Hexagonal-Phase Borophene Nanosheets Bifunctional Photoelectrode Capable of Driving High-Performance Solar-Assisted Li–O2 Battery

Inserting a photoelectrode into the cathode side of the Li–O2 battery has been considered as one of the effective ways to improve the reaction kinetics of Li2O2 and reduce the discharge/charge overpotential. Thus, the development of compatible bifunctional photoelectrode is of great significance for the realization of a solar-assisted Li–O2 battery. Herein, hexagonal-phase borophene nanosheets (HPBNs) were prepared by I2 oxidizing layered MgB2 in a mixture of CH3CN and HCl and followed by ultrasonic exfoliation. Based on the I2 oxidizing and chelating action along with precise thermodynamic regulation, 94% Mg2+ ions were maximally extracted from MgB2 for achieving hexagonal-phase borophene with typical parallel planes of (104) and (208). The obtained HPBNs were assigned to direct band gap semiconductor with a band gap of 2.61 eV enabling massive visible light absorption and displayed applicable potentials for Li2O2/O2 redox. The assembled photoassisted Li–O2 battery exhibited a high round-efficiency of 98% and a long cycle and superior rate performance at 1 mA cm–2. It is of particular concern that the HPBNs-based Li–O2 battery be enabled to operate under outdoor solar along with a comparable performance of photoassisted Li–O2 battery under Xe lamp. This work could provide promising insight for accelerating applications of bifunctional photoelectrodes in solar-assisted Li–O2 batteries.