Amine-Aldehyde Condensation-Derived N-Doped Hard Carbon Microspheres for High-Capacity and Robust Sodium Storage

Hard carbon is generally accepted as the choice of anode material for sodium-ion batteries. However, integrating high capacity, high initial Coulombic efficiency (ICE), and good durability in hard carbon materials remains challenging. Herein, N-doped hard carbon microspheres (NHCMs) with abundant Na⁺ adsorption sites and tunable interlayer distance are constructed based on the amine-aldehyde condensation reaction using m-phenylenediamine and formaldehyde as the precursors. The optimized NHCM-1400 with a considerable N content (4.64%) demonstrates a high ICE (87%), high reversible capacity with ideal durability (399 mAh g^-1 at 30 mA g^-1 and 98.5% retention over 120 cycles), and decent rate capability (297 mAh g^-1 at 2000 mA g^-1 ). In situ characterizations elucidate the adsorption-intercalation-filling sodium storage mechanism of NHCMs. Theoretical calculation reveals that the N-doping decreases the Na⁺ adsorption energy on hard carbon.