Coal-based hard carbon anodes for sodium-ion batteries: A review

Abstract Hard carbon is prioritized as the anodes candidate in sodium-ion batteries (SIBs) due to its excellent sodium storage performance. The precursors for hard carbon anodes are diverse, including biomass, organic macromolecule polymers, asphalt, and coal. Among these, coal is currently considered an ideal precursor owing to its high content of carbon, rich aromatic structural units, and low cost. This review systematically reviews the types and physicochemical characteristics of coal, preparation methods, advantages and drawbacks of hard carbon from coal (and their pros/cons) for coal-derived hard carbon, and the structural and sodium storage mechanisms of hard carbon. Some key strategies to improve sodium storage performance involving, such as carbon microcrystal orientation control, heteroatoms doping, and closed pores regulation, are highlighted. We have analyzed and summarized the synthesis methods and electrochemical properties of different types of coal, and proposed a coal-specific method for synthesizing hard carbon. By leveraging the characteristics of different coals, we selected the appropriate synthesis path for hard carbon, establishing a connection among raw materials, processes, properties, and industrialization. Furthermore, this review examined the challenges in coal-based hard carbon production, including complex molecular structure evolution of precursors, process controllability of adopted technology method optimization, while proposing future research directions.