Closed-Pore Engineering in Hard Carbon for Sodium-Ion Storage: Advances, Challenges and Future Horizons

As the global transition toward clean energy accelerates, the demand for sustainable, low-cost, and scalable energy storage technologies continues to grow. While lithium-ion batteries have dominated the market for decades, concerns over lithium resource availability and cost volatility are driving intense research into alternative chemistries. Sodium-ion batteries (SIBs), leveraging abundant and inexpensive sodium resources, are emerging as a promising solution, particularly for large-scale grid storage.

A new comprehensive review highlights recent progress in closed-pore engineering of hard carbon, one of the most promising anode materials for sodium-ion batteries. The study synthesizes current knowledge on how precisely designed nanoscale closed pores can significantly enhance sodium storage performance, offering a roadmap toward next-generation, energy-dense SIBs.

Hard carbons combine low cost, good electrical conductivity, and relatively high reversible capacity, making them attractive candidates for sodium storage. However, their complex and disordered microstructure has long obscured the fundamental mechanisms governing sodium insertion, especially in the low-voltage plateau region that contributes most of the total capacity.

Recent research increasingly points to the critical role of appropriately sized closed nanopores, which can host quasi-metallic sodium clusters and dramatically boost storage capacity and initial Coulombic efficiency. Yet until now, a systematic overview of how to design, characterize, and optimize these structures has been lacking.

The review outlines multiple strategies for tailoring closed-pore structures, including:

• High-temperature carbonization to tune interlayer spacing and pore evolution

• Chemical and physical activation to engineer pore volume and distribution

• Templating approaches using inorganic or biomass-derived templates

• Pore-entrance tightening via chemical vapor deposition

• Flash Joule heating for rapid, energy-efficient processing

• Molecular crosslinking to create stable three-dimensional carbon networks

Together, these methods enable fine control over pore size (typically ~1–2 nm), pore volume, and defect density—parameters that strongly influence sodium storage behavior.

The authors emphasize that sodium storage in hard carbon involves a complex interplay of processes, including surface adsorption, interlayer insertion, and pore filling. Increasing evidence suggests that the low-voltage plateau capacity arises largely from sodium cluster formation inside closed pores, although debates remain regarding the relative contributions of different mechanisms.

Advanced characterization techniques—such as solid-state NMR, SAXS, XPS, and in situ electrochemical analysis—are helping to unravel these processes, but quantitative understanding remains challenging due to the structural complexity of hard carbon.

Despite significant progress, several hurdles must be addressed before closed-pore-engineered hard carbons can be widely commercialized:

• Difficulty in precise, quantitative characterization of closed pores

• Trade-offs between capacity, rate capability, and efficiency

• High processing costs for some synthesis routes

• Limited studies at practical full-cell and industrial scales

• Safety considerations related to sodium clustering at low potentials

The review calls for integrated experimental and computational approaches, including machine learning-guided materials design, to accelerate optimization. It also highlights the promise of biomass-derived precursors as a pathway toward low-cost, sustainable production.

By advancing understanding of closed-pore structures and their role in sodium storage, the work provides valuable guidance for developing high-performance hard carbon anodes and accelerating the deployment of sodium-ion batteries in grid storage and beyond.