Nano-Space Confinement Drives Rational Closed Pore Design in Hard Carbons for High-Capacity and High-Rate Sodium Storage

Hard carbons are emerging as the most viable anodes for the commercialization of Na-ion batteries. However, their performance limits are far from being disclosed because of ambiguous Na-storage mechanism. Here, we report that nano-space confinement regulates heterogeneous nucleation of quasi-metallic Na clusters in closed pores, uncovering a coupled “intercalation-pore filling” and stage-wise storage mechanism for high capacities. Theoretical studies reveal that the energy barrier for Na-cluster growth decreases as the nanocavity size decreases; however, it remains energetically unfavorable at potentials (V vs. Na/Na+) > 0. Interestingly, in the coupled storage, Na-ion intercalation in nanoconfined orifices triggers stepwise pre-nucleation, reducing energy barriers for spontaneous Na-cluster growth in progressively larger cavities at positive potentials, thus enabling Na-cluster deposition into previously unused closed pores. This understanding guides the rational design of stage-wise closed pores, resulting in superior performance of 500 mAh g−1 at 50 mA g−1 and 344 mAh g−1 at 2000 mA g−1. Mechanistic studies further identify a new stage, where confined nano-spaces at 0.4–0.6 nm facilitate pre-desolvation and enhance Na-ion transport kinetics for high-rate capabilities. This work identifies the origin governing Na-storage behavior in the closed pores of hard carbons, boosting their overall performance beyond prior expectations.