Evaluation of Surface Carbon on Lithium-Ion Battery Cathode Active Materials Using a TOC Analyzer and Infrared/Raman Microscope

Significance of the topic

The surface carbon coating of lithium‑ion battery cathode active materials (notably LiFePO4‑based LMFP) plays a critical role in electronic conductivity, rate capability and overall cell performance. Quantitative and structural characterization of that carbon layer is essential for production quality control, material development and recycling workflows. Combining accurate total carbon quantitation with micro‑scale structural analysis enables correlation of carbon amount and its graphitic/order‑defect state with electrochemical function.

Objectives and study overview

This application study demonstrates a two‑pronged approach to evaluate surface carbon on LMFP cathode active material: (1) quantitative total carbon (TC) analysis of solid samples using a TOC solid sample measurement system (TOC‑L + SSM‑5000A) and (2) structural assessment of the carbon coating by Raman spectroscopy using the AIRsight infrared/Raman microscope. The work also validates TC quantitation by examining the sample residue after combustion with Raman.

Methodology

• Quantitation: Solid TC measurements were performed by high‑temperature combustion catalytic oxidation (980 °C) in oxygen with CO2 detection. Samples of 15 mg and 30 mg were weighed into sample boats and analyzed; a one‑point calibration with calcium carbonate powder was used. Carrier gas flow was 500 mL/min O2 and a short absorption cell was used. Reported instrument limit of quantitation (LOQ) for absolute carbon was 0.1 mgC (improvement possible with an optional cell‑switching valve).
• Structural analysis: Raman measurements of the LMFP surface and of residues after TC analysis were carried out using IRTracer‑100 coupled with AIRsight. Conditions included 532 nm excitation, 100× objective, ND filter at 7.5% to avoid laser‑induced damage, 5 accumulations of 25 s exposure each, and CCD detection.
• Validation: Post‑combustion Raman spectra of residues were compared with pre‑analysis spectra to confirm removal of surface carbon by combustion.

Instrumentation used

• TOC solid sample measurement system: TOC‑L (TOC‑LCPH) total organic carbon analyzer with SSM‑5000A solid sample combustion unit.
• AIRsight infrared/Raman microscope coupled with IRTracer‑100 for Raman/IR measurements.
• Measurement specifics: combustion furnace at 980 °C, O2 carrier gas 500 mL/min, Raman excitation 532 nm, 100× objective, ND filter 7.5%, accumulation 5×, exposure 25 s.

Main results and discussion

• Quantitative TC: Measured TC concentrations for the LMFP samples averaged approximately 1.69% carbon. Results for 15 mg and 30 mg aliquots were consistent, with coefficients of variation below 2%, demonstrating good reproducibility at low sample masses. Absolute carbon amounts measured were on the order of 0.25–0.52 mgC for individual runs; when planning analyses, sample mass should be increased if the expected absolute carbon falls below the instrument LOQ (~0.1 mgC).
• Raman structural evaluation: Raman spectra of the LMFP surface exhibited characteristic carbon bands: the G band near 1580 cm−1 and the D band near 1340 cm−1. The observed ID/IG ratio was ~0.8, indicating that the surface carbon layer is not highly crystalline graphite but contains significant structural defects—typical of conductive amorphous or turbostratic carbon coatings used to enhance electronic connectivity.
• Validation by residue analysis: Raman spectra collected from the LMFP residue after TC combustion showed disappearance of the D and G bands, indicating that surface carbon was effectively consumed during combustion and that the TC measurements reflected the surface carbon content. A new feature near ~1300 cm−1 appeared in the residue spectra, plausibly assigned to oxide species formed during high‑temperature oxidation.

Benefits and practical applications

• The TOC solid sample measurement approach enables rapid, accurate and quantitative assessment of surface carbon on cathode powders using minimal sample amounts—important when material is limited during development or qualification.
• Combining TC quantitation with Raman structural analysis provides both mass‑based and microstructural information: amount of carbon and its degree of graphitization/defectiveness, which together more directly link to electronic conductivity and expected electrochemical behavior.
• Applicable uses include production QC of carbon coatings, evaluation of coating optimization strategies, screening of cathode active materials, and monitoring of carbon‑related parameters in recycling streams (e.g., black mass, lithium extraction solutions, lithium carbonate) and downstream process control.

Future trends and potential applications

• Improved sensitivity and throughput: adoption of optional hardware (e.g., cell switching valves) and automated sample handling can lower LOQ and increase routine laboratory throughput for manufacturing QC.
• Multimodal mapping: combining Raman imaging with infrared maps and optical microscopy to spatially resolve carbon distribution, thickness variability and local structure on individual particles to better correlate coating uniformity with performance.
• In situ and operando analyses: development of near‑real‑time monitoring tools for coating processes and cell aging studies, using airtight/environmental cells with Raman/IR to track carbon evolution during charge/discharge cycles.
• Structure‑property correlation: systematic studies linking ID/IG and other spectral metrics to electronic conductivity and cell cycling metrics to enable predictive QC criteria based on combined TC and Raman fingerprints.
• Recycling process analytics: extending TC/TOC solid methods to analysis of process streams in battery recycling to quantify organic carbon contaminants and inform hydrometallurgical steps.

Conclusion

The combined use of a TOC solid sample combustion system and AIRsight Raman microscopy provides a robust workflow for evaluating surface carbon on LMFP cathode materials. Quantitative TC measurements are reproducible at low sample masses and are validated by post‑combustion Raman analysis showing effective removal of surface carbon. Raman structural metrics (e.g., ID/IG ≈ 0.8) reveal that the coating is defect‑rich rather than highly graphitized, consistent with conductive amorphous coatings commonly used for phosphate‑based cathodes. This integrated approach supports material development, quality control and recycling analytics in Li‑ion battery manufacturing and research.

Reference

• Evaluation of Organic Impurities in Lithium Carbonate by TOC Analysis, Application News No. 01-01098A-en (Shimadzu).
• Deterioration Evaluation of Lithium‑Ion Battery Components Using Infrared/Raman Microscope and Airtight Cells, Application News No. 01-00997A-en (Shimadzu).