Structural and Electrochemical Studies of 3 V Li x MnO2 Cathodes for Rechargeable Li Batteries

Abstract

X‐ray diffraction studies (XRD) were carried out for the investigation of the synthesis and electrochemical reduction of lithiated . The optimal Li:Mn ratio for a heat‐treated mixture of at 370°C (20 h) with a minimum of impurities, such as or spinel, was shown to be equal to 0.33. A combined application of the open‐circuit voltage (OCV), slow‐scan‐rate cyclic voltammetry and XRD measurements was used for the investigation of the intercalation mechanism. The initial compound, , was shown to undergo only one essential reversible transition during its electrochemical reduction to with a voltage plateau appearing around 3 V. It was conclusively demonstrated that both a thermal synthesis in a certain range of Li:Mn ratio and electrochemical reduction upon cycling result in the phase transition from to spinel. The characteristic feature of the latter reduction process is that it is essentially irreversible and occurs in a thin surface layer of the initial material. The formation of this thin layer seems to be responsible for a drop in the capacity of practical electrodes during their charge‐discharge cycling. A plausible explanation for this effect is discussed.