Coordination of dissolved transition metals in pristine battery electrolyte solutions determined by NMR and EPR spectroscopy

Abstract

The solvation of dissolved transition metal ions in lithium-ion battery electrolytes is not well-characterised experimentally, although it is important for battery degradation mechanisms governed by metal dissolution, deposition, and reactivity in solution. This work identifies the coordinating species in the Mn²⁺ and Ni²⁺ solvation spheres in LiPF₆/LiTFSI–carbonate electrolyte solutions by examining the electron–nuclear spin interactions, which are probed by pulsed EPR and paramagnetic NMR spectroscopy. These techniques investigate solvation in frozen electrolytes and in the liquid state at ambient temperature, respectively, also probing the bound states and dynamics of the complexes involving the ions. Mn²⁺ and Ni²⁺ are shown to primarily coordinate to ethylene carbonate (EC) in the first coordination sphere, while PF₆⁻ is found primarily in the second coordination sphere, although a degree of contact ion pairing does appear to occur, particularly in electrolytes with low EC concentrations. NMR results suggest that Mn²⁺ coordinates more strongly to PF₆⁻ than to TFSI⁻, while the opposite is true for Ni²⁺. This work provides a framework to experimentally determine the coordination spheres of paramagnetic metals in battery electrolyte solutions.