Understanding Li(+)-Solvent Interaction in Nonaqueous Carbonate Electrolytes with (17)O NMR.

To understand how Li(+) interacts with individual carbonate molecules in nonaqueous electrolytes, we conducted natural abundance (17)O NMR measurements on electrolyte solutions of 1 M LiPF6 in a series of binary solvent mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC). It was observed that the largest changes in (17)O chemical shift occurred at the carbonyl oxygens of EC, firmly establishing that Li(+) strongly prefers EC over DMC in typical nonaqueous electrolytes, while mainly coordinating with carbonyl rather than ethereal oxygens. Further quantitative analysis of the displacements in (17)O chemical shifts renders a detailed Li(+)-solvation structure in these electrolyte solutions, revealing that maximum six EC molecules can coexist in the Li(+)-solvation sheath, while DMC association with Li(+) is more "noncommittal" but simultaneously prevalent. This discovery, while aligning well with previous fragmental knowledge about Li(+)-solvation, reveals for the first time a complete picture of Li(+) solvation structure in nonaqueous electrolytes.