The effect of quaternary ammonium on discharge characteristic of Li/O 2 cells was studied by using Super-P carbon as air cathode, a 0.2 mol kg −1 LiSO 3CF 3 1:3 (wt.) PC/DME solution as baseline electrolyte, and tetrabutylammonium triflate (NBu 4SO 3CF 3) as an electrolyte additive or a co-salt. Results show that Li/O 2 cells can run normally in an electrolyte with NBu 4SO 3CF 3 as the sole conductive salt. However, such cells suffer lower voltage and capacity as compared with those using the lithium ionic baseline electrolyte. This is due to the larger molar volume of quaternary ammonium cation, which results in less deposition of oxygen reduction products on the surface of carbon. When used as an electrolyte additive or a co-salt, the ammonium is shown to increase capacity of Li/O 2 cells. The plot of differential capacity versus cell voltage shows that the Li/O 2 cell with ammonium added has broad and scatted differential capacity peaks between the voltages of two reactions of “2Li + O 2 → Li 2O 2” and “2Li + Li 2O 2 → 2Li 2O”. This phenomenon can be attributed to the phase transfer catalysis (PTC) property of quaternary ammonium on the second reaction. Due to inverse effects of the cation geometric volume and the PTC property of ammonium ions on the discharge capacity, there is an optimum range for the concentration of ammonium. It is shown that the addition of NBu 4SO 3CF 3 increases discharge capacity of Li/O 2 cell only when its concentration is in a range from 5 mol% to 50 mol% vs. the total of Li/ammonium mixed salt, and that the optimum concentration is about 5 mol%. In this work we show that the addition of 5 mol% NBu 4SO 3CF 3 into the baseline electrolyte can increase discharge capacity of a Li/O 2 cell from 732 mAh g −1 to 1068 mAh g −1 (in reference to the weight of Super-P carbon) when the cell is discharged at 0.2 mA cm −2.