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 1/3 Ni 1/3 Mn 1/3 O 2 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. An innovative incorporating modifier into lignite co-thermal carbonization strategy is proposed to precisely engineer pseudo-graphitic domains and closed pore in lignite-based hard carbon. The full-cell system based on the modified lignite-based hard carbon anode paired with a NaFe 1/3 Ni 1/3 Mn 1/3 O 2 cathode achieves a superior energy density of 240.8 Wh·kg -1 , which provides an efficient strategy for engineering advanced hard carbon anodes towards practical applications in high-energy-density SIB. • An innovative modifier-assisted co-thermal carbonization strategy is proposed to precisely tailor the pseudo-graphitic domains and closed pores in lignite-derived hard carbons (LHC). • The modified LHC possesses a high proportion of pseudo-graphitic domains and more closed pores, with some electronegative heteroatoms doping. • The modified LHC as anode for sodium-ion batteries (SIBs) delivers an enhanced Na + storage performance. • A synergistic sodium storage mechanism involving adsorption, intercalation, and pore filling was revealed by electrochemical kinetic investigations combined with ex-situ XRD analysis.