Importance of Fluorine in High Voltage Electrolytes for LNMO||SiGr Cell Chemistry

Lithium nickel manganese oxide (LNMO) and silicon/graphite (SiGr) are promising active materials for high voltage lithium ion batteries attributed to the high operating potential versus Li|Li⁺ of LNMO and the high specific discharge capacity of silicon. However, this cell chemistry exhibits rapid capacity fading, primarily attributed to electrolyte decomposition at the high operating voltage of 4.9 V. Here, a fluorinated electrolyte containing lithium hexafluorophosphate as conducting salt, as well as fluoroethylene carbonate and methyl (2,2,2-trifluoroethyl) carbonate as electrolyte solvents is introduced. The influence of the selected solvents on the interphase formation and galvanostatic cycling performance is analyzed using complementary electrochemical, spectroscopic, and safety-related techniques. The presence of fluorinated solvents enables a high oxidative stability of an electrolyte up to 5.0 V versus Li|Li⁺ and effective interphase formation. In comparison to cells with non-fluorinated electrolytes, the galvanostatic cycling performance demonstrates a considerable improvement, leading to a doubling of the achievable cycle life. Roll-over failure observed in the electrolyte with non-fluorinated solvents could be effectively suppressed for over 300 cycles and the resulting electrolyte formulation with fluorinated solvents is non-flammable. Additionally, by fine-tuning the electrolyte formulation, the extent of acetylcholinesterase inhibition, an indication of substance toxicity of the aged electrolyte could be reduced.