UC Irvine

Energy storage has been the biggest obstacle in the widespread adoption of renewable energy resources. The work presented in this thesis is aimed at developing and understanding the behavior of Niobium Pentoxide (Nb2O5) electrochemical energy storage devices. Nb2O5 is a Li+ intercalation metal oxide that is of current interest for lithium ion battery electrodes. In the first part of this thesis, electrophoretic deposition (ED) of Nb2O5 thin- films from aqueous NbOx colloidal solutions is described, which exhibits unusually high specific capacities for Li+ -based energy storage as a consequence of 70% porosity. The excellent energy storage metrics are attributed to augmentation of the faradaic capacity by high double-layer capacities enabled by the mesoporous structure of these films. In the second part of this thesis, the effect of Li+ intercalation on the conductivity of Nb2O5 has been explored. The electrical conductivity, σ, of battery and capacitor electrode materials is a factor determining the energy storage performance of these materials, but it is difficult to directly measure in-situ particularly for electrodeposited materials. Our approach exploits an array of nanoribbon of Nb2O5, fabricated using lithographically patterned nanoribbon electrodeposition (LPNE). σ of Nb2O5 nanoribbons is measured in-situ in a battery electrolyte as a function of the equilibrium potential and, separately, during repetitive lithiation/delithiation cycling. σ in the non-lithiated Nb2O5 is characteristic of semiconducting metal oxides, but it increases dramatically with lithiation. The last part of the thesis is aimed at uncovering the mechanism of capacity upon repetitive cycling for Nb2O5 based energy storage devices. Microscopy, spectroscopy and electrochemical characterization tools have been employed to gain insight into the electronic, structural, compositional and morphological evolution of Nb2O5 thin films as it undergoes thousands of cycles of charge-discharge. Overall, the work in this thesis elucidates a strategy towards fabrication of energy storage devices from materials that are difficult to electrodeposit using conventional redox reactions. Furthermore, it illustrates an approach to combine electrochemical and microscopy-based methods, to gain insights into the interactions between Li+ ions and the active material, during repetitive charge discharge cycling.