On discharge, the Li-O 2 battery can form a Li 2 O 2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li 2 O 2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the T he high theoretical specific energy of the rechargeable Li-O 2 battery has generated intense interest in the possibility of a practical device that could deliver energy storage significantly in excess of today's lithium-ion batteries 1-9 . However, major challenges hinder the development of such a technology 1-6,10-14 . Typically a Li-O 2 battery is composed of a lithium metal anode separated by an aprotic electrolyte solution from a porous O 2 cathode. The reaction at the cathode involves, on discharge, the reduction of O 2 to form Li 2 O 2 , with oxidation of the latter on charge. Growth of Li 2 O 2 on the cathode surface leads to low capacities, poor rates and early cell death [15][16][17] . In contrast, if Li 2 O 2 can be induced to grow in the electrolyte solution then high discharge capacities at relatively high rates and avoiding early cell death is possible 15 . It is clearly important to operate a Li-O 2 battery in which Li 2 O 2 grows in solution.A number of groups have elucidated the mechanism of O 2 reduction to Li 2 O 2 on discharge 15,16,[18][19][20][21] . The reduction proceeds through the following general steps: 22,[27][28][29][30] . High donor number salts have been shown to increase the capacity fourfold and reduce the discharge overpotential by ∼30-50 mV over low donor number salts 22 . Viologens 27,28 , phthalocyanines 29 and quinones 30 have been investigated as possible soluble reduction catalysts. Although the studies of such catalysts are important, in most cases there is little or no direct evidence demonstrating that they promote formation of Li 2 O 2 in solution and not on the electrode surface because they rely on electrochemical measurements alone. Yet past work on Li-O 2 batteries has shown how essential it is to provide more than electrochemical evidence in this field 31 . In some cases, soluble catalysts show an increase in discharge voltage (lower overpotential) as small as, for example, 40 mV (refs 28,29), which is very unlikely to be sufficient to shut off the direct reduction of O 2 to Li 2 O 2 , essential to stop detrimental Li 2 O 2 film formation. Also, none of the previous studies in low donor number solvents exhibited a significant increase in capacity on discharge at a relatively high rate, which is important for a successful Li-O 2 battery.Here we demonstrate that addition of DBBQ (2,5-di-tert-butyl-1,4-benzoquinone) to a weakly solvating (low donor number) electrolyte solution, LiTFSI in ether 22 , promotes O 2 reduction to Li 2 O 2 in solution while halving the discharge overpotential (increasing the discharge potential), suppressing the growth of a Li 2...