Rational Material Selection for Li-Ion-Based Thermogalvanic Harvesting Devices

Harvesting devices based on thermogalvanic principles, such as thermally regenerative electrochemical cycles (TREC), hold great promise for Internet of Things applications where autonomy is critical. One architecture that is particularly interesting for TREC cells with improved energy density, efficiency, and scalability is the thin-film Li-ion battery. In this work, a paradigm to guide the design of thin-film Li-ion-based TREC cells is established via three criteria for electrode selection. These requirements were distilled by translating the TREC principle into measurable material properties for Li-ion electrodes. More specifically, the identified criteria included a high thermogalvanic cell coefficient preferably exceeding 0.2 mV K–1, a weak dependence of the thermogalvanic coefficient on the lithiation state, and a favorable balance between the thermogalvanic cell coefficient and electrode kinetics as estimated from the lithiation overpotential. The material properties necessary to assess these criteria can all be obtained via a previously developed thermogalvanic characterization methodology. In the present work, this methodology was applied to a catalogue of five electrode materials, namely, LiMn2O4 (LMO), Li4Ti5O12 (LTO), LiFePO4 (LFP), anatase TiO2, and Cl-doped amorphous TiO2. The obtained results were organized into three separate blocks, each focused on a specific aspect, like electrolyte decomposition, nanoscaling effects, and kinetics, that warrants special consideration during thermogalvanic characterization. A material selection matrix was subsequently compiled by applying the aforementioned selection criteria to the characterization results. In this manner, LTO was identified as the most suitable candidate electrode, and recommendations for future investigations of thin-film Li-ion TREC devices could be delivered.