Remarkably Improved Electrochemical Performance of Li- and Mn-Rich Cathodes upon Substitution of Mn with Ni

Li- and Mn-rich transition-metal oxides of layered structure are promising cathodes for Li-ion batteries because of their high capacity values, ≥250 mAh g^-1. These cathodes suffer from capacity fading and discharge voltage decay upon prolonged cycling to potential higher than 4.5 V. Most of these Li- and Mn-rich cathodes contain Ni in a 2+ oxidation state. The fine details of the composition of these materials may be critically important in determining their performance. In the present study, we used Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ as the reference cathode composition in which Mn ions are substituted by Ni ions so that their average oxidation state in Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ could change from 2+ to 3+. Upon substitution of Mn with Ni, the specific capacity decreases but, in turn, an impressive stability was gained, about 95% capacity retention after 150 cycles, compared to 77% capacity retention for Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes when cycled at a C/5 rate. Also, a higher average discharge voltage of 3.7 V is obtained for Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ cathodes, which decreases to 3.5 V after 150 cycles, while the voltage fading of cathodes comprising the reference material is more pronounced. The Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ cathodes also demonstrate higher rate capability compared to the reference Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. These results clearly indicate the importance of the fine composition of cathode materials containing the five elements Li, Mn, Ni, Co, and O. The present study should encourage rigorous optimization efforts related to the fine composition of these cathode materials, before external means such as doping and coating are applied.