Comprehensive understanding of closed pores in hard carbon anode for high‑energy sodium‑ion batteries

As demand for scalable, low-cost energy storage surges, sodium-ion batteries (SIBs) are emerging as a front-runner for grid-scale applications. Yet their energy density still lags behind that of lithium-ion systems. Now, a team led by Prof. Hongshuai Hou at Central South University has published a landmark review revealing how “closed pores” in hard carbon anodes could be the key to unlocking high-energy, high-efficiency SIBs.

Why Closed Pores Matter

Hard carbon (HC) is the anode material of choice for SIBs due to its low cost and stable cycling. However, its amorphous microstructure has long puzzled researchers. While traditional models focused on open pores and graphitic interlayers, they failed to explain the low-voltage plateau capacity—a critical region for boosting energy density.

This review introduces a unified framework centered on closed pores: nanoscale voids that are inaccessible to gas adsorption but accessible to sodium ions. These pores enable the formation of quasi-metallic sodium clusters, dramatically increasing reversible capacity (up to 500 mAh g^-1) and initial Coulombic efficiency (ICE > 90%).

Key Insights and Innovations

1. From Open to Closed: A Pore Evolution Model
   The authors trace how open pores transform into closed pores during high-temperature carbonization, introducing concepts like:
   • Quasi-closed pores (partially accessible)
   • Fully closed pores (inaccessible even to electrolyte)
   • Pore necks and channels that control ion transport and SEI formation
2. Sodium Storage Mechanism Revealed
   Using solid-state NMR, SAXS, and in situ Raman, the team shows:
   • Sodium exists in ionic and quasi-metallic states inside closed pores
   • Desolvation occurs at pore entrances, enabling dense cluster formation
   • SEI is minimized inside closed pores, boosting ICE
3. Engineering Strategies for Closed Pores
   The review categorizes design strategies into:
   • Precursor modulation (cross-linking, esterification, component optimization)
   • Pore-forming agents (CO₂, KOH, metal oxides, carbon dots)
   • Carbonization control (two-step heating, CVD, flash Joule heating)

Examples include:

• • CO₂-etched starch microspheres delivering 487.6 mAh g^-1
  • ZnO-templated phenolic resin achieving 501 mAh g^-1
  • Flash Joule heating enabling ultrafast, tunable pore closure

Future Outlook

The authors propose a molecular-level design paradigm for hard carbon, integrating:

• Kinetic and thermodynamic control of pore formation
• Electrolyte engineering to optimize desolvation and SEI
• Unified active-site theory linking defects, interlayers, and pores

They emphasize that closed pores are not just structural features—they are electrochemical active sites. Mastering their design could bridge the energy density gap between sodium- and lithium-ion technologies.

Conclusion

This comprehensive review redefines how we understand and engineer hard carbon anodes. By shifting the focus from open porosity to closed-pore architecture, it offers a clear roadmap for designing next-generation SIBs with higher energy, longer life, and lower cost.

Stay tuned for more breakthroughs from Prof. Hongshuai Hou and the team at Central South University as they continue to push the boundaries of sodium-ion battery science.