Why Substitution Enhances the Reactivity of LiFePO4

The impact of substitution at the Fe site in LiFePO4 on reaction pathway, kinetics, and crystallographic changes upon electrochemical delithiation has been determined. Substitution was found by X-ray diffraction to reduce the lattice mismatch between the Li-rich and the Li-poor phases of the substituted samples as compared to the unsubstituted one. Substitution was also found, by monitoring the 200 reflection peaks of both the triphylite and heterosite phases, to increase the composition width of the single phase formed on lithium removal, Li1-xFePO4. A single phase was observed as high as x = 0.15 in Li1-xFe0.85V0.1PO4, whereas LiFePO4 at the same state of charge and of similar particle size show the existence of two phases. In addition, the temperature at which a single phase is observed for the composition range 0 <= x <= 1 is decreased from slightly above 300 degrees C to ca. 200 degrees C. This increased single-phase-like behavior explains the enhanced kinetics of substituted LiFePO4 and is consistent with a pseudosingle-phase reaction mechanism.