In the next decade, our reliance on batteries is predicted to increase five-fold [1]. Lithium-ion (Li-ion) batteries (LIBs) dominate the current market. LIBs operate by moving electrons from an anode to a cathode (discharging) and back (charging). The Li ions from the liquid electrolyte balance this flow [2].

Consequently, the lithium battery electrolyte composition is crucial for the performance and lifetime of the battery [3,4]. Li electrolytes are mostly composed of lithium hexafluorophosphate (LiPF₆) or lithium difluorophosphate (LiPO₂F₂) dissolved in organic carbonates. The content of LiPF₆ or LiPO₂F₂ significantly influences the ionic conductivity, electrolyte stability, and battery safety. Therefore, it is crucial to determine the LiPF₆ or LiPO₂F₂ content to ensure that Li-ion batteries meet performance, safety, and aging criteria [5,6].

Analysis is challenging for certain techniques due to solvent or salt effects. Ion chromatography provides an accurate, economic solution for battery electrolyte analysis. The Metrohm intelligent Partial-Loop Technique (MiPT) simplifies the analysis, improves reproducibility and accuracy, and decreases costs. This Application Note details an ion chromatographic approach to determine lithium-ion battery electrolyte composition, i.e., the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluoro(oxalato)borate (LiODFB), LiPF₆, and LiPO₂F₂.

Sample handling was performed with the 858 Professional Sample Processor and MiPT. MiPT enables the precise generation of a calibration curve out of a single standard. Therefore, the 800 Dosino accurately aspirates a specific volume of the given standard into the injection loop. Samples were injected with a volume of 4 μL.

After injection, the target analytes (ODFB^-, PO₂F₂^-, PF₆^- and TFSI^-) were separated using the high-capacity Metrosep A Supp 7 - 250/4.0 column and a mixture of 14.4 mmol/L Na₂CO₃ and 40 vol% acetone as eluent. For accurate conductivity measurement, the background conductivity is reduced via sequential suppression, followed by conductivity detection. The example flow path for this analysis is shown in Figure 1.

The target analytes, i.e., LIB electrolyte components (LiODFB, LiPO₂F₂, LiPF₆ and LiTFSI), are effectively separated in their anionic forms (i.e., ODFB^-, PO₂F₂^-, PF₆^-, and TFSI^-) within 29 minutes (Figure 2). The recovery from two-level spike experiments (Table 1) ranged from 90–100% and reveals the robustness of the analysis. The sample concentration ranges covered 0.52–1.1 mg/L for ODFB^- (Table 2), 0.28–0.76 mg/L for PO₂F₂^- (Table 3), 11.05–14.07 mg/L for PF₆^- (Table 4), and 0.45–1.05 mg/L for TFSI^- (Table 5). The samples were determined in triplicate and showed average RSD values of 2.8% for ODFB^-, 2.8% for PO₂F₂^-, 1.8% for PF₆^-, and 0.8% for TFSI^-. 

1. Zhao, Y.; Pohl, O.; Bhatt, A. I.; et al. A Review on Battery Market Trends, Second-Life Reuse, and Recycling. Sustainable Chemistry 2021, 2 (1), 167–205. DOI:10.3390/suschem2010011
2. Fathi, R. A Guide to Li-Ion Battery Research and Development.
3. Treptow, R. S. Lithium Batteries: A Practical Application of Chemical Principles. J. Chem. Educ. 2003, 80 (9), 1015. DOI:10.1021/ed080p1015
4. Liu, Y.-K.; Zhao, C.-Z.; Du, J.; et al. Research Progresses of Liquid Electrolytes in Lithium-Ion Batteries. Small 2023, 19 (8), 2205315. DOI:10.1002/smll.202205315
5. Palacín, M. R. Understanding Ageing in Li-Ion Batteries: A Chemical Issue. Chem. Soc. Rev. 2018, 47 (13), 4924–4933. DOI:10.1039/C7CS00889A
6. Wang, Q.; Jiang, L.; Yu, Y.; et al. Progress of Enhancing the Safety of Lithium Ion Battery from the Electrolyte Aspect. Nano Energy 2019, 55, 93–114. DOI:10.1016/j.nanoen.2018.10.035