Effects of nanoscale vacancy clusters on the electrochemical properties of cathodes critically depend on the dynamic characteristics of vacancies during the battery cycling. However, a fundamental understanding of vacancy clusters in the layer-structured cathode remains elusive. Here, using scanning transmission electron microscopy, we reveal a cycling-induced vacancy aggregation behavior in a layer-structured cathode. We discover that during the initial charging, vacancies aggregate to form nanoclusters at the outer layer of the secondary particle, which subsequently extend to the inner part of the particle when fully charged. With extended cycling, these nanoscale vacancy clusters become immobilized. We further reveal that the generation of these vacancy clusters is correlated to the material synthesis conditions. Our findings solve a long-standing puzzle on the origin, nature, and behavior of the commonly visible vacancy clusters in the layered cathode, providing insights into correlation between properties and dynamic behaviors of atomic-scale defects in layered oxide cathodes.
Keywords: Li-ion batteries; electron microscopy; layered oxide cathode; nanoscale vacancy cluster; vacancy migration.