Layered Materials with Metal Site Vacancies

Abstract

Layered Li–Mn–Ni–O materials were analyzed in a region of the phase diagram where a strange bump was seen in the boundary of the layered region. These structures were found to contain a significant amount of metal site vacancies. The maximum vacancy content was found to result in highly ordered monoclinic structures where manganese occupies two of the $\sqrt{3} \times \sqrt{3}$ superlattices on the transition metal layers while the third was randomly filled with nickel, lithium and vacancies. The resulting ordering predicted by a Monte Carlo simulation was consistent with the sharp ordering peaks seen in the XRD patterns. The vacancy concentrations were confirmed by Rietveld refinement, density measurements and redox titration; all of which were in good agreement. The role of the vacancies during electrochemical cycling, if there is one, remains unclear. The material with the greatest possible vacancy concentration, Li[Ni$_{1/6}\Box_{1/6}$Mn$_{2/3}$]O₂, showed electrochemical behavior consistent with lithium-rich layered materials, namely high irreversible capacity associated with the high voltage plateau and voltage fade associated with conversion to spinel. However, this material was not, in fact, lithium-rich given that the vacancies result in lithium occupying only 50 % of metal sites. The vacancy results also demonstrated that there were roughly 1 % vacancies in a stoichiometric lithium-rich material lying along the line from Li₂MnO₃ and LiNi_0.5Mn_0.5O₂. The Monte Carlo simulation suggested that this allows Ni$^{3+}$ to substitute for Mn$^{4+}$ on two of the superlattices. This has never been recognized before and a complete understanding of the starting material is crucial to fully understand the complex electrochemical behavior of the lithium-rich positive electrode materials. The exact shape of the other side of the bump (to the left of Li[Ni$_{1/6}\Box_{1/6}$Mn$_{2/3}$]O₂ in the Gibbs triangles) has not been determined. It is also unclear as to why the bump would be so sharp on both sides and this is worthwhile for further study.