Lithium Tetrafluoroborate-Based Ester Electrolyte Formulations to Improve the Operating Temperature Range in NCM 622 || Graphite Li-Ion Batteries

The use of alternative lithium salts, lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate, and lithium hexafluorophosphate (LiPF6), in the ester/fluoroethylene carbonate electrolyte results in improved low-temperature performance at −20 °C compared to LiPF6 in a carbonate-based electrolyte, but the room-temperature performance of the ester-based formulation has significant capacity fade due to the instability of the solid electrolyte interphase (SEI). Incorporation of electrolyte additives, lithium bis(oxalato)borate, lithium difluorophosphate, and lithium bis(trimethylsilyl) phosphate (LiTMSP), into lithium tetrafluoroborate (LiBF4) ester-based electrolytes (methyl acetate/fluoroethylene carbonate-MA/FEC) has been investigated in graphite//LiNi0.6Co0.2Mn0.2O2 cells to improve room-temperature cycling stability without diminishing low-temperature rate performance. The LiBF4-based ester electrolyte containing 1% LiTMSP delivers the highest reversible capacity at low temperature (−20 °C), while the room-temperature performance is significantly improved compared to that of the base ester-based electrolyte (LiBF4-MA) and almost comparable to that of the standard carbonate-based electrolyte. The improved electrochemical performance for 1% LiTMSP at −20 °C is due to the lower charge transfer resistance as supported by electrochemical impedance spectroscopy, and improved room-temperature performance can be ascribed to the generation of a stable phosphate-rich SEI on the graphite surface as revealed by X-ray photoelectron spectroscopy.