Hard Carbon Anode Delivers 288.01 mAh/g in a Sodium Metal Half Cell | atomfair

By ATOMFAIR LLC | Published July 27, 2026 | Updated August 18, 2026

HC-S03 provides a low-rate, sodium-metal half-cell baseline rather than a full-cell performance claim. Under the stated 0.1C/0.1C, 25 +/- 1 C test, the hard carbon electrode delivered 288.01 mAh/g with 91.73% first-cycle coulombic efficiency at 8.5 mg/cm2 loading. The useful question is therefore not simply whether the capacity is high: it is what this first-cycle curve says about HC-S03 as a practical starting point for sodium-ion anode screening and what still needs to be measured before electrode pairing.

What This HC-S03 Result Shows

The curve shows that HC-S03 can store sodium reversibly in the stated half-cell configuration while retaining a first-cycle efficiency above 90%. That combination is meaningful because irreversible sodium consumption during formation must be covered by the cathode inventory in any later full cell. At the same time, the test was run at a single low rate and on a single reported curve. It is useful for material screening and electrode-baseline work, but it does not establish rate capability, cycle life, or full-cell balance.

Test Conditions and Data Source

The following conditions are transcribed from the supplied HC-S03 charge-discharge chart. The chart labels the electrodes as HC and Na; this article describes the setup as HC-S03 tested against sodium metal because the analysis concerns the hard-carbon electrode under evaluation.

How to Read the Voltage-Capacity Profile

The discharge trace moves from a higher-voltage slope into the low-voltage region where hard carbon typically stores much of its sodium. In this dataset, the reported discharge capacity is 288.01 mAh/g. If the stated 8.5 mg/cm2 is active-material loading, that result corresponds to an estimated areal capacity of about 2.45 mAh/cm2. This conversion is useful for comparing the curve with cathode areal capacity during cell design, but the value should be confirmed against the electrode formulation and active-material fraction before it is used for N/P calculations.

The reported 91.73% first-cycle coulombic efficiency indicates that HC-S03 retains most, but not all, of the sodium inserted during the first cycle. For a full sodium-ion cell, the lost fraction is not a minor footnote: cathode loading, formation protocol, and sodium inventory must be selected with that irreversible demand in mind. The curve is therefore more valuable as a formation and balancing baseline than as an isolated headline capacity.

Why 8.5 mg/cm2 Changes the Interpretation

Areal loading changes what a specific-capacity number means in practice. The 8.5 mg/cm2 condition places the result beyond a very-low-mass proof-of-concept electrode, so electrolyte wetting, coating uniformity, and through-thickness transport already matter to the measured response. It still should not be read as a high-loading, application-ready validation. A second test at higher loading could introduce different polarization, impedance, and sodium-utilization behavior even when the active material is nominally the same.

For controlled comparison, use the same cutting diameter, separator, electrolyte volume, sodium source, stack pressure, rest period, formation current, and voltage window when evaluating HC-S03 against other hard-carbon conditions. The relevant product family is the sodium-ion anode electrode sheets category, where loading, coating architecture, current collector, and drying condition can be selected as test variables rather than treated as interchangeable details.

What This Single Test Supports – and What It Does Not

Supported by the reported curve: a first-cycle HC-S03 versus sodium-metal screening baseline at 0.1C; a reported capacity of 288.01 mAh/g under the stated conditions; and a reported first-cycle efficiency of 91.73% that can inform preliminary sodium-inventory planning.

Not established by the reported curve: cell-to-cell reproducibility, long-term capacity retention, high-rate behavior, impedance growth, electrode density effects, electrolyte optimization, or the N/P ratio required for a stable sodium-ion full cell. Those questions require a planned test matrix, not extrapolation from one half-cell profile.

Next Validation Steps for Full-Cell Design

The next experiment should repeat the half-cell result across multiple cells, then add rate capability, extended cycling, impedance tracking, and post-formation capacity measurement. Use battery test equipment and instruments that can preserve the same formation and measurement protocol across the comparison set. The wider Battery Research hub provides the parent topic for this data page, while the related NFM sodium cathode voltage-window analysis offers a useful cathode-side comparison before electrode matching.

Once HC-S03 and the intended cathode both have measured areal capacities and first-cycle losses, a sodium-ion full-cell experiment can be planned around an explicit N/P target rather than nominal material capacities. For a later format transition, an NFM hard-carbon dry pouch cell shows the type of paired-electrode platform that can be used after coin-cell variables have been narrowed. It is a follow-on validation route, not evidence that this HC-S03 half-cell result already represents pouch-cell performance.

Data Scope

This article interprets the supplied HC-S03 first-cycle chart only. It does not add unreported cycle-life, rate, impedance, density, or full-cell data. Researchers requiring a comparable design, custom electrode condition, or raw test context should specify the target loading, coating side, current collector, electrolyte, cell format, and validation objective when contacting ATOMFAIR at inquiry@atomfair.com.