1. What a hard-carbon anode is (the short version)

Hard carbon is a carbon that does not turn into graphite even when fired at high temperature (a non-graphitizing carbon). The roadmap explains that it is made from oxygen-rich precursors and cannot be converted into graphite no matter how high the carbonization temperatureSourced [1]. It is the most widely used anode for sodium-ion batteries.

• What the structure is: small carbon layers with a larger spacing than graphite (above 0.34 nm), in random orientations and joined by disordered carbon regions. In the gaps are “closed” pores that gas sorption cannot probeSourced [1].
• How much it stores: a 2000 paper by Stevens and Dahn reported a reversible sodium capacity of 300 mAh/g in hard carbonSourced [4]. Current reported values, according to the roadmap, are 200 to 450 mAh/g in coin-cell half cells, with most literature values averaging about 300 mAh/gSourced [1].
• What is hard: several theories of where and how sodium is stored remain in play, so structure and performance cannot be tied together in a single equationSourced [1, 2].

2. Structure — turbostratic domains and closed pores

(1) Graphite, soft carbon and hard carbon

When carbons are heat-treated, some turn readily into graphite (graphitizing, or soft carbon) and some do not (non-graphitizing, or hard carbon). Franklin's 1951 study of carbons of different origin treated at 1,000 to 3,000 °C showed that the two form two distinct and well-defined classes, with differences apparent from the earliest stages of carbonization. In non-graphitizing carbons, a strong system of cross-linking forms at low temperature to unite the crystallites, which leads to a random orientation of the crystallites in a rigid, finely porous mass. In graphitizing carbons the cross-linking is much weaker, neighboring crystallites tend to lie nearly parallel, and the structure is more compactSourced [3].

View the original SVG schematic and exact annotations

(2) Turbostratic domains and closed pores

The roadmap describes hard carbon as “randomly oriented graphitic domains with an interlayer spacing larger than graphite (above 0.34 nm), connected by disordered carbon regions.” Between those domains there are “closed” pores that cannot be probed by gas sorption. The larger the graphitic crystallites, the larger the pores between them. In addition there are residual heteroatoms (mainly oxygen-containing groups), curved layers and edge sitesSourced [1]. Bommier and co-authors call the regions where intercalation occurs “turbostratic nanodomains”Sourced [5].

At the same time, the exact structure is still unknown. The roadmap says that it depends on precursor and carbonization conditions, which give different interlayer spacing, crystallite size, pore domains and edge terminationSourced [1].

3. How sodium is stored — adsorption, intercalation and pore filling

As sodium is inserted into hard carbon, the potential changes in two regions. According to the roadmap these are a “sloping region” between 1 V and 0.1 V and a “plateau region” below 0.1 V (versus sodium metal)Sourced [1].

Where and how does sodium enter in the two regions? The roadmap and the review summarize the following theories.

All are Sourced (the roadmap's summary [1], Stevens and Dahn [4], Bommier et al. [5], the review [2]). Which is right is said to differ by material.

Recent work emphasizes “filling” of closed pores. The 2025 review expresses the view that high-density quasi-metallic sodium clusters accommodated in closed pores contribute most to plateau capacitySourced [2]. The roadmap also mentions reports that examined the metallic nature of sodium in pores by solid-state NMR and similar methodsSourced [1]. The review itself, however, states that there is still no rigorous scientific standard for distinguishing open from closed poresSourced [2].

4. A materials engineer's view (1): an unsettled mechanism means unsettled design rules

5. Our calculation: the upper limit of plateau capacity from pore volume, and how XRD sees d-spacing

(1) How much can the pores be filled with sodium?

Closed-pore volume is estimated from helium pycnometry with the true density of graphite, 2.26 g/cm³, as the reference, using V(closed) = 1/(particle density) − 1/2.26Sourced [2]. For a particle density of 1.5 g/cm³ this gives 0.224 cm³/g, and for 1.7 g/cm³ it gives 0.146 cm³/gOur calculation (the densities are examples). The review notes, however, that this method depends on the choice of reference structure and that computed values differ widely between studiesSourced [2].

(2) How XRD sees the interlayer spacing

The interlayer spacing d002 is obtained from the position of the (002) XRD peak using Bragg's law (2d sinθ = λ). For Cu Kα1 radiation (λ = 0.15406 nm, a standard value) the conversions are as followsOur calculation.

Calculated values are Our calculation. The manufacturer's value and the review's range in the Note column are Sourced. Because the (002) peak of hard carbon is broad, reading d002 involves an uncertainty from how the peak position is fitted (commentary by this article).

6. Precursors — biomass, resin and pitch

The properties of hard carbon split largely according to how readily the precursor cross-links. The review says that thermosetting precursors (plant biomass and resins) are oxygen-rich and have a developed cross-linked network from the outset, so graphitization on firing can be controlled and closed-pore-rich structures are easier to makeSourced [2]. If cross-linking is insufficient, free radicals are released violently during pyrolysis, causing excessive pore formation or melting of the carbon so that it loses its original formSourced [2].

All are Sourced (the 2025 review's summary [2]). They are general statements and do not describe any particular manufacturer's raw materials or process.

As an example of pre-oxidation, the review cites a study with waste foam as precursor in which the sample oxidized at 250 °C performed well, whereas the one oxidized at 280 °C showed a wider interlayer spacing but lower first-cycle efficiency and reversible capacity because of formation of an undesirable SEISourced [2]. A difference of a few tens of degrees in oxidation conditions decides performance.

7. Manufacturing and carbonization temperature

View the original SVG schematic and exact annotations

(1) How carbonization temperature changes the structure

The review summarizes the changes in pores as carbonization temperature rises in the following stagesSourced [2].

• Low temperature: pores are almost all open and exposed to the electrolyte. They contribute to sloping-region capacity, but first-cycle efficiency and plateau capacity are very low.
• Moderate temperature: gaps between crystallite layers partly shrink to form “quasi-closed pores.” First the larger N2 can no longer enter, then even the smaller CO2 is no longer detected. Part of the electrolyte is excluded, and some sloping capacity converts into plateau capacity.
• Higher temperature (beyond a critical temperature set by the precursor): some closed pores become “fully closed” pores that are closed even to the electrolyte, and plateau capacity falls instead.

The review reports that, according to Titirici and co-workers, BET surface area dropped dramatically on heating and reversible capacity shifted from slope-dominated to plateau-dominated. Hasegawa and co-workers showed from true-density measurements that samples treated above 1,600 °C with small BET surface areas still contained many closed pores that contributed to plateau capacitySourced [2]. On graphitization, the review explains that above 1,000 °C, soft carbon crystallites stack rapidly and interconnect, whereas thermosetting precursors mainly undergo crystallite growth, with little stacking or connection. Topological defects are said to persist even after treatment at 2,000 °CSourced [2].

(2) Kuraray's stated values

The Japanese materials maker Kuraray sells a plant-derived hard-carbon anode material, KURANODE™. The values on its product page are as follows.

All are Sourced (Kuraray KURANODE™ product page [6]; the page says the values are measured by Kuraray, for reference only and not guaranteed. No capacity or first-cycle efficiency for sodium-ion use is listed on that page, so none is given here).

The company cites for KURANODE™ a moisture resistance high enough to use water-based binders (SBR/CMC), input/output, low-temperature and cycle performance better than graphite, and cost competitiveness against conventional hard carbon, and says it suits the anode of lithium-ion capacitors and sodium-ion batteries as well as lithium-ion batteriesSourced [6, 7] (company claims). Whether this product is used in any specific sodium-ion battery, such as CATL's, could not be confirmed.

8. A materials engineer's view (2): the difficulty of keeping “the same hard carbon” from lot to lot

9. The trade-off between first-cycle efficiency and capacity

(1) Why first-cycle efficiency is a problem

First-cycle Coulombic efficiency (ICE) is the fraction of the first charge (sodium inserted) that comes back on the first discharge (sodium extracted). What does not come back is lost, because sodium becomes decomposition products of the electrolyte (the SEI) or is trapped inside the material. The review says that the large surface area of open pores makes a great deal of SEI and lowers first-cycle efficiency and cycling stability, and that some defects bind sodium so strongly that they trap it irreversiblySourced [2].

In a sodium-ion full cell, the only sodium that can go into the anode is what the cathode holds. For alloy and conversion anodes the roadmap says that a large excess of cathode is required to compensate for large initial irreversible capacitySourced [1]. The same logic applies to hard carbon (commentary by this article).

Our calculation (assumptions: anode 300 mAh/g, cathode 130 mAh/g). Active-material-only values, not cell weight or energy density.

A drop in first-cycle efficiency from 90% to 80% alone raises the active material needed for the same capacity by about 9% (12.95 ÷ 11.88 = 1.09)Our calculation. The calculation shows that, in a full cell, raising first-cycle efficiency can matter more for weight than raising the hard carbon's capacity. With a different pairing of cathode and anode capacities the numbers change.

(2) Do capacity and efficiency simply trade off?

Looking at the 15 laboratory values in the review's Table 1 (Fig. 7), higher capacity and higher first-cycle efficiency are not in a simple inverse relationship. Some entries have efficiency above 90% with total capacity around 450 mAh/g (Nos. 2 and 9), while the two entries near 500 mAh/g (Nos. 10 and 14) have efficiencies of 77.6% and 82.3%Sourced [2].

All are Sourced (Table 1 of the 2025 review [2]; current densities 20 to 50 mA/g). The original papers the review cites were not checked here.

Three readings follow (commentary by this article).

• Cases that achieve both have been reported. The table has several entries with efficiency above 90% and total capacity above 400 mAh/g (Nos. 2, 6, 7 and 9). They are, however, laboratory samples given elaborate treatments such as oxidation, pore formation or special precursors.
• The type of treatment changes the result greatly. Even for the same phenolic resin, efficiency differs by more than ten points: 71.6% with hydrothermal treatment, 83% with two-step carbonization and 88.5% with molecular regulation.
• Even so, the highest-capacity entry need not have high efficiency. The two entries near 500 mAh/g (Nos. 10 and 14) have efficiencies near or below 80%, yet No. 7 reaches 488 mAh/g at 90.6%. These 15 were not chosen systematically, so this is not a general law.

The physical reason for the trade-off lies in the carbonization-temperature section (Fig. 5). Carbonized at low temperature, a material has many open pores and defects and gives sloping capacity, but forms SEI and has low first-cycle efficiency. As temperature rises, open pores shrink and efficiency rises and closed pores increase, giving plateau capacity, but beyond the critical temperature fully closed pores grow and plateau capacity fallsSourced [2]. In other words, the temperature that maximizes first-cycle efficiency and the temperature that maximizes capacity need not coincide.

10. Cost

The roadmap says that, although hard carbon has kept ahead of other anode materials, its cost is significantly higher than that of graphite. In the same passage it states that graphite costs 500 to 2,000 US dollars per tonne, similar to the cost of common hard-carbon precursors such as cellulose and sucroseSourced [1]. A simple calculation shows why hard carbon can cost more even when the precursor price is similar.

This calculation shows one structural reason hard carbon is priced above graphite, but it does not show current market prices of hard carbon (commentary by this article). As other cost drivers the review lists the environmentally unfriendly impurity removal before carbonizing biomass, extra time and energy for two-step carbonization, and expensive resin precursors that are hard to industrializeSourced [2]. Kuraray says KURANODE™ is cost-competitive against conventional hard carbonSourced [6] (a company claim; no price is given). A market price of hard carbon (US$/kg) could not be confirmed from public-agency statistics or published material (an item that could not be confirmed).

11. Characterization — XRD, SAXS and BET

What makes hard carbon hard to evaluate is that each technique sees a different structure. The roadmap says that combining several techniques is needed to relate structure to performanceSourced [1].

Statements marked [1], [2] or [6] are Sourced (the sources' statements). Unmarked statements are commentary by this article.

The roadmap says that benchmarking the best-performing hard carbons and setting international standard protocols would be desirableSourced [1]. Because evaluation methods are not aligned, "capacity" and "first-cycle efficiency" figures from different papers or documents cannot be compared directly. Always check whether it is a half or full cell, the current density, the voltage window, the temperature and the electrolyte (commentary by this article).

12. What is still hard

• The mechanism is unsettled. Which theory applies to which material, and whether it differs by material, is not settled [1, 2]. This is why design rules do not converge.
• There is no scientific criterion for distinguishing open from closed pores. As the review says, a clear definition of closed pores and a systematic account of how open pores become closed are lacking [2].
• Combining first-cycle efficiency with capacity. Laboratory cases that achieve both exist, but they require elaborate treatments, and reproduction in mass production could not be confirmed in this investigation. First-cycle efficiency bears on full-cell weight (Chapter 9).
• Lot-to-lot reproducibility. Being amorphous and sensitive to precursor, pretreatment and carbonization conditions, it has limited reproducibility [2].
• Precursor screening, impurity removal and cost. Initial screening of biomass is difficult and pretreatment carries an environmental load. Hard carbon is said to cost significantly more than graphite [1, 2].
• Moisture and gas generation. A problem CATL names among the obstacles it overcame for mass production [8]. The cause and remedy have not been published.
• Standardization of evaluation. Measurement conditions are not aligned and international standards are not in place [1].

13. How to read evaluations and standards

Before comparing which number is larger, align the basis of comparison in the following order. This is an editorial reading checklist, not a set of test conditions or acceptance limits prescribed by a standard.

1. What was measured?

initial efficiency, reversible capacity and low-voltage capacity. Check the units and whether the result describes a material, component or complete system.

2. Under which conditions?

precursor, heat treatment, particle and electrode structure, electrolyte and counter-electrode. Treat results under different conditions as separate results before comparing them.

3. The comparison trap

Do not transfer half-cell capacity against sodium metal directly to a full cell.

4. What does the standard cover?

Check the identifier, title, edition and scope in the issuing body’s material. Distinguish test methods, safety, interoperability and management requirements. Naming a standard alone does not establish that a test, conformity assessment or certification has been completed.

Keep a comparison note containing the object, units, conditions, sample count and variation, comparator, standard edition and unresolved points. Do not invent numerical conditions or acceptance limits from a public abstract of a standard.

14. Glossary

15. References

1. Tapia-Ruiz et al. (NEXGENNA consortium of the Faraday Institution and others) "2021 roadmap for sodium-ion batteries," Journal of Physics: Energy 3, 031503 (2021) (open access) — doi.org/10.1088/2515-7655/ac01ef
2. Gan et al. (Central South University) "Comprehensive Understanding of Closed Pores in Hard Carbon Anode for High-Energy Sodium-Ion Batteries," Nano-Micro Letters 17, 325 (2025) (open access) — doi.org/10.1007/s40820-025-01833-x
3. Franklin "Crystallite growth in graphitizing and non-graphitizing carbons," Proceedings of the Royal Society A 209, 196-218 (1951) (abstract checked) — doi.org/10.1098/rspa.1951.0197
4. Stevens and Dahn "High Capacity Anode Materials for Rechargeable Sodium-Ion Batteries," Journal of The Electrochemical Society 147, 1271 (2000) (abstract checked) — doi.org/10.1149/1.1393348
5. Bommier, Surta, Dolgos and Ji "New Mechanistic Insights on Na-Ion Storage in Nongraphitizable Carbon," Nano Letters 15, 5888-5892 (2015) (abstract checked) — doi.org/10.1021/acs.nanolett.5b01969
6. Kuraray (Environmental Solutions Division) "Bio Hard Carbon Anode Material for Lithium-ion Batteries | KURANODE™" (product page, with grade values) — carbon-solutions.kuraray.com
7. Kuraray "KURANODE™" (product list page) — kuraray.com
8. CATL "CATL Unveils Six Major Innovations: Multi-Chemistry Systems to Redefine New Energy Mobility Experience," 21 April 2026 — catl.com

16. Claim-to-source audit

Last updated 10 October 2026. Sources are limited to primary material (peer-reviewed papers, open-access reviews and roadmaps, and official company product pages and press releases). Values from company materials are treated as company-stated values and company claims. Because the article includes structural readings and materials-design interpretations, those are marked as Commentary and kept separate from sourced fact. Kuraray's capacity and first-cycle efficiency for sodium-ion use, adoption in any specific battery such as CATL's, specific carbonization temperatures, times and yields, a market price of hard carbon and the cause of gas generation are not stated here because no published primary source could confirm them. The scatter plot in Fig. 7 restates numbers from the review's table; the original papers were not checked. All figures are explanatory concept graphics. Figs. 1, 2, 3, 4, 5, 7 and 8 are vector drawings; the hero image and Fig. 6 are AI-generated images, and none of them shows a real cross-section, micrograph, measured data or physical product.