A comparative study of LiNi 0.8Co 0.2O 2 cathode materials modified by lattice-doping and surface-coating

Abstract

Lithium nickel cobalt oxides as cathode materials for lithium ion batteries were modified by Ti-doping and TiO 2-coating. The effects of Ti-doping and TiO 2-coating on the structural and electrochemical properties of LiNi 0.8Co 0.2O 2 cathode material were investigated and compared. The results of X-ray diffraction (XRD) show that a solid solution phase (R-3 m) is observed in the range of 0≤ y≤0.1 for LiNi 0.8− y Ti y Co 0.2O 2 materials, and a new hexagonal phase LiTiO 2 appears on the surface of TiO 2-coated materials when the atomic ratio of Ti/(Ni+Co) is larger than 0.06. The ordering degree of layered structure was decreased for Ti-doped materials, and almost no changes of bulk structure occurred for TiO 2-coated materials. Charge–discharge experiments show both lattice-doping and surface-coating improve the cycling stability of LiNi 0.8Co 0.2O 2 cathode material. Ex-situ XRD of delithiated cathode materials confirms that suppression of lattice changes is attributed to the improved cycling stability of doped materials. But the similar suppression mechanism is not suitable for coated materials. Electrochemical Impedance Spectroscopy (EIS) experiments imply that the improvement of cycling performance for coated materials could be interpreted to the interface stability during cycling.