Gaining Insights into Li-ion Distribution and Diffusion in LiFePO4 Using Correlated (S)TEM Diffraction, EELS and Atomically Resolved iDPC Imaging

Lithium Iron Phosphate (LiFePO4) is a technologically highly relevant and widely used cathode material for rechargeable battery systems, due to its stability, long cycle-life and high charge and discharge efficiency. Despite its wide use and in-depth research performed on this material system, questions regarding phase-transition mechanisms between lithium rich and lithium poor regions remained difficult to explain. [1,2] In this work [3] we put special emphasis on correlative evaluation of data obtained from (S)TEM experiments. Selected Area Electron Diffraction (SAED), Electron Energy Loss Spectroscopy (EELS) and integrated Differential Phase Contrast imaging (iDPC) are obtained and compared to each other meticulously to confirm findings on LiFePO4, supported by multislice simulations. We further present a method for laterally resolved ion-quantification on de-vices without spectrometers and probe- or image correction by SAED and HR-(S)TEM, a method-ology that can in principle be applied to other material systems as well.
Using the methodology developed we show the phase transition mechanisms between regions with varying Li-content in LiFePO4 at unprecedented detail. We also observed the ionic diffusion of Li within the [010]-channels of LiFePO4, which further allowed for calculating an upper limit for the Li-diffusion coefficient in the [010]-direction. Observation of local crystal structure deviations based on this diffusion also gave the first experimental evidence for the ionic jump-relaxation model ex-plained by Funke.