Key Takeaways & Executive Findings
- •• • The optimized HC-10 sample achieves a reversible capacity of 450 mAh/g after 800 cycles at 1 A/g, demonstrating excellent long-term cycling stability for sodium-ion battery anodes. • • The dual-stabilization strategy (micropore confinement + C–S covalent bonding) suppresses sulfur migration and interfacial side reactions, which is critical for maintaining capacity retention over extended cycling. • • The introduction of stabilized sulfur species creates additional redox-active sites, enhancing the overall sodium storage capacity beyond that of pristine hard carbon. • • The two-step thermal process (pre-carbonization at 500 °C, sulfur loading at 600 °C) enables precise control over the sulfur distribution and bonding, offering a scalable route for coal-derived carbon anodes.
Abstract
Hard carbon anodes for sodium-ion batteries suffer from limited capacity, low initial Coulombic efficiency, and poor long-term cycling stability. To address these issues, we report a dual-stabilization strategy that combines micropore confinement and chemical bonding to control sulfur species in coal-derived hard carbon. Bituminous coal, with its naturally condensed aromatic framework, serves as the carbon precursor. A two-step thermal process first constructs a microporous carbon framework, followed by gas-phase sulfidation to introduce sulfur. The sulfur is confined within micropores and forms stable covalent C–S bonds with the carbon matrix, providing synergistic physical–chemical stabilization. This suppresses sulfur migration, prevents interfacial side reactions, and introduces additional redox-active sites. The optimized sample (HC-10) delivers a high reversible capacity of 450 mAh/g after 800 cycles at a current density of 1 A/g, with excellent rate capability and cycling stability. Mechanistic analysis reveals that the stabilized sulfur species reversibly participate in sodium-ion storage and improve interfacial kinetics. This work provides an effective strategy for stabilizing sulfur in coal-derived carbon materials and offers insights into the design of high-performance anodes for sodium-ion batteries.
1. Introduction
Sodium-ion batteries (SIBs) are emerging as a cost-effective alternative to lithium-ion systems, yet their commercial deployment is hindered by the lack of high-performance anode materials. Hard carbon derived from coal is attractive due to its low cost and abundant resources, but conventional hard carbon anodes exhibit limited reversible capacity, low initial Coulombic efficiency, and poor cycling stability, particularly at high current densities. The large ionic radius of Na+ induces severe volume expansion and sluggish solid-state diffusion, leading to electrode degradation and capacity fade.
This work addresses these bottlenecks by introducing a dual-stabilization strategy for sulfur species within coal-derived hard carbon. By combining micropore confinement with covalent C–S bonding, the sulfur is physically and chemically anchored, preventing its migration and side reactions at the electrode–electrolyte interface. This approach not only stabilizes the electrode structure but also introduces additional redox-active sites, thereby enhancing capacity and rate performance. The reported HC-10 sample achieves a high reversible capacity of 450 mAh/g after 800 cycles at 1 A/g, demonstrating the effectiveness of this strategy in overcoming the intrinsic limitations of hard carbon anodes.
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Zhu Hong, Cui Yu, Han Qi, Li Cunhe, Zhou Rui, Chi Xiaoyu, Shen Yanfeng, Wang Meijun, Guo Chunli, Chang Liping (2026). Stabilization of Sulfur Species in Coal-Derived Hard Carbon via Micropore Confinement and Chemical Bonding for Enhanced Sodium Storage. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61106-7
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Frequently Asked Questions
What is the specific role of micropore confinement versus chemical bonding in stabilizing sulfur species, and how does each contribute to the electrochemical performance?
Micropore confinement physically restricts sulfur migration, while C–S covalent bonding chemically anchors sulfur to the carbon matrix. Together, they suppress sulfur loss and interfacial side reactions, maintaining structural integrity. The optimized HC-10 sample exhibits a reversible capacity of 450 mAh/g after 800 cycles at 1 A/g, indicating that the dual stabilization effectively preserves active sites and enhances cycling stability.
How does the sulfur loading process affect the initial Coulombic efficiency and long-term cycling stability of the hard carbon anode?
The two-step thermal process (pre-carbonization at 500 °C, sulfur loading at 600 °C) ensures uniform sulfur distribution and strong C–S bonding. This minimizes irreversible side reactions, leading to high initial Coulombic efficiency. The stabilized sulfur species participate reversibly in sodium storage, contributing to the high capacity retention observed over 800 cycles.
What are the potential scalability challenges of this two-step thermal process for industrial production of coal-derived hard carbon anodes?
The process involves controlled pre-carbonization and gas-phase sulfidation, which are scalable unit operations. However, maintaining uniform sulfur distribution and micropore structure at scale requires precise temperature and atmosphere control. The use of bituminous coal as a low-cost precursor offers economic advantages, but batch-to-batch variability in coal composition may affect reproducibility.
How does the performance of HC-10 compare to state-of-the-art hard carbon anodes in terms of rate capability and cycling stability?
HC-10 achieves a reversible capacity of 450 mAh/g after 800 cycles at 1 A/g, which is competitive with or superior to many reported hard carbon anodes. The dual-stabilization strategy enhances rate capability by improving interfacial kinetics, as evidenced by the stable cycling at high current densities.
What is the underlying sodium storage mechanism in the sulfur-stabilized hard carbon, and how does it differ from pristine hard carbon?
The stabilized sulfur species introduce additional redox-active sites that reversibly react with sodium ions, contributing to capacity beyond the typical adsorption–intercalation mechanism of hard carbon. This also improves interfacial kinetics, as shown by the enhanced rate performance and cycling stability.
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