Ion-exchange properties of P2-NaxMnO2: evidence of the retention of the layer structure based on chemical reactivity data and electrochemical measurements of lithium cells

The ion-exchange properties of two P2-type layered NaxMnO2 bronzes (x=0.6, 0.75) with a differential microstructure were studied in LiCF3SO3 solutions in acetonitrile under ambient conditions. Na+ ions are readily exchanged with Li+, but the reaction causes a significant loss of crystallinity that results in some amorphization. The feasibility of the process increases with increasing structural disorder in the parent compound; conversion, however, is incomplete. The ability of the exchanged material to intercalate water in the air is consistent with the formation of an Li-Mn-O compound that retains the layered framework. Also, the electrochemical data obtained for this material as cathode in lithium cells are consistent with retention of the layer structure and exclude a potential spinel transition due to the ion-exchange reaction. However, the cycling properties of cells made from these layered compounds are quite modest, probably because of the strong structural disorder induced by the lithium reaction