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

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NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni2+ and Mn2+ 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 Ni2+ and Mn2+ in LiPF6 electrolyte solutions affect 1H, 19F, and 31P 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. Mn2+ is shown to cause far greater peak broadening than Ni2+, with the effect of Mn2+ observable at just 10 M. Generally, 19F peaks from PF6– degradation species are most affected by the presence of the paramagnetic metals, followed by 31P and 1H 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. Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI)2 or is dissolved from LiMn2O4. We show that electrolyte samples containing micromolar levels of dissolved Mn2+ will lose resolution of small 19F and 31P peaks, but this can be minimised by diluting electrolyte samples with a suitably coordinating NMR solvent. Li3PO4 addition to the sample is also shown to return 19F and 31P spectral resolution to close to its original value by precipitating Mn2+ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.



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Journal of Physical Chemistry C

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American Chemical Society (ACS)
Faraday Institution (Unknown)
Faraday Institution (FIRG001)
Faraday Institution (FIRG001)
Faraday Institution (FIRG001)
Natural Sciences and Engineering Research Council of Canada Faraday Institution [FIRG001} Royal Society - Professorship to Clare P. Grey