Thermogalvanic Harvesting With Thin-Film Li-Ion Materials: Experimental Reflections on Device Concepts

The Internet-of-Things (IoT) will require innovative solutions to enable power autonomy in miniaturized nodes. One possible strategy for these applications is to harvest energy using the thermogalvanic effect, which converts heat to electricity via an electrochemical reaction. In this work, three device concepts for thermogalvanic harvesting with thin-film Li-ion materials were considered, and a practical experiment demonstrating the operational limitations was presented for each approach. All demonstrations were executed using thin-film Li $_{4}$ Ti $_{5}$ O $_{12}$ (LTO) electrodes, which possess attractive thermogalvanic and kinetic properties. The first device concept was a thermogalvanic cell. This component harvests energy via the application and removal of a temperature difference between two identical LTO electrodes. In addition, a hybrid Thermally Regenerative Electrochemical Cycling (TREC) device was studied. Here, a cell with an LTO working electrode of variable temperature and a Li metal counter-electrode at constant temperature is charged at one LTO temperature and discharged at another temperature. The last concept was a thin-film TREC cell, which contains an LTO working electrode, a LiPON solid electrolyte, and a Li metal counter-electrode. Harvesting is accomplished by changing the temperature of the entire cell between the charge and discharge steps. By presenting an overview of the advantages and pitfalls of different device concepts, this work is a first step in the development of novel thermogalvanic harvesting components based on thin-film Li-ion materials.