Li_x(Mn_1-yCo_y)O ₂ 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 R (3) over barm). Materials synthesised from the precursors by ion exchange using LiBr in ethanol at 80 degreesC 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 degreesC 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.