Layered LixMn1−yLiyO2 intercalation electrodes: synthesis, structure and electrochemistry

Abstract

Layered Li_xMn_1−yLi_yO₂ materials with the O3 (α-NaFeO₂) structure have been synthesised by a low temperature ion exchange route from the corresponding sodium compounds. The effects on electrochemical performance (ability to store reversibly large quantities of lithium and hence charge) and crystal chemistry by partially substituting some of the Mn ions by Li have been investigated by structural and electrochemical techniques as well as chemical analysis. The materials deliver high discharge capacities; in excess of 200 mA h g⁻¹ at 25 mA g⁻¹ or C/8. On the first charge once all Mn is in the 4+ oxidation state further Li⁺ removal occurs by two unconventional mechanisms, one involving oxidation of O²⁻ with subsequent O loss (effective removal of Li₂O) and the other electrolyte oxidation generating H⁺ which exchanges for Li⁺ in the electrode. Although both mechanisms appear to occur at 30 and 55 °C, the former dominates at both temperatures. A mechanism for O loss involving O release at the surface with Mn migration into the bulk in order to fill up vacant octahedral sites in the transition metal layers is proposed, consistent with the neutron diffraction data. Evidence for tetrahedral Li and Li loss from the octahedral sites in the transition metal layers is also presented. The compounds in this study irreversibly transform to spinel-like materials on extended cycling. This is not, however, detrimental to their electrochemical performance and is analogous to the behaviour of other O3 layered lithium manganese oxides.