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

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 LiPF6, and lithium tetrafluoroborate, or LiBF4) dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) mixtures. With increasing salt concentration and DMC percentage in both systems, F-19 NMR experiences an upfield shift along with decreasing J(31p-19F) and J(11B-19F), which evidence the stronger interaction between the Li+ and F- in an environment with diminishing presence of high dielectric medium. The quadrupolar relaxation of B-11 dominates the F-19 relaxation mechanism and demonstrates that LiBF4 mainly exists as ion pairs in solution either at high salt concentration or in a medium of low polarity. Li-7, F-19, H-1 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 LiBF4 system than in the LiPF6 system.