Electrochemical performance of Sn-doped cobalt-free Li1.15Ni0.27Mn0.58-xSnxO2 cathode material for Li-ion batteries

Abstract

In this study, a series of cobalt-free Sn-doped cathode materials of Li1.15Ni0.27Mn0.58-xSnxO2 are prepared by the solvothermal method for the first time. The structural characterization of the material reveals that all the samples consist of hexagonal layered structural LiMO2 (M = Mn/Ni), monoclinic-layered structural Li2MnO3, and cubic spinel structural LiMn2O4. Moreover, the interplanar spacing of the layered structure increases in the material, and the electrochemical impedance of the material declines by Sn doping. As a result, all Sn-doped samples exhibit better electrochemical performance than pristine Li1.15Ni0.27Mn0.58O2 materials. Among them, Li1.15Ni0.27Mn0.56Sn0.02O2 delivers comprehensively improved electrochemical performance. The initial coulombic efficiency of the Li1.15Ni0.27Mn0.56Sn0.02O2 sample is 84.4%, which is nearly 10% higher than the pristine material, and Li1.15Ni0.27Mn0.56Sn0.02O2 exhibits an initial discharge specific capacity of 260.8 mAh/g at a current density of 0.1 C, and the capacity retention after first 100 cycles at 1 C reached 94.67%. Rate capability of Li1.15Ni0.27Mn0.58-xSnxO2 is significantly improved by Sn doping. The specific discharge capacity of the Li1.15Ni0.27Mn0.56Sn0.02O2 sample at 5 C is two times higher than that of Li1.15Ni0.27Mn0.58O2. Moreover, the Li1.15Ni0.27Mn0.56Sn0.02O2 material can still maintain a discharge capacity of 220 mAh/g when the current density returns to 0.1 C after a large current density cycling process. These results show that the proper amount of Sn doping can effectively improve the electrochemical performance of Li1.15Ni0.27Mn0.58O2, due to the fact that Sn ions have larger ionic radii than the transition metal (Mn/Ni) ions and Sn can partially replace the transition metal element ions in the layered structure, thereby expanding the lithium ion diffusion channel and inhibiting instability of the material structure during the cycle. However, an excessive amount of Sn (x > 0.03) generates a Li2SnO3 impurity in the material, resulting in deterioration of material properties.