• Hard carbons are promising anode materials for sodium-ion batteries due to their low cost, environmental friendliness, and abundant precursors.
• The sodium storage performance of hard carbons is critically influenced by their structural features, including graphitic domains, pores, and disordered carbon layers, which depend on the precursor.
• Functional groups (heteroatoms, oxygen-containing groups) and precursor microstructure significantly affect the physical and electrochemical properties of the resulting hard carbon.
• Carbonization conditions such as temperature, heating rate, and atmosphere play a key role in optimizing hard carbon anodes for high-performance sodium-ion batteries.