Phase Analysis and Atomically Resolved Delithiation Quantification of (Li)FePO4 by Integrated Differential Phase Contrast Imaging and Selected Area Electron Diffraction

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. Although previous work investigated crystal phases of partially delithiated battery cells with TEM based
techniques, such as Selected Area Electron Diffraction (SAED) and Bright-Field TEM [1], as well as Precession Electron Diffraction [2], the underlying charging and discharging mechanisms are still not well understood at the atomic level.
Our work shows that we are able to determine the local crystal phase and degree of delithiation on electrochemically delithiated LiFePO4 samples. Identification and analysis of LixFePO4 crystals was not only performed macroscopically, but also at an atomically resolved level (see figure 1).
Phase transition boundaries between lithiated and delithiated phases are depicted with High-Resolution integrated Differential Phase Contrast (iDPC) imaging, a technique with high sensitivity to light elements which is particularly well suited for beam sensitive materials like LiFePO4. Partially delithiated phases have also been observed among fully delithiated and fully lithiated phases. We also present a method to determine the quantitative delithiation grade on a nanometer scale using High-Resolution STEM-FFT analysis. These evaluations are further supported by SAED as well as extensive multislice STEM simulations.