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

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.