Advanced X-Ray Transmission Microscopy for Chemical and Fracture Imaging of Single LixFePO4 Particles at High Resolution

The demand for tools for the chemical imaging of battery processes with ever increasing chemical and spatial resolution is rising because the processes involved in the reaction occur at length scales that span the atomic to the macro scale. The observation of phase transformations relevant to battery electrodes in single crystals of varying sizes can help identify kinetic limitations to utilization and durability, as well as the existence of non-trivial size effects. Such detailed insight can be employed in the rational design of next generation materials which can achieve the theoretical limits of storage density and life of the materials. Conventional transmission X-ray microscopy (TXM) is an ideal tool that can probe large volumes of material at high spatial resolution with chemical and structural information. In turn, the spatial resolution of X-ray microscopes has been limited by the quality of Fresnel zone plate used to focus X-ray until the development of diffractive imaging methods.

In this study, soft X-ray ptychography (beamlines 11.0.2 and 5.3.2.1 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. 

[1] 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 Li_xFePO₄ 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.