Background
Driven by the rapid advancement of renewable energy and the global energy transition, sodium-ion batteries (SIBs) have emerged as a strong candidate for next-generation energy storage due to their resource abundance, low cost, and similarities to lithium-ion batteries. However, conventional graphite anodes suffer from poor sodium storage performance due to the larger ionic radius and specific thermodynamic properties of sodium. Hard carbon materials, with highly disordered structures, wide interlayer spacing, and high sodium storage capacity, are considered ideal anode candidates. A three-stage sodium storage mechanism, involving “adsorption-intercalation-pore filling”, provides a theoretical basis for performance optimization. However, hard carbon anodes face two key challenges: low initial Coulombic efficiency (ICE, typically 50%–80%) and insufficient reversible capacity (below 300 mAh g −1 ). Early improvements focused on precursor modulation and carbonization processes, but high surface areas often led to excessive interfacial side reactions, reducing initial efficiency. Recent strategies, such as constructing closed ultramicropores (< 0.7 nm), enhance sodium-ion transport by excluding solvent molecules, thus preventing excessive electrolyte decomposition and SEI overgrowth, but they reduce surface adsorption capacity. Heteroatom doping (e.g., with N and S) has been used to introduce active sites, widen interlayer spacing, and improve electron conductivity, boosting reversible capacity. However, excessive mesopore formation can still compromise initial efficiency. Reports of biomass-derived hard carbon materials achieving both high ICE (> 80%) and high reversible capacity (> 350 mAh g −1 ) remain scarce. The main challenge is to balance pore structure and surface chemistry to improve ICE without sacrificing reversible capacity.
Research Progress
A research team led by Professors Caichao Wan and Yiqiang Wu at Central South University of Forestry and Technology reports on synergistic strategies that combine ultramicropore confinement with electronic-state modulation in sustainable lignin-derived hard carbon (N-S@HDM) to achieve robust sodium-ion batteries. They utilized sodium lignosulfonate (SLS), a sulfonated polymer found in papermaking sludge, as a precursor for N-S@HDM. The rationale for selecting SLS as the precursor for N-S@HDM lies in its unique advantages: (1) its intrinsic sulfur groups act as a self-doping source for defect engineering and electronic modulation; (2) its cross-linkable structure enables preoxidation to create a carbon matrix with expanded interlayers and closed pores, key to reconciling capacity and ICE; and (3) SLS is abundant and low-cost, ensuring sustainability and scalability. A pre-oxidation treatment was firstly applied to SLS, and its effects were assessed using thermogravimetry–mass spectrometry (TG-MS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy (Fig. 1). The process involved several stages: from 30 to 180 °C, adsorbed and bound water were released, accompanied by demethylation; from 180 to 325 °C, esterification and cross-linking occurred; and above 325°C, aromatization took place. This pre-oxidation step tailored the chemical structure, enhancing cross-linking and structural stability, preventing the alignment of graphitic microcrystals and reducing parallel carbon layer stacking during subsequent carbonization, promoting the formation of hard carbon.
Subsequently, the pre-oxidized material underwent high-temperature pyrolysis with urea to simultaneously introduce N/S co-doping and create ultramicropores. The effects of N/S co-doping on the material's electronic states were analyzed using density functional theory (DFT). Heteroatom doping involves substituting carbon atoms with heteroatoms like N and S, altering charge and spin density, which tunes the work function, strengthens Na + adsorption, and creates active sites. DFT results show that N/S co-doping significantly enhances electrochemical performance. Specifically, the Na + adsorption energy of the N/S co-doped sample (N-S@HDM-1300, pyrolyzed at 1300 °C) reached −0.96 eV, a notably higher value than the −0.54 eV of the undoped carbon. Charge density plots reveal expanded electron delocalization in N-S@HDM-1300, lowering the migration barrier for Na + ions. Work function analysis shows a lower Φ value, facilitating electron transfer and Fermi level alignment. The density of states (DOS) profile indicates that N-S@HDM-1300 exhibits metallic characteristics, unlike pristine carbon, which remains semiconducting. In summary, heteroatoms play a crucial role in modulating the carbon framework’s electronic structure, enabling efficient sodium-ion insertion and extraction. (Fig. 2)
As an anode material for sodium-ion batteries (SIBs), N S@HDM 1300 exhibits outstanding electrochemical properties. During the first cycle, it achieves a reversible capacity of 401.5 mAh g − 1 and an exceptionally high initial Coulombic efficiency (ICE) of 90.6%, with a capacity contribution of 41.9% originating from the low-voltage plateau region. Rate performance evaluation indicates that the material maintains a capacity of 265 mAh g − 1 at a high current density of 5 A g − 1 , which represents 68.7% of the capacity delivered at 0.03 A g − 1 . Regarding long-term cycling stability, N S@HDM 1300 sustains a discharge capacity of 307.3 mAh g − 1 after 500 cycles at 300 mA g − 1 , achieving a high capacity retention of 95.0%. These results collectively demonstrate that the material synergistically combines high capacity and high initial efficiency with excellent rate capability and cycling stability. (Fig. 3)
Future Prospects
By pioneering a dual-modulation strategy that targets both pore architecture and electronic state in biomass-derived carbon, the research team has successfully developed a sodium-ion battery anode material that combines high initial Coulombic efficiency with high reversible capacity. To drive this material system toward practical use, the authors outline the following future research directions: (1) Investigating the interfacial evolution and performance degradation mechanisms under extreme conditions (e.g., ultra-low and high temperatures) to establish a theoretical foundation for broad-temperature operation; (2) Focusing on scaling up electrode fabrication and optimizing full-cell integration, with particular attention to validating long-term cycling stability and safety in practical pouch cell configurations; (3) Systematically evaluating the material performance limits under various operational conditions, targeting specific applications such as renewable energy storage and portable electronics. This work aims to bridge the gap between laboratory research and industrial application, accelerating the commercialization of high-performance, cost-effective sodium-ion batteries.
Sources: https://spj.science.org/doi/10.34133/research.1039
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Synergistic Ultramicropore-Confined and Electronic-State Modulation Strategies in Sustainable Lignin-Derived Hard Carbon for Robust Sodium-Ion Batteries
15-Jan-2026