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.