Unveiling aging mechanisms of electrolytes in commercial end-of-life lithium-ion batteries

In this study, 77 end-of-life (EOL) commercial lithium-ion batteries (LIBs) of various formats were systematically analyzed to investigate electrolyte degradation and the influence of pristine electrolyte compositions on aging behavior. Comprehensive chemical characterization was conducted using targeted and non-targeted mass spectrometry (MS), employing LC-MS/MS, GC-MS, and high-resolution MS (HRMS). This integrated approach enabled the identification of confirmed pristine components and complex degradation products. The results show that rechargeable pouch and cylindrical cells often deviate from conventional model systems, containing mixed lithium salt anions, ionic liquids (ILs), and high concentrations of triflates, triflimides, and bis(fluorosulfonyl)imide (FSI). These function as solvents, salts, or safety-enhancing additives. Specific IL degradation products were identified, and hypotheses formulated on previously unreported pathways. Furthermore, a novel series of oligomerization products of propylene carbonate (PC) was detected. In contrast, non-rechargeable coin cells revealed widespread use of per- and polyfluoroalkyl substances (PFAS) in their original electrolytes. Based on ex situ analyses, hypothetical PFAS degradation mechanisms are proposed here for the first time. The absence of carbonate oligomers and lithium salt-derived products, alongside the presence of standard carbonates, indicates lithium counterion coordination as a key factor in Lewis acid-catalyzed degradation. This study offers valuable insights into real-world battery aging.