The effects of ionic liquid (IL) N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Py14TFSI) based electrolyte on the electrochemical performance of cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O-2 have been investigated. The results of thermogravimetric analysis (TGA), flammability and differential scanning calorimetry (DSC) tests indicate that Py14TFSI addition enhances thermal stability of the electrolyte and reduces the safety concern of Li-ion battery. Electrochemical measurements demonstrate that the cathode material shows good electrochemical performance in Py14TFSI-added electrolyte. The cathode material is able to deliver high initial discharge capacity of 250 mAh g(-1) in electrolyte with Py14TFSI content up to 80% at 01 C. In addition, the cathode material delivers less initial irreversible capacity loss and higher initial coulombic efficiency in electrolyte with higher Py14TFSI content. However, increasing Py14TFSI content in the electrolyte affects rate capability of the cathode material distinctively. With 60% Py14TFSI-added electrolyte, Li[Li0.2Mn0.54Ni0.13Co0.13]O-2 shows better cycling stability with a capacity retention of 84.4% after 150 cycles at 1.0C than that in IL free electrolyte. The superior cycling performance of the cathode material cycled in Py14TFSI-added electrolyte is mainly ascribed to the formation of stable electrode/electrolyte interfaces, based on the results of scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and electrochemical impedance spectroscopy (EIS) investigations. (C) 2011 Elsevier Ltd. All rights reserved.