On the Na+ transport and electrochemical stability window in NaTFSI:NMA deep eutectic solvent

Abstract

Deep eutectic solvents (DES), such as the ▪:▪ (sodium bistrifluoromethane sulfonyl imide - N-methyl acetamide) system, have been considered as promising electrolytes for sodium-ion (▪) batteries. However, our current atomistic understanding of those systems is still in a state of evolution and remains incomplete. In this context, we combined classical force field molecular dynamics (MD) simulations and periodic density functional theory (DFT) calculations to enhance our atomistic understanding on the ▪ transport mechanisms, DES structure organization, hydrogen-bond network formation, and electrochemical stability windows (ESWs). Firstly, we obtained a substantial improvement in the description of the DES transport and structural properties using self-consistent MD/DFT calculations to assign effective charges on all atoms, while the remaining OPLS parameters were kept constant. From our analyses, an increasing in the ▪ mole fraction leads to different chemical environments, reflecting an electrolyte with larger aggregates, low conductivity, and high viscosity. The liquid structure seems to remain unchanged as a function of temperature. However, higher temperatures reduce the lifetime of ionic pairs and increase the mobility of ▪ ions. The disruption of the hydrogen-bonds formed between the ▪ species increases with the ▪ concentration. In addition, our periodic DFT calculations reveals the substitution of atomic sites from ▪ to ▪ during the reduction process. Furthermore, the ESW results provided evidence supporting the safeguarding of the solvent from degradation at higher salt concentrations.