New intercalation compounds for lithium batteries: layered LiMnO2

Abstract

The mechanism of lithium intercalation in layered LiMnO 2 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 Li 0.5 MnO 2 after 5 charge(extraction)-discharge(insertion) cycles, the electrochemical data only change significantly on the first charge(extraction), near Li 0.5 MnO 2 . A rationale is provided by a model in which, on first extracting 0.5 Li from layered LiMnO 2 , 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 LiMn 2 O 4 spinel indicates that the former does not convert to ‘normal’ spinel on cycling. By doping LiMnO 2 with as little as 10% Co the cooperative Jahn-Teller distortion due to localised high spin Mn 3+ (3d 4 ) disappears despite the high concentration of Mn 3+ and a substantial improvement in the ability to cycle lithium is obtained from 130 mAh g –1 to 200 mAh g –1 at 100 µA cm –2 .