Influence of the Fluorination Degree of Organophosphates on Flammability and Electrochemical Performance in Lithium Ion Batteries: Studies on Fluorinated Compounds Deriving from Triethyl Phosphate

Murmann P, Moennighoff X, von Aspern N, Janssen P, Kalinovich N, Shevchuk M, Kazakova O, Roeschenthaler G-V, Cekic-Laskovic I, Winter M

Abstract

Three symmetric substances originating from triethyl phosphate were specifically synthesized with varying degree of fluorinationat the side chain. Different concentrations of each phosphate were evaluated as co-solvent with regard to their flammability and theelectrochemical cycling performance.With higher degree of fluorination and a higher amount of the phosphate in the electrolyte, theself-extinguishing time (SET), a value to determine and compare the flammability of electrolytes, could be significantly lowered toyield a non-flammable electrolyte mixture. A specifically designed SET device is introduced, which offers more accurate results dueto lowered standard deviations by minimizing random and systematic errors. As the application of phosphates as co-solvents resultsin a trade-off in cycling performance, a thorough determination in regard to the ionic conductivity, the anodic oxidation stabilityand the compatibility with anode and cathode material was carried out in half- and full-cells. The manuscript strives to establish adeeper understanding of the influence that the utilization of phosphates as co-solvents entail with special focus on the fluorinationdegree. It could be shown that the partially fluorinated phosphate offers the best cycling results and therefore the lowest trade-off inperformance, while a severe improvement in SET could be achieved compared to the reference electrolyte.