Effect of crystallite size on the phase transition behavior of heterosite FePO₄

For advanced lithium-ion battery technology, olivine-based cathodes are considered to be the most dominant and technologically recognized materials. The extraction of lithium ions from olivine LiFePO₄ results in the two-phase mixture with heterosite FePO₄ exhibiting a deintercalation potential of 3.45 V vs. Li⁺/Li over a wide range of lithium content. Here, we report the synthesis and characterization of chemically deintercalated heterosite FePO₄ with varying crystallite sizes using different analytical techniques. The decrease in the crystallite size of heterosite FePO₄ leads to an increase in the lattice parameters including the unit cell volume. The characteristic behavior in the structural properties of heterosite FePO₄ shows a strong dependency on the crystallite size which is correlated with the change in the chemical bonding. The volume expansion of the nano-sized heterosite FePO₄ with respect to the bulk counterpart is suggested to be a direct consequence of reduced hybridization between the Fe3d and O2p states. Furthermore, the combined X-ray diffraction and Mössbauer spectroscopic studies reveal the appearance of a new phase namely trigonal FePO₄ at the lower crystallite sizes due to the enhanced surface energy kinetics. We also find that the observed trigonal FePO₄ phase is more magnetically active than the paramagnetic olivine FePO₄. For the unique structural advantage of the heterosite phase as an electrode material, the change in bonding characteristics is very useful and can have strong implications on the electronic properties of heterosite FePO₄ at the nanoscale level.