Key Takeaways & Executive Findings
- •• • The dual-functional pre-sodiation strategy elevates ICE from 54.2% (untreated HC) to 99.5% (pCH4-HC), directly addressing the primary bottleneck for commercial SIB anodes. • • pCH4-HC delivers a high reversible capacity of 321.7 mAh g−1, indicating effective compensation for initial sodium loss and enhanced active site utilization. • • Long-term cycling stability is demonstrated with 74.0% capacity retention after 1000 cycles at 300 mA g−1, underscoring the durability of the NaF-rich SEI. • • Full-cell integration with NFM cathode yields 81.9 mAh g−1 after 100 cycles, proving practical viability for large-scale energy storage applications.
Abstract
Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs) but suffers from low initial Coulombic efficiency (ICE) and unstable solid electrolyte interphase (SEI). Here, we report a dual-functional strategy combining surface engineering and solution chemical pre-sodiation. A graphitic carbon coating on HC acts as a conductive buffer network and shields surface defects, while sodium biphenyl (Na-Bp) pre-sodiation drives sodium ions into the material via a potential difference, inducing a pre-SEI layer that matures into a thin, dense, NaF-rich inorganic SEI during cycling. This approach compensates for irreversible sodium loss and enhances cycling stability. The pre-sodiated electrode (pCH4-HC) achieves an ICE of 99.5% and a reversible capacity of 321.7 mAh g−1, compared to 54.2% for untreated HC. Long-term cycling shows 74.0% capacity retention after 1000 cycles at 300 mA g−1. In full-cells with NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode, pCH4-HC||NFM delivers 81.9 mAh g−1 after 100 cycles, demonstrating excellent stability and rate performance. This dual-strategy approach validates the adaptability of pre-sodiation technology for high-performance SIBs.
1. Introduction
Hard carbon (HC) anodes for sodium-ion batteries (SIBs) face critical commercial hurdles: low initial Coulombic efficiency (ICE) and unstable solid electrolyte interphase (SEI) formation. These issues stem from surface defects that irreversibly trap sodium ions and from continuous SEI fragmentation during cycling, leading to excessive electrolyte consumption and capacity fade. Conventional surface coatings, such as Al2O3 via atomic layer deposition or pitch-based carbon via chemical vapor deposition, have improved ICE to 75% and 85.3%, respectively, but remain insufficient for practical deployment. The fundamental challenge is to simultaneously shield surface defects and pre-compensate for sodium loss without compromising electrode kinetics.
This study introduces a dual-functional strategy that synergizes graphitic carbon coating with solution chemical pre-sodiation using sodium biphenyl (Na-Bp). The carbon coating acts as a conductive buffer and defect shield, while Na-Bp pre-sodiation leverages a potential difference to insert sodium ions, forming a pre-SEI layer. This pre-SEI matures into a thin, dense, NaF-rich inorganic layer during cycling, which is crucial for stable long-term performance. The approach achieves an ICE of 99.5% and a reversible capacity of 321.7 mAh g−1, with 74.0% capacity retention after 1000 cycles. When paired with a NaNi1/3Fe1/3Mn1/3O2 cathode, the full-cell delivers 81.9 mAh g−1 after 100 cycles, demonstrating the practical viability of this pre-sodiation technology for high-performance SIBs.
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Siyuan Lin, Haihan Zhang, Zhenxin Huang, Qianyu Zhang, Yunhua Xu, Chengyong Shu, Yuping Wu, Wei Tang (2026). Dual-Functional Chemical Pre-Sodiation of Carbon-Coated Hard Carbon Anodes with Initial Coulombic Efficiency up to 99.5% for Sodium-Ion Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3944-9
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Frequently Asked Questions
What is the specific mechanism by which the Na-Bp pre-sodiation induces a pre-SEI layer, and how does this pre-SEI influence the composition and thickness of the mature SEI after cycling?
Na-Bp pre-sodiation creates a potential difference that drives sodium ions into the carbon-coated HC, forming a pre-SEI layer on the surface. This pre-SEI acts as a template, promoting the formation of a thinner, more homogeneous, and NaF-rich inorganic SEI during subsequent cycling. The NaF-rich composition enhances mechanical stability and ionic conductivity, reducing continuous electrolyte decomposition and improving long-term cycling performance.
How does the graphitic carbon coating specifically shield surface defects, and what is the quantitative impact on reversible capacity compared to uncoated HC?
The graphitic carbon coating, applied via CVD, forms a uniform, thin layer that physically covers surface defects on HC, reducing irreversible sodium trapping. This shielding effect increases the reversible capacity from an unspecified baseline to 321.7 mAh g−1 for pCH4-HC, while also contributing to the high ICE of 99.5% by minimizing side reactions.
What are the long-term cycling stability metrics under practical current densities, and how does the SEI composition evolve over 1000 cycles?
At a current density of 300 mA g−1, pCH4-HC retains 74.0% of its initial capacity after 1000 cycles. The SEI, initially formed as a pre-SEI, matures into a thin, dense, NaF-rich inorganic layer that remains stable, preventing continuous electrolyte consumption and capacity fade.
How does the full-cell performance with NFM cathode compare to half-cell metrics, and what are the implications for commercial device integration?
In full-cells with NFM cathode, pCH4-HC delivers a reversible capacity of 81.9 mAh g−1 after 100 cycles, demonstrating stable cycling and rate performance. This indicates that the pre-sodiation strategy effectively compensates for sodium loss in a practical full-cell configuration, making it suitable for large-scale SIB applications.
What is the scalability potential of the chemical pre-sodiation process in terms of cost, safety, and environmental impact compared to conventional pre-lithiation methods?
The Na-Bp pre-sodiation is a solution-based process that can be easily scaled up, similar to pre-lithiation techniques. It operates at room temperature and uses commercially available reagents, offering cost advantages. The process is safe under controlled conditions, and the dual-functional approach reduces the need for excess cathode material, improving overall cell energy density and cost-effectiveness.
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