Transition metal coordination to degradation products in battery electrolytes revealed by NMR and EPR spectroscopy

Abstract

The dissolution of transition metals from lithium-ion battery materials contributes to cell failure. Developing a better understanding of transition metal solvation, reactivity, and deposition will help mitigate transition metal dissolution and, thus, facilitate batteries with higher capacity and longer lifetime. In this work, Mn2+ and Ni2+ coordination in degraded LiPF6 carbonate electrolyte solutions are examined via 1H and 19F NMR relaxometry at ambient temperatures and pulsed (double resonance HYSCORE and ENDOR) EPR spectroscopy of the frozen solution. Critically, the solvation spheres of transition metals in heat degraded electrolytes are shown to differ from the solvation sphere in pristine electrolytes, with significant coordination to LiPF6-derived fluorophosphate degradation species—a factor that will impact the mechanisms and the extent of transition metal dissolution. The Mn2+ coordination environment is shown to be affected by adding a variety of species including water, ethylene glycol, and acetylacetonate, increasingly displacing EC and PF6– from inner and outer Mn2+ coordination shells. EPR and NMR studies of electrolytes containing the battery additive and proposed degradation product LiPO2F2 explicitly confirm that the transition metals coordinate to PO2F2– in both the inner and outer sphere. By contrast, in heat-degraded electrolytes, additional protonated (uncharged) fluorophosphate species are also present in the metal solvation shell, as clearly demonstrated via large 1H and 31P hyperfine interactions, seen in the EPR spectra of Mn2+ complexes. To probe transition metal deposition, Mn2+ and Ni2+ were exposed to a wide variety of salts found in the solid electrolyte interphase (SEI), revealing for the first time that the metathesis deposition pathway is viable with many different SEI species.