Characteristics of nanosized LiNi (x) Fe1-x PO4/C (x=0.00-0.20) composite material prepared via sol-gel-assisted carbothermal reduction method

Abstract

Pure LiFePO4 and LiNi (x) Fe1-x PO4/C (x = 0.00-0.20) nanocomposite cathode materials have been synthesized by cheap and convenient sol-gel-assisted carbothermal reduction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy, and inductively coupled plasma have been used to study the phase, morphology, and chemical composition of un-doped and Ni-doped materials. XRD patterns display the slight shrinkage in crystal lattice of LiFePO4 after Ni2+ doping. The SEM images have revealed that Ni-doped particles are not agglomerated and the particle sizes are practically homogeneously distributed. The particle size is found between 50 and 100 nm for LiNi0.20Fe0.80PO4/C sample. The discharge capacity at 0.2 C rate has increased up to 155 mAh g(-1) for the LiNi0.05Fe0.95PO4/C sample and good capacity retention of 99.1 % over 100 cycles, while that of the unsubstituted LiFePO4/C and pure LiFePO4 has showed only 122 and 89 mAh g(-1), respectively. Doping with Ni has a noticeable effect on improving its electrical conductivity. However, serious electrochemical declension will occur when its doping density is beyond 0.05 mol LiNi0.20Fe0.80PO4/C electrode shows only 118 mAh g(-1), which is less than un-doped LiFePO4/C sample at 0.2 C. The cycling voltammogram demonstrates that Ni-doped LiNi0.05Fe0.95PO4/C electrode has more stable lattice structure, enhanced conductivity, and diffusion coefficient of Li+ ions, in which Ni2+ is regarded to act as a column in crystal lattice structure to prevent the collapse during cycling process.