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

Abstract

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.