We here present a photoassisted rechargeable Li-O 2 battery by integrating a g-C 3 N 4 photocatalyst to address the overpotential issue of conventional non-aqueous Li-O 2 batteries. The high charging overpotential of a Li-O 2 battery is compensated by the photovoltage, and finally an ultralow charging voltage of 1.9 V is achieved, which is much lower than that of any other conventional non-aqueous Li-O 2 batteries. It is also worth noting that the charging voltage is even much lower than the discharging voltage (B2.7 V), resulting in 142% energy efficiency (output electric energy/input electric energy, not including solar energy).Rechargeable Li-O 2 batteries, offering a high theoretical specific energy of B3560 W h kg À1 , 1 are extremely attractive for electric vehicles and have attracted tremendous attention in recent years. 2-6 However, significant challenges still remain to be addressed for practical applications. 7-11 A typical non-aqueous Li-O 2 battery consists of a Li anode, a Li + ion conducting organic electrolyte, and a porous cathode. Upon discharging, O 2 is reduced to insoluble lithium peroxide (Li 2 O 2 ) on the surface of the cathode, and the process is reversed upon charging. The overall charge/discharge reaction can be written as 2Li + + O 2 + 2e À 2 Li 2 O 2 . The thermodynamic standard potential for the formation and decomposition of Li 2 O 2 is 2.96 V versus Li + /Li 12 (all potentials in this work are referenced to Li + /Li). Unfortunately, the electrochemical decomposition of the insoluble and poorly conducting Li 2 O 2 requires a high charging voltage (as much as 4 V in some cases). [13][14][15][16][17] The high charging voltage leads to a low energy efficiency, hampering the practical application of Li-O 2 batteries. Besides, the electrochemical decomposition of other battery components may occur at high charging voltage.In order to lower the charging voltage, two approaches were generally used: (1) constructing highly active electrochemical catalysts, such as Au, RuO 2 , Co 3 O 4 etc., to lower the overpotential required for the decomposition of Li 2 O 2 ; 5,18-21 (2) utilizing dissolved redox mediators (RMs) as mobile charge carriers between the electrode surface and Li 2 O 2 . 22-25 For example, Chen et al. introduced a solution of tetrathiafulvalene in dimethyl sulfoxide 22 and Lim et al. proposed I À ions in tetraglyme (G4) as a dissolved RM. 23 But so far, the charging voltages still remain higher than 3.5 V despite these efforts. A very recent study showed that incorporation of a dye-sensitized TiO 2 photoelectrode greatly reduced the charging voltage of Li-O 2 batteries, 26 delivering a charging voltage of 2.72 V.In this work, employing graphitic carbon nitride (g-C 3 N 4 ) as a photocatalyst, we demonstrate a photoassisted chargeable Li-O 2 battery with an ultralow charging voltage of 1.9 V.The non-aqueous Li-O 2 battery has attracted tremendous attention in recent years due to its outstanding theoretical energy density. However, a number of issues prevent the practical applicat...