Confined Phase Transition Infiltration Coupled With In Situ Pore Sealing: Constructing Hard Carbon Rich in Closed Pore for Plateau‐Capacity‐Dominated Sodium Storage

Biomass‐derived hard carbon (HC) is a promising candidate for anode materials in sodium‐ion batteries (SIBs). However, abundant open pores generated during activation process often lead to severe proliferation of the solid electrolyte interphase (SEI) and irreversible Na + consumption, severely compromising the initial Coulombic efficiency (ICE) and low‐voltage plateau capacity. This work proposes a synergistic strategy of confined phase transition infiltration and in situ pore sealing, using waste green tea as a precursor to construct a HC anode dominated by plateau capacity. Low dose of ZnCl 2 induces controllable hydrolysis and aromatization of cellulose and generate nanoscale open pores. These pores act as confined microreactors, inducing an in situ chemical vapor deposition like carbon growth that effectively “stitching” the open voids into solvent sieving closed pores. Consequently, it delivers a remarkable ICE of 79.1%, a high reversible capacity of 342.3 mAh g −1 at 0.1 A g −1 , and robust cycling stability (86.2% retention after 1000 cycles at 1.0 A g −1 ). In‐/ex situ analysis reveals that its reversible sodium storage mechanism encompasses a triple synergy of defect adsorption, interlayer intercalation, and closed pore filling with quasi metallic sodium clusters. The confined pore sealing engineering strategy provides a robust paradigm for designing high‐performance HC anodes.