Effect of Mn content on the microstructure and electrochemical performance of LiNi0.75-xCo0.25MnxO2 cathode materials

Newly developed LiNi0.75-xCo0.25MnxO2 (0.1 <= x <= 0.25) cathode materials were successfully synthesized by mixing Ni0.75-xCo0.25Mnx (OH)(2) and Li2CO3 via the solid-state reaction followed by heating to elevated temperatures. X-ray diffraction patterns showed that the samples calcined at 900 C exhibited a typical hexagonal alpha-NaFeO2 structure without any other impurities present. The ratio of peak intensities of (003) to (104) decreased with the increasing Mn content. The scanning electron microscopy micrograph revealed uniform primary particle size, and the small primary particles agglomerated to form the secondary ones with size distribution in the range of 6-9 mu m. The X-ray photoelectron spectroscopy measurement indicated that the valence of Ni ions was a mixture of 2+ and 3+. The amount of Mn substituted for Ni increased proportional to the ratio of Ni2+/Ni3+ in the samples was increased. Besides, for dynamic equilibrium (Ni2+ + Mn4+ <-> Ni3+ + Mn3+), small amount Mn4+ was reduced to Mn3+. After cycling, the cathode materials showed good electrochemical performance, with initial discharge capacity around 170 mAh/g at 0.1 C rate in the voltage range of 3-4.3 V. At a high rate of 1 C, 85% efficiency was still attainable in all samples. (c) 2005 The Electrochemical Society.