Preparation, electrochemical properties, and cycle mechanism of Li _1-x Fe_0.8Ni_0.2O₂-Li _x MnO₂ (Mn/(Fe+Ni+Mn)=0.8) material

A new type of Li_1-x Fe_0.8Ni_0.2O ₂-Li _x MnO₂ (Mn/(Fe+Ni+Mn)=0.8) material was synthesized at 350 °C in an air atmosphere by a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li _1-x FeO₂-Li _x MnO₂ ((Fe+Ni+Mn)=0.8) with much reduced impurity peaks. It was composed of many large particles of about 500-600 nm and small particles of about 100-200 nm, which were distributed among the larger particles. The Li/Li_1-x Fe_0.8Ni _0.2O₂-Li _x MnO₂ cell showed a high initial discharge capacity above 192 mAh/g, which was higher than that of the parent Li/Li_1-x FeO₂-Li _x MnO₂ (186 mAh/g). This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles. We suggest a unique role of doped nickel ion in the Li/Li_1-x Fe _0.8Ni_0.2O₂-Li _x MnO₂ cell, which results in the increased initial discharge capacity from the redox reaction of Ni²⁺/Ni³⁺ between 2.0 and 1.5 V region.