Liquid‐Phase Spatial Confinement Engineering of sp ² ‐Rich Closed‐Pore Hard Carbon for Sodium‐Ion Batteries

Porous‐carbon‐based hard carbons have attracted widespread attention as anodes for sodium‐ion batteries owing to the high plateau capacity; however, their practical application is constrained by an intrinsic trade‐off: constructing abundant closed pores enhances capacity but impedes charge transport and compromises structural stability. Herein, we introduce a solvent‐mediated liquid‐phase spatial confinement strategy that fundamentally addresses this dilemma. By incorporating an immiscible nonpolar solvent into a hydrothermal system, carbon precursor deposition and nucleation are precisely regulated within a confined, high‐tension liquid‐phase interface, yielding sp 2 ‐rich closed‐pore hard carbon with simultaneously enhanced capacity, kinetics, and durability. Combined in situ characterizations and theoretical calculations reveal that such sp 2 ‐rich carbon layer significantly promotes electronic and Na + transport, induces preferential anion decomposition to generate a thin and NaF‐rich solid electrolyte interphase, while reinforcing structural integrity. Consequently, the engineered hard carbon delivers high capacity (413.5 mA h g −1 at 20 mA g −1 ) together with excellent rate capability and cycling stability (224.1 mA h g −1 at 2 A g −1 after 4000 cycles). When assembled into Ah‐level 32700 cylindrical cells, it achieves high energy density of 124.0 Wh kg −1 and stable operation over 1200 cycles at 1C, demonstrating a new paradigm for the rational design of high‐performance porous‐carbon‐based hard carbon anodes.