N-Doping Activated Presodiation Enhances Sodium-Ion Provision in Hard Carbon Anodes

Hard carbon (HC) is considered the most promising anode material for commercial sodium-ion batteries (SIBs), yet it still faces critical challenges such as low initial Coulombic efficiency (ICE). Presodiation can improve ICE by compensating for sodium loss caused by irreversible defects in HC. However, research on how the active sodium content during presodiation affects the formation and stability of the preconstructed solid electrolyte interface (SEI) remains limited. Based on this background, this work introduces many pyrrolic nitrogen (N-5) sites into HC through pyrolysis of a urea-HC composite, working in synergy with chemical presodiation to enhance sodium-ion provision. Experimental and theoretical investigations reveal that the N-5 sites strengthen the adsorption capability for Na⁺ and its ability to capture PF₆ ^- from the electrolyte, promoting the preformation of a stable, inorganic-rich SEI layer. As a result, the PS-0.5NHC achieves an excellent ICE of 98.54%, a capacity retention of 90.6% after 1000 cycles at 1.5 A g^-1. In a full-cell configuration, the ICE is also significantly improved from 70.74% to 91.53%. This work provides a novel design concept for mitigating the inherent defects of the HC anode.