Identifying key electrolyte degradation products by chemical analysis can provide insight into elucidating mechanistic pathways that, if blocked, could potentially impart improved degradation resistance, leading to safer, longer-lasting lithium-ion batteries. Using ion chromatography (IC) on samples containing complex mixtures of anionic species can help in the elucidation of chemical structures of unknown components. The use of high-resolution accurate-mass spectrometry (HRMS) provides high-confidence analyte identification, while use of IC provides confirmatory information about ionizable functional groups. Together these two techniques bring a new view into the search for the identification of anionic degradation products in lithium-ion batteries.
Figure 1 shows the anion-exchange separation of components from the wash of an anode exhibiting 45% loss in capacity. Many of these analytes increase in concentration with anode capacity loss. Anions elute in this separation in order of valency on the anion exchange column but are detected as singly-charged ions in the mass spectrometer. Inorganic anions sulfate (-2 charge), phosphate (-3 charge) and pyrophosphate (-4 charge) are noted.
Table 1 provides the peak identifications for some species found in this sample HR/AM MS/MS detection shown in Figure 1.
It is important to correlate mass accuracy of a found mass to the proposed chemical formula in order determine if the proposed chemical formula is reasonable. For example, the anion with a m/z of 125.0009 must be monovalent at the found retention time, so the probable species is the monovalent phosphate ester, dimethylphosphate, as opposed to a divalent species ethylhydrogenphosphate.
- Ion Chromatography (IC) provides ion-exchange separations of anionic (or cationic) sample components.
- The IC with a conductivity detector is coupled to an HR/AM MS to provide information in the elucidation of unknowns.
- Analytes are eluted in the order of monovalent < divalent < trivalent < and higher so information is provided on key structural features.
- To date, we have found components from the aging of lithium-ion battery anodes in several classes, including carboxylic acids, esters, phosphate esters, sulfate esters, as well as inorganic anions.