The interaction between inorganic Li salts (LiTf, LiNTf2) and the surface of alumina particles as studied with solid state nuclear magnetic resonance

The interaction between Li salts {LiTf (Tf = CF3SO3) and LiNTf2 (NTf2 = N(SO2CF3)2)} with surface modified alumina particles (basic, neutral or acidic) is investigated employing a range of advanced solid state NMR methodologies. Utilizing 7Li MAS NMR, a new signal – in addition to the signal of the pure salt – could be identified in the composite samples, increasing with increasing basicity of the alumina surface. Employing 7Li–{1H} CPMAS NMR and 7Li–{1H}–CPMAS–{27Al} REAPDOR NMR spectroscopy, this new signal could be unequivocally assigned to an alumina-surface bound Li species. For the anions, 19F MAS NMR spectra clearly prove the existence of new anion sites. Employing 19F–{7Li} REDOR spectroscopy and 19F–{27Al} TRAPDOR NMR spectroscopy, the identified signals could be safely assigned to anions within the pristine Li salt and anions attached to the alumina surface. These results present direct evidence for the anion∙∙∙alumina surface and cation∙∙∙alumina surface interaction, suggested by several authors to aid in the interpretation of the effect of the ceramic additive on the ionic conductivity.