Structural and Electrochemical Studies of 3 V LixMnO2 Cathodes for Rechargeable Li Batteries

X-ray diffraction studies (XRD) were carried out for the investigation of the synthesis and electrochemical reduction of lithiated MnO2. The optimal Li:Mn ratio for a heat-treated mixture of LiNO3 + y-MnO2 at 370OC (20 h) with a mini-mum of impurities, such as y-4-MnO2 or spinel, was shown to be equal to 0.33. A combined application of the open-cir-cuit voltage (OCV), slow-scan-rate cyclic voltammetry and XRD measurements was used for the investigation of the inter-calation mechanism. The initial compound, Li,,, 33MnO2, was shown to undergo only one essential reversible transitionduring its electrochemical reduction to Li0 5,,MnO2, with a voltage plateau appearing around 3 V It was conclusivelydemonstrated that both a thermal synthesis in a certain range of Li:Mn ratio and electrochemical reduction upon cycling result in the phase transition from Li0 3MnO 2 to Li,,MnO2 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