Effects of Sn doping on the structural and electrochemical properties of Li1.2Ni0.2Mn0.8O2 Li-rich cathode materials

Micron spherical Sn doping Li1.2Ni0.2Mn0.8O2 cathode materials are synthesized via a carbonate co-precipitation method. The crystal structure and morphology of the pristine and Sn-doping samples are characterized by a joint X-ray diffraction (XRD) Rietveld refinement method and scanning electron microscopy (SEM). It is proved that Sn successfully doped into the aimed materials. X-ray photoelectron spectroscopy (XPS) results demonstrate that the chemical state of Sn is +4. Appropriate doping contents of Sn are beneficial to improve the electrochemical performance, including the capacities and cycling stability. The sample with 1.0 mol% Sn delivers high initial discharge capacities and improved rate performance (212.6 mAh/g at 0.2C and 137.8 mAh/g at 5C, respectively). The capacity retention is 96.2% after 50 cycles at 0.2C. It is very stable for the doped sample with 2.0 mol% Sn after 160 cycles at 0.2C, the capacity retention is 98.5%. The discharge capacity decreases markedly with the Sn contents increase to 3.0 mol%, since Sn4+ can not participate in electrochemical reaction. Electrochemical impedance spectroscopy (EIS) results demonstrate that the charge transfer resistance of the samples with 1.0-2.0 mol% Sn-doping decreases obviously. It reveals that doping with appropriate Sn substitution not only improves the structure stability, but also increases the conductivity, therefore improves their rate capability and cycling stability. (C) 2015 Elsevier Ltd. All rights reserved.