Synthesis and Characterization of Nanostructured, Mixed-Valent Compounds for Electrochemical Energy Storage Devices

Electrical energy storage systems play a vital role in efficient use of electricity in microgrids or smartgrids to bridge the gaps between demand and supply, especially for renewable energy sources of intermittent and cyclic nature (e.g., solar- or wind-based electrical generation). In the transportation sector, the transition from the current hybrid electric vehicles to all-electric vehicles hinges critically on the development of electrical energy storage systems with dramatically improved energy and power density, durability, and reduced cost.;However, the performance of current electrical energy storage systems (both batteries and electrochemical capacitors) are not capable of meeting tomorrow's energy storage requirements for advanced transportation, commercial, and residential applications. This is because batteries often suffer from slow power delivery, limited life-time, and long charging time whereas electrochemical capacitors suffer from low energy density. While extensive efforts have been made to the development of novel electrode materials for innovative electrical energy storage devices, progress has been hindered by the lack of a profound understanding of the complex charge storage mechanism. Therefore, the main objective of this research is to develop novel electrode materials which can exhibit both high energy and power density with prolonged life-time and to gain a fundamental understanding of their charge storage mechanism.;First, this thesis describes the controlled synthesis of thin, conformal coating of nanostructured, mixed-valent manganese oxides onto porous carbon fiber papers and their application in electrochemical capacitors. The novel electrodes exhibited dramatically enhanced pseudocapacitive behavior in aqueous electrolytes, demonstrating an order of magnitude increase in energy density without a penalty for power density when normalized to the weight of active materials.;Additionally, this thesis outlines the characterization and correlation of the composition, structure, and morphology of electrodes with their electrochemical performance, employing both in-situ and ex-situ synchrotron-enabled X-ray analysis (diffraction and absorption), and advanced electron microscopy.;Finally, the thesis presents new insights into the enhanced pseudocapacitance and the charge storage mechanism obtained by in-situ synchrotron-based X-ray absorption spectroscopy (XAS). The enhanced performance is attributed to the unique mixed-valent (2+, 3+, and 4+) manganese oxides of porous nano-architectures, which may facilitate rapid mass transport and enhance surface double-layer capacitance, while promoting facile redox reactions associated with charge storage believed to be contributed by both Mn and O sites, leading to unprecedented levels of energy and power density.;In particular, the implication that significant amount of charge may be stored at the O sites of nanostructured MnOx offers potential for taking advantages of these new energy storage mechanisms (in addition to those associated with redox reactions of cations) in rational design of a new-generation pseudocapacitor materials.