Confirmation of the Domino-Cascade Model by LiFePO4/FePO4 Precession Electron Diffraction

A recent transmission electron microscopy (TEM) method using precession electron diffraction (PED) was used to obtain LiFePO4 and FePO4 phase mapping at the nanometer-scale level on a large number of particles of sizes between 50 and 300 nm in a partially charged cathode. Despite the similarity of the two phases (the difference of lattice parameters is <5%), the method gives clear results that have been confirmed using high-resolution transmission electron microscopy (HRTEM) and energy-filtered transmission electron microscopy/electron energy loss spectroscopy (EFTEM/EELS) experiments. The PED maps show that the particles are either fully lithiated or fully delithiated and, therefore, bring a strong support to the domino-cascade model at the nanoscale level (scale of a particle). A core-shell model or spinodal decomposition at mesoscale (scale of agglomerates of particles) is possible. Size effects on the transformation are also discussed.