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.