Effects of Surface-Film Formation on the Electrochemical Characteristics of LiMn2O4 Cathodes of Lithium Ion Batteries

LiMn2O4 cathodes of Li ion batteries tend to develop native surface films in carbonate-based electrolytes. By combining dc voltammetry and ac electrochemical impedance spectroscopy, we study how these films react with an electrolyte of LiBF4 in ethylene and diethyl carbonates. We demonstrate that such reactions can affect the measurement of the characteristic electrochemical parameters of the cathode, namely, the intercalation capacitance, the initial capacity loss, and the solid-state diffusion coefficient of Li+. A generalized framework for data analysis, based on the considerations of electrode equivalent circuits, is used to combine the results of the dc and ac measurements. The Li+ conducting film on the cathode introduces an ohmic drop at the electrode surface. Oxidative dissolution of the electrode's native surface film occurs during the first charge cycle. The potential- and cycle-dependent effects of the associated faradaic reactions have been examined under voltage-controlled charge/discharge conditions at different rates.