Semiconducting Metal–Organic Polymer Nanosheets for a Photoinvolved Li–O2 Battery under Visible Light

Li-O2 batteries are considered the ultimate energy storage technology for their potential to store large amounts of electrical energy in a cost-effective and simple platform. Large overpotentials for the formation and oxidation of Li2O2 during discharging and charging have thus far confined this technology to a scientific curiosity. Herein, we consider the role of catalytic intervention in the reversibility of the cathode reactions and find that semiconducting metal-organic polymer nanosheets composed of cobalt-tetramino-benzoquinone (Co-TABQ) function as a bifunctional catalyst that facilitates the kinetics of the cathode reactions under visible light. Upon discharging, we report that O2 is first adsorbed on the Co atoms of Co-TABQ and accepts electrons under illumination from the dz2 and dxz orbitals of Co atoms in the π2p* orbitals, which facilitates reduction to LiO2. The LiO2 is further shown to undergo a second reduction to the discharge product of Li2O2. In the reverse charge, the holes generated in the dz2 orbitals of Co are mobilized under the action of the applied voltage to enable the fast decomposition of Li2O2 to O2 and Li+. Under illumination, the Li-O2 battery exhibits respective discharge and charge voltages of 3.12 and 3.32 V for a round-trip efficiency of 94.0%. Our findings imply that the orbital interaction of metal ions with ligands in Co-TABQ nanosheets dictates the light harvesting and oxygen electrocatalysis for the Li-O2 battery.