Distinct Microstructural Evolution of Li- and Mn-Rich Oxide Cathodes during Calendar and Cycle Aging of Lithium-Ion Batteries

The respective microstructural impacts of calendar and cycle aging of Li-ion battery cathodes remain difficult to distinguish because both processes often occur simultaneously. Here, we isolate the microstructural evolution of these aging modes on a Li- and Mn-rich (LMR) layered oxide cathode using aberration-corrected scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy-loss spectroscopy (STEM-EELS). Both aging modes induce similar near-surface degradation, including surface reconstruction, cation mixing, and activation of a (003)-facet phase transformation. However, notable differences emerge in the cathode–electrolyte interphase, indicating distinct electrolyte decomposition pathways under static and dynamic conditions. More significantly, nanovoid formation is exclusively observed in cycle-aged particles but is absent after calendar aging, suggesting that repeated Li transport promotes irreversible migration and loss of oxygen and transition-metal species, thus leading to vacancy condensation within the bulk lattice. These findings shed light on improving the long-term stability of LMR cathodes.