The effects of Ni doping on the performance of O3-lithium manganese oxide material

Li_0.7Li_1/6Mn_5/6]O₂ and Li_0.7Li_1/12Ni_1/12Mn_5/6]O₂ powders were synthesized by a sol-gel method. The powders had a typically rhombohedral layered O₃ structure. Both the samples were nanometer-sized powders and the size of Li_0.7Li_1/12Ni_1/12Mn_5/6]O₂ was smaller than that of Li_0.7Li_1/6Mn_5/6]O₂. The discharge curve shape of both the sample electrodes was almost equal to that of the layered structure. However, the electrode materials were transferred from layered to spinel structures with increasing the cycle number. Li/Li_0.7Li_1/6Mn_5/6]O₂ and Li_0.7Li_1/12Ni_1/12Mn_5/6]O₂ cells initially delivered a discharge capacity of 261 and 238 mAh/g, respectively. The capacities of Li/Li_0.7Li_1/6Mn_5/6]O₂ and Li_0.7 Li_1/12Ni_1/12Mn_5/6]O₂ after the 45th cycle were 174 and 221 mAh/g, respectively, corresponding to the retentions of 67% and 93%. The nanostructure of the synthesized powders seems to result in high initial discharge capacity as well as in the suppression of the discharge capacity fading by providing high surface area needed for Li ion reaction. In Ni doped-Li_0.7Li_1/12Ni_1/12Mn_5/6]O₂, the capacity fading was reduced by suppressing the oxidation state of Mn from 4⁺ to 3⁺ due to the role of Ni ion doped.