The Structural and Electrochemical Impact of Li and Fe Site Substitution in LiFePO₄

The crystal structure and delithiation mechanism of Li-site substituted LiFePO 4 have been revealed by investigation of supervalent V 3+ substitution. The combined X-ray and neutron powder diffraction data analysis surprisingly shows that the substituting aliovalent vanadium ions occupy the Fe site while some of the Fe resides at the Li site, probably as sarcopside, which leads to an increase in the unit cell volume. Such substitution reduces the miscibility gap at room temperature and also significantly lowers the solid solution formation temperature in the two-phase region. The effect of the phase diagram modification results in improved kinetics, leading to better rate performance. Such substitution, however, significantly lowers the LiFePO 4 capacity at moderate current densities.