Photo-energy conversion and storage in an aprotic Li-O2 battery.

A photo-involved Li-O-2 battery with carbon nitride (C3N4) is presented as a bifunctional photocatalyst to accelerate both oxygen reduction and evolution reactions. With illumination in a discharge process, photoelectrons generated in the conduction band (CB) of C3N4 are donated to O-2 for O-2(-), which undergoes a second electron reduction to O-2(2-) and gives the final product of Li2O2; in a reverse process, holes left behind in the valence band (VB) of C3N4 plus an applied lower voltage than the equilibrium drive the Li2O2 oxidation. The discharge voltage is significantly increased to 3.22 V, surpassing the thermodynamic limit of 2.96 V, and the charge voltage is reduced to 3.38 V. This leads to a record-high round-trip efficiency of 95.3 % and energy density increase of 23.0 % compared to that in the dark.