Tris(2,2,2-trifluoroethyl) phosphite as a co-solvent for nonflammable electrolytes in Li-ion batteries

In this work, we used tris(2,2,2-trifluoroethyl) phosphite (TTFP), in which the oxidization number of phosphorus was three (III), to formulate nonflammable electrolytes of the Li-ion batteries. Using 1 m (mole solute per kilogram solvent) LiPF6 3:3:4 (w) propylene carbonate (PC)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte as a baseline, the effect of TTFP on the flammability and conductivity of the electrolytes, as well as the cell performance was evaluated. It is observed that the addition of TTFP can substantially reduce flammability of the electrolytes at a small expense in the ionic conductivity. When the TTFP content reaches 15 wt.% versus the solvent, the electrolyte becomes nonflammable. In Li/graphite half-cell, TTFP not only suppresses PC decomposition and graphite exfoliation but also increases Coulombic efficiency (CE) of the lithiation and delithiation cycle. In Li/cathode (a lithium nickel-based mixed oxide cathode) half-cell, TTFP has negligible adverse impact on the cycling performance when the cells are cycled between 2.7 and 4.2 V. In graphite/cathode Li-ion cell using PC-based electrolytes, TTFP can improve cycling performance, especially at high temperature (60 °C), since its presence favors the formation of solid electrolyte interface (SEI) film on the graphite electrode and increases thermal stability of LiPF6-based electrolytes.