Nitrogen-Doped Hard Carbon Anode from Redwood Biomass for Sodium-Ion Batteries with High Initial Coulombic Efficiency...

Hard carbon (HC) is regarded as a promising anode candidate for sodium-ion batteries (SIBs) due to its natural abundance, high theoretical capacity, optimal operation potential, and superior cycling stability. However, it suffers from low initial coulombic efficiency (ICE) and poor rate capability. Herein, a series of HC-x are prepared by carbonizing chemically-treated waste African redwood at various temperatures of x °C under Ar for 2 h. The optimal HC-1300 is further nitrogen-doped by annealing with melamine at 800 °C under N₂ for 2 h to form N-HC-1300. The N-HC-1300 exhibits a high discharge capacity of 350.1 mAh g^-1 at 0.1 C, a high ICE of 90.1%, excellent rate capability (e.g., 260.4 mAh g^-1 at 1 C), and superior cycling stability (e.g., 262.7 mAh g^-1 after 1200 cycles at 1 C with 96.5% retention). When paired with an N-HC-1300 electrode, the Na₃V₂(PO₄)₃ (NVP) delivers a high initial discharge capacity of 108.7 mAh g^-1, an average operating voltage of 3.3 V, and an ICE of 92.9%. At 1 C, the NVP retains 80.2 mAh g^-1 after 300 cycles with 93.4% capacity retention. The enhanced performance is attributed to hierarchical pore and channel structures, enlarged carbon layer spacing, and nitrogen doping of N-HC-1300.