1,1-Difluoro-1-alkenes as new electrolyte additives for lithium ion batteries

► 1,1-Difluoro-1-alkenes bearing an aryl group are investigated as additives for high voltage lithium ion battery electrolytes. ► 1,1-Difluoro-4-phenylbut-1-ene (DF-1) shows good cycle performance at 4.45V charge. ► DF-1 forms an SEI on the cathode and anode. ► The cathode surface has a LiF-rich layer with unsaturated hydrocarbons. ► The anode surface is determined to include a LiF-rich layer. Five 1,1-difluoro-1-alkenes bearing aryl groups are investigated as electrolyte additives for high-charge-voltage lithium ion batteries with an aim to induce favorable solid electrolyte interphase (SEI) formation. Among them, 1,1-difluoro-4-phenylbut-1-ene (DF-1) forms appropriate SEIs on the cathode and anode by its reactions under oxidative and reductive conditions, respectively, which was suggested by linear sweep and cyclic voltammetry. The SEIs formed with the DF-1 containing electrolyte on the cathode and anode were analyzed by X-ray photoelectron spectroscopy and time of flight-secondary ion mass spectrometry, indicating the following: (i) the cathode surface has a LiF-rich layer with unsaturated hydrocarbons, and (ii) the anode surface also has a LiF-rich layer, which probably includes fluoromethylene (CHF) compounds. The formation of the SEIs on both electrodes with DF-1 results in low impedance being maintained, and thus, particularly good cycle performance even at high-charge-voltage.