Ether-functionalized ionic liquid electrolytes for lithium-air batteries

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 (LiOx) in these ionic liquids with supporting electrolytes. The DEME-TFSA system has the highest LiOx 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 LiOx generation properties and lithium ion supply.