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 LiFePO4 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 center dot H2O and finally LiFePO4. The initial olivine particles adopt an egglike shape and appear to form from the fusing of (NH4)FePO4 center dot 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 degrees 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 NH4+). 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.