Redefining the Functional Boundary of the Photothermal Effect in Flexible Energy Storage: From Dominant Driver in Supercapacitors to Kinetic Modulator in Batteries

Flexible energy-storage devices are key components of wearable electronics, yet they suffer from restricted ion transport, sluggish interfacial kinetics, and capacity loss under low-temperature, deformed, and complex service conditions. Photothermal effects, which convert light into localized heat in situ, offer a lightweight, self-powered alternative to external heating. Unlike previous reviews that treat photothermal behavior as an accompanying phenomenon of photovoltaic or photoelectrochemical processes, this review establishes the photothermal effect as an independent core mechanism and, for the first time, defines its functional boundary across device types. We show that the photothermal effect acts as a dominant enhancement mechanism in supercapacitors, where interfacial ion transport is rate-limiting. It directly regulates conductivity, polarization, and pseudocapacitive kinetics, typically restoring low-temperature capacitance by approximately 2-5 fold within seconds of illumination. In batteries, by contrast, it serves mainly as an auxiliary kinetic modulator bounded by bulk diffusion, phase transitions, and interfacial chemistry, generally yielding more modest (approximately 20%-50%) low-temperature improvements over minutes. Building on this boundary, we review key photothermal materials, device architectures, and failure mechanisms, and propose standardized protocols coupling illumination, temperature, deformation, and cycling. This framework aims to guide photothermal flexible energy storage from proof-of-concept toward all-weather wearable applications.