High Resolution 3D Spectro-Microscopy of Iron Fluoride Nanowires As Conversion Cathodes for High Capacity Li Ion Batteries
Authors: Farmand Maryam , Yu Young-Sang , Shapiro David 
Published: February 2020

There is an ever-increasing demand for 3D spectro-microscopy techniques for visualizing and characterizing quantitative morphological and chemical information with high spatial resolution. Such techniques are of crucial importance for studying the relations between electrochemical performance, functionality and chemical and morphological properties of materials. These techniques could eventually serve as a comprehensive and reliable basis for rational materials design.

Soft X-ray ptychography is a spectro-microscopy technique that extracts structural information from the inverse of diffraction data. Hence the spatial resolution of this technique is not dependent and limited by X-ray spot size and optics quality.1 At the ALS, through the use of a high performance scanning system, an in house developed fast CCD and reconstruction algorithms operated on a multi GPU cluster, ptychographic imaging of nano-materials with an unprecedented wave-length limited spatial resolution of less than 5 nm has been achieved.2 This exceptionally high spatial resolution is of crucial importance in characterization of nano-scale phenomena. In our previous work 2, ptychography was utilized to study phase transformation mechanism and its relation to particle size and structural defects in single particle LiFePO4cathodes.  In this work, high-resolution ptychographic imaging is utilized to visualize phase transformations in 3D in a class of high capacity cathodes.

Conversion materials are considered as viable candidates for future high capacity cathodes in Lithium ion batteries. In these cathodes, the electrode undergoes electrochemical conversion/deconversion as charge/discharge cycles progress. Iron triflouride (FeF3) is a conversion material that offers a 3-electron charge transfer upon discharge:

FeF3 + 3Li+ + 3e-
⇄ 3LiF + Fe      (4.5 − 1.5 V vs
. Li+/Li)

 This material possesses a high theoretical capacity of 712 mAh∙g-1 and a theoretical energy density of 1950 Whk∙g-1.3 However, it suffers from fast capacity fade and large voltage hysteresis upon charge/discharge cycling, due to structural rearrangement and drastic phase transformations.4 Fe nano-domains, embedded in LiF matrix, virtually decompose the cathode upon discharge. Here, ptychography is used to track the evolution and mechanism of phase segregation process in porous FeF3 nanowires in 3D. Composite cathodes comprising of FeF3 nanowires, carbon black, and polymer binder were discharged in a coin cell with Li as anode. Ex-situ measurements were then conducted on these composite cathodes at various discharge depths on both Fe L
- and F K-edges at beamline 5.3.2.1 of the Advanced Light Source. Figure1 shows preliminary measurements conducted in normal Scanning Transmission X-ray Microscopy (STXM) mode at the beamline. The cathode here was fully discharged to a 1.0 V and then subjected to X-ray measurements. Figure 1a and 1b show morphological and chemical maps, where figure 1c shows averaged XAS spectra obtained from each cluster through PCA and a k-means clustering method. Various concentrations and ratios of Fe and F are observed in each cluster, pointing to the separation of the two phases in a complex pattern, which needs to be elucidated in 3D. High-resolution ptychographic chemical maps and the correlation of the chemical composition to structural variations in these cathodes will be visualized in 3D through implementation of reconstruction and segmentation algorithms.

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Figure 1