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

To understand how Li+ interacts with individual carbonate molecules in nonaqueous electrolytes, we conducted natural abundance O-17 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 O-17 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 O-17 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.