The surface film formed on a lithium metal electrode in a new imide electrolyte, lithium bis(perfluoroethylsulfonylimide) [LiN(C2F5SO2)2]

A newly developed imide electrolyte salt, LiN(C{sub 2}F{sub 5}SO{sub 2}){sub 2} (LiBETI) was found to give very uniform, thin, and stable surface films on a lithium metal electrode in the propylene carbonate (PC) solution. LiBETI/PC was studied and compared to determine its ability to form such a stable surface film, with conventional electrolyte systems such as LiCF{sub 3}SO{sub 3}/PC, LiPF{sub 6}/PC, and LiN(CF{sub 3}So{sub 2}){sub 2}/PC (LiTFSI/PC). The surface film formed in LiBETI/PC system was a hemispherical, and the composition of the film consisted mainly of LiF, which is similar to that in a LiPF{sub 6}/PC system. Quartz crystal microbalance (QCM) and cyclic voltammetry (after the tenth cycle) indicated that the surface film formed in LiBETI/PC (ca. 50 nm) was thinner than those in LiPF{sub 6}/PC (ca. 90 nm), LiTFSI/PC (ca. 140 nm), or LiCF{sub 3}SO{sub 3}/PC (ca. 255 nm). The variation of the resonance resistance ({Delta}R) obtained from in situ CV/QCM measurement, which has been demonstrated to be a good measure of the surface roughness, also suggested that LiBETI/PC system gave a compact and smooth surface topology during lithium deposition-dissolution cycles. Impedance spectroscopy together with preliminary cycling tests showed that the LiBETI/PC system provides the highest cycling efficiency and improved cycleability among existing electrolyte salt systems in rechargeable battery systems employing lithium metal anodes.