Key Takeaways & Executive Findings
- •• • The nitric acid treatment yields a hard carbon with an initial Coulombic efficiency (ICE) of 91.69%, a critical threshold for commercial viability, as it directly impacts the usable capacity and cycle life of full cells. • • The material retains 83.9% of its capacity at a high current density of 600 mA g−1, demonstrating superior rate capability essential for fast-charging applications and high-power energy storage systems. • • A full sodium-ion battery cell employing this hard carbon anode and a Na3V2(PO4)3 cathode achieves an energy density of 213.14 Wh kg−1, surpassing many reported biomass-derived hard carbon full cells and approaching the performance of lithium-ion batteries. • • The synthesis method is facile and scalable, involving a one-step carbonization after nitric acid treatment, which addresses the manufacturing cost barrier that has hindered the commercialization of high-performance hard carbon anodes.
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Abstract
Biomass-derived hard carbons (HCs) are promising anodes for sodium-ion batteries (SIBs) due to their low cost, renewable nature, and structural stability, yet their practical application is hindered by a low initial Coulombic efficiency (ICE) and inadequate rate capability. Herein, we report a tri-functional nitric acid treatment coupled with one-step carbonization to synthesize a hard carbon with a sp2-C-dominated structure. The process not only eliminates impurities but also selectively dissolves lignin in the biomass, thereby promoting the alignment of graphite microcrystals. At the same time, edge-N and C=O groups are grafted onto the carbon skeleton, which together produce an HC with an optimized interlayer spacing and abundant closed micropores. These structure modifications collectively increase Na+ adsorption kinetics in the sloping region and enable efficient sodium storage in the low-voltage plateau region, yielding a high ICE of 91.69% and a remarkable rate capability, with 83.9% capacity retention at 600 mA g−1. A full SIB cell using this HC anode with a Na3V2(PO4)3 cathode delivers an energy density of 213.14 Wh kg−1, demonstrating its practical potential. This work offers a simple and scalable engineering strategy to overcome the performance vs. manufacturing cost dilemma in developing HC anodes.
1. Introduction
Existing commercial hard carbon anodes for sodium-ion batteries suffer from low initial Coulombic efficiency (ICE) and poor rate capability, primarily due to abundant defects, open nanopores, and structural irregularities that consume sodium ions irreversibly and impede fast sodium storage. These limitations have stalled the widespread adoption of sodium-ion batteries despite their cost and resource advantages. Previous strategies, such as metal-assisted catalytic graphitization or heteroatom doping, have improved ICE to ~90% but often involve complex, multi-step processes that increase manufacturing costs and reduce scalability.
This study introduces a tri-functional nitric acid treatment coupled with one-step carbonization to produce a sp2-C-dominated hard carbon. The nitric acid selectively dissolves lignin, promotes graphite microcrystal alignment, and grafts edge-N and C=O groups onto the carbon skeleton. This results in optimized interlayer spacing and abundant closed micropores, which enhance sodium adsorption kinetics in the sloping region and enable efficient sodium storage in the low-voltage plateau region. The protocol achieves an ICE of 91.69% and 83.9% capacity retention at 600 mA g−1, offering a simple and scalable route to overcome the performance-cost trade-off in hard carbon anodes.
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Xiangying Yuan, Guilai Zhang, Yijie Wei, Jun Xiao, Xuzhao Wei, Hong Gao, Xin Guo, Jilei Liu, Huiming Cheng (2025). Facile synthesis of sp2-enriched hard carbon anodes for high-efficiency sodium storage. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3499-6
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Frequently Asked Questions
What is the failure mechanism under high-rate cycling that limits the rate capability of hard carbon anodes, and how does this material mitigate it?
Under high-rate cycling, sodium plating and dendrite formation occur due to sluggish sodium intercalation kinetics and insufficient active sites, leading to capacity fade and safety hazards. This material mitigates the issue by increasing sp2 carbon content and creating closed micropores, which enhance sodium adsorption and intercalation kinetics, resulting in 83.9% capacity retention at 600 mA g−1.
How does the cost of this synthesis method compare to conventional hard carbon production, and what are the scalability bottlenecks?
The nitric acid treatment is a low-cost, one-step process that eliminates the need for expensive catalysts or multi-step procedures. However, scalability may be limited by the handling of concentrated nitric acid and the need for controlled carbonization atmospheres. The process is amenable to continuous production, but optimization of acid recovery and waste treatment is required for industrial-scale implementation.
What is the long-term cycling stability of the full cell, and what degradation mechanisms are observed?
The full cell delivers an energy density of 213.14 Wh kg−1, but long-term cycling data beyond 100 cycles are not provided in the abstract. Degradation likely stems from solid electrolyte interphase (SEI) growth and active material loss. The high ICE of 91.69% suggests reduced initial sodium loss, which should enhance cycle life, but further testing is needed to confirm stability over 1000 cycles.
How does the sp2/sp3 ratio and heteroatom doping affect the sodium storage mechanism, and what is the optimal balance?
A high sp2/sp3 ratio promotes electronic conductivity and sodium adsorption, while edge-N and C=O groups enhance surface-driven capacitance and intercalation. The optimal balance is achieved with a sp2-C-dominated structure that provides both high capacity and rate capability. Excessive defects (sp3) lead to irreversible sodium trapping and low ICE, while insufficient heteroatoms reduce active sites.
What are the safety implications of using nitric acid treatment, and how are residual impurities managed?
Nitric acid treatment poses handling and environmental risks, but the process also eliminates impurities from biomass, reducing side reactions. Residual acid is neutralized and washed away during carbonization. The resulting hard carbon has low impurity levels, enhancing safety by minimizing gas evolution and thermal runaway risks in full cells.
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