Multinuclear magnetic resonance investigation of cation-anion and anion-solvent interactions in carbonate electrolytes

Abstract The investigation of ion-solvent interaction in electrolytes is crucial for basic understanding of ion transport through the bulk electrolyte as well as interphase formation processes on both electrodes, which dictate performances of lithium ion batteries (LIBs). In this report, nuclear magnetic resonance (NMR) was used to study the solvation behaviors of two typical lithium salts (lithium hexafluorophosphate, or LiPF 6 , and lithium tetrafluoroborate, or LiBF 4 ) dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) mixtures. With increasing salt concentration and DMC percentage in both systems, 19 F NMR experiences an upfield shift along with decreasing J 31 P − 19 F and J 11 B − 19 F , which evidence the stronger interaction between the Li + and F − in an environment with diminishing presence of high dielectric medium. The quadrupolar relaxation of 11 B dominates the 19 F relaxation mechanism and demonstrates that LiBF 4 mainly exists as ion pairs in solution either at high salt concentration or in a medium of low polarity. 7 Li, 19 F, 1 H NMR diffusion measurements were conducted to characterize the relative mobility of cation, anion and solvent molecules, the results of which support the conclusion above. Salt concentration and solution polarity have a much stronger effect on cation-anion aggregation and solvation in the LiBF 4 system than in the LiPF 6 system.