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

The carbon-free LiNiPO4 and cobalt doped LiNi1 − xCoxPO4/C (x = 0.0-1.0) were synthesized and investigated for high voltage applications (> 4 V) 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 (50-80 nm) 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 (146.2 mA h g− 1) and good capacity retention of 85.1% at the end of the 60 cycles 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 (5.26 V)-reduction (5.08 V) peaks. With this study, characteristics of the LiNi1 − xCoxPO4/C series were deeply evaluated and discussed.