Closed Nanopore Engineering of Hard Carbon for Enhanced Sodium Plateau Storage: A Review and Prospects

Hard carbon stands as the most commercially viable anode material for sodium-ion batteries. Numerous studies confirm that closed nanopore design ensures the competitiveness of hard carbon compared to other anodes. However, the complex multi-scale microstructure of hard carbon poses challenges to the establishment of sodium storage mechanisms and quantitative relationships within closed nanopores. In this regard, closed nanopore engineering has been comprehensively summarized as a significant research direction in the field of hard carbon. Firstly, the sodium storage behavior of distinct nanopore types is compared. Subsequently, the structure and sodium storage behavior of closed nanopores are revealed through contact/non-contact techniques. Importantly, closed nanopore construction strategies are summarized based on their origins, which are divided into two categories: in situ formation and open nanopore conversion. Finally, perspectives on emerging frontiers are offered, including 3D pore topology visualization, defect modulation effects, multifield-coupled synthesis techniques, and the data-driven research paradigm for hard carbons. By synthesizing these advances and proposing a novel design framework, this review is anticipated to provide valuable insights for the design of high-performance hard carbon, thereby advancing the practical applications of sodium-ion batteries.