Sodium-ion batteries are attractive due to their potentially lower cost than lithium-ion systems. However, the larger ionic radius of Na⁺ means that candidate electrode materials often undergo more substantial volumetric changes during reaction when compared to Li-ion batteries, and these changes must be understood and controlled for the development of electrode materials with long cycle life. In this study, multiple in situ techniques are used to investigate nanoscale-to-macroscale transformation pathways in Cu₂S (a sulfide electrode material) during electrochemical reaction with Na and Li. In situ and ex situ x-ray diffraction reveal that the macroscale phase transformations in Cu₂S electrodes are similar within both Na and Li cells; the material undergoes initial intercalation followed by conversion in both cases. However, in situ transmission electron microscopy (TEM) shows that the nanoscale reaction pathways differ significantly. During sodiation, Cu₂S particles undergo a local phase separation that results in Cu nanocrystals surrounding a shell of Na₂S. This leads to complete destruction of the initial Cu₂S lattice, as opposed to the retention of the lattice during the displacement reaction in the lithiation case. These different reaction mechanisms likely contribute to observed differences in electrochemical signatures in Na and Li cells. Despite these dissimilarities, Na/Cu₂S electrochemical cells are shown to exhibit excellent cycle life (negligible capacity decay over 400 cycles), which is similar to the Li case. Thus, although the more substantial volume changes during the sodiation of Cu₂S induce a new reaction pathway, they do not cause accelerated capacity decay, as is commonly argued for Na-ion materials. These results suggest that other large-volume-change electrode materials may also be engineered for long cycle life in next-generation Na-ion batteries.