Novel ionic liquids (ILs), which were functionalized with ether oxygens to suppress crystallization and with bis(fluorosulfonyl)imide (FSI) anions to improve ionic conductivity, were synthesized and compared. Electrolyte mixtures were prepared by adding LiFSI to N-ethoxyethyl-N-methylpiperidinium FSI ([P1,2O2][FSI]) and N-ethoxyethyl-N-methylmorpholinium FSI ([M1,2O2][FSI]) in ratio of 1:9 (mol/mol). The electrolyte mixtures were found to exhibit a glass transition temperature between −71 and −93 °C and a decomposition temperature higher than 266 °C. Between these temperatures, the electrolytes were ion conductive and thermally stable liquids. [P1,2O2][FSI]-LiFSI exhibited a wider electrochemical stability window and possessed an ability to form solid electrolyte interfaces having a lower resistance compared to [M1,2O2][FSI]-LiFSI. Based on these results, the Li | LiFePO4 cells containing [P1,2O2][FSI]-LiFSI as electrolytes were assembled and found to possess a high capacity, reaching to the nominal capacity 150 mAh g−1 at C/10. Moreover, the cell retained flat potential profiles throughout 50 cycles.