Advanced X-Ray Transmission Microscopy for Chemical and Fracture Imaging of Single LixFePO4 Particles at High Resolution
In this study, soft X-ray ptychography (beamlines 11.0.2 and 18.104.22.168 at the Advanced Light Source) combined with X-ray absorption spectroscopy (XAS),1 complemented with scanning (SEM) and transmission (TEM) electron microscopies, is applied to assess the chemical and morphological consequences of delithiation in lithium iron phosphate (LiFePO4) plates of three different sizes, from micro to nanometric as shown in Figure 1. Single crystals were found to contain two phases, with a complex correlation between crystallographic orientation and phase distribution, contrary to the common expectation that FePO4 mainly grows along a clear crystallographic direction. In addition, while fracture was observed at the two larger crystal sizes, as expected from the lattice mismatch between LiFePO4 and FePO4, no mechanical damage was observed to occur in nanoplates. These results provide clear insight into the need to significantly reduce primary crystal size in LiFePO4 electrodes.
 D. A. Shapiro, Y.-S. Yu, T. Tyliszczak, J. Cabana, R. Celestre, W. Chao, K. Kaznatcheev, A. L. D. Kilcoyne, F. Maia, S. Marchesini, Y. S. Meng, T. Warwick, L. L. Yang, H. A. Padmore, Nat. Photon. 8, 765 (2014).
Figure 1. (A and B) Scanning electron microscopy (SEM) images and (C and D) Optical density (OD) maps obtained by ptychography of LixFePO4 crystals with different size. The darker areas in the OD maps indicate regions of relatively lower density, as expected near the surface and internal defects (cracks and voids). (E) Chemical phase information from difference map between ptychography images collected at 708 eV and 710 eV. The red and blue areas indicate the highest content of LiFePO4 and FePO4, respectively. (F) Chemical phase maps obtained by linear combination fits of XAS data at each pixel. The presence of the LiFePO4 (indicated as LFP) and FePO4 (indicated as FP) standards were assigned colors red and blue, respectively. The green outlined regions indicate areas of particle overlap, defined by the SEM images.