A review of the preparation and characterization techniques for closed pores in hard carbon and their functions in sodium-ion batteries

With the advantages of simple preparation, cost-effectiveness, abundant raw materials, and environmentally friendly properties, hard carbon is the only commercially available anode material for sodium-ion batteries. However, its unstable capacity is attributed to the complex physicochemical characteristics of the precursors, the intricate and difficult-to-control microstructure, and the debated mechanisms of sodium storage. Although recent reports have revealed a strong correlation between closed pores and the capacity of hard carbon in the low-voltage plateau region, systematic overviews of this relationship remain scarce. This review examines the microstructural properties and precursor selectivity of hard carbon materials and outlines the strategies for the research and development of closed pores, including design theory and characterization. Finally, it summarizes the technical bottlenecks faced by the closed pore research and looks forward to the future development directions.