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 d_z² and d_xz orbitals of Co atoms in the π_2p^* 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 d_z² 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.