Anions in lithium-ion battery solvents

Importance of the topic

N-methylpyrrolidone (NMP) is a key solvent in lithium-ion battery production, crucial for dissolving electrode binders and maintaining electrode stability. Monitoring low-level anionic impurities in NMP is vital to safeguard battery performance, safety, and lifecycle, particularly as NMP is recycled and reused.

Objectives and study overview

This application note presents a robust ion chromatography (IC) method with Metrohm_x0092_s intelligent Preconcentration Technique with Matrix Elimination (MiPCT-ME) for quantifying trace anions in NMP. The targeted analytes include fluoride, chloride, nitrite, bromide, nitrate, phosphate, and sulfate in the μg/L range without prior sample treatment.

Methodology and instrumentation

500 µL of undiluted NMP was directly injected into a Metrosep A PCC 2 HC/4.0 preconcentration column using the 930 Compact IC Flex system equipped with MiPCT-ME. After sample loading, the column was rinsed with 1.5 mL ultrapure water to remove the matrix. The analytes were eluted onto a Metrosep A Supp 7 - 250/4.0 column at 45 °C using an isocratic eluent of 3.2 mmol/L Na2CO3 and 1.0 mmol/L NaHCO3 at a flow rate of 0.7 mL/min. Sequential suppression and conductivity detection enabled sensitive measurement.

Results and discussion

Anions were baseline-separated in under 34 minutes with calibration from 5 to 100 µg/L. Measured concentrations in unspiked NMP ranged from 11 to 76 µg/L. Spike recoveries for added standards (30 µg/L) fell between 90 and 120 %, validating the method_x0092_s accuracy. Two early eluting unidentified peaks likely correspond to acetate and formate, indicating potential for expansion to organic acid analysis.

Benefits and practical applications

The presented IC method allows direct, dilution-free analysis of trace anions in complex NMP matrices, supporting quality control of newly produced and recycled solvents. By ensuring accurate impurity levels, manufacturers can optimize electrolyte composition, enhance battery lifespan, and improve safety.

Future trends and possibilities

Future developments may include extending the approach to additional battery solvents such as methanol, ethanol, acetone, and isopropanol, and refining separation parameters for broader impurity profiling, including organic acids. Integration with automated sample handling and advanced data analytics will further streamline routine quality assessments.

Conclusion

Metrohm_x0092_s IC method with MiPCT-ME provides a straightforward, sensitive, and reliable solution for trace anion determination in NMP without extensive preparation steps. Its adaptability makes it a valuable tool for ensuring the quality and safety of lithium-ion battery manufacturing solvents.

Reference

• Yue M, Azam S, Zhang N et al. Residual NMP and Its Impacts on Performance of Lithium-Ion Cells. Journal of The Electrochemical Society 2024;171(5):050515. DOI:10.1149/1945-7111/ad4396
• The role of NMP in the production process of lithium batteries. Shenyang East Chemical Science-Tech Co Ltd. Accessed August 16, 2024.
• Darcel C. What is NMP Solvent? Maratek. Accessed August 16, 2024.
• The Advanced Rechargeable & Lithium Batteries Association. Recommendation about N-Methyl-Pyrrolidone (NMP; CAS No. 872-50-4). Proposal for Inclusion in Annex XIV for Authorization, 2017.