Sodium-ion batteries (SIBs) are gaining importance as promising alternatives to lithium-ion batteries (LIBs) for grid and large-scale energy storage applications due to the high Earth abundance of the elements used in SIBs such as Na, Fe, and Mn. However, the energy density urgently needs further improvement to meet increasing energy storage demands. Stable and high-capacity positive electrodes are a prerequisite for increasing the energy density of SIBs. Among the various reported cathode materials, layered oxides are ideal candidates because they can have tunable compositions with reasonable capacity and the electrochemical performance can be easily tailored by introducing different elements into the host structures.