Nitrogen/Phosphorus Dual-Doped Hard Carbon Anode with High Initial Coulombic Efficiency for Superior Sodium Storage

Among anode materials for sodium-ion batteries (SIBs), hard carbon (HC) gains more attention due to its low cost, high electronic conductivity, and renewable resources. However, sluggish kinetics result in its low-rate capability and unfavorable cycle stability. Moreover, HC often presents low initial Coulombic efficiency (ICE), which also limits the utilization of the battery capacity and energy density. Herein, nitrogen and phosphorus dual-doped HC (denoted as NPHC) with network structure is synthesized by a facile interfacial polymerization method, which endows the NPHC with synergistic effects of the enlarged interlayer spacing and improved conductivity. Consequently, the obtained NPHC exhibits a high reversible capacity (286 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (144 mAh g⁻¹ at 10 A g⁻¹), high ICE (71 %), and remarkable cyclability over 2000 cycles at 1 A g⁻¹. Moreover, the storage mechanism of sodium ions is examined by a series of ex-situ characterizations. Additionally, when coupled with Na₃V₂(PO₄)₂F₃ as a cathode, the full cell delivers superior cyclability (capacity retains 90 % over 300 cycles at 1 A g⁻¹). This work may shed new insight into designing other carbon-based electrode materials for high-performance energy storage.