Quantifying Dissolved Transition Metals in Battery Electrolyte Solutions with NMR Paramagnetic Relaxation Enhancement.

Transition metal dissolution is an important contributor to capacity fade in lithium-ion cells. NMR relaxation rates are proportional to the concentration of paramagnetic species, making them suitable to quantify dissolved transition metals in battery electrolytes. In this work, 7 Li, 31 P, 19 F, and 1 H longitudinal and transverse relaxation rates were measured to study LiPF 6 electrolyte solutions containing Ni 2+ , Mn 2+ , Co 2+ , or Cu 2+ salts and Mn dissolved from LiMn 2 O 4 . Sensitivities were found to vary by nuclide and by transition metal. 19 F (PF 6 - ) and 1 H (solvent) measurements were more sensitive than 7 Li and 31 P measurements due to the higher likelihood that the observed species are in closer proximity to the metal center. Mn 2+ induced the greatest relaxation enhancement, yielding a limit of detection of ∼0.005 mM for 19 F and 1 H measurements. Relaxometric analysis of a sample containing Mn dissolved from LiMn 2 O 4 at ∼20 °C showed good sensitivity and accuracy (suggesting dissolution of Mn 2+ ), but analysis of a sample stored at 60 °C showed that the relaxometric quantification is less accurate for heat-degraded LiPF 6 electrolytes. This is attributed to degradation processes causing changes to the metal solvation shell (changing the fractions of PF 6 - , EC, and EMC coordinated to Mn 2+ ), such that calibration measurements performed with pristine electrolyte solutions are not applicable to degraded solutions-a potential complication for efforts to quantify metal dissolution during operando NMR studies of batteries employing widely-used LiPF 6 electrolytes. Ex situ nondestructive quantification of transition metals in lithium-ion battery electrolytes is shown to be possible by NMR relaxometry; further, the method's sensitivity to the metal solvation shell also suggests potential use in assessing the coordination spheres of dissolved transition metals.