Exploring Point Defects in Battery Materials with Electron Microscopy and Multislice Ptcyhography

In this talk, we will discuss how aberration-corrected scanning transmission electron microscopy (STEM) and multislice electron ptychography can be used to probe the atomic scale dynamics and structure of point defects in materials for battery applications. As a prototypical example, we will report on directly quantifying point defect formation and migration in MgCr2O4. In this system, we observe the dynamics of interstitial formation through STEM imaging, which is found to depend on electron dose and energy. The interstitials are observed to reversibly migrate back and forth from the bulk crystal structure to the interstitial positions. Spectroscopy and ptychography will provide evidence of preferential mass loss of the lighter species, i.e. forming vacancies within the structure.<br/><br/>We will also highlight how phase contrast methods, such as iDPC STEM and electron ptychography, can be used to observe the distortion of cation-anion tetrahedra and octahedral during imaging. For example, with the formation of interstitials in MgCr2O4, the atoms bend away from the mid-plane in response to the presence of charged point defects. Furthermore, we will use the intensities of the point defect positions to infer the exchange with neighboring vacancies or migration deeper into the crystal. Through the depth sensitivity of multislice electron ptychography, we will explore where within the sample the defects are formed by the electron beam, both at 300 kV and 60 kV. Finally, we will discuss how STEM imaging and ptychography provide direct insights into mechanisms of ionic conduction, particularly in non-stoichiometric material.