Shyang-Roeng Sheen

Effect of Mn Content on the Microstructure and Electrochemical Performance of LiNi0.75 − x Co0.25Mn x O2 Cathode Materials

Newly developed LiNi 0 . 7 5 - x Co 0 . 2 5 Mn x O 2 (0.1 ≤ x ≤ 0.25) cathode materials were successfully synthesized by mixing Ni 0 . 7 5 - x Co 0 . 2 5 Mn x (OH) 2 and Li 2 CO 3 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 α-NaFeO 2 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 μm. 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 Ni 2 + /Ni 3 + in the samples was increased. Besides, for dynamic equilibrium (Ni 2 + + Mn 4 + ↔ Ni 3 + Mn 3 + ), small amount Mn 4 + was reduced to Mn 3 + . 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.

Improving the Electrochemical Performance of LiCoO2 Cathode by Nanocrystalline ZnO Coating

The cycle life of LiCoO 2 cathodes cycled at a high cutoff voltage can be largely improved by surface modification with ZnO coating. The bright-field transmission electron microscopy (TEM) images and selected area diffraction patterns of as-coated LiCoO 2 particles reveal the existence of continuous ZnO films with a 10-nm thickness deposited on LiCoO 2 surfaces, indicating a Li-Zn-O phase formed on the surface region. Besides high-voltage cycleability, cycle-life degradation caused by inappropriate conductive carbon can also be moderated by ZnO coating. Furthermore, the rate capability at high current density can also be notably improved. The role ZnO coating played in the charge-discharge process is discussed from the viewpoint of surface chemistry with the aid of data from scanning electron microscopy, TEM, energy-dispersive X-ray, and impedance spectra. Moreover, the correlation between electrochemical performance and surface properties of ZnO-coated LiCoO 2 is explored.