Photoactive cobalt-bipyridine electron mediator promotes reversible Li2O2 formation in irradiated Li-O2 battery

Photo-assisted charge/discharge has witnessed marked success in Li-O2 batteries (LOBs) to overcome the sluggish kinetics of triphasic reactions at the cathode, but challenges remain to develop high-efficiency but low-cost photocathode catalysts. Herein, a photoactive electron mediator is applied, for the first time, as bifunctional photocathode catalyst in solar-assisted LOBs. The facilely synthesized tris(2,2-bipyridine) cobalt(II) complex (Co-bpy) manifests good electrocatalytic activity in dark and high photoactivity under illumination, concertedly affording an superior charge/discharge voltage gap of 0.12 V and round-trip efficiency of 97%. Moreover, a significantly enhanced charge/discharge reversibility was attained under illumination and attributed to the light-induced amorphization of Li2O2 that further extends the cycle life. A two-circuit model is proposed to rationalize the better reduction of charge/discharge polarization at lower operating current density. This work not only extends the family of photocathode catalysts to small-molecule complexes, but also offers new insights into the mechanism of photoenergy storage in LOBs.