An ionic liquid-based electrolyte consisted of N-methyl-N-propylpyrrolidine bis(trifluoromethanesulfonyl)imide (Pyr(13)TFSI), sulfolane (SL) and ethyl methyl carbonate (EMC) as co-solvent, lithium difluoro(oxalate)borate (LiDFOB) as Li salt for cathode material LiNi0.8Co0.1Mn0.1O2 (NCM811) is investigated in this study. The electrolyte with ionic liquid (IL) has improved safety characteristics and a wide electrochemical window that can reach 4.8V (vs. Li/Li+). The Li/NCM811 half-cells with the electrolyte display outstanding electrochemical performance, achieving a discharge capacity and capacity retention of 166.1 mAh g(-1) and 97.6% after 100 cycles between 3.0 and 4.3V at 25 degrees C. This excellent electrochemical performance is due to a thin and compact cathode electrolyte interphase (CEI) film formed on the surface of the LiNi0.8Co0.1Mn0.1O2 electrode derived from the electrolyte with IL and reduction of lithium dendrite formation. The film inhibits the volume change of the material during the charge-discharge cycles and maintains the structural integrity of the cathode material, which enhances electrochemical performance of the Li/NCM811 half-cells. (C) 2019 The Electrochemical Society.