Influence of coated MnO2 content on the electrochemical performance of Li1.2Ni0.2Mn0.6O2 cathodes

Layered cathode material Li1.2Ni0.2Mn0.6O2 has been synthesized using a coprecipitation method and coated by MnO2 with varying amounts (1, 3, 5, and 9 wt%). The physical properties and electrochemical performances of the materials are characterized by XRD, SEM, charge/discharge tests, cycle life, and rate capability tests. XRD patterns show that the pristine and coated Li1.2Ni0.2Mn0.6O2 powders exhibit layered structure. The discharge capacities and coulombic efficiencies of Li1.2Ni0.2Mn0.6O2 in the first cycle have been improved and increase with the increasing content of coated MnO2. The 9 wt% MnO2-coated Li1.2Ni0.2Mn0.6O2 delivers 287 mAhg(-1) for the first discharge capacity and 86.7 % for the first coulombic efficiency compared with 228 mAhg(-1) and 65.9 % for pristine Li1.2Ni0.2Mn0.6O2. However, the 5 wt% MnO2-coated Li1.2Ni0.2Mn0.6O2 shows the best capacity retention (99.9 % for 50 cycles) and rate capability (88.6 mAhg(-1) at 10 C), while the pristine Li1.2Ni0.2Mn0.6O2 only shows 91.5 % for 50 cycles and 25.3 mAhg(-1) at 10 C. The charge/discharge curves and differential capacity vs. voltage (dQ/dV) curves show that the coated MnO2 reacts with Li+ during the charge and discharge process, which is responsible for higher discharge capacity after coating. Electrochemical impedance spectroscopy results show that the R (ct) of Li1.2Ni0.2Mn0.6O2 electrode decreases after coating, which is responsible for superior rate capability.