An Attempt to Formulate Non-Carbonate Electrolytes for Sodium-Ion Batteries

Non-aqueous carbonate solvents have been the main choice for the development of lithium-ion batteries, and similarly most research on sodium-ion batteries have been performed using carbonate-based solvents. However, the differences between sodium and lithium batteries - in term chemistry/electrochemistry properties as well as electrode materials used - open up opportunities to have a new look at solvents that have attracted little attention as electrolyte solvent. This work investigates properties of a wide range of different solvent classes in the context of sodium-ion battery electrolytes and compares them to the performance of propylene carbonate. The thirteen solvents studied here include one or several members of glymes, carbonates, lactones, esters, pyrrolidones, sulfones, and alkyl phosphates. Out of those, five outperforming solvents of gamma-butyrolactone (GBL), gamma-valerolactone (GVL), N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), and trimethyl phosphate (TMP) were further investigated using additives of ethylene sulfite (ES), vinylene carbonate (VC), fluoroethylene carbonate (FEC), prop-1-ene-1,3-sultone (PES), sulfolane (TMS), tris(trimethylsilyl) phosphite (TTSPI), and sodium bis(oxalato)borate (NaBOB). The solvents TMS and tetraethylene glycol dimethyl ether (TEGDME) were tested in 1 : 1 mixtures by volume with the co-solvents; NMP, dimethoxyethane (DME), and TMP. All electrolytes used NaPF6 as the salt. Primary evaluation relied on electrochemical cycling of full-cell sodium-ion batteries consisting of Prussian white cathodes and hard-carbon anodes. Galvanostatic cycling was performed using both two- and three-electrode cells, in addition, cyclic and linear sweep voltammetry was used to further evaluate the electrolyte formulations. Moreover, the resistance was measured on the anode and cathode, using Intermittent current interruption (ICI) technique.