Effects of fluorine doping on structure, surface chemistry, and electrochemical performance of LiNi0.8Co0.15Al0.05O2

Abstract

The performance of LiNi0.8Co0.15Al0.05O2 is shown to be improved by fluorine doping via a modified low-temperature method. Scanning electron microscopy shows that fluorine doping catalyzes the growth of the primary particles. X-ray diffraction (XRD) results reveal that all the synthesized materials have typical hexagonal structure without impurities. The lattice parameters calculated from the XRD data by Rietveld refinement methods indicate that the interslab spacing distance is enlarged by fluorine doping. Inductively coupled plasma atomic emission spectroscopy and energy-dispersive X-ray spectroscopy show that fluorine atoms are preferentially enriched on the surface of the particle. X-ray photoelectron spectroscopy indicates that the valence state of nickel ions at the surface is lowered. The cycling performance of modified samples at room temperature, at high temperature (55°C), and at high upper cut-off potentials is improved. Electrochemical impedance spectroscopy measurements reveal that fluorine doping could lower the impedance rise during cycling and suppress the degradation of the cathode material.