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 + gamma-MnO2 at 370 degrees C (20 h) with a minimum of impurities, such as gamma-be ta-MnO2 or spinel, was shown to be equal to 0.33. A combined applicati on of the open-circuit voltage (OCV), slow-scan-rate cyclic voltammetr y and XRD measurements was used for the investigation of the intercala tion mechanism. The initial compound, Li0.33MnO2, was shown to undergo only one essential reversible transition during its electrochemical r eduction to Li0.75MnO2, with a voltage plateau appearing around 3 V. I t was conclusively demonstrated that both a thermal synthesis in a cer tain range of Li:Mn ratio and electrochemical reduction upon cycling r esult in the phase transition from Li0.3MnO2 to Li0.5MnO2 spinel. The characteristic feature of the latter reduction process is that it is e ssentially irreversible and occurs in a thin surface layer of the init ial material. The formation of this thin layer seems to be responsible for a drop in the capacity of practical electrodes during their charg e-discharge cycling. A plausible explanation for this effect is discus sed.