Ternary Crosslinking Builds Nanosheet Hard Carbon and Interfacial Polarization Regulates Na⁺ Kinetics in Hard Carbon Anodes

ABSTRACT

Hard‐carbon anodes for sodium‐ion batteries are often constrained by low initial Coulombic efficiency, severe interfacial polarization, and sluggish Na⁺ kinetics, especially when porosity engineering amplifies electrolyte consumption. This work reports a heteroatom‐engineered hard carbon (UNMC) that couples molecular‐level crosslinking regulation with ultrahigh N/O doping to program a chemically graded solid‐electrolyte interphase (SEI) and accelerate Na⁺ storage. A uniform UF/MA/2‐MIM crosslinked network, reinforced by Zn²⁺ coordination, is converted via hydrothermal restructuring, pre‐oxidation, and carbonization into a nanosheet‐bundle framework featuring moderately expanded turbostratic domains and dense closed nanopores. UNMC delivers a high first‐cycle charge capacity of 437.1 mAh g ⁻¹ with an initial coulombic efficiency of 80.93%, sustains 367.32 mAh g ⁻¹ after 2000 cycles at 0.1 A g ⁻¹ , and maintains 178.89 mAh g ⁻¹ over 10,000 cycles at 10 A g ⁻¹ . A pre‐sodiated UNMC//Na₃V₂(PO₄)₃ full cell achieves 95.14% initial Coulombic efficiency and an active‐material‐level energy density of 200 Wh kg ⁻¹ . Depth‐resolved/in situ characterizations and DFT consistently indicate that N/O synergy strengthens Na⁺ anchoring, promotes an inorganic‐rich inner SEI, and suppresses polarization, establishing a transferable structure–interface co‐design paradigm for durable, high‐rate hard‐carbon anodes.