In the relentless pursuit of sustainable and efficient energy storage solutions, sodium-ion batteries (SIBs) have rapidly garnered attention as a promising alternative to the widely used lithium-ion systems. This surge in interest is largely driven by sodium’s abundant availability, lower cost, and chemical similarities to lithium. However, realizing the full potential of SIBs hinges crucially on the development of high-performance anode materials that can overcome sodium’s larger ionic radius and distinct electrochemical properties. Traditional graphite anodes, while effective for lithium-ion batteries, underperform significantly in sodium storage applications, primarily due to these intrinsic material limitations. Researchers have thus turned their focus to hard carbons, whose disordered microstructures and expanded interlayer spacings provide a more accommodating framework for sodium ions. Yet, despite their promise, conventional hard carbons face persistent challenges: notably, low initial Coulombic efficiencies, typically ranging between 50% and 80%, which imply significant irreversible capacity loss, and reversible capacities often restricted below 300 mAh g⁻¹, limiting practical energy densities.
The sodium storage process in hard carbon anodes often follows a complex three-stage mechanism encompassing adsorption at defect sites, intercalation between graphene-like layers, and pore filling within the carbon matrix. This multi-faceted interaction offers a theoretical roadmap for structural optimization, aiming to enhance both efficiency and capacity. Early efforts concentrated on tailoring the carbon precursor materials and refining carbonization protocols to modulate the pore structure and electronic properties. Nevertheless, these approaches frequently resulted in materials with excessively high surface areas. This increased surface exposure exacerbates parasitic electrolyte decomposition and solid electrolyte interphase (SEI) growth, severely undermining the initial Coulombic efficiency. Recent advances have attempted to circumvent this dilemma by engineering closed ultramicropores smaller than 0.7 nm. Such confined pores act as molecular sieves, excluding solvent molecules and thereby minimizing electrolyte breakdown and SEI overgrowth. Although successful in reducing interfacial side reactions, these microporous architectures concurrently reduce the surface area available for sodium adsorption, potentially limiting capacity.
In parallel, heteroatom doping strategies have gained traction, particularly involving nitrogen and sulfur atoms. These dopants introduce electron-rich active sites and expand interlayer spacings, simultaneously enhancing electrical conductivity and ion storage capabilities. However, an overabundance of mesopores—larger than micropores but smaller than macropores—often accompanies heteroatom doping, which detrimentally impacts initial Coulombic efficiency by promoting excessive electrolyte interaction. Although biomass-derived hard carbons have shown promise owing to their renewable nature and intrinsic heteroatom content, achieving a balanced combination of high initial Coulombic efficiency (>80%) and elevated reversible capacity (>350 mAh g⁻¹) remains an elusive goal in the field.
A groundbreaking study by a research group led by Professors Caichao Wan and Yiqiang Wu at Central South University of Forestry and Technology presents an innovative dual-modulation strategy to tackle these challenges. They focused on leveraging sodium lignosulfonate (SLS)—a sulfonated polymer commonly found in papermaking sludge—as a sustainable precursor for synthesizing heteroatom-doped hard carbon with tailored pore structures. The uniqueness of SLS lies in its inherent sulfur-containing groups, which serve as an efficient self-doping source for defect engineering and electronic structure modulation. Additionally, its cross-linkable architecture facilitates controlled pre-oxidation, enabling the formation of a robust carbon matrix endowed with expanded interlayer spacings and closed ultramicropores. Such structural features are pivotal in reconciling the competing requirements of high reversible capacity and superior initial efficiency. Moreover, the abundant availability and low cost of SLS underscore the potential scalability and environmental friendliness of this approach.
The research commenced with a meticulous pre-oxidation process applied to SLS, systematically characterized through thermogravimetry–mass spectrometry (TG-MS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and solid-state carbon-13 nuclear magnetic resonance (NMR). This multi-step thermal treatment revealed distinct stages: initial water release and demethylation below 180 °C, followed by esterification and cross-linking between 180 and 325 °C, and finally a dominant aromatization process beyond 325 °C. The pre-oxidation step critically restructured the molecular framework, promoting covalent cross-linking that stabilizes the carbon network. Importantly, this chemical modification inhibits the typical graphitic microcrystal alignment during subsequent high-temperature carbonization, effectively preserving larger interlayer distances and closed ultramicroporous architectures conducive to sodium ion accommodation.
Subsequent pyrolysis at elevated temperatures in the presence of urea facilitated simultaneous nitrogen and sulfur co-doping, further enhancing the electronic and ionic transport properties of the resultant hard carbon material, designated as N-S@HDM. Density Functional Theory (DFT) calculations elucidated the profound impact of heteroatom doping on the electronic landscape. Replacing carbon atoms with nitrogen and sulfur atoms introduced localized modifications in charge and spin densities, which adjusted the work function of the carbon host. These changes manifested as stronger sodium-ion adsorption energies, measured at -0.96 eV for N-S@HDM—substantially more favorable than the -0.54 eV observed for undoped carbon. Additionally, charge density analyses showcased expanded electron delocalization within the doped framework, reducing sodium-ion migration barriers. Work function reductions further enabled enhanced electron transfer kinetics and better matching of Fermi levels between the electrode and electrolyte. A comprehensive density of states (DOS) analysis revealed a transition from semiconducting behavior in pristine carbon to metallic-like conductivity in N-S@HDM, underpinning markedly improved electrochemical performances.
Electrochemical evaluations highlighted the remarkable capabilities of N-S@HDM as a high-performance sodium-ion battery anode. The material delivered an outstanding initial reversible capacity of 401.5 mAh g⁻¹ and achieved an exceptional initial Coulombic efficiency of 90.6%, underscoring minimal irreversible capacity loss. Notably, 41.9% of this capacity derived from the low-voltage plateau region, indicative of efficient sodium-ion storage within the ultramicropores. The anode demonstrated excellent rate capabilities, retaining 265 mAh g⁻¹ at a high current density of 5 A g⁻¹—a robust 68.7% of its capacity at low rates—showcasing its suitability for fast-charging applications. Beyond rate performance, the material exhibited impressive cycling stability, maintaining a capacity of 307.3 mAh g⁻¹ after 500 cycles at 300 mA g⁻¹, corresponding to a capacity retention of 95.0%. Together, these results confirm that combining ultramicropore confinement with electronic state modulation offers a synergistic avenue to break the trade-off between capacity and initial efficiency, a persistent bottleneck in hard carbon development.
Looking ahead, the research team envisions several critical pathways to propel this promising material toward widespread commercialization. A foundational step involves in-depth investigation of interfacial evolution and degradation mechanisms under extreme operational environments, such as ultra-low and elevated temperatures, to underpin the design of durable batteries with reliable performance across diverse climates and applications. Scaling up synthesis methodologies and developing electrode fabrication processes compatible with industrial demands remain essential. This includes optimizing full-cell configurations, verifying long-term cycling stability, and rigorously assessing safety profiles, especially within pouch cell architectures that mimic real-world devices. Further, targeted evaluations of performance limits under varied operational parameters will align material capabilities with specific end-use scenarios, from grid-level renewable energy storage to portable electronics with fast charge and long cycle life requirements.
This visionary dual-regulation strategy, harnessing biomass-derived lignin as a sustainable carbon source, represents a paradigm shift in the design of sodium-ion battery anodes. By simultaneously engineering nanostructured pore environments and fine-tuning electronic states via heteroatom doping, the study surmounts longstanding limitations in initial Coulombic efficiency and reversible capacity without compromising rate or cycling performance. Such breakthroughs not only advance fundamental scientific understanding but also expedite the transition of sodium-ion technology from laboratory concepts to commercially viable energy storage solutions. The implications resonate profoundly in the context of global energy transitions, where low-cost, eco-friendly, and high-performance batteries are pivotal to integrating intermittent renewable resources and fostering energy equity worldwide.
In summary, this research vastly enriches the toolkit for hard carbon anode optimization and sets a new benchmark for sustainable sodium-ion battery technologies. The meticulous orchestration of chemical, structural, and electronic modifications—rooted in eco-abundant lignin precursors—paves the way for next-generation energy storage devices that marry performance, affordability, and environmental stewardship. As the sodium-ion battery landscape evolves, integrating such sophisticated materials will be central to meeting the escalating demands of electrification in transportation, grid stabilization, and portable electronics, propelling the vision of a cleaner and more resilient energy future.
Subject of Research: Not applicable
Article Title: Synergistic Ultramicropore-Confined and Electronic-State Modulation Strategies in Sustainable Lignin-Derived Hard Carbon for Robust Sodium-Ion Batteries
News Publication Date: 15-Jan-2026
Web References: http://dx.doi.org/10.34133/research.1039
Image Credits: Copyright © 2026 Yuzhong Xie et al.
Keywords
Sodium-ion batteries, hard carbon anodes, sodium lignosulfonate, ultramicropores, heteroatom doping, nitrogen doping, sulfur doping, initial Coulombic efficiency, reversible capacity, lignin-derived carbon, pre-oxidation, electronic-state modulation, density functional theory, renewable energy storage
