Ryan Jorn

Glyme-based electrolytes have emerged as promising candidates for sodium-ion batteries owing to their ability to form stable solvation structures around Na+ via multidentate coordination with ether oxygen atoms. While the solvation and transport properties of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in short-chain glymes have been systematically investigated, solutions of pentaglyme (G5) remain poorly understood and, to the best of our knowledge, are unexplored in binary glyme mixtures. Herein, molecular dynamics (MD) simulations are used to investigate the solvation structure, ion association, and transport properties of NaTFSI across six systems: monoglyme (G1), triglyme (G3), and pentaglyme (G5), as well as three binary G1:G3 mixtures of varying composition. A combination of experiments and simulations is employed to provide a comprehensive structural and dynamic characterization, including ionic conductivity, viscosity, van Hove correlation functions, and Walden analysis. The results reveal that G3 preferentially displaces G1 from the Na+ coordination shell, even when present in small amounts, thereby generating free G1 molecules that reduce viscosity in G1-containing systems. Analysis of the Na+–Na+ spatial correlations reveals a glyme-bridged Na+–G3–Na+ structural motif unique to G3 and persisting across all G3-containing mixtures. In contrast, G5 exhibits fundamentally different dynamical behavior from G1(0.5):G3(0.5), despite providing the same oxygen-to-Na+ ratio, displaying the highest viscosity and lowest conductivity among all systems studied while maintaining long-lived Na+-Na+ structural correlations. Within the binary mixtures, the G1(0.25):G3(0.75) electrolyte achieves the optimal balance, exhibiting the smallest deviation from the ideal Walden line. These findings demonstrate that molecular architecture, as well as chain length, governs electrolyte behavior, providing fundamental insights for the rational design of glyme-based sodium-ion battery electrolytes.

Recent experiments demonstrating the concentration-dependent stability of aqueous and nonaqueous electrolytes have renewed interest in previously discounted solvents for viable energy storage technologies. Propylene carbonate provides a clear example of a solvent whose electrochemical stability is radically altered by increasing salt concentration beyond the standard 1.0 M recipe; however, the molecular origins of this effect remain unresolved. Prior studies have focused on changes in lithium-ion bulk solvation structure to capture the onset of electrolyte stability, yet the connection between bulk solvation and the environment of the electrode surface has not been considered. In this work we reexamine the liquid structure of a prototypical LiPF₆+PC electrolyte over a range of concentrations (0.7–3.2 M) using nonreactive classical molecular dynamics to probe changes in ion solvation in transitioning from the bulk environment to the electrode interface. By application of a constant potential method to represent idealized electrodes, we show that the impact of the interface is to reduce ion pairing and that sufficient concentration is necessary to restore the strong ion association found in the bulk environment. Hence, we hypothesize that the switch in stability for PC electrolytes is dictated more so by the electrostatic interactions at the electrode interface, which can be overcome by sufficient salt content and is a direct result of the strong coordination of lithium ions by the PC solvent.

Ether-based electrolytes have gained increasing attention for energy storage based on their utility as solvate ionic liquids at high concentration and their role in forming effective cointercalation complexes at graphite electrodes. While transferrable atomistic models have been proposed to describe glyme ether solutions at varying concentration and backbone chain lengths, coarse-grained models have not been extensively explored. The need for coarse-grained descriptions is emphasized by the formation of potentially mesoscale aggregate structures which enable efficient ion transport. Herein we describe a simple approach to developing such models using a combination of a charge smearing for long-ranged electrostatics and Boltzmann Inversion to develop short-ranged tabulated potentials. The impact of long-ranged interactions on electrolyte structure and the ingredients to the coarse-grained models are discussed along with the importance of system selection for training the short-ranged portion. Overall, the final model shows good transferability for diglyme and monoglyme, which share an emphasis on ion association, but fails to capture the solvent separated ionic structure of triglyme electrolytes.

A generalized quantum master equation approach is introduced to describe electron transfer in molecular junctions that spans both the off-resonant (tunneling) and resonant (hopping) transport regimes. The model builds on prior insights from scattering theory but is not limited to a certain parameter range with regard to the strength of the molecule–electrode coupling. The framework is used to study the simplest case of energy and charge transfer between the molecule and the electrodes for a single site noninteracting Anderson model in the limit of symmetric and asymmetric coupling between the molecule and the electrodes. In the limit of elastic transport, the Landauer result is recovered for the current by invoking a single active electron Ansatz and a binary collision approximation for the memory kernel. Inelastic transport is considered by allowing the excitation of electron–hole pairs in the electrodes in tandem with charge transport. In the case of low bias voltages where the Fermi levels of the electrodes remain below the molecular state, it is shown that the current arises from tunneling and the molecule remains neutral. However, once the threshold is reached for aligning the fermi level of one electrode with the molecular orbital, a small amount of charge transfer occurs with a negligible amount of hopping current. While inelasticity in the current has a minimal impact on the shape of the current–voltage curve in the case of symmetric electrode coupling, the results for a slight asymmetry in coupling demonstrate complete charge transfer and a significant drop in current. These results provide encouraging confirmation that the framework can describe charge transport across a wide range of electrode–molecule coupling and provide a unique perspective for developing new master equation treatments for energy and charge transport in molecular junctions. An extension of this work to account for inelastic scattering from electron–vibrational coupling at the molecule is straightforward and will be the subject of subsequent work.

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.