Citric acid- and ammonium-mediated morphological transformations of olivine LiFePO4 particles

The effects of citric acid (CA) and ammonium (NH₄⁺) ions on the structural and morphological transformations of olivine LiFePO ₄ upon hydrothermal treatment are systematically investigated, as a function of reaction time, by using a combination of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magic-angle-spinning nuclear magnetic resonance (MAS NMR), and Fourier transform infrared absorption spectroscopy (FTIR). In the presence of both CA and NH₄⁺ ions, the structures evolve from amorphous precursors to crystalline (NH₄)FePO₄•H₂O and finally LiFePO₄. The initial olivine particles adopt an egglike shape and appear to form from the fusing of (NH₄)FePO ₄•H₂O plates. This metastable morphology evolves to form a mixture of cubic and rhombic particles. These particles are then etched, resulting in hollow structures and then ultimately barrel-like particles, after over 120 h of hydrothermal reaction at 180 °C. The final morphology is close to the equilibrium structure proposed by Islam et al. [Fisher, C. A. J.; Islam, M. S. J. Mater. Chem.2008, 18, 1209]. The presence of NH₄ ⁺ ions (as detected by FTIR) adsorbed on the surfaces of these particles, seems to slow growth along certain directions, resulting in cubic/rhombic-shaped particles. The formation of hollow particles is ascribed to the opposing effects of etching (from CA) and surface protection (from NH ₄⁺). The electrochemical performances vary significantly with particle shape. The hollow and roughened spindle-like particles (formed in the absence of NH₄⁺ ions) exhibit superior electrochemical properties, compared to the other particles, because of their higher specific surface areas and shorter Li⁺ ion diffusion lengths. The facile synthesis of olivine LiFePO₄ particles with very different morphologies provides an interesting platform for further fundamental investigation into the shape-dependent electrochemical performance and electrochemical lithium intercalation and deintercalation mechanisms of olivine LiFePO₄. © 2011 American Chemical Society.