Novel High Voltage Electrodes for Li-ion Batteries
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An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family.