Development of a Method for Thermogalvanic Characterization of Li-Ion Electrodes: A Case Study in Thin-Film Anatase TiO2

Abstract

The Internet-of-Things will require innovative solutions to power wireless or remote sensor nodes, for instance, via harvesting heat from the environment to produce electricity. Thermogalvanic and thermally regenerative electrochemical cycle (TREC) cells are an interesting approach for such systems due to their high temperature responses. Both of these strategies rely on the temperature dependence of an electrochemical equilibrium potential, i.e., the thermogalvanic effect. However, whereas thermogalvanic cells operate under a fixed temperature gradient, TREC harvesting is instead accomplished by charging and discharging an electrochemical cell at different temperatures. The use of thin-film Li-ion batteries in these devices can enable great advantages in terms of performance and scalability. However, existing approaches to determine thermogalvanic coefficients in Li-ion materials are not suitable as screening techniques that can provide reliable coefficient values for device design. This work presents a novel methodology to characterize thermogalvanic profiles, i.e., the dependence of the thermogalvanic coefficient on the lithiation state, for Li-ion electrodes. To this end, a thermogalvanic cell featuring thin-film electrodes and an electrochemically controllable lithiation state was used and operated with an iterative procedure. When preventing charge difference accumulation via compositional leveling, the presented method enabled precise and high-resolution measurements of the thermogalvanic profile. Validation of the methodology was performed by measuring the profile of thin-film anatase TiO2, a commonly studied Li-ion electrode material, and demonstrating it to be in excellent agreement with the accepted phase behavior. Moreover, the identification of nanoscaling effects in this profile highlighted the strength of this approach. This work may open the door to further research in the field of thermogalvanic harvesting using Li-ion materials for the Internet-of Things.