Closed-Pore Engineering in Hard Carbon for Sodium Ion Storage: Advances, Challenges and Future Horizons

Hard carbons (HCs) are considered one of the most promising anode materials for sodium-ion batteries (SIBs) due to their low cost, high reversible capacity, and low operational potential. However, due to the complex physicochemical and microstructural properties, the sodium storage mechanism remains debated in HCs, particularly in the low-potential plateau region, which has also hindered further improvements in reversible capacity and Initial Coulombic Efficiency (ICE). The state-of-the-art investigation manifests that appropriately-sized closed pores of HCs are critical for boosting the low-potential plateau capacity by Na metal nanocluster filling. Nevertheless, there is a lack of a comprehensive review on the design, construction, and Na⁺ storage mechanism in closed pores. In this contribution, design strategies of closed-pore pored and their effective characterization methods are systematically reviewed, with a particular emphasis on clarifying the main factors affecting closed-pore structures and correlation to Na⁺ storage performances. Afterward, in-depth sodium storage mechanism in closed pores is summarized, and the future research directions of closed-pore structures are discussed. This review is expected to provide a clear understanding of precise control over closed-pore structures and offer useful guidance for the rational design of HCs anode materials toward energy-dense SIBs.