Influence of gradual cobalt substitution on lithium nickel phosphate nano-scale composites for high voltage applications

Abstract

The carbon-free LiNiPO4 and cobalt doped LiNi1−xCoxPO4/C (x=0.0–1.0) were synthesized and investigated for high voltage applications (>4V) for Li-ion batteries. Nano-scale composites were prepared by handy sol–gel approach using citric acid under slightly reductive gas atmosphere (Ar-H2, 85:15%). Structural and morphological characteristics of the powders were revealed by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and inductively coupled plasma (ICP). Except for a small impurity phase (Ni3P), phase pure samples crystallized in the olivine-lattice structure with a linear relationship between lattice parameters (a, b and c) and chemical composition. The FE-SEM images proved that LiNiPO4/C particles (50–80nm) did not agglomerate, and showed that as the cobalt content was higher agglomeration had increased. The electrochemical properties of all electrodes were investigated by galvanostatic charge–discharge measurements. Substitution of Ni2+ by Co2+ caused higher electronic conductivities and showed more effective Li+ ion mobility. When the cobalt content is 100%, the capacity reached to a higher level (146.2mAhg−1) and good capacity retention of 85.1% at the end of the 60cycles was observed. The cycling voltammogram (CV) revealed that LiCoPO4/C electrode improved the electrochemical properties. The Ni3+–Ni2+ redox couple was not observed for carbon free LiNiPO4. Nevertheless, it was observed that carbon coated LiNiPO4 sample exhibits a significant oxidation (5.26V)–reduction (5.08V) peaks. With this study, characteristics of the LiNi1−xCoxPO4/C series were deeply evaluated and discussed.