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

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 and Mn 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 and Mn in LiPF electrolyte solutions affect H, F, and 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 is shown to cause far greater peak broadening than Ni, with the effect of Mn observable at just 10 μM. Generally, F peaks from PF degradation species are most affected by the presence of the paramagnetic metals, followed by P and 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) or is dissolved from LiMnO. We show that the weak F and P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn 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. LiPO addition to the sample is also shown to return F and P spectral resolution by precipitating Mn out of electrolyte samples, although this method consumes any HF in the electrolyte solution.