Rapid Assay of Sodium Hexafluorophosphate for Use in Sodium-Ion Batteries by ICP-OES

Importance of the Topic

The purity of sodium hexafluorophosphate (NaPF6) electrolyte is critical for reliable performance of sodium-ion batteries. Trace metal impurities can catalyze side reactions, degrade cell components, and undermine cycle life, safety, and efficiency. Monitoring impurity levels supports quality control in electrolyte production and battery manufacturing, helping to meet industry standards and ensure long-term device stability.

Study Objectives and Overview

This application note describes a rapid analytical method to quantify 26 trace elements in NaPF6 electrolyte samples using an Agilent 5800 Vertical Dual View (VDV) ICP-OES system operating in axial view mode. Two commercial NaPF6 lots (samples A and B) were evaluated for impurity content, method sensitivity, accuracy, and stability. A spiked sample was also measured to assess recovery performance.

Methodology and Used Instrumentation

Sample preparation involved dissolving NaPF6 salts in acidified de-ionized water under moisture-free conditions.

• Calibration standards were matrix-matched in 1% NaCl with multi-element mixes at 0.05–0.5 mg/L.
• A spiked solution of sample B contained all target elements at 0.025 mg/L.
• Instrument: Agilent 5800 VDV ICP-OES with MiraMist nebulizer, inert cyclonic spray chamber, Easy-fit VDV torch, and SSRF generator.
• Software: Agilent ICP Expert Pro with IntelliQuant Screening and Fitted Background Correction (FBC).

Operating parameters included 1.1 kW RF power, 12 L/min plasma flow, axial viewing mode, and three replicate readings per sample.

Main Results and Discussion

Calibration linearity exceeded R²>0.9999 for all 26 elements over the analytical ranges. Method detection limits ranged from 0.001 to 0.135 mg/kg. Quantitative analysis of samples A and B showed most impurities below 1 mg/kg; exceptions were Fe (~3.47 mg/kg), Li (~1.13 mg/kg), and S (~1.05 mg/kg). Spike recoveries for sample B fell between 98% and 108%, confirming method accuracy. A 90-minute stability test of the spiked sample demonstrated RSDs <2.5%, indicating robust performance under high-matrix conditions.

Benefits and Practical Applications

This ICP-OES method provides:

• High throughput screening for 26 impurities in NaPF6 electrolytes.
• Low ppb-level detection limits enabling early identification of contamination.
• Automated background correction and wavelength selection for improved reliability.
• Strong robustness against high sodium matrix interferences.

It supports quality control in electrolyte manufacturing and ensures consistent battery performance.

Future Trends and Possibilities for Use

As sodium-ion batteries gain commercial traction, demand for rapid and accurate electrolyte analysis will grow. Future developments may include:

• Integration with automated sample handling for high-throughput production labs.
• Application of ICP-OES screening tools alongside real-time process monitoring.
• Extension to new electrolyte chemistries and solid-state systems.
• Advanced data analytics for predictive maintenance of battery performance.

Conclusion

The Agilent 5800 VDV ICP-OES axial mode, combined with IntelliQuant and Fitted Background Correction, delivers a reliable, sensitive, and robust method for trace element quantification in NaPF6 electrolytes. The approach achieves excellent linearity, low detection limits, high recovery accuracy, and long-term stability, making it well suited for routine quality control in sodium-ion battery production.

References

1. Siddiqi S. Sodium-Ion Insights: Industry’s Key Queries, IDTechEx, May 2024.
2. GB/T 19282-2014. Analytical Method for Lithium Hexafluorophosphate, National Standard of the People’s Republic of China.
3. A Practical Guide to Elemental Analysis of Lithium Ion Battery Materials Using ICP-OES, Agilent, publication 5994-5489EN.
4. Yingping N., Wenkun F. Lithium Hexafluorophosphate Electrolysis Using Agilent 5110 ICP-OES, Agilent, publication 5994-1937EN.
5. Agilent IntelliQuant Screening: Smarter and Quicker Semiquantitative ICP-OES Analysis, publication 5994-1518EN.
6. Fitted Background Correction (FBC): Fast, Accurate, Fully Automated Background Correction, Agilent, publication 5991-4836EN.
7. Larsson F. et al. Toxic Fluoride Gas Emissions from Lithium-Ion Battery Fires, Scientific Reports 2017, 7(1), 10018.
8. Han J.Y., Jung S. Thermal Stability and the Effect of Water on Hydrogen Fluoride Generation in Lithium-Ion Battery Electrolytes Containing LiPF6, Batteries 2022, 8(7), 61.