Ligand Elucidation of Dissolved Transition Metal Ions in Pristine and Degraded Electrolytes in Li-Ion Batteries

Abstract

During operation of a Li-ion battery, layered transition metal oxides release metal ions into the electrolyte. In addition to capacity loss, dissolved metal ions are mobile in the electrolyte and can deposit at the anode, enhancing Faradaic losses and posing the risk of dendrite formation.[1] In the electrolyte solution, transition metal ions are characterized mainly by their oxidation state and complexation behavior, which dictate the transport through the electrolyte. Additionally, mechanistic insights, indirect information about electrolyte degradation, and design of counter-measures might be extracted thereof. However, particularly the coordination sphere is difficult to access, due to inherently low concentrations and scarcity of suitable analytics.In this work, the coordination sphere of dissolved transition metal ions is selectively probed by pulse Electron Paramagnetic Resonance (pEPR). pEPR selectively addresses isolated paramagnetic ions, masking analytical interference from other species in the battery. Because of its known impact on capacity fade, firstly Mn2+ is analyzed in premixed pristine and degraded electrolyte solutions. Carbonates with high dielectric constants such as ethylene carbonate (EC) are the dominant ligands, accompanied by electrolyte salt anions for charge compensation. If present, trace amounts of electrolyte degradation products such as fluorophosphates selectively form the first coordination sphere. The results from pEPR measurements, that have to be performed on frozen solutions at cryogenic temperatures, agree with paramagnetic NMR results at ambient temperatures. Furthermore, samples from batteries operating with vanadium-based cathodes were investigated. Depending on the storage conditions, V4+ ligands predominantly consist of decomposition products from the electrolyte solvent ethylene carbonate (e.g. ethylene glycol anions) or electrolyte salt LiPF6 (fluorophosphates).[2,3] [1] C. Zhan, T. Wu, J. Lu, K. Amine, Energy Environ. Sci. 2018, 11, 243–257.[2] C. Szczuka, R.-A. Eichel, J. Granwehr, ACS Appl. Energy Mater. 2022, 5, 449–460.[3] C. Szczuka, P. Jakes, R.-A. Eichel, J. Granwehr, Adv. Energy Sustain. Res. 2021, 2, 2100121.