NaPF6-Based Battery Electrolytes near a Sodium Metal Electrode: Role of Organic Solvents on Solid Electrolyte Interphase Formation

Abstract

The choice of electrolytes for improved sodium-metal batteries (SMBs) presents a major challenge owing to the complexity of the solid electrolyte interphase (SEI) between the electrode and electrolyte. Herein, we systematically examine and compare the dissociation mechanism of electrolytes consisting of sodium hexafluorophosphate (NaPF6) in three different organic solvents with varying polarity (tetrahydrofuran (THF), propylene carbonate (PC), and a mixture of ethylene carbonate and diethyl carbonate (EC:DEC)) on the sodium metal surface using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. For the NaPF6-THF and NaPF6-PC electrolyte systems, the study reveals the strong reducing nature of the PF6– anion, which readily dissociates and dominates over the solvent molecules present in the system. Our analysis reveals that the solvent molecules, THF and PC, are relatively stable against reductive dissociation over sodium metal in their respective electrolyte solutions. Conversely, in the EC:DEC-based electrolyte, SEI formation is derived from both the anion and EC, and DEC molecules remain undissociated within the simulation timescale. Further, the induced charge (electron) density difference approach captures the charge depletion and accumulation on the sodium electrode and electrolyte species, respectively. The analysis of atomic charges and bond order over time reveals the irreversible dissociation of electrolyte species and provides insights into the nature of chemical bonding in Na–F and Na–P species formed in the SEI layer. Furthermore, the results show that THF and EC:DEC-based electrolytes yield SEIs with well-distributed, inorganic-rich species primarily composed of noncovalently bonded Na–F and Na–P species.