Effects of Cosolvent and Nonsolvating Solvent on the Structural Dynamics of Organic Electrolytes in Sodium-Ion Batteries

In sodium-ion batteries (SIBs), the performance of a single solvent often does not meet actual requirements and a cosolvent or nonsolvating solvent is needed. However, the effect of these electrolyte additives on the solvation structure and dynamics of Na⁺ in SIBs is yet to be fully understood. Herein, electrolyte structural dynamics are examined for NaPF₆ in dimethyl carbonate (DMC) with 1,1,2,2-tetrafluoro-2,2,2-trifluoroethoxy ethane (HFE) as the nonsolvating solvent or propylene carbonate (PC) as the cosolvent using steady-state and time-resolved infrared (IR) spectroscopies. Molecular dynamics simulations show that the solvation size of Na⁺ decreases with a loosened structure upon adding the nonsolvating solvent, whereas its first solvation shell becomes denser and the second one becomes softened with decreased participation of the PF₆^- anion upon adding the cosolvent. While a decreased participation of DMC in the solvation layer of Na⁺ is suggested by linear IR results, an increased structural inhomogeneity (and hence overall more dynamical fluctuations) is found for Na⁺-coordinated DMC upon adding both additives by two-dimensional (2D) IR spectroscopy where the carbonyl stretch is used as a probe. The Na⁺/DMC/PC complex is found to structurally evolve slower than both Na⁺/DMC and Na⁺/DMC/HFE complexes on the picosecond time scales by spectral diffusion dynamics extracted from 2D IR diagonal signals. Frontier orbital theory calculations also indicate that both solvent additives are beneficial to increasing the stability of PF₆^-. The results obtained in this work provide important insights into the roles played by the solvent additives in influencing the solvation structures and dynamics of Na⁺, which are critical for understanding the Na⁺ transfer mechanism in SIBs.