Synthesis strategies of hard carbon anodes for sodium-ion batteries

Sodium-ion battery (SIB) is an ideal candidate for large-scale energy storage due to high abundant sodium sources, relatively high energy density, and potentially low costs. Hard carbons, as one of the most promising anodes, could deliver high plateau capacities at low potentials, which boosts the energy densities of SIBs. Their slope capacities have been demonstrated from the defect adsorption of sodium ions, while the plateau capacity depends highly on intercalation and pore filling. Nevertheless, the specific structures of sodium ions stored in hard carbons have not been clarified, namely active sites of adsorption, intercalation, and pore-filling mechanisms. Therefore, delicate synthesis methods are required to prepare hard carbons with controllable specific structures, along with elucidating the precise active sites for enhancing the Na-ion storage performance. To offer databases for future designs, we summarized the synthesis strategies of hard carbon anodes for constructing active sites of plateau capacities. Synthesis methods were highlighted with corresponding influences on the meticulous structures of hard carbons and Na-ion storage behaviors. Last but not least, perspectives were proposed for developing hard carbon anodes from the points of research and practical applications.