Modifier-assisted co-thermal carbonization of lignite for hard carbon with enriched pseudo-graphitic domains and closed pores toward enhanced sodium storage

Lignite, characterized by disordered aromatic lamellae, natural pores and microfractures, and surface-active functional groups, has emerged as a high-quality precursor for advanced hard carbon anodes in sodium-ion batteries (SIBs). In this work, we propose a modifier-assisted co-thermal carbonization strategy to precisely tailor the pseudo-graphitic domains and closed pores in lignite-derived hard carbons (LHC), aiming to enhance their Na⁺ storage capabilities. Through incorporating urea during carbonization, the resulting nitrogen-doped lignite-based hard carbon (N-LHC) possesses a high content of pseudo-graphitic domains (43.6 %), an optimized interlayer distance (0.373 nm), and a greater number of closed pores interconnected by short-range ordered microcrystals. Benefiting from these structural and chemical modifications, the N-LHC anode delivers a high reversible capacity of 380 mAh·g⁻¹, with the plateau capacity of 207 mA g⁻¹ and an improved initial Coulombic efficiency (ICE) of 79.1 %. When paired with a NaFe_1/3Ni_1/3Mn_1/3O₂ cathode, the full-cell achieves a notable energy density of 240.8 Wh·kg⁻¹ at 20 mA g⁻¹ and retains 157.5 Wh·kg⁻¹ at 200 mA g⁻¹ with a power density of 230.7 W kg⁻¹. Electrochemical kinetics combined with ex-situ X-ray diffraction analyses reveal a synergistic sodium storage mechanism involving adsorption, intercalation, and pore filling. DFT calculations further confirm the critical role of heteroatoms doping in enhancing Na⁺ adsorption kinetics and overall storage capacity. This work provides an effective strategy for engineering advanced hard carbon anodes toward practical high-energy-density SIBs.