Imaging and Spectroscopy of Pristine and Cycled Li2MnO3

Li-ion batteries have yet to reach their full application potential due to their inherent cycling performance issues, such as voltage instability and capacity fading, while fundamental questions remain unanswered regarding the mechanisms responsible. However, Mn-based layered oxides continue to be promising candidates for cathodes in high-energy-density Li batteries given their high voltage and high discharge capacities. A significant impediment to gaining insight into these layered oxides lies in their complexity and inhomogeneity. Thus, the focus of the present contribution will be on the parent material, Li₂MnO₃, which shows cycling characteristics similar to its more complex offspring. Scanning transmission electron microscopy (STEM) and spectroscopy are used to characterize structural and electronic properties of both pristine and cycled material. 

STEM-based methods are quickly becoming the most promising characterization tools for these and similar materials, owed largely to the wide-range of techniques available on advanced STEM instruments, including the direct imaging of both heavy and light elements, and both energy-dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies. Imaging modes such as high/low angle annular dark field (H/LAADF) and annular bright field (ABF) are exploited to image heavy atomic columns, strain contrast, and light atomic columns, respectively. Additionally, electron energy loss spectroscopy along with calculations based on density functional theory are used to probe the local electronic structure by monitoring the O K- and Mn L-edges, which can be used to track changes to both the O content and the Mn valence. Thus, the focus will remain on the structural and electronic evolution of the pristine layered oxide explored by combining spectroscopy and atomic-scale imaging with various in situ microscopy techniques and ex situ electrochemical cycling. Specifically, features such as the atomic ordering of Mn/Li atoms, O vacancy evolution, and Mn valence will be of particular interest.