Analysis of the Decomposition Products of Lithium Hexafluorophosphate in the Electrolytic Solution of Lithium-Ion Rechargeable Batteries by Column-Switching Ion Chromatography

Importance of Topic

The stability of lithium hexafluorophosphate (LiPF6) in electrolyte solutions is critical for the performance and safety of lithium-ion rechargeable batteries. Hydrolytic decomposition of LiPF6 generates fluoride and fluorophosphate ions, which can degrade cell components, reduce cycle life and impair battery efficiency. Reliable analytical methods to detect and quantify these decomposition products support quality control, accelerate electrolyte development and ensure battery reliability.

Objectives and Study Overview

This study aimed to develop a rapid, sensitive and robust column-switching ion chromatography (IC) method to separate and quantify fluoride (F–), monofluorophosphate (PO3F2–) and identify difluorophosphate (PO2F2–) in LiPF6 electrolytes. The method was optimized for standard solutions and applied to commercial battery electrolytes under fresh and aged (24-hour air exposure) conditions.

Methodology and Instrumentation

A dual-column IC system was built on the Shimadzu Prominence HIC-SP platform:

• Guard Column: Shim-pack IC-SA2 (10 mm × 4.6 mm ID)
• Analytical Column: Shim-pack IC-SA2 (250 mm × 4.0 mm ID)
• Mobile Phase: 12 mmol/L NaHCO3 + 0.6 mmol/L Na2CO3 in water
• Flow Rate: 1.0 mL/min on both pumps
• Injection Volume: 10 µL
• Column Temperature: 30 °C
• Detection: CDD-10ASP suppressor

The sample is first passed through the guard column to isolate low-retention decomposition products, which are then directed to the analytical column for high-resolution separation. A second pump elutes retained LiPF6 to waste and reconditions the columns.

Main Results and Discussion

Calibration curves for F– (0.1–10 mg/L) and PO3F2– (1–100 mg/L) exhibited excellent linearity (R2 > 0.999). Repeatability tests (n = 6) on 1 mg/L F– and 10 mg/L PO3F2– yielded RSDs of <0.05% for retention times and <1.2% for peak areas. Analysis of commercial electrolyte showed detectable levels of F– and PO2F2– in fresh samples; after 24 hours of air exposure, PO3F2– was also observed, indicating progressive hydrolysis.

Benefits and Practical Applications

This column-switching IC approach enables high-throughput monitoring of electrolyte purity and degradation. Its sensitivity and precision support quality assurance in battery manufacturing, facilitate formulation screening and aid in failure analysis of aged cells.

Future Trends and Possibilities

Advances may include coupling with mass spectrometry for structural confirmation of unknown fluorophosphate species, miniaturized flow-through systems for in-line process monitoring and extension to other electrolyte additives and degradation pathways.

Conclusion

The developed column-switching IC method offers a robust, sensitive and efficient tool for monitoring LiPF6 decomposition products in lithium-ion battery electrolytes. It meets the demands of routine quality control and research applications, providing critical insights into electrolyte stability and battery performance.

Reference

Shimadzu Corporation. High Performance Liquid Chromatography, No. L417, LAAN-A-LC-E195.