The development of sodium ion batteries (NIBs) can provide an alternative solution to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. Due to the many similarities between sodium and lithium chemistries, the development of sodium-ion batteries (NIBs) – in particular the discovery and development of electrode materials – has been benefited greatly from the knowledge developed in understanding lithium chemistries in LIBs. However, many electrode materials used in LIBs show an inferior performance in sodiation. For example, in transition metal oxides MO_x (M = Fe, Co and Ni), the sodiation reactions have shown much lower capacity and slower kinetics than those commonly observed in lithiation reactions. This could not be simply interpreted from the difference in the ionic size of Na⁺ and Li⁺ because the reaction happened in this system is not intercalation but conversion. For the conversion compounds, although there were many papers published to investigate their electrochemical performance, the origin of different reaction kinetics is still not clear which hinder the further study on this system. To address this issue, we present herein our comprehensive, systematic study of the sodiation and lithiation of a model MO_x conversion compound, viz., nanostructured sheets of NiO. We investigate the sodiation and lithiation processes using a combination of multiple temporal- and spatial-scaled characterization techniques, including electrochemical measurements, in situ transmission electron microscopy (TEM), and ab initio molecular dynamics (MD) simulations. From the in situ TEM, where our previous results showing the lithiation heterogeneous reaction pathways, we found the sodiation only go through a “shrinking-core” pathway and further sodiation was blocked by a crystalline passivation layer of Na₂O that formed at an early stage. Our MD simulation shows that the sodiation pathway originates from a layer-by-layer reaction that occurs during the insertion of Na ions; this was the rate-determining step for the entire process. In case of lithiation, however, the formation of Li anti-site defects significantly distorts the local NiO lattice, which thereby facilitates Li insertion and enhance the overall reaction rate. In conclusion, the intrinsic layer-by-layer sodiation reaction and the formation of a crystalline passivation layer of Na₂O play an important role for sluggish sodiation reactions.