Trends in Na-Ion Solvation with Alkyl-Carbonate Electrolytes for Sodium-Ion Batteries: Insights from First-Principles Calculations

Classical molecular dynamics (MD) simulations and M06-2X hybrid density functional theory calculations have been performed to investigate the interaction of various nonaqueous organic electrolytes with Na+ ion in rechargeable Na-ion batteries. We evaluate trends in solvation behavior of seven common electrolytes namely pure carbonate solvents (ethylene carbonate (EC), vinylene carbonate (VC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC)) and four binary mixtures of carbonates (EC:PC, EC:DMC, EC:EMC, and EC:DEC). Thermochemistry calculations for the interaction of pure and binary mixtures of carbonate solvents with Na+ ion, Na+ ion coordinated with carbonate clusters obtained from molecular dynamics simulations, show that the formation of Na-carbonate complexes is exothermic and proceeds favorably. Based on the highest binding energy (ΔEb), enthalpy of solvation (ΔH(sol)), and Gibbs free energy of solvation (ΔG(sol)) values for the interaction of Na+ ion with carbonate solvents, our results conclusively show that pure EC and binary mixture of (EC:PC) are the best electrolytes for sodium-ion based batteries. Quantum chemical analyses are performed to understand the observed trends in ion solvation. Quantum theory of atoms in molecules (QTAIM) analysis shows that the interactions in Na-carbonate complexes are classified as a closed-shell (electrostatic) interaction. The localized molecular orbital energy decomposition analysis (LMO-EDA) also indicates that the electrostatic term (ΔEele) in the interaction energy between Na+ ion and carbonate solvents has the highest value and confirms the results of QTAIM about the electrostatic nature of Na+ ion interaction. The noncovalent interaction (NCI) plots indicate that the noncovalent interactions responsible for the formation of Na-carbonate complexes are strong to weak attractive interactions. Density of state (DOS) calculations show that the HOMO–LUMO energy gap in the EC, VC, PC, BC, DMC, EMC, and DEC increases as they interact with Na+ ion, although the HOMO–LUMO energy gap decreases with the addition of EC as an electrolyte additive to PC, DMC, and EMC. Calculated trends based on these quantum chemical calculations suggest that EC and binary mixture of EC:PC emerge as the best electrolytes in sodium-ion batteries, which is in excellent agreement with previously reported in silico experimental results.