Photothermal and Photoelectric Effects in Lithium-Ion Batteries: Mechanistic Insights and Performance Enhancement

Abstract

The growing demand for reliable, sustainable off-grid power solutions is especially significant for Internet of Things (IoT) devices. Solar energy, as a widely available renewable resource, has advanced energy-harvesting and storage technologies. Traditional solar-to-electricity setups rely on separate components, leading to larger device footprints and challenges with output voltage mismatches. This research explores an optimized, integrated system that combines light-harvesting and energy storage using a shared electrode, reducing device size and enhancing efficiency. The key innovation lies in the material selection for the shared electrode, balancing electrochemical performance and photoactivity. To start with, graphitic carbon nitride and bismuth vanadate were selected as potential photo-active materials for photo-enhanced batteries. However, they were proven to be not suitable for the application from their electro- chemical measurement due to the low capacity and degradation. Prussian blue analogues (PBAs) were then selected for their photothermal heating efficiency and compatibility as cath- odes in photothermal-enhanced Li-ion batteries. Electrochemical testing under illuminated and dark conditions demonstrated that light-induced heating boosted battery performance, increasing capacity by up to 38% at high current densities. EIS measurements further con- firmed reduced charge transfer resistance with illumination, underscoring the critical role of photothermal effects. A novel measurement technique, named impedance-based internal temperature estimation, clarified the impact of photothermal effects on battery performance. This study provides insights into how material properties and bias voltages can balance pho- tothermal and photo-generated charge effects, advancing our understanding of photo-induced processes. Furthermore, an in-depth examination of band alignment between the photoelectrode and counter electrode highlighted the mechanisms of charge transport, differentiating between photothermal and photoelectric effects through ultraviolet photoelectron spectroscopy (UPS) and UV-Vis spectroscopy in Li-ion batteries by using semiconducting metal oxide including anatase/rutile TiO2 and Fe2O3. Results indicated that photothermal effects dominated at the applying voltage which is below the energy gap between the conduction band minimum and Li plating/stripping potential, while a higher bias voltage activated photoelectric effects by band-bending at the interface, revealing the influence of band alignment on charge transport. Overall, this work illustrates that the processes taking place in photo-batteries are intricate, and it offers new electrochemical protocols and techniques to gain insight into the mechanisms that govern the changes in behaviour when illuminating photo-batteries. In conclusion, this work contributes valuable insights into integrating light-harvesting with energy storage systems, emphasizing the importance of material selection, photothermal effects, and precise measurement techniques. These findings support future advancements in photo-rechargeable batteries and sustainable energy technologies.