Influence of coated MnO₂ content on the electrochemical performance of Li_1.2Ni_0.2Mn_0.6O₂ cathodes

Abstract

Layered cathode material Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.2Ni_0.2Mn_0.6O₂ powders exhibit layered structure. The discharge capacities and coulombic efficiencies of Li_1.2Ni_0.2Mn_0.6O₂ in the first cycle have been improved and increase with the increasing content of coated MnO₂. The 9 wt% MnO₂-coated Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.2Ni_0.2Mn_0.6O₂. However, the 5 wt% MnO₂-coated Li_1.2Ni_0.2Mn_0.6O₂ shows the best capacity retention (99.9 % for 50 cycles) and rate capability (88.6 mAhg⁻¹ at 10 C), while the pristine Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.2Ni_0.2Mn_0.6O₂ electrode decreases after coating, which is responsible for superior rate capability.