Full differentiation and assignment of boron species in the electrolytes Li2B6O9F2 and Li2B3O4F3 by solid-state 11B NMR spectroscopy

The syntheses of two new fluorooxoborates, Li2B3O4F3 and Li2B6O9F2, which possess considerable ion conductivity at higher temperatures, have been reported recently. Here, we describe the characterisation of these compounds by solid-state 11B NMR spectroscopy. The complex central-transition MAS spectra, resulting from overlap of sub-spectra contributed by the individual boron species in the crystal structures, could be clearly separated by acquisition and analysis of 3QMAS spectra. By numerical fit of these sub-spectra, the isotropic chemical shift δisoδiso, the quadrupolar coupling constant χχ, and the asymmetry ηη were determined. Using known relations between boron coordination and chemical shift as well as quadrupolar coupling, the individual 11B NMR resonances have been ascribed to boron species in tetrahedral or trigonal environment. To remove remaining assignment ambiguities, the response of the 11B resonances to 19F decoupling was qualitatively analysed. Thus, by using the combined information conveyed by chemical shift, quadrupolar and dipolar interaction, a complete assignment of the complex 11B line shapes exhibited by the fluorooxoborates has been achieved.