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4D-STEM Mapping of Nanoscale Structural Ordering in Cathode Materials

Scanning transmission electron microscopy (STEM) has found great opportunities in studying transformation heterogeneity at the nanoscale in energy storage materials, such as the chemical phase separation [1, 2], cation ordering and internal structures in electrode particles [4]. For example, our previous studies show that reduced particle size (< 100 nm) can effectively suppress the compositional heterogeneity in cathode particles during multivalent ion insertion [5]. Selected-area electron diffraction was employed to unravel the solid-solution phase formed in the cathode nanoparticles during phase transformation. Energy dispersive spectroscopy was used to visualize the composition non-uniformity in the cathode materials after electrochemical ion insertion, which is dependent on the particle size. Here, we build upon this earlier effort of ours to further apply scanning electron nanodiffraction, also known as four-dimensional (4D)-STEM, to study transformation heterogeneity in cathode nanoparticles While our previous studies focus on the composition non-uniformity in the cathode materials, in this work we study the structural heterogeneity, exemplified by the phase orientation and strain, in cathode nanoparticles. To map phase orientations in a transformation system, the conventional methods of template matching and correlation mapping [6] have limitations, as they are either designed for a single-phase system or consider multiple factors beyond phase orientation. To solve this problem, we define a parameter of “tetragonality” to describe the local distortion on the cathode structure induced by