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

Abstract

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