The ion diffusion process, shuttling of Li and Na-ions in and out of the host electrode materials is the basis on which Li-ion batteries (LIBs) and Na-ion batteries (SIBs) operate.[1] Its reversibility and the governing kinetics dictate many of the battery performance parameters such as cycling stability and rate capability.[2] However, it is challenging to understand the ion diffusion process at the atomic level for manipulation in practical rechargeable batteries.[3] Tracing the dynamic process of Li-ion transport at the atomic scale is a long-cherished wish in solid state ionics and essential for battery material engineering. Approaches via phase change, strain, and valence states of redox species are developed to circumvent the technical challenge of direct imaging Li, however, all are limited by poor spatial resolution and weak correlation with state-of-charge (SOC). Here, we adopt an ion-exchange approach by sodiating the de-lithiated cathode and probing Na distribution to trace the Li de-intercalation, which enables us to visualize the heterogeneous Li-ion diffusion down to atomic level through the utilization of techniques such as scanning transmission electron microscopy - high angle annular dark field (STEM-HAADF) imaging. In the model LiNi1/3Mn1/3Co1/3O2 cathode, dislocation-mediated ion diffusion is kinetically favorable at low SOC and planar diffusion along (003) layers dominates at high SOC, which work synergistically to determine the ion diffusion dynamics. Our work unveils the heterogeneous nature of ion diffusion in battery material and stresses the role of defect engineering in tailoring ion transport kinetics.[4] References [1] J. B. Goodenough, Y. Kim, Chem. Mater., 2010, 22, 587 [2] M. S. Whittingham, Chem. Rev., 2004, 104, 4271. [3] T. Shang and H. Li et al., Adv. Energy Mater. 2017, 7, 1700709. [4] B. Xiao and X. Li et al., submitted. Figure 1