Detection of surface layers using 7Li MAS NMR

Abstract

Magic angle spinning solid state NMR is generally used to characterize bulk materials. We show here that it is also a promising tool to detect and characterize the diamagnetic surface layer on a paramagnetic material, which is a novel development. We apply this technique to the very hot topic of positive electrode/electrolyte characterization in the field of lithium rechargeable batteries. We report a 7Li MAS NMR study of physisorbed surface layers on LiNi1/2Mn1/2O2, a positive electrode material for lithium ion batteries. 7Li MAS NMR signals arising from surface layers formed by mixing the material with lithium carbonate or from contact of the material with ambient atmosphere, as well as with electrolyte are collected and analyzed. The progressive broadening of the line shape of the MAS NMR spectra reflects the increasing intimacy of the surface layer or secondary phase with the bulk material and therefore gives extremely useful complementary structural information on the surface not available using XPS or IR. We show that relaxation time measurements can be used as a probe of surface layers, allowing for discrimination of interphases from different origins. We propose a detailed analysis of the relaxation curves with a stretched exponential model allowing the description of the distribution of environments inside the surface layer. Our work indicates that MAS NMR can provide useful information for fine surface characterization of materials.