Synthesis and characterization of olivine phosphate LiMPO4, M = Co(II), Ni (II) nanostructures in the solid state

Olivine phosphate as a most important family of cathode was showed high rechargeable capacity. In this paper, a mechanochemical method for synthesis of olivine nanostructures was presented. The compounds such as LiCoPO4 (LCP) and LiNiPO4 (LNP) were synthesized by solid state reaction. NH4CoPO4 (ACP) and NH4NiPO4 (ACP) as precursors were prepared in an aqueous solution. Mechanochemical reaction between precursors and Li2CO3 reaction occurred at room temperature. The chemical changes of LCP and LNP compounds were characterized by Fourier transform infrared spectroscopy (FT-IR), X-Ray Diffraction (XRD) analysis, and also morphological changes by Scanning Electron Microscopy (SEM), Energy Dispersion X-ray (EDX) analysis and surface analysis Brunauer-Emmett-Teller (BET). Quantitative analysis of LNP showed pure LiNiPO4, although LCP contained both LiCoPO4 76.3 at% and Co3O4 23.7 at% phases. Results showed that the original products of the mechanochemical reaction have an amorphous phase, which crystallized after annealing. FTIR spectra showed that NH4+ ions were replaced with Li+ ions. The SEM images of LCP and LNP show nanoparticles that have squamous morphologies with well-defined grain boundaries and uniform distribution. The particle size distributions were found to range from 15 to 80 nm and 62-237 nm for LCP and LNP respectively. The BET analysis of LiCoPO4 with Type III isotherm shows the surface area 3.3234 m2/g, total pore volume 0.013424 cm3/g and also, the average size of the porous 16.157 nm. Although, BET analysis diagram of LiNiPO4 shows Type IV isotherm with mesopores structure that has surface area 7.4696 m2/g, total pore volume 0.08079 and also, the average size of the porous 43.263 nm. The cyclic voltammetry data showed a higher specific capacitance, 346.18 Fg- 1 in LCP, whereas LNP has 307.69 Fg-1.