Effects of Cosolvent\nand Nonsolvating Solvent on the\nStructural Dynamics of Organic Electrolytes in Sodium-Ion Batteries

In sodium-ion batteries (SIBs), the performance of a\nsingle solvent\noften does not meet actual requirements and a cosolvent or nonsolvating\nsolvent is needed. However, the effect of these electrolyte additives\non the solvation structure and dynamics of Na<sup>+</sup> in SIBs\nis yet to be fully understood. Herein, electrolyte structural dynamics\nare examined for NaPF<sub>6</sub> in dimethyl carbonate (DMC) with\n1,1,2,2-tetrafluoro-2,2,2-trifluoroethoxy ethane (HFE) as the nonsolvating\nsolvent or propylene carbonate (PC) as the cosolvent using steady-state\nand time-resolved infrared (IR) spectroscopies. Molecular dynamics\nsimulations show that the solvation size of Na<sup>+</sup> decreases\nwith a loosened structure upon adding the nonsolvating solvent, whereas\nits first solvation shell becomes denser and the second one becomes\nsoftened with decreased participation of the PF<sub>6</sub><sup>–</sup> anion upon adding the cosolvent. While a decreased participation\nof DMC in the solvation layer of Na<sup>+</sup> is suggested by linear\nIR results, an increased structural inhomogeneity (and hence overall\nmore dynamical fluctuations) is found for Na<sup>+</sup>-coordinated\nDMC upon adding both additives by two-dimensional (2D) IR spectroscopy\nwhere the carbonyl stretch is used as a probe. The Na<sup>+</sup>/DMC/PC\ncomplex is found to structurally evolve slower than both Na<sup>+</sup>/DMC and Na<sup>+</sup>/DMC/HFE complexes on the picosecond time\nscales by spectral diffusion dynamics extracted from 2D IR diagonal\nsignals. Frontier orbital theory calculations also indicate that both\nsolvent additives are beneficial to increasing the stability of PF<sub>6</sub><sup>–</sup>. The results obtained in this work provide\nimportant insights into the roles played by the solvent additives\nin influencing the solvation structures and dynamics of Na<sup>+</sup>, which are critical for understanding the Na<sup>+</sup> transfer\nmechanism in SIBs.