Ionic liquids composed of N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME), N-methyl-N-methoxyethylpiperidinium (PP1.1o2) cations functionalized with ethers, N-methyl-N-propylpiperidinium (PP13), and N-butyl-N-methylpyrrolidinium (P14) cations and the bis(trifluoromethanesulfonyl)amide (TFSA) anion are investigated for application as electrolytes in non-aqueous lithium–oxygen (Li–O2) batteries. The PP13–TFSA, P14–TFSA and DEME–TFSA ionic liquids have high oxygen radical stability. A comparison of the lithium supply capacity measured using pulse-gradient spin-echo NMR for 7Li nuclei and the oxygen supply capacity measured using electrochemical methods indicates that the oxygen supply is the rate-limiting step for the generation of lithium–oxygen compounds (LiO x ) in these ionic liquids with supporting electrolytes. The DEME–TFSA system has the highest LiO x generation activity among the ionic liquid systems examined. We demonstrate the improved performance (output power, discharge–charge capacity and coulombic efficiency) of a Li–O2 battery using the DEME–TFSA system compared with that using the PP13–TFSA system. The improvements observed for the DEME–TFSA system are attributed to the high LiO x generation properties and lithium ion supply.