Hard carbon has become the leading anode candidate for sodium-ion batteries, but its path from laboratory curiosity to mass-produced industrial material has been slowed by a fundamental disagreement: scientists could not agree on where, inside the disordered carbon, the sodium atoms actually reside. A new review published in Frontiers in Chemistry argues that the field’s three competing storage models are not mutually exclusive at all, and that reconciling them provides a practical roadmap for designing better anodes and driving costs down toward parity with graphite.
The review, authored by a team at Yancheng Institute of Technology in China, traces the full pipeline from precursor chemistry to electrochemical performance to commercial scale-up. Its central claim is that the insertion-filling, adsorption-insertion, and adsorption-filling models converge on a unified multi-stage adsorption-intercalation-filling paradigm, in which the balance between intercalation and pore filling is governed by carbonization temperature, precursor chemistry, and pore architecture rather than by any single mechanism.
The disagreement has deep roots. In 2000, Stevens and Dahn introduced the House of Cards model, describing hard carbon as randomly oriented stacks of small aromatic fragments, like scattered playing cards. This picture gives rise to two kinds of structural sites: locally parallel graphene sheets that can host inserted ions, and nanoscale voids between misaligned layers. By comparing glucose-derived hard carbon in lithium and sodium half-cells, they showed that both exhibit a two-region voltage profile, with the nanoporosity estimated from sodium and lithium storage at roughly 20 and 21 percent by volume, and the ratio of plateau capacities matching the molar volume ratio of the two metals.
The charge-discharge profile of hard carbon typically shows a sloping region above 0.1 volts versus sodium metal and a low-potential plateau below 0.1 volts. The three competing models assign these regions differently. The insertion-filling model, originating with Stevens and Dahn, attributes the sloping region to sodium intercalation into graphitic interlayers and the plateau to pore filling. The adsorption-insertion model reverses the assignment, placing pseudocapacitive adsorption in the slope and intercalation at the plateau. The adsorption-filling model denies any substantive intercalation contribution to the plateau altogether, attributing it exclusively to sodium clustering within nanopores.
Recent experimental work has steadily eroded the exclusivity of each position. Alptekin and colleagues, using solid-state sodium-23 NMR, pair distribution function analysis, and small-angle X-ray scattering, found that within the sloping region sodium simultaneously occupies defect sites, interlayers, and pore surfaces, with closed-pore filling occurring only afterward. Dahn’s group later developed a wedge nanopore model in which sodium entering narrow closed pores desolvates and forms quasi-metallic layers along the pore walls, with density functional calculations pointing to an optimal pore size near one nanometer. On the adsorption-insertion side, Han and colleagues established a three-stage picture spanning disordered-carbon adsorption above 0.4 volts, graphitic defect adsorption between 0.1 and 0.4 volts, and interlayer intercalation below 0.1 volts. Operando NMR work by Ouyang and Zhong then captured a dynamic migration of sodium from interlayer and defect sites toward quasi-metallic clusters in closed pores, providing spectroscopic evidence that plateau capacity in low-defect hard carbon comes mainly from closed-pore clusters rather than sustained intercalation.
Theoretical calculations reinforced the convergence. Youn and colleagues found that single-atom adsorption on defective graphene is exothermic and accounts for sloping capacity, while sodium nanoclusters of three to six layers are energetically favored in micropores at potentials matching the plateau. Critically, their calculated capacity limit for pure intercalation was only about 140 milliampere-hours per gram, meaning the deficit to the more than 300 milliampere-hours per gram typical of hard carbon must be supplied by pore filling. Chen and Cao’s team then showed experimentally that two structurally distinct carbons, one rich in pseudo-graphitic domains and one rich in open micropores, draw their plateau capacity from fundamentally different sources, leading them to propose a hybrid mechanism in which the plateau hosts simultaneous intercalation and pore filling, weighted by the carbon’s microstructure.
From this synthesis, the review identifies four governing parameters: carbonization temperature, precursor selection, heteroatom doping, and pore architecture. Temperature acts as the primary lever. Between 800 and 1200 degrees Celsius, the carbon retains abundant defects and oxygen groups, producing sloping-dominated storage but poor initial Coulombic efficiency. In the intermediate window of 1200 to 1600 degrees, open micropores close into discrete closed nanopores while sufficient interlayer spacing survives, and plateau capacity rises sharply. Above roughly 1700 degrees, graphitization narrows the interlayers and eliminates closed porosity, causing a steep capacity decline.
Precursor chemistry supplies the second lever. The review introduces a three-dimensional classification framework spanning scalability, structural controllability, and performance ceiling across biomass, sugars, resins, and pitch. Within lignocellulosic biomass, cellulose, hemicellulose, and lignin follow divergent pyrolysis pathways: cellulose favors open microporous networks, while lignin’s crosslinked aromatic structure promotes closed-pore formation. Sugar precursors offer well-defined molecules amenable to crosslinking chemistry that programs closed pores before carbonization. Phenolic resins provide the greatest structural design freedom, and pitch, the cheapest option, requires pre-oxidation or crosslinking pretreatments to suppress its inherent graphitization tendency. Heteroatom doping adds atomic-scale control: sulfur expands interlayer spacing, nitrogen enhances sodium adsorption and conductivity, phosphorus promotes closed-pore nucleation, and co-doping strategies can produce synergistic gains exceeding the sum of single-dopant effects.
Pore structure emerges as the terminal interface with performance. Open micropores provide adsorption sites but drive irreversible solid-electrolyte interphase formation that can push initial Coulombic efficiency below 60 percent, while closed nanopores host plateau capacity without that penalty. Post-synthesis strategies such as chemical vapor deposition of a thin carbon overlayer have raised efficiency from below 60 to above 85 percent while largely preserving plateau capacity, though the review notes such treatments remain difficult to translate to industrial scale.
The final section assesses commercialization. After Kuraray long supplied small batches of plant-based hard carbon at prices above 300,000 yuan per ton, CATL’s 2021 launch of its first-generation sodium-ion battery pushed hard carbon to the center of industry attention. Chinese companies subsequently built thousand-ton and then 10,000-ton production lines, with straw-based routes cutting costs to around 40,000 yuan per ton. Industry average prices reportedly fell to about 50,000 yuan per ton in 2025 and approached 40,000 yuan in 2026, with coal- and straw-based routes projected to reach roughly 30,000 yuan per ton around 2027 to 2028, the threshold for parity with lithium-battery graphite anodes. The review projects global demand of 200,000 to 300,000 tons by 2031, with China accounting for over 80 percent of production and sales. It concludes that remaining challenges lie in consistent precursor quality, precise pore and surface control, low-energy carbonization, and reproducible performance across large batches, with future progress depending on integrated process engineering rather than material screening alone.
Subject of Research: A review of hard carbon anode mechanisms, precursor classification and commercialization for sodium-ion batteries
Article Title: Hard carbon anodes for sodium-ion batteries: from mechanistic reconciliation, precursor-tailored microstructure regulation to industrial scale-up
Article References: Sun, Y.-G., Ding, J.-Y., Xiong, J., Zhang, Y.-H., Fan, H.-J., Xie, Y.-C., Zhang, Q.-R., Zhang, K.-L., Li, F.-C., & Zhang, B.-B. (2026). Hard carbon anodes for sodium-ion batteries: from mechanistic reconciliation, precursor-tailored microstructure regulation to industrial scale-up. Frontiers in Chemistry, 14, Article 1978945. https://doi.org/10.3389/fchem.2026.1978945
Image Credits: AI Generated
DOI: 10.3389/fchem.2026.1978945
Keywords: hard carbon, sodium-ion batteries, anodes, sodium storage mechanism, precursors, pyrolysis, heteroatom doping, closed pores, commercialization, energy storage, Hard, carbon
News Source: Bethany Barker. (October 29, 2026). Hard Carbon Anodes Move From Disputed Mechanisms Toward Industrial Scale-Up. Scienmag.
