Hexagonal-Phase\nBorophene Nanosheets Bifunctional\nPhotoelectrode Capable of Driving High-Performance Solar-Assisted\nLi–O<sub>2</sub> Battery

Inserting a photoelectrode into the cathode side of the\nLi–O<sub>2</sub> battery has been considered as one of the\neffective ways\nto improve the reaction kinetics of Li<sub>2</sub>O<sub>2</sub> and\nreduce the discharge/charge overpotential. Thus, the development of\ncompatible bifunctional photoelectrode is of great significance for\nthe realization of a solar-assisted Li–O<sub>2</sub> battery.\nHerein, hexagonal-phase borophene nanosheets (HPBNs) were prepared\nby I<sub>2</sub> oxidizing layered MgB<sub>2</sub> in a mixture of\nCH<sub>3</sub>CN and HCl and followed by ultrasonic exfoliation. Based\non the I<sub>2</sub> oxidizing and chelating action along with precise\nthermodynamic regulation, 94% Mg<sup>2+</sup> ions were maximally\nextracted from MgB<sub>2</sub> for achieving hexagonal-phase borophene\nwith typical parallel planes of (104) and (208). The obtained HPBNs\nwere assigned to direct band gap semiconductor with a band gap of\n2.61 eV enabling massive visible light absorption and displayed applicable\npotentials for Li<sub>2</sub>O<sub>2</sub>/O<sub>2</sub> redox. The\nassembled photoassisted Li–O<sub>2</sub> battery exhibited\na high round-efficiency of 98% and a long cycle and superior rate\nperformance at 1 mA cm<sup>–2</sup>. It is of particular concern\nthat the HPBNs-based Li–O<sub>2</sub> battery be enabled to\noperate under outdoor solar along with a comparable performance of\nphotoassisted Li–O<sub>2</sub> battery under Xe lamp. This\nwork could provide promising insight for accelerating applications\nof bifunctional photoelectrodes in solar-assisted Li–O<sub>2</sub> batteries.