Improved electrochemical performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material by fluorine incorporation

High capacity cathode materials Li[Li0.2Mn0.54Ni0.13Co0.13]O2-xFx (x = 0, 0.05 and 0.10) have been synthesized by a sol-gel method using NH4F as F source. The effects of fluorine content on the structure, morphology and electrochemical performance of the Li[Li0.2Mn0.54Ni0.13Co0.13]O2-xFx have been extensively studied. With fluorine doping, cycling stability of Li[Li0.2Mn0.54Ni0.13Co0.13]O2-xFx is significantly improved because of the stabilization of the host structure. Li[Li0.2Mn0.54Ni0.13Co0.13]O1.95F0.05 shows a capacity retention of 88.1% after 50 cycles at 0.2 C at room temperature, much higher than that of 72.4% for pristine one. The improvement mechanism of fluorine doping has been investigated by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that fluorine incorporation stabilizes the electrode/electrolyte interface by suppressing the formation of poorly conducting LiF in the SEI layer and thus maintains stable interfacial resistances. As compared to the enhanced material structure, the stabilized electrode/electrolyte interface is the primary factor contributing to the improved electrochemical performance. In addition, the thermal stability of fully delithiated electrode is also greatly improved by fluorine doping. (C) 2013 Elsevier Ltd. All rights reserved.