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.

Diffusion and structure of propylene carbonate-metal salt electrolyte solutions for post-lithium-ion batteries: From experiment to simulation

The diffusion of cations in organic solvent solutions is important for the performance of metal-ion batteries. In this article, pulsed field gradient nuclear magnetic resonance experiments and fully atomistic molecular dynamic simulations were employed to study the temperature-dependent diffusive behavior of various liquid electrolytes representing 1M propylene carbonate solutions of metal salts with bis(trifluoromethylsulfonyl)imide (TFSI-) or hexafluorophosphate (PF6-) anions commonly used in lithium-ion batteries and beyond. The experimental studies revealed the temperature dependence of the diffusion coefficients for the propylene carbonate (PC) solvent and for the anions following an Arrhenius type of behavior. It was observed that the PC molecules are the faster species. For the monovalent cations (Li+, Na+, K+), the PC solvent diffusion was enhanced as the cation size increased, while for the divalent cations (Mg2+, Ca2+, Sr2+, Ba2+), the opposite trend was observed, i.e., the diffusion coefficients decreased as the cation size increased. The anion diffusion in LiTFSI and NaTFSI solutions was found to be similar, while in electrolytes with divalent cations, a decrease in anion diffusion with increasing cation size was observed. It was shown that non-polarizable charge-scaled force fields could correspond perfectly to the experimental values of the anion and PC solvent diffusion coefficients in salt solutions of both monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations at a range of operational temperatures. Finally, after calculating the radial distribution functions between cations, anions, and solvent molecules, the increase in the PC diffusion coefficient established with the increase in cation size for monovalent cations was clearly explained by the large hydration shell of small Li+ cations, due to their strong interaction with the PC solvent. In solutions with larger monovalent cations, such as Na+, and with a smaller solvation shell of PC, the PC diffusion is faster due to more liberated solvent molecules. In the salt solutions with divalent cations, both the anion and the PC diffusion coefficients decreased as the cation size increased due to an enhanced cation-anion coordination, which was accompanied by an increase in the amount of PC in the cation solvation shell due to the presence of anions.

Investigating the Role of Surface Roughness and Defects on EC Breakdown, as a Precursor to SEI Formation in Hard Carbon Sodium-Ion Battery Anodes

Hard carbon (HC) anodes together with ethylene carbonate (EC)-based electrolytes have shown significant promise for high-performing sodium-ion batteries. However, questions remain in relation to the initial contact between the carbon surface and the EC molecules. The surface of the HC anode is complex and can contain both flat pristine carbon surfaces, curvature, nanoscale roughness, and heteroatom defects. Combining density functional theory and experiments, the effect of different carbon surface motifs and defects on EC adsorption are probed, concluding that EC itself does not block any sodium storage sites. Nevertheless, the EC breakdown products do show strong adsorption on the same carbon surface motifs, indicating that the carbon surface defect sites can become occupied by the EC breakdown products, leading to competition between the sodium and EC fragments. Furthermore, it is shown that the EC fragments can react with a carbon vacancy or oxygen defect to give rise to CO₂ formation and further oxygen functionalization of the carbon surface. Experimental characterization of two HC materials with different microstructure and defect concentrations further confirms that a significant concentration of oxygen-containing defects and disorder leads to a thicker solid electrolyte interphase, highlighting the significant effect of atomic-scale carbon structure on EC interaction.