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

Abstract

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, and 1H longitudinal and transverse relaxation rates were measured to study LiPF6 electrolyte 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 –) and 1H (solvent) measurements were more sensitive than 7Li and 31P 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 for 19F and 1H measurements. Relaxometric analysis of a sample containing Mn dissolved from LiMn2O4 at ∼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 LiPF6 electrolytes. 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 during operando NMR studies of batteries employing widely-used LiPF6 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.

Understanding and Mitigating the Dissolution and Delamination Issues Encountered with High-Voltage LiNi0.5Mn1.5O4

“…The 1 H NMR and 19 F NMR spectra of the electrolytes collected after the powd ing test (shown in Figure 6a,b, respectively) displayed similar peaks shifting and ening with an order of LNMO (Targary or NEI Corporation) > LMO > NMC442 NMC/NMC811. The ICP-MS data (Figure 7) were also plotted in this order, an crease in Mn dissolution content was observed along this order, further demon that the NMR peaks shifting and broadening was caused by the paramagnetic M solution [13,14]. Based on the results from NMR and ICP-MS measurements, i concluded that compared to layered structure NMC, spinel LNMO and LMO much more severe Mn dissolution when in contact with the carbonate-based ele This indicates that the electrode/electrolyte interaction for LNMO was harshe could explain the delamination issue observed in its high-voltage cycled cells.…”

Beyond Composition: Surface Reactivity and Structural Arrangement of the Cathode–Electrolyte Interphase

“…However, there are a few factors that the user should be aware of. First, large amounts of transition metal dissolution, particularly dissolved Mn 2+ , can broaden NMR signals, making it difficult to detect electrolyte decomposition products . Second, the presence of HF in LiPF 6 -based solutions requires that the cell be placed into a chemically resistant NMR tube as mentioned above.…”

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

“…20 Later simulations of the Mn 2+ solvation shell showed that Mn 2+ interaction energies follow the order EC > PF 6 − > linear carbonates. 21 Our previous NMR studies have suggested that dissolved transition metals may favour coordination to EC over ethyl methyl carbonate 22,23 and may coordinate preferentially to PF 6 − degradation products over pristine electrolyte components. 23,24…”

“…We have previously shown that the presence of dissolved paramagnetic ions can result in differential enhancement of the NMR relaxation rates of the signals from a variety of species in both pristine and degraded battery electrolyte solutions. 23,24 We have used these relaxation rates, coupled with the accompanying bulk magnetic susceptibility shifts, to quantify metal concentrations and investigate binding to different electrolyte solvents. We have also used EPR to, for example, establish the solvation shell of dissolved vanadyl ions.…”

“… 21 Our previous NMR studies have suggested that dissolved transition metals may favour coordination to EC over ethyl methyl carbonate 22,23 and may coordinate preferentially to PF 6 − degradation products over pristine electrolyte components. 23,24 …”