13C NMR estimation of preferential solvation of lithium ions in non-aqueous mixed solvents

The C-13 NMR spectra of organic electrolyte solutions containing 1 mol dm(-3) LiPF6 in the three binary mixed solvent systems, methyl propionate (MP)-ethyl methyl carbonate (EMC), N,N-dimethylacetamide (DMA)-propylene carbonate (PC) and MP-ethyl propionate (EP) were measured at various solvent compositions. The difference in the chemical shifts of a solvent with and without the electrolyte was attributed to the coordination of the solvent to the lithium ion. The order of the normalized coordination shift which represents the coordinating ability for each mixed solvent system was MP > EMC, DMA > PC and MP > EP. An equation was derived which estimates the solvent ratio in the first coordination shell of the lithium ion in the binary mixed solvents for any solvent composition. The estimated ratio at a 1:1 volume composition was 1.6 for MP-EMC, 15 for DMA-PC and 1.3 for MP-EP.