Nonstoichiometric layered Li_xMn_yO₂ intercalation electrodes: a multiple dopant strategy

Layered Li_xMn_yO₂ materials with the O3 (α-NaFeO₂) structure have been synthesised by a low temperature ion exchange route from the corresponding sodium compounds. Previous studies have concentrated upon the undoped and singly doped families of O3 layered lithium manganese oxides. This is the first report of multiply doped analogues. The effects on electrochemical performance (ability to store reversibly large quantities of lithium and hence charge) and crystal chemistry of partially substituting some of the Mn ions with two dopants have been investigated. A range of structural (X-ray and neutron diffraction) and electrochemical (galvanostatic cycling and a.c. impedance) techniques as well as chemical analyses were utilised. The new materials, Li_xMn _y-2zM_zM′_zO₂ (M, M′ = Li, Cu, Mg, Ni, Zn, Al and Co), offer high capacities in excess of 200 mA h g ^-1 at a rate of 50 mA g^-1 (C/4) with the highest values being for the z = 0.025 series. The main advantage of the multiply doped materials over the previously reported undoped and singly doped O3 layered lithium manganese oxides is their far superior rate capability. All 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 lightly doped O3 layered lithium manganese oxides.