New Intercalation Compounds for Lithium Batteries : Layered LiMnO₂

The mechanism of lithium intercalation in layered LiMnO2 has been investigated by combining data from a variety of techniques, including powder X-ray and neutron diffraction, cyclic voltammetry and galvanostatic cycling. Whereas the diffraction data indicate the coexistence-of layered and spinel phases at Li0.5MnO2 after 5 charge (extraction)-discharge (insertion) cycles, the electrochemical data only change significantly on the first charge(extraction), near Li0.5MnO2. A rationale is provided by a model in which, on first extracting 0.5 Li from layered LiMnO2, displacement of Mn ions occurs into the lithium layers, forming regions with the local structure and composition of spinel. This can explain the presence of a 4 V peak in the cyclic voltammogram on the first charge. Long range order only develops on more extended cycling and since this does not alter significantly the Li+ or e(-) energies, the electrochemistry does not change further. Load curves show significant hysteresis and this is linked to a domain-like microstructure with spinel imbedded in layered material. The marked difference between load curves for this material and LiMn2O4 spinel indicates that the former does not convert to 'normal' spinel on cycling. By doping LiMnO2 with as little as 10% Co the cooperative Jahn-Teller distortion due to localised high spin Mn3+(3d(4)) disappears despite the high concentration of Mn3+ and a substantial improvement in the ability to cycle lithium is obtained from 130 mAh g(-1) to 200 mAh g(-1) at 100 mu A cm(-2).