Nanoconfinement-induced high-rate performance of hard carbon for densified sodium cluster storage

Hard carbon is recognized as a promising anode material for sodium-ion batteries, but its practical application is constrained by low initial Coulombic efficiency (ICE), insufficient reversible capacity, and poor rate performance, which are rooted in inadequate pseudo-graphitic domain structure and uncontrolled sodium cluster formation. Herein, we propose a nanoconfinement strategy via graphene orientation-guided graphitization to achieve high-rate performance of cellulose-derived hard carbon. The oxygen-functional groups on graphene form a stable cross-linking structure with cellulose to suppress disordered defects, while the sp²-hybridized carbon skeleton guides the directional arrangement of carbon layers, synergistically constructing a confined structure with abundant pseudo-graphitic domains and size-tunable closed pores. Benefiting from this optimized structure, the resultant electrode achieves a high specific capacity of 323.9 mAh g^-1, an ICE of 89.9%, and excellent rate performance (226.8 mAh g^-1 at 3.0 A g^-1). More importantly, sodium metal clusters are for the first time observed via nanoconfinement induction with the filling stage achieving their controllable densification through enhanced micropore confinement. This further validates and reinforces the new adsorption-intercalation-pore filling mechanism for sodium cluster densification. This work highlights nanoconfinement induction for high-rate hard carbon anodes, promoting the application of sodium-ion batteries in large-scale energy storage systems.