Implementation of ion exclusion chromatography for characterization of lithium ion battery materials

Inorganic compounds such as lithium fluoride (LiF) and lithium carbonate (Li₂CO₃) as well as weakly acidic lithium salts like lithium acetate (LiCH₃CO₂) or lithium formate (LiHCO₂) are reported decomposition products in lithium ion batteries (LIBs). The simultaneous analysis of these compounds is challenging due to the complex system consisting of conductive salt, organic carbonates, additives and their decomposition variety. Ion exclusion chromatography with conductivity detection (IEC-CD) seems to be predestinated for this analytical task due to its ability to separate and determine weakly acidic anions, which are the relevant species arising from lithium salts and electrolyte decomposition processes. One important chromatographic method to analyze ionic decomposition products is ion exchange chromatography (IC), which is currently a state-of-the-art (SOTA) technique for fluoride (F^-) quantification in LIBs. However, the calibration curve of F^- by IC hyphenated to a conductivity detection (CD) provides a small linear range for low concentrations and an analyte dependent retention shift occurs. IEC-CD represents a substantial upgrade in this respect and generated benefits for electrolyte analysis by an improved linear range for F^- (up to several 100 ppm). Furthermore, especially in complex samples, the IEC-CD method provides a more reliable chromatographic separation. In this study, IEC-CD is implemented to investigate decomposition pathways of fluor-releasing electrolyte additives such as fluoroethylene carbonate (FEC). The quantification of formate (HCO₂^-), acetate (CH₃CO₂^-) and carbonate (CO₃^2-) was also possible to gain deeper understanding of electrolyte additive decomposition in LIBs.