Tailoring the Bulk and Interfacial Environments of Hard Carbons for High‐Rate and Low‐Temperature Sodium‐Ion Batteries

ABSTRACT Hard carbons (HCs) are promising anodes for sodium‐ion batteries (SIBs) but suffer from irreversible Na ⁺ trapping, inadequate rate capability, and compromised low‐temperature performance, primarily due to microstructural defects and suboptimal surface chemistry. Herein, an in situ‐transformation carbonization strategy is proposed to synthesize surface low‐concentration N, P‐doped hard carbons (NP‐HCs) for high‐rate and low‐temperature SIBs. A heteroatom‐enriched polyphosphazene is conformally coated onto poplar wood precursors, with triethylamine playing a dual‐function role in facilitating polymerization and precursor modification. This strategy endows the NP‐HCs with a tailored interfacial environment for fast Na ⁺ desolvation and transport, while establishing a bulk environment featuring abundant closed pores and expanded interlayer spacings. Consequently, NP‐HCs deliver an ultrahigh reversible capacity of 428.8 mAh g ⁻¹ and outstanding rate capability (272.6 mAh g ⁻¹ at 10 C). Notably, remarkable low‐temperature performance is achieved, with exceptional rate capability and cycling stability (93.1% capacity retention over 1200 cycles) at ‐20°C, underscoring their robustness under extreme conditions. Operando/ex situ characterizations coupled with computational studies reveal Na‐storage mechanisms and accelerated kinetics, offering critical insights for high‐performance HCs.