| Summary: | The rapid emergence of the Internet-of-Things (IoT) is driving the demand for chipbased self-powered sensors that require energy harvesters and energy storage devices, i.e. “thin film energy devices”, as key components.
The first section of this thesis introduces the working principle of a new type of thermal energy harvester, a “Multi-cell Thermogalvanic System” (MTS), that provides an alternative to other thermal energy harvesters that cannot be miniaturized or require materials that cannot be used in chip-based IOT devices. Using coin cells, several proofs of concept showed that MTS devices have efficiencies that are comparable to other thermal energy harvesters and have sufficient energy and power output for IOT devices, without requiring large heat sinks and with less stringent constraints on materials selection. It is also shown that by making several improvements in materials and processing methods, MTS devices can be used as thin film energy harvesters in chip-based IoT sensors.
The second section of this thesis focuses on the mechanisms of cyclic lithiation and delithiation of RuO2. RuO2 is a candidate cathode material for next-generation thin film lithium ion batteries (TF-LIBs), due to its relatively large capacity (~5x LiCoO2) and its very good cyclability and rate capability, as well as compatibility with integration with silicon-based microelectronic circuits (all-room-temperature processing). Like other electrode materials that store Li through reversible conversion reactions, RuO2 was found to have a relatively large voltage hysteresis, which limits its cycling energy efficiency. Investigation of the mechanism of this reaction provided insights for further optimization of RuO2 for TF-LIBs. The methods developed in this study can also be used to investigate other high-capacity conversion-reaction electrode materials.
The third section demonstrates a method for improving the mechanical stability of RuO2 thin films by making arrays of lithographically-patterned notches within them. Additionally, it was found that this approach can be used to form regular arrays of channels with widths that can be modulated by the state of lithiation and that can be reversibly opened and closed by delithiation and re-lithiation. Therefore, this method may also be applied to microfluidic devices or sensors.
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