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

Transition metal dissolution is an important contributorto capacityfade in lithium-ion cells. NMR relaxation rates are proportional tothe concentration of paramagnetic species, making them suitable toquantify dissolved transition metals in battery electrolytes. In thiswork, Li-7, P-31, F-19, and H-1 longitudinal and transverse relaxation rates were measured to studyLiPF(6) electrolyte solutions containing Ni2+,Mn2+, Co2+, or Cu2+ salts and Mndissolved from LiMn2O4. Sensitivities were foundto vary by nuclide and by transition metal. F-19 (PF6 (-)) and H-1 (solvent) measurementswere more sensitive than Li-7 and P-31 measurementsdue to the higher likelihood that the observed species are in closerproximity to the metal center. Mn2+ induced the greatestrelaxation enhancement, yielding a limit of detection of & SIM;0.005mM for F-19 and H-1 measurements. Relaxometricanalysis of a sample containing Mn dissolved from LiMn2O4 at & SIM;20 & DEG;C showed good sensitivity and accuracy(suggesting dissolution of Mn2+), but analysis of a samplestored at 60 & DEG;C showed that the relaxometric quantification isless accurate for heat-degraded LiPF6 electrolytes. Thisis attributed to degradation processes causing changes to the metalsolvation shell (changing the fractions of PF6 (-), EC, and EMC coordinated to Mn2+), such that calibrationmeasurements performed with pristine electrolyte solutions are notapplicable to degraded solutions a potential complication forefforts to quantify metal dissolution during operando NMR studies of batteries employing widely-used LiPF6 electrolytes. Ex situ nondestructive quantification of transition metalsin lithium-ion battery electrolytes is shown to be possible by NMRrelaxometry; further, the method's sensitivity to the metalsolvation shell also suggests potential use in assessing the coordinationspheres of dissolved transition metals.