Identity of passive film formed on aluminum in Li-ion battery electrolytes with LiPF6

The passive film that forms on aluminum in 1:1 ethylene carbonate+ethylmethyl carbonate with 1.2 M LiPF6 and 1:1 ethylene carbonate+dimethyl carbonate with 1.0 M LiPF6 was investigated by a combination of electrochemical quartz crystal microbalance measurements (EQCM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy. During anodic polarization of aluminum a film of AlF3 forms on top of the air-formed oxide, creating a duplex, or two-layered film. The thickness of the AlF3 increases with the applied potential. Independent measurements of film thickness by EQCM and EIS indicate that at a potential of 5.5 V vs Li/Li+, the thickness of the AlF3 is approximately 1 nm.