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and Li cell data. The Li cycling performance and the accompanying structural effects on α- and β-LiVOPO4 were also studied under normal and deep discharge conditions using XAS to probe the local vanadium environment. XANES and EXAFS confirm reversible changes in the two structures when intercalation does not exceed 1Li/VOPO4. Deeper discharge revealed more disruption to both structures, including an additional 0.25 Å increase to the V=O bond length. A greater range in the VO6 symmetry of α-LiVOPO4 resulted in a more flexible accommodating host, in agreement with its improved low voltage performance compared to β-LiVOPO4.

A new Fe-V mixed metal phosphate of the composition Li1+xFe0.5(VO)0.5(PO4)F0.5 has been synthesized and characterized as a single phase Li insertion/extraction cathode material for rechargeable lithium batteries and is the focus of chapter 3. Its tetragonal crystal structure revealed from X-ray diffraction and absorption spectral data exhibits little change with Li extraction and subsequent Li insertion. The charge/discharge cycling capacities obtained from Li cells is consistent with the structure. The presence of F in the material is essential to prepare a mixed metal phosphate with equal amounts of Fe and V in the crystal structure and is probably the key to our success in preparing one of the first single phase metal phosphate cathode materials.

Studies on ionic liquid electrolytes for Li-O2 applications are the subject of chapters 4 and 5. Initial work discussed in chapter 4 covers oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) on glassy carbon (GC) and gold electrodes, investigated in a neat and a Li+-containing room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis-(triflouromethanesulfonyl)imide (EMITFSI). The presence of Li+ significantly changes the ORR mechanism. While similar one-electron O2/O2.- reversible couples result on both electrodes in neat EMITFSI, in the presence of added LiTFSI the initially formed LiO2 decomposes to Li2O2. In addition, the ORR and OER in the Li+ doped solution exhibits strong distinctions between the Au and GC electrodes. The voltammetric data on the Au electrode revealed a highly rechargeable ORR yielding LiO2 and Li2O2, which undergoes multiple cycles without electrode passivation.

Oxygen reduction and evolution reactions (ORR and OER) were also studied in ionic liquids containing other singly charged cations having a wide range of ionic radii, or charge densities. Specifically, ORR and OER mechanisms were studied using cyclic and rotating disk electrode voltammetry in the neat ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and 1-methyl-1-butyl-pyrrolidinium bis-(triflouromethanesulfonyl)imide (PYR14TFSI) and in their solutions containing LiTFSI, NaPF6, KPF6 and tetrabutylammonium hexafluorophosphate (TBAPF6). A strong correlation was found between the ORR products and the ionic charge density, including those of the ionic liquids. The observed trend is explained in terms of the Lewis acidity of the cation present in the electrolyte using an acidity scale created from 13C NMR chemical shifts and spin lattice relaxation (T1) times of 13C=O in solutions of these charged ions in propylene carbonate (PC). The ionic liquids lie in a continuum of a cascading Lewis acidity scale with respect to the charge density of alkali metal, IL and TBA cations with the result that the ORR products in ionic liquids and in organic electrolytes containing any conducting cations can be predicted on the basis of a general theory based on the Hard Soft Acid Base (HSAB) concept.

Conclusions and direction for future research are presented in Chapter 6.

Document Type

Dissertation

Rights Information

copyright 2012

Rights Holder

Christopher J. Allen


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