Solvation Structure of Sodium Bis(fluorosulfonyl)imide-Glyme Solvate Ionic Liquids and Its Influence on Cycling of Na-MNC Cathodes

Abstract

Electrolytes consisting of sodium bis(fluorosulfonyl)imide (NaFSI) dissolved in glymes (monoglyme, diglyme, and triglyme) were characterized by FT-Raman spectroscopy and ¹³C, ¹⁷O, and ²³Na NMR spectroscopy. The glyme:NaFSI molar ratio was varied from 50:1 to 1:1, and it was observed that, in the dilute electrolytes, the sodium salt is completely dissociated into solvent separated ion pairs (SSIPs). However, contact ion pairs (CIPs) and aggregates (AGGs) become the predominant species in more concentrated solutions. Some of the electrolytes with the highest concentrations can be classified as solvate ionic liquids (SILs), where all of the solvent molecules are coordinated to sodium cations. Therefore, these electrolytes are fundamentally different from more dilute electrolytes which are typically used in commercially available secondary batteries. The melting point or glass transition temperature, dynamic viscosity, density, sodium concentration, and ionic conductivity of these solvate ionic liquids are reported as well as the crystal structures of [Na(G3)][FSI] and [Na(G3)₂][FSI]. Galvanostatic cycling experiments were performed in coin-type cells with a Na_2/3[Mn_0.55Ni_0.30Co_0.15]O₂ cathode to study the influence of these electrolytes on the electrochemical stability and charge/discharge behavior.