New intercalation compounds for lithium batteries: layered LiMnO2

The mechanism of lithium intercalation in layered LiMnO2 has been investiga ted by combining data from a variety of techniques, including powder X-ray and neutron diffraction, cyclic voltammetry and galvanostatic cycling. Wher eas the diffraction data indicate the coexistence-of layered and spinel pha ses at Li0.5MnO2 after 5 charge (extraction)-discharge (insertion) cycles, the electrochemical data only change significantly on the first charge(extr action), near Li0.5MnO2. A rationale is provided by a model in which, on fi rst extracting 0.5 Li from layered LiMnO2, displacement of Mn ions occurs i nto the lithium layers, forming regions with the local structure and compos ition 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 ex tended cycling and since this does not alter significantly the Li+ or e(-) energies, the electrochemistry does not change further. Load curves show si gnificant hysteresis and this is linked to a domain-like microstructure wit h spinel imbedded in layered material. The marked difference between load c urves for this material and LiMn2O4 spinel indicates that the former does n ot convert to 'normal' spinel on cycling. By doping LiMnO2 with as little a s 10% Co the cooperative Jahn-Teller distortion due to localised high spin Mn3+(3d(4)) disappears despite the high concentration of Mn3+ and a substan tial 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).