Graduate Thesis Or Dissertation
 

New Graphite Intercalation Chemistry with Alkaline Earth Metals

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  • Novel graphite intercalation compounds (GICs) and associated chemistry have been explored, specifically: (1) containing alkyl- substituted imidazolium cations; (2) a series containing alkaline earth metal (Mg, Ca, Sr, Ba) cations with ethylenediamine (en) co-intercalates; (3) showing reversible intragallery M-M bonding and (4) via electrochemical intercalation of Mg cations with potential application as Mg-ion battery anodes. New GICs containing 1-alkyl-2,3-dimethyl imidazolium cations (Im1-1-a, a=alkyl length) are obtained from [Na(en)1.0]C15 by cation exchange. Exchange reactions occur rapidly, but require a higher temperature than for substituted alkylammoniums. Powder X-ray diffraction, thermogravimetry and structural modeling indicate that [Im1-1-4]C47 is a stage-2 GIC with 0.36 nm monolayer galleries and cations oriented parallel to graphene sheets. [Im1-1-12]C44 is a stage-1 GIC with a gallery expansion of 0.40 nm. The lower sheet charge density for [Im1-1-12]C44 is commensurate with its larger dimensions. Imidazolium cations without alkyl substitution at the imidazolium ring C2 do not form stable GICs by this route. The first structural and compositional details of a low-stage graphite interaction compound (GIC) containing Mg are reported, with the GIC obtained by combining magnesium metal and graphite powder in en at 100 °C under an inert atmosphere. Thermal analyses indicate the bottle-green stage 1 product has a composition of [Mg(en)1.0]C13. X-ray diffraction shows a c-axis expansion of 0.55 nm, indicating the presence of intercalate monolayers with the en co-intercalate oriented perpendicular to the encasing graphene layers. Redox titration indicates two electrons are transferred per Mg. A structural model is proposed with dimeric [Mg2(en)2]2+ intercalate species. A series of new ternary GICs containing alkaline earth metal cations (M=Mg, Ca, Sr, Ba) and ethylenediamine (en) are reported. These GICs are deep blue to green in color and can be prepared as phase-pure compounds by the direct reaction of graphite powder with the metal in liquid en at mild temperatures (25-100 °C) under an inert atmosphere. X-ray diffraction and thermal analyses were employed to determine the structural and compositional details. [Mg2(en)2.0]C26 and [Ba2(en)2.0]C34 can be obtained as stage-1 GICs with gallery expansions of 0.55 and 0.46 nm, respectively, indicating the presence of intercalate monolayers with en co-intercalates oriented perpendicular to the encasing graphene layers. Reactions with Ca and Sr metals form [Ca(en)2.0]C26 and [Sr(en)2.0]C22, which are stage-1 GICs with intercalate bilayers and gallery expansions of 0.76 nm. Titration indicates that each metal intercalate is associated with a 2-electron reduction process. Details on the effects of reaction time, temperature, and starting stoichiometry on reaction rates and product staging are reported. The alkaline earth metals (M=Mg, Ca, Sr, and Ba) exhibit a +2 oxidation state in nearly all known stable compounds, but M(I) dimeric complexes with M-M bonding, [M2(en)2]2+, of all these metals can be stabilized within the galleries of donor-type GICs. These metals can also form GICs with more conventional metal (II) ion complexes, [M(en)2]2+. We report here facile interconversion between dimeric-M(I) and monomeric-M(II) intercalates upon the addition/removal of en. Thermogravimetry, powder X-ray diffraction, and pair distribution function analysis of total scattering data support the presence of either [M2(en)2]2+ or [M(en)2]2+ guests. This phase conversion requires coupling graphene and metal redox centers, with associated reversible M-M bond formation within graphene galleries. This chemistry allows the facile isolation of unusual oxidation states, reveals M0 →M2+ reaction pathways, and present new opportunities in the design of hybrid conversion / intercalation materials for applications such as charge-storage. Following on these chemical syntheses, we have further explored the electrochemical intercalation reactions for Mg-en complexes in graphite. Using an Mg(SO3CF3)2 / en electrolyte, galvanostatic charge and discharge (GCD) profiles show capacities of 810 and 455 mAh/g during the first reduction and oxidation steps, respectively. After this formation cycle, approx. 105 mAh/g capacity can be retained for over 40 cycles with a columbic efficiency of >93%. Ex-situ X-ray diffraction confirms that stage-1 GICs are produced at the fully reduced state. Cyclic voltammetry (CV) and first-cycle differential capacity measurements (dQ/dV) both show that four redox couples arise in first charge/discharge process. Very large polarizations (2.2V) are observed, suggesting that there may be changes in Mg-N bonding in this redox chemistry. In summary, stage-1 Mg GICs can be obtained by electrochemical intercalation - these reactions deliver a capacity of 455 mAh/g on initial discharge, and show longer cycling of 105 mAh/g. Ex-situ XRD studies confirm the GIC phases formed are related to those previously reported by chemical reduction. This work may provide a new paradigm toward charge storage in intercalation electrodes.
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  • Pending Publication
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  • 2020-07-27 to 2021-08-28

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