Photo-energy Conversion and Storage in an Aprotic Li-O2 Battery.

A photo-involved Li-O2 battery with carbon nitride (C3 N4 ) 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 C3 N4 are donated to O2 for O2 - , which undergoes a second electron reduction to O2 2- and gives the final product of Li2 O2 ; in a reverse process, holes left behind in the valence band (VB) of C3 N4 plus an applied lower voltage than the equilibrium drive the Li2 O2 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.