Iodine-Tuned Ultramicropores Push Hard Carbon Anodes Forward in Sodium-Ion Batteries

Sodium-ion batteries have long promised a cheaper, more sustainable alternative to today’s lithium-ion technology. Yet the electrochemistry of sodium carries a stubborn penalty: the sodium ion is significantly larger than lithium, which makes it harder to shuttle efficiently and reversibly through the tight carbon frameworks that store charge in battery anodes. Hard carbon, a disordered form of carbon that can accommodate sodium between its layers and inside its pores, is widely regarded as one of the most promising anode materials for these batteries. The problem is that no one has been able to maximize all of its desirable traits at once: high initial Coulombic efficiency, large low-voltage plateau capacity, fast charge-transfer kinetics, and long-term cycling stability have tended to trade off against one another, forcing researchers to sacrifice one performance metric whenever they optimize another.

A research team has now reported a molecular-level strategy that appears to break this conventional trade-off. Writing in the journal Nano-Micro Letters, the researchers describe an iodine-mediated oxidative cross-linking approach applied to starch precursors, yielding spherical hard carbon with abundant ultramicropores and carbonyl-rich surfaces. The resulting architecture does something unusual: it regulates the electrolyte itself at the interface, confining solvation structures inside pores smaller than a nanometer and steering the formation of an inorganic-rich solid electrolyte interphase. The optimized material delivers a reversible capacity of 352.9 mAh per gram at 0.1C, an initial Coulombic efficiency of 88.4 percent, a plateau contribution of 64.1 percent of the total discharge capacity, a capacity of 288.9 mAh per gram even at a demanding 5C rate, and 95.6 percent capacity retention after 200 cycles at 0.33C.

The choice of precursor is central to the design. Starch is an attractive feedstock for hard carbon because it is inexpensive, naturally abundant, rich in carbon, and intrinsically spherical in morphology. But there is a catch that has limited its direct use: when starch is pyrolyzed without modification, extensive cleavage of its glycosidic bonds generates volatile intermediates. The result is severe foaming, structural collapse, and dramatic carbon loss, which destroys the spherical framework before it can become a useful electrode material. The volatile molecule levoglucosan, formed during thermal decomposition, escapes into the gas phase and takes much of the carbon with it, leaving behind a collapsed, low-yield residue.

The researchers’ solution was to introduce iodine during the precursor oxidation stage. Iodine promotes oxidative aromatization and interchain cross-linking at the molecular level, converting the starch framework into a more thermally stable conjugated network before carbonization begins. This stabilization suppresses the formation and volatilization of levoglucosan during subsequent heat treatment, allowing the spherical morphology to survive intact. The effect on carbon yield is striking: the iodine-mediated strategy raises the yield to 45.5 percent, compared with less than 10 percent for iodine-free samples. That improvement in yield is not merely an efficiency gain in the laboratory; it preserves the precursor’s engineered geometry so that the final pore architecture can be controlled deliberately rather than left to chance.

The resulting hard carbon possesses a distinctive structural signature. It contains abundant ultramicropores smaller than 0.9 nanometers, with the pore distribution concentrated mainly around 0.5 to 0.6 nanometers, while exhibiting an exceptionally low external surface area of only 1.5 square meters per gram. This combination matters because it pairs a nearly inaccessible external surface with abundant internal confined spaces. Excessive electrolyte penetration into the bulk material is reduced, while the internal ultramicropores provide numerous confined spaces for reversible sodium storage. The material also carries abundant carbonyl functionalities, which contribute additional sodium-storage sites and play an important role in regulating the local interfacial chemistry where carbon meets electrolyte.

The most consequential finding is that these ultramicropores behave as molecular sieves for the electrolyte. Density functional theory calculations show that when the pore diameter falls below 0.804 nanometers, ethylene carbonate molecules—the bulky solvent component of the electrolyte—are sterically excluded from the confined space, whereas sodium ions maintain strong adsorption within the same pores. In other words, the carbon framework can selectively restrict solvent molecules while still granting sodium ions access to storage sites. Meanwhile, adsorption of the PF6 anion becomes increasingly favorable in appropriately sized ultramicropores, and the carbonyl groups further strengthen the interaction with PF6. Together, these effects create an anion-enriched interfacial environment, shifting the electrolyte structure toward contact ion pairs and aggregates. This confined, anion-rich environment favors anion-derived interfacial reactions and promotes the formation of inorganic-rich interphases on the carbon surface.

That regulation of electrolyte solvation connects directly to the formation of the solid electrolyte interphase, the thin reaction layer that governs how well any battery anode performs. The combination of ultramicropore confinement and carbonyl-rich surfaces promotes PF6 accumulation and decomposition, facilitating the growth of a NaF-rich inorganic interphase. Such an interphase reduces the interfacial energy barrier and facilitates sodium-ion transport across it. The study thus traces a chain from nanoscale carbon structure to electrochemical behavior: ultramicropore confinement drives selective solvation regulation, which produces anion enrichment, which constructs an inorganic-rich interphase, which in turn speeds sodium transport. It is this chain, the researchers argue, that allows the material to overcome the long-standing trade-off among initial Coulombic efficiency, plateau capacity, rate capability, and cycling stability.

In situ X-ray diffraction and Raman spectroscopy reveal how sodium is actually stored within the framework, resolving the process into three stages. In the first stage, between 2.5 and 0.6 volts, sodium-ion adsorption dominates: sodium ions interact with external surface sites, open pores, edge defects, structural defects, and carbonyl functionalities. In the second stage, between 0.6 and 0.01 volts, sodium progressively enters the graphitic carbon layers, forming NaCx species. In the third stage, below 0.01 volts, the abundant ultramicropores facilitate pore filling at the low-voltage plateau, resulting in the formation of quasi-metallic sodium clusters. Critically, these clusters are highly reversible. Phenolphthalein experiments provide visual evidence of sodium-cluster formation at potentials below 0.01 volts, and the coloration disappears during subsequent charging, consistent with the clusters being removed on charge rather than accumulating irreversibly.

The electrochemical numbers underline why the design is significant. The plateau capacity accounts for 64.1 percent of the total discharge capacity, highlighting the contribution of ultramicropore filling to sodium storage in the low-voltage region that matters most for the energy a full cell can deliver. The rate capability is equally notable: the material retains 288.9 mAh per gram at 5C, and when the current density returns to 0.1C the original capacity is fully recovered, indicating that the fast-cycling processes do not damage the storage sites. Long-term cycling demonstrates structural and electrochemical stability, with 95.6 percent capacity retention after 200 cycles at 0.33C. The high initial Coulombic efficiency of 88.4 percent is particularly important for practical cells, because the first charge in a sodium-ion battery consumes sodium to build the interphase, and efficiency lost in that first cycle reduces the cell’s available energy.

Beyond the specific material, the work carries a broader design lesson for the field. High-performance hard carbon anodes, the study concludes, cannot be engineered solely by controlling total pore volume or surface area. Pore size, surface functionality, electrolyte solvation, and interphase chemistry must be considered together, because each one shapes the others. The iodine-mediated cross-linking strategy provides a way to establish that connection at the molecular level: by controlling precursor chemistry, the researchers simultaneously regulate the carbon framework, generate ultramicropores, introduce carbonyl functionalities, modify electrolyte solvation, and promote the formation of an inorganic-rich interphase. The result is a synergy between nanoconfinement, interfacial chemistry, and sodium-storage behavior in which precursor engineering determines the final pore architecture and surface chemistry, which in turn regulate electrolyte behavior and sodium-storage kinetics. For a technology racing to compete with lithium-ion batteries on cost and sustainability, that molecular-level control may prove to be the template that next-generation sodium-ion anodes are built around.

Subject of Research: Molecular-level precursor engineering of hard carbon anodes with ultramicropore-confined solvation and interphase regulation for sodium-ion batteries

Article Title: Ultramicropore‑confined solvation and interphase regulation unlock high‑performance hard carbon anodes for sodium‑ion batteries

Article References: Ultramicropore‑confined solvation and interphase regulation unlock high‑performance hard carbon anodes for sodium‑ion batteries. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: sodium-ion batteries, hard carbon, anode materials, ultramicropores, solid electrolyte interphase, electrolyte solvation, energy storage, starch precursor, iodine cross-linking, Nano-Micro Letters, Ultramicropore, confined