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 is examined via ¹H and ¹⁹F NMR relaxometry at ambient temperatures and pulsed (double resonance HYSCORE and ENDOR) EPR spectroscopy of the frozen solutions. Critically, the solvation spheres of transition metals in heat-degraded electrolytes are shown to differ from those found 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.