Distinguishing between Photothermal and Photoelectric Effects in Li-Ion Batteries.

Over the past decades, photo-enhanced batteries where light is used to improve the rate performance or recharge batteries have received increased attention in the academic community. However, the underlying mechanisms that contribute to performance enhancement in several photo-enhanced batteries are still under debate. For instance, photothermal effects, resulting from light absorption and subsequent conversion to heat through non-radiative relaxation, and photo-induced charge transfer, involving the generation and separation of electron-hole pairs under the illumination resulting in charge carrier transport, can be challenging to disentangle. This study aims to distinguish between the photothermal and photo-induced charge transfer in TiO and FeO as model systems because of their photoactivity and ability to store Li-ions. Using ultraviolet photoelectron spectroscopy (UPS) and UV-vis spectroscopy, we measure the band positions of these materials, and by a combination of different electrochemical processes, we demonstrate the transition from photothermal dominated to photoelectric effects in these materials. These results further illustrate the fact that different processes take place in photo-batteries, and this work provides a workflow to investigate these complex interactions.