Investigation of Electrolyte Salts in Non-Flammable Triethyl Phosphate for Sodium-Ion Batteries

Five different electrolyte salts, namely NaBF₄, NaClO₄, NaDFOB, NaFSI and NaPF₆, were evaluated in non-flammable triethyl phosphate (TEP) based electrolyte solutions in sodium-ion full-cells using high-mass loading Prussian white and hard carbon electrodes. Their impact on the viscosity, ionic conductivity and solvation structure was analyzed, revealing that NaFSI-based electrolytes exhibited a stronger interaction with TEP and less ion-pairing than the other salts, resulting in the highest ionic conductivity at a concentration of 0.8 m (mol/kg). Galvanostatic cycling experiments showed that none of the electrolyte salts dissolved in TEP forms an efficient passivation layer. However, adding 1 wt.% vinylene carbonate (VC) significantly improved cycling performance for the cells with NaBF₄, NaDFOB or NaFSI, but not with NaClO₄ or NaPF₆. Additionally, NaFSI in TEP with 1 wt.% VC electrolyte solution showed minimal gas evolution during the formation cycling (<8 mbar). In a 1 Ah multilayer pouch cell, 0.8 m NaFSI in TEP with 1 wt.% VC showed promising results with 88 % capacity retention after 200 cycles. X-ray photoelectron spectroscopy analysis indicated that the addition of VC results in the formation of a thin SEI and minimized TEP decomposition, particularly for 0.8 m NaFSI TEP with 1 wt.% VC. This study lays the groundwork for safer liquid electrolytes and integrating them into near-to-commercial cell setups.