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,7Li,31P,19F, and1H longitudinal and transverse relaxation rates were measured to study
LiPF6electrolyte solutions containing Ni2+,
Mn2+, Co2+, or Cu2+salts and Mn
dissolved from LiMn2O4. Sensitivities were found
to vary by nuclide and by transition metal.19F (PF6–) and1H (solvent) measurements
were more sensitive than7Li and31P measurements
due to the higher likelihood that the observed species are in closer
proximity to the metal center. Mn2+induced the greatest
relaxation enhancement, yielding a limit of detection of ∼0.005
mM for19F and1H measurements. Relaxometric
analysis of a sample containing Mn dissolved from LiMn2O4at ∼20 °C showed good sensitivity and accuracy
(suggesting dissolution of Mn2+), but analysis of a sample
stored at 60 °C showed that the relaxometric quantification is
less accurate for heat-degraded LiPF6electrolytes. This
is attributed to degradation processes causing changes to the metal
solvation shell (changing the fractions of PF6–, EC, and EMC coordinated to Mn2+), such that calibration
measurements performed with pristine electrolyte solutions are not
applicable to degraded solutions—a potential complication for
efforts to quantify metal dissolution duringoperandoNMR studies of batteries employing widely-used LiPF6electrolytes.Ex situnondestructive 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.