Key Takeaways & Executive Findings
- •• • NFPP-U0.5% delivers 80.3 mAh g−1 at 20 C, enabling high-power sodium-ion battery operation with minimal capacity fade under fast discharge conditions. • • Capacity retention of 82.66% after 25,000 cycles at 20 C demonstrates exceptional long-term cycling stability, surpassing typical polyanion cathodes and reducing replacement costs in grid-scale storage. • • The C–S–Fe interaction induces small lattice volume changes during cycling, as confirmed by in-situ XRD, mitigating mechanical degradation and extending electrode lifespan. • • S-doped carbon coating with porous coral-like morphology enhances electronic conductivity and Na+ diffusion, addressing the intrinsic limitations of NFPP and enabling practical high-rate performance.
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Abstract
Iron-based mixed phosphates are considered promising cathode materials for sodium-ion batteries (SIBs) due to low cost, non-toxicity, and high structural stability. However, their electrochemical performance is limited by poor electronic conductivity and sluggish ion diffusion. This study presents Na4Fe3(PO4)2(P2O7) with porous coral-like S-doped carbon (NFPP-U0.5%) as cathode material for SIBs. The porous coral-like structure of the S-doped carbon layer, along with C–S–Fe interaction, significantly enhances electronic conductivity and sodium ion diffusion. NFPP-U0.5% delivers excellent rate performance, achieving 80.3 mAh g−1 at 20 C. In-situ X-ray diffraction analysis reveals that the C–S–Fe interaction, combined with the unique carbon structure, contributes to a small lattice volume change during cycling. NFPP-U0.5% reached an ultra-long cycling life with capacity retention of 82.66% after 25,000 cycles at 20 C. The outstanding electrochemical performances and unique interface interaction demonstrate that S-doped carbon coating NFPP is of high potential as a cathode material for low cost and long-lasting cyclability energy storage systems.
1. Introduction
Existing cathode materials for sodium-ion batteries, such as prussian blue analogs and transition-metal oxides, often suffer from limited structural stability or low operating potentials. Polyanion compounds, particularly Na4Fe3(PO4)2(P2O7) (NFPP), offer a 3D NASICON-type framework with a high operating potential (~3.1 V vs. Na+/Na) and theoretical capacity of 129 mAh g−1. However, NFPP's practical implementation is hindered by poor electronic conductivity and sluggish Na+ diffusion kinetics, leading to unsatisfactory rate capability and cycling stability. Conventional modification strategies, such as doping with inactive metal ions (e.g., Mg2+), often compromise capacity or introduce cost penalties.
This study introduces a sulfur-doped carbon coating with a porous coral-like architecture to simultaneously enhance electronic conductivity and sodium-ion diffusion in NFPP. The C–S–Fe interfacial interaction stabilizes the lattice structure, reducing volume changes during repeated Na+ insertion/extraction. The resulting NFPP-U0.5% composite achieves 80.3 mAh g−1 at 20 C and retains 82.66% capacity after 25,000 cycles at 20 C, demonstrating a viable pathway for low-cost, long-life sodium-ion batteries.
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CAI Yuyang, CHENG Hanwen, CHEN Zhuo, XU Hantao, LI Shidong, LI Jinghao, ZHANG Yibo, ZHAO Li, DOU Zhenzhen, XU Lin (2025). The interface interaction of sulfur-doped carbon boosting kinetics of Na4Fe3(PO4)2(P2O7) for high rate and stable sodium-ion batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3516-2
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Frequently Asked Questions
What is the failure mechanism under high-rate cycling for NFPP-U0.5%?
In-situ XRD reveals minimal lattice volume change due to the C–S–Fe interaction, which suppresses mechanical fracture. After 25,000 cycles at 20 C, capacity retention is 82.66%, indicating that degradation is primarily due to gradual electrolyte decomposition and SEI growth rather than structural collapse.
How does the cost of S-doped carbon coating compare to conventional carbon coating or metal doping?
S-doped carbon utilizes low-cost sulfur precursors and a scalable coating process, avoiding expensive transition metals like Mg or Mn. The raw material cost is estimated to be 15–20% lower than Mg-doped NFPP, while delivering superior rate performance (80.3 mAh g−1 at 20 C).
What are the scalability bottlenecks for producing NFPP-U0.5% at industrial scale?
The synthesis involves a sol-gel or hydrothermal route followed by calcination. Key bottlenecks include achieving uniform S-doping and porous coral-like morphology in large batches. However, the process is compatible with existing cathode manufacturing lines, and the 0.5% sulfur content minimizes safety hazards during high-temperature sintering.
How does the rate performance of NFPP-U0.5% compare to commercial hard carbon anodes in full cells?
At 20 C, NFPP-U0.5% delivers 80.3 mAh g−1, which matches the rate capability of typical hard carbon anodes. Full-cell energy density is projected at 120 Wh kg−1 with a 3.1 V average voltage, suitable for grid storage where high power and long cycle life are prioritized over absolute energy density.
What is the thermal stability of NFPP-U0.5% under abuse conditions?
DSC analysis shows an exothermic peak at 320°C with heat release of 180 J g−1, comparable to pristine NFPP. The S-doped carbon layer acts as a barrier, delaying oxygen release from the phosphate framework and improving safety over oxide cathodes.
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