Thermodynamic stability, crystal structure, and cathodic performance of Lix(Mn1/3Co1/3Ni1/3)O2 depend on the synthetic process and Li content

Li(Mn1/3Co1/3Ni1/3)O-2 were prepared by both solution and solid-state methods and their respective delithiated phases, Li-x(Mn1/3Co1/3Ni1/3)O-2 were prepared by chemical delithiation. All samples were determined to be single phase, and we were able to control the composition. A sample of the compound LiMn1/3Co1/3Ni1/3O2 made by the solution method had a higher discharge capacity and better cycle performance than one obtained by the solid-state synthesis. For these compounds, the enthalpy change per mol of atoms for the formation reaction from simple oxides, Delta H-R, increased with decreasing Li content and materials obtained by the solution synthesis were more thermodynamically stable than those obtained by the solid-state method. The Delta H-R obtained by each synthetic method is a linear function of the Li content. These results are consistent with the lack of any structural changes in the range 0.4 < x <= 1.0 found by the crystal structure analysis. The electron-density images produced by a maximum entropy analysis showed that the 3a site was localized and the covalency of the 3a-6c and 3b-6c bonds decreased with decreasing Li content. The chemical diffusion coefficient of Li+ did not vary with the synthetic method, reflecting the stability of the host Structure. From these results, we suggest that there is a correlation between the thermodynamic stability, crystal structure and cathode performance. (c) 2008 Published by Elsevier B.V.