Sodium-ion batteries are considered as a favorable alternative to the widely used lithium-ion batteries for applications such as grid-scale energy storage. However, to meet the energy density and reliability that is necessary, electrodes that are structurally stable and well characterized during electrochemical cycling need to be developed. Here, we report on how the applied discharge current rate influences the structural evolution of Na0.67Mn0.8Mg0.2O2 electrode materials. A combination of ex situ and in situ X-ray diffraction (XRD) data were used to probe the structural transitions at the discharged state and during charge/discharge. Ex situ data shows a two-phase electrode at the discharged state comprised of phases that adopt Cmcm and P63/mmc symmetries at the 100 mA/g rate but a predominantly P63/mmc electrode at 200 and 400 mA/g rates. In situ synchrotron XRD data at 100 mA/g shows a solely P63/mmc electrode when 12 mA/g charge and 100 mA/g discharge is used even though ex situ XRD data shows the p...