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

Significance of the Topic

Lithium-ion battery cathode active materials require careful surface engineering to deliver target performance in electric vehicles and consumer applications. For olivine phosphate materials such as LiFe0.4Mn0.6PO4 (LMFP), intrinsic electronic conductivity is low and a thin carbon coating on particle surfaces is routinely applied to improve electronic pathways. Quantitative control of carbon content and an understanding of its microstructure (degree of graphitization and defects) are therefore critical for reproducible cell performance, rate capability, and lifecycle behavior. Rapid, low‑consumption analytical workflows that provide both the mass fraction and structural quality of surface carbon are valuable in development, QA/QC, and recycling process monitoring.

Objectives and Study Overview

This study illustrates a combined analytical approach to evaluate carbon coatings on LMFP cathode particles. Two complementary techniques were employed: (1) a Total Organic Carbon (TOC) solid sample measurement system (TOC-L + SSM-5000A) for quantitative total carbon (TC) determination by high‑temperature combustion/CO2 detection, and (2) the AIRsight infrared/Raman microscope for structural characterization of the carbon layer via Raman spectroscopy. The work also validates carbon removal during combustion by Raman analysis of the post‑combustion residue.

Methodology

• Sample type: LMFP (LiFe0.4Mn0.6PO4) cathode active material with a surface carbon coating.
• Quantitation: TC analysis by catalytic combustion at 980 °C in an oxygen carrier gas stream with CO2 detection. One‑point calibration was performed using calcium carbonate powder reagent. Two sample masses were tested per single measurement: ~15 mg and ~30 mg. The instrument LOQ for absolute carbon was 0.1 mgC (improvable with optional cell switching valve).
• Reproducibility: For practical use, triplicate measurements (total sample consumption 45–90 mg per material) were considered given the absolute carbon levels close to the LOQ.
• Raman structural analysis: Microscope Raman measurements were performed on intact LMFP particles and on residues remaining after TC analysis to assess removal of carbon and to characterize carbon structural defects. Acquisition settings were chosen to minimize laser-induced damage: 532 nm excitation, 100× objective, ND filter at 7.5%, 25 s exposure, 5 accumulations, CCD detector.

Used Instrumentation

• TOC-L (total organic carbon analyzer) combined with SSM-5000A solid sample combustion unit — catalytic combustion TC measurement at 980 °C; O2 carrier gas at 500 mL/min; short cell; one‑point calibration with calcium carbonate.
• AIRsight infrared/Raman microscope paired with IRTracer-100 — confocal Raman spectroscopy capability, CCD detector, Raman excitation at 532 nm used for structural characterization of surface carbon.

Main Results and Discussion

• Quantitative TC results: Single‑measurement absolute carbon contents were approximately 0.254–0.269 mgC for ~15 mg samples and ~0.514–0.522 mgC for ~30 mg samples, yielding TC concentrations close to 1.69–1.72% carbon by weight. Reported averages were 1.694% (CV 1.30%) for the 15 mg set and 1.705% (CV 0.94%) for the 30 mg set, demonstrating good reproducibility across tested sample masses.
• Analytical limits and sample consumption: The instrument LOQ of 0.1 mgC dictates minimum sample mass when absolute carbon content is low; the study recommends increasing sample mass if the expected absolute carbon approaches or falls below the LOQ.
• Raman structural characterization: Raman spectra of the as‑coated LMFP showed the expected carbon bands: the G band near 1580 cm−1 and the D band near 1340 cm−1. The measured intensity ratio ID/IG was 0.8, indicating carbon with notable structural defects (i.e., non‑highly crystalline, partially disordered carbon). Such defect density affects electronic conductivity and therefore battery rate performance.
• Validation by post‑combustion Raman: Raman spectra of the LMFP residue after TC combustion showed disappearance of the carbon D and G bands, indicating that the carbon coating was effectively oxidized and quantified by the TOC solid sample method. A new feature observed near ~1300 cm−1 in the residue was attributed to oxide species formed during high‑temperature oxidation.

Benefits and Practical Applications

• Combined quantitative and structural assessment: The TOC solid sample system provides accurate, low‑consumption measurement of total surface carbon, while AIRsight Raman microscopy yields structural information (defect level, graphitization) that correlates with electronic conductivity.
• Minimal sample requirement: Achievable quantitation with single measurements using as little as 15 mg sample (subject to absolute carbon amount) makes the workflow practical for development-scale materials where sample is limited.
• Versatility for battery research and recycling: The TOC system can be configured for TC/TOC/IC in both solid and liquid matrices, enabling applications from cathode powder QC to analysis of water‑extractable organics in black mass, Li extraction solutions, and lithium carbonate in recycling streams. AIRsight allows investigation of degradation in electrodes and separators using both Raman and infrared modes.

Future Trends and Potential Applications

• Tighter process control: As EV battery production scales, inline or near‑line versions of low‑consumption TC analysis could be integrated into manufacturing QC to monitor carbon coating uniformity and target specifications.
• Advanced structural correlation: Combining Raman mapping with complementary techniques (TEM, XPS, conductivity mapping) will better correlate carbon defect metrics (e.g., ID/IG) with macroscopic electrode conductivity and cycle performance.
• Recycling analytics: Expanded TOC workflows for liquid and solid streams in recycling could help quantify organic contaminants and carbon residues, guiding hydrometallurgical unit operations and material recovery efficiency.
• Automation and sensitivity improvements: Instrument options such as cell switching valves can lower LOQs, enabling reliable analysis with even smaller sample sizes; automated sample handling could support higher throughput QA/QC.

Conclusion

The combined use of a TOC solid sample measurement system (TOC-L + SSM-5000A) and AIRsight infrared/Raman microscopy provides a robust, complementary approach for evaluating carbon coatings on LMFP cathode active materials. The TOC system offers quantitative measurement of surface carbon with good reproducibility at low sample masses, while Raman spectroscopy characterizes carbon structural disorder (ID/IG ≈ 0.8), which is directly relevant to electronic conductivity. Post‑combustion Raman confirmed near‑complete oxidation of the carbon coating, validating the TC quantitation. Together, these methods support material development, manufacturing QC, and recycling analytics for lithium‑ion battery technologies.

References

1. Jiajie Du and Yoshiyuki Tange. Evaluation of Surface Carbon on Lithium‑Ion Battery Cathode Active Materials Using a TOC Analyzer and Infrared/Raman Microscope. Shimadzu Application News, First Edition Jul 2026, document 01-01122-EN.
2. Evaluation of Organic Impurities in Lithium Carbonate by TOC Analysis. Shimadzu Application News No. 01-01098A-en.
3. Deterioration Evaluation of Lithium‑Ion Battery Components Using Infrared/Raman Microscope and Airtight Cells. Shimadzu Application News No. 01-00997A-en.