A review of hard carbon anodes for rechargeable sodium-ion batteries

Hard carbons (HCs) are recognized as potential anode materials for sodium-ion batteries (SIBs) because of their low cost, environmental friendliness, and the abundance of their precursors. The presence of graphitic domains, numerous pores, and disordered carbon layers in HCs plays a significant role in determining their sodium storage ability, but these structural features depend on the precursor used. The influence of functional groups, including heteroatoms and oxygen-containing groups, and the microstructure of the precursor on the physical and electrochemical properties of the HC produced are evaluated, and the effects of carbonization conditions (carbonization temperature, heating rate and atmosphere) are also discussed.