Molecular Simulations of Electric Double Layers and Solid Electrolyte Interphase Formation in High Concentration Sodium-Ion Battery Electrolytes

Sodium-ion batteries (SIBs) represent a promising avenue for next-generation energy storage solutions leveraging abundant sodium resources to address challenges in large-scale renewable energy storage. This study investigates the interfacial physicochemical properties of SIB electrolytes through molecular dynamics (MD) simulations. Our model comprises a prototypical electrolyte, sodium bis(fluorosulfonyl)imide (NaFSI) in dimethyl carbonate (DME), sandwiched between two electrodes modeled as sodium metal lattices. We examine the solvation structure surrounding the sodium ion at the electric double layer (EDL) adjacent to the negatively charged electrode, focusing on variations in salt concentration (from 1 M to 5 M) and electrode potential difference (from 0 V to 4 V). Our findings reveal that at 1 M concentration, increasing the potential difference leads to a decrease in the coordination number of FSI ions around sodium ions in the first peak at ~0.6 nm from the negatively electrified electrode, with the majority of sodium ions at the interface coordinated by six oxygen atoms from three DME molecules under a 4 V potential. At higher concentrations (3 M), a distinct peak of sodium ions at ~0.3 nm from the electrode surface emerges, intensifying with increased potential. Here, sodium ions partially shed their solvation shell, transitioning from coordination with three oxygen atoms from FSI and two from DME, to two oxygen atoms from each. This adjustment is more pronounced at 5 M, illustrating that sodium ions are more likely to detach from their solvation shell and migrate towards the electrode at higher salt concentrations and potential. This observation highlights the challenge of predicting redox potentials and electrochemical reactions based on bulk solvation structures. Additionally, using ReaxFF MD simulations and the Electrochemical Dynamics with Implicit Degrees of Freedom (EChemDID) method, we explore the solid-electrolyte interphase (SEI) formation mechanism. We observe that the reduction of FSI ions and DME molecules is sensitive to the solvation structures and their distribution. The decomposition of DME molecules is mitigated by the concurrent decomposition of FSI ions in high-concentration electrolytes. This study elucidates the interplay between the physical nanostructure of EDLs and the SEI formation mechanism in SIBs, providing molecular insights of interfacial physicochemical properties of SIB electrolytes.