Photo-Assisted Zn-Iodine Battery via Bifunctional Cathode with Iodine Host and Solar Response Boost

The aqueous photo-assisted battery is considered an efficient means of converting and storing solar energy in one device. However, identifying a suitable photocathode with excellent iodine capture capabilities for photo-assisted Zn-iodine batteries still remains challenging. In this work, bifunctional BiOI is prepared as sole cathode material for a photo-assisted Zn-iodine battery while simultaneously realizing an iodine host and solar responsiveness. The as-presented BiOI with abundant vacancies offers highly reversible photo-assisted iodine redox reactions. Meanwhile, the dual reaction routes involving vacancy iodine storage and reversible two-steps iodine redox are confirmed by in/ex situ characterization techniques during the energy storage process. Consequently, the assembled battery exhibits areal capacity of 0.24 mAh cm−2 at 1 mA cm−2 with coulombic efficiency exceeding 96.5%. More impressively, benefiting from the wide visible light absorption of the BiOI cathode, the battery demonstrates a much enhanced specific areal capacity of 0.4 mAh cm−2 at 1 mA cm−2 under sun illumination, representing a remarkable increment of 60% compared to that in the dark environment. This work expands the utility of cathode materials for a photo-assisted Zn-iodine battery.