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

Abstract Five different electrolyte salts, namely NaBF 4 , NaClO 4 , NaDFOB, NaFSI and NaPF 6 , 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 NaBF 4 , NaDFOB or NaFSI, but not with NaClO 4 or NaPF 6 . 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.