Solution NMR of Battery Electrolytes: Assessing and Mitigating Spectral Broadening Caused by Transition Metal Dissolution

Abstract

NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni 2+ and Mn 2+ ions from cathode materials may affect the NMR spectra of the electrolyte solution, with the unpaired electron spins in these paramagnetic solutes inducing rapid nuclear relaxation and spectral broadening (and often peak shifts). This work establishes how dissolved Ni 2+ and Mn 2+ in LiPF 6 electrolyte solutions affect 1 H, 19 F, and 31 P NMR spectra of pristine and degraded electrolyte solutions, including whether the peaks from degradation species are at risk of being lost and whether the spectral broadening can be mitigated. Mn 2+ is shown to cause far greater peak broadening than Ni 2+ , with the effect of Mn 2+ observable at just 10 μM. Generally, 19 F peaks from PF 6 − degradation species are most affected by the presence of the paramagnetic metals, followed by 31 P and 1 H peaks. Surprisingly, when NMR solvents are added to acquire the spectra, the degree of broadening is heavily solvent-dependent, following the trend of solvent donor number (increased broadening with lower solvent donicity). Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI) 2 or is dissolved from LiMn 2 O 4 . We show that the weak 19 F and 31 P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn 2+ are broadened to an extent that they are no longer visible, but this broadening can be minimized by diluting electrolyte samples with a suitably coordinating NMR solvent. Li 3 PO 4 addition to the sample is also shown to return 19 F and 31 P spectral resolution by precipitating Mn 2+ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.

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

“…It is possible that the relaxation measurement predicts only 55% of the actual Mn concentration because the other 45% of Mn ions that are bound to EC in the pristine electrolyte solution are instead bound to another species in the degraded solution (such as PO 2 F 2 – ). This is consistent with our previous work showing that the addition of different solvents to electrolyte samples containing dissolved transition metals may dramatically affect transverse relaxation enhancement of electrolyte components, presumably by altering the metal coordination shell . Preferential coordination of transition metals by PF 6 – degradation species has also been previously proposed by us and by others; , upcoming work will also explore transition metal coordination in detail using NMR and electron paramagnetic resonance (EPR) spectroscopy.…”

“…This is consistent with our previous work showing that the addition of different solvents to electrolyte samples containing dissolved transition metals may dramatically affect transverse relaxation enhancement of electrolyte components, presumably by altering the metal coordination shell . Preferential coordination of transition metals by PF 6 – degradation species has also been previously proposed by us and by others; , upcoming work will also explore transition metal coordination in detail using NMR and electron paramagnetic resonance (EPR) spectroscopy. Theoretically, if the solvation shell surrounding the 4.67 mM Mn (the concentration present in the degraded electrolyte solution) comprised 6 EC molecules, this would correspond to a total displacement of 12.6 mM EC, likely by 12.6 mM of one or several other species.…”

“…− degradation species has also been previously proposed by us 128 and by others; 72,140 upcoming work will also explore transition metal coordination in detail using NMR and electron paramagnetic resonance (EPR) spectroscopy. Theoretically, if the solvation shell surrounding the 4.67 mM Mn (the concentration present in the degraded electrolyte solution) comprised 6 EC molecules, this would correspond to a total displacement of 12.6 mM EC, likely by 12.6 mM of one or several other species.…”

“…This is consistent with our previous work showing that the addition of different solvents to electrolyte samples containing dissolved transition metals may dramatically affect transverse relaxation enhancement of electrolyte components, presumably by altering the metal coordination shell. 128 Preferential coordination of transition metals by PF 6…”

“…This work follows from our previous studies: in the first, we explored the use of bulk magnetic susceptibility shifts to identify the oxidation states and concentrations of dissolved paramagnetic ions. 127 In the second, we examined the line broadening induced by paramagnetic ions in pristine and degraded battery electrolytes, 128 describing methods to mitigate this. Here, we analyze relaxation phenomena in greater detail, measuring and analyzing both transverse and longitudinal relaxation rates; we show that they are highly sensitive to the presence of Mn 2+ , especially with 19 F and 1 H measurements.…”