Probing the Electrode–Electrolyte Interface of Sodium/Glyme-Based Battery Electrolytes

Sodium-ion batteries (NIBs) are promising systems for large-scale energy storage solutions; yet, further enhancements are required for their commercial viability. Improving the electrochemical performance of NIBs goes beyond the chemical description of the electrolyte and electrode materials as it requires a comprehensive understanding of the underlying mechanisms that govern the interface between electrodes and electrolytes. In particular, the decomposition reactions occurring at these interfaces lead to the formation of surface films. Previous work has revealed that the solvation structure of cations in the electrolyte has a significant influence on the formation and properties of these surface films. Here, an experimentally validated molecular dynamics study is performed on a 1 M NaTFSI salt in glymes of different lengths placed between two graphite electrodes having a constant bias potential. The focus of this study is on describing the solvation environment around the sodium ions at the electrode–electrolyte interface as a function of glyme chain length and applied potential. The results of the study show that the diglyme/TFSI system presents features at the interface that significantly differ from those of the triglyme/TFSI and tetraglyme/TFSI systems. These computational predictions are successfully corroborated by the experimentally measured capacitance of these systems. In addition, the dominant solvation structures at the interface explain the electrochemical stability of the system as they are consistent with cyclic voltammetry characterization.

triglyme/NaTFSI, (c) tetraglyme/NaTFSI systems and compared at three applied potential differences, 0V, 1V, 3V (shown top to down in panels in each window, respectively).Comparison of the distributions near the electrode surfaces for (d) diglyme/NaTFSI, (e) triglyme/NaTFSI, and (f) tetraglyme/NaTFSI (solid lines represent the distribution from the negative electrode and dashed lines represent the distribution from the positive electrode and TFSI -, Na + , diglyme, triglyme, and tetraglyme are represented by orange, blue, green, purple, and cyan lines, respectively).The positions of the electrode surfaces in figures a, b and c are depicted by vertical lines.The first interfacial layers at 0V (top panels in Figures S1d-f) span between 1.6 and 5.6 Å from the electrode surfaces, with a first broad peak of TFSI oxygen atoms appearing around 2.8 Å and a clear first peak of glyme oxygen atoms around 4.4 Å.A second interfacial layer is between 4 and 7.2 Å from the electrode surfaces, with the first peak of Na + showing up at 6 Å.When examining from left to right, the peak heights of TFSI anions and Na + ions at 0V show an increase from diglyme to tetraglyme.At 1V (middle panels in Figures S1d-f), the first peak positions remain the same for Na + ions at both positive and negative electrodes, with a slight increase in peak height for the diglyme/TFSI system and considerable changes in the Na + peak heights for the triglyme/TFSI and tetraglyme/NaTFSI systems at both interfaces.The glyme distributions at 1V

Comparison of the normalized density profiles along the z-axis at the interfacial regions
indicate that even though the peak positions representing glymes remain the same at the negative electrode (solid lines), a shoulder appears around 2.8 Å from the positive electrode (dashed lines), which is an indication of glyme oxygens tendency to compete with TFSI oxygen atoms to get closer to the positively charged surface.The distribution for TFSI oxygen atoms at 1V shows clear differences between the two interfaces.While the peak height for TFSI appearing at 2.8 Å away from the positive electrode increases (orange dashed lines), the peak at the negative electrode depletes significantly (orange solid lines).

Figure S1 .
Figure S1.Normalized probability density distributions of each species (considering the oxygen atoms of glymes and TFSI) across the cell as a function of z in (a) diglyme/NaTFSI , (b)

Figure
Figure S1d-f provides a complete comparison of the salient features of the interfacial layers

Figure
Figure S4.(a) Distribution of the fractions of TFSI anions (left panels) and triglyme molecules (right panels) in the first solvation shell of sodium as a function of applied voltages (panels showing top to bottom) for the triglyme/NaTFSI system.(b) A snapshot of the minor sodium solvation structure presents in the triglyme/NaTFSI system, 2 TFSI/ 1 triglyme.

Figure
Figure S5.(a) Distribution of the fractions of TFSI anions (left panels) and tetraglyme molecules (right panels) in the first solvation shell of sodium as a function of applied voltages (panels showing top to bottom) for the tetraglyme/NaTFSI system.(b) A snapshot of the major sodium solvation structure present in the tetraglyme/NaTFSI system, 0 TFSI/ 2 tetraglyme.