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)O2 were prepared by both solution and solid-state methods and their respective delithiated phases, Lix(Mn1/3Co1/3Ni1/3)O2 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, ΔHR, 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 ΔHR 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.