The layered intercalation compounds Li(Mn_1-yCo_y)O₂: positive electrode materials for lithium batteries

The layered intercalation compounds Li(Mn1-yCoy)O-2; 0 less than or equal to y less than or equal to 0.5 cannot be prepared by conventional solid state reaction but have been synthesized using a solution-based route coupled with ion exchange. A continuous range of solid solutions with rhombohedral symmetry exists for 0.1 less than or equal to y less than or equal to 0.5. Consideration of transition metal to oxygen bond lengths indicates that Mn3+ is replaced by cobalt in the trivalent state, Localized high spin Mn3+ (3d(4)) induces a cooperative Jahn-TelIer distortion in layered LiMnO2, lowering the symmetry from rhombohedral R (3) over bar m to monoclinic (C2/m). Substitution of as little as 10% Mn by Co is sufficient to suppress the distortion in Li-0.9(Mn0.9Co0.1)O-2, whereas half the Li must be extracted from LiMnO2 to achieve a single undistorted rhombohedral phase. On removing and reinserting Li in LiMnO2 only half the quantity of Li (equivalent to a specific charge of 130 mAhg(-1)) may be reinserted on the first cycle; this substantial drop in capacity is eliminated with only 10% Co substitution. The latter material can sustain a large capacity on cycling (200 mAhg(-1)). Higher Co contents have somewhat lower capacities but fade less at higher cycle numbers, The marked improvement in capacity retention of the Co-doped materials compared with pure LiMnO2 may be related in part to the absence of the Jahn-Teller distortion. Electrochemical data indicate conversion to a spinel-like structure on cycling. Such conversion is progressively slower with increasing Co content. Cycling of this spinel-like material is significantly better than Co-doped spinel of the same composition. These materials are of interest as electrodes in rechargeable lithium batteries. (C) 1999 Academic Press.