Analysis of Residual N-Methyl-2-Pyrrolidone (NMP) in Lithium-Ion Battery Electrodes

Importance of the Topic

Residual N-Methyl-2-pyrrolidone (NMP) is widely used as a solvent in lithium-ion battery electrode manufacturing to dissolve polymer binders and ensure uniform slurry application. Incomplete removal of NMP during electrode drying can impair battery performance, safety, and long-term stability. Accurate monitoring of trace residual NMP supports quality control and regulatory compliance in high-throughput production.

Objectives and Study Overview

This study evaluates a headspace (HS) gas chromatography method for quantifying residual NMP in cathode and anode materials. Using the Agilent 8697 headspace sampler coupled with an Agilent 8860 GC system and FID detector, the method is benchmarked against a traditional liquid extraction (LE) protocol. Key goals include method sensitivity, precision, detection limits, carryover performance, and comparability between HS and LE approaches across multiple electrode types.

Methodology and Instrumentation

Sample Preparation and Calibration

• Blank electrodes were prepared by baking 0.5 g foil pieces at 200 °C for 3–7 h. Real samples from two cathode and two anode sources were weighed at 0.5 g.
• Calibration standards (200–50 000 µg/mL NMP in water) were spiked onto blanks to establish matrix-matched curves: linear models for anodes and cathode type 1, quadratic for cathode type 2.

Headspace Extraction and GC Conditions

• Vial: 20 mL PTFE/silicone septa, 0.5 g sample cut into small pieces.
• HS incubation: 190 °C, 20 min, 25 shakes/min.
• Transfer line and loop at 190 °C, 1 mL loop volume, N₂ pressurization.
• GC inlet: splitless quartz liner at 220 °C. Column: DB-WAX UI (30 m×0.25 mm, 0.25 µm). Carrier N₂ at 1.0 mL/min, split 20:1.
• Oven program: 60 °C (2 min) → 250 °C at 25 °C/min, hold 3 min. FID at 250 °C, gases: air 400 mL/min, H₂ 30 mL/min, N₂ 25 mL/min.

Liquid Extraction Protocol

• Ultrasound-assisted extraction with ethyl acetate or ethanol, followed by filtration prior to GC injection.

Main Results and Discussion

Calibration and Linearity

• Anodes: matrix-matched linearity (R²>0.999) over 200–10 000 µg/mL NMP; cathode type 1: linear (R²=0.9989); cathode type 2: best fit quadratic.

Precision and Detection Limits

• Repeatability (n=6) at 4, 20, 200 µg/g: anode RSD 2.16%, 1.45%, 0.68%; cathode RSD 3.81%, 0.98%, 0.99%.
• Method detection limit (MDL) for anodes: ~0.025 µg/g (S/N=3:1); LOQ ~0.08 µg/g (S/N=10:1).
• Carryover: no detectable NMP in blank runs following a 200 µg/g spiked sample.

Comparison of HS and LE Methods

• Anode recoveries by HS were 89–92% of LE values; cathode recoveries by HS were 65–78% of LE values.
• HS provides rapid, solvent-free sample prep and precise quantitation suitable for routine process control.
• LE offers more complete extraction and is recommended during method development or when dealing with new electrode formulations.

Benefits and Practical Applications

The HS-GC-FID approach simplifies sample handling, eliminates solvent disposal, and delivers low detection limits with robust precision. It is ideal for high-throughput monitoring in established electrode production, enabling quick feedback for process adjustments. LE remains valuable for troubleshooting, validation of new materials, and confirming HS results when matrix effects vary significantly.

Future Trends and Potential Applications

Advances may include coupling headspace sampling with mass spectrometric detection for enhanced selectivity, automated vial handling for greater throughput, and inline monitoring solutions integrated into roll-to-roll electrode coating lines. Expanding the method to quantify other residual solvents or additives could further streamline battery manufacturing quality control.

Conclusion

The Agilent 8697 HS sampler combined with the 8860 GC-FID provides a sensitive, precise, and low-carryover method for residual NMP analysis in lithium-ion battery electrodes. While HS meets routine monitoring needs in established processes, LE extraction is advised for method development and critical validation. This complementary approach ensures comprehensive control of residual solvent levels to maintain electrode quality.

Reference

1. Liu Y., Zhang R., Wang J., Wang Y. Current and Future Lithium-Ion Battery Manufacturing. iScience 2021, 24, 102332.
2. Zhang X., Han G., et al. Effect of NMP Addition on Battery Performance in Negative Electrodes. Henan Chemical Industry 2021, 38, 23–25.
3. Shang H. T., Zhang J., Jiang F. Analysis of N-Methyl-2-Pyrrolidone (NMP) in Battery Electrodes. Agilent Technologies Application Note, 2024.