Determination of Sulfur and Carbon in Graphite and Carbon-Based Battery Materials

Importance of the Topic

Accurate measurement of carbon and sulfur in graphite and carbon-based battery materials is critical for predicting and ensuring electrochemical performance, electronic conductivity and long-term reliability of lithium-ion battery anodes. Carbon purity (often >99.95 % for high-purity graphite) strongly correlates with conductivity, while trace sulfur concentrations—commonly introduced during mining or processing—can degrade conductivity and accelerate cell degradation. Routine, robust analytical methods to quantify both elements support refining processes, incoming material quality control and R&D optimization of composite and alloy anode materials.

Objectives and Study Overview

This application note demonstrates a validated workflow using the LECO SC832DR macro combustion system to determine sulfur and carbon in graphite and carbon-based battery materials. Objectives include: establishing sample preparation and instrument parameters, describing calibration and blanking procedures, and presenting representative results across a range of carbon materials (graphite, hard carbon, SiOx/C composites, expanded graphite and silicon–carbon alloys) to illustrate method precision and applicable concentration ranges.

Methodology

The analytical approach is high-temperature oxygen combustion of weighed sample masses, followed by gas cleanup and non-dispersive infrared (NDIR) detection of combustion products: SO2 for sulfur and CO2 for carbon. Key procedural elements:

• Combustion in a horizontal ceramic furnace at 1450 °C under pure oxygen.
• Combustion gases dried with an anhydrone desiccant prior to detection.
• NDIR detection uses concentration-dependent absorption at characteristic IR wavelengths. The SC832DR model employs dual-range IR cells for sulfur (short and long path lengths) to extend linear dynamic range and allow automatic cell selection by software.
• Typical nominal sample mass used in presented data: ~0.100 g (macro sample mass capability allows higher masses if required).

Sample Preparation and Quality Controls

Samples should be homogeneous powders or granules. Combustion boats (LECO 528-203 ceramic) must be prebaked at 1000 °C for at least one hour and stored in a desiccator; re-bake if unused after 24 hours. Use clean tongs only to avoid contamination. Recommended QC steps include:

• System conditioning with three replicates of a matrix-matched sample.
• Blank measurements: a minimum of five blank replicates; the standard deviation of the last three blanks should be ≤0.001 % (10 ppm) for both S and C.
• Calibration/drift correction using certified reference materials (minimum five replicates per calibration point) and verification with separate reference materials to confirm acceptable tolerance.

Instrument Configuration and Parameters

Key instrument settings used in the work:

• Instrument model: LECO SC832DR macro combustion Sulfur and Carbon determinator.
• Furnace temperature: 1450 °C.
• Lance on delay: 20 s; nominal mass used in routine: 1.0000 g was documented as a general parameter, but typical sample masses in this study were ≈0.100 g.
• Element-specific integration and timing: sulfur integration times 120–250 s; carbon 180–360 s. Baseline stability waits and comparator-based QC were employed.

Used Instrumentation

• LECO SC832DR combustion system with dual-range NDIR cells (for SO2 and CO2 detection).
• Accessories: 528-203 ceramic combustion boats, 761-929 crucible tongs, 501-614 spatula.

Main Results and Discussion

Results reported illustrate the method's applicability across diverse carbon materials with typical precision values (reported as standard deviation) and concentration ranges:

• SiOx/C composite anode powder: carbon ≈96.9 % (s ≈0.2), sulfur ≈0.014 % (s ≈0.001).
• Hard carbon powder: carbon ≈93.4 % (s ≈0.3), sulfur ≈0.009 % (s ≈0.001).
• Medium purity graphite: carbon ≈99.5 %; sulfur below the method lower detection limit (<0.004 %).
• Graphite powder (higher purity example): carbon ≈100.0 % (apparent values near 100 % reflect instrument calibration and sample matrix; s ≈0.4 % relative units), sulfur ≈0.019 % (s ≈0.002).
• Conductive expanded graphite: carbon ≈78.1 %; sulfur ≈2.27 % (s ≈0.03), demonstrating the method's capability for higher sulfur matrices.
• Silicon–carbon alloy: carbon ≈58.5 %; sulfur ≈0.026 % (s ≈0.001).

Calibration details: linear, forced-through-origin calibrations were used. Carbon calibration reference: LECO metallurgical coke (89.1 % C). Sulfur calibration: LECO coal reference materials (1.93 % S and 0.223 % S used for verification). The method reliably quantified sulfur down to a few ppm for many samples, though some medium-purity graphite results were reported as below the method detection limit (<0.004 % S). Reported repeatability (sample standard deviations) indicates good precision for QC purposes.

Key discussion points:

• Dual-range NDIR detection for sulfur extends measurable dynamic range and reduces the need for dilution or multiple instrument configurations across diverse matrices.
• Strict blank control and pre-baked boats are essential to reach low-ppm sulfur precision and avoid contamination artifacts.
• Matrix variability (e.g., silicon content in SiOx/C composites) affects combustion gas yields and necessitates matrix-appropriate calibration and verification.

Benefits and Practical Application

The presented LECO SC832DR workflow provides a rapid, robust pathway for simultaneous macro determination of carbon and sulfur in graphite and carbon-based battery materials. Practical benefits include:

• High throughput for routine QC when combined with autoloaders.
• Sufficient sensitivity for trace sulfur monitoring critical to battery anode performance.
• Wide dynamic range for sulfur via dual-range NDIR, enabling analysis of both low-ppm impurities and high-sulfur materials without method changeover.
• Relatively simple sample preparation (powdered samples into pre-baked boats) and straightforward calibration using certified reference materials.

Future Trends and Applications

Anticipated developments and opportunities in elemental analysis of battery materials include:

• Lower detection limits via improved gas handling and optical detection to better control ultratrace impurities affecting next-generation high-energy anodes.
• Automation and inline/at-line integration for continuous process monitoring during refining or electrode production.
• Integration of combustion elemental analysis with complementary techniques (e.g., ICP-OES/MS for metal contaminants, XPS or Raman for surface characterization) to provide a holistic quality profile.
• Enhanced software-driven data validation and instrument diagnostics to streamline large-scale QC deployments across battery supply chains.

Conclusion

The LECO SC832DR macro combustion NDIR system offers a validated, practical method for simultaneous carbon and sulfur determination in a variety of graphite and carbon-based battery materials. With appropriate sample preparation, blank control and matrix-matched calibration, the method delivers repeatable results across a wide concentration range and supports routine QC and R&D in battery material development and refinement.

References

LECO Corporation. Determination of Sulfur and Carbon in Graphite and Carbon-Based Battery Materials. Application Note, Instrument: SC832DR. Form No. 203-821-715. 11/25-REV0. LECO Corporation, St. Joseph, MI, USA. © 2025 LECO Corporation.