Lix(Mn1-yCoy)O2 intercalation compounds as electrodes for lithium batteries: influence of ion exchange on structure and performance

LixMn1-yCoyO2 compounds were synthesised by a low temperature route involving ion exchange from sodium precursors. Neutron diffraction confirmed that the structures are layered (space group Rm). Materials synthesised from the precursors by ion exchange using LiBr in ethanol at 80°C possess vacancies on the transition metal sites which pin residual Na+ ions. Such transition metal vacancies and Na+ ions are not observed on refluxing at 160 °C in hexanol. We show that lithium intercalation accompanies the ion exchange process. The presence of Na+ in the Li+ layered materials induces disorder perpendicular to the layers and this has been modelled. The performance of the materials depends on the ion exchange conditions. The y=0.025 compound obtained in ethanol exhibits a particularly high capacity to cycle lithium. The initial discharge capacity is 200 mA h g-1 with a fade rate of only 0.08% per charge/discharge cycle on extended cycling. This performance is delivered despite conversion to a spinel-like phase during cycling and is markedly superior to the cycling ability of directly prepared spinels over a similar composition range.