Capturing Reversible Cation Migration in Layered Structure Materials for Na‐Ion Batteries

Abstract Na‐ion batteries are promising for large‐scale energy storage due to the low cost and earth abundance of the sodium resource. Despite tremendous efforts being made to improve the battery performance, some fundamental mechanism issues still are not sufficiently understood. One such issue in the most popularized layered structure materials is the potential cation migration into sodium layers, which would highly affect the energy efficiency thus hindering its widespread use in the future. Here a systematic study of the cation migration in layered structure materials is presented and its relationship with voltage hysteresis is disclosed. Using the high‐angle annular dark field‐scanning transmission electron microscopy and in operando X‐ray diffraction as implements, for the first time, the reversible migration of the transition metal between transition metal layers and sodium layers is captured. The research inspires a novel insight into the cation migration related layered materials, which should be considered for future battery design toward conventional use.