Organic Electrolyte Composed of Strongly and Weakly Coordinating Molecules for Sodium-Ion Battery. A Molecular Dynamics Simulation Study

Abstract

Molecular dynamics simulations were conducted at temperatures of 298.15, 273.15, 253.15, and 233.15 K on three organic electrolytes, composed of 1 m NaPF₆ dissolved in strongly coordinating diglyme (DG), a mixture of DG and weakly coordinating Tetrahydrofuran (THF) with a 2:8 volume ratio, and a mixture of DG, THF, and weakly coordinating 1,3-dioxolane (DOL) with a 2:4:4 volume ratio, respectively, hereafter denoted as ND, NDT, and NDTD electrolytes for sodium-ion batteries. The studies indicate strong Na⁺–DG coordination that leads to a vehicular mechanism, in the sense that Na⁺ persists in migrating together with strongly coordinating DG in the first coordination shell at all the temperature ranges. Such a vehicular mechanism hinders Na⁺ migration in the ND electrolyte. In contrast, the introduction of weakly coordinating molecules, such as THF in the NDT electrolyte and THF/DOL in the NDTD electrolyte, considerably perturbs Na⁺ solvation with various coordinating configurations that include Na⁺–THF and/or Na⁺–DOL as well as Na⁺–${\mathrm{PF}}_6^{-}$ contact-ion pairs. Such diversity of the coordinating configurations significantly improves Na⁺ migration, especially in the NDTD electrolyte, which has the highest ionic conductivity as well as the fractional ionic conductivity of Na⁺ of 3.68 ± 0.36 and 1.32 ± 0.11 mS·cm⁻¹, respectively, even at a low temperature of 233.15 K.